Electronic structure, molecular orientation, charge transfer dynamics and solar cells performance in donor/acceptor copolymers and fullerene: Experimental and theoretical approaches (original) (raw)
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).
New acceptor–donor–acceptor (A–D–A) type copolymers for efficient organic photovoltaic devices
Journal of Physics and Chemistry of Solids, 2015
Three new conjugated systems alternating acceptor-donor-acceptor (A-D-A) type copolymers have been investigated by means of Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) at the 6-31g (d) level of theory. 4,4 0-Dimethoxy-chalcone, also called the 1,3-bis(4-methoxyphenyl)prop-2en-1-one (BMP), has been used as a common acceptor moiety. It forced intra-molecular S⋯O interactions through alternating oligo-thiophene derivatives: 4-AlkylThiophenes (4-ATP), 4-AlkylBithiophenes (4-ABTP) and 4-Thienylene Vinylene (4-TEV) as donor moieties. The band gap, HOMO and LUMO electron distributions as well as optical properties were analyzed for each molecule. The fully optimized resulting copolymers showed low band gaps (2.2-2.8 eV) and deep HOMO energy levels ranging from À 4.66 to À 4.86 eV. A broad absorption [300-900 nm] covering the solar spectrum and absorption maxima ranges from 486 to 604 nm. In addition, organic photovoltaic cells (OPCs) based on alternating copolymers in bulk heterojunction (BHJ) composites with the 1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]-C 61 (PCBM), as an acceptor, have been optimized. Thus, the band gap decreased to 1.62 eV, the power conversion efficiencies (PCEs) were about 3-5% and the open circuit voltage V oc of the resulting molecules decreased from 1.50 to 1.27 eV.
Macromolecules, 2007
We designed and synthesized a series of conjugated polymers containing alternating electrondonating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5′-[2,2′]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-4,7-(2,1,3benzothiadiazole)]), and their absorption characteristics can be tuned by adjusting the ratio of the two electrondonating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene) and 5,5′-[2,2′]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as electron transporters. Field-effect transistors made of these materials show hole mobility in the range of 5 × 10-4-3 × 10-3 cm 2 /(V s), which promises good device fill factor. Because of the combination of these characteristics, power conversion efficiencies up to 3.5% and an external quantum efficiency of at least 25% between 400 and 800 nm with a maximum of 38% around 700 nm were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. Further improvement of the materials will include the modification of both the side chains and the backbone to effect change to the active layer morphology to maintain good charge carrier mobility in the composite.
Advanced Energy Materials, 2014
conjugated polymer or small molecule as the donor material and a fullerene derivative as the electron acceptor. Efforts to raise the power conversion effi ciency by increasing the open-circuit voltage (V OC) have primarily focused on fi nding donor materials with lower-lying highest occupied molecular orbital (HOMO) levels [ 4 ] or fullerene derivatives with higher-lying lowest unoccupied molecular orbital (LUMO) levels. [ 5 ] Several research groups [ 6 ] have shown that a maximum of approximately 1.0 V for the V OC exists for effi cient OPV devices using fullerene derivatives as the electron acceptor. This limit is due to the inability to effi ciently split excitons on the fullerene molecule when the energy of the charge transfer (CT) state is less than 0.15 eV below the fullerene singlet excited state energy of 1.7 eV. In polymerfullerene systems where the fullerene has the smaller singlet energy, excitons formed in the polymer can reach the fullerenes via energy transfer, and ineffi cient hole transfer from the fullerenes to the polymer results in a large current loss in devices with V OC values exceeding 1.0 V. In order to relax this ceiling on the V OC and fi nd effi cient devices with voltages that can yield high effi ciencies in both single and multi-junction devices, new electron acceptors are needed with higher energy singlet excited states. [ 3b , 6d , 7 ] In addition to this restriction on the V OC , fullerene derivatives are relatively expensive [ 8 ] and C 60 derivatives do not absorb light well. One study [ 8a ] has shown that the PC 60 BM commonly used in bulk heterojunction (BHJ) organic solar cells could account for 12% of the overall OPV module cost and that C 70-based derivatives would be even more expensive. Current research into new electron acceptors [ 9 ] has covered an array of polymers [ 10 ] and small molecules. [ 11 ] While most devices prepared with these acceptors have effi ciencies near or below 2%, a few, including those based on evaporated devices incorporating halogenated boron subphthalocyanine molecules, [ 11p ] dimeric perylene diimide small molecules, [ 11r ] and solution-processed all-polymer devices based on naphthalene diimide [ 10a ] have achieved effi ciencies greater than 4%. V OC values approaching and even exceeding 1.0 V have been achieved in a few of these devices [ 11b , 11h ] but the typical values for the short-circuit current (J SC) and fi ll factor are There is a need to fi nd electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open-circuit voltage (V OC), do not absorb strongly and are expensive. Here, a phenylimide-based acceptor molecule, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (HPI-BT), that can be used to make solar cells with V OC values up to 1.11 V and power conversion effi ciencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum effi ciency of 56%, compared to 75-90% for polymer-fullerene devices, results from less effi cient separation of geminate charge pairs. While favorable energetic offsets in the polymer-fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI-BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.