Tetrakis(phthalocyaninato) terbium–cadmium quadruple-decker liquid crystals with good semiconducting properties

Neutral and mono-oxidized states of novel sandwich-type tetrakis[2,3,9,10,16,17,23,24-octa(dodecanoyloxy)phthalocyaninato] terbium–cadmium quadruple-decker complex {[Pc(OC₁₂H₂₅)₈]Tb[Pc(OC₁₂H₂₅)₈]Cd[Pc(OC₁₂H₂₅)₈]Tb[Pc(OC₁₂H₂₅)₈]} (1) and {[Pc(OC₁₂H₂₅)₈]Tb[Pc(OC₁₂H₂₅)₈]Cd[Pc(OC₁₂H₂₅)₈]Tb[Pc(OC₁₂H₂₅)₈]}·SbCl₆ (2) were synthesized and spectroscopically characterized. Polarized optical microscope (POM) together with differential scanning calorimeter (DSC) measurement revealed their similar two rectangular columnar mesophases over a relatively lower temperature range and higher temperature range, respectively, within their wide liquid crystal temperature range of 19–266 °C for 1 and 4–249 °C for 2. Temperature-dependent X-ray diffraction (XRD) analysis result disclosed the slight difference in terms of the neighboring quadruple-decker π–π stacking between these two mesophases, which in turn accounts for their electric conducting behavior along with the change in temperature. In addition, due to the ionic conductive nature, the mono-oxidized liquid crystals of 2 display more than 2 order of magnitude higher electric conductivity than that for 1, with the highest value 4.1 × 10⁻⁴ S cm⁻¹ achieved at 140 °C.     

Small molecules based on bithiazole for solution-processed organic solar cells

A series of donor-acceptor-donor small molecules (1-3) with bithiazole as acceptor unit, triphenylamine as donor unit and thiophene with different number (0, 1, 2) as bridge were synthesized by palladium(0)-catalyzed Suzuki or Stille coupling reactions. The thermal, optical, electrochemical, charge transport, and photovoltaic properties of these small molecules were examined. All compounds exhibit excellent thermal stability with decomposition temperatures (5% weight loss) over 390 °C in nitrogen atmosphere. As increasing the number of thiophene and π-conjugation length of molecule, the absorption maximum in film red shifts from 406 to 498 nm, the extinction coefficient increases from 1.35 × 10⁴ to 7.66 × 10⁴ M⁻¹ cm⁻¹, and the optical band gap decreases from 2.6 to 2.0 eV. The electron-donating thiophene and bithiophene in compounds 2 and 3 up-shift HOMO energy level from −5.42 (1) to −5.24 eV (2) or −5.22 eV (3), and down-shift LUMO energy level from −2.48 (1) to −2.84 eV (2) or −2.81 eV (3). The hole mobility of compound 3 is up to 3.6 × 10⁻⁴ cm² V⁻¹ s⁻¹, which is one order of magnitude higher than that of compound 2, but compound 1 shows no field-effect transistor performance. Solution-processed bulk heterojunction organic solar cells based on 1-3:PC₇₁BM (1:4, w/w) blend films exhibit increasing power conversion efficiency (up to 2.61%) as increasing thiophene unit number.     

Comparison of short and long wavelength absorption electron donor materials in C60-based planar heterojunction organic photovoltaics

Buckminsterfullerene, C₆₀-based planar heterojunction (PHJ) organic photovoltaics (OPVs) have been created using a short wavelength absorption (λ_max = 490 nm) electron-donating bis(naphthylphenylaminophenyl)fumaronitrile (NPAFN). NPAFN exhibits a hole mobility greater than 0.07 cm² V⁻¹ s⁻¹ as determined by its field-effect transistor. It can be attributed to such hole mobility that enables a thin layer (<10 nm) NPAFN in PHJ OPV, ITO/NPAFN/C₆₀/bathocuproine/Al. Because of the low lying HOMO energy level (5.75 eV) of NPAFN and relatively high ionization potential ITO (∼5.58 eV), such OPVs exhibit a very high open circuit voltage of ∼1.0 V, relatively high fill factor of 0.60, and a relatively high shunt resistance of 1100 Ω cm⁻², which all compensate for a relatively low short circuit current of 3.15 mA cm⁻² due to the short absorption wavelength and inferred short exciton diffusion length of NPAFN. Altogether, NPAFN OPVs display a power conversion efficiency (η_PC) of 2.22%, which is better than other long wavelength absorption materials in similar PHJ OPVs, such as pentacene (λ_max 670 nm, HOMO 5.12 eV, η_PC 1.50%) and copper phthalocyanine (λ_max 624, 695 nm, HOMO 5.17 eV, η_PC 1.43%).     

Triphenylamine modified bis-diketopyrrolopyrrole molecular donor materials with extended conjugation for bulk heterojunction solar cells

Two new solution-processable enlarged π-conjugated donor–acceptor (D–A) organic small molecules consisting of dialkoxysubstituted benzo[1,2-b:4,5-b′]dithiophene (BDT) or dioctyltertthiophene (3T) as the central donor units, diketopyrrolopyrrole (DPP) as the acceptor unit and triphenylamine (TPA) as the terminal conjugated segment, TPA–DPP–BDT and TPA–DPP–3T, were designed and synthesized. Both small molecules possess broad absorption ranging from 300 to 800 nm with an optical band at approximately 1.50 eV and relatively low HOMO energy levels from −5.12 to approximately −5.16 eV. Expectedly, the UV–Vis absorption onset (810 nm) of TPA–DPP–BDT is largely red-shifted (60 nm) relative to that (750 nm) of previously reported BDT(TDPP)₂, which consists of BDT and DPP units. Unlike most of the TPA based molecules, strong molecular aggregation was observed in the solid state for both small molecules. In addition, atomic force microscopy (AFM) and X-ray diffraction (XRD) investigations indicated that TPA–DPP–3T and TPA–DPP–BDT exhibit good miscibility with fullerene derivatives. The organic solar cells based on TPA–DPP–BDT/PC₆₁BM(1:1) demonstrated power conversion efficiencies as high as 4.04% with a short-circuit current density (J_sc) of 11.40 mA cm⁻² and a fill factor (FF) of 53.2% when the active layer of the cell was annealed at 130 °C for 10 min.     

Relationship between HOMO energy level and open circuit voltage of polymer solar cells

A series of π-conjugated polymers (PDHF-BT and PDHF-TBT) with 4-(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (BT), 4,7-bis(3,4-ethylenedioxythienyl)-2,1,3-benzothiadiazole (TBT), and 9,9′-dihexylfluorene were synthesized by the Suzuki coupling reaction. The HOMO energy level of PDHF-BT was −5.47 eV, which was lower than that of PDHF-TBT (−5.22 eV), while the LUMO energy level of PDHF-BT (−3.45 eV) was very similar to that of PDHF-TBT (−3.42 eV). These energy levels of PDHF-BT and PDHF-TBT were also supported by a DFT calculation. The power conversion efficiency (PCE) of the polymer solar cell (PSC) with a structure of ITO/PEDOT:PSS/PDHF-BT:PCBM (1:1)/Al was determined as 0.34% and it was larger than that of the device based on PDHF-TBT (0.22%). Correspondingly, the V_oc of the PSC based on PDHF-BT (0.71 V) was much larger than that of the device based on PDHF-TBT (0.40 V). The results support that the V_oc of polymer based PSCs is strongly related to the HOMO energy level of the active polymers.     

High efficiency blue PhOLEDs using spiro-annulated triphenylamine/fluorene hybrids as host materials with high triplet energy,high HOMO level and high Tg

Two spiro-annulated triphenylamine/fluorene oligomers, namely 4′-(9,9′-spirobifluoren-4-yl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene] (NSF-SF), and 4,4′-di(spiro(triphenylamine-9,9′-fluorene)-2-yl)-spiro(triphenylamine-9,9′-fluorene) (NSF-NSF), are designed and synthesized. Their thermal, electrochemical and photophysical properties were investigated. The introduction of spiro-annulated triphenylamine moieties assurances the high HOMO energy levels of NSF-NSF and NSF-SF at −5.31 eV and −5.33 eV, respectively, which accordingly facilitates the hole injection from nearby hole-transporting layer. Meanwhile, the perpendicular arrangement of the spiro-conformation and the full ortho-linkage effectively prevents the extension of the π-conjugation and consequently guarantees their high triplet energies of 2.83 eV. Phosphorescent organic light-emitting devices (PhOLEDs) with the configurations of ITO/MoO₃/TAPC/EML/TmPyPB/LiF/Al were fabricated by using the two compounds as host materials and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^2′]iridium(III) picolate (FIrpic) as the dopant. The turn-on voltage of the device B based on NSF-NSF was 2.8 V. Simultaneously, the device exhibited excellent performance with the maximum current efficiency of 41 cd A⁻¹, the maximum power efficiency of 42 lm W⁻¹ and the maximum external quantum efficiency (EQE) of 19.1%. At a high brightness of 1000 cd m⁻², the device remained EQE of 16.2% and the roll-off value of external quantum efficiency is 15%.     

A crystalline D-π-A organic small molecule with naphtho[1,2-b:5,6-b′]dithiophene-core for solution processed organic solar cells

In this work, we have designed and synthesized a new naphtho[1,2-b:5,6-b′]dithiophene-containing enlarged π-conjugated donor–acceptor (D–A) small molecule, NDT(TTz)₂, for use in solution-processed organic photovoltaics. NDT(TTz)₂, which contains a thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene as the central fused core and triphenylamine-flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties of NDT(TTz)₂ have been evaluated by differential scanning calorimetry, thermogravimetry, UV–Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. As desired for photovoltaic applications, NDT(TTz)₂ possesses good solubility, thermal stability, and a well-ordered, π–π stacked, crystallinity. The optical band gap and HOMO level of NDT(TTz)₂ were determined to be 2.0 eV and −5.23 eV, respectively. In addition to organic thin film transistor studies, application of NDT(TTz)₂ to preliminary photovoltaic devices has also been investigated by fabricating solution-processed bulk heterojunction solar cells together with PC₇₁BM in a typical layered device structure, ITO/PEDOT:PSS/NDT(TTz)₂:PC₇₁BM/LiF/Al. Without extensive optimization of the device, NDT(TTz)₂ in these devices shows a maximum power conversion efficiency of 1.44% under AM 1.5 illumination at a 100 mW/cm² intensity.     

An easily made thienoacene comprising seven fused rings for ambient-stable organic thin film transistors

A novel easily made thienoacene-based organic semiconductor, i.e., dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2), was synthesized in high yield, and its thermal stability, electrochemical properties, thin-film morphology and field-effect mobility were investigated. Ph5T2 exhibit excellent thermal stability with a decomposition temperature (T_d) of 427 °C. Thin-film X-ray diffraction (XRD) and atomic force microscopy (AFM) characterizations indicate that Ph5T2 can form highly ordered films with large domain size on the para-sexiphenyl (6P)-modified substrates. Organic thin-film transistors (OTFTs) with top-contact geometry based on Ph5T2 exhibit mobilities up to 1.2 cm² V⁻¹ s⁻¹ in ambient. The devices are highly stable and exhibit almost no performance degradation during 3 months storage under ambient conditions with relative humidity up to 80%.     

Enhanced performance of dye-sensitized solar cells with novel 2,6-diphenyl-4H-pyranylidene dyes

Novel 2,6-diphenyl-4H-pyranylidene derivatives were designed and synthesized as dyes for dye-sensitized solar cells (DSSC). Dyes 2a, b with a phenyl substituent showed high DSSC energy conversion efficiencies of 5.3% (J_sc = 10.3 mA/cm², V_oc = 0.72 V, FF = 0.72) and 4.7% (J_sc = 8.9 mA/cm², V_oc = 0.73 V, FF = 0.72) at 100 mW/cm² under simulated AM 1.5 G solar light conditions. These values are twice better than that of dye 1 without the phenyl substituent under the same conditions. Both the photocurrent density (J_sc) and open circuit voltage (V_oc) of DSSCs based on dyes 2a, b are increased compared with 1. It can be attributed to their twisted structures, absorption abilities and proper energy levels. This result shows that the tetraphenylpyranylidene is a promising electron-donor unit for high-efficiency DSSCs.     

Synthesis and characterization of triphenylamine flanked thiazole-based small molecules for high performance solution processed organic solar cells

Two new small molecules, 5,5-bis(2-triphenylamino-3-decylthiophen-2-yl)-2,2-bithiazole (M1) and 2,5-bis(2-triphenylamino-3-decylthiophen-2-yl)thiazolo[5,4-d]thiazole (M2) based on an electron-donor triphenylamine unit and electron-acceptor thiophene-thiazolothiazole or thiophene-bithiazole units were synthesized by a palladium(0)-catalyzed Suzuki coupling reaction and examined as donor materials for application in organic solar cells. The small molecules had an absorption band in the range of 300-560 nm, with an optical band gap of 2.22 and 2.25 for M1 and M2, respectively_. As determined by cyclic voltammetry, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of M1 were −5.27 eV and −3.05 eV, respectively, which were 0.05 eV and 0.02 eV greater than that of M2. Photovoltaic properties of the small molecules were investigated by constructing bulk-heterojunction organic solar cell (OSC) devices using M1 and M2 as donors and fullerene derivatives, 6,6-phenyl-C61-butyric acid methyl ester (PC₆₁BM) and 6,6-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as acceptors with the device architecture ITO/PEDOT:PSS/M1 or M2:PCBM/LiF/Al. The effect of the small molecule/fullerene weight ratio, active layer thickness, and processing solvent were carefully investigated to improve the performance of the OSCs. Under AM 1.5 G 100 mW/cm² illumination, the optimized OSC device with M1 and PC₇₁BM at a weight ratio of 1:3 delivered a power conversion efficiency (PCE) of 1.30%, with a short circuit current of 4.63 mA/cm², an open circuit voltage of 0.97 V, and a fill factor of 0.29. In contrast, M2 produced a better performance under identical device conditions. A PCE as high as 2.39% was recorded, with a short circuit current of 6.49 mA/cm², an open circuit voltage of 0.94 V, and a fill factor of 0.39.     

A new thermally crosslinkable hole injection material for OLEDs

A new thermal cross-linkable hole injection monomer VB-DATA derived from famous m-MTDATA as core peripherally functionalized with styryl (vinylbenzene) moiety as polymerizable group has been synthesized and characterized. The propensity of VB-DATA thin films formation is sensitive to the nature of solvent, in which the dichloroethane solution gave smooth polymeric thin films with surface roughness of RMS ∼0.84 nm by spin-casting followed by thermal treatment at 190 °C. The introduction of oxygen-linked vinylbenzene group shifted HOMO energy level of VB-DATA to −5.1 eV along with good nondispersive hole transport property (μ_h ∼ 10^–6 cm² V^–1 s^–1) makes it suitable for serving as HIL on top of ITO electrode. The replacement of PEDOT:PSS by thermally cross-linked VB-DATA films showed comparable OLEDs performance, giving more flexibility on material selection for future OLEDs applications, especially solution-processed ones.     

Benzodithiophene and benzotrithiophene-based conjugated polymers for organic thin-film transistors application: Impact of conjugated- and acyl-side chain

A series of benzodithiophene (BDT) and benzotrithiophene (BTT)-based conjugated polymers (P1–P4), with/without conjugated- and acyl-side chain, have been synthesized by Stille cross-coupling reaction. Their thermal, photophysical, electrochemical properties, devices performances, and microstructure have been investigated. Conjugated-side chain can significantly raise the thermal stability and acyl-side chain can lower HOMO/LUMO energy levels. Organic thin-film transistors (OTFTs) based on conjugated polymers were fabricated and the transistor electrical characterization showed the device performance was sensitive to the conjugated- and acyl-side chain substituent of polymers. A maximum hole mobility of 1.70 × 10⁻³ cm² V⁻¹ s⁻¹ was obtained for P1-based devices, which is an order of magnitude higher than those of P3 and P4-based devices. The corresponding microstructures were investigated by grazing-incidence X-ray diffraction (GIXD) to correlate with conjugated- and acyl-side chain dependent carrier mobility of P1–P4. The results showed that the conjugated- and acyl-side chain have an impact on the polymer packing models and device performances.     

Efficient nondoped blue organic light-emitting diodes based on phenanthroimidazole-substituted anthracene derivatives

A series of new blue materials based on highly fluorescent di(aryl)anthracene and electron-transporting phenanthroimidazole functional cores: 2-(4-(anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (ACPI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1-p-henyl-1H-phenanthro[9,10-d]imidazole (1-NaCPI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (2-NaCPI) were designed and synthesized. These materials exhibit good film-forming and thermal properties as well as strong blue emission in the solid state. To explore the electroluminescence properties of these materials, three layer, two layer and single layer organic light-emitting devices were fabricated. With respect to the three layer device 4 using ACPI as the emitting layer, its maximum current efficiency reaches 4.36 cd A⁻¹ with Commission Internationale del’Eclairage (CIE) coordinates of (0.156, 0.155). In the single layer device 10 based on ACPI, maximum current efficiency reaches 1.59 cd A⁻¹ with Commission Internationale del’Eclairage (CIE) coordinates of (0.169, 0.177). Interestingly, both device 4 and 10 has low turn on voltage and negligible efficiency roll off at high current densities.     

Optimizing the conjugation between N,N′-dicarbazolyl-3,5-benzene and triphenylphosphine oxide as bipolar hybrids for highly efficient blue and single emissive layer white phosphorescent OLEDs

Two bipolar host materials, mCPpPO and mCPmPO have been synthesized by Ni(II)/Zn-catalyzed cross-coupling of diphenylphosphine oxide and corresponding aryl bromide. The photophysical properties, HOMO/LUMO orbital distribution and triplet levels of these host materials are investigated and optimized by tuning the linking modes between electron acceptor triphenylphosphine oxide and electron donor N,N′-dicarbazolyl-3,5-benzene (mCP). When mCP is linked to the meta-position of benzene of triphenylphosphine oxide, the hybrid (mCPmPO) shows much higher steric hinderance than the para-position linked analogue (mCPpPO) so that it possesses a higher triplet energy. Equipped with the bipolar transport properties, mCPmPO-based blue PhOLED doped FIrpic shows a maximum current efficiency (η_c,max) of 40.0 cd/A, a maximum power efficiency (η_p,max) of 39.7 lm/W, corresponding the maximum external quantum efficiency (η_EQE,max) of 20.3%, and the current efficiency still maintain to 34.8 cd/A even at 1000 cd/m². Based on the optimized triplet energy level, the single emission layer white PhOLED hosted by mCPmPO shows η_c,max, η_p,max and η_EQE,max of 46.9 cd/A, 39.7 lm/W and 17.6%, respectively.     

UV light-emitting electrochemical cells based on an ionic 2,2′-bifluorene derivative

UV light-emitting electrochemical cells (LECs) were, for the first time, achieved by the ionic 2,2′-bifluorene derivative, 1, which was synthesized through covalent tethering of methylimidazolium moieties as pendent groups. LEC devices incorporating ionic bifluorene 1 without (Device I) and with (Device III) the presence of poly(methyl methacrylate) (PMMA) exhibited UV EL emissions centered at 388 and 386 nm with maximum external quantum efficiencies and power efficiencies of 1.06% and 7.44 mW W⁻¹ for Device III and 0.15% and 1.06 mW W⁻¹ for Device I, respectively. Transmission electron microscopy (TEM) images showed that 1 tends to form nanospheres due to amphiphilic nature. The presence of PMMA unified the size of nanospheres which greatly reduced the void area in films, suppressing the current leakage and enhancing the device efficiency. Furthermore, thicker thickness of the emissive layer of LECs increases the distance between carrier recombination zone and electrodes to avoid exciton quenching. Thus, a sevenfold increase in device efficiency was obtained in thicker UV LECs containing PMMA (Device III) as compared to thinner UV LECs based on neat films of 1 (Device I). The EL emissions in the UV region are successfully achieved by LECs based on 1, which are so far the shortest emission wavelength achieved in LECs.     

Influence of spatial arrangements of π-spacer and acceptor of phenothiazine based dyes on the performance of dye-sensitized solar cells

Three phenothiazine based organic dyes PTA, PDTA and PTDA with D–π–A, π–D–π–A and A–π–D–π–A frameworks were designed and synthesized for the dye sensitized solar cells (DSSCs). Phenothiazine with octyloxyphenyl moiety acts as donor while thiophene and cyanoacetic acid units act as a π-spacer and an acceptor, respectively. The effects of the molecular structures of the dyes on the performance of the DSSCs were investigated systematically along with their photophysical and photoelectrochemical properties. The dye PTDA with A–π–D–π–A framework exhibited a better light harvesting capacity and an effective electron extraction pathway from the electron donor to the TiO₂ surface, leading to an efficiency of 6.82% under 100 mW cm⁻² light illumination, while the dyes PTA and PDTA with D–π–A and π–D–π–A frameworks delivered efficiencies of 6.34% and 5.12%, respectively.     

Efficient blue emitters based on 1,3,5-triazine for nondoped organic light emitting diode applications

Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and triphenylamine have been designed and synthesized. Organic light emitting diodes (OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabricated and tested. The OLEDs exhibited good performances with low turn-on voltage of 3 V, maximum luminance of ca. 8990 cd/m² for TTT-1 and 15,980 cd/m² for TTT-2, and maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.     

A diketopyrrolopyrrole containing molecular semiconductor: Synthesis,characterization and solution-processed 1D-microwire based electronic devices

Highly oriented 1D-microwires of a diketopyrrolopyrrole (DPP) based semiconductor i.e. DPP(CBZ)₂ (Carbazole capped DPP) were synthesized, characterized and applied to the fabrication of organic optoelectronic devices. 1D-microwires of DPP(CBZ)₂ were prepared by solution processing on capillary tubes serving to pin solution. A bottom-gate, top-contact field-effect transistor employing 1D-microwire and polymeric gate dielectric showed a hole mobility of 1.24 × 10⁻² cm²/V s, an on-to-off drain current ratio (I_on/I_off) of 4.7 × 10³ and subthreshold slopes of 4 V/dec under ambient conditions. Under white light, a photosensitivity of 800 at V_G = −40 V and photoresponsivity of 830 mW/A were achieved. This work demonstrates the potential of this new molecule and the solution method for use in various opto-electronic devices such transistors, photosensors and photovoltaics.     

Porphyrins modified with a low-band-gap chromophore for dye-sensitized solar cells

Two novel porphyrin dyes (PMBTZ and PHBTZ) modified with alkyl-thiophene and 2,1,3-benzothiadiazole (BTZ) moieties were designed and synthesized. The optical and electrochemical properties were characterized by UV-visible, fluorescence spectroscopy and cyclic voltammetry. With the introduction of the low-band-gap chromophore onto the porphyrins, the absorption spectra of the two porphyrin dyes in the range of 450-600 nm were broadened and the maximum wavelength was red-shifted compared with P_Zn as expected. The first oxidation potentials (E_ox1) were altered to the negative, which lowered from 1.27 to 1.11 and 1.15 eV, respectively. For a typical solar cell device based on dye PMBTZ, the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach to 65%, with a broad respondent region of 350-800 nm. Under standard global AM 1.5 solar condition, the dye-sensitized solar cell (DSSC) based on the dye PMBTZ showed the best photovoltaic performance: a short-circuit photocurrent density (J_sc) of 14.11 mA/cm², an open-circuit photo voltage (V_oc) of 0.59 V, and a fill factor (ff) of 0.66, corresponding to solar-to-electric power conversion efficiency (η) of 5.46%.     

Photovoltaic properties of low band gap copolymers based on phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units

Two novel copolymers P1 and P2 having phenylenevinylene donor and cyanovinylene 4-nitrophenyl acceptor units, were synthesized by heck coupling and employed as electron donor along with PCBM or modified PCBM (F) as electron acceptor for the fabrication of bulk heterojunction (BHJ) photovoltaic devices. These copolymers P1 and P2 showed broad band absorption around 640-700 nm and optical band gap of 1.60 eV and 1.72 eV, respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) estimated from cyclic voltammetry measurement reveals that these values are well suitable for the use of these copolymers as electron donor along with PCBM derivatives as electron acceptor for BHJ active layer. The suitable LUMO off set allows efficient photo-induced charge transfer at the donor/acceptor interfaces in the BHJ photovoltaic device and resulting power conversion efficiency (PCE) of 2.8% and 3.29% for P1 and P2, respectively, when PCBM is used as acceptor. This value has been improved up to 3.52% and 4.36% for the devices based on P1 and P2 when F is used as electron acceptor, instead of PCBM. We have also investigated the effect of solvent annealing on the photovoltaic performance of device based on P1: F and P2: F blends and found that the over all PCE of the devices is 4.36% and 4.88%, respectively. The increase in PCE is mainly due to the improvement in the J_sc, which is due to the increased charge transport in the annealed device as compared to as cast device.