Towards stable and commercially available perovskite solar cells

PERSPECTIVE PUBLISHED: 17 OCTOBER 2016 | ARTICLE NUMBER: 16152 | DOI: 10.1038/NENERGY.2016.152

Towards stable and commercially available perovskite solar cells Nam-Gyu Park1*, Michael Grätzel2, Tsutomu Miyasaka3, Kai Zhu4 and Keith Emery4 Solar cells employing a halide perovskite with an organic cation now show power conversion efficiency of up to 22%. However, these cells are facing issues towards commercialization, such as the need to achieve long-term stability and the development of a manufacturing method for the reproducible fabrication of high-performance devices. Here, we propose a strategy to obtain stable and commercially viable perovskite solar cells. A reproducible manufacturing method is suggested, as well as routes to manage grain boundaries and interfacial charge transport. Electroluminescence is regarded as a metric to gauge theoretical efficiency. We highlight how optimizing the design of device architectures is important not only for achieving high efficiency but also for hysteresis-free and stable performance. We argue that reliable device characterization is needed to ensure the advance of this technology towards practical applications. We believe that perovskite-based devices can be competitive with silicon solar modules, and discuss issues related to the safe management of toxic material.

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rganic–inorganic halide perovskite was first used as a potential light harvester in a liquid-electrolyte-based dyesensitized solar cell structure in 20061. Already by 2009, there were hints of the potential of MAPbI3 (MA = CH3NH3) and MAPbBr3 for photovoltaic (PV) applications, although their power conversion efficiencies (PCEs) were only as high as 3–4% (ref. 2). In a subsequent study, in 2011, the PCE was doubled by optimizing precursor solution concentration and annealing temperature3. The absorption coefficient of MAPbI3 was measured to be one order of magnitude higher than that of ruthenium-based dye (coded N719), thus pointing to a very promising PV material. However, the high solubility of MAPbI3 in polar liquid electrolytes prevented follow-up studies on perovskite solar cells (PSCs) until solid-state PSCs were first reported4, confirming the longterm stability of the unencapsulated device in a 500 h ex situ lightsoaking test. Such stability was attributed to a moisture-tolerant morphology consisting of wrapping nanoscale MAPbI3 dots with a hole-transporting material, spiro-MeOTAD (2,2’,7,7’-tetrakis(N,N-di-4-methoxyphenylamino)-9,9’-spirobifluorene). Shortly thereafter, a meso-superstructured solid-state PSC employing Al2O3 as a scaffold layer was reported5, where the thin MAPbI3 layer deposited on Al2O3 transported photoexcited electrons that were collected by an electrode via a thin hole-blocking TiO2 layer. Reports on solid-state PSCs triggered a wave of research on perovskite PVs. As a result, a certified PCE of 22.1% was reported in 2016 on the National Renewable Energy Laboratory’s best efficiencies chart 6. Unlike other devices on such a chart, PSCs are marked as ‘not stabilized’, indicating that validations with stable devices are still required. In addition to stability problems, the observed discrepancy in reverse and forward scanned current– voltage (I–V) curves — so-called I–V hysteresis — is an obstacle to commercialization because of the imprecise estimation of PV parameters. In this Perspective, we survey recent developments in the field to propose a strategy for commercially available PSCs. To obtain insight on a realistic path towards stable and commercially viable

cells, various aspects are considered, including the relationships between fabrication and efficiency, material and device engineering for efficient, stabile and reproducible solar cells, the correlation between interface properties and device stability, and issues related to hysteresis, efficiency and stability. We also attempt a cost analysis compared with a silicon solar cell. Finally, we discuss the multifunctionality of organic–inorganic halide perovskite to extend applications beyond PVs.

Controlling grains and grain boundaries

Regardless of device configuration, the quality of the perovskite layer is of critical importance with regard to obtaining high efficiency. Several methodologies have been proposed to obtain high-quality perovskite films, with the most-studied method being solutionbased crystallization via spin coating of a precursor solution. Post-treatment with the antisolvent of MAPbI3 or an intermediate compound is state-of-the-art technology 7,8. However, very recently, a simple alternative vacuum flash-assisted deposition method was developed that avoids the use of antisolvents. It enabled a certified record PCE of 19.6% for a 1 cm2-sized device to be reached, opening a new promising path to production on an industrial scale9. The quality of the perovskite grains is the primary factor influencing PV performance, with crucial importance being ascribed to minimizing non-radiative recombination. The Lewis acid–base adduct approach can serve to reduce such recombination, because highquality perovskite films can be prepared reproducibly via the adduct MAI·PbI2·DMSO (DMSO = dimethylsulfoxide)10. Figure 1a–c schematically displays the procedures of the antisolvent (including the fast deposition–crystallization) and adduct approaches, as well as the morphology of the resultant perovskite grains. Stoichiometric control is an advantage of the adduct approach. Lewis basicity plays an important role in controlling grain size and crystallinity. For instance, thiourea was found to be better than DMSO for producing FAPbI3 (FA = HC(NH2)2) (ref. 11). Solution-processed perovskite films are polycrystalline and thus cannot avoid the formation of grain boundaries, as seen in

School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea. 2Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland. 3Toin University of Yokohama, Graduate School of Engineering, Aoba, 225-8503 Yokohama, Kanagawa, Japan. 4National Renewable Energy Laboratory, Golden, Colorado 80401, USA. *e-mail: [email protected] 1

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PERSPECTIVE

NATURE ENERGY DOI: 10.1038/NENERGY.2016.152

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Figure 1 | Perovskite films obtained with different synthesis methods. a–c, Schematics of perovskite coating procedures by the antisolvent method (a), fast deposition–crystallization method (b) and Lewis acid–base adduct approach (c). Insets in each panel are plane-view scanning electron microscope images of the perovskite film morphologies obtained by each coating method. CBZ, chlorobenzene; FDC, fast deposition-crystallization. In panel c, arrows in the infrared spectra correspond to S=O stretching frequencies, where decrease in S=O stretching frequency is due to decrease in bond strength between sulfur and oxygen as a consequence of the adduct formation. d, Conductive atomic force microscopy before and after self-formed grain boundaries (GBs) by 6 mol% excess MAI in precursor solution with MAI, PbI2 and DMSO. Panels reproduced from: a, ref. 7, NPG; b, ref. 8, Wiley; c, ref. 10, American Chemical Society; d, ref. 15, NPG.

the scanning electron microscopy (SEM) images in Fig. 1a–c. Non-adiabatic molecular dynamics combined with time-domain density functional theory have suggested that grain boundaries significantly accelerate the electron–hole recombination in MAPbI3 (ref. 12); this conclusion is, however, not predicted from first-principles calculations. Studies on time-resolved photoluminescence and transient absorption also suggested that grain boundaries are responsible for non-radiative recombination13. Moreover, stronger I–V hysteresis was observed at grain boundaries than in grain interiors because of faster ion migration at the boundaries14. Minimizing the grain boundary via, for example, Ostwald ripening might be a viable approach. On the other hand, recent studies have revealed that the grain-boundary layer that was self-formed by excess MAI in adduct formation actually enhanced carrier lifetime, improving charge carrier conduction at the grain boundary (Fig. 1d) and charge separation between MAPbI3 and the hole-transporting layer, which had a benign impact on opencircuit voltage (VOC) and fill factor 15. Moreover, conductive atomic force microscopy (AFM) revealed that the grain boundary provided a pathway for charge conductance. Thus, understanding the 2

effects of grain boundaries on performance and grain-boundary engineering are very important aspects in PSC research.

Mesoscopic versus planar structure

As the first PSCs were developed from dye-sensitized solar cells (DSCs), their architecture employed a mesoporous TiO2 film as a scaffold. Its role was to support the perovskite nanoparticles that replaced the molecular sensitizer as a light harvester and to selectively extract electrons from the photoexcited pigment 16. However, the observation that organic–inorganic halide perovskite can on its own conduct electrons and holes led to further evolution of the device architecture. The mesoscopic embodiment employs a semiconductor oxide scaffold, whose pores are fully infiltrated with the perovskite that protrudes above the nanostructure to form a capping layer. In planar embodiments of a PSC, a compact perovskite film a few hundred nanometres thick is sandwiched between electron and hole selective contacts16,17. The simple structure of planar PSCs would suggest that ultimately all devices may adopt this configuration. However, at present, mesoscopic systems — with a certified PCE of 22% — still NATURE ENERGY | VOL 1 | NOVEMBER 2016 | www.nature.com/natureenergy


PERSPECTIVE

NATURE ENERGY DOI: 10.1038/NENERGY.2016.152 b Reactive boundary c(r = R) = 0

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Figure 2 | Photocarrier generation and collection. a, Schematic showing photoinduced charge carrier (e–, electron; h+, holes) generation and collection in a spherical perovskite particle infiltrated in the mesoporous scaffold of a wide band gap oxide semiconductor such as TiO2. The wavy arrow presents the diffusion length L of an exciton or a charge carrier. The symbol e–…h+ represents an electron–hole pair. The arrow points in the direction of electron flow because the direction of the flow of holes plays no role as the scaffold is selective for electron capture only. Electrons are captured by injection in the conduction band of the oxide. R, pore radius; c, reactive boundary; r, radius of reactive boundary. Panels b and c compare carrier diffusion and capture in cell configurations with spherical and planar geometry. b, The perovskite is infiltrated in the spherical pores of an oxide scaffold acting as electron acceptor. The collection efficiency (ηcoll) of photogenerated charge carriers is plotted as a function of the ratio R/L, calculated from Equation (1), which was derived by integrating the continuity equation expressed in spherical coordinates. c, Carrier collection efficiency as a function of xf /L for planar architecture, which was calculated from the same continuity equation expressed with a linear coordinate. Film thickness is denoted xf. The carrier has to diffuse across a slab of thickness xf before being captured. Curves are drawn for various values of the parameter αL, where α is the reciprocal absorption length.

hold a substantial lead over planar embodiments whose highest certified PCE is 15.6% (ref. 18). Using a mesoporous TiO2 scaffold to host the perovskite greatly assists the charge carrier collection, in particular for absorber materials where the light absorption length is longer than the carrier diffusion length. This applies, for example, to tin halide perovskites that reach external quantum efficiency (EQE) near 80% with mesoscopic scaffolds, while planar geometries produce no significant photocurrents19,20. Figure 2a depicts the scenario where the perovskite is infiltrated into the spherical pores of an electron or hole capturing scaffold. The charge carrier collection efficiency (ηcoll) is given by the expression: ηcoll =

R R 3L coth − R L L

where L is the diffusion length of the charge carriers (exciton) and R is the pore radius. The mesoporous architecture greatly reduces the length of the diffusion path of the charge carriers to the particle boundary while the radial symmetry of the pore accelerates diffusion with respect to a planar geometry, facilitating their extraction. Figure 2b,c compares the collection efficiency for photogenerated charge carriers in the presence and absence of a mesoscopic scaffold. Assuming a charge carrier (exciton) diffusion length of at least 100 nm and a pore radius of 10 nm, the collection efficiency by the mesoporous PSC would be greater than 99%. To collect more than 99% of photogenerated carriers for a planar film of 500 nm thickness and a light absorption length 1 / α = 250 nm (where α denotes the absorption coefficient), the diffusion length should be longer than 2.5 μm, which is difficult to reach with PSCs over large areas. Thus, the power of the mesoscopic PSC architecture is that it greatly facilitates collection of photogenerated charge carriers enhancing the potential of perovskites as light-harvesting materials in future photovoltaic deployment. It should be noted that the line between planar and mesoscopic cell architectures is often difficult to draw. Frequently, ‘planar’ embodiments employ nanocrystalline oxide films as an electron capture layer 21 and, moreover, the fluoride tin oxide (FTO)covered glass substrates supporting the perovskite are themselves highly corrugated. Conversely, the efficiency of PSCs endowed

with a mesoscopic scaffold is boosted further by endowing it with a compact perovskite capping layer. At present, the mesoscopic architecture gives a certified PCE of 21% (ref. 6).

Electroluminescence and efficiency

The electroluminescence features of fully operational PSCs allow important insights on the key performance metrics of the device to be derived, in particular its VOC. For perovskites of composition FA0.85MA0.15PbI2.55Br0.45 the maximum obtainable VOC measured under standard air mass 1.5 solar radiation is 1.32 V (ref. 22), which is about 230 mV smaller than their band gap due to entropic losses23. These losses result from a gain in spatial disorder as the direct light from the Sun is received within a very small spatial angle, whereas it is reemitted in all directions when the cell is held at open circuit where no photocurrent (JSC) can flow. This ideal value, VOC(ideal), is reached only if the external quantum efficiency for light emission by the solar cell measured at VOC is 100% (ref. 24). Each decrease in ηext by a factor of ten entails a 60 mV loss of the VOC according to the formula: VOC = VOC(ideal) + (kT / q)ln(ηext)(2) where k is the Boltzmann constant, T is temperature, q is the elementary charge and ηext is the external quantum efficiency of electroluminescence from the cell at a forward bias generating the same dark current as the short circuit photocurrent. In line with these theoretical predictions, the electroluminescence quantum yield of PSCs has been found to correlate with its VOC value. To study the electronic quality of the device and identify the recombination mechanisms, the electroluminescence was recorded under forward bias in the dark with the device operating as a light-emitting diode22. The external electroluminescence quantum efficiency (EQEEL) increased linearly with injection current as expected for a device with an ideality factor of two and approaches 0.5% for currents in the range of JSC. This value translates into a voltage loss of kT ln(1 / EQEEL) = 0.14 V, confirming the measured VOC = 1.32 V – 0.14 V = 1.18 V, where 1.32 V is the theoretical maximum for VOC obtained in the radiative limit 22. An EQEEL of 0.5% is a record for solution-processed solar cells matching that of the best silicon solar cells25. The electroluminescence is orders of magnitude more intense than that of organic

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PERSPECTIVE c-TiO2

NATURE ENERGY DOI: 10.1038/NENERGY.2016.152

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Figure 3 | Carrier extraction and recombination in perovskite materials. Illustration of the kinetic scheme for electron–hole extraction and recombination using MAPbI3 perovskite and a s-SWCNT HTL and TiO2 ETL. hv, incident photon energy; kr, recombination rate; De and Dh, electron and hole diffusion coefficients, respectively; kCT,e and kCT,h, electron and hole transfer rates, respectively; kBT,e and kBT,h, electron and hole back-transfer rates, respectively. Green spheres represent electrons and blue spheres represent holes. Details of kinetic measurements and characteristic times (or rates) are given in ref. 29. The relevant timescales for the two measurement techniques of time-resolved microwave conductivity and transient absorption spectroscopy are shown at the bottom of the figure. Adapted from ref. 29, RSC.

solar cells, where the EQEEL is commonly