Improved performance and stability of perovskite solar cells with bilayer electron-transporting layers

Zinc oxide nanoparticles (NPs) are very promising in replacing the phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as electron-transporting materials due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures. The perovskite/ZnO NPs heterojunction has also demonstrated much better stability than perovskite/PC₆₁BM, however it shows lower power conversion efficiency (PCE) compared to the state-of-art devices based on perovskite/PCBM heterojunction. Here, we demonstrated that the insufficient charge transfer from methylammonium lead iodide (MAPbI₃) to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSC with MAPbI₃/ZnO NPs heterojunction. When PC₆₁BM/ZnO NPs bilayer electron transporting layers (ETLs) were used with a device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, which can combine the advantages of efficient charge transfer from MAPbI₃ to PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrodes, highly efficient PSCs with PCE as high as 17.2% can be achieved with decent stability.

Perovskite solar cells with PC₆₁BM/ZnO nanoparticles bilayer electron-transporting layers were achieved with a power conversion efficiency of 17.2% and decent stability.

Organic–inorganic hybrid perovskite solar cells (PSCs) have recently attracted tremendous attention because of their excellent photovoltaic efficiencies.^1–4 Since the initial results published in 2009 with efficiencies about 4% using a typical dye-sensitized solar cell structure with liquid electrolyte,⁵ significant progress has been made in device performance through developing high quality film processing methods,^6–10 tuning the perovskite composition,^11–15 optimizing the device architectures^16,17 and synthesizing new hole/electron transport materials.^18–21 Recently, a certified record power conversion efficiency (PCE) of 22.7% was achieved.²² Despite of the success in obtaining dramatically improved PCE, there are certain concerns about the stability and cost towards commercialization. For the state-of-the-art PSCs, perovskites are susceptible to degradation in moisture and air, thus the charge transport materials should prevent the perovskite from exposure to such environments.^20,23–25 One the other hand, PSCs also suffer from the high cost of widely used organic charge transport materials such as 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD), phenyl-C_61/71-butyric acid methyl ester (PC_61/71BM).^3,18,26 As alternatives, inorganic materials such as CuSCN,²⁷ CuI,²⁸ CuGaO₂,²⁰ and NiO_x ^29,30 which can be acted as hole transport materials and ZnO,^31,32 SnO₂ ^12,33,34 and TiO₂ ^10,35 which can be acted as electron transport materials are widely studied. Among them, metal oxide nanoparticles (NPs) are very promising in replacing the organic counterparts due to the high carrier mobilities, superior stability, low cost and solution processability at low temperatures.^16,31,33The perovskite/ZnO NPs heterojunction has been demonstrated much better stability than perovskite/PCBM,²³ however it shows lower PCE compared to the state-of-art devices based on perovskite/PCBM heterojunction.^36–38 Thus in this paper, we systematically studied the charge transfer and recombination at CH₃NH₃PbI₃ (MAPbI₃) and ZnO NPs or PC₆₁BM interfaces and tried to fabricate devices with high PCE and super stability simultaneously. We demonstrated that insufficient charge transfer from MAPbI₃ to ZnO NPs and significant interface trap-states lead to the poor performance and severe hysteresis of PSCs based on MAPbI₃/ZnO NPs heterojunction, while the devices based on MAPbI₃/PC₆₁BM show high PCE and negligible hysteresis due to the efficient charge transfer from MAPbI₃ to PC₆₁BM and less recombination at the interface. On the other hand, the MAPbI₃/ZnO NPs devices show excellent stability in air because of the excellent capping ability of ZnO NPs while the stability of MAPbI₃/PC₆₁BM devices is very poor. Thus, we fabricated the PSCs with bilayer electron-transporting layers (ETLs) with the device structure of ITO/poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) (PTAA)/MAPbI₃/PC₆₁BM/ZnO NPs/Al, trying to combine the advantages of efficient charge extraction ability of PC₆₁BM and excellent blocking ability of ZnO NPs against oxygen, water and electrode, and finally device with PCE as high as 17.2% was achieved with decent stability.