Guoqiang Ren

Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics

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.

Polymer Nanowire/Fullerene Bulk Heterojunction Solar Cells: How Nanostructure Determines Photovoltaic Properties

We report studies of bulk heterojunction solar cells composed of self-assembled poly(3-butylthiophene) nanowires (P3BT-nw) as the donor component with a fullerene acceptor. We show that the nanostructure of these devices is the single most important variable determining their performance, and we use a combination of solvent and thermal annealing to control it. A combination of conductive and photoconductive atomic force microscopy provides direct connections between local nanostructure and overall device performance. Films with a dense random web of nanowires cause the fullerene to aggregate in the interstices, giving a quasi-ordered interpenetrating heterojunction with high short-circuit current density (10.58 mA/cm(2)), but relatively low open circuit voltage (520 mV). Films with a low density of nanowires result in a random bulk heterojunction composed of small crystalline PCBM and P3BT phases. Fewer nanowires result in higher open circuit voltage (650 mV) but lower current density (6.02 mA/cm(2)). An average power conversion efficiency of 3.35% was achieved in a structure which balances these factors, with intermediate nanowire density. The best photovoltaic performance would be realized in a material structure which maintains the interpenetrating network of nanowires and fullerene phases (high current density), but avoids the device bridging we observe, and the recombination and shunt losses associated with it (high open-circuit voltage).

Solar Cells Based on Block Copolymer Semiconductor Nanowires: Effects of Nanowire Aspect Ratio

The solution-phase self-assembly of nanowires (NWs) from diblock copolymer semiconductors, poly(3-butylthiophene)-block-poly(3-octylthiophene), of different block compositions gave crystalline NWs of similar width (13-16 nm) but a tunable average aspect ratio (length/width) of 50-260. The power conversion efficiency of bulk heterojunction solar cells comprising the diblock copolythiophene NWs and PC(71)BM was found to increase with increasing aspect ratio, reaching 3.4% at the highest average aspect ratio of 260. The space charge limited current mobility of holes in neat films of the copolymer NWs and in copolymer NWs/PC(71)BM films (∼1.0 × 10(-4) cm(2)/(V s)) was invariant with aspect ratio, reflecting the parallel orientation of the NWs to the substrate. The enhancement of photovoltaic efficiency with increasing aspect ratio of NWs was explained in terms of increased exciton and charge photogeneration and collection in the bulk heterojunction solar cells.

High-Mobility n-Type Conjugated Polymers Based on Electron-Deficient Tetraazabenzodifluoranthene Diimide for Organic Electronics

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly(tetraazabenzodifluoranthene diimide)s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm(2)/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107.

Efficient solar cells based on a new phthalimide-based donor–acceptor copolymer semiconductor: morphology, charge-transport, and photovoltaic properties

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

Nanowires of oligothiophene-functionalized naphthalene diimides: self assembly, morphology, and all-nanowire bulk heterojunction solar cells

We report the solution-phase self-assembly of nanowires of a series of six oligothiophene-functionalized naphthalene diimides (NDI-nTH and NDI-nT) and their use as electron acceptors in all-nanowire bulk heterojunction organic solar cells. The dimensions and detailed morphology of the nanowires varied greatly with molecular structure of the NDI molecules and the solution concentration and solvent composition. Nanowires of NDI-3TH have typical dimensions of 80–250 nm in width, 2–10 μm in length, and 14.2–15.3 nm in thickness. Significant red-shift and enhancement of optical absorption in the low energy region (>600 nm) were observed in the nanowire suspension compared with solution and thin film, suggesting an increased supramolecular order in the nanowires. All-nanowire bulk heterojunction solar cells fabricated using NDI-3TH nanowires as electron acceptor and poly(3-hexylthiophene) nanowires as donor had a power conversion efficiency of 1.15%. External quantum efficiency of the photovoltaic cells showed that the n-type organic semiconductor nanowires contributed significantly to light harvesting. Our results represent the first demonstration of all-nanowire organic solar cells and show that non-fullerene n-type organic semiconductor nanowires can be effectively incorporated into bulk heterojunction solar cells.