In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe(450 °C)) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe(450 °C) films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe(RT)/organic interface.

We demonstrate hybrid organic photovoltaic (HOPV) bilayer devices with very high open circuit voltages (VOC) of 1.18 V based on a sol–gel processed zinc oxide (ZnO) acceptor and a vacuum deposited boron subphthalocyanine chloride (SubPc) donor layer. X-ray photoelectron spectroscopy (XPS) and Kelvin Probe (KP) measurements of the ZnO/SubPc interface show that the ZnO preparation conditions have a significant impact on the film composition and the electronic properties of the interface, in particular the work function and interface gap energy. Low temperature processing at 120 °C resulted in a ZnO work function of 3.20 eV and the highest VOC of 1.18 V, a consequence of the increased interface gap energy.

•The conductivity of PEDOT:PSS electrodes was improved using dimethyl sulfoxide.•DMSO was added to the PEDOT:PSS solution or a PEDOT:PSS film was immersed in DMSO.•Large amount of PSS were removed from the surface of the immersed electrode.•Devices with immersed PEDOT:PSS electrodes out-performed the other DMSO treatment.•Demonstrates the potential of PEDOT:PSS electrodes as an alternative to ITO.Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSSADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSSIMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSSIMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSSIMM electrode out-performed the PEDOT:PSSADD electrode, primarily due to an increase in short circuit current density from 6.62 mA cm−2 to 7.15 mA cm−2. The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.Graphical abstract

In this paper, we discuss the use of the typical electron-donor (donor) material boron subphthalocyanine chloride (Cl-BsubPc) and a chlorinated derivative (hexachloro boron subphthalocyanine chloride, Cl–Cl6BsubPc) to act as electron-accepting (acceptor) materials and as replacements for C60, when coupled with tetracene and pentacene as the electron-donor materials in organic photovoltaics (OPVs). A large decrease in photocurrent was observed when C60 was replaced in the pentacene OPVs, although there was evidence of the harvesting of some triplets for the pentacene/Cl–Cl6BsubPc OPV. Large increases in Voc and stability were observed. Photoluminescence quenching, electron mobilities, and photovoltaic device characteristics are also presented and indicate the ambipolar quality of these small molecule organic semiconductors.

•Optical spacer layer used to optimise a tandem organic photovoltaic device.•α-NPD:MoOX used as a high transparency conductive optical spacer layer.•Optical spacer layer modified absorptance in front sub-cell balancing photocurrents.•An 80% improvement in power conversion efficiency was achieved using spacer layer.We report an improvement in power conversion efficiency in a small molecule tandem organic photovoltaic (OPV) device by the optimisation of current balancing of the sub-cells using an optical spacer layer. A co-deposited layer of N,N’-bis(1-naphthyl)-N,N′-diphenyl-1,1’-biphenyl-4,4’-diamine (α-NPD) and molybdenum oxide was used as the optical spacer layer and provided a highly transparent and conductive layer. Optical simulations showed the addition of the optical spacer in a boron subphthalocyanine (SubPc)/C60 based tandem OPV device increased the SubPc absorption in the front sub-cell and resulted in current balancing through the device. Fabricated tandem OPV devices showed similar trends, with the power conversion efficiency increasing from 2.3% to 4.2% with the addition of an optimised optical spacer thickness. External quantum efficiency and total absorption efficiency measurements back up the optical model data which attribute the increased performance to improved SubPc absorption in the front sub-cell, balancing the photocurrents of the two sub-cells.Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slide

We report a method of fabricating a high work function, solution processable vanadium oxide (V2Ox(sol)) hole-extracting layer. The atmospheric processing conditions of film preparation have a critical influence on the electronic structure and stoichiometry of the V2Ox(sol), with a direct impact on organic photovoltaic (OPV) cell performance. Combined Kelvin probe (KP) and ultraviolet photoemission spectroscopy (UPS) measurements reveal a high work function, n-type character for the thin films, analogous to previously reported thermally evaporated transition metal oxides. Additional states within the band gap of V2Ox(sol) are observed in the UPS spectra and are demonstrated using X-ray photoelectron spectroscopy (XPS) to be due to the substoichiometric nature of V2Ox(sol). The optimized V2Ox(sol) layer performance is compared directly to bare indium–tin oxide (ITO), poly(ethyleneoxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and thermally evaporated molybdenum oxide (MoOx) interfaces in both small molecule/fullerene and polymer/fullerene structures. OPV cells incorporating V2Ox(sol) are reported to achieve favorable initial cell performance and cell stability attributes.

We report that through the incorporation of polar self-assembled monolayers (SAMs) at the indium tin oxide (ITO)/donor interface in chloroaluminium phthalocyanine (ClAlPc)/C60 discrete heterojunction organic photovoltaic (OPV) cells, the power conversion efficiency can be dramatically increased. This enhanced performance is due to better alignment between the hole-extracting electrode Fermi level and the highest occupied molecular orbital (HOMO) of the ClAlPc donor, as well as an improved surface compatibility which provides a more optimised electrode/donor interface. Optimised cells demonstrate an increase of ∼85% in open circuit voltage which results in a near three-fold increase in power conversion efficiency from 1.3% to 3.3% under 1 sun illumination. Comparative studies are made using cells based on two other organic donor materials, copper phthalocyanine (CuPc) and boron subphthalocyanine (SubPc).

A substantial increase in device performance and operational stability in solution processed inverted bulk heterojunction (BHJ) organic photovoltaic devices (OPV) is demonstrated by introducing a zinc oxide (ZnO) interlayer between the electron collecting bottom electrode and the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). The structure and morphology of the dense, planar ZnO layers were controlled either by electro-deposition or spray pyrolysis techniques. Metal oxide sandwich OPV devices based on the photoactive blend on an electro-deposited ZnO interlayer with a (100) preferential crystal orientation, and using a tungsten oxide (WOx) interlayer on the opposite electrode, resulted in a remarkable increase in power conversion efficiency with a value of 4.91% under AM1.5 illumination and an external quantum efficiency of 74%. Electro-deposition of the ZnO at low temperature proved to be the most promising method for forming the ZnO interlayers, allowing the highest control of film structure and morphology, as well as leading to significantly improved device efficiency and stability.

Nanowire-array films of the molecular semiconductor, copper hexadecafluorophthalocyanine (F16CuPc), have been fabricated by templated growth on a pre-deposited planar perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) layer. The PTCDA template induces the F16CuPc molecules to adopt a new type of lying-down arrangement with the molecular plane oriented parallel to the substrate instead of the conventional standing-up configuration which occurs on weakly interacting substrates. The resulting nanowire-array films exhibit a structure and morphology that has significant potential for application in a wide range of organic electronic devices.

We present a detailed investigation of the molecular orientation transition and resulting morphology of copper hexadecafluorophthalocyanine (F16CuPc) thin films induced by solvent annealing. The F16CuPc molecules reorganize from small spherical or fibre-like crystals to large-size ribbon crystals which then dominate the resulting film properties. This reorganization and the formation of the ribbon crystals are closely related to the evaporation of solvent molecules and Ostwald ripening. The resulting thin films demonstrate morphological and structural characteristics with significant potential for application in high-performance organic electronic devices.

Organic photovoltaic (OPV) planar heterojunction devices based on an aqueous solution processed 3,4′,4′′,4′′′-copper(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (TSCuPc) donor layer and a fullerene (C60) acceptor have been fabricated demonstrating power conversion efficiencies of up to 0.32% and an open circuit voltage (VOC) of almost 0.6 V. The VOC is significantly larger than that produced by similar cells based on the commonly studied copper phthalocyanine (CuPc)/C60 heterojunction (VOC = 0.46 V). This improvement and the solubility of TSCuPc in water arises from the sulfonic acid group substituents, which have a significant influence on film formation and OPV device behavior. In contrast to other solution-processed molecular semiconductors, TSCuPc forms nanocrystals in solution resulting in dense crystal thin film networks on the substrate surface. This unusual thin film morphology was studied in detail using ultraviolet−visible absorption spectroscopy, X-ray diffraction, and atomic force microscopy. Combined with current density−voltage analysis under 1 sun solar illumination, a deeper understanding of the molecular arrangement of TSCuPc thin films is presented as well as its impact on the resulting device behavior.

We report a ∼60% increase in open circuit voltage (Voc) and power conversion efficiency in a chloroaluminium phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction photovoltaic device after insertion of a MoO3 hole-extracting layer at the interface between the indium tin oxide (ITO) electrode and the ClAlPc donor layer, with an associated improvement in device stability. A similar improvement was observed in heterojunction devices based on mixed ClAlPc/C60 layers. We propose that the improvements in device performance are due to the pinning of the ITO Fermi level to the valance band of the MoO3 interlayer, where the latter is closely aligned with the highest occupied molecular orbital of ClAlPc.

The operation of discrete heterojunction organic photovoltaic (OPV) cells employing chloro-aluminium phthalocyanine (ClAlPc) as the electron donor and C60 as the electron acceptor is reported and the characteristics are correlated with the energy level structure of the devices determined using X-ray photoelectron spectroscopy. The results give new insight into the origin of the open circuit voltage (Voc) in discrete heterojunction OPVs. The measured Voc in this system is found to be determined by: (i) the frontier orbital energy offsets between the donor and acceptor materials, accounting for the likely formation of an abrupt vacuum level shift at the heterojunction interface and (ii) the degree of alignment between the hole-extracting electrode Fermi level and the highest occupied molecular orbital energy of the electron donor material. The generality of the findings is demonstrated by rationalising the Voc in OPVs employing the archetypal electron donor, copper phthalocyanine.

We report a significant increase in the open circuit voltage (Voc) and power conversion efficiency in both chloroaluminium phthalocyanine (ClAlPc)/fullerene (C60) and boron subphthalocyanine chloride (SubPc)/C60 organic photovoltaic (OPV) cells with the insertion of a thin molybdenum oxide (MoOx) hole-extracting layer. This improvement was not seen with copper phthalocyanine (CuPc)/C60, and the addition of the MoOx layer leads to reduced device performance for pentacene/C60 cells. Cells containing the MoOx layer demonstrated significantly improved stability compared to the cells deposited on bare indium-tin oxide (ITO). External quantum efficiency (EQE) measurements taken before and after constant AM1.5G illumination for 60 min showed reduced current losses for all cells containing the MoOx layer, especially in spectral regions where the donor layer contributes. We attribute this improvement to the increased stability at the MoOx/donor interface.

We demonstrate that the molecular semiconductor, copper hexadecafluorophthalocyanine (F16CuPc), can be used as an electron accepting layer in bilayer small molecule organic photovoltaic (OPV) cells using boron subphthalocyanine chloride (SubPc) as the electron donor. The F16CuPc/SubPc cells show good overall performance and stability, with a power conversion efficiency of 0.56% obtained for optimised F16CuPc layer thickness and only 10% degradation in cell performance under constant illumination for 60 min under ambient conditions. External quantum efficiency measurements demonstrate that the photocurrent derives from excitons generated in both the SubPc and F16CuPc layers, although the latter makes a relatively small contribution. The results demonstrate that F16CuPc is a promising electron accepting material for small molecule OPV cells.

The structure−function relationship of organic photovoltaic (OPV) cells based on the chloroaluminum phthalocyanine (ClAlPc)/fullerene (C60) planar heterojunction are explored. Optimization is achieved with the use of a molybdenum oxide (MoOx) and an underlying 3,4,9,10-perylene tetracarboxylic acid (PTCDA) interlayer at the hole extracting electrode, the latter acting as a structural template for the subsequent growth of the ClAlPc donor layer. OPV cells demonstrate power conversion efficiencies of 3.0% under simulated AM1.5G (air mass 1.5 global) illumination, with the short-circuit current (Jsc) showing an ∼25% improvement relative to a device without a templating layer. Results from X-ray diffraction and electronic absorption spectroscopy suggest an improved packing and crystallinity in the ClAlPc layer when deposited on the PTCDA template, which suggests an enhancement in charge transport through the film. External quantum efficiency measurements confirm an overall improvement in Jsc in the ClAlPc spectral region upon templating. The effect of the MoOx interlayer is to minimize losses in the open-circuit voltage and fill factor caused by significant band bending and pinning of the adjacent organic layer highest occupied molecular orbital levels to nonstoichiometric defect states in the near Fermi level region of MoOx. The results present an improved strategy for the development of higher-performance OPV cells based on small molecule heterojunctions.

The thickness-dependent morphology and structure transitions of pristine and thermally annealed F16CuPc thin films deposited from the vapor phase by organic molecular beam deposition have been studied with use of atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Pristine films show clear morphology transitions with increasing thickness, changing from spherical-like crystals to flexible-fiber-like crystals via standing-up needle-like crystals. Two different roughening processes are identified: kinetic roughening for films <30 nm thick (scaling exponent, β ≈ 0.123) and rapid roughening for films ≥30 nm thick (β ≈ 3.089). Thicker pristine films are composed of two crystal layers: a spherical-crystal layer and a flexible-fiber-crystal layer. Controlled thermal annealing leads to further reorganization of the packing geometry and large crystals dominate the resulting film morphology. Mechanisms are proposed for the structure and morphology transitions.

Vertical co-deposition of sub-100 nm polystyrene sphere templates with water-soluble small molecule or polymeric semiconductors, followed by solvent vapour assisted sphere removal, is shown to be an excellent method for generating porous large area organicsemiconductorthin films with sub-100 nm open-cellular networks, with numerous potential applications in areas such as sensing and photovoltaics.

We demonstrate power conversion efficiencies of 1.5% from molecular photovoltaic devices based on bilayer pentacene/fullerene heterojunctions under 1 sun AM1.5G simulated irradiation. Importantly, we demonstrate the independence of the device performance on active area in the range 0.05–0.65 cm2, a critical consideration for future scaling up to manufacture. A degradation study under constant solar illumination shows two parallel mechanisms for degradation; a photooxidation resulting in a drop of the generated photocurrent, and a UV annealing effect reducing the fill factor, both of which can be eliminated by careful choice of analysis conditions.

Vertical co-deposition of sub-100 nm polystyrene sphere templates with water-soluble small molecule or polymeric semiconductors, followed by solvent vapour assisted sphere removal, is shown to be an excellent method for generating porous large area organicsemiconductorthin films with sub-100 nm open-cellular networks, with numerous potential applications in areas such as sensing and photovoltaics.

A substantial increase in device performance and operational stability in solution processed inverted bulk heterojunction (BHJ) organic photovoltaic devices (OPV) is demonstrated by introducing a zinc oxide (ZnO) interlayer between the electron collecting bottom electrode and the photoactive blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM). The structure and morphology of the dense, planar ZnO layers were controlled either by electro-deposition or spray pyrolysis techniques. Metal oxide sandwich OPV devices based on the photoactive blend on an electro-deposited ZnO interlayer with a (100) preferential crystal orientation, and using a tungsten oxide (WOx) interlayer on the opposite electrode, resulted in a remarkable increase in power conversion efficiency with a value of 4.91% under AM1.5 illumination and an external quantum efficiency of 74%. Electro-deposition of the ZnO at low temperature proved to be the most promising method for forming the ZnO interlayers, allowing the highest control of film structure and morphology, as well as leading to significantly improved device efficiency and stability.

We demonstrate hybrid organic photovoltaic (HOPV) bilayer devices with very high open circuit voltages (VOC) of 1.18 V based on a sol–gel processed zinc oxide (ZnO) acceptor and a vacuum deposited boron subphthalocyanine chloride (SubPc) donor layer. X-ray photoelectron spectroscopy (XPS) and Kelvin Probe (KP) measurements of the ZnO/SubPc interface show that the ZnO preparation conditions have a significant impact on the film composition and the electronic properties of the interface, in particular the work function and interface gap energy. Low temperature processing at 120 °C resulted in a ZnO work function of 3.20 eV and the highest VOC of 1.18 V, a consequence of the increased interface gap energy.

In this work, a thin ZnSe layer was deposited in a vacuum and then thermally annealed in air to provide an efficient electron extraction layer for an inverted organic photovoltaic (OPV) cell. Annealing the ZnSe film at 450 °C (ZnSe(450 °C)) increased the device performance and gave an efficiency of 2.83%. X-ray photoelectron spectroscopy (XPS) measurements show that the increased device performance upon annealing at 450 °C is due to the thermal conversion of ZnSe to ZnO. ZnO has a wider band gap than ZnSe, which allows for more light to reach the photoactive layer. The electronic structures of the treated ZnSe films were explored by ultraviolet photoemission spectroscopy (UPS) which showed that the ZnSe(450 °C) films had a Fermi level close to the conduction band edge, allowing for efficient electron extraction compared to the energetic barrier for extraction formed at the ZnSe(RT)/organic interface.

Nanowire-array films of the molecular semiconductor, copper hexadecafluorophthalocyanine (F16CuPc), have been fabricated by templated growth on a pre-deposited planar perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) layer. The PTCDA template induces the F16CuPc molecules to adopt a new type of lying-down arrangement with the molecular plane oriented parallel to the substrate instead of the conventional standing-up configuration which occurs on weakly interacting substrates. The resulting nanowire-array films exhibit a structure and morphology that has significant potential for application in a wide range of organic electronic devices.

The operation of discrete heterojunction organic photovoltaic (OPV) cells employing chloro-aluminium phthalocyanine (ClAlPc) as the electron donor and C60 as the electron acceptor is reported and the characteristics are correlated with the energy level structure of the devices determined using X-ray photoelectron spectroscopy. The results give new insight into the origin of the open circuit voltage (Voc) in discrete heterojunction OPVs. The measured Voc in this system is found to be determined by: (i) the frontier orbital energy offsets between the donor and acceptor materials, accounting for the likely formation of an abrupt vacuum level shift at the heterojunction interface and (ii) the degree of alignment between the hole-extracting electrode Fermi level and the highest occupied molecular orbital energy of the electron donor material. The generality of the findings is demonstrated by rationalising the Voc in OPVs employing the archetypal electron donor, copper phthalocyanine.