Efficient Charge Carrier Generation Research Articles

Enhancing the organic solar cell efficiency by combining plasmonic and Förster Resonance Energy Transfer (FRET) effects

Here, we combine two strategies i.e., Förster resonance energy transfer and plasmonic effect, to enhance the photovoltaic performance in organic solar cells by introducing gold nanoparticles and squaraine in a binary mixture of poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester. In this configuration, the Förster resonance energy transfer between poly(3-hexylthiophene) and squaraine reduces the electrical loss arising from the exciton recombination in poly(3-hexylthiophene), while gold nanoparticles enable an efficient charge carrier generation in poly(3-hexylthiophene):squaraine mixture as optically confirmed by photoluminescence measurement and transient absorption spectroscopy. The multicomponent organic solar cells demonstrate an enhancement of ~36% in power conversion efficiency over the reference device.

Acene-Modified Small-Molecule Donors for Organic Photovoltaics.

A series of acene-modified small molecules have been designed and synthesized, and their photovoltaic characteristics were studied by using the small molecules in organic photovoltaics (OPVs). Different cores were introduced to modulate the conjugation lengths of the small molecules and the bulk heterojunction (BHJ) morphologies. Three small-molecule donors were prepared, namely Ph-TTR, Na-TTR, and An-TTR, which have phenyl, naphthalene, and anthracene moieties, respectively, as conjugated cores. These donors were synthesized in a few steps and exhibited favorable BHJ morphologies, thereby giving promising power conversion efficiencies (PCEs). The donors showed excellent miscibility with the acceptor PC71 BM, and the use of the additive 1,8-diiodooctane (DIO) led to a remarkable increase in crystallinity, thereby increasing the PCEs of their OPVs. Of the three donors, Na-TTR showed the most efficient charge carrier generation and favorable molecular packing structures; hence, of the three types of devices tested, the Na-TTR:PC71 BM devices exhibited the highest PCE, specifically 6.27 %, without pre- or post-treatments. The promising PCEs achieved from these easily synthesized acene-modified small molecules suggested that acene-modified small molecules can be useful materials in OPVs.

(Invited) Efficient Charge Ggeneration Dynamics in Hematite Photoanodes Decorated with Gold Nanostructures

Photochemical water splitting offers a useful solution for efficient energy conversion into hydrogen gas. Hematite has been focused on this purpose due to the advantage of low-cost, chemical stability, and suitable band gap. This photocatalytic ability, however, is limited by the short-lived carriers, the poor conductivity, and the short hole diffusion length. As one solution, combination with noble metal can enhance the photocatalytic performance. In particular, gold nanoparticles (AuNP) indicate characteristic absorption in the visible region and injection of electrons into the semiconductor. In this study, a hybrid material of hematite photoanodes with gold nanostructures was fabricated and the carrier dynamics was elucidated by femtosecond transient absorption (TA) spectroscopy. The hybrid material was prepared by immersing the photoanode into the AuNP solution for 16 h, followed by heating at 400℃ for 4 h to remove the organic matter. Femtosecond pump-probe TA spectroscopy was carried out with a Ti:sapphire laser (800 nm wavelength, 130 fs fwhm pulse width, 0.7 mJ/pulse intensity, 1 kHz repetition; Spectra Physics, Hurricane) using pump pulses at 400 nm and probe pulses in the Vis/NIR region. FE-SEM images of hematite and hematite decorated with AuNP (Au/Hematite) indicated that 40 nm AuNPs were decorated on the hematite surface. TA signal of hematite exhibited the positive absorption peak at the 585 nm, while the signal of Au/Hematite did bleaching at 580-620 nm overlapping with broad absorption whose apparent peak shifts from 670 nm towards 580 nm with time. The negative signal at the plasmon band indicates heat generation within the time resolution and cooling in the picosecond timescale. The observed strong positive absorption is induced by the increased electron density in the hematite due to electron transfer from AuNP, demonstrating efficient charge carrier generation in hematite by the decorated gold nanostructure.

Synthesis of Tungsten Trioxide/Hematite Core‐Shell Nanoarrays for Efficient Photoelectrochemical Water Splitting

AbstractHematite, with a band gap of 2.0∼2.2 eV suitable for visible light absorption, has emerged to be a promising photoanode material for photoelectrochemical (PEC) catalysis of the oxygen evolution reaction (OER). Herein, we proposed the design and fabrication of a WO3/α‐Fe2O3 core‐shell heterojunction structure, aiming at alleviating the severe mismatch between the relatively long light penetration depth and the extremely short holes diffusion length in α‐Fe2O3. The WO3 nanoarray underlayer is grown directly on the FTO substrate, serving as an effective electron transfer layer and additional light absorber. The α‐Fe2O3 layer is further prepared via spin‐coating and a subsequent calcination process as a thin and uniform shell. The loading amount was regulated with different spin coating times of the precursor. The optimized WO3/α‐Fe2O3 core‐shell nanoarrays display an excellent performance with lower onset potential of 0.65 V vs. RHE and increased photocurrent response of 1.29 mA cm−2 at 1.23 V vs. RHE, much superior than the pristine α‐Fe2O3 or WO3 photoanode. Characterizations demonstrate that both improved light absorption and formation of heterojunction account for the excellent performance. After coating a NiFe‐LDH co‐catalyst layer, the oxygen evolution reaction is further enhanced, and high stability is achieved, benefiting from the efficient charge carrier generation, promoted charge separation in the semiconductor and the rapid consumption of holes for surface reaction.

Key Tradeoffs Limiting the Performance of Organic Photovoltaics

Abstract2017 saw the publication of several new material systems that challenge the long‐held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer stateEctand the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (Voc) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies.

Field enhancement in plasmonic nanostructures

Efficient generation of charge carriers from a metallic surface is a critical challenge in a wide variety of applications including vacuum microelectronics and photo-electrochemical devices. Replacing semiconductors with vacuum/gas as the medium of electron transport offers superior speed, power, and robustness to radiation and temperature. We propose a metallic resonant surface combining optical and electrical excitations of electrons and significantly reducing powers required using plasmon-induced enhancement of confined electric field. The properties of the device are modeled using the exact solution of the time-dependent Schrödinger equation at the barrier. Measurement results exhibit strong agreement with an analytical solution, and allow us to extract the field enhancement factor at the surface. Significant photocurrents are observed using combination of optical power and 10 V DC excitation on the surface. The model suggests optical field enhancement of 3 orders of magnitude at the metal interface due to plasmonic resonance. This simple planar structure provides valuable evidence on the electron emission mechanisms involved and it can be used for implementation of semiconductor compatible vacuum devices.

(Invited) Amphiphilic Block Copolymers and Their Hybrids for Efficient Aqueous-Processed Solar Cells

Due to their low cost and high efficiency, polymer/nanocrystal hybrid solar cells have attracted much attention in recent years. Additionally, aqueous-processed solar cells are attractive because they exhibit the potential to produce clean energy via the use of nontoxic solvents. The use of water instead of toxic organic solvents during the fabrication process of these devices is capable of significantly reducing the environmental impact as well as the production cost. In aqueous-processed solar cells, hybrid solar cells employing water-soluble conjugated polymers and inorganic nanocrystals as photovoltaic materials have gained much attention. These hybrid materials take advantage of both water-soluble conjugated polymers and nanocrystals, which exhibit adjustable absorption properties through structural modification, efficient charge carrier generation, and high charge carrier mobility. Water-soluble conjugated polymers are typically homopolymers with ionic (i.e., sulfonate and phosphonate moieties) or nonionic, hydrophilic (i.e., hydroxyl and glycol moieties) side chains. Unfortunately, the use of hydrophilic side chains in these conventional water-soluble conjugated polymers adversely affected the stacking of the polymer backbone, which can lead to the deterioration of their optoelectronic properties and thus a lower power conversion efficiency in solar cells. Furthermore, these side chains may cause charge trapping when applied in the active layer, which results in the further reduction in performance In this presentation, the use of a water-soluble hybrid material consisting of amphiphilic block copolymers and cadmium telluride nanocrystals as the active layer in hybrid solar cells will be discussed. The amphiphilic block copolymers composed of poly(3-hexylthiophene) (P3HT) and poly(acrylic acid) (PAA) self-assembled into ordered nanostructured micelles which then transformed to nanowires by co-micellization with P3HT additives. Furthermore, after annealing, the hybrid materials formed an interpenetrating network. The properties of the hybrid materials and the film morphology, and how they correlate to the device performance will be discussed.

Ternary polymer solar cells based on two highly efficient fullerene acceptors with high efficiency and stability under long-time thermal annealing treatment

By introducing a dihydronaphthyl-based C60 bisadduct (NCBA) small molecule acceptor as an third component materials to typical polymer donor:fullerene acceptor binary polymer solar cells (PSCs), we demonstrate that the short-circuit current density (JSC), open circuit voltage (VOC), fill factor (FF), power conversion efficiency (PCE), and thermal stability can be enhanced simultaneously. The ternary photoactive layer enhances photogenerated charge-carrier generation efficiency through complementary short wavelength visible-light absorption between the donor molecule and fullerene acceptors. Simultaneously, the improved PSC performance is mainly attributed to the enhanced exciton dissociation and charge-carrier transport. Furthermore, we find that the NCBA as third component materials can reduce charge-carrier recombination and enhancement of thermal stability in ternary PSCs as well. The results bring new insight into the future development of high-efficiency ternary PSCs.

(Invited) Reflections on Rust: Iron Oxide Photoelectrodes for Solar Energy Conversion and Storage

Solar water splitting is a promising route for sustainable and scalable hydrogen production from renewable sources: water and sunlight. The hydrogen can be used as a clean fuel for refueling fuel cell electric vehicles with zero emissions. Hydrogen can also serve as a feedstock for the production of drop-in liquid fuels by CO2 hydrogenation, and ammonia via the Haber–Bosch process. The greatest challenges towards viable solar water splitting technology lay in the selection and design of photocatalytic materials and devices that harvest sunlight and split water efficiently and produce hydrogen at a competitive cost. To this end photocatalytic materials that are stable in alkaline or acidic aqueous solutions, absorb visible light, promote water oxidation or reduction reactions and comprise of abundant materials must be employed. Iron oxide (α-Fe2O3, hematite), aka rust, is one of few materials meeting these criteria, but its poor transport properties and fast charge recombination present challenges for efficient charge carrier generation, separation and collection. We explore innovative solutions to these challenges using ultrathin (20-30 nm) films on specular back reflectors. This simple optical cavity design effectively traps the light in otherwise nearly translucent ultrathin films, amplifying the intensity close to the surface wherein photogenerated charge carriers can reach the surface and split water before recombination takes place. This is the enabling key towards the development of high-efficiency iron oxide photoelectrodes whose structure and properties can be tailored by design. However, the design rules for these devices remain elusive because iron oxide behaves differently than conventional semiconductors such as silicon, probably due to the strong correlation between the electrons that give rise to the current and the ions that form the crystalline lattice. Our first goal is to discover the physico-chemical processes that limit the performance of iron oxide photoelectrode and devise design principles that enable overcoming these limitations through material design at the nanoscale that combines iron oxide together with selective underlayers and overlayers that separate positive and negative charge carriers, thereby reducing recombination losses and increasing efficiency. The second goal is to apply these design principles to construct high-efficiency solar water splitting devices. The third and final goal is to invent new device architectures and operation schemes that enable affordable solar hydrogen production in large-scale applications, closing the gap between basic research on lab-scale devices and a new solar energy technology that provides stable power on demand. Figure 1

(Invited) Charge Generation in Bulk-Heterojunctions Based on Fullerene Heterodimers

Organic photovoltaic (OPV) cells remain to be one of the most attractive approaches for utilization of solar energy showing gradually increasing conversion efficiency of 11%. Significant efforts were directed to the improvement of the electron donating (polymer) part of the bulk-heterojunction (BHJ) solar cells. On the other hand, less advancement has been made on the acceptor side. Recently, a few examples of soluble fullerene hetero- and homodimers based on C60 and C70 linked through chemical bridges have been developed and were shown to competitively perform in solar cells. However, the electronic structure of these fullerene dimers is not well understood. Thus, it is not known how those two fullerene molecules are electronically coupled in the dimer, i.e., whether the excited neutral state or anion state of the dimer is delocalized over the whole dimer or still localized only on one fullerene cage. We have experimentally shown that fullerene molecules in the C60-C70-heterodimer in solid films are strongly electronically coupled and the spin densities of the anion and excited triplet states are delocalized over the whole molecular dimer. However, in the frozen diluted solutions the fullerene cages in heterodimers are instead weakly coupled. The anion and triplet states of the dimers show the signature of individual C60 or C70 molecules. We explain this phenomenon by the presence of two different conformers in the solid-state films and in frozen solutions. Since the electronic coupling between fullerene molecules in the dimers is influenced by their packing in the blends, our observation provides insight into how to tune the electronic properties of the fullerene acceptors by proper adjustment of molecular bridge structures between fullerene cages to fix the desired conformation, which in turn may have an impact on the charge carrier generation efficiency in organic solar cells.

Photoprecursor Approach Enables Preparation of Well-Performing Bulk-Heterojunction Layers Comprising a Highly Aggregating Molecular Semiconductor.

Active-layer morphology critically affects the performance of organic photovoltaic cells, and thus its optimization is a key toward the achievement of high-efficiency devices. However, the optimization of active-layer morphology is sometimes challenging because of the intrinsic properties of materials such as strong self-aggregating nature or low miscibility. This study postulates that the "photoprecursor approach" can serve as an effective means to prepare well-performing bulk-heterojunction (BHJ) layers containing highly aggregating molecular semiconductors. In the photoprecursor approach, a photoreactive precursor compound is solution-deposited and then converted in situ to a semiconducting material. This study employs 2,6-di(2-thienyl)anthracene (DTA) and [6,6]-phenyl-C71-butyric acid methyl ester as p- and n-type materials, respectively, in which DTA is generated by the photoprecursor approach from the corresponding α-diketone-type derivative DTADK. When only chloroform is used as a cast solvent, the photovoltaic performance of the resulting BHJ films is severely limited because of unfavorable film morphology. The addition of a high-boiling-point cosolvent, o-dichlorobenzene (o-DCB), to the cast solution leads to significant improvement such that the resulting active layers afford up to approximately 5 times higher power conversion efficiencies. The film structure is investigated by two-dimensional grazing-incident wide-angle X-ray diffraction, atomic force microscopy, and fluorescence microspectroscopy to demonstrate that the use of o-DCB leads to improvement in film crystallinity and increase in charge-carrier generation efficiency. The change in film structure is assumed to originate from dynamic molecular motion enabled by the existence of solvent during the in situ photoreaction. The unique features of the photoprecursor approach will be beneficial in extending the material and processing scopes for the development of organic thin-film devices.

Hexagonal Ceria Located at the Interface of Anatase/Rutile TiO2 Superstructure Optimized for High Activity under Combined UV and Visible-Light Irradiation

The photocatalytic activity of the system critically depends on the catalyst’s architecture and composition. In a ceria–titania system a proper ratio and location of ceria on titania might be crucial to have an optimum level of physicochemical properties for efficient charge carrier generation and separation in the system to initiate redox reaction at its surface. Herein, a novel architecture of the ceria-doped TiO2 system has been developed to study its photoactivity and the mechanism of charge transfer. The photocatalytic activity with optimum cerium content Ce/Ti = 0.2 wt % shows the highest degradation of methylene blue under combined (UV + Vis) irradiation compared to the pristine titania. The obtained result shows that catalyst composition is an important parameter in modeling and optimizing the system to have the desired impact on photoactivity. The proposed unique architecture and composition combination appears to be an ideal model for design of novel photoactive materials having activity in a wi...

Diketopyrrolopyrroles with a Distinct Energy Level Cascade for Efficient Charge Carrier Generation in Organic Solar Cells

Three structurally different low molecular weight diketopyrrolopyrroles (DPPs) are synthesized in order to provide donors with a precise offset in their energy levels. The DPPs are characterized for optical, electrochemical, and thermal properties. By changing the terminal aryl groups attached to the DPP core from phenyl over m‐pyridine to p‐pyridine, different solid state packing is observed in thin film studies using UV/VIS absorption spectra and X‐ray diffraction. Most importantly it is shown that both, reduction as well as oxidation potentials can be precisely tuned with a gradual stepping of about 100 meV by changing the terminal groups attached to the DPP core. Exploiting this energy level modification, these materials are tested in planar cascade organic photovoltaic devices using C60 as acceptor. A sub nm thick interlayer of a suitable DPP derivative is introduced to obtain a distinct energy level cascade at the donor/acceptor interface. Power conversion efficiency as well as short‐circuit current density is doubled with respect to the reference bilayer devices lacking the interface cascade. Spectrally resolved analysis of external quantum efficiency reveals that this enhancement can mainly be attributed to destabilization of bound charge transfer states formed in the C60 layer at the interlayer interface, thus reducing geminate recombination losses.

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Efficient quantum dot-sensitized solar cell based on CdSxSe1-x/Mn-CdS/TiO2 nanotube array electrode

Translucent TiO2 nanotube (NT) array film has been used as supporter materials for the fabrication of CdSxSe1-x/Mn-CdS quantum dot sensitized solar cell (QDSSC). The TiO2 NT array electrodes are sensitized with Mn-CdS and CdSxSe1-x QDs by employing a two-stage sensitization strategy combining successive ionic layer adsorption and reaction (SILAR) techniques and hydrothermal process. The photoresponse region of CdSxSe1-x/Mn-CdS/TiO2 NT electrodes could be controlled by the ratio of S and Se. By systematical investigation the optimal composition of CdSxSe1-x, a high power conversion efficiency of 2.41% is obtained with CdS0.47Se0.53/Mn-CdS/TiO2 NT QDSSC prepared with feed molar ratio of S:Se=0:4. After being coated with 3 SILAR cycles of CdSe on the CdS0.47Se0.53/Mn-CdS/TiO2 NT electrode, the power conversion efficiency could be further improved to the highest value of 3.26%. The enhancement of power conversion efficiency is mainly attributed to the significant increase of short-circuit current density (Jsc) which resulted from the improved light harvesting ability caused by expanded light absorption range and efficient electrons collection caused by mid-gap states created by Mn-CdS. These results are well evidenced with the photovoltaic performance studies (J-V behaviors), incident photon to charge carrier generation efficiency (IPCE), and electrochemical impedance spectroscopy (EIS).

Charge Transfer: Electronic Structure of Fullerene Heterodimer in Bulk-Heterojunction Blends (Adv. Energy Mater. 7/2014)

As reported by Oleg G. Poluektov, Jens Niklas, Vladimir Dyakonov, and co-workers in article number 1301517, electronic coupling between fullerene cages in the dimer is influenced by their packing in the active organic photovoltaic (OPV) blends. This provides a new pathway to tune the electronic properties of the fullerene acceptors for better charge carrier generation efficiency in organic solar cells.

Electronic Structure of Fullerene Heterodimer in Bulk‐Heterojunction Blends

To increase performance of organic solar cells, the optimization of the electron‐accepting fullerenes has received less attention. Here, an electronic structure study of a novel covalently linked C60‐C70‐heterodimer in blend with the polymer PCDTBT (poly[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) is presented. Upon optical excitation of polymer:heterodimer solid films, the unpaired electron is shared between both C60 and C70 cages. In contrast, in the solution the electron is localized on one half of the dimer. Electronic structure calculations reveal that for the C60‐C70‐heterodimer two nearly isoenergetic minima exist, essentially the cis and trans conformers, which are separated by a thermodynamically accessible rotational barrier. In the cis conformation, the edge‐to‐edge distance between the two cages is ca. 4 Å and an unpaired electron is shared between two dimer halves, while in the trans conformation the separation between the fullerene cages is larger and favors electron localization on one half of the heterodimer. By comparison with the experimental data, it is concluded that the cis conformation is preferable in films, and the trans conformation in solution. Modification of the linking molecular bridge opens the possibility to influence the electronic properties of fullerene dimers, which in turn may have an impact on the charge carrier generation efficiency in solar cells.

Charge generation in polymer-fullerene bulk-heterojunction solar cells.

Charge generation in organic solar cells is a fundamental yet heavily debated issue. This article gives a balanced review of different mechanisms proposed to explain efficient charge generation in polymer-fullerene bulk-heterojunction solar cells. We discuss the effect of charge-transfer states, excess energy, external electric field, temperature, disorder of the materials, and delocalisation of the charge carriers on charge generation. Although a general consensus has not been reached yet, recent findings, based on both steady-state and transient measurements, have significantly advanced our understanding of this process.

Phase transition, optical and photoconductive properties of bay-substituted benzoporphyrin derivatives

Two benzoporphyrin derivatives substituted with long side chains at the bay positions have been synthesized through a facile route. They were confirmed to show two separate columnar phases upon cooling, i.e., a hexagonal columnar phase and a rectangular columnar phase. In particular, the temperature dependence of the electronic spectra revealed that the Q-band split into two peaks, indicating the existence of typical herringbone-type dimers. Moreover, their photoconductive properties were studied by steady-state and transient photocurrent measurements. It was found that both benzoporphyrins exhibited very fast hole mobility over 0.1 cm2 V−1 s−1 in both columnar phases and a high charge carrier generation efficiency up to 2%, demonstrating that they are potential p-type organic semiconductors for photovoltaic devices.

Temperature Activation of the Photoinduced Charge Carrier Generation Efficiency in Quaterthiophene:C60 Mixed Films

We measure photoinduced excitations in a dicyanovinyl end-capped methylated quaterthiophene derivative in blends with the electron acceptor C60, as already employed in organic photovoltaics. By usi...