How band tail recombination influences the open‐circuit voltage of solar cells

Max Hilaire Wolter,Thomas Paul Weiss,Thomas Feurer,Stephan Buecheler,Philip Jackson,Susanne Siebentritt,Romain Carron,Enrico Avancini,Wolfram Witte,Benjamin Bissig,Shiro Nishiwaki

doi:10.1002/pip.3449

Abstract

AbstractThe power conversion efficiency of solar cells strongly depends on the open‐circuit voltage which, in turn, depends on the recombination activity within the device. A possible source of detrimental charge carrier recombination is band tails. An empirical linear relationship between loss and the Urbach energy of the band tails has been shown in the past. Here we discuss how band tails influence the radiative recombination and the nonradiative recombination in the bulk of the absorber. First, we show through photoluminescence that the band tails can be willfully tuned in state‐of‐the‐art thin‐film Cu (In,Ga)Se2 (CIGSe) absorbers and solar cells on a 20% efficiency level and beyond through the incorporation of alkali atoms. In the second part, we compare our CIGSe results to published results from other solar cell technologies. This comparison reveals that CIGS solar cells follow the previously described empirical trend: an increase in the open‐circuit voltage with decreasing band tails. Finally, we model the influence of tail states on the radiative and nonradiative recombination losses: Radiative recombination is increased because carriers thermalize into the tail states and nonradiative recombination of free carriers in the bands is increased because of Shockley–Read–Hall recombination through the tail states. The comparison with experimental data shows that the influence of tail states is even worse than the increase in radiative and SRH recombination predicted by our model. Our results thus suggest that band tails act as one of the main remaining voltage limitations in the majority of state‐of‐the‐art solar cells.

Highlights

The efficiency of a solar cell defines how much solar energy can be converted into electricity and strongly depends on the solar cell's open-circuit voltage VOC
Since band tails constitute a density of states (DOS) that extend into the band gap, they induce additional radiative and nonradiative recombination channels that contribute to losses in the open-circuit voltage VOC of solar cell devices
The correlation between voltage losses and Urbach energy has recently been investigated for various solar cell technologies

Summary

Introduction

The efficiency of a solar cell defines how much solar energy can be converted into electricity and strongly depends on the solar cell's open-circuit voltage VOC. The latter is a strong simplification as the absorption coefficient follows a square root behavior at energies above the band gap energy.[55] It should be noted that Equation 1 is a useful approximation for the absorption behavior of tail states to calculate voltage losses only in the case of Urbach energies smaller than kBT.

Results

Conclusion