Fabricate Bulk Heterojunction Research Articles

High quality graphene-semiconducting oxide heterostructure for inverted organic photovoltaics

In this work, we demonstrate damage-free direct growth of high-quality semiconducting ZnO thin films on thermal chemical vapor deposition (CVD)-grown graphene transferred onto a plastic substrate. A mist pyrolysis CVD (MPCVD) method based on a non-vacuum process at a low growth temperature of 160 °C is introduced for the successful direct growth of ZnO on graphene without any additional treatments or processes. ZnO thin films that did not exhibit damage to the structural and electrical properties of graphene were successfully grown on graphene. The MPCVD-grown ZnO thin films revealed more uniform surface morphology and better structural property than ZnO thin films deposited using sol–gel and sputtering. Using the ZnO/graphene heterostructure, we fabricated bulk heterojunction inverted organic photovoltaics with a power conversion efficiency of 1.55%.

Efficient bulk heterojunction solar cells using an alternating phenylenevinylene copolymer with dithenyl(thienothiadiazole) segments as donor and PCBM or modified PCBM as acceptor

A novel low band gap alternating phenylenevinylene copolymer, P, with dithenyl (thienothiadiazole) segments was synthesized by Heck coupling. It was soluble in common organic solvents, showed broad absorption curve with long-wavelength absorption maximum at 580–598 nm and optical band gap of 1.74 eV, which is comparable with the electrochemical band gap of about 1.80 eV. P (electron donor) was blended with PCBM or modified PCBM i.e. F (electron acceptor) to fabricate bulk heterojunction (BHJ) solar cells. The power conversion efficiency (PCE) of the devices based on P:PCBM and P: F cast from 1.2-dichlorobenzene (DCB) was found to be 1.40% and 2.32%, respectively. The higher value of PCE for the device with P: F as compared to P:PCBM is attributed to the increase in both short circuit current ( J sc) and open circuit voltage ( V oc). The increase in the J sc is due to the stronger light absorption of F in visible region as compared to PCBM, i.e. more exciton generation in the blend. On the other hand, the higher difference between the LUMO levels of P and F, as compared to P and PCBM is responsible for the enhancement in the V oc. A maximum overall PCE of 4.20% was obtained for the BHJ polymer cell based on the active layer ( P: F) deposited from mixed solvents 1-chloronapthalene/1,2-dichlorobenzene (CN/DCB) and subsequent thermal annealing at 120 °C. This improvement in the PCE has been attributed to the enhanced crystallinity of the blend and more balanced charge transport in the device due to the thermal treatment.

Investigation of Annealing and Blend Concentration Effects of Organic Solar Cells Composed of Small Organic Dye and Fullerene Derivative

We have fabricated bulk heterojunction organic solar cells using coumarin 6 (C6) as a small organic dye, for light harvesting and electron donation, with fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), acting as an electron acceptor, by spin-coating technique. We have investigated thermal annealing and blend concentration effects on light harvesting, photocurrent, and performance parameters of the solar cells. In this work, we introduced an experimental method by which someone can easily detect the variation in the contact between active layer and cathode due to thermal annealing after cathode deposition. We have showed, in this work, unusual behavior of solar cell composed of small organic molecules under the influence of thermal annealing at different conditions. This behavior seemed uncommon for polymer solar cells. We try from this work to understand device physics and to locate a relationship between production parameters and performance parameters of the solar cell based on small organic molecules.

Patternable brush painting process for fabrication of flexible polymer solar cells

The brushing painting method is one of the typical solution processes, which makes it possible to fabricate electronic devices by simply using polymers. For the fabrication of modules and large-area devices, however, a patterning process is required for each layer. Due to the problems associated with the conventional patterning, many studies have been conducted to develop new patterning . In this study, we successfully fabricated bulk heterojunction (BHJ) structured polymer solar cells (PSCs) on a plastic substrate with an active layer through the brushing painting process. In particular, flexible PSCs with hole transporting and photo-active layers formed by the repositionable adhesive-based patterning method were fabricated to produce large-area solar cells. The fabricated PSCs exhibited Jsc, Voc, FF and power conversion efficiency (PCE) values of 8.5 mA/cm 2, 0.636 V, 48.8% and 2.6%, respectively. In other words, they were more efficient than those fabricated by the spin coating method. In particular, an increase in the Jsc and FF values was observed when the series resistance (Rs) decreased to 29 Ω cm 2 while the shunt resistance (Rsh) increased to 2018 Ω cm 2.

π-Conjugated Main Chain Polymers Containing Bis(bithiophenyl dithienothiophene)-based Repeating Group and their Application to Polymer Solar Cells

New dithienothiophene (DTT)-containing conjugated polymers, such as poly(2,6-bis(2-thiophenyl-3-dodecylthiophene-2-yl)dithieno[3,2-b;2′,3′-d]thiophene, 1 and poly(2,6-bis(2-thiophenyl-4-dodecylthiophene-2-yl)dithieno[3,2-b;2′,3′-d]thiophene, 2 have been successfully synthesized via Stille coupling reactions using dodecyl-substituted thiophene-based monomer, bistributyltin dithienothiophene, and bistributyltin bithiophene. The main difference between two polymers is the substitution position of dodecyl side chains in a repeating group. The semiconducting properties of the two polymers have been evaluated in organic photovoltaics (OPVs). Two conjugated polymers are also well mixed with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) for fabricating bulk heterojunction PV devices. As a result, the polymer solar cell devices made of polymers 1 and 2 had the best preliminary results with an open-circuit voltage of 0.43–0.44 V, a short-circuit current density of 5.95–5.96 mA/cm2, and a fill factor of 0.34, offering an overall power conversion efficiency (PCE) of 0.9%.

Soluble functionalised fullerenes for photovoltaics

Two mono-adducts of fullerene C 60 have been synthesised using new aryl azides in good yields. These were characterized with spectroscopic techniques and then used to fabricate bulk heterojunction solar cells with regioregular poly (3-hexylthiophene) (RR-P3HT). A photocurrent density of 3.89 mA/cm 2, open-circuit voltage of 0.53 V and power conversion efficiency of 0.89% for adduct I and photocurrent density of 2.93 mA/cm 2, open-circuit voltage of 0.53 V and power conversion efficiency of 0.71% for adduct II have been achieved.

Side chain engineering of fused aromatic thienopyrazine based low band-gap polymers for enhanced charge carrier mobility

A strategic side-chain engineering approach leads to the two orders of magnitude enhancement of charge carrier mobility in phenanthrene based fused aromatic thienopyrazine polymers. Hole carrier mobility up to 0.012 cm2/Vs can be obtained in thin film transistor devices. Polymers were also utilized to fabricate bulk heterojunction photovoltaic devices and the maximum PCE obtained in these OPV's was 1.15%. Most importantly, performances of the devices were correlated with thin morphological analysis performed by atomic force microscopy and grazing incidence X-ray scattering.

Conjugated small molecules with broad absorption containing pyridine and pyran units: Synthesis and application for bulk heterojunction solar cells

Two new conjugated small molecules (SMs), M1 and M2, of low band gap with pyridine and pyran units, respectively, were synthesized by a convenient two-step reaction. They were soluble in common organic solvents and showed broad absorption ranging from 300 to 750 nm. Their long-wavelength absorption maximum was at 601–640 nm and the optical band gap was 1.60–1.67 eV. These SMs and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were used as donor and acceptor respectively to fabricate bulk heterojunction organic photovoltaic (OPV) devices through solution processing. The power conversion efficiencies (PCEs) for the OPV devices based on the as cast M1:PCBM and M2:PCBM are 1.42% and 1.80%, respectively. The higher PCE for the M2:PCBM based devices is attributed to the higher hole mobility, broader absorption spectra and lower band gap of M2 as compared to M1. The PCEs have been improved up to 1.75% and 2.18% for the solvent treated M1:PCBM and M2:PCBM blends, respectively. Finally, they have been further enhanced to 2.03% and 2.77% for the thermally annealed solvent treated M1:PCBM and M2:PCBM blends, respectively. This improvement has been attributed to the increase in the crystallinity of the blend and balance charge transport of the devices based on the solvent treated and thermally annealed solvent treated blends.

Synthesis and photovoltaic properties of an alternating phenylenevinylene copolymer with substituted-triphenylamine units along the backbone for bulk heterojunction and dye-sensitized solar cells

We have fabricated bulk heterojunction (BHJ) photovoltaic devices based on the as cast and thermally annealed P:[6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) blends and found that these devices gave power conversion efficiency (PCE) of about 1.15 and 1.60% respectively. P is a novel alternating phenylenevinylene copolymer which contains 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid units along the backbone and was synthesized by Heck coupling. This copolymer was soluble in common organic solvents and showed long-wavelength absorption maximum at 390–420 nm with optical band gap of 1.94 eV. The improvement of PCE after thermal annealing of the device based on the P:PCBM blend was attributed to the increase in hole mobility due to the enhanced crystallinity of P induced by thermal treatment. In addition, we have fabricated BHJ photovoltaic devices based on the as cast and thermally annealed PB: P:PCBM ternary blend. PB is a low band gap alternating phenylenevinylene copolymer with BF 2–azopyrrole complex units, which has been previously synthesized in our laboratory. We found that the device based on this ternary blend exhibited higher PCE (2.56%) as compared to either P:PCBM (1.15%) or PB:PCBM (1.57%) blend. This feature was associated with the well energy level alignment of P, PB and PCBM, the higher donor–acceptor interfaces for the exciton dissociation and the improved light harvesting property of the ternary blend. The further increase in the PCE with thermally annealed ternary blend (3.48%) has been correlated with the increase in the crystallinity of both P and PB. Finally, we used copolymer P as sensitizer for quasi solid state dye-sensitized solar cell and we achieved PCE of approximately 3.78%.

Synthesis, characterization and photovoltaic behavior of platinum acetylide polymers with electron-deficient 9,10-anthraquinone moiety

A new class of soluble, solution-processable platinum(II) acetylide polymers functionalized with electron-deficient 9,10-anthraquinone spacer and their corresponding diplatinum model complexes were synthesized and characterized. The organometallic polymers exhibit good thermal stability and show low-energy broad absorption bands in the visible region. The effect of the presence of thiophene rings along the polymer chain on the optical and photovoltaic properties of these metallated materials was examined. The low-bandgap polymer with thiophene–anthraquinone–thiophene (donor–acceptor–donor) fragment can serve as a good electron donor for fabricating bulk heterojunction polymer solar cells by blending with a methanofullerene electron acceptor. At the same donor:acceptor blend ratio of 1:4, the light-harvesting ability and solar cell efficiency notably increase when the anthraquinone ring is sandwiched by two thiophene units. Photoexcitation of such polymer solar cells results in a photoinduced electron transfer from the π-conjugated metallopolymer to [6,6]-phenyl C 61-butyric acid methyl ester with power conversion efficiency up to ∼ 0.35%. For safety concern, these metallopolymers were also tested for possible cytotoxicity and they do not show significant cytotoxic activity on human liver derived cells and skin keratinocytes at reasonable doses, rendering these functional materials safe to use in practical devices.

Low Band Gap Polymers Having Multi-Fused Heterocycles

The authors reported the synthesis and characterization of novel D-A-type copolymers that consist of a heptacyclic donor and 2,1,3-benzothiadiazole as an acceptor. The core structure of the ladder-type monomer can be efficiently assembled by Brønsted acid mediated intramolecular Friedel-Crafts cyclization. It is noteworthy that the synthesized polymers show good solubility to common organic solvents, lower band gaps than those of corresponding non-fused polymers, and high hole mobilities. Furthermore, the authors demonstrated the validity of the polymers by fabricating bulk heterojunction (BHJ) solar cells, which exhibit high power conversion efficiencies (PCE) of 2.8% for P1 and 3.7% for P2, ­respectively.

Organic molecules based on dithienyl-2,1,3-benzothiadiazole as new donor materials for solution-processed organic photovoltaic cells

Polymers based on dithienyl-2,1,3-benzothiadiazole (TBT) have received strong attention as the donor materials of polymer photovoltaic cells (PVs), since they can have a low band gap. But soluble small organic molecules based on TBT have been rarely studied. This paper reports the synthesis of two small organic molecules based on TBT and their application as the donor materials of solution-processed bulk heterojunction organic photovoltaic cells (OPVs). These compounds were soluble in common organic solvents, such as chloroform, chlorobenzene and tetrahydrofuran. They have band gaps comparable to poly(3-hexylthiophene) (P3HT) and lower HOMO and LUMO (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) levels than P3HT. These molecules and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were used as the donors and acceptor to fabricate bulk heterojunction OPVs through solution processing. After optimization of the experimental conditions, power conversion efficiency (PCE) of 0.66% was achieved on the solution-processed OPVs under AM 1.5G, 100 mW cm −2 illumination.

Symmetrical molecules of low band gap with a central spacer connected via ether bond with terminal 4-nitro-α-cyanostilbene units: Synthesis and application for bulk heterojunction solar cells

Two new soluble and symmetrical molecules M1 and M2 of low band gap with a central spacer which was connected at both sides via ether bond with terminal 4-nitro-α-cyanostilbene units were synthesized. The spacer of M1 and M2 consisted of dihexyloxyphenylene and n-hexylene, respectively. Their long-wavelength absorption maximum was at 590–640 nm. The thin film absorption onset for both molecules was located at 742 nm which corresponds to an optical band gap of 1.67 eV. We have fabricated bulk heterojunction (BHJ) photovoltaic devices using these molecules as donor and PCBM as acceptor. We have investigated the solvent vapor treatment effect of these blends on their morphology and photovoltaic properties. We found that the overall power conversion efficiency (PCE) for the devices based on the solvent vapor treated M1:PCBM and M2:PCBM blends is 3.05% and 1.90%, respectively, higher than those of pristine blends. This increase of PCE has been attributed to the increase in surface roughness of the blend and better balance in charge transport. The PCE has been further increased up to 3.57% and 2.42% with the thermally annealed solvent treated M1:PCBM and M2:PCBM blend, respectively. This increase of PCE may be attributed to the enhanced crystallinity of the blend and reduction of the space charge effect, improving the charge transport and collection efficiency.

Solution-Processable Crystalline Platinum-Acetylide Oligomers with Broadband Absorption for Photovoltaic Cells

A series of solution-processable and crystalline platinum−acetylide oligomers containing a thienyl−benzothiadiazole−thienyl core and oligothiophene alkynyl ligands are synthesized and characterized. X-ray crystallography analysis indicates a two-dimensional arrangement of oligomers through CH−π interactions in single crystals. These oligomers show two intense and broad absorption bands in the visible spectral region, with the short-wavelength absorption band being strongly dependent on the oligothiophene length. In neat films, all the oligomers form large crystalline domains of several hundred nanometers in size upon thermal treatment and exhibit space-charge limited current (SCLC) mobilities on the order of 10−5−10−4 cm2 V−1 s−1. The photovoltaic properties of these oligomers were evaluated by fabricating bulk heterojunction devices with fullerene derivatives (PC61BM and PC71BM) and some of these devices showed high-power conversion efficiencies (PCEs) of up to 3% and a peak external quantum efficiency (EQE) to 50% under AM 1.5 simulated solar illumination. The present work suggests that well-defined platinum oligomers with desirable light-absorbing and self-assembly properties have potential for solution-processed organic photovoltaics.

Photovoltaic characterization of poly(2,5-bis(3-dodecylthiophen-2-yl)-2′,2″-biselenophene) for organic solar cells

A novel regioregular selenophene and thiophene-based poly(2,5-bis(3-dodecylthiophen-2-yl)-2′,2″-biselenophene) (PBTBS), was chosen as promising electron donor materials blended with [6,6]-phenyl-C61 butyric acid methyl ester(PCBM) to fabricate bulk heterojunction solar cells. The optical and photovoltaic properties of PBTBS were characterized and compared to those of poly(3,3″′-didedocyl-quaterthiophene)( PQT-12). The PBTBS showed a higher ionization potential (IP) and improved oxidative stability than PQT-12 through the cyclic voltammetry (CV) measurements, which was due to stronger electron-donating properties of selenophene than that of thiophene analogue. A comparison of photovoltaic properties of cells between two polymer donors PBTBS and PQT-12 were also carried out. The result showed that cells based on the PBTBS had the comparable photovoltaic parameters with those of the PQT-12. An open voltage ( V OC ) of 0.46 V, a short-circuit current ( J SC ) of 0.17 mA/cm 2, a fill factor ( FF) of 0.32 and a power conversion efficiency ( η) of 0.34% were achieved at 100 mW/cm 2 (AM 1.5). The external quantum efficiency of cells based on the PBTBS exhibited much better performance with around 3.4% at 540 nm compared with that of the PQT-12.

Enhancement of power conversion efficiency of photovoltaic cell fabricated with 1,2,4,5-tetra(6-hexyl-2-vinyl-dithieno [3,2-b;2′,3′-d]thiophene)-benzene and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester

With regard to the application of organic semiconductors in electronic and optoelectronic devices, the organic photovoltaic cell (OPV) in particular is an important device for developing future energy resources. Recently, organic photovoltaic devices employing conjugated polymer-fullerene heterojunctions have been shown to have a power conversion efficiency approaching 5.0-6.0%. This range of value is obtained by introducing variations in the processing techniques. Oligothiophenes are also wellknown as good hole-transport materials and by changing the number of thiophene rings and making chemical modifications or substitutions, its characteristics relevant to PV applications can be controlled. However, most of the linear oliogothiophenes are not well soluble in organic solvents. Compared with linear organic conjugated oligomers and polymers used in organic field-effect transistors (OFETs) or OPVs, multibranched molecules have a number of advantages, including the ability to spatially control the active components, and thus, the interlayer carrier mobility properties owing to ready crystallization. In this work, we demonstrated new organic photovoltaic devices fabricated with 1,2,4,5-tetra(6-hexyl-2-vinyl-dithieno [3,2-b;2',3'-d]thiophene)-benzene (HPDTT), In our previous report, a similar molecule of 1,2,4,5-tetrakis((E)-2-(5-hexyl2,2-bithiophen-5-yl)vinyl)benzene (HPBT) having bithiophene peripheral groups was employed to PV device, it exhibits power conversion efficiency (PCE) of around unity. HPDTT is also well mixed with methanofullerene [6,6]phenyl C61-butyric acid methyl ester (PCBM) for fabricating bulk heterojunction PV devices. The device properties were investigated and compared with those of the device fabricated with HPBT and PCBM.

Harvesting solar energy using conjugated metallopolyyne donors containing electron-rich phenothiazine–oligothiophene moieties

A new family of soluble, solution-processable metallopolyynes of platinum(II) functionalized with electron-rich phenothiazine–oligothiophene rings and their corresponding dinuclear model complexes were synthesized and characterized. The organometallic polymers show different degrees of absorption capabilities in the solar spectral region, rendering some of them good electron donors for fabricating bulk heterojunction polymer solar cells by blending with a methanofullerene electron acceptor. The influence of the number of thienyl rings along the polymer chain on the optical and photovoltaic properties of these metallopolymers was studied. At the same donor:acceptor blend ratio of 1:4 or 1:5, the light-harvesting capability and solar cell efficiency notably increase as the number of thienyl rings is doubled. Photoexcitation of the polymer solar cells results in a photoinduced electron transfer from the π-conjugated metallopolyyne to [6,6]-phenyl C 61-butyric acid methyl ester and the best-performing polymer showed a power conversion efficiency (PCE) up to ∼1.3% with a corresponding peak external quantum efficiency of 63% under air mass (AM1.5) simulated solar illumination even at shorter absorption wavelength regime. The power dependencies of the solar cell parameters (including the short-circuit current density, open-circuit voltage, fill-factor and PCE) were also tested in detail.

Synthesis of MDMO-PPV capped PbS quantum dots and their application to solar cells

Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) capped PbS quantum dots about 3–6nm in diameter were synthesized with a novel method. Unlike the synthesis of oleic acid capped PbS quantum dots, the reactions were carried out in solution at room temperature, with the presence of a capping ligand species, MDMO-PPV. The quantum dots were used to fabricate bulk heterojunction solar cells with an indium tin oxide (ITO)/polyethylenedioxythiophene/polystyrenesulphonate (PEDOT: PSS)/MDMO-PPV: PbS/Al structure. Current density–voltage characterization of the devices showed that after the addition of the MDMO-PPV capped PbS quantum dots to MDMO-PPV film, the performance was dramatically improved compared with pristine MDMO-PPV solar cells.

Thermal diffusion processes in bulk heterojunction formation for poly-3-hexylthiophene/C60 single heterojunction photovoltaics

We have fabricated bulk heterojunction (BHJ) photovoltaic (PV) devices by thermal annealing of poly-3-hexylthiophene (P3HT)/C60 single heterojunction (HJ) PV devices at near the melting point of P3HT. The BHJ PV devices exhibited an increased efficiency of 12 times compared with single HJs. We found that the annealing of HJ devices produces an interpenetrated network of interfaces between the P3HT and C60 layers. This plays a major role in carrier separation and mobility enhancement. Also the formation of crystalline C60 domains, concurrent with polymer crystallinity, contributes to an increase in the overall external conversion efficiency. Surprisingly, the heterojunction morphology, as inferred through device performance, strongly depends on the thermal gradient across the film.