Paper EH-MoM6
Nanoscale Characterization of Defects and Interfaces in Thin Film Solar Cells

Monday, December 8, 2014, 10:20 am, Room Lehua

Thin-film solar cells based on polycrystalline CdTe, Cu(In,Ga)Se₂ (CIGS), and Cu₂ZnSnSe₄ (CZTSe) have demonstrated high solar-to-electricity conversion efficiencies. Typically, polycrystalline thin-film solar cells are expected to exhibit poor performance compared to their single-crystalline counterparts, due to the existence of unavoidable structural defects such as dislocations and grain boundaries (GBs). It is surprising, therefore, that CdTe-, CIGS-, and CZTSe-based polycrystalline thin-film solar cells have achieved higher efficiencies than their single-crystalline counterparts.

Here, we present our results on the study of the atomic structure and electronic properties of structural defects including stacking faults, twins, dislocations, and GBs in CdTe, CIGS, and CZTSe using a combination of aberration-corrected scanning transmission electron microscopy (STEM) and first-principles density-functional theory calculation. Polycrystalline CdTe thin films were grown by closed-space sublimation, whereas CIGS and CZTSe thin films were deposited by co-evaporation. Atomic resolution scanning transmission electron microscopy (STEM) images were acquired with Nion UltraSTEM 100 and 200 microscopes. We found that intrinsic GBs in CdTe, CIGS, and CZTSe create deep levels mainly due to the anion-anion “wrong” bonding. However, these deep levels can be removed through extrinsic and/or intrinsic passivation. I addition to high resolution structural and chemical characterization, we will present data on the electrical and optoelectronic properties of defects obtained by a correlative approach that involves STEM, cryogenic cathodoluminescence defect spectroscopy and electron beam induced current imaging.