Shockley-Queisser theory based calculation of efficiency limit of heterojunction solar cells
Authors: ZHU Zengwei , ZHANG Meirong , QIAO Baorong , CHEN Jia , YANG Huiyong , ZHOU Dayong ,   Gusu Laboratory of Materials, Suzhou 215123, China
Published: December 2024

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 avoided, and even exceed or approach the 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, the assumptions of S-Q theory based on the original S-Q theory of photovoltaic cells are revised in this work. The carrier mobility in the barrier region is assumed to be finite, and the infinite mobility in the S-Q model is abandoned. But the mobility in the N-type and the P-type neutral region are still infinite. The lumped relationship between carrier mobility and resistance in the barrier region is derived. Therefore, the physical process of charge transport is described in detail in this paper based on the continuity equation for semiconductors by 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 is 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. The calculation results under the condition of 5780 K blackbody radiation and 300 K cell temperature with N-type wide bandgap (<i>E</i><sub>H</sub>) and P-type narrow bandgap (<i>E</i><sub>L</sub>) materials show that the highest conversion efficiency is about 31% with a hole resistance of 0.01 Ω·cm<sup>2</sup> and electronic resistance of 0.01 Ω·cm<sup>2</sup>. The calculations show that the electronic resistance has a more negative and complicated effect on solar cell performance than hole resistance. When <i>R</i><sub>e</sub> and <i>R</i><sub>h</sub> are small, the best conversion efficiency is in a range between 1.22 eV and 1.32 eV of the narrow bandgap. Increasing <i>R</i><sub>e</sub> can increase the open circuit voltage of solar cells, but there are losses in efficiency and fill factor of solar cells. When <i>R</i><sub>e</sub> is large enough, for example, <i>R</i><sub>e</sub> = 1000 Ω·cm<sup>2</sup>, the open circuit voltage of solar cells is not limited by EL and can exceed the bandgap limit of the narrow bandgap material. Increasing <i>R</i><sub>h</sub> will also reduce efficiency, but the effect is not so great as <i>R</i><sub>e</sub>. The change of absorption coefficient can cause the photogenerated current of L and H branches to change, and the radiation recombination losses of both branches can be regulated.