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The backbone composition of conjugated copolymers is of great importance in determining the conjugated structure and intermolecular assembly and in manipulating their optical, electrochemical, and electronic properties. However, limited attention has been directed at controlling the backbone composition of donor–acceptor (D–A) type low bandgap polymers. Herein, we developed a series of D–A random copolymers (P(BDTT-r-DPP)) composed of different compositions of electron-rich (D) thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and electron-deficient (A) pyrrolo[3,4-c]pyrrole-1,4-dione (DPP). The optical and electrical properties of D–A random copolymers could be controlled by tuning the ratios of BDTT to DPP (4:1, 2:1, 1:1, 1:2, and 1:4) in the polymer backbone; an increase in BDTT resulted in increased absorption in the range of 400–600 nm and a lower-lying highest occupied molecular orbital energy level, while a higher proportion of DPP induced stronger absorption in the range of 700–900 nm. The... more...

A novel 56 π-electron fullerene acceptor material, benzocyclobutene-C 60 bisadducts (BCBCBA), was developed via the [2+2] cycloaddition of in-situ generated benzyne with C 60 to raise the lowest unoccupied molecular orbital (LUMO) energy level of the fullerene. This four-membered linkages to C 60 of BCBCBA resulted in the fullerene bisadducts with very addends, thus minimizing the adverse effects of their regioisomers on the intermolecular packing between fullerene bisadducts. In addition, the LUMO level of BCBCBA was as high as −3.70 eV, which is 0.16 eV higher than that of phenyl-C 61 -butyric acid methyl ester (PCBM) (−3.86 eV). This result indicates the significant potential for improving open-circuit voltage ( V OC ) values in polymer solar cells (PSCs). The poly(3-hexylthiophene) (P3HT):BCBCBA based PSCs show V OC of 0.73 and power conversion efficiency (PCE) of 3.51%, which are similar to the P3HT:bis-PCBM based PSCs. more...

Polymer/small-molecule parallel-connected tandem organic solar cells are explored as a simple platform for high-efficiency tandem solar cells that are not subject to complications associated with complex current matching or successive solution processing. In particular, MoOx/Ag/MoOx (MAM) trilayer electrodes are investigated as an intermediate electrode that connects a front subcell composed of a solution-processed bulk heterojunction of poly{2,6′-4,8-di(5-ethylhexylthienyl) benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDTT-DPP) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and a back subcell composed of a thermally evaporated C70 layer doped with 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC). The effect of each MoOx layer is analyzed in terms of both optical and electrical properties. A joint theoretical and experimental study indicates that the electric field is distributed along the cell depth such that the photogenerated current of the front subcell depends on the thickness of both MoOx layers, with a varied interference effect, and that of the back subcell depends primarily on the thickness of the adjacent MoOx layer, with a varied cavity resonance effect. Based on optimized conditions of MAM electrodes, parallel tandem hybrid organic solar cells having higher power conversion efficiency than that of single junction cells are successfully realized. more...

Conjugated homopolymers can be synthesized more simply and reproducibly at lower cost than widely developing donor–acceptor (D–A) alternating copolymers. However, except for well-known poly(3-hexylthiophene) (P3HT), almost no successful homopolymer-based polymer solar cells (PSCs) have been reported because of their relatively wide band gap and unoptimized energy levels that limit the values of short circuit current (JSC) and open-circuit voltage (VOC) in PSCs. Herein, we report the development of poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene) (PBDTT) homopolymer that has high light absorption coefficients and nearly perfect energy alignment with that of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Therefore, we were able to produce high-performance PSCs with the power conversion efficiency (PCE) of 6.12%, benefiting from both high VOC (0.93 V) and JSC (11.95 mA cm–2) values. To the best of our knowledge, this PCE value is one of the highest values reported for the homo... more...

A new series of donor–acceptor (D–A) conjugated random terpolymers (PBDTT–DPP–TPD) were synthesized from electron-rich thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT), in conjugation with two electron-deficient units, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) and thieno[3,4-c]pyrrole-4,6-dione (TPD), of different electron-withdrawing strengths. The optical properties of these random terpolymers can be easily controlled by tuning the ratio between DPP and TPD; an increase in TPD induced increased absorption between 400 and 650 nm and a lower highest occupied molecular orbital energy level, while higher DPP contents resulted in stronger absorption between 600 and 900 nm. The best power conversion efficiency (PCE) of 6.33% was obtained from PBDTT–DPP75–TPD25 with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) due to the improved light absorption and thus a short-circuit current density (JSC) higher than 16 mA/cm2. Interestingly, the trend observed in the PCE values differed from that of optical be... more...

Fullerene tris-adducts have the potential of achieving high open-circuit voltages (V(OC)) in bulk heterojunction (BHJ) polymer solar cells (PSCs), because their lowest unoccupied molecular orbital (LUMO) level is higher than those of fullerene mono- and bis-adducts. However, no successful examples of the use of fullerene tris-adducts as electron acceptors have been reported. Herein, we developed a ternary-blend approach for the use of fullerene tris-adducts to fully exploit the merit of their high LUMO level. The compound o-xylenyl C60 tris-adduct (OXCTA) was used as a ternary acceptor in the model system of poly(3-hexylthiophene) (P3HT) as the electron donor and the two soluble fullerene acceptors of OXCTA and fullerene monoadduct (o-xylenyl C60 monoadduct (OXCMA), phenyl C61-butyric acid methyl ester (PCBM), or indene-C60 monoadduct (ICMA)). To explore the effect of OXCTA in ternary-blend PSC devices, the photovoltaic behavior of the device was investigated in terms of the weight fraction of OXCTA (W(OXCTA)). When W(OXCTA) is small (0.3), OXCTA can generate a synergistic bridging effect between P3HT and the fullerene monoadduct, leading to simultaneous enhancement in both V(OC) and short-circuit current (J(SC)). For example, the ternary PSC devices of P3HT:(OXCMA:OXCTA) with W(OXCTA) of 0.1 and 0.3 exhibited power-conversion efficiencies (PCEs) of 3.91% and 3.96%, respectively, which were significantly higher than the 3.61% provided by the P3HT:OXCMA device. Interestingly, for W(OXCTA)0.7, both V(OC) and PCE of the ternary-blend PSCs exhibited nonlinear compositional dependence on W(OXCTA). We noted that the nonlinear compositional trend of P3HT:(OXCMA:OXCTA) was significantly different from that of P3HT:(OXCMA:o-xylenyl C60 bis-adduct (OXCBA)) ternary-blend PSC devices. The fundamental reasons for the differences between the photovoltaic trends of the two different ternary-blend systems were investigated systemically by comparing their optical, electrical, and morphological properties. more...

Polymer solar cells (PSCs) consisting of fullerene bis-adduct and poly(3-hexylthiophene) (P3HT) blends have shown higher efficiencies than P3HT:phenyl C(61)-butyric acid methyl ester (PCBM) devices, because of the high-lying lowest unoccupied molecular orbital (LUMO) level of the fullerene bis-adducts. In contrast, the use of fullerene bis-adducts in donor-acceptor (DA) copolymer systems typically causes a decrease in the device's performance due to the decreased short-circuit current (J(SC)) and the fill factor (FF). However, the reason for such poor performance in DA copolymer:fullerene bis-adduct blends is not fully understood. In this work, bulk-heterojunction (BHJ)-type PSCs composed of three different electron donors with four different electron acceptors were chosen and compared. The three electron donors were (1) poly[(4,8-bis-(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene)-2,6-diyl-alt-(5-octylthieno[3,4-c]pyrrole-4,6-dione)-1,3-diyl] (PBDTTPD), (2) 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), and (3) P3HT polymers. The four electron acceptors were (1) PCBM, (2) indene-C(60) monoadduct (ICMA), (3) indene-C(60) bis-adduct (ICBA), and (4) indene-C(60) tris-adduct (ICTA). To understand the difference in the performance of BHJ-type PSCs for the three different polymers in terms of the choice of fullerene acceptor, the structural, optical, and electrical properties of the blends were measured by the external quantum efficiency (EQE), photoluminescence, grazing incidence X-ray scattering, and transient absorption spectroscopy. We observed that while the molecular packing and optical properties cannot be the main reasons for the dramatic decrease in the PCE of the DA copolymers and ICBA, the value of the driving force for charge transfer (ΔG(CT)) is a key parameter for determining the change in J(SC) and device efficiency in the DA copolymer- and P3HT-based PSCs in terms of fullerene acceptor. The low EQE and J(SC) in PBDTTPD and PBDTTT-C blended with ICBA and ICTA were attributed to an insufficient ΔG(CT) due to the higher LUMO levels of the fullerene multiadducts. Quantitative information on the efficiency of the charge transfer was obtained by comparing the polaron yield, lifetime, and exciton dissociation probability in the DA copolymer:fullerene acceptor films. more...

Systematic control of the chemical structure of conjugated polymers is critically important to elucidate the relationship between the conjugated polymer structures and properties and to optimize their performance in bulk heterojunction (BHJ) polymer solar cell (PSC) devices. Herein, we synthesized three new copolymers, i.e., P0, P1, and P2; these copolymers contain the same benzodithiophene donor unit but have different acceptor units with different numbers of nitrogen atoms in the range of 0–2. The effects of the introduced nitrogen atoms on the structural, optical, electrical, and photovoltaic properties of the conjugated polymers were investigated; the structural properties of the polymers, in particular, were studied using both experimental (grazing-incidence X-ray scattering (GIXS) measurements) and computational methods (molecular simulation). As the number of introduced nitrogen atoms increased, the planarity of the main chain conformation increased in the order of P0 < P1 < P2. Additionally, the P... more...

The ability to tune the lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels of fullerene derivatives used as electron acceptors is crucial in controlling the optical/electrochemical properties of these materials and the open circuit voltage (V(oc)) of solar cells. Here, we report a series of indene fullerene multiadducts (ICMA, ICBA, and ICTA) in which different numbers of indene solubilizing groups are attached to the fullerene molecule. The addition of indene units to fullerene raised its LUMO and HOMO levels, resulting in higher V(oc) values in the photovoltaic device. Bulk-heterojunction (BHJ) solar cells fabricated from poly(3-hexylthiophene) (P3HT) and a series of fullerene multiadducts-ICMA, ICBA, and ICTA showed V(oc) values of 0.65, 0.83, and 0.92 V, respectively. Despite demonstrating the highest V(oc) value, the P3HT:ICTA device exhibited lower efficiency (1.56%) than the P3HT:ICBA device (5.26%) because of its lower fill factor and current. This result could be explained by the lower light absorption and electron mobility of the P3HT:ICTA device, suggesting that there is an optimal number of the solubilizing group that can be added to the fullerene molecule. The effects of the addition of solubilizing groups on the optoelectrical properties of fullerene derivatives were carefully investigated to elucidate the molecular structure-device function relationship. more...

All-polymer solar cells have shown great potential as flexible and portable power generators. These devices should offer good mechanical endurance with high power-conversion efficiency for viability in commercial applications. In this work, we develop highly efficient and mechanically robust all-polymer solar cells that are based on the PBDTTTPD polymer donor and the P(NDI2HD-T) polymer acceptor. These systems exhibit high power-conversion efficiency of 6.64%. Also, the proposed all-polymer solar cells have even better performance than the control polymer-fullerene devices with phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor (6.12%). More importantly, our all-polymer solar cells exhibit dramatically enhanced strength and flexibility compared with polymer/PCBM devices, with 60- and 470-fold improvements in elongation at break and toughness, respectively. The superior mechanical properties of all-polymer solar cells afford greater tolerance to severe deformations than conventional polymer-fullerene solar cells, making them much better candidates for applications in flexible and portable devices., All-polymer solar cells have advantages over fullerene-based solar cells due to improved stability and tunable chemical and electronic properties. Here, Kim et al. develop highly efficient and robust solar cells based on PBDTTTPD and P(NDI2HD-T), highlighting their potential in flexible and portable electronics. more...

Remarkable progress has been realized in improving the power conversion efficiency (PCE) of polymer solar cells (PSCs) over the past few years, raising the possibility of their commercialization as an alternative counterpart to conventional inorganic solar cells. The development of conjugated polymers that contain various alternately bonded electron-rich (D) and electron-deficient (A) units, called D–A alternating copolymers, plays a crucial role in improving the PCE of PSCs, which has now exceeded 8–9%. However, further development of conjugated polymers is essential to produce highly efficient and stable PSCs with a PCE of more than 10% for commercialization. In this regard, terpolymers, which consist of three different units in the polymer backbone, are promising candidates because they can show synergetic effects of each unit by tuning the composition ratio, thereby enabling full-range absorption, high charge mobility, and good solubility. Substantial progress in the development of terpolymers has been achieved with regard to PCE. Herein, we review the development of terpolymers for PSCs and discuss the major factors that need to be considered in the design of new terpolymers. more...

A simple and efficient approach of controlling the side-chain density in the electron donating polymers has been demonstrated to tune their 3-D packing structure and HOMO level, which increases the hole mobility and V(oc) values, thus improving the solar cell performance. more...