Pabitra Shakya Tuladhar

Thieno[3,2-b]thiophene-Diketopyrrolopyrrole-Containing Polymers for High-Performance Organic Field-Effect Transistors and Organic Photovoltaic Devices

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm(2) V(-1) s(-1), which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC(71)BM gave a power conversion efficiency of 5.4%.

Synthesis of novel thieno[3,2-b]thienobis(silolothiophene) based low bandgap polymers for organic photovoltaics

Thieno[3,2-b]thienobis(silolothiophene), a new electron rich hexacyclic monomer has been synthesized and incorporated into three novel donor-acceptor low-bandgap polymers. By carefully choosing the acceptor co-monomer, the energy levels of the polymers could be modulated and high power conversion efficiencies of 5.52% were reached in OPV devices.

Polaron pair mediated triplet generation in polymer/fullerene blends

Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.

Optimisation of diketopyrrolopyrrole:fullerene solar cell performance through control of polymer molecular weight and thermal annealing

Poly-thieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T) is a promising low bandgap donor polymer for organic solar cells. In this study we employ two different approaches to improve the device efficiency via optimisation of the morphology of the active layer: tuning of the molecular weight of the polymer and thermal annealing. In the former case, a higher molecular weight was found to yield a more intermixed morphology, resulting in enhanced exciton dissociation and charge separation, as confirmed by atomic force microscopy, and photoluminescence and transient absorption spectroscopies. In the later case, thermal annealing prior to metal electrode deposition increased the photon conversion efficiency to as high as 6.6%, with this enhanced efficiency being maintained even with prolonged annealing (240 hours at 80 °C). This enhancement in performance with thermal annealing was correlated with increased polymer crystallinity.

A comparison of charge separation dynamics in organic blend films employing fullerene and perylene diimide electron acceptors

We report a comparison of charge carrier dynamics and device performance for low band gap polymer PBDTTT-CT in blends with the fullerene acceptor PC71BM and a PDI derivative with similar electron affinities. Charge separation and recombination dynamics are found to be remarkably similar for these two acceptors, with both blends exhibiting efficient, ultrafast charge separation (time constants of 1.6 and 1.4 ps, respectively). The lower device performance for the PDI acceptor (1.75% compared to 3.5% for the equivalent PC71BM device) is shown to result from slower charge transport, increasing nongeminate recombination losses during charge collection.

Diselenogermole as a novel donor monomer for low band gap polymers

We report the synthesis of a new diseleno[3,2-b:2′,3′-d]germole (DSG) monomer containing branched 2-ethylhexyl sidechains on the bridging germanium group. The utility of this new electron rich monomer is explored by co-polymerisation of a distannylated DSG monomer with N-octylthienopyrrolodione to afford a soluble low band gap polymer. The DSG containing polymer shows a broader and more red-shifted absorption spectrum compared its thiophene analogue. Polymer solar cells are fabricated from blends with PC71BM exhibiting initial power conversion efficiencies of 5.2% in inverted bulk heterojunction solar cells.

The effect of thiadiazole out-backbone displacement in indacenodithiophene semiconductor polymers

We describe the synthesis and characterisation of two new polymers consisting of an electron-rich backbone containing indacenodithiophene (IDT) and dithiophene (DT) with the electron-poor units benzothiadiazole (BT) and benzopyrazolothiadiazole (BPT) fused on top of DT. The effect of this substitution has been studied and discussed by optical, electrochemical and computational means. Despite having very similar molecular distribution as well as thermal and electrochemical properties, the addition of the stronger electron-withdrawing BPT unit leads to a substantial change on the absorption properties by promoting the intramolecular charge transfer (ICT) band alongside the π–π*. Furthermore, we also report organic field effect transistor and solar cells device results, giving hole mobilities of 0.07 cm2 V−1 s−1 with low threshold voltage (<10 V) and power conversion efficiencies of up to 2.2%.

Cyano substituted benzotriazole based polymers for use in organic solar cells

A new synthetic route to the electron accepting di-cyano substituted benzo[d][1,2,3]triazole (BTz) monomer 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-dicarbonitrile (dTdCNBTz) is reported. The cyano substituents can be easily introduced to the BTz unit in one step via the nucleophilic aromatic substitution of the fluorine substituents of the fluorinated precursor 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-difluoro (dTdFBTz). Co-polymers were prepared with distannylated benzo[1,2-b:4,5-b′]dithiophene (BDT) monomers containing either 2-ethylhexylthienyl (T-EH) side chains or 2-butyloctylthienyl (T-BO) side chains via Stille coupling to yield the novel medium band gap polymers P1 and P2 respectively. Whilst the organic photovoltaic (OPV) performance of P1 was limited by a lack of solubility, the improved solubility of P2 resulted in promising device efficiencies of up to 6.9% in blends with PC61BM, with high open circuit voltages of 0.95 V.

Electronic structure tuning of new fused thieno[3,2-b]thieno bisthiophene based polymers via alkyl chain and Group IV heteroatom modulation

Herein we present an extension of our work on indacenodithiophenes (IDT) by replacing the central benzene ring with a thieno(3,2-b) thiophene unit. This newly developed thieno[3,2-b]thieno bisthiophene (4T) donor moiety was synthesized from commercially available reagents and incorporated into a series of donor-acceptor polymers. We will discuss the pronounced donating character of 4T compared to IDT and the choice of bridging atom in those new polymers with an emphasis on field effect transistor and photovoltaic device performance.