Improving performance of Cs₂AgBiBr₆ solar cell through constructing gradient energy level with deep-level hole transport material

The valence band offset between Cs₂AgBiBr₆ and hole transport layer (HTL) is approximately 1.00 eV, which results in high energy loss and is identified as one of the bottle necks of Cs₂AgBiBr₆ perovskite solar cell (PSC) for achieving high power conversion efficiency (PCE). To tackle this problem, we propose the optimization of the energy level alignment by designing and synthesizing novel deep-level hole transport materials (HTMs). The sole introduction of deep-level HTMs successfully reduces the valence band offset between Cs₂AgBiBr₆ and HTL, but induces the increased valence band offset at HTL/Au interface, limiting the PCE improvement. To further solve the problem and improve the PCE, the gradient energy level arrangement is constructed by combining the newly developed deep-level HTM 6,6'-(3-((9,9-dimethyl-9H-fluoren-3-yl)(4-methoxyphenyl)amino)thiophene-2,5-diyl)bis(N-(9,9-dimethyl-9H-fluoren-2-yl)-N,9-bis(4-methoxyphenyl)-9H-carbazol-3-amine) (TF) with 2,2',7,7'-tetrakis(N,N’-di-pmethoxyphenylamine)-9,9-spirobifluorene (Spiro-OMeTAD). Through optimization, an impressive PCE of 3.50% with remarkably high open-circuit voltage (V_oc) and fill factor (FF) is achieved, qualifying it among the best pristine Cs₂AgBiBr₆ PSCs.