Formation Of Free Charge Carriers Research Articles

Self-assembled lead-free double perovskite-MXene heterostructure with efficient charge separation for photocatalytic CO2 reduction

Lead-free double perovskites with superior stability have been considered as promising non-toxic substitutes to their lead-contained counterparts in photocatalysis. However, the severe charge recombination greatly restricts their potential as high-performance photocatalysts. Herein, for the first time, we present a self-assembled heterostructure of lead-free double perovskite Cs2AgBiBr6 nanocrystals (NCs) on the surface of MXene nanosheets via mutual electrostatic attraction. The presence of MXene nanosheets effectively promotes the formation of free charge carriers in Cs2AgBiBr6 NCs via reducing the exciton binding energy. Additionally, the ultrafast photogenerated electron transfer from Cs2AgBiBr6 to MXene with a timescale of 1.1 ps largely prolongs the charge carrier lifetime by two times. As a result of the efficient charge separation and electron extraction, the Cs2AgBiBr6/MXene heterostructures achieve a high photoelectron consumption yield of 50.6 µmol g−1 h−1 for photocatalytic CO2 reduction, which surpasses most previously reported lead-free perovskite-based catalysts.

Triplet‐Charge Annihilation in a Small Molecule Donor: Acceptor Blend as a Major Loss Mechanism in Organic Photovoltaics

AbstractOrganic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. Herein, a small molecule donor, DRCN5T, is studied using a variety of morphology and spectroscopy techniques, and blended with both fullerene and non‐fullerene acceptors. Specifically, grazing incidence wide‐angle X‐ray scattering and transient absorption, Raman, and electron paramagnetic resonance spectroscopies are focused on. It is shown that despite DRCN5T's ability to achieve OPV efficiencies of over 10%, it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet‐charge annihilation. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form. Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet‐charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non‐fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA.

Comparative Investigation of Fullerene PC71BM and Non-fullerene ITIC-Th Acceptors Blended With P3HT or PBDB-T Donor Polymers for PV Applications

Fundamentally, organic solar cells (OSCs) with a bulk-heterojunction active layer are made of at least two electronically dissimilar molecules, in which photoabsorption in one (donor) generates Frenkel excitons. The formation of free charge carriers emerge after exciton dissociation at the donor:acceptor interface. In the past decade, most of the progress in enhanced device performance has been steered by the rapid development of novel donor and acceptor materials and on device engineering. Among these donor materials, regioregular poly(3-hexylthiophene) (P3HT) produced better performance despite the mismatch of its absorption coefficient with the solar emission spectrum. Comparatively the donor PBDB-T exhibits an outstanding absorption coefficient with a deeper-lying highest occupied molecular orbital (HOMO) level. Previously most of the efficient acceptors were based on fullerene molecules characterized by limited photoabsorption and stability. In contrast, the recently developed non-fullerene OSCs have a tunable absorption spectrum and exhibit improved stability. In this work, we explore the fundamental sources of the differences in the device performance for different blend compositions made of fullerene derivative (PC71BM) and non-fullerene (ITIC-Th) when paired with the polymer donors P3HT and PBDB-T. The characteristic changes of the optical properties of these blends and their roles in device performance are also investigated. We also studied charge generation where PBDB-T:PC71BM showed the highest maximum exciton generation rate (Gmax) of 3.22 × 1028 s–1 while P3HT: ITIC-Th gave the lowest (0.96 × 1028 s–1). Also noted, PC71BM based counterparts gave better charge transfer capabilities as seen from the lower PL quenching and higher charge carrier dissociation plus collection probability P(E,T) derived from a plot of Jph/Jsat ratio under short-circuit conditions against the effective voltages.

Detection of the organic solvent vapors by the optical gas sensors based on polyaminoarenes

The important problem to ensure the safety of life is the development of effective methods for monitoring of toxic gases in the atmosphere and industrial premises. To ensure such control, various gas sensors are developed that work on the effects of changes in electrical resistance, optical absorption or radiation in a certain spectral range. Most known gas sensors have high operating temperatures, which creates some difficulties in their operation, so more and more attention is paid to sensors based on conjugated electrically conductive polymers, in particular, polyaminoarenes. In result of absorption of inorganic polar gases such as ammonia, nitrogen oxide, hydrogen chloride and others the significant changes in conductivity, optical absorption and morphology of polyaminoarene films are observed. However influence of organic solvent vapors on the optical spectra of polyaminoarene films for today is poorly studied. In the present work the functional polymer films of polyanisidine (PoA) and polytoluidine (PoTI) are proposed as sensitive elements of optical sensors operating at room temperatures. The sensitive films on the transparent SnO2 surface were prepared by electrodeposition. The influence of vapors of organic solvents (dimethylformamide, tetrahydrofuran, chloroform, nitrobenzene, toluene) on the optical characteristics of PoTi and PoA films was established. The optical absorption spectra investigated PoA film was characterized by two band with maximum near 360–410 nm (π–π* transition) and broad band in the range of 620–950 nm which is a superposition of the second and third bands. Influence of organic vapors causes the changes in films coloration. The maximum of sensitivity to the organic vapors for PoTI films in all cases is observed at λ > 550 nm and extends to near-infra-red area indicating a formation of free charge carriers of polaron type. Nonpolar solvent vapors insignificantly affect the optical properties of polyaminoarene films responсes to spectral changes in the visible and NIR range of spectrum. A highest gas sensitivity of optical signal was observed under influence of dimethylformamide and tetrahydrofuran vapors. The time to establish the steady-state value of the optical response is 30–60 s for PoTi, while for PoA reaches 120–180 s depending on the nature of organic vapors.

Monitoring Electron-Phonon Interactions in Lead Halide Perovskites Using Time-Resolved THz Spectroscopy.

Lead halide perovskite semiconductors have low-frequency phonon modes within the lead halide sublattice and thus are considered to be soft. The soft lattice is considered to be important in defining their interesting optoelectronic properties. Electron-phonon coupling governs hot-carrier relaxation, carrier mobilities, carrier lifetimes, among other important electronic characteristics. Directly observing the interplay between free charge carriers and phonons can provide details on how phonons impact these properties (e.g., exciton populations and other collective modes). Here, we observe a delicate interplay among carriers, phonons, and excitons in mixed-cation and mixed-halide perovskite films by simultaneously resolving the contribution of charge carriers and phonons in time-resolved terahertz photoconductivity spectra. We are able to observe directly the increase in phonon population during carrier cooling and discuss how thermal equilibrium populations of carriers and phonons modulate the carrier transport properties, as well as reduce the population of carriers within band tails. We are also able to observe directly the formation of free charge carriers when excitons interact with phonons and dissociate and to describe how free carriers and exciton populations exchange through phonon interactions. Finally, we also time-resolve how the carriers are screened via the Coulomb interaction at low and room temperatures. Our studies shed light on how charge carriers interact with the low-energy phonons and discuss implications.

Metal–organic frameworks for electronics and photonics

Abstract

The impact of donor-acceptor phase separation on the charge carrier dynamics in pBTTT:PCBM photovoltaic blends.

The effect of donor-acceptor phase separation, controlled by the donor-acceptor mixing ratio, on the charge generation and recombination dynamics in pBTTT-C14:PC70 BM bulk heterojunction photovoltaic blends is presented. Transient absorption (TA) spectroscopy spanning the dynamic range from pico- to microseconds in the visible and near-infrared spectral regions reveals that in a 1:1 blend exciton dissociation is ultrafast; however, charges cannot entirely escape their mutual Coulomb attraction and thus predominantly recombine geminately on a sub-ns timescale. In contrast, a polymer:fullerene mixing ratio of 1:4 facilitates the formation of spatially separated, that is free, charges and reduces substantially the fraction of geminate charge recombination, in turn leading to much more efficient photovoltaic devices. This illustrates that spatially extended donor or acceptor domains are required for the separation of charges on an ultrafast timescale (<100 fs), indicating that they are not only important for efficient charge transport and extraction, but also critically influence the initial stages of free charge carrier formation.

Long-lived charge carrier generation in ordered films of a covalent perylenediimide-diketopyrrolopyrrole-perylenediimide molecule.

The photophysics of a covalently linked perylenediimide-diketopyrrolopyrrole-perylenediimide acceptor-donor-acceptor molecule (PDI-DPP-PDI, 1) were investigated and found to be markedly different in solution versus in unannealed and solvent annealed films. Photoexcitation of 1 in toluene results in quantitative charge separation in τ = 3.1 ± 0.2 ps, with charge recombination in τ = 340 ± 10 ps, while in unannealed/disordered films of 1, charge separation occurs in τ < 250 fs, while charge recombination displays a multiexponential decay in ∼6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH2Cl2 vapor annealing films of 1, grazing-incidence X-ray scattering shows that the molecules form a more ordered structure. Photoexcitation of the ordered films results in initial formation of a spin-correlated radical ion pair (electron-hole pair) as indicated by magnetic field effects on the formation of free charge carriers which live for ∼4 μs. This result has significant implications for the design of organic solar cells based on covalent donor-acceptor systems and shows that long-lived, charge-separated states can be achieved by controlling intramolecular charge separation dynamics in well-ordered systems.

High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors.

The study of the photophysical properties of organic-metallic lead halide perovskites, which demonstrate excellent photovoltaic performance in devices with electron- and hole-accepting layers, helps to understand their charge photogeneration and recombination mechanism and unravels their potential for other optoelectronic applications. We report surprisingly high photoluminescence (PL) quantum efficiencies, up to 70%, in these solution-processed crystalline films. We find that photoexcitation in the pristine CH3NH3PbI3-xClx perovskite results in free charge carrier formation within 1 ps and that these free charge carriers undergo bimolecular recombination on time scales of 10s to 100s of ns. To exemplify the high luminescence yield of the CH3NH3PbI3-xClx perovskite, we construct and demonstrate the operation of an optically pumped vertical cavity laser comprising a layer of perovskite between a dielectric mirror and evaporated gold top mirrors. These long carrier lifetimes together with exceptionally high luminescence yield are unprecedented in such simply prepared inorganic semiconductors, and we note that these properties are ideally suited for photovoltaic diode operation.

Possible mechanisms of charge separation during exiton decay in the polymer matrices of third-generation solar cells

An important stage in the functioning of third-generation solar cells based on polymers is considered: the dissociation of exitons generated by solar light and the separation of the resulting electrons and holes. Various molecular mechanisms for the formation of free charge carriers are studied by means of quantum mechanics and statistical physics. Some processes are associated with polymer matrices; others with systems typical of ideal neat matrices.

Dynamics of exciton dissociation in donor-acceptor polymer heterojunctions

Exciton dissociation in a donor-accepter polymer heterojunction has been simulated using a nonadiabatic molecular dynamics approach, which allows for the coupled evolution of the nuclear degrees of freedom and the electronic degrees of freedom described by multiconfigurational electronic wavefunctions. The simulations reveal important details of the charge separation process: the exciton in the donor polymer first dissociates into a "hot" charge transfer state, which is best described as a polaron pair. The polaron pair can be separated into free polaron charge carriers if a sufficiently strong external electric field is applied. We have also studied the effects of inter-chain interaction, temperature, and the external electric field strength. Increasing inter-chain interactions makes it easier for the exciton to dissociate into a polaron pair state, but more difficult for the polaron pair to dissociate into free charge carriers. Higher temperature and higher electric field strength both favor exciton dissociation as well as the formation of free charge carriers.

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

In this feature article, we review and examine evidence that the primary photoexcited species in conjugated polymers is considerably delocalized. Localization occurs via a series of complex relaxation mechanisms on the <200 femtosecond time scale. We show that short-lived delocalization in the neutral excited state and charge separated state of bulk heterojunction blends might play an essential role in ensuring efficient formation of free charge carriers for photovoltaic applications. Finally, the additional parameter of intramolecular charge transfer character in the excited state of more recently developed donor–acceptor copolymers is discussed. Both delocalization and partial charge transfer reduce the binding of the electron and hole in photoexcited organic semiconductors and can help to overcome the bottleneck to macroscopic current generation in polymer solar cells.

Diketopyrrolopyrrole-based acceptor polymers for photovoltaic application

Developing new acceptor materials as alternatives to fullerene acceptors remains a challenge in the field of organic photovoltaics. We report on the synthesis and optoelectronic properties of three acceptor polymers bearing diketopyrrolopyrrole units in the main chain (PA, PB and PC). Their performance as the acceptor material in bulk heterojunction solar cells using P3HT as the donor material has been tested. The solar cells show relatively high open-circuit voltages (≥0.9 V) but low fill factors and short-circuit current densities limit the photovoltaic device performance. Formation of free charge carriers and low electron mobility are identified as the major obstacles. In blends of P3HT with PA or PB charge formation is limited, while for the P3HT:PC blend photogenerated charges recombine into the PC triplet state before they can separate, unless assisted by a reverse electric field.

The Relationship between the Electric Field-Induced Dissociation of Charge Transfer Excitons and the Photocurrent in Small Molecular/Polymeric Solar Cells

A bulk heterojunction system consisting of a soluble poly(p-phenylenevinylene) donor and a small molecular electron acceptor based on Vinazene (2-vinyl-4,5-dicyanoimidazole) has been investigated with respect to the correlation between field-induced emission quenching and free charge carrier formation. Transient and steady-state emission properties measured at different voltages indicate that emissive charge transfer (CT) excitons formed at the heterojunction are prone to direct field-induced dissociation. However, the split-up of these CT excitons cannot fully account for the photocurrents observed on the very same devices. It is proposed that the formation of free carriers involves a nonemissive precursor and that the recombination of this precursor does not proceed via the formation of CT excitons.

Polarons in wide-band-gap molecular materials: Polysilanes

Illumination of polysilanes leads to the formation of free charge carriers. Due to the charge–phonon coupling and molecule deformation, polarons are formed. Their occurrence can be detected by thermostimulated luminescence (TSL). Polaron binding energy Ep can be determined by fitting experimental dependences of charge carrier mobility vs. applied electric field measured at different temperatures. Ep for poly[methyl(phenyl)silylene] films was found to be 0.29eV.

Time-Resolved Study of Fullerene Doping Effects on Photoluminescence and Photoconductivity of Poly(3-alkylthiophene)

We have carried out time-resolved measurements of photoluminescence and photocurrent in poly(3-octylthiophene) (PAT8) doped with fullerene (C60). For photocurrent study, we applied a laser two-correlated-pulse technique, which permitted the selection of electronic processes connected with recombination of intermediate short-living species responsible for the formation of free charge carriers. The wavelength and pulse width of the laser pulses were 400 nm and 150 fs, respectively, and delay time (Δt) between correlated pulses was controlled within the ps time domain. It was shown that the average photocurrent in undoped samples has a prominent component originating from interpair recombination of polaron pairs. This component, however, changed with doping: with low doping (C60 less than 0.1 mol%) it increased, but with higher doping (C60 up to 0.5 mol%) it disappeared. It was shown that the electron transfer from π-conjugated segments of PAT8 to C60 takes about 10 ps, and leads to the formation of polaron pairs which dissociate into free charge carriers much faster (lifetime is much shorter than 3 ps) than the pairs formed without doping.

Time-resolved study of polaron pairs in conjugated polymers by two-correlated-pulses technique

Formation of polaron pairs is studied in photoconducting conjugated polymers such as substituted polythiophenes. The polaron pairs show themselves in the second order processes that were the main supplier of free charge carrier at high excitation density provided by femtosecond laser pulses. Two-correlated-pulses technique are used, and permitted to resolve kinetics of the free charge carrier formation within the picosecond time domain. The external electrical field is shown to change a relative amount of free charge carriers formed in the first and second order processes (i.e., by dissociation of primarily formed polaron pairs and by dissociation of the pairs formed as a result of recombination of primary pairs). A method of estimation of intercharge distances in the pairs based on the action of the electrical field is suggested and shows that polaron pairs of two intercharge distances (about 4 and 10 nm) are formed at the photoexcitation of polythiophenes studied.

Time-resolved optical and electrical study of second-order processes responsible for the formation of free polarons in conjugated polymers

A technique based on the action of pairs of correlated laser light pulses (400 nm wavelength) with a controlled delay time between them was applied for studying second-order processes responsible for the formation of free charge carriers in a conjugated polymer poly(2,5-dioctyloxy-$p$-phenylenevinylene). Time-resolved experiments within the picosecond time domain enabled the attainment of direct evidence of the existence of intermediate states preceding free charge carriers at the photoexcitation of conjugated polymers. These states are shown to take part in second-order processes responsible for the generation of free charge carriers in conjugated polymers at a sufficiently high excitation density. The short lifetime of species involved in the processes led to the speculation that they are primarily formed polaron pairs, the majority of them being generated without any delay at the absorption of the light.

Interaction of intermediate excited states and formation of free charge carriers under laser excitation of a conjugated polymer

Picosecond time-resolved technique, based on correlated action of periodically repeated pairs of femtosecond laser pulses of 400 nm wavelength and recording of average photocurrent, was applied for studying the mechanism of free charge carrier formation in ladder type poly( p -phenylene) (MeLPPP). The technique distinguishes selectively components of the photoconductivity caused by second-order processes between intermediate species formed under the light absorption and which are responsible for cw photoconductivity. It was shown that though intra-chain excitons take part in those processes, an essential part of charge carriers is generated very quickly (within 1 ps). These primarily formed charge carriers are however not free, belonging to polaron pairs. The latter are demonstrated to be precursors of free polarons. It is because of their ability to suffer electric-field-assisted dissociation and to take part in random recombination reactions that generate free polarons though at a price of destroying more polarons inside the pairs. General scheme of free charge carrier formation, based on the results obtained, is presented.

Formation of polaron pairs and time-resolved photogeneration of free charge carriers inπ-conjugated polymers

We have performed in the present work time-resolved experiments on poly(3-dodecyl-thiophene) (P3DDT) and poly(2,5-dioctyloxy-$p$-phenylene vinylene) (OO-PPV) films by directly probing the formation of charge carriers responsible for the cw photoconductivity within the time domain of -10 ps to 1 ns. Laser light pulses of 400 nm wavelength, 150 fs width, induced photoconductivity in a sample with a frequency 1 kHz. Red 800 nm light pulses delayed in respect to blue ones were revealed to affect the photoconductivity. The effect of the second pulses increased with the delay time. Red light induced changes of the photoconductivity were positive in OO-PPV, and negative in P3DDT. These results are rationalized as an evidence of delayed not immediate formation of free charge carriers. The carriers seem to be formed within 10 ps after the pumping pulse. A mechanism of formation of free polarons from polaron pair is suggested, which has permitted to explain main feature of the results including different signs of the effect of the red light in different polymers.