Indium-free transparent conductive oxides for improved solar cell performance and reliability

The rising adoption of solar cells worldwide necessitates reducing solar cell costs, enhancing cell efficiency, and long-term module reliability. New solar cell architectures such as silicon heterojunction (HJT) and thin-film technology like an organic solar cell, perovskite, III-V, copper indium gallium selenide (CIGS), etc. are actively being investigated. The majority of them implement a transparent conductive oxide (TCO), predominantly indium tin oxide (ITO), in their device structure. With the rising cost and depleting indium reserves, it is essential to find alternatives. This thesis focuses on developing and analysing indium-free TCOs fabricated using atomic layer deposition (ALD) and explores various applications of TCOs for solar cells. It begins with a detailed examination of the evolving relevant literature, followed by a detailed description of the techniques used throughout the thesis. ALD grown ZnO based TCO is studied. Firstly, a DFT analysis of various dopants of ZnO is presented. Subsequently, Zr doped ZnO is fabricated, characterized, and implemented as an electron selective layer for organic photovoltaic cells (OPV). The introduction of Zr as a dopant increased electron mobility and a reduction of sheet resistance. This was translated into an OPV device which demonstrated an increase in 1% abs efficiency due to increased carrier collection. Graphene is a promising TCO due to its high conductivity and transparency. Unfortunately, the transfer process hinders its implementation in a solar cell as a top or bottom contact. In this work, the first transfer-free method was developed by growing graphene directly onto an ALD-grown functional layer (NiOx). It will be shown that the NiOx layer gets partially reduced to Ni by carbon, which subsequently catalysis the graphene growth. Potential induced degradation (PID) has once again become a major reliability issue for solar manufacturers. But the determination and identification of PID before module fabrication is still a challenge. This work presents a novel method to provide accelerated lamination free PID testing at a solar cell level. This method was validated by implementing it on cells from different manufacturers. In addition, this novel method was used to test the effectiveness of ALD films, mainly ZnO and Al-doped ZnO (AZO), to prevent PID at a solar cell device level. It will be shown that the addition of a 5 nm TCO thin film can prevent the Na diffusion into the solar cell, thus protecting the cell from PID. Finally, the techno-economic analysis of ALD TCOs for solar cells is presented. It was shown that with the current tools in the market and indium costs, ALD based TCOs are an economical alternative. Furthermore, a Levelized costs of electricity (LCOE) study on the ALD capping layer demonstrated a >1% improvement in LCOE, suggesting that PID prevention using ALD capping layer is technologically and economically advantageous