Multi-junction solar cell | EPFL Graph Search

Multi-junction (MJ) solar cells are solar cells with multiple p–n junctions made of different semiconductor materials. Each material's p-n junction will produce electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's sunlight to electrical energy conversion efficiency. Traditional single-junction cells have a maximum theoretical efficiency of 33.16%. Theoretically, an infinite number of junctions would have a limiting efficiency of 86.8% under highly concentrated sunlight. As of 2023 the best lab examples of traditional crystalline silicon (c-Si) solar cells had efficiencies up to 26.81%, while lab examples of multi-junction cells have demonstrated performance over 46% under concentrated sunlight. Commercial examples of tandem cells are widely available at 30% under one-sun illumination, and improve to around 40% under concentrated sunlight. However, this

Development of high band gap protocrystalline material for p-i-n thin film silicon solar cells

Fabio Maurizio

High band gap thin film silicon solar cells are of high interest for the use as top cell in triple junctions solar cells. Protocrystalline silicon, in the transition phase between amorphous and microcrystalline silicon, is such a high band gap material. During this project, protocrystalline intrinsic layers have been developed and optimized in p-i-n single junction solar cells by plasma-enhanced chemical vapour deposition (PECVD) in a new multi-chamber deposition system. In the first series, cells and layers were deposited with different dilutions going from silane to hydrogen flux ratios of 1:1 to 1:64. At the dilutions 1:16 and 1:32, protocrystalline ma-terial was obtained which showed amorphous characteristics only up to a certain thickness, where crystallites started to evolve. With dilution 1:16, a second cell series has been deposited with variation of the intrinsic layer thick-ness. The best cells in terms of open-circuit voltage and fill factor product were obtained with an i-layer thickness of 100 nm. These cells have open-circuit voltages and fill factors up to 0.975 V and 75.8 % (0.932 V and 69.9 %) for the initial (degraded) state. These are promising values for the use of this material in top cells. However, the low current density means that the optimum i-layer thickness is probably higher for triple cell application. Further cells have been deposited at different temperatures. These results imply that there is still room for further optimization, when current limitations of the heating system will be overcome.

2012

Bulk and surface engineering to improve performance and stability of perovskite solar cells

Thomas Paul Baumeler

Human society is craving new and renewable energy sources. Indeed, the need for novel sources is an obligation in order to cope with the exponentially increasing global energy demand. Also, environmentally friendly alternatives to fossil fuels are more crucial than ever in order to limit the effects of global warming. Finally, the current gas and oil crisis underlies the importance for each state to have some alternatives in case of shortage, i.e. to gain some energetic independence. In that regard, solar energy represents the best candidate, given its huge potential and its universal character. In particular, perovskite solar cells (PSCs) show the highest promise amongst emerging technologies. From their first report in 2009, the power conversion efficiency rocketed, to cross 25 % recently. Unfortunately, several important issues remain.Chapter 2. Its lower bandgap makes formamidinium lead iodide (FAPbI3) a more suitable candidate for single-junction solar cells than pure methylammonium lead iodide (MAPbI3). However, its structural and thermodynamic stability is improved by introducing a significant amount of MA and bromide, both of which increase the bandgap and amplify trade-off between the photocurrent and photovoltage. Here, we simultaneously stabilized FAPbI3 into a cubic lattice and minimized the formation of photoinactive phases, such as hexagonal FAPbI3 and PbI2, by introducing 5% MAPbBr3, as revealed by synchrotron X-ray scattering. We were able to stabilize the composition (FA0.95MA0.05Cs0.05)Pb(I0.95Br0.05)3, which exhibits a minimal trade-off between the photocurrent and photovoltage. This material shows low energetic disorder and improved charge-carrier dynamics as revealed by photothermal deflection spectroscopy (PDS) and transient absorption spectroscopy (TAS), respectively. This allowed the fabrication of operationally stable perovskite solar cells yielding reproducible efficiencies approaching 22%.Chapter 3. Herein, we successfully engineered a multi-cation halide composition of perovskite solar cells via the two-step sequential deposition method. Strikingly we find that adding mixtures of 1D polymorphs of orthorhombic d-RbPbI3 and d-CsPbI3 to the PbI2 precursor solution induces the formation of porous mesostructured hexagonal films. This porosity greatly facilitates the heterogeneous nucleation and the penetration of FA/MA cations within the PbI2 film. Thus, the subsequent conversion of PbI2 into the desired multication cubic a-structure by exposing it to a solution of formamidinium methylammonium halides is greatly enhanced. During the conversion step, the d-CsPbI3 also is fully integrated into the 3-D mixed cation perovskite lattice, which exhibits high crystallinity and superior optoelectronic properties. The champion device shows a power conversion efficiency (PCE) over 22%, with an open-circuit voltage of 1.15V, a short-circuit photocurrent of 24.82 mA·cm-2, and a fill factor of 78% at a bandgap of 1.55 eV. Furthermore, these devices exhibit enhanced operational stability, with the best device retaining more than 90% of its initial value of PCE under 1-Sun illumination with maximum power point tracking for 400 h.Chapter 4. A promising approach for the production of highly efficient and stable hybrid perovskite solar cells is employing mixed ion materials. Remarkable performances have been reached by materials comprising a stabilized mixture of methylammonium (MA+) and formamidinium (FA+) as the monovalent cat

EPFL2022

Solar-Electrochemical Devices for Water Splitting and Chlorine Generation

Enrico Chinello

Solar technologies hold the potential to ultimately counteract the effects of climate change. Nevertheless, the intermittency of sunlight poses technical challenges regarding the storage of the surplus production and the supply of the deficit. The generation of chemical fuels is of great interest to store energy for prolonged periods. Hydrogen has been a key candidate to address the issue. The possibility to generate hydrogen exploiting sunlight has been widely investigated in recent years. Fully decoupled devices seem the most promising pathway. The exploitation of earth-abundant elements is indispensable for the future of solar hydrogen, as they are cost-effective and they are inclined to meet the requirements of generation at a global scale. We have designed and tested a solar hydrogen generator solely employing earth-abundant materials; its efficiency is the highest reported to date for such class of devices. The solar-to-hydrogen efficiency we have recorded is > 14%, improving by more than 40% the prior state of the art. We have employed tailor-made silicon hetero-junction solar cells, coupled to an alkaline electrolyzer. The main hurdle that prevents the large deployment of solar hydrogen technologies is their cost. Those financial barriers can be overcome by coupling hydrogen evolution with a more cost-viable oxidation reaction. Generation of halides is a promising pathway to substitute oxygen evolution. The chlor-alkali process is a major electrochemical operation in the industry, and generates hydrogen, chlorine and sodium hydroxide. In my work, I have designed and tested a novel solar chlor-alkali reactor, which integrates a novel solar concentrator developed by a local startup and multi-junction solar cells. I have recorded solar-to-chemical conversion efficiencies > 25% under real outdoor illumination, doubling prior values reported in literature. Chlorine and chlorinated products can be deployed for disinfection of drinking water. Liquid solutions of sodium hypochlorite can be preferred over chlorine gas for such applications. I have demonstrated and compared the performances of solar-powered devices for hypochlorite generation, employing multi-junction technologies or silicon hetero-junction solar cells. I have characterized the efficiencies under real outdoor conditions, revealing solar-to-chemical conversion efficiencies of 10 â 14% for silicon-based cells, and confirming > 25% values for multi-junction cells. I have then developed a computational model that predicts efficiencies, throughputs and production cost in three locations of choice. The results suggest that solar-hypochlorite can outcompete traditional hypochlorite. Deployment of solar hypochlorite generators in developing countries can address the lack of access to drinking water those communities suffer. I have evaluated the performances of a simpler and more robust batch-type hypochlorite generator. The results highlight that the throughput can be enhanced by structuring the electrode plates. I have assessed the economics of hypochlorite production via solar-powered batch reactors; the cost is higher, but is still competitive with respect to the supply of bulk hypochlorite. My work holds the potential to inspire and spur further innovation in the field of solar-electrochemical operation, in which the platforms I have demonstrated and assessed can represent a starting point towards a new generation of devices.

EPFL2019