Co-solvent enhanced zinc oxysulfide buffer layers in Kesterite copper zinc tin selenide solar cells

Abstract

A co-solvent, dimethylsulfoxide (DMSO), is added to the aqueous chemical "bath" deposition (CBD) process used to grow ZnOS buffer layers for thin film Cu2ZnSnSe4 (CZTSe) solar cells. Device performance improves markedly as fill factors increase from 0.17 to 0.51 upon the co-solvent addition. X-ray photoelectron spectroscopy (XPS) analyses are presented for quasi-in situ CZTSe/CBD-ZnOS interfaces prepared under an inert atmosphere and yield valence band offsets equal to -1.0 eV for both ZnOS preparations. When combined with optical band gap data, conduction band offsets exceed 1 eV for the water and the water/DMSO solutions. XPS measurements show increased downward band bending in the CZTSe absorber layer when the ZnOS buffer layer is deposited from water only. Admittance spectroscopy data shows that the ZnOS deposited from water increases the built-in potential (Vbi) yet these solar cells perform poorly compared to those made with DMSO added. The band energy offsets imply an alternate form of transport through this junction. Possible mechanisms are discussed, which circumvent the otherwise large conduction band spike between CZTSe and ZnOS, and improve functionality with the low-band gap absorber, CZTSe (Eg = 0.96 eV).

Restraining the Band Fluctuation of CBD‐Zn(O,S) Layer: Modifying the Hetero‐Junction Interface for High Performance Cu2ZnSnSe4 Solar Cells With Cd‐Free Buffer Layer

“…Because of no surface Ar ion cleaning, all the O 1s peaks contain a fraction arising from C‐O bonds with binding energy of 533.3 eV, which should be attributed to the organic contamination. The other peaks appearing in the untreated sample are assigned as Zn(OH) 2 (532.7 eV), Zn(O,S) alloy (531.7 eV), and ZnO (530.4 eV), respectively . Besides the Zn(O,S) alloy, the untreated Zn(O,S) film is rich in ZnO and Zn(OH) 2 , as a result of competition growth of Zinc hydroxide and Zinc ammonium complex .…”

“…Application of Zn(O,S) buffer layer in kesterite‐based thin film solar cells also attracts increasing attention. Several institutes have reported ∼5% efficiency kesterite solar cells based on CBD‐Zn(O,S) buffer layer . Recently, M. Neuschitzer et al reported a 6.5% efficiency CZTSe solar cell which is close to the 6.9% efficiency CdS reference by adjusting the sulfur content in ultra thin CBD‐Zn(O,S) buffer layers which are less than 10 nm .…”

“…To replace the toxic CdS buffer layer for CIGS and CZTSSe thin film solar cells, extensive studies have been carried out on the alternative Cd‐free buffer layers, such as In 2 S 3 , ZnS, (Zn,Sn)O, Zn(O,S), among which Zn(O,S) takes advantages of large and tunable band gap (2.6–3.8 eV), high transparency, low toxicity, and earth abundant source materials . Recently, Cu(In,Ga)Se 2 /CBD‐Zn(O,S) based thin film solar cells have reached a remarkable 22% power conversion efficiency which is comparable with the world record of the Cu(In,Ga)Se 2 /CdS thin film solar cells .…”

“…While the CZTSSe/CBD‐Zn(O,S) solar cells with general thickness buffer layer (20–50 nm) have suffered from the poor interface performance, which may arise from the large positive conduction band offset (CBO) between CZTSSe and Zn(O,S), the low conductivity of Zn(O,S) layer, and the ZnO and Zn(OH) 2 secondary phases in the Zn(O,S) layer . Because the relative sulfur content S/(S+O) of the CBD‐Zn(O,S) is often as high as close to 0.7 and CBD‐Zn(O,S) films are often characterized as amorphous or poor crystalline, the measured optical band gap of CBD‐Zn(O,S) films is often large, and there is a large difference of conduction band edge between CBD‐Zn(O,S) and ZnO secondary phase, which is much likely to cause a large band fluctuation of buffer layer, thus deteriorating the device performance. It is necessary to find a way to eliminate the ZnO and Zn(OH) 2 secondary phases in CBD‐Zn(O,S) layer and modify the hetero‐junction interface to improve the device performance of cadmium‐free thin film solar cells .…”

Quantitative Study on the Chemical Solution Deposition of Zinc Oxysulfide

“…8 Moreover, spectroscopic characterization of Cu 2 ZnSnSe 4 /ZnOS heterojunctions showed that including DMSO during CBD growth of ZnOS affected the band energy line up but resulted in no definitive compositional changes. 9 Films shown in Figure 1 were deposited with standard CBD of ZnOS on quartz substrates in an open reaction vessel that is similar for solar cell buffer layer depositions. Films grown on the quartz slides exhibit different coverage resulting in the distinctly different transmission spectra.…”