Photoconductive Charge Transfer Complexes as Charge Transport Layers for High Performance Inverted Perovskite Solar Cells

Charge transport layers (CTLs) are critical for achieving high power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). Herein, the p-type bulk heterojunction (p-BHJ, i.e., PCBM doped PTAA) and n-type BHJ (n-BHJ, i.e., PBDTTT-C-T doped PCBM) charge transfer complexes are employed as hole and electron transport layers, respectively, to fabricate inverted PSCs. The photo-induced charge transfer between p-type and n-type organic semiconductors in the BHJ layers provides extra photoconductivity for enhanced charge transport and quasi-Fermi level splitting, hence enhancing the fill factor and open-circuit voltage of PSCs. The p-BHJ layer helps to improve the crystallinity and light absorption of perovskite, whilst the n-BHJ layer provides extra light absorption and charge generation to boost the short-circuit current. The combination of p-BHJ and n-BHJ CTLs in Cs-0.05(FA(0.92)MA(0.08))(0.95)Pb (I0.92Br0.08)(3) based inverted PSCs synergistically enhances the PCE from 18.3% to 22.6% with superior operational and thermal stabilities, and showing a negligible dependence on the thickness of these BHJ CTLs. Density functional theory simulations show that the formation energy of BHJ complex is critical in determining the doping effect and the ultimate performance enhancement of PSCs.