Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells
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1012-Y12-08
Highly Arsenic Doped CdTe Layers for the Back Contacts of CdTe Solar Cells Vincent Barrioz1, Yuri Y. Proskuryakov2, Eurig W. Jones1, Jon D. Major2, Stuart J.C. Irvine1, Ken Durose2, and Dan A. Lamb1 1 School of Chemistry, University of Wales Bangor, Gwynedd, Bangor, LL57 2UW, United Kingdom 2 Department of Physics, Durham University, South Road, Durham, DH1 3LE, United Kingdom
ABSTRACT In an effort to overcome the lack of a suitable metal as an ohmic back contact for CdTe solar cells, a study was carried out on the potential for using a highly arsenic (As) doped CdTe layer with metallization. The deposition of full CdTe/CdS devices, including the highly doped CdTe:As and the CdCl2 treatment, were carried out by metal organic chemical vapour deposition (MOCVD), in an all-in-one process with no etching being necessary. They were characterized and compared to control devices prepared using conventional bromine-methanol back contact etching. SIMS and C-V profiling results indicated that arsenic concentrations of up to 1.5 × 1019 atoms·cm-3 were incorporated in the CdTe. Current-voltage (J-V) characteristics showed strong improvements, particularly in the open-circuit voltage (Voc) and series resistance (Rs): With a 250 nm thick doped layer, the series resistance was reduced from 9.8 Ω·cm2 to 1.6 Ω·cm2 for a contact area of 0.25 cm2; the J-V curves displayed no rollover, while the Voc increased by up to 70 mV (~ 12 % rise). Preliminary XRD data show that there may be an As2Te3 layer at the CdTe surface which could be contributing to the low barrier height of this contact.
INTRODUCTION The back contact of CdTe solar cells has long been a challenge in which even a noble metal with high work function (Φm), such as gold suffers from the J-V “rollover” effect due to the metal/CdTe Schottky junction. As no common metals have Φm high enough for an ohmic contact with CdTe, two main approaches have been pursued so far; (1) increasing the p-type doping at the back to make a thin p+ layer enabling the majority carriers to tunnel through the Schottky barrier (and lower surface carrier recombination); and/or (2) reduce the band gap energy (Eg) at the back contact, by depositing an intermediate layer, so that the barrier is effectively reduced. p-type ZnTe is often used either with Cu [1] or N [2] as dopant. The use of stable Cu-Te phases [3] is also studied despite Cu being a fast diffuser in CdTe. An example of the low gap approach is the use of Sb2Te3 (Eg = 0.3 eV), first reported by Romeo et al. [4]. Its chemical stability [5] and effect on device performances [6] were thoroughly studied when used in combination with Mo or Ni metallization. Generally however, device performance stability and repeatability must be the defining attributes of a viable contender for commercial use. A further consideration is the ease of processing – most back contact designs require a wet etch which has the effect of terminating the back surface of the CdTe layer with a Te-rich surface for metallization.
Although CdTe is known to be a
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