Polymer Solar Cell Devices Research Articles

Eco‐Friendly Solvent‐Processed Fullerene‐Free Polymer Solar Cells with over 9.7% Efficiency and Long‐Term Performance Stability

AbstractA wide‐bandgap polymer, (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(2,5‐(methyl thiophene carboxylate))]) (3MT‐Th), is synthesized to obtain a complementary broad range absorption when harmonized with 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (ITIC). The synthesized regiorandom 3MT‐Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT‐Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT‐Th‐based PSCs exhibit excellent shelf‐life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene‐3‐carboxylate in 3MT‐Th is a promising new accepting unit for constructing p‐type polymers used for high‐performance nonfullerene‐type PSCs.

High-performance fluorine-containing BDT-based copolymer for organic solar cells with a high open circuit voltage

A new donor–acceptor (D-A) conjugated copolymer (PBDTT(ff)-ttTPD) based on fluorine-substituted benzodithiophene (BDT) and 6-alkylthienothienyl thieno[3,4-c]pyrrole-4,6-dione (ttTPD) has been synthesized via a Stille cross-coupling reaction. As a control, the nonfluorinated BDT-based ttTPD copolymer (PBDTT-ttTPD) was also synthesized by using the same polymerization method. The number-average molecular weights (Mn) of PBDTT(ff)-ttTPD and PBDTT-ttTPD were found to be 48,000 g/mol (Đ = 2.2) and 43,000 g/mol (Đ = 2.1), respectively. The HOMO levels of PBDTT(ff)-ttTPD and PBDTT-ttTPD were calculated to be −5.65 and −5.45 eV, respectively. The inclusion of fluorinated BDT units is a very effective approach to lowering the polymer's HOMO level. The SCLC mobilities of PBDTT(ff)-ttTPD and PBDTT-ttTPD were determined to be 5.9 × 10−4 and 3.0 × 10−4 cm2/Vs, respectively. Polymer solar cell devices prepared with PBDTT(ff)-ttTPD and PBDTT-ttTPD as their active layers were found to exhibit power conversion efficiencies of 7.45 and 6.79% with open circuit voltages of 0.98 and 0.84 V, respectively. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2506–2512

Effects of Regioisomers in Fullerene Derivatives on Photovoltaic Properties of Bulk Heterojunction Solar Cells

During the last decade, bulk heterojunction (BHJ) polymer solar cells (PSCs) revealed rapid progress and achieved a power conversion efficiency (PCE) of 6-10%, owing to the development of new high-performance low-bandgap polymers as electron donors. Meanwhile, although new electron-accepting materials for PSC devices habe been explored, [70]fullerene derivatives are still prevalent as electron acceptors. An important aspect of [70]fullerene derivatives, which is always neglected in PSC applications, is that as-prepared [70]fullerene mono-adducts consist of a mixture of regioisomers due to the lower symmetry of rugby-ball shaped C70 over spherical C60. As contrasted with C60, C70 has four distinct types of [6,6]-bonds (a-, b-, e-, and k-type bonds), and thereby [70]fullerene derivatives including a prevalent, high-performance [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) have been employed as a regioisomer mixture in BHJ PSC devices. Considering that the miscibility with conjugated polymers and the molecular arrangement of [70]fullerene derivatives in the active layer have a large impact on charge separation and charge-transporting properties, pristine isomers of [70]fullerene derivatives rather than the isomer mixtures would yield a more desirable donor-acceptor network toward highly efficient BHJ PSCs. Indeed, the regioisomer separations of fullerene bis-adducts have been demonstrated to exhibit positive effects on BHJPSCs. In this study, the regioisomers of [70]fullerene mono-adducts have been separated for the first time to evaluate the pure isomer effect on the photovoltaic properties. To assess the substitution position effect on C70 accurately, first we designed new dihydronaphthyl-substituted [70]fullerenes with two butoxycarbonyl groups ([70]NCMA) as a simplified system. The symmetrical dihydronaphthyl group was selected as the substituent to eliminate the plausible isomers such as enantiomers and diastereomers. Then, we have further extended this strategy to separate the regioisomers of a prevalent, high-performance [70]fullerene mono-adduct acceptor, [70]PCBM, for the first time. [1] S. Kitaura, K. Kurotobi, M. Sato, Y. Takano, T. Umeyama, and H. Imahori, Chem. Commun., 48, 8550-8552 (2012). [2] R. Tao, T. Umeyama, K. Kurotobi, and H. Imahori, ACS Appl. Mater. Interface, 6, 17313-17322 (2014). [3] R. Tao, T. Umeyama, T. Higashino, T. Koganezawa, and H. Imahori, Chem. Commun., 51, 8199-8388 (2015). [4] R. Tao, T. Umeyama, T. Higashino, T. Koganezawa and H. Imahori, ACS Appl. Mater. Interfaces, 7, 16676-16685 (2015). [5] T. Umeyama, T. Miyata, A. C. Jakowetz, S. Shibata, K. Kurotobi, T. Higashino, T. Koganezawa, M. Tsujimoto, , S. Gélinas, W. Matsuda, S. Seki, R. H. Friend, and H. Imahori, Chem. Sci., in press.

Efficient Fullerene-Free Polymer Solar Cells Based on Alkylthio Substituted Conjugated Polymers

A series of medium and wide band gap polymers with different backbones and side-chains were designed and synthesized as electron donors for fullerene-free polymer solar cells (PSCs). The relationship between the molecular structures of the polymer donors and the photovoltaic properties of the fullerene-free PSC devices was fully investigated. The optical band gaps and the energy levels of the polymers were modulated by the copolymerization of the benzodithiophene (BDT) unit with the various comonomers. Moreover, by incorporating alkylthio substituents, the highest occupied molecular orbital (HOMO) levels of the polymer donors were effectively reduced, and thus the open-circuit voltages (Voc) of the fullerene-free PSC devices were strongly improved. Simultaneously, the extinction coefficients of the polymers were also enhanced by introducing the alkylthio groups, which ensured sufficient light-harvesting capability. The exciton dissociation probability P(E,T) in the blend films of the alkylthio substituted...

Ruthenium based metallopolymer grafted reduced graphene oxide as a new hybrid solar light harvester in polymer solar cells

A new class of pyridyl benzimdazole based Ru complex decorated polyaniline assembly (PANI-Ru) was covalently grafted onto reduced graphene oxide sheets (rGO) via covalent functionalization approach. The covalent attachment of PANI-Ru with rGO was confirmed from XPS analysis and Raman spectroscopy. The chemical bonding between PANI-Ru and rGO induced the electron transfer from Ru complex to rGO via backbone of the conjugated PANI chain. The resultant hybrid metallopolymer assembly was successfully demonstrated as an electron donor in bulk heterojunction polymer solar cells (PSCs). A PSC device fabricated with rGO/PANI-Ru showed an utmost ~6 fold and 2 fold enhancement in open circuit potential (Voc) and short circuit current density (Jsc) with respect to the standard device made with PANI-Ru (i.e., without rGO) under the illumination of AM 1.5 G. The excellent electronic properties of rGO significantly improved the electron injection from PANI-Ru to PCBM and in turn the overall performance of the PSC device was enhanced. The ultrafast excited state charge separation and electron transfer role of rGO sheet in hybrid metallopolymer was confirmed from ultrafast spectroscopy measurements. This covalent modification of rGO with metallopolymer assembly may open a new strategy for the development of new hybrid nanomaterials for light harvesting applications.

Imidazole-Functionalized Fullerene as a Vertically Phase-Separated Cathode Interfacial Layer of Inverted Ternary Polymer Solar Cells.

By using a facile one-pot nucleophilic addition reaction, we synthesized a novel imidazole (IMZ)-functionalized fullerene (C60-IMZ), and applied it as a third component of inverted ternary polymer solar cells (PSCs), leading to dramatic efficiency enhancement. According to FT-IR, XPS spectroscopic characterizations, and elemental analysis, the chemical structure of C60-IMZ was determined with the average IMZ addition number estimated to be six. The lowest unoccupied molecular orbital (LUMO) level of C60-IMZ measured by cyclic voltammetry was -3.63 eV, which is up-shifted relative to that of 6,6-phenyl C61-butyric acid methyl ester (PC61BM). Upon doping C60-IMZ as a third component into an active layer blend of poly(3-hexylthiophene) (P3HT) and PC61BM, the power conversion efficiency (PCE) of the inverted ternary PSCs was 3.4% under the optimized doping ratio of 10 wt %, dramatically higher than that of the control device ITO/P3HT:PC61BM/MoO3/Ag based on the binary P3HT:PC61BM blend (1.3%). The incorporation of C60-IMZ results in enhancement of the absorption of P3HT:PC61BM blend film, increase of the electron mobility of the device, and rougher film surface of the P3HT:PC61BM active layer beneficial for interfacial contact with the Ag anode. Furthermore, C60-IMZ doped in P3HT:PC61BM blend may migrate to the surface of ITO cathode via vertical phase separation as revealed by XPS depth analysis, consequently forming a cathode interfacial layer (CIL), which can lower the work function (WF) of ITO cathode. Thus, the interfacial contact between the active layer and ITO cathode is improved, facilitating electron transport from the active layer to ITO cathode. The effectiveness of C60-IMZ as a vertically phase-separated CIL on efficiency enhancement of inverted ternary PSCs is further verified by doping it into another active layer system comprised of a low-bandgap conjugated polymer, poly(thieno[3,4-b]-thiophene/benzodithiophene) (PTB7), blended with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM). Under the optimized C60-IMZ doping ratio of 10 wt %, the PCE of the PTB7:PC71BM-based inverted ternary PSC device reaches 5.3%, which is about 2 times higher than that of the control binary device (2.6%).

Regioisomer effects of [70]fullerene mono-adduct acceptors in bulk heterojunction polymer solar cells.

Despite numerous organic semiconductors being developed during the past decade, C70 derivatives are predominantly used as electron acceptors in efficient polymer solar cells (PSCs). However, as-prepared C70 mono-adducts intrinsically comprise regioisomers that would mask individual device performances depending on the substituent position on C70. Herein, we separate the regioisomers of C70 mono-adducts for PSC applications for the first time. Systematic investigations of the substituent position effect using a novel symmetric C70 mono-adduct ([70]NCMA) and a prevalent, high-performance one ([70]PCBM) reveals that we can control the structures of the blend films with conjugated polymers and thereby improve the PSC performances by regioisomer separation. Our approach demonstrates the significance of exploring the best-matching regioisomer of C70 mono-adducts with high-performance conjugated polymers, which would achieve a remarkable progress in PSC devices.

Mechanism of inverted polymer solar cells based on poly(dopamine)/ZnO as composite cathode buffer layer

Inverted polymer solar cells with P3HT:PCBM as active layer are fabricated based on poly(dopamine)/ZnO (PDA/ZnO) as composite cathode buffer layer. Effects of PDA/ZnO composite cathode buffer layer with the different self-polymerization times on the device performance are investigated. According to the results, the short circuit current and photoelectric conversion efficiency of polymer solar cells first increase then decrease with the increase of the self-polymerization time of PDA. For 10-min PDA self-polymerization, the photovoltaic performance of the device achieves the optimal values:open circuit voltage 0.66 V, short circuit curent density 9.70 mA/cm2, fill factor 68.06%, and power conversion efficiency 4.35% under irratiation of light with a strength of 100 mW/cm2. We conclude that the improvement of device performance is due to the PDA/ZnO composite cathode buffer layer reduced the contact resistance between the ZnO and ITO, at the same time, the presence of a large number of nitrogen groups in PDA is advantageous for the electronic collection of the inverted polymer solar cells. Meanwhile, polymer solar cell with PDA/ZnO as composite cathode buffer layer also exhibits excelent stability. In addition, PDA has a strong adhesive force that makes the ZnO interface layer on its surface not easy to fall off. This provides a new way of fabricating the flexible polymer solar cell devices.

High-performance fullerene-free polymer solar cells with solution-processed conjugated polymers as anode interfacial layer

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.

Polymer solar cells with open-circuit voltage of 1.3 V using polymer electron acceptor with high LUMO level

Open-circuit voltage (VOC) of polymer solar cells (PSCs) is always in the range of 0.6–0.9V and rarely exceeds 1.0V. The common strategy to enhance VOC of PSCs is to use polymer donors with low-lying highest occupied molecular orbital energy level (EHOMO). In this manuscript, we report high VOC of 1.3V for PSCs by further using a polymer acceptor with high-lying lowest unoccupied molecular orbital energy level (ELUMO). With poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDTBT) as the donor and poly[(N,N′-bis(2-hexyldecyl)-diamine-bis(difluoro-borane)−2,2′-bipyridine)-alt-(2,5-thiophene)] (P-BNBP-T) as the acceptor, the PSC devices show the VOC of 1.30V and power conversion efficiency (PCE) of 3.20%. This VOC is higher than that of the conventional acceptor materials by 0.4V owing to the high-lying ELUMO of P-BNBP-T. We have also investigated the effect of processing solvents on the blend morphology and the device performance. Compared with high boiling-point solvent o-dichlorobenzene, low boiling-point solvent chloroform gives finer phase separation size, improved exciton dissociation and consequently better photovoltaic device performance. The demonstration of high VOC in this work indicates that there is large room for PCE enhancement of PSCs.

Synthesis of fluorinated benzotriazole (BTZ)- and benzodithiophene (BDT)-based low-bandgap conjugated polymers for solar cell applications

A series of donor–acceptor (D–A) polymers (P1–P3) based on benzodithiophene (BDT) and electron-accepting benzotriazole (BTZ) units containing thiophene linkers with/without alkyl side-chains were designed and synthesized via Stille coupling polymerization method. The effects of polymers with multiple fluorinated BTZ groups on their thermal, optical, electrochemical, and photovoltaic properties were investigated. These polymers possessed the highest occupied molecular orbital (HOMO) levels ranged −5.38 to −5.6 eV and the lowest unoccupied molecular orbital (LUMO) levels ranged −3.55 to −3.57 eV, which covered broad absorption ranges with low optical bandgaps. The bulk heterojunction (BHJ) polymer solar cell (PSC) devices containing an active layer of D-A polymers blended with different weight ratios of electron-acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were explored under 100 mW cm−2 of AM 1.5 white-light illumination, where the maximum power conversion efficiency (PCE) value of 3% (with Jsc = 7.70 mA/cm2, FF = 54.04, and Voc = 0.72 V) was obtained in the PSC device consisting of polymer P3.

A Novel pH Neutral Self-Doped Polymer for Anode Interfacial Layer in Efficient Polymer Solar Cells

Three pH neutral conjugated polymers based on thiophene and benzene derivatives, namely PCP-Na, PCF-Na, and PFS-Na, were designed and synthesized. In the three polymers, PCP-Na exhibited a self-doping effect in aqueous solution, and the thin film of PCP-Na demonstrated a high electrical conductivity of 1.66 × 10–3 S/cm and a deep HOMO level of −5.22 eV; these properties make it an ideal anode interfacial layer (AIL) material in polymer solar cells (PSCs). The single-junction PSC modified with PCP-Na AIL showed a PCE of 9.89%, which is comparable to that of the PEDOT:PSS-modified device (PCE = 9.53%). Notably, PCP-Na AIL exhibited excellent thickness insensitivity in fabricating PSC devices; i.e., the PSC devices with very thick PCP-Na interlayers over 300 nm still demonstrated a high fill factor over 70%, indicating the PCP-Na layer has excellent charge collection and transport properties. Furthermore, the homo-tandem PSC device was fabricated using a new connecting interlayer, which consists of PCP-Na as...

Control of Mesoscale Morphology and Photovoltaic Performance in Diketopyrrolopyrrole‐Based Small Band Gap Terpolymers

Morphology control is one of the key strategies in optimizing the performance of organic photovoltaic materials, particularly for diketopyrrolopyrrole (DPP)‐based donor polymers. The design of DPP‐based polymers that provide high power conversion efficiency (PCE) presents a significant challenge that requires optimization of both energetics and morphology. Herein, a series of high performance, small band gap DPP‐based terpolymers are designed via two‐step side chain engineering, namely introducing alternating short and long alkyls for reducing the domain spacing and inserting alkylthio for modulating the energy levels. The new DPP‐based terpolymers are compared to delineate how the side chain impacts the mesoscale morphology. By employing the alkylthio‐substituted terpolymer PBDPP‐TS, the new polymer solar cell (PSC) device realizes a good balance of a high Voc of 0.77 V and a high Jsc over 15 mA cm−2, and thus realizes desirable PCE in excess of 8% and 9.5% in single junction and tandem PSC devices, respectively. The study indicates better control of domain purity will greatly improve performance of single junction DPP‐based PSCs toward 10% efficiency. More significantly, the utility of this stepwise side chain engineering can be readily expanded to other classes of well‐defined copolymers and triggers efficiency breakthroughs in novel terpolymers for photovoltaic and related electronic applications.

Efficiency enhancement of polymer solar cells by applying an alcohol-soluble fullerene aminoethanol derivative as a cathode buffer layer

Cathode buffer layer (CBL) introduced between the active layer and cathode is crucial for selectively transporting electrons and blocking holes for polymer solar cells (PSCs). Calcium (Ca) is the most commonly used CBL in conventional-structure bulk heterojunction (BHJ) PSC devices, but is prone to oxidation due to its high reactivity, inhibiting its practical applications. Herein, we applied an alcohol-soluble fullerene aminoethanol derivative (C60-ETA) as an efficient CBL surpassing Ca in conventional-structure BHJ-PSC devices, leading to obvious efficiency enhancement with the best power conversion efficiency (PCE) reaching 9.66%. C60-ETA CBL was applied in PSC devices based on three different photoactive layer systems, including poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl C71-butyric acid methyl ester (PTB7-Th:PC71BM), polythieno[3,4-b]thiophene-co-benzodithiophene (PTB7):PC71BM and poly(4,8-bis-alkyloxybenzo(l,2-b:4,5-b′)dithiophene-2,6-diylalt-(alkylthieno(3,4-b)thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C):PC71BM, affording the best PCE of 9.66%, 8.51% and 7.19%, respectively, which are all higher than those of the corresponding devices based on the commonly used Ca CBL. The mechanism of efficiency enhancement of C60-ETA CBL relative to Ca is studied, revealing that C60-ETA CBL may induce improvements on both the interfacial contact between the active layer/cathode and electron transport, facilitating electron extraction by the Al cathode, and consequently leading to the increase of short-circuit current density (Jsc), which contributes primarily to the PCE improvement.

Benzodithiophene based ternary copolymer containing covalently bonded pyrrolo[3,4-c]pyrrole-1,3-dione and benzothiadiazole for efficient polymer solar cells utilizing high energy sunlight

A new electron deficient monomer (PDBT) unit incorporating a weak electron accepting unit, pyrrolo[3,4-c]pyrrole-1,3-dione (DPPD), and a strong electron accepting unit, 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BT), with a configuration of DPPD-BT-DPPD was prepared, and copolymerized with a benzodithiophene (BDTT) derivative to afford a new alternating ternary copolymer P(BDTT-PDBT). The estimated optical band gap (Eg) and highest occupied/lowest unoccupied (HOMO/LUMO) molecular orbital energy levels of P(BDTT-PDBT) were 1.72 eV and −5.45 eV/−3.62 eV, respectively. A field effect transistor made from P(BDTT-PDBT) exhibited a hole mobility of 3.2 × 10−3 cm2 V−1 s−1. Conventional single layer polymer solar cells (PSCs) prepared using P(BDTT-PDBT) with an additive offered a maximum power conversion efficiency (PCE) of 5.73% with an open-circuit voltage (Voc) of 0.90 V, a short-circuit current (Jsc) of 9.73 mA/cm2, and a fill factor (FF) of 0.66. Interestingly, the PSC device prepared without an additive also showed a similar PCE of 4.84% (Voc ∼ 0.92 V, Jsc ∼ 8.08 mA/cm2, and FF ∼ 0.65). This paper reports the preparation of an efficient ternary copolymer for polymer solar cells utilizing high energy sunlight, as well as the property modulation of an alternating binary copolymer, P(BDTT-DPPD), containing BDTT and DPPD derivatives via the insertion of a strong electron accepting BT unit on the P(BDTT-DPPD) backbone.

Highly efficient two-dimensional conjugated benzodithiophene-based photovoltaic polymers

In order to improve the photovoltaic properties of polymer solar cells (PSC), varieties of donor and acceptor units were designed and synthesized to construct the highly efficient conjugated polymers. Recent years, materials based on benzo [1.2-b:4,5-b ¢ ]dithiophene (BDT) units have showed promising applications in the field of organic photovoltaics for its excellent properties. As compared to BDT-based polymers with flexible side chains, polymers based on two-dimensional conjugated BDT (2D-BDT) usually had better thermal stability, slightly red-shifted of absorption spectra, lower HOMO levels, higher hole mobility and consequently improved photovoltaic properties. By using novel 2D-BDT-based polymers, the power conversion efficiency (PCE) of the PSC devices has been boosted to over 10%. In this article, we first briefly introduce the synthesis methods of 2D-BDT unit, and then focus our attention to summarize the numerous of 2D-BDT-based polymers and their applications in highly efficient PSC devices.

Hybrid Silver Mesh Electrode for ITO-Free Flexible Polymer Solar Cells

Herein, we demonstrate a tailored poly(3,4-ethylenedioxythiophene) PEDOT polymer-coated silver (Ag) mesh electrode on a flexible polyethylene terephthalate (PET) substrate. By introducing this highly conductive polymer, the existing problems of Ag mesh-type transparent conductive electrodes, such as high pitch, roughness, current inhomogeneity and adhesion problems between Ag mesh grid and PEDOT polymer or PET substrate, were successfully solved, resulting in excellent electron spreading from the discrete Ag mesh onto the entire surface without sacrificing sheet conductivity and optical transparency. Based on this hybrid anode, we demonstrate highly efficient flexible polymer solar cells (PSCs) with a high fill factor of 67.11 %, resulting in a power conversion efficiency (PCE) of 6.9 % based on a PTB7:PC71BM bulk heterojunction device. Furthermore, the PSC device with the Ag mesh electrode also exhibits good mechanical bending stability, as indicated by a 70% retention of the initial PCE after 500 bending cycles compared with the PSC device with a PET/indium tin oxide (ITO) electrode, which retained 0% of the initial PCE after 300 bending cycles.

Correlation of intermolecular packing distance and crystallinity of D-A polymers according to π-spacer for polymer solar cells

Abstract Donor–acceptor-type polymers, PBDPBT-T, PBDPBT-TT, and PBDPBT-biT, were polymerized via Stille coupling reactions. In the unfavorable mode of DFT calculations, the distance between BDP in the two different polymer main chains showed different distance of repeating unit (Dn) according to the π-spacers type. Especially, PBDPBT-TT caused larger D2 than PBDPBT-T and PBDPBT-biT. The UV–vis absorption spectra of PBDPBT-T and PBDPBT-biT films were red-shifted compared with the solution absorption, whereas that of a PBDPBT-TT film was blue-shifted. PBDPBT-T and PBDPBT-biT had high crystallinity and a tendency of face-on orientation. In contrast, PBDPBT-TT had low crystallinity. Owing to differences in packing formation, the polymers had different JSC values. When a polymer solar cell device was fabricated through a solution process, PBDPBT-biT (in a 1:1.5 ratio with PC70BM) exhibited a PCE of 4.50%, with a JSC value of 8.6 mA/cm2, a VOC value of 0.88 V, and an FF value of 59.4%.

Improved Morphology and Efficiency of Polymer Solar Cells by Processing Donor–Acceptor Copolymer Additives

A novel wide‐bandgap conjugated polymer PBTA‐FPh based on benzodithiophene‐alt‐benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in the side chain has been designed and synthesized. In comparison to the pristine polymer PBTA‐BO that consists of nonpolar alkyl side chains, the resulting PBTA‐FPh exhibits less pronounced aggregation while possessing analogous optical and electrochemical bandgaps. Contact angle measurements demonstrate that the surface energy can be enhanced by incorporating FPh moiety, leading to a better miscibility of PBTA‐BO with PC71BM in the presence of a certain amount of PBTA‐FPh. The photoactive layer of PBTA‐BO:PC71BM:PBTA‐FPh with weight ratio of 1:1.2:0.02% exhibits a percolated network with the fibrous features, as revealed by transmission electron microscopy measurements. Of particular interest is the significantly improved photovoltaic performances of polymer solar cell devices for which the power conversion efficiency is enhanced from 6.46% for the control device to 7.91% for device processed with PBTA‐FPh as the polymeric additive. These observations indicate that introducing donor–acceptor type of polymeric additive comprising of polar groups in the side chain can be a promising strategy for the fabrication of high‐performance polymer solar cells.

Imide-linked alkyl chain influence on the properties of pyrrole-based imide-functionalized polymers containing pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione and benzodithiophene units for polymer solar cells

A series of pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (DPPD)-based polymers incorporating different alkyl side chains on the imide nitrogen were prepared to investigate the imide-linked alkyl side chain effects on the properties of DPPD-based polymers. DPPD derivatives with heptyl, octyl, decyl and dodecyl substituents on the imide nitrogen were prepared and copolymerized with 2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene (BDT) to offer polymers P(BDT-C7-DPPD), P(BDT-C8-DPPD), P(BDT-C10-DPPD), and P(BDT-C12-DPPD), respectively. All four polymers displayed identical optical band gaps (Eg∼2.11eV) and highest occupied/lowest unoccupied molecular orbital energy levels (HOMO/LUMO∼−5.39eV/–3.28eV). The conventional single layer polymer solar cell (PSC) fabricated with a device structure of ITO/PEDOT:PSS/polymer:PC70BM (1:1.5wt%)/Al offered a power conversion efficiency (PCE) of 1.65%, 1.49%, 1.24%, and 0.87%, respectively. The PSC device performance was improved to 5.67%, 5.29%, 4.06%, and 2.46%, respectively, with the use of processive additive such as 1,8-diiodoocate (DIO).