The UV/vis spectra were measured with a JASCO V-550 UV-Vis spectrophotometer, the emission spectra with a CARY Eclipse fluorescence spectrophotometer. IR spectra were recorded on a JASCO FT/IR-4200 Fourier Transform spectrometer. NMR spectra ( 1 H and 13 C) were measured with a Bruker ARX 400 spectrometer using solutions in CDCl 3 ; J values are given in Hz. Mass spectra were obtained using a Varian MAT 311A instrument with an electro spray ionization source (ESIMS). The microwave-assisted synthesis was carried out in a Discover reaction unit (CEM) using sealed reaction vials. The temperature inside the vial was monitored by an IR sensor; the pressure by a hydraulic system. The polymerizations were performed under temperature control with a maximum microwave power of 300 W. MaterialsAll reactions were carried out under an argon atmosphere using the usual Schlenk techniques.TLC was carried out on dry silica gel plates. For liquid chromatography, silica gel with a pore size 0.06-0.2 nm was used. All solvents were of reagent grade and used as received, unless otherwise specified. 5,8-Dibromo-2,3-dioctylquinoxaline, 1 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-cyclopenta- [2,1-b:3,4-b`]dithiophene 2-4 , 2-(tri-n-butylstannyl)thiophene, 5 and 4,7-dibromobenzo-2,1,3-thiadiazole 6 were prepared according to literature procedures. Synthesis 4,7-Bis(thiophen-2-yl)benzo-2,1,3-thiadiazole 4,7-Dibromobenzo-2,1,3-thiadiazole (17.01 mmol, 5 g), 2-(tri-n-butylstannyl)thiophene (37.4 mmol, 13.96 g), KF (136 mmol, 7.91 g) and PdCl 2 (PPh 3 ) 2 (1.36 mmol, 0.955 g) were placed in a 100 ml-Schlenk tube. After addition of dry THF (100 ml) the reaction mixture was stirred for 48 h at 80 °C. Then, the reaction mixture was poured into chloroform (200 ml). The organic phase was washed with water (2 × 200 ml), dried over anhydrous MgSO 4 and the solvent removed under reduced pressure. Purification by column chromatography (silica gel, toluene/n-hexane 1/4 v/v) gave 2.19 g (7.29 mmol; 42.9 %) of red crystals.

The electrical conductivity of organic semiconductors can be enhanced by orders of magnitude via doping with strong molecular electron acceptors or donors. Ground-state integer charge transfer and charge-transfer complex formation between organic semiconductors and molecular dopants have been suggested as the microscopic mechanisms causing these profound changes in electrical materials properties. Here, we study charge-transfer interactions between the common molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane and a systematic series of thiophene-based copolymers by a combination of spectroscopic techniques and electrical measurements. Subtle variations in chemical structure are seen to significantly impact the nature of the charge-transfer species and the efficiency of the doping process, underlining the need for a more detailed understanding of the microscopic doping mechanism in organic semiconductors to reliably guide targeted chemical design

Polythiophenes are being intensively studied for use in polymer/fullerene bulk-heterojunction photovoltaic (PV) devices because they absorb light throughout the entire visible spectrum and show high hole mobility, which are the two most important parameters for a successful donor in these devices. A large body of anecdotal information exists about what morphological features lead to a high efficiency bulk-heterojunction PV device, but few design parameters for improved polymers exist. We compare two polythiophene isomers, poly(3hexylthiophene) (P3HT) and poly(3,3′′′-didodecylquaterthiophene) (PQT-12) to determine which parameters make these two polymers more and less suitable for use in bulk-heterojunction solar cells. We find that although they have nearly identical absorption spectra and hole mobilities, the PV devices made using P3HT are far superior to those made using PQT-12 because the PQT-12 forms crystalline nanoparticles in solution that do not form electrical connections in the thin films. The use of heat treatment for P3HT/fullerene films improves the layer morphology for PV applications but the same treatment for PQT-12/fullerene layers forces material separation on the micrometer length scale and further degradation of the electrical properties of the device.