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The ideal solar cell defined by the Shockley-Queisser (S-Q) theory is an important milestone in the analysis of photovoltaic devices based on some assumptions. One or more of the above assumptions are gradually evaded and even exceed or close to S-Q efficiency limit, so the development and improvement of S-Q theory is necessary. Heterojunction solar cells are one of the hot research fields in photovoltaics. In order to address the hindering effect of energy band discontinuity in the spatial barrier region of heterojunction solar cells on the transport of photogenerated carriers, this paper revised the assumptions of S-Q theory based on the original S-Q theory of photovoltaic cells. It is assumed that the carrier mobility in the barrier region is finite and the infinite mobility in the S-Q model is abandoned. But the mobility in the N-type and P-type neutral region is still infinite. The lumped relationship between carrier mobility and resistance in the barrier region is derived. Thus the physical process of charge transport is described in detail in this paper based on the continuity equation for semiconductors considering the effect of absorption coefficients to prevent the quasi-Fermi level from crossing the conduction or valence band. Thus, the revised S-Q theoretical limit model of heterojunction solar cell was constructed. The diode equivalent circuit diagram is deduced and the photovoltaic conversion efficiency is evaluated eventually. The loss effects of charge transmission and band gap mismatch on the performance of heterojunction solar cells are analyzed in detail in this paper. The calculation results under the condition of 5780K blackbody radiation and 300K cell temperature with N-type wide bandgap(EH) and P-type narrow bandgap(EL) materials show that the highest conversion efficiency is about 31% with hole resistance of 0.01 Ω·cm^2 and electronic resistance of 0.01 Ω·cm^2. The electronic resistance has more negative and complicated effects on solar cell performance than hole resistance based on the results of the calculation. When Re and Rh are small, the best conversion efficiency is achieved between 1.22 and 1.32 of the narrow bandgap. Increasing Re can increase the open circuit voltage of solar cells, but there are losses in efficiency and fill factor of solar cells. When Re is large enough, for example, Re=1000, the open circuit voltage of solar cells is not limited by EL and can exceed the bandgap limit of the narrow bandgap material. Increasing Rh also reduces efficiency and fill factor but has less effects than Re. The change of absorption coefficient makes the photogenerated current of L and H branches change, and the radiation recombination loss of both branches can be regulated.
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Keywords:
- Shockley-Queisser model /
- heterojunction solar cell /
- carrier mobility /
- absorptivity
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