Small Atoms Doping—A Strategy to Reduce Sn_Zn Recombination Center Concentration in CZTSe

Kesterite Cu₂ZnSnS_xSe_4-x (CZTSSe) is nowadays among the most promising earth-abundant thin film photovoltaic absorbers. Nevertheless, the high open circuit voltage (V_oc) deficit remains an unsolved issue preventing kesterite solar cells to be closer in performance to more mature thin film technologies. In this sense, most of the latest reports agree on CZTSSe bulk defect structure as the main limitation for the low V_oc.<br/><i>Ab initio </i>publications have identified Sn-related defects as the main culprits for the intrinsic high recombination rates in CZTSSe bulk. In particular, the relatively low formation energy of Sn_Zn defects (associated to the ambivalent Sn oxidation state, Sn²⁺ and Sn⁴⁺) implies and absurdly high concentration of Sn_Zn antisites. Additionally, Sn antisites present a giant capture cross section (associated to the huge lattice deformation upon a change of Sn charge state). Thus, the high concentration and cross section in addition to the mid-gap energy position of Sn_Zn, Sn_Cu and its associated defect clusters make Sn-antisites extremely efficient recombination centers, limiting to large extent the efficiency of CZTSSe based devices.<br/>Recently, modifications in the growth conditions have been recurrently reported to impact the optoelectronic performance of CZTSSe devices, emphasizing the importance of a careful growth control in order to reduce deep defect concentration, which could be related to an increased E_f during growth needed to improve the device properties. In the same vein, transient n-type doping has been suggested as a methodology to reduce the Sn-related recombination center concentration without altering the secondary phase formation and the p-type conductivity of the material. The donor activity of Sn antisites is the reason why an increased E_f during growth (when Sn mobility is enhanced) would increase the formation energy of Sn deep defects and in turn reduce their concentration. Nevertheless, achieving transient n-type doping is a technologically challenging task since there is a need to find donor impurities soluble at high temperatures, which are able to outgas or become electrically inert at room temperature. Thus, small and light atoms are the most interesting, hydrogen being probably the most suitable candidate for transient n-type doping due to the low formation energy of H_idonor defects. Besides, other small atoms like Li, B and Na could have a similar effect. In this case, the small atoms are expected to outgas during the cooling down of the sample, slightly affecting the carrier concentration of the material, but maintaining the p-type conductivity.<br/>In this work, the use of complex alkali hydrides is proposed as an innovative approach to achieve n-type transient doping during the kesterite formation. Specifically, the effects of LiAlH₄ decomposition during the reactive annealing are thoroughly explored, knowing that this hydride decomposes to its own elemental components at sufficiently high temperatures. This approach shows a strong beneficial influence on the CZTSe crystal growth and solar cell devices performance, especially the open circuit voltage and fill factor. In the first optimization, a V_oc of 469 mV is achieved, resulting in efficiencies about 9%, without antireflective coating. Compositional characterization reveals no small atom incorporation in the CZTSe matrix. The optoelectronic characterization suggests a reduction of bulk non-radiative recombination along with improvements in the CZTSe/CdS and CZTSe/MoSe₂ interface quality. Additionally, effects on sulfo-selenide CZTSSe and pure sulfide CZTS will also be presented. Finally, a mechanism on how small atoms (Li and H) interact to reduce the concentration of deep Sn_Zn recombination centers while keeping doping relatively unchanged will be presented.