Enhanced Solar Cell Performance of Fluorinated Small-Molecule and Polymer DonorsPublic Deposited
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Photovoltaic technology can help meet the exploding world energy demand in a sustainable and environmentally friendly way. Organic photovoltaics (OPVs) are especially attractive because they can be constructed with low-cost organic content and are amenable to high-throughput large-scale fabrication techniques. Synthetic modification of organic semiconductors has paved way for the tremendous progress that has been made in the field of OPVs. This thesis demonstrates the benefits of rational fluorination on small molecule (SM) and polymer OPV donors. The material and photovoltaic properties of three SMs are characterized: uncapped T-bBDT, methyl end-capped MT-bBDT and fluorine end-capped FT-bBDT. The two end-capped materials, MT-bBDT and FT-bBDT, show enhanced thin-film crystallinity and field-effect mobility versus the uncapped T-bBDT. However, only FT-bBDT yields enhanced OPV power conversion efficiency (PCE) of 4.92% to that of T-bBDT (3.85%), with MT-bBDT affording the lowest PCE of 3.39%. Despite its high mobility and crystallinity, MT-bBDT based devices yield low PCE because of unfavorable frontier energy levels and poor blend morphology. This study thus establishes the advantages of fluorine end-capping of SM donors while concurrently highlighting the benefits and limits of alkyl end-capping. The optoelectronic and photovoltaic properties of PBTZF4 (novel polymer with tetrafluorinated repeating unit) is compared to that of its bifluorinated analogue PBTZF2. Additional fluorination in PBTZF4 planarizes polymer backbone and enhances π-π stacking aggregation, leading to ~50 –150-fold higher field-effect mobility than that of PBTZF2. This, in addition to the lowered energy levels, results in high PCE of 5.55% when PBTZF4 is paired with molecular acceptor bPDI2P, versus only 3.68% for PBTZF2. Femtosecond transient absorption (fsTA) spectroscopy and light intensity based measurements show that geminate recombination is negligible in both blends. This is a significant finding since the performance of non-fullerene acceptors, unlike fullerenes, had been generally thought to be limited by geminate recombination.