Photovoltaic Performance Improvement of D–A Copolymers Containing Bithiazole Acceptor Unit by Using Bithiophene Bridges

Two bithiophene-bridged D–A copolymers, PDTSBTBTz and PBDTBTBTz, based on bithiazole acceptor unit and dithienosiole (DTS) or benzodithiophene (BDT) donor unit, were synthesized by the Pd-catalyzed Stille-coupling reaction. The two copolymers exhibit good thermal stability, strong absorption in the visible region, and relatively lower HOMO energy level at ca. −5.10 eV. The hole mobilities of PDTSBTBTz and PBDTBTBTz measured by SCLC method are 1.85 × 10^–3 and 1.77 × 10^–3 cm²/(V s), respectively. Power conversion efficiency (PCE) of the polymer solar cell (PSC) based on PDTSBTBTz: PC₇₀BM (1:1, w/w) was 3.82% with <i>J</i>_sc = 8.68 mA/cm², <i>V</i>_oc = 0.72 V, and FF = 0.611, under the illumination of AM1.5, 100 mW/cm². In contrast, the PCE of the PSC based on PBDTBTBTz:PC₇₀BM (1:1, w/w) reached 4.46% with <i>J</i>_sc = 9.01 mA/cm², <i>V</i>_oc = 0.82 V, and FF = 0.603. These results indicate that bithiophene-bridged D–A copolymers are promising photovoltaic donor materials for the application in PSCs

Efficient Polymer Solar Cells Based on Poly(3-hexylthiophene) and Indene–C₆₀ Bisadduct Fabricated with Non-halogenated Solvents

The photovoltaic performance of poly­(3-hexylthiophene) (P3HT) has been improved greatly by using indene–C₆₀ bisadduct (ICBA) as acceptor instead of phenyl-C₆₁-butyric acid methyl ester (PCBM). However, the solvent of dichlorobenzene (DCB) used in fabricating polymer solar cells (PSCs) limited the application of the PSCs, because of the environmental problem caused by the harmful halogenated solvent. In this work, we fabricated the PSCs based on P3HT/ICBA processed with four low-harmful non-halogenated solvents of toluene, <i>o</i>-xylene, <i>m</i>-xylene, and <i>p</i>-xylene. The PSCs based on P3HT/ICBA (1:1, w/w) with toluene as the solvent exhibit the optimized power conversion efficiency (PCE) of 4.5% with open-circuit voltage (<i>V</i>_oc) of 0.84 V, short circuit current density (<i>J</i>_sc) of 7.2 mA/cm², and fill factor (FF) of 71%, under the illumination of AM 1.5G at 100 mW/cm². Upon using 1% <i>N</i>-methyl pyrrolidone (NMP) as a solvent additive in the toluene solvent, the PCE of the PSCs was greatly improved to 6.6% with a higher <i>J</i>_sc of 10.3 mA/cm² and a high FF of 75%, which is even higher than that of the devices fabricated with halogenated DCB solvent. The X-ray diffraction (XRD) measurement shows that the crystallinity of P3HT increased with the NMP additive. The investigations on morphology of the active layers by atomic force microscopy (AFM) and transmission electron microscopy (TEM) indicate that the NMP additive promotes effective phase separation and formation of nanoscaled interpenetrating network structure of the active layer, which is beneficial to the improvement of <i>J</i>_sc and PCE for the PSCs fabricated with toluene as the solvent

Conjugated Side-Chain Isolated Polythiophene: Synthesis and Photovoltaic Application

A design concept of “side chain isolation” was proposed for developing new polythiophene derivatives with conjugated side chain (CSC-PTs), and <b>PT5TPA</b> with styryl–triphenylamine (TPA) side chain and unsubstituted tetrathienyl spacer was designed and synthesized. Compared to previously reported CSC-PTs, side chain isolated <b>PT5TPA</b> showed red–shifted and enhanced π–π* transition absorption of the polymer backbone along with the shoulder peak and steep absorption edge, indicating improved planarity of the backbone. In addition, the unsubstituted thiophene spacer along the polymer backbone of the side chain isolated <b>PT5TPA</b> results in a lower HOMO energy level of the polymer at −5.1 eV. The polymer solar cell based on <b>PT5TPA</b> as donor and indene–C₆₀ bisadduct as acceptor displayed a power conversion efficiency of 3.6% with a high open circuit voltage of 0.94 V, under the illumination of AM1.5G, 100 mW/cm². The results indicate that the side chain isolated CSC-PTs could open a new way for developing high performance photovoltaic polymers

Conjugated and Nonconjugated Substitution Effect on Photovoltaic Properties of Benzodifuran-Based Photovoltaic Polymers

In order to investigate the influence of two-dimensional (2D) conjugated structure on photovoltaic properties of benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­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 pendants on dithienyl-BDF are more coplanar than it on dithienyl-BDT. These results indicate that the 2D-conjugated structure is helpful for molecular structure design of the BDF-based polymers in enhancing the intermolecular π–π stacking and improving charge transport property. Furthermore, the photovoltaic devices based on these two polymers show similar short circuit density and fill factor values, while the open circuit voltage of the PBDFTT-CF-T-based device is 0.78 V, which is 0.15 V higher than that of the PBDFTT-CF-O-based device. Therefore, the efficiencies of the devices based PBDFTT-CF-T/PC₇₁BM and PBDFTT-CF-O/PC₇₁BM are 6.26% and 5.22%, respectively. The results in this work demonstrate that the weak electron-donating ability of alkylthienyl side groups can be seen as an effective strategy to improve photovoltaic properties of the BDF-based polymers and the 2D-conjugated molecular structure is favorable to improve hole mobility

Remove the Residual Additives toward Enhanced Efficiency with Higher Reproducibility in Polymer Solar Cells

Undesirable efficiency reproducibility was sometimes observed in fabrication of high performance polymer solar cell devices incorporating high boiling point additives. The anomalous results originated from the slow drying of additives not only reduced the controllability of device performance but also impeded the studies of device physics and material design. How to remove the residual additives and achieve stable interface properties is crucial for both the academic and industrial community. Herein, we demonstrated that the morphological stability is enhanced and efficiency reproducibility is increased obviously from 7.07 ± 0.27% to 7.53 ± 0.12% after spin-coating inert solvents for the PBDTTT-C-T/PCBM system. The relationship between processing conditions and photovoltaic performance was well explored and demonstrated via multiple techniques including atomic force microscopy, Kelvin probe force microscopy, transmission electron microscopy, and X-ray photospectroscopy. Most importantly, this method was successfully employed in more than five representative donor polymers. Our study suggested that the slow drying process of the residual high boiling point additives could induce undesirable morphological variation as well as unfavorable interfacial contact, and by washing with low boiling point “inert” solvent, like methanol, the negative influence caused by the residual additive can be avoided and hence the additives would perform more efficiently in the optimization of device performance of highly efficient PSCs

Poly(thieno[3,2‑<i>b</i>]thiophene-<i>alt</i>-bithiazole): A D–A Copolymer Donor Showing Improved Photovoltaic Performance with Indene‑C₆₀ Bisadduct Acceptor

A new D–A copolymer, poly­(thieno­[3,2-<i>b</i>]­thiophene-<i>alt</i>-bithiazole) (<b>PTTBTz</b>), based on thieno­[3,2-<i>b</i>]­thiophene donor unit and bithiazole acceptor unit, was synthesized by the Pd-catalyzed Stille-coupling reaction for the application as donor material in polymer solar cells (PSCs). <b>PTTBTz</b> film possesses high thermal stability with 5% weight-loss temperature at 450 °C, a lower-lying HOMO energy level at −5.20 eV, a higher hole mobility of 6.45 × 10^–3 cm²/(V s), and a crystalline structure. Photovoltaic performance of the polymer was investigated with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM) or indene-C₆₀ bisadduct (ICBA) as acceptor and with 3% DIO additive. The power conversion efficiency (PCE) of the PSC based on <b>PTTBTz</b>:ICBA (1:1 w/w) reached 5.35% with a high <i>V</i>_oc of 1.03 V, a <i>J</i>_sc of 8.55 mA/cm², and an FF of 0.608, whereas the PCE of the PSC based on <b>PTTBTz</b>:PC₇₀BM (1:1 w/w) was 4.57% with a <i>V</i>_oc of 0.82 V, a <i>J</i>_sc of 9.89 mA/cm², and an FF of 0.563, under the illumination of AM1.5, 100 mW/cm². <b>PTTBTz</b> is one of the D–A copolymers that shows better photovoltaic performance with ICBA as acceptor than PC₇₀BM. <b>PTTBTz</b>/ICBA could be a promising front active layer for high-efficiency tandem PSC because of its high <i>V</i>_oc

Effects of π-Conjugated Bridges on Photovoltaic Properties of Donor-π-Acceptor Conjugated Copolymers

A series of conjugated donor (D)-π-acceptor (A) copolymers, <b>P­(BDT-F-BT)</b>, <b>P­(BDT-T-BT)</b>, and <b>P­(BDT-TT-BT)</b>, based on benzodithiophene (BDT) donor unit and benzothiadiazole (BT) acceptor unit with different π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridges between the BDT donor unit and BT acceptor unit are furan (<b>F</b>) in <b>P­(BDT-F-BT)</b>, thiophene (<b>T</b>) in <b>P­(BDT-T-BT)</b> and thieno­[3,2-<i>b</i>]­thiophene (<b>TT</b>) in <b>P­(BDT-TT-BT)</b>. It was found that the π-bridges significantly affect the molecular architecture and optoelectronic properties of the copolymers. With the π-bridge varied from furan to thiophene, then to thieno­[3,2-<i>b</i>]­thiophene, the shape of the molecular chains changed from z-shaped to almost straight line gradually. Band gaps of <b>P­(BDT-F-BT)</b>, <b>P­(BDT-T-BT)</b> and <b>P­(BDT-TT-BT)</b> were tuned from 1.96 to 1.82 to 1.78 eV with HOMO levels up-shifted from −5.44 to −5.35 to −5.21 eV, respectively. Bulk heterojunction solar cells with the polymers as donor and PC₇₁BM as acceptor demonstrated power conversion efficiency varied from 2.81% for <b>P­(BDT-F-BT)</b> to 3.72% for <b>P­(BDT-T-BT)</b> and to 4.93% for <b>P­(BDT-TT-BT)</b>. Compared to furan and thiophene, thieno­[3,2-<i>b</i>]­thiophene π-bridge in the copolymers shows superior photovoltaic performance. The results indicate that the photovoltaic performance of some high efficiency D–A copolymers reported in literatures could be improved further by inserting suitable π-bridges

Selenium-Containing Medium Bandgap Copolymer for Bulk Heterojunction Polymer Solar Cells with High Efficiency of 9.8%

In this work, a new D–A copolymer based on <i>m</i>-alkoxyphenyl substituted benzodithiophene (BDT-<i>m</i>-OP) as donor unit and benzo­[1,2-<i>c</i>:4,5-<i>c</i>′]­dithiophene-4,8-dione (BDD) as acceptor unit was designed and synthesized, in which selenophene unit as π-conjugated spacer was incorporated into the polymer backbone to broaden the absorption spectrum, enhance the charge transport properties, and even improve the photovoltaic properties. Compared with PBPD-Th with thiophene as π-conjugated spacer, PBPD-Se exhibits an evidently extended absorption spectrum and an enhanced hole mobility with a slightly raised HOMO energy level. The PBPD-Se:PC₇₁BM-based PSCs exhibits a significantly improved PCE of 9.8% with an enhanced <i>J</i>_sc of 14.9 mA cm^–2 and a slightly lower <i>V</i>_oc of 0.90 V in comparison with a PCE of 8.4% with a <i>V</i>_oc of 0.95 V and a <i>J</i>_sc of 12.4 mA cm^–2 for PBPD-Th:PC₇₁BM-based devices. These results indicate that the rational selection of π-conjugated spacer in the D–A copolymer backbone is very essential to achieve high efficiency PSCs

Diketopyrrolopyrrole-Based Conjugated Polymers with Perylene Bisimide Side Chains for Single-Component Organic Solar Cells

Diketopyrrolopyrrole-Based Conjugated Polymers with Perylene Bisimide Side Chains for Single-Component Organic Solar Cell