Bulk-heterojunction Cells Research Articles

Solvent‐Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells

It is demonstrated that squaraine:PC70BM blends can result in solar cells with high efficiency and fill factor when the nano­structure scale is increased to lead to conduction of photogenerated carriers to the electrodes. Morphological control occurs by a combination of thermal and solvent annealing of these small molecule, solution-processed bulk heterojunction cells. Peak efficiencies of a population of devices is 5.2% at fill factors of 0.5 are achieved under optimized conditions.

Carbazole and thienyl benzo[1,2,5]thiadiazole based polymers with improved open circuit voltages and processability for application in solar cells

The preparation of low energy gap carbazole based main-chain polymers having improved open circuit voltages (Voc) and solubility is described. Poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P1 and poly[9-(heptadecan-9-yl)-9H-carbazole-2,7-diyl-alt-(5,6-bis(octyloxy)-4,7-di(2,2′-bithiophen-5-yl)benzo[c][1,2,5]thiadiazole)-5,5-diyl] P2 were prepared in good yields using Suzuki coupling methodologies. The polymers were characterized by NMR spectroscopy; UV-Vis absorption spectroscopy, cyclic voltammetry and their molecular weights were estimated using gel permeation chromatography. Introduction of octyloxy substituents on the benzothiadiazole acceptor repeat units on polymers P1 and P2 enhances their solubility compared to the polymer PCDTBT and alters their electronic and photophysical properties. Investigation of the photovoltaic properties of the two new polymers side by side with those of PCDTBT in bulk heterojunction cells using PC70BM as a molecular acceptor indicated that both polymers provide higher Voc values than that of PCDTBT (0.96 and 0.90 V for P1 and P2 respectively vs. 0.82 V for PCDTBT) and good power conversion efficiencies with PCE values of 4.22 and 4.12% respectively for P1 and P2 and a PCE of 4.30% for PCDTBT.

Investigating morphology and electronic properties of self-assembled hybrid systems for solar cells

We investigate self-assembled network morphologies of hybrid materials and their application in solar cells. We show the use of a semiconducting functional block copolymer as a structure directing agent. The application of a functional block copolymer circumvents an additional filling step of the templated inorganic part with a hole conductor. Within a one-pot synthesis various morphologies adequate for active materials in photovoltaic devices are prepared. The solar cell performance was found to depend on the morphological design of the hybrid material since the formation of percolating networks is of general importance. Using conductive scanning force microscopy on a cross-section of a functional solar cell device, we proved that the titania in our system forms a percolating network. Measurements showed that percolation is not a limiting factor for the device performance. In combination with a functionalized titania-precursor, we were able to double the power conversion efficiency of our hybrid bulk heterojunction cells in comparison to our previous results.

Recombination in polymer-fullerene bulk heterojunction solar cells

Recombination of photogenerated charge carriers in polymer bulk heterojunction (BHJ) solar cells reduces the short circuit current (Jsc) and the fill factor (FF). Identifying the mechanism of recombination is, therefore, fundamentally important for increasing the power conversion efficiency. Light intensity and temperature dependent current-voltage measurements on polymer BHJ cells made from a variety of different semiconducting polymers and fullerenes show that the recombination kinetics are voltage dependent and evolve from first order recombination at short circuit to bimolecular recombination at open circuit as a result of increasing the voltage-dependent charge carrier density in the cell. The "missing 0.3V" inferred from comparison of the band gaps of the bulk heterojunction materials and the measured open circuit voltage at room temperature results from the temperature dependence of the quasi-Fermi-levels in the polymer and fullerene domains - a conclusion based upon the fundamental statistics of Fermions.

Universality of non-Ohmic shunt leakage in thin-film solar cells

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited (SCL) current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism.

Bulk-Heterojunction Photovoltaic Cells with Donor–Acceptor Ratio Varied across the Film Prepared by Evaporative Spray Deposition Using Ultradilute Solution Method

We fabricated three types of bulk-heterojunction (BHJ) photovoltaic cells with the electron donor and acceptor ratio varied across the film. One has a stacked layer structure of a donor-rich layer (2:1) the near cathode and an acceptor-rich layer (1:2) near the anode (DA-BHJ), another has a stacked structure with the inverse order (AD-BHJ), and the other has a constant ratio (1:1) as a reference. These were fabricated by evaporative spray deposition using ultradilute solution (ESDUS) method. The DA-BHJ cell showed significantly higher energy conversion efficiency than the AD-BHJ cell and the reference BHJ cell where the three exhibited almost identical light absorption.

Effective Light Harvesting of Tandem Polymer Solar Cell

A novel soluble conjugated polymers, P2, with coplanar thiophene-phenylene-thiophene unit is designed and synthesized as suitable active material used in tandem cells to compensate the poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) bulk-heterojunction cell in this paper. P2 polymer bears advantages in both low optical bandgap (1.7 eV) and high hole mobility properties (3.4×10−3 cm2/V-s from field-effect transistor measurement). Furthermore, the electrochemical studies of P2 indicate desirable highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) band structure that enables a high open circuit voltage when pairing with PCBM acceptor. The best power conversion efficiency of this polymer solar cell thus far based on P2/PC71BM system with a weight ratio of 1:3 reached 4.4% with a short circuit current density (Jsc) of 10.2 mA/cm2, an open circuit voltage (Voc) of 0.81 V, and a fill factor (FF) of 0.53 under air mass (AM) 1.5 G (100 mW/cm2). The preliminary data of the tandem cell with indium tin oxide (ITO) glass/PEDOT:PSS/P2:PC71BM/TiOx/PEDOT:PSS/P3HT:PC71BM/TiOx/Al configuration has reached Jsc of 6.2 mA/cm2, Voc of 1.33 V, FF of 0.56 and an overall efficiency of 4.6% under AM 1.5 G (100 mW/cm2).

Effects of concentrated sunlight on organic photovoltaics

We report the effects of concentrated sunlight on key photovoltaic parameters and stability of organic photovoltaics (OPV). Sunlight collected and concentrated outdoors was focused into an optical fiber and delivered onto a 1 cm2 bulk-heterojunction cell. Sunlight concentration C was varied gradually from 0.2 to 27 suns. Power conversion efficiency exhibited slow increase with C that was followed by saturation around 2% at C=0.5–2.5 suns and subsequent strong reduction. Possible OPV applications in stationary solar concentrators (C≤2 suns) are discussed. Finally, experiments at C=55–58 suns demonstrated potential of our approach for accelerated studies of light induced mechanisms in the OPV degradation.

폴리(옥틸티오펜)/풀러렌 벌크 이종접합의 광기전성에 미치는 CIS 입자의 블렌딩 효과

초록·키워드 목차 오류제보하기 등록된 정보가 없습니다. #3-octylthiophene #CIS #fullerene #photovoltaic cell #poly 등록된 정보가 없습니다.

Comparative Study on Structural Properties of Poly(3-hexylthiophene) and Poly(3-hexylthiophene):6,6-Phenyl-C61 Butyric Acid Methyl Ester Thin Films Using Synchrotron X-ray Diffraction

The structural properties of a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl-C61 butyric acid methyl ester (PCBM) was investigated using synchrotron X-ray diffraction and atomic force microscope techniques. The growth of anisotropic crystals of P3HT components in the bulk heterojunction was observed. The π–π stacking structure between the facing thiophene rings of P3HT was disturbed by the introduction of PCBM, whereas the side chain crystallization along the a-axis of P3HT occurred smoothly. We discuss a possible mechanism of the anisotropic crystallization on the basis of the configuration of P3HT. The importance of controlling the π–π stacking and phase separation of the composed materials on the performance of the bulk heterojunction cell is suggested.

Cyclometalated Platinum Polymers: Synthesis, Photophysical Properties, and Photovoltaic Performance

The synthesis and characterization of platinum-containing conjugated polymers in which the platinum atom is attached to the conjugated backbone via a C∧N ligand is presented. The newly designed platinum-containing monomer can be polymerized under both Stille and Suzuki conditions. The polymers exhibit optical bandgaps between 2.1 and 1.65 eV depending on the choice of comonomer. Triplet exciton formation is detected indirectly by measuring photosensitized emission of singlet oxygen in both solution and in film. The ability of the materials to sensitize formation of singlet oxygen varies both with excitation wavelength and with the change from solution to solid state. This study provides design principles for developing conjugated polymers with significant triplet yields in the solid state. The photovoltaic performance of these polymers was also evaluated in preliminary experiments with power conversion efficiencies as high as 1.3% obtained for a bulk heterojunction cell with PCBM.

Bulk heterojunction photovoltaic cells based on vacuum evaporated cadmium sulfide–phthalocyanine hybrid structures

Bulk heterojunction (BHJ) photovoltaic (PV) cells have been prepared by vacuum co-deposition of inorganic cadmium sulfide (CdS) and organic zinc phthalocyanine (PcZn) materials. Characteristics of the BHJ cells were compared with the cells based on vacuum evaporated bilayer CdS/PcZn heterojunctions, and it was found that the BHJ structure possesses higher PV performance due to higher interface area at which excitons can dissociate. Variation of the component ratio in the BHJ devices and comparison of their photocurrent spectra allowed us to distinguish the contribution of CdS, CdS/PcZn interface and PcZn to the PV response. The mechanism of charge transport was attributed to the model of space-charge-limited current with traps; however, the concentration of traps was different in the bilayer and BHJ structures.

Strategies for Increasing the Efficiency of Heterojunction Organic Solar Cells: Material Selection and Device Architecture

Thin-film blends or bilayers of donor- and acceptor-type organic semiconductors form the core of heterojunction organic photovoltaic cells. Researchers measure the quality of photovoltaic cells based on their power conversion efficiency, the ratio of the electrical power that can be generated versus the power of incident solar radiation. The efficiency of organic solar cells has increased steadily in the last decade, currently reaching up to 6%. Understanding and combating the various loss mechanisms that occur in processes from optical excitation to charge collection should lead to efficiencies on the order of 10% in the near future. In organic heterojunction solar cells, the generation of photocurrent is a cascade of four steps: generation of excitons (electrically neutral bound electron-hole pairs) by photon absorption, diffusion of excitons to the heterojunction, dissociation of the excitons into free charge carriers, and transport of these carriers to the contacts. In this Account, we review our recent contributions to the understanding of the mechanisms that govern these steps. Starting from archetype donor-acceptor systems of planar small-molecule heterojunctions and solution-processed bulk heterojunctions, we outline our search for alternative materials and device architectures. We show that non-planar phthalocynanines have appealing absorption characteristics but also have reduced charge carrier transport. As a result, the donor layer needs to be ultrathin, and all layers of the device have to be tuned to account for optical interference effects. Using these optimization techniques, we illustrate cells with 3.1% efficiency for the non-planar chloroboron subphthalocyanine donor. Molecules offering a better compromise between absorption and carrier mobility should allow for further improvements. We also propose a method for increasing the exciton diffusion length by converting singlet excitons into long-lived triplets. By doping a polymer with a phosphorescent molecule, we demonstrate an increase in the exciton diffusion length of a polymer from 4 to 9 nm. If researchers can identify suitable phosphorescent dopants, this method could be employed with other materials. The carrier transport from the junction to the contacts is markedly different for a bulk heterojunction cell than for planar junction cells. Unlike for bulk heterojunction cells, the open-circuit voltage of planar-junction cells is independent of the contact work functions, as a consequence of the balance of drift and diffusion currents in these systems. This understanding helps to guide the development of new materials (particularly donor materials) that can further boost the efficiency of single-junction cells to 10%. With multijunction architectures, we expect that efficiencies of 12-16% could be attained, at which point organic photovoltaic cells could become an important renewable energy source.

Thin-film encapsulation of polymer-based bulk-heterojunction photovoltaic cells by atomic layer deposition

This study demonstrated thin-film encapsulation of bulk-heterojunction polymer photovoltaic cells, utilizing a process based on atomic layer deposition (ALD) that both prevented degradation caused by ambient gases and served as an annealing step that increased the initial efficiency of the cells. With the ALD temperature set at 140 °C and the total deposition time set at 1 h, the photovoltaic cells, based on blended poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C 61 butyric acid methylester (PCBM), were optimally annealed during encapsulation, achieving a power conversion efficiency (PCE) of 3.66%. Encapsulating the cells with a 26 nm Al 2O 3/HfO 2 nanolaminated film overcoated with an epoxy resin protection layer enabled the cells to obtain an in-air degradation rate that was similar to cells that were stored in nominally O 2/H 2O-free atmosphere. The nanolaminated structure of the encapsulation film resolved the issue of hydrolysis-induced aging observed with Al 2O 3 films, owing to the hydrophobicity of the HfO 2 layers. Additionally, extended exposure of the ALD precursors during the ALD process significantly improved the coverage of the ALD films over the P3HT/PCBM active layer at the perimeter of the cells.

Molecular Bulk Heterojunctions: An Emerging Approach to Organic Solar Cells

The predicted exhaustion of fossil energy resources and the pressure of environmental constraints are stimulating an intensification of research on renewable energy sources, in particular, on the photovoltaic conversion of solar energy. In this context, organic solar cells are attracting increasing interest that is motivated by the possibility of fabricating large-area, lightweight, and flexible devices using simple techniques with low environmental impact. Organic solar cells are based on a heterojunction resulting from the contact of a donor (D) and an acceptor (A) material. Absorption of solar photons creates excitons, Coulombically bound electron-hole pairs, which diffuse to the D/A interface, where they are dissociated into free holes and electrons by the electric field. D/A heterojunctions can be created with two types of architectures, namely, bilayer heterojunction and bulk heterojunction (BHJ) solar cells. BHJ cells combine the advantages of easier fabrication and higher conversion efficiency due to the considerably extended D/A interface. Until now, the development of BHJ solar cells has been essentially based on the use of soluble pi-conjugated polymers as donor material. Intensive interdisciplinary research carried out in the past 10 years has led to an increase in the conversion efficiency of BHJ cells from 0.10 to more than 5.0%. These investigations have progressively established regioregular poly(3-hexylthiophene) (P3HT) as the standard donor material for BHJ solar cells, owing to a useful combination of optical and charge-transport properties. However, besides the limit imposed to the maximum conversion efficiency by its intrinsic electronic properties, P3HT and more generally polymers pose several problems related to the control of their structure, molecular weight, polydispersity, and purification. In this context, recent years have seen the emergence of an alternative approach based on the replacement of polydisperse polymers by soluble, conjugated single molecules as donor materials in BHJ cells. In fact, molecular donors present specific advantages in terms of structural definition, synthesis, and purification. In this Account, we present a brief survey of recent work in this nascent field of new single-molecule donors in organic solar cells. Various series of three-dimensional donors built by the attachment of different kinds of conjugated branches on a central node, including silicon, twisted bithiophene, triphenylamine, and borondipyrromethene (BODIPY), are discussed in relation to the performances of the resulting solar cells. Furthermore, it is shown that the concept of a molecular donor with internal charge transfer leads at the same time to improved light-harvesting properties, red-shifted photoresponse, and a higher open-circuit voltage, resulting in a considerable increase of conversion efficiency, up to values now approaching 3%. These results show that soluble molecular donors can lead to BHJ cells that combine high conversion efficiency with the distinct advantages of working with single molecules, including structural definition, synthesis, purification, and reproducibility.

“Plastic” Solar Cells: Self-Assembly of Bulk Heterojunction Nanomaterials by Spontaneous Phase Separation

As the global demand for low-cost renewable energy sources intensifies, interest in new routes for converting solar energy to electricity is rapidly increasing. Although photovoltaic cells have been commercially available for more than 50 years, only 0.1% of the total electricity generated in the United States comes directly from sunlight. The earliest commercial solar technology remains the basis for the most prevalent devices in current use, namely, highly-ordered crystalline, inorganic solar cells, commonly referred to as silicon cells. Another class of solar cells that has recently inspired significant academic and industrial excitement is the bulk heterojunction (BHJ) "plastic" solar cell. Research by a rapidly growing community of scientists across the globe is generating a steady stream of new insights into the fundamental physics, the materials design and synthesis, the film processing and morphology, and the device science and architecture of BHJ technology. Future progress in the fabrication of high-performance BHJ cells will depend on our ability to combine aspects of synthetic and physical chemistry, condensed matter physics, and materials science. In this Account, we use a combination of characterization tools to tie together recent advances in BHJ morphology characterization, device photophysics, and thin-film solution processing, illustrating how to identify the limiting factors in solar cell performance. We also highlight how new processing methods, which control both the BHJ phase separation and the internal order of the components, can be implemented to increase the power conversion efficiency (PCE). The failure of many innovative materials to achieve high performance in BHJ solar cell devices has been blamed on "poor morphology" without significant characterization of either the structure of the phase-separated morphology or the nature of the charge carrier recombination. We demonstrate how properly controlling the "nanomorphology", which is critically dependent on minute experimental details at every step, from synthesis to device construction, provides a clear path to >10% PCE BHJ cells, which can be fabricated at a fraction of the cost of conventional solar cells.

Buffer layer formation in organic photovoltaic cells by self-organization of poly(dimethylsiloxane)s

A surface-segregated thin layer of poly(dimethylsiloxane)- block-poly(methylmethacrylate) (PDMS- b-PMMA) formed by self-organization during spin-coating was utilized as an interfacial buffer layer in organic photovoltaic cells. X-ray photoelectron spectroscopy revealed that PDMS- b-PMMA mixed into the coating solution spontaneously accumulated at the surface of the active layer due to the low surface energy of PDMS. The introduction of the PDMS- b-PMMA layer in bulk-heterojunction cells of poly(3-hexylthiophene):[6,6]-phenyl C 61-butyric acid methyl ester resulted in an improvement of the power conversion efficiency (PCE) of the cells from 3.05% to 3.56% on average. The highest PCE of 3.86% was achieved with 0.4 mg ml −1 of PDMS- b-PMMA. These results indicate that the PDMS- b-PMMA layer formed by self-organization provides a facile and versatile approach to improve the photovoltaic performance, possibly by suppressing charge carrier recombination at the organic/metal interface.

Trannulenes: a new class of photoactive materials for organic photovoltaic devices

We report the first large-scale preparation of fluorinated trannulene C60F15[C(COOMe)3]3 (TMCMT) and its application as a photoactive material in organic solar cells. Two types of photovoltaic devices were fabricated: bulk heterojunction cells comprising blends of TMCMT with a polyfluorene polymer and bilayer cells based on a planar heterojunction of TMCMT with a perylene diimide. Photoluminescence quenching studies suggested that a photoinduced electron transfer occurs from polyfluorene and perylenediimide to TMCMT, therefore it behaves as photoinduced electron acceptor. Both types of solar cells comprising TMCMT as photoactive material showed clear photovoltaic effects. This proves for the first time the feasibility of the previously anticipated idea of using trannulenes as light-harvesting materials in photovoltaic devices.

Detailed balance theory of excitonic and bulk heterojunction solar cells

A generalized solar cell model for excitonic and classical bipolar solar cells describes the combined transport and interaction of electrons, holes, and excitons in accordance with the principle of detailed balance. Conventional inorganic solar cells, single-phase organic solar cells and bulk heterojunction solar cells, i.e., nanoscale mixtures of two organic materials, are special cases of this model. For high mobilities, the compatibility with the principle of detailed balance ensures that our model reproduces the Shockley-Queisser limit irrespective of how the energy transport is achieved. For less ideal devices distinct differences become visible between devices that are described by linear differential equations and those with nonlinear effects, such as a voltage-dependent collection in bipolar $p\text{\ensuremath{-}}i\text{\ensuremath{-}}n$-type devices. These differences in current-voltage characteristics are also decisive for the validity of the reciprocity theorem between photovoltaic quantum efficiency and electroluminescent emission. Finally, we discuss the effect of band offset at the heterointerface in a bulk heterojunction cell and the effect of the average distances between these heterointerfaces on the performance of a solar cell in order to show how our detailed balance model includes also these empirically important quantities.

Efficient Squaraine-Based Solution Processable Bulk-Heterojunction Solar Cells

We report the fabrication of organic bulk-heterojunction solar cells based, for the first time, on squaraine/PCBM blends. The most efficient device, solution-processed in air, exhibits J(sc) = 5.70 mA/cm(2), V(oc) = 0.62 V, fill-factor = 0.35, and power conversion efficiency = 1.24%, one of the highest to date for a small molecule solution-processed bulk-heterojunction cell.