Bulk Heterojunction Photovoltaic Devices Research Articles

Conjugated random copolymers of benzodithiophene–benzooxadiazole–diketopyrrolopyrrole with full visible light absorption for bulk heterojunction solar cells

We have used Stille coupling polymerization to synthesize a series of new donor–acceptor (D–A) conjugated random copolymers—PBDTT-BO-DPP—that comprise electron-rich alkylthienyl-substituted benzodithiophene (BDTT) units in conjugation with electron-deficient 2,1,3-benzooxadiazole (BO) and diketopyrrolopyrrole (DPP) moieties that have complementary light absorption behavior. These polymers exhibited excellent thermal stability with thermal degrading temperatures higher than 340 °C. Each of these copolymers exhibited (i) broad visible light absorption from 400 to 900 nm and (ii) a low optical band gap that is smaller than 1.4 eV and a low-lying highest occupied molecular orbital that is deeper than −5.22 eV. As a result, bulk heterojunction photovoltaic devices derived from these polymers and fullerenes provided a high short-circuit current density that is larger than 12 mA cm−2. In particular, a photovoltaic device prepared from the PBDTT-BO-DPP (molar ratio, 1 : 0.5 : 0.5)/PC71BM (w/w, 1 : 2) blend system with 1-chloronaphthalene (1 volume%) as an additive exhibited excellent photovoltaic performance, with a value of Voc of 0.73 V, a high short-circuit current density of 17 mA cm−2, a fill factor of 0.55, and a promising power conversion efficiency of 6.8%, indicating that complementary light-absorption random polymer structures have great potential for increasing the photocurrent in bulk heterojunction photovoltaic devices.

Donor conjugated polymers-based on alkyl chain substituted oligobenzo[c]thiophene derivatives with well-balanced energy levels for bulk heterojunction solar cells

We designed and synthesized two donor conjugated polymers (poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-(3-(3-hexylthiophen-2-yl)benzo[c] thiophen-1-yl)thiophen-2-yl)acrylonitrile (CN-PHTBTPB) and poly(2Z,2′Z)-2,2′-(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-(3-(3-octylthiophen-2-yl)benzo[c]thiophen-1-yl)thiophen-2-yl)acrylonitrile (CN-POTBTPB)), comprised of hexyl and octyl chains substituted at the 3-position of 1,3-di(thiophene-2-yl)benzo[c]thiophene moieties as a donor unit coupled with a 1,4-bis-(cyano-2-thienylvinylene) phenylene based precursor by means of Knoevenagel polycondensation followed by a chemical oxidative polymerization method. Both the synthesized polymers (CN-PHTBTPB and CN-POTBTPB) exhibited reduced optical band gaps of about 1.86 eV. The resulting polymers were experimentally found to possess deep HOMO (about −5.50 to 5.78 eV) and high-lying LUMO (about −3.78 to 3.82 eV) energy levels. Theoretical calculations revealed that the variation in the HOMO and LUMO energy levels are consistent with cyclic voltammetry measurements. Using these polymers, the bulk heterojunction photovoltaic devices with the structure of ITO/PEDOT:PSS/Synthesized donor polymer:PCBM/LiF/Al was fabricated in open air. The device based on the CN-PHTBTPB and CN-POTBTPB polymers delivered respectively power conversion efficiencies of 1.65% and 1.98% under the illumination of AM 1.5G, 100 mW cm−2, which indicates that this kind of conjugated polymer is a highly promising material for organic solar cell application. The clear correlation between the blend film morphology and the power conversion efficiency of the devices was observed via atomic force microscopic studies.

Solution-processed benzotrithiophene-based donor molecules for efficient bulk heterojunction solar cells

In this study we used convergent syntheses to prepare two novel acceptor–donor–acceptor (A–D–A) small molecules (BT4OT, BT6OT), each containing an electron-rich benzotrithiophene (BT) unit as the core, flanked by octylthiophene units, and end-capped with electron-deficient cyanoacetate units. The number of octylthiophene units affected the optical, electrochemical, morphological, and photovoltaic properties of BT4OT and BT6OT. Moreover, BT4OT and BT6OT possess low-energy highest occupied molecular orbitals (HOMOs), providing them with good air stability and their bulk heterojunction (BHJ) photovoltaic devices with high open-circuit voltages (Voc). A solar cell device containing BT6OT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in a 1:0.75 ratio (w/w) exhibited a power conversion efficiency (PCE) of 3.61% with a short-circuit current density (Jsc) of 7.39 mA cm−2, a value of Voc of 0.88 V, and a fill factor (FF) of 56.9%. After adding 0.25 vol% of 1-chloronaphthalene (CN) as a processing additive during the formation of the blend film of BT6OT:PC71BM (1:0.75, w/w), the PCE increased significantly to 5.05% with values of Jsc of 9.94 mA cm−2, Voc of 0.86 V, and FF of 59.1% as a result of suppressed nanophase molecular aggregation.

Directly probing the molecular order of conjugated polymer in OPV blends induced by different film thicknesses, substrates and additives

In organic bulk heterojunction photovoltaic (OPV) devices, formation of a phase-separated morphology of the blend thin film with a high degree of molecular order is required for efficient device performance. Using resonant Raman spectroscopy we monitor in situ the P3HT molecular order in P3HT:PCBM blend films influenced by the substrate, film thickness and additives. We report that molecular order depends on substrate for as-cast films, consistent with vertical phase separation driven by a surface energy gradient, but is standardised to a highly ordered state by thermal annealing. In situ Raman spectroscopy reveals this phase transition to a more ordered state begins at 40–60 °C for ∼120 nm thick blend films, which corresponds to the glass transition temperature (Tg). Ultra-thin (<10 nm thick) blend films had greater P3HT order than the bulk and reorganised at lower temperatures, which we propose is due to a P3HT-rich interfacial layer at the film/air interface, and that extra disordered component retained despite annealing is due to P3HT trapped in a disordered state within the corresponding PCBM-rich substrate interface. Finally we probe how the 1,8-octanedithiol (ODT) additive improves P3HT molecular order in blends by increasing phase separation during deposition, finding that 3% ODT by volume presents a saturation point for improving molecular order, and the improvement is comparable to that by thermal annealing. Through in situ experiments and varied fabrication conditions, we have built an understanding of how processing conditions determine conjugated polymer molecular order in blends, with the aim of controlling morphology for higher OPV efficiencies.

Controllable fabrication of ternary ZnIn2S4 nanosheet array film for bulk heterojunction solar cells

Large-scale uniform ternary ZnIn2S4 nanosheet array films have been grown on indium-tin oxide (ITO) glass substrates effectively by a simple direct elemental reaction route. Inorganic–organic bulk heterojunction photovoltaic devices were fabricated with ZnIn2S4 film as an electron acceptor and poly (2-methoxy-5-(2/-ethylhexoxy)-1,4-phenylene vinylene (MEH-PPV) polymer as electron donor. The formation of the ZnIn2S4 nanosheet film was characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV-VIS-NIR spectroscopy. The as-prepared ZnIn2S4 nanosheet film was demonstrated as an n-type semiconductor exhibiting an apparent photocurrent response and good photostability. The photovoltaic performance of the constructed solar cell device with the active structure of ITO/ZnIn2S4:MEH-PPV/Au was evaluated at room temperature under AM1.5G (100 mW cm−2) simulated globe sun illumination from a filtered Xenon lamp.

Isoindigo-based small molecules for high-performance solution-processed organic photovoltaic devices: the electron donating effect of the donor group on photo-physical properties and device performance

Five solution processable isoindigo-based donor-acceptor-donor (D-A-D) small molecules with different electron donating strengths have been designed and synthesized. The variation in the electron donating strength of the donor group strongly affected the optical, thermal, electrochemical and photovoltaic device performances of the isoindigo organic materials. The highest power conversion efficiency of ~3.2% was achieved in the bulk heterojunction photovoltaic device consisting of ID3T as the donor and PC70BM as the acceptor. This work demonstrates the potential of isoindigo moieties as electron-deficient units and presents guidelines for the synthesis of D-A-D small molecules for producing highly efficient, solution-processed organic photovoltaic devices.

Thiophene spacers impart crystallinity and enhance the efficiency of benzotrithiophene-based conjugated polymers for bulk heterojunction photovoltaics

In this study we synthesized the donor–acceptor conjugated copolymers PBTT4BT and PBTT4BO featuring benzotrithiophene (BTT) units as donors and benzothiadiazole (BT) and benzoxadiazole (BO) units, respectively, as acceptors, linked through 4-dodecylthiophene spacers. The presence of the spacer units enhanced not only the solubility of the synthesized polymers but also their molecular packing in the solid state; both of these polymers exhibited good crystallinity, as evidenced by a d-spacing of 23.8 A in the (100) plane in their X-ray diffraction curves. When we used these synthesized polymers in bulk heterojunction photovoltaic device applications, the optimal device incorporating PBTT4BO/PC61BM as the active layer exhibited a low efficiency of 3.2%, due to the poor solubility of PBTT4BO, whereas the optimal device incorporating the more-soluble PBTT4BT and PC71BM displayed an efficiency of 4.4%, which is substantially 1.5% higher than that for the PBTTBT/PC71BM device, where PBTTBT was formed by copolymerizing BTT and BT units without any spacer. After thermal annealing, the efficiency of the PBTT4BT/PC71BM device improved further to 5.6%, with a VOC value of 0.72 V, a JSC value of 11.58 mA cm−2 and a fill factor of 67%. The annealed PBTT4BT/PC71BM active layer possessed a nanoscaled network-like morphology with rod-like PBTT4BT domains that were beneficial for charge separation and transport; accordingly, the power conversion efficiency of the annealed PBTT4BT/PC71BM photovoltaic device was enhanced greatly over that of the as-cast PBTT4BT/PC71BM device.

Aluminum nanoparticles for efficient and stable organic photovoltaics

Enhancement of both the efficiency and stability of bulk heterojunction photovoltaic devices is demonstrated by the addition of highly stable Al NPs, generated via laser ablation, into the photoactive layer. An efficiency enhancement of 30% is demonstrated, which can be attributed to multiple scattering effects and enhanced structural stability of the photoactive blend. Surprisingly, the devices with Al NPs die after ∼150 h of operation under continuous illumination, compared with the pristine devices which die after ∼30 h.

Tuning zinc oxide/organic energy level alignment using mixed triethoxysilane monolayers

Interfacial energy level alignment influences several critical organic optoelectronic device characteristics including charge transfer barriers, turn-on voltage, and open circuit voltage (Voc). Introduction of dipolar molecular monolayers on metal oxide surfaces has allowed improvements in device performance as well as fundamental studies of energy level alignment in these devices. We demonstrate that dipolar mixed monolayers can be covalently bonded to zinc oxide (ZnO) through the triethoxysilane chemical attachment scheme, and that these monolayers tune the work function of the ZnO surface over 0.6 eV. We then employ mixed monolayer-treated ZnO surfaces as the electron-selective contact in inverted bulk heterojunction photovoltaic devices to determine the correlation between the Voc and the work function of the contact. We find the relationship of −0.14 V eV−1 between the Voc and contact work function is consistent with current results and theories of contact influence on Voc.

Doping Effect of CBP in Bulk-heterojunction Photovoltaic Devices Composed of P3HT and Soluble Perylene Bisimide

The effect of doping carrier transporting materials being used for organic light emitting diodes was investigated in bulk-heterojunction photovoltaic devices composed of poly(3-hexylthiophene) (P3HT) and a perylene bisimide (PBI) derivative. We found that the doping of 4,4’-bis(carbazol-9-yl)biphenyl (CBP) can improve the short circuit current and fill factor with a doping concentration of 2.5wt%, resulting in an increase of power conversion efficiency from 0.14% to 0.28%. This effect is mainly attributable to the improvement of miscibility between P3HT and PBI.

Charge transport in polythiophene:fullerene:nanotube bulk heterojunction photovoltaic devices investigated by impedance spectroscopy

P3HT:PCBM bulk heterojunction devices incorporating SWNTs, which are predominantly metallic in character, have been analyzed using impedance spectroscopy to understand the effect of SWNTs on their charge carrier transport properties. SWNTs reduce the effective lifetime of injected charge carriers. Frequency dependence of capacitance and conductance of P3HT:PCBM devices show monotonic variations without any clear peak positions. Simulations of the complex admittance of the P3HT:PCBM devices under trap free space charge limited current within the framework of Scher–Montrol theory are used to qualitatively show that such characteristics are a signature of charge transport which is highly dispersive in nature. The position of peak τpeak in the imaginary part of impedance Im(Z), which is essentially same as the first transition frequency of Cole–Cole plot, has a direct relation with the effective dc mobility of charge carriers which varies with dispersion parameters. Using the dc mobility values and the voltage variation of peak frequency of Im(Z), the ratio of τdc to τpeak has been calculated. The magnitude of this ratio is indicative of the degree of dispersiveness in transport. It has been shown that, SWNTs at low concentrations tend to reduce the dispersiveness in charge transport.

Conjugated moiety effect on blend film phase separation and photovoltaic properties of benzo[1,2-b:4,5-b′]dithiophene-containing coplanar donor–acceptor copolymers

A series of high coplanar alternative copolymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) donor unit and alkoxy substituted electron-accepting 2,1,3-benzothiadiazole moiety (PBDT-BT), high coplanar electron-donating thieno[3,2-b]thiophene moiety (PBDT-TT), or thieno[3,2-b]thiophene attached to both sides of the benzothiadiazole moiety (PBDT-TTBTT) were designed and synthesized. The influences of the conjugated moiety on polymer optical and electrochemical properties, phase separation in blend film with (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM), and photovoltaic properties for the application as donor materials in polymer photovoltaic cells (PVCs) were studied. The optical and electrochemical properties of copolymers showed that the HOMO and LUMO energy levels were determined by BDT and BT moiety in the alternative copolymers, respectively. The TT moiety does not have significant influence on the energy level when incorporated with BDT and BT units in alternative copolymers, but can significantly affect the phase separation behavior when mixed with PC61BM by improving the crystal structural order. Interestingly, hierarchical lamella structures were observed in the blend film containing copolymer and PC61BM: one is crystalline copolymer’s ordered lamella packing structure with size about 2nm, as revealed by the XRD experiments; the other is the lamella phase separation structure with width about 15nm. The crystal grain size in blend film was estimated by means of Scherrer’s equation, and the value coincided well with that from TEM observation. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and PC61BM or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as acceptors. The results indicate larger crystal domain size will give better power conversion efficiency (PCE) in our system, where the domain size is smaller than the exciton diffusion length (∼20nm). The photovoltaic device based on PBDT-TTBTT:PC71BM showed a Jsc of 9.15mA/cm2 and a PCE of 3.54%.

Benzotriazole-based donor–acceptor type semiconducting polymers with different alkyl side chains for photovoltaic devices

A series of low band gap polymers composed of benzotriazole and benzo[1,2-b:4,5-b′]dithiophene derivatives were synthesized using a Stille cross-coupling reaction for use in organic photovoltaic devices. Linear or branched alkyl groups were incorporated into the benzothiazole-based accepting monomer part, and alkoxy or alkylthiophene groups were introduced to benzo[1,2-b:4,5-b′]dithiophene-based donating monomer part. Changes in photo-physical properties of the polymers by the structural modification of the donor–acceptor type low band gap polymers were investigated. The synthesized polymers were soluble in common organic solvents, and the resulting polymer solutions could be used to form smooth and uniform thin films by spin-casting. The synthesized polymers were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to approximately 300°C. The intra-molecular charge transfer interaction between the electron donating and electron accepting blocks in the polymeric backbone induced a broad absorption from 300 to 650nm. The optical band gap energies of the polymers were measured to be 2.03–1.90eV depending on the polymer structure. Solution-processed field-effect transistors were fabricated and characterized using the polymers as p-type channel materials. Bulk hetero-junction photovoltaic devices were fabricated using the polymers with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the electron acceptor. One of the fabricated device showed the maximum power conversion efficiency of 3.20% under AM 1.5G (100mW/cm2) conditions.

Yet another poor man's green bulk heterojunction photocells: Annealing effect and film composition dependence of photovoltaic devices using poly(3-hexylthiophene):C70 composites prepared with chlorine-free solvent

Chemically modified fullerenes and chlorinated solvents have been predominantly chosen in the recent studies of the photovoltaic devices based on polymer bulk heterojunction composites. However, these items seem to be undesirable because of their potential impacts on the environment as well as the consumption of resources. In this context, a systematic study on the annealing effect and film composition dependence of bulk heterojunction photovoltaic devices based on composites consisting of poly(3-hexylthiophene) (P3HT) and neat C70 prepared with a chlorine-free solvent 1,2,4-trimethylbenzene has been carried out. For the device using P3HT:C70 composite (2:1 by weight) film as an active layer, power conversion efficiency of 1.47%, with open-circuit voltage of 0.52V, short-circuit current density of 6.2mA/cm2 and fill-factor of 45%, has been obtained after post-production annealing at 160°C. The combination of neat fullerenes with naturally obtained solvents will open up a new way to produce cost-effective and environmentally-friendly photovoltaic devices based on polymer:fullerene bulk heterojunction composites.

Synthesis, Properties, and Photovoltaic Performance of Copolymers Based on Difluorodioxocyclopentene-Annelated Thiophene

New π-conjugated polymers containing difluorodioxocyclopentene-annelated thiophenes as an electron-accepting unit have been synthesized for application to p-type organic semiconducting materials in organic photovoltaics. The photophysical and electrochemical properties of these polymers were investigated by UV–vis absorption spectra and cyclic voltammetry measurements. The carrier mobility measurement of the copolymer on organic field-effect transistor devices revealed p-channel behavior. Bulk heterojunction photovoltaic devices made from blends of the polymer and [6,6]-phenyl-C71-butyric acid methyl ester showed moderate photovoltaic characteristics with a power conversion efficiency of 0.77%.

New Donor Materials Based on Thiazole and Triphenylamine for Photovoltaic Devices

New photovoltaic donor materials, 4,4'-(2,2'-bithiazole-5,5'-diyl)bis(N,N-diphenylbenzenamine) (BDT) and 4-(2,2'-bithiazol-5-yl)-N,N-diphenylbenzenamine (BT), were synthesized. A solution processable triphenylamine-containing bithiazole (BDT and BT) was blended with a [6,6]-phenyl butyric acid methyl ester (PCBM) acceptor to study the performance of small-molecule-based bulk heterojunction (BHJ) photovoltaic devices. Optimum device performance was achieved after annealing, for device with a BDT/PCBM ratio of 1:4. The open-circuit voltage, short-circuit current, and power conversion efficiency of the device with the aforementioned BDT/PCBM ratio were 0.51 V, 4.10 , and 0.68%, respectively, under simulated AM 1.5 solar irradiation (100 ).

Synthesis of conjugated perylene diimide-based copolymer with 5,5′-bis(4-aminophenyl)-2-2′-bifuryl moiety as an active material for organic photovoltaics

Abstract In this study, we have synthesized conjugated n-type perylene tetracarboxylic diimide-based copolymer with 5,5′-bis(4-aminophenyl)-2-2′-bifuryl segment (PTCDI-PFDA) as an active material for the fabrication of bulk heterojunction photovoltaic device of type ITO/PEDOT:PSS/rr-P3HT:PTCDI-PFDA/Al. The synthesized n-type copolymer (an oligomer with a relatively low molecular weight) was characterized by 1 H NMR, gel permeation chromatography and thermo gravimetric analysis. The electronic and structural properties of the copolymer were investigated by UV–vis and fluorescence spectroscopy. The copolymer exhibited an optical band gap of about 1.59 eV. The HOMO and LUMO energy levels were calculated using cyclic voltammetry and density functional theory (DFT). The resulting copolymer was experimentally found to possess low-lying HOMO ( ca . −5.24 eV) and high-lying LUMO ( ca . −3.9 eV) energy levels. The fabricated device ( D1 ) delivered an efficiency ( η ) of about 0.34% with a high open circuit voltage ( V oc = 0.92 V), current density ( J sc = 2.83 mA/cm 2 ), and fill factor (FF = 13%) under 100 mW/cm 2 white light.

Organic Bulk Heterojunction Photovoltaic Devices Based on Polythiophene–Graphene Composites

A solution-processed graphene content was synthesized by treatment of graphite oxide (GO) with phenyl isothiocyanate (PITC) by taking advantage of the functional carboxyl groups of graphene oxide. The GO was prepared by the oxidation of natural graphite powder and was expanded by ultrasonication in order to exfoliate single or/and few-layered graphene oxide sheets. The functionalized graphene oxide, GO-PITC, can be dispersed within poly-(3-hexylthiophene) (P3HT) and can be utilized as the electron acceptor in bulk heterojunction polymer photovoltaic cells. When P3HT is doped with GO-PITC, a great quenching of the photoluminescence of the P3HT occurred, indicating a strong electron transfer from the P3HT to the GO-PITC. The utilization of GO-PITC as the electron acceptor material in poly-(3-hexylthiophene) (P3HT) bulk heterojunction photovoltaic devices was demonstrated, yielding in a power conversion efficiency enhancement of 2 orders of magnitude compared with that of pristine P3HT.

Synthesis of seleno[3,4-c]pyrrole-4,6-dione-based polymers for polymer solar cells

Novel π-conjugated polymers PSePDBDT1 and PSePDBDT2 containing seleno[3,4-c]pyrrole-4,6-dione and benzo[1,2-b:4,5-b′]dithiophene units were synthesized, and their thermal, optical and photovoltaic properties were compared with those of the corresponding thiophene-based polymer, PTPDBDT2, containing sulfur instead of selenium. PSePDBDT2 exhibited higher thermal stability and a narrower bandgap than PTPDBDT2. Using PSePDBDT2 or PTPDBDT2 as an electron donor and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as an electron acceptor, inverted-type bulk heterojunction photovoltaic devices with the configuration ITO/TiOx/PSePDBDT2 or PTPDBDT2:PC61BM (1:2, w/w)/PEDOT:PSS/Au (ITO=indium tin oxide, TiOx=titanium oxide, PEDOT:PSS=poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonic acid)) were fabricated. The preliminary power conversion efficiencies of the devices containing PSePDBDT2 and PTPDBDT2 were 0.28% and 1.54%, respectively, under simulated AM1.5 solar irradiation (77.8mW/cm2).

Solution-processable graphene linked to 3,5-dinitrobenzoyl as an electron acceptor in organic bulk heterojunction photovoltaic devices

3,5-Dinitrobenzoyl chloride was covalently linked to graphene oxide (GO) nanosheets prepared by a modified Hummers’ method, using ethylenediamine as a spacer. The linkage of the GO with the small molecule was confirmed by spectroscopic (e.g., Fourier transform infrared, Raman) and microscopic analyses. The resultant GO-ethylene-dinitro-benzoyl (GO-EDNB) consists of a controlled scale of different graphene structures and is highly dispersable in common organic solvents. The GO-EDNB was used as the electron acceptor material in poly-(3-hexylthiophene) (P3HT) bulk heterojunction photovoltaic devices to significantly improve the performance, yielding a power conversion efficiency improvement of two orders and one order of magnitude compared with the pristine P3HT and the P3HT-GO devices respectively.