The role of interfaces in thin-film CdTe solar cells
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The role of interfaces in thin-film CdTe solar cells M. J. Romero, T. A. Gessert, M. M. Al-Jassim, R. G. Dhere, D.S. Albin, and H. R. Moutinho National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393 Phone: 303-384-6653, Fax: 303-384-6604, Email: [email protected] ABSTRACT Thin-film CdTe solar cells are very promising for future cost-effective photovoltaics. The photovoltaic effect in these cells is based on the extraction of photoexcited carriers by the field provided by the CdTe/CdS heterojunction. An additional interface with non-rectifying characteristics is needed to close the external circuit on the other side of the CdTe thin film. Finally, the transport of photoexcited carriers is influenced by the presence of grain boundaries (GBs). In this contribution, we investigate several aspects of these interfaces and their effects on the operation of CdTe solar cells by electron-beam-induced current (EBIC) and cathodoluminescence (CL) measurements.
INTRODUCTION Thin-film CdTe solar cells are promising for Ti future, cost-effective, terrestrial photovoltaics. Furthermore, the success in processing highly ZnTe:Cu efficient thin-film solar panels on flexible substrates has raised the interest in these technologies in the CdTe space community. Photoconversion efficiencies up to 16.5% have recently been attained from CdTe/CdS cells by the improved optical transmission provided by Cd2SnO4 (CTO) and/or Zn2SnO4 (ZTO) as transparent conducting oxides (TCO) for the front contact [1]. Additionally, it has CdS been shown that interdiffusion of CdS and ZTO SnO2 1 µm consumes the CdS film and improves the internal quantum efficiency for high-energy photons [2]. On Figure 1. Cross-sectional view of a the other hand, deviations from the ideal behavior SnO2/CdS/CdTe/ZnTe:Cu/Ti structure. for diodes, such as rollover and crossover, are often measured on CdTe/CdS solar cells due to the back surface field in the CdTe, near the interface with the back contact [3]. These examples illustrate how critical interfaces are for improving efficiency in thinfilm photovoltaics. The CdTe solar cell shown in Figure 1 consists of a SnO2 (500 nm)/CdS (300 nm)/CdTe (4 µm)/ZnTe:Cu (500 nm)/Ti (500nm) structure. For this particular cell, both CdS and CdTe films were deposited by vapor-transport deposition (VTD) and subsequently treated with a dry CdCl2 process. The back contacts were fabricated by a sequential process involving r.f.-sputter deposition of ZnTe:Cu (~6 at.% Cu), and d.c.-sputter deposition of Ti. Fill factors approaching 77% have been demonstrated by this contacting scheme [4]. The photovoltaic effect is based on the extraction of photoexcited carriers in the CdTe by the field provided by the CdTe/CdS heterointerface. The ZnTe:Cu/Ti back contact is needed to close the external circuit. Aspects related to the interfaces that are the most critical for further improving efficiency, are described below (follow Fig. 1): (a) Grain boundaries might be visualized as CdTe/CdTe homointerfaces. The transport of photoexcited ca
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