Bulk Heterojunction Photovoltaic Cells Research Articles

Effect of low concentrations of carbon black in organic solar cells

We demonstrate for the first time the efficiency improvement of poly(3-hexylthiophene) (P3HT)-fullerene (C60) bulk heterojunction photovoltaic cells by the introduction of trace concentrations of carbon black (CB) into the photoactive layer. As compared to control devices with only C60, the addition of low concentration (12.5ppm) of CB resulted in 35% improvement in short circuit current density (Jsc), and 79% improvement in power conversion efficiency (PCE). CB served the dual function of enhancing electron transport as well as optimizing phase separation leading to high current density and PCE. The results indicate that CB is a promising additive for the performance enhancement of polymer photovoltaic cells and may work with diverse donor–acceptor systems.

Increased performance of inverted organic photovoltaic cells using a cationically functionalized fullerene interfacial layer

The incorporation of a cationically functionalized fullerene interfacial layer (NMFP-Br) into an inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction photovoltaic cell results in a significant power conversion efficiency (PCE) improvement of 67%, from 2.1% to 3.5%, relative to cells without an interfacial layer. The incorporation of NMFP-Br as an ITO modifying interfacial layer results in a 190mV increase in the open-circuit voltage, 13% increase in fill factor, and 250% reduction in series resistance. Cell efficiencies are greater than or comparable to other reported cells based on organic electron injection layers with the same active layer and electrode configuration. The orthogonal solubility afforded by ionic functionality allows for sequential solution phase deposition without the necessity of chemical cross-linking of the fullerene based interfacial layer. Inverted devices incorporating a single phase, ionically functionalized fullerene interfacial layer have not been previously demonstrated. The unusually high conductivity of NMFP-Br films is investigated and shown to be ~3 orders of magnitude higher than PCBM and contributes to the substantial reduction in series resistance.

Triphenylamine-based amorphous polymers for bulk-heterojunction photovoltaic cells

In this paper, the recent research progress on triphenylamine (TPA)-based donor-acceptor (D-A) amorphous polymers including our developed polymers is reviewed. TPA has three-dimensional branched structures and can provide D-A polymers containing D and A units in the main chain or side chain. The use of TPA-based amorphous polymers in the fabrication of organic photovoltaics (OPVs) offers great advantages over the use of a polycrystalline film in terms of high reproducibility of the OPV performance. The amorphous polymer design using TPA, therefore, indicates a promising direction for the development of new donor materials in OPVs.

Enhanced Charge-Carrier Transport through Shorter Carbon Nanotubes in Organic Photovoltaics

We demonstrate for the first time the efficiency improvement of organic photovoltaics by the addition of shorter multiwalled carbon nanotubes (MWNTs) generated by size sorting. The different size MWNTs were generated by size sorting a batch of carboxylated MWNTs and were introduced as charge carriers in poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction photovoltaic cells. As compared to a control with only PCBM, the addition of the long and short MWNT resulted in 12 and 34% improvement in short circuit current density (Jsc) respectively. The results indicate that length of carbon nanotubes is an important consideration in photovoltaic and possibly other nanoelectronic devices.

A Series of Squaraine Dyes: Effects of Side Chain and the Number of Hydroxyl Groups on Material Properties and Photovoltaic Performance

Squaraine dyes are considered an important group of photoactive materials in the field of organic photovoltaic devices. In this work, we purposely tuned the side chains and number of hydroxyl (OH) groups in a series of squaraine (SQ) dyes, i.e., SQ1–4, to investigate the effect of structural variations on the material properties as well as the performance of these dyes as donor materials in bulk heterojunction (BHJ) photovoltaic cells. The material structure and properties of these SQs were systematically characterized using various tools. Solution-processed BHJ photovoltaic cells based on these SQ compounds combined with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor gave efficient power conversion efficiencies greater than 4.0% under AM 1.5G solar illumination at 100 mW/cm2. Our observations show that the OH groups deepened the highest occupied molecular orbital (HOMO) of the donor and thus enhanced the open-circuit voltage, whereas the linear chain improved the charge transport prop...

Thieno[3,2-b]thiophene-Substituted Benzo[1,2-b:4,5-b′]dithiophene as a Promising Building Block for Low Bandgap Semiconducting Polymers for High-Performance Single and Tandem Organic Photovoltaic Cells

We designed and synthetized a new poly{4,8-bis((2-ethylhexyl)thieno[3,2-b]thiophene)-benzo[1,2-b:4,5-b′]dithiophene-alt-2-ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]thiophene-2-carboxylate} (PTTBDT-FTT) comprising bis(2-ethylhexylthieno[3,2-b]thiophenylbenzo[1,2-b:4,5-b′]dithiophene (TTBDT) and 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate (FTT). The optical bandgap of PTTBDT-FTT was 1.55 eV. The energy levels of the highest occupied and lowest unoccupied molecular orbitals of PTTBDT-FTT were −5.31 and −3.73 eV, respectively. Two-dimensional grazing-incidence X-ray scattering measurements showed that the film’s PTTBDT-FTT chains are predominantly arranged with a face-on orientation with respect to the substrate, with strong π–π stacking. An organic thin-film transistor fabricated using PTTBDT-FTT as the active semiconductor showed high hole mobility of 2.1 × 10–2 cm2/(V·s). Single-junction bulk heterojunction photovoltaic cells with the configuration ITO/PEDOT:PSS/PTTBDT-FTT:PC71BM/Ca/Al were...

How disorder controls the kinetics of triplet charge recombination in semiconducting organic polymer photovoltaics.

Recent experiments by Rao et al. (Nature, 2013, 500, 435-439) indicate that recombination of triplet charge-separated states is suppressed in organic polymer-fullerene based bulk-heterojunction (BHJ) photovoltaic cells exhibiting a high degree of crystallinity in the fullerene phase relative to systems with more disorder. In this paper, we use a series of Frenkel-exciton lattice models to rationalize these results in terms of wave-function localization, interface geometry, and density of states. In one-dimensional co-linear and co-facial models of the interface, increasing local energetic disorder in one phase localizes the interfacial triplet charge-transfer ((3)CT) states and increases the rate at which these states relax to form lower-energy triplet excitons. In two dimensional BHJ models, energetic disorder within the fullerene phase plays little role in further localizing states pinned to the interface. However, inhomogeneous broadening introduces strong coupling between the interfacial (3)CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher degrees of energetic disorder.

Ternary blend bulk heterojunction photovoltaic cells with an ambipolar small molecule as the cascade material

Download options Please wait... Article information DOI https://doi.org/10.1039/C3RA46105J Article type Paper Submitted 29 Aug 2013 Accepted 30 Oct 2013 First published 31 Oct 2013 Download Citation RSC Adv., 2014,4, 1087-1092 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Ternary blend bulk heterojunction photovoltaic cells with an ambipolar small molecule as the cascade material L. Ye, H. Xia, Y. Xiao, J. Xu and Q. Miao, RSC Adv., 2014, 4, 1087 DOI: 10.1039/C3RA46105J To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Lei Ye Hai Xia Yubin Xiao Jianbin Xu Qian Miao

A C70-carbon nanotube complex for bulk heterojunction photovoltaic cells

A C70 fullerene-multi-walled carbon nanotube (C70-CNT) complex has been used as a component of the photoactive layer in a bulk heterojunction photovoltaic cell. As compared to a control device with only C70, the addition of CNTs led to improvements in short circuit current density (Jsc), open circuit voltage (Voc), and power conversion efficiency by 31.8, 17.5, and 69.5%, respectively. This device takes advantage of both the electron accepting feature of C70 and the high electron transport capability of CNTs. These results indicate that C70 decorated CNT is a promising additive for performance enhancement of polymer photovoltaic cells.

A dual-functional additive improves the performance of molecular bulk heterojunction photovoltaic cells

In this paper we report a simple and effective approach towards improving the performance of molecular bulk heterojunction (BHJ) photovoltaics through incorporation of a new, nonvolatile processing additive; (3-chloropropyl)trimethoxysilane (CP3MS). A small amount of the additive CP3MS combined with post-annealing treatment significantly enhanced the power conversion efficiency (PCE) of dialkylated diketopyrrolopyrrole chromophore (SMDPPEH)-based molecular BHJ solar cells. The PCE increased from 2.75% for a device prepared without the additive or annealing to 4.55% for a device containing 0.1% CP3MS that had been subjected to post-annealing treatment at 100 °C for 10 min. CP3MS performed an interesting dual function when incorporated as an additive in molecular BHJ devices. The first was that it controlled the morphology: the addition of 0.1% CP3MS to the SMDPPEH:PC61BM blend was sufficient to improve the film's crystallinity and morphology. The second function was the spontaneous migration of the CP3MS molecules from the bulk to the interface between the active layer and the Al cathode, forming an ultrathin interlayer that acted as a buffer layer suppressing charge recombination and enhancing charge transport at the interface. Analyses using atomic force microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, together with examinations of device performance, confirmed the dual-functional nature of CP3MS as an additive. CP3MS also showed its promising ability to enhance the performance of different blend systems. The performance of benzodithiophene (BDT)-based molecular solar cells enhanced from 3.05% without additive to 3.8% with the incorporation of 0.1% CP3MS.

Effects of single-walled carbon nanotubes on the optical and photo-conductive properties of their composite films with regio-regular poly(3-hexylthiophene)

The effect of a small admixture of single-walled carbon nanotubes (SWNTs) HiPCO (high pressure carbon monoxide) (from 0.5 to 2 wt%) on the supramolecular structure in regio-regular poly(3-hexylthiophene) (RR-P3HT) thin films is studied and their optical and photoconductivity properties are investigated. It is demonstrated that the presence of such small amounts of nanotubes improves the structural organization in the films as evidenced by X-ray diffraction (XRD) studies. This is confirmed by UV–visible optical absorption investigations which clearly show a better conjugation of P3HT in the presence of nanotubes. In Raman spectra of composites, changes in intensities and frequencies of the radial breathing modes are observed upon addition of nanotubes. This can be rationalized by a modification of the resonance conditions caused by a selective dispersion and wrapping of SWNTs via π-interaction (π-stacking). As a consequence of these interactions, a dramatic photoluminescence (PL) quenching is observed which becomes more and more pronounced with increasing the nanotube content. This implies a fast photo-induced electron transfer favoured by a large area of the SWNTs/P3HT interface and strong interactions between these two components. An increase in the composite photocurrent by at least one-order of magnitude, as compared to the case of pure P3HT film, is the most pronounced effect of this electron transfer. These two effects are of crucial importance for the application of the investigated composites in bulk hetero-junction photovoltaic cells (BHJPCs) and organic photo-detectors (OPDs).

Recent Approaches to Controlling the Nanoscale Morphology of Polymer-Based Bulk-Heterojunction Solar Cells

The need for clean, inexpensive and renewable energy has increasingly turned research attention towards polymer photovoltaic cells. However, the performance efficiency of these devices is still low in comparison with silicon-based devices. The recent introduction of new materials and processing techniques has resulted in a remarkable increase in power-conversion efficiency, with a value above 10%. Controlling the interpenetrating network morphology is a key factor in obtaining devices with improved performance. This review focuses on the influence of controlled nanoscale morphology on the overall performance of bulk-heterojunction (BHJ) photovoltaic cells. Strategies such as the use of solvents, solvent annealing, polymer nanowires (NWs), and donor–acceptor (D–A) blend ratios employed to control the active-layer morphologies are all discussed.

Ternary blends for polymer bulk heterojunction solar cells

The objective of this mini-review is to outline current major ternary blends used in the active layer of polymer bulk heterojunction photovoltaic solar cells and to give an insight into the direction of the field. The use of a third-component material in polymer − fullerene blends is described in two sections. On the one hand, the first family of solid state additives enables us to enlarge photon collection by expanding the action spectra of the solar cells. The second section deals with materials used to engineer bulk heterojunction morphology at the nanoscale. The different approaches explored for many of the ternary blend systems suggest the great potential of such mixtures to significantly improve the optoelectronic properties of solar cells on a long-term basis. © 2013 Society of Chemical Industry

Chloroboron (III) subnaphthalocyanine as an electron donor in bulk heterojunction photovoltaic cells

In this work, chloroboron (III) subnaphthalocyanine (SubNc) was used as an electron donor, combined with a [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) or fullerene C70 acceptor in bulk heterojunction photovoltaic cells. In spite of the limited solubility of SubNc in organic solvents, the solution processed device exhibited an efficiency of 4.0% under 1 sun, AM1.5G solar irradiation at room temperature, and 5.0% at 80 ° C due to the temperature-dependence of the carrier mobilities. SubNc:C70 bulk heterojunctions were also fabricated via thermal co-evaporation, demonstrating an efficiency of 4.4%. This result shows that SubNc is a promising material for photovoltaic applications via various processing techniques, such as vacuum deposition and wet coating.

Bulk Heterojunction Photovoltaic Devices Based on a Poly(2-Methoxy, 5-Octoxy)-1, 4-Phenylenevinylene-Single Walled Carbon Nanotube-ZnSe Quantum Dots Active Layer

A solution-processed bulk heterojunction photovoltaic cell is fabricated based on poly[(2-methoxy, 5-octoxy)-1,4-phenylenevinylene](MOPPV)-single walled carbon nanotube(SWNT)-ZnSe quantum dots. The surface morphology shows the formation of an interpenetrating network between well-dispersed SWNTs and ZnSe in the MOPPV matrix. A blue-shifted absorption band indicates the strong electron interaction between SWNTs, ZnSe and MOPPV. A marked increase in the short-circuit current and power conversion efficiency (PCE) of ITO/PEDOT:PSS/MOPPV-SWNT-ZnSe/LiF/Al devices was achieved and compared with that without SWNTs. Results indicate that the enhanced performance is contributed by a high photocurrent due to efficient exciton dissociation and increased mobility for carrier transport in the SWNT pathway.

Quantifying space charge accumulation in organic bulk heterojunctions by nonlinear optical microscopy

In this work, we apply electric field induced second harmonic generation microscopy to directly observe and quantify space charge accumulation in operating organic bulk heterojunction photovoltaic cells comprised of poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) mixed with phenyl-C61-butyric acid methyl ester (PCBM). We adjust the effective electron and hole mobility within these blends by altering the relative composition of PSBTBT and PCBM, and observe dramatic shifts in space charge accumulation. The PSBTBT rich device shows strong electron accumulation (2.8×1014e−/cm3) and the PCBM rich device shows strong hole accumulation (5.5×1014h+/cm3).

Investigation of the Role of the Acceptor Molecule in Bulk Heterojunction Photovoltaic Cells Using Impedance Spectroscopy

An investigation of the recombination kinetics and the density of states distribution in bulk heterojunction organic photovoltaic cells was undertaken using impedance spectroscopy measurements under varying levels of illumina- tions. Three organic polymer solar cells were investigated which differed only by the p-type polymer used in the active layers while the device architecture and acceptor molecule (PC70BM) were kept the same. We found that the density of states profiles for the three cells are similar and can be superimposed by a horizontal shift due to a difference between the polymers' HOMO and the PC70BM's LUMO levels. The recombination mechanisms for the three cells showed the same behavior, suggesting an important role of the acceptor in this process; however, large differences in the absolute value of carrier lifetime τeff related to the choice of the polymer are evident, indicating that the recombination kinetics depends on a combination of the properties of both materials and the morphology of their blend.

High Open Circuit Voltage Solution-Processed Tandem Organic Photovoltaic Cells Employing a Bottom Cell Using a New Medium Band Gap Semiconducting Polymer

Two donor–acceptor (D–A) copolymers, based on the donor unit TIPS substituted benzodithiophene (TIPSBDT) and the acceptor quinoxaline-based units with or without fluorine substitution (PTIPSBDT-DTQX and PTIPSBDT-DFDTQX), were designed and synthesized as a donor material for bulk-heterojunction (BHJ) photovoltaic cells. The introduction of F atoms with high electron affinity to be quinoxailine moieties is effective in further lowering both the HOMO and LUMO energy levels of PTIPSBDT-DFDTQX to attain higher open-circuit voltage (Voc). Single junction photovoltaic cells were fabricated, and the polymers:PC71BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In a single layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 2–6%. The solution process inverted tandem photovoltaic cells, in which two photovoltaic cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer pro...

Fabrication of nanostructured ZnO film as a hole-conducting layer of organic photovoltaic cell

We have investigated the effect of fibrous nanostructured ZnO film as a hole-conducting layer on the performance of polymer photovoltaic cells. By increasing the concentration of zinc acetate dihydrate, the changes of performance characteristics were evaluated. Fibrous nanostructured ZnO film was prepared by sol-gel process and annealed on a hot plate. As the concentration of zinc acetate dihydrate increased, ZnO fibrous nanostructure grew from 300 to 600 nm. The obtained ZnO nanostructured fibrous films have taken the shape of a maze-like structure and were characterized by UV-visible absorption, scanning electron microscopy, and X-ray diffraction techniques. The intensity of absorption bands in the ultraviolet region was increased with increasing precursor concentration. The X-ray diffraction studies show that the ZnO fibrous nanostructures became strongly (002)-oriented with increasing concentration of precursor. The bulk heterojunction photovoltaic cells were fabricated using poly(3-hexylthiophene-2,5-diyl) and indene-C60 bisadduct as active layer, and their electrical properties were investigated. The external quantum efficiency of the fabricated device increased with increasing precursor concentration.

Enhanced carrier mobility and photon-harvesting property by introducing Au nano-particles in bulk heterojunction photovoltaic cells

In this study, polymer solar cells (PSCs) doped with Au nanoparticles (Au NPs) were successfully fabricated to maximize the photon-harvesting properties on the photoactive layer. In addition, a conductivity-enhanced hybrid buffer layer was introduced to improve the photon absorption properties and effectively separate the generated charges by adding Au NPs and dimethylsulfoxide (DMSO) to the PH 500 as a buffer layer. The PSC performance was optimized with a 88% improvement over the conventional PSCs (photoactive area: 225mm2, power conversion efficiency (PCE): 3.2%) by the introduction to the buffer layer of Au NPs and DMSO at 10wt% and 1.0wt%, respectively, and with 15wt% Au NP doping in the photoactive layer. The internal resistance was decreased due to the increased photocurrent caused by the localized surface plasmon resonance (LSPR) effect of the Au NPs in the photoactive layer and by the improvement of carrier mobility induced by the DMSO doping of the buffer layer. As a result, the series resistance (RS) deceased from 42.3 to 19.7Ωcm2 while the shunt resistance (RSH) increased from 339 to 487Ωcm2.