oa Nanoalloying - A Novel Method For The Fabrication Of Solar Cell Back Contacts

It is well known that for solar cell back contacts CdTe forms a Schottky barrier when it is contacted directly to a metal.Therefore an interlayer is needed to provide low ohmic contact resistance. Due to its strong p-type character, narrowing the Schottky barrier and therefore allowing holes to tunnel through, Sb2Te3 is a promising material. Furthermore, the use of such a Cu-free back contact material might enhance the stability of the solar cell. However, the fabrication of Sb2Te3 can be challenging due to Te re-evaporation from the hot substrate, significantly complicating stoichiometry and property control. Therefore, the nanoalloying growth method was used for the first time to fabricate the back contact layers. This method is based on the stoichiometric deposition of element layers on a cold substrate and afterwards the application of a low-temperature annealing process in which compound formation takes place. We discuss the properties of single Sb2Te3 films and present performance studies on superstrate-based CdTe solar cells with nanoalloyed Sb2Te3 back contacts. High quality single-phase Sb2Te3 with large grain sizes, carrier mobilities > 400 cm²/Vs and large Seebeck coefficients was obtained. Changing deposition parameters allows to influence the texture and to adjust the carrier concentration, demonstrating the flexibility of the method. An efficiency of 11.7% was achieved with Sb2Te3/Cu/Mo back contact and with Sb2Te3/Cu/Au contact in a first experimental series with significant potential for further optimization.