Opto-electrical surface engineering of wafer based c-Si solar cells

Climate changes due to increase of CO2 emission are becoming a serious issue for this planet. The so called climate crisis has been the main topic of the last United Nations Climate Change Conference (COP 21) . Direct conversion of sunlight into electricity is one of the most promising technology for achieving the COP 21 agreement. Wafer-based crystalline silicon (c-Si) solar cells account for more than 90% of the total PV market because silicon is a non-toxic and abundant material and Si-based PV modules have demonstrated long term stability and high durability. To maintain this technology dominant also in the coming years, continuous improvement in conversion efficiency without increasing processing costs are required. In this thesis novel solutions based on opto-electrical surface engineering are presented as potential solutions to increase conversion efficiency and/or decrease the costs of wafer-based c-Si solar cells. In particular, advanced light management techniques were developed to enhance light absorption in thin c-Si absorber and to fabricate customized PV products for building integrated photovoltaic (BIPV) applications. This thesis begins with introducing, theoretical limits (Chapter 1), working principles and current status (Chapter 2) of waferbased c-Si solar cells. In Chapter 3 the losses analysis of industrial multi-crystalline silicon (mc-Si) solar cell was performed by using the ASA simulation tool. Such analysis pointed out the main opto-electrical losses for a mc-Si solar cell which were tackled in the next Chapters. In particular, Chapter 4 deals with design and fabrication of advanced light trapping scheme for minimizing optical losses of state-of-the-art c-Si solar cells. To this aim a combination of surface textures with different geometrical scales were used in order to trigger several optical effects. In particular, nano-texturing fabricated via reactive ion etching (RIE) on the front side and micro texturing based on alkaline etching on the rear side were used providing broadband light-in coupling and light scattering. Almost ideal back reflectors such as Ag or Distributed Bragg reflectors (DBR) were applied on the rear side. By using such light trapping scheme, the so-called 4n2 absorption enhancement limit, which has been elusive for more than 30 years was experimentally demonstrated on a broad wavelength range. The interdigitated back contact (IBC) c-Si solar cell was indicated as the most promising solar cell architecture to apply such light trapping scheme. This technology was not available within the PVMD group. Therefore, in Chapter 5 a simplified self-aligned process for fabrication high efficiency IBC c-Si solar cells was demonstrated. The process involved the combination of ion implantation and epitaxial growth of in-situ doped Si. The process flow was optimized to minimize the thermal budget and the number of lithographic steps. By using only two lithographic steps, a conversion efficiency equal to 20.2% on 9 cm2 device was demonstrated. For such solar cell architecture it was shown that a lightly doped front surface field improves carrier collection. After developing a process flow for fabricating IBC c-Si solar cells, the application of the advanced light management technique to IBC was presented in Chapter 6. To this aim two major issues were tackled. The first was related to the removal of surface defects induced by the RIE process to decrease surface recombination. To achieve this goal a cost effective process was developed. The second dealt with adapting the light trapping scheme to the IBC process integration. To this aim, the decoupled front (nano-textured) and rear side (micotexturing) light trapping scheme of Chapter 4 was modified by superposing both texture scales on the front side of the wafer. This approach is called modulated surface texture (MST). The combination of the advanced light trapping and surface passivation schemes was employed in IBC c-Si solar cells. Top efficiency of 19.8% for MST-IBC solar cell was demonstrated. Advanced light management techniques were also applied to bifacial c-Si solar cells. The objective of this study was twofold: (i) enhancing cell efficiency by increasing the internal rear internal reflectance and (ii) providing novel solutions for BIPV applications. In particular, DBR and TiO2 particles in the form of white paint were used as back reflectors of bifacial c-Si solar cells. The DBR enabled the possibility of fabricating rear side coloured bifacial modules, which can be attractive for BIPV applications.