Electronic structure, molecular orientation, charge transfer dynamics and solar cells performance in donor/acceptor copolymers and fullerene: Experimental and theoretical approaches (original) (raw)
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2012
Conjugated polymers with donor−acceptor architectures have been successfully applied in bulk heterojunction solar cell devices. Tuning the electron-withdrawing capability in donor−acceptor (D−A) conjugated polymers allows for design of new polymers with enhanced electrical and optical properties. In this paper, a series of D−A copolymers, PBDFDTBT (P1a), PBDTDTBT (P2a), PNDTDTBT (P3a), and PQDTDTBT (P4a), were selected and theoretically investigated using PBE0/6-311G** and TD-PBE0/6-311G**//PBE0/6-311G** methods. The calculated results agree well with the available experimental data of HOMO energy levels and band gaps. We further designed and studied four novel copolymers, P1b, P2b, P3b, and P4b, by substituting the 2,1,3-benzothiadiazole (BT) unit in P1a−P4a with a stronger unit of naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT), respectively. Compared with P1a−P4a, the newly designed polymers of P1b−P4b show better performance with the smaller band gaps and lower HOMO energy levels. The PCEs of ∼5%, ∼7%, ∼7%, and ∼7% for P1b−P4b, predicted by Scharber diagrams, are much higher than those of P1a−P4a when used in combination with PCBM. These results clearly reveal that tuning the electron-withdrawing capability in D−A conjugated polymers is an effective way to improve the electrical and optical properties and the efficiency of the photovoltaic device.
Donor-acceptor semiconducting polymers for organic solar cells
Journal of Polymer Science Part A: Polymer Chemistry, 2013
This review describes the synthesis and photovoltaic performance of donor-acceptor (D-A) semiconducting polymers that have been reported during the last decade. 9,9-Dialkyl-2,7fluorene, 2,7-carbazole, cyclopenta[2,1-b:3,4-b 0 ]dithiophene, dithieno[3,2-b:2 0 ,3 0 -d]silole, dithieno[3,2-b:2 0 ,3 0 -d]pyrrole, benzo [1,2b:4,5-b 0 ]dithiophene, benzo[1,2 b:4,5 b 0 ]difuran building blocks, and their D-A copolymers are described in this review.
Polym. Chem., 2015
Donor-acceptor conjugated polymers with 2-(2-ethylhexyl)-3-hexyl thienyl substituted benzo [1,2-b:4,5b']dithiophene (BDT) as donor building block and 5,6-difluorobenzo[c][1,2,5]thiadiazole as acceptor building block have been synthesized by Stille coupling polymerization. The polymerization conditions were optimized to achieve high molecular weight polymers (number-average molecular weight, M n , up to 139 kg mol −1 ). The molecular weight dependent polymer properties were studied and compared. Photovoltaic applications of the polymers in bulk heterojunction (BHJ) solar cells revealed that the power conversion efficiency increased significantly (from 0.9% to 4.1%) as the M n increased from 10 kg mol −1 to 73 kg mol −1 while further increase of the molecular weight had less influence on the solar cell performance. † Electronic supplementary information (ESI) available. See
Dyes and Pigments, 2018
Structural modification of benzo[c]-1,2,5-thiadiazole (BT) has been proved to be the prominent way to fine-tune the frontier energy levels and the intermolecular and intramolecular interactions in organic conjugated materials. In this study, a new acceptor unit, alkyl benzo[c][1,2,5]thiadiazole-5-carboxylate (BT-Est), was designed and synthesized by drafting mono alkoxy-carboxylate substituent on 5-position of BT core. Its compatibility in the conjugated system was investigated by co-polymerizing BT-Est with well-known benzo[1,2-b:4,5-b']dithiophene monomers containing either 2-(2ethylhexyl)thienyl or 2-((2-ethylhexyl)thio)thienyl side chains to form two new polymers, P1 and P2, respectively. The BT-Est yielded polymers with good solubility, medium bandgap (~1.71 eV), and deep highest occupied molecular orbital energy levels (−5.48 to −5.54 eV). Among the polymers, P1 exhibited broader absorption, compact molecular packing, high charge carrier mobility, and effective exciton dissociation, despite of the torsion angle caused by the free rotation of the carboxylate group in the polymer backbone. Consequently, the best power-conversion efficiency of 6.9%, with a J SC of 14.6 mA cm −2 , V OC of 0.9 V, and FF of 52.5% were obtained for P1-based devices with the well-known non-fullerene acceptor ITIC. We systematically expounded the structureproperty relationship of the BT-Est polymers using diverse characterization methods. Our results demonstrated that the mono carboxylate-substitution on the BT core can be used as the alternate strategy to modulate the optoelectronic properties and control the aggregation in the conjugated polymers. Thus, BT-Est has the potential to produce new donor-acceptor conjugated polymers and small molecules for application in organic electronics.
New Donor-Acceptor polymers with a wide absorption range for photovoltaic applications
Solar Energy, 2020
For conjugated polymers of interest in photovoltaic applications, control of the bandgap as well as the energy levels of the molecules are of great importance to improve the efficiency and performance of the resulting polymer solar cells. A general tactic for adjusting these properties via modification of the conjugated polymer structure is by using different and chosen molecular groups for copolymerization. This communication presents the synthesis of conjugated donor-acceptor type polymers that have the same benzotrithiophene (BTT) donor and different acceptor units, i.e. DPP and fluoro-carbazole substituted thieno[3,4-b]pyrazine (FCTP) denoted as P(BTT-DPP) (P1) and P(BTT-FCTP) (P2), respectively, and their photovoltaic performance using as donor, and non-fullerene acceptor (PDIF) in the bulk heterojunction active layer. The bandgaps as well as the HOMO and LUMO energy levels are effectively tuned. The P(BTT-FCTPZ) structure exhibits a smaller bandgap as compared to P(BTT-DPP) that results from the lower LUMO energy and higher HOMO energy due to the FCTP unit. Having optimized the active layers, the PSCs that were based on P(BTT-DPP) and P(BTT-FCTPZ gave an overall power conversion efficiency of about 9.77% and 10.97%, respectively, using a wide bandgap PDIF non-fullerene acceptor, and 8.38% and 9.05, respectively, using PC 71 BM as electron acceptor.
Low Band Gap Donor-Acceptor Conjugated Polymers toward Organic Solar Cells Applications
Macromolecules, 2007
Mixtures of conjugated polymers and fullerenes command considerable attention for application in organic solar cells. To increase their efficiency, the design of new materials that absorb at longer wavelengths is of substantial interest. We have prepared such low band gap polymers using the donor-acceptor route, which is based on the concept that the interaction between alternating donors and acceptors results in a compressed band gap. Furthermore, for application in photovoltaic devices, sufficient polymer solubility is required. We have prepared four low band gap conjugated polymers, with a bis(1-cyano-2-thienylvinylene)phenylene base structure, and achieved an excellent solubility by the introduction of long alkoxy and alkyl side chains. The polymers were synthesized via an oxidative polymerization. Their electronic properties were determined from electrochemical and optical measurements, which confirm that they indeed have a low band gap. In the blend of such a low band gap polymer with PCBM, evidence for efficient charge transfer was obtained from PL and EPR measurements. However, bulk heterostructure solar cells made of such blends display only low efficiencies, which is attributed to low charge carrier mobilities.
The Journal of Physical Chemistry C, 2011
became attractive due to their ease of processing, formation of large areas, flexibility, and low cost. 1,2 To this end, the development of polymer solar cells has received much attention from both academic and industrial laboratories. In the past two years, great progress has been made in bulk-heterojunction (BHJ) PSCs, 3À5 but power conversion efficiency (PCE) is still a big challenge toward commercialization. To improve the PCE, it is very important to develop p-type conjugated polymers with low bandgap, high hole mobility, and deep lying HOMO energy level.
Synthetic Metals, 2013
Full donor-type conjugated polymers containing benzodithiophene and thiophene derivative units were synthesized as electron donors for organic photovoltaic devices. The alkoxy-substituted benzo[1,2b:4,5-b ]dithiophene (BDT) monomer, 2,6-bis(trimethyltin)-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5b ]dithiophene, was polymerized with 2,5-dibromothiophene through a Pd(0)-catalyzed Stille coupling reaction. To enhance the interchain interactions between polymers chains, an alkylselenophenesubstituted BDT derivative was newly synthesized, and copolymerized with the same counter monomer parts. The two newly synthesized polymers were characterized for use in organic photovoltaic devices as electron donors. Measured optical band gap energies of the polymers were 2.10 and 1.96 eV, depending on polymer structure. Field-effect transistors were fabricated using the polymers to measure their hole mobilities, which ranged from 10 −3 to 10 −5 cm 2 V −1 s −1 depending on the polymer structure. Bulk heterojunction organic photovoltaic cells were fabricated using conjugated polymers as electron donors and a [6,6]-phenyl C 71-butyric acid methyl ester (PC 71 BM) as an electron acceptor. One fabricated device showed a power conversion efficiency of 2.73%, an open-circuit voltage of 0.72 V, a short-circuit current of 7.73 mA cm −2 , and a fill factor of 0.46, under air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm −2).