Cadmium Tin Oxide and Zinc Magnesium Oxide Prepared by Hollow Cathode Sputtering for CdTe Photovoltaics
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Cadmium Tin Oxide and Zinc Magnesium Oxide Prepared by Hollow Cathode Sputtering for CdTe Photovoltaics Alan E. Delahoy1, Shou Peng2,3, Payal Patra4, Surya Manda1, Akash Saraf1, Yunfei Chen1, Xuehai Tan1, and Ken K. Chin1 1CNBM
New Energy Materials Research Center, Department of Physics, New Jersey Institute of Technology, Newark, NJ, USA 2 Bengbu Design and Research Institute for Glass Industry, Bengbu, China 3 China Triumph International Engineering Co. Ltd., Shanghai, China 4 New Jersey Innovation Institute, Newark, NJ, USA
ABSTRACT This work reports the fabrication and characterization of superstrate-type Zn1-xMgxO/CdTe heterojunction solar cells on both CdxSnyO and commercial SnO2:F transparent conducting oxides (TCOs) in which the ZMO and CTO layers are produced for the first time by hollow cathode sputtering. The sputtering is conducted in a reactive mode using metal or alloyed metal targets fitted to a custom-made linear cathode. It is notable that the CdS buffer layer conventionally employed in CdTe solar cells is entirely replaced by the ZMO window layer. The use of ZMO is found to eliminate the blue loss associated with CdS optical absorption and further results in a higher open-circuit voltage. Key parameters were found to be the conduction band offset at the ZMO/CdTe interface and the ZMO thickness. It was discovered that the ZMO exhibits intense photoluminescence even at room temperature. Most of the solar cells were fabricated in the FTO/ZMO/CdTe configuration although CTO/ZMO/CdTe solar cells were also demonstrated. The CTO was produced with an electron mobility of 46 cm2 V-1s-1 without any post-deposition annealing or treatment.
INTRODUCTION Photovoltaic modules based on thin-film polycrystalline cadmium telluride are now being manufactured at the multi-GW/year scale. The highest reported efficiencies at the module and cell level are 18.6% (aperture) and 22.1%, respectively. Some of the important factors that determine the attainable efficiency are: TCO properties such as conductivity and transmittance [1]; buffer layer properties such as bandgap and the band offset at the absorber – buffer interface [2,3]; the CdTe absorber properties such as the minority carrier (electron) lifetime and mobility, and the majority carrier (hole) concentration [4]; bandgap engineering [5]; and the quality and stability of the back contact. Until 2013, the open-circuit voltage Voc of CdTe solar cells generally fell in the range 800 - 875 mV [6], representing a significant deficit relative to the Shockley-Queisser limit. (Recently, a Voc > 1.0 V was achieved using single-crystal CdTe doped with P [7], and a Voc of 1.1 V was achieved in a CdTe/Cd1-xMgxTe double-heterostructure cell [8].) Furthermore, the short-circuit current density Jsc suffered significant losses due to optical absorption in the CdS (or CdS:O) buffer layer. The line of work reported here focusses on optimization of the front side of the solar cell, viz. the TCO and buffer layers, and addresses both
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