Improvement of the open-circuit voltage of Cu₂ZnSnS₄solar cells using a two-layer structure

Abstract

In Cu 2 ZnSnS 4 (CZTS) photovoltaic cells, a low open-circuit voltage (V OC ) principally causes low conversion efficiency. We investigated the deposition of a CZTS layer by a two-layer process to improve the V OC of the CZTS cells. In this process, the first CZTS layers near a Mo electrode have a high Cu content and the second layer near the surface has a low Cu content. The two-layer process improved the V OC of the CZTS cells from 0.66 to 0.78 V. Finally, the best CZTS cell showed a conversion efficiency of 8.8%. R ecently, the production of compound thin-film photovoltaic cells, for example, the Cu(In,Ga)Se 2 and CdTe types, has been increasing annually. However, the materials used to make these cells include rare metals (In and Ga), which significantly limits the volume that can be produced. The thin-film photovoltaic material Cu 2 ZnSnS x Se 4−x (CZTSSe) has recently become a candidate material for new photovoltaic cells. Wang et al. recently reported the production of CZTS 0.2 Se 0.8 thin-film solar cells with a conversion efficiency of 12.6%. 2) However, CZTSSe contains Se, a harmful element, and has a band-gap energy (E g ) of 1.1 eV, which is smaller than the optimum E g of 1.4-1.5 eV for the solar spectrum. On the other hand, Cu 2 ZnSnS 4 (CZTS) has been attracting increasing attention as a candidate material 1) because it contains no rare (In and Ga) or harmful (Se and Te) elements, and has a suitable E g of 1.4 eV. However, the CZTS cells that were previously reported have efficiencies below 10%. [3][4][5][6] Consequently, for the mass production of photovoltaic cells, it is clear that the development of CZTS cells with high efficiency is required.One of the main reasons for the poor photovoltaic properties of CZTS cells is their open-circuit voltage (V OC ), which is lower than that expected from the E g . The low V OC directly results in low conversion efficiency. To understand the V OC of the CZTS cells, the conduction band offset (CBO) at the interface between the buffer and the CZTS layers should be examined. In general, the interface between the buffer layer and the absorber layer is categorized into either a "cliff " or a "spike" type, where a large cliff lowers V OC and a large spike reduces the short-circuit current density (J SC ). 7,8) Researchers previously reported on the evaluation of the CBO value at the interface between the CdS and CZTS layers using methods such as hard X-ray photoelectron spectroscopy, 9) pump=probe methods [ultraviolet photoelectron spectroscopy (UPS)], 10) and UPS=inverse photoelectron spectroscopy (IPES), 11) and showed it to be sufficiently small, in the range from +0.4 to −0.2 eV, to realize a high-performance cell. Therefore, the CBO value is not the main reason for the low V OC in the CZTS cells.Another possible reason for the low V OC is the influence of an inner potential between the CZTS and window layers. In other words, the lower inner potential between the CZTS and window layers can cause a lower V OC when the hole carrier concentration (N c ) of the CZ...

Improving the photovoltaic performance of co‐evaporated Cu2ZnSnS4 thin‐film solar cells by incorporation of sodium from NaF layers

“…The E a values for the cells A, B, and C were determined to be 1.18, 1.16, and 1.02 eV, respectively. The values for the cells A and B agree well with those for previously reported CZTS cells [3,18]. Because the obtained E a values were significantly lower than E g (1.40-1.50 eV, refer to section 3.5), charge-carrier recombination would occur predominantly at the CZTS/CdS interface [19], especially for the cell C. Figures 4(b) and (c) show the temperature dependence of n and J 0 , extracted from the dark and light J-V curves, respectively [13,14].…”

“…To date, the highest η for a solar cell fabricated by using directly evaporated CZTS layers is 4.61%, as reported by Mise [7]. Thus, co-evaporated CZTS cells tend to exhibit lower efficiencies than the two-step-processed cells [3,4]. The reason for the lower η in directly evaporated cells has not been completely determined.…”

Interface band gap narrowing behind open circuit voltage losses in Cu2ZnSnS4 solar cells

“…In the solar cell with interface states, the open circuit voltage extrapolates to 1.11 V, which matches the value of the transport gap minus 0.2 eV narrowing at the interface as defined for the interface region (1.15 eV). The corresponding recombination energy deficit is 0.39 eV, which fits very well the recombination energy deficits of 0.3 eV, 0.4 eV, and 0.4 eV found experimentally in the highestefficiency CZTS/CdS solar cells [3][4][5] . As long as the simulated device is dominated by interface recombination, the simulated value of the recombination energy deficit is robust with respect to changes in various device parameters, including defect characteristics.…”

Formation of copper tin sulfide films by pulsed laser deposition at 248 and 355 nm

“…However, due to the limited availability of elements like In and Te and the toxicity of Cd, alternative absorbers such as Cu 2 ZnSnS 4 (CZTS) are being investigated [1], and recently a thin film solar cell based on a CZTS absorber layer has reached an efficiency of 8.8% [2].…”