J. López-García

ZnSe Etching of Zn‐Rich Cu2ZnSnSe4: An Oxidation Route for Improved Solar‐Cell Efficiency

Cu2ZnSnSe4 kesterite compounds are some of the most promising materials for low-cost thin-film photovoltaics. However, the synthesis of absorbers for high-performing devices is still a complex issue. So far, the best devices rely on absorbers grown in a Zn-rich and Cu-poor environment. These off-stoichiometric conditions favor the presence of a ZnSe secondary phase, which has been proved to be highly detrimental for device performance. Therefore, an effective method for the selective removal of this phase is important. Previous attempts to remove this phase by using acidic etching or highly toxic organic compounds have been reported but so far with moderate impact on device performance. Herein, a new oxidizing route to ensure efficient removal of ZnSe is presented based on treatment with a mixture of an oxidizing agent and a mineral acid followed by treatment in an aqueous Na2S solution. Three different oxidizing agents were tested: H2O2, KMnO4, and K2Cr2O7, combined with different concentrations of H2SO4. With all of these agents Se(2-) from the ZnSe surface phase is selectively oxidized to Se(0), forming an elemental Se phase, which is removed with the subsequent etching in Na2S. Using KMnO4 in a H2SO4-based medium, a large improvement on the conversion efficiency of the devices is observed, related to an improvement of all the optoelectronic parameters of the cells. Improvement of short-circuit current density (J(sc)) and series resistance is directly related to the selective etching of the ZnSe surface phase, which has a demonstrated current-blocking effect. In addition, a significant improvement of open-circuit voltage (V(oc)), shunt resistance (R(sh)), and fill factor (FF) are attributed to a passivation effect of the kesterite absorber surface resulting from the chemical processes, an effect that likely leads to a reduction of nonradiative-recombination states density and a subsequent improvement of the p-n junction.

Single-Step Sulfo-Selenization Method to Synthesize Cu2ZnSn(SySe1−y)4 Absorbers from Metallic Stack Precursors

Pentenary Cu2ZnSn(S(y)Se(1-y))4 (kesterite) photovoltaic absorbers are synthesized by a one-step annealing process from copper-poor and zinc-rich precursor metallic stacks prepared by direct-current magnetron sputtering deposition. Depending on the chalcogen source--mixtures of sulfur and selenium powders, or selenium disulfide--as well as the annealing temperature and pressure, this simple methodology permits the tuning of the absorber composition from sulfur-rich to selenium-rich in one single annealing process. The impact of the thermal treatment variables on chalcogenide incorporation is investigated. The effect of the S/(S+Se) compositional ratio on the structural and morphological properties of the as-grown films, and the optoelectronic parameters of solar cells fabricated using these absorber films is studied. Using this single-step sulfo-selenization method, pentenary kesterite-based devices with conversion efficiencies up to 4.4 % are obtained.

High V OC Cu 2 ZnSnSe 4 /CdS:Cu based solar cell: Evidences of a metal-insulator-semiconductor (MIS) type hetero-junction

In this work we report the Cu doping of chemical bath deposited CdS for the preparation of Cu2ZnSnSe4/CdS:Cu hetero-junction. We demonstrate that increasing the Cu concentration in the reaction bath several fundamental properties of CdS are changed (optical, crystalline, morphological), positively impacting in the resulting devices. The improvement of the efficiency from 1.6% to 6.1% is mainly explained by the large increases on VOC, obtaining the higher voltage value reported to now in the literature for this hetero-junction. We propose the formation of a metal-insulator-semiconductor (MIS) type device to explain our experimental results, which opens the possibility to use the MIS structure to solve, at least in part, the voltage deficit problems of kesterites.

UV-Raman scattering assessment of ZnO:Al layers from Cu(In, Ga)Se 2 based solar cells: Application for fast on-line process monitoring

In this paper, we present a non destructive optical methodology based on the use of pre-resonant UV-Raman scattering measurements, which allows for the on-line fast characterization of ZnO:Al (AZO) transparent conductive oxide (TCO). The experimental data corroborate the possibility to obtain by this methodology reliable information about the electrical properties of the TCO layers. The UV-Raman spectra of AZO are characterized by the presence of a defect-induced band. The relative intensity of this band in the spectra shows a clear correlation with the TCO resistivity. This methodology has been tested in real case devices, showing its potential for its implementation as on-line control tool for TCOs in the photovoltaic industry.