# A Novel Ge-Doping Approach for Grain Growth and Recombination Suppression in Buffer-Free CIGSe Solar Cells

## TL;DR

A new method of adding germanium to CIGSe solar cells improves their structure and performance by reducing defects and recombination.

## Contribution

A novel Ge-doping approach is introduced to enhance grain growth and suppress recombination in buffer-free CIGSe solar cells.

## Key findings

- Ge doping promotes grain growth in CIGSe absorbers without introducing impurity phases.
- Ge doping enhances band bending at grain boundaries and suppresses carrier recombination.
- Ge-doping improves the performance of buffer-free CIGSe solar cells by reducing recombination rates.

## Abstract

What are the main findings?
Ge doping promotes grain growth in CIGSe absorbers.Ge does not introduce notable impurity phases as a result of its loss via volatile Ge-Se compounds.Ge-doping enhances band bending at grain boundaries and suppresses carrier recombination.Ge-doping leads to the device performance enhancement of CIGSe buffer-free solar cells.

Ge doping promotes grain growth in CIGSe absorbers.

Ge does not introduce notable impurity phases as a result of its loss via volatile Ge-Se compounds.

Ge-doping enhances band bending at grain boundaries and suppresses carrier recombination.

Ge-doping leads to the device performance enhancement of CIGSe buffer-free solar cells.

What are the implications of the main findings?
Provides a novel approach to enhancing the crystallinity of CIGSe thin films.Demonstrates the effects of grain boundary passivation on suppressing recombination.Offers a method for improving the performance of CIGSe cells with a simplified structure.

Provides a novel approach to enhancing the crystallinity of CIGSe thin films.

Demonstrates the effects of grain boundary passivation on suppressing recombination.

Offers a method for improving the performance of CIGSe cells with a simplified structure.

Ge-doped CIGSe absorbers were fabricated using a two-step process of depositing sputtered stacked Ge-doped CIGSe precursors and selenization annealing. The effects of Ge doping on the crystallinity as well as defects of CIGSe absorbers and the performance of CIGSe buffer-free solar cells were investigated. The results show that Ge doping significantly promotes the grain growth of CIGSe absorbers. Due to Ge loss via volatilization during selenization annealing, Ge residue is undetectable in Ge-doped absorbers. Ge doping offers an effective approach to improve CIGSe crystallinity without introducing notable impurity phases or Ge-related defects. However, Ge doping also induces Se loss, and excessive Se vacancy defects adversely affect the performance of the absorber. In addition, Ge doping increases the contact potential difference at CIGSe grain boundaries and is beneficial for reducing carrier recombination at these sites. Analysis of recombination rates in Ge-doped CIGSe buffer-free solar cells reveals that the combined effects of enhanced crystallinity and optimized electrical properties at grain boundaries effectively suppress the recombination in the space charge region, at the interface, and in the quasi-neutral region, leading to improved device performance.