BDT Unit Research Articles

Carboxylic-substituted polymer design strategy enabled halogen-free solvent processed efficient organic solar cells

Carboxylic ester groups can be rationally utilized to regulate the energy levels and crystallinity in molecular design of conjugated polymer donors for organic solar cells (OSCs). In this work, a new polymer named PB4T was designed and prepared by copolymerization of a carboxylate substituted tetra-thiophene unit with the commonly used BDT unit. Due to the strong electron-withdrawing capability and the symmetric conformation of carboxylic ester substitution, PB4T exhibits appropriate HOMO energy level and stable planar geometry. When matching PB4T with five different NFAs with varied energy levels and bandgaps, including eC9, eC9-2Cl, IT-4F, BTA3, and A4T-3, all the OSCs processed with the halogen-free solvent show good photovoltaic performance. The PB4T:eC9- and PB4T:eC9-2Cl-based OSCs can realize high PCEs of 15.50 % and 15.32 %, respectively. This work demonstrates that PB4T has good universality to work well with newly-emerging NFAs, and carboxylate substituted thiophene units show great potential in constructing high-efficiency polymer donors.

New wide bandgap conjugated D-A copolymers based on Benzo[d]thiazole for efficient photovoltaics

Two wide bandgap copolymers based on benzo[d]thiazole unit, named PBTz-F and PBTz-Cl, containing benzo[1,2-b:4,5-b′]dithiophene (BDT) as electron donor units are developed. The resulting polymers lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels via substituting halogen atoms (fluorine or chlorine atoms) on the thiophene side chain of the BDT unit. However, the chlorine atom in the copolymers has a stronger capacity to downshift energy levels than the fluorine atom. PBTz-Cl:Y6-BO-based OSC delivers a higher PCE of 12.36% with simultaneously enhanced VOC of 0.90 V, JSC of 22.78 mA cm−2, and FF of 0.61 when compared to that based on PBTz-F:Y6-BO (PCE = 11.38%, JSC = 22.48 mA cm−2, VOC = 0.88 V, and FF = 0.58) due to its stronger absorption, higher charge mobilities, deeper HOMO energy level, and favorable blend film morphology. These results emphasized that chlorination is a practical strategy to obtain high PCE and BTz-based polymers are potential donor candidates for high-performance OSCs.

P-Type Conjugated Polymers Containing Electron-Deficient Pentacyclic Azepinedione.

Bisthienoazepinedione (BTA) has been reported for constructing high-performing p-type conjugated polymers in organic electronics, but the ring extended version of BTA is not well explored. In this work, we report a new synthesis of a key building block to the ring expanded electron-deficient pentacyclic azepinedione (BTTA). Three copolymers of BTAA with benzodithiophene substituted by different side chains are prepared. These polymers exhibit similar energy levels and optical absorption in solution and solid state, while significant differences are revealed in their film morphologies and behavior in transistor and photovoltaic devices. The best-performing polymers in transistor devices contained alkylthienyl side chains on the BDT unit (pBDT-BTTA-2 and pBDT-BTTA-3) and demonstrated maximum saturation hole mobilities of 0.027 and 0.017 cm2 V-1 s-1. Blends of these polymers with PC71BM exhibited a best photovoltaic efficiency of 6.78% for pBDT-BTTA-3-based devices. Changing to a low band gap non-fullerene acceptor (BTP-eC9) resulted in improved efficiency of up to 13.5%. Our results are among the best device performances for BTA and BTTA-based p-type polymers and highlight the versatile applications of this electron-deficient BTTA unit.

Benzodithiophene unit copolymerization to improve the stability of thiophene-based organic solar cells

With the increasing performance of organic solar cells (OSCs), the design of stable donor materials is of paramount importance. In this work, we synthesised a simple alternating copolymer with thiophene and BDT (P3T-BDT) units to understand the impact of incorporation of BDT on the thermal stability of the commonly used thiophene-based OSCs. OSCs fabricated with P3T-BDT:PC71BM and P3HT:PC71BM with 1:4 ratios displayed a different degradation kinetics despite their similar initial power conversion efficiency (PCE). The PCE of P3HT-based device degraded by half while the P3T-BDT-based device lost only 15% of its initial PCE after thermally being degraded at 85 ∘C for 48 h mainly due to the decrease in J . Thermally-induced degradation of P3HT caused chain breakage while the P3T-BDT remained intact. The surface morphology of P3HT:PC71BM film exhibited a large increase in roughness in the first few hours leading to a loss of more 40% of its initial PCE while the P3T-BDT based OSC had a similar roughness throughout the accelerated ageing. This is also confirmed by the growth of the PL intensity of P3HT:PC71BM while P3T-BDT:PC71BM remained the same confirming a suppression of demixing of the donor and acceptor in the latter. In summary, the addition a BDT unit in the thiophene-based homopolymer was found to improve the thermal stability thiophene-based OSCs.

A Benzo[1,2‐b:4,5‐b′]Difuran Based Donor Polymer Achieving High‐Performance (>17%) Single‐Junction Organic Solar Cells with a Fill Factor of 80.4%

AbstractIn the field of non‐fullerene organic solar cells (OSCs), most of the promising polymer donors are based on benzo[1,2‐b:4,5‐b′]dithiophene (BDT) units while benzo[1,2‐b:4,5‐b′]difuran (BDF)‐based polymers have drawn less attention since the efficiencies of BDF polymer‐based devices are generally lower than those of BDT polymer‐based ones. In this contribution, the BDT unit in a polymer donor named D18 is replaced with a BDF unit, and a new polymer named D18‐Fu is synthesized. As a highly‐crystalline molecule named Y6‐1O is chosen as the acceptor, the efficiency of binary devices based on D18‐Fu can reach 16.38%. Furthermore, when one of fullerene derivatives PC71BM is added, the ternary devices based on D18‐Fu achieve an efficiency of 17.07% and a high fill factor (FF) of 80.4%, both of which are the highest values among those of BDF polymer‐based devices. For comparison, D18‐based ternary devices show an inferior efficiency of 15.61% mainly due to the lower FF of 73.9%. Subsequent characterization reveals that D18‐Fu possesses a more coplanar molecular geometry, leading to better morphology and higher charge mobility for a promising FF. The high performance shown in this work demonstrates the potential role of BDF units in the design of polymer donors for highly efficient OSCs.

Recent Progress of Benzodifuran‐Based Polymer Donors for High‐Performance Organic Photovoltaics

At present, a variety of benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based polymer donors are designed and synthesized, which have made great progress toward achieving high power conversion efficiencies (PCEs) of polymer solar cells (PSCs). Despite many advantages of benzo[1,2‐b:4,5‐b′]difuran (BDF) compared with BDT unit, the overall performance of BDF polymer‐based PSCs lags far behind that of the counterpart BDT‐based polymers. Recently, great advances have been achieved in BDF polymer‐based PSCs with the highest PCEs over 16%, suggesting that BDF‐based polymers have the significant potential to catch up or even surpass the performance of BDT‐based polymers. In this review, the recent advances of BDF polymers in PSCs are first summarized and then the design strategies and the chemical structure–performance relationships of BDF polymers are discussed. Finally, perspectives on the future development of BDF polymers for high‐efficiency PSCs are provided.

Synergistic enhancement in open-circuit voltage and photovoltaic performance via linear naphthyldithiophene building block

Among various photovoltaic building blocks, fused aromatic units such as benzo[1,2-b:4,5-b']dithiophene (BDT) based conjugated polymers have exhibited excellent photovoltaic properties in organic solar cells (OSCs) due to their better delocalization along polymeric backbones. However, further extending conjugation in fused aromatic units appears some unavailable drawbacks such low-solubility, low-antioxidation ability, high-lying HOMO level, etc., which cause negative effect on the corresponding open-circuit voltage (Voc) and power conversion efficiency (PCE). Here, we designed and synthesized two novel wide band gap polymers, PBDT-3T and PNDT-3T, using benzodithiophene and naphthyldithiophene as donor units and ester-substituted triple thiophenes as acceptor units. Comparing to the referenced BDT based polymer, it was found that NDT unit not only extends the conjugated degree of the main chain and increase polymeric crystallinity and carrier mobility, but also increases the Voc of NDT based photovoltaic device. The final short-circuit current and fill factor are 15.90 mA cm−2 and 63.7%, and finally the PCE reached 9.72%, which exhibited better advantages than the BDT unit polymers. Therefore, the adoption of NDT as photovoltaic building block is proved to be an effective strategy in synergistic improvement in Voc and PCE and could be referenced to other photovoltaic materials.

Biaxially extended side-chain conjugation of benzodithiophene-based polymer dots for superior photocatalytic stability under visible-light irradiation

Conjugated polymer dots (Pdots) based on a series of BDT-based polymers with varied biaxially extended side-chain conjugations were prepared and employed for photocatalytic hydrogen production under visible light irradiation from aqueous solutions. The BDT-based polymers consisted of a dithiophedifluorobenzo[c][1,2,5]thiadiazole (DFBT) moiety as the acceptor unit and a BDT unit with different biaxially extended side-chains, including a alkyl-monothienyl group (T), a linear alkyl-dithiophene (2T) group, a branched alkyl-terthiophene (3T) group, and a fused alkyl-benzotrithiophene (B3T) group. The as-prepared Pdots based on these four polymers (PBDT-T, PBDT-2T, PBDT-3T, and PBDT-B3T) showed varied photophysical and electrochemical properties along with distinctive photocatalytic activity and long-term stability. Among them, the PBDT-B3T Pdots were showed to deliver a high photocatalytic efficiency of 14.1 mmol g−1 h−1 and a superior photocatalytic stability over 375 h in the presence of Pt as cocatalyst and ascorbic acid as sacrificial electron donors. Our results revealed that varying the structures of the biaxially extended side-chains endows a great flexibility in the photocatalytic activity of the polymers and the biaxially-extended conjugated side-chains can be a promising design strategy for the conjugated polymer to enhance the photocatalytic activity and stability.

Calculated optical properties of donor molecules based on benzo[1,2-b:4,5-b′]dithiophene and its derivatives

In this work, we study the light absorption properties of different novel molecules based on benzo[1,2-b:4,5-b′]dithiophene (BDT), namely, BT-2F, BTRCl, and BTTzR, which hold great promise as electron-donor materials in organic solar cells. By employing density functional theory, we study the electronic states and related transitions in these systems. Moreover, in the case of BTTzR, we observe that the addition of two and three oligothiophene chains to the central benzene ring of the BDT unit leads to both a red-shift of the existing peaks and, interestingly, the development of new blue-shifted features, an effect that permits to enhance the panchromaticity of the molecule in the visible spectral range, thus rendering these new derivatives highly appealing as light absorbers in organic solar cells.

Effect of donor and acceptor on optoelectronic properties of benzo[1,2-b:4,5-b′]dithiophene

A series of acceptor and donor groups anchored to benzo[1,2-b:4,5-b′]dithiophene (BDT) molecule have been systematically investigated at the density functional theory (DFT) and time-dependent density functional theory (TDDFT) level to reveal structure–property relationships, charge transfer, and fluorescence lifetimes. The DFT optimization shows that the hetero atom in the ring induces the polarity from central ring to both ends of the thiophene ring, participating in the conjugation. The donor and acceptor groups were anchored at the terminals of the BDT at two different positions to fine-tune the properties according to the requirement and study the push–pull effect. All the models studied in this work retain their aromaticity as estimated from NICS(0) and NICS(1) aromaticity index in ground and excited states. The results show that the hardness, softness, HOMO–LUMO gaps, ionization potentials (IP), and electron affinities (EA) of the BDTs are significantly affected by the electron-withdrawing and electron-donating groups. The 1H and 13C NMR chemical shift values have been computed to quantify the push–pull effect. Further, the charge transfer properties in these BDTs were explored based on reorganization energies and diagnostic descriptors derived from hole–electron theory that present different electron excitation behavior. The relationship between the computed variables such as highest occupied molecular orbital, lowest unoccupied molecular orbital, oscillator strength, dipole moment, absorption, and fluorescence energy correlates the system with one another and also to extend the possible applications of the system in optical devices. Structure–property relationship of various BDTs reveal that, upon optical excitation, the resonance effect plays an important role changing the bonding character between the substituent and BDT unit, enabling efficient electron delocalization. The examination of TDDFT results indicates that among the various models studied in this work, nitro-substituted model is better candidate for optoelectronic properties with relatively large absorption wavelength and long fluorescence lifetime.

Development of new nonacyclic small-molecule acceptors involving two benzo[1,2-b:4,5-b′]dithiophene moieties for efficient polymer solar cells

Benzo[1,2-b:4,5-b′]dithiophene (BDT) is demonstrated as a very effective building block to construct acceptor-donor-acceptor (A-D-A) small-molecule acceptors (SMAs) involving one BDT unit for high-performance polymer solar cells (PSCs). Herein, we successfully developed two new A-D-A SMAs, namely IDBDT-O and IDBDT-T, with the same BDT-fused nonacyclic indacenobis(benzo[1,2-b:4,5-b′]dithiophene) (IDBDT) central D unit and 1,1-dicyanomethylene-3-indanone (IC) terminal A group, but different side chains (alkoxyl vs alkylthiophene) attached on the BDT units. The IDBDT central core is formed by the fusion of one indacene segment and two BDT moieties, which obviously differs from the previously studied BDT-fused SMAs. Compared to IDBDT-T with alkylthiophene side chains, IDBDT-O with alkoxyl side chains exhibits a broader absorption, smaller band gap, and higher-lying HOMO energy level. Moreover, these IDBDT-based SMAs have upshifted LUMO energy levels relative to the reported BDT-fused SMAs, which is beneficial to increase open-circuit voltage (Voc). Bulk-heterojunction PSCs fabricated from polymer donor PBDB-T and IDBDT-T achieve the best power conversion efficiency (PCE) of 7.14% with a high Voc of 0.96 V, a short-circuit current density (Jsc) of 15.20 mA cm−2, much larger than a PCE of 1.83%, a Voc of 0.89 V and a Jsc of 5.21 mA cm−2 for the counterpart of PBDB-T:IDBDT-O. The enhanced device performance could be ascribed to higher and more balanced carrier mobility, more efficient exciton dissociation, and weaker bimolecular recombination as well as better active layer morphology in the IDBDT-T-based PSCs relative to IDBDT-O. Our results provide a new molecular design strategy to exploit BDT-fused A-D-A SMAs for high-performance PSCs.

Enhanced photovoltaic performance of benzodithiophene-alt-bis(thiophen-2-yl)quinoxaline polymers via π–bridge engineering for non-fullerene organic solar cells

A series of novel alternating polymers, namely P(BDTO-TTFQ), P(BDTT-TTFQ), and P(BDTSi-TTFQ), incorporating electron-rich benzo [1,2-b:4,5-bʹ]dithiophene (BDT) derivatives, namely 4,8-bis(2-butyloctyloxy)benzo [1,2-b:4,5-bʹ]dithiophene (BDTO), 4,8-bis(5-(2-butyloctyl)thiophen-2-yl)benzo [1,2-b:4,5-bʹ]dithiophene (BDTT), and 4,8-bis(triisopropylsilylethynyl)-benzo [1,2-b:4,5-ʹ]dithiophene (BDTSi), as well as electron-deficient 5,8-bis(5-(4-hexylthiophen-2-yl)thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TTFQ) units were prepared. The photo-physical, electrochemical, crystallinity, curvature, charge transport, and photovoltaic properties of the TTFQ-based polymers were investigated thoroughly and compared briefly to those of structurally similar 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (TFQ)-based polymers, namely P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ), containing BDTO, BDTT, and BDTSi. This study confirmed that the incorporation of additional π˗bridges (3-hxeylthiophene) between the BDT and TFQ units of P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ) do not significantly alter the properties of the polymers P(BDTO-TFQ) and P(BDTT-TFQ), but do significantly alter the properties of P(BDTSi-TFQ). Consequently, the polymers P(BDTO-TTFQ) and P(BDTT-TTFQ) exhibit comparable power conversion efficiencies (PCEs, 3.99% and 6.69%, respectively) to those of P(BDTO-TFQ) and P(BDTT-TFQ) (3.49% and 7.06%, respectively), but P(BDTSi-TTFQ) exhibits a significantly improved PCE of 6.21% compared to that of P(BDTSi-TFQ) (0.75%).

Triphenylamine-based hole transporting materials with thiophene-derived bridges for perovskite solar cells

A triphenylamine-based compound containing benzoyl[1,2-b:4,5-b’]dithiophene (BDT) derivative (BTPA-2) has been synthesized and employed in FA0.85MA0.15PbI3 perovskite solar cells (PSCs) as the hole transporting material (HTM). The reference BTPA-1 containing thiophene bridge was also prepared for a comparative study. Compared to the thiophene unit in BTPA-1, the hexyloxy substituted BDT unit in BTPA-2 made the HOMO energy level lower. The more conjugated system with the BDT unit can improve the conductivity and hole mobility. The BTPA-1 and BTPA-2 based FA0.85MA0.15PbI3 PSCs showed the best power conversion efficiencies (PCE) of 12.76 % and 13.97 %, respectively. Larger Voc and higher Jsc for the BTPA-2 based PSC were attributed to its reduced interfacial recombination and more conjugated system compared to the BTPA-1 based PSCs. More hydrophobic property of BTAP-2 made its PSC exhibit better environmental stability than BTPA-1.

A systematic evaluation of triisopropylsilylethynyl-substituted thienyl side chain effects on series of benzo[1,2-b:4,5-b′]dithiophene based polymer donors and their photovoltaic performances

In this report three new donor copolymers based on alternate octyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate as acceptor and benzo[1,2-b:4,5-b′]dithiophene (BDT) derivatives having triisopropylethynylsilane (TIPS), triisopropyl((thiophen-2-yl)ethynyl)silane (TIPS-Th) and triisopropyl((3-octylthiophen-2-yl)ethynyl)silane (TIPS-ThC8) side chains as donors, (respectively named as P1, P2 and P3) are synthesized to assess whether insertion of thiophene or alkyl thiophene units between TIPS side chains and BDT backbones can improve their properties and photovoltaic performance. Indeed, the modification of TIPS side chains on BDT backbone found to distinctly affect their opto-electrochemical properties, molecular orientations and nanoscale morphologies in resultant polymers. Among the polymers, optimized organic solar cells (OSCs) of P1:PC71BM exhibited best power conversion efficiency (PCE) of 6.55% with a very high VOC of 1.02 V, thereby outperforming P2 and P3 (PCE of 4.72% and 5.47%, respectively). The detailed characterization of optimized blends revealed that this superior device performance of P1:PC71BM blend films attributed to their compact and “face-on” molecular orientation, high hole mobility and superior nanoscale morphology, which minimized voltage loss and leading to an efficient charge transfer with low recombination losses in OCSs. In contrast, lower performances of P2 and P3 were mainly originating from their decreased solubilities caused by the insertion of TIPS-Th or TIPS-ThC8, thereby negatively affecting their molecular weights, morphology and charge carrier mobilities. Thus, these results provide a valuable guideline for designing polymers based on 2-TIPSTh substituted BDT units.

Asymmetrical side-chain engineering of small-molecule acceptors enable high-performance nonfullerene organic solar cells

Three new small molecules based on the benzo [1,2-b:4,5-b’]dithiophene (BDT) fused central core with different side-chains, namely DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl, are designed and synthesized to investigate the side-chain effect on the properties of nonfullerene acceptors. DPBDT-4Cl has symmetrical phenylalkyl side-chains on the central BDT unit. In order to narrow the bandgap and reduce the steric hindrance, the phenylalkyl chains are systematically replaced with the flexible electron-donating alkoxy side-chain (POBDT-4Cl) and alkyl side-chain (COBDT-4Cl). As a result, POBDT-4Cl and COBDT-4Cl are characterized with asymmetry-featured side-chains. From DPBDT-4Cl to POBDT-4Cl to COBDT-4Cl, their light absorption abilities, molecular packing behaviors and crystallinity are gradually enhanced. The devices based on these three acceptors all show power conversion efficiencies (PCEs) over 11% with energy loss below 0.55 eV. Compared to DPBDT-4Cl, POBDT-4Cl and COBDT-4Cl obviously exhibit enhanced device performance with improved short-circuit current densities (Jsc) and fill factors (FFs), which mainly ascribe to their reduced charge recombination and enhanced charge transport. In addition, the COBDT-4Cl achieved a high efficiency of 13.5% with a Jsc of 21.8 mA cm−2 and an FF of 0.71. This result is among the best performance obtained from asymmetry-featured small molecules.

Wide band-gap organic molecules containing benzodithiophene and difluoroquinoxaline derivatives for solar cell applications

Two new semiconducting organic small molecules, namely BDTQ-BDT(EH) and BDTQ-BDT(OC), were prepared by attaching electron accepting 2,3-didodecyl-6,7-difluoro-5,8-di(thiophen-2-yl)quinoxaline (DTQ) unit on 2,6-position of electron donating 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(EH)) and 4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (BDT(OC)) units. Molecule BDTQ-BDT(EH) showed higher thermal stability (5% weight loss temperature, Td “349 оC), slightly lower band-gap (Eg “2.10 eV) and deeper highest occupied molecular orbital energy level (HOMO “–5.36 eV) level compared to those (Td “336 оC, Eg “2.11 eV, and HOMO “–5.30 eV, respectively.) of the molecule BDTQ-BDT(OC). The organic solar cells (OSCs) made with the synthesized molecules as an electron donor and [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) as an electron acceptor gave a maximum power conversion efficiency (PCE) of 1.20% and 0.83%, respectively, for BDTQ-BDT(EH) and BDTQ-BDT(OC). This study confirmed that the substituents attached on the 4,8-position of BDT unit greatly alter the properties of the resulting molecules.

Influence of the backbone structure of the donor material and device processing conditions on the photovoltaic properties of small molecular BHJSCs

Small molecule donor materials, ICT18, ICT19 and ICT20 are synthesized to explore the effect of donor material architecture/backbone length and nature of central donor moiety on their thermal, optical, electronic and photovoltaic properties. Compared to ICT18 (ε, 0.83 × 105 mol−1 cm−1), ICT19 (2.26 × 105 mol−1 cm−1) has higher light harvesting ability indicating that higher conjugation length is essential for better exciton generation and is further enhanced by replacing the BDT unit of ICT19 with DTP unit of ICT20 (2.50 × 105 mol−1 cm−1). Thermal stability of these materials is enhanced and their HOMO level is destabilized with an increase in their conjugation length. BHJSCs are fabricated using these materials as donor and PC71BM as acceptors. Under the optimized conditions, BHJSCs with ICT18, ICT19 and ICT20 donors show PCE of 5.69, 6.92 and 8.13%, respectively. ICT19 and ICT20 with large conjugation length and symmetrical A-D1-D2-D1-A architecture show higher hole conductivity over ICT18 with low conjugation length and unsymmetrical D2-D1-A structure. Photo transient experiments are performed to understand the excited state dynamics of the synthesized materials. The enhancement in the PCE is due to higher light harvesting ability of the active layer with efficient and balanced charge transport and effective exciton dissociation at the donor-acceptor interface.

Facile synthesized benzo[1,2-b:4,5-b']difuran based copolymer for both fullerene and non-fullerene organic solar cells

A novel D-A conjugated polymer, PFBT-T, consisting of alkyl substituted benzo[1,2-b:4,5-b']difuran (BDF) as electron donor, alkyl substituted benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) as electron acceptor and thiophene as the π-bridge has been designed and synthesized through facile Stille coupling reaction. Compared with its reported similar polymer based on benzo[1,2-b:4,5-b']thiophene (BDT) unit, PFBT-T has deeper LUMO and HOMO energy level and more intense absorption in 350–550 nm, leading to higher short-circuit current density (Jsc) and open circuit voltages (Voc). Organic solar cells (OSCs) based on PFBT-T with architecture of ITO/PEDOT:PSS/PFBT-T:(PC71BM or IT-M)/PDINO/Al were fabricated. OSCs using PC71BM or IT-M as acceptors respectively obtained the power conversion efficiencies (PCEs) of 5.83% and 5.64%, which both were better than OSCs based on its BDT-based analogues. These results demonstrated that BDF was a promising donor constructing block which may replace BDT unit in some cases.

Side-chain influences on the properties of benzodithiophene-alt-di(thiophen-2-yl)quinoxaline polymers for fullerene-free organic solar cells

Three new benzodithiophene-alt-di(thiophen-2-yl)quinoxaline polymers were prepared with the aim of exploring 5,8-bis(thiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (TFQ)-based polymers on fullerene-free organic solar cells (OSCs). The electron-rich benzo[1,2-b:4,5-bʹ]dithiophene unit contains three different substituents, namely 2,6-bis(trimethyltin)-4,8-bis(2-butyloctyloxy)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTO), 2,6-bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophen-2-yl)benzo[1,2-b:4,5-bʹ]dithiophene (Distannyl BDTT), and 2,6-bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-ʹ]dithiophene (Distannyl BDTSi). These were polymerized with electron deficient 5,8-bis(5-bromothiophen-2-yl)-2,3-didodecyl-6,7-difluoroquinoxaline (Dibromo TFQ) to afford the polymers P(BDTO-TFQ), P(BDTT-TFQ), and P(BDTSi-TFQ). The estimated band-gaps (Egs) and the highest occupied molecular orbital levels (HOMOs) were 1.88, 1.86, and 1.94 eV, and −5.35, −5.44, and −5.58 eV, respectively. The OSCs made with a blend of polymer and ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2ʹ,3′-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene) gave the maximum power-conversion efficiencies (PCEs) of 3.49, 7.06, and 0.75%, respectively. Overall, the substituents attached at the 4- and 8-positions of the BDT unit of the TFQ-based polymers were found to exhibit greatly altered properties, such as their absorption, energy levels, and crystallinity. This resulted in dissimilar photovoltaic performance for the resulting polymers.

Electronic Characters and Synthesis Method of Novel Conjugated System Based on Benzodithiophene Groups

Benzodithiophene based conjugated small molecules (SMBDTs) are usually used in organic photovoltaic (OPV), Organic Filed Effection Transistor (OFET), Organic Phototransistor (OPT) and Non-Linear Optical (NLO) chromophores. Band-gap engineering is one of the key design principles for π-conjugated materials and this can be done by altering the structures of SMBDTs with sidechain and backbone reactions. In this way, scientists develop several kinds of SMBDTs with different electron donors and acceptors. The alkoxyl and aromatic substituted BDT units are mostly used as the donors, while the alkyl cyanoacetate, dicyano, rhodamine, indenedione, thieno[3,4-c]pyrrole-4,6(5H)-dione, benzothiadiazole and diketopyrrolopyrrole groups are used as the acceptors. The electronic characters of SMBDTs including the HOMO and LUMO energy level are listed and discussed. The synthesis methods of SMBDTs are mostly in common, especially with the backbone reaction. There are about four coupling methods for the backbone reaction, mostly used is the Stille coupling methods. In this review paper, the common synthesis methods and the electronic characters by several samples are summarized to provide researchers an overview of SMBDTs’ synthesis, structures and applications.