Design and Simulation of Electron and Hole Transport Layer for Lead-Free Perovskite Solar Cell Application

Abstract

The perovskite solar cells are an emerging photovoltaic technology as it reaches 25.5% efficiency within a decade. Due to this rapid advancement researchers have developed various perovskite absorber layers, charge transport layers, transparent conducting oxides and metal contacts. However, it is quite complicated to fabricate numerous combinations of these layers and hence the simulation is an efficient way to analyse the best possible combination which can result in higher device performance. In the present thesis work, the numerical simulation of lead-free perovskite solar cells is performed using SCAPS 1D software. The current study brings forth the numerical simulation of various lead-free perovskite alternatives having narrow and wide bandgap configurations. For the narrow bandgap configuration, the FA0.75MA0.25Sn0.25 Ge0.5I3 and CsSnGeI3 based perovskite layers are considered. However, for the wide bandgap application, the CH3NH3GeI3 based perovskite layer is considered. The perovskite solar cells using these layers are optimized based on charge transport layers, perovskite absorber layer thickness, perovskite absorber defect density and their energy band alignment with respect to the contacts. To investigate the effect of charge transport layers (i.e., hole transport layer and electron transport layer), the work shows that the correlation of VOC with the built-in potential (Vbi). The results disclosed that to attain better VOC and PV performance, the device should exhibit the better Vbi and proper band alignment that allows the efficient facilitation of charge carriers, along with good charge carrier mobility. Thus, it is suggested that for the proper transport of electrons, the conduction band minimum of the electron transport layer (ETL) must lie below the conduction band minimum of the perovskite layer. Similarly, in the case of hole transport, the valence band maximum of the hole transport layer must lie above that of the perovskite layer. Thus, the simulation study mainly suggests the possible combination of the ETL and HTL alternatives for less explored perovskite solar cell configuration. Further, it is found that to optimize the solar cells the thickness of the absorber layer should be carefully chosen. Also, the study shows that the defects of the absorber layer exhibit a significant impact on the device performance as the higher defect led to the creation of more recombination centres and thus reduces the overall PV performance. Thus, it is obtained that the defect density of the perovskite layers should not be greater than 1×1014 cm-3.

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The front and the back contact metal work functions depict a crucial role in determining the device efficiency. If the back metal work function is less than 5 eV it provides a barrier for the charge carrier however high metal work function saturates its PCE and thus 5 eV is recommended. In addition, for the front contact, it is suggested to have the work function should be below 4.4 eV. After, optimizing the standalone configuration, the study is extended to the lead-free all- perovskite multijunction solar cell configuration. This present work proposes a novel realization of (CH3NH3GeI3-CsSnGeI3) and (CH3NH3GeI3-FA0.75MA0.25Sn0.25Ge0.5I3 (FAMASnGeI3)) for the first time. It is obtained that by proper control of the perovskite layer thickness and defect density, by selecting the suitable charge transport layer, the PV performance of multijunction solar cells can be improved. In addition, the simulated multijunction solar cell depicts the device efficiency greater than 26% which is considered as a significant improvement in the field of the lead-free all-perovskite multijunction solar cell. The study and analysis provide a significant insight to the researchers fabricating the highly efficient lead-free all-perovskite multijunction solar cells. Furthermore, the present study is extended towards the realization of lead–free double– perovskite (Cs2AgBi0.75Sb0.25Br6, i.e., mixed antimony bismuth halide double-perovskite) solar cells. The double-perovskite solar cell is optimized with respect to their charge transport layer and it has been found that the device PV performance improves significantly. However, the proposed lead-free double perovskite solar cell depicts the device efficiency up to 18.18 % which is considered as a significant alternative for the lead-free perovskite solar cell along with the satisfactory device photovoltaic performance. All the proposed structures for perovskite solar cell configuration are novel structures and provide better parameters in most cases when compared to the latest existing perovskite structures.