Two donor–acceptor polymers (P1 and P2) based on dithienobenzoquinoxaline (M1) and dithienobenzopyridopyrazine (M2) as acceptor and indacenodithiophene as donor were synthesized via Stille polycondensation. The fused dithienobenzene unit in M1 and M2 units can improve the intermolecular stacking of polymer and also decrease the steric hindrance. P1, with dithienobenzoquinoxaline acceptor, shows a band gap of 1.61 eV. The band gap of P2 was reduced to 1.48 eV after changing to dithienobenzopyridopyrazine as the acceptor unit. The mobilities of P1 and P2 reach 5.6 × 10–2 and 1.5 × 10–2 cm2 V–1 s–1, respectively. The results from photovoltaic measurements showed a very promising PCE of 6.06% for the P1/PC71BM blend system without any thermal or solvent treatments, showing a great offer for the roll-to-roll manufacturing of PSCs.
Two new low-band-gap conjugated polymers based on the polymerization between indacenodithiophene and 2,3-diphenylquinoxaline or phenanthrenequinoxaline were synthesized. Due to the fused phenanthrenequinoxaline unit, the polymer (PIDT-phanQ) possesses better planarity than PIDT-diphQ, resulting in an improved hole mobility in organic field-effect transistors and power conversion efficiency in polymer solar cells.
A series of cyclopentadithiophene-based low band gap conjugated polymers with varied side-chain patterns and F-substituents were synthesized. By replacing the shorter 2-ethylhexyl (EH) side-chain with the longer 3,7-dimethyloctyl (DMO) side-chain, it resulted in significant changes to the optical, electrochemical, and morphological properties of the polymers, as well as the subsequent performance of devices made from these materials. Solar cells fabricated from polymer with 2-ethylhexyl (EH) side-chain and monofluoro substituent exhibits increased open circuit voltage, short circuit current and fill factor, resulting in the highest power conversion efficiency (5.5%) in this series of polymers. This finding provides valuable insight for making more efficient low band gap polymers.
A series of fullerene acceptors have been selected for the systematic study of their electron-transporting properties on a standardized field-effect transistor (FET) platform. It was found that small structural alternations, functional patterns, and number of addends on fullerene derivatives strongly affect their mobilities. The measured charge mobilities correlate well with structural features of these materials and provide useful insights into designing better fullerene-based semiconductors for organic electronics.
Abstract Although high power conversion efficiencies (PCE) have already been demonstrated in conventional structure polymer solar cells (PSCs), the development of high performance inverted structure polymer solar cells is still lagging behind despite their demonstrated superior stability and feasibility for roll‐to‐roll processing. To address this challenge, a detailed study of solution‐processed, inverted‐structure PSCs based on the blends of a low bandgap polymer, poly(indacenodithiophene‐ co ‐phananthrene‐quinoxaline) (PIDT‐PhanQ) and [6,6]‐phenyl‐C 71 ‐butyric acid methyl ester (PC 71 BM) as the bulk heterojunction (BHJ) layer is carried out. Comprehensive characterization and optical modeling of the resulting devices is performed to understand the effect of device geometry on photovoltaic performance. Excellent device performance can be achieved by optimizing the optical field distribution and spatial profiles of excitons generation within the active layer in different device configurations. In the inverted structure, because the peak of the excitons generation is located farther away from the electron‐collecting electrode, a higher blending ratio of fullerene is required to provide higher electron mobility in the BHJ for achieving good device performance.
A series of low band-gap conjugated polymers (PDTC, PDTSi and PDTP) containing electron-rich C-, Si-, and N-bridged bithiophene and electron-deficient thienopyrroledione units were synthesized viaStille coupling polymerization. All these polymers possess a low-lying energy level for the highest occupied molecular orbital (HOMO) (as low as −5.44 eV). As a result, photovoltaic devices derived from these polymers show high open circuit voltage (Voc as high as 0.91 V). These rigid polymers also possess respectable hole mobilities of 1.50 × 10−3, 6.0 × 10−4, and 3.9 × 10−4 cm2 V−1s−1 for PDTC, PDTSi, and PDTP, respectively. The combined high Voc and good hole mobility enable bulk hetero-junction photovoltaic cells to be fabricated with relatively high power conversion efficiency (PCE as high as 3.74% for the PDTC-based device).
A partially fluorinated low bandgap polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b′]dithiophene)-alt-4,7-(5-fluoro-[2,1,3]-benzothiadiazole)] (PCPDTFBT) was synthesized through a microwave-assisted Stille polymerization. It was found that PCPDTFBT has better π–π stacking in solution than its nonfluorinated analogue, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-([2,1,3]-benzothiadiazole)] (PCPDTBT), resulting in 2 times higher hole mobility. Power conversion efficiency (PCE) of the device using PCPDTFBT/PC71BM as active layer (5.51%) is much higher than the device using PCPDTBT/PC71BM (2.75%) that was fabricated under the same condition without using any solvent additive to modify the morphology. The significantly enhanced PCE is the result of improved open circuit voltage and short circuit current coming from the lower lying HOMO energy level and the appropriate morphology of PCPDTFBT. In addition, the device with PCPDTFBT/PC71BM could also be processed from nonchlorinated organic solvents such as o-xylene to obtain high PCE of 5.32% (which is the highest value for PCPDTBT type polymers processed without using chlorinated solvents). Further device optimization by inserting a thin layer of fullerene-containing surfactant between the active layer and Ag cathode resulted in even higher PCE of 5.81%. These encouraging results showed that PCPDTFBT has the potential to be used as a low bandgap polymer to provide complementary absorption in tandem solar cells.
A simple method was developed using metal grid/conducting polymer hybrid transparent electrode to replace indium tin oxide (ITO) for the fabrication of inverted structure polymer solar cells. The performance of the devices could be tuned easily by varying the width and separation of the metal grids. By combining the appropriate metal grid geometry with a thin conductive polymer layer, substrates with comparable transparency and sheet resistance to those of ITO could be achieved. Polymer solar cells fabricated using this hybrid electrode show efficiencies as high as ∼3.2%. This method provides a feasible way for fabricating low-cost, large-area organic solar cells.
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