Interface Engineering for Highly Efficient Perovskite Solar Cells: Role of Surface Passivation

Abstract

This thesis focuses on developing high-efficiency perovskite solar cells via interface engineering; and on understanding the correlation between open-circuit voltage and surface recombination. This work can be divided into four parts: (i) introducing an indium-doped TiOx electron transport layer; (ii) developing a one-side passivation layer (PMMA:PCBM) to passivate the ETL/perovskite interfaces, where PMMA, PCBM and ETL are poly(methyl methacrylate), phenyl-C61-butyric acid methyl ester and electron transport layer, respectively; (iii) establishing a double-sided passivation design to passivate both ETLs/perovskite and perovskite/HTLs interfaces, where HTL is hole transport layer; (iv) exploiting a set of combination ETLs (ZnO/MgF2) for substituting the fullerene-based ETLs in inverted perovskite solar cells. We first show that the extrinsic indium-doping improves both the conductivity of the transport layer and the band alignment at the ETL/perovskite interface compared to pure TiO2, boosting the fill-factor and voltage of perovskite cells. Using the optimized transport layers, we demonstrate a high steady-state efficiency of 17.9% for CH3NH3PbI3-based cells and 19.3% for Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3-based cells, corresponding to absolute efficiency gains of 4.4% and 1.2% respectively as compared to TiO2-based control cells. In addition, we report a steady-state efficiency of 16.6% for a semi-transparent cell and use it to achieve a four-terminal perovskite-silicon tandem cell with a steady-state efficiency of 24.5%. Second, we demonstrate an ultrathin passivation layer consisting of a PMMA:PCBM mixture that can effectively passivate defects at or near to the perovskite/TiO2 interface, significantly suppressing interfacial recombination. The passivation layer increases the open circuit voltage of mixed-cation perovskite cells by as much as 80 mV, with champion cells achieving Voc ~1.18 V. As a result, we obtain efficient and stable perovskite solar cells with a steady-state PCE of 20.4% and negligible hysteresis over a large range of scan rates. In addition, we show that the passivated cells exhibit very fast current and voltage response times of less than 3 s under cyclic illumination. Third, we introduce a double-side passivating contact design using ultrathin PMMA films. We demonstrate very high-efficiency (~20.8%) perovskite cells with some of the highest open circuit voltages (1.22 V) reported for the same 1.6 eV bandgap. Photoluminescence imaging and transient spectroscopic measurements confirm a significant reduction in non-radiative recombination in the passivated cells, consistent with the voltage increase. Analysis of the molecular interactions between perovskite and PMMA reveals that the carbonyl (C=O) groups on the PMMA are responsible for the excellent passivation via Lewis-base electronic passivation of Pb2+ ions. At last, we demonstrate a set of effective combination ETLs comprising ZnO (~70 nm)/MgF2 (~1.5 nm) for the purpose of enhancing the performance of MAPbI3-based inverted perovskite solar cells. The outstanding work function, conductivity and hole-blocking properties of ZnO nanomaterials along with the extra contribution from the ultrathin insulating MgF2 layer (diminished the injection barrier of ETLs/Al interfaces, thus reducing the contact resistance), make them ideal for use as excellent ETLs that can replace and outperform the fullerene-based ETLs in inverted perovskite solar cells. Combining the ZnO/MgF2 ETL with an ultrathin PMMA:PCBM passivation film, a high PCE of 17.5% with a high FF (~0.795) and negligible hysteresis was achieved for the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based inverted perovskite cells.