Significantly improved efficiency of organic solar cells

Significantly improved efficiency of organic solar cells incorporating Co3O4 NPs in the active layer S. Amber Yousaf, M. Ikram & S. Ali

Applied Nanoscience ISSN 2190-5509 Appl Nanosci DOI 10.1007/s13204-018-0726-8

1 23

Your article is protected by copyright and all rights are held exclusively by SpringerVerlag GmbH Germany, part of Springer Nature. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com”.

1 23

Author's personal copy Applied Nanoscience https://doi.org/10.1007/s13204-018-0726-8

ORIGINAL ARTICLE

Significantly improved efficiency of organic solar cells incorporating Co3O4 NPs in the active layer S. Amber Yousaf1 · M. Ikram1 · S. Ali1,2 Received: 27 October 2017 / Accepted: 14 March 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract Effect of various concentrations of fabricated cobalt oxide (Co3O4) nanoparticles (NPs) in the active layer of different donors and acceptors based hybrid organic bulk heterojunction-BHJ devices were investigated using inverted architecture. The organic active layer comprising different donors P3HT (poly(3-hexylthiophene-2,5-diyl) and PTB7 (Poly[[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl]] [6,6]) and acceptor (phenyl-C-butyric acid methyl ester (PC60BM and PC70BM) materials. The incorporation of NPs in the binary blends of P3HT:PC60BM and PTB7:PC70BM enhanced the power conversion efficiency (PCE) of resulting ternary devices. This increase in PCE is mostly due to decrease in series resistance (Rs) with an optimum amount of Co3O4 in the organic photoactive layer. It was found that the ternary blend has higher absorbance relative to a binary blend of P3HT:PCBM. Addition of NPs in the active materials blend increased film roughness and form interpenetrating network to facilitate charges transportation in the active layer. The higher amount of NPs enhanced agglomerations which caused to break down the BHJ led to decrease the performance of the ternary devices. Keywords Active layer · P3HT · PTB7 · Film roughness · Co3O4 · Absorbance

Introduction In view of the rapidly declining natural reserves of energy, the most desirable goal of modern science is perhaps yoking clean and freely available solar energy in the most resourceful, vigorous, and cost-effective manner, on a large scale (Lee et al. 2008; Bhalla and Tyagi 2016). Because of nontoxic materials, tunable optical and electronic properties, mechanical flexibility, solution processing, colorful and lightweight (Kozma et al. 2013; Zhou et al. 2014; Lin et al. 2017); organic photovoltaics (OPVs) have attracted a great interest in research since last two decades. If developed into a mature technology, OPVs offer new horizons of how to produce and use energy. In preceding years many conjugated * M. Ikram [email protected] 1

Solar Cell Applications Research Lab, Department of Physics, Government College University, Lahore 54000, Punjab, Pakistan

Department of Physics, Riphah Institute of Computing and Applied Sciences (RICAS), Riphah International University, 14 Ali Road, Lahore 54000, Punjab, Pakistan

2

polymers such as P3HT, PTB7, (PCDTBT)-poly[[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl) carbonyl] thieno[3,4-b] thiophenediyl]], (PCPDTBT)-Poly[2,6-(4,4-bis-(2-ethylhexyl)-4Hcyclopenta [2,1-b;3,4-b′] dithiophene)-alt-4,7 (2,1,3-benzothiadiazole)], (TBTIT)-thieno[3,2-b][1]benzothiophene Isoindigo thiophene (Kadem et al. 2016; Etxebarria et al. 2014; Zhang et al. 2016; Ahmad et al. 2017; Yue et al. 2015) etc., have been employed in OPVs with fullerene and non-fullerene based acceptors (Ganesamoorthy et al. 2017; Liang et al. 2017). Although the devices demonstrate excellent power conversion efficiencies (PCEs) for single junction devices (~ 10%) and tandem devices (~ 11%) (Zhang et al. 2015; Chen et al. 2014) but PCE relative to conventional silicon-based solar cells is still very low. These low PCEs are owed to amorphous nature, low carrier mobility and short exciton diffusion length of the organic materials employed in devices (Ikram et al. 2015a; Tian et al. 2017). To minimize these issue, transition metal oxides (TMO) have been widely studied accredit to their properties and applications in electronics and optoelectronics (Abdullah et al. 2014; Greiner et al. 2011; Bera and Saha 2016). The merger of conjugated polymers and TMOs in

13

Vol.:(0123456789)

Author's personal copy

Applied Nanoscience

organic–inorganic hybrid solar cells (HSC) (Zhong et al. 2012) uses the advantages of both types of materials. Inorganic part of the ternary devices, having an ideal band gap, high electron mobility, environmentally stable and friendly; enhance absorption, facilitates charges in the active layer and support to overcome photodegradation (Wright and Uddin 2012; Beek et al. 2004). Various research groups used NPs in the polymer active layer with different architecture as CdS NPs in PTB7:PCBM based inverted devices increased PCE from 6.30 to 7.01% (Sharma et al. 2016). Many TMOs such as ZnO, TiO2, CuO, NiO, SnO2, PbS, CdSe have been employed in the active layer of OSCs with different morphologies that improved the PCE from 15 to 47% of (Gong et al. 2010; Yang et al. 2010; Jotterand and Jobin 2011; Fu et al. 2012; Oh et al. 2013; Yoon et al. 2013; Wanninayake et al. 2015). These NPs have also been used to replace P3HT or PCBM in ternary and quaternary blend based devices to lower the cost and efficient solar cells (Yang et al. 2013b; Ikram et al. 2015a, b, 2016). In this research, the influence of mixing C o 3O 4 in P3HT:PCBM and PTB7:PCBM based devices were prepared using dichlorobenzene (DCB) and chlorobenzene (CB) as solvent, respectively. C o3O4 being p-type semiconductor, non-toxic, and abundant relative to other is one of the most favorable metal oxide in technological fields (Mahmoud 2016; Allaedini and Muhammad 2013). Co3O4 can be considered as a potential candidate in photovoltaic (PV) research due to its two visible range optical band gaps, i.e., 1.5 and 2.2 eV (Majhi et al. 2016). It has been reported that Co3O4 had been used as a light absorber and also as a hole transporting layer in all-oxide PV and polymer solar cells, respectively (Kupfer et al. 2015; Wang et al. 2015).

Experimental details

impurities. The washed precipitates were dried by aging at 60 °C for 48 h:

CoCl3 + 3NaOH → Co(OH)3 + 3NaCl. Dried Co(OH)3 was further heated in a muffle furnace at 400 °C with a ramp rate of 10 °C/min for 60 min:

Co(OH)3 → Co2 O3 + H2 O 3Co2 O3 + H2 O → 2Co3 O4 + H2 O.

Device fabrication The ITO substrates were cleaned prior to use with acetone and iso-propanol (IPA). To deposit zinc oxide (ZnO) layer, ZnO NPs were diluted with IPA (1:1). The diluted NPs solution was spin coated on cleaned ITO at 5000 rpm/45 s and heated at 150 °C/20 min. The hybrid polymers and nanocomposite-based solutions were prepared by dissolving P3HT, PC60BM and PTB7, PC70BM in DCB and CB with 1:0.8 and 1:1.5 M, respectively, to achieve 36 mg/ml. Solutions were stirred for 24 h at 70 °C prior to use for device fabrication. Co3O4-NPs were dispersed in DCB, CB separately and added to the polymer solutions with different wt. ratio (1, 2, 3, 4 and 5) %. The different ratios of photoactive solutions were spun cast on ZnO coated ITO substrates at 1500 rpm for 1-min. The P3HT:PCBM based devices were heated at 70 °C for 15 min. Finally, a thin layer of MoO3 (thickness—10 nm) and Ag (thickness—100 nm) back contact were deposited at high vacuum (10−7 mbar) by the thermal evaporator. Hereafter, the inverted devices structures ITO/ZnO/ P3HT:Co3O4:PC60BM/MoO3/Ag are labeled as 0%—D1, 1%—D2, 2%—D3, 3%—D4, 4%—D5 and ITO/ZnO/ PTB7:Co3O4:PC70BM/MoO3/Ag are labeled as 0%—D6, 1%—D7, 2%—D8, 3%—D9, 4%—D10.

Characterizations

Materials Indium-doped tin oxide (ITO) (8–12 Ω) coated glass substrates were obtained from Delta Technologies, USA. N11 ZnO NPs (8–16 nm) were received from Sigma-Aldrich. P3HT was purchased from Ossila, UK and PTB7 from one-material. PCBM of 99.99% purity was received from Solenne and cobalt chloride ( CoCl3) was purchased from Sigma-Aldrich (99.99%). Sodium hydroxide (NaOH— 99.9%) was purchased from Panreac.

Synthesis of Co3O4 nanoparticles Co3O4-NPs were prepared by mixing an aqueous solution of CoCl3 and NaOH under vigorous stirring at room temperature. Dark green precipitates of Co(OH)2 were collected and washed several times with distilled water to remove

13

Structural and phase information of Co3O4 NPs was collected using X-ray diffractometer (XRD) (model: PAN Analytical X’pert PRO) with Cu-Kα radiation (1.54 Å) and 2θ values (25°–75°). solar simulator CT 100 AAA under AM 1.5 G with 1000 W/cm2 irradiation intensity and Keithley 2420 source meter unit were used to evaluate electrical parameters. Optical absorption profiles of NPs as well as films were obtained by UV–visible spectrophotometer (model: Genesys 10S). Ambios Multimode Microscope (AFM) and Hitachi S4160 were deployed to check the microstructure, roughness and morphology of the films. External quantum efficiency (EQE) measurements were recorded via QEX 10PV.

Author's personal copy Applied Nanoscience

Results and discussion

Fig. 1 XRD pattern of Co3O4 nanoparticles

Fig. 2 SEM Images of C o3O4 nanoparticles

XRD pattern of annealed cobalt oxide is shown in Fig. 1. The observed peaks are identified and well matched by JCPDS card no. 01-080-1545, showing the absence of impurity in the prepared sample. The C o3O4 exists in cubic nature (a = b = c = 8.1691 Å, α = β = γ 90°) belonging to space group Fd-3m. The peak positions appear at 2θ° = 31.29°, 36.82°, 44.81°, 55.62°, 59.32° and 65.25° are indexed to the corresponding planes (220), (311), (400), (422), (511) and (440) respectively. The calculated average crystallite size related to these peaks found to vary between 15 and 25 nm by Debye–Scherrer’s formula. To check the morphology of Co3O4 NPs scanning electron microscopy (SEM) was used as shown in Fig. 2. The image shows the formation of small and large clusters of Co3O4 NPs. Inset pinpoints few spherical structures of NPs have particle size between 29.3 and 36.7 nm. The absorption spectrum of Co3O4 NPs was obtained by UV–Vis spectroscopy as shown in Fig. 3a. From absorption spectra, the absorption peaks of Co3O4 are around 485 and 780 nm Fig. 3a. These are associated with the charge transfer from O 2− → Co2+ and O 2− → Co3+, respectively (Farhadi et al. 2013; Makhlouf et al. 2013; Salavati-Niasari and Khansari 2014). The calculated band gaps of C o3O4-NPs were around 1.3 and 2.0 nm using Tauc plot (Fig. 3b). Figure 4a, b represent the energy band diagram of the prepared devices with inverse geometry. To ensure high exciton dissociation and charge transfer, the energy levels of the active materials should be aligned (Stylianakis et al. 2017). In these devices system, the energy levels of Co3O4 NPs (conduction band = 4.2 eV, valance band = 6.1 eV, band gap = 1.3 eV) matches well with active layer polymers donor (D) and acceptor (A) based systems in this study and believed will participate to improve device parameters (Sharma et al. 2016; Zhang et al. 2014; Wu et al. 2013; Kim et al. 2007). Open circuit voltage (Voc) of the device

Fig. 3 a UV–Vis spectrograph of Co3O4, b bandgap calculation for Co3O4 nanoparticles

13


Applied Nanoscience

Fig. 4 a Skeleton of inverted device structure. b Energy band diagram of P3HT:Co3O4:PCBM. c PTB7: C o3O4:PCBM

profoundly depends on the band position of active materials (LUMOA–HOMOD). In ternary devices, conduction band of NPs can effectively change the upper limit of Voc (Gershon 2011). Upon light incidence, excitons generated in the photoactive layer are separated at “D-A” interface. In this structure, holes-h+ move towards MoO3/Ag while electrons (e−s) have two paths to approach ITO, either travel through PCBM or higher electron motility NPs. The possible pathways for e−s transport are P3HT/PTB7:PCBM, P3HT/PTB7:Co3O4 Fig. 5 J–V graphs a P3HT:Co3O4:PCBM. b PTB7:Co3O4:PCBM

13

and Co3O4:PCBM may enhance the charge separation and transportation (Ikram et al. 2015a, b, c, d). Furthermore, once e−s are injected to ETL, the appropriate energy levels of ZnO collects e−s and block holes. The current density (Jsc) vs voltage (V) characteristics of binary (P3HT/PTB7:PCBM) and ternary (P3HT/ PTB7:Co3O4:PC60BM/PC70BM) system based devices are presented in Fig. 5a, b. Electrical parameters extracted from J–V curves under one sun are summarized in Table 1. Upon

Author's personal copy Applied Nanoscience Table 1 J–V data of P3HT:Co3O4:PCBM and PTB7:Co3O4:PCBM based inverted devices Devices

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Rs (Ω-cm2)

D1 D2 D3 D4 D5 D6 D7 D8 D9 D10

8.82 8.94 9.29 9.20 8.66 15.23 15.39 15.43 15.17 14.83

0.56 0.57 0.58 0.57 0.55 0.71 0.71 0.71 0.71 0.71

59 61 63 61 58 56 57 60 57 56

2.91 3.10 3.39 3.19 2.76 6.05 6.22 6.57 6.13 5.89

2.85 2.55 2.37 2.52 4.25 3.40 3.29 3.20 2.74 3.56

mixing of NPs, an increase in fill factor (FF) and Jsc was found lead up PCEs from 2.91 to 3.39% and 6.05–6.57% for D1–D3 and D6–D8, respectively. A significant increase in FF was observed with the addition of Co3O4-NPs in the P3HT and PTB7 based devices. Presence of NPs form percolation network in active layer which facilitates charge transfer due to high electron mobility at “D-A” interfaces causing a decrease in series resistance (Rs) and increase in Jsc (Lin et al. 2013; Yang et al. 2013a; Ikram et al. 2015c). For PTB7 based devices, Rs decreases but no appreciable change in Jsc was found which suggests absorption of the higher amount of PC70BM relative to PTB7 (1.5:1) dominate C o3O4. Likewise, Voc for the D1–D3 increases due to the energy level alignment between HOMO of “D” and LUMO of “A” in the presence of Ec of NPs (Ikram et al. 2014; Feng et al.

Fig. 6 SEM images of P3HT:Co3O4:PC60BM (D1–D5) and PTB7:Co3O4:PC70BM (D6–D10)

13


Applied Nanoscience

Fig. 7 AFM images of P3HT:Co3O4:PCBM (D1–D5) and PTB7:Co3O4:PCBM (D6–D9)

2014). For PTB7 based devices (D6–D10) exhibit constant Voc as doping might be acting as electrons cascade (Sharma et al. 2016). The decrease in Rs and increase in Jsc, FF and Voc were observed with small amount (2%) of Co3O4 in the active layer, beyond this ratio trend reverse due to the aggregation of NPs that breaks down the BHJ structure and removes percolation network (Ikram et al. 2014). Figure 6 represents the SEM images of the films with and without C o3O4 NPs. A smooth surface can be observed for the binary system film (Fig. 6, D1 and D6). Incorporation of NPs in the active layer demonstrates the formation of percolation network for ternary blend films marked with red circles in Fig. 6, D3 and D8). With the optimum amount, NPs link the PCBM domain causing increased mobility and reduced resistance of the devices. However, at higher concentrations, small and large agglomeration of NPs are formed which are distributed randomly on film

13

surface (Fig. 6, D5 and D10). The presence of agglomerates discontinues the favorable percolation network and breaks down the BHJ. This results in a decreased Jsc (Table 1) as agglomerates also act as recombination centers/charge traps and limits the charge collection (Ikram et al. 2016). Active layer morphology plays a crucial role in OSCs performance, therefore, to further investigate the effect of Co3O4 NPs to polymer active layer, the micromorphology and film roughness of binary and ternary systems were performed using atomic force microscopy (AFM) as shown in Fig. 7. For control devices Fig. 7, D1 and D6, film surfaces are smooth with root mean square (RMS) value 3.21 and 2.1 nm, respectively. Conjugation of NPs increased the roughness of films relative to NPs free films (Fig. 7, D2–D5, D7–D9) with RMS values 7.16, 8.27, 10.84, 11.5 nm and 2.7, 3.04, 3.84 nm, respectively. Increase in roughness corresponds to the formation of small and large agglomerates with Co3O4 resulting in charge transport distance reduction

Author's personal copy Applied Nanoscience Fig. 8 UV-Vis absorption spectra of films a P3HT:Co3O4: PCBM. b PTB7:Co3O4:PCBM

Fig. 9 EQE profile a P3HT:Co3O4:PCBM. b PTB7:Co3O4:PCBM

and photocurrent increment. However, larger agglomerates of NPs cause phase separation and removes pathways for charge carriers Fig. 7, D5 (Imran et al. 2017). Figure 8a, b illustrate UV–Vis absorption spectra of the binary and ternary blend based films. With the increasing amount of C o3O4 NPs, an increase in absorption was observed Fig. 8, D2–D5 and D7–D10. This increment in the visible region is attributed to C o3O4 absorption as it has band gap energies 1.3 and 2 eV (Fig. 3a). The higher surface roughness of the films lead to stronger scattering of light which predominantly enhanced absorption in the visible region (Lim et al. 2017). Relative to binary films, ternary blend films show higher and wide range of absorption with the addition of Co3O4-NPs but for PTB7:PCB70M system, strong absorption is occurring around 400–500 nm indicating that PCBM dominates Co3O4-NPs. Thus, absorption spectrum supports the argument that increase in Jsc is basically due to the better morphology. The measured external quantum efficiency (EQE) of binary and ternary devices are shown in Fig. 9a, b. Inverted

OSCs based devices P3HT:PC60BM and PTB7:PC70BM show broad and strong response from 390 to 625 nm and 400–725 nm, respectively. An increase in EQE with the incorporation of C o 3O 4 in the P3HT:PC 60BM and PTB7:PC70BM blend devices were observed. This increase is credited to increase in absorption with the NPs incorporation in the different polymer-based devices.

Conclusion Various concentrations of C o3O4 NPs were incorporated into the active layers of P3HT:PCB60M and PTB7:PC70BM based inverted BHJ solar cells dissolved in CB and DCB as solvents. Mixing of the optimum amount of Co3O4 in the polymer binary blend devices enhanced PCE from 2.91 to 3.39% and 6.05–6.57%, respectively. Increase in PCE of the ternary system was mostly due to decrease in Rs correspond to increase in FF. Addition of NPs increased the surface roughness and also provides pathways for charges to

13


Applied Nanoscience

transport in active layer towards their respective electrodes. Device performance of ternary system increased with optimum amount of C o3O4 NPs and this benefit reverse with higher concentration of NPs. The higher concentrations of metal oxides in the active layers showed agglomerations thus lower the device efficiency. Acknowledgements The authors thankful to higher education commission (HEC), Pakistan for financial support through PAK-US joint Project.

Compliance with ethical standards Conflict of interest The authors confirm that this manuscript has no conflict of interest.

References Abdullah MM, Rajab FM, Al-Abbas SM (2014) Structural and optical characterization of C r2O3 nanostructures: evaluation of its dielectric properties. AIP Adv. https://doi.org/10.1063/1.4867012 Ahmad Z, Touati F, Muhammad FF et al (2017) Effect of ambient temperature on the efficiency of the PCPDTBT: pC 71 BM BHJ solar cells. Appl Phys A Mater Sci Process 123:3–8. https://doi.org/10.1007/s00339-017-1098-8 Allaedini G, Muhammad A (2013) Study of influential factors in synthesis and characterization of cobalt oxide nanoparticles. J Nanostruct Chem 3:77. https://doi.org/10.1186/2193-8865-3-77 Beek WJE, Wienk MM, Janssen RAJ (2004) Efficient hybrid solar cells from zinc oxide nanoparticles and a conjugated polymer. Adv Mater 16:1009–1013. https://doi.org/10.1002/adma.20030 6659 Bera SR, Saha S (2016) Fabrication of CdTe/Si heterojunction solar cell. Appl Nanosci 6:1037–1042. https://doi.org/10.1007/s1320 4-015-0516-5 Bhalla V, Tyagi H (2016) Solar energy harvesting by cobalt oxide nanoparticles, a nanofluid absorption based system. Sustain Energy Technol Assess. https: //doi.org/10.1016/j. seta.2017.01.011 Chen CC, Chang WH, Yoshimura K et al (2014) An efficient triplejunction polymer solar cell having a power conversion efficiency exceeding 11%. Adv Mater 26:5670–5677. https://doi. org/10.1002/adma.201402072 Etxebarria I, Guerrero A, Albero J et al (2014) Inverted vs standard PTB7:PC70BM organic photovoltaic devices. The benefit of highly selective and extracting contacts in device performance. Org Electron Phys Mater Appl 15:2756–2762. https : //doi. org/10.1016/j.orgel.2014.08.008 Farhadi S, Safabakhsh J, Zaringhadam P (2013) Synthesis, characterization, and investigation of optical and magnetic properties of cobalt oxide (Co3O4) nanoparticles. J Nanostruct Chem 3:9. https ://doi.org/10.1186/2193-8865-3-69 Feng W, Rangan S, Cao Y et al (2014) Energy level alignment of polythiophene/ZnO hybrid Solar cells. J Mater Chem A 2:7034. https ://doi.org/10.1039/c4ta00937a Fu H, Choi M, Luan W et al (2012) Hybrid solar cells with an inverted structure: nanodots incorporated ternary system. Solid State Electron 69:50–54. https://doi.org/10.1016/j.sse.2011.12.009 Ganesamoorthy R, Sathiyan G, Sakthivel P (2017) Review: fullerene based acceptors for efficient bulk heterojunction organic solar cell

13

applications. Sol Energy Mater Sol Cells 161:102–148. https: //doi. org/10.1016/j.solmat.2016.11.024 Gershon T (2011) Metal oxide applications in organic-based photovoltaics. Mater Sci Technol 27:1357–1371. https : //doi. org/10.1179/026708311X13081465539809 Gong Z, Karandikar S, Zhang X et al (2010) Composite nanomaterial thin film-based biosensors. Proc IEEE Sens. https://doi. org/10.1109/ICSENS.2010.5690979 Greiner MT, Helander MG, Tang W-M et al (2011) Universal energylevel alignment of molecules on metal oxides. Nat Mater 11:76– 81. https://doi.org/10.1038/nmat3159 Ikram M, Murray R, Hussain A et al (2014) Hybrid organic solar cells using both ZnO and PCBM as electron acceptor materials. Mater Sci Eng B 189:64–69. https: //doi.org/10.1016/j. mseb.2014.08.005 Ikram M, Ali S, Murray R et al (2015a) Influence of fullerene derivative replacement with T iO2 nanoparticles in organic bulk heterojunction solar cells. Curr Appl Phys 15:48–54. https://doi. org/10.1016/j.cap.2014.10.026 Ikram M, Imran M, Nunzi JM et al (2015b) Replacement of P3HT and PCBM with metal oxides nanoparticles in inverted hybrid organic solar cells. Synth Met 210:268–272. https://doi.org/10.1016/j. synthmet.2015.10.005 Ikram M, Imran M, Nunzi JM et al (2015c) Efficient and low cost inverted hybrid bulk heterojunction solar cells. J Renew Sustain Energy. https://doi.org/10.1063/1.4929603 Ikram M, Imran M, Nunzi JM, Ali S (2015d) Efficient inverted hybrid solar cells using both CuO and P3HT as an electron donor materials. J Mater Sci Mater Electron 26:6478–6483. https://doi. org/10.1007/s10854-015-3239-1 Ikram M, Murray R, Imran M et al (2016) Enhanced performance of P3HT/(PCBM:ZnO:TiO2) blend based hybrid organic solar cells. Mater Res Bull 75:35–40. https://doi.org/10.1016/j.mater resbull.2015.11.031 Imran M, Ikram M, Dilpazir S et al (2017) Towards efficient and cost-effective inverted hybrid organic solar cells using inorganic semiconductor in the active layer. Appl Nanosci. https://doi. org/10.1007/s13204-017-0618-3 Jotterand SA, Jobin M (2011) Characterization of P3HT:PCBM:CdSe hybrid solar cells. Energy Proc 31:117–123. https : //doi. org/10.1016/j.egypro.2012.11.173 Kadem B, Hassan A, Cranton W (2016) Efficient P3HT:PCBM bulk heterojunction organic solar cells; effect of post deposition thermal treatment. J Mater Sci Mater Electron 27:7038–7048. https ://doi.org/10.1007/s10854-016-4661-8 Kim JY, Lee K, Coates NE, Moses D, Nguyen TQ, Dante M, Heeger AJ (2007) Efficient tandem polymer solar cells fabricated by allsolution processing. Science 317(5835):222–225. https://doi. org/10.1126/science.1141711 Kozma E, Kotowski D, Catellani M et al (2013) Synthesis and characterization of new electron acceptor perylene diimide molecules for photovoltaic applications. Dye Pigment 99:329–338. https:// doi.org/10.1016/j.dyepig.2013.05.011 Kupfer B, Majhi K, Keller DA et al (2015) Thin film Co3O4/TiO2 heterojunction solar cells. Adv Energy Mater 5:2–6. https://doi. org/10.1002/aenm.201401007 Lee W, Shin S, Han SH, Cho BW (2008) Manipulating interfaces in a hybrid solar cell by in situ photosensitizer polymerization and sequential hydrophilicity/hydrophobicity control for enhanced conversion efficiency. Appl Phys Lett 92:20–23. https://doi. org/10.1063/1.2929368 Liang N, Jiang W, Hou J, Wang Z (2017) New developments in nonfullerene small molecule acceptors for polymer solar cells. Mater Chem Front 1:1291–1303. https: //doi.org/10.1039/C6QM00 247A Lim JW, Hwang DK, Lim KY et al (2017) ZnO-morphology-dependent effects on the photovoltaic performance for inverted polymer

Author's personal copy Applied Nanoscience solar cells. Sol Energy Mater Sol Cells 169:28–32. https://doi. org/10.1016/j.solmat.2017.04.046 Lin J-F, Tu G-Y, Ho C-C et al (2013) Molecular structure effect of pyridine-based surface ligand on the performance of P3HT:TiO2 hybrid solar cell. ACS Appl Mater Interfaces 5:1009–1016. https ://doi.org/10.1021/am302700c Lin K, Wang J, Hu Z et al (2017) Novel cross-linked films from epoxyfunctionalized conjugated polymer and amine based small molecule for the interface engineering of high-efficiency inverted polymer solar cells. Sol Energy Mater Sol Cells 168:22–29. https ://doi.org/10.1016/j.solmat.2017.03.035 Mahmoud KH (2016) Synthesis and spectroscopic investigation of cobalt oxide nanoparticles. Polym Compos 37:101–113. https:// doi.org/10.1002/pc.23364 Majhi K, Bertoluzzi L, Keller DA et al (2016) Co3O4 based all-oxide PV: a numerical simulation analyzed combinatorial material science study. J Phys Chem C 120:9053–9060. https://doi. org/10.1021/acs.jpcc.6b01164 Makhlouf SA, Bakr ZH, Aly KI, Moustafa MS (2013) Structural, electrical and optical properties of C o3O4 nanoparticles. Superlattices Microstruct 64:107–117. https : //doi.org/10.1016/j. spmi.2013.09.023 Oh SH, Heo SJ, Yang JS, Kim HJ (2013) Effects of ZnO nanoparticles on P3HT:PCBM organic solar cells with DMF-modulated PEDOT:PSS buffer layers. ACS Appl Mater Interfaces 5:11530– 11534. https://doi.org/10.1021/am4046475 Salavati-Niasari M, Khansari A (2014) Synthesis and characterization of Co3O4 nanoparticles by a simple method. Comptes Rendus Chim 17:352–358. https://doi.org/10.1016/j.crci.2013.01.023 Sharma R, Bhalerao S, Gupta D (2016) Effect of incorporation of CdS NPs on performance of PTB7: PCBM organic solar cells. Org Electron Phys Mater Appl 33:274–280. https://doi.org/10.1016/j. orgel.2016.03.030 Stylianakis MM, Konios D, Petridis C et al (2017) Ternary solutionprocessed organic solar cells incorporating 2D materials. 2D Mater. https://doi.org/10.1088/2053-1583/aa8440 Tian Y, Zhang X, Geng H-Z et al (2017) Carbon nanotube/polyurethane films with high transparency, low sheet resistance and strong adhesion for antistatic application. RSC Adv 7:53018–53024. https://doi.org/10.1039/C7RA10092B Wang X, Peng Q, Zhu W, Lei G (2015) High performance of inverted polymer solar cells with cobalt oxide as hole-transporting layer. Semicond Sci Technol 30:55001. https://doi.org/10.1088/02681242/30/5/055001 Wanninayake AP, Gunashekar S, Li S et al (2015) Performance enhancement of polymer solar cells using copper oxide nanoparticles. Semicond Sci Technol 30:64004. https://doi. org/10.1088/0268-1242/30/6/064004

Wright M, Uddin A (2012) Organic-inorganic hybrid solar cells: a comparative review. Sol Energy Mater Sol Cells 107:87–111. https://doi.org/10.1016/j.solmat.2012.07.006 Wu J, Yue G, Xiao Y et al (2013) An ultraviolet responsive hybrid solar cell based on titania/poly(3-hexylthiophene). Sci Rep 3:1283. https://doi.org/10.1038/srep01283 Yang P, Zhou X, Cao G, Luscombe CK (2010) P3HT:PCBM polymer solar cells with T iO2 nanotube aggregates in the active layer. J Mater Chem 20:2612. https://doi.org/10.1039/b921758d Yang S, Hou Y, Xing J et al (2013a) Ultrathin SnO2 scaffolds for TiO2-based heterojunction photoanodes in dye-sensitized solar cells: oriented charge transport and improved light scattering. Chem A Eur J 19:9366–9370. https: //doi.org/10.1002/chem.20130 0524 Yang S, Hou Y, Zhang B et al (2013b) Highly efficient overlayer derived from peroxotitanium for dye-sensitized solar cells. J Mater Chem A 1:1374–1379. https://doi.org/10.1039/C2TA00688J Yoon S, Heo SJ, Kim HJ (2013) Hybrid polymer/inorganic nanoparticle blended ternary solar cells. Phys Status Solidi Rapid Res Lett 7:534–537. https://doi.org/10.1002/pssr.201307157 Yue W, Ashraf RS, Nielsen CB et al (2015) A thieno[3,2-b][1]benzothiophene isoindigo building block for additive- and annealingfree high-performance polymer solar cells. Adv Mater 27:4702– 4707. https://doi.org/10.1002/adma.201501841 Zhang H, Pokhrel S, Ji Z et al (2014) PdO doping tunes band-gap energy levels as well as oxidative stress responses to a C o 3O 4 p-type semiconductor in cells and the lung. J Am Chem Soc 136:6406–6420. https://doi.org/10.1021/ja501699e Zhang J, Zhang Y, Fang J et al (2015) Conjugated polymer-small molecule alloy leads to high efficient ternary organic. Sol Cells. https ://doi.org/10.1021/jacs.5b03449 Zhang Y, Bovill E, Kingsley J et al (2016) PCDTBT based solar cells: 1 year of operation under real-world conditions. Sci Rep 6:21632. https://doi.org/10.1038/srep21632 Zhong M, Yang D, Zhang J et al (2012) Improving the performance of CdS/P3HT hybrid inverted solar cells by interfacial modification. Sol Energy Mater Sol Cells 96:160–165. https: //doi.org/10.1016/j. solmat.2011.09.041 Zhou N, Lin H, Lou SJ et al (2014) Morphology-performance relationships in high-efficiency all-polymer solar cells. Adv Energy Mater. https://doi.org/10.1002/aenm.201300785 Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

13