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Study of thin film poly-crystalline CdTe solar cells presenting high acceptor concentrations achieved by in-situ arsenic doping

Kartopu, G.
Oklobia, O.
Turkay, D.
Diercks, D. R.
Gorman, B. P.
Barrioz, V.
Campbell, S.
Major, J. D.
Al Turkestani, M. K.
Yerci, Selçuk
Barnes, T. M.
Beattie, N. S.
Zoppi, G.
Jones, S.
Irvine, S. J. C.
Doping of CdTe using Group-V elements (As, P, and Sb) has gained interest in pursuit of increasing the cell voltage of CdTe thin film solar devices. Studies on bulk CdTe crystals have shown that much higher acceptor concentration than the traditional copper treatment is possible with As, P or Sb, enabled by high process temperature and/or rapid thermal quenching under Cd overpressure. We report a comprehensive study on in-situ As doping of poly-crystalline CdTe solar cells by MOCVD, whereby high acceptor densities, approaching 3 x 10(16) cm(-3) were achieved at low growth temperature of 390 degrees C. No As segregation could be detected at grain boundaries, even for 10(19) As cm(-3). A shallow acceptor level (+ 0.1 eV) due to As-Te substitutional doping and deep-level defects were observed at elevated As concentrations. Devices with variable As doping were analysed. Narrowing of the depletion layer, enhancement of bulk recombination, and reduction in device current and red response, albeit a small near infrared gain due to optical gap reduction, were observed at high concentrations. Device modelling indicated that the properties of the n-type window layer and associated interfacial recombination velocity are highly critical when the absorber doping is relatively high, demonstrating a route for obtaining high cell voltage.