On Quantifying Large Lattice Relaxations in Photovoltaic Devices

TL;DR

This study investigates how large lattice relaxations affect the stability and electrical properties of Cu(In,Ga)Se$_2$ photovoltaic devices under various conditions, revealing the role of sodium content and defect dynamics.

Contribution

It applies the kinetic theory of large lattice relaxations to analyze photovoltaic device behavior, providing new insights into defect-related metastability and sodium's stabilizing effects.

Findings

Lattice relaxation explains voltage and doping trends during light exposure.

Higher sodium content increases initial doping and device stability.

Revised activation energies suggest specific defect complexes are not responsible for metastability.

Abstract

Temporal variations of Cu(In,Ga)Se$_2$ photovoltaic device properties during light exposure at various temperatures and voltage biases for times up to 100 h were analyzed using the kinetic theory of large lattice relaxations. Open-circuit voltage and p-type doping increased with charge injection and decreased with temperature at low injection conditions. Lattice relaxation can account for both trends and activation energies extracted from the data were approximately 0.9 and 1.2 eV for devices with lower and higher sodium content, respectively. In these devices, increased sodium content resulted in higher initial p-type doping with greater stability. First principles calculations providing revised activation energies for the ($V_{Se}-V_{Cu}$) complex suggest that this defect does not account for the metastability observed here.