Effect of 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) as well as bathocuproine (BCP) and Ag interlayer thickness on the performance of organic tandem solar cells

- PTCBI in CRZ achieved much higher FF than BCP thus higher ηPCE.
- Ultrathin Ag layer was helpful due to flatter surface and plasmonic effect.
- Slightly thicker Ag could eliminate the S-shaped J-V curves in BCP based CRZ.
- The overall ηPCE of normal tandem cell was improved to 3.49% by thickness-tuning.

Firstly, multi-fold subphthalocyanine (SubPc) homo-tandem cells were fabricated. When complementary absorbing SubPc and chloroaluminum phthalocyanine (ClAlPc) were used to prepare tandem cells, both short circuit current (JSC) and fill factor (FF) are significantly improved relative to the SubPc double tandem cell. 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI) as electron transporting layer (ETL) in the charge recombination zone (CRZ) achieves a much higher FF than bathocuproine (BCP) thus higher power conversion efficiency (ηPCE) in both the normal and reverse tandem cells, ascribed to the matched energy levels, very smooth film surface, and ohmic contact with Ag interlayer. The effect of Ag interlayer thickness was also investigated. Ultrathin Ag layer with isolated clusters is helpful for obtaining higher photocurrent in both PTCBI and BCP based CRZs, originating from a flatter interface, less optical loss, and a plasmonic effect induced absorption enhancement of C60 in bottom subcell. By optical modeling for current matching, the performance of normal tandem cell is improved, exhibiting a high open circuit voltage of 1.80 V and an overall ηPCE of 3.49%.

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