• Energy Research

In the search for a nontoxic replacement of the commonly employed CdS buffer layer for Cu(In,Ga)(S,Se) $_\mathrm{2}$ based solar cells, chemically deposited Zn(O,S) thin films are a most promising choice. In this paper, we address the usually slow deposition speed of Zn(O,S) in a newly developed ammonia-free chemical bath process, resulting in a deposition of 30 nm in 3 min with good homogeneity on 30 cm × 30 cm sized substrates. Solar cells with buffer layers prepared from this process match the efficiency of CdS reference cells. In a second step, we address the light-soaking post-treatment, still needed for maximum efficiencies. By addition of aluminum to the deposition process, the initial efficiencies can be increased slightly. With the addition of boron, the light-soaking post-treatment is rendered unnecessary, while maintaining high efficiencies above 15%, surpassing reference cells with CdS buffer.

Evaluating interfering capacitance steps in admittance spectroscopy for solar cell defect analysis is still a problem which needs to be solved. While the common analysis developed by Walter et al.[1] is capable of extracting defect distributions from the capacitance data, it results in erroneous defect densities in the presence of overlapping capacitance steps. We derive an expression for the capacitance step caused by defects with a density of states distributed in energy. By adding several of these defect distributions, interfering capacitance steps can be described. Thus, it is possible to fit the entire capacitance spectrum simultaneously for all temperatures. We apply the presented method to Cu 2 ZnSnSe 4 -based solar cells with power conversion efficiencies between 5% and 7%. Comparing the obtained defect parameters with the ones obtained by the method from Walter et al. reveals that the Walter method overestimates the defect densities in the case of overlapping capacitance steps.

In this paper, we show that CuInSe2 (CIS) absorbers grown under Cu-excess have better collection efficiencies compared to Cu-poor ones. We also show that an ex situ potassium fluoride postdeposition treatment leads to an improvement in V OC for CIS absorbers grown under both Cu-excess and Cu-poor conditions. Additionally, for absorbers grown under Cu-excess, the junction breakdown, which is observed in reverse bias of untreated cells, is removed. This improvement is based mainly on improving the interface of the CIS absorber grown under Cu-excess to the cadmium sulphide buffer layer through moving the dominant recombination from the interface to the bulk. In contrast to observations in the literature, the treated surface is not completely Cu-free.

We present a high-temperature Cu $_2$ ZnSnSe $_4$ coevaporation study, where solar cells with a power conversion efficiency of 7.1% have been achieved. The process is monitored with laser light scattering in order to follow the incorporation of the Sn into the film. We observe the segregation of ZnSe at the Mo/CZTSe interface. Optical analysis has been carried out with photoluminescence and spectrophotometry. We observe strong band tailing and a bandgap, which is significantly lower than in other reported efficient CZTSe absorbers. The photoluminescence at room temperature is lower than the bandgap due to the existence of a large quantity of tail states. Finally, we present effects of low-temperature postannealing of the absorbers on ordering of the Cu/Zn atoms in CZTSe and solar cell parameters. We observe strong changes in all solar cell parameters upon annealing. The efficiency of the annealed devices is significantly reduced, although ordering is improved compared with ones made from nonannealed absorbers.

Abstract Low temperature (100 and 250 °C) post-deposition annealings were applied to Cu 2 ZnSnSe 4 (CZTSe) absorbers to modify the ordering degree of the kesterite. Spectrophotometry and photoluminescence revealed a blue or red shift of the band gap and the luminescence peak after ordering or disordering post-treatments, respectively. For most solar cells, the open circuit voltage was found to be limited by bulk recombination and to follow band gap variations, leading to a constant open circuit voltage deficit. For the range of ordering degree investigated, the large open circuit voltage deficit, typically encountered in kesterite solar cells, could not be attributed to disorder. Capacitance–voltage measurements demonstrated that the net carrier concentration of ordered CZTSe is reduced by one order of magnitude compared to disordered CZTSe. Besides its beneficial effects on the open circuit voltage, the ordering post-treatment was also found to increase the collection of photo-generated carriers, resulting in a high current density and a significant improvement in device efficiency.

AbstractPost‐device heat treatment (HT) in chalcopyrite [Cu (In,Ga)(S,Se)2] solar cells is known to improve the performance of the devices. However, this HT is only beneficial for devices made with absorbers grown under Cu‐poor conditions but not under Cu excess. We present a systematic study to understand the effects of HT on CuInSe2 and CuInS2 solar cells. The study is performed for CuInSe2 solar cells grown under Cu‐rich and Cu‐poor chemical potential prepared with both CdS and Zn(O,S) buffer layers. In addition, we also study Cu‐rich CuInS2 solar cells prepared with the suitable Zn(O,S) buffer layer. For Cu‐poor selenide device, low‐temperature HT leads to passivation of bulk, whereas in Cu‐rich devices, no such passivation was observed. The Cu‐rich devices are hampered by a large shunt. The HT decreases shunt resistance in Cu‐rich selenides, whereas it increases shunt resistance in Cu‐rich sulfides. The origin of these changes in device performance was investigated with capacitance–voltage measurement, which shows the considerable decrease in carrier concentration with HT in Cu‐poor CuInSe2, and temperature‐dependent current–voltage measurements show the presence of barrier for minority carriers. Together with numerical simulations, these findings support a highly doped interfacial p+ layer device model in Cu‐rich selenide absorbers and explain the discrepancy between Cu‐poor and Cu‐rich device performance. Our findings provide insights into how the same treatment can have a completely different effect on the device depending on the composition of the absorber.

ABSTRACTThin‐film solar cells reach high efficiencies and have a low carbon footprint in production. Tandem solar cells have the potential to significantly increase the efficiency of this technology, where the bottom‐cell is generally composed of a Cu(In,Ga)Se2 absorber layer with bandgaps around 1 eV or higher. Here, we investigate CuIn(Se1 − xTex)2 absorber layers and solar cells with bandgaps below 1 eV, which will bring the benefit of an additional degree of freedom for designing current‐matched two‐terminal tandem devices. We report that CuIn(Se1 − xTex)2 thin films can be grown single phase by co‐evaporation and that the bandgap can be reduced to the optimum range (0.92–0.95 eV) for a bottom cell. From photoluminescence spectroscopy, it is found that no additional non‐radiative losses are introduced to the absorber when adding Te. However, losses occur in the final solar cell due to non‐optimized interfaces. Nevertheless, a device with 9% power conversion efficiency is demonstrated with a bandgap of 0.97 eV and , the highest efficiency so far for chalcopyrites with band gap <1 eV. Interface recombination is identified as a major recombination channel for larger Te contents. Thus, further efficiency improvements can be expected with improved absorber/buffer interfaces.

We study defects in CuInSe $_{\text{2}}$ (CIS) grown under Cu-excess. Samples with different Cu/In and Se/metals flux ratios were characterized by thermal admittance spectroscopy, capacitance–voltage (C–V) measurements, and temperature-dependent current–voltage (IVT) measurements. All samples showed two different capacitance responses, which we attribute to defects with energies around 100 and 220 meV, plus the beginning of an additional step that we attribute to a freeze-out effect. By application of the Meyer–Neldel rule, the parameters of the two defects can be assigned to two different groups, both lying within the energy region of the so-called N1-defect that has been observed for Cu-poor absorbers.

Kesterite solar cells are low-cost alternatives for photovoltaics, based only on abundant metals, but they exhibit limited voltages. A new wide-gap kesterite solar cell provides a much higher voltage at a good efficiency.