BDT Unit Research Articles

Fluorobenzotriazole (FTAZ)‐Based Polymer Donor Enables Organic Solar Cells Exceeding 12% Efficiency

AbstractThe fluorobenzotriazole (FTAZ)‐based copolymer donors are promising candidates for nonfullerene polymer solar cells (PSCs), but suffer from relatively low photovoltaic performance due to their unsuitable energy levels and unfavorable morphology. Herein, three polymer donors, L24, L68, and L810, based on a chlorinated‐thienyl benzodithiophene (BDT‐2Cl) unit and FTAZ with different branched alkyl side chain, are synthesized. Incorporation of a chlorine (Cl) atom into the BDT unit is found to distinctly optimize the molecular planarity, energy levels, and improve the polymerization activity. Impressively, subtle side chain length of FTAZ realizes a dramatic improvement in all the device parameters, as revealed by the short‐current density (Jsc) improved from 7.41 to 20.76 mA cm−2, fill‐factor from 36.3 to 73.5%, and even the open‐circuit voltage (Voc) from 0.495 to 0.790 V. The best power conversion efficiency (PCE) of 12.1% is obtained from the L810‐based device, which is one of the highest values reported for FTAZ‐based PSCs so far. Notably, the corresponding external quantum efficiency curve keeps a very prominent value up to 80% from 500 to 800 nm. The notable performance is discovered from the reduced energy loss, improved molecular face‐on orientation, the down‐shifted energy levels, and optimized absorption coefficient regulated by side‐chain engineering.

Low-Energy-Loss Polymer Solar Cells with 14.52% Efficiency Enabled by Wide-Band-Gap Copolymers

SummaryTwo wide band-gap copolymers poly[4,8-bis(5-(2-butylhexylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-TZNT] (PBDTS-TZNT) and poly[4,8-bis(4-fluoro-5-(2-butylhexylthio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-TZNT] (PBDTSF-TZNT) based on naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) and benzo[1,2-b:4,5-b']dithiophene (BDT) with different conjugated side chains have been developed for efficient nonfullerene polymer solar cells (NF-PSCs). The rigid planar backbone of BDT and TZNT units imparted high crystallinity and good molecular stacking property to these copolymers. Using 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC) as the acceptor, PBDTSF-TZNT devices showed a high Voc of 0.98 V with an Eloss of 0.61 eV. On selecting 3,9-bis(2-methylene-(5,6-difluoro-(3-(1,1-dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b’]-dithiophene (IT-4F) instead of ITIC, the devices maintained the high Voc of 0.93 V with an even lower Eloss of 0.59 eV. The combination of the above-mentioned low Eloss, broadened absorption, better matched energy level, improved crystallinity, and fine-tuned morphology promoted the power conversion efficiency (PCE) of PBDTSF-TZNT:IT-4F devices from 12.16% to 13.25%. Homo-tandem devices based on PBDTSF-TZNT:IT-4F subcells further enhanced the light-harvesting ability and boosted the PCE of 14.52%, which is the best value for homo-tandem NF-PSCs at present.

π‐Dimerization ability of conjugated oligomer dication diradicaloids composed of dithienylpyrrole and benzodithiophene units

AbstractStable conjugated oligomer dications 12+ and 22+ composed of 2,5‐di(2‐thienyl)‐N‐methylpyrrole (DTP) units at the both ends of the π‐systems with methoxy substituents at the 3‐position of thienyl units and methylthio end‐capping group and benzo[2,1‐b:3,4‐b′]dithiophene (BDT) unit(s) at the central part with sterically demanding phenyl substituents were synthesized. From DFT calculations and ESR experiments, it was concluded that the diradical character in 12+ and 22+ was reduced in comparison with the radical character of their reference compound 3·+ composed of one DTP and one BDT units. In addition, the shorter 12+ was considered to have a further lower diradical character than that of 22+. Nevertheless, judging from the results of VT‐UV‐vis‐NIR measurements, both 12+ and 22+ showed a comparable π‐dimerization ability at the DTP moiety in dichloromethane solution with 3·+.

Overcoming the trade-off between Voc and Jsc: Asymmetric chloro-substituted two-dimensional benzo[1,2-b:4,5-b′]dithiophene-based polymer solar cells

To achieve the high PCE and simultaneously overcome the trade-off between open circuit voltage (VOC) and short-circuit current density (JSC), the reasonable design of the donor material is still a great challenge in non-fullerene polymer solar cells (PSCs). Herein, a pair of asymmetric 2D BDT-based wide band gap polymers, named PBPTBz-1Cl and PBPTBz-2Cl, respectively, were designed and synthesized through simultaneously introducing a chloro-substituted thiophene and another alkoxylphenyl groups as side chains onto the same BDT unit. As expected, the polymers PBPTBz-1Cl and PBPTBz-2Cl exhibit the deeper HOMO levels of −5.32 and −5.41 eV, respectively, compared with the HOMO value of −5.27 eV for chlorine-free substituted polymer PBPTBz-0Cl. Therefore, the PBPTBz-2Cl:IT-M based PSCs exhibited a higher Voc of 0.99 V, with an enhanced Jsc of 14.92 mA cm−2 and PCE of 7.18%. The Voc, Jsc and PCE were simultaneously enhanced from PBPTBz-0Cl:IT-M to PBPTBz-1Cl:IT-M, then to PBPTBz-2Cl:IT-M based PSCs successively. These results demonstrate that the challenge of the trade-off between Voc and Jsc could be overcome through the asymmetric strategy of the chloro-substituted thiophene and alkylphenyl groups as different side chains onto the same BDT unit. Hence, considering the low cost of starting materials and convenient introduction of chlorine atoms, the asymmetric chlorination of the side chain of BDT unit would be the great potential for achieving the high-performance PSCs.

Impact of Polymer Side Chain Modification on OPV Morphology and Performance

Efficiencies of organic photovoltaic (OPV) devices have been steadily climbing, but there is still a prominent gap in understanding the relationship between fabrication and performance. Side chain substitution is one processing parameter that can change OPV device efficiency considerably, primarily because of variations in morphology. In this work, we explain the morphological link between side chain selection and device performance in one polymer to aid in the development of design rules more broadly. We study the morphology of an OPV active layer using a PBDTTPD-backbone polymer with four different side chain configurations, which are shown to change device efficiency by up to 4 times. The optimal device has the smallest domain sizes, the highest degree of crystallinity, and the most face-on character. This is achieved with two branched 2-ethylhexyl (2EH) side chains placed symmetrically on the BDT unit and a linear octyl (C8) side chain on the TPD unit. Substituting either side chain (C14 on BDT and/or 2EH on TPD) makes the orientation less face-on, while the TPD side chain primarily affects domain size. For all side chains, the addition of fullerene increases polymer crystallization compared to the neat film, but the degree of mixing between polymer and fullerene varies with side chain. Interestingly, the optimal device has a negligible amount of mixed phase. The domain sizes present in the optimal system are remarkably unchanged with a changing fullerene ratio between 10 and 90%, hinting that the polymer preferentially self-assembles into 10-20 nm crystallites regardless of concentration. The formation of this crystallite may be the key factor inhibiting mixed phase.

Multichloro-Substitution Strategy: Facing Low Photon Energy Loss in Nonfullerene Solar Cells

The large photon energy loss (Eloss) in the conversion from photons to electrons is still one of the critical factors in limiting the performance of polymer solar cells (PSCs). In order to simultaneously obtain the decreased Eloss and expanded spectral response, the reasonable design of wide band gap conjugated polymer with a low-lying HOMO level is still a great challenge in nonfullerene PSCs. In this work, four D–A type wide band gap conjugated polymers with introduced chlorine atoms on the conjugated thienyl-side chains of the BDT units, namely, PBT1Cl-Bz, PBT2Clas-Bz, PBT2Cls-Bz, and PBT4Cl-Bz, were synthesized and characterized. Because of the large dipole moment of carbon–chlorine (C–Cl) and the strong noncovalent interaction of Cl···S and Cl···π, the tetrachloro-substituted PBT4Cl-Bz exhibits a much deeper HOMO level (−5.64 eV) than the other three polymers. Therefore, a high open-circuit voltage (Voc) of 0.96 V was achieved while keeping a decent Jsc of 16.04 mA cm–2 when matched with nonfullerene...

Highly Efficient Non‐Fullerene Polymer Solar Cells Enabled by Wide Bandgap Copolymers With Conjugated Selenyl Side Chains

In this work, the authors design and synthesize two novel wide bandgap copolymers based on selenophene substituted benzo[1,2‐b:4,5‐b']dithiophene (BDTSe) as the donor unit and fluorinated benzotriazole as the acceptor unit for high performance non‐fullerene polymer solar cells (NF‐PSCs). A larger maximum molar extinction coefficient (ϵ) of 8.54 × 104 M−1 cm−1 is achieved when introducing sulfur atom onto the two‐dimensional (2D) BDTSe units, which should realize the better complementary absorption with ITIC as the acceptor, leading to a higher Jsc of 19.51 mA cm−2. Furthermore, a lower highest occupied molecular orbital (HOMO) energy level with almost no change in bandgap can be also achieved after inserting the sulfur atoms, thus resulting in an enhanced open‐circuit voltage (Voc) of 0.84 V without sacrificing the short‐current density (Jsc). In addition, the higher crystallinity and optimized morphology are found to be beneficial to more efficient exciton dissociation and charge transport, giving rise to a higher fill factor (FF) of 75.1% and an elevated power conversion efficiency (PCE) of 12.31%. The results indicate that the strategy of alkylthioselenyl side chains on the BDT unit for constructing the donor‐acceptor (D‐A) copolymer donor materials is an excellent approach for realizing highly efficient NF‐PSCs.

Open-Circuit Voltage Modulations on All-Polymer Solar Cells by Side Chain Engineering on 4,8-Di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-Based Donor Polymers

In recent years, all-polymer solar cells (all-PSCs), incorporating active layers based on blends of electron-donor (D) and acceptor (A) polymers, have drawn attention because of the advantages they hold in the flexibility of choosing the D:A combinations to modulate their energy levels and to improve their overall open-circuit voltages (V oc ) and power conversion efficiencies (PCE)s. V oc is one of the key parameters for the determination of the PCEs of PSCs. In this work, we synthesized six donor polymers with three different side chains appended to the 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT) units. By substituting carbon with sulfur and silicon atoms at the 5-position of the thiophenes attached to the BDT units, the highest occupied molecular orbital (HOMO) levels of the donor polymers could be successfully lowered. As anticipated, the V oc values of the resulting all-PSCs increased along with the lowering of the HOMO levels of the donor polymers. Among the six all-PSCs, the PBDT-BDD:PNDI-T10 all-PSC realized a balance between the photovoltage and photocurrent, where a decent PCE of 5.6% was obtained with a V oc of 0.9 V and a photocurrent of 10.5 mA/cm 2 .

A wide-bandgap polymer based on alkylthio-naphthyl- substituted benzo[1,2-b:4,5-b′]dithiophene units for efficient fullerene-based and fullerene-free polymer solar cells

A wide-bandgap polymer PBDTNS-TPD based on alkylthio-naphthyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) units was designed and synthesized. Benefiting from the prolonged conjugation of the conjugated side groups on BDT units, the PBDTNS-TPD exhibited better light-harvesting properties and charge transport properties than that of alkylthio-phenyl-substituted counterpart PBDTBS-TPD. The PBDTNS-TPD:PC71BM solar cell devices yield a power conversion efficiency (PCE) of 9.09%, which exhibits an increase of 29% in comparison with that of PBDTBS-TPD-based PSCs (PCE 7.02%). Moreover, the PBDTNS-TPD:ITIC based PSCs also showed a higher PCE of 8.19% compared to the PBDTBS-TPD:ITIC based PSCs (PCE 5.93%). This work demonstrates that the replacement of the alkylthio-phenyl moiety with alkylthio-naphthyl unit on BDT is a useful method to design high-performance wide-bandgap polymers for fullerene-based and fullerene-free PSCs.

Theoretical investigation on the effect of fluorine and carboxylate substitutions on the performance of benzodithiophene-diketopyrrolopyrrole-based polymer solar cells

Three donor–acceptor (D–A) polymers 2–4 were designed and investigated based on the reported polymer 1 with benzo[1,2-b:4,5-b′]dithiophene (BDT) as D fragment and diketopyrrolopyrrole (DPP) as A fragment. The fluorine substitutions on the BDT unit in molecule 2 have less influence on the lowest unoccupied molecular orbital (LUMO) compared with the carboxylate substitutions on the BDT unit in 3 and 4. The charge transfer rate (kinter-CT) of molecule 4 is the largest, which determines that molecule 4 has a priority in the interfacial process among these investigated molecules with the same acceptor PC 61 BM. The designed molecules 2–4 show larger open-circuit voltages (Voc), relatively narrower bandgaps and higher value of kinter-CT/kinter-CR than 1. Moreover, the results demonstrate that fluorine and carboxylate substitutions on molecule 4 show a synergistic effect on the FMO energy levels and electron interfacial process, which is expected to help the further understanding of the design rules for polymer donor materials in polymer solar cells.

Synthesis and characterization of highly conjugated side‐group‐substituted benzo[1,2‐b:4,5‐b′]dithiophene‐based copolymer for use in organic solar cells

ABSTRACTA new donor–acceptor (D–A) conjugated copolymer based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT) and thieno[3,4‐c]pyrrole‐4,6‐dione (TPD) was synthesized via a Stille cross‐coupling reaction. A highly conjugated thiophene‐based side group, tris(thienylenevinylene) (TTV), was incorporated into each BDT unit to generate the two‐dimensional D–A copolymer (PBDT‐TTV). An alkoxy‐substituted BDT‐based TPD copolymer (PBDT‐OR) was synthesized using the same polymerization method for comparison. PBDT‐TTV thin films produced two distinct absorption peaks. The shorter wavelength absorption (458 nm) was attributed to the BDT units containing the TTV group, and the longer wavelength band (567–616 nm) was attributed to intramolecular charge transfer between the BDT donor and the TPD acceptor. The highest occupied molecular orbital energy levels of PBDT‐OR and PBDT‐TTV were calculated to be −5.53 and −5.61 eV, respectively. PBDT‐TTV thin films harvested a broad solar spectrum covering the range 300–700 nm. A comparison with the PBDT‐OR films revealed stronger interchain π–π interactions in the PBDT‐TTV films and, thus, a higher hole mobility. A polymer solar cell device prepared using PBDT‐TTV as the active layer was found to exhibit a higher power conversion efficiency than a device prepared using PBDT‐OR under AM 1.5 G (100 mW/cm2) conditions. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 653–660

A New Polymer Electron Acceptor Based on Thiophene-S,S-dioxide Unit for Organic Photovoltaics.

For polymer solar cells (PSCs), efficient polymer electron acceptors are always based on strong electron-withdrawing imide unit or boron-nitrogen coordinative bond (B←N). In this paper, a new polymer electron acceptor based on thiophene-S,S-dioxide (TDO) unit is reported. The polymer electron acceptor, PBDT-TDO, consists of alternating TDO unit and 4,8-bis(alkylthienyl)-2-yl]benzo[1,2-b:4,5-b']dithiophene (BDT) unit. For comparison, a control polymer with alternating BDT unit and thiophene unit has also been synthesized. Replacing thiophene unit with TDO unit in the polymer backbone leads to large downshift of lowest unoccupied molecular orbital/highest occupied molecular orbital energy levels by 0.9 eV/0.4 eV, which is attributed to the dearomatization and electron deficiency of TDO unit. The replacement also leads to redshift of absorption spectra by ≈110 nm. PSC device with PBDT-TDO as the electron acceptor shows photovoltaic response with the preliminary power conversion efficiency of 0.64%. This work suggests a new approach to design polymer electron acceptors using the TDO unit.

Alternating polymers based on fluorinated alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene and isoindigo derivatives for polymer solar cells

To effectively modulate the molecular energy levels of isoindigo-based polymers through side-chain modification on the donor segments, three alkoxyphenyl-modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and used as electronic-donor units, which were copolymerized with the electron-withdrawing segment 2-ethylhexyl-substituted isoindigo (IID) to construct donor-acceptor (D-A) polymers. The obtained three polymers are named as P-IID, mF-IID and oF-IID, which possess 4-(2-octyldodecaneoxy)-phenyl, 2-fluoro-4-(2-octyldodecaneoxy)-phenyl and 3-fluoro-4-(2-octyldodecaneoxy)-phenyl side-chains in the BDT unit, respectively. All polymers display broad absorption regions with the film absorption edges of 759 nm (P-IID), 742 nm (mF-IID) and 726 nm (oF-IID). The fluorination on the phenyl side-chains of BDT unit results in obvious blue-shift in the absorption spectra, especially for the ortho-position fluorinated polymer oF-IID because of the enlarged dihedral angles between the phenyl rings and the BDT backbones. Gradient reduction in the highest occupied molecular orbital (HOMO) energy levels are observed for the three polymers, with HOMO values of −5.35 eV for P-IID, −5.48 eV for mF-IID and −5.53 eV for oF-IID, separately. As the result of the HOMO energy levels, polymer solar cells (PSCs) with inverted device configuration by blending the polymers with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) show continually increased open circuit voltage (Voc) from 0.84 V for P-IID to 0.88 V (mF-IID) and then a notable value of 0.96 V for oF-IID. However, the fluorination leads to intensively reduced short-circuit current density (Jsc) of the PSCs, 9.68 mA cm−2 for P-IID, 5.61 mA cm−2 for mF-IID and a very low value of 1.61 mA cm−2 for oF-IID, respectively. Consequently, moderate power conversion efficiency (PCE) of 5.23% is achieved for the non-fluorinated polymer P-IID as well as a decreased value of 2.50% for the meta-fluorinated polymer and a very low PCE of 0.93% for the ortho-fluorinated polymer. These results imply that although fluorine substituted side-chain in the donor unit can effectively deepen the HOMO energy levels of the polymers to obtain enlarged Voc, the absorption losses of the fluorinations caused low Jsc should be taken into consideration to design high efficient electron-donating alternating polymers for polymer solar cells.

Small Molecules with Asymmetric 4-Alkyl-8-alkoxybenzo[1,2-b:4,5-b′]dithiophene as the Central Unit for High-Performance Solar Cells with High Fill Factors

A small molecule named DRBDTCO based on benzo[1,2-b:4,5-b′]dithiophene (BDT) with an asymmetric side chain and its dimer, dDRBDTCO, with the octamethylene connector were designed and synthesized as donor materials for solution-processed bulk heterojunction solar cells. The optimized power conversion efficiency of a DRBDTCO-based device was 8.18% under AM 1.5 G irradiation (100 mW cm–2), which is higher than that of its dDRBDTCO-based device. Both molecule-based devices exhibited high fill factors of >73%, which are attributed to their optimized morphologies with a feature size of 15–20 nm, highly efficient charge collection, less bimolecular recombination, and well-balanced charge transport properties. The results demonstrate that the asymmetric BDT unit could be a promising building block for donor materials with high efficiencies and high fill factors.

9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor

In the last two years, polymer solar cells (PSCs) developed quickly with n-type organic semiconductor (n-OSs) as acceptor. In contrast, the research progress of nonfullerene organic solar cells (OSCs) with organic small molecule as donor and the n-OS as acceptor lags behind. Here, we synthesized a D-A structured medium bandgap organic small molecule H11 with bithienyl-benzodithiophene (BDTT) as central donor unit and fluorobenzotriazole as acceptor unit, and achieved a power conversion efficiency (PCE) of 9.73% for the all organic small molecules OSCs with H11 as donor and a low bandgap n-OS IDIC as acceptor. A control molecule H12 without thiophene conjugated side chains on the BDT unit was also synthesized for investigating the effect of the thiophene conjugated side chains on the photovoltaic performance of the p-type organic semiconductors (p-OSs). Compared with H12, the 2D-conjugated H11 with thiophene conjugated side chains shows intense absorption, low-lying HOMO energy level, higher hole mobility and ordered bimodal crystallite packing in the blend films. Moreover, a larger interaction parameter (χ) was observed in the H11 blends calculated from Hansen solubility parameters and differential scanning calorimetry measurements. These special features combined with the complementary absorption of H11 donor and IDIC acceptor resulted in the best PCE of 9.73% for nonfullerene all small molecule OSCs up to date. Our results indicate that fluorobenzotriazole based 2D conjugated p-OSs are promising medium bandgap donors in the nonfullerene OSCs.

High‐Performance Non‐Fullerene Polymer Solar Cells Based on Fluorine Substituted Wide Bandgap Copolymers Without Extra Treatments

A new wide bandgap conjugated polymer, PFBZ, based on fluorination thienyl conjugated benzodithiophene (BDT‐2F) as a donor unit and difluorinated benzotriazole (BTz‐2F) as an acceptor unit, was synthesized for non‐fullerene polymer solar cells (PSCs) application. Compared with the analogue based on non‐fluorinated BDT unit (PBZ), PFBZ shows a lower HOMO level, a similar wide optical bandgap with a higher extinction coefficient, and a slightly higher hole mobility. As a result, without any extra treatments, the PFBZ:ITIC‐based device exhibits a higher PCE of 10.4% with Voc of 0.89 V, Jsc of 18.8 mA cm−2, and FF of 62% in comparison with the PCE of 8.0% with Voc of 0.79 V, Jsc of 17.2 mA cm−2, and FF of 59% for PBZ:ITIC‐based devices. Furthermore, the photovoltaic performance of the PFBZ: ITIC‐based devices are relatively insensitive to the thickness of the active layer and the PCE still remains 8.9% with a thickness of up to 200 nm. In addition, the PFBZ: ITIC‐based devices exhibit high tolerance to the thermal annealing, and still show a high PCE of 9.3% with annealing at 150 °C for 1 hour. These results indicate that PFBZ is a promising candidate for the large scale manufacturing and practical application of high efficient PSCs.

Small-Molecule Acceptor Based on the Heptacyclic Benzodi(cyclopentadithiophene) Unit for Highly Efficient Nonfullerene Organic Solar Cells.

A new nonfullerene small molecule with acceptor-donor-acceptor (A-D-A) structure, namely, NFBDT, based on a heptacyclic benzodi(cyclopentadithiophene) (FBDT) unit using benzo[1,2-b:4,5-b']dithiophene as the core unit, was designed and synthesized. Its absorption ability, energy levels, thermal stability, as well as photovoltaic performances were fully investigated. NFBDT exhibits a low optical bandgap of 1.56 eV resulting in wide and efficient absorption that covered the range from 600 to 800 nm, and suitable energy levels as an electron acceptor. With the widely used and successful wide bandgap polymer PBDB-T selected as the electron donor material, an optimized PCE of 10.42% was obtained for the PBDB-T:NFBDT-based device with an outstanding short-circuit current density of 17.85 mA cm-2 under AM 1.5G irradiation (100 mW cm-2), which is so far among the highest performance of NF-OSC devices. These results demonstrate that the BDT unit could also be applied for designing NF-acceptors, and the fused-ring benzodi(cyclopentadithiophene) unit is a prospective block for designing new NF-acceptors with excellent performance.

Systematic Investigation of Benzodithiophene-Benzothiadiazole Isomers for Organic Photovoltaics.

Two new donor-acceptor small molecules based on benzo[1,2-b:4,5-b']dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT) were designed and synthesized. Small molecules 4,4'-[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(2,2'-bithiophene)-5,5'-diyl]bis(benzo[c][1,2,5]thiadiazole) (BDT-TT-BT) and 4,4'-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis[7-(2,2'-bithiophene-5-yl)benzo[c][1,2,5]thiadiazole] (BDT-BT-TT) are structural isomers with the 2,2-bithiophene unit placed either between the BDT and BT units or at the end of the BT units. This work is targeted toward finding the effect of structural variation on optoelectronic properties, morphology, and photovoltaic performance. On the basis of theoretical calculations, the molecular geometry and energy levels are different for these two molecules when the position of the 2,2-bithiophene unit is changed. Optical and electrochemical properties of these two small molecules were characterized using UV-vis and cyclic voltammetry. The results showed that BDT-BT-TT has broader absorption and an elevated HOMO energy level when compared with those of BDT-TT-BT. The performance of these two isomers in solar cell devices was tested by blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Power conversion efficiencies as high as 3.22 and 3.71% were obtained in conventional solar cell structures for BDT-TT-BT and BDT-BT-TT, respectively. The morphology was studied using grazing incident wide-angle X-ray scattering and transmission electron microscopy, which revealed different phase separations of these two molecules when blended with PC71BM.

Unsubstituted Benzodithiophene-Based Conjugated Polymers for High-Performance Organic Field-Effect Transistors and Organic Solar Cells.

Unsubstituted benzo[1,2-b:4,5-b']dithiophene (BDT) was used to construct a high-performance conjugated polymer with 5,6-difluoro-4,7-bis[4-(2-octyldodecyl)thiophene-2-yl]benzo[c][1,2,5] thiadiazole (DTFFBT), named PBDT-DTFFBT. The polymer shows the low-lying highest occupied molecular orbital (HOMO) energy level (-5.40 eV) and a broad absorption spectra with strong vibronic absorption peak. Pure polymer films exhibit good crystallinity and edge-on orientation, partially attributed to the BDT units without any side chains, and as a result, the corresponding thin-film transistor showed excellent hole mobility over 1 cm(2) V(-1) s(-1). Interestingly, a well-distributed nanofibrillar polymer aggregation with face-on orientation was obviously formed when blending with PC71BM, which was in favor of the charge transportation. Consequently, the bulk heterojunction polymer solar cells based on the blends showed high power conversion efficiency of 9.29% with large short-current density (14.56 mA cm(-2)) and high fill factor (0.751) without any process additives or thermal annealing.

Synthesis of D–A low-bandgap polymer-based thieno[3,4-b]pyrazine and benzo[1,2-b:4,5-b′]dithiophene for polymer solar cells

Three D–A low-bandgap polymer-based benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,4-b]pyrazine (TP) unit with different substituents were synthesized using Stille coupling. The substituent effects were explored. It is found that attaching the phenyl group at the 2,3-positions of thienopyrazine unit is beneficial to good coplanarity and regularity of molecular packing. Meanwhile, attaching the longer alkoxyl side chain at BDT unit or phenyl group at the 2,3-positions of thienopyrazine unit had the relatively deep HOMO (LUMO) energy levels. The three D–A polymers showed good thermal stability and exhibited broad absorption, low bandgap, and order packing structure. As a result, the highest PCE of 1.50 % was achieved for the polymer P2 with the substituents, including hexyldecyloxyl at the BDT unit and phenyl at the TP unit.