Surface sulfidation constructing gradient heterojunctions for high-efficiency (approaching 18%) HTL-free carbon-based inorganic perovskite solar cells

Due to the advantages of cost-effectiveness and tunable band gap, hole transport layer (HTL)-free CsPbI_XBr_3−X carbon-based inorganic perovskite solar cells (C-IPSCs) are emerging candidates for both single junction and tandem solar cells. Because of the direct contact between the carbon electrode and the perovskite surface, energy barriers and defects at the interface limit the enhancement of power conversion efficiency (PCE). In this work, we first reported a preparation method of CsPbI_2.75Br_0.25 HTL-free C-IPSCs and developed an effective surface sulfidation regulation (SSR) strategy to promote hole extraction and inhibit non-radiative recombination of inorganic perovskite by 2-(thiocyanomethylthio)benzothiazole (TCMTB) surface modification. The introduced S²⁻ anions form strong binding with uncoordinated Pb ions, inhibit the perovskite degradation reaction, and effectively passivate the surface defects. In addition, PbS formed by the SSR strategy constructed a gradient heterojunction, which promoted the arrangement energy levels and enhanced hole extraction. An additional back-surface field is induced at the interface of perovskite by energy band bending, which increases the open-circuit voltage (V_OC). As a result, the SSR-based CsPbI_2.75Br_0.25 HTL-free C-IPSCs showed a PCE of 17.88% with a fill factor of 81.56% and V_OC of 1.19 V, which was among the highest reported values of CsPbI_2.75Br_0.25 HTL-free C-IPSCs.