Abstract
This work aims to improve CuInS2-thin film solar cells by means of Gallium incorporation into the absorber layer (partial, isovalent substitution of Indium).
For this purpose the structural properties of the Cu(In,Ga)S2 (CIGS) thin films and the growth process (reactive annealing of metallic precursors) have been investigated by X-ray diffraction (XRD), by depth resolved micro-Raman spectroscopy and by secondary neutral atoms mass spectroscopy (SNMS). The electronic properties have been investigated by j-V characteristics and spectral response measurements of CIGS/CdS/ZnO hetero junctions.
The thin film growth process can be divided into three steps:
1.) alloying of the sequentially deposited metallic precursor films and formation of binary Cu-In and Cu-Ga phases,
2.) the incorporation of sulfur from a reactive atmosphere (sulfur vapor Sx or H2S/Ar),
3.) recrystallization induced by Cu-S binary phases.
Ga-free precursors are characterized by the direct formation of the ternary CuInS2 phase straight from the metallic film. The addition of Ga to influences all three growth steps.
A CuGaS2 phase forms first during sulfurization. This is in accordance with equilibrium considerations. With ongoing chalcopyrite film formation the CuGaS2 phase is overgrown by CuInS2.
In the following the two ternary phases partially intermix by thermally activated diffusion leading to a quaternary Cu(In,Ga)S2 film with an inhomogeneous Ga-depth distribution. The experimental results are summarized in a model of the growth process.
Chemical diffusion coefficients for In in CuGaS2 and Ga in CuInS2 are derived on the basis of a 2-dimensional interdiffusion model which considers bulk diffusion through crystal grains as well as rapid diffusion along grain boundaries.
The incorporation of Ga in the near surface region of the CIGS absorber film leads to a widening of the band gap as could be deduced from spectral response measurements.
This leads to a significant and reproducable improvement in open circuit voltage Voc of the heterojunction. In contrast to earlier works the gain in Voc is not accompanied by any degradation of other photovoltaic device parameters.
The obtained experimental correlations of open circuit voltage, Ga-alloying and absorber band gap will be discussed in the framework of the current understanding of the dominating recombination mechanism at the absorber/buffer interface.
On the basis of the already mentioned diffusion model a higher band gap widening close to the hetero interface will be postulated which explains the Ga-related increase in open circuit voltage.
The experimental tools and models developed in this work form a good basis for further improvements of the CuInS2-solar cell by Ga-induced band gap engineering. |
