Assessment of off-axis and in-line electron holography for measurement of potential variations in Cu(In,Ga)Se2 thin-film solar cells

Debora Keller,Rolf Erni,Marta D Rossell,Ayodhya N Tiwari,Christophe Gatel,Etienne Snoeck,Patrick Reinhard,Fabian Pianezzi,Stephan Buecheler

doi:10.1186/s40679-015-0015-5

Abstract

AbstractElectron holography is employed to study variations of the electrostatic crystal potential in Cu(In,Ga)Se2 (CIGS) thin-film solar cells at different length scales: Long-range potential variations across the layer structure of the solar cell as well as inhomogeneities within the layers are analyzed by off-axis holography. In-line holography is applied to examine the local potential variation across a CIGS grain boundary. The phase reconstruction from a focal series is performed by a modified transport of intensity equation (TIE) which is optimized to reduce common artifacts. For comparison, three different microscopes of different optical configurations were used for in-line holography. Based on the results, the impact of the used microscope as well as further acquisition parameters on the in-line holography measurement is assessed. The measured potential variations are discussed considering the effect of different possible sources that may cause potential fluctuations. It is found that most of the variations are best explained by mean inner potential fluctuations rather than by inhomogeneities of the electronic properties. Finally, the present resolution limit of both methods is discussed regarding the feasibility of future electronic characterization of CIGS by holography.

Highlights

Thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers have demonstrated conversion efficiencies of up to 21.7 % [1]
Potential variations across the pn‐junction Off-axis holograms of the two solar cells were acquired in order to analyze the phase variation across the upper part of the solar cell layer stack, especially the CIGS/CdS interface region
High-angle annular dark-field (HAADF-) and bright-field (BF-) STEM micrographs of the corresponding area are shown in Fig. 4c, d, where the stacking of the different solar cell layers (CIGS, CdS, ZnO, MgF2) is indicated

Summary

Introduction

Thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorbers have demonstrated conversion efficiencies of up to 21.7 % [1]. Despite the high efficiencies, many loss mechanisms are not yet clearly understood. The absorber layer and the interfacial areas in addition build highly complex structures exhibiting nanoscale gradients and fluctuations in electronic, structural, and compositional properties. Inhomogeneities in the electronic properties of CIGS can be divided into two groups [2, 3]. Cu-deficient CIGS composites show strong compensation, i.e., the net charge introduced by ionized, acceptorolifkedoCnuorv-alcikaencInie2Cs+u(dVe−Cfeu)ctiss. Compensated by the formation it is expected that such defects provoke inhomogeneous charge distributions and may cause potential fluctuations of up to 150 mV in. To the potential fluctuations in the bulk, considerable potential variations are expected at grain boundaries in the polycrystalline CIGS

Methods

Results

Conclusion