Enhancement of photovoltaic characteristics using a PEDOT interlayer in TiO2/MEHPPV heterojunction devices

The effect of inserting a PEDOT interlayer between the MEHPPV layer and the Au electrode of a nanocrystalline ITO/TiO₂/MEHPPV/Au heterojunction device on the photovoltaic characteristics of the device has been studied. The MEHPPV layer has both a light-sensitizing role and a hole-transporting function. The overall conversion efficiency of the device with a PEDOT layer is better by more than 80% than that obtainable without a PEDOT layer. The modified device shows improved photocurrent density–voltage (J–V) characteristics, in that there is a strong reduction of the roll-over behavior in the forward bias region, and an increase in the fill factor. These improvements are due to the reduction of junction resistance across the MEHPPV/Au interface in the presence of the PEDOT interlayer, which results in improved hole injection.     

Zinc-phthalocyaninetetraphosphonic acid as a novel transparent-conducting-oxide passivation for organic photovoltaic devices

A novel monolayer chemical passivation improving the surface electronic properties of indium-tin oxide (ITO), used as an electrode in organic solar cells (OSC), is reported. Deposition of zinc-phthalocyaninetetraphosphonic acid on ITO substrates, from a water solution, creates a chemically bound organic monolayer passivation, which improves the charge transfer through the ITO/zinc-phthalocyanine (ZnPc) interface in ZnPc/C₆₀ OSC. Current–voltage measurements on devices produced on such substrates show improved serial and parallel resistances as well as fill factor, compared to OSC on non-passivated substrates. The use of this novel passivation for electrodes allows to dispose off the additional conventional PEDOT:PSS buffer layer.     

Engineering of hybrid interfaces in organic photovoltaic devices

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide (ITO) anode (front-side electrode) as well as at the Mg−Ag cathode (back-side electrode) in metal phthalocyanine (MePc)/C₆₀ organic solar cells (OSCs).For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocyanine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages, thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the possibility to engineer the hybrid interface without additional buffer.For the back-side electrode we report the formation of buffer-free charge carrier selective Mg−Ag cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocyanine (CuPc) donor and fullerene C₆₀ acceptor materials. The chemical and structural properties of the CuPc:C₆₀/Mg−Ag interface are investigated by element depth profiling using secondary ion mass spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS.We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode structures, where the Mg:Ag alloy layer has a composition close to that of Ag₃Mg. In addition, Mg diffusion into CuPc:C₆₀ layer is observed. As a result, an interaction between Mg and Cu²⁺ with a concurrent change in oxidation state of both metals takes place. However, no formation of MgPc is observed.The findings of this work are discussed against the background of the performance and electrical properties of the corresponding MePc/C₆₀-based organic solar cells.     

A study of the factors influencing the performance of ternary MEH-PPV:porphyrin:PCBM heterojunction devices: A steric approach to controlling charge recombination

A series of three porphyrins varying only in the steric bulk of their peripheral groups have been synthesised and integrated into the active layer of bulk heterojunction solar cells. The porphyrins broaden the spectral response of the device and contribute to the total photocurrent generated. More importantly, the device characteristics change systematically with increasing steric bulk on the peripheral meso phenyl groups of the tetra-phenyl porphyrin. Optical, scanning probe and scanning transmission X-ray microscopy are used to demonstrate that the observed changes do not arise from morphological differences in film structure. Cyclic voltammetry, UV-vis spectroscopy and DFT calculations are used to establish that the porphyrin LUMO, HOMO and bandgap are independent of side group. We conclude that the variations in open-circuit voltage with side group are the result of the porphyrin acting as a bimolecular recombination centre, with an efficiency that is dependent on the side group type. The possibility of designing optimised macromolecules for OPV devices based on an understanding the effect of porphyrin steric bulk upon device performance is discussed.     

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.     

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.     

Interface engineering in chalcopyrite thin film solar devices

Successful interface engineering requires compositional and electronic material characterization as a prerequisite for understanding and intentionally generating interfaces in photovoltaic devices. The paper gives an overview with several examples, all referring to Cu(In,Ga)(S,Se)₂ (“CIGSSe”)-based solar cells, with an emphasis on characterization using highly specialized methods, such as elastic recoil detection analysis, X-ray emission spectroscopy and photoelectron spectroscopy using synchrotron and ultraviolet light for excitation, inverse photoemission spectroscopy and Kelvin probe force microscopy. First, the determination of the depth profile of the band gap energy E_g in the absorber layer is demonstrated. The modification of E_g towards both interfaces is discussed in terms of beneficial electronic effects. Next, the interface between absorber and buffer layers with alternative and promising non-toxic materials is considered. Between CIGSSe and a ZnSe buffer deposited by the metalorganic chemical vapor deposition (MOCVD) method a buried ZnS interface was found. For a Zn(O,OH) buffer processed with an ion layer gas reaction (ILGAR) the correlation of surface composition, valence band maximum and efficiency of the resulting solar cell is shown. In addition, another approach is considered where a ZnMgO window layer is sputtered directly on the absorber omitting any buffer layer. The determination of the potential distribution at the ZnMgO/CIGSSe interface supports the understanding of this new and simpler way to get good cell performances even without any buffer. Finally, monolithically integrated solar modules without encapsulation were investigated before and after accelerated aging tests and changes at the interconnects were identified.     

Semi-transparent metal electrode of Cu-Ni as a replacement of an ITO in organic photovoltaic cells

We demonstrate an indium-free organic photovoltaic cell that incorporates an ultrathin metal film as a semitransparent anode. In the proposed device structure, the indium tin oxide electrode is replaced by an ultrathin Cu-Ni bilayer. When an NiO is used as the hole transporting layer, the characteristic photovoltaic parameters of the cell fabricated with the metal electrode are similar to those of the device fabricated with the indium tin oxide (ITO). Despite the fact that the metal electrode exhibits a transparency that is 65% of the ITO electrode, the short-circuit current for the metallic anode based cell is 77% of the ITO based one, indicating that the photon absorption could be enhanced by the optical microcavity formed between the Cu-Ni and Al electrodes. The overall photo-conversion efficiency for the metallic electrode based cell is 76% of the ITO based one, which was measured to be 3.3%. The obtained performances of ultrathin metals when included in the cell architecture used here, combined with their low cost, high compatibility with other materials, and mechanical flexibility, confirm their potentials for organic photovoltaics.     

Spatial distribution of light absorption in organic photovoltaic devices

In this paper we present methods for the optimization of light absorption of organic photoelectric bilayer devices like organic photodetectors and organic solar cells, which show the best performance if it is ensured that the spatial density of the absorbed light energy reaches its maximum in certain “active” areas. Mathematical simulations show interesting spatial distributions of light absorption depending on the thicknesses and the optical constants of the individual device layers. Our methods permit to dispose the maxima of absorption density to the area near the interface between the active layers of the bilayer device. We built photovoltaic devices according to the simulated configurations and gained improvements of the power conversion efficiencies of more than one magnitude. Parametric studies were carried out, which give us a suggestion for the potential active materials. Furthermore we analyzed the correlation between the photocurrent and the absorption density in given areas around the p–n junction, which will lead to better understanding of the diffusion range of dissociated charge carriers.     

Hybrid inverted bulk heterojunction solar cells with nanoimprinted TiO2 nanopores

Photovoltaic devices with highly ordered nanoporous titanium dioxide (titania; TiO₂) were fabricated to improve the photovoltaic performances by increasing TiO₂ interface area. The nanoimprinting lithography technique with polymethyl methacrylate (PMMA) mold was used to form titania nanopores. The solar cell with poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) active layer on nanoporous titania showed higher power conversion efficiency (PCE) of 1.49% than on flat titania of 1.18%. The improved efficiency using nanoporous titania is interpreted with the enhanced-charge separation and collection by increasing the interface area between TiO₂ and active layer.     

Dye sensitized photovoltaic cells: Attaching conjugated polymers to zwitterionic ruthenium dyes

The synthesis of a zwitterionic ruthenium dye that binds to anatase surfaces and has a built-in functionality that allows for the attachment of a conjugated polymer chain is presented. The system was found to adsorb on the surface of anatase anchored by the ruthenium dye. Two types of devices were prepared: standard photoelectrochemical (PEC) solar cells and polymer solar cells. The PEC solar cells employed a sandwich geometry between TiO₂ nanoporous photoanodes and Pt counter electrodes using LiI/I₂ in CH₃CN as an electrolyte. The polymer solar cells employed planar anatase electrodes and the complex was adsorbed onto the surface before evaporation of gold electrodes. Alternative devices were obtained by spincoating of the polymer solution onto PEDOT:PSS covered indium-doped tin oxide substrates. PEC solar cells gave the best results and the main finding was that the polymer chain served as a light harvesting antenna for the ruthenium dye.     

Indium-free, acid-resistant anatase Nb-doped TiO2 electrodes activated by rapid-thermal annealing for cost-effective organic photovoltaics

Indium-free and acid-resistant anatase Nb-doped TiO₂ (NTO) electrodes are promising as economical substitutes for high-cost Sn-doped In₂O₃ (ITO) films used in organic photovoltaics. By rapid-thermal annealing under an ambient vacuum, an insulating amorphous NTO film of low transparency was changed dramatically into a transparent and conductive anatase NTO electrode. Metallic conductivity of the annealed NTO electrode could be attributed to formation of the anatase phase and activation of the Nb dopant. Based on synchrotron X-ray scattering and high-resolution transmission electron microscopy, the electrical properties of the NTO electrode could be correlated with the microstructure of the NTO film. The acid-stability of NTO film also supports its use as a substitute for ITO electrode. Unlike Ga:ZnO and Al:ZnO films, which were easily etched by acidic PEDOT:PSS solution, the NTO film was stable against this reagent. Importantly, the annealing temperature influenced the performance of the organic solar cell fabricated with the NTO electrode. This indicates that activation of Nb dopants and formation of the anatase phase play an important role in the extraction of carrier from the organic layer to the anode electrode.     

Improved power conversion efficiency of bulk heterojunction poly(3-hexylthiophene):PCBM photovoltaic devices using small molecule additive

We report that the power conversion efficiency (PCE) of the bulk heterojunction organic photovoltaic device based on poly(3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) blend was improved by incorporating a small molecule SM having absorption band in the longer wavelength region. SM is a small molecule containing thienothiadiazole central unit with terminal cyanovinylene 4-nitrophenyl at both sides, which were connected to the central unit via a thiophene ring. The combination of SM with P3HT and PCBM allows not only a broad band absorption up to longer wavelength, but also tuning the inter-energy level leading to a higher short circuit current (J_sc) and open circuit voltage (V_oc). The device based on the as cast P3HT:PCBM:SM exhibits a PCE of 3.69%, which is higher than the device based on P3HT:PCBM and SM:PCBM blends. The overall PCE of the device based on thermally annealed blend is further improved up to 4.1%. The improvement of the PCE has been attributed to a better charge transport in the device, due to the increased crystallinity of the blend through thermal annealing.     

An Arrhenius approach to estimating organic photovoltaic module weathering acceleration factors

A mathematical model based on the Arrhenius equation is used to determine the acceleration of temperature dependent degradation processes affecting the performance of polymeric PV modules in artificial weathering over two benchmark climates. To take into account the natural variability of stress factors in outdoor environments, equivalent temperatures corresponding to photothermally and thermally activated degradation processes were calculated using detailed temperature and radiation data for these modules. Temperature and radiation data for the accelerated laboratory weathering part were derived from the implementation of an international standard for the weathering of plastics. Depending on the value of the activation energy and the reference outdoor location, acceleration factors ranging from 3 to 11 were calculated.     

New amorphous small molecules—Synthesis, characterization and their application in bulk heterojunction solar cells

We successfully synthesized a series of novel solution processible small molecules (2TAPM, 4TAPM and 2BTAPM) consisting of electron-accepting unit (2-pyran-4-ylidenemalononitrile) (PM) and electron-donating unit (Triphenylamine and different thiophene units). Differential scanning calorimetry (DSC) measurement indicates that these small molecules are amorphous. UV-vis absorption spectra show that the combination of PM with moieties having gradually increased electron-donating ability results in an enhanced intramolecular charge transfer (ICT) transition, leading to an extension of the absorption spectral range and a reduction of the band gap of the molecules. Both cyclic voltammetry measurement and theoretical calculations show that the highest occupied molecular orbital (HOMO) energy levels of the molecules could be fine-tuned by changing the electron-donating ability of the electron-donating moieties. The bulk heterojunction (BHJ) photovoltaic devices with a structure of ITO/PEDOT:PSS/small molecules:PC₇₁BM/LiF/Al were fabricated by using the small molecules as donors and (6,6)-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) as acceptor. Power conversion efficiencies of 1.76% and 2.47% were achieved for the photovoltaic devices based on 2TAPM:PC₇₁BM and 4TAPM:PC₇₁BM under simulated air mass 1.5 global irradiation (100 mW/cm²), respectively.     

Bulk-heterojunction photovoltaic devices based on donor–acceptor organic small molecule blends

We present a systematic study on photovoltaic devices that combine an organic small molecule photoactive donor–acceptor bulk heterojunction system with controlled doping of the charge transport layers. The doped transport layers are formed using high vacuum co-evaporation deposition technique (i.e. co-sublimation of matrix and dopant). Solar cell devices have been fabricated based on zinc-phthalocyanine (ZnPc) as donor (D) and fullerene (C₆₀) as electron acceptor (A) with doped charge transport layers. The cells show a short circuit current, I_sc=1.5 mA/cm², an open circuit voltage, V_oc=450 mV, a fill factor, FF=0.5, and a power conversion efficiency, η_e=3.37% under sun (10 mW/cm²) white light illumination. In addition, these bulk-heterojunction photovoltaic devices were characterized under 1 sun (100 mW/cm²) white light illumination showing I_sc=6.3 mA/cm², V_oc=500 mV, and η_e=1.04%. We have observed that the performance of such ‘bulk-heterojunction’ photovoltaic devices is critically dependent on the transport properties of the interpenetrating network D/A system and doped charge transport layers.     

Organic heterojunction photovoltaic cells: role of functional groups in electron acceptor materials

We have fabricated and characterized some donor/acceptor type photovoltaic devices. While zinc phthalocyanine has been the donor material, a series of acceptor materials with identical backbone but different strength has been chosen. The role of function groups in acceptor material on photovoltaic device characteristics has been studied under different illumination intensities. Origin of the open-circuit voltage has been discussed considering ionization potential of donor and electron affinity of the acceptor material. Nonlinear increase of short-circuit current on light intensity has been observed and discussed in terms of conductance switching of acceptor material via electroreduction.     

Controlled p-doping of pigment layers by cosublimation: Basic mechanisms and implications for their use in organic photovoltaic cells

We present a systematic study on doping of vanadyl- and zinc-pathalocyanine by a fully fluorinated form of tetracyano-quinodimethane as an example of controlled doping of thin organic films by cosublimation of matrix and dopant. The films are characterized in situ by temperature dependent Seebeck and conductivity measurements. We observe a drastic increase of conductivity and a corresponding shift of the Fermi level towards the valence states with increasing dopant concentration. We thus conclude that doping has the potential of both reducing the series resistance and increasing the photovoltage of organic solar cells. As a first step to exploit this potential, we present two different ways of preparing diodes with rectification ratios in excess of 10⁴ using doped phthalocyanines. By adding an undoped interlayer between the contact and the doped layer, we have produced diodes which work already in the strict absence of oxygen and are stable in air. To increase the efficiency of charge carrier generation in photovoltaic cells, we need to use photoactive donor–acceptor-heterojunctions. We present here first examples of pn- and pin-type heterojunctions combining p-doped and nominally undoped layers.     

Flexible polymer photovoltaic modules with incorporated organic bypass diodes to address module shading effects

We present experimental results on large-area low-cost processed flexible organic photovoltaic (OPV) modules incorporating organic bypass diodes to eliminate the negative effects of shading on the module power output. A fully organic-based structure (organic solar module combined with an organic bypass diode) is essential to allow monolithic interconnection of the bypass diode during the solar module production within the same printing steps. The origin of shading losses in organic photovoltaic modules is analyzed in detail, and guidelines for the design and architecture of flexible OPV modules are derived. Inorganic and organic diodes were tested on their functionality as bypass diodes, and a set of diode specifications to minimize shading losses is summarized. Organic bypass diodes were found to efficiently reduce the adverse shading effects in OPV modules.     

Photoluminescence enhancement and atmosphere-dependent photovoltaic performance in CdS nanorod arrays/MEH-PPV hybrid

Performance improvement of CdS nanorod arrays (NRA)/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) hybrid solar cells (HSCs) during the storage in ambient atmosphere has been demonstrated. An unusual photoluminescence (PL) enhancement was observed after MEH-PPV hybridizing with CdS NRA due to the abundance of S-vacancy-induced interface electron traps in CdS NRA, which was confirmed by temperature-dependent PL and charge transport analysis. By investigating on the time-resolved photoluminescence (TRPL) and current-voltage characteristics, we demonstrated that the performance improvement of CdS NRA/MEH-PPV HSCs was contributed to the co-effects of interface electron trap-oxygen interaction and MEH-PPV-oxygen interaction, and the contribution ratio between these two interactions was varied over time in air.