| AVS 61st International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Wednesday Sessions |
| Session EN+AS+EM+SE-WeM |
| Session: | Thin Film Photovoltaics |
| Presenter: | Heayoung Yoon, NIST |
| Authors: | H. Yoon, NIST P. Haney, NIST P. Koirala, University of Toledo J.I. Basham, NIST Y. Yoon, NIST R.W. Collins, University of Toledo N.B. Zhitenev, NIST |
| Correspondent: | Click to Email |
Thin film CdTe solar cells are a promising photovoltaic (PV) technology in today’s market due t o their high optical absorption and inexpensive fabrication processes . However, the current module efficiency is well below the theoretically estimated maximum efficiency (13 % vs. 30 %) . Recent studies have suggested that inhomogeneity of the PV materials is mainly responsible for the low power conversion efficiency. In this work, we investigate the variation of local PV properties of CdTe solar cells, focusing on grain bulk, grain boundaries, and n-CdS / p-CdTe junctions. The window ( ≈ 120 nm thick CdS) and absorber ( ≈ 2.2 µm thick CdTe) layers were sputtered on a TCO (transparent conductive oxide) coated glass substrate followed by CdCl2 treatment. The back contact metals (3 nm Cu / 30 nm Au) were deposited and annealed, creating 256 devices in a 15 cm by 15 cm solar panel. Following light and dark current-voltage measurements, we performed local characterizations using electron beams for high (> 13 %) and low efficiency (< 6 %) devices within the panel. Electron beam induced current (EBIC) was used to measure the local carrier collection efficiency with a spatial resolution of ≈20 nm exciting carriers either from the top surface or the cross-sections of the devices. Cross-sectional EBIC data reveals that the peak of efficiency is in the middle of CdTe layer in the low efficiency devices, while the carrier collection is maximal near the p-n junction in the high efficiency devices. The EBIC contrasts at grains/grain boundaries in these devices are also compared. The measured local electronic properties are correlated to microstructural morphology (Transmission Electron Microscopy), orientation (Electron Back Scattered Diffraction), and chemical composition (Energy Dispersive X-ray spectroscopy). We perform 2D model drift-diffusion simulations to determine the magnitude of downward band-bending near grain boundaries (with typical magnitude of 0.2 eV). We will discuss the impact of carrier generation rate (high level injection vs. low level injection) in EBIC analysis.