Thin-film Organic Photovoltaic Devices Research Articles

Light intensity dependence of the photocurrent in organic photovoltaic devices

The competition between recombination and extraction of carriers defines the charge collection efficiency and, therefore, the overall performance of organic photovoltaic devices, including solar cells and photodetectors. In this work, we describe different components of the steady-state light intensity-dependent photocurrent (IPC) and charge collection efficiency under operational conditions. Further, we demonstrate how different loss mechanisms can be identified based on their unique signatures in the IPC. In particular, we show how IPC can be used to distinguish first-order, trap-assisted recombination from other first-order photocurrent loss mechanisms, which dominate at the low-intensity characteristic of indoor light-harvesting applications. The theoretical framework is presented and verified by a one-dimensional drift-diffusion device model. Finally, the extended IPC methodology is validated on organic thin-film photovoltaic devices. We conclude that the relatively straightforward measurement of IPC over a large dynamic range can be a powerful tool for understanding solar and indoor device fundamentals. • IPC is a tool for understanding photovoltaic device fundamentals • Photocurrent loss mechanisms are identified based on their signatures in IPC • The theoretical framework is verified with a drift-diffusion device model • Experimental demonstration of IPC method on state-of-the-art organic solar cells Zeiske et al. present a combined theoretical and experimental study of intensity-dependent photocurrent (IPC), a tool for understanding solar and indoor device fundamentals, to identify different photovoltaic device performance-limiting photocurrent loss mechanisms based on their unique signatures in IPC.

Low-bandgap push–pull molecules in polymer matrices for use in thin-film organic photovoltaic devices

Conjugated polymers are widely used in thin-film organic photovoltaic devices to absorb light and serve as electron donors or acceptors. Small molecular analogues are attractive substitutes because they have fully defined structures, can be purified rigorously, and are typically more soluble and volatile. However, producing active films composed primarily of small molecules remains challenging. We have devised bulk heterojunction solar cells in which poly(3-hexylthiophene-2,5-diyl) and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] are used as matrices to prepare films containing low-bandgap push–pull molecules as electron donors and (6,6)-phenyl-C61-butyric acid methyl ester or (6,6)-phenyl-C71-butyric acid methyl ester as electron acceptors. Compared with reference devices devoid of push–pull molecular additives, increases in power conversion efficiencies up to 30.4% were measured.

Morphology and electrical characteristics of polymer: Fullerene films deposited by electrospray

Electrospray (ES) has emerged as an attractive approach to fabricate thin-film organic photovoltaic devices. However, the morphological properties and their implication to electrical characteristics of polymer:fullerene blends deposited by this method have not been clearly understood yet. In order to uncover the interplay of nanoscale morphology and electrical properties of ES-deposited films, both the residues of single droplets and thin films from the overlapping of multiple droplets were investigated by various characterization techniques. The surface morphology was demonstrated in details by the optical microscope (OM) and atom force microscope (AFM). The ordering structures and aggregation of the polymer were identified by the Raman and UV–vis spectroscopic investigations. The spatially distribution map of the P3HT aggregation states was analyzed by fitting the C˭C mode of P3HT with Lorentzian functions. The conductive atomic force microscopy (C-AFM) was used to quantify local currents and reveal the correlation between the nanostructure and charge-transport mobility. Both surface morphology and aggregation of the P3HT were found to strongly depend on the overlapping boundaries formed by the dry residues of individual droplets. More P3HT aggregation present at the boundaries. However, boundaries also show significantly higher charge-transport resistance thus lower current. Thin films deposited with less droplet evaporation exhibit more homogenous morphology, more uniform phase segregation, and consequently higher charge mobility.

Recycling indium tin oxide (ITO) anodes for use in organic light-emitting diodes (OLEDs)

Electrodes in many thin-film optoelectronic devices are made of layers of indium tin oxide (ITO), which are suitably conductive, can be deposited by established methods, and are nearly transparent and colorless. Regrettably, indium is an uncommon element, its price continues to rise steeply, and producing ITO electrodes consumes a large fraction of the total energy needed to make thin-layer devices. Nevertheless, ITO electrodes are widely used in devices, both in research and industrial production. The growing commercial importance of these devices creates an urgent need for ways to ensure sustainability, such as by recovering ITO from devices no longer needed or even by recycling intact ITO electrodes. Recent work has shown that patterned ITO electrodes can be recovered from organic thin-film photovoltaic devices (OPVs) after simple exposure to water or dilute aqueous base, and then reused in new devices with essentially no decrease in performance. We now report that similar approaches can be used to recover and reuse ITO electrodes in organic light-emitting diodes (OLEDs), which are currently manufactured in much greater numbers than OPVs. Use of atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to study the surfaces of ITO electrodes recovered from OLEDs revealed minor changes in morphology, but the performance of the recycled electrodes in new OLEDs proved to be virtually unchanged, even after multiple cycles of recovery and reuse.

Thin-film photovoltaic devices incorporating low-bandgap push-pull molecules dispersed in passive polymeric matrices

Active layers in many thin-film organic photovoltaic devices (OPVs) contain light-absorbing polymers that serve as electron donors, mixed with appropriate electron acceptors. In principle, the polymers can be replaced by small molecules with suitable bandgaps, which offer multiple advantages, including well-defined structures and methods of synthesis and purification that provide uniform samples. However, such materials often undergo separation of phases and crystallization, so making long-lived films that remain smooth, homogeneous, flexible, and transparent is not easy. We have found that effective OPVs can be made by dispersing mixtures of low-bandgap push–pull small molecules as electron donors and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptor in matrices of optoelectronically passive conventional polymers, including polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(ethylene glycol), and poly(dimethylsiloxane). By varying the identity of the matrix, its molecular weight, the loading of active components, and the conditions of annealing, we have produced efficient OPVs from components that would otherwise have undergone phase separation and crystallization, leading to poor performance. Layers with up to 35% matrix were found to be effective and could be fabricated at room temperature by simple processes. To probe the role of the polymers as dispersants, morphologies of composite films were examined by atomic force microscopy and electron microscopy. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017

(Invited) Understanding the Energy Band Alignment in Inverted Organic Thin-Film Photovoltaic Devices

The energy band alignment in thin-film and low-dimensional devices is extremely important. However, experimental determination or theoretical prediction of the energy level alignment has been difficult to realize, especially under typical operating conditions. Following a recent breakthrough in our group, where SKPM technique was employed to image the vacuum level profile across the depth direction of an operando organic PV device with a conventional stacking structure, here we present the vacuum level alignment measurement on an inverted structure OPV device. Previously, there had been conflicting reports on the built-in field and the device operation polarity. In this talk, we will present a comprehensive understanding of the energy level alignment in inverted structure OPVs, and how the measured parameters can be adopted in numerical modeling of the device physics.

Evaluation of the charge transfer efficiency of organic thin-film photovoltaic devices fabricated using a photoprecursor approach.

Recently, a unique 'photoprecursor approach' was reported as a new option to fabricate a p-i-n triple-layer organic photovoltaic device (OPV) through solution processes. By fabricating the p-i-n architecture using two kinds of photoprecursors and a [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the donor and the acceptor, the p-i-n OPVs afforded a higher photovoltaic efficiency than the corresponding p-n devices and i-devices, while the photovoltaic efficiency of p-i-n OPVs depended on the photoprecursors. In this work, the charge transfer efficiency of the i-devices composed of the photoprecursors and PC71BM was investigated using high-sensitivity fluorescence microspectroscopy combined with a time-correlated single photon counting technique to elucidate the photovoltaic efficiency depending on the photoprecursors and the effects of the p-i-n architecture. The spatially resolved fluorescence images and fluorescence lifetime measurements clearly indicated that the compatibility of the photoprecursors with PC71BM influences the charge transfer and the photovoltaic efficiencies. Although the charge transfer efficiency of the i-device was quite high, the photovoltaic efficiency of the i-device was much lower than that of the p-i-n device. These results imply that the carrier generation and carrier transportation efficiencies can be increased by fabricating the p-i-n architecture.

Maximal light-energy transfer through a dielectric/metal-layered electrode on a photoactive device

We report the fabrication of an optimized low reflective dielectric/metal-layered electrode that provides significant electrical conductivity and light transparency in the near-infrared wavelength regime. By making the metal film thickness thick enough and choosing a proper dielectric layer with a certain thickness, we show that our suggested electrode significantly reduces the light reflection while preserving high electrical conductivity. We demonstrate our optimized electrodes present a highly conductive surface with a sheet resistance of 5.2 Ω/sq and a high light transmittance of near 85% in the near-infrared regime. We also apply our optimized electrode to thin-film organic photovoltaic devices and show the electrode helps in absorbing light energy inside an active layer. We believe that this simple but powerful layered electrode will pave the way for designing transparent electrodes on photoactive devices.

Properties of polymethyl methacrylate-based nanocomposites: Reinforced with ultra-long chitin nanofiber extracted from crab shells

Ultra-long chitin nanofibers were incorporated into polymethyl methacrylate (PMMA) resin to prepared PMMA/Chitin nanocomposites with improved properties. Transmission electron microscopy (TEM) images showed that through the introduced 4-step all-mechanical treatment, the average aspect ratio of the obtained chitin fiber was up to 1000 with the length at dozens of micron range. Due to the laminated structure formed by “layer-by-layer” effect, tensile strength and Young’s modulus of the prepared composite were significantly enhanced after the filling of chitin nanofibers, as compared with neat PMMA. Light transmittance test indicated that increasing the fiber content causes little light scattering because the nano-scalar network which is smaller enough than the visible wavelength could well preserve the original transparency of PMMA. Furthermore, chitin nanofiber film with extremely low thermal expansion improved the thermal stability of PMMA in a great degree. This could lead to various commercial applications including flexible electronic printing, organic thin-film photovoltaic devices, and is a significantly environmental move towards the sustainable utilization of marine-river crab shell wastes.

Wide-Angle and Polarization-Insensitive Perfect Absorber for Organic Photovoltaic Layers

In this letter, we computationally explore a wide-angle and polarization-insensitive perfect absorber based on hybrid metal-dielectric-metal structures. By introducing an organic photovoltaic layer between top metallic nanopatterns and a continuous metal bottom plate, an enhanced angle-and polarization-insensitive absorption can be obtained in the spectral range 400–700 nm, which is promising to realize improved thin-film organic photovoltaic devices. The physical mechanism of the perfect absorber is explained theoretically and numerically by the critical coupling principle.

ChemInform Abstract: Development of Fullerene Derivatives with High LUMO Level Through Changes in π‐Conjugated System Shape

AbstractReview: 15 refs.

Design Concept for High-LUMO-level Fullerene Electron-acceptors for Organic Solar Cells

Abstract This review article describes design concept, synthesis, and features of fullerene derivatives having high lowest unoccupied molecular orbital (LUMO) levels to achieve high open-circuit voltage in organic thin-film photovoltaic devices. Installation of organic electron-donating groups onto fullerene and decrease of the size of the fullerene π-electron-conjugated system raise the LUMO levels, affording high-performance organic solar cells. Addition of the methano group as the smallest carbon addend to fullerene to obtain 56π-electron fullerene derivatives is likely a promising strategy for this purpose.

Development of fullerene derivatives with high LUMO level through changes in π-conjugated system shape

This article describes a concept for designing fullerene-based electron-accepting materials to obtain high open-circuit voltage (V OC) in organic thin-film photovoltaic devices without an accompanying decrease in short-circuit current density. The keys to this concept are (1) reducing the size of the fullerene π-conjugated system to realize high V OC and (2) shortening the inter-fullerene distance in the solid-state packing structure to achieve high short-circuit current density (J SC), which is made possible by well-designed supramolecular organization or a small organic addend. In this article, two representative examples are discussed. One is 1,4-bis(silylmethyl)[60]fullerene (SIMEF), which forms a columnar fullerene-core array for high electron mobility and undergoes thermal crystallization for good phase separation with the electron-donating material. The other is a 56π-electron fullerene derivative bearing a dihydromethano group, the smallest carbon addend, which does not disrupt fullerene–fullerene contact in the solid state.

Investigation on Schottky contacts in organic thin film photovoltaic devices by transient photocurrent

The organic thin film photovoltaic device with a structure of tin indium oxide (ITO)/organic semiconductor/metal is fabricated. A rectifying behavior of the device is observed from the current-voltage characteristics. However, it is hard to judge the direction of internal electric field between the organic semiconductor and electrodes under Schottky contacts. In order to investigate the characteristics of Schottky contacts between the organic semiconductor and electrodes, the devices with the structures of ITO/organic insulator layer/organic semiconductor/metal and ITO/organic semiconductor/organic insulator layer/metal are fabricated. It is easy to judge the direction of internal electric field between the organic semiconductor and electrodes under Schottky contacts by the direction of transient photocurrent which is produced under the irradiation from the modulated laser. The correctness of judgement is further proved by the change of transient photocurrent intensity with a bias voltage applied.

Development of New Fullerene-based Electron Acceptors for Efficient Organic Photovoltaic Cells

ABSTRACTThis article describes design of fullerene-based electron-accepting materials to obtain high performance in organic thin-film photovoltaic devices. A 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene gives higher open circuit voltage than 1,2-diadduct because of smaller π-conjugated systems, and enables columnar fullerene-core array for high electron mobility and thermal crystallization for ideal phase separation with electron-donor materials. A 56π-electron fullerene derivative possessing the dihydromethano group as the smallest carbon addend does not disrupt fullerene-fullerene contact in solid state, giving high open-circuit voltage without decreasing of short-circuit current density and fill factor.

Tetracene Dicarboxylic Imide and Its Disulfide: Synthesis of Ambipolar Organic Semiconductors for Organic Photovoltaic Cells

We have designed and synthesized a new donor/acceptor-type tetracene derivative by the introduction of dicarboxylic imide and disulfide groups as electron-withdrawing and -donating units, respectively. The prepared compounds, tetracene dicarboxylic imide (TI) and its disulfide (TIDS) have high chemical and electrochemical stability as well as long-wavelength absorptions of up to 886 nm in the thin films. The crystal packing structure of TIDS molecules features face-to-face π-stacking, derived from dipole-dipole interactions. Notably, TIDS exhibited ambipolar properties of both electron-donating and -accepting natures in p-n and p-i-n heterojunction organic thin-film photovoltaic devices. Accordingly, TI and TIDS are expected to be promising compounds for designing new organic semiconductors.

Electrode Considerations for the Optical Enhancement of Organic Bulk Heterojunction Solar Cells

AbstractIn this paper, we consider the optical effects in conventional and inverted bulk heterojunction organic solar cells associated with various electrodes, and perform a systematic study on the anode and cathode buffer layers commonly used in high performance devices. In the devices produced here, we determine that parasitic absorption by low work function metals such as calcium reduces photocurrent by 25%, and that parasitic reflection at interfaces between the transparent conducting oxide and metal oxide buffer layers can reduce photocurrent by more than 10%. We also quantify the impact of an optical spacer, and determine that this accounts for only a fraction of the improvement that can be gained through alternative electrode optimization routes. It is therefore our intention that this study serves as a guideline for the optimization of the electrodes of organic thin film photovoltaic devices.

Organic Thin Film Photovoltaic Devices from Discotic Materials

Since the first demonstration of organic photovoltaic devices much progress has been made. Organic solar cells reach now power conversion efficiencies of up to 3% over the solar spectrum. The morphology of the active film is very important for efficient devices. Films spin-coated from blend solutions phase separate. The scale of the phase separation depends on the solvent, solubility of the materials and parameters of the spin-coating process such as speed, temperature, etc. If the morphology could be controlled on a molecular scale the efficiency of charge separation and transport could be expected to be substantially higher. The use of discotic liquid crystalline materials might help to reach this goal, because of their capacity to self-organise into columnar stacks. In this work we describe photovoltaic devices made with discotic liquid crystalline hexabenzocoronene and perylene dye molecules. Thin films have been produced by spin coating blends directly from solution. Devices with an external quantum efficiency (incident photon to current efficiency) of up to 34% at monochromatic illumination at 490 nm have been achieved with a blend of hexaphenyl-substituted hexabenzocoronene (HBC-PhC 12 ) and a perylene diimide. Photovoltaic devices with other hexabenzocoronene derivatives as hole conductor show lower efficiencies. We attribute the lower device performance of the latter to the different film morphology occurring from spin coating of these materials. The aim of this work is to exploit the advantageous self-organising properties of HBC-perylene blends for solar cells.