Design and synthesis of soluble dibenzosuberane-substituted fullerene derivatives for bulk-heterojunction polymer solar cells

Two new dibenzosuberane-substituted fullerene derivatives, dibenzosuberane-C₆₀ mono-adduct (DBSCMA) and bis-adduct (DBSCBA) were synthesized using a classical cyclopropanation reaction via a tosylhydrazone route for application as acceptor materials in polymer solar cells (PSCs). DBSCBA shows good solubility in common organic solvents and both derivatives were characterized by ¹HNMR, ¹³C NMR, MALD-TOF, elemental analysis and UV–vis absorption measurements. The shift of fullerene energy levels induced by the dibenzosuberane substitution was investigated by using theoretical simulations and ultraviolet photoelectron spectroscopy. Bulk-heterojunction PSCs based on poly (3-hexylthiophene) (P3HT) and dibenzosuberane-C₆₀ derivatives were fabricated and optimized by adjusting the donor/acceptor ratio and using thermal annealing and solvent additive. The morphologies of the active layers processed under different conditions were also examined by atomic force microscopy. When tested under an illumination of AM 1.5 G at 100 mW/cm², the highest power conversion efficiency of the devices using DBSCBA is 3.70% which is superior to that of conventional P3HT:PCBM devices.     

Degradation of self-assembled monolayers in organic photovoltaic devices

Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C₆₀-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.     

Electronic structure of fullerene derivatives in organic photovoltaics

The electronic structures of the fullerene derivatives [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), [6,6]-diphenyl C₆₂ bis (butyric acid methyl ester) (bisPCBM), C₇₀, [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₀BM), [6,6]-phenyl-C₆₁-butyric acid butyl ester (PCBB), [6,6]-phenyl-C₆₁-butyric acid octyl ester (PCBO), [6,6]-thienyl-C₆₁-butyric acid methyl ester (TCBM), and indene-C₆₀ bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain.     

Partitioning of the organic layers for the fabrication of high efficiency organic photovoltaic devices

Lateral partitioning of hole extraction layer with insulating walls improved the power conversion efficiency of organic photovoltaic device. When the conductivity of the hole extraction layer is low, no improvement is obtained by partitioning. However, when the conductivity is high, a significant improvement was obtained in the partitioned cells, showing the estimated power conversion efficiency of 4.58% compared to the 3.54% of the single cell structure. This improvement, carefully corrected by masking at measurement, could be explained by the reduction of series resistance. Although accurate estimation of device area at partitioned device might be difficult, its effectiveness on the properties of large area organic photovoltaic device is clear, as shown in the result of 1 cm²-size cell with the same manner.     

Integrating nanostructured electrodes in organic photovoltaic devices for enhancing near-infrared photoresponse

We introduce a simple methodology to integrate prefabricated nanostructured-electrodes in solution-processed organic photovoltaic (OPV) devices. The tailored “photonic electrode” nanostructure is used for light management in the device and for hole collection. This approach opens up new possibilities for designing photonically active structures that can enhance the absorption of sub-bandgap photons in the active layer. We discuss the design, fabrication and characterization of photonic electrodes, and the methodology for integrating them to OPV devices using a simple lamination technique. We demonstrate theoretically and experimentally that OPV devices using photonic electrodes show a factor of ca. 5 enhancement in external quantum efficiency (EQE) in the near infrared region. We use simulations to trace this observed efficiency enhancement to surface plasmon polariton modes in the nanostructure.     

Electronic and chemical properties of ZnO in inverted organic photovoltaic devices

Photo-conversion efficiency of inverted polymer solar cells incorporating pulsed laser deposited ZnO electron transport layer have been found to significantly increase from 0.8% to up to 3.3% as the film thickness increased from 4 nm to 100 nm. While the ZnO film thickness was found to have little influence on the morphology of the resultant ZnO films, the band structure of ZnO was found to evolve only for films of thickness 25 nm or more and this was accompanied by a significant reduction of 0.4 eV in the workfunction. The films became more oxygen deficient with increased thickness, as found from X-ray photoelectron spectroscopy (XPS) and valence band XPS (VBXPS). We attribute the strong dependence of device performance to the zinc to oxygen stoichiometry within the ZnO layers, leading to improvement in the band structure of ZnO with increased thickness.     

Understanding the effect of triplet sensitizers in organic photovoltaic devices

The effect of PtOEP as a dopant on the performance of MEH-PPV/C₆₀ photovoltaic devices was studied. Bilayer heterojunction devices with various compositions and layer structures were used to determine the possible pathways by which the photogeneration efficiency is enhanced. A key finding is that photocurrent generation enhancement always occurs in the MEH-PPV absorption region, regardless of the PtOEP dopant concentration or the MEH-PPV layer thickness. This result suggests that the presence of PtOEP in the donor MEH-PPV layer is primarily responsible for increasing the triplet exciton diffusion length of MEH-PPV by acting as a triplet sensitizer, rather than as an additional absorber for direct photogeneration. Values obtained from simulation show that the enhancement of exciton diffusion length of MEH-PPV can be more than a factor of 2 with optimal PtOEP concentrations. Further support for the role of PtOEP as a triplet sensitizer in MEH-PPV was obtained in experiments incorporating a blocking layer between MEH-PPV and C₆₀, whereby the various exciton transfer processes can be differentiated.     

Photogenerated charge carrier transport and recombination in polyfluorene/fullerene bilayer and blend photovoltaic devices

Using extraction of photogenerated charge carriers by linearly increasing voltage (photo-CELIV), we investigated two key transport parameters in photovoltaic materials based on the donor APFO-3 and acceptor PCBM: the mobility and lifetime of photogenerated charge carriers, in bilayers of varying geometry and in blends with various acceptors loading. We find that mobility depends strongly on delay time for shorter delay time in all devices. The observed recombination kinetics is found to be monomolecular. The mean lifetime of charge carriers is 2–3 μs in blends and is slightly greater than 4 μs in bilayer devices. In addition, the implications of mobility and lifetime values on the collection efficiency of the devices are presented.     

Reversible photoinduced bi-state polymer solar cells based on fullerene derivatives with azobenzene groups

[6,6]-Phenyl-C61-butyric acid-4′-hydroxyl-azobenzene ester (PCBAb) was synthesized and used as the acceptor in the fabrication of reversible UV–VIS response bi-state polymer solar cells (PSCs) based on the photoinduced cis–trans isomerization of PCBAb. The device can be switched between “active” and “sleep” by the irradiation of UV and visible light, respectively. The active device has a PCE of 2.0%. With UV irradiation, the device goes to “sleep” with a lowered PCE (0.4%), and simultaneously decreased J_sc, V_oc and FF, while after visible light treatment, the device is made “active” again. The mechanism of the bi-state process involves the different electron mobilities of the isomers.     

Electrical aspects of photovoltaic devices based on bi-layer organic semiconducting materials

Here we have investigated the opto-electrical properties of bi-layer sandwich devices based on polymer/C₆₀ hetero junctions. In this type of device the polymer acts as an electron donor to the second layers the C₆₀ which plays the role of an electron acceptor.     

Control over fibril width via different solubility additives for diketopyrrolopyrrole-based photovoltaic devices

Control over polymeric bulk heterojunction (BHJ) morphology is one of the key factors in obtaining high-efficiency devices. The domain size influence on device performance is widely considered critical. In this paper, the fibril width of 3,6-bis-(thiophen-2-yl)-N,N′-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT):[6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) blend thin film was adjusted by different processing additives. By decreasing the solubility of PDBT-TT in different additives, the fibril width can be decreased from 65.7 nm to 14.8 nm. It is possible that the PDBT-TT seed-crystallite nuclei concentration is higher in the relatively low solubility solvents than that in the relatively high solubility solvents, thus leading to the formation of narrower fibrils. The PDBT-TT/PC71BM narrow fibrillar interpenetrating network structure was beneficial to exciton separation and charge transport processes. As a result, the solar cell with the narrowest fibril width has a higher short circuit current (J_sc) and fill factor (FF), thus achieving optimized device performance from less than 1% to 4.75%.     

Synthesis and electroluminescent properties of highly efficient anthracene derivatives with bulky side groups

Three new asymmetric light emitting organic compounds were synthesized with diphenylamine or triphenylamine side groups; 10-(3,5-diphenylphenyl)-N,N-diphenylanthracen-9-amine (MADa), 4-(10-(3,5-diphenylphenyl)anthracen-9-yl)-N,N-diphenylaniline (MATa), and 4-(10-(3′,5′-diphenylbiphenyl-4-yl)anthracen-9-yl)-N,N-diphenylaniline (TATa). MATa and TATa had a PL_max at 463 nm in the blue region, and MADa had a PL_max at 498 nm. MADa and MATa had T_g values greater than 120 °C, and TATa had a T_g of 139 °C. EL devices containing the synthesized compounds were fabricated in the configuration: ITO/4,4′,4′′-tris(N-(2-naphthyl)-N-phenyl-amino)-triphenylamine (2-TNATA) (60 nm)/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (15 nm)/MADa or MATa or TATa or 9,10-di(2′-naphthyl)anthracene (MADN) (30 nm)/8-hydroxyquinoline aluminum (Alq₃) (30 nm)/LiF (1 nm)/Al (200 nm). The efficiency and color coordinate values (respectively) were 10.3 cd/A and (0.199, 0.152; bluish-green) for the MADa device, 4.67 cd/A and (0.151, 0.177) for the MATa device, and 6.07 cd/A and (0.149, 0.177) for the TATa device. The TATa device had a high external quantum efficiency (EQE) of 6.19%, and its luminance and power efficiencies and life-time were more than twice those of the MADN device.     

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

Series resistance is one of the key parameters affecting the performance of organic photovoltaic devices. Several electronic mechanisms arising from different structures within the solar cell can contribute to increasing it. We focus on the series resistance origin by altering the acceptor transport properties trough the incorporation of fullerene traps located at energies below the transporting electron levels. Indene-C₆₀ bisadduct as acceptor molecule blended with poly(3-hexylthiophene) forms the active layer in which small amounts of [6,6]-phenyl-C₆₁-butyric acid methyl ester have been added as trapping sites. A complete analysis of the impedance response has allowed identifying bulk transport resistive circuit elements in the high-frequency part of the spectra. Series resistance is observed to be dependent on the concentration of fullerene traps, thus indicating a connection between bulk transport processes and resistive elements. By comparing different contacts it has been discarded that outer cathode interfaces influence the series resistance experimentally extracted from impedance spectroscopy.     

Organic materials for photovoltaic and light-emitting devices

Materials for organic photovoltaic cells and light-emitting devices are reviewed. Dye-sensitized systems represent the most studied of these materials as they offer high efficiency of photoelectric energy conversion. Systems demonstrating efficient luminescence were identified; they are based on conjugated polymers, complexes of rare-earth elements with organic ligands, and dyes. To achieve efficient photoelectric conversion, different types of sensitizing dyes will be tested. Phthalocyanines and pentacenes are of special interest. Phthalocyanines are the most promising materials: they are easily synthesized and nontoxic, and their electric characteristics are widely investigated. Harnessing the unique electron-acceptor properties of the C₆₀ molecule, one can attain considerable enhancement in the efficiency of solar-energy conversion into electricity. Organic photovoltaic cells are often made on the basis of aromatic and heterocyclic polymers—poly(p-phenylene-vinylene), polyanilines, polypyrroles, and polythiophenes. Organic photoconducting materials offer high photosensitivity and low dark current. They are readily available and can be easily deposited on a substrate, which make them suitable for the fabrication of relatively cheap photovoltaic cells.     

Effects of fullerene solubility on the crystallization of poly(3-hexylthiophene) and performance of photovoltaic devices

The substantial crystallization suppression of poly(3-hexylthiophene) (P3HT) in the untreated P3HT:C60 composite film prepared from o-dichlorobenzene (ODCB) solution has been revealed. Besides, the effective conjugation length of P3HT in this composite has been nearly maintained to that in the solution. The different crystallization behaviors of P3HT in its composites with C60 and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) are mainly attributed to the relative solubility of C60 and PCBM with respect to P3HT in ODCB. The solution to overcome this disadvantage of chain conformation and crystallinity of P3HT in the composite with C60 is thus proposed and finalized by resorting to the addition of low volatile solvent with much higher solubility of C60 than P3HT into the main solvent used, so as P3HT can crystallize before C60 forms crystallites in the solution. The feasibility of this approach has been proven by the improved efficiency of devices based on composites of P3HT and the low cost C60 without resorting to post-treatments. Our results demonstrated in this study could further benefit development of new electron acceptor materials, particularly based on fullerenes and their derivatives, by considering the role of the new materials in determining the crystallization of the other components involved in the composite film.     

Effect of side groups on the vacuum thermal evaporation of polythiophenes for organic electronics

The vacuum thermal evaporation of poly(3-hexylthiophene) (P3HT) and poly(thiophene) (PTh) conductive polymers with and without side groups is reported. The role of side groups in relation to structural and electronic properties is examined. FT-IR and GPC analysis is used to study the effect deposition has on conjugation of the polymer. Topography and grain structure of the polymer films are studied using MicroXAM, AFM and TEM. XRD and TEM data reveal enhanced molecular packing and crystallinity of PTh. This results in significantly improved charge transport properties with relatively high hole mobilities (10⁻⁴ cm²/Vs). Evaluation of PTh and P3HT electronic properties is performed on simple geometry planar C₆₀ heterojunction solar cells. PTh/C₆₀ devices exhibit almost a 70% increase in efficiency as compared to P3HT/C₆₀ devices, demonstrating enhanced charge collection in PTh films through improved molecular order.     

Enhancement of the power conversion efficiency for organic photovoltaic devices due to an embedded rugged nanostructural layer

Organic photovoltaic (OPV) cells utilizing a rugged nanostructural layer were fabricated by using a mixed solution method. The charge separation at the heterointerface between the poly(3-hexylthiophene) (P3HT) nanostructural layer with a rugged surface and the C60 layer was increased due to an increase in the interfacial region between the donor and the acceptor layers, resulting in an increase in the short-circuit current density and the power conversion efficiency (PCE) of the OPV cells with a P3HT nanostructural layer. The PCE of the OPV cells with a nanostructural rugged layer is 30% higher than that without a rugged layer.     

Plasmonic Au nanoparticles for enhanced broadband light absorption in inverted organic photovoltaic devices by plasma assisted physical vapour deposition

We use a low vacuum plasma assisted physical vapour deposition (PAPVD) method to deposit a Au nanoparticles (NPs) thin film onto the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer in inverted poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methylester (P3HT:PCBM) organic photovoltaic (OPV) devices. The Au NPs that incorporated into the PEDOT:PSS layer and reached to the active P3HT:PCBM layer can provide significant plasmonic broadband light absorption enhancement to the active layer. An approximately 50–90% improvement in short-circuit current density and in power convention efficiency has been achieved compared with those OPV devices without the plasmonic light absorption enhancement. This technique can be adopted and easily fit into most OPV device fabrication processes without changing other layers’ processing methods, morphologies, and properties.     

Performance and lifetime improvement of polymer/fullerene blend photovoltaic cells with a C60 interlayer

Polymer solar cell based on the blend of poly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) with C₆₀ insertion layer were fabricated. The solar cell structure was ITO/poly(3,4-ethylenedioxythiopene) (PEDOT):poly(styrenesulfonate) (PSS)/MEH-PPV:PCBM/C₆₀/Al. It was found that the C₆₀ inserting layer could increase the device performance and lifetime. The energy conversion efficiency (ECE) of the solar cell with C₆₀ layer reached about 1.89% under Air Mass 1.5, 100 mW cm⁻² illumination, which is enhanced in comparison with that of the device without C₆₀ layer. Mechanisms of the solar cell performance and lifetime dependence on the C₆₀ layer are discussed.     

Influence of functional derivatives of an amino-coumarin/MWCNT composite organic hetero-junction on the photovoltaic characteristics

A layer of a coumarin derivativemultiwall carbon nanotube (MWCNT) composite was sandwiched between an indium tin oxide (ITO) layer and a top metal electrode. This layer acted as the active hetero-junction (middle hetero-junction) in a triple hetero-junction organic solar cell. The composition of the organic material in the middle hetero-junction component, second from the ITO layer, was varied, keeping the weight percentage of the MWCNTs constant, to investigate the photovoltaic properties of the cell. The choice of amino-coumarin and its derivatives were dictated by the need for a high efficiency organic up-converter. Amino-coumarin is a two photon absorbing organic donor whereas CNTs are acceptors and ballistic charge transport media. For those reasons, amino-coumarin-CNT composite materials were selected as a photo active layer to will absorb the sub-band photons effectively. Additionally, the radial self-assembling of the CNTs inside the organic matrix (during synthesis) enhances the photovoltaic characteristics of the material. The photo induced charge transport mechanism between the MWCNTs and organics was analyzed using photo luminescence (PL) measurements. By the use of 15 wt% of CNTs with the organics, more than 90% of the PL signal was quenched, indicating an ultrafast transport mechanism between the donor and acceptor materials.