Dissociation Of Photogenerated Excitons Research Articles

(Invited) Signatures of Charge Carrier (De)Localization in Semiconducting Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are attractive materials for a variety of opto-electronic devices and applications due their strong and energetically tunable optical absorption, and high charge carrier mobilities that result from the delocalized nature of the π-electron system. To realize the full potential of s-SWCNTs in these applications, it is critical to understand the generation, transport, interfacial transfer, and recombination of free charge carriers (electrons and holes). As with many pi-conjugated semiconductors, there remain uncertainties regarding the degree of (de)localization for charge carriers generated in s-SWCNTs either through photogenerated exciton dissociation or through charge injection by adsorbed redox dopants or gating. In this presentation, I will compare the various signatures of charge carrier localization and delocalization that appear in spectroscopic and transport studies of s-SWCNTs in a variety of different environments and over several decades of carrier density. These comparisons help to elucidate the important roles of intrinsic and extrinsic factors on carrier (de)localization and the information that spectroscopic signatures in the visible, near-IR, and mid-IR can provide on such (de)localization.

PbSe nanorod-quantum dot bulk nano-heterojunction solar cells generating multiple excitons with record photo conversion efficiencies

Multi-exciton-generation (MEG) process is critical due to its potential to push solar cell photoconversion efficiencies (PCEs) beyond theoretical limits. However, cells with this feature are rare, as the demonstration of this characteristic property in a working cell is challenging. Herein, a donor-acceptor type bulk nano-heterojunction layer, prepared by blending two nanostructures known to have high MEG yields, i.e., PbSe quantum dot-QDs and PbSe nanorods-NRs, is utilized as the light absorbing active layer. Transmission electron microscopy images show that NR-donors are surrounded by QD-acceptors, indicating the formation of optimal donor-acceptor interfaces that allow efficient dissociation of photo-generated excitons. Effective charge separation across the NR/QD interface is also demonstrated with valence and conduction band offsets measured in the range of 0.20–0.25 eV. It is found that the type of the hole transport layer has a profound effect in reducing the electron back injection and increasing the MEG efficiency/yield. Cells with 0.97 eV NR-donor band gap exhibit a PCE of 4.09% with a peak internal quantum efficiency greater than 100%. Cells, outperforming the previously reported MEG-based cells comprising PbSe QDs or NRs, demonstrate a staircase-like response in external quantum efficiency, which is considered as the most obvious indicator of an ideal MEG system.

Ligand-free CsPbBr3 with calliandra-like nanostructure for efficient artificial photosynthesis

The low-efficiency CO2 uptake capacity and insufficient photogenerated exciton dissociation of current metal halide perovskite (MHP) nanocrystals with end-capping ligands extremely restrict their application in the field of artificial photosynthesis. Herein, we demonstrate that ligand-free CsPbBr3 with calliandra-like nanostructure (LF-CPB CL) can be synthesized easily through a ligand-free seed-assisted dissolution-recrystallization growth process, exhibiting significantly enhanced CO2 uptake capacity. More specifically, the abundant surface bromine (Br) vacancies in ligand-free MHP materials are demonstrated to be beneficial to photogenerated carrier separation. The electron consumption rate of LF-CPB CL for photocatalytic CO2 reduction increases 7 and 20 times over those of traditional ligand-capping CsPbBr3 nanocrystal (L-CPB NC) and bulk CsPbBr3, respectively. Moreover, the absence of ligand hindrance can facilitate the interfacial electronic coupling between LF-CPB CL and tetra(4-carboxyphenyl)porphyrin iron(III) chloride (Fe-TCPP) cocatalyst, bringing forth significantly accelerated interfacial charge separation. The LF-CPB CL/Fe-TCPP exhibits a total electron consumption rate of 145.6 μmol g−1 h−1 for CO2 photoreduction coupled with water oxidation, which is over 14 times higher than that of L-CPB NC/Fe-TCPP.

PbI2-TiO2 Bulk Heterojunctions with Long-Range Ordering for X-ray Detectors.

High-performance X-ray detectors are usually based on single crystals, due to the long-range ordering and hence outstanding electronic properties. On the other hand, bulk heterojunctions (BHJs) that can effectively enhance photogenerated exciton dissociation are widely used for photodetectors. The benefits of both spur investigation into how to combine these two strategies to enhance X-ray detection. Here, TiO2 networks are incorporated into PbI2 crystals to form interpenetrating type II heterojunctions, namely BHJs. These BHJs exhibit long-range ordering in molecular packing similar to that of single crystals. Compared with single crystals, the long-range ordered BHJs facilitate the separation of photogenerated electrons and holes to inhibit recombination, extend the mobility lifetime product by 6.4 times, and consequently improve X-ray sensitivity by 5.8 times. Hence, this work provides a new strategy using gel-grown crystals to fabricate high-performance X-ray detectors as well as a new platform for studying the behavior of X-ray-generated carriers in BHJs with long-range ordering.

Efficient ternary polymer solar cells by optimizing photogenerated exciton dissociation and photon harvesting with a short wavelength absorption polymer acceptor as the third component

Ternary blend films, obtained by introducing a third component (a second acceptor as the third component) to a binary polymer solar cell (PSC), are a promising ternary strategy because the light absorption range, surface morphology, and charge carrier transport of the photoactive layer may be optimized, as can the energy level alignment between the donor and the acceptor. In this work, acceptors such as the short-wavelength-absorption polymer N2200 and the long-wavelength-absorption small molecule FOIC were combined with the donor PBDB-T-2F to construct ternary blends. The optimized ternary PSC could achieve a power conversion efficiency (PCE) of 13.98%, which is higher than the efficiencies of binary PSCs based on PBDB-T-2F:FOIC (12.65%) and PBDB-T-2F:N2200 (9.36%). The enhanced PCE of the ternary PSC is based on the high electron mobility, balanced charge transport, optimized surface morphology and charge carrier kinetics and the extended light absorption of the ternary photoactive layer, realized by adjusting the ratio of FOIC:N2200. Our results indicate that mixing a polymer acceptor into a binary photoactive layer to form a ternary blend photoactive layer is a valuable strategy for improving photovoltaic performance. • Ternary PSCs were fabricated with F–N2200 as third component materials. • The improvement of charge dynamics process of ternary photoactive layer by ternary strategy. • The improved photon harvesting at the donor–blend acceptor interface.

Edge Influence on Charge Carrier Localization and Lifetime in CH3NH3PbBr3 Perovskite: Ab Initio Quantum Dynamics Simulation.

The distribution of charge carriers in metal halide perovskites draws strong interest from the solar cell community, with experiments demonstrating that edges of various microstructures can improve material performance. This is rather surprising because edges and grain boundaries are often viewed as the main source of charge traps. We demonstrate by ab initio quantum dynamics simulations that edges of the CH3NH3PbBr3 perovskite create shallow trap states that mix well with the valence and conduction bands of the bulk and therefore support mobile charge carriers. Charges are steered to the edges energetically, facilitating dissociation of photo-generated excitons into free carriers. The edge-driven charge separation extends carrier lifetimes because of decreased overlap of the electron and hole wave functions, which leads to reduction of the nonadiabatic coupling responsible for nonradiative electron-hole recombination. Reduction of spatial symmetry near the edges activates additional vibrational modes that accelerate coherence loss within the electronic subsystem, further extending carrier lifetimes. Enhanced atomic motions at edges increase fluctuations of edge energy levels, enhancing mixing with band states and improving charge mobility. The simulations contribute to the atomistic understanding of the unusual properties of metal halide perovskites, generating the fundamental knowledge needed to design high-performance optoelectronic devices.

Built-in electric field promotes photoexcitation separation and depletion of most carriers in TiO2:C UV detectors

TiO2 has been widely used in ultraviolet (UV) photodetectors, but due to the large number of structural defects and strong band-to-band recombination of the exciton in TiO2, the devices usually have large dark current (Id) and low light current (Il), which seriously reduces the sensitivity and responsivity (R) of the TiO2 based devices. In this work, carbon (C) quantum dots (QDs) are introduced into TiO2 film to ameliorate these issues. Due to the difference of work function between TiO2 nanoparticles and C QDs, the built-in electric field (Ebi) can be formed, which effectively facilitates the photogenerated exciton dissociation in the TiO2 film under UV illumination. Meanwhile, the constructed depletion region in dark reduces the majority carrier density, thus decreasing the Id of the photodetector. Moreover, the Ebi and depletion region will also contribute to the faster charge collection under UV illumination and recombination of the electron in dark, which is beneficial for the improved response/recovery speed of the device.

Influence of the intramolecular donor-acceptor distance on the performance of double-cable polymers

A series of double-cable polymers PFT-C4-PDI, PFT-C6-PDI and PFT-C8-PDI, composed of the poly(fluorene-alt-thiophene) (PFT) backbone, the perylene diimide (PDI) pendants and the length-various (four-, six- and eight-carbon) covalent alkoxy linkers, were presented. The backbone polymer chain created the hole-transporting channel and the inner-chain aggregation of PDI units created the electron-transporting channel, but the aggregation became weaker along with the longer linker, as proven by the UV–Vis absorption and fluorescence quenching. The polymers were non-conducting, but functioned as efficient compatibilizers. The doping of the polymers induced the formation of the bi-continuous networks inside P3HT:PCBM blends, facilitated photo-generated exciton dissociation and charge transporting. PFT-C4-PDI more efficiently increased the absorption coefficient and the charge-carrier mobility of the P3HT:PCBM film. The power conversation efficiency (PCE) of the P3HT:PCBM bulk-heterojunction solar cells with 3 wt% PFT-C4-PDI, PFT-C6-PDI and PFT-C8-PDI doping were improved by 16.9%, 9.2% and 8.0%, respectively, relative to the non-doped reference device.

Modulated charge transport characteristics in solution-processed UV photodetector by incorporating localized built-in electric field

The application of ZnO in the field of ultraviolet (UV) photodetectors is hindered by the lower responsivity, which is attributed to the strong band-to-band direct recombination caused by its large exciton binding energy and defect-assisted charge recombination related with the presence of defect states. Here, a solution-processed ZnO:Poly[N-9′-heptadecanyl-2,7- carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) composite photosensitive layer is employed to overcome these shortcomings. By incorporating PCDTBT, the localized built-in electric field can be constructed, which effectively facilitates the photogenerated exciton dissociation in annealing-free ZnO layer. Meanwhile, the formed depletion region in dark reduces the majority carrier density, thus decreasing the dark current of the photodetector. Furthermore, the absorption spectrum of PCDTBT perfectly overlaps the photoluminescence of ZnO, which is beneficial for the reutilization of carrier recombination energy by fluorescence resonance energy transfer. This study reveals that the charge recombination loss in ZnO limits the photoresponse, and points a direction to improve the light detection capacity of UV photodetectors.

Efficient charge separation at multiple quantum well perovskite/PCBM interface

Low-dimensional organometal halide perovskites have attracted more and more attention because of their good optoelectronic properties and improved stability compared to three-dimensional analogues. In this work, we investigated the charge separation mechanism in multiple quantum well (MQW) perovskite films, which are composed of a mixture of layered perovskites (or quantum wells) with different bandgaps. Despite inefficient dissociation of photo-generated excitons in large-bandgap quantum wells due to the large exciton binding energy, efficient charge separation can occur at the MQW perovskite/electron-extracted-layer interface via energy and/or charge transfer from large-bandgap quantum wells to small-bandgap quantum wells. The MQW perovskite solar cell exhibits a 25-fold improvement in device efficiency, as compared to a pure 2D analogue.

Facilitated extrinsic majority carrier depletion and photogenerated exciton dissociation in an annealing-free ZnO:C photodetector.

Applications of ZnO in photodetectors are limited by the great quantity of extrinsic majority carriers due to structural defects and difficult exciton dissociation due to the large exciton binding energy; these generally lead to a higher dark current (Id) and lower light current (Il), severely degrading the responsivity and detectivity. C dots are incorporated into an annealing-free ZnO layer to innovatively construct a local built-in electric field (Ebi) using the difference in the work functions; this simultaneously overcomes the drawbacks of the pristine ZnO photosensitive layer. In dark, the extrinsic majority carrier of ZnO is depleted around the incorporated C dots due to the self-depleting effect; thus, the Id decreases. Under UV illumination, the photogenerated exciton driven by the local Ebi is easily dissociated into a free charge carrier, contributing to the improved Il. This study paves a universal way to effectively improve the detection characteristics of photoconductive devices by incorporating the local Ebi.

Single-junction organic solar cells based on a novel wide-bandgap polymer with efficiency of 9.7%.

A novel wide-bandgap copolymer (PDBT-T1) is developed and applied in organic solar cells, which yield a high efficiency of 9.74% and a high fill factor of 75%. The high photovoltaic performance is due to efficient photogenerated exciton dissociation and charge collection in PDBT-T1-based solar cells. The results show that PDBT-T1 is an outstanding candidate as a wide-bandgap material for tandem (or multi-junction) organic solar cells. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Enhanced performance of isotype planar heterojunction photoresponsive organic field-effect transistors by using Ag source-drain electrodes

Pentacene/lead phthalocyanine (PbPc)-based isotype and fullerene /PbPc-based anisotype planar heterojunction near-infrared photoresponsive organic field-effect transistors (PhOFETs) were fabricated. We compared the performance of the isotype planar heterojunction PhOFET and the anisotype one, and dominantly investigated the effect of contact on the performance of the isotype planar heterojunction devices. The results showed that the isotype planar heterojunction device exhibits a comparable maximum photoresponsivity of 107 mA/W and a comparable maximum photo/dark current ratio of to the anisotype one ( of 109 mA/W and of ), and exhibits superior air stability to the anisotype one. Moreover, it is surprising to find that Ag source-drain electrodes replacing Au ones yield a performance enhancement in isotype PHJ devices, this is mainly because the use of Ag source-drain electrodes enhances the photo-generated exciton dissociation efficiency at the metal/PbPc interface.

Charge-carrier relaxation dynamics of poly(3-hexylthiophene)-coated gold hybrid nanoparticles

Poly(3-hexylthiophene)-coated gold (Au@P3HT) hybrid nanoparticles have been prepared via a “grafting-to” approach, and compared with pristine P3HT to investigate the dynamics of exciton relaxation using time-resolved transient-absorption and fluorescence spectroscopy. In efforts to facilitate the efficient dissociation of photo-generated excitons, we have incorporated gold nanoparticles having surface-plasmon resonances into thiol-terminated P3HT to fabricate Au@P3HT nanocomposites. The first-excited singlet state of Au@P3HT decays faster than that of pristine P3HT due to interactions with surface-plasmon resonances; excitons undergo dissociation via energy transfer from P3HT to the surface-plasmon state of a gold nanoparticle in Au@P3HT nanocomposites. The lowest triplet state of P3HT is also less populated due to the energy transfer while its lifetime is slightly reduced by the presence of gold nanoparticles. Thus, it is suggested that our incorporation of gold nanoparticles into P3HT reduces the recombination of geminate excitons and, thereby, increases the probability of exciton dissociation.

Ubiquitous carrier harvesting in organic solar cells with embedded indium–tin-oxide nano-electrodes

Bulk heterojunction in organic solar cells allows adequate interfaces for the dissociation of photo-generated excitons. However, the intermixed donor- and acceptor-like materials do not guarantee successful carrier transport and collections. In this work, we demonstrate ubiquitous carrier harvesting characteristics by using embedded indium–tin-oxide nano-electrodes (NEs) with a conformal electro-polymerized coating of a hole conducting layer. Compared to its planar electrode counterpart, this device exhibits internal quantum efficiency insensitive to the distribution of carrier generation, as determined by angle-resolved absorption spectroscopy and photocurrent characteristics. The ubiquitous carrier harvesting characteristics offer a viable solution to organic thin-film devices fabricated on flexible substrates or operated under diffused ambient light.

Ag纳米颗粒对有机小分子太阳能电池性能的改善

Enhanced performance of small molecular weight organic solar cells based on CuPc/C60 and TiOPc/C60 with Ag nanoparticles fabricated on the ITO anode and MoO3 as the anode buffer layer has been demonstrated.Surface plasmon induced by the incorporation of Ag nanoparticles results in the increased absorption efficiency and photogenerated exciton dissociation probability of the photoactive layers.Meanwhile,the quenching of the photogenerated excitons at the organic/metal interface can be successfully restricted by the MoO3 anode buffer layer.Consequently,the short-circuit current is improved and the other parameters maintain unaffected,which leads to an enhanced power conversion efficiency of the devices.

Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies

Internanocrystal coupling induced excitons dissociation in lead salt nanocrystal assemblies is investigated. By combining transient photoluminescence spectroscopy, grazing incidence small-angle X-ray scattering, and time-resolved electric force microscopy, we show that excitons can dissociate, without the aid of an external bias or chemical potential gradient, via tunneling through a potential barrier when the coupling energy is comparable to the exciton binding energy. Our results have important implications for the design of nanocrystal-based optoelectronic devices.