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Bodipy-Based Panchromatic (pi) Conjugated Polymers For Organic Photovoltaics

In a highly interdisciplinary field, such as organic photovoltaics (OPVs), developing a predictive understanding of the relationship between molecular structures, morphology of the photoactive layer and the ultimate device performance is the key to unlocking the vast potential of this field. Although isolated examples of high-performance organic molecules are prevalent in the literature, the reasons for their superior performance are not well understood. The function of an OPV device is dependent of four key processes: (i) light absorption, (ii)charge separation, (iii) charge transport, and (iv) charge collection. While the first three are material-dependent factors, charge collection depends on the nature of the interfaces involved. We have thus investigated a new class of semiconductor molecules based on BODIPY dyes with the aim of understanding how variations in the molecular structure affect the optoelectronic and transport properties of the molecules. First-generation pi-conjugated polymers based on the BODIPY core possess broad and intense absorption spectra. Additionally, the frontier molecular orbital (FMO) energy levels of the polymers can be tuned by a judicious choice of the comonomers. Electron-deficient comonomers with electron affinities higher than that of the BODIPY core, predominantly afford n -type polymers. A unique feature of these semiconductors is their panchromatic absorption spectrum that spans throughout the visible region. Thus these polymers can be considered to be potential electron acceptors in all-polymer solar cells. Copolymerization of BODIPY with electron-rich comonomers, on the other hand, only results in p -type semiconductors. Furthermore, the highest occupied molecular orbital (HOMO) of these polymers is found to correlate with the ionization potential of the electron-rich monomer. Having said that, the lowest unoccupied molecular orbital (LUMO) energy level does not change. Thus for the first time, a correlation between theoretical calculations and experimental observations has been demonstrated for predicting the FMO energy levels of BODIPY-based semiconducting polymers. Second-generation copolymers based on an unsubstituted BODIPY core retain the broad absorption characteristics of the first-generation polymers. In addition, due to reduced electron density on the BODIPY core, the HOMO energy level of the resulting polymers is reduced thereby imparting enhanced oxidative stability to these polymers. Charge transport measurements through thick films (∼1 micron) reveal only p -channel activity with hole mobilities comparable to some of the high-performance polymers reported in literature. Preliminary bulk-heterojunction OPV devices fabricated with these polymers show modest power conversion efficiencies. We believe that understanding the morphology of the active layer in relation to the polymer structure will help improve future molecular designs and eventually, device performance.