Band gap grading and photovoltaic performance of solution-processed Cu(In,Ga)S₂ thin-film solar cells

Abstract

The photophysical properties of CuInxGa1-xS2 (CIGS) thin films, prepared by solution-based coating methods, are investigated to understand the correlation between the optical properties of these films and the electrical characteristics of solar cells fabricated using these films. Photophysical properties, such as the depth-dependent band gap and carrier lifetime, turn out to be at play in determining the energy conversion efficiency of solar cells. A double grading of the band gap in CIGS films enhances solar cell efficiency, even when defect states disturb carrier collection by non-radiative decay. The combinational stacking of different density films leads to improved solar cell performance as well as efficient fabrication because a graded band gap and reduced shunt current increase carrier collection efficiency. The photodynamics of minority-carriers suggests that the suppression of defect states is a primary area of improvement in CIGS thin films prepared by solution-based methods.

Band Gap Grading of Stacked Cu(In,Ga)S2 Thin Films

“…However, V oc of A + B + A was smaller than that of B + A, where V oc was primarily affected by E g of the front layer, the shunt resistance, and the defect states . However, the identical front layer (A) in both films led to similar E g and shunt resistance of the solar cells, suggesting that the defect states influenced V oc of solar cells, because a strong temperature dependence was observed in A + B + A. Indeed, the lifetime of the carriers in B + A also exhibited the temperature dependence (Figure (c)), indicating the non‐negligible density of the defect states .…”

“…The solar cell efficiency is improved by the graded band gap in the absorber layer due to the enhanced carrier collection and the reduced carrier recombination . In our previous study, the photovoltaic performance of solar cells was affected by the stacking combination of layers, where the solar cell with dense‐bottom and porous‐top layers showed better performance than that with a reversely stacked structure . In this Note, the combination of layers is examined further to investigate the correlation between the optical properties and the photovoltaic characteristics of solar cells in more detail.…”

“…The solar cell efficiency of A + B + A was expected to increase, compared to B + A, because the higher degree of the band gap grading at the back side could improve the carrier diffusion length (Figure (b)) . Indeed, J sc of A + B + A was higher than that of B + A (Table ), due to the steep grading.…”

“…Indeed, the optical intensity of the absorption and photoluminescence spectra was subjected to an exponential attenuation along the depth . Thus the observed E g of both films was determined by the dense‐top (paste A) layer, where E g of the dense layer (1.56 eV) was higher than that of the porous layer (1.53 eV) due to the grain size difference . In fact, the sequential stacking did not influence E g of the top layer, because the layer density and crystal structure were not affected by stacking .…”

“…In fact, the sequential stacking did not influence E g of the top layer, because the layer density and crystal structure were not affected by stacking . Accordingly, the depth dependence of E g was also assumed to be similar in the stacked films, as observed in the top layer, which implied that E g of the stacked films was not identical along the depth of the films, due to the difference in E g between the dense and the porous layer, as well as the strain distribution and chemical interaction at the interface . When the change of E g was shown by the variation of the conduction band minimum, it was doubly graded (Figure (a)).…”