Ahmed Iraqi

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.

Synthesis and characterisation of telechelic regioregular head-to-tail poly(3-alkylthiophenes)

An investigation into the Stille cross-coupling reaction as an alternative synthetic route to regioregular head-to-tail poly(3-alkylthiophenes) is presented. 2-Iodo-3-alkyl-5-tri-n-butylstannylthiophene derivatives were made, isolated and characterised and subsequently used in Pd0 catalysed cross-coupling reactions in different solvents. Short and long chain regioregular poly(3-hexylthiophenes) with over 96% head-to-tail coupling ratios between adjacent thiophene rings were obtained. The materials were analysed by gel permeation chromatography and NMR spectroscopy. Polymers with varying degrees of polymerisation were obtained depending on the conditions used. End group analysis has shown that polymers, functionalised with both iodo and tri-n-butyltin end groups can be obtained under certain reaction conditions. Preliminary homocoupling reactions were also conducted.

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.

Molecular weight dependent vertical composition profiles of PCDTBT:PC71BM blends for organic photovoltaics

We have used Soxhlet solvent purification to fractionate a broad molecular weight distribution of the polycarbazole polymer PCDTBT into three lower polydispersity molecular weight fractions. Organic photovoltaic devices were made using a blend of the fullerene acceptor PC71BM with the molecular weight fractions. An average power conversion efficiency of 5.89% (peak efficiency of 6.15%) was measured for PCDTBT blend devices with a number average molecular weight of Mn = 25.5 kDa. There was significant variation between the molecular weight fractions with low (Mn = 15.0 kDa) and high (Mn = 34.9 kDa) fractions producing devices with average efficiencies of 5.02% and 3.70% respectively. Neutron reflectivity measurements on these polymer:PC71BM blend layers showed that larger molecular weights leads to an increase in the polymer enrichment layer thickness at the anode interface, this improves efficiency up to a limiting point where the polymer solubility causes a reduction of the PCDTBT concentration in the active layer.

Covalent binding of redox active centres to preformed regioregular polythiophenes

The synthesis of regioregular head-to-tail poly[3-(6-bromohexyl)thiophene] is reported, together with its reaction with 2-carboxyanthraquinone (Anth) to give an example of a regioregular polythiophene containing pendant functional groups (87% loading). NMR data on the two soluble polymers are reported together with preliminary studies of some of their physical properties. Cyclic voltammetric studies of anthraquinone polymer coated electrodes show that the observed response is coverage dependent: thin films display four redox couples due to the Anth0/–/2– processes and the p- and n-doping of the conjugated thiophene backbone. Thick films are rectifying in the sense that reduction of the Anth groups is inhibited on the negative sweep. Spectroelectrochemical studies confirm the nature of the anodic p-doping process (the film turns red to nearly colourless) and show characteristic changes on reduction (red to black).

Anthracene-based donor–acceptor low band gap polymers for application in solar cells

The preparation of anthracene-based low band gap conjugated polymers comprising 2,6-linked anthracene and dithienyl-benzo[c][1,2,5]thiadiazole or dibithiophenyl-benzo[c][1,2,5]thiadiazole alternate repeat units is presented. The photophysical, electrochemical and photovoltaic properties of the polymers in bulk heterojunction solar cells using PC(71)BM as an acceptor are discussed.

PCDTBT based solar cells: one year of operation under real-world conditions.

We present measurements of the outdoor stability of PCDTBT:PC71BM based bulk heterojunction organic solar cells for over the course of a year. We find that the devices undergo a burn-in process lasting 450 hours followed by a TS80 lifetime of up to 6200 hours. We conclude that in the most stable devices, the observed TS80 lifetime is limited by thermally-induced stress between the device layers, as well as materials degradation as a result of edge-ingress of water or moisture through the encapsulation.

Air processed organic photovoltaic devices incorporating a MoOx anode buffer layer

Molybdenum oxide (MoOx) has been shown to act as an efficient hole extraction layer in organic photovoltaic (OPV) devices. However, exposing MoOx films to air is problematic as it is hygroscopic; the uptake of moisture having a negative impact on its electronic properties. Here, we use spectroscopic ellipsometry to characterise the uptake of water, and fabricate PCDTBT:PC70BM based OPVs to determine its effects on device performance. We then show that thermally annealing MoOx reduces its hygroscopicity, permitting it to be processed in air. Using this process, we create air-processsed OPVs having PCEs (power conversion efficiencies) of up to 5.36%.

Polyketones via oxidation of polybutadienes catalysed by complexes of platinum(II)

Abstract Polybutadienes containing different ratios of 1,4- and 1,2-polymerized units are converted to polyketones by oxidation with tBuOOH in the presence of catalytic quantities of [Pt(diphoe)CF3(CH2Cl2)]+ (diphoe = Ph2PCHCHPPh2); reactivity ratios for the different double bonds are determined as pendant/trans/cis, 6: 5: 3.

The role of the hole-extraction layer in determining the operational stability of a polycarbazole:fullerene bulk-heterojunction photovoltaic device

We have made a comparative study of the relative operational stability of bulk-heterojunction organic photovoltaic (OPV) devices utilising different hole transport layers (HTLs). OPV devices were fabricated based on a blend of the polymer PCDTBT with the fullerene PC70BM, and incorporated the different HTL materials PEDOT:PSS, MoOx and V2O5. Following 620 h of irradiation by light from a solar simulator, we find that devices using the PEDOT:PSS HTL retained the highest efficiency, having a projected T80 lifetime of 14 500 h.