Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based Thin-Film Solar Cells

Abstract

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.

Rubidium Fluoride Post-Deposition Treatment: Impact on the Chemical Structure of the Cu(In,Ga)Se2 Surface and CdS/Cu(In,Ga)Se2 Interface in Thin-Film Solar Cells

“…Moreover, the CdS/CIGSe samples exhibit a feature at ~162.5 eV that is not present in the CdS reference spectrum. At or near this energy, S 3d  S 2p transitions are typically found for sulfate or sulfite compounds 36,46 . The sulfate reference spectra show two sharp lines at lower emission energy, attributed to S 3s  S 2p transitions.…”

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

“…The measured time-resolved PL (TRPL) recorded at 1.99 eV reveals a carrier lifetime of ≈11 ns from the red curve fit to Equation (S1) (Supporting Information). [18,19,28] A detailed measurement of the S 2p region ( Figure S4c , [17] suggesting that our sputtered CdS:O buffers consist of three components: CdS, CdSO 4 , and CdSO 3 . This film was characterized by XRD and Raman spectroscopy, showing a pure trigonal phase ( Figure S2, Supporting Information).…”

“…[24] A 200 nm thick CBTS film was deposited on a Si wafer substrate for spectroscopic ellipsometry (SE) measurement. [17,19] Although CdS:O exhibits the multiphase feature, there is only one apparent absorption edge observed in the optical transmission spectra ( Figure S5a, Supporting Information). SE measurements were performed over the spectral range from 225 nm to 1800 nm (0.7-5.5 eV) at angles of incidence of 55°, 60°, and 65° ( Figure S3a, Supporting Information).…”

“…As shown in Figure S6d and Table S2 (Supporting Information), CdS buffer creates a cliff-like (type II) conduction band alignment with the CBTS absorber, that is, a CBM ≈0.57 V lower than that of CBTS, usually associated with higher interface recombination rates. [13,17,19] Thus, CdS:O buffers sputtered using higher O 2 contents enable higher V OC 's by reducing interface recombination. As a result, interface recombination greatly limits the V OC 's in CdS buffered cells, which are much less than the maximum attainable V OC (≈1.65 V) for the absorber with a ≈2.0 eV bandgap based on the Shockley-Queisser limit.…”

“…[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. [13,15,[17][18][19] The highly resistive CdSO 4 phases can also act as passivators of the misfit CdTe/CdS interface. [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.…”