Tuning the LUMO level of the acceptor to increase the open-circuit voltage of polymer-fullerene solar cells: A quantum chemical study

In the present study, semiempirical and density functional molecular orbital calculations are performed on fullerene derivatives with varying reduction potentials, successfully used as an electron acceptor in bulk heterojunction solar cells. The geometries of all the compounds were optimized with the semiempirical PM3 method. Density functional theory (DFT) single-point calculations, B3LYP/3-21G*, have been carried out with the aim to investigate the energy levels of the frontier orbitals. We have correlated the theoretical lowest unoccupied molecular orbital (LUMO) levels of different fullerenes with the open-circuit voltage of the photovoltaic device based on the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)]–1,4-phenylenevinylene (MDMO–PPV) with the acceptor molecules. We have also investigated the influence of new substituents on the LUMO level of the parent fullerene showing the possibility to further increase the open-circuit voltage of the MDMO–PPV:fullerene device.     

Structural effect of electron acceptor on charge transfer in poly(3-hexylthiophene)/methanofullerene bulk heterojunctions

Radical pairs, polarons and fullerene anion radicals photoinduced by photons with energy of 1.98-2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) with bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6.6]C₆₂ (bis-PCBM) methanofullerene have been studied as compared with P3HT/PCBM composite by direct light-induced EPR (LEPR) method in a wide temperature range. A part of spin polarons and methanofullerene anion radicals are pinned in trap sites which number and depth are governed by an ordering of the polymer/fullerene system and energy of initiating photons. It has been shown that dynamics and recombination of mobile polarons and counter methanofullerene anion radicals are governed by their exchange- and multi-trap assisted diffusion. Relaxation and dynamics of both the charge carriers determined by the steady-state saturation method are governed by structure and conformation of their microenvironment as well as by the photon energy. Longitudinal diffusion of polarons was shown to depend on lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The energy barrier required for polaron interchain hopping is higher than its intrachain diffusion. Pseudorotation of fullerene derivatives in a polymer matrix was shown to follow the activation Pike model. The replacement of PCBM by bis-PCBM provides higher anisotropy of polaron dynamics and decreases its selectivity to the photon energy. This makes spin dynamics easier and minimizes energy dispersion at charge transfer.     

A stabilization effect of [60]fullerene in donor–acceptor organic solar cells

In this paper we compare the behavior of solar cells based on pure regioregular poly(3-hexylthiophene) with those in which the same polymer is blended with a functionalized fullerene, under continuous illumination. We found that the decay of both photocurrent and photovoltage signals with time is substantially reduced when the blend is used.     

Towards optimization of device performance in conjugated polymer photovoltaics: Charge generation, transfer and transport in poly(p-phenylene-vinylene) polymer heterojunctions

We fabricate photovoltaics comprised layers and blends of a hole-transporting derivative of poly(p-phenylene-vinylene) with a variety of electron-transporters: titanium dioxide, a cyano-substituted PPV, and a fullerene derivative (PCBM) to enhance device performance. Photovoltaic device characterization is combined with time-resolved and steady-state photoluminescence to understand the nature of the excited state and its effect upon device performance. We find that morphological differences, such as chain conformation or domain size, often overshadow the effect of charge transfer, so that device performance is not necessarily correlated with rapid decay times. Exciton generation is found to be a similarly important factor in most devices. These results provide insight into non-optimized device morphologies.     

Photovoltaic solar cells using poly(3,3-didodecylquaterthiophene)

The fabrication and the optimization of photovoltaic solar cells based on poly(3,3-didodecylquaterthiophene) (12-PQT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are reported. On the one hand, the effect of the blend composition shows the necessity to increase the amount of fullerene derivative compared to conventional poly(3-hexylthiophene)-based systems. On the second hand, thermal annealing of devices is optimized and discussed. Overall, in this work, the highest power conversion efficiencies are shown in the range of 0.3–0.4% which represents a lower value than that reported with poly(3-hexylthiophene). Results are discussed in terms of charge carrier mobility and phase segregation in this bicontinuous donor-acceptor network.     

Network structure organic photovoltaic devices prepared by electrochemical copolymerization

Characteristics of the organic photovoltaic devices prepared by in-situ electrochemical copolymerization of substituted fullerene with alkylthiophene were studied. The active layer was formed by direct electrochemical copolymerization between thiophene moiety substituted fullerene and 3-octylthiophene on PEDOT-coated indium tin oxide (ITO) glass, where fullerene is anchored within conducting polymer chain through covalent bonding. Since two monomers utilized in this study had an opposite oxidation potential, the step potential bias method was used to copolymerize these two monomers. Bias potential and concentration of monomers were carefully controlled so that blend ratio of composite film was exactly same as monomer feed. Power conversion efficiency of electrochemically copolymerized photovoltaic device changed within experimental error ranges for before and after heat treatment. Meanwhile, composite films prepared by spin coating of blend of poly(3-octylthiophene) and PCBM showed morphological changes after heat treatment at 8 °C for 1 h according to field emission scanning electron microscopy (FE-SEM) study. This study suggests a new approach for preventing migration of fullerene from its optimized morphology.     

Dynamics of charge carriers photoinduced in poly(3-dodecylthiophene)/fullerene bulk heterojunction

Magnetic and dynamics properties of mobile polarons and fullerene anion-radicals photoinduced by laser beam in the poly(3-dodecylthiophene)/fullerene bulk heterojunction were studied. The number of these charge carriers was found to decrease with the growth in the photon energy. Monotonic temperature dependence for main electron relaxation parameters was obtained. The 1D polaron diffusion along the polymer chain and the fullerene rotation was shown to follow activation Elliot hopping model and to govern by the photon energy. The deviation in activation energies for polaron and anion-radical motion and the difference in their dependence on the laser irradiation photon energy prove a non-interacting character of these charge carriers photoinduced in the polymer/fullerene bulk heterojunction.     

Precursor route poly(thienylene vinylene) for organic solar cells: Photophysics and photovoltaic performance

Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65 eV (750 nm), which corresponds to the absorption spectrum of the polymer.     

一种聚合物/C60异质结有机太阳电池

用MEH-PPV为给体(空穴传输)、C60为受体(电子传输)首先制备了分层和体异质结结构的两种器件,器件结构为ITO/PEDOT:PSS/MEH-PPV/C60/Al和ITO/PEDOT:PSS/MEH-PPV:C60/Al。之后又制备了结构为ITO/PE-DOT:PSS/MEH-PPV:C60/C60/Al的第3个器件。作者比较了这3种器件的光伏性质,发现器件3的短路电流密度(JSC)比器件1和器件2的分别增加了300%和150%,开路电压(VOC)分别增加了100%和20%。这主要是由于C60层增加了电子由受体传输到负电极的通道并增大了给体受体界面面积。另一原因是此C60层一定程度地阻挡了空穴从有机物向负极的传输,从而有效地改善了太阳电池的性能。     

The role of conformational transitions on the performance of poly(3-hexylthiophene)/fullerene solar cells

Poly(3-hexylthiophene) formed gels and showed liquid crystalline structures at high concentrations. The absorption properties of poly(3-hexylthiophene) showed dramatic changes during gelation, which is an indication of strong intermolecular π-electronic coupling of the ordered self-assembled poly(3-hexylthiophene) gels. The effect of conformational transitions on the photovoltaic properties of solution-processed poly(3-hexylthiophene)/fullerene blends have been studied in this paper. It is shown that the photovoltaic performance is strongly affected by gelation, which alters the morphology of the photoactive layer. Device optimization yields solar cells with a power conversion efficiency of 4.0% under standard test conditions (AM 1.5, 100 mW/cm²).     

Improving power conversion efficiency in polythiophene/fullerene-based bulk heterojunction solar cells

In the present work, we have studied photovoltaic devices fabricated from a blend of regioregular poly (3-hexlthiophene) (P3HT) and Buckminster fullerene. The solvent and composite ratio have been selected to obtain best morphology and minimum degradation. Buffer layers of poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) at the anode and of LiF at the cathode were used to improve the device performance. It was further found that post-annealing of the devices for an optimum duration and temperature improves the solar cells, and the power conversion efficiency of the devices increases to 2.1% at AM1.5. Though the efficiency using [6,6]-phenyl C60 butyric acid methyl ester (PCBM) as the composite acceptor instead of C₆₀ can be higher, it was not used because of its very high cost as compared to C₆₀.     

Higher fullerenes as electron acceptors for polymer solar cells: A quantum chemical study

In the present study, we have used quantum chemical methods to study the energy levels of the frontier orbitals of higher fullerene derivatives (from C₇₀ to C₈₄ and having the same addend as in [6,6]-phenyl C₆₁-butyric acid methyl ester) with the aim to understand if they can be used as electron acceptors in bulk heterojunction polymer–fullerene solar cells. Higher fullerenes have a stronger and broader absorption compared to C₆₀ and they can improve the current output of the corresponding devices. The geometries of all the compounds were optimized with the density functional theory at the B3LYP/3-21G* level of calculation. The lowest unoccupied molecular orbital (LUMO) levels of the investigated compounds correlate well with the reduction potentials (obtained by cyclic voltammetry) of the already prepared species. We found that the LUMO level depends not only on the fullerene size (number of carbons of the cage) and constitutional isomer, but also on the position and, in some cases, the addend orientation. This issue should be considered because for a proper device operation, a well-defined LUMO is required. The position of the LUMO level of some higher fullerene derivatives can be suitable for low-bandgap polymers.     

Solid-state D102 dye sensitized/poly(3-hexylthiophene) hybrid solar cells on flexible Ti substrate

Flexible solid-state dye sensitized solar cell is an important milestone for low-cost, large scale fabrication of dye-sensitized solar cells. Flexible solid-state dye-sensitized solar cell is fabricated for the first time on titanium substrates using D102 sensitizer and a sputtered platinum semi-transparent cathode. Devices are illuminated from the cathode side since titanium substrates are non-transparent. Due to rear-side illumination, significant proportion of radiation is absorbed and scattered by poly(3-hexylthiophene) and platinum, respectively. Limiting the amount of platinum and poly(3-hexylthiophene), up to a point, is found to enhance device efficiency. The amount of platinum and poly(3-hexylthiophene) is optimized on glass substrates before fabrication of flexible devices on titanium substrates. The rough surface of titanium substrates is smoothened until a mirror finish and the growth of a thin layer of native oxide enhanced the device efficiency. Under optimized conditions, an efficiency of 1.20% is obtained for devices fabricated on titanium foil substrates. The lower efficiency as compared to conventional devices is mainly due to light absorption/scattering from the poly(3-hexylthiophene) and platinum layers.     

Bulk heterojunction organic photovoltaic based on polythiophene–polyelectrolyte carbon nanotube composites

It is shown that carbon nanotubes can be used to enhance carrier mobility for efficient removal of the charges in thin film polymer-conjugated/fullerene photovoltaic devices. The fabricated photovoltaic devices consist of poly(3-octylthiophene) (P3OT) polymer blended with undoped multiwalled carbon nanotubes (MWNTs) and carbon nanotubes doped with nitrogen (CNx-MWNTs). Nanophase formation and dispersion problems associated with the use of carbon nanotubes in polymer devices were addressed through the generation of functional groups and electrostatic attaching of the polyelectrolyte poly(dimethyldiallylamine) chloride (PDDA) in both MWNTs and CNx-MWNT systems. The resultant nanophase was highly dispersed allowing for excellent bulk heterojunction formation. Our results indicate that CNx-MWNTs enhance the efficiency of P3OT solar cells in comparison with MWNTs.     

Fabrication and photovoltaic properties of multilayered thin films designed by layer-by-layer assembly of poly(p-phenylenevinylene)s

Ultrathin films of poly(p-phenylenevinylene) (PPV) were fabricated by the layer-by-layer (LbL) deposition technique with a cationic PPV precursor and an anionic PPV. The hole mobility of the PPV-based LbL film was as high as 4×10⁻⁴ cm² V⁻¹ s⁻¹. Multilayered polymer solar cells with the PPV-based LbL film as a light-harvesting layer were fabricated by a combination of the spincoating and the LbL deposition technique, exhibiting a power conversion efficiency of 0.28% under AM1.5G simulated solar illumination with 100 mW cm⁻². The high performance is ascribed to the high light-harvesting efficiency and hole mobility of the PPV-based LbL layer.     

Synthesis and photovoltaic properties of soluble fulleropyrrolidine derivatives for organic solar cells

Organic solar cells made from bi-layer thin-film heterojunctions having poly((2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene)vinylene) (MEH-PPV) as an electron donor and fulleropyrrolidine derivatives as an electron acceptor were investigated. We synthesized soluble fulleropyrrolidine derivatives substituted different chain lengths for the organic solar cell. Due to the high solubility and sufficiently long chain length of fulleropyrrolidine derivatives, though those are monomers, a thin film (about 80–90 nm) could be fabricated individually by the spin-coating method. The fill factor of the bi-layer device was achieved to be 0.46, which is higher than that of the single-layer device by a polymer/fulleropyrrolidine derivative blend film of 0.37, due to the decrease of the recombination.     

Fabricating red-blue-switching dual polymer electrochromic devices using room temperature ionic liquid

In this work, room temperature ionic liquid (RTIL)—1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF₆)—was employed to fabricate dual polymer electrochromic devices (DPECDs). [BMIM]PF₆ was used as the electrolyte both in the electrochemical synthesis of conducting polymers (CPs) and in the fabrication of DPECDs. The electrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methylthiophene) (PMeT) were employed to serve as two complementary coloring electrochromic thin films. Through combining these two electrochromic layers, the assembled DPECDs were found to switch between deep red and deep blue, which are two primary colors for a display. By employing RTIL as electrolyte, the devices retained 65% of their optical contrast and electroactivity after 5×10³ deep double potential steps, showing enhanced stability and durability. The DPECDs also exhibited stable electrochromic performance, with a maximum optical contrast of 26% at 665 nm, and achieved a high coloring efficiency of 460 cm² C^-1.     

Triphenylamine-substituted methanofullerene derivatives for enhanced open-circuit voltages and efficiencies in polymer solar cells

A new class of triphenylamine substituted methanofullerene derivatives, bis(4'-(diphenylamino)biphenyl-4-yl)methanofullerene (1) and the bisadduct (2), were synthesized. The incident photon to current efficiency (IPCE) studies revealed that the diphenylamino components have contribution to the photocurrent that expands the light harvesting window around 400 nm. When being blended with poly (3-hexylthiophene) (P3HT) to fabricate the solar cell, the device of P3HT:1 (1:0.7) shows high open circuit voltage (V_oc) of 0.69 V under the illumination of AM 1.5, 100 mW/cm² with high power conversion efficiency (PCE) of 3.16%, which is about 0.1 V higher than that of the corresponding [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) devices. This indicates that the arylamine substituents on 1 have played some special roles on the high V_oc performance. Similar effects are also observed for 2. The device of P3HT:2 (1:1) shows even higher V_oc of 0.87 V with the PCE of 1.83%. These results indicate that 1 and 2 are alternative high performance acceptors.     

Methanol oxidation on the polymer coated and polymer-stabilized Pt nano-particles: A comparative study of permeability and catalyst particle distribution ability of the PANI and its derivatives

The electrosynthesis and characterization of poly (aniline and aniline derivatives) on the pretreated aluminum (Al/Pt) and platinum plated aluminum (Al–Pt) electrodes were investigated. The results show that the both of them are suitable substrates as well as smooth Pt and platinum plated Pt (Pt–Pt) for fabrication of polymer modified electrodes. Comparative study of permeability of the freshly deposited polyaniline and polyaniline derivatives on the smooth Pt and Al–Pt or Pt–Pt substrates towards the methanol were done. The electrooxidation of methanol was studied by cyclic voltammetry in 0.1 M sulfuric acid as supporting electrolyte. Enhanced electrocatalytic activity towards the methanol oxidation was noticed when the bulk Pt and Al–Pt or Pt–Pt were covered by polyaminoanilines and polyhydroxyanilines films due to the permeability of their films for methanol molecules. A high catalytic current related to oxidation of methanol was found for the polymer modified electrodes when Pt nano-particles were embedded into the polymer films referred to as polymers-stabilized Pt nano-particles. The enhanced electrocatalytic activity might be due to the dispersion of Pt nano-particles into the polymer matrixes and the synergistic effects between the dispersed Pt particles and their matrixes.     

Photoinduced FT-IR spectroscopy and CW-photocurrent measurements of conjugated polymers and fullerenes blended into a conventional polymer matrix

In this work we present an investigation of the photoexcited states in conjugated polymer (donor) – fullerene (acceptor) interpenetrating networks embedded into conventional polymer hosts like polystyrene (PS), polyvinylcarbazole (PVK) or polyvinylbenzylchloride (PVBC) (guest – host approach), using photoinduced FT-IR spectroscopy. We discuss the influence of the specific host polymer matrices on the photoexcited states of the photoactive guests and investigate the photoinduced electron transfer by analysis of the infrared activated vibrational (IRAV) bands of poly-3-octylthiophene (P3OT) in comparative studies. Solar cells based on mixtures of poly [2-methoxy, 5-(3′,7′-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV), a highly soluble fullerene derivative [6 and 6]-phenyl C₆₁-butyric acid methyl ester (PCBM) and a conventional polymer (PS, PVK or PVBC) are characterized. We studied the current–voltage characteristics of devices and determined the energy conversion and electron/photon conversion efficiencies.