Designed Bithiophene-Based Interfacial Layer for High-Efficiency Bulk-Heterojunction Organic Photovoltaic Cells. Importance of Interfacial Energy Level Matching

This contribution describes the design, synthesis, characterization, and organic photovoltaic (OPV) device implementation of a novel interfacial layer (IFL) for insertion between the anode and active layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The IFL precursor, 5,5′-bis[( p -trichlorosilylpropylphenyl)phenylamino]-2,2′-bithiophene (PABTSi 2 ), covalently anchors to the Sn-doped In 2 O 3 (ITO) surface via the −SiCl 3 groups and incorporates a bithiophene unit to align the highest occupied molecular orbital (HOMO) energy with that of P3HT (5.0 eV). The synthesis and subsequent electrochemical analysis of PABTSi 2 indicates a HOMO energy of 4.9 eV, while the lowest uoccupied molecular orbital level remains sufficiently high, at 2.2 eV, to effectively block electron leakage to the OPV ITO anode. For the P3HT:PCBM OPV fabrication, PABTSi 2 is used as a spin-coated cross-linked (via −SiCl 3 hydrolysis and condensation) 1:2 blend with poly[9,9-dioctylfluorene- co - N -[4-(3-methylpropyl)]-diphenylamine] (TFB). Such devices exhibit an average power conversion efficiency of 3.14%, a fill factor of 62.7%, an open-circuit voltage of 0.54 V, and a short-circuit current of 9.31 mA/cm 2, parameters rivaling those of optimized PEDOT:PSS-based devices.