The study of inorganic perovskite solar cells

Abstract

Energy harvesting is a highly concerned issue in our energy-demanded community. Solar energy is one of the energy sources to provide sustainable energy and a perovskite solar cell (PSC) is a new kind of solar cells emerging in 2009 with a power conversion efficiency (PCE) of 3.8%. Due to the intrinsic potential of PSCs and a great amount of effort by researchers, the PCE of PSCs has been greatly enhanced to 22.7% in 2017 and is already higher than that of some commercial silicon solar cells. However, the highly efficient hybrid organic-inorganic PSCs suffered from instability due to the organic component. Therefore, developing inorganic perovskite solar cells, for example, caesium lead halide, is one of the most promising strategies to solve the problem. In this thesis, inorganic caesium lead halide PSCs were studied and our works were mainly divided into two parts. The first one was to reduce the high annealing temperature of caesium lead bromide (CsPbBr₃) from 250 °C to 160 °C by a pyridine treatment. A high-temperature annealing process increased the fabrication cost, reduced the material choices for different parts of the devices and limited the possibility of flexible devices. The pyridine-treated PSCs demonstrated the highest PCE of 6.04%, which was comparable to that of the control devices prepared at a high temperature. The mechanism was that pyridine vapour reacted with a PbBr₂ film to form an intermediate phase PbBr₂.(Py)x which reduced the thermal activation energy for the formation and crystallization of CsPbBr₃ perovskite. The pyridine-treated devices without encapsulation can also exhibit high stability in the ambient air with relative humidity up to 70%. This work provides a low-temperature technique for the fabrication of other inorganic PSCs. Another work was to apply a copper(I) thiocyanate (CuSCN) as a hole transport layer in the CsPbI₂Br PSCs to improve the efficiency and stability of devices. The top 10% of photon flux of the sunlight AM 1.5 is on the range of 550 nm to 810 nm. CsPbI₂Br perovskite has a lower bandgap (1.92 eV) than CsPbBr₃, so higher short-circuit current density (Jsc) can be generated. By replacing the hole transport layer of spiro-OMeTAD with CuSCN, the PCE of the devices were improved from 7.31% to 10.36% and Jsc was enhanced from 13.1 mA/cm² to 14.1 mA/cm². The encapsulated devices with CuSCN can sustain 93% of its initial efficiency after one month. For film degradation, we have also identified that the degradation started from the grain boundaries of the films. These findings pave a way for realizing PSCs with better stability and higher efficiency.