Controlling Band Alignment at the Back Interface of Cadmium Telluride Solar Cells using ZnTe and Te Buffer Layers
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.31
Controlling Band Alignment at the Back Interface of Cadmium Telluride Solar Cells using ZnTe and Te Buffer Layers Fadhil K. Alfadhili, Adam B. Phillips*, Geethika K. Liyanage, Jacob M. Gibbs, Manoj K. Jamarkattel, and Michael J. Heben Wright Centre for Photovoltaics Innovation and Commercialization, Department of Physics and Astronomy, University of Toledo, Toledo, OH, 43606, USA
ABSTRACT Formation of a low barrier back contact plays a critical role in improving the photoconversion efficiency of the CdTe solar cells. Incorporating a buffer layer to minimize the band bending at the back of the CdTe device can significantly lower the barrier for the hole current, improving open circuit voltage (VOC) and the fill factor. Over the past years, researchers have incorporated the both ZnTe and Te as buffer layers to improve CdTe device performance. Here we compare device performance using these two materials as buffer layers at the back of CdTe devices. We show that using Te in contact to CdTe results in higher performance than using ZnTe in contact to the CdTe. Low temperature current densityvoltage measurements show that Te results is a lower barrier with CdTe than ZnTe, indicating that Te has better band alignment, resulting in less downward bending in the CdTe at the back interface, than ZnTe does.
INTRODUCTION Over the past few years, significant improvement in CdTe device efficiency has occurred with record values currently over 20% [1]. Most of this performance improvement is due to increased current collection by changing the traditional window layer (CdS) of CdTe to a wider bandgap material with better band alignment [2]. However, the open circuit voltage (VOC) remains well below the theoretical limit due to the large barrier formed at the back of the device caused by the deep valence band edge of CdTe ( -5.8 eV) [3]. Researchers have implemented different types of back-buffer layers (BBLs) to reduce the barrier, band bending, and carrier recombination at the back surface [4]. Among the candidates for buffer layers, zinc telluride (ZnTe) has been used due to the expected low valence band offset (VBO) between CdTe and ZnTe ( -0.14 eV) [5] which
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should result in better band alignment and a lower barrier to holes [5, 6]. However, previous experimental work shows that the barrier height between the CdTe and ZnTe:Cu is -0.4 eV [7], far higher than the expected value. Tellurium (Te) has also been used as a buffer layer at the back of CdTe, and been shown to improved device performance through, presumably, reduced band bending [4]. Te can be formed either by wet chemical etching- techniques [8, 9, 10], chemical bath deposition (CBD) [11], or physical vapor deposition- methods such as sublimation [12] or evapora
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