Effects of Na and MoS₂ on Cu₂ZnSnS₄ thin‐film solar cell

Abstract

Cu2ZnSnS4 (CZTS)‐based materials have a useful band gap and a high absorption coefficient; however, their power conversion efficiency is low compared with that of CdTe and Cu(In,Ga)Se2‐based solar cells. Two of the factors that strongly affect CZTS solar cell characteristics are the MoS2 layer and the presence of defects. In this study, Mo back‐contact layers were annealed to control MoS2 layer formation and the Na content in the Mo layer before the absorber precursor layer was deposited. The increase in oxygen content in the Mo layer suppressed MoS2 layer formation. In addition, the increase in Na diffusion during the initial stage of the absorber precursor deposition decreased the defect density in the absorber layer and in the absorber–buffer interface. These results were verified through measurements of the external quantum efficiency, the temperature dependence of the open‐circuit voltage (VOC), and admittance spectra. The current densities (JSC) and VOC, as well as the power conversion efficiencies, improved as the annealing temperature of the Mo layer increased, which suggests that CZTS solar cell characteristics can be improved by suppressing MoS2 layer formation and increasing Na content in the Mo layer before deposition of the absorber precursor layer. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of a pre-annealing treatment (PAT) on Cu2ZnSn(S,Se)4 thin films prepared by rapid thermal processing (RTP) selenization

“…Details of the sputtering processes are thoroughly described elsewhere [28]. Out of the many possible precursor stack orders, in this study, we chose Cu/SnS/ZnS/Mo-Glass, which showed good adhesion and reasonable photovoltaic characteristics in our previous report [30]. After the deposition by sputtering, a 4-μm layer of selenium was deposited by thermal evaporation onto the Cu/ SnS/ZnS/Mo stack, which provided a final precursor structure of Se/Cu/SnS/ZnS/Mo-Glass.…”

Growth of Cu2ZnSnS4 absorber layer on flexible metallic substrates for thin film solar cell applications

“…In the case of Mo back contact, high temperature sulfurization process leads to the decomposition of the CZTS absorber layer, resulting in MoS 2 phase formation [22,23], which restricts the hole transport from CZTS to Mo layer [24]. The interfacial MoS 2 layer may affect open circuit voltage (V oc ) and change the band alignment between the CZTS absorber layer and Mo substrate.…”

Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu2ZnSnS4 solar cells

“…The commercialization of thin-film solar cells should be preceded by the development of a low-cost, high-efficiency technology [1][2][3]. The power conversion efficiencies (PCEs) of thin-film solar cells reported to date are 21.0% for CdTe [4], 21.7% for Cu(In,Ga)Se 2 (CIGS) [4,5], and 12.6% for Cu 2 ZnSn(S,Se) 4 (CZTSSe) [6].…”

“…The power conversion efficiencies (PCEs) of thin-film solar cells reported to date are 21.0% for CdTe [4], 21.7% for Cu(In,Ga)Se 2 (CIGS) [4,5], and 12.6% for Cu 2 ZnSn(S,Se) 4 (CZTSSe) [6]. Among these, thin-film solar cells based on CZTS and related materials (e.g., Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and CZTSSe) represent a significant improvement in solar cell technology because they use Earth-abundant elements, have useful band gap energies (E g ) of 1.0-1.5 eV, and have absorption coefficients greater than 10 4 cm -1 [1,3,7,8]. However, their PCEs are low compared with those of CdTe and CIGS solar cells.…”

“…In general, CZTS-based absorbers that have Cu-poor and Zn-rich compositions exhibit high PCEs [6,22,23]; however, the Zn-related secondary phase formed in the Zn-rich zone becomes a factor that diminishes the solar cell characteristics [19,[24][25][26][27]. Furthermore, defects are one of the main causes of recombination losses [1,21,22]. Minority carrier (electrons) collection occurs at grain boundaries (GBs) in the absorber layer, and GBs act as current flow channels [1,16,17,28]; thus, GBs are important in determining the characteristics of solar cells.…”