Selfconsistent Model of Photoconversion Efficiency for Multijunction Solar Cells

TL;DR

This paper presents a semi-analytical model for calculating and optimizing the efficiency of multijunction solar cells, considering various recombination mechanisms and thermal effects, aligning well with experimental data.

Contribution

The authors develop a self-consistent formalism that incorporates multiple recombination processes and thermal balance, enabling accurate efficiency predictions and optimization of multijunction solar cells.

Findings

Re-absorption effects are negligible under non-concentrated light for typical SRH lifetimes.

Efficiency can be significantly improved by enhancing heat dissipation and emissivity.

The model's predictions align with experimental and other numerical results.

Abstract

To accurately calculate efficiencies $\eta$ of experimentally produced multijunction solar cells (MJSCs) and optimize their parameters, we offer semi-analytical photoconversion formalism that incorporates radiative recombination, Shockley-Read-Hall (SRH) recombination, surface recombination at the front and back surfaces of the cells, recombination in the space charge region (SCR) and the recombination at the heterojunction boundaries. Selfconsistent balance between the MJSC temperature and efficiency was imposed by jointly solving the equations for the photocurrent, photovoltage, and heat balance. Finally, we incorporate into the formalism the effect of additional photocurrent decrease with subcell number increase. It is shown that for an experimentally observed Shockley-Read-Hall lifetimes, the effect of re-absorption and re-emission of photons on MJSC efficiency can be neglected for non-concentrated radiation conditions. A significant efficiency $\eta$ increase can be achieved by improving the heat dissipation using radiators and bringing the MJSC emissivity to unity, that is closer to black body radiation rather than grey body radiation. Our calculated efficiencies compare well with other numerical results available and are consistent with the experimentally achieved efficiencies. The formalism can be used to optimize parameters of MJSCs for maximum photoconversion efficiency.