Solidstate dyesensitized and bulk heterojunction solar ...

Review Received: 5 April 2011

Revised: 6 July 2011

Accepted: 7 July 2011

Published online in Wiley Online Library: 23 September 2011

(wileyonlinelibrary.com) DOI 10.1002/pi.3157

Solid-state dye-sensitized and bulk heterojunction solar cells using TiO2 and ZnO nanostructures: recent progress and new concepts at the borderline ¨ Ackermannb Johann Boucle´ a∗ and Jorg Abstract In the field of photovoltaic energy conversion, hybrid inorganic/organic devices represent promising alternatives to standard photovoltaic systems in terms of exploiting the specific features of both organic semiconductors and inorganic nanomaterials. Two main categories of hybrid solar cells coexist today, both of which make much use of metal oxide nanostructures based on titanium dioxide (TiO2 ) and zinc oxide (ZnO) as electron transporters. These metal oxides are cheap to synthesise, are non-toxic, are biocompatible and have suitable charge transport properties, all these features being necessary to demonstrate highly efficient solar cells at low cost. Historically, the first hybrid approach developed was the dye-sensitized solar cell (DSSC) concept based on a nanostructured porous metal oxide electrode sensitized by a molecular dye. In particular, solid-state hybrid DSSCs, which reduce the complexity of cell assembly, demonstrate very promising performance today. The second hybrid approach exploits the bulk heterojunction (BHJ) concept, where conjugated polymer/metal oxide interfaces are used to generate photocurrent. In this context, we review the recent progress and new concepts in the field of hybrid solid-state DSSC and BHJ solar cells based on TiO2 and ZnO nanostructures, incorporating dyes and conjugated polymers. We point out the specificities in common hybrid device structures and give an overview on new concepts, which couple and exploit the main advantages of both DSSC and BHJ approaches. In particular, we show that there is a trend of convergence between both DSSC and BHJ approaches into mixed concepts at the borderline which may allow in the near future the development of hybrid devices for competitive photovoltaic energy conversion. c 2011 Society of Chemical Industry Keywords: hybrid solar cells; nanocrystals; ZnO; TiO2 ; solid-state dye-sensitized solar cells; hybrid bulk heterojunctions; blends; conjugated polymers; dyes; ligands; organic semiconductors

INTRODUCTION

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visible sensitization with molecular dyes. This approach, which uses organic dyes and inorganic metal oxide nanostructures, can be classified as the first hybrid organic/inorganic approach, and demonstrates power conversion efficiencies close to those of silicon-based photovoltaics. Also, photovoltaic cells based on organic semiconductors have attracted much interest in the last two decades,6 – 9 which have seen the advent of the bulk heterojunction (BHJ) concept. Based on the intimate mixing at the nanoscale of an electron donor – usually a p-type conjugated polymer – and an electron acceptor – usually an n-type organic semiconductor such as a fullerene derivative – organic BHJs have shown regular improvements due to a

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∗

Correspondence to: Johann Bouclé, XLIM UMR 6172, Université de Limoges/CNRS, 123 avenue Albert Thomas, 87060 Limoges Cedex, France and Jörg Ackermann, Centre Interdisciplinaire de Nanosciences de Marseille (CINAM), UPR CNRS 3118, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France. E-mail: [email protected]

a XLIM UMR 6172, Université de Limoges/CNRS, 123 avenue Albert Thomas, 87060 Limoges Cedex, France b Centre Interdisciplinaire de Nanosciences de Marseille (CINAM), UPR CNRS 3118, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France

c 2011 Society of Chemical Industry

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In the global context of rising world energy demand and sustainable development, solar energy appears as a relevant alternative to energy derived from fossil fuels. Taking benefit from a quasi-infinite and renewable resource, photovoltaic conversion in particular has been the subject of much research effort worldwide for the demonstration of efficient and realistic alternatives to conventional energy production systems. Taking advantage of the strong development of the semiconductor industry, inorganic photovoltaic cells have dominated the market for several decades, but the use of energy-consuming processing steps forces today the requirement for cheaper alternatives. Among the technologies that have been developed, the use of organic materials, such as conjugated polymers or low-molecular-weight molecules, is very promising for optoelectronics due to the possibility of using soft processing technologies from solution at low temperature.1 – 3 Two main strategies in the field of organic solar cells have been developed as alternatives to standard photovoltaic systems. Historically, the first significant approach was demonstrated by Mickeal Grätzel in the late 1980s, with the so-called dye-sensitized solar cell (DSSC).4,5 This breakthrough resulted from the possibility of obtaining nanostructured titanium dioxide (TiO2 ) electrodes presenting a very large surface area, well adapted for their

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J Bouclé, J Ackermann

After a PhD in Physics in France in 2004, Dr. Johann Boucle´ specialized in hybrid solar cells based on conjugated polymers and metal oxide nanocrystals as research associate at Imperial College London in 2005 with Prof. J. Nelson, then at the University of Cambridge in 2006 with Dr. N. C. Greenham. Since his nomination in 2007 as Associate Professor at the XLIM Institute (CNRS, UMR 6172)/University of Limoges (France), his main interests focus on solid-state dyesensitized solar cells and hybrid optoelectronics. J. Ackermann worked first as engineer at the Bavarian Center for Applied Energy Research, Germany on silicon thin film solar cells until 2000, before he obtained his PhD degree on hybrid organic-inorganic thin film solar cells at the University of Aix-Marseille 2 in France in 2002. After two years as postdoc fellow in the group of Prof. P. Dumas at the CNRS in Marseille, he joined the CNRS as a researcher in 2004. His primary research, located at the Center of Interdisciplinary Center of Nanoscience of Marseille (CINAM CNRS UPR 3118) since 2008, is focusing on hybrid organic-inorganic nanomaterials for low cost photovoltaics and electronics. In 2010, J. Ackermann founded the start-up company Genes’Ink, which develops and commercializes hybrid nanoparticle based inks for printed electronics and photovoltaics.

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better control of the nanophase separation10 and to the development of new materials absorbing a larger fraction of the solar spectrum, such as low-bandgap polymers.11,12 Power conversion efficiencies up to 8.3% have been demonstrated and the technology is now being scaled up for commercial applications by several industrial companies such as Konarka (USA), Solarmer (USA) and Heliatek (Germany). In all organic device geometries, the most employed organic electron acceptors are fullerene derivatives, as they present a very high electron affinity and a strong ability for charge transport. However, one of the main difficulties related to high photon-to-electron conversion yields is the necessity to achieve an interpenetrated and percolating network of both donor and acceptor materials within the exciton diffusion length of the polymer donor, typically of the order of a few tens of nanometres. In this context, the advent of inorganic nanocrystals, having original and controllable electronic and optical properties,13,14 has rapidly resulted in the demonstration of several hybrid devices in which the organic acceptor material is replaced by inorganic semiconducting nanoparticles. In particular, hybrid BHJs based on the association of transition metal oxide compounds with organic semiconductors have shown promise for more than two decades, and among the metal oxides, TiO2 and zinc oxide (ZnO) are at the core of intense research efforts concerning photovoltaic energy conversion (Fig. 1). In this context, we propose a synthetic review of the main achievements of solid-state hybrid devices based on TiO2 and ZnO nanoparticles or nanostructures with organic dyes and semiconductors for photovoltaic energy conversion. DSSCs have been much investigated and all device components have been

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Figure 1. Evolution of the number of publications devoted to TiO2 and ZnO for photovoltaic energy conversion, either in inorganic or hybrid device concepts. (Source: ISI Web of Science, Thomson Reuters).

studied and improved in turn, from the metal oxide electrode, to the organic dye to the electrolyte. Several reviews have been published on various aspects of device functions, including general trends for material and commercial developments for DSSCs,15 – 18 or, more specifically, concerning nanostructured metal oxides;19 – 22 molecular dyes;23 – 25 electrolytes;26 or electron dynamics and charge transport.27 – 30 Among all DSSC components, the nanostructured metal oxide electrode is crucial, as its morphology drives many physical processes that control the overall device performance: the light-harvesting properties are directly dependent on the amount of interface available for dye grafting; the generation yield of free charge carriers, and especially electrons, is driven by the electronic configuration of the metal oxide; and the collected photocurrent is limited by the ability of photogenerated charges to flow in the percolating nanostructured electrode. Important developments have been reported on the nanostructuring of DSSC photoanodes since the use of a nanocrystalline porous film processed from randomly organized TiO2 nanocrystals. Especially, much work has targeted the achievement of an ideal interdigitated structure, taking benefit from the important advances in processing by soft techniques of inorganic materials such as TiO2 and ZnO. Another important aspect of DSSCs relates to the possibility of the realization of solid-state cells, by replacing the conventional liquid electrolyte by solid organic hole transporters. In parallel with the development of DSSCs, metal oxide nanostructures have been rapidly developed for hybrid heterojunction solar cells based on conjugated polymers.31 – 34 A critical issue, however, was associated with an additional constraint resulting from the excitonic nature of the photoexcited states in organic materials. Indeed, photon absorption in organic semiconductors creates electron–hole pairs bound at room temperature by a nonnegligible coulombic interaction. As a consequence, free charge carriers can only be produced if the formed excitons can reach an interface with an acceptor material before recombining. Thus, the first hybrid approaches, which were based on bilayer devices containing a planar inorganic/organic interface, gave poor conversion efficiencies. As a consequence, two different concepts of hybrid BHJs were developed. The first concept uses porous nanostructured metal oxide layers as electron acceptors filled with a p-type polymer. This allows one simultaneously to increase the hybrid interface while controlling the morphology of the active layer at


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Solid-state DSSCs and BHJ solar cells using TiO2 and ZnO

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Figure 2. Schematic of the two hybrid solar cell approaches based on the metal oxides TiO2 and ZnO described in the present review. Both solid-state DSSC and polymer/metal oxide BHJ approaches are considered, and novel mixed device concepts exploiting both strategies are discussed.

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dye-sensitized metal oxide DSSC photoanodes. Considering that these organic components present in addition significant optical absorption, we emphasize that such solid-state DSSC approaches are very similar to hybrid BHJ solar cells that use interfacial modifications using dyes. Although interfacial modifications are well known as a tool to control the electron dynamics at the organic/inorganic interface,56,57 these new strategies illustrate that additional organic components can also influence the light-harvesting properties of active layers. Finally, these novel ‘borderline’ strategies exploit the specificities and advantages of both DSSC and hybrid BHJ approaches and we believe this trend to be a very promising strategy towards the demonstration of efficient and competitive hybrid solar cells. In the context of actual hybrid photovoltaic research, this review presents recent progress and new concepts of hybrid solar cells based on metal oxide nanostructures, incorporating dyes and conjugated polymers. By focusing our discussions on TiO2 and ZnO nanostructures only, we will point out their specificities in common hybrid geometries and give an overview of new borderline concepts, which couple and exploit the main advantages of both DSSC and BHJ approaches (Fig. 2). We emphasize that only solidstate approaches will be presented, as they reduce the complexity for cell assembly and manufacturing. Conventional DSSCs that use a liquid electrolyte will thus not be considered specifically, but only recalled when required. The next section of this review presents the most significant achievements of solid-state DSSC approaches. The main features of nanostructured TiO2 and ZnO metal oxides will be pointed out, from the historical randomly organized nanocrystalline electrodes to more advanced hierarchically or vertically oriented electrodes. We then review the association of metal oxides with conjugated polymers, starting from bilayer devices, then



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the nanoscale. In the second concept, semiconducting nanocrystals are blended from solution with a polymer donor to form the required phase-separated bi-continuous network compatible with high charge separation yields.35 – 37 Here, inorganic nanocrystals take advantage of tunable electronic properties, as well as soft synthesis procedures from solution. Recently, highly efficient hybrid devices presenting power conversion efficiencies up to 3.13% have been reported using CdSe nanocrystals blended with a low-bandgap polymer.38 Although various inorganic nanocrystals such as CdSe,39 – 41 CdTe,42,43 silicon44 or PbS45,46 have been studied intensively in the field of hybrid BHJ solar cells, metal oxide nanoparticles, and especially TiO2 and ZnO, are of particular interest due to their ease of fabrication, non-toxicity and relatively low production costs. Again, several reviews have been devoted to the main aspects of hybrid BHJ approaches,35 – 37,47 – 49 including metal oxide/polymer solar cells.31 – 34 For both approaches of hybrid heterojunction solar cells, the nature of surface ligands was found to be crucial for device performance.50 – 53 This is particularly the case for nanocrystal/polymer blends, as the ligand has to allow efficient particle dispersion in organic solvents without preventing charge separation and transport in the final active layer. In general, ligand exchange procedures are found to significantly improve charge separation yields while reducing interfacial recombination. Based on this observation, some recent work goes even further by using dyes not only as interfacial modifiers, but also as additional optically active components, either at the metal oxide particle surface or blended in a metal oxide/polymer composite, in an attempt to increase light absorption towards higher efficiencies.54,55 In the case of solid-state DSSCs, much research effort has been focusing on the replacement of the liquid electrolyte by a solid-state hole transporter. In this context, conjugated polymers and oligomers have been introduced in

www.soci.org discussing nanostructured metal oxide/polymer systems and metal oxide/polymer blends. Finally, we give an overview of recent developments of hybrid solar cells at the borderline of both solid-state DSSC and hybrid BHJ devices. In particular, we focus on strategies that demonstrate the contribution of an additional organic component – either conjugated polymer or dye – to the generation of photocurrent.

TIO2 AND ZNO METAL OXIDES FOR SOLIDSTATE DSSC APPROACHES

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The first evidence of the photoactivity of wide-bandgap metal oxide electrodes such as TiO2 was reported more than thirty years ago for the electrolysis of water.58 It was only a few years later that efficient visiable sensitizations of metal oxide anodes were demonstrated using transition metal complexes adsorbed on metal oxide surfaces.59,60 Highly efficient photoinduced electron transfers were evidenced in these systems,61,62 opening the possibility of using photoelectrochemical cells for the conversion of photons into electrons. At the same time, and to further enhance the light-harvesting properties – hence charge transfer rates – of dye-sensitized TiO2 films, nanostructured electrodes of very large surface area were developed from colloidal nanocrystals,63 leading to the well-known demonstration of the DSSC concept.4 DSSCs, also referred to as photoelectrochemical cells or ‘Grätzel’s cells’, consist of a nanostructured TiO2 electrode several micrometers thick providing enough specific area for the chemisorption of a light-absorbing dye monolayer (Fig. 2 shows a schematic structure of a DSSC).5 Following photoexcitation of the dye, ultrafast electron transfer to the metal oxide conduction band is observed within several hundreds of femtoseconds.64 Current collection occurs via charge transport through the percolated metal oxide network, where trapping/detrapping mechanisms are important limiting processes.27,65 The dye is regenerated through electron exchange reactions with a redox couple in a liquid electrolyte, which was initially based on the iodide/triiodide couple. The overall cell efficiency is driven by the kinetics involved, from electron injection (ca 100 fs), to dye regeneration (ca 100 ps), to electron transport (100 µs to 10 ms), to charge recombination (100 µs to 1s).66,67 Especially, limiting charge recombination is crucial to ensure efficient current collection. Today’s DSSC power conversion efficiencies reach more than 11% for laboratory cells,68,69 and over 8% for sub-modules.70,71 Since their discovery by Grätzel, DSSCs have rapidly showed their potential for commercial applications, compared to inorganic solar cells, due to the possibility of using soft printing technologies in ambient conditions,72 – 74 and due to their significant performance especially in diffuse illumination conditions both indoors and outdoors. DSSC products are now being commercialized by G24 Innovations (Cardiff, UK) which manufactures modules, using roll-to-roll technologies, on flexible substrates for various applications. Nevertheless, replacement of the volatile liquid electrolyte remains a major issue of DSSCs to significantly reduce inherent leakages and improve device lifetimes, while reducing the number of processing steps involved. Several approaches have been proposed using either quasi-solid75 – 78 or solid-state components.18,77,79 In particular, solid-state approaches demonstrate a strong potential for industrial scale-up at low cost. In 2010, Oxford Photovoltaics (Oxford, UK) was created in order to develop and commercialize solid-state DSSC cells and modules, illustrating the true potential of DSSC technology for solar energy conversion.


J Bouclé, J Ackermann One of the main differences between DSSCs and other types of solar cells is that photon absorption and charge transport are distinct processes. In this context, the nanocrystalline metal oxide photoanode is a major component, which drives device performance through light harvesting, charge separation, charge transport and interfacial recombination. Although several alternative metal oxide compounds20,22 have been regularly assessed, such as SnO2 ,80,81 Nb2 O5 ,82,83 Zn2 SnO4 ,84,85 TiO2 still demonstrate the best device efficiencies so far, either in liquid or solid-state geometries. Moreover, having better transport properties, ZnO has also been much studied. In the two following subsections, we emphasize the main specificities of TiO2 and ZnO for DSSC applications by focusing on approaches that use a solid-state organic electrolyte. We initially discuss conventional nanocrystalline porous electrodes before discussing more recent approaches based on various electrode morphologies. TiO2 nanocrystals for solid-state DSSCs TiO2 has rapidly shown its potential for DSSCs for several reasons. Being non-toxic, abundant, biocompatible and cheap, TiO2 is a 3d transition metal oxide showing a different parity of electrons in the valence and conduction bands (hybridization of oxygen 2p states with titanium 3p states in the valence band, and pure 3d states in the conduction band).22 This electronic configuration reduces the probability for electron–hole recombination, favouring efficient current collection. Among the three stable TiO2 polymorphs, anatase and rutile are more easily synthesized in the laboratory than brookite, and both have been mostly implemented in DSSCs. Slight improvements in specific area and charge transport,86 as well as a slight increase of the electron Fermi level, make anatase more suitable than rutile for solar energy conversion, although very close performances are observed for both polymorphs. For both conventional DSSCs based on liquid electrolytes and solid-state DSSCs, TiO2 porous photoanodes are basically processed starting from nanocrystals synthesized by hydrothermal methods,87 where particle growth occurs in an autoclave under 70 atm (7.1 MPa), at 200–250 ◦ C for 12 h. Alternative synthesis methods have been reported,88,89 such as flame or aqueous synthesis,90,91 in order to reduce the number of steps involved or to achieve crystal growth at lower temperatures. Laser pyrolysis has also been recently used,92 enabling the production in large quantities of TiO2 nanocrystals presenting well-controlled properties suitable for DSSC applications. Such considerations are crucial for industrial scale-up at low cost. The typical nanocrystal morphology, illustrated for particles grown under hydrothermal conditions in Figs 3(a) and (b), exhibits mainly facets with (101) orientation (lowest surface energy for anatase).93,94 Subsequently, re-dispersion of the nanocrystals, evaporation and conversion of the solvent to ethanol, followed by addition of ethyl cellulose and/or poly(ethylene glycol) lead to a metal oxide paste suitable for screen-printing deposition.95,96 A short sintering step at 500 ◦ C is required in order to induce the electrical interconnection of particles and remove the organic phase from the film.87,95 In this conventional process, surfactants are not critical components as they are mostly removed after this calcination step and do not interfere in the final solar cell under normal operation. The situation is very different when TiO2 nanocrystals are self-organized during film deposition, or blended with a p-type semiconductor, as will be discussed in that part of the review devoted to polymer/nanocrystal blends. Nanocrystalline electrodes (Fig. 3(c)) can develop high surface areas up to 150 m2 g−1 , compatible with particle mean diameters


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Figure 3. Typical morphology of anatase TiO2 nanocrystals synthesized under hydrothermal conditions at 240 ◦ C in nitric acid solution observed using transmission electron microscopy at (a) low and (b) high magnification. (Reprinted with permission from Wu et al.94 Copyright 2007 American Chemical Society.) (c) Resulting morphology of a typical nanocrystalline porous TiO2 electrode after sintering at 500 ◦ C. (Reprinted from Barbé et al.87 with permission from John Wiley & Sons. Copyright 1997).

Table 1. Summary of photovoltaic parameters (short-circuit current density JSC , open-circuit voltage VOC , fill factor FF and power conversion efficiency η) corresponding to conventional solid-state DSSCs based on TiO2 and ZnO

Conventional solid-state DSSC approaches FTO/porous TiO2 /N3/CuSCN/Au FTO/porous TiO2 /N719/spiro-OMeTAD/Au FTO/porous TiO2 /D102/spiro-OMeTAD/Au FTO/porous TiO2 /C220/spiro-OMeTAD/Au FTO/porous ZnO/P-Ru dyea /CuSCN/Au FTO/porous ZnO/N719/spiro-OMeTAD/Au

JSC (mA cm−2 )

VOC (V)

FF (%)

η (%)

Irradiance (mW cm−2 )

Ref.

7.80 4.60 7.70 9.73 4.50 2.05

0.60 0.93 0.87 0.88 0.55 0.39

44 71 61 71 57 53

2.10 3.20 4.10 6.08 1.50 0.50

100 100 100 100 100 100

111 115 114 120 121 122

0.87 0.70 0.84 0.57 0.47

54 72 63 42 57

1.70 1.67 4.90 0.25 0.14

100 100 100 100 100

123 124 125 126 127

Hierarchically aligned nanostructures for solid-state DSSC approaches FTO/gyroid TiO2 /D149/spiro-OMeTAD/Ag 3.66 FTO/nt-TiO2 b array/C203/spiro-OMeTAD/Au 3.30 FTO/TiO2 3D fibers/C218/spiro-OMeTAD/Au 8.50 FTO/ZnO array/D149/spiro-OMeTAD/Au 1.05 AZOc /ZnO array/N719/spiro-OMeTAD/Au 0.20 a

Phosphonated ruthenium polypyridyl dye, see O’Regan et al.121 ‘nt’, nanotubes. c Aluminium-doped zinc oxide. b

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properties of the interface.105 Several strategies can be used to reduce these loss mechanisms, such as interfacial modifications of TiO2 with insulating or wider bandgap materials.106 Considering the focus of this review on metal oxides for photovoltaic energy conversion, the reader is referred to reports dedicated to these aspects for further details.22,107 In solid-state DSSCs, the liquid electrolyte is replaced by a p-type semiconductor, which is oxidized to regenerate the photoexcited dye and transports the positive charges to the electrode. The first attempts were made in the late 1990s, using solutionprocessed inorganic compounds such as p-type copper iodide (CuI)108,109 or CuSCN.110,111 Moderate efficiencies up to 2.1% were demonstrated due to the difficulty of completely filling a thick porous metal oxide electrode of several micrometers in thickness (Table 1). Polypyrrole has also been assessed as a hole transporter due to the possibility of photoelectrochemically polymerizing it directly in the pores of a dye-sensitized TiO2 electrode.112,113 A major breakthrough was reported in 1998 by Grätzel’s group using an organic p-type molecular glass (2,2 ,7,7 -tetrakis-(N,N-dip-methoxyphenylamine)-9,9 -spirobifluorene or spiro-OMeTAD) associated with a ruthenium complex (N3).114 The demonstration



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of 10 to 20 nm. A high porosity of the film is evidenced, with a pore volume to film volume ratio in the range 0.5–0.7, highly favourable for dye adsorption. One important aspect of nanocrystalline TiO2 electrodes relates to the strong influence of the electronic states on both electron transport and charge recombination, which was initially evidenced and studied in liquid DSSCs27,97,98 before being characterized for solid-state approaches.99 – 103 In DSSCs, TiO2 nanocrystals present bulk-like electronic properties as the mean particle diameter is usually much larger than the first exciton Bohr radius (ca 1.5 nm). However, nanometric size effects (dangling bonds, presence of impurities, local disorder, etc.) favour the occurrence of numerous ‘trap’ sites for electrons, often located near the particle surface and associated with sub-bandgap states below the TiO2 conduction band edge. Multiple trapping/detrapping processes have been modelled using Monte Carlo approaches, for example, to rationalize the electron diffusion mechanisms in the percolating metal oxide network.27,65,97,104 Other mechanisms have been theoretically described in order to understand and address the main limitations to device performance, such as the adsorption geometry of dyes on the TiO2 surface and the optoelectronic

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of a significant power photon-to-electron conversion yield of 33% was attributed to the ability of the spiro-OMeTAD glass to infiltrate the dye-sensitized electrode. The use of additional doping agents, such as lithium salts or silver ions, rapidly pushed the efficiencies up to 3.2%,115 placing solid-state DSSCs as realistic alternatives to organic BHJs. However, solid-state approaches require thinner mesoporous metal oxide electrodes to achieve complete pore filling, which is detrimental to light harvesting as less dye can absorb the incident photons. This constraint has been overcome by using organic indoline dyes presenting very high molar extinction coefficients (>50 × 103 L mol−1 cm−1 ) compared to ruthenium complexes (ca 10 to 15 × 103 L mol−1 cm−1 ), enabling the use of only 1.5–2 µm thick porous TiO2 electrodes.116 A high power conversion efficiency of 4.1% was demonstrated due to the high optical absorption of the cell, associated with JSC = 7.7 mA cm−2 , VOC = 0.87 V and fill factor (FF) = 61%. Since then, many improvements of solid-state DSSC efficiencies have been reported by systematically improving the spiro-OMeTAD pore filling,57,117 and using new ruthenium118 or organic dyes119 presenting very high extinction coefficients. A certified 6.08% efficient solid-state DSSC, with JSC = 9.74 mA cm−2 , VOC = 0.88 V and FF = 71%, was reported in 2011, setting a new threshold in the development of DSSCs.120 From the metal oxide point of view, device performance is directly associated with the ability of electrons to diffuse over large distances in the inorganic network. The diffusion length of electrons results from a balance between their recombination with dye cations or the oxidized electrolyte, and their diffusion coefficient in the TiO2 network. Considering this crucial role, much effort has been devoted to improve the charge transport properties in TiO2 by tuning the morphology of the metal oxide nanostructure, in an attempt to reach an ideal interdigitized n-type–dye–p-type junction. These developments, which have been very intense in the last few years, allow the development of strategies for the self-organization of nanocrystals, as well as strategies for the growth of anisotropic nanoparticles or vertically aligned nanorods and nanowires, pushing ahead the knowledge of titanium oxide materials. Self-organization of TiO2 nanocrystals was first reported using block copolymers as structure-templating agents,128,129 leading to highly porous TiO2 networks, one of the main difficulties being to produce thick porous electrodes. Such a strategy has been employed in solid-state DSSC approaches, as thinner porous films usually favour pore infiltration by a molecular glass. Fine control of the TiO2 metal oxide structure was demonstrated by Crossland et al. using diblock copolymers, leading to thin gyroid mesostructured TiO2 electrodes (Fig. 4(a)).123 By using only 400 nm of such an organized porous layer, a high power conversion efficiency without liquid component of 1.7% under full sunlight has been reported showing the relevance of the approach (Table 1). In a related strategy for the control of electrode morphology, nanocrystalline TiO2 networks have been prepared using spherical polyelectrolyte brushes as templates. Fine control of the pores and the wall thickness can be achieved, leading to preliminary 0.8% efficient solid-state cells.130 Concerning the growth of elongated nanocrystals or nanowires, two main effects are anticipated. Better charge transport is expected along a nanowire or nanorod long axis compared to a spherical particle. Moreover, the use of longer nanocrystals in a porous electrode is associated with less interparticle barriers for electron transport, leading, in principle, to higher photocurrents. Several reviews have been published


J Bouclé, J Ackermann on these aspects for liquid DSSCs,19,131 – 134 and we give a brief overview of the recent efforts that focus on solid-state approaches. The first significant report on the use of TiO2 nanotubes for solid-state DSSCs was made in 2007 using a plasticized polymer electrolyte.135 A cell with an efficiency of 4% was demonstrated, with the benefits of a high surface area and easy polymer electrolyte processing. Another attempt at improved charge collection in TiO2 was made by Grätzel’s group using highly ordered TiO2 nanotube arrays grown on fluorine-doped tin oxide (FTO) substrates by potentiostatic anodization of titanium.124 A promising efficiency of 1.7% was demonstrated using an organic dye (C203) and spiro-OMeTAD, the high FF observed for this cell being a direct consequence of a flat top surface reducing the risk for current shunts (Fig. 4(b)). In a related approach using TiO2 nanotube arrays treated by TiCl4 , Bandara et al. demonstrated a cell with an efficiency of 1.94% using a ruthenium antenna dye.136 Finally, highly efficient solid-state cells based on spiroOMeTAD and self-assembled three-dimensional fibrous networks of TiO2 nanowires have been reported. This structure offers a high roughness factor, light scattering and much faster electron transport than conventional nanocrystalline electrodes, leading to a device with an impressive 4.9% efficiency under full sunlight.125 The major advantage of the fibrous network is associated with a longer electron lifetime compared to conventional particle electrodes (Fig. 4(c)), illustrating the possibility of controlling the electron dynamics through the morphology of the metal oxide. Several alternative strategies to improve TiO2 -based DSSC photoanodes have been reported in the last decade, including the use of more exotic TiO2 nanostructures (nanospheres, nanoflowers, dendritic nanorods, etc.) or core–shell structure geometries.20 However, the best device efficiencies remain associated with the historical randomly organized isotropic TiO2 nanocrystalline electrodes. Although the theoretical efficiency limit for TiO2 -based liquid DSSCs has not been reached so far,27 alternative metal oxide electrodes have been assessed to replace TiO2 . In this context, ZnO is probably the most employed metal oxide after TiO2 for photovoltaic applications.21,137 We give in the following subsection a brief overview of the use of ZnO for solid-state DSSCs. ZnO in solid-state DSSCs ZnO is a wide-bandgap (ca 3.3 eV) binary transition metal oxide that preferentially crystallizes in the hexagonal wurtzite phase. The conduction band edge of ZnO is found to be very close to that of TiO2 (ca −4.4 eV). Moreover, having a higher electron mobility in the bulk (200–300 cm2 V−1 s−1 ) than TiO2 (0.1 cm2 V−1 s−1 ),21 ZnO has been much investigated as a relevant alternative to TiO2 for DSSC applications. The numerous studies reported in the last few decades illustrate this trend.21,137 – 139 The first visible sensitization of ZnO was reported four decades ago using merocyanine dye,140 and the first photovoltaic properties of ZnO-based photoelectrochemical devices were reported slightly later.141 – 143 Since then, the electronic properties of nanostructured ZnO electrodes and their operation in DSSCs have been studied and compared to TiO2 .144,145 In particular, electron lifetimes are found to be longer in nanocrystalline ZnO films than in TiO2 (Fig. 5(a)), as there are fewer traps in the nanostructure. Charge recombination is found to be slightly accelerated in ZnO compared to TiO2 electrodes in reverse bias conditions, resulting in a significantly smaller open-circuit voltage.145,147 Moreover, despite improved transport properties, ZnO remains difficult to sensitize due to the intrinsic instability of the metal oxide in acidic


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Figure 4. (a) SEM images of a gyroid TiO2 mesoporous electrode grown from diblock copolymer templating at high (left) and low (right) magnification. (Adapted with permission from Crossland et al.123 Copyright 2009 American Chemical Society.) (b) Current–voltage characteristics in the dark and under simulated solar emission (AM 1.5G) of a TiO2 nanotube solid-state DSSC with C203 dye. (Reproduced from Chen et al.124 by permission of the Royal Society of Chemistry.) (c) Comparison of electron lifetime estimated from electrochemical impedance spectroscopy between a conventional TiO2 nanoparticle film (filled squares) and a three-dimensional fibrous network (filled circles) as a function of potential. (Adapted with permission from Tétreault et al.125 Copyright 2010 American Chemical Society).

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sensitized by the ruthenium N719 dye, demonstrated modest efficiencies up to 0.50%. In a related approach, highly porous ZnO films prepared at low temperature have been associated with an organic indoline dye (D149) and CuSCN.151 Suitable pore filling was observed using the impregnation method, leading to cells with efficiencies of 0.46%. Complex sensitization procedures as well as low open-circuit voltages are the main factors limiting device performance. In addition to nanocrystalline ZnO films processed from randomly organized nanocrystals, the regular progress made on the preparation of ZnO nanostructures from soft synthesis techniques was rapidly exploited to convert solar photons into electrons using more controlled nanostructures. As for TiO2 , nanorods, nanotubes and novel hierarchically grown ZnO nanostructures were implemented as DSSC photoanodes, in order to better exploit the transport properties of bulk ZnO.20 – 22,137 In particular, ZnO nanowires were first demonstrated as a relevant alternative to nanoparticle films in liquid DSSCs due to higher electron diffusion coefficients (by two orders of magnitude), and better ability for charge collection.20,152 Similarly, ZnO nanotubes obtained by chemical etching153 or through filling of an anodic alumina template154 are expected to show beneficial effects associated with a large amount of dye loading at both the outer



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dye solutions.148 Consequently, DSSCs based on nanocrystalline ZnO films demonstrate relatively modest efficiencies compared to TiO2 , but the possibility to use various soft synthetic procedures such as sol–gel hydrolysis at low temperature has driven much effort towards their development. A significant 5% efficient liquid cell based on a nanocrystalline ZnO layer was demonstrated on glass substrate using the N719 ruthenium sensitizer (Table 1).149 Very recently, the use of a low-temperature compression method for the preparation of ZnO porous films, associated with an organic sensitizer having a high extinction coefficient (D149), led to efficient and flexible DSSCs with power conversion efficiencies just over 4%.150 Regarding solid-state approaches, issues similar to those for TiO2 have to be addressed. The first significant report on the use of electrodeposited porous ZnO electrodes for solid-state DSSCs was made in 2000 by O’Regan et al. using CuSCN as solid electrolyte and a ruthenium dye.121 Taking advantage of a columnar electrode structure, CuSCN was directly grown in the ZnO pores by electrodeposition, leading to a solid-state cell of 1.5% efficiency under full sunlight. More recently, nanocrystalline ZnO electrodes have been processed from colloidal nanocrystals easily grown at low temperature from solution.122 Using the molecular hole transporter spiro-OMeTAD, the resulting porous electrodes,

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J Bouclé, J Ackermann

Figure 5. (a) Comparison of electron lifetime, estimated by transient photovoltage decay, between nanocrystalline TiO2 and ZnO photoanodes as a function of open-circuit potential. The electrolyte is a conventional liquid redox couple in this case. (Reprinted with permission from Quintana et al.145 Copyright 2007 American Chemical Society.) (b) SEM image of a ZnO nanowire array grown by electrodeposition. (Reproduced from Plank et al.146 with permission from the authors and from IOP Publishing Ltd).

and inner nanotube surfaces, as well as light-scattering effects. Although nanotubes or nanowires have not been successful in improving the performance of ZnO-based DSSCs so far, significant improvements have been demonstrated using hierarchically grown ZnO nanostructures. The presence of an organic species during electrodeposition induces self-organization, leading to the formation of mesoporous nanostructures. Following this strategy, a liquid DSSC of 5.6% efficiency was demonstrated using electrodeposited films processed from ZnO/dye hybrid precursors.155 Another important breakthrough was achieved by using solution-processed hierarchical aggregates consisting of 15 nm ZnO nanocrystals assembled in larger clusters (100 to 500 nm diameter), as depicted. Important light-scattering effects, associated with a large surface area, lead to a power conversion efficiency of 6.1%.156 Concerning solid-state approaches, hierarchical ZnO electrodes have also been widely investigated in the last few years.157 – 159 A significant power conversion efficiency of 0.25% was reported using electrodeposited ZnO nanowire arrays with an organic dye (D149) and spiro-OMeTAD (Fig. 5(b)), which was further improved to 0.71% using an additional ZrO2 capping.126,146 A similar strategy was reported for the growth, using a catalyst-free vapour-transport method, of highly crystalline ZnO nanowire arrays directly on a sputtered aluminium-doped ZnO electrode, presenting limited device efficiencies up to 0.14% (Table 1).127 These specific examples illustrate the important advances that have been made regarding the self-assembly of metal oxide nanostructures. In particular, a very wide range of synthesis techniques can be used to further improve the operation of hybrid devices. Beside DSSC approaches, these tools have also been widely applied to hybrid metal oxide/conjugated polymer devices. These developments are discussed in the next section

HYBRID BHJ SOLAR CELLS BASED ON TIO2 AND ZNO AS ACCEPTORS

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The possibility of solution processing combined with a large choice of possible chemical structures makes semiconducting polymers promising materials for low-cost printed photovoltaics. Hybrid heterojunction solar cells aim to combining the above mentioned advantages of polymers with the electronic performance, nanostructure and stability of metal oxide semiconductors. Metal oxides


are high-bandgap semiconductors, which are transparent in the visible, and thus light absorption in hybrid heterojunction devices mainly occurs in the organic phase. Photoexcitation of organic semiconductors leads to the creation of bound electron–hole pairs, so called excitons. Photocurrent generation in hybrid solar cells occurs via exciton dissociation in the vicinity of the hybrid interface followed by injection of the free hole and electron into the polymer and the metal oxide, respectively. Therefore hybrid heterojunction solar cells are excitonic devices and as a consequence have to take into account the limited exciton diffusion length, which is typically ca 5–10 nm.160 – 163 There are three approaches to hybrid solar cells (Fig. 2), where the organic acceptor is replaced by a metal oxide n-type semiconductor: • Planar hybrid heterojunction bilayers. The corresponding solar cells have in general a low efficiency, but give important information about charge carrier generation processes and hybrid interface physics. • Nanostructured metal oxide/polymer heterojunctions. The aim of this approach is to create a BHJ in which the electron acceptor phase is a nanostructured electrode with a characteristic dimension in the range of the exciton diffusion length of the organic donor. A disadvantage of this structure is the need to grow metal oxide nanostructures on substrates before processing the solar cells, which may be difficult to realize on a large area and thus may present a limitation for low-cost solar cells. • BHJs based on blends of a polymer with metal oxide nanoparticles. The blends are obtained either by mixing metal oxide nanoparticles with a polymer or by in situ synthesis of the nanoparticles in the polymer layer. Solar cells based on this approach can be totally solution processed, which is the major advantage of this approach. However, the general problem of BHJs relates to morphology control of the blend, which may limit the efficiency of devices. Photocurrent generation inside the three device types is controlled by the exciton diffusion length of the organic phase. Therefore, the thickness of the polymer layer and the polymer domain size should be in the range of the exciton diffusion length. This fact limits the efficiency of bilayer devices because light absorption of the polymer layer leads to photocurrent generation only within a few nanometres from the interface. In the case


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Table 2. Summary of photovoltaic parameters for the main polymer/metal oxide approaches based on bilayer or BHJ geometries, including porous metal oxide/polymer systems and polymer/nanocrystal blends JSC (mA cm−2 )

VOC (V)

FF (%)

η (%)


Ref.

Bilayer device structures ITO/TiO2 /MEH-PPV/Hg ITO/TiO2 /M3EH-PPV/Au FTO/TiO2 /P3HT/Ag FTO/TiO2 (N3+TBP)/P3HT/Ag ITO/Zn1−x Mgx O/P3HT/Ag (x = 0.25) FTO/TiO2 /P3HT/Ag

0.32 1.20 0.85 1.86 1.27 1.22

0.92 0.65 0.60 0.57 0.70 0.72

52 40 67 57 56 51

0.15 0.40 0.34 0.60 0.49 0.45

100 80 100 100 100 100

162 167 56 56 168 169

Nanostructured metal oxide/polymer approaches FTO/ns-TiO2 a /P3HT/Ag ITO/TiO2 /MEH-PPV/Au ITO/ns-TiO2 /(TPD)-MEH-PPV/PEDOT : PSS/Au eITO/ZnO fibers/P3HT/Ag ITO/nr-ZnO array/Z907/P3HT/PEDOT : PSS/Au ITO/ns-ZnO/P3HT/Au

1.22b 3.25 2.10 2.17 1.73 2.18

0.72 0.86 0.64 0.44 0.30 0.36

51 28 43 56 39 44

0.45b 0.71 0.58 0.53 0.20 0.35

100 100 100 100 100 100

169 170 171 172 173 174

Nanocrystal/polymer blends ITO/PEDOT : PSS/nc-TiO2 c :P3HT/Al ITO/PEDOT : PSS/nr-TiO2 d :P3HT/Al ITO/PEDOT : PSS/nr-TiO2 :N3:P3HT/Al ITO/PEDOT-PSS/TiO2 -(oligo-3HT-(Br)COOH/Al ITO/PEDOT : PSS/nr-TiOx :MDMO-PPV/LiF/Al ITO/PEDOT : PSS/nc-ZnO : MDMO-PPV/Al ITO/PEDOT : PSS/nc-ZnO : P3HT/Al ITO/PEDOT-PSS/P3HT : ZnO/Li/Al ITO/PEDOT : PSS/ZnO : P3HT/Al ITO/PEDOT : PSS/ZnO : P3HT-Ee /Al ITO/nc-ZnO/nc-ZnO : P3CT/PEDOT-PSS/Ag

2.76 2.62 4.40 2.82 0.60 2.40 2.19 3.50 5.20 2.10 1.00

0.44 0.69 0.78 0.65 0.52 0.81 0.69 0.83 0.75 1.02 0.52

40 63 65 64 42 59 55 50 52 40 35

0.42 1.14 2.20 1.19 0.20 1.60 0.92 1.4 2.0 0.83 0.18

100 100 100 100 60 71 75 50 50 50 100

175 176 53 177 178 179 180 181 182 183 184

a

‘ns’, nanostructured metal oxide. Estimated by the authors from the IPCE data. c ‘nc’, isotropic nanocrystals or nanoparticles. d ‘nr’, nanorods. e P3HT-E, an ester-functionalized P3HT; see Oosterhaut et al.183 for details. b

of hybrid solar cells using nanostructured inorganic acceptors, theoretical investigations have shown that size and spacing of the nanostructures must be of the order of the exciton diffusion length.164 In the case of metal oxide/polymer blends, the domain size of the organic donor phase should be in the same range. Hybrid polymer solar cells that use metal oxides as acceptors have been recently reviewed164,165 and another publication is dedicated to this subject.166 Therefore, this review only briefly discusses the historical evolution of the three approaches. We mainly focus on the recent progress in the field of hybrid heterojunction solar cells in order to emphasize how the control of morphology and the modification of the hybrid interface via surfactants have contributed to increase the efficiencies of these devices within the last few years.

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TiO2 bilayers The first proof of the formation of a hybrid heterojunction leading to exciton dissociation was demonstrated by Savenije et al. in 1998.162 These devices are based on a TiO2 /MEH-PPV interface sandwiched between a transparent ITO electrode and a mercury drop contact, and showed JSC of 0.32 mA cm−2 , VOC of 0.92 mV and FF of 52% under AM 1.5 white light illumination resulting in a total solar energy conversion efficiency of 0.15% (Table 2). Time-resolved microwave conductivity (TRMC) measurements with nanosecond photoexcitation at 544 nm were



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Bilayer devices Hybrid bilayer solar cells are generally produced by spin-coating a polymer layer on top of a metal oxide layer that is deposited onto a transparent electrode (indium tin oxide (ITO) or FTO) beforehand. A hybrid bilayer using a TiO2 flat film covered with a poly[2-methoxy5-(2 -ethylhexyloxy)-1,4-phenylvinylene] (MEH-PPV) layer was first reported in 1998 (Table 2 gives the corresponding photovoltaic parameters).162

During almost a decade, research in the field of hybrid bilayer devices was focused on TiO2 -based devices and a maximum efficiency of 0.60% has been reached so far.56 Since 2007, research on bilayer solar cells using ZnO as electron acceptor has been carried out. The bilayer approach allows investigation of the hybrid interface in detail because of the simple fabrication of each layer and well-knows interface morphology and can give important information about charge carrier generation and photophysics of the metal oxide/polymer interface. In the following, the main improvements and important scientific discoveries relating to solar cells using TiO2 /polymer bilayers are discussed briefly, while ZnO/polymer bilayers are covered in more detail.

www.soci.org used to study the charge carrier generation of the TiO2 /MEHPPV interface in detail.185 The experiments showed that charge separation persists into the microsecond range guarantying efficient charge collection to the electrode as well as a quantum efficiency of charge carrier injection into the TiO2 layer of 6%. The next improvement came by replacing MEH-PPV with poly[2,5-dimethoxy-1,4-phenylene-1,2-ethylene-2-methoxy5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene] (M3EH-PPV) with higher hole mobilities, which led to TiO2 /M3EH-PPV heterojunction solar cells with η = 0.4%.167 Hybrid bilayer solar cells using poly(3-hexylthiophene) (P3HT) as electron donor have been reported by several groups, but the first solar cell with significant ´ efficiency was produced by Frechet and co-workers in 2004.186 They used a polythiophene derivative with thermally removable branched ester solubilizing groups and compared the devices with those containing standard P3HT. After thermal treatment the ester groups are transformed into carboxylic acid groups, which can interact with the TiO2 surface and thus increase the interface interaction between the polymer and the metal oxide. The solar cell using the P3HT derivative has an external quantum efficiency (EQE) of 12.6%, associated with VOC = 0.52 V, JSC = 2.06 mA cm−2 , FF = 40% and power conversion efficiency of 1.10%, while the control cell using P3HT has an EQE of 4.2%, and VOC = 0.66 V, JSC = 0.68 mA cm−2 , FF = 60% and η = 0.69%. The better performance of the P3HT derivative was assigned to an enhanced hybrid interface as well as to better light absorption and energy transfer compared to the control sample using P3HT. The impact of the carboxylic acid groups and other surface modification on the operation of hybrid TiO2 /P3HT solar cells was then further studied by McGehee and co-workers.56,187 They showed that the grafting of the carboxylic acid group generates a dipole moment at the surface of TiO2 , which reduces the open-circuit voltage of the devices when using the P3HT derivative. By using a series of para-substituted benzoic acids with varying dipoles and a series of multiply substituted benzene carboxylic acids, they further demonstrated that the energy offset at the TiO2 /polymer interface and thus the open-circuit voltage of devices can be tuned systematically by 0.25 V.56 Photoluminescence quenching measurements showed that TiO2 substrates modified by these molecules quenched more excitons in regioregular P3HT than bare TiO2 surfaces. The EQE values of the corresponding devices were effectively increased from 5 to 10–14% with certain surface modifiers, and it was estimated that all quenched excitons could contribute to the photocurrent. The best efficiency was obtained with a ruthenium(II) sensitizing dye, which improved the efficiency of optimized TiO2 /P3HT bilayer devices up to 0.6% under AM 1.5 solar illumination (Table 2).

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ZnO bilayers Hybrid solar cells using bilayer polymer/ZnO heterojunctions were reported for the first time by Olson et al. in 2007.168 The devices were used to study VOC as a function of magnesium substitutions into the ZnO crystal structure, which aimed at increasing the bandgap and the position of the conduction band edge with increasing magnesium content. They demonstrated that VOC was improved from 500 to 900 mV in Zn1−x Mgx O/P3HT devices with x up to 0.35. A concomitant increase in overall device efficiency was seen as x increases from 0 to 0.25, with a maximum power conversion efficiency of 0.5%. These bilayers were further studied using TRMC with the aim of determining the locus of free charge carrier generation in the bilayer system.188 The TRMC signal for Zn1−x Mgx O/P3HT layers for x ≥ 0.2 was the same as that for


J Bouclé, J Ackermann pure P3HT layers, indicating that free charge carrier generation predominantly occurs in the bulk of the polymer and not at the hybrid interface, followed by electron injection to the oxide to yield photocurrent. The proposed model for photoinduced charge carrier generation is interesting, but the results presented are so far the only example confirming this model, which stands in contrast to common organic photovoltaic accretions of exciton dissociation at the donor/acceptor interface. Lloyd et al. conducted a similar study by incorporating lithium during sol–gel processing of the metal oxide layer.189 The Zn1−x Lix O/P3HT devices showed an improved VOC and JSC on average by a factor of 40 and 90%, respectively, leading to an efficiency of 0.44%. Interface modification of hybrid ZnO/P3HT bilayer devices by grafting various alkylenethiol self-assembled monolayers (SAMs) at the ZnO surface demonstrated that the SAMs increase the performance of the devices. This was an unexpected results due to the fact that the alkylenethiol SAM is insulating and increases the serial resistance of the solar cells.190 It was found that P3HT layers in direct contact with a ZnO surface are disordered. Such formation of a disordered P3HT layer in contact with a ZnO surface was also observed in a theoretical work by Saba et al.191 However, P3HT films spin-coated onto the SAM-modified ZnO layers were more crystalline than the control samples as determined through a less pronounced hypsochromic shift and more pronounced low-energy vibronic structures in the absorption spectrum of P3HT. This could improve the exciton diffusion length and charge transport properties of the polymer, and predominate over the insulating effect of the SAMs. Most of these experimental results for ZnO/P3HT bilayers have been recently summarized by Hsu and Lloyd.192 Friend and co-workers demonstrated performance enhancement of ZnO/P3HT bilayer devices achieved by using interfacial modification with phenyl-C61 -butyric acid (PCBA) monolayers.193 EQE of PCBA-modified solar cells reached 9%, which corresponds to a full charge separation of generated excitons, compared to an EQE of 3% for the unmodified devices. The grafted PCBA is found to form a dipole moment pointing away from the ZnO surface leading to improved exciton dissociation as well as reduced recombination of charge carriers at the interface. Hybrid heterojunctions using nanostructured TiO2 and ZnO The use of nanostructured metal oxide layers is a promising approach towards control of BHJ morphology, which is generally achieved at the cost of the loss of complete solution processing of the solar cells. At first, nanoporous TiO2 layers, which are usually used in DSSCs,87,95 were combined with polymers. The efficiency of the corresponding devices was low and limited due to poor infiltration of the polymer inside the pores.166 The complete infiltration of the polymer presents a major challenge and affords optimized nanostructured morphologies with good wetting properties.164 To benefit from nanostructured acceptors and thus to achieve high device performance, an improved nanoscale morphology is needed that allows (i) infiltration of the polymer within the pore and (ii) efficient charge transport channel to collect the electrons to the electrodes. It is a general opinion that the ideal case of nanostructuring corresponds to a densely packed array of vertically aligned nanorods with dimensions of the order of the exciton diffusion length of the polymer donor. Furthermore, an optimized interface between the nanostructured metal oxide and the polymer allowing efficient exciton dissociation and high open-circuit voltages is needed. In the following subsections, the most relevant improvements in the field of nanostructured


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Figure 6. SEM image of a mechanically fractured 4 mm long TiO2 nanotube array sample. (Reprinted with permission from Shankar et al.194 Copyright 2007 American Chemical Society).

metal oxide/polymer heterojunction solar cells are presented, with particular attention being paid to surface modification. Hybrid nanostructured TiO2 /polymer solar cells Porous TiO2 films of various morphologies have been used to produce hybrid polymer solar cells. Mesoporous structures of TiO2 with pore size in the range of the typical exciton diffusion length (ca 10 nm) were obtained through self-assembly using a structure-directing block copolymer, and corresponding devices show a maximum EQE of 10%.169 A TiO2 interconnected network structure was obtained using a polystyrene-block-polyethylene oxide diblock copolymer as the templating agent. The pore size of the structure is controlled by the amount of titanium precursor provided. The heterojunction solar cells consisting of such a TiO2 porous network with MEH-PPV gave high short-circuit current densities of 3.25 mA cm−2 , VOC = 0.86 V, FF = 28% and η = 0.71% (a)

www.soci.org under AM 1.5 solar illumination, as well as a maximum EQE value of 34%.170 TiO2 nanotube arrays fabricated by anodizing titanium foils in an ethylene glycol-based electrolyte were combined with a carboxylated polythiophene derivative, which self-assembled onto the TiO2 nanotube arrays by immersing them in a solution of the polymer (Fig. 6). Liquid- and solid-state solar cells were made with these hybrid heterojunctions and showed high photocurrent densities JSC of 5.5 and 2.0 mA cm−2 , respectively.194 It should be mentioned that TiO2 nanorods, which may be the most suitable morphology for efficient photocurrent generation, have not been demonstrated so far. Another strategy to improve the efficiency of nanostructured TiO2 /polymer solar cells is the optimization of the device structure. By inserting a poly(3,4-ethylenedioxythiophene) (PEDOT):poly(sodium styrenesulfonate) (PSS) layer between the polymer and the metal electrode, a 20–30% increase in energy conversion efficiency was obtained leading to maximum η of 0.4%.195 Surface modification of nanostructured TiO2 /polymer interfaces via grafting of surfactants, as already applied to bilayer heterojunctions, has also been used as an important tool to improve device performance. Nelson and co-workers studied the influence of surface modification with molecules bearing permanent dipole moments in nanostructured TiO2 /P3HT solar cells.196 They concluded that the monolayer not only controls the energy level of the hybrid interface but also has a barrier function by spatially separating the charges. In 2009, Ramakrishna and co-workers demonstrated for the first time hybrid nanostructured TiO2 /polymer solar cells exceeding efficiencies of 2% (Fig. 7).197 They compared nanostructured TiO2 /P3HT heterojunctions treated with a metal-free organic dye (D102) with those treated with a ruthenium dye (N719). Bis(trifluoromethylsulfonyl)amine lithium salt (Li(CF3 SO2 )2 N) and 4-tert-butylpyridine (TBP) were further incorporated prior to the deposition of the P3HT layer. The results show that the efficiency and the effect of the TBP and lithium salt treatment strongly depend on the grafted dye. The best performance was obtained using D102 in combination with lithium salt and TBP, which in(b)

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Figure 7. (a) Chemical structures of P3HT, N719 and D102 organic materials and schematic structure of the hybrid device tested by Zhu et al.197 An SEM image of the nanostructured TiO2 electrode is also displayed. (b) The energetic cascades illustrate the various physical processes involved. (Reproduced with permission from Zhu et al.197 Copyright Wiley-VCH Verlag.).

www.soci.org creases the efficiency from η = 0.04% for a device grafted only with D102 by a factor of 60 to η = 2.63%. The incorporation of lithium salt is essential for the improvement, with the results indicating that it affects not only the transport properties of the interface but also the absorption of the dye. Hybrid nanostructured ZnO/polymer solar cells In the case of ZnO, hybrid nanostructurated solar cells were first prepared with vertically oriented nanorod arrays by several groups.172,198,199 These nanorod ZnO/P3HT devices show much higher photocurrent generation than planar devices, illustrating the need for large heterojunction interfaces with polymer domains in the range of the exciton diffusion length. Interestingly, the nanorod ZnO/P3HT devices gave much higher performance after storage in air for some days compared to storage in argon. A maximum efficiency of 0.53% under simulated solar illumination (1 sun) was reached.172 The importance of molecular interlayers to improve the performance of nanorod ZnO/P3HT heterojunctions was first demonstrated by Nelson’s group.199 Surface modification with a ruthenium dye (Z907) improved the power conversion efficiency by a factor of four compared to untreated nanorod ZnO/P3HT devices. The best devices showed JSC of 2 mA cm−2 , VOC of ca 0.22 V and an overall conversion efficiency of 0.2% under AM 1.5 illumination. The recombination kinetics of the nanorod ZnO : Z907/P3HT devices was found to be very slow compared to dense layers of nanoparticles, of the order of milliseconds. Dag and Wang studied, using ab initio calculations, the atomic structure of the ZnO/P3HT interface and the effect of the electronic coupling between both materials.200 They focused on non-polar ZnO (1010) surfaces, which are the primary surfaces of ZnO nanorods and thus relevant for nanostructured hybrid interfaces. Their results show that the P3HT dimer stacking occurs perpendicularly to the nanorod direction, which reduces charge carrier mobility in this direction. Furthermore, strong electronic coupling between the P3HT chain and the ZnO surface in the conduction band states was demonstrated. A simple approach to produce hybrid nanostructured ZnO/polymer heterojunction completely by solution processing was demonstrated by Boucle´ et al.174 Spin-coating of ZnO nanocrystals onto ITO/dense ZnO substrates, followed by sintering into a nanoporous ZnO layer led to a hybrid nanoporous ZnO/P3HT solar cell with a conversion

J Bouclé, J Ackermann efficiency of 0.35% under AM 1.5 illumination of 100 mW cm−2 . It was found to be crucial to choose properly the organic ligands as well as the sintering procedure to achieve nanoporous ZnO layers having suitable charge transport properties. Hybrid solar cells using TiO2 and ZnO metal oxide/polymer blends Polymer/nanocrystal blends offer the major advantage of complete solution processability and thus good compatibility with low-cost printing techniques. Additionally compared with allorganic blends, improved electronic and thermal stability of the bulk heterojunction morphology can be expected. The major challenge for the polymer blend approach is thus the control of the nanoscale morphology of the heterojunction during film processing. This would guarantee a large hybrid interface as well as efficient charge transport properties inside the hybrid network. Furthermore charge carrier generation via exciton dissociation at the polymer/nanocrystal interface has to be controlled without altering the charge transport properties of the blend. In particular, surface modification of nanocrystals, although it constitutes a promising tool in the case of planar and nanostructured hybrid heterojunctions, can be incompatible with good charge transport properties for hybrid blends. TiO2 /polymer blends The first attempts to fabricate nanocrystalline TiO2 /polymer blend solar cells led to poor solar energy conversion efficiencies.201,202 In 2004, the first significant efficiencies were reported by Kwong et al.175 The combination of a high concentration of TiO2 nanocrystals in the polymer (above 50% by weight) and the use of xylene instead of chloroform led to ITO/PEDOT : PSS/nanocrystalline TiO2 /P3HT/Al solar cells with η = 0.42%. Another step towards higher efficiencies was the use of TiO2 nanorods.176 Hybrid solar cells based on nanorod TiO2 /P3HT blends showed short-circuit current densities of 2.62 mA cm−2 , open-circuit voltage of 0.69 V and FF of 63% under simulated A.M. 1.5 illumination (100 mW cm−2 ), associated with a power conversion efficiency of 1.14% (Table 3). This high efficiency could be obtained by optimizing the TiO2 nanorod concentration in the blend and by thermal annealing. More importantly, the removal of insulating surfactants at the nanorod surface increased the short-circuit current density and FF significantly. Nelson and co-workers studied the influence of the organic

Table 3. Summary of photovoltaic parameters of selected hybrid devices illustrating novel concepts at the borderline between solid-state DSSCs and hybrid BHJs, where an additional organic photoactive material is employed for the generation of photocurrent

New mixed hybrid concepts FTO/porous TiO2 /P3HT-COOH/spiro-OMeTAD/Au FTO/porous TiO2 /SQ1/spiro-OMeTAD : dye/Au FTO/porous TiO2 /TT1/P3HT/Au FTO/porous TiO2 /Sb2 S3 /P3HT/Au ITO/ZnO : dye : PPHT/Al ITO/PEDOT-PSS/nr-ZnOa :dye : P3HT/Al ITO/PEDOT-PSS/nr-ZnO : dye/Al FTO/TiO2 /dye/PEDOT-PSS/graphite FTO/nt-TiO2 b :SQ-1:P3HT/PEDOT-PSS/Au a

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b

JSC (mA cm−2 )

VOC (V)

FF (%)

η (%)


Ref.

3.70 3.87 2.86 13.02 1.45 0.21 1.63 1.10 11.0

0.54 0.79 0.74 0.64 0.62 0.40 0.52 0.86 0.60

45 59 48 61 61 54 37 30 58

0.90 1.80 1.01 5.13 0.55 0.06 0.32 0.28 3.80

100 100 100 99.8 – 75 100 100 100

219 220 221 222 223 224 54 225 55

‘nr’, nanorods. ‘nt’, nanotubes.



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Solid-state DSSCs and BHJ solar cells using TiO2 and ZnO (a)

www.soci.org (b)

Figure 8. (a) Schematic of P3HT/TiO2 nanorod hybrid after interface modification, and chemical structures of various interfacial molecules. (b) Current–voltage characteristics of photovoltaic devices using the various interface ligand molecules under AM 1.5 (100 mW cm−2 ) irradiation. (Reprinted with permission from Lin et al.53 Copyright 2009 American Chemical Society).

ligand exchange of TiO2 nanorod surfaces on the charge carrier generation and device performances of ITO/PEDOT : PSS/nanorod TiO2 :P3HT/Al solar cells.196 They demonstrated that a partial replacement of the insulating trioctylphosphine oxide ligands by an amphiphilic ruthenium dye (Z907) increases the charge separation yield. However, photocurrent generation was poor due to poor charge transport which was found to be trap-limited within the TiO2 nanorods. Chen and co-workers further studied the interfacial modification of TiO2 nanorods with various organic dyes.53 As a result, ITO/PEDOT : PSS/nanorod TiO2 :P3HT/Al bulk heterojunction solar cells with JSC = 4.4 mA cm−2 , VOC = 0.78 V, FF = 65% and η = 2.2% under simulated AM 1.5 illumination (100 mW cm−2 ) were demonstrated, as depicted in Fig. 8. All dyes tested reduced the charge carrier recombination rates and improved the exciton dissociation at the hybrid interface compared to pyridine-bearing control samples. The most efficient solar cells were obtained with the N3 dye, indicating that three-dimensional and bulky interface molecules slow down recombination more efficiently than planar molecules. In similar work, the impact of amphiphilic interfacial modifiers on the performance of TiO2 nanorod/P3HT blends was studied, and an efficiency of 1.19% has been reached for optimized devices.177 An interesting alternative to the blend approach was demonstrated by Janssen’s group in 2003 using in situ synthesis of titanium oxide nanoparticles inside a polymer matrix.178 Titanium(IV) isopropoxide (Ti(OC3 H7 )), a TiO2 precursor, was mixed with poly[2-methoxy-5-(3 ,7 -dimethyloctyloxy)1,4-phenylenevinylene] (MDMO-PPV) and converted into a TiO2 network inside the polymer at room temperature in the dark. The corresponding devices show JSC = 0.6 mA cm−2 , VOC = 0.52 V, FF = 42% and η = 0.20%. The efficiency was rather low, related to the poorly defined amorphous nature of the TiOx phase present at low crystallization temperature. Better results for this approach can be obtained from replacing TiOx by ZnO, which can form a highly crystalline phase even at low temperature.

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ZnO/polymer blends Contrary to nanocrystalline TiO2 /polymer blends, hybrid solar cells using ZnO nanocrystals mixed with conjugated polymers show early in high solar conversion efficiencies. Beek et al. demonstrated BHJ solar cells using blends of ZnO nanocrystals with MDMOPPV with maximum efficiency of 1.6% in 2004.179 In subsequent

work, they studied photogeneration and decay of charge carriers inside the blends, as well as the influence of ZnO concentration, ligand exchange and nanoparticle size and shape.32,203,204 They showed that under optimized conditions, the best performance is obtained with nanocrystals of 4.9 nm in diameter. The use of n-propylamine as surfactant to improve the solubility and thus the miscibility of the nanocrystals inside the blends led, for both nanospheres and nanorods, to performance losses. This was attributed to the introduction of energetic barriers between the nanoparticles. Combining ZnO nanocrystals with P3HT led to solar cells with JSC = 2.19 mA cm−2 , VOC = 0.69 V, FF = 55% and η = 0.92%.180 The lower efficiency compared to nanocrystalline ZnO : MDMO-PPV devices was assigned to a coarse mixing of the blend and high film roughness generating current leakages. To overcome the problems of mixing nanoparticles and polymers together, in situ generation of a ZnO network inside a polymer constitutes a promising approach. Using diethylzinc as ZnO precursor in combination with P3HT produced hybrid BHJ solar cells with JSC = 3.5 mA cm−2 , VOC = 0.83 V, FF = 50% and η = 1.4%, a considerable improvement over the efficiency of nanocrystalline ZnO : P3HT blends.181 Using threedimensional electron tomography to investigate in situ generated ZnO : P3HT blends, Janssen and coworkers visualized for the first time the morphology of the ZnO network inside the organic phase.182 A direct correlation between the three-dimensional morphology, photophysical data and solar cell performance has been demonstrated, which allow differentiation between device limitation related to photoinduced charge carrier generation and transport properties of the BHJ (Fig. 9). The best hybrid ITO/PEDOT : PSS/ZnO : P3HT/Al devices were obtained with a 225 nm thick active layer and showed JSC = 5.2 mA cm−2 , VOC = 0.75 V, FF = 52% and η = 2.0%. Interestingly, spatial modelling of the three-dimensional morphology of these films based on electron tomography data was developed and compared to experimental data.205 An elegant strategy to improve the miscibility of the in situ generated nanoparticles with the P3HT phase was demonstrated by functionalizing the polymer with ester side-groups, enabling better compatibility with the polar surface of ZnO.183 The resulting nanocrystalline ZnO : P3HT-E blends show a higher degree of miscibility as expected, but give lower efficiency compared to the ZnO : P3HT control samples, which could be explained by the lower hole mobility of the P3HT

www.soci.org derivatives. To produce air-stable polymer photovoltaics free from vacuum steps and fullerene, hybrid heterojunctions using blends of ZnO nanocrystals and poly(3-carboxydithiophene) (P3CT) have been studied by Krebs and co-workers.184 By using an inverted device structure and printed silver layers, they demonstrated that the solution-processed ITO/nanocrystalline ZnO/nanocrystalline ZnO : P3CT/PEDOT-PSS/Ag(printed) has an accelerated lifetime of ca 100 h without any encapsulation under continuous AM 1.5G illumination (100 mW cm−2 , 72 ± 2 ◦ C, ambient atmosphere, 35 ± 5% humidity).

NOVEL CONCEPTS AT THE BORDERLINE OF SOLID-STATE DSSCS AND HYBRID BHJ SOLAR CELLS Several examples were discussed for both solid-state DSSC and hybrid BHJ approaches, in which metal oxide nanostructures were combined with dyes and organic semiconductors to form a hybrid solar cell. In both cases, the use of organic dyes as interfacial modifiers is either compulsory for efficient charge generation as in DSSCs, or is a strategy to improve the charge carrier generation at the metal oxide/polymer interface as in BHJs. Recently, new approaches of hybrid BHJ solar cells have been reported where organic dyes are introduced in the system to further increase light absorption and thus the photogeneration of charge carriers. These trends illustrate in our opinion the close interface between both solid-state DSSC and hybrid BHJ approaches, as these novel device concepts mix the specific features of both (Fig. 2 shows a schematic overview of such mixed device designs). Moreover, these aspects are evidenced from both the DSSC and BHJ strategies. We mentioned above that much work has been carried out to demonstrate an efficient solid-state DSSC using a more suitable organic hole transporter, towards cheap processing and compatibility for industrial scale-up. Although the spiro-OMeTAD molecular glass demonstrates very efficient performance today,206 various alternative materials have also been investigated for use as hole transporters. Conjugated polymers have shown such potentialities.207,208 Among them, several candidates have demonstrated promising device efficiencies, such as polyaniline,209 polypyrrole,113 PEDOT210 – 212 or polythiophene derivatives.213 – 216 Several small conjugated molecules, such as pentacene217 or triphenylamine derivatives,218 have also been tested. Although several constraints have to be fulfilled, such as a suitable energetic configuration for dye regeneration and charge collection or efficient pore

J Bouclé, J Ackermann filling, additional effects can be expected from such approaches. In particular, the possibility of exploiting the light-harvesting properties of the organic hole transporter was demonstrated early on by Grätzel’s group,216 illustrating clearly the narrow gap between the DSSC and BHJ concepts. This result paved the way towards novel device concepts, where conjugated polymers can be specifically designed to improve the charge generation of solid-state DSSCs. For example, Lohwasser et al. specifically designed functional P3HT with carboxylic acid end-groups to sensitize TiO2 photoanodes (Fig. 10(a)).219 Using spiro-OMeTAD as hole transporter, they achieved a 0.9% efficient solid-state DSSC under full sunlight (a summary of photovoltaic parameters is given in Table 3). In parallel, other approaches have tried to introduce additional relay dyes in an attempt to exploit additional energy cascades. Antenna dyes based on ruthenium sensitizers have been demonstrated by Karthikeyan et al. and applied to solid-state DSSCs.226 A remarkable performance improvement has been observed up to 3.4% under illumination compared to the reference N719 dye (0.7%) for a ruthenium/vinytetraphenylbenzidine (TPD)/NCS dye, due to multistep charge transfer cascades following optical absorption by the TPD antenna. Other strategies have used inorganic low-bandgap nanocrystals such as PbS and CdS to directly sensitize TiO2 films, leading to significant efficiencies up to 1% using spiro-OMeTAD as hole transporter.227 Pursuing this route, several groups have recently been attempting to demonstrate extended photocurrent generation using two absorbing species. Yum et al. associated a near-infrared sensitizer (SQ1) adsorbed on a TiO2 film with a mixture of spiro-OMeTAD and a highly phosphorescent phenanthroline ruthenium(II) relay dye in a solid-state DSSC.220 They found a nearly 30% performance improvement compared to the cell without relay dye due to the transfer of blue ¨ photons absorbed by the relay dye to the SQ1 dye by Forster resonance energy transfer. A cell with an efficiency of 1.8% was demonstrated using an optimized energy relay dye concentration (Table 3). A similar result was reported by Mor et al. in 2010 using SQ1 and a visible-light-absorbing 4-(dicyanomethylene)-2methyl-6-(4-dimethylaminostyryl)-4H-pyran donor dye, showing an excitation transfer efficiency of approximately 67%.228 Another step was taken very recently on novel borderline concepts with the demonstration of a panchromatic response of a solid-state DSSC based on a near-infrared dye and P3HT as hole transporter. In addition to its role for hole transport, a clear contribution of the conjugated polymer to the optical absorption of the cell is demonstrated (Fog. 10(b)), leading to a power conversion

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Figure 9. Reconstructed volumes of P3HT/ZnO layers obtained by electron tomography for three samples with thicknesses of 57 nm (left), 100 nm (centre) and 167 nm (right). (Reprinted from Oosterhaut et al.182 by permission from Macmillan Publishers Ltd. Copyright 2009).



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(b)

(a)

Figure 10. (a) Novel DSSC device concept where light sensitization of a TiO2 film is induced by grafting a modified P3HT polymer. (Reproduced from Lohwasser et al.219 by permission from the Royal Society of Chemistry.) (b) Absorption spectra of P3HT and TT1 near-infrared dye used in a solid-state DSSC concept presenting a panchromatic response. (Reproduced from Lee et al.221 Copyright 2011, with permission from Elsevier).

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phology control of nanocrystal/polymer blends, a subsequent study presented a new concept of dye-sensitized BHJ solar cells.54 The assembly of oligothiophene derivative dyes at the surface of ZnO nanorods led to hybrid coaxial heterojunctions, which were solution-processed into hybrid solar cells without the need of additional blending with a polymer. The use of coaxial heterojunction nanorods as active layer led to a morphology controlled at the nanoscale independently of the deposition conditions. The resulting ITO/PEDOT : PSS/ZnO : dye/Al devices show photovoltaic parameters with JSC = 1.63 mA cm−2 , VOC = 0.52 V, FF = 37% and η = 0.32% under simulated AM 1.5 illumination (Table 3). Another approach using only dye molecules for both light absorption and charge transport was reported by Unger et al.225 Star-shaped [tris(dicyanovinyl-2-thienyl)phenyl]amine dye layers of various thicknesses were spin-cast from solution onto dense TiO2 layers. The corresponding FTO/TiO2 :dye/PEDOT : PSS/graphite/FTO devices showed efficiencies up to 0.3% with a dye layer thickness of about 8 nm. Although the use of dyes as additional absorber materials in BHJ solar cells was first demonstrated for hybrid blends, it is obvious that nanostructured metal oxide layers are potentially more suitable to combine the advantages of both the DSSC and conventional BHJ approaches. In 2009, Grimes and co-workers demonstrated a TiO2 nanotube array/P3HT-based heterojunction solar cell which uses an unsymmetric squaraine dye (SQ-1) that absorbs in the red and near-infrared regions (Fig. 11(a)).55 The FTO/TiO2 nanotube : SQ-1:P3HT/PEDOT : PSS/Au gave JSC = 11 mA cm−2 , VOC = 0.60 V, FF = 58% and η = 3.8% for the best device under simulated AM 1.5 illumination. The EQE spectra of the devices highlight the positive impact of the dye (Fig. 11(b)), as it not only increases the overall device efficiency over the whole sensitivity range of the cell, but also generates highly efficiently photocurrents beyond the P3HT absorption threshold.

CONCLUSIONS Hybrid photovoltaic devices based on metal oxide nanostructures have been much reported over the last two decades. In this context, TiO2 and ZnO demonstrate a strong potential due to their favourable electronic properties and ease of fabrication



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efficiency of up to 1% under full sun.221 Another study was performed by Chang et al. in 2010, where inorganic stibnite (Sb2 S3 ) nanocrystals were used to sensitize a porous TiO2 electrode. By using P3HT as both hole transporter and light absorber, they demonstrated a very efficient system with power conversion efficiencies up to 5.1% under AM 1.5G standard solar irradiance (using in addition suitable interfacial modifiers).222 These trends clearly reduce the gap between DSSCs and hybrid BHJ solar cells. Regarding hybrid BHJ concepts, the dye modification of the metal oxide is mainly conducted to improve charge carrier injection and reduce recombination rates of the hybrid interface. However, being used originally for DSSCs, these dyes have potentially strong absorption properties and can also contribute actively to photocurrent generation. An example of such a dye-sensitized BHJ solar cell was demonstrated in 2008 by adding alizarin dyes into a blend of ZnO nanocrystals and poly(3-phenylhydrazonethiophene) (PPHT).223 The resulting ITO/nanocrystalline ZnO : dye : PPHT/Al devices showed strongly improved efficiency compared to the control devices using nanocrystalline ZnO : PPHT blends. EQE analysis clearly indicates that the dye contributes actively to photocurrent generation. In similar work, a perylene derivative (PDI) was used to produce nanocrystalline ZnO : PDI : P3HT and nanocrystalline TiO2 :PDI : P3HT heterojunctions, without a clear proof of dye contribution to photocurrent generation.229,230 While both examples introduce the dye by simple blending into the ZnP/polymer heterojunction, direct grafting of the dye onto the metal oxide surface seems more suitable for efficient electron injection due to the proximity to the metal oxide surface as in the case for DSSCs. A first demonstration of such a dye-sensitized BHJ was shown by Said et al. by using tetra(4-carboxyphenyl)porphyrin (TCPP) in nanorod ZnO : TCPP : P3HT blends.224 The four carboxylic acid groups of TCPP lead to efficient grafting of the dye onto the ZnO nanorods. EQE analysis of the corresponding ITO/PEDOT : PSS/nanorod ZnO : TCPP : P3HT devices clearly demonstrates additional photocurrent generation due to light absorption of the dye. However, dye grafting reduces the overall efficiency of the devices. This was assigned to strong aggregation of the nanorods inside the blend layer and increased recombination rates, both arising from the dye grafting. To circumvent the problems related to mor-

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J Bouclé, J Ackermann (b)

Figure 11. (a) Schematic of a hybrid solid-state solar cell based on dye-sensitized TiO2 nanotubes with polymer intercalation. (b) Incident photon to collected electron efficiency (IPCE) of a FTO/TiO2 nanotube array/SQ-1 dye/P3HT/PEDOT : PSS/Au solar cell, where the P3HT layer was prepared with various degrees of surface wetness prior to 150 ◦ C post-annealing. (Reprinted with permission from Mor et al.55 Copyright 2011 American Chemical Society).

through soft synthetic routes from solutions, and their use is further supported by their non-toxicity, biocompatibility and cheapness. Two main strategies have generally been explored for their association with organic photoactive materials in the solid state: the DSSC concept based on a nanostructured porous electrode sensitized by a molecular dye and the BHJ concept based on conjugated polymer/metal oxide associations. In both cases, charge photogeneration is intimately associated with the inorganic/organic interface, as well as with the transport properties of the inorganic metal oxide used. While the former aspect can be controlled by tailoring specific dye molecules or ligands towards improved charge separation, the latter aspect can be addressed by tuning the morphology of the metal oxide, going from isotropic spheres to hierarchically oriented nanorods or nanotubes. Although hybrid device performances have shown regular improvement over the years, reaching up to 6% for solidstate DSSCs and just over 3% for hybrid BHJs, novel strategies are required to demonstrate competitive alternatives to all-organic devices such as polymer/fullerene blends. In this context, new approaches at the borderline between both concepts have been investigated, reducing the gap between them. In particular, the use of conjugated polymers has been found to further extend the light-harvesting properties of solid-state DSSCs, while additional optically active dyes can efficiently broaden the photoresponse of hybrid BHJs, pushing their efficiencies to just below 4%. It is noticeable that there is a trend of convergence between both DSSC and BHJ approaches into mixed concepts at the borderline. We believe that such mixed DSSC/BHJ strategies may allow in the near future the realization of hybrid devices for competitive photovoltaic energy conversion at low cost.

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