The lack of suitable acceptor (n-type) polymers has limited the photocurrent and efficiency of polymer/polymer bulk heterojunction (BHJ) solar cells. Here, we report an evaluation of three naphthalene diimide (NDI) copolymers as electron acceptors in BHJ solar cells which finds that all-polymer solar cells based on an NDI-selenophene copolymer (PNDIS-HD) acceptor and a thiazolothiazole copolymer (PSEHTT) donor exhibit a record 3.3% power conversion efficiency. The observed short circuit current density of 7.78 mA/cm(2) and external quantum efficiency of 47% are also the best such photovoltaic parameters seen in all-polymer solar cells so far. This efficiency is comparable to the performance of similarly evaluated [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM)/PSEHTT devices. The lamellar crystalline morphology of PNDIS-HD, leading to balanced electron and hole transport in the polymer/polymer blend solar cells accounts for its good photovoltaic properties.

New electron-acceptor materials are long sought to overcome the small photovoltage, high-cost, poor photochemical stability, and other limitations of fullerene-based organic photovoltaics. However, all known nonfullerene acceptors have so far shown inferior photovoltaic properties compared to fullerene benchmark [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM), and there are as yet no established design principles for realizing improved materials. Herein we report a design strategy that has produced a novel multichromophoric, large size, nonplanar three-dimensional (3D) organic molecule, DBFI-T, whose π-conjugated framework occupies space comparable to an aggregate of 9 [C60]-fullerene molecules. Comparative studies of DBFI-T with its planar monomeric analogue (BFI-P2) and PC60BM in bulk heterojunction (BHJ) solar cells, by using a common thiazolothiazole-dithienosilole copolymer donor (PSEHTT), showed that DBFI-T has superior charge photogeneration and photovoltaic properties; PSEHTT:DBFI-T solar cells combined a high short-circuit current (10.14 mA/cm(2)) with a high open-circuit voltage (0.86 V) to give a power conversion efficiency of 5.0%. The external quantum efficiency spectrum of PSEHTT:DBFI-T devices had peaks of 60-65% in the 380-620 nm range, demonstrating that both hole transfer from photoexcited DBFI-T to PSEHTT and electron transfer from photoexcited PSEHTT to DBFI-T contribute substantially to charge photogeneration. The superior charge photogeneration and electron-accepting properties of DBFI-T were further confirmed by independent Xenon-flash time-resolved microwave conductivity measurements, which correctly predict the relative magnitudes of the conversion efficiencies of the BHJ solar cells: PSEHTT:DBFI-T > PSEHTT:PC60BM > PSEHTT:BFI-P2. The results demonstrate that the large size, multichromophoric, nonplanar 3D molecular design is a promising approach to more efficient organic photovoltaic materials.

A new solution processable n-type polymer semiconductor is synthesized and characterized for use as an electron acceptor material in all-polymer bulk heterojunction solar cells. The new crystalline copolymer, poly(naphthalene diimide-alt-biselenophene) (PNDIBS), has a high field-effect electron mobility (0.07 cm2/(V s)) and broad visible-near-infrared absorption band with an optical band gap of 1.4 eV. All-polymer bulk heterojunction solar cells comprised of PNDIBS acceptor and poly(3-hexylthiophene) donor have a photovoltaic power conversion efficiency of 0.9%. The external quantum efficiency spectrum of the all-polymer solar cells shows that about 19% of the photocurrent comes from the near-infrared (700–900 nm) light harvesting by the new n-type polymer semiconductor.

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BiFeO 3 (BFO), BiFe1−xTixO3, and BiFe0.9Ti0.05Co0.05O3 thin films were deposited on Pt/TiO2/SiO2/Si substrates by chemical solution deposition. BFO film has distorted rhombohedral R3c structure and in BiFe1−xTixO3 (104)/(110) reflections broadened suggesting limited grain growth with Ti substitution. The surface roughness (rms) decreased in the case of Ti substituted BFO. Up to 5% Ti in the lattice reduces the leakage current substantially. For BiFe1−xTixO3, the leakage current qualitatively followed the same trend and the behavior resembles to space charge limited current conduction. The magnetic properties were completely lost by Ti substitution and slightly recovered upon cosubstitution with magnetically active Co. The disappearance of ferromagnetic hysteresis of BFO with Ti substitution and its reappearance with Co is suggestive of the origin of magnetic properties consequential from the BFO lattice itself and hence support it as an intrinsic property of BFO. Capacitance-voltage characteristics of BFO, BiFe0.95Ti0.05O3, and BiFe0.9Ti0.05Co0.05O3 showed butterfly loop indicating ferroelectric property at room temperature as well as at low temperature. However, saturated polarization-voltage hysteresis was not observed in all cases and BiFe0.9Ti0.05Co0.05O3 films showed very poor ferroelectricity compared to BFO and BiFe0.95Ti0.05O3.