PCBM Blend Research Articles

Photocathodic hydrogen evolution from catalysed nanoparticle films prepared from stable aqueous dispersions of P3HT and PCBM

Photo-assisted hydrogen evolution is achieved on photocathodes comprising of nanoparticles of poly(3-hexylthiophene) (npP3HT) and nanoparticles of phenyl-C61-butyric acid methyl ester (npPCBM) onto which ultra-low loadings of Pt nanoparticles are deposited. The nanoparticles, npP3HT and npPCBM, are prepared individually via miniemulsion using surfactants of opposite head group polarity. Aqueous dispersions of npP3HT:npPCBM, devoid of organic solvent, are cast conformally onto ITO-coated glass to yield water-insoluble bulk-heterojunction films. Pt (1 μg/cm2) catalyst is deposited photoelectrochemically onto ITO/npP3HT:npPCBM photocathodes and found to nucleate preferentially on PCBM nanoparticles. ITO/npP3HT:npPCBM/Pt photocathodes produce 65 μA/cm2 photocurrent under 100 mW/cm2 of visible light at 0.0 VSHE and liberate H2 gas. The photocurrents observed for electrodes prepared using npP3HT:npPCBM are twice as large, and the onset potential is ∼0.4 V more positive than analogous photocathodes cast from nanoparticles each comprising an intimate blend of P3HT and PCBM. These are encouraging results for large scale synthesis of organic photoelectrochemical devices, given the simplicity of the photoelectrode, i.e., prepared from aqueous solutions and devoid of vacuum-deposited films such as charge transport layers and protective films.

Two TPD-Based Conjugated Polymers: Synthesis and Photovoltaic Applications as Donor Materials

Two conjugated thienopyrrolodione-thiophene-vinyl-thiophene (TPDTVT)-based polymers, with one of them having an additional cyano (CN) group attached to the TVT building block, are synthesized and incorporated in photovoltaic device as electron-donating materials. Both TPD-TVT and TPD-TVTCN showed good thermal stability and appropriate solubility in chlorinated solvents, but they exhibited quite different optoelectrochemical and photovoltaic properties. The stronger intermolecular interactions of TPD-TVTCN, compared to TPD-TVT, lead to enhanced aggregation in blends with PC71BM. The domain sizes within 15–20 nm and improved molecular packing in TPD-TVTCN:PC71BM blend films result in better photovoltaic performance of TPD-TVTCN:PC71BM device compared to the device employing TPD-TVT.

Charge carrier transport in poly(p-phenylene vinylene):methanofullerene photovoltaic blends

The charge carrier transport and energetic disorder in OC1C10-PPV:PCBM blends with different fullerene concentrations are investigated. We demonstrate that the temperature-dependent current density versus voltage (J–V) characteristics of PCBM electron-only device and OC1C10-PPV hole-only device can be accurately described using our recently introduced improved mobility model. Furthermore, the $$J-V$$ characteristics of OC1C10-PPV:PCBM blends that were measured in electron-only and hole-only devices for different wt% of PCBM can also be accurately described by the improved mobility model. Additionally, we find that the width of the Gaussian density of states $$\sigma$$ and zero-field mobility of electron and hole in OC1C10-PPV:PCBM blends are the function of wt% PCBM. For electron-only devices based on OC1C10-PPV:PCBM blends, the electron mobility gradually increases with increase in the PCBM weight ratio, while the width of the Gaussian density of states $$\sigma$$ gradually decreases with increase in the PCBM weight ratio. Surprisingly, the hole mobility and the width of the Gaussian density of states $$\sigma$$ in hole-only devices based on OC1C10-PPV:PCBM blends show an identical behavior. These results provide information about the energetic disorder and a simplified modeling of the charge transport in disordered organic semiconductors.

Synthesis of organic molecule donor for efficient organic solar cells with low acceptor content

Abstract A new planar A-D-A structured organic small molecule semiconductor (O-SMS) with dialkyl-thiophene substituted benzodithiophene (BDT) as central electron-rich core flanked by relatively electron-deficient units of [1,2,5]thiadiazolo[3,4-c]pyridine (PTz) and terminated with alkyl-bithiophene as π-conjugated end-caps, BDTDPTz, was designed and synthesized for the application as donor material in organic solar cells (OSCs). BDTDPTz possesses wider absorption spectra with an optical bandgap of 1.65 eV, lower the highest occupied molecular orbital (HOMO) energy level of −5.42 eV and highly crystalline structures in solid films. The OSCs based on BDTDPTz:PC71BM blend film with a lower PC71BM content of 40% demonstrate a power conversion efficiency (PCE) of 6.28% with a relatively higher open-circuit voltage of 0.868 V and short circuit current density of 12.83 mA cm−2. These results indicate that highly coplanar and crystalline structure of BDTDPTz can effectively reduce the content of fullerene acceptor in the active layer and then enhance the absorption and PCE of the OSCs.

Exceeding 14% Efficiency for Solution-Processed Tandem Organic Solar Cells Combining Fullerene- and Nonfullerene-Based Subcells with Complementary Absorption

For a highly efficient tandem organic solar cell, it is important for the subcells to minimize the absorption overlap and generate high and balanced currents. Considering the strong absorption and high external quantum efficiency at the short wavelength, developing a highly efficient blend system with a wide-bandgap (WBG) polymer as the donor and a fullerene derivative as the acceptor in the front cell would be an effective strategy. However, it is a challenge to obtain a high short-current density (Jsc) for this blend system. Here, we develop a WBG polymer (PBD1) with an optical bandgap of 1.88 eV. The PBD1:PC71BM blend system with a thickness of 230 nm achieves a power conversion efficiency (PCE) of 9.8% with a high Jsc of 14.6 mA cm–2. When tandem devices are fabricated with PBD1:PC71BM in the front cell, a PCE of 14.2% with a high Jsc of 12.3 mA cm–2 is achieved.

The effect of dihydronaphthyl-based C60 bisadduct as a third-component material on Charge-Carrier dynamics, photovoltaic performance, and stability

We describe a ternary photoactive layer based on PffBT4T-2OD as a donor with a blend of dihydronaphthyl-based C60 bisadduct (NCBA) and PC71BM as the acceptor. It offers ideal exciton dissociation, an ideal charge-carrier recombination and photovoltaic performance relationship, and improves the power-conversion efficiency (PCE) and stability of polymer solar cells (PSCs). NCBA has a broad visible-light absorption spectrum and shallower lowest unoccupied molecular orbital (LUMO) energy levels, which enhance exciton dissociation and charge-carrier transport and suppress charge-carrier recombination. This remarkably increases the photocurrent density and fill factor. An optimized PCE of 9.79% was achieved for the ternary PffBT4T-2OD:NCBA:PC71BM(1:0.24:0.96)-based PSCs. The mechanism was exciton dissociation at the PffBT4T-2OD/NCBA, PffBT4T-2OD:PC71BM, and NCBA/PC71BM interfaces. This simultaneously enhances the exciton-dissociation ratio and charge-carrier transport. Meanwhile, PSCs with 20% NCBA as third-component materials exhibit improved thermal stability and photo-stability leading to a lower ratio of PCE under long-time thermal-annealing and visible-light-illuminated treatment. This work indicates that NCBAs are third-component materials that significantly improve PSC performance.

Functionalized Graphene Oxide Enables a High-Performance Bulk Heterojunction Organic Solar Cell with a Thick Active Layer.

Novel functionalized graphene oxide π-π stacking with conjugated polymers (P-GO) is fabricated via a simple ethanol-mediated mixing method, leading to better dispersion in organic nonpolar solvents and bypassing the inherent restrictions of hydrophilicity and oleophobicity. We analyze the mechanism of the incorporation of P-GO into inverted organic solar cells (OSCs) based on a poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4- b]thiophenediyl]] (PTB7):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) system to investigate the possibility of high-performance thick-film OSC fabrication. It is verified that the incorporation of P-GO into the PTB7:PC71BM blend films leads to a decreased π-π stacking distance, enlarged coherence length for polymer, and optimized phase separation, resulting in more effective charge dissociation, reduced bimolecular recombination, and more balanced charge transport. The OSCs with 1% P-GO incorporation demonstrate a thickness-insensitive fill factor (57.8%) and power conversion efficiency (PCE) (7.31%) even with 250 nm thick photoactive layers, leading to a dramatic PCE enhancement of 34% compared with the control devices with the same thickness.

Physicochemical Studies on Nafion® Modified ZnO Interlayers for Enhanced Electron Transport in the Inverted Polymer Solar Cells

AbstractElectron selective intermediate layers are crucial for both efficiency and stability of the inverted polymer solar cells (I‐PSCs). In this paper, an established low‐temperature (<130 °C) recipe to fabricate nano zinc oxide (ZnO) layers (L‐ZnO) exhibited typical values of power conversion efficiency (PCE∼3.3%) with the standard P3HT:PC61BM blend. However, PCEs showed further improvement up to ∼24% (∼ 4.08%) when these layers were annealed at 300 °C (H‐ZnO). This annealing step can be bypassed by adopting a slightly different chemistry using Nafion as a modifier (NM‐ZnO). I‐PSCs fabricated with NM‐ZnO showed nearly 40% increase in the PCE (∼4.62%) with PEDOT:PSS hole transport layers (HTL) that matches the so far highest recorded value for such a system. For the low‐bandgap polymer blend PBDTT‐FTTE:PC71BM, a peak efficiency of ∼6.77% was obtained compared to ∼5.34% with L‐ZnO (∼28% increase). Our experiments revealed that these enhancements were due to a combination of benefits obtained from Nafion modification such as favourable positioning of work function (WF), high band gap (Eg) and better transport characteristics through the layer interfaces.

Ternary blend organic solar cells with a non-fullerene acceptor as a third component to synergistically improve the efficiency

Ternary blends, where an additional component is added to a donor/acceptor blend, represent a promising route to improve upon the performance of organic solar cells. Non-fullerene acceptor materials are particularly suited for this application due to their relatively stronger absorption coefficients and tuneable molecular energy levels. This paper demonstrates a ternary blend solar cell by incorporating a promising non-fullerene acceptor, IDFBR, into a PffBT4T-2OD:PC71BM blend. The addition of IDFBR to this blend leads to the formation of a cascading energy band structure in the device, which is required for the operation of an efficient ternary system. Ternary blend solar cells with an IDFBR concentration of 5 wt% demonstrated improved photovoltaic performance, when compared to the host binary blends. This improved performance was attributed to a reduction in energetic disorder and improved charge carrier transport in the ternary blend.

Photophysical and photoconductive aspects of donor-acceptor low band gap conjugated copolymer

This article presents the photophysical and photoconductive properties of a newly synthesized low band gap Donor-Acceptor conjugated poly (benzothiadiazole-triphenylamine) co-polymer labelled as P(BTZ-TPA) in which 2,1,3 benzothiadiazole (BTZ) act as the electron deficient moiety and triphenylamine (TPA) is used as donor moiety. Photophysical properties of this synthesized polymer were analyzed by recording steady state absorption and emission spectra of dilute solution and thin films respectively. In addition to this, solvatochromic experiments were carried out using a binary mixture consisting of toluene and acetonitrile to understand the solvent dependence on the ground state and excited states of this polymer. The optical characterization revealed high degree of intramolecular charge transfer through the polymer backbone. The P(BTZ-TPA) thin films exhibit good fluorescence emission with emission peak around 615 nm. Quantum yield of the fluorescence emission is measured in relative method. Also, fluorescence decay parameters were examined using time correlated single-photon counting (TCSPC) technique. Carrier generation and transport of the generated carriers upon photo excitation is studied in pristine polymer films as well as in P(BTZ-TPA):PCBM blend films, by performing photoconductivity studies using broad spectral source and lasers of wavelengths 488 nm and 632 nm. P(BTZ-TPA) thin films exhibited good photoresponse over the entire visible region, with promising internal photocurrent efficiency. Quenching of fluorescence is observed in PCBM blend films. This drastic quenching of fluorescence emission is assigned to the charge transfer interaction between co-polymer and PCBM, and is confirmed by substantial increment in photoconductivity of the blend films. An internal photocurrent efficiency of 17.4% was obtained in the blend films for a biasing field of 10V/μm.

Optimization of conjugated polymer blend concentration for high performance organic solar cells

Recently, conjugated polymer poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2″;5″,2′′′-quaterthiophen-5,5′′′-diy)] (PffBT4T-2OD), gained immense attention among the researchers for the photovoltaic device—owing to high temperature processability, high crystallinity and superior charge transport characteristics. In addition, PffBT4T-2OD displays a unique aggregation property which plays a crucial role in determining the quality of the photoactive blend film and concomitantly influencing the organic photovoltaic (OPV) device performance substantially. Here we demonstrate the detailed investigation into the mechanisms governing the aggregation properties of PffBT4T-2OD:PC71BM blend and its role in determining the interfacial properties—material and electronic; together influencing the device performance as a whole. Spectroscopic analysis (XRD and FTIR) indicate that increasing the blend composition influences the aggregation properties in the film, as a function of increased side chain and polymer backbone interactions. Contact angle measurements showed that this, in turn, greatly influences the wettability of the photoactive layer with the adjacent electron transporting layer (ETL) surface. Impedance spectroscopy measurements revealed that the modified surface properties significantly result in the variation of charge transport characteristics across the ETL/polymer interface. The OPV devices employing the optimized blend concentration 33 mg/ml with favourable aggregation properties exhibits high power conversion efficiency of about 9.6% which is 45% higher than the reference device. A detailed relationship between the aggregation characteristics and the related variation in the interfacial properties is correlated with the device performance.

Solid solution phenomenon in the amorphous conjugated polymer:fullerene bulk heterojunction structure

Abstract The solid solution phenomenon in the amorphous PCDTBT:PC71BM blend was investigated. It has been disclosed that the PCDTBT:PC71BM blend with less than 20% PC71BM may form a kind of solid solution structure, which demonstrates a deeper HOMO level, a higher hole mobility, more free charges and less trap density than those of the pure PCDTBT film. It may well explain why the typical PCDTBT:PC71BM bulk heterojunction solar cells require higher PC71BM ratio to achieve a high power conversion efficiency. In that case, a portion of PC71BM is well intermixed with PCDTBT to form a solid solution phase and others form a pure PC71BM phase. It is the PCDTBT:PC71BM solid solution but not the pure PCDTBT to serve as the donor phases and to form interpenetrating networks with PC71BM phases. The PCDTBT:PC71BM solid solution phases favor to increase the VOC and enhance the charge collection leading to a high photovoltaic performance for the PCDTBT:PC71BM bulk heterojunction solar cells.

Operando Direct Observation of Charge Accumulation and the Correlation with Performance Deterioration in PTB7 Polymer Solar Cells.

Polymer solar cells are one of the promising energy sources because of the easy solution-processable production with large area at a low cost without toxicity. Among the polymer materials, a donor-acceptor conjugated copolymer PTB7 has been extensively studied because of the typical high-performance polymer solar cells. Here, we show operando direct observation of charge accumulation in PTB7:PC71BM blend solar cells from a microscopic viewpoint using electron spin resonance spectroscopy. The accumulation of ambipolar charges in the PTB7-based cells is directly observed for the first time, which shows a clear correlation with the performance deterioration during device operation. The sites of the ambipolar charge accumulation are elucidated at the molecular level, whose information would be useful for improving the cell durability in addition to the performance improvement.

Effect of the Polymer Chain Arrangement on Exciton and Polaron Dynamics in P3HT and P3HT:PCBM Films

Arrangement of polymer chains at the interface with an acceptor is one of the key issues which influences the interfacial charge transfer. Films of P3HT and P3HT:PCBM blends with the different polymer chain arrangement have been prepared from one-component and binary solvents representing a mixture of chloroform and different poor solvents with high boiling temperature. Electronic absorption and photoluminescence spectra evidenced in favor of reduced disorder of P3HT films as a result of use of poor solvents; the ordering was displayed through structuring and narrowing the spectral bands, indicative of decreasing width of Gaussian distribution of molecular transition frequencies. At the same time, transient absorption of singlet excitons showed that exciton decay in highly ordered P3HT films slows down as compared to the disordered film, and this effect was reproduced for the different poor solvents used. A more pronounced effect was revealed in P3HT:PCBM blends where much faster decay of excitons was found in disordered as-prepared P3HT:PCBM film from chloroform as compared to the annealed film or films prepared from the binary solvents. A complex behavior of polarons in the different regions of the blend film, i.e., within crystalline domains of P3HT and at the interface of P3HT/PCBM, was observed. The conclusion was drawn that chain disorder induces easier exciton dissociation and charge transfer at the P3HT/PCBM interface.

Realization of Large‐Scale Polymer Solar Cells Using Ultrasonic Spray Technique Via Solvent Engineering (Solar RRL 7∕2018)

Reliability and reproducibility remain as main challenges in the realization of large-scale polymer solar cells. In article no. 1800064, Jiang Cheng, Lu Li and co-workers demonstrate a smart method to prototyping high yield and largescale PTB7:PC71BM solar cells using ultrasonic spray coating. Marangoni-effect related coffee-rings and pin holes are successfully overcome via tuning the drying processes. The formation of PTB7-enriched surface, confirmed in solvent engineering, led to an improved PTB7:PC71BM blend film, and the influence on the device characteristic performance was systematically studied. The power conversion efficiency of the device is improved to 8.23%, and the preparation of a large-scale active area solar cell of 18 cm2 with a maximum power output of 74.8 mW is demonstrated promising for industrial applications.

Synthesis and Characterization of Cyclopentadithiophene and Thienothiophene-Based Polymers for Organic Thin-Film Transistors and Solar Cells

Novel donor-donor type alternating copolymers (8CDT-TT and 16CDT-TT) derived from cyclopentadithiophene (CDT) and thienothiophene (TT) moieties that differ from solubilizing side chains were successfully synthesized and characterized. After the synthesis of CDT-TT-based conjugated polymers with dioctyl and dihexadecyl side chains, their optical, thermal, structural and semiconducting properties were investigated. Organic thin-film transistors fabricated from 8CDT-TT and 16CDT-TT exhibit carrier mobilities as high as 3.92×10-4 and 1.05×10-3 cm2V-1s-1, respectively. Bulk heterojunction solar cells fabricated using a polymer:PCBM blend ratio of 1:3 exhibit power conversion efficiencies of 2.12 and 1.84% for 8CDT-TT and 16CDT-TT, respectively.

Triptycene as a Supramolecular Additive in PTB7:PCBM Blends and Its Influence on Photovoltaic Properties.

Additives play an important role in modifying the morphology and phase separation of donor and acceptor molecules in bulk heterojunction (BHJ) solar cells. Here, we report triptycene (TPC) as a small-molecule additive for supramolecular control of phase separation and concomitant improvement of the power conversion efficiency (PCE) of PTB7 donor and fullerene acceptor-based BHJ polymer solar cells. An overall 60% improvement in PCE is observed for both PTB7:PC61BM and PTB7:PC71BM blends. The improved photovoltaic (PV) performance can be attributed to three factors: (a) TPC-induced supramolecular interactions with donor:acceptor components in the blends to realize a nanoscale phase-separated morphology; (b) an increase in the charge transfer state energy that lowers the driving force for electron transfer from donor to acceptor molecules; and (c) an increase in the charge carrier mobility. An improvement in efficiency using TPC as a supramolecular additive has also been demonstrated for other BHJ blends such as PBDB-T:PC71BM and P3HT:PCBM, implying the wide applicability of this new additive molecule. A comparison of the photostability of TPC as an additive for PTB7:PCBM solar cells to that of the widely used 1,8-diiodooctane additive shows ∼30% higher retention of PV performance for the TPC-added solar cells after 34 h of AM 1.5G illumination. The results obtained suggest that the approach of using additives that can promote supramolecular interactions to modify the length scale of phase separation between donor and acceptor is very promising and can lead to the development of highly efficient and stable organic photovoltaics.

Solution processed graphene as electron transport layer for bulk heterojunction based devices

The exceptional electrical properties of the graphene offer many opportunities for the performance improvement in the devices. In this study, the importance of incorporation of solution processed graphene as an electron transport layer (ETL) in bulk heterojunction (BHJ) devices is elucidated by measuring the current-voltage characteristics under dark and illumination conditions. Raman spectra P3HT:PCBM blend thin films display the same modes as of pristine P3HT. The UV/Vis analysis confirms the formation of P3HT:PCBM blend. The three different devices of architectures: standard structure, standard structure with ETL and inverted structure with ETL, comprising active layer of as-prepared P3HT:PCBM blends are fabricated and compared. Various Schottky diode parameters like ideality factor, series resistance, and barrier height are determined from current-voltage curves using Cheung and Cheung approach. The incorporation of graphene ETL in the device shows the enhancement in photocurrent resulting from the better extraction and improved charge collection efficiency. The photovoltaic effect is also observed for inverted device configuration. The charge transport analysis of the devices show overall power law trend, i.e. Ohmic current at low voltage followed by space charge limited conduction in existence of discrete trap levels at higher voltage range.

The effects of solvent treated PEDOT:PSS buffer layer in organic solar cells

Various treatments on the PEDOT:PSS films were carried out to investigate it’s influence on the conductivity, morphology, transmittance and the corresponding impact of the performance of the organic photovoltaic devices based on the PCPDTBT:PCBM and P3HT:PCBM blends. These processing including doping PEDOT:PSS with DMF and ME solvents and exposing these films to the vapor of DMF and ME solvents, separately. A considerable enhancement of the conductivity and transmittance of PEDOT:PSS was observed after doping solvent into the PEDOT;PSS solution followed by solvent treatment through exposing these films to solvents environment. The best organic PV doped devices based on either PCPDTBT:PCBM or based on P3HT:PCBM with power conversion efficiency were 2.93% compared to 1.87% for the pristine PV devices or 2.79% compared to 1.77% for the pristine devices, respectively. The conductivity improvement was highly influenced by solvent treatment.

Insight Into the Role of PC71BM on Enhancing the Photovoltaic Performance of Ternary Organic Solar Cells.

The development of non-fullerene acceptor molecules have remarkably boosted power conversion efficiency (PCE) of polymer solar cells (PSCs) due to the improved spectral coverage and reduced energy loss. An introduction of fullerene molecules into the non-fullerene acceptor-based blend may further improve the photovoltaic performance of the resultant ternary PSCs. However, the underlying mechanism is still debatable. Herein, the ternary PSCs based on PBDB-T:ITIC:PC71BM blend were fabricated and its PCE was increased to 10.2% compared to 9.2% for the binary PBDB-T:ITIC devices and 8.1% for the PBDB-T:PC71BM PSCs. Systematic investigation was carried out to disclose the effect of PC71BM on the blend morphology and charge transport behavior. It is found that the PC71BM tends to intermix with the PBDB-T donor compared to the ITIC counterpart. A small amount of PC71BM in the ternary blend is helpful for ITIC to aggregate and form efficient electron-transport pathways. Accordingly, the electron mobility is increased and the density of electron traps is decreased in the ternary blend in comparison with the PBDB-T:ITIC blend. Finally, the suppressed bimolecular recombination and enhanced charge collection lead to high PCE for the ternary solar cells.