Hole-selective passivating contacts: from process development to application

Abstract

Increasing energy demand and need to reduce greenhouse gas emissions are major concerns for this century. In this context, photovoltaic (PV) technology has risen as one of the primary solutions to overcome these challenges. Therefore, it is important to seek ways to further reduce costs and improve performance of c-Si solar panels. Current state-of-the-art solar panels utilize so-called passivating contacts, a key technology to enable high-efficiency solar cells. Passivating contacts comprise a single layer or stack of materials designed to induce carrier-selectivity and passivation between the c-Si and metal electrodes, minimizing recombination losses. Despite enabling high performances, current passivating contact technologies, based on amorphous or polycrystalline silicon, suffer from one or a combination of drawbacks, such as high parasitic absorption, high-thermal budget or thermal instability, and process complexity. In this context, transition metal oxides (TMOs) have emerged as alternative passivating contact materials mainly due to their improved transparency. In this thesis, the development and application of hole-selective passivating contacts based on TMOs is explored. The thesis can be divided into two sections. In the first part, the development of novel atomic layer deposition (ALD) processes for potential hole-selective contact materials based on copper oxide is reported. In the second, a novel ALD interlayer is employed together with existing hole-selective materials and the resulting structures are investigated as hole-selective contacts in c-Si solar cells. Novel ALD processes are developed for the binary oxides Cu2O and CuO using thermal and plasma processes, respectively. The complete process development is reported, including saturation curves and growth behaviour. Self-limiting reactions are achieved for both processes, producing uniform thin films with low roughness. In a third ALD work, a ternary oxide deposition process to form CuxCryOz is investigated using the super-cycle approach. A wide range of compositions is explored, ranging from pure CuOx to CrOx. An unexpected dependency of growth rate as a function of the CuOx/CrOx sub-cycle ratios is found. Through modelling, it is shown that CuOx enhances the growth of CrOx by a factor of 9, working as a catalyst to the reaction. The properties of the various CuxCryOz compositions are investigated both as-deposited and after annealing, and some tested as potential hole-selective contacts. As-deposited Cr-rich CuxCryOz thin films appear to be strong hole-selective candidates as they present a wide band gap and the lowest contact resistivity among different compositions. In separate works, Cu2O and MoOx hole-selective materials are investigated in novel stack structures containing an ALD-deposited AlyTiOx/TiO2 stack as a passivating interlayer. Both stacks are optimised to achieve the best trade-off between passivation and contact resistivity. Record-low recombination is achieved for both materials while maintaining low contact resistivity and high optical transparency. Additionally, the AlyTiOx/TiO2/MoOx stack demonstrates self-healing behaviour after sputtering of a transparent conductive oxide layer, without requiring an additional annealing step. The optimized stacks of AlyTiOx/TiO2/Cu2O and AlyTiOx/TiO2/MoOx are implemented as full-area contacts at the rear side of proof-of-concept c-Si homojunction solar cells. A significant improvement in open-circuit voltage confirms the boost in surface passivation provided by the interlayer. A record efficiency for devices incorporating full-area TMO-based contacts (including electron-selective materials) without an a-Si:H interlayer is achieved for the stack containing MoOx, providing an important stepping stone towards the development of high-efficiency cells employing TMO-based contacts fabricated using simple, low-temperature processes.