CdSe thin film solar cells utilizing a nanostructured back contact

Thin films of CdSe have been grown on a variety of carbon nanotube substrates using a chemical bath deposition (CBD) process at room temperature. The cnts forming the substrate have two functions: 1) they serve as nucleation sites to initiate the CdSe film growth and 2) they serve as a nanostructured back contact to assist with carrier removal from the resulting CdSe layer. The initial CdSe deposit is a conformal crystalline film on each cnt and is formed from an ion-by-ion deposition of the Cd and Se ions. Once the entire cnt layer has been covered growth proceeds in a radial direction from each cnt or cnt bundle until the growing interfaces meet and fill in the interstitial spaces. From that point a solid film grows vertically from the substrate and forms a CdSe absorber layer with a nanostructured back contact buried within it. Devices exhibiting small area efficiencies of up to 4% have been made using a CuSe window layer (also grown using a CBD process) directly on the CdSe absorber layer. CdSe is a favorable choice for the absorber layer both because of its high bandgap and the use of Se rather than the scarcer Te that is used for CdTe thin film cells. The goals of this work are to 1) demonstrate large area device efficiencies approaching 15% at AM1.5 and 2) scale up the aqueous CBD processes for the CdSe and CuSe depositions for use in existing but unused roll-to-roll film processing facilities. It is anticipated that the low capital expense associated with this manufacturing approach will meet the DoE's LCOE targets.