A transient photovoltage study into the voltage dependence of high LUMO organic layers and band gap tuning in high efficiency low hysteresis hybrid-OPV-perovskite solar cells

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Oral, Daniel Bryant, presentation 137
Publication date: 5th February 2015

Over the past few years the organic-inorganic lead halide ABX₃ perovskite crystal structure, where commonly A=CH₃NH₃ B=Pb and X=I, has been shown to be a promising competitor to commercial silicon solar cells due to their unique light harvesting and transporting properties combined with a lower cost and ease of processing [1-2]. One particularly appealing property of CH₃NH₃PbI₃ perovskite is their compatibility with a wide range of polymers commonly used in the OPV field such as PCBM and Poly-TPD [3]. Owing to their large diffusion lengths and suitable bands gaps a CH₃NH₃PbI₃ perovskite layer has been used as a donor in standard OPV architectures with efficiencies now reaching 15.3% [4-6]. It has also been shown that iodine can be replaced at the X site with varying amounts of bromine leading to a tunable band gap and LUMO level leading to enhanced Voc as well as improved stability [7]. More recently some of the methylammonium A site cations have been replaced with formamidinium in combination with bromine at some of the X sites to yield an efficient but stable 18% perovskite solar cell [8]. Here we use the flexible variation in the A and X sites to tune the band gap of the perovskite layer such that high LUMO level electron accepting C60 derivative polymers can be used, producing cells with high open-circuit voltages (>1.2V) whilst maintaining a high operational currents (>17mA) from cells that have negligible hysteresis. We note, as others have, that the voltage within these cells cannot be predicted by the HOMO and LUMO levels of the hole and electron accepting layers alone but can be limited by them. The low hysteresis within the cells has allowed characterisation and a chance investigate the origin and factors that influence voltage through a series of charge extraction and transient photovoltage measurements. We further utilise the formulation diversity combined with specially developed processing techniques to show how the hybrid-OPV-Perovskite cell’s properties can be influenced when cells with the same components are built in either in inverted or non-inverted architecture for optimal device engineering. This has allowed us (in collaboration) to design both electron accepting and hole accepting layers which can be used in conjunction with organic-inorganic lead halide perovskites with not only performance enhancement but also from a commercial point of view offer a cost benefit.
Energy level diagram for a hybrid-OPV-perovskite solar cell employing either CH3NH3PbI3 or CH3NH3Pb(I(1-x)Br(x))3 and the potential for increased LUMO level polymer inclusion for increased voltage output
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