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Fadhil Alfadhili1 Suneth Watthage1 Geethika Liyanage1 Jacob Gibbs1 Adam Phillips1 Michael Heben1

1, University of Toledo, Toledo, Ohio, United States

By adding Zn or Mg to CdTe, the band gap increases from 1.5 eV for pure CdTe to > 2 eV. This, coupled with the fact that CdTe can be produced at low cost, suggests that CdZnTe or CdMgTe could be an ideal top cell for mass produced tandem devices. The addition of the Zn or Mg into the CdTe is through direct substitution for the Cd. As a result, the change in band gap is due to an increase in the conduction band energy level, leaving the valence band close to that of CdTe. Because of the low valence band energy of CdTe, the options for back contacts have been limited. This issue will be exacerbated by requiring the back contact to be near infrared (NIR) transparent when the CdZnTe or CdMgTe is incorporated into a tandem device. Recently we fabricated a low barrier, transparent back contact to CdTe. This was accomplished by treating a CdCl2 activated CdTe sample with methylammonium iodide (CH3NH3I, MAI) solution followed with a mild thermal treatment to produce a layer of Te at the back surface. An indium tin oxide (ITO) overlay was applied to the Te layer as the transparent back contact. Improved open circuit voltage (VOC) of this device over the standard Cu/Au back contacted device was achieved. The champion device efficiency from the MAI treated CdTe with the ITO back electrode was 12.2% with VOC of 823 mV, short circuit current (JSC) of 21.4 mA/cm2, and fill factor (FF) of 69.3%. The best performing device from the standard Cu/Au back contacted devices showed VOC, JSC, FF, and power conversion efficiency (PCE) of 811 mV, 21.3 mA/cm2, 75.6%, and 13.0%, respectively. Note that the FF was slightly lower due to the reduced sheet resistance of the ITO compared to Au. In monolithically integrated tandem devices, this will not be an issue as lateral conductivity is unnecessary. Further, the NIR transmittance reduction due to the MAI treatment was only ~6%. Previous work has shown that the interaction between MAI and Zn is similar to that of MAI and Cd – the Zn2+ or Cd2+ ions react with MA+ and I- ions, resulting Zn- or Cd-based perovskites. Due to this reason, Cd was selectively remove from a CdTe surface by reacting a MAI thin film with the CdTe surface. The formed Cd-based perovskites can easily rinse out leaving a Te behind. Since the Zn2+ reacts with MAI similar to the Cd2+, this process should work for CdZnTe films as well. Furthermore, Te layers formed through the MAI process will be compared to deposited Te layers.

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