Improved Ga grading of sequentially produced Cu(In,Ga)Se2 solar cells studied by high resolution X-ray fluorescence

Abstract

There is particular interest to investigate compositional inhomogeneity of Cu(In,Ga)Se2 solar cell absorbers. We introduce an approach in which focused ion beam prepared thin lamellas of complete solar cell devices are scanned with a highly focused synchrotron X-ray beam. Analyzing the resulting fluorescence radiation ensures high resolution compositional analysis combined with high spatial resolution. Thus, we are able to detect subtle variations of the Ga/(Ga + In) ratio down to 0.01 on a submicrometer scale. We observed that for sequentially processed solar cells a higher selenization temperature leads to absorbers with almost homogenous Ga/(Ga + In) ratio, which significantly improved the conversion efficiency.

Adjusting the Ga grading during fast atmospheric processing of Cu(In,Ga)Se2 solar cell absorber layers using elemental selenium vapor

“…A very similar impact of an intermediate annealing step on the Ga in-depth distribution was found by Schöppe et al [20], who achieved a more uniform in-depth Ga profile during a faster selenization (>20 min) in vacuum. The intermediate annealing step they describe was, however, performed at comparably lower temperatures (about 390°C) and under a Se supply with a deposition rate of about 200 nm/min.…”

Interplay of Performance‐Limiting Nanoscale Features in Cu2ZnSn(S,Se)4 Solar Cells

“…Not only the layered geometry of the sample is clearly visible by SEM and EDS, but also contrast variations within the absorber, small voids, and a thin Pt layer that was deposited to protect the absorber during the lamella preparation. [ 22 ] The Mo back contact, onto which the CZTSSe absorber was grown, is visible at the bottom of the SEM image (Figure 1a) and seems to be also present in the S map (Figure 1b). However, the alleged S signal in the Mo layer actually originates from the Mo L α lines, which strongly overlap with the S K α lines (see Figure S2, Supporting Information).…”

ID16B: a hard X-ray nanoprobe beamline at the ESRF for nano-analysis

“…The observed Se signal is not limited to the absorber layer, but is also clearly detected inside the Mo back contact, especially under the detached regions of the absorber layer. Such findings help to optimize the preparation condi- tions required to further increase the conversion efficiency and to fully exploit the potential of these thin film solar cells (Schö ppe et al, 2015). The approach of measuring thin lamellas prepared by focused ion beam using nano-XRF can also be applied to other multilayer structures or inhomogeneous materials such as kesterites.…”