Poly(benzodithiophene) Homopolymer for High-Performance Polymer Solar Cells with Open-Circuit Voltage of Near 1 V: A Superior Candidate To Substitute for Poly(3-hexylthiophene) as Wide Bandgap Polymer

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 (<i>J</i>_SC) and open-circuit voltage (<i>V</i>_OC) 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 <i>V</i>_OC (0.93 V) and <i>J</i>_SC (11.95 mA cm^–2) values. To the best of our knowledge, this PCE value is one of the highest values reported for the homopolymer donor-based PSCs. Significantly, the optimized condition of the device was achieved without any solvent additive or thermal treatment. Therefore, PBDTT is a promising candidate to take over the role of P3HT in tandem solar cells and ternary blend solar cells

Importance of Optimal Composition in Random Terpolymer-Based Polymer Solar Cells

A new series of donor–acceptor (D–A) conjugated random terpolymers (PBDTT–DPP–TPD) were synthesized from electron-rich thienyl-substituted benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDTT), in conjugation with two electron-deficient units, pyrrolo­[3,4-<i>c</i>]­pyrrole-1,4-dione (DPP) and thieno­[3,4-<i>c</i>]­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 (<i>J</i>_SC) higher than 16 mA/cm². Interestingly, the trend observed in the PCE values differed from that of optical behavior of the PBDTT–DPP–TPD in terms of the DPP to TPD ratio, showing nonlinear compositional dependence from 2 to 6%. Density functional theory calculations showed that the small portions of strong electron-withdrawing DPP in PBDTT–DPP25–TPD75 and PBDTT–DPP10–TPD90 could provide trap sites, which suppress efficient charge transfer. In contrast, for PBDTT–DPP90–TPD10 and PBDTT–DPP75–TPD25, the effect of minor portions of TPD on electron density distribution was found to be minimal. In addition, the polymer packing and nanomorphology were investigated by grazing-incidence X-ray scattering and atomic force microscopy. The findings suggested that controlling the ratio of electron-deficient units in the random terpolymers is critical for optimizing their performance in polymer solar cells because it affects the polymer packing structure, the optical and electrical properties, and the electron distribution in the polymers

Effect of Incorporated Nitrogens on the Planarity and Photovoltaic Performance of Donor–Acceptor Copolymers

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., <b>P0</b>, <b>P1</b>, and <b>P2</b>; 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 <b>P0</b> < <b>P1</b> < <b>P2</b>. Additionally, the <b>P0</b>, <b>P1</b>, and <b>P2</b> polymers showed increased interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively, with increased intermolecular ordering. These results were in excellent agreement with the simulation results. In addition, the enhanced planarity resulted in a red-shifting at the onset of absorption in the polymer film from 544 to 585 nm, a downshift in the lowest unoccupied molecular orbital (LUMO) energy level from −3.02 to −3.26 eV, and an increase in the hole mobility from 2.33 × 10^–6 to 3.78 × 10^–5 cm²/(V s). As a result, we observed dramatically enhanced performance of the PSCs in the order of <b>P0</b> < <b>P1</b> < <b>P2</b>. For example, the <b>P2</b>:PC₆₁BM device exhibited a 3.5-fold improvement in power conversion efficiency (PCE) compared to that of <b>P0</b>:PC₆₁BM. The further optimization of <b>P2</b> with PC₇₁BM showed the PCE of 3.22%

Effect of Fullerene Tris-adducts on the Photovoltaic Performance of P3HT:Fullerene Ternary Blends

Fullerene tris-adducts have the potential of achieving high open-circuit voltages (<i>V</i>_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 <i>o</i>-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 (<i>o</i>-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 (<i>W</i>_OXCTA). When <i>W</i>_OXCTA is small (<0.3), OXCTA can generate a synergistic bridging effect between P3HT and the fullerene monoadduct, leading to simultaneous enhancement in both <i>V</i>_OC and short-circuit current (<i>J</i>_SC). For example, the ternary PSC devices of P3HT:(OXCMA:OXCTA) with <i>W</i>_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 <i>W</i>_OXCTA > 0.7, both <i>V</i>_OC and PCE of the ternary-blend PSCs exhibited nonlinear compositional dependence on <i>W</i>_OXCTA. We noted that the nonlinear compositional trend of P3HT:(OXCMA:OXCTA) was significantly different from that of P3HT:(OXCMA:<i>o</i>-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

Controlling Number of Indene Solubilizing Groups in Multiadduct Fullerenes for Tuning Optoelectronic Properties and Open-Circuit Voltage in Organic Solar Cells

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 (<i>V</i>_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 <i>V</i>_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 <i>V</i>_oc values of 0.65, 0.83, and 0.92 V, respectively. Despite demonstrating the highest <i>V</i>_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

Photoinduced Charge Transfer in Donor–Acceptor (DA) Copolymer: Fullerene Bis-adduct Polymer Solar Cells

Polymer solar cells (PSCs) consisting of fullerene bis-adduct and poly­(3-hexylthiophene) (P3HT) blends have shown higher efficiencies than P3HT:phenyl C₆₁-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 (<i>J</i>_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-<i>b</i>:4,5-<i>b</i>′]­dithiophene)-2,6-diyl-<i>alt</i>-(5-octylthieno­[3,4-<i>c</i>]­pyrrole-4,6-dione)-1,3-diyl] (PBDTTPD), (2) poly­[(4,8-bis-(2-ethylhexyloxy)benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene)-2,6-diyl-<i>alt</i>-(4-(2-ethylhexanoyl)-thieno­[3,4-<i>b</i>]­thiophene)-2,6-diyl] (PBDTTT-C), and (3) P3HT polymers. The four electron acceptors were (1) PCBM, (2) indene-C₆₀ monoadduct (ICMA), (3) indene-C₆₀ bis-adduct (ICBA), and (4) indene-C₆₀ 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 (Δ<i><i>G</i></i>_CT) is a key parameter for determining the change in <i>J</i>_SC and device efficiency in the DA copolymer- and P3HT-based PSCs in terms of fullerene acceptor. The low EQE and <i>J</i>_SC in PBDTTPD and PBDTTT-C blended with ICBA and ICTA were attributed to an insufficient <i>Δ<i>G</i></i>_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