Rational Design of High Performance Conjugated Polymers for Organic Solar Cells

Morphology-driven high-performance polymeric photodetector

The influence of polymer/fullerene morphology on photodetector performance is reported. Various morphologies of spin-coated films are generated by different blending ratio. Morphological study combined with measurement of charge carrier mobility reveals that blend films with an excess content of crystalline fullerene have a phase-separated morphology, resulting in enhanced charge carrier mobility. Under this phase separated morphology, photovoltaic performance is enhanced because of the generation of percolating pathways for charge carriers. Interestingly, however, a homogeneous morphology is found to be more beneficial for photodetector application than this phase separated morphology. An optimized device displayed 3 dB bandwidth up to 1 kHz and detectivities up to D = 1.1 × 10(10) cm Hz(1/2) W(-1). These results emphasize the importance of developing an independent strategy for designing high performance photodetectors separately from solar cell devices both in terms of materials and device geometry. Possible relations between morphology and various figures of merit of photodetectors are discussed.

Small molecules based on benzo[1,2-b:4,5-b']dithiophene unit for high-performance solution-processed organic solar cells

Small molecules, namely, DCAO(3)TBDT and DR(3)TBDT, with 2-ethylhexoxy substituted BDT as the central building block and octyl cyanoacetate and 3-ethylrhodanine as different terminal units with the same linkage of dioctyltertthiophene, have been designed and synthesized. The photovoltaic properties of these two molecules as donors and fullerene derivatives as the acceptors in bulk heterojunction solar cells are studied. Among them, DR(3)TBDT shows excellent photovoltaic performance, and power conversion efficiency as high as 7.38% (certified 7.10%) under AM 1.5G irradiation (100 mW cm(-2)) has been achieved using the simple solution spin-coating fabrication process, which is the highest efficiency reported to date for any small-molecule-based solar cells. The results demonstrate that structure fine turning could cause significant performance difference and with that the performance of solution-processed small-molecule solar cells can indeed be comparable with or even surpass their polymer counterparts.

Precision Synthesis of n-Type π-Conjugated Polymers in Catalyst-Transfer Condensation Polymerization

Recent developments in catalyst-transfer condensation polymerization, which proceeds in a chain-growth polymerization manner, have made it possible to synthesize well-defined π-conjugated polymers with controlled molecular weight and low polydispersity, as well as block copolymers and gradient copolymers. However, catalyst-transfer condensation polymerization has been limited to the polymerization of donor monomers (such as thiophene) for the synthesis of p-type π-conjugated polymers. Here, we highlight several recent advances in catalyst-transfer condensation polymerization leading to n-type π-conjugated polymers. The Kumada-Tamao coupling polymerization of Grignard pyridine monomers yields well-defined poly(pyridine-3,5-diyl) and poly(pyridine-2,5-diyl) with a broad molecular weight distribution. Monomers consisting of strong acceptor and weak donor moieties also undergo catalyst-transfer polymerization; well-defined poly(fluorene benzothiaziazole) was obtained by Suzuki-Miyaura coupling polymerization and poly(bithiophene naphthalene diimide) was obtained by an unusual Ni-catalyzed coupling polymerization of an anion radical generated from a bromothiophene naphathalene diimide bromothiophene monomer and activated zinc.

Donor-Acceptor-Type Low Bandgap Polymer Carrying Phenylazomethine Moiety as a Metal-Collecting Pendant Unit: Open-Circuit Voltage Modulation of Solution-Processed Organic Photovoltaic Devices Induced by Metal Complexation

A novel low-bandgap conjugated polymer with the phenylazomethine moiety as a pendant metal-collecting unit (PImCDTBT) was synthesized via Suzuki cross-coupling copolymerization. Its basic physicochemical properties were revealed, and the optical bandgap and highest occupied molecular orbital (HOMO) energy levels were estimated to be 1.73 eV and -5.36 eV, respectively. PImCDTBT successfully assembles metal ions on its phenylazomethine site, as evidenced by a spectral change similar to that of its monomeric model compounds. In a bulk heterojunction photovoltaic device, the open-circuit voltage was clearly enhanced from 0.46 to 0.52 V by complexation of PImCDTBT, with only 1 wt % of SnCl₂. This was due to a change in either the electronic state of the polymer or its environment by complexing with the cationic Sn²⁺ ion.

Synthesis of Poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b']dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT) in a Direct Arylation Scheme

Poly(4,4-dialkyl-cyclopenta[2,1-b:3,4-b']dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT), a potentially interesting low bandgap donor copolymer for bulk heterojunction-type organic solar cells with a power conversion efficiency >5.5%, can be now synthesized in a direct arylation scheme starting from 4,4-dialkyl-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT) and 4,7-dibromo-2,1,3-benzothiadiazole (4,7-dibromo-BT) as monomers. The direct arylation procedure leads to PCPDTBT with an M_n of up to 40 000 and circumvents the use of costly diboronic acid/ester or distannyl monomers.

Precise Synthesis of Amphiphilic Multiblock Copolymers by Combination of Acyclic Diene Metathesis (ADMET) Polymerization with Atom Transfer Radical Polymerization (ATRP) and Click Chemistry

Various block (graft) copolymers have been prepared by combination of acyclic diene metathesis (ADMET) polymerization of 9,9-dialkyl-2,7-divinyl-fluorene with Cu-catalyzed atom transfer radical polymerization (ATRP) of styrene using macroinitiators prepared by introduction of initiating functionalities into poly(9,9-dialkylfluorene-2,7-vinylene)s (PFVs) chain ends: a precise synthesis of the amphiphilic ABCBA-type block copolymers has also been attained by subsequent combination with click reaction after modification of the chain end with NaN₃.