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1. Exploring the practical efficiency limit of silicon solar cells using thin solar-grade substrates

   https://doi.org/10.1039/d0ta04575f  

   Augusto, A.; Karas, J.; Balaji, P.; Bowden, S. G.; King, R. R. (August 2020, Journal of Materials Chemistry A)

   null (Ed.)

   Multiple silicon solar cell technologies have surpassed or are close to surpassing 26% efficiency. Dielectric and amorphous silicon-based passivation layers combined with minimal metal/silicon contact areas were responsible for reducing the surface saturation current density below 3 fA cm −2 . At open-circuit, in passivated contact solar cells, the recombination is mainly from fundamental mechanisms (Auger and radiative) representing over 3/4 of the total recombination. At the maximum power point, the fundamental recombination fraction can drop to half, as surface and bulk Shockley–Read–Hall step in. As a result, to further increase the performance at the operating point, it is paramount to reduce the bulk dependence and secure proper surface passivation. Bulk recombination can be mitigated either by reducing bulk defect density or by reducing the wafer thickness. We demonstrate that for commercially-viable solar-grade silicon, thinner wafers and surface saturation current densities below 1 fA cm −2 , are required to significantly increase the practical efficiency limit of solar cells up to 0.6% absolute. For a high-quality n-type bulk silicon minority-carrier lifetime of 10 ms, the optimum wafer thickness range is 40–60 μm, a very different value from 110 μm previously calculated assuming undoped substrates and solely Auger and radiative recombination. In this thickness range surface saturation current densities near 0.1 fA cm −2 are required to narrow the gap towards the fundamental efficiency limit. We experimentally demonstrate surface saturation currents below 0.5 fA cm −2 on pi/CZ/in structures across different wafer thicknesses (35–170 μm), with potential to reach open-circuit voltages close to 770 mV and bandgap-voltage offsets near 350 mV. Finally, we use the bandgap-voltage offset as a metric to compare the quality of champion experimental solar cells in the literature, for the most commercially-relevant photovoltaic cell absorbers and architectures. 

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2. Improving the Passivation of Molybdenum Oxide Hole‐Selective Contacts with 1 nm Hydrogenated Aluminum Oxide Films for Silicon Solar Cells

   https://doi.org/10.1002/pssa.202000093  

   Gregory, Geoffrey; Feit, Corbin; Gao, Zhengning; Banerjee, Parag; Jurca, Titel; Davis, Kristopher O. (June 2020, physica status solidi (a))

   The intrinsic and doped amorphous silicon layers in silicon heterojunction solar cells parasitically absorb light in the short wavelength region of the solar spectrum, lowering the generation current available to the device. Herein, a promising alternative to the hole‐selective amorphous silicon contact layers using only wide bandgap, transparent oxide materials is presented. Using thermal atomic layer deposition, a 1 nm hydrogenated aluminum oxide layer is deposited followed by a 4 nm molybdenum oxide layer on n‐type crystalline silicon. This contact stack provides an effective carrier lifetime of 1.14 ms. It is shown that the molybdenum oxide layer is successfully deposited with a high work function, which facilitates efficient hole extraction and repels majority carriers from the c‐Si surface. Then the implied open‐circuit voltage, saturation current density, and contact resistivity are recorded as a function of contact annealing temperature and show that they are relatively stable up to 200 °C. 

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3. Photovoltaic Characteristics of PERC-Like CdTe Solar Cells

   Roy, E.; Powell, K.; Yoon, H. (June 2023, Electronic Materials Conference)

   Rapid progress has been achieved in thin film CdTe solar cells, reaching a power conversion efficiency of 22.1 %. Researchers demonstrated a short-circuit current density (Jsc) of ≈ 31 mA/cm2 and a fill factor (FF) of ≈ 79 %, close to the theoretically calculated maximum values. However, the open-circuit voltage (Voc) remains below 0.9 V, much lower than the estimated Voc of 1.2 V. One strategy to improve the Voc is to implement a passivated back-contact on CdTe that can reduce the recombination by repelling minority carriers at the surface (i.e., electrons in CdTe). An aluminum oxide thin film (Al2O3) is an attractive candidate owing to its innate fixed negative charges (1012 ~ 1013 cm-2). Here, we use a patterned Al2O3 layer on CdTe to produce PERC-like CdTe solar cells (CdTe PERC). 

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4. Micromachined Flexible Silicon Solar Cells as a Power Supply for Smart Contact Lenses

   https://doi.org/10.1109/PowerMEMS56853.2022.10007084  

   Pourshaban, Erfan; Karkahnis, Mohit U.; Deshpande, Adwait; Hasan, Md. Rabiul; Rock, Nathan D.; Banerjee, Aishwaryadev; Ghosh, Chayanjit; Kim, Hanseup; Mastrangelo, Carlos H. (December 2022, 2022 21st International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications (PowerMEMS))

   This article reports the fabrication, characterization, implementation, and microsystem integration of micromachined flexible silicon solar cells to supply electric power to smart contact lenses. Single silicon solar cell shows the open circuit voltage (V oc ) of 0.5 and 0.55 V Under indoor and outdoor lighting conditions, respectively. The V oc enhanced to 1.25 and 1.65 V after making series connections between three cells. The maximum power output of 50 µW and 2.7 mW are recorded under indoor and outdoor lighting conditions. Furthermore, a power management IC is used to boost up the voltage to 3.3 V and efficiently store or use the generated energy. 

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5. >10% solar-to-hydrogen efficiency unassisted water splitting on ALD-protected silicon heterojunction solar cells

   https://doi.org/10.1039/c9se00110g  

   Tan, Chor Seng; Kemp, Kyle W.; Braun, Michael R.; Meng, Andrew C.; Tan, Wanliang; Chidsey, Chris E.; Ma, Wen; Moghadam, Farhad; McIntyre, Paul C. (May 2019, Sustainable Energy & Fuels)

   Solar water splitting using photoelectrochemical cells (PEC's) is a promising pathway toward clean and sustainable storage of renewable energy. Practical realization of solar-driven synthesis of hydrogen and oxygen integrating light absorption and electrolysis of water has been challenging because of (1) the limited stability of good photovoltaic materials under the required electrochemical conditions, and (2) photovoltaic efficiency losses due to light absorption by catalysts, the electrolyte, and generated bubbles, or reflection at their various interfaces. Herein, we evaluate a novel integrated solar water splitting architecture using efficient silicon heterojunction photovoltaic cells that avoids such losses and exhibits a solar-to-hydrogen (STH) efficiency in excess of 10%. Series-connected silicon Heterojunction with Intrinsic Thin layer (HIT) cells generate sufficient photovoltage for unassisted water splitting, with one of the cells acting as the photocathode. Platinum is deposited on the back (dark) junction of this HIT cell as the catalyst for the hydrogen evolution reaction (HER). The photocathode is protected from corrosion by a TiO 2 layer deposited by atomic layer deposition (ALD) interposed between the HIT cell and the Pt, enabling stable operation for >120 hours. Combined with oxygen evolution reaction (OER) catalysts deposited on a porous metal dark anode, these PEC's achieve stable water splitting with a record high STH efficiency for an integrated silicon photosynthesis device. 

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