Low Band Gap Donor Acceptor Copolymer Research Articles

Difluoroterthiophene as promising block to build highly planar conjugated polymer for polymer photovoltaic cells

Developing highly planar semiconducting polymer is essential for achieving high mobility and improving the photovoltaic performance of bulk-heterojunction polymer solar cells. In this contribution, two novel low-bandgap donor-acceptor copolymers, P1-3T and P2-3T2F, consisting of electron-accepting benzothiadiazole acceptor segment and electron-donating terthiophene donor segment with and without fluorine atoms were designed and synthesized. The density functional theory calculations and ultraviolet–visible absorption studies demonstrate that the fluorinated P2-3T2F polymer has more planar backbone conformation, deeper HOMO level, broader absorption spectrum with a vibronic shoulder peak, and higher absorption coefficient compared with the non-fluorinated analogue polymer, P1-3T. Furthermore, P2-3T2F exhibits good crystallinity and high hole mobility, presumably due to the well-ordered lamellar packing and the π-π stacking interaction between the backbones of the conjugated polymers. The optimized polymer solar cell based on P2-3T2F:PC61BM exhibits a short-circuit current density (Jsc) of 12.77 mA cm−2, a fill factor (FF) of 68.99%, an open-circuit voltage (Voc) of 0.80 V, and a maximum power conversion efficiency (PCE) of 7.14%, which is approximately 46% higher than that of the P1-3T:PC61BM device. The enhanced PCE is primarily due to increased light-harvesting ability and interconnected morphology with finely dispersed polymer-rich and PC61BM-rich domains, which improves the efficiency of exciton dissociation and rises the mobility of charge carriers, thus boosting the short-circuit current density and the FF value. Moreover, the relatively deep HOMO of P2-3T2F effectively increases the Voc.

Temperature-Modulated Optimization of High-Performance Polymer Solar Cells Based on Benzodithiophene–Difluorodialkylthienyl–Benzothiadiazole Copolymers: Aggregation Effect

A novel series of low band gap donor–acceptor copolymers derived from 4,5-bis(2-ethylhexyloxy)-benzo[2,1-b:3,4-b′]dithiophene (BDT) and 5,6-difluoro-4,7-bis(4-alkylthien-2-yl)benzo[c][1,2,5]thiadiazole bearing various alkyl side chains, such as PffBB-n (n = 10, 12, 14, and 16), were developed for high-performance bulk-heterojunction (BHJ) polymer solar cells (PSCs). PffBB-n exhibited not only strong and wide absorption but also controllable aggregation behavior in solution and thin films, in which aggregation behavior was greatly influenced by the length of alkyl side chains attached, temperature applied, and solvent used. Aggregation-induced spectral broadening further extended the absorption cut-off to ∼780 nm in thin films, leading to a narrow optical band gap of ∼1.6 eV. Because of the strong aggregation strength, PffBB-n equipped with long alkyl side chains shows enhanced ordered molecular packing and good crystallinity as revealed by X-ray diffraction studies. In addition, temperature-dependent aggr...

Investigation of donor–acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman microspectroscopy

Thin films made of three low-band gap donor–acceptor copolymers (CDTF, CDTDP and CDTDOP) composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5′,5′-diyl as an electron-acceptor structural unit and various electron-donor structural units, such as 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyl-1,4-phenylene and 2,5-didodecyloxy-1,4-phenylene, respectively, and thin films of their blends with various ratios of a soluble fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM) as an active layer for bulk heterojunction solar cells were studied by means of UV–vis absorption spectroscopy and Raman microspectroscopy. The molecules of CDTDP and CDTDOP possess the same main chains; they differ in the side-chain oxygen only, which changes the donor strength of the donor units. UV–vis and Raman studies allow us to show differences in the hindering of molecule planarization and aggregation in the blends. Absorption of the polymer films covered the whole visible spectral region and extended up to near infrared for CDTDOP. The absorption behavior of the CDTDP blend films qualitatively differed from the absorption behavior of the blend films of CDTF or CDTDOP. The Raman measurements were performed at two different laser excitation wavelengths (633 and 785 nm), which enabled the photoluminescence of both components in the Raman spectra to be distinguished. The Raman study was performed in different parts of the films, including the separated areas. It was proven that the separated areas in the blend films had higher contents of [60]PCBM than the rest of the films.

Construction of Donor–Acceptor Polymers via Cyclopentannulation of Poly(arylene ethynylene)s

A one-step postpolymerization modification that converts three high bandgap poly(arylene ethynylene)s into low bandgap donor–acceptor copolymers is described. The strategy relies on a palladium-catalyzed cyclopentannulation reaction between the main-chain ethynylene functionality and a small molecule aryl bromide (6-bromo-1,2-dimethylaceanthrylene). The reaction installs new cyclopenta[hi]aceanthrylene electron-accepting groups between the electron rich arylenes along the polymer backbone. The modified polymers include poly(9,9-didodecyl-fluorene-2,7-ethynylene), poly(9-dodecyl-carbazole-2,7-ethynylene), and poly(2,5-dioctyloxyphenylene-1,4-ethynylene). The functionalization efficiency was evaluated via isotopic 13C labeling of the polymeric ethynylene carbons and then monitoring the chemical environment of those carbons via NMR spectroscopy. Near complete conversion of the sp carbon species to sp2 carbon species was observed, which demonstrates the high efficiency of the modification strategy. Gel permea...

Electrical Instability Induced by Electron Trapping in Low-Bandgap Donor-Acceptor Polymer Field-Effect Transistors.

The mechanism of electrical instability and the double slope of p-type organic field-effect transistors (OFETs) fabricated from low-bandgap donor-acceptor copolymers are resolved. Those polymers enable electron conduction in the device, which leads to electron trapping and consequent formation of -SiO(-). This causes a turn-on voltage shift, hole-mobility increase, and double-slope occurrence. These findings tremendously impact the molecular design and device engineering of OFETs.

Influence of charge carrier mobility and morphology on solar cell parameters in devices of mono- and bis-fullerene adducts

Herein, we analyze charge carrier mobility and morphology of the active blend layer in thin film organic solar cells and correlate them with device parameters. A low band gap donor–acceptor copolymer in combination with phenyl-C61-butyric acid methyl ester (PCBM) or two bis-adduct fullerenes, bis-PCBM and bis-o-quino-dimethane C60 (bis-oQDMC), is investigated. We study the charge transport of polymer:fullerene blends in hole- and electron-only devices using the space-charge limited current method. Lower electron mobilities are observed in both bis-adduct fullerene blends. Hole mobility, however, is decreased only in the blend containing bis-oQDMC. Both bis-adduct fullerene blends show very high open circuit voltage in solar cell devices, but poor photocurrent compared to the standard PCBM blend for an active layer thickness of 200 nm. Therefore, a higher short circuit current is feasible for the polymer:bis-PCBM blend by reducing the active layer thickness in order to compensate for the low electron mobility, which results in a PCE of 4.3%. For the polymer:bis-oQDMC blend, no such improvement is achieved due to an unfavorable morphology in this particular blend system. The results are supported by external quantum efficiency measurements, atomic force microscopy, transmission electron microscopy and UV/vis spectroscopy. Based on these results, the investigations presented herein give a more scientific basis for the optimization of solar cells.

Synthesis and characterization of wide range light absorbing poly(dithieno[3,2-b:2′,3′-d]thiophene-alt-3,6-bis(thiophen-2-yl)-2,5-di-n-octyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for polymer solar cells

We designed and synthesized a new low band gap donor–acceptor copolymer (PDTDPP-Oct) composed of alternating dithieno[3,2-b:2′,3′-d]thiophene and bis-thienyl-diketopyrrolo[3,4-c]pyrrole units. The polymer films showed a very broad absorption band that extended up to 950nm. Thermogravimetric analysis revealed good thermal stability and a high glass transition temperature of 128°C. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymer were estimated to be −5.01 and −3.70eV, respectively. Bulk heterojunction polymer solar cells were fabricated with the structure: ITO/PEDOT:PSS/polymer:PCBM/Al. The introduction of 1,8-octanedithiol as a processing additive to the polymer/PCBM blend solution significantly improved the device performance relative to devices prepared from the blend solution without the additive. This cell gave a short circuit current of −1.8mA/cm2, an open circuit voltage of 0.60V, and a fill factor of 0.52, yielding a power conversion efficiency of 0.55%. The performance improvements were mainly attributed to the fine morphology of the blend film.

Thienoisoindigo-based low-band gap polymers for organic electronic devices

We synthesized a series of new low-band gap donor–acceptor copolymers containing 4,4′-bis(alkyl)-[6,6′-bithieno[3,2-b]pyrrolylidene]-5,5′(4H,4′H)-dione. This acceptor unit, so-called dithienoketopyrrole (DTKP), is an analogue of isoindigo, the phenyl rings of which are replaced by thiophenes. Donor moieties such as benzodithiophene, cyclopentadithiophene, fluorene, and dithienothiophene are polymerized with DTKP in an alternating fashion by Stille or Suzuki–Miyaura coupling methods. Exceedingly low-band gaps (Eg = 1.0–1.6 eV) were achieved in these copolymers through internal charge transfer interactions between the donor and acceptor moieties. The structural, photophysical, and electrochemical properties of the resultant copolymers were characterized, and field-effect transistor (FET) mobilities were measured. The copolymers showed electronic absorption spectra extending to the near infrared region (600–1400 nm) with absorption maxima at 745–971 nm, along with a low-lying LUMO of −3.8 eV. Density functional theory (DFT) calculation indicated high planarity for the copolymer backbone when compared to that of its phenyl-isoindigo counterparts. FET hole mobilities on the order of 10−4 to 10−3 cm2 V−1 s−1 were obtained, demonstrating a feasibility to use them in organic photovoltaic cells.

Low band-gap donor–acceptor copolymers based on dioxocyclopenta[c]thiophene derivatives as acceptor units: synthesis, properties, and photovoltaic performances

New donor–acceptor type copolymers containing dihexyldioxocyclopenta[c]thiophene or (dihexylmethylidene)dioxocyclopenta[c]thiophene as acceptor units have been designed and synthesized for the application as hole-transporting (p-type) organic semiconducting materials in organic photovoltaics (OPVs). The investigation of photophysical and physicochemical properties revealed that these copolymers featured low optical band gaps (1.56–1.73 eV) and low-lying HOMO energy levels (−5.40 to −5.02 eV). Bulk heterojunction OPV devices based on these copolymers and [6,6]-phenyl-C61-butyric acid methyl ester as active layers showed moderate power conversion efficiencies (PCEs) of between 1.36 and 2.68% under air mass 1.5 simulated solar illumination. Space-charge-limited current measurements and atomic force microscopy measurements of the blend films revealed that both charge-transporting characteristics and film morphologies have significant influences on the photovoltaic performances. OPV devices based on the copolymers with [6,6]-phenyl-C71-butyric acid methyl ester showed a PCE of up to 5.17% with a short circuit current of 10.1 mA cm−2, an open circuit voltage of 0.80 V and a fill factor of 0.64.

An extremely low bandgap donor–acceptor copolymer for panchromatic solar cells

An extremely low bandgap donor–acceptor copolymer has been designed and synthesized as the donor material of the polymer solar cells via Stille coupling reaction. The donor–acceptor alternating structure consisted of 4,4-diethylhexyl-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) donor unit and 2,3-bis(2-ethylhexyl)thieno[3,4-b]pyrazine (TP) acceptor unit. Since both units have attached branched alkyl chains, the polymer was well dissolved in common organic solvents. UV–vis spectrum of the polymer film exhibited a panchromatic absorption ranging from 280 to 1285nm and a low bandgap of 1.20eV. Compared to that in solution, solid-state UV–vis absorption spectrum of the polymer showed a strong bathochromic shift, indicating more efficient π-stacking and stronger intermolecular interactions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymer were estimated to be 5.07 and 3.87eV, respectively. Based on the ITO/PEDOT:PSS/Polymer:PC61BM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100mW/cm2) was 0.149%. The effects of annealing temperature (100–200°C) for 30min on the device performance were studied as well. It was found that PCE of 0.264% could be acquired under the annealing condition at 175°C for 30min. The improved device efficiency under the optimal condition was confirmed by the higher light harvest in UV–vis spectra, the enhanced quenching of photoluminescence (PL) emission, and the increase in external quantum efficiency.

Excitation Dynamics in Low Band Gap Donor–Acceptor Copolymers and Blends

AbstractDonor–acceptor (D–A) type copolymers show great potential for the application in the active layer of organic solar cells. Nevertheless the nature of the excited states, the coupling mechanism and the relaxation pathways following photoexcitation are yet to be clarified. We carried out comparative measurements of the steady state absorption and photoluminescence (PL) on the copolymer poly[N‐(1‐octylnonyl)‐2,7‐carbazole]‐alt‐5,5‐[4′,7′‐di(thien‐2‐yl)‐2′,1′,3′‐benzothiadiazole] (PCDTBT), its building blocks as well as on the newly synthesized N‐(1‐octylnonyl)‐2,7‐bis‐[(5‐phenyl)thien‐2‐yl)carbazole (BPT‐carbazole). The high‐energy absorption band (HEB) of PCDTBT was identified with absorption of carbazoles with adjacent thiophene rings while the low‐energy band (LEB) originates instead from the charge transfer (CT) state delocalized over the aforementioned unit with adjacent benzothiadiazole group. Photoexcitation of the HEB is followed by internal relaxation prior the radiative decay to the ground state. Adding PC70BM results in the efficient PL quenching within the first 50 ps after excitation. From the PL excitation experiments no evidence for a direct electron transfer from the HEB of PCDTBT towards the fullerene acceptor was found, therefore the internal relaxation mechanisms within PCDTBT can be assumed to precede. Our findings indicate that effective coupling between copolymer building blocks governs the photovoltaic performance of the blends.

The Effect of Solvent Additives on Morphology and Excited-State Dynamics in PCPDTBT:PCBM Photovoltaic Blends

The dependence of the thin film morphology and excited-state dynamics for the low-bandgap donor-acceptor copolymer poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) in pristine films and in blends (1:2) with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) on the use of the solvent additive 1,8-octanedithiol (ODT) is studied by solid-state nuclear magnetic resonance (NMR) spectroscopy and broadband visible and near-infrared pump-probe transient absorption spectroscopy (TAS) covering a spectral range from 500-2000 nm. The latter allows monitoring of the dynamics of excitons, bound interfacial charge-transfer (CT) states, and free charge carriers over a time range from femto- to microseconds. The broadband pump-probe experiments reveal that excitons are not only generated in the polymer but also in PCBM-rich domains. Depending on the morphology controlled by the use of solvent additives, polymer excitons undergo mainly ultrafast dissociation (<100 fs) in blends prepared without ODT or diffusion-limited dissociation in samples prepared with ODT. Excitons generated in PCBM diffuse slowly to the interface in both samples and undergo dissociation on a time scale of several tens of picoseconds up to hundreds of picoseconds. In both samples a significant fraction of the excitons creates strongly bound interfacial CT states, which exhibit subnanosecond geminate recombination. The total internal quantum efficiency loss due to geminate recombination is estimated to be 50% in samples prepared without ODT and is found to be reduced to 30% with ODT, indicating that more free charges are generated in samples prepared with solvent additives. In samples prepared with ODT, the free charges exhibit clear intensity-dependent recombination dynamics, which can be modeled by Langevin-type recombination with a bimolecular recombination coefficient of 6.3 × 10(-11) cm(3) s(-1). In samples prepared without ODT, an additional nanosecond recombination of polaron pairs is observed in conjunction with an increased intensity-independent trap-assisted nongeminate recombination of charges. Furthermore, a comparison of the triplet-induced absorption spectra of PCPDTBT with the charge-induced absorption in PCPDTBT:PCBM blends reveals that triplets have a very similar excited-state absorption spectrum compared to the free charge carriers, however, in contrast have a distinct intensity-independent lifetime. Overall, our results suggest that whether free charges or strongly bound CT states are created upon dissociation of excitons at the PCPDTBT:PCBM interface is determined instantaneously upon exciton dissociation and that once formed strongly bound CT states rapidly recombine and thus are unlikely to dissociate into free charges. The observation of a significantly larger bimolecular recombination coefficient than previously determined for poly(3-hexylthiophen-2,5-diyl):PCBM (P3HT:PCBM) and PCDTBT:PCBM samples indicates that nongeminate recombination of free charges considerably competes with charge extraction in PCPDTBT:PCBM photovoltaic devices.

Novel ladder-type heteroheptacene-based copolymers for bulk heterojunction solar cells

Four semiconducting copolymers were designed and synthesized based on a novel ladder-type heteroheptacene building block, which was prepared in four steps from 3,7-dibromodibenzo[b,d]thiophene. The copolymers were characterized by 1H NMR spectroscopy, UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Field effect transistors (FETs) fabricated from these four polymers exhibit semiconducting behavior in air. The highest mobility of 2.21 × 10−4 cm2 V−1 s−1 was found in the polymer copolymerized from the ladder-type heteroheptacene and terthiophene. Low band-gap donor–acceptor copolymers with a deep-lying HOMO energy level were synthesized by a copolymerization between the ladder-type heteroheptacene donor and the 2,1,3-benzothiadiazole acceptor. Polymer solar cells made from one of these polymers gave a power conversion efficiency of 4.17%, a current density of 9.2 mA cm−2, an open-circuit voltage of 0.79 V, and a fill factor of ∼57.4% under 100 mW cm−2 AM 1.5 G sunlight illumination.

Synthesis and characterization of dithienothiophene/benzothiadiazole based low band gap donor–acceptor copolymers for bulk hetero junction photovoltaic cells

Two newly designed low band gap copolymers P2TDTT-BT and P2TDTT-DTBT, comprised of a substituted dithienyl-dithieno[3,2-b:2′,3′-d]thiophene derivative as a donor and a benzothiadiazole or dithienyl benzothiadiazole based acceptor, respectively, were synthesized by Stille polymerization. The UV–visible absorption of P2TDTT-BT and P2TDTT-DTBT films ranged from the UV to NIR regions. The HOMO/LUMO energy levels of P2TDTT-BT and P2TDTT-DTBT were estimated to be −5.41/−3.74eV and −5.34/-3.65eV, corresponding to energy band gaps of 1.67 and 1.69eV, respectively. Bulk heterojunction photovoltaic cells were fabricated using a blend of P2TDTT-BT or P2TDTT-DTBT and PC61BM in a 1:1 ratio and delivered power conversion efficiencies of 0.31% and 0.92%, respectively.

Tuning the electronic coupling in a low-bandgap donor–acceptor copolymer via the placement of side-chains

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

A low band gap donor–acceptor copolymer containing fluorene and benzothiadiazole units: synthesis and photovoltaic properties

A new low band gap copolymer containing dialkylfluorene and 4,7-dithienyl-2,1,3-benzothiadiazole (TBT), poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PF-TBT) was synthesized by Heck cross-coupling polymerization. The copolymer is soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzene. The TGA result indicated that the copolymer possesses good thermal stability. The absorption, electrochemical and photovoltaic properties of PF-TBT were investigated and compared with those of poly(fluorenevinylene-alt-4,7-diphenyl-2,1,3-benzothiadiazole) (PF-DBT) whose structure is similar to PF-TBT. The copolymer exhibited a broad absorption band with an absorption edge close to 700 nm and an optical band gap of 1.82 eV. Cyclic voltammetry studies indicated that the relatively low HOMO energy level assured a higher open circuit voltage (Voc) when PF-TBT is used as the donor material in a photovoltaic cell. The bulk heterojunction (BHJ) solar cell using PF-TBT as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor with the structure of ITO/PEDOT : PSS/copolymer : PCBM/LiF/Al, exhibited a Voc of 0.86 V, short-circuit current density (Jsc) of 3.97 mA cm−2, fill factor (FF) of 0.35, and a power conversion efficiency (PCE) of 1.18% under one sun of AM 1.5 solar simulator illumination (100 mW cm−2).

Efficient solar cells based on a new phthalimide-based donor–acceptor copolymer semiconductor: morphology, charge-transport, and photovoltaic properties

Bulk heterojunction solar cells based on blends of the new low band gap donor–acceptor copolymer, poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide) (PhBT12), and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were systematically investigated. The PhBT12/fullerene blend films were found to exhibit a crystalline nanoscale morphology with space-charge-limited mobility of holes as high as 4.0 × 10−4 cm2/Vs without thermal annealing, leading to moderately efficient devices. The performance of the solar cells varied significantly with PhBT12/fullerene composition, reaching a power conversion efficiency of 2.0% with a current density of 6.43 mA/cm2 and a fill factor of 0.55 for the 1:1 PhBT12/PC71BM blend devices. However, thermally annealed (120 °C) PhBT12/fullerene blend devices had negligible photovoltaic properties due to micrometer scale phase separation of the blends which is attributed to the long side chains. We expect that better photovoltaic performance can be achieved by modifying the polymer side chain length and the device processing as well. These results show that phthalimide-based donor–acceptor copolymer semiconductors, exemplified by PhBT12, are promising low band gap materials for developing efficient bulk heterojunction solar cells.

Conjugated Polymer Photovoltaic Materials with Broad Absorption Band and High Charge Carrier Mobility

AbstractPolymer solar cells (PSCs) have attracted great attention in recent years because of their advantages of easy fabrication, low cost, light weight, and potential for flexible devices. However, the power conversion efficiency (PCE) of the PSCs needs to be improved for future commercial applications. Factors limiting the PCE of the PSCs include the low exploitation of sunlight due to the narrow absorption band of conjugated polymers, and the low charge‐transport efficiency in the devices due to the lower charge‐carrier mobility of the polymer photovoltaic materials. In this Research News article, recent progress in new conjugated polymer photovoltaic materials fabricated by our group and others is reviewed, including polythiophene (PT) and poly(thienylene vinylene) derivatives with conjugated side chains for a broad absorption band, crosslinked PT derivatives with conjugated bridges for higher hole mobility, and low‐bandgap donor–acceptor copolymers for broad, red‐shifted absorption to match the solar spectrum.