Thiophene/Phenylene/Thiophene-Based Low-Bandgap Conjugated Polymers for Efficient Near-Infrared Photovoltaic Applications

We have prepared thiophene/phenylene/thiophene (TPT)-based low bandgap conjugated polymers exhibiting tunable energy levels and investigated their application in solar cells. By incorporating various electron-withdrawing comonomers through Stille coupling reactions, we obtained TPT-based donor/acceptor copolymers having bandgaps ranging from 1.0 to 1.8 eV. We compared the absorption spectra, electrochemistry, field effect hole mobility, and photovoltaic properties of these low bandgap TPT derivatives with those of poly(3-hexylthiophene) (P3HT). The absorption coefficients of the thin films fell in the range from 0.77 × 105 to 1.4 × 105 cm−1. These materials displayed sufficiently high hole mobilities (>10−3 cm2 V−1 s−1) for efficient charge extraction and good fill-factors for organic photovoltaic applications. Electrochemical studies indicated desirable HOMO/LUMO levels, with a good correlation between the HOMO energy levels and the open circuit voltage (Voc) when the polymers were blended with fullerene derivative as an electron acceptor. Power conversion efficiencies of up to 4.3% were achieved under AM 1.5G simulated solar light (100 mW cm−2). Our findings suggest that TPT derivatives presenting suitable electron-withdrawing groups are promising photovoltaic materials

New Two-Dimensional Thiophene−Acceptor Conjugated Copolymers for Field Effect Transistor and Photovoltaic Cell Applications

We report the synthesis, properties, and optoelectronic device applications of two-dimensional (2D) like conjugated copolymers, P4TBT, P4TDTBT, P4TDTQ, and P4TDPP, consisting of 2′,5′′-bis(trimethystannyl)-5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′;4′′,2′′′]quarterthiophene (4T) with the following four acceptors of 4,7-dibromo-2,1,3-benzothiodiazole (BT), 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT), 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5′-bromodithien-2-ylquinoxalines] (DTQ), and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP). The optical band gaps (eV) of the studied conjugated copolymers are in the order of P4TDPP (1.29) P4TDTBT (1.60) P4TDTQ (1.83) (1.88). The 2D-like conjugated copolymers exhibited high field effect (FET) hole mobilities in the range of 10−1−10−4 cm2 V−1 s−1. On the other hand, the FET electron mobilities were observed for P4TDTBT and P4TDPP because of their relatively low-lying LUMO level suitable for electron injection. In particular, P4TDPP showed the ambipolar characteristics with the hole and electronic mobilities of 0.115 cm2 V−1 s−1 (on/off ratio: 2.49 × 104) and 3.08 × 10−3 cm2 V−1 s−1 (on/off ratio: 7.34 × 102), respectively, which was strongly related to its ordered intermolecular chain packing based on the DSC and XRD results. The power conversion efficiencies (PCE) of the prepared polymer/PC71BM (1:3) based photovoltaic cells were in the range 1.28−1.67% under the illumination of AM 1.5G (100 mW/cm2). The PCE could be enhanced up to 2.43% of the P4TDPP/PC71BM (1:2) based device because of the balanced hole/electron mobility. The above results indicate that these two-dimensional 4T−acceptor conjugated copolymers could enhance the charge-transport characteristics and are promising materials for organic optoelectronic devices

Morphology Evolution of Spin-Coated Films of Poly(thiophene−phenylene−thiophene) and [6,6]-Phenyl-C₇₁-butyric Acid Methyl Ester by Solvent Effect

This paper describes the influence of the solvent on the morphological evolution and performance of polymer solar cells (PSCs) based on blended films of poly(thiophene−phenylene−thiophene) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The blends are spin-coated with solvents exhibiting various evaporation rates, including o-dichlorobenzene (DCB), chlorobenzene (CB), chloroform (CF), and tetralin. The changing morphologies of these blended films are monitored using atomic force microscopy (AFM) and transmission electron microscopy (TEM). A solvent having a higher boiling point [1,8-octanedithiol (OT)] is also introduced as an additive to further fine-tune the morphology of the bulk heterojunction (BHJ). We demonstrate herein that the morphology of a blendand, hence, the performance of a BHJ devicecan be manipulated by controlling the rate of solvent evaporation during film formation. The resulted fine-scale phase separation leads to enhanced performance of such organic photovoltaic devices. The highest power efficiency for our PSCs (5.8%, AM 1.5G irradiation (100 mW/cm2)) resulted when we use DCB as the solvent with OT as a processing additive