Performance Of Bulk Heterojunction Research Articles

Power losses in conventional and inverted non-polymeric donor:fullerene bulk heterojunction solar cells - The role of vertical phase separation in BQR:PC71BM blends

The performance of bulk heterojunction (BHJ) organic solar cells can be affected by a range of factors including the materials combination, processing solvent, post deposition annealing, and/or whether they are used in a conventional or inverted architecture. In this study we compared conventional and inverted BHJ solar cells composed of a non-polymeric donor (5 Z ,5′ Z )‐5,5'‐[(5‴,5‴''''‐{4,8‐bis[5‐(2‐ethylhexyl)‐4‐ n -hexylthiophen‐2‐yl]benzo[1,2‐b:4,5‐b']dithiophene‐2,6‐diyl}bis{3′,3'',3‴‐trihexyl‐[2,2':5′,2'':5'',2‴‐quaterthiophene]‐5‴,5‐diyl})bis(methanylylidene)]bis[3‐ n -hexyl‐2‐thioxothiazolidin‐4‐one] (BQR) and [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor. It was found that the conventional device structure had power conversion efficiencies nearly two and a half times that of the inverted device, 9.0% versus 4.0%. Through a combination of Shockley equivalent circuit fitting, optical modelling, and light intensity dependent photocurrent measurements we identified that the origin of the power losses for the inverted architecture relative to the conventional device structure arose from a larger component of bimolecular recombination. Neutron reflectometry measurements showed that the origin of the larger bimolecular recombination losses for the inverted device was due to the PC 71 BM phase separating, with a PC 71 BM rich layer located near the anode reducing the hole extraction efficiency. • BQR and PC 71 BM conventional and inverted BHJ solar cells are compared. • The conventional device structure was two and a half times more efficient. • Increased bimolecular recombination identified as the origin of the power losses for the inverted architecture. • A PC 71 BM rich layer near the anode reduced the hole extraction efficiency in inverted device.

Influence of PEDOT:PSS morphology on performance of bulk heterojunction solar cells

Influence of morphology of poly(3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer on performance of bulk heterojunction (BHJ) organic solar cells is analysed. Morphology depends on next technological factors: dispersity of PEDOT:PSS solution, substrate treatment, drying, PEDOT:PSS treatment. It is shown that samples with layer roughness less 10 nm have the best performance. Layer roughness was studied by atomic force microscopy (AFM).

Carrier losses in non-geminate charge-transferred states of nonfullerene acceptor-based organic solar cells

To understand the current limitations of nonfullerene-based organic solar cells (OSCs), the early-time dynamics of the carrier generation in the high performance bulk heterojunction (BHJ) blend of a semiconducting polymer, PBDB-T, and the low bandgap nonfullerene acceptor, ITIC-m, are investigated. After photoexcitation, photo-induced excitons are separated through the ultrafast (~200 fs) electron transfer process from PBDB-T to ITIC-m and through the fast (3–6 ps) hole transfer process from ITIC-m to PBDB-T. However, a part of the separated charges recombines in the non-geminate (long-range) charge-transferred (CT) states. The yield of mobile carriers is correspondingly decreased by recombination in the CT states. In our measurements, the carrier recombination loss in the CT state is decreased by optimizing the BHJ morphology, especially for showing better electron mobility using a processing additive (1,8-diiodooctane) during the fabrication of the composite film, as evidenced by the decreased CT band intensity at ~30 ps and the increased polaron band intensity, which eventually improve power conversion efficiencies (PCEs).

Characterization and Performance of PAni-TiO2 Photovoltaic Cells Treated by RF Plasma

Severe research attempts are still in progress to improve the performance of polyaniline (PAni) based photoactive layers as one of the cheapest materials used for manufacturing organic solar cells. Herein, polymer solar cells were fabricated with ITO/(PAni-TiO2)/Au system. The photoactive layers (PAni-TiO2) were treated with a hydrogen-plasma discharge for low processing time of 0, 3 and 5 min to enhance the synthesized solar cells efficiency. The morphology, micro structure and optical properties of the prepared samples and plasma treated nanocomposite layers were investigated and discussed. The performance of bulk heterojunction (BHJ) cell samples have been systematically investigated before and after plasma treatment. The absorption and optical band gap energy is increased after the treated PAni-TiO2 photoactive layers. It is found that, the efficiency was enhanced to 0.7% after 5 min of hydrogen plasma process compared to 0.36% for the pristine cell. The efficiency increase is ascribed to a structural change that accompanied by a rapid increase in surface roughness, which leaded to a decrease in the reflected photons and in turn an increase in the produced charge carriers.

Optoelectronic properties of cyclopentadithiophene-based donor–acceptor copolymers as donors in bulk heterojunction organic solar cells: A theoretical study

Cyclopentadithiophene and benzothiadiazole derivatives have promising characteristics for use in organic solar cells. In this study, we theoretically investigated two donor–acceptor copolymers comprising 2-(benzo[c][1,2,5]thiadiazol-4-yl)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-one and 2-(2-(benzo[c][1,2,5]thiadiazol-4-yl)-4H-cyclopenta[2,1-b:3,4-b']dithiophen-4-ylidene)malononitrile with alternating donor–acceptor building units. Different copolymer chain lengths (n = 1–5) were extended to investigate their structural, electronic, and photo-physical properties based on density functional theory calculations. The copolymers exhibited high planarity, reduced band gap energies, and broad absorption in the visible range, and thus they may enhance the performance of organic solar cells. The transition density matrix plots were simulated for the copolymers and used to evaluate hole–electron localization interactions, donor–acceptor interactions, and the electronic excitation processes in their excited states. The charge transfer properties, fill factors, and open-circuit voltages were estimated. Based on our simulations, we found that the trimer was the most suitable conformation among the copolymers for designing high performance bulk heterojunction (BHJ) solar cells. The power conversion efficiency of the BHJ-based blended copolymers-[70]PCBM was estimated as 9.5%. Thus, the appropriate design of conjugated materials could considerably enhance the donor properties to further improve the performance of BHJ solar cells.

Tailoring Morphology Compatibility and Device Stability by Adding PBDTTPD-COOH as Third Component to Fullerene-Based Polymer Solar Cells

The crystallinity and morphology of polymer and fullerene have a profound influence on the performance of bulk heterojunction (BHJ) organic photovoltaic devices. The poor compatibility of donor and...

The improved performance of BHJ organic solar cells by random dispersed metal nanoparticles through the active layer

In this paper, the performance of bulk heterojunction (BHJ) organic solar cells (OSCs) in the presence of random dispersed Ag nanoparticles (NPs) into the active layer is investigated. By the well-known Maxwell-Garnett effective medium theory, we have analyzed the optical absorption of P3HT:PCBM and PCDTBT:PCBM photoactive blends when spherical Ag NPs are randomly embedded in them. The photocurrent enhancement of OSCs based on the blend:AgNP composites has been examined by an analytical drift-diffusion method. Our theoretical analysis demonstrates a considerable enhancement in the optical absorption of the blends with Ag NPs resulted in the power conversion efficiency (PCE) improvement of the devices up to 65.6% in comparison with the reference blends. In addition, the spectrally correlation of simulated external quantum efficiency (EQE) and the absorption coefficient enhancements proves the influence of localized surface plasmon resonance (LSPR) of the Ag NPs in boosting the performance of the OSC devices.

Thermally induced degradation of PBDTTT-CT:PCBM based polymer solar cells

Thermally induced degradation of photovoltaic performance in bulk-heterojunction (BHJ) polymer solar cells as a result of changes either in the active layer morphology or at interfaces during operation at elevated temperature is a common phenomenon. In this work, we have studied the thermal stability of a high performance polymer:fullerene BHJ PSCcomprising a conjugated polymer poly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b0]dithiophene-2,6-diyl]-alt-[2-(20-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-CT) and a fullerene derivative [6,6]-phenyl-C70-butyric acid methyl ester photoactive layer within a conventional device architecture of glass/ITO/PEDOT:PSS/active layer/Mg/Al. By varying the temperature exposure conditions, the degradation path has been identified as an interfacial change in the device rather than a bulk effect. Furthermore, charge carrier dynamics studied by open circuit corrected charge carrier extraction has shown that post-annealed devices suffer from charge extraction due to the development of interfacial changes as compared to the non-treated devices in both pristine and with 1,8-Diiodooctane added scenarios.

Effects of neat C60 doping on the performance of bulk-heterojunction solar cells based on P3HT:PCBM

ABSTRACTWe explored the effect of neat C60 doping on the performance of bulk heterojunction (BHJ) organic photovoltaic cells (OPVs) based on poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The power conversion efficiencies (PCEs) were found to gradually decrease as the relative concentration of C60 increased. This PCE decrease is mainly due to the reduction in short-circuit current density (Jsc). Atomic force microscopy images revealed that C60-doped BHJ films consisted of unformed large P3HT crystals and many small PCBM aggregates. This morphology leads to a decrease in the effective P3HT conjugation length and an enhancement of absorption bleaching in neutral P3HT chains. These results explain well the blue-shift absorption spectra, the decrease of absorption coefficients, and the reduction in Jsc found in C60-doped BHJ OPVs.

Decakis(arylthio)corannulenes: Transferable Photochemical and Redox Parameters and Photovoltaic Device Performance

Absorption and redox properties of seven decakis(phenylthio)corannulene derivatives 3a–3g lead to a model for estimating material properties in persulfurated aromatic compounds. The same series is evaluated for performance in bulk heterojunction and perovskite photovoltaic devices.

The Study of Crystallization of Polyfluorene and Fullerene Derivatives in Semiconducting Layer of Organic Solar Cells

This research was focused on the effect of solid crystallization additive namely 1,4-dichlorobenzene (PDCB) in the 1:2 (w/w) active layer of benzothiadiazole/thiophene-based copolymers (PFTBzTT) to [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) on the morphology and performance of bulk heterojunction (BHJ) organic solar cells. The active layer was deposited by spin-coating with chloroform solutions by different PDCB additive concentrations from 0-52 mg/ml. The inclusion of additive into the polymer solution was able to improve the performance of BHJ solar cells. The maximum power conversion efficiency (PCE) of 0.84% achieved for a cell with PDCB concentration of 36 mg/ml after annealing at 180 °C for 20 min. The XRD and TEM techniques used to analyse the crystal structure and morphology of the thin films. From these results were found that PDCB additive presented higher level of PCBM crystal structure by more aggregation of PCBM and a larger extent of phase separation than those of the films without additive. The AFM results demonstrated that the optimum PDCB concentration and annealing process helped PCBM aggregated into micron sized crystal rods.

Effects of electron-withdrawing group and electron-donating core combinations on physical properties and photovoltaic performance in D-π-A star-shaped small molecules

The first representatives of star-shaped molecules having 3-alkylrhodanine (alkyl-Rh) electron-withdrawing groups, linked through bithiophene π-spacer with electron-donating either triphenylamine (TPA) or tris(2-methoxyphenyl)amine (m-TPA) core were synthesized. The physical properties and photovoltaic performance of these novel molecules with 3-ethylrhodanine groups were comprehensively studied and compared to their full analogs having dicyanovinyl (DCV) units as the other type of well-known and frequently used acceptor groups. On one hand, the former demonstrate several advantages such as higher solubility and better photovoltaic performance in bulk-heterojunction (BHJ) organics solar cells (OSCs) as compared to the latter. Nevertheless, the former have slightly lower optical/electrochemical bandgaps and higher thermooxidation stability. On the other hand, molecules of both series based on m-TPA core along with higher solubility and higher position of HOMO energy levels have more pronounced tendency to crystalize as compared to the TPA-based molecules. Detailed investigation of the structure-property relationships for these series of molecules revealed that donor and acceptor unit combinations influence both charge generation and charge transport/recombination properties, as demonstrated by the ultrafast photoinduced absorption spectroscopy, space charge limited current measurements and transient photovoltage technique. These results give more insight how to fine-tune and predict physical properties and photovoltaic performance of small molecules having either alkyl-Rh or DCV units in their chemical structures and thus providing a molecular design guideline for the next generation of high-performance photovoltaic materials.

Effects of thermal-annealing and processing-additive treatment on crystallization-induced phase separation in organic solar cells utilizing octapentyl tetrabenzotriazaporphyrins

Effects of thermal-annealing and processing-additive treatment on crystallization-induced phase separation and photovoltaic performance in bulk heterojunction (BHJ) organic solar cells utilizing non-peripherally substituted octapentyl tetrabenzotriazaporphyrin (C5TBTAPH2) mixed in 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C71 ([70]PCBM) were reported. By using either thermal-annealing or processing-additive treatment, the C5TBTAPH2-crystallite sizes in the BHJ films were enlarged and the power conversion efficiencies (PCEs) were increased due to improvement of the crystallization-induced phase separation. However, the optimum PCEs (4.2%) of the processing-additive treated devices were markedly higher than those PCEs (1.4%) of devices treated by thermal annealing. These results were discussed by taking the thermal properties and molecular aggregation of [70]PCBM into consideration.

Effects of alkyl chain length on the optoelectronic properties and performance of pyrrolo-perylene solar cells.

While the impact of alkyl side-chain length on the photovoltaic properties of conjugated polymers and their performance in bulk heterojunction (BHJ) solar cells has been studied extensively, analogous studies on pyrrolo-perylene-based polymers have not received adequate attention. To explore these effects, we synthesized two copolymers based on N-annulated pyrrolo-perylene and consisting of cyano group substituents on thiophene vinylene thiophene units with two different alkyl groups of 2-decyltetradecyl and 7-decylnonadecyl, and we studied them with regard to chemical structure and photovoltaic performance. UV-vis spectroscopy and cyclic voltammetry studies showed that variations in alkyl chain length affect crystallization, light absorption, and the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels. These factors have a pronounced impact on the morphology of BHJ thin films and their charge carrier separation and transportation characteristics, which, in turn, influences photovoltaic properties.

N-Type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells.

Knowledge of the critical factors that determine compatibility, blend morphology, and performance of bulk heterojunction (BHJ) solar cells composed of an electron-accepting polymer and an electron-donating polymer remains limited. To test the idea that bulk crystallinity is such a critical factor, we have designed a series of new semiconducting naphthalene diimide (NDI)-selenophene/perylene diimide (PDI)-selenophene random copolymers, xPDI (10PDI, 30PDI, 50PDI), whose crystallinity varies with composition, and investigated them as electron acceptors in BHJ solar cells. Pairing of the reference crystalline (crystalline domain size Lc = 10.22 nm) NDI-selenophene copolymer (PNDIS-HD) with crystalline (Lc = 9.15 nm) benzodithiophene-thieno[3,4-b]thiophene copolymer (PBDTTT-CT) donor yields incompatible blends, whose BHJ solar cells have a power conversion efficiency (PCE) of 1.4%. However, pairing of the new 30PDI with optimal crystallinity (Lc = 5.11 nm) as acceptor with the same PBDTTT-CT donor yields compatible blends and all-polymer solar cells with enhanced performance (PCE = 6.3%, Jsc = 18.6 mA/cm(2), external quantum efficiency = 91%). These photovoltaic parameters observed in 30PDI:PBDTTT-CT devices are the best so far for all-polymer solar cells, while the short-circuit current (Jsc) and external quantum efficiency are even higher than reported values for [70]-fullerene:PBDTTT-CT solar cells. The morphology and bulk carrier mobilities of the polymer/polymer blends varied substantially with crystallinity of the acceptor polymer component and thus with the NDI/PDI copolymer composition. These results demonstrate that the crystallinity of a polymer component and thus compatibility, blend morphology, and efficiency of polymer/polymer blend solar cells can be controlled by molecular design.

Improvement of bulk heterojunction organic solar cells based on PTB7:PC61BM with small amounts of P3HT

The effect of a small amount of poly(3-hexylthiophene) (P3HT) additive on the performance of bulk heterojunction (BHJ) organic solar cells based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C61-butyric acid methyl ester (PTB7:PC61BM) was investigated. The optimized solar cell with 5% P3HT weight fraction showed a marked improvement, with an open-circuit voltage of 0.76 V, a short-circuit current density of 17 mA/cm2, a fill factor of 0.40, and a power conversion efficiency of 5.11%. A notable reduction in the domain sizes of the active layers with 5% P3HT weight fraction was observed by atomic force microscopy. This suggests that the interface area between the PTB7 and PC61BM domains increased, resulting in an enhanced exciton dissociation. This also suggests that the small amount of P3HT in the PTB7:PC61BM active layer acted as a mediator to enhance the hole current from PTB7 domains to the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) anode buffer layer. Possible mechanisms for these phenomena are discussed on the basis of the results of surface energy analysis.

Bulk Heterojunction Photovoltaic Cells with Triphenylamine-Based Amorphous Polymer and Non-Halogenated Solvent Processing Provide Reproducible Performance

The performance of bulk heterojunction (BHJ) organic photovoltaics (OPVs) based on an amorphous polymer, poly(3HTBT-TPA), and fabricated using either halogenated or non-halogenated solvents was investigated. All the BHJ OPVs exhibited almost the same power conversion efficiencies of around 2.2%, indicating that their performance is independent of the casting solvent. These experimental results indicate that the use of amorphous π-conjugated polymers in the fabrication of BHJ OPVs offers two advantages over the use of polycrystalline π-conjugated polymers: highly reproducible OPV performance and the possibility of an environmentally friendly process for fabricating OPVs.

Side chain modification: an effective approach to modulate the energy level of benzodithiophene based polymers for high-performance solar cells

Over the past few years, it has been proven that deepening the highest occupied molecular orbital (HOMO) levels of conjugated polymers is one of the most successful strategies to develop novel materials for high performance bulk heterojunction polymer solar cells.

Development of non-halogenated binary solvent systems for high performance bulk-heterojunction organic solar cells

Halogenated solvents such as dichlorobenzene (DCB) are commonly used in the fabrication of bulk-heterojunction (BHJ) organic solar cells. However, most halogenated solvents are very toxic. This report provides a route to eliminate halogenated solvents based on Hansen Solubility Parameters (HSPs) for manufacturing high efficiency BHJ solar cells. The device performances fabricated from several different binary solvent systems were compared to that from DCB. For the investigation of different gel behavior of solvent mixtures, Hansen solubility parameters and viscosity were mainly considered for the selection of binary solvent mixture candidates. Upon the addition of 20vol.% benzaldehyde (BA) to p-xylene (p-XL), the device performances were improved in power conversion efficiency (PCE) from 3.1% to 3.8% and external quantum efficiency (EQE) from 63% to 70%. The solar cell devices fabricated using p-XL/BA binary solvent mixtures exhibited PCE comparable to that from DCB.

New insights into morphology of high performance BHJ photovoltaics revealed by high resolution AFM.

Direct imaging of the bulk heterojunction (BHJ) thin film morphology in polymer-based solar cells is essential to understand device function and optimize efficiency. The morphology of the BHJ active layer consists of bicontinuous domains of the donor and acceptor materials, having characteristic length scales of several tens of nanometers, that reduces charge recombination, enhances charge separation, and enables electron and hole transport to their respective electrodes. Direct imaging of the morphology from the molecular to macroscopic level, though, is lacking. Though transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy (AFM) in the tapping and nanomechanical modes to investigate the BHJ active layer morphology that, when combined with Ar(+) etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a "face-on" orientation and the morphology at the anode interface was markedly different.