Bulk Heterojunction Organic Photovoltaics Research Articles

Photocrosslinkable liquid–crystalline polymers for stable photovoltaics by adjusting side-chains spacing and fullerene size to control intercalation

We report a novel copolymer system with high crystallinity and photocrosslinkable building blocks for π–π intermolecular interactions that is, an alternating copolymer with liquid–crystalline nature, and heat/solvent resistance. By copolymerization of 2,5-bis(3-bromododecylthiophen-2-yl)thieno[3,2-b]thiophene (BbTTT) monomer with thiophene and thieno[3,2-b]thiophene via Stille reaction, two novel copolymers PBbTTT-T and PBbTTT-TT have been synthesized. The balanced space between fullerene size and the side-chains of the polymer is crucial to determine the optimum polymer:fullerene blending ratios and the formation of intercalated nanostructure, in which the lamellar arrangement can be controlled by adjusting the fullerene size. This pre-optimum bimolecular crystal morphology can be frozen and preserved with long term performance after UV treatment, a clear advantage for the photo-crosslinking strategy. Furthermore, the free space in the intercalation impacts greatly on the stability of the donor–acceptor bicontinuous network, especially after photocrosslinking. The bulk-heterojunction organic photovoltaics based on PBbTTT-TT:PC71BM at 1:3.5 by weight shows an stable, well-ordered and intercalated nanostructure with an efficiency higher than 2.4% after 40h annealing at an elevated temperature of 150°C.

Pyridalthiadiazole-Based Narrow Band Gap Chromophores

π-Conjugated materials containing pyridal[2,1,3]thiadiazole (PT) units have recently achieved record power conversion efficiencies of 6.7% in solution-processed, molecular bulk-heterojunction (BHJ) organic photovoltaics. Recognizing the importance of this new class of molecular systems and with the aim of establishing a more concrete path forward to predict improvements in desirable solid-state properties, we set out to systematically alter the molecular framework and evaluate structure-property relationships. Thus, the synthesis and properties of 13 structurally related D(1)-PT-D(2)-PT-D(1) compounds, where D represents a relatively electron-rich aromatic segment compared to PT, are provided. Physical properties were examined using a combination of absorption spectroscopy, cyclic voltammetry, thermal gravimetric analysis, differential scanning calorimetry, and solubility analysis. Changes to end-capping D(1) units allowed for fine control over electronic energy levels both in solution and in the bulk. Substitution of different alkyl chains on D(2) gives rise to controllable melting and crystallization temperatures and tailored solubility. Alterations to the core donor D(2) lead to readily identifiable changes in all properties studied. Finally, the regiochemistry of the pyridal N-atom in the PT heterocycle was investigated. The tailoring of structures via subtle structural modifications in the presented molecular series highlights the simplicity of accessing this chromophore architecture. Examination of the resulting materials properties relevant for device fabrication sets forth which can be readily predicted by consideration of molecular structure and which lack a systematic understanding. Guidelines can be proposed for the design of new molecular frameworks with the possibility of outperforming the current state of the art OPV performance.

High-Performance Semitransparent Bulk-Heterojunction Organic Photovoltaics with Ag Interfacial Layer

Semitransparent bulk-heterojunction organic photovoltaics with power conversion efficiency (PCE) of 1.63±0.39% was achieved by using poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) active layer and Ag/Ca/Ag transparent cathode. Insertion of a thin Ag interfacial layer (1 nm) between the P3HT/PCBM active layer and Ca/Ag transparent cathode dramatically improved the PCE from 0.83±0.33 to 1.63±0.39% as well as other photovoltaic properties such as fill factor, short-circuit current, and series resistance. In this experiment, it is considered that the improved device performance is possibly due to improved contact property by the Ag interfacial layer at the active/cathode interface.

High-Performance Semitransparent Bulk-Heterojunction Organic Photovoltaics with Ag Interfacial Layer

Semitransparent bulk-heterojunction organic photovoltaics with power conversion efficiency (PCE) of 1.63±0.39% was achieved by using poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PCBM) active layer and Ag/Ca/Ag transparent cathode. Insertion of a thin Ag interfacial layer (1 nm) between the P3HT/PCBM active layer and Ca/Ag transparent cathode dramatically improved the PCE from 0.83±0.33 to 1.63±0.39% as well as other photovoltaic properties such as fill factor, short-circuit current, and series resistance. In this experiment, it is considered that the improved device performance is possibly due to improved contact property by the Ag interfacial layer at the active/cathode interface.

Measuring Domain Sizes and Compositional Heterogeneities in P3HT‐PCBM Bulk Heterojunction Thin Films with 1H Spin Diffusion NMR Spectroscopy

AbstractThe application of 1H spin diffusion nuclear magnetic resonance (NMR) is expanded to polymer‐fullerene blends for bulk heterojunction (BHJ) organic photovoltaics (OPV) by developing a new experimental methodology for measuring the thin films used in poly‐3‐hexylthiophene–phenyl C61‐butyric acid methyl ester (P3HT‐PCBM) OPV devices and by creating an analysis framework for estimating domain size distributions. It is shown that variations in common P3HT‐PCBM BHJ processing parameters such as spin‐coating speed and thermal annealing can significantly affect domain size distributions, which in turn affect power conversion efficiency. 1H spin diffusion NMR analysis reveals that films spin‐cast at fast speeds in dichlorobenzene are primarily composed of small (<10 nm) domains of each component; these devices exhibit low power conversion efficiencies (η = 0.4%). Fast‐cast films improve substantially by thermal annealing, which causes nanometer‐scale coarsening leading to higher efficiency (η = 2.2%). Films spin‐cast at slow speeds and then slowly dried exhibit larger domains and even higher efficiencies (η = 2.6%), but do not benefit from thermal annealing. The 1H spin diffusion NMR results show that a significant population of domains tens of nanometers in size is a common characteristic of samples with higher efficiencies.

Synthesis and Photovoltaic Properties of Novel Monoadducts and Bisadducts Based on Amide Methanofullerene

Four new [6,6]-phenyl-C(61) and C(71) butylsaure n-dibutyl amides (PCBDBA) with mono- and bis-adduction on C(60) and C(70) cages, respectively, have been synthesized as models to study the effect of the mono- and bis-adduction on fullerene cages on device performance when used as electron acceptors with the donor of regioregular P3HT in bulkheterojunction organic photovoltaics (BHJ-OPV). The optoelectronic, electrochemistry, and photovoltaic properties of these mono- and bis-products were fully investigated. The best device performance of these fullerene derivatives were obtained from the two monoadducts with power conversion efficiency (PCE) of 1.77% for C(60) derivative and 1.90% for C(70) derivative, respectively, which are close to PCBM's 2.43%. The results revealed the structure-function relationship among the monoadduct and bisadduct derivatives of C(60) and C(70) with the BHJ-OPV performance.

Comparison of thiophene- and selenophene-bridged donor–acceptor low band-gap copolymers used in bulk-heterojunction organic photovoltaics

We report a detailed comparison of absorption spectroscopy, electrochemistry, DFT calculations, field-effect charge mobility, as well as organic photovoltaic characteristics between thiophene- and selenophene-bridged donor–acceptor low-band-gap copolymers. In these copolymers, a significant reduction of the band-gap energy was observed for selenophene-bridged copolymers by UV-visible absorption spectroscopy and cyclic voltammetry. Field-effect charge mobility studies reveal that the enhanced hole mobility of the selenophene-bridged copolymers hinges on the solubilising alkyl side chain of the copolymers. Both cyclic voltammetry experiments and theoretical calculations showed that the decreased band-gap energy is mainly due to the lowering of the LUMO energy level, and the raising of the HOMO energy level is just a secondary cause. These results are reflected in a significant increase of the short circuit current density (JSC) but a slight decrease of the open circuit voltage (VOC) of their bulk-heterojunction organic photovoltaics (BHJ OPVs), of which the electron donor materials are a selenophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl) benzo[c][1,2,5]-thiadiazole} (pCzSe) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(selenophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTSe), or a thiophene-bridged donor–acceptor copolymer: poly{9-dodecyl-9H-carbazole-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pCzS) or poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;4,5-b′]dithiophene-alt-5,6-bis(dodecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]-thiadiazole} (pBDTS); the electron acceptor material is [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Judging from our device data, the potential Se–Se interactions of the selenophene-bridged donor–acceptor copolymers, which is presumably beneficial for the fill factor (FF) of BHJ OPVs, is rather susceptible to the device fabrication conditions.

Yield of exciton dissociation in a donor–acceptor photovoltaic junction

A simple model is constructed to describe dissociation of charge transfer excitons in bulk heterojunction solar cells, and its dependence on the physical parameters of the system. In bulk heterojunction organic photovoltaics (OPVs), exciton dissociation occurs almost exclusively at the interface between the donor and acceptor, following one-electron initial excitation from the HOMO to the LUMO levels of the donor, and charge transfer to the acceptor to make a charge-transfer exciton. After exciton breakup, and neglecting the trapping of individual carriers, the electron may undergo two processes for decay: one process involves the electron and/or hole leaving the interface, and migrating to the electrode. This is treated here as the electron moving on a set of acceptor sites. The second loss process is radiationless decay following recombination of the acceptor electron with the donor cation; this is treated by adding a relaxation term. These two processes compete with one another. We model both the exciton breakup and the subsequent electron motion. Results depend on tunneling amplitude, energetics, disorder, Coulomb barriers, and energy level matchups, particularly the so-called LUMO-LUMO offset.

Poly(3-hexylthiophene): synthetic methodologies and properties in bulk heterojunction solar cells

Poly(3-hexylthiophene) (P3HT) remains of significant importance as a prototypical benchmark hole conductor material in Organic Photovoltaics (OPVs). In this review we discuss synthetic strategies to P3HT, particularly focusing on those leading to the regioregular form and discussing key physical and morphological properties. Finally, a survey and a brief discussion of P3HT performance in bulk-heterojunction (BHJ) OPVs are also provided.

Solution processed small molecule bulk heterojunction organic photovoltaics based on a conjugated donor–acceptor porphyrin

Due to the π-conjugation of the whole molecule along with the push–pull property of a newly designed conjugated donor–acceptor porphyrin, the solution processed solar cells using the blend of the porphyrin small molecule with PC71BM exhibit a promising performance with a power conversion efficiency of up to 4.02%.

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.

Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoOx) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N2 atmosphere and annealed in air yield continuous thin films of MoOx. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoOx and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoOx thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoOx HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo5+ gap states in the MoOx HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoOx HCL.

The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics

The domain size, domain purity and interfacial width between domains for a bulk heterojunction are controllably altered through use of Cahn–Hilliard modeling and their relative effect on OPV performance is predicted using Monte Carlo modeling. It is found that locally sharp, well-connected domains of only 4 nm extent out perform morphologies with broadened interfaces and/or impure domains even when domain sizes were at the ‘optimum’ size of ∼10 nm. More generally, these data provide information on the most effective method to optimize the as-cast bulk heterojunction morphology depending upon initial domain purity and the nature of interfaces between domains. Further, it indicates why morphology optimization is more effective for some blends than others. It is shown that the quench depth of the blend can be used as a general technique to control the interfacial structure of the morphology and realize substantial increases in short circuit photocurrent.

Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles

A key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet-excitons) in the donor polymer, and charge-transfer-excitons at the donor–acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of singlet-excitons and charge-transfer-excitons. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester are fabricated. Upon incorporating a ferroelectric polymer as additive in the active-layer, power conversion efficiencies increase by nearly 50%, and internal quantum efficiencies approach 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric poling on device behavior show that improved dissociation is due to ferroelectric dipoles rather than any morphological change. Enhanced singlet-exciton dissociation is also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.

Solvent-Resistant Organic Transistors and Thermally Stable Organic Photovoltaics Based on Cross-linkable Conjugated Polymers

Conjugated polymers, in general, are unstable when exposed to air, solvent, or thermal treatment, and these challenges limit their practical applications. Therefore, it is of great importance to develop new materials or methodologies that can enable organic electronics with air stability, solvent resistance, and thermal stability. Herein, we have developed a simple but powerful approach to achieve solvent-resistant and thermally stable organic electronic devices with a remarkably improved air stability, by introducing an azide cross-linkable group into a conjugated polymer. To demonstrate this concept, we have synthesized polythiophene with azide groups attached to end of the alkyl chain (P3HT-azide). Photo-cross-linking of P3HT-azide copolymers dramatically improves the solvent resistance of the active layer without disrupting the molecular ordering and charge transport. This is the first demonstration of solvent-resistant organic transistors. Furthermore, the bulk-heterojunction organic photovoltaics (B...

Effect of carbon nanotube-fullerene hybrid additive on P3HT:PCBM bulk-heterojunction organic photovoltaics

The use of carbon nanotube-fullerene hybrid additives to the standard P3HT:PCBM blend bulk-heterojunction organic photovoltaic device is presented. The effects of incorporating single-wall carbon nanotubes, functionalized with linked C 60 molecules by amination on device characteristics are detailed. The concentration of the carbon nanotube-fullerene hybrid in the active layer blend was varied to ascertain their cumulative impact on device performance in terms of open circuit voltage, short circuit current, fill factor and efficiency. We found that decreasing the length of the carbon nanotubes to ∼60 nm through fluorination and subsequent thermal treatment was beneficial in terms of eliminating shorted devices allowing much improved diode formation. A trend of improving device performance as a function of concentration of the carbon nanotube-hybrids in a heterojunction of P3HT:PCBM was observed.

Sputtered nickel oxide thin film for efficient hole transport layer in polymer–fullerene bulk-heterojunction organic solar cell

Bulk-heterojunction (BHJ) organic photovoltaics (OPV) are very promising thin film renewable energy conversion technologies due to low production cost by high-throughput roll-to-roll manufacturing, an expansive list of compatible materials, and flexible device fabrication. An important aspect of OPV device efficiency is good contact engineering. The use of oxide thin films for this application offers increased design flexibility and improved chemical stability. Here we present our investigation of radio frequency magnetron sputtered nickel oxide (NiOx) deposited from oxide targets as an efficient, easily scalable hole transport layer (HTL) with variable work-function, ranging from 4.8 to 5.8eV. Differences in HTL work-function were not found to result in statistically significant changes in open circuit voltage (Voc) for poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ device. Ultraviolet photoemission spectroscopy (UPS) characterization of the NiOx film and its interface with the polymer shows Fermi level alignment of the polymer with the NiOx film. UPS of the blend also demonstrates Fermi level alignment of the organic active layer with the HTL, consistent with the lack of correlation between Voc and HTL work-function. Instead, trends in jsc, Voc, and thus overall device performance are related to the surface treatment of the HTL prior to active layer deposition through changes in active layer thickness.

Evidence for near-Surface NiOOH Species in Solution-Processed NiOx Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics

The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiOx) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O2-plasma treatment of the as-deposited NiOx films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O2-plasma treated NiOx films are compared using both electron-only and BHJ devices. O2-plasma-treated NiOx interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiOx interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiOx films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.

Air-stable triarylamine-based amorphous polymer as donor material for bulk-heterojunction organic solar cells

A new air-stable triarylamine-based amorphous polymer, TSP-T11, which consists of thiophene and triarylamine units, can be successfully utilized to fabricate bulk-heterojunction organic photovoltaics (OPVs) using PC 60BM or PC 70BM as acceptor materials. The highest level of performance of OPVs optimized at TSP-T11:PC 70BM (weight ratios of 1:4) with thicknesses of 68 nm exhibited an open circuit voltage ( V oc) of 0.75 V, a short circuit current ( J sc) of 8.03 mA cm −2, and a power-conversion efficiency (PCE) of 2.22% under simulated air mass 1.5 solar irradiation at 100 mW cm −2. Although TSP-T11 has a lower hole mobility (1.5×10 −4 cm 2 V −1 s −1) than P3HT, the use of amorphous film of TSP-T11 as a donor material for OPVs offers advantages over the use of polycrystalline film of P3HT in terms of its air-stability and pinhole-free homogeneous morphology.

Bulk Heterojunction Solar Cells – Tuning of the HOMO and LUMO Energy Levels of Pyrrolic Squaraine Dyes

AbstractThe optimization of organic photovoltaic (OPV) devices requires, amongst several factors mainly related to thin‐film blend morphology, the capability to finely tune the characteristic energy levels of both the donor and the acceptor materials constituting the active layer. Squaraine compounds represent one of the most promising small molecule materials to be employed as the donor in the bulk heterojunction (BHJ) OPV configuration. In this study a series of arylhydrazone end‐capped symmetric squaraine compounds bearing electron‐withdrawing substituents on the aryl ring has been synthesized with the aim of increasing the device open circuit voltage by shifting the highest occupied molecular orbital (HOMO) energy level. These new semiconductors have been characterized by optical spectroscopy (UV/Vis absorption and IR transmittance) and electrochemical measurements (cyclic voltammetry and differential pulsed voltammetry). Our data enable the establishment of interesting structure–property relationships relating the nature, position and number of electron‐withdrawing substituents to the position of the donor HOMO and lowest unocupied molecular orbital (LUMO) levels. The low energy shift effect on the HOMO level with respect to the parent unsubstituted squaraine can be as high as 0.3 eV. The effect is so strong that the derivative bearing the strongest acceptor no longer behaves as a donor semiconductor. Preliminary results on OPV device preparation and characterization are also discussed.