Passivation of CdS/Cu₂ZnSnS₄ Interface from Surface Treatments of Kesterite‐Based Thin‐Film Solar Cells

Abstract

Surface treatments of Cu2ZnSnS4 have shown a beneficial effect on the solar cells performance due to a reduction in the open‐circuit voltage (VOC) deficit. Several reasons have been suggested for the VOC deficit, including an unfavorable band alignment at the buffer/Cu2ZnSnS4 interface. Herein, the influence of Cu2ZnSnS4 surface treatment (air exposure and air anneal) on the electronic and chemical properties of Cu2ZnSnS4 and CdS/Cu2ZnSnS4 interfaces is investigated. Using hard X‐ray photoelectron spectroscopy, it is shown that the band alignment at the CdS/Cu2ZnSnS4 interface is not significantly altered by the applied surface treatment. The device enhancement is instead connected to interface passivation for the surface‐treated Cu2ZnSnS4 samples due to the formation of SnOx, which is shown to not be fully removed upon KCN etching prior to the buffer layer deposition. In addition, a surface treatment of the Cu2ZnSnS4 absorber prior to buffer layer deposition influences the growth of CdS buffer, as a thicker CdS‐overlayer is observed to grow on a surface‐treated Cu2ZnSnS4 sample as compared with a nontreated sample. This suggests that a reoptimization of the CdS thickness for a given Cu2ZnSnS4 surface treatment is required.

Atomic Layer Grown Zinc–Tin Oxide as an Alternative Buffer Layer for Cu2ZnSnS4-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth

“…Comparing the influence of the CZTS surface treatment on the device properties, it is clear that an air annealing treatment (AA) resulted in increased device properties as compared to that of a non-treated (F) CZTS when similar ZTO films are deposited (700 cycles at 120 °C yielding a similar XRF composition), in agreement with previous reports for CdS/CZTS . However, the AE CZTS sample shows the lowest device performance when the ZTO buffer is employed, in contrast to our previous work using CdS buffer where the efficiency of the surface-treated CZTS devices has been found to increase with the surface treatment of CZTS (F < AE < AA) . No significant differences between the differently treated absorbers are observed for the bulk band gap values, as determined from the quantum efficiency measurements (Figure b,c).…”

“…21,22 The analyzer transmission function has not been taken into account, which may introduce an error in the calculation of the absolute values, but the aim of this study is to compare relative amounts between the investigated samples, and the method was therefore found to give reliable results, as we previously discussed. 8,23 The depth profile of CZTS anneal measurements were performed using HAXPES at beamline I09 (Diamond) by exposing the sample to 1 × 10 −5 mbar N 2 at temperatures between 100 and 200 °C for 30 min (including warming up and cooling time). Photoemission spectra were recorded before and after the N 2 anneal treatment employing a VG Scienta EW4000 energy electron analyzer and both soft and hard X-rays.…”

“…However, the absorber surface structure can change during the subsequent annealing steps in the device stack formation. A surface structure and composition change driven by the environment may induce strong changes in the solar cells’ properties, and increased solar cell performance has already been reported for some annealing treatments of both sulfide- and selenide CZTS­(Se)-based thin film solar cells. − In particular, in our recent publication, we showed that a surface treatment of the CZTS absorber prior to the CdS buffer layer deposition results in increased device performance, which was linked to the formation of interface species at the CdS/CZTS heterojunction . Further, the results show increased device performance upon a surface treatment of the absorber (air exposure or air annealing) as compared to a non-treated surface.…”

Thermoelectric properties of CZTS thin films: effect of Cu–Zn disorder

“…This feature is observed in nanocrystalline bulk 17,18,51 and thin film samples from different preparation routes. Figure 4 shows measurements performed on thin film samples grown by sulfurization of precursors deposited via with doctor blade (University of Trento, Figure 4a-b) or physical vapor deposition (PVD) methods: co-sputtering of SnS, ZnS and Cu (ENEA Rome, Figure 4c) 8 and co-sputtering of CuS, SnS and ZnS (Ångström Laboratory, Uppsala University, Figure 4d) [52][53][54][55] . The Seebeck coefficient and electrical resistivity (available only for the doctor blade sample) present a trend comparable to the spin-coated samples.…”

Design optimization and efficiency enhancement of axial junction nanowire solar cells utilizing a forward scattering mechanism

“…55,56 Al 2 O 3 and SiO 2 are two of the most investigated materials used for the passivation of these cells due to their superior properties and the technological feasibility of using them for passivation using several processes including chemical bath deposition, gas phase epitaxy and sputtering. 53,[55][56][57][58][59][60][61][62][63][64][65][66] For nanowire solar cells, the enhanced surface area that improves utilization of solar spectrum also presents the challenge of enhanced surface recombination resulting in deterioration of external quantum efficiency of the solar cell due to lost charge carriers at the surface. 67 Surface passivation is needed for the alleviation of surface recombination which can signicantly improve the carrier collection efficiency.…”