Fabrication and characterization of ZnS and ZnSe absorber layers for UV-selective transparent photovoltaics.

We have fabricated ZnS, ZnSe and mixed Zn(S,Se) thin film layers for application as
solar cell absorbers in ultraviolet (UV)-selective solar cells. We fabricated the thin film
layers using a two-step process involving evaporation of the Zn layer followed by an
anneal in an H2S and/or H2Se containing atmosphere. A study of the annealing
conditions was performed to minimize hole formation in the thin film layer. A mixed
Sulfur and Selenium containing layer was also developed in order to reach a 3 eV band
gap, such that the layer absorbs all the UV light and leaves the visible light passing
through. Next, we tried to increase the conductivity of the layer by adding extrinsic
doping elements. Cu was found to increase the conductivity by more than three orders
of magnitude, while leaving the transparency of the layer mostly unchanged. Finally,
solar cell devices were fabricated using transparent back and front contacts and a NiO
buffer layer. We could measure a very small photocurrent of about 100 A/cm2 and an
open circuit voltage of about 500 mV under AM1.5G illumination.