Optimization of CdTe thin‐film solar cell efficiency using a sputtered, oxygenated CdS window layer

Abstract

A major source of loss in cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells results from light absorbed in the CdS window layer, which is not converted to electrical current. This film can be made more transparent by oxygen incorporation during sputter deposition at ambient temperature. Prior to this work, this material has not produced high-efficiency devices on tin oxide-coated soda-lime-glass substrates used industrially. Numerous devices were fabricated over a variety of process conditions to produce an optimized device. Although the material does not show a consistent increase in band gap with oxygenation, absorption in this layer can be virtually eliminated over the relevant spectrum, leading to an increase in short-circuit current. Meanwhile, fill factor is maintained, and open-circuit voltage increases relative to baseline devices with sublimated CdS. The trend of device parameters with oxygenation and thickness is consistent with an increasing conduction band offset at the window/CdTe interface. Optimization considering both initial efficiency and stability resulted in a National Renewable Energy Laboratory verified 15.2%-efficient cell on 3.2-mm soda-lime glass. This window material was shown to be compatible with SnO 2 -based transparent conducting oxide and high resistance transparent coated substrates using in-line compatible processes.

Chemical and mechanical techniques enabling direct characterization of the CdS/CdTe heterojunction region in completed devices

“…However, directly interrogating these processes is quite difficult. For example, as-deposited CdS:O re crystallizes to form hexagonal CdS with a 2.4 eV optical band gap after annealing in an inert environment [4,5]. We have observed similar behavior, but it is not clear that this is representative of the reactive environment of CSS deposition at high temperature.…”

Oxygenated CdS Buffer Layers Enabling High Open‐Circuit Voltages in Earth‐Abundant Cu2BaSnS4 Thin‐Film Solar Cells

“…As shown in Figure 1, CBTS has a very large lattice constant; therefore, it is difficult to find suitable heteropartner materials to make ideally lattice-matched junctions. [12,13] However, blue light with photon energies E > 2.4 eV is strongly absorbed by the CdS layer and so does not contribute to the photocurrents. Via these trap levels, the photogenerated holes and electrons can recombine at the heterointerface, which could be even more detrimental to open-circuit voltages (V OC 's) than recombination in the absorber bulk for wide-bandgap solar cells.…”

“…Misfit dislocations may produce dangling and wrong bonds at the interface that may create deep trap states. [13][14][15][16][17][18] Notably, oxygenation can upshift the conduction band minimum (CBM) compared to that of CdS, keeping the electrons away from the Fermi level, which therefore serves to reduce the effective interface recombination rate wileyonlinelibrary.com between the CdTe and the window layer. [9][10][11] Among the conventional buffer materials shown in Figure 1, CdS, with a bandgap E g ≈2.4 eV, appears to show the smallest lattice mismatch with most of the absorbers.…”

CdTe Photovoltaics for Sustainable Electricity Generation

“…11 Deposition of CdS and CdTe layers and CdCl 2 passivation treatment are carried out using closed-space sublimation from sources integrated into a single-vacuum system. 6,12 Thereafter, the substrate is transferred to another tool for co-sublimation of Cd 1Àx Mg x Te which would create the conduction band offset due to its higher band gap. This tool allows for similar temperature control as the ARDS and the substrate here is preheated before the deposition of Cd 1Àx Mg x Te is carried out.…”