Bulk Heterojunction Polymer Solar Cells Research Articles

A universal processing additive for high-performance polymer solar cells

Diphenyl ether acts as a predominant processing additive regardless of the polymer crystallinity in bulk-heterojunction polymer solar cells.

Quantitative correlation of the effects of crystallinity and additives on nanomorphology and solar cell performance of isoindigo-based copolymers.

The high power conversion efficiency of bulk heterojunction (BHJ) polymer solar cells can be achieved from either low crystallinity (P3TI) or high crystallinity (P6TI) of isoindigo-based donor-acceptor alternating copolymers blended with PC71BM by controlling nanophase separation using additives. P3TI shows similar device performance regardless of the type of additives, while P6TI is significantly affected by whether the additive is aliphatic or aromatic. To understand the interplays of crystallinity of polymers and the type of additive on the formation of nanomorphology of BHJ, we employed the simultaneous grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS) technique to perform the quantitative investigation. By incorporating additives, the PC71BM molecules can be easily intercalated into the P3TI polymer-rich domain and the size of the PC71BM clusters is reduced from about 24 nm to about 5 nm by either aliphatic 1,8-diiodooctane (DIO) or aromatic 1-chloronaphthalene (CN). On comparison, it is found to be more difficult for PC71BM molecules to be intercalated into the highly crystalline P6TI dense domain, and the PC71BM molecules have a higher tendency to be self-aggregated, which results in a larger size of PC71BM clusters of about 58 nm. The clusters can be reduced to about 7 nm by DIO and 13 nm by CN. The presence of crystallites in the P6TI domain can interact with the additive to tailor the crystallization of PC71BM clusters to a size similar to that of P6TI crystallites (∼12 nm) and form a connected network for efficient charge transportation. Thus, the power conversion efficiency of P6TI:PC71BM reaches its maximum of 7.04% using aromatic CN additives. This is a new finding of the effect of crystallinity, which is not observed in the common low crystalline donor-acceptor alternating copolymers such as PTB7. Our results provide a useful guideline to manipulate the desired morphology of BHJ films constructed from alternating copolymer with different crystallinity, which is critical for achieving high power conversion efficiency of solar cells.

Random D–A1–D–A2terpolymers based on benzodithiophene, thiadiazole[3,4-e]isoindole-5,7-dione and thieno[3,4-c]pyrrole-4,6-dione for efficient polymer solar cells

New efficient D–A1–D–A2random terpolymers for photovoltaic applications.

Water- and alcohol-soluble cationic phenanthroline derivatives as efficient cathode interfacial layers for bulk-heterojunction polymer solar cells

In recent years, cathode interfacial layers (CILs), as versatile functional layers, have been extensively investigated for use in organic electronics. Many excellent CILs have been designed and prepared, and these have provided a distinct improvement in device performance. In this study, two bathophenanthroline-based cationic CILs (Bphen-Et and Bphen-Pr) were synthesized via a simple intramolecular cyclization reaction with 1,2-dibromoethane and 1,3-dibromopropane, respectively, for use in polymer solar cells (PSCs), and their chemical, thermal, photophysical, electrochemical, and photovoltaic properties were characterized. Bphen-Et and Bphen-Pr exhibited high thermal stability, and their glass transition temperatures exceeded 100 °C. Additionally, Bphen-Et has a glass transition temperature above 181 °C. The electrochemical characterization shows that Bphen-Et and Bphen-Pr have very deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, where the HOMOs and LUMOs of Bphen-Et and Bphen-Pr are −6.63 and −6.67 and −4.16 and −4.18 eV, respectively. These very low HOMOs and LUMOs enhance the photovoltaic performance and decrease interfacial energy loss. The PSCs based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7): [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) system with Bphen-Et and Bphen-Pr CILs exhibit simultaneous enhancement in open-circuit voltage, short-circuit current density, and fill factor, and their power conversion efficiency increases from 3.98% to 8.05%, relative to that of the bare Al device. This type of cationic aromatic compound shows promise as a candidate CIL for use in PSCs.

A novel random terpolymer for high-efficiency bulk-heterojunction polymer solar cells

A new random terpolymer, coded LGC-D013, based on TPD as the acceptor and BDT and terthiophene as the donor units, has been synthesized and characterized for donor material in bulk-heterojunction (BHJ) organic photovoltaic (OPV) application.

Enhanced performance in inverted polymer solar cells employing microwave-annealed sol-gel ZnO as electron transport layers

In this work, we propose a facile microwave-assisted approach for annealing sol-gel derived ZnO films to serve as electron transport layers (ETLs) for inverted bulk heterojunction polymer solar cells. We have demonstrated an impressive enhancement in performance for devices based on a poly (3-hexylthiophene) (P3HT): (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) system employing the microwave-annealed ZnO (ZnO (MW)) ETLs in comparison to the cases using the conventional hotplate-annealed ZnO (ZnO (HP)) ones. The better electron transport in the device with the ZnO (MW) ETL is mainly ascribed to the preferable interfacial contact as evidenced by the morphology characteristics. Furthermore, the comprehensive analyses conducted from the light intensity dependent photocurrent and photovoltage measurements, the capacitance-voltage characteristics, and the alternating current impedance spectra suggest that the utilization of the ZnO (MW) ETLs can effectively suppress trap-assisted recombination as well as charge accumulation at the interface between P3HT: PC61BM layers and ZnO layers, which is responsible for the enhanced device performance.

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.

Novel Copolymers Based Tetrafluorobenzene and Difluorobenzothiadiazole for Organic Solar Cells with Prominent Open Circuit Voltage and Stability.

Two novel copolymers based on benzothiadiazole (BT) or difluorobenzothiadizole (ffBT) with 2,2'-(perfluoro-1,4-phenylene)dithiophene (2TPF4), namely PBT-2TPF4 and PffBT-2TPF4, are synthesized for applications in polymer solar cells (PSCs). A noticeably high open-circuit voltage (Voc ) of 1.017 and 0.87 V are achieved for PffBT-2TPF4 and PBT-2TPF4-based devices, respectively. Although only a moderate efficiency (5.7%) of PBT-2TPF4-based devices is obtained, it is first demonstrated that 2TPF4 is a promising acceptor block for construction of the donor copolymers which possess high Voc , prominent crystallinity, and long-term stability, simultaneously. Besides, two thienyl flanking the tetrafluorophenylene can decrease torsion angle between conjugated units, resulting in a high coplanar structure of copolymers to enhance their charge carrier mobility. The findings may open a promising and practical way to accelerate the commercialization of PSCs by developing a series of new donor copolymers for efficient and long-term stable thickness bulk heterojunction PSCs.

Benzothiadiazole-pyrrolo[3,4-b]dithieno[2,3-f:3′,2′-h]quinoxalindione-based random terpolymer incorporating strong and weak electron accepting [1,2,5]thiadiazolo[3,4g]quinoxalinefor polymer solar cells

We report the synthesis of a D-A random terpolymer denoted as P2 consists of one thiophene donor unit and three acceptor benzothiadiazole (BT), pyrrolodithienoquinoxalinedione (PDQD) and thiadiazoloquinoxaline (TDQ) units by Stille-coupling reaction and investigated its optical and electrochemical properties. We have compared its properties with the parent copolymer P1. The P2 exhibits bandgap of about 1.18 eV which is lower than that of P1 (1.50 eV), indicating strength of accepting units controls both the optical and electrochemical bandgap. We have used terpolymer P2 as electron donor along with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as electron acceptor for the fabrication of solution processed bulk heterojunction polymer solar cells (PSCs). PSC based on an optimized P2:PC71BM (1:2 by weight) active layer processed with 3v % DIO/DCB solution, displayed a power conversion efficiency (PCE) of 7.22%, which is higher than that for P1 based polymer solar cell (PCE = 6.56%) processed under same conditions. The higher value of PCE for P2:PC71BM may be related to more favorable phase separated morphology of active layer as compared to P1:PC71BM, beneficial for the exciton dissociation and charge transport, as evidenced from the larger hole mobility.

Intrinsic Delocalization during the Decay of Excitons in Polymeric Solar Cells.

In bulk heterojunction polymer solar cells, external photoexcitation results in localized excitons in the polymer chain. After hot exciton formation and subsequent relaxation, the dipole moment drives the electron to partially transfer to extended orbitals from the original localized ones, leading to self-delocalization. Based on the dynamic fluorescence spectra, the delocalization of excitons is revealed to be an intrinsic property dominated by exciton decay, acting as a bridge for the exciton to diffuse in the polymeric solar cell. The modification of the dipole moment enhances the efficiency of polymer solar cells.

Alkoxyphenyl-thiophene, -selenophene and -furan substituted benzodithiophene based 2D π-conjugated polymers for polymer solar cells and effect of chalcogen on optoelectronic properties

In order to analyze the correlation between the optoelectronic properties and chalcogen present in the conjugated side chain of benzodithiophene (BDT) based 2D π-conjugated polymers, we synthesized a new series of 2D π-conjugated polymers P1 (S), P2 (Se), and P3 (O), in which alkoxyphenyl-thiophene, alkoxyphenyl-selenophene and alkoxyphenyl-furan substituted BDT as an electron rich donor unit and thieno[3,4-c]pyrrole-4,6-dione as an electron deficient acceptor unit, respectively. The P1-P3 showed varied optoelectronic properties with respect to the chalcogen present in the conjugated side chains of donor unit. The bulk heterojunction (BHJ) polymer solar cells (PSCs) based on P1-P3 showed maximum power conversion efficiency of 3.37, 3.53 and 1.54%, respectively. In particular, open-circuit voltages (Voc) of P1-P3 based PSCs were (0.92, 0.96 and 0.80V) significantly affected upon chalcogen exchange, which was further analyzed by Mott-Schottky analysis and, the obtained flat-band potential values are well matched with the Voc of P1-P3 based devices.

Properties of inverted polymer solar cells based on novel small molecular electrolytes as the cathode buffer layer

Novel small-molecule electrolytes were designed and synthesized for use in the cathode interlayer in bulk-heterojunction polymer solar cells (PSCs). The synthesized materials consist of polar quaternary ammonium bromide with the addition of multiple hydroxyl groups, which are N,N,N,N,N,N-hexakis(2-hydroxyethyl)butane-1,4-diaminium bromide (C4) and N,N,N,N,N,N-hexakis(2-hydroxyethyl)hexane-1,6-diaminium bromide (C6). The materials generate a favorable interface dipole through the quaternary ammonium bromide. In addition, the multiple polar hydroxyl groups increased the interface dipole magnitude. The power conversion efficiency of the devices with the interlayer was up to 9.20% with a Jsc of 17.22 mA/cm2, a Voc of 0.75 V, and an FF of 71.3%. The PCE of devices with an interlayer show better long-term stability than a device without an interlayer. Our strategy shows that it is possible to enhance the efficiency of PSCs by simple approaches without complicated syntheses.

Improved efficiency of bulk heterojunction polymer solar cells by doping with iridium complex

To study the effect of iridium complexes on performance of polymer solar cells, we report the ternary bulk heterojunction (BHJ) polymer solar cells (PSCs) by doping an iridium complex material of bis (1-phenylisoquinoline) acetylacetonate iridium(III) [Ir(piq)2acac] into the conventional active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The results show that the power conversion efficiency (PCE) of P3HT:PC71BM based ternary devices is improved from 2.99% to 4.44% by doping 1wt% Ir(piq)2acac, which is benefited from the enhanced exciton harvesting by Förster resonance energy transfer (FRET) from Ir(piq)2acac to P3HT and optimized self-organized morphology within ternary blends.

Semi-crystalline A1–D–A2-type copolymers for efficient polymer solar cells

A series of acceptor1–donor–acceptor2 (A1–D–A2)-type copolymers was designed and synthesized using thiophene as an electron-rich unit and benzothiadiazole (BT) and benzotriazole (BTz) as electron-deficient moieties. A weaker acceptor, BTz, was incorporated as a solubilizing moiety with three tetradecyl (or tetradecyloxy) side chains, and a stronger acceptor, BT, was substituted with different numbers of fluorines to modulate the intramolecular charge transfer interaction and the resulting electronic structures. The similar molecular structures of BT and BTz did not disrupt the interchain organization significantly, and the absence of solubilizing alkyl substituents on the electron-rich thiophene did not increase the highest occupied molecular orbital of the resulting polymers. The polymers showed broad absorption in the range of 350–750 nm. Intra- and/or interchain non-covalent coulombic interactions (for example, dipole–dipole interactions via S···O, S···F) ensured a planar backbone and a semi-crystalline morphology in the film. Bulk heterojunction polymer solar cells were fabricated using blends of the polymers and phenyl-C71-butyric acid methyl ester (PC71BM). For a difluoro-BT containing polymer (BTzDT2FBT), a power conversion efficiency up to 7% was achieved with a short circuit current density of 14.64 mA cm−2, open circuit voltage of 0.75 V and fill factor of 0.64. Three types of A1–D–A2 conjugated polymers were synthesized based on thiophene as an electron-rich unit and two similarly structured electron-deficient moieties of benzothiadiazole (BT) and benzotriazole (BTz). The weaker acceptor, BTz, with three alkyl (or alkyloxy) substituents took on a solubilizing role without deteriorating the chain planarity. The absence of alkyl substituents on thiophene induced a deep valence band. Increasing the number of fluorine substituents on BT decreased the frontier orbitals and enhanced the interchain packing. The chain planarity and crystalline interchain organization were facilitated via intra- and/or interchain non-covalent S···O and S···F coulombic interactions. The best power conversion efficiency of 7% was measured for BTzDT2FBT:PC71BM.

Importance of varying electron-accepting moieties in regular conjugated terpolymers for use in polymer solar cells

Three A1-D-A2-D regular conjugated terpolymers containing diketopyrrolopyrrole, isoindigo, or thienoisoindigo moieties were synthesized. Each terpolymer with the combination of two different acceptors and thiophene as common donor displayed strong absorption from 500 nm to the near-IR region as well as different molecular energy levels. Among three terpolymers, bulk heterojunction polymer solar cell made of the terpolymer containing diketopyrrolopyrrole, isoindigo, and thiophene moieties and phenyl-C71-butyric acid methyl ester showed the highest power conversion efficiency of 6.95% due to its appropriate internal morphology and the high open circuit voltage conferred by the relatively low-lying highest occupied molecular orbital level of the polymer.

Effect of substituent groups on quinoxaline-based random copolymers on the optoelectronic and photovoltaic properties

Abstract In this study, three low band gap donor-acceptor (D-A) type quinoxaline-based random copolymers were synthesized. The effect of varying substituent groups on quinoxaline groups on optoelectronic properties and the performance of bulk-heterojunction polymer solar cells was investigated. Electrochemical and optical studies indicate that these copolymers are promising materials for both electrochromic and polymer solar cells applications. Two random polymers were found as excellent candidates for NIR electrochromic devices owing to their 60% and 94% optical contrast values respectively in NIR region with less than 1 s switching times. Polymer solar cells were constructed using the copolymers as the donor components together with PC71BM as the acceptor in the active layer. Among all, the highest PCE was reported as 2.13%.

Efficient Solution Processable Polymer Solar Cells Using Newly Designed and Synthesized Fullerene Derivatives

Two modified PC60BM and PC70BM fullerene derivates denoted as C60DAM and C70DAM, respectively, were synthesized and characterized. Through the structural modification, the lowest unoccupied molecular orbital (LUMO) energy level of these fullerene derivatives can be raised as much as 0.15 eV with respect to that of their parent fullerene derivatives, which is possibly ascribed to the electron-donating nature of their substituted moiety. Bulk heterojunction (BHJ) polymer solar cells (PSCs) fabricated with poly(3-hexylthiophene) (P3HT) as electron donor and either C60DAM or C70DAM as electron acceptor showed both higher open-circuit voltage (Voc) and short-circuit current (Jsc), resulting in enhanced power conversion efficiency (PCE) (3.23% for P3HT:C60DAM and 4.45% for P3HT:C70DAM) as compared to corresponding PC60BM. The enhanced Jsc and higher Voc values have been ascribed to the stronger absorption in the visible region and increase in LUMO level of the modified fullerene derivatives compared to their pr...

Synthesis and photovoltaic properties of donor-acceptor type narrow bandgap copolymers based on benzo[def]carbazole

Three kinds of narrow bandgap copolymers were synthesized by combination of a donor unit of benzo[def]carbazole with each acceptor unit of benzothiadiazole, thienopyrrole-4,6-dione, and 1,4-diketopyrrolopyrrole. Basic properties of these polymers were investigated for applications in bulk heterojunction polymer solar cells. These copolymers had broad absorption bands in UV and visible regions from 350 to 800nm that overlapped with the major region of the solar spectrum, and their optical band gaps were in the range of 1.68–2.11eV. The highest occupied molecular orbitals of these copolymers were situated in the range of −5.22eV to −5.34eV, which could provide good air stability and high open circuit voltages in photovoltaic applications. Initial performances of these copolymers in bulk heterojunction photovoltaic devices revealed that the power conversion efficiencies exceeded 1.4% under irradiation of AM 1.5 solar-simulated light (100mWcm−2).

Unsubstituted Benzodithiophene-Based Conjugated Polymers for High-Performance Organic Field-Effect Transistors and Organic Solar Cells.

Unsubstituted benzo[1,2-b:4,5-b']dithiophene (BDT) was used to construct a high-performance conjugated polymer with 5,6-difluoro-4,7-bis[4-(2-octyldodecyl)thiophene-2-yl]benzo[c][1,2,5] thiadiazole (DTFFBT), named PBDT-DTFFBT. The polymer shows the low-lying highest occupied molecular orbital (HOMO) energy level (-5.40 eV) and a broad absorption spectra with strong vibronic absorption peak. Pure polymer films exhibit good crystallinity and edge-on orientation, partially attributed to the BDT units without any side chains, and as a result, the corresponding thin-film transistor showed excellent hole mobility over 1 cm(2) V(-1) s(-1). Interestingly, a well-distributed nanofibrillar polymer aggregation with face-on orientation was obviously formed when blending with PC71BM, which was in favor of the charge transportation. Consequently, the bulk heterojunction polymer solar cells based on the blends showed high power conversion efficiency of 9.29% with large short-current density (14.56 mA cm(-2)) and high fill factor (0.751) without any process additives or thermal annealing.

Luminescent GdVO4:Sm3+ quantum dots enhance power conversion efficiency of bulk heterojunction polymer solar cells by Förster resonance energy transfer

In this work, we report enhanced power conversion efficiency (PCE) of bulk heterojunction polymer solar cells by Förster resonance energy transfer (FRET) from samarium-doped luminescent gadolinium orthovanadate (GdVO4:Sm3+) quantum dots (QDs) to polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7) polymer. The photoluminescence emission spectrum of GdVO4:Sm3+ QDs overlaps with the absorption spectrum of PTB7, leading to FRET from GdVO4:Sm3+ to PTB7, and significant enhancements in the charge-carrier density of excited and polaronic states of PTB7 are observed. This was confirmed by means of femtosecond transient absorption spectroscopy. The FRET from GdVO4:Sm3+ QDs to PTB7 led to a remarkable increase in the power conversion efficiency (PCE) of PTB7:GdVO4:Sm3+:PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester) polymer solar cells. The PCE in optimized ternary blend PTB7:GdVO4:Sm3+:PC71BM (1:0.1:1.5) is increased to 8.8% from 7.2% in PTB7:PC71BM. This work demonstrates the potential of rare-earth based luminescent QDs in enhancing the PCE of polymer solar cells.