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Science News: 2022 Adv. Energy Mater., 27.6% Perovskite/C-si Tandem Solar Cell! Topcon Structure for Industrial Production?

  Organic-inorganic metal halide perovskites show great potential in a variety of applications. Especially in photovoltaic (PV) devices, organic-inorganic metal halide perovskites can achieve excellent optoelectronic properties through solution processing. By changing the composition, the band gap of the perovskite film can also be easily changed. Compared with III-V high-bandgap semiconductor materials, perovskite thin films are low-cost and easy to fabricate, making perovskite cells a potential candidate for single- and multi-junction solar cells. In the past 12 years, the power conversion efficiency of perovskite solar cells has rapidly increased from 3.8% to 25.7%, exceeding 80% of its thermodynamic limit. In addition to the huge advances in single-junction devices, tandem devices have also made impressive achievements, with efficiencies clearly likely to exceed 30%.

  Currently the most promising solution to overcome the single-junction Shockley-Queisser limit is the tandem cell structure. Klaus Weber et al. from Australia used a c-Si cell with a tunneling oxide passivating contact (TOPCon) structure as the bottom cell of the tandem device and a solution-processed perovskite film as the top cell of the tandem device. c-Si cells use n-type silicon substrates. Compared with cells based on p-type substrates, cells based on n-type substrates generally show higher lifetimes and less degradation, and thus are attractive for high-efficiency devices. The c-Si cell features a damaged etched (but not textured) top surface. The research team conformally fabricated perovskite subcells on the damage-etched front surface to mitigate the negative effects of rough c-Si substrates, preventing shunting paths across the carrier transport layer, absorber layer, and their associated interfaces. In addition, the research team revealed the source of the electrical losses in the TOPCon subcell and demonstrated localized contact to the back of the mesa structure, which is feasible for large-scale production.

Perovskitec-Si tandem solar cell TOPCon structure
a) Schematic diagram of the monolithic perovskite/c-Si tandem cell. b–d) illustrating the mitigation of rough surface, reflectivity management and TOPCon structure design, respectively. e) The IV curve of the series device under the illumination of the AM1.5G solar simulator, the effective area is 1-cm2.

  To improve the efficiency of the tandem device, the research team analyzed the main losses of the tandem device. The research team found that the voltage loss mainly comes from perovskites. As shown in the figure below, the team fabricated three test structures and used a photoluminescence (PL) and luminescence quantum yield (PLQY) test system to evaluate implied VOC. For quartz/perovskite, the measured implied VOC is 1.25 V. This is a ≈430 mV shortfall of its bandgap (1.68 eV), or a ≈150 mV shortfall compared to the Schockley-Queisser limit. In this high-bandgap mixed cation/halide perovskite, the reason for this large voltage deficit may be related to halide or cation segregation and defects within the perovskite absorber.

PLQY Voc loss analysis PL spectrum Perovskite and c-Si solar cell

a) PL spectrum of the perovskite top cell test structure using a photoluminescence and luminescence quantum yield (PLQY) test system. b) Recombination Voc loss analysis of the device simulated c-Si bottom cell.

  Through the method mentioned, it was experimentally confirmed that an efficiency of 27.6% (1-cm2) was achieved using a monolithic tandem device using an industrially produced bottom c-Si cell with a TOPCon structure.

c-Si solar cell IV curve under solar simulator

Schematic diagram of the c-Si cell test structure. d) Simulated IV curve of the c-Si device under solar simulator illumination. e) The reverse scan IV curve of the tandem device under the illumination of a solar simulator. f) Reflectivity and external quantum efficiency measurement curves of tandem cells.

Keywords: Perovskite, c-Si, Voc Loss Analysis, Photoluminescence and Luminescence Quantum Yield Test System, PLQY, Quantum Efficiency

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