Non-destructive determination of phase, size, and strain of individual grains in polycrystalline photovoltaic materials

Mariana Mar Lucas,Tiago Ramos,Peter S Jørgensen,Stela Canulescu,Peter Kenesei,Jonathan Wright,Henning F Poulsen,Jens W Andreasen

doi:10.1016/j.jallcom.2021.161364

Abstract

The non-destructive structural characterization of individual grains in thin-films photovoltaics based on polycrystalline materials is a powerful tool for revealing important details of the microstructure of solar cell absorbers. Here, we use three-dimensional X-ray diffraction (3DXRD) to obtain statistics on the phase, size, orientation and strain tensors of the grains, as well as their twin relations in Cu2ZnSnS4 (CZTS) absorbers. Moreover, this powerful approach allows the non-ambiguous determination of phases with distinct optical and electrical properties but similar lattice parameters, such as CZTS and ZnS. Our analysis over cumulative statistics of nearly 600 grains in polycrystalline CZTS reveals that a fraction of 2.5% corresponds to the ZnS secondary phase. Statistics of the strain distribution in the polycrystalline CZTS layer indicate an average tensile stress in the plane of the film of ~70 MPa and a compressive stress along the normal to the film of ~145 MPa. We found that 41% of the total number of grains in CZTS absorbers are Σ3 twins. We calculate the frequency of the six types of Σ3 boundaries, revealing that the 180° rotation along the axis<221>is the most frequent. Accessing the microstructure opens the possibility to study its influence on the properties of the film, such as bandgap variations due to strain or the role of twin boundaries in the charge transport mechanisms.

Highlights

Photovoltaic thin-film technology is increasingly targeting alternative materials to meet the triple challenge of sustainability, low energy payback time, and scalability
1 Abstract We demonstrate a non-destructive approach to provide structural properties on the grain level for the absorber layer of kesterite solar cells
To maximize the signal to noise ratio of the diffracted intensity originating from the 1 μm sized grains, we reduced the 1 mm thickness of the molybdenum coated soda-lime glass substrate (Mo-SLG) by mechanical polishing and milling by a focused ion beam (FIB) down to 4 μm thickness

Summary

Introduction

Photovoltaic thin-film technology is increasingly targeting alternative materials to meet the triple challenge of sustainability, low energy payback time, and scalability. A relatively new but promising candidate is Cu2ZnSnS4 (CZTS), with an efficiency of 11% [3], and the selenized version, Cu2ZnSn(S, Se), where efficiency has reached 12.6% [4]. All of these materials still perform below the Shockley–Queisser limit [5]. For CZTS with a typically Cu-poor and Zn-rich composition, secondary phases with different bandgaps form, such as the high bandgap ZnS, increasing series resistance when situated in the back contact of the solar cell or acting as a barrier to the charge carriers at the p-n junction [10]–

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