Yield Of Charges Research Articles

Surface hopping simulations on charge separation in an organic donor-acceptor system.

Charge separation in organic solar cells is a long-lasting and heavily debated issue. Here, we use the surface hopping method based on the Pariser-Parr-Pople (PPP) Hamiltonian and configuration interaction singles (CIS) approximation to simulate the charge separation process in an organic donor-acceptor system. The system is composed of one donor polymer chain and four acceptor polymer chains, and they are all stacked face-to-face. We let the system to relax from a photoexcited state, and then we observed that the electron is transferred from the donor chain to different acceptor chains and the hole is left on the donor chain, forming polaron pairs with different electron-hole distances. By performing statistical analysis on a number of trajectories, we found that the electron and the hole are fully separated before the system relaxes to its lowest excited state. The yield of free charges shows a significant dependence on the donor-acceptor band offset which provides the driving force for charge separation, while showing negligible dependence on the photoexcitation energy. The external electric field has a remarkable effect on the yield of free charges.

Traps in the spotlight: How traps affect the charge carrier dynamics in Cs2AgBiBr6 perovskite

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs 2 AgBiBr 6 . However, efficiencies of Cs 2 AgBiBr 6 -based photovoltaics are still modest. To elucidate the loss mechanisms, in this report, we investigate charge dynamics in Cs 2 AgBiBr 6 films by double-pulse excitation time-resolved microwave conductivity (DPE-TRMC). By exciting the sample with two laser pulses with identical wavelengths, we found a clear photoconductance enhancement induced by the second pulse even 30 μs after the first laser pulse. Modeling the DPE-TRMC results, complemented by photoluminescence and transient absorption, we reveal the presence of deep emissive electron traps, while shallow hole trapping is responsible for the long-lived transient absorption signals. These long-lived carriers offer interesting possibilities for X-ray detectors or photocatalysis. The DPE-TRMC methodology offers unique insight into the times involved in charge trapping and depopulation in Cs 2 AgBiBr 6 . • Double-pulse excitation TRMC is developed to study trapping processes • Cs 2 AgBiBr 6 shows high photoconductance up to 30 μs after the first excitation • A high yield of free charges results from populated trap states by the first pulse • Emission of holes residing in shallow states extends over many microseconds Trap-assisted electron recombination in Cs 2 AgBiBr 6 thin films, combined with low hole mobility and shallow trapping, can be a major drawback of Cs 2 AgBiBr 6 in photovoltaic devices. Caselli et al. reveal that the long-lived charge carriers in Cs 2 AgBiBr 6 can offer a suitable, lead-free alternative for other applications, such as X-ray detectors or photocatalysis.

Origin of poor photovoltaic performance of bis(tetracyanoantrathiophene) non-fullerene acceptor

Abstract Light-induced EPR and out-of-phase electron spin echo techniques were applied to identify the origin of low photovoltaic performance of novel non-fullerene antrathiophene-based acceptor 2,2′-[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitrile (AT1), compared to conventional fullerene acceptors. These techniques revealed that average trapping energy of electrons in PCDTBT/AT1 blend is higher than that in the blends of PCDTBT with fullerene acceptors, while the distance of the initial photo-induced electron transfer is decreased. These two effects are suggested to enhance the efficiency of geminate recombination in PCDTBT/AT1 blend and reduce the yield of free charges, which is supposed to be the main reason for poor OPV performance of AT1-containing blends. Low electron mobility μe = 3.5 *10-6 cm2V-1s−1 is estimated for AT1 from photo-CELIV experiment. Strong localization of the electrons on terminal acceptor units of AT1 consistently explains both efficient geminate recombination in blends of antrathiophene-based acceptor with polymer donors and low electron mobility in AT1.

BODIPY based A-D-A molecules: Effect of CF3 group substitution at meso phenyl group

Two pairs of A-D-A molecules have been synthesized with fluorene and benzodithiophene as the central donor subunits and terminal BODIPY units, functionalized with either a 4-methylphenyl or 4-trifluoromethylphenyl group at the meso position. The effect of the para substituent of the meso phenyl group on the photophysical properties of these molecules is studied through steady state absorption and fluorescence spectroscopy as well as femtosecond transient absorption and time resolved fluorescence spectroscopy techniques. Applicability of these molecules as donors in solution processed solar cell active layers was investigated through time resolved microwave conductivity measurements on blends with PC60BM acceptor, which shows a varying yield of charge transfer with choice of substituent. Transient absorption spectroscopy is then employed to investigate the role of the 4-trifluoromethylphenyl group in altering the efficiency of charge transfer from these A-D-A molecules to PC60BM. The results show a consistent picture of picosecond charge transfer and a component of a few hundred ps geminate recombination that results in a small yield of long-lived free charges optimized for the methylphenyl derivatives.

Excited-State Dynamics in Fully Conjugated 2D Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are a highly versatile group of porous materials constructed from molecular building blocks, enabling deliberate tuning of their final bulk properties for a broad range of applications. Understanding their excited-state dynamics is essential for identifying suitable COF materials for applications in electronic devices such as transistors, photovoltaic cells, and water-splitting electrodes. Here, we report on the ultrafast excited-state dynamics of a series of fully conjugated two-dimensional (2D) COFs in which different molecular subunits are connected through imine bonds, using transient absorption spectroscopy. Although these COFs feature different topologies and chromophores, we find that excited states behave similarly across the series. We therefore present a unified model in which charges are generated through rapid singlet-singlet annihilation and show lifetimes of several tens of microseconds. These long-lived charges are of particular interest for optoelectronic devices, and our results point toward the importance of controlling the singlet-singlet annihilation step in order to increase the yield of separated charges.

Exciton Emission and Light induced Charge Separation in colloidal ZnO Nanocrystals.

Adsorption of organic molecules at ZnO nanoparticle surfaces enables the transfer of energy or charge across resulting organic-inorganic interfaces and, consequently, determines the optoelectronic performance of ZnO based hybrids. We investigated on aqueous colloidal ZnO dispersions adsorption-induced changes with photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. Citrate and acetate ion adsorption increases or decreases radiative exciton annihilation at hν = 3.3 eV and at room temperature, respectively. Searching for a correspondence between PL emission and the yield of trapped charge carriers originating from exciton separation - using photon energies of hν = 4.6 eV and fluxes of = 1014 cm-2 s-1 for excitation - we found that there is a negligible fraction of paramagnetic products that originate from exciton separation. Upon polychromatic excitation with significantly higher photon fluxes (Ṅ ph = 1016 cm-2·s-1), ZnO specific shallow defects trap unpaired electrons in citrate and acetate functionalized samples. The adsorption dependent PL intensity changes and the excitation parameter dependent yield of separated charges (EPR) in colloidal ZnO nanoparticles underline that the distribution over the different exciton annihilation channels sensitively depends on interface composition and the intensity of the photoexcitation light.

Influence of neutron emission on the charge, mass and kinetic energy distribution of final fragments from 235U(nth, f) reaction

Using a Monte Carlo method we simulate an experiment that measures the mass, charge and kinetic energy of final fragments (after neutron emission) from the 235U(nth, f) reaction. As input data for the simulation, for primary fragments (before neutron emission), we assume (i) a distribution of mass and kinetic energy; (ii) an average number of emitted neutron as a decreasing linear function of kinetic energy and (iii) for each mass, a constant yield of charges as a function of kinetic energy, equal to that obtained by W Lang et al for the highest measured kinetic energy window (108.5 MeV) which corresponds to the lowest measured excitation energy, that corresponds to the so called cold fission. The output of the simulation is the distribution of mass, charge and kinetic energy of final fragments. From this output we obtain that, for a given mass, the charge that has the highest yield in cold fission region has a yield obeying an increasing function of kinetic energy in all other region. Conversely, the yield of the less probable charge in cold fission is a decreasing function of kinetic energy in all other region. Our results of simulation suggest that, for two primary isobaric fragmentations with similar Q-value, the preference for more asymmetric charge splits (called Coulomb effect), observed in cold fission, is valid in all region, but neutron emission shadows this property in final fragments distribution.

What Controls the Rate of Ultrafast Charge Transfer and Charge Separation Efficiency in Organic Photovoltaic Blends.

In solar energy harvesting devices based on molecular semiconductors, such as organic photovoltaics (OPVs) and artificial photosynthetic systems, Frenkel excitons must be dissociated via charge transfer at heterojunctions to yield free charges. What controls the rate and efficiency of charge transfer and charge separation is an important question, as it determines the overall power conversion efficiency (PCE) of these systems. In bulk heterojunctions between polymer donor and fullerene acceptors, which provide a model system to understand the fundamental dynamics of electron transfer in molecular systems, it has been established that the first step of photoinduced electron transfer can be fast, of order 100 fs. But here we report the first study which correlates differences in the electron transfer rate with electronic structure and morphology, achieved with sub-20 fs time resolution pump-probe spectroscopy. We vary both the fullerene substitution and donor/fullerene ratio which allow us to control both aggregate size and the energetic driving force for charge transfer. We observe a range of electron transfer times from polymer to fullerene, from 240 fs to as short as 37 fs. Using ultrafast electro-optical pump-push-photocurrent spectroscopy, we find the yield of free versus bound charges to be weakly dependent on the energetic driving force, but to be very strongly dependent on fullerene aggregate size and packing. Our results point toward the importance of state accessibility and charge delocalization and suggest that energetic offsets between donor and acceptor levels are not an important criterion for efficient charge generation. This provides design rules for next-generation materials to minimize losses related to driving energy and boost PCE.

Slower carriers limit charge generation in organic semiconductor light-harvesting systems.

Blends of electron-donating and -accepting organic semiconductors are widely used as photoactive materials in next-generation solar cells and photodetectors. The yield of free charges in these systems is often determined by the separation of interfacial electron–hole pairs, which is expected to depend on the ability of the faster carrier to escape the Coulomb potential. Here we show, by measuring geminate and non-geminate losses and key transport parameters in a series of bulk-heterojunction solar cells, that the charge-generation yield increases with increasing slower carrier mobility. This is in direct contrast with the well-established Braun model where the dissociation rate is proportional to the mobility sum, and recent models that underscore the importance of fullerene aggregation for coherent electron propagation. The behaviour is attributed to the restriction of opposite charges to different phases, and to an entropic contribution that favours the joint separation of both charge carriers.

(Invited) Photoinduced Electron Transfer Processes of Trifluorinated Molecules Dispersed in Conjugated Polymer Films and at Interfaces with Swcnts

The use of a Marcus formulation to describe photo-induced electron transfer processes in the complex blends of conjugated polymers and fullerenes associated with organic photovoltaics is rare. The conventional approach considers the polymer and fullerene to form a type II band offset similar to a picture for two inorganic semiconductors. A motivation for this presentation is to ask whether this simple model is correct and only the energetics need be considered when examining the driving force for the photo-induced electron transfer process. Or whether a re-organization energy is also required to describe the effect and therefore an optimum driving force is required to realize high charge separation yields. This presentation will describe two studies using a series of trifluoromethylfullerenes (TMFs) dispersed at low concentration in a conjugated polymer film or in a bilayer forming an interface with a layer of chiral-pure single-walled carbon nanotubes (SWCNTs). The fullerene series provides exquisite control over a wide range of reduction potentials of the fullerene, and can thus influence the driving force for photo-induced electron transfer studies. In both experiments, the use of the electrodeless technique of flash photolysis, time-resolved microwave conductivity (fp-TRMC) which provides a sensitive probe of the yield and kinetics of charges generated in both systems.

Distance distributions of photogenerated charge pairs in organic photovoltaic cells.

Strong Coulomb interactions in organic photovoltaic cells dictate that charges must separate over relatively long distances in order to circumvent geminate recombination and produce photocurrent. In this article, we measure the distance distributions of thermalized charge pairs by accessing a regime at low temperature where charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The exponential attenuation of tunneling rate with distance provides a sensitive probe of the distance distribution of primary charge pairs, reminiscent of electron transfer studies in proteins. By fitting recombination dynamics to distributions of recombination rates, we identified populations of charge-transfer states and well-separated charge pairs. For the wide range of materials we studied, the yield of separated charges in the tunneling regime is strongly correlated with the yield of free charges measured via their intensity-dependent bimolecular recombination dynamics at room temperature. We therefore conclude that populations of free charges are established via long-range charge separation within the thermalization time scale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we estimate critical charge separation distances of ∼3-4 nm in all materials. In some blends, large fullerene crystals can enhance charge separation yields; however, the important role of the polymers is also highlighted in blends that achieved significant charge separation with minimal fullerene concentration. We expect that our approach of isolating the intrinsic properties of primary charge pairs will be of considerable value in guiding new material development and testing the validity of proposed mechanisms for long-range charge separation.

Photovoltaic performance and charge recombination dynamics of P3HT/PCBM blend heterojunction

We prepared the polymer solar cell based on poly(3-hexylthiophene)(P3HT)/fullerene derivative PCBM(PCBM=[6,6]-phenyl-C61-butyric acid methyl ester) heterojunction and investigated the irradiation intensity-dependent charge recombination dynamics of heterojunction employing nanosecond transient absorption spectroscopy with bias light so as to simulate the photophysical process in heterojunction when the photovoltaic device is on operation. The experimental data exhibit that the yield of free charges gradually decreases and the loss of mobile carriers originated from bimolecular recombination simultaneously increases as the irradiation intensity gradually enhances. This indicates that the polymer solar cell is much suitably used at a low irradiation intensity.

Charge Photogeneration in Neat Conjugated Polymers

The origin and yield of charges in neat conjugated polymers has long been controversial. In this paper, we review the body of literature that has been created over the past three decades of research in this field and provide insight from our own recent work highlighting the importance of polymer microstructure in understanding the photophysics of these materials. We focus primarily on polythiophene, poly(p-phenylene vinylene), and ladder-type poly(p-phenylene) derivatives, as these three prototypical polymer backbone structures have undergone the most complete study. We find compelling evidence that the primary photoexcitations in conjugated polymers include both intrachain excitons and excimers, that charges are produced in a secondary process, primarily from breakup of intrachain excitons, and that the locus of long-lived charge generation is at the interface between amorphous and crystalline domains of the polymer. Interestingly, the existence of interchromophore complexes that we refer to as excimers ...

Phonons Do Not Assist Carrier Multiplication in PbSe Quantum Dot Solids

Carrier multiplication (CM)—the Coulomb scattering whereby a sufficiently energetic charge excites a valence electron—is of interest for highly efficient quantum dot (QD) photovoltaics. Using time-resolved microwave conductivity experiments on 1,2-ethanedithiol-linked PbSe QD solids infilled with Al2O3 or Al2O3/ZnO by atomic layer deposition, we find that CM and hot-carrier cooling are temperature independent from 90–295 K and that spontaneous phonon emission limits the yield of charges resulting from the CM–cooling competition.

The influence of solid‐state microstructure on the origin and yield of long‐lived photogenerated charge in neat semiconducting polymers

AbstractThe influence of solid‐state microstructure on the optoelectronic properties of conjugated polymers is widely recognized, but still poorly understood. Here, we show how the microstructure of conjugated polymers controls the yield and decay dynamics of long‐lived photogenerated charge in neat films. Poly(3‐hexylthiophene) was used as a model system. By varying the molecular weight, we drive a transition in the polymer microstructure from nonentangled, chain‐extended, paraffinic‐like to entangled, semicrystalline (MW = 5.5–347 kg/mol). The molecular weight range at which this transition occurs (MW = 40–50 kg/mol) can be deduced from the drastic change in elongation at break found in tensile tests. Linear absorption measurements of free‐exciton bandwidth and time‐resolved microwave conductivity (TRMC) measurements of transient photoconductance track the concomitant evolution in optoelectronic properties of the polymer as a function of MW. TRMC measurements show that the yield of free photogenerated charge increases with increasing molecular weight in the paraffinic regime and saturates at the transition into the entangled, semicrystalline regime. This transition in carrier yield correlates with a sharp transition in free‐exciton bandwidth and decay dynamics at a similar molecular weight. We propose that the transition in microstructure controls the yield and decay dynamics of long‐lived photogenerated charge. The evolution of a semicrystalline structure with well‐defined interfaces between amorphous and crystalline domains of the polymer is required for spatial separation of the electron and hole. This structural characteristic not only largely controls the yield of free charges, but also serves as a recombination center, where mobile holes encounter a bath of dark electrons resident in the amorphous phase and recombine with quasi first‐order kinetics. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT:PCBM Photovoltaic Blends

The precise mechanism and dynamics of charge generation and recombination in bulk heterojunction polymer:fullerene blend films typically used in organic photovoltaic devices have been intensively studied by many research groups, but nonetheless remain debated. In particular the role of interfacial charge-transfer (CT) states in the generation of free charge carriers, an important step for the understanding of device function, is still under active discussion. In this article we present direct optical probes of the exciton dynamics in pristine films of a prototypic polycarbazole-based photovoltaic donor polymer, namely poly[N-11''-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), as well as the charge generation and recombination dynamics in as-cast and annealed photovoltaic blend films using methanofullerene (PC(61)BM) as electron acceptor. In contrast to earlier studies we use broadband (500-1100 nm) transient absorption spectroscopy including the previously unobserved but very important time range between 2 ns and 1 ms, which allows us not only to observe the entire charge carrier recombination dynamics but also to quantify the existing decay channels. We determine that ultrafast exciton dissociation occurs in blends and leads to two separate pools of products, namely Coulombically bound charge-transfer (CT) states and unbound (free) charge carriers. The recombination dynamics are analyzed within the framework of a previously reported model for poly(3-hexylthiophene):PCBM (Howard, I. A. J. Am. Chem. Soc. 2010, 132, 14866) based on concomitant geminate recombination of CT states and nongeminate recombination of free charge carriers. The results reveal that only ~11% of the initial photoexcitations generate interfacial CT states that recombine exclusively by fast nanosecond geminate recombination and thus do not contribute to the photocurrent, whereas ~89% of excitons create free charge carriers on an ultrafast time scale that then contribute to the extracted photocurrent. Despite the high yield of free charges the power conversion efficiency of devices remains moderate at about 3.0%. This is largely a consequence of the low fill factor of devices. We relate the low fill factor to significant energetic disorder present in the pristine polymer and in the polymer:fullerene blends. In the former we observed a significant spectral relaxation of exciton emission (fluorescence) and in the latter of the polaron-induced ground-state bleaching, implying that the density of states (DOS) for both excitons and charge carriers is significantly broadened by energetic disorder in pristine PCDTBT and in its blend with PCBM. This disorder leads to charge trapping in solar cells, which in turn causes higher carrier concentrations and more significant nongeminate recombination. The nongeminate recombination has a significant impact on the IV curves of devices, namely its competition with charge carrier extraction causes a stronger bias dependence of the photocurrent of devices, in turn leading to the poor device fill factor. In addition our results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer:fullerene organic photovoltaic devices.

Photoinduced Charge Carrier Generation in Blends of Poly(Thienothiophene) Derivatives and [6,6]-Phenyl-C61-butyric Acid Methyl Ester: Phase Segregation versus Intercalation

The morphology, optical properties, and photoconductance of blends of the poly(thienothiophene) derivatives poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT), poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT), and poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[2,3-b]thiophene) (pBTCT) with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied. After thermal annealing, the pATBT:PCBM blend exhibits formation of phase-segregated polymer and PCBM domains. Annealing of pBTTT:PCBM and pBTCT:PCBM yields a layered structure with PCBM molecules intercalated between layers of π-stacked polymers. In the intercalated systems the photoluminescence is almost completely quenched, in contrast to the phase-segregated pATBT:PCBM blend. The higher degree of exciton quenching in the intercalated systems likely results in a higher initial yield of charges. However, on longer time scales (>10 ns), the microwave photoconductance for the layered systems is lower than for pATBT:PCBM blend system...

Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends

The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were studied with ultrafast optical pump−probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about ha...

Generation and transport of charge carriers in polyvinylcarbazole

The processes of generation and transport of excess charge carriers in polyvinylcarbazole at room temperature and at 353 K were experimentally studied. The polymer was charged with pulses of electrons at energies of 7 and 50 keV, which differed in both linear energy transfer and track structure. A universal method of investigation based on the combination of the time-of-flight technique in the both (surface and bulk) modes with the measurement of radiation-induced conductivity was used. The radiation-chemical yield of free charges was measured using two independent procedures. It was shown that the radiation chemical yield of free charges at 293 K was somewhat smaller for 7-keV than for 50-keV electrons; in the latter case, G = 1.1 in an electric field of 2 × 107 V/m. The parameters of the generalized physical model were determined for polyvinylcarbazole.

Mechanism of Radiation Damage in SiO2/Si Induced by vuv Photons

The generation yield (Rf) of effective positive charges at an SiO2/Si interface, induced by a vacuum ultraviolet (vuv) photon, is measured as a function of the substrate temperature by a low-temperature microwave plasma etching apparatus. The Rf decreases monotonously with decreasing temperatures from 300 K down to 120 K. The model is verified by analyzing the experimental results according to the effective positive charge generation model previously proposed. From this analysis, it is proposed that an ultra low-damage plasma process becomes possible by eliminating the holes generated by the vuv photons from the SiO2 film keeping the specimens at low temperatures.