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The inorganic CsPbI2Br perovskite with excellent thermal stability has become one of the potential candidates for commercial perovskite solar cells (PSC) in the future. However, the low efficiency and high humidity instability of CsPbI2Br PSC hindered its further development. To improve the device's performance, we use a bifunctional organic polymer material (PFDTBT) in the carbon-based CsPbI2Br PSC as an interface modification layer to achieve the function of improving the efficiency of the PSC. The application of PFDTBT reduces the energy level difference between the anode and CsPbI2Br and blocks the electron transfer from the perovskite layer to the carbon electrode. It also passivates the surface defects of the CsPbI2Br layer and improves the charge separation and extraction at the CsPbI2Br/anode interface. Meanwhile, the incorporation of the PFDTBT layer avoids the device's instability caused by the oxidation process and complex doping. After modification with PFDTBT, we achieve a high efficiency of 12.98%, which is the highest efficiency of devices with an undoped organic interface modification layer. Besides, the unencapsulated CsPbI2Br PSC with a dopant-free interface modification layer exhibited improved long-term stability in dry air for 25 days (relative humidity of 25%). This research provides a simple method for improving the performance of PSCs.

To pursue effective charge transport and high fill factor (FF) in photovoltaic devices, adopting ordered polymers with enhanced hole mobility is greatly desired. Currently, a popular strategy in improving charge transport has been applied by enforcing the coplanarity of fused-ring conjugated structures. However, their synthetic complexity hinders their further development. Here, we successfully developed an effective strategy to introduce soluble alkenyl side chains in D–A type photovoltaic polymer backbones to improve the charge transport ability and photovoltaic performance. The introduction of the trans-vinylene (CHCH) linkage in alkenyl side chains strengthens the polymeric crystalline properties and charge transport ability, and further improves the photovoltaic performance. Meanwhile, a high FF of 0.73 can still be obtained for a device with a film thickness of 200 nm. The low cost, feasible chemical strategy could potentially be applied for other thickness-insensitive organic photovoltaic materials design.

Organic photovoltaics (OPVs) were fabricated with J71, PBT1-C or their blends as donor and IT-2F as acceptor. The power conversion efficiency (PCE) of J71 and PBT1-C based binary OPVs arrive to 10.45% and 10.23%, respectively. Although the very similar photovoltaic performance of two binary OPVs, three photovoltaic parameters are distinctly different. Meanwhile, three materials also exhibit complementary absorption range to cover solar light as wide as possible. Integrating the advantages of materials and binary OPVs, the optimized ternary OPVs are obtained with 12.26% PCE and 75.53% fill factor by incorporating 20 wt% PBT1-C in donors. The enhanced photovoltaic performance should result from enhanced photon harvesting and optimized phase separation. The two polymer donors should prefer to form alloyed state, which can reasonably interpret the enhanced photovoltaic performance of ternary OPVs with PBT1-C content from 10 wt% to 50 wt% in donor.

Terpolymers comprising three different components in the backbone have emerged as a new design strategy for conjugated polymers. However, compared with the backbone modification, less effort has been devoted to the alkyl side chain engineering, especially the subtle side chain modifications of random polymers. In this contribution, we designed and synthesized a series of random terpolymers, in which the subtle side chain regioregularity is used to finely tune the optical, electronic, and morphological properties. PB55 was found to outperform the other copolymers with better mixing with fullerene and higher photovoltaic performance. Moreover, the utilization of these random terpolymers in nonfullerene solar cells was investigated. A well-known nonfullerene acceptor, ITCPTC, was used as the acceptor. Among the terpolymers, PB55 yielded the best photovoltaic performance with an impressive PCE of ∼12.1%, representing the highest value reported in the literature so far for nonfullerene OSCs based on random ter...

With a fused terthiophene, dithieno[3,4-b:2′,3′-d]thiophene (DTT) is used as a thienoquinoidal building block for conjugated polymers. However, due to its highly extended heteroarene structure, chemical synthesis of the functional DTT unit is a challenge. All known DTT-based polymer films were prepared by an electrochemical polymerization approach. In this study, we first synthesized the functional DTT monomer and DTT-based conjugated polymer, PDBT-DTT, via a typical Stille coupling reaction. With another fused thiophene ring, DTT shows increased aromatic (weaker quinoid) character compared to the thieno-[3,4-b]thiophene (TT) unit. Moreover, PDBT-DTT showed much higher photovoltaic performance as well as better thermal and photostability than its TT-based analogue (PDBT-TT). The results demonstrate that DTT is a potential quinoidal building block for constructing high-performance conjugated polymers.

Polymer solar cells (PSCs) have garnered considerable attention in recent years. However, improving the fill factor (FF) remains a big challenge. The FF is closely related to the mobilities of the donor. Thus, improving the mobilites of the donor by constructing an enlarged π-conjugated fused ring in polymeric backnones could improve FF and power conversion efficiency (PCE). Here, a fused-ring dithienosilolodithiophene (DTTS)-based low-bandgap polymer of PDTTS with an enlarged π-conjugated system was created and its thermal stability, absorption range, molecular energy, and the corresponding photovoltaic properties were investigated. UV-vis absorption spectra showed that PDTTS with enlarged conjugation on polymeric backbones exhibited a more obvious absorption shoulder peak compared with a similar polymer of PDTS containing a dithienosilole (DTS) unit. Hence, PDTTS had a more planar structure than PDTS and resulted in stronger intermolecular π–π interactions. X-ray diffraction revealed PDTTS to exhibit greater crystallinity than PDTS, which suggested more ordered π–π stacking in PDTTS backbones. The mobility of PDTTS was more than one order of magnitude higher than that of PDTS, and benefited from the more planar conjugated structure of PDTTS. Photovoltaic devices based upon PDTTS/PC71BM blends exhibited efficiencies up to 6.33% with a higher FF of 65% for as-cast films without processing additives. Under optimized processing conditions, PDTS/PC71BM blends exhibited an efficiency of 3.22% with a FF of 53%. These preliminary results suggest that polymers with an enlarged π-conjugated system possess higher mobility and facilitate effective carrier transportation, and boost FF and PCE values.

Perylenediimide (PDI)-based small molecules have significantly contributed to the development of non-fullerene acceptors, whereas the development of PDI-based polymer acceptors is relatively lagging behind. In this study, we designed and synthesized two PDI-based n-type polymers named as PF-PDI and PBDT-PDI, in which PDI was used as electron-deficient unit and fluorene (F) or benzodithiophene (BDT) were used as electronrich components. The density functional theory (DFT) calculations and grazing incidence wide-angle X-ray scattering (GIWAXS) results indicate that the PF-PDI shows larger steric hindrance and relatively weaker lamellar packing than that of PBDT-PDI. Comparing with PBDT-PDI, PF-PDI shows red-shift absorption and lower-lying HOMO level, which agrees well with the DFT results. A well-known wide bandgap polymer donor, PDBT-T1 was employed to fabricate polymer solar cells (PSCs) with the two acceptors. The all polymer solar cells (all-PSCs) based on PDBT-T1:PF-PDI showed a high power conversion efficiency (PCE) of 4.47%, which is approximately 2-fold larger than that of devices with PDBT-T1:PBDT-PDI (PCE = 2.70%).

Ternary bulk heterojunction (BHJ) blends have been demonstrated as a promising approach to increase the power conversion efficiencies (PCEs) of organic solar cells. Currently, most studies of ternary organic solar cells are based on blends of two donors and one acceptor, because of the limitation in acceptor materials. Here, we report that high-performance ternary solar cells have been fabricated with a wide-bandgap polymer donor (PDBT-T1) and two acceptor materials, phenyl-C70-butyric acid methyl ester (PC70BM), and nonfullerene acceptor (ITIC-Th). The addition of ITIC-Th into the BHJ blends dramatically increases the light absorption. Consequently, the champion ternary solar cell shows a high PCE of ∼10.5%, with an open-circuit voltage (Voc) of 0.95 V, a short-circuit current (Jsc) of 15.60 mA/cm2, and a fill factor (FF) of 71.1%, which largely outperforms their binary counterparts. Detailed studies reveal that the ternary solar cells work in a parallel-like device model (ITIC-Th and PC70BM form their o...

Over the past 20 years, significant progress has been made in organic photovoltaics (OPVs) due to its advantages of being cost-effective, being lightweight, and having flexible manufacturability. The optical-active layer of OPVs consists of a p-type polymer as the donor and an n-type small molecule as the acceptor. An efficient design strategy of a polymer donor is based on an alternating electron-donating unit (D) and an electron-accepting unit (A). Among numerous electron-accepting units, an emerging annelated thiophene of benzodithiophenedione (BDD) has exhibited a distinguished photovoltaic performance because of its planar molecular structure, low-lying highest occupied molecular orbit (HOMO) level and good self-assembly property. In this review article, we summarize the most recent developments in BDD-based photovoltaic materials. Special attention is paid to the chemical structure-property relationships, such as the absorption, bandgap, energy levels, mobilities, and photovoltaic performances. The empirical regularities and perspectives on the future development of BDD-based photovoltaic materials are included. Development of a polymer that has exciting potential for solar-energy conversion has been reviewed by researchers in China. Traditional solar cells are made using silicon, but their efficiency has not yet reached a level where they can compete with fossil fuels. In the last few decades, organic solar cells have emerged as an alternative, offering simpler fabrication and lighter, cost-effective and flexible devices. Lijun Huo from Beihang University, Beijing, and colleagues summarise recent progress in organic photovoltaic cells made using benzodithiophenedione (BDD) based polymeric donors. The authors review the properties of this kind of material that are key to optimising photovoltaic performance such as optical absorption, the electron energy levels, the charge mobility, the crystallinity and morphology. Careful control of these properties has enabled BDD-based devices to achieve power conversion efficiencies of over 15%. An emerging annelated thiophene of benzodithiophenedione (BDD) has exhibited its distinguished photovoltaic performance since its planar molecular structure, low-lying highest occupied molecular orbit (HOMO) level and well self-assembly property. In recent 7 years, BDD-based polymer donor have shown a rapid and incredible advancement by utilizing different acceptor materials. Considering the potentials of BDD-based materials, we summarize the most recent advances in the BDD-based photovoltaic materials and highlight the relations between BDD-based molecular structures and photovoltaic properties.

Extending the π-conjugation length of the polymeric backbone is an effective way to enhance the photovoltaic performance of polymer solar cells (PSCs). Here, the donor–donor–acceptor (D–D–A) molecular strategy has been used to design and synthesize two wide-bandgap conjugated copolymers, in which 2,2-bithiophene (BT) or thieno[3,2-b] thiophene (TT) is introduced to the D–A polymer as a third component to investigate the influence of the conjugation backbone on photovoltaic properties. The structure–property relationship and photovoltaic performance of the polymer have been investigated. Compared to P2 (TT as the third unit), P1 (BT as the third unit) exhibits a deeper highest occupied molecular orbital (HOMO) level and a more planar backbone structure with slightly higher mobility. Based on a conventional device structure with PC70BM as the acceptor material, P1-based solar cells exhibit a maximum power conversion efficiency (PCE) of 6.93%, an open-circuit voltage (VOC) of 0.86 V, a short-circuit current (JSC) of 11.06 mA cm−2, and a fill factor (FF) of 72.9%, which are much better than those of P2-based solar cells (PCE 3.92%). These results demonstrate that extending the effective π-conjugation structure of the polymer backbone could improve the photovoltaic performance of PSCs by inserting an additional appropriate donor unit in the D–A polymer.

A series of propeller-shaped triperylene hexaimide (TPH) non-fullerene acceptors, featuring branched alkyl side chains with different lengths (TPH-4, TPH-5, TPH-6, and TPH-7), have been designed and synthesized. The effects of the branched alkyl chain length on the physical properties, thin-film microstructure, molecular packing, charge transport and the resulting photovoltaic properties of these materials have been systematically investigated. It was found that TPH-7 with the longest alkyl side chain showed the best photovoltaic performance compared with three other TPH molecules, and an outstanding power conversion efficiency (PCE) of 8.6% under AM 1.5G irradiation (100 mW cm−2) has been obtained using a wide-band-gap polymer PDBT-T1 as the electron donor. These results demonstrate that finely tailoring alkyl substituents on TPH-based small molecular acceptors critically impacts the structural order of thin films and molecular orientation, and thus the photovoltaic performance.

In this work, we demonstrated the synthesis and characterization of two novel donor–acceptor type conjugated polymers, IDT-T1 and IDTT-T1, which consist of indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) donor and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c]dithiophene-4,8-dione (T1) acceptor building blocks. The resulting copolymers exhibited a wide bandgap of about 1.90 eV with a comparatively low-lying highest occupied molecular orbital (HOMO) energy level. However, IDTT-T1 with an extended fused-ring unit was found to have better coplanarity and higher carrier mobility (0.11 cm2 V−1 s−1). As a consequence, organic solar cells employing IDTT-T1 as the donor material afforded a power conversion efficiency (PCE) of 6.58%, while IDT-T1-based devices yielded a relatively lower PCE of 6.26%.

The open-circuit voltage ( V oc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high V oc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)- T1 ) exhibited a HOMO value of –5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)- T1 ). As a result, POBDT(S)- T1 -based solar cells show a high V oc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)- T1 -based devices show a relatively lower V oc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.

Extensive efforts have been focused on the study of wide-band gap (WBG) polymers due to their important applications in multiple junction and ternary blend organic solar cells. Herein, three WBG copolymers named PBDT(X)-T1 (X = O, S, Se) were synthesized based on the benzodithiophene (BDT) donor unit and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) acceptor unit. Different aromatic heterocycle groups (furan, thiophene and selenophene) were introduced to modify the BDT unit to investigate the influence of conjugated side chains on the photovoltaic properties of conjugated polymers. Photophysical properties, electrochemistry, charge transport and crystalline properties of the polymers were studied to discuss the role of chalcogen atoms on the performance of conjugated polymers. Solar cells based on these three WBG copolymers were fabricated. Among them, the PBDT(Se)-T1-based solar cell shows the best photovoltaic performance with the highest power conversion efficiency (PCE) of 8.52%, an open-circuit voltage (Voc) of 0.91 V, and a high fill factor (FF) of 72%. The high crystallinity and preferential face-on orientation in the blend film partially explain the superior photovoltaic performance achieved in PBDT(Se)-T1-based solar cells. The results indicate the important role of chalcogen atoms in conjugated side chains and that high photovoltaic performance can be realized through side chain engineering of BDT-based WBG conjugated polymers.

A novel optical band gap of 1.57 eV isoindigo (IID)-containing polymer of PBDP-IID was synthesized and the HOMO of the polymer descends to −5.44 eV due to strong electron-deficient isoindigo unit. Bulk heterojunction solar cells made from PBDP-IID exhibited a high Voc of 0.95 V, which shows the highest value among the PSCs based on the narrow bandgap polymer donors with Eg < 1.6 eV.

Attaching meta-alkoxy-phenyl groups as conjugated side chains is an easy and effective way to modulate the molecular energy level of D-A polymer for photovoltaic application, and the polymer solar cells based on the polymer consisting meta-alkoxy-phenyl groups as conjugated side chain, PBT-OP, shows an enhanced open circuit voltage and thus higher efficiency of 7.50%, under the illumination of AM 1.5G, 100 mW/cm(2) .

The correlation among molecular conformation, the crystallinity of the morphology, propensity for π-π stacking, J- versus H-aggregation, and photovoltaic performance have been studied based on two newly designed polymers, PBDTTT-S-T and PDT-S-T. The results show that more linear backbone structure is helpful to improve photovoltaic properties of the polymer, and therefore, molecular conformation should be considered for molecular design of photovoltaic polymers.

To extend the π conjugation along backbone units is an effective way to tune the optoelectronic properties of conjugated polymers. In this work, to investigate the influence of the extended conjugation on photovoltaic properties of the polymer, we employed a two-step molecular optimization process, and a series of diketopyrrolopyrrole (DPP)-based conjugated polymers, PBDPP-1, PBDPP-2, and PBDPP-3, was prepared and applied in polymer solar cells. After this two-step optimization of molecular structure, the compatibility between the polymer and PC71BM becomes much better; as a result, bicontinuous phase separation with appropriate domain size can be formed in the blend of PBDPP-3/PC71BM. Therefore, the PBDPP-3/PC71BM-based device shows a short circuit current density (Jsc) of 16.25 mA/cm 2 , which is more than two times of that of the PBDPP-1/PC71BM- based device. In addition, the blend films processed by DIO/CB showed smoother surfaces as well as smaller domain size compared with the blend films processed by pure CB. The results in this work indicate that when conjugation of the DPP- polymer's backbone is extended in two dimensions the morphology of the active layers in photovoltaic device can be optimized effectively, and hence the overall efficiency can be improved from 2.83 to 6.18%.

A new polymer based on thieno[3,4-f]isoindole-5,7-dione (TID) and benzo[1,2-b:4,5-b]dithiophene was designed and synthesized. The copolymer was characterized by elemental analysis, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), ultraviolet-visible (UV-vis) absorption spectra as well as electrochemical cyclic voltammetry (CV) tests. Compared to PBDTTPD, introducing the quinoid unit could elevate the HOMO energy level and lower the LUMO energy level of the polymer, and consequently, the band gap can be reduced very effectively. The electronic structures of the two polymers were also studied by DFT calculations at the B3LYP/6-31G level. When a polymer solar cell (PSC) device was fabricated with an active layer of a blend of PBDTTID and PC61BM using 3% (v/v) diiodooctane (DIO) as a solvent additive, a high Voc of 0.80 V was obtained, with a PCE of 3.12% under AM 1.5G conditions. The PBDTTID-based PSC device shows a slightly lower Voc but better Jsc and FF than the PBDTTPD-based PSC device, and the preliminary results indicate that TID will be a desirable building block for designing photovoltaic polymers with low-lying HOMO energy levels, as well as low band gaps.

A novel polymer donor (PBDTS‐Se) is designed to match with a non‐fullerene acceptor (SdiPBI‐S). The corresponding solar cells show a high efficiency of 8.22%, which result from synergetic improvements of light harvesting, charge carrier transport and collection, and morphology. The results indicate that rational design of novel donor materials is important for non‐fullerene organic solar cells.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

In order to investigate the influence of two-dimensional (2D) conjugated structure on photovoltaic properties of benzo[1,2-b:4,5-b′]difuran (BDF)-based polymers, two low band gap photovoltaic polymers, named PBDFTT-CF-O and PBDFTT-CF-T, were designed and synthesized. These two polymers have the same backbones and different side groups. Although these two polymers show similar optical band gaps (ca. 1.5 eV), the polymer with alkylthienyl side groups, PBDFTT-CF-T, exhibits stronger absorption in long wavelength direction than the polymer with alkoxyl side groups, PBDFTT-CF-O. Meanwhile, PBDFTT-CF-T exhibits a HOMO level of −5.21 eV, which is 0.23 eV lower than that of PBDFTT-CF-O due to weaker electron-donating ability of alkylthienyl side groups than that of aloxyl side groups. The hole mobility of the blend of PBDFTT-CF-T/PC71BM (1:1.5, w/w) is 0.128 cm2 V–1 s–1, which is 1 order of magnitude higher than that of the blend of PBDFTT-CF-O/PC71BM. Density functional theory (DFT) model shows thiophene pendant...

A systematic molecular design process from PBDTTT-S to PBDTDTTT-S-T, a high-performance semiconducting polymer for organic photovoltaics, has been achieved by enhancing structural order, self-assembly and carrier mobility. Solar cells made from PBDTDTTT-S-T blended with PC(70) BM show a power conversion efficiency (PCE) of 7.81%, which is 25% higher than that of the parent PBDTTT-S.

Two new donor–acceptor (D–A) alternative copolymers, PBDTDTQx-T and PBDTDTQx-O, were designed and synthesized to investigate the influence of two-dimensional conjugated structure on photovoltaic properties of conjugated polymers. In these two polymers, PBDTDTQx-O was used as control material, which is an alternative copolymer based on a quinoxaline derivative (DTQx) and alkoxy-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-O) unit; PBDTDTQx-T has an identical conjugated backbone as PBDTDTQx-O, but a simple two-dimensional conjugated BDT unit (BDT-T) was used to replace the alkoxy-BDT. The polymers were characterized by TGA, UV–vis absorption, electrochemical cyclic voltammetry, hole mobility of space-charge-limited current (SCLC) model, and photovoltaic measurements. It was found that PBDTDTQx-T exhibits similar molecular energy levels and higher hole mobility than PBDTDTQx-O. The power conversion efficiency (PCE) of the polymer solar cells (PSCs) based on PBDTDTQx-T: [6,6]-phenyl-C-71-butyric acid methy...

Polymer solar cells (PSCs) have attracted much attention because of their potential application in flexible, light-weight, and low-cost large-area devices through roll-to-roll printing. The bulk heterojunction PSCs showed advanced features in realizing high efficiencies and solution-processible devices. The active layer in this kind of device consists of an interpenetrating network formed by an electron-donor material blended with an electron-acceptor material. 3] Typically, conjugated polymers are used as electron donors and fullerene derivatives are used as the electron acceptors in the PSCs. Recently, power conversion efficiencies (PCEs) of 6–7% have been realized by using new conjugated polymer donors or new fullerene-derived acceptors. Short circuit current density (Jsc), open circuit voltage (Voc), and fill factors (FF) are key parameters for a PSC device, because the PCE of the device is proportional to the values of the three parameters. To broaden the response wavelength range of a PSC device by using conjugated side chains or narrowband-gap conjugated polymers is an effective way to realize high Jsc values. Conjugated polymers with lower HOMO levels are helpful in realizing high Voc and PCE values, as the Voc value of PSCs is directly proportional to the offset between the HOMO level of electron donor and the LUMO level of electron acceptor. PSiFDTBT, PFDTBT, and PCDTBT are three excellent examples for this concept. Consequently, by using conjugated polymers with lower HOMO levels and also narrow band gaps, high PCEs were realized in different families of conjugated polymers. Conjugated polymers based on benzo[1,2-b :4,5-b’]dithiophene (BDT) units have attracted interest as electron donors in the PSC field in recent years, since the report of Hou and Yang et al. on the synthesis and photovoltaic properties of a series of copolymers based on BDT. Many copolymers of BDT with different conjugated units, such as thieno[3,4b]thiophene (TT), 4,7-dithiophene-2-yl-2,1,3-benzothiadiazole (DTBT), N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD), and bithiazole, etc. were synthesized, and the copolymers showed promising photovoltaic properties. In these BDT-based polymers, the alternative copolymers of BDT and TT, namely PBDTTTs, are an important family of photovoltaic materials. For additional improvements in the photovoltaic performance of the PBDTTTs, structural modifications brought about by using different substituents on BDT, or the copolymerized moieties is of great importance. For example, Liang et al. introduced a fluorine atom into the TT unit of the PBDTTTs, and the HOMO level of the resulting polymer was successfully lowered by approximately 0.12 eV, and thus a higher Voc value was achieved, resulting in a great improvement of PCE. Hou et al. optimized PBDTTTs further by replacing the alkoxycarbonyl group on the TT unit with the alkylcarbonyl groups. The structural modification can also be carried out on the BDT units. In this work, we designed an 5-alkylthiophene-2-yl-substituted BDT monomer and synthesized two new PBDTTT-based polymers having either the thienylsubstituted BDT with alkoxycarbonyl-substituted thieno[3,4b]thiophene (TT-E) or the alkylcarbonyl-substituted thieno[3,4-b]thiophene (TT-C); that is PBDTTT-E-Tand PBDTTTC-T, respectively (Scheme 1). To fully investigate the effect of the thienyl-substituted BDTon the photovoltaic properties of the polymers, two corresponding PBDTTT polymers based on the alkoxy-substituted BDT (BDT-O), PBDTTT-E and PBDTTT-C (Scheme 1), were also prepared. The synthetic route of the thienyl-substituted BDT monomer (BDT-T) is shown in Scheme 1. The branched alkyl group 2-ethylhexyl was employed as the side chain on the thiophene to guarantee high solubility of the target polymers. The TT-E and TT-C monomers are commercially available. The polymers were prepared through a Stille coupling reaction between the bis(trimethyltin) BDT monomers (BDT-T and BDT-O) and the bromides (TT-E and TTC) as shown in Scheme 1. All the polymers are soluble in chloroform (CHCl3), chlorobenzene, and dichlorobenzene. Thermogravimetric analysis (TGA) measurements were employed to evaluate the thermal stability of the polymers. We found that the two-dimentional (2D) conjugated polymers based on alkylthienyl-substituted BDTs are much more stable than their analogues, the alkoxy-substituted BDTs. The TGA plots of these four polymers are shown in Figure 1. It can be seen that the decomposition temperatures [*] Dr. L. Huo, S. Zhang, F. Xu, Prof. J. Hou State Key Laboratory of Polymer Physics and Chemistry Beijing National Laboratory for Molecular Sciences Institute of Chemistry, Chinese Academy of Sciences Beijing 100190 (China) E-mail: hjhzlz@iccas.ac.cn

Efficient ternary organic solar cells (OSCs) are fabricated by employing a polymer PBT1-C as the donor and two non-fullerene materials, MeIC and MeIC2, as one alloyed acceptor. The optimized ternary OSCs with 30 wt% MeIC2 in acceptors achieve a power conversion efficiency (PCE) of 12.55%, which is much higher than that of 11.47% for MeIC-based binary OSCs and 11.41% for MeIC2-based binary OSCs. The >9.4% improvement in PCE is mainly attributed to the optimized photon harvesting and morphology of ternary active layers, resulting in the simultaneously improved short-circuit current and fill factor. Furthermore, good compatibility and similar lowest unoccupied molecular orbital energy levels of MeIC and MeIC2 are beneficial to form one alloyed acceptor for efficient electron transport in the ternary active layers. This work may provide new insight when selecting the third component for preparing efficient ternary OSCs.

Although there is a consensus on the development of solar energy as a green energy resource, the importance of development of the polymer solar cell (PSC) has only been realized in recent years owing to its advantages of low cost and flexibility in large-area applications. Bulk heterojunction (BHJ) type polymer solar cells now play a leading role in realizing high power conversion efficiency (PCE). The development of new polymer materials to further improve the efficiencies of BHJ-type PSCs will accelerate their commercial application. At present, P3HT-PCBM-based PSCs (P3HT= poly(3-hexylthiophene), PCBM=methanofullerene (6,6)-phenyl-C61-butyric acid methyl ester) have exhibited high efficiencies of 4–5%. However, the narrow absorption spectrum of P3HT in 300–650 nm is one of the main hindrances to the further improvement of the efficiencies of P3HT-based devices. To overcome this problem, some low-band-gap polymer materials as donors have been synthesized successfully and applied to photovoltaic devices. Large short-circuit current density (Jsc) and higher efficiencies have been demonstrated using this strategy. In spite of the high Jsc achievable with low-band-gap polymers, the device performance suffers owing to the low open-circuit voltage (Voc) of 0.5–0.7 V. According to published models and experimental results, theVoc of a PSC is closely related to the offset between energy levels of the highest occupied molecular orbital (HOMO) of the donor and lowest unoccupied molecular orbital (LUMO) of the acceptor. Thus, to enhance the opencircuit voltage of these devices, some building blocks with lower-lying HOMOs should be introduced in donor molecules to lower their HOMO levels. Polyfluorene and its derivatives are a well-known class of wide-band-gap organic semiconducting materials. Many donor polymers containing fluorene on the main chain were reported to show lower HOMO levels and correspondingly exhibited higher open-circuit voltages. However, the wider band gaps of these polymers resulted in lower values of Jsc and consequently lower efficiencies. To enhance the efficiency, there is a need for a low-band-gap polymers that can have a high Voc. Few polymers with high open-circuit voltages (above 0.8 V) and consequently high PCEs (above 5%) have been reported to date, except for two kinds of polymers, poly[(2,7silafluorene)-alt-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PSiFDBT) and poly[N-9’’-heptadecanyl-2,7-carbazole-alt5,5-(4’,7’-di-2-thienyl-benzothiadiazole) (PCDTBT), both of which contain a fluorene unit in the polymer main chain. Considering the similarity in the molecular structures of PSiFDBTand PCDTBTwith analogous fluorene units in both of their main chains, it is necessary to explore new molecular structures with favorable characteristics for higher opencircuit voltages and conversion efficiencies. Recently, some benzo[1,2-b :4,5-b’]dithiophene-containing polymers were applied to field-effect transistor (FET) and PSC devices (Table 1). 6, 7] A high mobility of 0.15–

A series of low-bandgap polymers based on a soluble chromophore of 3,6-dithiophen-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP) unit were synthesized by introducing of different electron-rich building blocks copolymerized with DPP unit. Four new DPP-based polymers, PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP, were characterized by GPC, TGA, NMR, UV−vis absorption, and electrochemical cyclic voltammetry. The results indicate that their bandgaps as well as their molecular energy levels are readily tuned by copolymerizing with different conjugated electron-donating units. In order to investigate their photovoltaic properties, polymer solar cell (PSC) devices based on PDPP-DTS, PDPP-F, PDPP-BDT, and PDPP-BDP were fabricated with a structure of ITO/PEDOT:PSS/polymers:PC70BM(1:2,w/w)/Ca/Al under the illumination of AM 1.5G, 100 mW/cm2. The power conversion efficiencies (PCE) of the four DPP-based PSC devices were measured and shown in this paper. The best performance of the PSC device was obtained by using...

Two polythiophene derivatives with electron-donating alkylthio side chains, poly[(3-hexyl-thio)thiophene] (P3HST) and poly[(3-hexylthio)thiophene-alt-thiophene] (P3HST-co-Th) have been synthesized and characterized. Both P3HST and P3HST-co-Th show broader absorption peaks than poly(3-hexylthiophene). Meanwhile, the alkylthio side chains decrease the HOMO energy level of the polymers, which benefits the higher open circuit voltage of the polymer solar cells (PSCs) based on the polymer as donor. PSCs have been fabricated with the polymers as donor and [6,6]-phenyl C61 butyric acid methyl ester as acceptor (1: 1, w/w). The devices based on P3HST and P3HST-co-Th show an open circuit voltage of 0.63 V, and a power conversion efficiency of 0.34% and 0.5%, respectively, under the illumination of AM1.5, 80 mW · cm ―2 .

Two novel n-type conjugated polymers based on perylene diimide (PDI), poly(PDI-vinylene) (PDIV), and poly(PDI-thienylene) (PDITh), have been designed and synthesized by the Stille coupling reaction. In comparison with the PDI monomer (compound M1), PDIV and PDITh films show a significantly broad absorption band from 380 to 720 nm, and a narrower bandgap of ≈1.71 and 1.74 eV, respectively. Cyclic voltammograms of the two polymers display a couple of reversible reduction/re-oxidation (n-doping/ dedoping) peaks. The onset reduction (n-doping) potentials of PDIV and PDITh are at -0.62 V and -0.66 V versus Ag/Ag + respectively, which correspond to the electron affinities (EAs) of 4.09 eV for PDIV and 4.05 eV for PDITh. The EA values of the two polymers are the highest among the n-type conjugated polymers reported to date. The results indicate that PDIV and PDITh could be used as polymer acceptors in all polymer solar cells.

A class of cross-linked polythiophenes with different ratio (2%, 4%, 8%) of vinylene−terthiophene−vinylene (VTThV) conjugated bridges (PT−VTThV2, PT−VTThV4, and PT−VTThV8) were synthesized. The cross-linking influenced the absorption spectra of the polymer solutions very little, but it resulted in blue-shift of the absorption spectra of the polymer films by ca. 28−39 nm. Cyclic voltammograms display that the p-doping/dedoping and n-doping/dedoping processes of all the cross-linked polythiophenes are reversible and the electrochemical bandgaps increased a little with the increase of the content of the conjugated bridges. The hole mobility values determined from the space-charge-limited current (SCLC) model reached 4.70 × 10-3 for PT−VTThV2, 2.58 × 10-3 for PT−VTThV4, and 9.48 × 10-4 cm2/(V s) for PT−VTThV8, respectively. The hole mobility of PT−VTThV2 with 2% VTThV conjugated bridges is about three orders higher than that of the corresponding polymer P1 without the conjugated bridges. The power conversion ...

A series of alternating copolymers of electron-rich arylamine and electron-deficient 2,1,3-benzothiadiazole (BT), PV-BT, DP-BT, and TP-BT, were synthesized by Heck coupling reaction. UV–vis absorption and fluorescence spectra show that the copolymerization of electron-rich diphenylamine (DP), triphenylamine (TP), MEH-PV (PV), and electron-deficient BT results in low-bandgap conjugated polymers. Within the three copolymers of PV-BT, DP-BT, and TP-BT, TP-BT possesses the highest hole mobility of 4.68 × 10− 5 cm2/V, as determined from the space charge limited current (SCLC) model. The bulk heterojunction-typed polymer solar cells (PSCs) were fabricated with the blend of the copolymers and PCBM as the photosensitive layer. The power conversion efficiencies (PCE) of the PSCs based on PV-BT, DP-BT, and TP-BT reached 0.26%, 0.39%, and 0.52%, respectively, under the illumination of AM 1.5, 100 mW/cm2. The results indicate that TP-BT is a promising photovoltaic polymer for PSCs. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3861–3871, 2007

Four branched polythiophenes (PTs) with different ratios of conjugated terthiophene-vinylene side chains, PT-TThV10 to PT-TThV40, were synthesized by Stille coupling reaction. The polymers exhibited reversible p-doping/dedoping (oxidation/re-reduction) and n-doping/dedoping (reduction/reoxidation) processes. The absorption spectra, hole mobility, and photovoltaic properties of the polymers were much improved in comparison with the PT derivative without the terthiophene-vinylene side chain and were influenced by the content of the conjugated side chains. With the increase of the content of the conjugated side chains, the absorption peak of the branched PTs enhanced and blue-shifted. The maximum hole mobility reached 6.35 x 10(-4) cm2/V.s (SCLC method) and the maximum power conversion efficiency of the polymer solar cell reached 1.91% under the illumination of AM 1.5, 100 mW/cm2, for the polymer with 20% terthiophene-vinylene side chains. The results indicate that the branched PTs with suitable content of the terthiophene-vinylene side chains could be promising photovoltaic materials.

A new conjugated polymer, poly{[1′,4′-bis-(thienyl-vinyl)]-2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene-vinylene} (PTVMEH-PPV) was synthesized via Grim polymerization. The polymer is soluble in common organic solvents such as chloroform and tetrahydrofuran, and possesses adequate thermal stability ( T d > 246 °C). The absorption spectrum of PTVMEH-PPV film shows a broader absorption peak covering the wavelength range from 380 nm to 620 nm, which is red-shifted and broadened in comparison with that of MEH-PPV. The onset oxidation potential of the polymer is 0.12 V versus Ag/Ag + , ca. 0.2 V lower than that of MEH-PPV. The band gap of the polymer measured by cyclic voltammetry is 1.82 eV, which basically agrees with that obtained from the onset wavelength of the absorption spectra. Polymer solar cell was fabricated based on the blend of PTVMEH-PPV and PCBM with a weight ratio of 1:1. The device shows the maximum external quantum efficiency of 14% at ca. 520 nm, an open circuit voltage of 0.67 V and a power conversion efficiency of 0.32% under the illumination of AM1.5, 80 mW/cm 2 .

The past decade has witnessed significant advances in the field of organic solar cells (OSCs). Ongoing improvements in the power conversion efficiency of OSCs have been achieved, which were mainly attributed to the design and synthesis of novel conjugated polymers with different architectures and functional moieties. Among various conjugated polymers, the development of wide-bandgap (WBG) polymers has received less attention than that of low-bandgap and medium-bandgap polymers. Here, we briefly summarize recent advances in WBG polymers and their applications in organic photovoltaic (PV) devices, such as tandem, ternary, and non-fullerene solar cells. Addtionally, we also dissuss the application of high open-circuit voltage tandem solar cells in PV-driven electrochemical water dissociation. We mainly focus on the molecular design strategies, the structure-property correlations, and the photovoltaic performance of these WBG polymers. Finally, we extract empirical regularities and provide invigorating perspectives on the future development of WBG photovoltaic materials.

PBDTTTZ: A Broad Band Gap Conjugated Polymer with High Photovoltaic Performance in Polymer Solar Cells Lijun Huo, Xia Guo, Shaoqing Zhang, Yongfang Li,* and Jianhui Hou* State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

As researchers continue to develop new organic materials for solar cells, benzo[1,2-b:4,5-b']dithiophene (BDT)-based polymers have come to the fore. To improve the photovoltaic properties of BDT-based polymers, researchers have developed and applied various strategies leading to the successful molecular design of highly efficient photovoltaic polymers. Novel polymer materials composed of two-dimensional conjugated BDT (2D-conjugated BDT) have boosted the power conversion efficiency of polymer solar cells (PSCs) to levels that exceed 9%. In this Account, we summarize recent progress related to the design and synthesis of 2D-conjugated BDT-based polymers and discuss their applications in highly efficient photovoltaic devices. We introduce the basic considerations for the construction of 2D-conjugated BDT-based polymers and systematic molecular design guidelines. For example, simply modifying an alkoxyl-substituted BDT to form an alkylthienyl-substituted BDT can improve the polymer hole mobilities substantially with little effect on their molecular energy level. Secondly, the addition of a variety of chemical moieties to the polymer can produce a 2D-conjugated BDT unit with more functions. For example, the introduction of a conjugated side chain with electron deficient groups (such as para-alkyl-phenyl, meta-alkoxyl-phenyl, and 2-alkyl-3-fluoro-thienyl) allowed us to modulate the molecular energy levels of 2D-conjugated BDT-based polymers. Through the rational design of BDT analogues such as dithienobenzodithiophene (DTBDT) or the insertion of larger π bridges, we can tune the backbone conformations of these polymers and modulate their photovoltaic properties. We also discuss the influence of 2D-conjugated BDT on polymer morphology and the blends of these polymers with phenyl-C61 (or C71)-butyric acid methyl ester (PCBM). Finally, we summarize the various applications of the 2D-conjugated BDT-based polymers in highly efficient PSC devices. Overall, this Account correlates the molecular structures of the 2D-conjugated BDT-based polymers with their photovoltaic properties. As a result, this Account can guide the molecular design of organic photovoltaic materials and the development of organic materials for other types of optoelectronic devices.

The structure evolution of oligomer fused-ring electron acceptors (FREAs) toward high efficiency of as-cast polymer solar cells (PSCs) is reported. First, a series of FREAs (IC-(1-3)IDT-IC) based on indacenodithiophene (IDT) oligomers as cores are designed and synthesized, effects of IDT number (1–3) on their basic optical and electronic properties are investigated, and more importantly, the relationship between device performance of as-cast PSCs and donor(D)/acceptor(A) matching (absorption, energy level, morphology, and charge transport) of IC-(1-3)IDT-IC acceptors and two representative polymer donors, PTB7-Th and PDBT-T1 is surveyed. Then, the most promising D/A system (PDBT-T1/IC-1IDT-IC) with the best D/A harmony among the six D/A combinations, which yields a power conversion efficiency (PCE) of 7.39%, is found. Finally, changing the side-chains in IC-1IDT-IC from alkylphenyl to alkyl enhances the PCE from 7.39% to 9.20%.

A new copolymer PIDTDTQx based on indacenodithiophene and quinoxaline is synthesized and characterized. The correlation between the D/A ratio, mobility, and photovoltaic properties, as well as morphology of the D/A blend based on a PIDTDTQx:PC(70) BM system is investigated. The power conversion efficiency of the polymer solar cells based on PIDTDTQx/PC(70) BM (1:4, w/w) reaches 7.51%.

A new building block of benzo[1,2-b:4,5-b′]difuran (BDF) was firstly designed and synthesized. The newly designed unit was applied for constructing a new photovoltaic low band gap polymer, PBDFDTBT, which exhibited promising power conversion efficiency of 5.0%.

A strong electron-withdrawing group, sulfonyl, was firstly introduced to a semiconducting polymer, PBDTTT-S. The PCE of the PBDTTT-S device reached 6.22% with a high open-circuit voltage of 0.76 V. The sulfonyl group is thus a promising candidate as a strong electron-withdrawing group applied to high-efficiency PSCs.

A series of low band gap silole-containing polymers were synthesized with different alkyl side chains and a power conversion efficiency (PCE) of 3.43% was obtained.

Currently, most of the promising organic solar cells (OSCs) are based on low bandgap polymer donors with deep-lying highest occupied molecular orbit (HOMO) levels, which impose the challenges for device architecture design. In terms of fast charge extraction and suppression of bimolecular recombination, elaborate interface design in low bandgap OSCs is of significance to further boost their ultimate efficiency. In this work, a facile solution-processed functionalized single wall carbon nanotube (f-SWCNT) mesh/self-assembled molecule (SAM) hybrid structure is reported as hole transport layer (HTL) in low bandgap OSCs. The effectiveness of such hybrid HTL originates from two aspects: (i) SAM layer can effectively realize Ohmic contact between f-SWCNT and low bandgap polymer donors with deep-lying HOMO levels due to the reduction of interface energy barrier; (ii) f-SWCNT mesh can provide fast hole extraction pathways to quickly sweep out photogenerated charges. As a consequence of synergic effects of such hybrid HTL, both photocurrent and fill factor are greatly enhanced due to the reduced bimolecular recombination. Together with careful light management by using ZnO optical spacer, a high efficiency of 10.5% has been achieved. This work offers an excellent choice for large-scale processable and effective HTL toward the application in low bandgap OSCs with deep-lying energy levels.

A feasible synthetic route to alkoxy-substituted DTBDT was designed and the first DTBDT-based polymer was prepared and applied in organic solar cells. The preliminary results indicate that the DTBDT unit can be used as a potential building block in organic semiconductor materials.

To pursue high power conversion efficiency (PCE) of polymer solar cells (PSCs), more and more new active layer materials, especially polymeric photovoltaic materials, have been developed in the past several years, and benzo[1,2-b:4,5-b′]dithiophene (BDT) plays a very important role in these polymer materials. The applications of these new polymer materials boost the PCE of polymer solar cells greatly, and recent results indicate that 7–8% PCEs have been achieved by using BDT-based polymers. In this review, we summarize the recent works related to BDT-based polymers for the applications in PSCs and try to reveal the correlations of molecular structures of BDT-based polymers with their band gaps and molecular energy levels.

A planar benzodithiophene with lower HOMO was copolymerized with the thieno[3,4-b]thiophene unit to obtain a new low band gap polymer of PBDPTT-C, which exhibited a higher open-circuit voltage (V(oc)) of 0.8 V and a promising efficiency of 5.2%.