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.     

Interface modification to optimize charge separation in cyanine heterojunction photovoltaic devices

We investigate heterojunction photovoltaic devices using the carbocyanine 1,1′-diethyl-3,3,3′,3′-tetramethylcarbocyanine perchlorate (Cy5) as donor and buckminsterfullerene (C₆₀) as acceptor. We find that photocurrent generation occurs at the interface between CY5 and indium tin oxide (ITO) as well as at the organic heterointerface. By analyzing the spectral dependence of the photocurrent as a function of applied voltage, we were able to demonstrate that poly(3,4-ethylenedioxythiophene) (PEDOT) inhibits electron injection from the cyanine into ITO. Since the photocurrent generation at the ITO electrode is opposite to the one generated at the organic heterojuncion, the use of PEDOT leads to increased short-circuit current and open-circuit voltage.     

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.     

Surface passivation for germanium photovoltaic cells

Passivation of a germanium surface has proved to be challenging. Various materials have been examined for this purpose, like for example silicon nitride and amorphous silicon. In this work the optimisation of PECVD amorphous silicon and the influence of the preliminary surface treatment for passivation purposes are described. Furthermore, experiments done to extract the surface recombination velocity and the bulk lifetime of a germanium substrate will be presented. Optimisation of the deposition parameters, in combination with a pre-deposition in situ hydrogen plasma, results in a surface recombination velocity of 17 cm/s on a lowly doped germanium substrate.     

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.     

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.     

Integration of an M-phthalocyanine layer into solution-processed organic photovoltaic cells for improved spectral coverage

Photovoltaic devices made from blended poly(3,3?-didodecylquaterthiophene) (PQT-12) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) incorporating an additional interlayer of M-phthalocyanine (M-Pc) have been characterized using current-voltage response, UV-visible absorption and external quantum efficiency. The introduction of H₂Pc, CuPc, ClInPc or TiOPc layers improves device performance compared to conventional bulk-heterojunction PQT-12:PCBM cells without M-Pc. Devices with M-Pc show increased absorption and free charge generation at longer wavelengths and have higher open circuit voltage. Polymorphic changes from solvent interaction are observed in TiOPc films during fabrication. Power conversion efficiencies of 0.79% are achieved for this modified bulk-heterojunction solar cell.     

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.     

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.     

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

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

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.     

Organic photovoltaic devices with a crosslinkable polymer interlayer

Organic photovoltaic devices with a photo-crosslinkable interlayer were fabricated. This photo-crosslinkable interlayer acted as a leakage current reducing buffer layer. The performance of the small area OPV cell (0.04 cm²) was enhanced by the increase in the short circuit current and the fill factor. When a larger area cell (1 cm²) was used, the performance of OPV cell increased when the appropriate interlayer thickness was used. In the case of a 10 cm×10 cm module, the power conversion efficiency was about double than that without the interlayer. The insertion of the interlayer increased the current extraction by lowering the barrier height and attenuated the fill factor reduction by enhancing the rectification with a better leakage current sealing. From this study, it is clearly proved that the insertion of the appropriate photo-crosslinkable layer improves the performance of OPV devices, the effect was especially evident for large area cells.     

A novel approach for the modeling of advanced photovoltaic devices using the SILVACO/ATLAS virtual wafer fabrication tools

In this paper a new method for developing a realistic model of any type of solar cell is presented. Taking into account the high cost of research and experimentation involved with the development of advanced cells, we present here this novel methodology. In our opinion, the introduction of this modeling technique to the photovoltaic community will prove to be of great importance in aiding in the design and development of advanced solar cells. Two models of a single GaAs and an InGaP/GaAs/Ge multi-junction solar cell are prepared and are fully simulated. The major stages of the process are explained and the simulation results are compared to published experimental data to demonstrate the accurate results produced by the model utilizing this technique. The flexibility of the proposed methodology is illustrated and example results are shown throughout the whole process, demonstrating some of the different parameters effects on the model performance.     

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.     

Heteromorphic chloroindium phthalocyanine films for improved photovoltaic performance

A new structure incorporating multiple phases of chloroindium phthalocyanine (ClInPc) is fabricated and tested in photovoltaic devices. This so-called heteromorphic structure includes as-deposited and THF vapor treated ClInPc films to improve absorption and photovoltaic (PV) performance in devices. Absorption of the polymorphic phases of ClInPc are complementary and lead to improved current generation. Short circuit current is improved by over 70% using the heteromorphic structure, while power conversion efficiency (PCE) improves by more than 40% versus solely as-deposited devices. Advantages of the heteromorphic structure include broader spectral response, improved interfacial contact area and an intermediary open circuit voltage (V_oc).     

Surface passivation in high efficiency silicon solar cells

Surface passivation for crystalline silicon solar cells is particularly important for devices with open-circuit voltages in excess of 650 mV. Thick passivating thermal oxides, originally developed for use with buried contact solar cells, are shown to produce the most effective and stable surface passivation particularly in conjunction with lightly phosphorus diffused surfaces. However, for improved optical performance, antireflection coatings are only effective with surface oxide thicknesses reduced to 100–200 Å. Thinner passivating oxides cause significant voltage loss, most of which can be recovered through hydrogen passivation. Throughout this study, variation in surface passivation approaches has produced open-circuit voltages ranging from 620 mV to record voltages of 720 mV.     

Synthesis and photovoltaic properties of a star-shaped molecule with triphenylamine as core and benzo[1,2,5]thiadiazol vinylene as arms

A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D-A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D-A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.     

Analytical study of PPV-oligomer- and C60-based devices for optimising organic solar cells

A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C₆₀, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I–V characteristics of several other devices were investigated. It is found that the presence of C₆₀ in a bulk heterojunction solar cell introduces fill factor reducing shunting paths.     

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.     

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.