Suppressing the CuZn antisite defects and band tailing states is paramount for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. To this end, cation doping has been widely adopted and investigated. Nevertheless, to the best of our knowledge, there is a scarcity of reports specifically addressing the doping of group IIIA elements to date. Herein, we present a viable strategy, incorporating the Al element, to achieve superior absorber quality and device performance. The addition of Al-doping not only improves the selenization degree but also enhances surface smoothness and induces a more Cu-deficient CZTSSe surface, thereby elevating both the quality of the absorber and heterojunction interface. Furthermore, we observe that the CuZn defect can be effectively mitigated through the substitution of Al dopants at the Zn site, consequently resulting in a reduction of band tailing states. Consequently, it significantly improves the efficiency from 6.24 to 9.25% with a low open-circuit voltage deficit of ∼0.291 V relative to the doping-free case, one of the highest values for the type of IIIA element doping cells. Our results demonstrate that Al incorporation is also a feasible pathway to suppress the antisite defects and band tailing states in solution-processed CZTSSe, thereby facilitating the realization of efficient kesterite solar cells.
Zihan Chen,Yaohong Zhang,Zhi Li Teh,Jianfeng Yang,Lin Yuan,Gavin J. Conibeer,Robert J. Patterson,Qing Shen,Shujuan Huang,Zhilong Zhang
Quantum dots (QDs) have a wide range of applications in the field of optoelectronics. They have been playing multiple roles within the configuration of a device, by serving as the building blocks for both the active layer and the carrier transport layer. While the performance of various optoelectronic devices has been steadily improving via developments in passivating the QD active layer, the possible improvement via passivation of the QD-based carrier transport layer has been largely overlooked. Here, with lead sulfide QD photovoltaics as the platform of study, we demonstrate that the device performance can be significantly improved by passivating the QD hole transport layer (HTL) with zinc salt post-treatments. The power conversion efficiency is improved from 8.7% of the reference device to 10.2% and 9.5% for devices with zinc acetate (ZnAc)- and zinc iodide (ZnI2)-treated HTLs, respectively. Transient absorption spectroscopy confirms that both treatments effectively reduce band-tail states and increase carrier lifetime of the HTLs. Further elemental analysis shows that ZnAc provides a higher amount of Zn2+ for passivation while maintaining the function of HTL by allowing essential p-doping oxidation. In contrast, the additional I– passivation from ZnI2 inhibits p-doping oxidation and limits the function of HTL. This work demonstrates the potential of improving device performance by passivating the QD-based HTLs, and the method developed is likely applicable to other optoelectronic devices.
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