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2. Modelling & simulations of Cu2ZnSnS4 thin film solar cell devices

Author

Pu, Aobo

Subjects
Thin film solar cells, Photovoltaic, Simulation
Abstract

Thin film Photovoltaic (PV) is a promising green technology in meeting future energy demand at trivial carbon footprints. Cu2ZnSnS4 (CZTS) solar cell might well be the black horse among thin film PV candidate due to advantageous attributes such as earth abundant & non-toxic constituents, large absorption coefficients and tuneable bandgap for monolithic integration. Nevertheless, the development progress of CZTS has been sluggish owing to the large voltage deficit. Although several limiting mechanisms behind voltage deficit have been proposed, the full picture is still missing without linking them together in physical device model. This thesis intends to i) fill some of the gaps between theoretical hypotheses and experimental discoveries by device simulation; ii) guide the development of emerging techniques by predictive modelling. In the first step, a 6.9% CZTS baseline model has been constructed based on comprehensive characterization and verified through reproduction of measured outputs. At the buffer/absorber interface, a surface recombination velocity of 1×105 cm/s is identified to correlate with measured spectral response. A bulk lifetime of ~19 ns is extracted from effective lifetime modelling. The key underlying Voc deficit is pinpointed to disorder induced bandgap fluctuation, which is backed by both QE tail simulation. The hetero-interface and disorder contribute to over 90% of Voc loss based on our model. Secondly, the numerical optimization of hetero-interface engineering has been conducted by simulating Zn1-xCdxS/CZTS device. The reported Voc boost from using of Zn0.35Cd0.65S buffer is found to be related to suppression of interface recombination. However device performance is partially offset as the increasing Zn fraction is also accompanied by a lower buffer electron concentration. Based on optimization simulation, the optimum range of Zn ratio is 0.3-0.5 so interface recombination can be mitigated without creation of blocking spike barrier. Lastly, double grading profile is implemented and designed in CuZnSn(S1-x,Sex)4 (CZTSSe) model. Based on computational design, front & back grading should be moderate, minimum bandgap should be slightly above 1.0 eV and located close to junction edge. Optimal designed grading profiles helps achieve 2-7% efficiency gain and reduce voltage deficit to 531 mV.

Published
2018
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3. Solution-processed ultrathin SnO2 passivation of Absorber/Buffer Heterointerface and Grain Boundaries for High Efficiency Kesterite Cu2ZnSnS4 Solar Cells

Author

Sun, Heng, primary, Stride, John A., additional, Green, Martin, additional, Hao, Xiaojing, additional, Huang, Jialiang, additional, Yun, Jae S., additional, Sun, Kaiwen, additional, Yan, Chang, additional, Liu, Fangyang, additional, Park, Jongsung, additional, Pu, Aobo, additional, and Seidel, Jan, additional

Published
2019
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4. Flexible kesterite Cu2ZnSnS4 solar cells with sodium-doped molybdenum back contacts on stainless steel substrates

Author

Sun, Kaiwen, primary, Liu, Fangyang, additional, Huang, Jialiang, additional, Yan, Chang, additional, Song, Ning, additional, Sun, Heng, additional, Xue, Chaowei, additional, Zhang, Yuanfang, additional, Pu, Aobo, additional, Shen, Yansong, additional, Stride, John A., additional, Green, Martin, additional, and Hao, Xiaojing, additional

Published
2018
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8. Exploring Inorganic Binary Alkaline Halide to Passivate Defects in Low‐Temperature‐Processed Planar‐Structure Hybrid Perovskite Solar Cells

Author

Liu, Xu, primary, Zhang, Yuanfang, additional, Shi, Lei, additional, Liu, Ziheng, additional, Huang, Jialiang, additional, Yun, Jae Sung, additional, Zeng, Yiyu, additional, Pu, Aobo, additional, Sun, Kaiwen, additional, Hameiri, Ziv, additional, Stride, John A., additional, Seidel, Jan, additional, Green, Martin A., additional, and Hao, Xiaojing, additional

Published
2018
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12. Beyond 8% ultrathin kesterite Cu2ZnSnS4 solar cells by interface reaction route controlling and self-organized nanopattern at the back contact

Author

Liu, Fangyang, primary, Huang, Jialiang, additional, Sun, Kaiwen, additional, Yan, Chang, additional, Shen, Yansong, additional, Park, Jongsung, additional, Pu, Aobo, additional, Zhou, Fangzhou, additional, Liu, Xu, additional, Stride, John A, additional, Green, Martin A, additional, and Hao, Xiaojing, additional

Published
2017
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19. Improvement of Jscin a Cu2ZnSnS4Solar Cell by Using a Thin Carbon Intermediate Layer at the Cu2ZnSnS4/Mo Interface

Author

Zhou, Fangzhou, Zeng, Fangqin, Liu, Xu, Liu, Fangyang, Song, Ning, Yan, Chang, Pu, Aobo, Park, Jongsung, Sun, Kaiwen, and Hao, Xiaojing

Abstract

Back contact modification plays an important role in improving energy conversion efficiency of Cu2ZnSnS4(CZTS) thin film solar cells. In this paper, an ultrathin carbon layer is introduced on molybdenum (Mo)-coated soda lime glass (SLG) prior to the deposition of CZTS precursor to improve the back contact and therefore enhance CZTS solar cell efficiency. By introducing this layer, the short circuit current (Jsc) and device conversion efficiency increase for both nonvacuum (sol–gel) and vacuum (sputtering) methods. Specifically, for the sol–gel based process, Jscincreases from 13.60 to 16.96 mA/cm2and efficiency from 4.47% to 5.52%, while for the sputtering based process, Jscincreases from 17.50 to 20.50 mA/cm2and efficiency from 4.10% to 5.20%. Furthermore, introduction of this layer does not lead to any deterioration of either open circuit voltage (Voc) or fill factor (FF).

Published
2015
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20. Beyond 8% ultrathin kesterite Cu2ZnSnS4solar cells by interface reaction route controlling and self-organized nanopattern at the back contact

Author

Liu, Fangyang, Huang, Jialiang, Sun, Kaiwen, Yan, Chang, Shen, Yansong, Park, Jongsung, Pu, Aobo, Zhou, Fangzhou, Liu, Xu, Stride, John A, Green, Martin A, and Hao, Xiaojing

Abstract

Highly efficient, ultrathin (~400 nm) pure sulfide kesterite Cu2ZnSnS4(CZTS) solar cells have been realized by interface reaction route controlling and self-organized nano-pattern at the back contact. The Al2O3intermediate layer introduced at the Mo/CZTS interface can effectively inhibit the detrimental interfacial reaction between CZTS and Mo in the initial stage of sulfurization, and then turns into a self-organized nanopattern yielding a nanoscale opening for electrical contact. With this interface modification, the traditional issues of phase segregation (secondary phases) and voids at the back contact region can be well addressed, which greatly improves uniformity and reduces back contact recombination. As a result, this interface modification not only leads to beyond 8% ultrathin CZTS solar cells but also yields two certificated world record efficiencies: 9.26% for 0.237 cm2small area and 7.61% for 1 cm2standard kesterite CZTS solar cells (normal thickness).

Published
2017
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