Photovoltaic properties of poly(benzothiadiazole-thiophene-co-bithiophene) as donor in polymer solar cells

We report the photovoltaic properties of a D-A copolymer, poly(benzothiadiazole-thiophene-co-bithiophene) (PBTTbT), containing the donor (D) unit of oligothiophene with a hexyl side chain and the acceptor (A) unit of 2,1,3-benzothiadiazole (BT) with a methyl side chain. The geometry, electronic and absorption spectroscopic properties of bithiophene-benzothiadiazole-thiophene monomer (M1) of the polymer were investigated theoretically by the density functional theory (DFT) method for deep understanding the relationship of the structure and properties of the polymer. Polymer solar cells (PSCs) were fabricated with PBTTbT as an electron donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₀BM) as an electron acceptor. The power conversion efficiency (PCE) of PSC is 0.87% for an optimized PBTTbT:PC₇₀BM weight ratio of 1:3, under the illumination of AM 1.5, 100 mW/cm². With the additive of 1% 1,8-dioctanedithiol and thermal annealing at 130 °C for 15 min, the PCE of the device was improved to 1.98%. The efficiency improvement of the device was ascribed to a better morphology of the PBTTbT:PC₇₀BM active layer with the additive and thermal annealing.     

Improving the efficiency of organic solar cell with a novel ambipolar polymer to form ternary cascade structure

This study describes the utilization of a novel conjugated copolymer, namely, poly[2,3-bis(thiophen-2-yl)-acrylonitrile-9,9′-dioctyl-fluorene] (FLC8) for organic solar cell application for the first time. The highest occupied molecular orbital and the lowest unoccupied molecular orbital of FLC8 are −5.68 and −3.55 eV, respectively, which lie between the corresponding values of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM). In addition, both electron and hole mobilities of FLC8 are in the range of 10⁻⁴ (cm²/V s), making it an excellent ambipolar polymer. Such unique properties make FLC8 a good candidate to form a ternary cascade bulk-heterojunction organic solar cell when blending with P3HT and PCBM. The power conversion efficiency (PCE) of the ternary cascade solar cell can be increased by up to 30% as compared with the reference cell without FLC8. We suspect that this enhancement of PCE is caused by the additional charge separation offered by the cascade structure and the fast charge transfer due to the ambipolarity of FLC8.     

Electrosyntheses, characterizations and electrochromic properties of a copolymer based on 4,4′-di(N-carbazoyl)biphenyl and 2,2′-bithiophene

Electrochemical copolymerization of 4,4′-di(N-carbazoyl)biphenyl (CBP) with 2,2′-bithiophene (BT) is carried out in acetonitrile (ACN)/dichloromethane (DCM) (1:1, by volume) solution containing sodium perchlorate (NaClO₄) as a supporting electrolyte. Characterizations of the resulting copolymer P(CBP-co-BT) are performed by cyclic voltammetry (CV), UV-vis spectroscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and thermogravimetry (TG). The P(CBP-co-BT) film has distinct electrochromic properties and exhibits four different colors (orange yellow, blue, yellowish green and greenish blue) under various potentials. Maximum contrast (ΔT%) and response time of the copolymer film are measured as 51.6% and 0.94 s at 667 nm. An electrochromic device (ECD) based on P(CBP-co-BT) and poly(3,4-ethylenedioxythiophene) (PEDOT) is constructed and characterized. The optical contrast (ΔT%) at 700 nm is found to be 28.6% and the response time is measured as 0.47 s. The coloration efficiency (CE) of the device is calculated to be 234 cm² C⁻¹ at 700 nm. An ECD also has good optical memories and redox stability.     

Long-lived bulk heterojunction solar cells fabricated with photo-oxidation resistant polymer

We report the fabrication of long-lived polymer solar cells using a new donor-acceptor type alternating copolymer, poly(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-α]indene-2,7-diyl)-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole (PININE-DTBT) in bulk heterojunction composites with the fullerene derivative [6,6]-phenyl C₇₀-butyricacidmethyl ester (PC₇₀BM). The PININE-DTBT:PC₇₀BM solar cells exhibit an extended device lifetime (as compared with other polymer systems) with a reasonable power conversion efficiency of ∼2.7% under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW/cm². The long-lived feature of the devices originates from the photo-oxidation resistant backbone unit and the deep HOMO (highest occupied molecular orbital) level of PININE-DTBT.     

Polymer solar cell based on poly(2,6-bis(3-alkylthiophen-2-yl)dithieno-[3,2-b;2′,3′-d]thiophene)

Polymer photovoltaic solar cells using poly (2,6-bis(3-alkylthiophen-2-yl)dithieno-[3,2-b;2′,3′-d]thiophene) (PBTDT) as the donor is demonstrated. The UV-vis spectra show that PBTDT has strong absorption in the visible spectrum. By adjusting the ratio of PBTDT to [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and optimizing the annealing temperature, the PBTDT-based polymer solar cells show a power conversion efficiency of 0.42% under 100 mW/cm² AM1.5G simulated sunlight.     

Synthesis and characterization of a novel fullerene derivative for use in organic solar cells

A novel fullerene derivative with an N-hexylphenothiazine moiety, PTZ-C_60, was synthesized and characterized. The new synthesized fullerene showed good solubility in common organic solvents such as toluene, chlorobenzene and 1, 2 dichlorobenzene. The synthetic product PTZ-C₆₀ was characterized by ¹H and ¹³C NMR, FT-IR and UV-vis spectroscopy. Photovoltaic devices were fabricated using the new fullerene derivative as the electron acceptor and P3HT as the electron donor. The configuration of the device was as follows: ITO/PEDOT:PSS/active layer/LiF/Al. The weight ratios of the electron donor to the acceptor in the active layer were 1:0.5, 1:0.7, and 1:1. The open-circuit voltage (V_oc) of the fabricated devices was found to be higher than that of devices based on C₆₀ because the LUMO energy level of the new fullerene derivative was higher than that of C₆₀. Further, the power conversion efficiency (PCE) of these devices was observed to be high when annealing was carried out at 150 °C for 5 min and the thickness of the active layer was 80 nm. The maximum V_oc, short-circuit current density, and PCE of the best device were 0.608 V, 4.393 mA/cm², and 1.29%, respectively.     

Low bandgap carbazole copolymers containing an electron-withdrawing side chain for solar cell applications

A new series of low bandgap carbazole copolymers containing an electron-withdrawing moiety as a side chain, via Suzuki, Yamamoto, and Stille polymerization reactions has been synthesized. Their bandgaps and molecular energy levels can be tuned by copolymerizing with different conjugated electron-donating units. The resulting copolymers have low optical and electrochemical bandgaps. The optical bandgaps of the copolymers range from 1.79 to 1.24 eV. In order to investigate their photovoltaic properties, polymer solar cell devices based on low bandgap copolymers were fabricated with a structure of ITO/PEDOT:PSS/copolymers:PCBM/Al, under the illumination of AM 1.5 G, 100 mW/cm². The power conversion efficiencies (PCE) of the polymer solar cells based on these low bandgap copolymers were measured. The best performance was obtained by using PC-CARB as the electron donor and 6,6-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron acceptor. The PCE of the solar cell based on PC-CARB/P₇₁CBM (1:4) was 1.27% with an open-circuit voltage (V_oc) of 0.65 V, and a short-circuit current (J_sc) of 6.69 mA/cm².     

Influence of n-type chemical doping layer on the performance of organic photovoltaic solar cells

We report the performance improvement of organic solar cell by addition of an n-type chemical doping layer in organic bulk heterojunction device. The power conversion efficiency (PCE) of P3HT and PCBM-71 based polymer solar cells increases by adding a mixture of TCNQ (7,7,8,8-tetracyanoquinodimethane) and LCV (Leucocrystal violet) between active layer and cathode electrode. The PCE of the cell increases by 14% compared to the control cell with Al-only cathode electrode. The device with an organic n-doped layer shows the J_SC of 8.88 mA/cm², V_OC of 0.51 V, FF of 60.1%, and thus the PCE of 2.72% under AM1.5 illumination of 100 mW/cm².     

Synthesis and photovoltaic properties of biindene-C70 monoadduct as acceptor in polymer solar cells

A new fullerene derivative, biindene-C₇₀ monoadduct (BC₇₀MA), was synthesized by [4+2] cycloaddition reaction between 1,1′-biindene and C₇₀, for the application as acceptor in polymer solar cells (PSCs). BC₇₀MA is soluble in common organic solvents such as tetrahydrofuran, chloroform, toluene, o-dichlorobenzene, etc., and shows stronger absorption in the visible region and a slightly up-shifted lowest unoccupied molecular orbital (LUMO) energy level than that of PCBM. PSCs were fabricated with BC₇₀MA as acceptor and poly(3-hexylthiophene) (P3HT) as donor for investigating the photovoltaic properties of BC₇₀MA. The power conversion efficiency of the PSC based on P3HT/BC₇₀MA (1:1, w/w) with the additive of 3% octane-1,8-dithiol and thermal annealing at 110 °C for 10 min reached 3.44% with open circuit voltage of 0.64 V, short circuit current of 8.02 mA/cm² and fill-factor of 0.67, under the illumination of AM1.5, 100 mW/cm².     

Effect of thiophene donor units on the optical and photovoltaic behavior of fluorene-based copolymers

A series of novel conjugated polymers containing alternating electron-donating and electron-accepting units based on 9,9-dioctylfluorene, 4,7-dithienyl-2-yl-2,1,3-benzothiadiazole, and (oligo)thiophene were synthesized. The polymers were synthesized by the Suzuki cross-coupling polymerization of 9,9-dioctylfluorene-2,7-diboronic acid and 4,7-di(2-bromothien-5-yl)-2,1,3-benzothiadiazole with dibromo(oligo)thiophene (thiophene, bithiophene, and terthiophene). Optical properties of the copolymers were highly dependent on the length of the (oligo)thiophene unit. With the incorporation of three thiophene units in the polymer backbone, the copolymer absorption covers a broad range of the visible spectrum from 300 to 700 nm. The band gap energies derived from the absorption edge of the thin film spectra were in the range of 1.83-1.94 eV. The photovoltaic performance increases as the length of the (oligo)thiophene segments in the polymer backbone increases. The best performance of photovoltaic device was obtained by PFTBzTTT as the electron donor material with PCE of 1.25% under an AM 1.5 solar simulator.     

Improved the efficiency of small molecule organic solar cell by double anode buffer layers

Small molecule organic solar cell with an optimized hybrid planar-mixed molecular heterojunction (PM-HJ) structure of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) doped with 4 wt% sorbitol/ pentacene (2 nm)/ copper phthalocyanine (CuPc) (10 nm)/ CuPc: C₆₀ mixed (20 nm)/ fullerene (C₆₀) (20 nm)/ bathocuproine (BCP) (10 nm)/Al was fabricated. PEDOT: PSS layer doped with 4 wt% sorbitol and pentacene layer were used as interlayers between the ITO anode and CuPc layer to help the hole transport. And then the short-circuit current (J_sc) of solar cell was enhanced by inserting both the PEDOT: PSS (4 wt% sorbitol) and the pentacene, resulting in a 400% enhancement in power conversion efficiency (PCE). The maximum PCE of 3.9% was obtained under 1sun standard AM1.5G solar illumination of 100 mW/cm².     

Solution processable quinacridone based materials as acceptor for organic heterojunction solar cells

Two series of novel quinacridone (QA) based materials that combined a strong absorption over a broad range in visible region with good electrical characteristics, which were used as the new electron-accepting materials for organic solar cells, are explored. Unique cyclic compounds 1-6 are synthesized by incorporating electron withdrawing groups (CN, COOH) at carbonyl position of alkyl substituted quinacridones, which lead to the compounds possessing the characteristics of solution-processed and being suitable for photovoltaic applications. Heterojunction solar cells with simple device configuration using these soluble materials as acceptor and effective donor poly (3-hexyl thiophene) (P3HT) were fabricated. The maximum power conversion efficiency (PCE) achieved in the solar cell based on compound 5 is 0.42% under simulated AM 1.5 solar irradiation with J_sc=1.80 mA cm⁻², V_oc=0.50 V and FF=47%. Although the aimed devices just exhibit moderate PCE, our results clearly suggest that the new-type electron-accepting materials different from fullerene have great potential as acceptor in heterojunction solar cell due to many advantages of the QA derivatives such as relatively inexpensive, good electrochemical stability and could be readily modified.     

Organic solar cells with 2-Thenylmercaptan/AU self-assembly film as buffer layer

The interface between an electrode and the organic active layer is an important factor in organic solar cells (OSCs) that influences the power conversion efficiency (PCE). In this report, a buffer layer of 2-thenylmercaptan/Au self-assembly film is introduced into OSCs as a substitute for the poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) layer. The electrode/active layer interface is meliorated by Au-S coordinate bond of self assembly after applying this buffer layer. The series resistance reduces from 20 Ω cm² in a device based on PEDOT:PSS to 10.2 Ω cm². Correspondingly, the fill factor (FF) increases from 0.50 to 0.64. Moreover, due to the dipole of this self-assembled layer, the open circuit voltage (V_oc) also increases slightly from 0.54 V to 0.56 V and the PCE reaches 2.5%.     

Synthesis and characterization of crosslinked sulfonated poly(arylene ether sulfone) membranes for high temperature PEMFC applications

Sulfonated poly(arylene ether sulfone) copolymers containing carboxyl groups are prepared by an aromatic substitution polymerization reaction using phenolphthalin, 3,3′-disulfonated-4,4′-dichlorodiphenyl sulfone, 4,4′-dichlorodiphenyl sulfone and 4,4′-bisphenol A as polymer electrolyte membranes for the development of high temperature polymer electrolyte membrane fuel cells. Thin, ductile films are fabricated by the solution casting method, which resulted in membranes with a thickness of approximately 50 μm. Hydroquinone is used to crosslink the prepared copolymer in the presence of the catalyst, sodium hypophosphite. The synthesized copolymers and membranes are characterized by ¹H NMR, FT-IR, TGA, ion exchange capacity, water uptake and proton conductivity measurements. The water uptake and proton conductivity of the membranes are decreased with increasing the degree of crosslinking which is determined by phenolphthalin content in the copolymer (0-15 mol%). The prepared membranes are tested in a 9 cm² commercial single cell at 80 °C and 120 °C in humidified H₂/air under different relative humidity conditions. The uncrosslinked membrane is found to perform better than the crosslinked membranes at 80 °C; however, the crosslinked membranes perform better at 120 °C. The crosslinked membrane containing 10 mol% of phenolphthalin (CPS-PP10) shows the best performance of 600 mA cm⁻² at 0.6 V and better performance than the commercial Nafion^® 112 (540 mA cm⁻² at 0.6 V) at 120 °C and 30 % RH.     

Efficient bulk heterojunction solar cells with copolymers based on fluorene, dithienylbenzothiadiazole, and thiophene derivatives

Three copolymers, PFTpBt, PFbCNTpBt, and PFC6TpBt, which are based on dioctylfluorene, dithienylbenzothiadiazole, and thiophene derivatives with different functional groups, were synthesized via a Suzuki reaction. The copolymers were characterized by NMR, GPC, TGA, UV-vis absorption, and electrochemical cyclic voltammetry. Thermogravimetric analysis showed that the copolymers exhibited good thermal stability. The optical, electrochemical, photovoltaic properties, and hole mobility of the copolymers were investigated and discussed. The polymer solar cell based on PFTpBt had the best performance, with a power conversion efficiency (PCE) of 2.17% under the illumination of AM 1.5, 100 mW/cm².     

Air-stable triarylamine-based amorphous polymer as donor material for bulk-heterojunction organic solar cells

A new air-stable triarylamine-based amorphous polymer, TSP-T11, which consists of thiophene and triarylamine units, can be successfully utilized to fabricate bulk-heterojunction organic photovoltaics (OPVs) using PC₆₀BM or PC₇₀BM as acceptor materials. The highest level of performance of OPVs optimized at TSP-T11:PC₇₀BM (weight ratios of 1:4) with thicknesses of 68 nm exhibited an open circuit voltage (V_oc) of 0.75 V, a short circuit current (J_sc) of 8.03 mA cm⁻², and a power-conversion efficiency (PCE) of 2.22% under simulated air mass 1.5 solar irradiation at 100 mW cm⁻². Although TSP-T11 has a lower hole mobility (1.5×10⁻⁴ cm² V⁻¹ s⁻¹) than P3HT, the use of amorphous film of TSP-T11 as a donor material for OPVs offers advantages over the use of polycrystalline film of P3HT in terms of its air-stability and pinhole-free homogeneous morphology.     

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 BF₂-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%.     

Thiocyanate ligand substitution kinetics of the solar cell dye Z-907 by 3-methoxypropionitrile and 4-tert-butylpyridine at elevated temperatures

The dye-sensitized solar cell dye Z-907, [RuLL′(NCS)₂] may loose a thiocyanate ligand at elevated temperatures (80-100 °C) by ligand exchange with the solar cell additive 4-tert-butylpyridine (4-TBP) or the electrolyte solvent 3-methoxypropionitrile (3-MPN). The mechanism in homogeneous solution proceeds via three equilibrium reactions Eqs. (1)-(3) with the solvent complex [RuLL′(NCS)(3-MPN)] as an intermediate:[RuLL′(NCS)₂]+3-MPN=[RuLL′(NCS)(3-MPN)]⁺+NCS⁻ (1)[RuLL′(NCS)(3-MPN)]⁺+4-TBP=[RuLL′(NCS)(4-TBP)]⁺+3-MPN (2)[RuLL′(NCS)₂]+4-TBP=[RuLL′(NCS)(4-TBP)]⁺+NCS⁻ (3)A similar mechanism describes the heterogeneous substitution reactions of Z-907 attached to the surface of TiO₂ particles with 3-MPN and 4-TBP. All the six homogeneous and heterogeneous rate constants were obtained at 100 °C by monitoring the decay of Z-907 and product formation in test-tube experiments by HPLC coupled to UV/vis and electrospray mass spectrometry.A half-lifetime t_1/2=150 h was obtained for the Z-907 dye bound to TiO₂ nanocrystalline particles at 85 °C in the presence of 4-TBP and 3-MPN. Dye-sensitized solar cells (DSC) with Z-907 as a sensitizer and application of the so-called “non-robust” electrolytes containing 4-TBP and 3-MPN is therefore not expected to be able to pass a 1000 h thermal stress test at 85 °C. Addition of thiocyanate to the cell electrolyte may however, eliminate or reduce the problems caused by dye thiocyanate ligand substitution in DSC cells.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

Ionomers based on multisulfonated perylene dianhydride: Synthesis and properties of water resistant sulfonated polyimides

A novel locally and densely sulfonated dianhydride with four sulfonic acid groups, 1,6,7,12-tetra[4-(sulfonic acid)phenoxy]perylene-3,4,9,10-tetracarboxylic dianhydride (SPTDA), was successfully synthesized by direct sulfonation of the parent dianhydride, 1,6,7,12-tetraphenoxyperylene-3,4,9,10-tetracarboxylic dianhydride (PTDA). Sulfonated copolyimides were prepared from SPTDA, nonsulfonated dianhydride 4,4′-binaphthyl-1,1′,8,8′-tetracarboxylic dianydride, 4,4′-diaminodiphenyl ether (a) or dodecane-1,12-diamine (b). The synthesized copolymers, with the -SO₃H group on the polymer side chain, possess high molecular weights and high viscosities, and they form tough, flexible membranes. The copolymer membrane with an ion exchange capacity of 2.69 mequiv. g⁻¹ had a proton conductivity of 0.126 S cm⁻¹ at 20 °C and 0.292 S cm⁻¹ at 100 °C; the latter is much higher than that of Nafion^® 117 under the same conditions. The mechanical properties of the copolymer membranes were almost unchanged after accelerated water stability testing at 140 °C for 100 h; this indicates excellent hydrolytic stability of the synthesized copolyimides.