Darren C. Watters

Carbazole and thienyl benzo[1,2,5]thiadiazole based polymers with improved open circuit voltages and processability for application in solar cells

The preparation of low energy gap carbazole based main-chain polymers having improved open circuit voltages (Voc) and solubility is described. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P1 and poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(2,2′-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P2 were prepared in good yields using Suzuki coupling methodologies. The polymers were characterized by NMR spectroscopy; UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Introduction of octyloxy substituents on the benzothiadiazole acceptor repeat units on polymers P1 and P2 enhances their solubility compared to the polymer PCDTBT and alters their electronic and photophysical properties. Investigation of the photovoltaic properties of the two new polymers side by side with those of PCDTBT in bulk heterojunction cells using PC70BM as a molecular acceptor indicated that both polymers provide higher Voc values than that of PCDTBT (0.96 and 0.90 V for P1 and P2 respectively vs. 0.82 V for PCDTBT) and good power conversion efficiencies with PCE values of 4.22 and 4.12% respectively for P1 and P2 and a PCE of 4.30% for PCDTBT.

Selenophene vs. thiophene in benzothiadiazole-based low energy gap donor–acceptor polymers for photovoltaic applications

A series of low energy gap polymers comprising 2,7-linked carbazole or fluorene units flanked by thiophene or selenophene repeat units as alternating donor units and benzothiadiazole with or without alkoxy substituents as alternating acceptor repeat units is reported. The effects of replacing thiophene with selenophene in this series of polymers on their optical, electrochemical and photovoltaic device performance when fabricated into bulk heterojunction solar cells using PC70BM as an acceptor are investigated. Power conversion efficiencies (PCEs) ranging from 3.34 to 5.41% are obtained with these systems. Thiophene-based polymers are found to have higher efficiency compared to comparable selenophene-based polymers. We tentatively explain such differences on the basis of reduced molar absorbance and reduced charge-carrier mobility in the selenophene-based polymers.

Fluorene-based co-polymer with high hole mobility and device performance in bulk heterojunction organic solar cells.

A new donor-acceptor polymer based on 9,9-dioctylfluorene is synthesized and tested in organic photovoltaic devices. Results show that the polymer exhibits good solubility in a range of organic solvents and has a high hole mobility. When blended with a PC70 BM acceptor and fabricated into a bulk heterojunction, photovoltaic devices having a maximum power conversion efficiency (PCE) of 6.2% and a peak external quantum efficiency of 74% are created. Such efficiencies are realized without any necessity for solvent additives or thermal annealing protocols.

Competition between substrate-mediated π-π stacking and surface-mediated T(g) depression in ultrathin conjugated polymer films.

We report surface and interface effects in dynamics and chain conformation in the thin film of conjugated polymer PCDTBT. To probe dynamic anomalies, we measure the glass transition temperature (Tg) of PCDTBT films as a function of thickness, and find that there is a significant depression in Tg for films less than 100nm thick; a result qualitatively similar to that observed in many other polymer film systems. However, for films less than 40nm, the Tg converges to a constant value of 20K below its bulk value. Grazing incidence X-ray diffraction shows depth-dependent molecular organization that is associated with the unusual thickness-dependent dynamics.Graphical abstract

Impact of dithienyl or thienothiophene units on the optoelectronic and photovoltaic properties of benzo[1,2,5]thiadiazole based donor–acceptor copolymers for organic solar cell devices

We report a comparative study on four donor–acceptor benzothiadiazole-based copolymers containing dithienyl or thienothiophene moieties for application in organic photovoltaic (OPV) devices. Bulk-heterojunction OPV devices are fabricated having power conversion efficiencies ranging between 4 and 6%. Morphological, spectroscopic and charge-transport measurements are used to investigate the influence of either the dithienyl or thienothiophene moieties on the structure and photophysical properties of the copolymer and copolymer:PC71BM blend films and rationalise the solar cell characteristics. Although all copolymer:PC71BM blends exhibit comparable hole polaron yields, solar cell devices with the highest power conversion efficiencies are correlated with increased charge-carrier mobility of the copolymer and enhanced aggregation of PC71BM in the blend.