Polymer-based Photovoltaic Devices Research Articles

Relating free energy and open-circuit voltage to disorder in organic photovoltaic systems.

Efficient exciton dissociation into mobile charge carries a crucial factor underscoring the performance of organic polymer-based bulk-heterojunction photovoltaic devices. In this paper, we compute the energies of charge-transfer (CT) states of the model donor-acceptor lattice system with varying degrees of structural disorder to investigate how fluctuations in the material properties affect electron-hole separation. We also demonstrate how proper statistical treatment of the CT energies recovers the experimentally observed "hot" and "cold" exciton dissociation pathways. Using a quantum mechanical model for a model heterojunction interface, we recover experimental values for the open-circuit voltage at 50 and 100 meV of site-energy disorder. We find that energetic and conformational disorder generally facilitates charge transfer; however, due to excess energy supplied by photoexcitation, highly energetic electron-hole pairs can dissociate in unfavorable directions, potentially never contributing to the photocurrent while "cold" excitons follow the free energy curve defined at the operating temperature of the device.

In situ space-resolved X-ray diffraction and time-resolved EDXD on efficient polymer-based photovoltaic devices: Microstructural properties and aging effects

Abstract

Erratum to: “In-situ investigation of the bulk heterojunction formation processes in the active layers of organic solar cells”

Conjugated polymer-based organic photovoltaic devices (OPVs) have been intensively studied, since they are advantageous over regular inorganic devices. However, there is still no complete understanding of the functionality of conjugated polymer-based organic active layers. Due to this, research into the processes of microstructure formation will improve morphology control and optimization and enhance performance and productivity. In this study we used in situ measurements of conductivity and X-ray diffraction analysis in grazing incidence X-ray diffraction (GIXD) geometry to investigate the structure of light-sensitive layers produced by coating from solutions and based on poly(3-hexylthiophene) (P3HT) mixtures as donors with various fullerene acceptors: [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and indene-C60 bisadduct (ICBA). The structure of films produced by coating from solutions was studied by large-angle GIXD. Ex situ and in situ methods (under external voltage applied) were used to detect high compatibility between the donor and acceptor for ICBA-based films. The time dependences of changes in current and intensity of the (100) peak of P3HT demonstrate that the maximum emerges simultaneously. A two-step increase in intensity of the (100) peak of P3HT was observed for films containing PCBM as an acceptor. In situ studies of the structure confirm that the joint donor–acceptor structure is formed as the solvent evaporates. Atomic force microscopy studies of the surface roughness of films containing different acceptors demonstrate that they are characterized by different morphologies.

Increased thermal stabilization of polymer photovoltaic cells with oligomeric PCBM

Both crystalline and amorphous polymer-based organic photovoltaic devices are stabilized against thermal degradation by an ATRAP prepared PCBM oligomer.

Effect of copper oxide oxidation state on the polymer-based solar cell buffer layers.

Transporting buffer layers are important components of polymer-based organic photovoltaic devices. In this study, we have investigated the effects of the oxidation state in copper oxide based buffer layer in conjunction to its role in device performance. We have shown that variation in the oxidation state affects the band alignment and built-in voltage of the device, therefore leading to variation in device performance. Specifically, the fully oxidized copper oxide buffer layer has a valence band position at 5.12 eV, much closer to the highest occupied molecular orbital of poly(3-hexylthiophene-2,5-diyl) (P3HT) (∼5.2 eV), giving a best fill factor and efficiency at 57% and 4.06%, respectively. Lastly, we also demonstrate significant enhancement in device stability, with power conversion efficiency maintained at 75% of the original value even after 40 days, and propose a strategy for recovering the device performance based on the observed property of the oxide buffer layer.

Molecular weight dependence of the morphology in P3HT:PCBM solar cells.

In polymer-based photovoltaic devices, optimizing and controlling the active layer morphology is important to enhancing the device efficiency. Using poly(3-hexylthiophene) (P3HT) with well-defined molecular weights (MWs), synthesized by the Grignard metathesis (GRIM) method, we show that the morphology of the photovoltaic active layer and the absorption and crystal structure of P3HT are dependent on the MW. Differential scanning calorimetry showed that the crystallinity of P3HT reached a maximum for intermediate MWs. Grazing-incidence wide-angle X-ray diffraction showed that the spacing of the (100) planes of P3HT increased with increasing MW, while the crystal size decreased. Nonlinear crystal lattice expansions were found for both the (100) and (020) lattice planes, with an unusual π-π-stacking enhancement observed between 50 and 100 °C. The melting point depression for P3HT, when mixed with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and, hence, the Flory-Huggins interaction parameter depended on the MW. PCBM was found to perturb the ordering of P3HT chains. In photovoltaic devices, P3HT with a MW of ∼20K showed the best device performance. The morphologies of these blends were studied by grazing-incidence small-angle X-ray scattering (GISAXS) and resonant soft X-ray scattering. In GISAXS, we observed that the low-molecular-weight P3HT more readily crystallizes, promoting a phase-separated morphology.

Design considerations for electrode buffer layer materials in polymer solar cells.

Electrode buffer layers in polymer-based photovoltaic devices enable highly efficient devices. In the absence of buffer layers, we show that diode rectification is lost in ITO/P3HT:PCBM/Ag (ITO = indium tin oxide; P3HT = poly(3-hexylthiophene); PCBM = phenyl C61-butyric acid methyl ester) devices due to nonselective charge injection through the percolated phase pathways of a bulk heterojunction active layer. Charge-selective injection, and thus rectification and device function, can be regained by placing thin, polymeric buffer layers that break the direct electrode-active layer contact. Additionally, we show that strong active layer-buffer layer interactions lead to unwanted vertical phase separation and a kinked current-voltage curve. Device function is regained, increasing power conversion efficiency from 3.6% to 7.2%, by placing a noninteracting layer between the buffer and active layer. These results guide the design and selection of future polymeric electrode buffer layers for efficient polymer solar cell devices.

Formation of charge-transfer complexes significantly improves the performance of polymer solar cells based on PBDTTT-C-T: PC71 BM

By adding appropriate proportions of nitrobenzene (C6H5NO2) to the blended solution of poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]}:[6,6]-phenyl-C71-butyric acid methyl ester (PBDTTT-C-T: PC71BM), we substantially improved the power conversion efficiency from the best reported value of 7.48–8.88%. Experiments and simulations show that nitrobenzene and PBDTTT-C-T form stable coplanar charge-transfer complexes through hydrogen bonds. Formation of the PBDTTT-C-T-C6H5NO2 complex simultaneously increases the external quantum efficiency. The underlying mechanisms of increased external quantum efficiency are attributed to the following: (i) higher lowest unoccupied molecular orbital (LUMO) of PBDTTT-C-T-C6H5NO2 for more efficient photoinduced electron transfer to the LUMO of PC71BM and (ii) efficient quenching of fluorescence in the active layers due to formation of the PBDTTT-C-T-C6H5NO2 complex. This discovery clearly illustrates the potential of hydrogen-bonded complexes as a new route for efficient polymer-based photovoltaic devices. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of drying time for the qualitative active layer of an organic photovoltaic device

We investigated the effect of the drying time on the active layer based on a regioregular poly(3-hexylthiophene) (RR-P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM), as well as the performance of polymer-based organic photovoltaic (OPV) devices. The devices were fabricated with different combinations of spin speed and solution concentration at a fixed thickness of the active layer. These conditions varied the properties of the active layer in terms of the polymer crystallinity and the phase separation in the blend. We identified distinct differences for different drying times of the solvent during the spin-coating process. The active layers were characterized by using analysis methods such as X-ray diffraction (XRD), ultraviolet-visible (UV-visible) spectroscopy and atomic force microscopy (AFM). The qualitative property of the active layer and the highest power conversion efficiency were obtained with the slowest coated active layer with the lowest solution concentration under AM 1.5 G spectral illumination of 100 mW/cm2.

Application of helium ion microscopy to nanostructured polymer materials

AbstractHelium ion microscopy (HIM) is a relatively new high-resolution nanotechnology imaging and nanofabrication tool. HIM offers a near-molecular resolution (approaching that of TEM) combined with a simplicity of sample preparation and high depth of field similar to SEM. Simultaneously, the technique is not limited by the surface roughness as scanning probe microscopy (SPM) techniques or by the surface charging or radiation damage like SEM. In our review, we consider general principles, advantages, and prospects of HIM application in polymer science. Examples of high-resolution imaging of polymer-based nanocomposites, polymer nanoparticles, nanofibers, nanoporous materials, polymer nanocrystals, biopolymers, and polymer-based photovoltaic and sensor devices are presented. We compare the HIM’s applicability with other modern imaging techniques: SPM and SEM.

Functional Polymer Nanocomposites Enhanced by Nanorods

Because of their shape anisotropy, nanorods are attractive components for creating functional polymer nanocomposites. In many cases, this anisotropy is the basis of physical properties that are distinct from those obtained from isotropic particles, such as nanospheres. For instance, the shape of gold nanorods makes them ideal candidates for applications involving the manipulation of incident light and sensitive molecular spectroscopy due to enhanced polarizability of the particle. On the other hand, semiconducting nanorods, such as those composed of CdSe, have shown promise in polymer-based photovoltaic devices as sites for electron transport and charge transfer. In this Perspective, we motivate the fabrication of functional polymer nanocomposites based specifically upon the inclusion of nanorods over other nanoparticle shapes and discuss ways in which the anisotropy of the individual nanoparticles enhances assembly in and the properties of polymer nanocomposites in comparison to spherical nanoparticles. ...

Impact of thermal stability of poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) used as buffer layer in organic solar cells

We compared the performances of polymer-based photovoltaic devices prepared from different formulations of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS). The PEDOT:PSS buffer layer is incorporated between the indium tin oxide (ITO) electrode and the active layer, which is composed of a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A highest efficiency of 3.86% (under AM1.5 solar illumination) was achieved for device prepared from a PEDOT:PSS trade-named high conductivity grade. However, annealing devices at a temperature over 120 °C results in decreased photovoltaic performance. This study shows that attention has to be paid to chemicals used to formulate high conductive PEDOT:PSS to become compatible with the production of solar cells involving thermal processing.

Aqueous-solution-processed hybrid solar cells with good thermal and morphological stability

To fabricate efficient solar cells, it is crucial to control the nanoscale morphology of the bulk-heterojunction (BHJ) film, in which the thermal and morphological stability is an important issue. In this article, aqueous-solution-processed hybrid photovoltaic devices from poly(1,4-naphthalene vinylene) (PNV) and CdTe nanocrystals (NCs) are fabricated and a power conversion efficiency (PCE) of 1.74% under AM1.5G illumination is achieved. Furthermore, we demonstrate the high thermal stability of hybrid solar cells by investigating the evolution of hybrid films with prolonged annealing time and elevated temperature. Thermal annealing is necessary for converting PNV precursor to PNV and removing the ligands on CdTe NCs, which leads to the improvement of device performance. In particular, the prolonged annealing time and elevated temperature do not lower the PCE, because the interconnected CdTe networks are highly rigid and thereby suppress the excess phase separation of PNV, which is one of the main issues determining the long-term stability of polymer-based photovoltaic devices. This advantage is considered to promote the development of hybrid solar cells from various aqueous-solution-processed polymers and NCs.

Computer simulation of heterogeneous polymer photovoltaic devices

Polymer-based photovoltaic devices have the potential for widespread usage due to their low cost per watt and mechanical flexibility. Efficiencies close to 9.0% have been achieved recently in conjugated polymer based organic solar cells (OSCs). These devices were fabricated using solvent-based processing of electron-donating and electron-accepting materials into the so-called bulk heterojunction (BHJ) architecture. Experimental evidence suggests that a key property determining the power-conversion efficiency of such devices is the final morphological distribution of the donor and acceptor constituents. In order to understand the role of morphology on device performance, we develop a scalable computational framework that efficiently interrogates OSCs to investigate relationships between the morphology at the nano-scale with the device performance.In this work, we extend the Buxton and Clarke model (2007 Modelling Simul. Mater. Sci. Eng. 15 13–26) to simulate realistic devices with complex active layer morphologies using a dimensionally independent, scalable, finite-element method. We incorporate all stages involved in current generation, namely (1) exciton generation and diffusion, (2) charge generation and (3) charge transport in a modular fashion. The numerical challenges encountered during interrogation of realistic microstructures are detailed. We compare each stage of the photovoltaic process for two microstructures: a BHJ morphology and an idealized sawtooth morphology. The results are presented for both two- and three-dimensional structures.

Electronic structures and chemical reactions at the interface between Li and regioregular poly (3-hexylthiophene)

Interfaces between metals and π-conjugated polymers play an important role in the organic electronic and optoelectronic devices such as polymer-based light-emitting diodes (PLEDs) and photovoltaic devices. In this study, synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS) have been applied to in situ investigate the chemical reactions and electronic structure during the interface formation of Li on the regioregular poly(3-hexylthiophene) (rr-P3HT) thin films. Upon Li adsorption onto P3HT at 300K, Li dopes electrons into P3HT, inducing the occurrence of the P3HT band bending. Moreover, Li can diffuse into the subsurface and react with both S and C atoms in the thiophene rings, leading to the formation of Li2S and Li–C complex. Compared to the interface of Ca/P3HT, the diffusion/reaction depth of Li is much larger at the Li/P3HT interface. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cutoff an energy level alignment diagram at the Li/rr-P3HT interface is derived.

Ultrathin and lightweight organic solar cells with high flexibility

Application-specific requirements for future lighting, displays and photovoltaics will include large-area, low-weight and mechanical resilience for dual-purpose uses such as electronic skin, textiles and surface conforming foils. Here we demonstrate polymer-based photovoltaic devices on plastic foil substrates less than 2 μm thick, with equal power conversion efficiency to their glass-based counterparts. They can reversibly withstand extreme mechanical deformation and have unprecedented solar cell-specific weight. Instead of a single bend, we form a random network of folds within the device area. The processing methods are standard, so the same weight and flexibility should be achievable in light emitting diodes, capacitors and transistors to fully realize ultrathin organic electronics. These ultrathin organic solar cells are over ten times thinner, lighter and more flexible than any other solar cell of any technology to date.

Photoconduction action spectra of regio-regular poly(3-hexylthiopene):TiO2 diodes

The development of polymer-based photovoltaic devices brings the promise of low-cost and lightweight solar energy conversion systems. This technology requires new materials and device architectures with enhanced efficiency and lifetime, which depends on the understanding of charge-transport mechanisms. Organic films combined with electronegative nanoparticles may form systems with efficient dissociation of the photogenerated excitons, thus increasing the number of carriers to be collected by the electrodes. In this paper we investigate the steady-state photoconductive action spectra of devices formed by a bilayer of regio-regular poly(3-hexylthiophene) (RRP3HT) and TiO2 sandwiched between ITO and aluminum electrodes (ITO/TiO2:RRP3HT/Al). Photocurrents were measured for distinct bias voltages with illumination from either side of the device. Heterojunction structures were prepared by spin coating a RRP3HT film on an already deposited TiO2 layer on ITO. Symbatic and antibatic curves were obtained and a model for photocurrent action spectra was able to fit the symbatic responses. The quantum yield increased with the electric field, indicating that exciton dissociation is a field-assisted process as in an Onsager mechanism. Furthermore, the quantum yield was significantly higher when illumination was carried out through the ITO electrode onto which the TiO2 layer was deposited, as the highly electronegative TiO2 nanoparticles were efficient in exciton dissociation.

Synthesis of Poly(fluorene-alt-thieno[3,4-b]thiophene) for Photovoltaic Devices

Polymer-based bulk heterojunction solar cells have been fabricated using blended films composed of an electrondonating conjugated polymer and an electron-accepting material such as a fullerene derivative; they are promising low-cost, flexible renewable energy sources. A solar cell’s power conversion efficiency (PCE) is the product of the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) divided by the incident light power density. The Voc of polymer solar cells is generally proportional to the energy difference between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor, suggesting that polymers with deep HOMO levels are advantageous if conditions are otherwise similar. Photovoltaic cells have had PCEs of ca. 5% reported when a mixture of poly(3-hexylthiophene) (P3HT) and a fullerene derivative were respectively used as the electron donor and acceptor. Interesting low band gap polymers consisting of alternating ester or ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units have been developed. Among them, a polymer consisting of a fluorinated ketone-substituted thieno[3,4-b]thiophene and alkoxybenzo[1,2-b:4,5-b']dithiophene units, whose band gap and HOMO level were 1.77 eV and −5.22 eV, respectively, exhibited a maximum PCE of 7.73%. We have also synthesized conjugated polymers for photovoltaic applications. Especially, a polymer consisting of alternating carbazole and ester group-substituted thieno[3,4b]thiophene units (PCTT1) showed a band gap of 2.1 eV and HOMO level of −5.52 eV. This polymer, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), when used as the photoactive layer of solar cells, yielded a maximum PCE of 2.53%. Polyfluorenes have advantages such as thermal and oxidation stabilities. They can also form films easily and have good hole-transporting properties. The band gaps and HOMO levels of fluorene-containing polymers have been reported to be similar to those of carbazole-containing counterpart polymers, though different PCEs were shown by resulting polymer-based solar cells. Thus poly(fluorenealt-thieno[3,4-b]thiophene polymer) (PFTT) was synthesized as a counterpart of PCTT1. This work describes the synthesis and characterization of the polymer along with preliminary tests of resulting polymer-based photovoltaic devices. Experimental Section

Organic−Inorganic Nanocomposites by Placing Conjugated Polymers in Intimate Contact with Quantum Rods

Recent advances in the synthesis [ 1 ] and assembly [ 2 ] of nanocrystals (NCs) provide unique opportunities to exploit NCs for the development of next generation organic/inorganic hybrid solar cells as one of the most promising alternatives to Si solar cells to deliver effi cient energy conversion with inexpensive fabrication. [ 1 , 2 ] These conjugated polymer-based photovoltaic devices capitalize on the advantages peculiar to conjugated polymers (CPs), such as light weight, fl exibility, processability, roll-to-roll production, low cost, and large area, in conjunction with the high electron mobility and tunable optical properties of inorganic NCs. Of the organic/inorganic hybrids, poly(3-hexylthiopene) (P3HT) is one of the most extensively utilized CPs due to its excellent solution processability, environmental stability, high charge carrier mobility, and tailorable electrochemical properties. [ 3 , 4 ] CdSe quantum dots (QDs) are the most commonly investigated NCs because of their quantum-confi ned nature and well-matched energy level with P3HT. [ 5–11 ]

P3HT/PCBM Bulk Heterojunction Organic Photovoltaics: Correlating Efficiency and Morphology

Controlling thin film morphology is key in optimizing the efficiency of polymer-based photovoltaic (PV) devices. We show that morphology and interfacial behavior of the multicomponent active layers confined between electrodes are strongly influenced by the preparation conditions. Here, we provide detailed descriptions of the morphologies and interfacial behavior in thin film mixtures of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), a typical active layer in a polymer-based PV device, in contact with an anode layer of PEDOT-PSS and either unconfined or confined by an Al cathode during thermal treatment. Small angle neutron scattering and electron microscopy show that a nanoscopic, bicontinuous morphology develops within seconds of annealing at 150 °C and coarsens slightly with further annealing. P3HT and PCBM are shown to be highly miscible, to exhibit a rapid, unusual interdiffusion, and to display a preferential segregation of one component to the electrode interfaces. The ultimate morphology is related to device efficiency.