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In recent years, photocatalytic technology has attracted wide attention in environmental treatment, exploring non-toxic and metal-free photocatalysts is imminent to meet sustainable development. However, semiconductors with wide spectral response are rarely studied and applied in the field of photocatalysis. Herein, a new narrow band-gap polymer PFBDT-DPP (P3) with wide absorption from 500 to 860 nm was synthesized and further constructed heterostructure with g-C3N4 for photocatalytic sterilization and degradation of organic pollutant Rhodamine B (RhB). The optimal antibacterial rate for Escherichia coli reached 99.8% after 190 min of light irradiation and for Staphylococcus aureus reached 96.8% after 120 min of irradiation, and the highest degradation efficiency of RhB by P3/g-C3N4 was 98.9% within 60 min light irradiation, while g-C3N4 displayed an unsatisfactory sterilization and photodegradation performance. This is mainly attributed to the broadened light absorption range and enhanced carrier separation efficiency of P3/g-C3N4. This work could provide a new strategy to fabricate metal-free photocatalysts with high utilization of sunlight and excellent photocatalytic performance.

The employment of an intrinsic quinoidal building block, benzodipyrrolidone (BDP), on constructing conjugated polymers (PBDP-2F and PBDP-2CN) with high electron mobility and unipolar transport characteristic in polyethylenimine ethoxylated (PEIE) modified organic field-effect transistors (OFETs) is reported. The intrinsic quinoidal characteristic and excellent co-planarity of BDP can lower the lowest unoccupied molecular orbital (LUMO) levels and improve ordered interchain packing of the resulting polymers in solid states, which are favorable for electron-injection and transport. By using PEIE as the interlayer to block the hole injection, unipolar n-type transport characteristics with high electron mobility of 0.58 and 1.01 cm2 V−1 s−1 were achieved by the OFETs based on PBDP-2F and PBDP-2CN, respectively. More importantly, the extracted mobilities are highly reliable with the reliability factor of above 80%. To the best of our knowledge, PBDP-2CN is the very first quinoid-based conjugated polymer with reliable electron mobility exceeding 1 cm2 V−1 s−1. This work represents a significant step in exploring intrinsic quinoidal CPs for application in n channel OFETs and logic complementary circuits.

Interpenetrating polymer networks (IPNs) based on triisocyanate-terminated poly(urethane-imide)s (PUIs) were prepared by in situ interpenetrating reactions between modified polyurethane (PU) with different ratios of polyimide (PI). The effects of PU, which was made from hydroxyl-terminated polybutadiene modified with triisocyanate, and the amounts of PI on the mechanical properties, thermal properties, and crystalline character of the IPNs were discussed. Triisocyanate-terminated PUI showed that the highest tensile strength was 38 times that of the diisocyanate-terminated materials. Supramolecular cross-linking from an additional hydrogen-bonding network of modified PU and the degree of interpenetration with a regular imide structure of PI were introduced, which accounted for the remarkable improvement in mechanical properties of IPNs. Preferable thermal stability and glass transition temperature for the hard segment of IPNs were rewarded with increasing PI content. X-ray diffraction revealed vigorous segmental mixing between the soft and hard segments of modified PUI. Scanning electron micrographs showed the "fibrous assembly" morphology and short-range-ordered structure of modified PUI.

For oligothiophene-based donor polymers, the reports of pentathiophene are rare, while other oligothiophenes (bi-, ter-, and quaterthiophene) have been broadly studied. Here, we synthesized one wid...

The D-π-A conjugated polymers with a benzotriazole (BTz) unit as the A moiety have been intensively investigated as donor materials in nonfullerene solar cells. However, these BTz even the fluorinated-BTz constructed D-π-A polymers mostly suffered from upward highest occupied molecular orbital (HOMO) energy levels, leading to inferior open-circuit voltage (VOC) and efficiencies in the fabricated solar cells. Herein, we explored a new approach in response to this issue via the strategy of π-bridge fusion to A moiety. As a result, the medium band gap D-π-A polymer PY2 was evolved into wide band gap D-A polymer PY1 with fused-DTBTz as the new A moiety, accompanied with a greatly declined HOMO energy level by 0.26 eV, a remarkable blue-shifted absorption onset by about 51 nm, and concurrently moderately enhanced face-on stacking orientations in neat polymer and donor/acceptor blend films. The synergetic optimizations in energy level, absorption characteristic and molecular stacking feature via the π-bridge fusion design witness an all-round improvement in photovoltaic parameters including the focused VOC, short-circuit current density (JSC), and fill factor (FF), with narrow band gap ITIC as the acceptor material. Specifically, the PY1-based solar cells produce an optimal power conversion efficiency (PCE) of 12.49%, with superior VOC of 0.94 V, JSC of 18.46 mA cm-2, and FF of 0.72, significantly surpassing those of PY2-based optimal device with a PCE of 7.39%, VOC of 0.77 V, JSC of 14.54 mA cm-2, and FF of 0.66 and even the reported classical fluorinated-BTz based polymer J51 (VOC of 0.82 V, PCE of 9.26%). Promisingly, there is a huge room for improvement in photovoltaic properties with rational fluorination or chlorination of the fused-DTBTz unit or the D moiety of the D-A polymers.

A medium-band-gap donor–acceptor (D–A) polymer (PBDTS-DTBO) was designed and synthesized with alkoxyl-substituted benzo[c][1,2,5]oxadiazole (BO) as A unit and benzo[1,2-b:4,5-b']dithiophene (BDT) derivative (BDTTS) as D moiety. The resulting polymer not only possesses a high number-average molecular weight (Mn) of 53.0 kDa, but also exhibits a deep highest occupied molecular orbital (HOMO) energy level of −5.43 eV, which is in favor of a higher VOC of 0.88 V in the photovoltaic devices. Consequently, due to excellent phase separation and weak bimolecular recombination, the efficiency of the fullerene-based solar cells can reach 8.47% with a high current density (JSC) of 14.93 mA cm−2 and FF of 0.661, which is one of highest values for currently reported benzoxadiazole-based materials. In addition, considering the polymer also shows a medium optical bandgap of 1.75 eV, which can be well complementary with the classical non-fullerene acceptor ITIC. The best performance device exhibits a good efficiency of 6.45% in fullerene-free devices, which could be ascribed to the serious bimolecular recombination. This work revealed that alkoxyl-substituted benzoxadiazole could be a promising moiety to construct efficient light-harvesting polymer.

The instinct feature of π-bridges of donor-π-acceptor (D-π-A) conjugated polymers plays a critical role in tailoring the polymer basic properties as well as the photovoltaic performance. In this work, we designed two new D-π-A polymers PSeBDDIDT and PTzBDDIDT based on donor unit indacenodithiophene (IDT), acceptor unit benzo[1,2-c:4,5-c’]dithiophene-4,8-dione (BDD) with varying π-conjugated bridges of selenophene and thiazole units. Compared to the counterpart polymer PThBDDIDT with thiophene π-bridge, polymers PSeBDDIDT and PTzBDDIDT exhibited enhanced intermolecular interactions, expanded absorption range and better molecular planarity arising from the suppressed steric hindrance between the respective bridge and the adjacent BDD moiety. Conventional polymer solar cells were fabricated with PC71BM as the acceptor. The results witness a greatly improved efficiency for PSeBDDIDT based solar cell compared to PThBDDIDT based counterpart device (7.04%). As a result, the champion PCE of 8.65% was recorded with VOC of 0.899 V, JSC of 16.04 mA cm−2 and FF of 0.60. It is worth highlighting that this is the highest efficiency among all IDT-based polymeric donors reported till now. As a contrast, the devices fabricated from PTzBDDIDT demonstrated a low efficiency of 2.75%, with increased VOC of 0.968 V, JSC of 8.12 mA cm−2 and FF of 0.35. The inferior photocurrent and fill factor of PTzBDDIDT based device was in accord with the poor exciton dissociation probability (38%) and low hole and electron charge transport inside the polymer solar cell. In conclusion, this work demonstrates a state of the art IDT-based polymeric donor with selenophene π-bridge and the importance of rational regulation of π-bridges on polymer properties and photovoltaic efficiency of polymer solar cells.

Most polymer donors developed so far for high-performance polymer solar cells (PSCs) are designed in planar molecular geometries containing benzodithiophene (BDT) units. In this study, three two-dimensional conjugated polymers, PTBFEH-BDD, PTBFSDO-BDD and PTBFSEH-BDD, based on thieno[2,3-f]benzofuran (TBF) building blocks with different side-chains are designed and synthesized. It is found that the TBF unit can inherit and integrate the advantages of both the BDT and benzo[1,2-b:4,5-b′]difuran (BDF) building blocks. Due to the synergistic effects of oxygen and sulfur in the backbone and the optimized side chain, PTBFEH-BDD shows a higher absorption coefficient, more suitable aggregation and improved hole mobility in comparison with PTBFSDO-BDD and PTBFSEH-BDD, which improves the JSC and FF in PSCs. The PSCs based on PTBFEH-BDD:ITIC achieve a power conversion efficiency (PCE) of 11.13% with a JSC of 17.76 mA cm−2, a VOC of 0.893 V, and a FF of 70.16%, which is comparable with its BDT counterparts. Moreover, the PSCs based on these three TBF-based devices do not need any extra post treatments or additives. This study demonstrates that the TBF-based polymers are promising candidates for highly efficient non-fullerene PSCs.

Extending π-conjugation in the benzodithiophene (BDT) side chains has been proven useful to improve the efficiencies of the BDT-based polymer solar cells (PSCs). Herein, combined with a symmetry-breaking strategy of a BDT unit, we further designed a new asymmetric 1D–2D (one dimensional–two dimensional) monomer asy-BDTBP with an alkoxyl group as the 1D part and a π-extending alkoxybiphenyl as the 2D substituted group. Medium band-gap donor–acceptor (D–A) conjugated polymer P1 was synthesized with asy-BDTBP and 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) as the donor and acceptor unit, respectively. Encouragingly, P1 blended with PC71BM exhibited an obviously enhanced power conversion efficiency (PCE) compared to the reported symmetric analogue PBDTBP–DTffBT (6.70%). The PCE increased to 8.45% with an open-circuit voltage (VOC) of 0.838 V, a short-circuit current density (JSC) of 14.35 mA cm−2 and a fill factor (FF) of 70.27%. However, P1 coupled with a classical non-fullerene acceptor ITIC revealed a relatively poor efficiency of 6.35% due to the bad complementarity of absorption spectra. To match the absorption of ITIC, a wide band-gap D–A polymer P2 was further designed with a weak electron-withdrawing group benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) instead of DTffBT as the acceptor unit. As a result, P2 possessed a complementary absorption spectrum with ITIC, and the resulting devices presented an excellent photovoltaic performance. The optimal efficiency boosted to 10.04% with VOC of 0.873 V, JSC of 17.60 mA cm−2 and FF of 65.37%. This work demonstrates the great potential of asymmetric BDTs for high efficient PSCs and the importance of the rational design of polymers for different types of PSCs.

In this work, we reported efficient polymer solar cells with balanced hole/electron mobilities tuned by the acceptor content in bulk heterojunction blend films. The photovoltaic cells were fabricated with two new wide band-gap D-A polymers PBDDIDT and PBDDIDTT as the donor material. The molecular conformations of new polymers are carefully evaluated by theoretical calculations. The results of photovoltaic studies show that two devices reach their optimal conditions with rich PC 71 BM content up to 80% in blend films, which is uncommon with most of reported PSCs. The as-cast devices based on PBDDIDT and PBDDIDTT reveal good photovoltaic performance with PCE of 7.04% and 6.40%, respectively. The influence of PC 71 BM content on photovoltaic properties is further detailed studied by photoluminescence emission spectra, charge mobilities and heterojunction morphology. The results exhibit that more efficient charge transport between donor and acceptor occurs in rich PC 71 BM blend films. Meanwhile, the hole and electron mobilities are simultaneously enhanced and afford a good balance in rich PC 71 BM blend films (D/A, 1:4) which is critical for the improvement of current density and fill factors.

The recent evolution of non-fullerene acceptors yields brilliant progress for organic solar cells (OSCs). However, high-performance copolymer donors with simple chemical structures for new acceptors are a few. In this work, a series of donors based on BDT are developed with ester-substituted trithiophene as a repeating acceptor unit. Copolymerized polymers exhibit slightly red-shifted absorption and down-shifted the highest occupied molecular orbital (HOMO) as the side chains turning from alkyl (3TCO-R) and oxyalkyl (3TCO-OR) to thienyl (3TCO-Th) and chlorine-substituted thienyl (3TCO-Th-Cl), respectively. As a result, the corresponding power conversion efficiency (PCE) of 3TCO-Th-Cl based devices shows the most excellent performance (PCE = 7.70%) especially in open-circuit voltage (VOC = 1 V) when blending with ITIC and its PCE makes a breakthrough with Y6 (PCE = 10.30%). Moreover, the fabricated semitransparent OSCs exhibit a rational PCE of 8.16% with average visible transparency of 16.40%.

In this paper, to study the two-dimensional (2D) conjugated side chain effect on the polymer donor for the non-fullerene polymer solar cells (NFPSCs), the thiophenothiophene (TT) side-chained benzodithiophene (BDT) and bithiophene copolymer (PBDTDT-TT) has been prepared by the Stille coupling reaction and compared with the copolymer (PBDTDT-O) with an alkoxy side group and the same polymer backbone. The 2D conjugated polymer PBDTDT-TT exhibits better thermal stability and wider and red-shifted ultraviolet–visible absorption spectra. These two copolymers were used as the donor materials blended with a new non-fullerene receptor ITM, successfully fabricating the positive devices with an ITO/PEDOT:PSS/polymer:ITM/PFN-Br/Al structure. Compared with the PBDTDT-O:ITM blend film, the PBDTDT-TT:ITM blend film shows a lower LUMO energy level, a narrower optical band gap, and a higher electron and hole mobility in the devices. The best power conversion efficiency (PCE) is 8.43% for the PBDTDT-TT polymer, while the ...

Here we design and synthesize one novel fluorene side-chained benzodithiophene (BDT) monomer for polymer solar cells (PSCs) donor. By copolymerizing this monomer with 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) or 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT), two donor–acceptor (D–A) conjugated polymers PFBDT–DTBT and PFBDT–DTffBT are prepared. PSCs are prepared with these polymers as donor and PC71BM as acceptor. The maximum power conversion efficiency (PCE) of the two polymers PFBDT–DTBT and PFBDT–DTffBT based PSCs is 7.13% (VOC = 0.90 V, JSC = 13.26 mA cm–2, and FF = 0.598) and 7.33% (VOC = 0.96 V, JSC = 13.24 mA cm–2, and FF = 0.577). The UV–vis absorption and electrochemical cyclic voltammetry test results show that F atoms in DTffBT unit present an obvious influence on intermolecular effect and molecular energy levels of polymers. Furthermore, the energy loss of two PSCs devices in this work is confirmed to be 0.78 and 0.71 eV, lower than most results based...

The micro-structure ordering of the π–conjugated polymer film self-assembling from solution governs its electrical characteristics and performances for organic electronics. The side chains appended on the conjugated backbone could impact the self-assembling characteristics of the polymer, which show us how to design novel substituents to optimize the polymer arrangement and packing in its solid state. Here, cyclic alkyl chains are proven to be excellent ones for designing of narrow bandgap polymers, the design feature of which with preferential conformations can promote the polymer self-assembly and result in higher degree of lamellar order as well as tighter lamellar packing in the active layer, comparing with linear alkyl chains. The linear ones have poorer ordering in the polymer:fullerene blends as well as in the single crystals of the monomers. The well-organized polymer micro-structures facilitate polymer:fullerene phase separation together with balanced hole/electron transporting, and ultimately, improve the power conversion efficiency impressively compared to linear chains.

Four dithienoindaceno[1,2-b:5,6-b′]dithiophene (DT-IDT) based small molecules IT-O1, IT-O2, IT-O3 and IT-O4 with increasing alkoxyl chain length from methoxy to butoxy on the terminal-groups were synthesized to investigate the end-group side-chain effects on these acceptor–donor–acceptor-type small molecule electron acceptors. The optical absorption and energy levels of the four molecules, blend morphologies, carrier mobilities, and photovoltaic performances of the devices blended with the polymer PBDB-T are systematically investigated. Interestingly, both the solubility and electron mobility are enhanced for these materials with decrease in side chain length, which lead to ideal morphologies and balanced charge transport. Hence, increased Jsc and FF, and thus distinctly higher PCEs of 11.6% were obtained from the PBDB-T:IT-O1-based devices.

Two novel copolymer donors based on one-dimensional (1D)–two-dimensional (2D) asymmetrical benzodithiophene (BDT) units (BDTPH-H and BDTPH-OR) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (DTQx) were synthesized and compared with one-dimensional or two-dimensional symmetric BDT unit based photovoltaic polymers. Both asymmetrical polymers exhibited promising photovoltaic performance compared with 1D and 2D symmetric BDT polymers. The power conversion efficiency (PCE) of PBDTPH-DTQx based polymer solar cells is 5.6%, with balanced VOC = 0.7 V, JSC = 11.89 mA cm−2 and FF = 67.3%, which is almost the highest PCE compared with similar BDT unit and fluorine-free substituted DTQx based polymer. What is more, PBDTPH-DTQx shows better performance than PBDTPHO-DTQx due to the single phenyl making DTQx more planar and with better π–π stacking than alkoxy phenyl. These findings demonstrate that the 1D–2D asymmetrical BDT units are also applicable to poor planarity fluorine-free substituted quinoxaline acceptor system.

To study which strategy (extending side chain π-conjugation with single-bonded or fused aromatic rings) is more effective in improving the photovoltaic properties of benzodithiophene (BDT)-based polymers, naphthyl (NP) and biphenyl (BP) units were introduced to the BDT core as conjugated side chains. Two polymers PBDTNP-DTBO and PBDTBP-DTBO based on NP or BP-substituted BDT and 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]oxadiazole (DTBO) were designed. These two polymers only exhibited small differences in the chemical structure, but great differences in photovoltaic performance. PBDTNP-DTBO showed a high power conversion efficiency (PCE) of 8.79%, with an open-circuit voltage (Voc) of 0.82 V, a short-circuit current density (Jsc) of 14.34 mA cm−2, and a fill factor (FF) of 74.73%, while PBDTBP-DTBO only showed a maximum PCE of 6.69%, with a Voc of 0.75 V, a Jsc of 13.55 mA cm−2, and a FF of 65.86%. Hence, extending the side chain π-conjugation system of a BDT-based polymer with naphthyl is a more effective method to enhance the photovoltaic properties.

A series of conjugated polymers based on PFS derivatives with π-conjugated 5-(9H-fluoren-2-yl)-2,2′-bithiophene (fluorene-alt-bithiophene) backbones, namely PFS-3C, PFS-4C and PFS-6C, were synthesized for their use as the anode interfacial layers (AILs) in the efficient fullerene-free polymer solar cells (PSCs). Alkyl sulfonate pendants with different lengths of alkyl side chains were introduced in the three polymers in order to investigate the effect of the alkyl chain length on the anode modification. The obtained three polymers exhibited similar absorption bands and energy levels, indicating that changing the length of the alkyl side chains did not affect the optoelectronic properties of the conjugated polymers. Based on the PBDB-T:ITIC active layer, we fabricated the fullerene-free PSCs using the three polymers as the AILs. The superior performance of the fullerene-free PSC device was achieved when PFS-4C was used as the AIL, showing a power conversion efficiency (PCE) of 10.54%. The high performance of the PFS-4C-modified device could be ascribed to the high transmittance, suitable work-function (WF) and smooth surface of PFS-4C. To the best of our knowledge, the PCE obtained in the PFS-4C-modified device is among the highest PCE values in the fullerene-free PSCs at present. These results demonstrate that the PFS derivatives are promising candidates in serving as the AIL materials for high-performance fullerene-free PSCs.

In this work, four wide-bandgap (WBG) conjugated polymers based on asymmetrical building block thieno [2,3f]benzofuran (TBF) and fluorobenzotriazole (BZ), are synthesized and applied as donor materials in polymer solar cells (PSCs). The effect of conjugated side chains (alkylthiothiophenyl, alkylthiophenyl, alkylphenyl and fluoroalkoxyphenyl) on the optical absorption, electrochemical and photovoltaic properties are investigated in detail. These polymers exhibit similar properties, strong absorption from 300 to 650 nm with wide bandgaps ranging from 1.87 to 1.96 eV, excellent thermal stability, the high and balanced carrier mobility. Notably, the open-circuit voltages (VOC) of PTSDO-BZ increases by 0.16 V compared with PTDO-BZ. Similarly, larger VOC changes have also occurred in PPEH-BZ and PPFOEH-BZ comparation. The enhanced VOC can be ascribed to the low-lying highest occupied molecular orbital (HOMO) energy level by incorporating sulfur and fluorine substituents on the conjugated side chains. The as-cast devices of PTSDO-BZ:ITIC exhibit the highest power conversion efficiency (PCE) of 8.66%, with a VOC of 0.88 V, a short-circuit current density (JSC) of 15.06 mA cm−2, and a fill factor (FF) of 65.12%. The results indicate that the modification of the side chains of the TBF unit can effectively lower the HOMO energy levels and thereby obtaining high VOC.

Herein, a series of random donor copolymers were reported, by incorporating different contents of electron-rich 4,8-bis(9,9-bis(2-ethylhexyl)-9H-fluoren-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDTF) units into PBDB-T (poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione))]) polymer to fine-tune the highest occupied molecular orbital (HOMO) energy level. As we expected, downshifted HOMO of corresponding copolymers was observed with increased the ratios of BDTF unit on PBDB-T polymer, causing by the synergy of weak electron-donating ability of the fluorene side chains and the enhanced dihedral angles. Notably, compared with PBDB-T, the HOMO energy level of P91 containing 10% BDTF was shifted down by 0.07 eV. Among them, the devices based on P55:ITIC reached a high open-circuit voltage (VOC) of 1.00 V with incorporation of 50% of BDTF units. The result indicated that the BDTF unit, as an excellent building unit in donor materials, could play a crucial role to realize high-VOC polymer solar cells.

In this work, one novel asymmetric non-fullerene acceptor ITIC-3T was designed and synthesized. ITIC-3T molecule shows 1.58 eV Eg, with −3.95 eV LUMO level. Utilizing P3 (one asymmetric 2D conjugated benzodithiophene polymer) as donor material, bulk heterojunction solar cells were fabricated. The optimal device yields a PCE of 6.47% with a VOC of 0.64 V, a FF of 55.5% and a high JSC of 18.18 mA cm−2, which may be attributed to the complementary optical absorption of active blend (P3 and ITIC-3T). In general, this work demonstrates that asymmetric polymer donor and non-fullerene acceptor active layer blend is potential for fabricating efficient photovoltaic cells.

It is known that fluorination on π-conjugated donor-acceptor (D-A) polymers can significantly affect the optoelectronic properties and fluorination on A moiety has been well established for design of efficient photovoltaic materials. For example, polymers based on 4,7-dithienyl-5,6-difluorobenzothiadiazole (DTffBT) have been intensively investigated and exhibited excellent performance, but the corresponding DTBT-based polymers without fluorine often display an unfavorable efficiency. With the purpose of improving photovoltaic efficiency of DTBT-based D-A polymers, we design three polymers PDTBT-TxfBT (x = 0, 1, 2) with fluorination on D moiety (TxfBT) and systematically investigate fluorination on the photophysical/electrochemical and photovoltaic properties. The results show that polymer solar cells (PSCs) based on PDTBT-TBT exhibit moderate power conversion efficiency (PCE) of 5.84%. However, the bis-fluorination on TffBT moiety (PDTBT-TffBT) can greatly enhance the molecular planarity and intermolecular interaction, improve the charge transport and heterojunction morphology, and further suppress the charge recombination losses. PSCs based on PDTBT-TffBT demonstrate obviously improved photovoltaic efficiency with the best PCE up to 7.53% without any processing additives, which ranks among the top DTBT-based PSCs. However, it should be noted that unsymmetrical fluorination on TfBT moiety (PDTBT-TfBT) impairs the regularity of polymer backbone and intermolecular interaction, increases the recombination losses, and seriously reduces the short-circuit current density and efficiency (5.44%). The results exhibit that fluorination on D moiety is a helpful strategy for design high-performance photovoltaic materials and the regularity of fluorination is crucial to improving efficiencies.

In this paper, a novel donor based on alkylthiophenyl side-chained benzodithiophene (BDT) (M1) was first designed and synthesized to construct a series of conjugated polymers (P1, P2, P3) used in polymer solar cells (PSCs). P1, P2, and P3 were prepared by copolymerizing M1 with 4,7-di(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothiadiazole (DTBT), quinoxaline, and diketopyrrolopyrrole (DPP), respectively. The optical electrochemical and photovoltaic properties of these polymers were investigated. Alkylthiophenyl substitution on the BDT building block is favorable for the low-lying highest occupied orbital (HOMO) level of polymers. The HOMO of these copolymers is clearly lowered to −5.49 eV, −5.34 eV, and −5.50 eV, respectively. Compared with reference polymers, our polymers show lowered HOMO energy levels (by 0.14 eV at least). This is very helpful to enhance the open-circuit voltage (Voc) of the devices. Further, P2:PC61BM exhibited an optimized power conversion efficiency (PCE) of 4.28 %, which is clearly higher than that of P1 (1.48 %) and P3 (1.9 %).

A diketopyrrolopyrrole (DPP)-based π-conjugated polymer is a very promising low band gap electron donor material for polymer solar cells. We have incorporated alkyl groups into the 4-position of the thiophene rings connected to the DPP fragment, which is proven to be beneficial for improving the open-circuit voltage (VOC) and short-circuit current density (JSC). Two DPP-based polymers are synthesized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron-donating segment. Both the polymers show good solubility, slightly wide optical band gaps and deep HOMO energy levels when incorporating the alkyl groups. Heterojunction solar cells are fabricated with polymer:PC71BM as the active layer. VOC and JSC are simultaneously enhanced compared to the device performance of traditional DPP-BDT alternating polymers. A power conversion efficiency (PCE) of 8.11% was obtained, which indicates that rational utilization of backbone torsion is a promising strategy to improve the photovoltaic performance.

Two wide band gap (WBG) polymers based on thiophene-thiazolothiazole (TTz) units, PBT-TTz and PBT-S-TTz, were synthesized. Both polymers showed absorption onsets at 635 nm in solid films. Although PBT-TTz and PBT-S-TTz are WBG materials with relatively narrow absorption spectra, they have great potential for constructing high-performance polymer solar cells (PSCs). By replacing the alkyl side chain of PBT-TTz with an alkylthiol side chain, the HOMO level of PBT-S-TTz was lowered to −5.45 eV. A PCE of 7.92% was then obtained in a single-junction PSC device based on a PBT-S-TTz:PC71BM active layer. Moreover, high-performance fullerene-free PSCs were fabricated using these polymers and a high PCE of 8.22% was achieved. This work demonstrates that TTz-based polymers PBT-TTz and PBT-S-TTz are promising candidates as efficient WBG polymers for constructing high-performance PSC devices.

A new monomer dithieno-[3′,2′:3,4;2′′,3′′:5,6]benzo[1,2-c]xadiazole (fDTBO) is first used as an electron-deficient acceptor to build D–A copolymers in a photovoltaic field. Two polymers PBDTT-fDTBO and PBDTO-fDTBO consist of fDTBO with thienyl-substituted-benzodithiophene (BDTT) or alkoxy-substituted benzodithiophene (BDTO). Both polymers show a deep HOMO around −5.5 eV with a wide band gap of over 1.9 eV. The polymer solar cells (PSCs) based on two polymers both show over 1 V high open circuit voltage (Voc) independent of polymer/PCBM ratios and solvent additives content in the PSCs active layer. The power conversion efficiency (PCE) based on PBDTT-fDTBO devices is 4.5% for single junction PSCs, and these polymers can be applied in tandem PSCs due to their wide band gap (up to 1.99 eV). This work demonstrates that the fDTBO unit is a promising building block to design wide band gap photovoltaic polymers with high Voc.

The conformational change of the polymer backbone has an obvious influence on the blended film morphology and the photovoltaic properties. A planar polymer backbone can provide strong intermolecular interaction, but excessive self-aggregation usually disadvantages a favorable film morphology. Here, a dithienothiophene (DTT) unit and a terthiophene (T3) unit were introduced to construct a pair of new conjugated polymers, PDTT-DTBT and PT3-DTBT, respectively, to investigate the correlation between the conformation of the polymer backbone and the photovoltaic properties. The highly planar structure of the DTT unit made PDTT-DTBT exhibit a stronger molecular aggregation than PT3-DTBT. Compared to the DTT unit, the T3 unit changed the molecular structure from fused thiophenes to linked ones, which relaxed the rigidity of the polymer backbone and reduced the degree of polymer self-aggregation, and a more suitable blended film morphology was formed. Due to the weaker electron-donating ability of the DTT unit, PDTT-DTBT/PC71BM-based devices showed a higher open circuit voltage (Voc) of 0.75 V, and a power conversion efficiency (PCE) of 8.01% was obtained with a short-circuit current (Jsc) of 14.85 mA cm−2 and a filled factor (FF) of 71.62%. This PCE value was slightly higher than for PT3-DTBT/PC71BM-based devices, which exhibited a PCE of 7.68% (along with a Voc of 0.66 V, a Jsc of 16.62 mA cm−2 and a FF of 69.69%).

In order to investigate the effect of an (E)-1,2-di(thiophen-2-yl)ethene (TVT) unit on the hole mobility and photovoltaic properties of dithienyl-difluorobenzothiadiazole (DTBT) based polymers, two conjugated polymers PDT-DTBT-DT (thiophene backboned) and PTVT-DTBT-DT (TVT backboned) were synthesized. Compared to PDT-DTBT-DT, the backbone conformation of PTVT-DTBT-DT could be well modulated by the TVT unit, leading to an extended conjugation length and strengthened intermolecular interaction. Interestingly, it's found that the ultraviolet-visible (UV-vis) absorption peaks of the PTVT-DTBT-DT film was blue-shifted compared to that of the solution. The organic field-effect transistor (OFETs) characterization showed that PTVT-DTBT-DT possessed a high hole mobility of 0.12 cm2 V−1 s−1, which was higher than that of the counterpart PDT-DTBT-DT (0.04 cm2 V−1 s−1). Through simplified device optimization without any additives and annealing treatment, a power conversion efficiency (PCE) of 7.86% was achieved for PTVT-DTBT-DT with a short-circuit current density (Jsc) of 16.33 mA cm−2 and a fill factor (FF) of 68.92%, which is higher the PCE of 7.29% of PDT-DTBT-DT with a Jsc of 15.60 mA cm−2 and a FF of 66.62%. The PCE of 7.86% is among the highest PCEs reported for devices fabricated without any additives and thermal annealing treatment. The results revealed that PTVT-DTBT-DT as an ideal conjugated polymer could provide a greater possibility for the commercial application of PSCs, especially in terms of low cost and manufacturing convenience.

Thiadiazolo[3,4-c]pyridine (PT) has great potential in the construction of high-performance narrow bandgap (NBG) photovoltaic polymers. But to date the best power conversion efficiencies (PCEs) for PT-containing polymers are only around 6%. Herein, we report two PT-containing NBG polymers PDTPT-2T and PDTPT-2TF based on 2,2′-bithiophene (2T) and 3,3′-difluoro-2,2′-bithiophene (2TF), respectively. The effects of a fluorine substituent on optoelectronic properties are thoroughly investigated. The film absorption onset of PDTPT-2TF is 855 nm, bathochromic-shifted by 24 nm in comparison with that (831 nm) of PDTPT-2T. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of PDTPT-2TF are down-shifted by 0.15 and 0.11 eV relative to those of PDTPT-2T, respectively. X-ray diffraction (XRD) patterns indicate that a more ordered structure is formed in the solid film of PDTPT-2TF. Furthermore, the miscibility between the polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is significantly improved through the introduction of fluorine. Consequently, PDTPT-2TF exhibits a high PCE of 8.01%, while PDTPT-2T only shows a maximum PCE of 2.65%. The efficiency of 8.01% is the highest one for PT-containing polymers, and more importantly, it is achieved without any processing additives or post-treatments. This work indicates that PT would have great potential as a building block to construct high-performance photovoltaic polymers.

Two novel donor–π–acceptor (D–π–A) type polymers based on indacenodithiophene (IDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) with thiophene (PIDT-t-TPD) or thieno[3,2-b]thiophene (PIDT-tt-TPD) as π-bridges were designed and synthesized. The π-conjugated backbone length in the repeating unit of the polymer chain was varied by different π-bridges. Both polymers exhibit planar molecular structures, similar broad optical band gaps (Eoptg) and low-lying highest occupied molecular orbital (HOMO) energy levels. However, PIDT-tt-TPD with the more extended π-conjugated backbone shows higher hole mobility and better miscibility with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM). Polymer solar cells (PSCs) based on PIDT-tt-TPD:PC71BM show the highest power conversion efficiency (PCE) of 5.23% with a short-circuit current density (Jsc) of 11.22 mA cm−2. Considering the limited absorption of PIDT-tt-TPD (400–650 nm), the Jsc value is very impressive. In contrast, PIDT-t-TPD: PC71BM only shows a PCE of 2.47% with a low Jsc of 5.52 mA cm−2. The lower Jsc value of PIDT-t-TPD is consistent with its limited external quantum efficiency (EQE) response (lower than 40%). The present results demonstrate that extending the length of a π-conjugated backbone by an appropriate π-bridge in the D–A polymer improves the photovoltaic performance of PSCs.

Recently, we have used terthiophene side chain to modify benzo[1,2-b:4,5-b']dithiophene (BDT) to form novel building block for BDT polymers. In this paper, this building block is used to copolymerized with thieno[3,4-c]pyrrole-4,6-dione (TPD) and thieno[3,4-b]thiophene (TT). This building block and TPD- or TT-based polymers (P1 and P3) show high open circuit voltage (V-OC) (ca. 0.9-0.95 V) and low energy loss (E-g-eV(OC)) in solar cells devices compared with similar polymers without bulky side chain. We further introduce thiophene pi bridge into these polymers backbone to form two other polymers (P2 and P4). We find this thiophene pi bridge does contribute to this bulky side chained benzodithiophene polymer photovoltaic performances, especially for power conversion efficiencies (PCEs). The polymer solar cells (PSCs) performances are moderate in this article due to the serious aggregation in the PSCs active layer. (C) 2015 Wiley Periodicals, Inc.

In this work, two novel thieno[2,3-f]benzofuran (TBF) unit, with fluorinated alkoxyphenyl or alkylphenyl side chain, were synthesized and characterized. Based on these TBF donor unit and benzodithiophene-4,8-dione (BDD) acceptor, two novel polymers, PTBFPF-BDD and PTBFP-BDD, were prepared by Stille coupling reaction. The conjugated side chain effects on the thermogravimetric, optical, electrochemical, and photovoltaic properties of these polymers were systematically investigated. Both polymers exhibited desirable power conversion efficiency (PCE) of more than 8% after thermal annealing. PTBFP-BDD:ITIC-based devices achieved higher PCE of 8.50% with an open-circuit voltage (VOC) of 0.88 V, a short-circuit current density (JSC) of 16.01 mA/cm2 and a fill factor (FF) of 59.80%. In addition, the devices based on PTBFPF-BDD:ITIC showed an enhanced VOC of 0.92 V due to down-shifted the highest occupied molecular orbital (HOMO) energy levels. The results indicate that the alkylphenyl/alkoxyphenyl substitution of TBF donor units is promising strategy for improving PCE of PSCs.

The 2D asymmetric benzodithiophene (BDT) unit is used as a donor unit to construct one new polymer PBDTBDD-Th with benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDD) as acceptor building block. In comparison to the polymer PBDTsTh-BDD with a side chain containing a sulfur atom, the devices based on PBDTBDD-Th/ITIC show better performance due to the introduction of carbon atoms in the side chain, which could weaken the self-aggregations of polymer chains. As a result, the devices based on PBDTBDD-Th/ITIC blends yield power conversion efficiencies (PCEs) over 10%, much higher than those based on PBDTsTh-BDD/ITIC blends (7.09%). The exciton dissociation probabilities (P diss ) of a device based on PBDTBDD-Th/ITIC blends is 95.3%, which suggests that the device achieves good exciton dissociation and charge transfer. In general, the polymer PBDTBDD-Th shows capability to increase the PCEs of polymer solar cells (PSCs) with a non-fullerene acceptor.

In this article, pendent thiophene (2-butyl-5-octylthiophene) side chain is used to modify the backbone of the polymers containing benzo[1,2-b:4,5-b]dithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD). Compared with the dodecyloxy side-chained polymer (P1), pendent thiophene-based polymers (P2 and P3) show similar number-average molecular weight (M-n), polydispersity index, thermal stability (T-d approximate to 334-337 degrees C), and optical band gaps) (approximate to 1.8 eV). Polymer (P2)-based BDT with pendent thiophene and ethylhexyl-modified TPD shows relatively low-lying HOMO energy level (-5.52 eV) and nearly 1 V high open circuit voltage (V-OC). The polymer solar cell devices based on three copolymers show power conversion efficiencies from 2.01% to 4.13%. The hole mobility of these polymers tested by space charge limited current method range from 3.4 x 10(-4) to 9.2 x 10(-4) cm(2)V(-1)s(-1). (c) 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1558-1566

In this contribution, a novel cathode interlayer material (NDIO) based on naphthalene diimide was successfully prepared by a facile two-step reaction from commercially available compounds. NDIO exhibited excellent water solubility, high transparency in the visible light region, and well-matched molecular energy levels. By incorporating this novel water processed cathode interlayer, a high power conversion efficiency of 9.51% was recorded in PBDT-TS1/PC71BM-based polymer photovoltaic cells (PPCs), and the value is nearly 2-fold the device efficiency of PPCs without the cathode interlayer. More importantly, the insertion of the NDIO interlayer promotes the device efficiency of polymer/polymer photovoltaic cells based on PBDTTT-EFT/N2200 from 3.23% up to 5.77%. The successful applications in both polymer/PCBM and polymer/polymer blend-based inverted PPCs make NDIO a promising cathode interlayer for realizing aqueous processed polymer photovoltaic cells with high performance.

A novel benzodithiophene-substituted benzodithiophene (crossed-BDT) with an enlarged conjugated skeleton was designed and synthesized as an electron-rich unit for the construction of donor-acceptor copolymers. Different acceptors, including 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (DTBT) and 4,7-di(thiophen-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) were used as electron-deficient units for the target copolymers to investigate the effect of the inclusion of two fluorine atoms on the acceptor unit of the polymer. The crossed-BDT and DTBT based polymers show high open circuit voltages (V-oc) of over 0.9 V with the decent power conversion efficiency (PCE) of 3.74%. The crossed-BDT and DTffBT based polymers show nearly 1 V (0.95 V) V-oc due to the fluorine induced low-lying HOMO energy level. The 0.9-0.95 V is an impressive V-oc result for a 1.7 eV band gap (E-g) polymer. To the best of our knowledge, this is the first report about a crossed conjugated benzodithiophene building block design, and this building block is effective in improving the V-oc of low band gap DBT polymers.

Dopant-free poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b] thiophene)-2,6-diyl] (PBDTTT-C) polymer is used as hole transporting material (HTM) in electron transporting material (ETM) free planar perovskite solar cells (PSCs). The devices with a PBDTTT-C HTM show higher power conversion efficiency (PCE = 9.95%) than the devices with a P3HT HTM (PCE = 6.17%) with enhanced short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) in a simple device configuration (ITO/CH3NH3PbI3/PBDTTT-C/MoO3/Ag), due to the suitable energy level, better carrier mobility and lower interfacial charge recombination.

In this work, a hexylthiophene substituted thiophene-bridge is used to connect benzodithiophene (BDT) and benzothiadiazole (BT) segments to build a new photovoltaic polymer (PBDT-DTTBT). The highest occupied molecular orbital (HOMO) level of the polymer is decreased by 0.2 eV to −5.47 eV compared to the reference polymer without a hexylthiophene side chain, which may be ascribed to the conformational torsion caused by the steric hindrance from the bulky hexylthiophene side group. The band gap of the polymer shows no obvious change compared to a polymer with a same backbone without a hexylthiophene side chain (around 1.7 eV). Polymer solar cells (PSCs) based on PBDT-DTTBT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency (PCE) of 6.19% with an open-circuit voltage (Voc) of 0.80 V, a short-circuit current density (Jsc) of 12.72 mA cm−2 and a fill factor (FF) of 60.97%. The work provides a new method to design new conjugated polymers with a deep HOMO level and a low band gap for high Voc PSCs.

Power conversion efficiencies (PCEs) of state-of-the-art polymer solar cells (PSCs) have been promoted to over 9%. However, halogenated solvents like chlorobenzene (CB), o-dichlorobenzene (DCB), 1,8-diiodooctane (DIO) or their mixtures are still predominately used in the fabrication of these high performance PSCs. With the rapid progress in PCEs, removing the halogenated solvents from the fabrication processes of PSCs becomes an urgent task for the practical utilization of PSC technology. In this study, a halogen-free solvent system consisting of o-xylene (XY) and N-methylpyrrolidone (NMP) was successfully applied in the fabrication of the PSCs based on a variety of highly efficient polymers including PBDT-TS1 and other eight types of photovoltaic polymers. Notably, utilizing an XY–2% NMP mixture as a processing solvent, the PBDT-TS1/PC71BM-based PSC realized a PCE of 9.47%, which has been the highest value in halogen-free solvent processed PSCs until now. The photovoltaic properties and nanoscale morphology clearly indicated that the halogen-free solvent system featuring the XY–NMP mixture can replace the role of the widely utilized halogenated solvents in fabricating environmentally friendly PSCs with high efficiency.

Taking into account the effect of planarity, alkyl chain steric hindrance and molecular weight towards high performance polymer solar cells, a planar semiconducting polymer PTOBDTDTffBT is designed. Minimal intra- and inter-molecular steric hindrances are realized by removing the alkyl chains at the 4,4'-positions of the DTffBT monomer and introducing shorter octyloxy chains on the BDT unit, respectively. Such a steric minimization strategy endows PTOBDTDTffBT with a high number average molecular weight of 343.37 kg mol(-1), a planar conjugated backbone and strong inter-molecular aggregation characteristics as well. The optical properties indicate that the aggregation can only be partially broken even at 160 C; theoretical calculations indicate that the polymer backbone has small distortion and the thiophene-phenyl bridge can move the octyloxy chains outward by 6.77 A from the polymer backbone, both of which ensure the strong inter-chain aggregation. In spite of the high planarity and strong aggregation of the polymer, PTOBDTDTffBT is relatively amorphous in its film form, indicated by grazing incidence X-ray diffraction analysis. PTOBDTDTffBT exhibits a narrow bandgap of 1.71 eV together with a high ionization potential of 5.46 eV. Because of the strong aggregation of the polymer, the PTOBDTDTffBT/PC71BM active layer exhibits temperature-dependent photovoltaic performance. When the active layer is spin-coated at a relatively low temperature below 100 degrees C, PTOBDTDTffBT/PC71BM exhibits a stable power conversion efficiency above 6.6%. However, when the temperature is elevated from 120 degrees C to 160 degrees C, the power conversion efficiency decreased gradually to 3.58% because of the decreased short-circuit current density and fill factor. The temperature-dependent photovoltaic performance is explained by the fact that the PC71BM is easily spun out from the solution at higher temperatures as confirmed by the UV-vis absorption and X-ray photoelectron spectroscopy analysis. High temperatures can also lead to coarse morphology and large phase separation in the active layer observed from the atomic force microscopy and transmission electron microscopy images. Finally, a maximum power conversion efficiency of 7.68% is obtained when the active layer is spin-coated at 80 degrees C, which is a significant improvement of the power conversion efficiency for the planar PBDTDTffBT series.

Two benzodithiophene (BDT)-based homopolymers which have different mole ratios of poly-ether side chain substitute were synthesized by Stille coupling reaction. The polymers show decomposition temperature (T d) around 317 °C and optical band gap around 2.2 eV. Solar cell devices with bulk heterojunction structure and field-effect transistors devices were fabricated to evaluate the photovoltaic properties of resultant polymers. Solar cell devices based on the polymer with 100 % poly-ether side chain (P1) show low power conversion efficiencies (PCEs) of 0.71 % resulting from the poor morphology of active layer which has rough surface and fairly large domain size due to the high aggregation tendency of P1:PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) blend thin film as active layer in the structure of devices. Polymer with alternating poly-ether and alkoxy chained BDT (P2) and PCBM blend film shows smooth surface and appropriate domain size, which help to enhance the hole transportation and photovoltaic performances. The PCEs of the devices based on P2 reached 2.00 % which is a decent result for BDT-based homopolymer donor with relatively large band gap (ca. 2.2 eV). These two polymers exhibited mobilities of 3.95 × 10−4 and 6.18 × 10−4 cm2/Vs in field-effect transistors, respectively.

The beta, beta'-positions carbonyl or dicarbonyl bridged dithiophenes were prepared and electrochemically polymerized into corresponding polymers, polycyclopenta[2,1-b;3,4-b']dithiophen-4-one (PCDK) or polybenzo[1,2-b:6,5-b']dithiophene-4,5-dione (PCD2K), in boron trifluoride diethyl etherate based electrolyte. Potentiostatic and potentiodynamic method's confirmed that the PCDK and PCD2K films can be successfully electrodeposited in high quality with comparable intermolecular spacings at around 3.1 angstrom-3.2 angstrom. The mass changes and ion transport of the two polymers during redox processes were studied by electrochemical quartz crystal microbalance and in situ spectroelectrochemical methods, to investigate the formation of the two polymers. In situ UV-visible spectroscopy evidenced the PCDK and PCD2K evolved into polarons in the near infrared region under applied potentials. Electrochromic behaviors showed that the PCDK film appeared reddish brown in neutral state and cyan or green in oxidized state while the PCD2K showed green to dark green from neutral to oxidized states. Potential switching between oxidized and neutral states of the two films revealed that maxima transmittance changes were 38.23% and 36.64% for the PCDK and PCD2K. Thermal degradation of the two polymers showed their robust stability that allows for the application in a wide temperature scale. (C) 2014 Elsevier Ltd. All rights reserved.

Copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c]phenazine (TQ2), were synthesized via a Stille coupling reaction. By increasing the conjugation in the TQ2 unit, the polymer based on TQ2 showed a narrower band gap (Eg), a lower highest occupied molecular orbital energy level and enhanced interchain π–π interactions. Polymer solar cells based on TQ2 showed a simultaneous enhancement of the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) compared with polymer solar cells based on the TQ1 polymer. A good power conversion efficiency of 4.24% was achieved by solar cells based on the TQ2 polymer and [6,6]-phenyl-C71-butyric acid methyl ester composite. These preliminary results indicate that increasing the acceptor unit (quinoxaline) conjugation is an effective way of improving the performance of polymer solar cells.

In order to improve the solution processability of 4,7-bis(thiophen-2-yl) benzo[c][1,2,5] thiadiazole (DTBT)-based polymers, novel donor-acceptor polymer PTOBDTDTBT containing DTBT and benzo[1,2-b: 4,5-b']dithiophene (BDT) with conjugated side chain is designed and synthe-sized with narrow band gap 1.67 eV and low lying HOMO energy level -5.4 eV. The blend film of PTOBDTDTBT and PC 71 BM exhibits uniform and smooth film with root-mean-square (RMS) surface roughness 1.15 nm because of the excellent solubility of PTOBDTDTBT when six octyloxy side chains are introduced. The hole mobility of the blend film is measured to be 4.4 x 10(-5) cm(2) V-1 s(-1) by the space-charge-limited current (SCLC) model. The optimized polymer solar cells (PSCs) based on PTOBDTDTBT/PC71BM exhibits an improved PCE of 6.21% with V-oc = 0.80 V, J(sc) = 11.94 mA cm(-2) and FF = 65.10%, one of the highest PCE in DTBT containing polymers.

A series of triple bond side-chained benzodithiophene copolymers, derived from 4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene, were synthesized by Stille coupling reactions. Several electron acceptors are introduced into the polymer backbone to tune the photo-electronic properties of the copolymers. The obtained copolymers are readily soluble in common organic solvents, such as toluene, THF and chloroform. The number average molecular weights of these polymers were determined by GPC using a polystyrene standard, ranging from 15 400 to 28 300 which are decent results for polymers with a rigid triple bond side chain. These polymers show high decomposition temperature (403–419 °C), narrow band gap (1.27–1.89 eV), and low-lying HOMO energy level (−5.4 to −5.6 eV). The best polymer solar cell (PSC) device based on the copolymer PBDTPA-TPD and PC61BM showed a power conversion efficiency (PCE) of 0.93% with a Voc = 0.96 V, Jsc = 1.9 mA cm−2, and FF = 51%, which is one of the highest Voc values achieved by a benzodithiophene polymer.

Two tetradodeoxyphenyl-substituted indacenodithiophene (IDT) based polymers, PIDT3T and PIDTDTBT, were achieved by copolymerizing IDT with terthiophene (3T) or di-2-thienyl-2′,1′,3′-benzothiadiazole (DTBT). Although these two polymers show significantly different UV-vis absorption spectra and band gaps (2.08 eV and 1.75 eV), the HOMO levels (−5.35 eV and −5.30 eV) of these polymers are almost the same. Polymer solar cells (PSCs) based on polymers with the benzothiadiazole (BT) unit show relatively high short-circuit current density (Jsc) due to the relatively wide and high photo-electronic response and high hole mobility. Thanks to the four long aryl side chains on IDT, the polymer thin film shows an amorphous nature, and the AFM root-mean-square roughness (RMS) value of the polymer/PCBM blend film is only around 0.3 nm which can contribute to the homogenous bulk heterojunction structures without significant phase separation. Finally, decent power conversion efficiency (PCE) of 4.52% is achieved by the benzothiadiazole based polymer and PC71BM composite. By comparison study, we demonstrate why BT is an excellent acceptor unit for indacenodithiophene-based PSCs.

A donor–acceptor (D–A) polymer, PTBFDO-BDD, based on thieno[2,3-f]benzofuran (TBF) with 4-dodecyl thienyl chains, was designed and synthesized. The optical, electrochemical, photovoltaic and device active layer morphology properties of the new polymer were investigated. With the structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/PTBFDO-BDD:ITIC/PDINO/Al, polymer solar cells device exhibited a power conversion efficiency (PCE) of 7.15% (AM1.5G, 100 mW cm−2) with VOC of 0.803 V, JSC of 14.71 mA cm−2, and FF of 60.57%. This work demonstrates that thieno[2,3-f]benzofuran-based conjugated polymers are promising as polymer solar cells donor materials.

In this work, one novel carbazole side chained benzodithiophene (BDT) monomer is designed and synthesized, and this new donor monomer based two D−A conjugated polymers with electron-withdrawing acceptor 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) (P5) and benzodithiophene-4,8-dione (BDD) (P7) are prepared. The polymers show 1.65–1.85 eV band gap, which is similar to the polymer with same backbone. The P5 based polymer solar cells show acceptable PCE around 8% and low enegy loss around 0.7 eV, which is similar to the fluorene side-chained polymer. P7 based polymer shows 1 V open-circuit voltage in both fullerene based and fullerene-free polymer solar cells. Molecular conformations, photophysical/electrochemical properties and film morphology of the device active layer blend films were also systematically investigated to reveal the structure−property relationships. This work suggests that carbazole unit is a promising π-conjugated side chain group for BDT monomer to construct efficient two-dimensional (2D) D−A conjugated photovoltaic polymers.