HIGH-EFFICIENT ZnO/PVD-CdS/Cu(In,Ga)Se2 THIN FILM SOLAR CELLS:PREPARATION AND NEW INSIGHTS IN THE HETEROJUNCTION ELECTRONIC STRUCTURE

摘要

We report on highly efficient (> 14%, AM1.5) ZnO/PVD-CdS/Cu(In,Ga)Se2 solar cells. Investigations were carried out into several directions, such as (I) condition of the absorber surface – air-exposed and ultrahigh vacuum Se-decapped absorber surfaces, (ii) optimum CdS thickness and (iii) thermal annealing of the devices. A comparative study of the electric and photoelectric properties of the ZnO/CdS/Cu(In,Ga)Se2 devices with both physical vapor deposition (PVD) and reference chemical bath deposition (CBD) buffer layers showed that it is the condition of the interface absorber/buffer which determines the initial diode quality of the devices. The thermal annealing of the cells with PVD-CdS buffers converts the buffer/absorber interface to an electronic structure, characteristic for devices with CBD-CdS buffer layers. The beneficial effect of the annealing seems to originate from a formation in the absorber of a region with higher band gap than that of the absorber bulk, close to the CdS/Cu(In,Ga)Se2 interface. Formation of such a region is especially beneficial for solar cells from Se-decapped absorbers which leads to a dramatic increase of the open-circuit voltage. A 14.1% total area (14.5% active area) efficient solar cell under AM1.5 conditions was achieved when combining preparation of the devices from Sedecappedabsorbers with the thermal annealing of the as-prepared devices in air.