Solution-processed Nanocrystal Based Thin Films as Hole Transport Materials in Cadmium Telluride Photovoltaics
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.349
Solution-processed Nanocrystal Based Thin Films as Hole Transport Materials in Cadmium Telluride Photovoltaics Ebin Bastola, Kamala Khanal Subedi, Khagendra P. Bhandari, and Randy J. Ellingson Wright Center for Photovoltaics Innovation and Commercialization (PVIC), Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA
ABSTRACT
The cadmium telluride (CdTe) photovoltaic (PV) comprise an efficient and cost-effective technology for harvesting solar energy. However, device efficiency remains limited in part by low-open circuit voltage (VOC) and fill factor (FF) due to inefficient transport of photogenerated charge carriers. Given the deep valence band of CdTe, the use of copper/gold (Cu/Au) as a back contact serves primarily to narrow the width of the inherent Schottky junction evident in CdTe solar cells (in our laboratory, Cu/Au has been used as a standard back contact). For efficient transport of carriers to and into the back contact, a hole transport layer (HTL) is desired with valence band edge comparable to that of CdTe (~ -5.9 eV). Here, we report solution-processed nanocrystal (NCs) based thin films as HTLs in CdTe solar cells. The earth abundant materials we discuss include iron pyrite (FeS 2), nickel-alloyed iron pyrite (NixFe1-xS2), zinc copper sulfide (ZnxCu1-xS) nanocomposites, and perovskite-based films. The FeS2 and NixFe1-xS2 NCs are synthesized by a hot-injection route, and thin films are fabricated by drop-casting, and spin-coating techniques using colloidal NCs. ZnxCu1-xS thin films are fabricated by chemical bath deposition. These NC-based thin films are applied and studied as the HTLs in CdTe devices. On using these materials, the device performance can be increased up to 10% compared to the standard Cu/Au back contact. Here, we discuss the benefits, challenges, and opportunities for these back contact materials in CdTe photovoltaics.
1. BACKGROUND The solution-processed devices fabricated by using colloidal NCs are promising in several applications including energy harvesting[1, 2], catalysis[3, 4], light-emitting devices[5], and biological applications[6]. In the case of energy harvesting applications, these materials are used for many purposes such as light absorbing materials, window layer materials and contact materials. Very efficient, and stable photovoltaic devices have been fabricated using these colloidal NCs.[2] Applications of these colloidal NCs as the contact materials include hole transport and electron transport materials in solar cells.[7, 8] For example, iron pyrite (FeS2) NCs have been applied as the hole transport materials in cadmium telluride (CdTe) and perovskite solar cells.[7, 9] Cadmium telluride (CdTe) is one of the leading thin film materials used for the fabrication of very efficient solar cells. However, the efficiency of CdTe solar cells is
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