Heterojunction Organic Photovoltaic Devices Research Articles

Multiscale simulation of organic heterojunction light harvesting devices

Purpose – The purpose of this paper is to develop a computational model for the simulation of heterojunction organic photovoltaic devices with a specific application to a light harvesting capacitor (LHC) consisting of a double layer of organic materials connected in series with two insulating layers and an external resistive load. Design/methodology/approach – The model is based on a coupled system of nonlinear partial and ordinary differential equations describing current flow throughout the external resistive load as the result of exciton generation in the bulk, exciton dissociation into bonded pairs at the acceptor-donor material interface, and electron/hole charge generation and drift-diffusion transport in the two device materials. Findings – Numerical simulation results are shown to be in good agreement with measured on-off transient currents and allow for novel insight on the microscopical phenomena which affect the external LHC performance, in particular, the widely different time scales at which such phenomena occur and their relation to the overall device dynamics. Originality/value – The LHC demonstrates the viability of a novel approach for converting light energy into an electric current without a steady state flow of free charge carriers through the semiconducting layers. The new insight about the microscopic working principles that determine the macroscopically observed behavior of the LHC obtained via the model proposed in this paper are expected to serve as a basis for studying techniques for exploiting the full potential of the LHC.

Acceptor blending ratio dependence of bulk heterojunction organic photovoltaic devices

Bulk heterojunction (BHJ) organic photovoltaic (OPV) devices are generally composed of a blend film of a π-conjugated polymeric electron donor and a C60 derivative electron acceptor intercalated between an anode and a cathode, typical of a two-dimensional device structure. Often, a regioregular poly(3-hexylthiophene) (rr-P3HT) and indene-C60 bisadduct (ICBA) blend film is used as the BHJ active layer instead of the conventional P3HT and phenyl-C61-butyric acid methyl ester (PCBM) blend film due to the ICBA device’s higher power conversion efficiency (PCE), which is correlated with its higher open-circuit voltage (VOC). To determine the quantitative influences of those electron acceptors in the BHJ OPV devices, we prepared various devices of acceptors, either PCBM or ICBA, with a wide range of blending ratios (from 0 to 100%) for estimating their typical optoelectronic properties, e.g., UV-visible absorption spectra, photoluminescence intensity, surface morphology, and photovoltaic device parameters. Obtained data were compared to each other and were analyzed as a function of their acceptor blending ratio.

Universal Formation of Compositionally Graded Bulk Heterojunction for Efficiency Enhancement in Organic Photovoltaics

A universal method is reported to form graded bulk heterojunction (BHJ) organic photovoltaic devices (OPVs) by a simple solvent-fluxing process. Donors are enriched at the anode and acceptors are enriched at cathode side, matching the gradient electron and hole current across the film. Efficiency enhancements by 15-50% are achieved for all BHJ systems tested compared with the optimized regular BHJ OPVs.

Solution processed nickel oxide anodes for organic photovoltaic devices

Nickel oxide thin films have been prepared from a nickel acetylacetonate (Ni(acac)) precursor for use in bulk heterojunction organic photovoltaic devices. The conversion of Ni(acac) to NiOx has been investigated. Oxygen plasma treatment of the NiO layer after annealing at 400 °C affords solar cell efficiencies of 5.2%. Photoelectron spectroscopy shows that high temperature annealing converts the Ni(acac) to a reduced form of nickel oxide. Additional oxygen plasma treatment further oxidizes the surface layers and deepens the NiO work function from 4.7 eV for the annealed film, to 5.0 eV allowing for efficient hole extraction at the organic interface.

Improving the performance of subphthalocyanine/C60 planar heterojunction organic photovoltaic device through the insertion of molybdenum oxide anodic buffer

Here, an efficient subphthalocyanine (SubPc)/C60 heterojunction organic photovoltaic device is demonstrated by using MoO3 as the anodic buffer. In comparison with the device without any treatments, the insertion of MoO3 leads to a significant increase in open-circuit voltage due to a better energy level alignment of the SubPc, which is similar to the use of oxygen-plasma. In addition, MoO3 serves as an optical spacer to tune the SubPc/C60 interface at the optimum optical field distribution. As a result, the short-circuit current density is considerably improved as predicted using the simulation model based on the transfer matrix. A slightly increased fill factor implies the efficient hole extraction after the insertion of MoO3. Moreover, the device with MoO3 as anodic buffer shows an elongated lifetime as compared with the device with oxygen-plasma treatment.

Determination of Fullerene Scattering Length Density: A Critical Parameter for Understanding the Fullerene Distribution in Bulk Heterojunction Organic Photovoltaic Devices

Fullerene derivatives are commonly used as electron acceptors in combination with (macro)molecular electron donors in bulk heterojunction (BHJ) organic photovoltaic (OPV) devices. Understanding the BHJ structure at different electron donor/acceptor ratios is critical to the continued improvement and development of OPVs. The high neutron scattering length densities (SLDs) of the fullerenes provide effective contrast for probing the distribution of the fullerene within the blend in a nondestructive way. However, recent neutron scattering studies on BHJ films have reported a wide range of SLDs ((3.6-4.4) × 10(-6) Å(-2)) for the fullerenes 60-PCBM and 70-PCBM, leading to differing interpretations of their distribution in thin films. In this article, we describe an approach for determining more precisely the scattering length densities of the fullerenes within a polymer matrix in order to accurately quantify their distribution within the active layers of OPV devices by neutron scattering techniques.

Bimolecular Recombination in Organic Photovoltaics

The recombination of electrons and holes is a major loss mechanism in photovoltaic devices that controls their performance. We review scientific literature on bimolecular recombination (BR) in bulk heterojunction organic photovoltaic devices to bring forward existing ideas on the origin and nature of BR and highlight both experimental and theoretical work done to quantify its extent. For these systems, Langevin theory fails to explain BR, and recombination dynamics turns out to be dependent on mobility, temperature, electric field, charge carrier concentration, and trapped charges. Relationships among the photocurrent, open-circuit voltage, fill factor, and morphology are discussed. Finally, we highlight the recent emergence of a molecular-level picture of recombination, taking into account the spin and delocalization of charges. Together with the macroscopic picture of recombination, these new insights allow for a comprehensive understanding of BR and provide design principles for future materials and devices.

Consequences of hydrogen bonding on molecular organization and charge transport in molecular organic photovoltaic materials

Reported is a systematic molecular structure–property relationship study to evaluate the consequences of dedicated H-bonding interactions between molecular electron donors on molecular assembly, absorption, charge collection, and performance in small-molecule bulk heterojunction organic photovoltaic devices. Three families of branched quaterthiophene donor chromophores have been synthesized with members that share nearly identical electronic and optical properties in the molecularly dispersed state but are either capable or incapable of self-association by hydrogen bonding (H-bonding). Phthalhydrazide-functionalized quaterthiophenes are H-bond “active” and show signatures of H-bond promoted assembly in solution (by 1H NMR) and in both neat and blended (with C60) films (by IR). Compared to control compounds with H-bonding “turned off”, the H-bonded derivatives show red-shifted thin film absorption (neat and as blends with C60), different colors as bulk solids, and increased decomposition and melt temperatures. Photovoltaic devices made from blends of H-bonded donor molecules with C60 as the electron acceptor show improved charge collection length and external quantum efficiency resulting in a more than two-fold enhancement in power conversion efficiency relative to non-H-bonding controls, from 0.49% to 1.04%. We anticipate this approach could be generalized to include other donor chromophores with lower optical gap to harvest more longer-wavelength photons and achieve higher power conversion efficiencies.

Origin of the Mixing Ratio Dependence of Power Conversion Efficiency in Bulk Heterojunction Organic Solar Cells with Low Donor Concentration

We studied the origin of the improvement in device performance of thermally evaporated bulk heterojunction organic photovoltaic devices (OPVs) with low donor concentration. Samples with three different donor-acceptor mixing ratios, 0:10 (C70-only), 1:9 (low-doped) and 3:7 (high-doped), were fabricated with 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC):C70. The power conversion efficiencies (PCEs) of these samples were 1.14%, 2.74% and 0.69%, respectively. To determine why the low-doped device showed a high PCE, we measured various properties of the devices in terms of the effective energy band gap, activation energy, charge carrier mobility and recombination loss. We found that the activation energy for charge carrier transport was increased as we increased the TAPC concentration in the blends whereas the hole and electron mobilities became more balanced as the TAPC concentration was increased. Furthermore, the recombination loss parameter alpha (from the light intensity dependence) remained alpha to approximately 0.9 in the low-doped device, but it decreased to alpha to approximately 0.77 in the high-doped device, indicating a large recombination loss as a result of space charge. Therefore, the improved PCE of low-doped OPVs can be attributed to the balance between carrier mobilities with no increase in recombination loss.

Phosphole‐ and Benzodithiophene‐Based Copolymers: Synthesis and Application in Organic Photovoltaics

AbstractTwo kinds of phosphole‐ and benzodithiophene‐based copolymers bearing PV=O or PV=NSO2C8H17 functions were prepared by Pd–CuI‐promoted Stille coupling reactions, and their optical and electrochemical properties were investigated. Bulk heterojunction organic photovoltaic devices comprised of the new P,S‐bridged copolymers and (6,6)‐phenyl‐C71‐butyric acid methyl ester (PC71BM) exhibited power conversion efficiencies of up to 0.65 % under AM1.5 irradiation at 100 mW cm–2.

Improvement in the open-circuit voltage of an organic photovoltaic device through selection of a suitable and low-lying highest occupied molecular orbital for the electron donor layer

Abstract

Empirically based device modeling of bulk heterojunction organic photovoltaics

We develop an empirically based optoelectronic model to accurately simulate the photocurrent in organic photovoltaic (OPV) devices with novel materials including bulk heterojunction OPV devices based on a new low band gap dithienothiophene-DPP donor polymer, P(TBT-DPP), blended with PC70BM at various donor-acceptor weight ratios and solvent compositions. Our devices exhibit power conversion efficiencies ranging from 1.8% to 4.7% at AM 1.5G. Electron and hole mobilities are determined using space-charge limited current measurements. Bimolecular recombination coefficients are both analytically calculated using slowest-carrier limited Langevin recombination and measured using an electro-optical pump-probe technique. Exciton quenching efficiencies in the donor and acceptor domains are determined from photoluminescence spectroscopy. In addition, dielectric and optical constants are experimentally determined. The photocurrent and its bias-dependence that we simulate using the optoelectronic model we develop, which takes into account these physically measured parameters, shows less than 7% error with respect to the experimental photocurrent (when both experimentally and semi-analytically determined recombination coefficient is used). Free carrier generation and recombination rates of the photocurrent are modeled as a function of the position in the active layer at various applied biases. These results show that while free carrier generation is maximized in the center of the device, free carrier recombination is most dominant near the electrodes even in high performance devices. Such knowledge of carrier activity is essential for the optimization of the active layer by enhancing light trapping and minimizing recombination. Our simulation program is intended to be freely distributed for use in laboratories fabricating OPV devices.

Nanoscale phase domain structure and associated device performance of organic solar cells based on a diketopyrrolopyrrole polymer

We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform : dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor–acceptor interfacial area and resulting greater capacity for charge carrier generation.

Influence of gradient doping on photoelectric conversion efficiency of organic photovoltaic devices

On the basis of the traditional bulk heterojunction structure, we introduce a method of gradient doping in the mixing layer to improve photoelectric conversion efficiency (ηp) of organic photovoltaic device (OPV). One of the devices with the structure of ITO/CuPc (10 nm)/CuPc:C60 (2:1) (4 nm)/CuPc:C60 (1.5:1) (4 nm)/CuPc:C60 (1:1) (4 nm)/CuPc:C60 (1:1.5) (4 nm)/CuPc:C60 (1:2) (4 nm)/C60 (30 nm)/Bphen (8 nm)/Al (100 nm) show the improvement on performance:short-circuit current JSC = 9.18 mA/cm2, photoelectric conversion efficiency ηp = 1.35% under AM1.5 solar illumination. Compared with ηp of the traditional bulk heterojunction OPV ITO/CuPc(10 nm)/CuPc:C60 (1:1) (20 nm)/C60(30 nm)/Bphen(8 nm)/Al(100 nm), the ηp of the present structure is improved by 25%, which is attributed to the improvement on percolating paths of donor and acceptor molecules across the entire mixing layer, thus increasing the charge collection efficiency. Consequently, the overall device series resistance is reduced and the photoelectric conversion efficiency is enhanced.

Efficient Organic Photovoltaic Device Using a Sublimated Subphthalocyanine as an Electron Donor

In this letter, an efficient subphthalocyanine (SubPc)/C60 heterojunction organic photovoltaic device was demonstrated. The power conversion efficiency was improved from 2.7% to 4.3% by the sublimation of SubPc. The atomic force microscopic images showed that the crystallinity was improved with the sublimation of the SubPc, leading to the significantly enhanced hole mobility of SubPc in orders of magnitude as estimated by the space-charge limited current. The external quantum efficiency spectra further confirmed the improved charge collection efficiency as a result of the increase in the hole mobility of SubPc due to the unchanged spectral shape over the entire spectrum.

Effect of a calcium cathode on water-based nanoparticulate solar cells

Water-based nanoparticulate (NP) and bulk heterojunction (BHJ) organic photovoltaic (OPV) devices based on blends of poly(9,9-dioctylfluorene-co-N,N-bis(4-butylphenyl)-N,Ndiphenyl-1,4-phenylenediamine) (PFB) and poly(9,9-dioctylfluorene-co-benzothiadiazole (F8BT) have been fabricated with aluminium and calcium/aluminium cathodes. The NP devices exhibit power conversion efficiencies (PCEs) that are double that of the corresponding BHJ device. Moreover, the addition of calcium into the cathode structure results in a dramatic increase in open circuit voltage and PCEs approaching 1% for water-based polyfluorene OPV devices.

Realization of solution processed multi-layer bulk heterojunction organic solar cells by electro-spray deposition

Electro-spray deposition (ESD) was applied to fabricate solution processed donor–acceptor bulk heterojunction organic photovoltaic devices with multi-layer structure. Solvent effect was observed when using different organic solvents. Power conversion efficiency (PCE) of the devices prepared from dichlorobenzene increased dramatically comparing to the ones from chloroform, owing to improved homogeneity of the films. ESD enabled us to fabricate solution processed multi-layer (donor/donor:acceptor/acceptor) devices with simple successive deposition steps. Energy Dispersive X-ray Reflectometry analysis confirmed distinct three layered structure of the active layers. Solar cell device parameters of the trilayer devices were compared to single layer devices and those of spin coated devices with the same donor:acceptor ratio and film thickness. Post-thermal treatment results showed that after annealing at 125°C, trilayer devices exhibited best performance with the maximum PCE of 2.17%.

CuI interlayers in lead phthalocyanine thin films enhance near-infrared light absorption

The insertion of a CuI interlayer improved the preferential alignment of lead phthalocyanine (PbPc) molecules in the monoclinic phase, as demonstrated by x-ray diffraction analysis of ultra-thin (5 nm) and thin (30 nm) PbPc films. CuI triclinic phases were not observed. The improved preferential alignment and crystallinity in the monoclinic phase enhanced near-infrared photon absorption. The power conversion efficiency of a PbPc/C60 planar heterojunction organic photovoltaic device fabricated using the CuI templating layer structure could be improved from 1.3% to 2.5%.

Improvement in power conversion efficiency by blending of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) into poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester active layer

The improvement in power conversion efficiency (PCE) by the addition of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) into regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM)-based bulk heterojunction (BHJ) organic photovoltaic (OPV) devices was investigated. The addition of PEDOT:PSS into the P3HT:PCBM active layer was found to promote a significant enhancement of the fill factor, which ultimately increased the PCEs of the corresponding OPV devices, although the exciton generation rates of the BHJ layers were virtually the same, as observed by UV-Vis absorption spectra. Therefore, we conclude that the reduction of internal series resistance (RS) was the most crucial reason for the PCE improvement by the addition of PEDOT:PSS.

Organic bulk heterojunction photovoltaic devices with surfactant-free Au nanoparticles embedded in the active layer

The effect of embedding uncapped Au nanoparticles, produced by ultrafast laser ablation in liquids, in the active layer of bulk heterojunction organic photovoltaic devices was systematically studied. The nanoparticles incorporation leads to a power conversion efficiency improvement of 40%. This efficiency gain was attributed to enhanced light harvesting and subsequent exciton generation rate into the photoactive layer due to localized surface plasmon resonance and scattering effects. This argument was supported by the combinatorial study of spectroscopic analysis and photon to-electron conversion efficiency measurements of the devices.