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The Kagan group is tailoring organic-inorganic and organic-organic donor-acceptor heterojunctions and their interfaces to develop new materials for solar photovoltaics. The group has also designed and constructed new optical and electrical measurement techniques to probe and correlate the properties of the materials with their performance in devices.

We reported the synthesis, properties, and photovoltaic applications of a new conjugated push-pull copolymer (C12DPP-π-BT) containing a donor group (bithiophene) and an acceptor group (2,5-didodecylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione), bridged by a phenyl group. Using cyclic voltammetry, we found the energy levels of C12DPP-π-BT are intermediate to common electron donor and acceptor photovoltaic materials, poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), respectively. Whereas P3HT and PCBM are exclusively electron donating or accepting, we predicted C12DPP-π-BT may uniquely serve as either an electron donor or an acceptor when paired with PCBM or P3HT forming junctions with large built-in potentials. We confirmed the ambipolar nature of C12DPP-π-BT in space charge limited current measurements and in C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT bulk heterojunction solar cells, achieving power conversion efficiencies of 1.67% and 0.84%, respectively, under illumination of AM 1.5G (100 mW/cm²). Adding diiodooctane to C12DPP-π-BT:PCBM improved donor-acceptor inter-mixing and film uniformity, as seen in spatially resolved maps of solar cell short circuit currents and through electron microscopy, and therefore enhanced charge separation and overall device efficiency. Using higher molecular weight polymer C12DPP-π-BT in both C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT devices improved charge transport and hence the performance of the solar cells. In addition, we compared the structural and electronic properties of C12DPP-π-BT:PCBM and C12DPP-π-BT:P3HT blends, representing the materials classes of polymer:fullerene and polymer:polymer blends. In C12DPP-π-BT:PCBM blends, higher short circuit currents were obtained, consistent with faster charge transfer and balanced electron and hole transport, but lower open circuit voltages may be reduced by trap-assisted recombination and interfacial recombination losses. In contrast, C12DPP-π-BT:P3HT blends exhibit higher open circuit voltage, but short circuit currents were limited by charge transfer between the polymers. In conclusion, C12DPP-π-BT is a promising material with intrinsic ambipolar characteristics for organic photovoltaics and may operate as either a donor or acceptor in the design of bulk heterojunction solar cells.

Semiconductor nanocrystals continue to receive attention as promising materials in organic-inorganic solar photovoltaics. But a common problem in the fabrication of devices incorporating nanocrystaline materials is that the ligands commonly used to stabilize the nanocrystals typically give rise to large spacings between nanocrystal-and-polymer and nanocrystal-and-nanocrystal and therefore limit charge transfer and transport. We have recently shown that the organic surfactants capping a variety of semiconducting and metallic nanoparticles can be nearly quantitatively removed by treating colloidal nanocrsytal solutions with ammonium thiocyanate, allowing close interparticle spacing and good electrical conductivity between nanocrystals. Furthermore the addition of the thiocyanate to the nanocrystal surface is achieved under mild processing conditions, with environmentally benign starting materials, at room temperature, and is thus compatible with a wide range of device fabrication modalities. We have applied this new ligand chemistry to the fabrication of organic-inorganic solar cells. We have fabricated organic-inorganic solar cells from thiocyanate exchanged CdSe nanocrystals and the polymer P3HT. To date we have fabricated bilayer solar cells in both normal (direct) and inverted solar cells geometries. CdSe nanocrystals capped with thiocyanate are disperable in polar solvents whereas P3HT is deposited from chlorinated solventes, readily permitting deposition of one layer on top of the other. We show the device efficiency increases with increasing nanocrystal size as the increases in short circuit current are more significant than the decreases in open circuit voltage with increases nanocrystal size. To date, power conversion efficiencies in excess of 1% have been achieved in bilayer solar cells, although we continue to optimize the deposition conditions to improve cell performance. This approach is readily expandable to a wide-range of organic and inorganic building blocks. For example, we have similarly demonstrated organic-inorganic solar cells using a solution-processable precursor to pentacene in combination with the thiocyanate capped CdSe nanocrystals. 

The grant has supported the graduate studies of student Wenting Li and during the lifetime of the grant, the work of postdoctoral fellow Taegweon Lee, who is now employed by Samsung Corporation. The grant has also fostered collaborative research in understanding and developing materials, devices, and measurements of solar photovoltaics in the Kagan group that is impacting a cohort of undergraduate, masters, and graduate students as well as other postdoctoral fellows.