Fullerene Bulk Heterojunction Research Articles

Improving the Efficiency of Polymer:Fullerene Bulk Heterojunction Solar Cells by Varying the Material Concentration in the Photoactive Layer

Planar structure organic solar cells (ITO/PEDOT:PSS/P3HT:PCBM/Al) were studied to determine the optimal concentration of polymer and fullerene in the photoactive layer blend. The bulk heterojunction of these devices consisted of the electron donating polymer poly-3(hexylthiophene-2,5-diyl) (P3HT), and the electron accepting fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Four unique photoactive layer solutions were prepared by dissolving various amounts of the organics in 2 mL chlorobenzene, while keeping the weight ratio of P3HT:PC61BM at 1:0.8. The fabricated solar cells were characterized electrically to determine power conversion efficiency, fill factor, and current density. Subsequent characterization techniques including atomic force microscopy, ultraviolet-visible spectrophotometry, and variable angle spectroscopic ellipsometry were performed on P3HT:PC61BM films to determine surface roughness, optical absorption, and extinction coefficient. Through this systematic study, it was determined that an appropriate selection of P3HT:PC61BM concentration allows for significant increases in the overall device performance of organic solar cells.

Functionalized nanodiamond as a charge transporter in organic solar cells

We demonstrate for the first time the efficiency improvement of poly(3-hexylthiophene) (P3HT)–fullerene (C60) bulk heterojunction photovoltaic cells by the introduction of functionalized nanodiamonds (NDs) into the photoactive layer. A novel covalently bonded C60ND composite was synthesized via a microwave induced functionalization approach. As compared to control devices with only C60, the addition of carboxylated ND resulted in 53% improvement in short circuit current density Jsc. Such a device design takes advantages of both C60 for electron accepting and ND for efficient electron transport. The results indicate that C60 decorated NDs are promising additives for the performance enhancement of polymer photovoltaic cells.

Energy Level Alignment and Sub‐Bandgap Charge Generation in Polymer:Fullerene Bulk Heterojunction Solar Cells

Using charge modulated electroabsorption spectroscopy (CMEAS), for the first time, the energy level alignment of a polymer:fullerene bulk heterojunction photovoltaic cell is directly measured. The charge-transfer excitons generated by the sub-bandgap optical pumping are coupled with the modulating electric field and introduce subtle changes in optical absorption in the sub-bandgap region. This minimum required energy for sub-bandgap charge genreation is defined as the effective bandgap.

Nanomorphology of polythiophene–fullerene bulk-heterojunction films investigated by structured illumination optical imaging and time-resolved confocal microscopy

Structured illumination microscopy (SIM) and time-resolved confocal fluorescence microscopy are applied to investigate the nanomorphology of thin films comprising typical blends of the conjugated polymer, poly (3-hexylthiophene) (P3HT), and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM), used for organic photovoltaic applications. SIM provides evidence for the presence of a thin emissive region around the crystalline regions of PCBM and at the tips of rod-like domains. The time-resolved measurements show that the emission surrounding the PCBM rods is longer lived than the bulk of the film. The two modes of microscopy provide complementary evidence indicating that electron–hole separation is inhibited between the polymer and the large PCBM-rich domains in these regions. We show here that structured illumination microscopy is a viable method of gaining additional information from these photovoltaic materials, despite their weak emission.

Temperature Dependence of the Diffusion Coefficient of PCBM in Poly(3-hexylthiophene)

Interest in new functional small molecule and polymer blends, such as polymer–fullerene bulk heterojunction (BHJ) organic solar cells motivates the development of new methods to measure the diffusion coefficient of molecular species (e.g., PCBM) in polymers. The aim of this study is to systematically improve our understanding of the relevant material and processing parameters needed to control the microstructure of BHJ organic solar cells in order to develop a more complete understanding of how to improve its power conversion efficiency. Here, we fabricate a terraced monolayer–bilayer sample of P3HT and P3HT/PCBM and use this structure to quantify both the volume fraction of miscible PCBM in P3HT and the diffusion coefficient of disordered PCBM in disordered P3HT. Our findings reveal that the diffusion coefficient for disordered PCBM in P3HT is strongly dependent on the annealing temperature (i.e., increasing by 3 orders of magnitude when doubling the annealing temperature) and weakly dependent on the PCBM concentration. The temperature-dependent diffusion coefficients were fit with an Arrhenius relationship to determine an activation energy for the diffusion of disordered PCBM through P3HT. Ultimately, this report demonstrates that the self-assembly of the P3HT:PCBM BHJ solar cell during annealing and cooling is not limited by the diffusion of deuterated PCBM in P3HT with the nanostructure of PCBM being controlled by the relative volume fractions of ordered and disordered P3HT.

One‐Step Macroscopic Alignment of Conjugated Polymer Systems by Epitaxial Crystallization during Spin‐Coating

AbstractThe one‐step preparation of highly anisotropic polymer semiconductor thin films directly from solution is demonstrated. The conjugated polymer poly(3‐hexylthiophene) (P3HT) as well as P3HT:fullerene bulk–heterojunction blends can be spin‐coated from a mixture of the crystallizable solvent 1,3,5‐trichlorobenzene (TCB) and a second carrier solvent such as chlorobenzene. Solidification is initiated by growth of macroscopic TCB spherulites followed by epitaxial crystallization of P3HT on TCB crystals. Subsequent sublimation of TCB leaves behind a replica of the original TCB spherulites. Thus, highly ordered thin films are obtained, which feature square‐centimeter‐sized domains that are composed of one spherulite‐like structure each. A combination of optical microscopy and polarized photoluminescence spectroscopy reveals radial alignment of the polymer backbone in case of P3HT, whereas P3HT:fullerene blends display a tangential orientation with respect to the center of spherulite‐like structures. Moreover, grazing‐incidence wide‐angle X‐ray scattering reveals an increased relative degree of crystallinity and predominantly flat‐on conformation of P3HT crystallites in the blend. The use of other processing methods such as dip‐coating is also feasible and offers uniaxial orientation of the macromolecule. Finally, the applicability of this method to a variety of other semi‐crystalline conjugated polymer systems is established. Those include other poly(3‐alkylthiophene)s, two polyfluorenes, the low band‐gap polymer PCPDTBT, a diketopyrrolopyrrole (DPP) small molecule as well as a number of polymer:fullerene and polymer:polymer blends.

Tracing charge transfer states in polymer:fullerene bulk-heterojunctions

Charge transfer state emission in organic bulk-heterojunctions has been demonstrated to be an important loss mechanism for this reason a better understanding of the nature and origin of the charge transfer state is fundamental for the improvement of organic solar cells. Here, the relationship between photophysical and morphological features of a prototypical organic bulk-heterojunction is investigated in blends with different donor-acceptor ratios. By correlating imaging with photoluminescence spectra measured in different areas of the blends, the charge transfer state emission is unambiguously assigned to microscopical regions in which the intermixing of the two organic semiconductors is higher.

Correlating molecular morphology with optoelectronic function in solar cells based on low band-gap copolymer:fullerene blends

We review recent progress in the development of organic bulk heterojunction (BHJ) solar cells employing donor–acceptor copolymers as the electron-donor and fullerene derivatives as the electron-acceptor. We discuss the role of the donor and acceptor moieties, side-chains, bridging units and atomic substitutions of the copolymers on their optoelectronic functionality. The physical properties, e.g. molecular conformation, miscibility, phase-separated lateral and vertical morphology, of various photovoltaic blends prepared via solution casting and post-treatments are reviewed and correlated with photovoltaic device performance. Factors influencing the morphological stability of polymer:fullerene BHJ thin-films are briefly discussed. Finally, we address the use of thin organic interlayers to increase the efficiency of BHJ solar cells.

Solution-processed small molecule:fullerene bulk-heterojunction solar cells: impedance spectroscopy deduced bulk and interfacial limits to fill-factors

Using impedance spectroscopy, we demonstrate that the low fill factor (FF) typically observed in small molecule solar cells is due to hindered carrier transport through the active layer and hindered charge transfer through the anode interfacial layer (IFL). By carefully tuning the active layer thickness and anode IFL in BDT(TDPP)2 solar cells, the FF is increased from 33 to 55% and the PCE from 1.9 to 3.8%. These results underscore the importance of simultaneously optimizing active layer thickness and IFL in small molecule solar cells.

Quantifying organic solar cell morphology: a computational study of three-dimensional maps

Establishing how fabrication conditions quantitatively affect the morphology of organic blends opens the possibility of rationally designing higher efficiency materials; yet such a relationship remains elusive. One of the major challenges stems from incomplete three-dimensional representations of morphology, which is due to the difficulties of performing accurate morphological measurements. Recently, three-dimensional measurements of mixed organic layers using electron tomography with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) provided maps of morphology with high resolution and detail. Using a simple, yet powerful, computational tool kit, these complex 3D datasets are converted into a set of physically meaningful morphology descriptors. These descriptors provide means for converting these large, complicated datasets (∼5 × 107 voxels) into simple, descriptive parameters, enabling a quantitative comparison among morphologies fabricated under different conditions. A set of P3HT:endohedral fullerene bulk-heterojunctions, fabricated under conditions specifically chosen to yield a wide range of morphologies, are examined. The effects of processing conditions and electrode presence on interfacial area, domain size distribution, connectivity, and tortuosity of charge transport paths are herein determined directly from real-space data for the first time. Through this characterization, quantitative insights into the role of processing in morphology are provided, as well as a more complete picture of the consequences of a three-phase morphology. The analysis demonstrates a methodology which can enable a deeper understanding into morphology control.

Power efficiency enhancement of solution-processed small-molecule solar cells based on squaraine via thermal annealing and solvent additive methods

A small-molecule donor material of 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) was used to fabricate the solution-processed solar cells with fullerene. The thermal annealing process and the solvent additive method were systematically investigated to illuminate their effects on the performance of solar cells. The optimized device exhibited a power conversion efficiency (PCE) of 3.31% under 1 sun, AM 1.5G simulated solar irradiation with thermal annealing at 50°C and a small amount of 1,8-diiodooctane (DIO) additive with a volume ratio of 0.4%. A reduced surface roughness and improved charge carrier mobility were ascribed to the enhancement of PCE in DIO incorporated devices. The simulation using a theoretical model of charge transfer state based on Onsager–Braun theory was carried out, and the results revealed that thermal annealing and solvent additive could facilitate charge transfer dissociation, increase charge carrier generation, and thereby improve the performance of solar cells based on SQ: fullerene bulk heterojunction.

CuPc/C60 bulk heterojunction photovoltaic cells with evidence of phase segregation

This work presents organic photovoltaic (OPV) device study based on cupper phthalocyanine (CuPc) and fullerene (C60) bulk heterojunction (BHJ) structure. By varying blend composition, the optimized performances were obtained in 75%-CuPc containing devices with anode buffer of either CuPc or HPCzI (1,3,4,5,6,7-hexaphenyl-2-{3′-(9-ethylcarbazolyl)}-isoindole). It was discovered by scanning electron microscopy that 75%-CuPc containing film possessed phase separation, which is beneficial to charge transport via percolation process. Additionally, electronic absorption measurement and hole only device study showed that, depending on the mixing ratio, the absorption and the hole transport ability were different. The blend film containing 75% CuPc had the largest integrated absorption with the most CuPc dimmer aggregate and the least C60 aggregate. Moreover, the 75% CuPc blend film also possessed the highest hole transport ability. Thus, the best performance of the 75% CuPc BHJ device is mainly attributed to its good carrier transport originating from phase segregation. The present work highlights the phase separation in CuPc:C60 mixing film with optimized ratio, as well as its corresponding electronic absorption and carrier transport properties, which are essential for OPV device performance. Hence, insights are inferred for further engineering of BHJ OPV devices based on small molecules.

The Importance of Fullerene Percolation in the Mixed Regions of Polymer–Fullerene Bulk Heterojunction Solar Cells

AbstractMost optimized donor‐acceptor (D‐A) polymer bulk heterojunction (BHJ) solar cells have active layers too thin to absorb greater than ∼80% of incident photons with energies above the polymer's band gap. If the thickness of these devices could be increased without sacrificing internal quantum efficiency, the device power conversion efficiency (PCE) could be significantly enhanced. We examine the device characteristics of BHJ solar cells based on poly(di(2‐ethylhexyloxy)benzo[1,2‐b:4,5‐b′]dithiophene‐co‐octylthieno[3,4‐c]pyrrole‐4,6‐dione) (PBDTTPD) and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) with 7.3% PCE and find that bimolecular recombination limits the active layer thickness of these devices. Thermal annealing does not mitigate these bimolecular recombination losses and drastically decreases the PCE of PBDTTPD BHJ solar cells. We characterize the morphology of these BHJs before and after thermal annealing and determine that thermal annealing drastically reduces the concentration of PCBM in the mixed regions, which consist of PCBM dispersed in the amorphous portions of PBDTTPD. Decreasing the concentration of PCBM may reduce the number of percolating electron transport pathways within these mixed regions and create morphological electron traps that enhance charge‐carrier recombination and limit device quantum efficiency. These findings suggest that (i) the concentration of PCBM in the mixed regions of polymer BHJs must be above the PCBM percolation threshold in order to attain high solar cell internal quantum efficiency, and (ii) novel processing techniques, which improve polymer hole mobility while maintaining PCBM percolation within the mixed regions, should be developed in order to limit bimolecular recombination losses in optically thick devices and maximize the PCE of polymer BHJ solar cells.

Yet another poor man's green bulk heterojunction photocells: Annealing effect and film composition dependence of photovoltaic devices using poly(3-hexylthiophene):C70 composites prepared with chlorine-free solvent

Chemically modified fullerenes and chlorinated solvents have been predominantly chosen in the recent studies of the photovoltaic devices based on polymer bulk heterojunction composites. However, these items seem to be undesirable because of their potential impacts on the environment as well as the consumption of resources. In this context, a systematic study on the annealing effect and film composition dependence of bulk heterojunction photovoltaic devices based on composites consisting of poly(3-hexylthiophene) (P3HT) and neat C70 prepared with a chlorine-free solvent 1,2,4-trimethylbenzene has been carried out. For the device using P3HT:C70 composite (2:1 by weight) film as an active layer, power conversion efficiency of 1.47%, with open-circuit voltage of 0.52V, short-circuit current density of 6.2mA/cm2 and fill-factor of 45%, has been obtained after post-production annealing at 160°C. The combination of neat fullerenes with naturally obtained solvents will open up a new way to produce cost-effective and environmentally-friendly photovoltaic devices based on polymer:fullerene bulk heterojunction composites.

Controlling band gap and hole mobility of photovoltaic donor polymers with terpolymer system

Energy level, band gap, and hole mobility of photovoltaic donor polymers are crucial factors in achieving high power conversion efficiency. Controlling these characteristics, however, requires complicated synthesis routes. This work reports an effective way to control these properties simultaneously by introducing a simple terpolymer system. Simulation of energy level (band gap), synthesis of donor polymers, and calculation of solubility were systematically coordinated to explain the advantages of the terpolymer system. Anthradithiophene was introduced to elucidate the effect of the extended conjugation length on band gap and hole mobility of donor polymers. Benzodithiophene and thienothiophene which have shown good properties in bulk heterojunction organic solar cells were used as comonomers. Based on the result of the energy level calculation, terpolymers with various mole ratios of the monomers were synthesized; as the ratio of anthradithiophene to benzodithiophene increased, band gap of the terpolymers decreased while hole mobility increased. In addition, we explained the morphology of the terpolymer:fullerene bulk heterojunction films using solubility parameters of materials. With this method, we investigated the relationship between the properties of donor polymers and device performances systematically. The terpolymer system can be widely utilized for tuning the material properties in a systematic way which is quite similar to the band gap engineering in the inorganic semiconductors.

Polymer-Fullerene Miscibility: A Metric for Screening New Materials for High-Performance Organic Solar Cells

The improvement of the power conversion efficiency (PCE) of polymer bulk heterojunction (BHJ) solar cells has generally been achieved through synthetic design to control frontier molecular orbital energies and molecular ordering of the electron-donating polymer. An alternate approach to control the PCE of a BHJ is to tune the miscibility of the fullerene and a semiconducting polymer by varying the structure of the fullerene. The miscibility of a series of 1,4-fullerene adducts in the semiconducting polymer, poly(3-hexylselenophene), P3HS, was measured by dynamic secondary ion mass spectrometry using a model bilayer structure. The microstructure of the bilayer was investigated using high-angle annular dark-field scanning transmission microscopy and linked to the polymer-fullerene miscibility. Finally, P3HS:fullerene BHJ solar cells were fabricated from each fullerene derivative, enabling the correlation of the active layer microstructure to the charge collection efficiency and resulting PCE of each system. The volume fraction of polymer-rich, fullerene-rich, and polymer-fullerene mixed domains can be tuned using the miscibility leading to improvement in the charge collection efficiency and PCE in P3HS:fullerene BHJ solar cells. These results suggest a rational approach to the design of fullerenes for improved BHJ solar cells.

Halogen-free solvent processing for sustainable development of high efficiency organic solar cells

Eliminating processing with halogenated solvents is desirable to achieve sustainable large-scale fabrication of organic solar cells. This work demonstrates a device processing approach completely free of halogenated solvents to yield high-performance (power conversion efficiency, ηP>6%) polymer:fullerene bulk-heterojunction solar cells comprising a conjugated polymer PIDT-phanQ and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Introducing 2% 1-methylnaphthalene (Me-naph) as a processing additive to toluene alleviates PC71BM solubility problems, reduces phase domain size by two orders of magnitude, and boosts efficiency from ηP=0.02% to 6.10%. Both AFM and TEM imaging show that the Me-naph additive promotes a more finely phase-separated morphology in spin-coated films, while photoluminescence quenching and photoinduced absorption spectroscopy confirm that this finer morphology results in both better exciton quenching and more efficient charge separation.

Long-range exciton dissociation in organic solar cells.

It is normally assumed that electrons and holes in organic solar cells are generated by the dissociation of excitons at the interface between donor and acceptor materials in strongly bound hole-electron pairs. We show in this contribution that excitons can dissociate tens of angstroms away from the interface and generate partially separated electrons and holes, which can more easily overcome their coulombic attraction and form free charges. We first establish under what conditions long-range exciton dissociation is likely (using a kinetic model and a microscopic model for the calculation of the long-range electron transfer rate). Then, defining a rather general model Hamiltonian for the donor material, we show that the phenomenon is extremely common in the majority of polymer:fullerene bulk heterojunction solar cells.

Impact of Coumarin Dye Doping on Photovoltaic Properties of Bulk Heterojunction Device

In this article, we describe the photovoltaic properties of polymer–fullerene bulk-heterojunction films doped with Coumarin dyes. Although the two kinds of dye used in this study have similar absorption bands, distinct differences in both short-circuit current density and open-circuit voltage have been observed. In particular, we found that open-circuit voltage could be enhanced by doping with Coumarin 307. This finding implies that doping with organic dyes is a potentially effective scheme for improving the open-circuit voltage as well as the short-circuit current of bulk heterojunction devices.

Combined alternative electrodes for semi-transparent and ITO-free small molecule organic solar cells

We investigate various electrode combinations of bottom and top contacts for organic photovoltaic (OPV) cells. Silver (Ag), indium tin oxide (ITO), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires (AgNW) are used as bottom electrodes. As top electrodes, thin silver layers (t-Ag) and free-standing carbon nanotube (f-CNT) sheets are employed. The manufactured zinc phthalocyanine (ZnPc): fullerene C60 small molecule bulk heterojunction OPV cells with different kinds of bottom electrodes show efficiencies of 1.9∼2.2% and 1.1∼1.5%, when comprised of t-Ag and f-CNT top contacts, respectively. We demonstrate alternative electrodes beyond ITO, silver, and aluminum, which can be readily used for organic photovoltaics technology.