Cu₂ZnSnSe₄ Thin‐Film Solar Cells by Thermal Co‐evaporation with 11.6% Efficiency and Improved Minority Carrier Diffusion Length

Abstract

In this communication, we study the pure selenide phase CZTSe thin fi lm solar cells prepared by vacuum co-evaporation and demonstrate a power conversion effi ciency of 11.6%, which is the highest certifi ed effi ciency among CZTSe-based thin-fi lm solar cells reported to date. Device characterizations indicate that the higher effi ciencies appear to be related to signifi cantly enhanced transport properties for photogenerated carriers. The measured minority carrier diffusion length yields a record value of ≈2.1 µm, approaching that of state-of-the-art CIGS devices. The device also shows a record V OC -defi cit of 0.578 V, which can be attributed to reduced band tailing or the occurrence of shallower defect related states in the bandgap of the CZTSe layer. A comparative study of room-temperature photoluminescence (PL) of CZTSe and CZTS devices indicates that the PL emission peak from CZTSe is much closer in energy to the band edge than the case in CZTS. The improved device results are also consistent with the observation of a high-quality microstructure with large grains of ≈2.6 µm size.Polycrystalline CZTSe thin-fi lms with a thickness of ≈2.2 µm were deposited at 150 °C on 0.7-µm-thick Mo-coated soda-lime glass substrates using a vacuum co-evaporation technique, as described in earlier publications. [ 5,7 ] Knudsen-type elemental sources of Cu, Zn, and Sn, and a valved Se cracking source were used for the CZTSe co-evaporation. A ≈30-nm-thick NaF layer was deposited in a separate chamber prior to the CZTSe co-evaporation to promote grain coarsening of CZTSe thinfi lms. [ 16 ] Recrystallization and grain growth were carried out by annealing the co-evaporated CZTSe fi lm at ≈590 °C on a hot plate with excess selenium under N 2 atmosphere; similar to a process described elsewhere. [ 5,17,18 ] Inductively coupled plasma measurements showed a Cu-poor and Zn-rich elemental composition of the annealed fi lm: [Cu] Figure 1 a shows a θ -2 θ X-ray diffraction spectrum of the annealed CZTSe fi lm. The spectrum shows high crystallinity of the annealed fi lm with a major peak of (112)-orientation at ≈27.3°. The inset of Figure 1 a shows a top-view scanning electron microscopy (SEM) image of the annealed CZTSe fi lm, indicating dense and pinhole-free fi lm morphology. The average and standard deviation of grain size are determined to be 2.6 and 1.0 µm, respectively, estimated via digital image processing of SEM images.Using the co-evaporated CZTSe fi lms, solar cells with a total device area ( A ) of ≈0.43 cm 2 were fabricated with the following structure: MgF 2 /indium-tin-oxide (ITO)/ZnO/CdS/CZTSe/ Mo bottom electrode, as shown in Figure 1 b. The thicknesses Kesterite Cu 2 ZnSn( S x S e 1-x ) 4 (CZTSSe) has emerged as a promising candidate for scalable photovoltaic applications due to its earth-abundant elemental constituents and bandgap ( E g ) range between ≈1.0 eV ( x = 0) and ≈1.5 eV ( x = 1) with predicted theoretical maximum effi ciencies of over 30%. [ 1,2 ] To date, among thin-fi lm solar cells based on the kesteri...

Influence of compositionally induced defects on the vibrational properties of device grade Cu2ZnSnSe4 absorbers for kesterite based solar cells

“…15,16 The maximum values of the optoelectronic properties are achieved for a very narrow interval of compositions around Cu/(Zn þ Sn) % 0.74 and Zn/ Sn % 1.23, which agree with previously reported results. 2 More detailed investigation of the optoelectronic properties with composition is presented elsewhere.…”

Impact of annealing on electrical properties of Cu2ZnSnSe4 absorber layers

“…Currently, the record CZTSSe device has a power conversion efficiency of 12.6%, 1 and recently the pure selenide compound Cu 2 ZnSnSe 4 (CZTSe) achieved an efficiency of 11.6%. 2 These record devices rely on a sequential process for the absorber growth, 3 i.e., the growth of a precursor followed by an annealing at elevated temperatures in chalcogenide atmosphere. 4 The details of the reported annealing procedures vary among the groups and little is known on their impact on electrical device characteristics.…”

High efficiency bifacial Cu2ZnSnSe4 thin-film solar cells on transparent conducting oxide glass substrates

“…1,2 To date, CZTSSe thin-film solar cells with 10%-12.6% conversion efficiency have been realized. [3][4][5] However, the conversion efficiency of CZTSSe thin-film solar cells is still too low to replace commercial CuInGaSe 2 (CIGS) thin-film solar cells (21.6%). 6 Since conventional CZTSSe devices employ a Mo back contact that reflects light from the rear side, devices can only be illuminated from the front side.…”