Cadmium telluride solar cells: Selenium diffusion unveiled

Nat. Commun. 7, 12537 (2016)

Solar cells based on CdTe now reach competitive efficiencies of up to 22.1%, mostly due to increased short circuit current. Although cells with this record-high efficiency are proprietary technology, it is thought that such increases are achieved by bandgap engineering, using Se diffusion from a CdSe window layer that gives rise to a CdTe_xSe_1−x layer with graded composition and bandgap. However, a microscopic understanding of the mechanisms of formation and photoresponse of the resulting CdTe_xSe_1−x layers is still lacking. Now, Jonathan Poplawsky and colleagues from Oak Ridge National Laboratory, the University of Toledo and Vanderbilt University show that the photoactivity of the CdTe_xSe_1−x layers depends on the Se content and on the crystalline structure of the alloy and they confirm an optimal thickness of 100 nm for the CdSe window.

Credit: NPG

The researchers investigate the structure, composition and photoactivity of CdTe_xSe_1−x alloyed layers with nanoscale resolution in four solar cells with CdSe window thickness of 50, 100, 200 and 400 nm. For cells with 100-nm-thick windows, an average Se composition of 25% diffuses into the CdTe layer, and the entire alloyed layer has a zincblende structure with columnar grains that span from the front to the back contact, making it photoactive across the whole cross-section. As the Se composition varies through the alloyed layer, the bandgap is progressively reduced, increasing the photoresponse to less energetic photons.