Polymer-fullerene Blends Research Articles

Ultrafast Terahertz Photoconductivity of Photovoltaic Polymer-Fullerene Blends: A Comparative Study Correlated with Photovoltaic Device Performance.

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

Vibrational coherence probes the mechanism of ultrafast electron transfer in polymer-fullerene blends.

The conversion of photoexcitations into charge carriers in organic solar cells is facilitated by the dissociation of excitons at the donor/acceptor interface. The ultrafast timescale of charge separation demands sophisticated theoretical models and raises questions about the role of coherence in the charge-transfer mechanism. Here, we apply two-dimensional electronic spectroscopy to study the electron transfer process in poly(3-hexylthiophene)/PCBM (P3HT/PCBM) blends. We report dynamics maps showing the pathways of charge transfer that clearly expose the significance of hot electron transfer. During this ultrafast electron transfer, vibrational coherence is directly transferred from the P3HT exciton to the P3HT hole polaron in the crystalline domain. This result reveals that the exciton converts to a hole with a similar spatial extent on a timescale far exceeding other photophysical dynamics including vibrational relaxation.

Role of Oxygen Adsorption in Nanocrystalline ZnO Interfacial Layers for Polymer–Fullerene Bulk Heterojunction Solar Cells

Colloidal zinc oxide (ZnO) nanoparticles are frequently used in the field of organic photovoltaics for the realization of solution-producible, electron-selective interfacial layers. Despite of the widespread use, there is a lack of detailed investigations regarding the impact of structural properties of the particles on the device performance. In this work, ZnO nanoparticles with varying surface-area-to-volume ratio were synthesized and implemented into polymer-fullerene bulk heterojunction solar cells with a gas-permeable top electrode. By comparing the electrical characteristics before and after encapsulation, it was found that the internal surface area of the ZnO layer plays a crucial role under conditions where oxygen can penetrate the solar cells. The adsorption of oxygen species at the nanoparticle surface causes band bending and electron depletion next to the surface. Both effects result in the formation of a barrier for electron injection and extraction at the ZnO/bulk heterojunction interface and were more pronounced in case of small ZnO nanocrystals (high surface-area-to-volume ratio). Different transport-related phenomena in the presence of oxygen are discussed in detail, i.e., Ohmic losses, expressed in terms of series resistance, as well as the occurrence of space-charge-limited currents, related to charge accumulation in the polymer-fullerene blend. Since absorption of UV light can cause desorption of adsorbed oxygen species, the electrical properties depend also on the illumination conditions. With the help of systematic investigations of the current versus voltage characteristics of solar cells under different air exposure and illumination conditions as well as studies of the photoconductivity of pure ZnO nanoparticle layers, we gain detailed insight into the role of the ZnO nanoparticle surface for the functionality of the organic solar cells.

Dielectric effect on the photovoltage loss in organic photovoltaic cells.

The VOC loss in several polymer-fullerene solar cells is determined. Based on these data, a major source of photovoltage loss is attributed to the low dielectric constants of the polymers. Such loss is close to zero if the dielectric constant of the polymer-fullerene blend is close to 5.

Nongeminate recombination in neat P3HT and P3HT:PCBM blend films

The slow decay of charge carriers in polymer–fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.

Quantification of Nano‐ and Mesoscale Phase Separation and Relation to Donor and Acceptor Quantum Efficiency, Jsc, and FF in Polymer:Fullerene Solar Cells

Two characteristic length scales are revealed and quantified in a complex hierarchical polymer–fullerene blend by combining different X-ray scattering techniques. Anti-correlated composition variations between meso- and nanoscale separation are observed and impacted by the solvent mixture. Due to competition between the impact of the two length scales, the relation to device performance is complex and an ideal morphology is yet to be delineated.

Spatial degradation mapping and component-wise degradation tracking in polymer–fullerene blends

P3HT degrades homogeneously and the degradation rate is reduced by a factor of 3 in the presence of PCBM. Spatial photodegradation maps are obtained by scanning transmission X-ray microscopy near the carbon absorption edge.

Direct comparison of charge transport and electronic traps in polymer–fullerene blends under dark and illuminated conditions

A direct comparison of charge transport and electronic traps in representative polymer–fullerene blend, poly (3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM), is carried out in dark and illuminated conditions based on the measurements of temperature-dependent current–voltage characteristics. In dark condition, the charge transport presents a transition from Ohmic to trap-limited current. While the trap-filled space charge limited current is observed under illumination at the same applied bias. From evaluations of trap density and energy distribution by a differential method, it is reveal that the diverse charge transport in dark and illuminated conditions is mainly caused by the different trap states distribution, which strongly affects the space charges and the electrical field in P3HT: PCBM blends.

Nanostructured conformal hybrid solar cells: a promising architecture towards complete charge collection and light absorption

We introduce hybrid solar cells with an architecture consisting of an electrodeposited ZnO nanorod array (NRA) coated with a conformal thin layer (<50 nm) of organic polymer-fullerene blend and a quasi-conformal Ag top contact (Thin/NR). We have compared the performance of Thin/NR cells to conventional hybrid cells in which the same NRAs are completely filled with organic blend (Thick/NR). The Thin/NR design absorbs at least as much light as Thick/NR cells, while charge extraction is significantly enhanced due to the proximity of the electrodes, resulting in a higher current density per unit volume of blend and improved power conversion efficiency. The NRAs need not be periodic or aligned and hence can be made very simply.

Tuning organic magnetoresistance in polymer-fullerene blends by controlling spin reaction pathways

Harnessing the spin degree of freedom in semiconductors is generally a challenging, yet rewarding task. In recent years, the large effect of a small magnetic field on the current in organic semiconductors has puzzled the young field of organic spintronics. Although the microscopic interaction mechanisms between spin-carrying particles in organic materials are well understood nowadays, there is no consensus as to which pairs of spin-carrying particles are actually influencing the current in such a drastic manner. Here we demonstrate that the spin-based particle reactions can be tuned in a blend of organic materials, and microscopic mechanisms are identified using magnetoresistance lineshapes and voltage dependencies as fingerprints. We find that different mechanisms can dominate, depending on the exact materials choice, morphology and operating conditions. Our improved understanding will contribute to the future control of magnetic field effects in organic semiconductors.

Morphology control in co-evaporated bulk heterojunction solar cells

Bulk heterojunction solar cells made by vacuum co-evaporation of polythiophene (PTh) and fullerene (C60) are reported and the blend morphology control through donor–acceptor composition and post-situ annealing demonstrated. Co-deposited heterojunctions are shown to generate about 60% higher photocurrents than their thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the power conversion efficiency is improved by as much as 80%. UV–vis, XRD and AFM analysis shows that the enhanced photoresponse is a result of favorable morphological development of PTh upon annealing. Annealing-induced improvement of the donor–acceptor network is demonstrated across a wide range of compositions (20–80% PTh). This illustrates that good morphology control is possible in co-evaporated polymer–fullerene blends through multiple parameters and differentiates this donor–acceptor combination from the standard small-molecule systems.

Effects of Soft Insulating Polymer Doping on the Photovoltaic Properties of Polymer–Fullerene Blend Solar Cells

The blending of organic semiconductor and insulating polymer as an active layer of organic transistors has been assessed by several studies and may open new possibilities for advantageous functions. However, studies on the use of insulating polymer doping in organic solar cells are rarely conducted. In this research, a blending film comprising regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) was doped with a soft insulating poly(methyl methacrylate) (PMMA) to act as an active layer of a solar cell. The fill factor (FF) and open-circuit voltage (VOC) of P3HT:PCBM solar cells were improved after PMMA doping. The microstructure-dependent photovoltaic properties of devices with different fractions of PMMA were investigated using spectroscopy and quantum chemical calculations. We highlight that PMMA doping leads to a more homogeneous conformation of P3HT chains, fewer vacancies, and fewer leakage pathways in blending films. Consequently, charges can be efficiently transported to the electrodes of devices, resulting in enhanced FF and VOC. The results of theoretical calculations at the microscopic molecular level also confirmed the enhanced electrical performance of devices from PMMA doping.

Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer‐Fullerene Solar Cells While Electron Transfer Remains Active

AbstractDevice performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM); [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM); and the indene‐C60 bis‐adduct IC60BA). Power conversion efficiency improves from 1.52% in IC60BA‐based solar cells to 3.75% in PC71BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC71BM or PC61BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC60BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC61BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC60BA blend effectively turns off hole transfer from IC60BA excitons to the host polymer, even while electron transfer from the polymer to the IC60BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.

Fullerenes and carbon nanotubes as acceptor materials in organic photovoltaics

The development of new acceptor materials has played an important role in advancing the efficiency of organic photovoltaics (OPV). In this review we survey two types of acceptors, fullerene derivatives and single-walled carbon nanotubes (SWCNTs). The former has been the most successful type of acceptor in OPV, while the latter is a new type of acceptor that has shown encouraging spectroscopic results but its implementation in efficient devices is work in progress. We present some of the essential properties of an OPV acceptor, and the main experimental methodology used to characterize the photoconductivity of an OPV active layer comprising a donor-acceptor blend. We then discuss different aspects of donor-acceptor blends, or bulk heterojunctions, with a polymer donor and a fullerene or SWCNT acceptor. We show the basic charge-generation and recombination properties of a prototypical polymer-fullerene system, and discuss the significance of the molecular structure of the fullerene in terms of the mixing of the two components and its effect on photocarrier dynamics. In the case of the fullerene acceptors we also compare to OPV device efficiency. We discuss the role of charge and energy transfer from the polymer in exciton quenching in polymer-SWCNT blends, and show that the former results in the generation of long-lived charge carriers, similar to observations in polymer-fullerene blends. We show that SWCNT enrichment, to yield samples with only a few percent of metallic SWCNTs results in an enhancement of the photoconductance and carrier lifetime in P3HT-SCWNT blends. We also discuss the role that SWCNT diameter plays on the driving force for carrier generation, and suggest that smaller SWCNTs may be critical for efficient OPV device performance. Finally, we discuss the implications of the results of the photoconductivity studies on OPV device performance, and outline future challenges and open questions pertaining to the optimization of acceptors in OPV.

Direct detection of photoinduced charge transfer complexes in polymer fullerene blends

We report transient electron paramagnetic resonance (trEPR) measurements with sub-microsecond time resolution performed on a P3HT:PCBM blend at low temperature. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g-values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e. originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes which form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.

Phase Morphology and Molecular Structure Correlations in Model Fullerene-Polymer Nanocomposites

In this work we have performed a systematic study of blends of [6,-phenyl C61 butyric acid methyl ester (PCBM) with the following amorphous and semi-crystalline polymers: atactic polystyrene (PS), syndiotactic polystyrene (syn-PS), poly (2-vinyl-naphthalene) (P2VN), poly (9-vinyl-phenanthrene) (P9VPh), poly (vinylidene-fluoride) (PVdF) and poly (3-hexyl-thiophene) (P3HT). Experimental measurements using DSC, x-ray and neutron scattering coupled with molecular modeling (MD and DFT) have been utilized to determine the solubility and phase morphology of these model polymer-fullerene blends.

Comparison of large scale coating techniques for organic and hybrid films in polymer based solar cells

Polymer based solar cells (PSC) can be manufactured in a continuous roll to roll process as a low cost regenerative energy source. Coating ink properties and film thicknesses of 30–200nm are challenging with respect to the manufacturing process, which itself has an important impact on film properties and cell efficiencies. In this paper we compare the large area coating methods: knife coating, slot-die coating, and spray coating with laboratory spin coating. Properties of coating inks and a viscosity model for commercial PEDOT:PSS types are discussed. The significantly smaller viscosity to surface tension ratio, of typical coating inks for PSC compared to conventional coating inks, causes a different behavior during the coating process. Wet film thickness, homogeneity, and process stability and their dependence on process parameters are addressed for each coating method. Hole-conductive and photoactive layers, consisting of polymer-fullerene and polymer-nanoparticle blends, are then compared with respect to homogeneity, AFM topography and absorption spectra. First results indicate that the coating method itself has an impact on polymer-fullerene film morphology and opto-electric properties.

Inkjet-printed polymer–fullerene blends for organic electronic applications

Organic field effect transistors, photodiodes and solar cells based on polymer-fullerene blend active layers are printed electronic applications under intense studies. We show that the viscosity of the inkjet formulation, substrate temperature, drop spacing and the height of the droplet in relation to the surface are critical parameters to achieving high quality inkjet-printed polymer-fullerene based active layers. The effect of the above processing parameters on the performance of polymer-fullerene based organic solar cells is presented.

Enhancement of photo/thermal stability of organic bulk heterojunction photovoltaic devices via gold nanoparticles doping of the active layer

This study focuses on the crucial problem of the stability of organic photovoltaic (OPV) devices, aiming to shed light on the photo and thermal degradation mechanisms during prolonged irradiation under ambient conditions. For this purpose, the stability enhancement of bulk heterojunction OPV devices upon embedding surfactant free Au nanoparticles (NPs) into the photoactive layer is investigated by in situ time-resolved energy dispersive X-ray reflectometry (EDXR), photoluminescence (PL) and Raman spectroscopy as well as device degradation electrical measurements. It is shown that besides the improved cell efficiency attributed to plasmon absorption and scattering effects, the embedded NPs act as performance stabilizers, giving rise to enhanced structural stability and, in turn, to reduced photodegradation rate of the respective OPV devices. It is particularly clarified that, in addition to further stabilization of the polymer-fullerene blend, the observed improvement can be ascribed to a NP-mediated mitigation of the photooxidation effect at the cathode-active layer interface. Our work suggests the exploitation of surfactant free NPs to be a successful approach to address aging effects in OPV devices.

Moving through the Phase Diagram: Morphology Formation in Solution Cast Polymer–Fullerene Blend Films for Organic Solar Cells

The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.