Solution Processed PbS Quantum Dot Tandem Solar Cells

Abstract

Colloidal quantum dot solar cells have drawn great attention in the past decade due to their easy synthesis, tunable bandgaps and low cost. However, the quantum dots’ low absorbance together with limits placed on the thickness of the absorbing layer due to poor transport properties is hindering the development of quantum dot solar cells. In this research, for further developing quantum dot solar cells, a tandem solar cell strategy was utilised to address the above problem. A tandem solar cell stacks materials with different bandgaps to more efficiently convert the energy of a wider range of photons to electricity for higher solar cell efficiency. This project developed high performance monolithic tandem solar cells based on PbS colloidal quantum dots. Firstly, single-junction colloidal quantum dot cells were optimised as the building blocks of the tandem cells. MgCl2 passivated ZnO-NP electron transporting layers were applied to improve single-junction colloidal quantum dot cells by increasing both fill factor FF and short circuit current Jsc. Significant efficiency improvement has been observed based on single-junction PbS solar cells. Secondly, an optical modelling tool was created for the first time to simulate tandem colloidal quantum dot solar cells. This model could offer exact simulation on photon absorbance and current generation in tandem cells’ functional layers. The model provided very instructive and quite essential guidance for designing the tandem cell structure and experimental works of fabricating quantum dot tandem cells. Thirdly, based on improved single-junction PbS solar cell and an established directive model, high performance tandem colloidal quantum dot solar cells were successfully fabricated with different interlayers of both Au and ITO. A graded band alignment employing both ZnO-doping and quantum dot size tuning has been applied to further improve tandem cell performance. Due to improved carriers extraction, this strategy successfully pushed the efficiency of a tandem quantum dot solar cell to be around 7%.