Design of point contact solar cell by means of 3D numerical simulations

Abstract

Nikola Tesla said that "the sun maintains all human life and supplies all human energy". As a matter of fact, sun furnishes with energy all forms of living, e.g., starting from the photosynthesis process, plants absorb solar radiation and convert it into stored energy for growth and development, thus supporting life on earth. For this reason, sun is considered one of the most important and plentiful sources of renewable energies. This star is about 4.6 billion years old with another 5 billion years of hydrogen fuel to burn in its lifetime. This characteristic gives to all living creatures a sustainable and clean energy source that will not run out anytime soon. In particular, solar power is the primary source of electrical and thermal energy, produced by directly exploiting the highest levels of the irradiated energy from the sun to our planet. Therefore, solar energy offers many benefits such as no-releasing greenhouse gases (GHGs) or other harmful gases in the atmosphere, it is economically feasible in urban and rural areas, and evenly distributed across the planet. Moreover, as it was mentioned above, solar power is also essentially infinite, reason why it is close to be the largest source of electricity in the world by 2050. On the other hand, most of the energy forms available on earth arise directly from the solar energy, including wind, hydro, biomass and fossil fuels, with some exceptions like nuclear and geothermal energies. Accordingly, solar photovoltaic (PV) is a technology capable of converting the inexhaustible solar energy into electricity by employing the electronic properties of semiconductor materials, representing one of the most promising ways for generating electricity, as an attainable and smart option to replace conventional fossil fuels. PV energy is also a renewable, versatile technology that can be used for almost anything that requires electricity, from small and remote applications to large, central power stations. Solar cell technology is undergoing a transition to a new generation of efficient, low-cost products based on certain semiconductor and photoactive materials. Furthermore, it has definite environmental advantages over competing electricity generation technologies, and the PV industry follows a pro-active life-cycle approach to prevent future environmental damage and to sustain these advantages. An issue with potential environmental implications is the decommissioning of solar cell modules at the end of their useful life, which is expected to about 30 years. A viable answer is recycling or re-used them in some ways when they are no longer useful, by implementing collection/recycling infrastructure based on current and emerging technologies. Some feasibility studies show that the technology of end-of-life management and recycling of PV modules already exists and costs associated with recycling are not excessive. In particular, Photovoltaic is a friendly and an excellent alternative to meet growing global energy-demand by producing clean and sustainable electricity that can replace conventional fossil fuels and thus reducing the negative greenhouse effects (see section 1.1). Reasoning from this fact, solar cell specialists have been contributing to the development of advanced PV systems from a costly space technology to affordable terrestrial energy applications. Actually, since the early 1980s, PV research activities have been obtaining significant improvements in the performance of diverse photovoltaic applications. A new generation of low-cost products based on thin films of photoactive materials (e.g., amorphous silicon, copper indium diselenide (CIS), cadmium telluride (CdTe), and film crystalline silicon) deposited on inexpensive substrates, increase the prospects of rapid commercialization. In particular, the photovoltaic industry has focused on the development of feasible and high-efficiency solar cell devices by using accessible semiconductor materials that reduce production costs. Nonetheless, photovoltaic applications must improve their performance and market competitiveness in order to increase their global install capacity. In this context, the design of innovative solar cell structures along with the development of advanced manufacturing processes are key elements for the optimization of a PV system. Nowadays, TCAD modeling is a powerful tool for the analysis, design, and manufacturing of photovoltaic devices. In fact, the use of a properly calibrated TCAD model allows investigating the operation of the studied solar cells in a reliable and a detailed way, as well as identifying appropriate optimization strategies, while reducing costs, test time and production. Thereby, this Ph.D. thesis is focused on a research activity aimed to the analysis and optimization of solar cells with Interdigitated Back Contact (IBC) crystalline silicon substrate c-Si, also known as Back Contact-Back Junction (BC-BJ). This type of solar cell consists of a design where both metal contacts are located on the bottom of the silicon wafer, simplifying the cell interconnection at module-level. Characteristics that guarantee high-conversion efficiency due to the absence of front-contact shadowing losses. In particular, the main purpose of this thesis is to investigate the dominant physical mechanisms that limit the conversion efficiency of these devices by using electro-optical numerical simulations. Three-dimensional (3D) TCAD-based simulations were executed to analyze the performance of an IBC solar cell featuring point-contacts (PC) as a function of the metallization fraction. This scheme was also compared with a similar IBC structure featuring linear-contacts (LC) on the rear side of the device. In addition, the impact of introducing a selective emitter scheme (SE) in the PC cell was evaluated. The analyses were carried out by varying geometric and/or process parameters (for example, the size and shape of metalcontacts, doping profiles, carrier lifetime, and recombination rates). This approach provides a realistic and an in-depth view of the behavior of the studied IBC solar cells and also furnishes with useful information to optimize the architecture design of the device in order to enhance the conversion efficiency and minimize production costs.