Armin Richter

Bio: Armin Richter is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Silicon & Passivation. The author has an hindex of 29, co-authored 85 publications receiving 3577 citations.

Improved quantitative description of Auger recombination in crystalline silicon

Abstract: An accurate quantitative description of the Auger recombination rate in silicon as a function of the dopant density and the carrier injection level is important to understand the physics of this fundamental mechanism and to predict the physical limits to the performance of silicon based devices. Technological progress has permitted a near suppression of competing recombination mechanisms, both in the bulk of the silicon crystal and at the surfaces. This, coupled with advanced characterization techniques, has led to an improved determination of the Auger recombination rate, which is lower than previously thought. In this contribution we present a systematic study of the injection-dependent carrier recombination for a broad range of dopant concentrations of high-purity $n$-type and $p$-type silicon wafers passivated with state-of-the-art dielectric layers of aluminum oxide or silicon nitride. Based on these measurements, we develop a general parametrization for intrinsic recombination in crystalline silicon at 300 K consistent with the theory of Coulomb-enhanced Auger and radiative recombination. Based on this improved description we are able to analyze physical aspects of the Auger recombination mechanism such as the Coulomb enhancement.

Reassessment of the Limiting Efficiency for Crystalline Silicon Solar Cells

Abstract: Recently, several parameters relevant for modeling crystalline silicon solar cells were improved or revised, e.g., the international standard solar spectrum or properties of silicon such as the intrinsic recombination rate and the intrinsic carrier concentration. In this study, we analyzed the influence of these improved state-of-the-art parameters on the limiting efficiency for crystalline silicon solar cells under 1-sun illumination at 25°C, by following the narrow-base approximation to model ideal solar cells. We also considered bandgap narrowing, which was not addressed so far with respect to efficiency limitation. The new calculations that are presented in this study result in a maximum theoretical efficiency of 29.43% for a 110-μm-thick solar cell made of undoped silicon. A systematic calculation of the I-V parameters as a function of the doping concentration and the cell thickness together with an analysis of the loss current at maximum power point provides further insight into the intrinsic limitations of silicon solar cells.

Design rules for high-efficiency both-sides-contacted silicon solar cells with balanced charge carrier transport and recombination losses

Abstract: The photovoltaic industry is dominated by crystalline silicon solar cells. Although interdigitated back-contact cells have yielded the highest efficiency, both-sides-contacted cells are the preferred choice in industrial production due to their lower complexity. Here we show that omitting the layers at the front side that provide lateral charge carrier transport is the key to excellent optoelectrical properties for both-sides-contacted cells. This results in a conversion efficiency of 26.0%. In contrast to standard industrial cells with a front side p–n junction, this cell exhibits the p–n junction at the back surface in the form of a full-area polycrystalline silicon-based passivating contact. A detailed power-loss analysis reveals that this cell balances electron and hole transport losses as well as transport and recombination losses in general. A systematic simulation study led to some fundamental design rules for future >26% efficiency silicon solar cells and demonstrates the potential and the superiority of these back-junction solar cells. Front- and back-junction silicon photovoltaics dominate the market thanks to a lower manufacturing complexity compared with that of other device designs yet advances in efficiency remain elusive. Richter et al. now present an optimized design for the front and back junctions that leads to a 26.0%-efficient cell.

The Irresistible Charm of a Simple Current Flow Pattern – 25% with a Solar Cell Featuring a Full-Area Back Contact

Abstract: Screen-printed Al-BSF silicon solar cells have dominated the PV market for decades. Their long-term success is based on a low-complexity cell architecture and a robust production sequence. The full-area rear contact allows a simple and effective one-dimensional current flow pattern in the base resulting in high fill factors. Some of the successor technologies of this simple but yet successful cell architecture, i.e. partial rear contact (PRC) and interdigitated back contact (IBC) solar cells, have a significantly higher process and pattern complexity. This paper discusses a cell structure with an architecture very similar to the classical screen-printed aluminium back surface field (Al-BSF) solar cell but with a higher efficiency potential. This is achieved by substituting the full-area doped back surface region by a passivated contact scheme consisting of a tunnel oxide covered by a heavily doped silicon film, called TOPCon. The champion efficiency of 25.1% on n-type silicon shows that this structure has a high potential while keeping the process effort low and the current flow pattern simple. The very high open circuit voltage of 718 mV and fill factor of 83.2% results from both, the very low recombination and transport losses caused by this contact scheme and cell architecture.

Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26

Abstract: The efficiency of silicon solar cells has a large influence on the cost of most photovoltaics panels. Here, researchers from Kaneka present a silicon heterojunction with interdigitated back contacts reaching an efficiency of 26.3% and provide a detailed loss analysis to guide further developments.

Photovoltaic materials: Present efficiencies and future challenges

Abstract: Recent developments in photovoltaic materials have led to continual improvements in their efficiency. We review the electrical characteristics of 16 widely studied geometries of photovoltaic materials with efficiencies of 10 to 29%. Comparison of these characteristics to the fundamental limits based on the Shockley-Queisser detailed-balance model provides a basis for identifying the key limiting factors, related to efficient light management and charge carrier collection, for these materials. Prospects for practical application and large-area fabrication are discussed for each material.

Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency.

Abstract: Tandem devices combining perovskite and silicon solar cells are promising candidates to achieve power conversion efficiencies above 30% at reasonable costs. State-of-the-art monolithic two-terminal perovskite/silicon tandem devices have so far featured silicon bottom cells that are polished on their front side to be compatible with the perovskite fabrication process. This concession leads to higher potential production costs, higher reflection losses and non-ideal light trapping. To tackle this issue, we developed a top cell deposition process that achieves the conformal growth of multiple compounds with controlled optoelectronic properties directly on the micrometre-sized pyramids of textured monocrystalline silicon. Tandem devices featuring a silicon heterojunction cell and a nanocrystalline silicon recombination junction demonstrate a certified steady-state efficiency of 25.2%. Our optical design yields a current density of 19.5 mA cm−2 thanks to the silicon pyramidal texture and suggests a path for the realization of 30% monolithic perovskite/silicon tandem devices.

Monolithic perovskite/silicon tandem solar cell with >29% efficiency by enhanced hole extraction

Abstract: Tandem solar cells that pair silicon with a metal halide perovskite are a promising option for surpassing the single-cell efficiency limit. We report a monolithic perovskite/silicon tandem with a certified power conversion efficiency of 29.15%. The perovskite absorber, with a bandgap of 1.68 electron volts, remained phase-stable under illumination through a combination of fast hole extraction and minimized nonradiative recombination at the hole-selective interface. These features were made possible by a self-assembled, methyl-substituted carbazole monolayer as the hole-selective layer in the perovskite cell. The accelerated hole extraction was linked to a low ideality factor of 1.26 and single-junction fill factors of up to 84%, while enabling a tandem open-circuit voltage of as high as 1.92 volts. In air, without encapsulation, a tandem retained 95% of its initial efficiency after 300 hours of operation.