On the origin of tail states and VOC losses in Cu(In,Ga)Se2

TL;DR

This paper investigates the origin of tail states in Cu(In,Ga)Se2 solar cells, showing alkali metals reduce these states through doping and electrostatic effects, thereby improving voltage output.

Contribution

It reveals that alkali metals decrease tail states independently of grain boundaries and introduces a model linking doping, tail states, and voltage loss.

Findings

Alkali metals reduce tail states even without grain boundaries.

Doping increases due to alkali incorporation, lowering tail states.

Voltage loss can be explained by doping effects on tail states.

Abstract

The detrimental effect of tail states on the radiative and non-radiative voltage loss has been demonstrated to be a limiting factor for the open circuit voltage in Cu(In,Ga)Se2 solar cells. A strategy that has proven effective in reducing tail states is the addition of alkali metals, the effect of which has been associated with the passivation of charged defects at grain boundaries. Herein, tail states in Cu(In,Ga)Se2 are revisited by studying the effect of compositional variations and alkali incorporation into single crystals. The results demonstrate that alkalis decrease the density of tail states despite the absence of grain boundaries, suggesting that there is more to alkalis than just grain boundary effects. Moreover, an increase in doping as a result of alkali incorporation is shown to contribute to the reduced tail states, which are demonstrated to arise largely from electrostatic potential fluctuations and to be determined by grain interior properties. By analyzing the voltage loss in high-efficiency polycrystalline and single crystalline devices, this work presents a model that explains the entirety of the voltage loss in Cu(In,Ga)Se2 based on the combined effect of doping on tail states and VOC.