Probing the microscopic properties of Cu(In,Ga)Se₂ absorbers in thin film solar cells via correlative scanning electron microscopy techniques

Abstract text (incl. figure legends and references)

There are several challenges in the field of solar-cell research which cannot be solved without an in-depth investigation of the materials and devices. In the case of Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells, the band-gap energy of the polycrystalline photoabsorber can be tuned by varying the [Ga]/([Ga]+[In]) (GGI) ratio. However, there are several obstacles that prevent from achieving the desired device efficiency[1]. We aim to assess the issues that deteriorate the device performance on the sub-micrometer scale. In the present work, we employ several scanning electron microscopy (SEM) techniques to examine the microscopic properties of the CIGSe photoabsorber layer of the solar cell [2]. Electron backscatter diffraction (EBSD) is used for the crystallographic characterization to determine the average grain size (d_grain) and identify features such as twin grain boundaries (GBs), surface texture and preferred crystallographic orientation of the grains [3]. The local radiative recombination activities are characterized via cathodoluminescence (CL) spectroscopy. We extract information of the charge carrier recombination velocity (S_GB) at the grain boundaries from hyperspectral CL images in combination with time-resolved photoluminescence spectroscopy. Energy-dispersive X-ray spectroscopy (EDS) is applied to determine the distribution pattern of the elements within the absorber depth. The presence of secondary phases can also be detected via EDS mapping.

As shown in the Figure below, we apply these scanning electron microscopy techniques in a correlative manner on identical sample positions. Thus, we are able to detect changes in the microscopic properties when varying the GGI ratio in the CIGSe absorber layer and to investigate the microstructure-property relationships in several CIGSe thin-film solar cells. Indeed, it is found that the changes in the microscopic properties of the CIGSe photoabsorber have a substantial effect on the macroscopic device performance.

References

[1] Stanbery BJ, Abou-ras D, Yamada A, Mansfield L. CIGS Photovoltaics : Reviewing a New Paradigm, Phys. D: Appl. Phys. 55 (2022) 173001.

[2] Abou-Ras D, Kirchartz T, Rau U. Advanced Characterization Techniques for Thin Film Solar Cells. Weinheim, Germany: Wiley; 2016.

[3] Abou-Ras D, Kavalakkatt J, Nichterwitz M, Schäfer N, Harndt S, Wilkinson AJ, et al. Electron backscatter diffraction: An important tool for analyses of structure-property relationships in thin-film solar cells. Jom. 2013;65(9):1222–8.

Figure 1 : A) SEM cross section of the CIGSe absorber layer. B) EBSD image showing the local grain orientation and grain sizes. C) EDS map of the Ga-L series D) CL emission energy distribution, σ_lat.CL and σ_ver.CL are the fluctuations in the CL emission energy in the lateral and vertical directions.