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

Abstract

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.

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

“…It seems clear that to improve CZTSSe devices changes in the processing conditions to maximize V OC must be performed, with the bulk and interfaces of the material as paramount working areas. Whereas chemical etchings have proved to be an important tool to modify the properties of the p-n junction, and therefore improve V OC among other parameters, changing the back contact interface led as well to significant improvements [15][16][17][18][19][20][21]. The present work is devoted to the modification of the properties of the Mo/CZTSe interface by using different Mo configurations (monolayer, bi-layer and tri-layer) and an innovative intermediate ultrathin MoO 2 layer in CZTSe solar cells.…”

Analysis of Failure Modes in Kesterite Solar Cells

“…Insufficient buffer coverage can also lead to decreased VOC 58 . The position of Fermi level close to the middle of hetero-interface due to Fermi level pinning 52 , the absence of charge inversion at hetero-interface 13,59 , the high density of interface defects 59 and the presence of secondary phases at the interface 61,62 are mentioned as well to explain the VOC deficit compared to CIGSSe solar cells. Concerning pure sulfide CZTS absorber, it has been demonstrated that bandgap narrowing at the front interface reduces VOC 60 .…”

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

“…Accordingly, changes in this spectral region of the Raman spectra measured with 532 nm excitation from samples with different compositions cannot be attributed to the presence of ZnSe, in agreement with the previous work showing Raman analysis before and after ZnSe etching. 20 Figure 2(a) presents a comparison of Raman spectra for samples corresponding to different compositions when moving along the A-type defect line ([Zn Cu þ V Cu ]) in the direction away from the stoichiometric point. Frequencies of the modes are not affected with this kind of compositional change, which implies that the concentration of defects in this region is not so high as to cause a notable overall distortion of the crystalline lattice.…”