Heterojunction Photovoltaic Cells Research Articles

Gold cluster/titanium dioxide heterojunction photovoltaic cell

Metal clusters have recently been applied as photosensitizers to wet-type photovoltaic cells. However, there are some practical issues including instability of the clusters in a liquid phase and leakage of electrolyte. To address these issues, we fabricated a heterojunction photovoltaic cell with solid-state layers of glutathione-protected Au25 clusters and titanium dioxide (TiO2). The ITO/TiO2/Au25/Au cell responded to visible and near infrared light even at 900 nm. Short-circuit photocurrent was ∼14 μA cm−2 and open-circuit photovoltage was 0.53 V under a xenon lamp (>480 nm, 75 mW cm−2).

Ternary Bulk Heterojunction Photovoltaic Cells Composed of Small Molecule Donor Additive as Cascade Material

To explore the potential of ternary blend bulk heterojunction (BHJ) solar cells as a general platform for improving the performance of organic photovoltaics, we studied a ternary BHJ system based on poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), and DTDCTB. The optimized ternary structure containing a weight ratio of 20% DTDCTB as the cascade material demonstrates a ∼25% improvement of the power conversion efficiency (PCE) as compared to the binary P3HT/PC61BM solar cells. A systematic spectroscopic study is carried out to elucidate the underlying mechanism of charge transfer in the ternary system. Wavelength-dependent external quantum efficiency measurement confirms the contribution of DTDCTB to the enhanced photocurrent. Photoinduced absorption spectroscopy and transient photovoltage measurement reveal unambiguously that charges generated in DTDCTB are efficiently transferred to and subsequently transported in P3HT and PC61BM. The results also suggest that despite th...

Molecular packing and electronic processes in amorphous-like polymer bulk heterojunction solar cells with fullerene intercalation.

The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c]125 thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

Annealing temperature dependence of the efficiency and vertical phase segregation of polymer/polymer bulk heterojunction photovoltaic cells

We report observation of annealing temperature-induced simultaneous vertical phase segregation and large enhancement of power conversion efficiency (PCE) of all-polymer bulk heterojunction (BHJ) solar cells composed of a poly(3-hexylthiophene) (P3HT) donor and a naphthalene diimide-selenolo[3,2-b]selenophene copolymer (PNDISS) acceptor. The PCE of P3HT:PNDISS BHJ devices increased over 50-fold from 0.04% to 2.03% when the annealing temperature was increased from 50 to 150 °C. Absorption spectroscopy and photoluminescence quenching experiments provide evidence of increasing phase segregation of the polymer/polymer blend films with increasing annealing temperature. Field-effect charge transport, contact angle, surface energy, and variable angle ellipsometry measurements on the P3HT:PNDISS blend films showed that thermal annealing induced vertical phase segregation, whereby the low surface energy polymer (P3HT) migrated to the bulk, while the high surface energy polymer (PNDISS) enriches at the substrate/blend interface.

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.

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...

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).

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.

Electrical and photovoltaic properties of SnSe/Si heterojunction

Thin film of SnSe is deposited on n-Si single crystal to fabricate a p-SnSe/n-Si heterojunction photovoltaic cell. Electrical and photoelectrical properties have been studied by the current density–voltage (J–V) and capacitance–voltage (C–V) measurements at different temperatures. The fabricated cell exhibited rectifying characteristics with a rectification ratio of 131 at ±1V. At low voltages (V<0.55V), the dark forward current density is controlled by the multi-step tunneling mechanism. While at a relatively high voltage (V>0.55V), a space charge-limited-conduction mechanism is observed with trap concentration of 2.3×1021cm−3. The C–V measurements showed that the junction is of abrupt nature with built-in voltage of 0.62V which decreases with temperature by a gradient of 2.83×10−3V/K. The cell also exhibited strong photovoltaic characteristics with an open-circuit voltage of 425mV, a short-circuit current density of 17.23mAcm−2 and a power conversion efficiency of 6.44%. These parameters have been estimated at room temperature and under light illumination provided by a halogen lamp with an input power density of 50mWcm−2.

Microwave plasma-assisted regeneration of carbon nanosheets with bi- and trilayer of graphene and their application to photovoltaic cells

Vertically aligned carbon nanosheets (CNSs) with bi- and trilayer graphene have been achieved on various metal substrates from solid carbon sources by irradiation with H2 and Ar plasma. The resulting graphene structures have fewer layers, bigger sizes, and higher graphitization compared to previous reports obtained from gas sources, as confirmed by scanning and transmission electron microscopes, Raman and X-ray photoelectron spectroscopes. The results suggest that the interaction between the plasma (of H2 and Ar) and carbon sources favors the formation of bigger and thinner nucleation cites in the initial stage and atomic H etching dominates during the whole thinner CNS growth. The vertically aligned graphene film can be directly transferred on as-patterned SiO2/Si to form a heterojunction photovoltaic cell with a power conversion efficiency of 0.9%.

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.