Improving the Interface of Silicon and Perovskite Solar Cells

Abstract

In recent years, multijunction solar cells have shown outstanding capability to achieve remarkably high efficiencies. Among them perovskite-silicon tandem solar cell is a promising candidate with an efficiency surpassed that of single junction silicon cell. Most of the power loss in solar cells originates from surface recombination, hence one potential challenge in fabricating high efficiency perovskite-silicon tandem cell is modifying the interfaces, in order to reduce interfacial recombination. This thesis focuses on surface passivation in perovskite-silicon tandem cell, accompanied by modelling of perovskite solar cells to explore the effect of charge selective layers on interface recombination, device performance, and self-doping within the absorber layer. We first demonstrate a unique and straightforward interlayer-free approach to passivate highly boron-doped low resistivity n-Si using a thin layer of TiO2 fabricated by atomic layer deposition and a suitable pre-treatment of the silicon surface. The resultant passivation is both highly effective and also conducting. This is significant as it will allow direct fabrication of perovskite solar cells (PSCs) on standard homo-junction silicon solar cells, increasing the efficiency of photovoltaics while avoiding the need for costlier heterojunction silicon cells or complex integration. This passivation technique is also of interest for standalone silicon solar cells, as we achieve efficacious passivation even for highly doped layers. Second, we design, fabricate, and characterize an effective surface passivation layer for perovskite solar cells. In this project, we apply a dimensional engineering technique using a mixture of two different spacer cations to decrease non-radiative recombination at perovskite/HTL interface. The dual cation passivation layer can provide open-circuit voltage of 1.21V with a power conversion efficiency of 23.13%, which is superior to their single cation counterparts. The mixed cation passivation layer forms a 1D/2D perovskite film on top of 3D perovskite, leading to a more hydrophobic and smoother surface. Space charge-limited current measurement illustrates three times lower trap-filled limit voltage in mixed cation passivated sample than the reference cell, indicating fewer trapped states in this device. The shelf-life stability test in ambient with 60% relative humidity also reveals the highest stability for the device with dual cation surface passivation. Third, we employ numerical PSC simulations exploiting COMSOL Multiphysics to investigate the effect of varying ion concentration and transport layers' work function on voltage and power loss. These simulations are extremely helpful to unravel the physical principles behind the photovoltaic performance of various device structures. We show that the equilibrium electrostatics of the perovskite-transport layer heterojunctions, which are determined by the work function difference between the two materials, can result in increased rates of recombination for any given concentration of interface defects. Finally, following the prior modelling work, we investigate the self-doping feature of the perovskite layer by considering the impact of the device structure. Our modelling results show that the doping occurs due to the formation of net concentration of ions, and because the ions move in response to the electrostatic of the whole cell, the doping in the perovskite can depend on the band alignment at the interfaces. As a case study we compare two different device structures with p- and n-type perovskite doping in terms of their work function impact on the charge distribution, which in turn determines the doping. In summary, in this thesis we present some surface passivation techniques for both silicon and perovskite solar cells and present comprehensive simulation studies on the effect of perovskite/transport layer interfaces and device structure on cell performance. Show more