Abstract
AbstractCdTe is the leading commercial thin film photovoltaic technology with current record laboratory efficiency (22.1%). However, there is much potential for progress toward the Shockley‐Queisser limit (32%). The best CdTe devices have short‐circuit current close to the limit but open‐circuit voltage has much room for improvement. Back contact optimization is likely to play a key role in any improvement. Back contact material choice is also influenced by their applicability in more complex architectures such as bifacial and tandem solar cells, where high visible and/or near‐infrared transparency is required in conjunction with their electrical properties. The CdTe research community has employed many back contact materials and processes to realize them. Excellent reviews of back contacts were published by McCandless and Sites (2011) and Kumar and Rao (2014). There have been numerous publications on CdTe back contacts since 2014. This review includes both recent and older literature to give a comprehensive picture. It includes a categorization of back contact interface materials into groups such as oxides, chalcogenides, pnictides, halides, and organics. The authors attempt to identify the more promising material groups. Attention is drawn to parallels with back contact materials used on other thin film photovoltaics such as perovskites and kesterites.
Highlights
Photovoltaics (PV) using thin film CdTe as a photon absorber have been studied for several decades
CdTe PV can be constructed under two device architectures (Figure 1); the PV thin films can be deposited onto a substrate material in the general order electrode, p-type CdTe absorber, n-type buffer layer, electrode or the reverse order
Zinc (II) oxide (ZnO) heavily doped with aluminum (AZO) is a well-known transparent conducting oxide (TCO), and has been used both as a front contact,143 and as a back contact material (Parikh (2007),144 Heisler et al (2013)145) for CdTe absorber solar cells
Summary
Photovoltaics (PV) using thin film CdTe as a photon absorber have been studied for several decades. Lead telluride (PbTe) has been suggested as a back contact material in 1D simulation studies.97 XPS has suggested a low VBO between PbTe and CdTe(111) surfaces.98 It was expected that a two-dimensional electron gas can form at the PbTe/CdTe (111) interface, probably increasing recombination but no experimental solar cell results were available until the work of Swartz et al (2019).99 They found that despite indications that the CdTe/PbTe:Tl contact was Ohmic, that the PbTe:Tl layer appeared to be photoconductive.
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