Nonradiative Charge Recombination Research Articles

Dark-blue titanium dioxide: Effect of phenothiazine on structural and optical properties of nanocrystalline anatase TiO2

We studied the effect of the phenothiazine (PTZ) surface doping on the structural and optical properties of nanocrystalline titanium dioxide powder with a single anatase phase (A-TiO2). We found that the mixing and interaction of PTZ and A-TiO2 leads to the intense dark-blue color of PTZ-doped A-TiO2 (A/PTZ). The appearance of new bands in the Fourier transform IR spectrum indicated the formation of PTZ•+ radical cations and reduced Ti3+species as well as extra oxygen vacancies (VO) in the TiO2 matrix. The UV-vis absorption spectrum of A/PTZ exhibited an increase in absorption in the visible region (>400 nm). The doping of A-TiO2 with PTZ causes a noticeable redshift of the absorption edge and a significant narrowing of the band gap by 0.24 and 0.49 eV for direct and indirect electronic transitions, respectively. The photoluminescence spectra show that the photoluminescence originating from Ti3+ and [VO-Ti3+] states for A/PTZ is stronger than for A-TiO2, but the excitonic photoluminescence of A/PTZ is quenched. An increased number of surface defects in A/PTZ can essentially increase the nonradiative charge recombination, and therefore may considerably enhance the photoactivity of the dark-blue TiO2 in the visible range of the spectrum.

In Situ Grain Boundary Modification via Two-Dimensional Nanoplates to Remarkably Improve Stability and Efficiency of Perovskite Solar Cells.

Even though a record efficiency of 23.3% has been achieved in organic-inorganic hybrid perovskite solar cells, their stability remains a critical issue, which greatly depends on the morphology of perovskite absorbers. Herein, we report a practical grain boundary modification to remarkably improve the humidity and thermal stability by gradually growing in situ two-dimensional nanoplates between the grain boundaries of perovskite films using phenylethylammonium iodide (PEAI). The experimental results show that PEAI nanoplates play a critical role in stabilizing perovskite thin films by reducing the moisture sensitivity and suppressing phase transition at the grain boundaries. In addition to the significant improved ambient stability, the grain boundary modification by PEAI can effectively suppress the nonradiative charge recombination at grain boundaries. As a result, the efficiency of perovskite solar cells is up to 20.34% with significant humidity and thermal stability.

Control of Charge Recombination in Perovskites by Oxidation State of Halide Vacancy.

Advances in perovskite solar cells require development of means to control and eliminate the nonradiative charge recombination pathway. Using ab initio nonadiabatic molecular dynamics, we demonstrate that charge recombination in perovskites is extremely sensitive to the charge state of the halogen vacancy. A missing iodine anion in MAPbI3 has almost no effect on charge losses. However, when the vacancy is reduced, the recombination is accelerated by up to 2 orders of magnitude. The acceleration occurs due to formation of a deep hole trap in the singly reduced vacancy, and both deep and shallow hole traps for the doubly reduced vacancy. The shallow hole involves a significant rearrangement of the Pb-I lattice, leading to a new chemical species: a Pb-Pb dimer bound by the vacancy charge, and under-coordinated iodine bonds. Hole trapping by the singly reduced iodide vacancy operates parallel to recombination of free electron and hole, accelerating charge losses by a factor of 5. The doubly reduced vacancy acts by a sequential mechanism-free hole, to shallow trap, to deep trap, to free electron, and accelerates the recombination by a factor of 50. The study demonstrates that iodine anion vacancy can be beneficial to the performance, because it causes minor changes to the charge carrier lifetime, while increasing charge carrier concentration. However, the neutral iodine and iodine cation vacancies should be strongly avoided. The detailed insights into the charge carrier trapping and relaxation mechanisms provided by the simulation are essential for development of efficient photocatalytic, photovoltaic, optoelectronic and related devices.

Increased Lattice Stiffness Suppresses Nonradiative Charge Recombination in MAPbI3 Doped with Larger Cations: Time-Domain Ab Initio Analysis

Hybrid organic–inorganic perovskites have attracted great attention as promising photovoltaic materials due to their excellent electronic and optical properties and facile synthesis. Experiments report that halide perovskites containing several organic cations exhibit much longer charge carrier lifetimes than pristine CH3NH3PbI3 (MAPbI3). Using time-domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we demonstrate that partial replacement of MA with formamidinium (FA) or guanidinium (GA) significantly reduces the NA electron–vibrational coupling. The effect arises due to stiffening of the inorganic Pb–I lattice, which exhibits strongly decreased fluctuations while maintaining its original structure and electronic band gap. In addition to extending charge carrier lifetimes, reduced lattice motions, and particularly those of iodides, are indicative of decreased ion diffusion that is responsible for formation of defects, such as iodine interstitials and vacancies, and curren...

Long Carrier Lifetimes in PbI2-Rich Perovskites Rationalized by Ab Initio Nonadiabatic Molecular Dynamics

Hybrid organic–inorganic perovskites have attracted considerable interest due to their impressive performance in solar energy applications. Many experiments show that a slight excess of PbI2 significantly enhances the properties of the most studied CH3NH3PbI3 compound. We use real-time time-dependent density functional theory and nonadiabatic molecular dynamics to demonstrate that the effect arises due to decreased electron–phonon interactions responsible for nonradiative charge recombination. The fast organic CH3NH3+ (MA) cations, present on surfaces of stochiometric and MAI-rich perovskites, are particularly mobile and introduce high-frequency phonons and strong electric fields that couple to the charge carriers and create large nonadiabatic coupling. Excess PbI2 decreases MA surface coverage, reduces the nonadiabatic coupling by up to an order of magnitude, and extends the charge carrier lifetime. Generally, charges in perovskites are long-lived because the nonadiabatic coupling is very small, less tha...

Boosting Perovskite Light-Emitting Diode Performance via Tailoring Interfacial Contact.

Solution-processed perovskite light-emitting diodes (LEDs) have attracted wide attention in the past several years. However, the overall efficiency and stability of perovskite-based LEDs remain inferior to those of organic or quantum dot LEDs. Nonradiative charge recombination and the unbalanced charge injection are two critical factors that limit the device efficiency and operational stability of perovskite LEDs. Here, we develop a strategy to modify the interface between the hole transport layer and the perovskite emissive layer with an amphiphilic conjugated polymer of poly[(9,9-bis(3'-( N, N-dimethylamino)propyl)-2,7-fluorene)- alt-2,7-(9,9-dioctylfluorene)] (PFN). We show evidences that PFN improves the quality of the perovskite film, which effectively suppresses nonradiative recombination. By further improving the charge injection balance rate, a green perovskite LED with a champion current efficiency of 45.2 cd/A, corresponding to an external quantum efficiency of 14.4%, is achieved. In addition, the device based on the PFN layer exhibits improved operational lifetime. Our work paves a facile way for the development of efficient and stable perovskite LEDs.

Lewis Base Passivation of Hybrid Halide Perovskites Slows Electron-Hole Recombination: Time-Domain Ab Initio Analysis.

Nonradiative electron-hole recombination plays a key role in determining photon conversion efficiencies in solar cells. Experiments demonstrate significant reduction in the recombination rate upon passivation of methylammonium lead iodide perovskite with Lewis base molecules. Using nonadiabatic molecular dynamics combined with time-domain density functional theory, we find that the nonradiative charge recombination is decelerated by an order of magnitude upon adsorption of the molecules. Thiophene acts by the traditional passivation mechanism, forcing electron density away from the surface. In contrast, pyridine localizes the electron at the surface while leaving it energetically near the conduction band edge. This is because pyridine creates a stronger coordinative bond with a lead atom of the perovskite and has a lower energy unoccupied orbital compared with thiophene due to the more electronegative nitrogen atom relative to thiophene's sulfur. Both molecules reduce two-fold the nonadiabatic coupling and electronic coherence time. A broad range of vibrational modes couple to the electronic subsystem, arising from inorganic and organic components. The simulations reveal the atomistic mechanisms underlying the enhancement of the excited-state lifetime achieved by the perovskite passivation, rationalize the experimental results, and advance our understanding of charge-phonon dynamics in perovskite solar cells.

Spatially inhomogeneous photoluminescence-voltage hysteresis in planar heterojunction perovskite-based solar cells

Spatially resolved photoluminescence (PL) of methylammonium lead iodide (MAPbI3) films in planar heterojunction solar cells is probed by time-resolved confocal microscopy to study the interface effect on PL intensity-voltage (PL-V) hysteresis. Negligible PL-V hysteresis is observed at the interfacial area, while significant hysteresis is observed in the bulk film. PL lifetime imaging of the perovskite device reveals inhomogeneous charge extraction due to variation of the interfacial contact quality. Poor interfacial contact leads to more severe PL-V hysteresis in the bulk perovskite film. The PL-V characteristics also suggest that voltage-driven ion migration may lead to redistribution of charge traps, and consequently affect the nonradiative charge recombination and the PL intensity in MAPbI3 films.

Strain compensated superlattices on m-plane gallium nitride by ammonia molecular beam epitaxy

The results of tensile strained AlN/GaN, AlGaN/GaN, and compressive strained InGaN/GaN superlattices (SLs) grown by Ammonia MBE (NH3-MBE) are presented. A combination of atom probe tomography and high-resolution X-ray diffraction confirms that periodic heterostructures of high crystallographic quality are achieved. Strain induced misfit dislocations (MDs), however, are revealed by cathodoluminescence (CL) of the strained AlN/GaN, AlGaN/GaN, and InGaN/GaN structures. MDs in the active region of a device are a severe problem as they act as non-radiative charge recombination centers, affecting the reliability and efficiency of the device. Strain compensated SL structures are subsequently developed, composed of alternating layers of tensile strained AlGaN and compressively strained InGaN. CL reveals the absence of MDs in such structures, demonstrating that strain compensation offers a viable route towards MD free active regions in III-Nitride SL based devices.

Efficient ambient-air-stable solar cells with 2D–3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

Perovskite solar cells are remarkably efficient; however, they are prone to degradation in water, oxygen and ultraviolet light. Cation engineering in 3D perovskite absorbers has led to reduced degradation. Alternatively, 2D Ruddlesden–Popper layered perovskites exhibit improved stability, but have not delivered efficient solar cells so far. Here, we introduce n-butylammonium cations into a mixed-cation lead mixed-halide FA0.83Cs0.17Pb(IyBr1−y)3 3D perovskite. We observe the formation of 2D perovskite platelets, interspersed between highly orientated 3D perovskite grains, which suppress non-radiative charge recombination. We investigate the relationship between thin-film composition, crystal alignment and device performance. Solar cells with an optimal butylammonium content exhibit average stabilized power conversion efficiency of 17.5 ± 1.3% with a 1.61-eV-bandgap perovskite and 15.8 ± 0.8% with a 1.72-eV-bandgap perovskite. The stability under simulated sunlight is also enhanced. Cells sustain 80% of their ‘post burn-in’ efficiency after 1,000 h in air, and close to 4,000 h when encapsulated. Various strategies are developed to combine high efficiency and stability in perovskite solar cells. Here, Wang et al. mix 2D and 3D mixed-cation and mixed-halide perovskite phases in solar cells with stabilized efficiencies up to 19.5% and improved stability under full illumination and ambient air.

Two-Dimensional Materials for Halide Perovskite-Based Optoelectronic Devices.

Halide perovskites have high light absorption coefficients, long charge carrier diffusion lengths, intense photoluminescence, and slow rates of non-radiative charge recombination. Thus, they are attractive photoactive materials for developing high-performance optoelectronic devices. These devices are also cheap and easy to be fabricated. To realize the optimal performances of halide perovskite-based optoelectronic devices (HPODs), perovskite photoactive layers should work effectively with other functional materials such as electrodes, interfacial layers and encapsulating films. Conventional two-dimensional (2D) materials are promising candidates for this purpose because of their unique structures and/or interesting optoelectronic properties. Here, we comprehensively summarize the recent advancements in the applications of conventional 2D materials for halide perovskite-based photodetectors, solar cells and light-emitting diodes. The examples of these 2D materials are graphene and its derivatives, mono- and few-layer transition metal dichalcogenides (TMDs), graphdiyne and metal nanosheets, etc. The research related to 2D nanostructured perovskites and 2D Ruddlesden-Popper perovskites as efficient and stable photoactive layers is also outlined. The syntheses, functions and working mechanisms of relevant 2D materials are introduced, and the challenges to achieving practical applications of HPODs using 2D materials are also discussed.

Ferroelectric Alignment of Organic Cations Inhibits Nonradiative Electron-Hole Recombination in Hybrid Perovskites: Ab Initio Nonadiabatic Molecular Dynamics.

Hybrid organic-inorganic perovskites show impressive potential for photovoltaic applications and currently give rise to one of the most vibrant research areas in the field. Until recently, the electrostatic interactions between their organic and inorganic components were considered mostly for stabilization of the fragile perovskite structure. We study the effect of local interactions of polar C-N bonds in the organic layer on the nonradiative electron-hole recombination in the recently reported room-temperature ferroelectric hybrid perovskite, (benzylammonium)2PbCl4. Using nonadiabatic molecular dynamics and real-time time-dependent density functional theory, we show that ferroelectric alignment of the polar groups weakens the electron-phonon nonadiabatic coupling and inhibits the nonradiative charge recombination. The effect is attributed to suppression of contributions of higher frequency phonons to the electron-phonon coupling. The coupling is dominated in the ferroelectric phase by slower collective motions. We also demonstrate the importance of van der Waals interactions for the charge-phonon relaxation in the hybrid perovskite systems. Combined with the long-range charge separation achievable in the ferroelectric phase, the weakened electron-phonon coupling indicates that ferroelectric order in hybrid perovskites can lead to increased excited-state lifetimes and improved solar energy conversion performance.

Recombination dynamics in aerotaxy-grown Zn-doped GaAs nanowires

In this paper we have investigated the dynamics of photo-generated charge carriers in a series of aerotaxy-grown GaAs nanowires (NWs) with different levels of Zn doping. Time-resolved photo-induced luminescence and transient absorption have been employed to investigate radiative (band edge transition) and non-radiative charge recombination processes, respectively. We find that the photo-luminescence (PL) lifetime of intrinsic GaAs NWs is significantly increased after growing an AlGaAs shell over them, indicating that an AlGaAs shell can effectively passivate the surface of aerotaxy-grown GaAs NWs. We observe that PL decay time as well as PL intensity decrease with increasing Zn doping, which can be attributed to thermally activated electron trapping with the trap density increased due to the Zn doping level.

Energy dissipation pathways in Photosystem 2 of the diatom, Phaeodactylum tricornutum, under high-light conditions.

To prevent photooxidative damage under supraoptimal light, photosynthetic organisms evolved mechanisms to thermally dissipate excess absorbed energy, known as non-photochemical quenching (NPQ). Here we quantify NPQ-induced alterations in light-harvesting processes and photochemical reactions in Photosystem 2 (PS2) in the pennate diatom Phaeodactylum tricornutum. Using a combination of picosecond lifetime analysis and variable fluorescence technique, we examined the dynamics of NPQ activation upon transition from dark to high light. Our analysis revealed that NPQ activation starts with a 2-3-fold increase in the rate constant of non-radiative charge recombination in the reaction center (RC); however, this increase is compensated with a proportional increase in the rate constant of back reactions. The resulting alterations in photochemical processes in PS2 RC do not contribute directly to quenching of antenna excitons by the RC, but favor non-radiative dissipation pathways within the RC, reducing the yields of spin conversion of the RC chlorophyll to the triplet state. The NPQ-induced changes in the RC are followed by a gradual ~ 2.5-fold increase in the yields of thermal dissipation in light-harvesting complexes. Our data suggest that thermal dissipation in light-harvesting complexes is the major sink for NPQ; RCs are not directly involved in the NPQ process, but could contribute to photoprotection via reduction in the probability of (3)Chl formation.

Modeling of the redox state dynamics in photosystem II of Chlorella pyrenoidosa Chick cells and leaves of spinach and Arabidopsis thaliana from single flash-induced fluorescence quantum yield changes on the 100ns-10s time scale.

The time courses of the photosystem II (PSII) redox states were analyzed with a model scheme supposing a fraction of 11-25% semiquinone (with reduced [Formula: see text]) RCs in the dark. Patterns of single flash-induced transient fluorescence yield (SFITFY) measured for leaves (spinach and Arabidopsis (A.) thaliana) and the thermophilic alga Chlorella (C.) pyrenoidosa Chick (Steffen et al. Biochemistry 44:3123-3132, 2005; Belyaeva et al. Photosynth Res 98:105-119, 2008, Plant Physiol Biochem 77:49-59, 2014) were fitted with the PSII model. The simulations show that at high-light conditions the flash generated triplet carotenoid (3)Car(t) population is the main NPQ regulator decaying in the time interval of 6-8μs. So the SFITFY increase up to the maximum level [Formula: see text]/F 0 (at ~50μs) depends mainly on the flash energy. Transient electron redistributions on the RC redox cofactors were displayed to explain the SFITFY measured by weak light pulses during the PSII relaxation by electron transfer (ET) steps and coupled proton transfer on both the donor and the acceptor side of the PSII. The contribution of non-radiative charge recombination was taken into account. Analytical expressions for the laser flash, the (3)Car(t) decay and the work of the water-oxidizing complex (WOC) were used to improve the modeled P680(+) reduction by YZ in the state S 1 of the WOC. All parameter values were compared between spinach, A. thaliana leaves and C. pyrenoidosa alga cells and at different laser flash energies. ET from [Formula: see text] slower in alga as compared to leaf samples was elucidated by the dynamics of [Formula: see text] fractions to fit SFITFY data. Low membrane energization after the 10ns single turnover flash was modeled: the ∆Ψ(t) amplitude (20mV) is found to be about 5-fold smaller than under the continuous light induction; the time-independent lumen pHL, stroma pHS are fitted close to dark estimates. Depending on the flash energy used at 1.4, 4, 100% the pHS in stroma is fitted to 7.3, 7.4, and 7.7, respectively. The biggest ∆pH difference between stroma and lumen was found to be 1.2, thus pH- dependent NPQ was not considered.

Near-unity quantum yields from chloride treated CdTe colloidal quantum dots.

Colloidal quantum dots (CQDs) are promising materials for novel light sources and solar energy conversion. However, trap states associated with the CQD surface can produce non-radiative charge recombination that significantly reduces device performance. Here a facile post-synthetic treatment of CdTe CQDs is demonstrated that uses chloride ions to achieve near-complete suppression of surface trapping, resulting in an increase of photoluminescence (PL) quantum yield (QY) from ca. 5% to up to 97.2 ± 2.5%. The effect of the treatment is characterised by absorption and PL spectroscopy, PL decay, scanning transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. This process also dramatically improves the air-stability of the CQDs: before treatment the PL is largely quenched after 1 hour of air-exposure, whilst the treated samples showed a PL QY of nearly 50% after more than 12 hours.

Supercritical CO2-Assisted Electrochemical Deposition of ZnO Mesocrystals for Practical Photoelectrochemical Applications

We have successfully developed an effective supercritical CO2 (sc-CO2) emulsion-assisted electrochemical approach for cathodic deposition of ZnO mesocrystals. The sc-CO2 was introduced along with a nonionic surfactant in the process to form emulsified electrolyte, which significantly increased the supersaturation degree and promoted the molecular diffusion to affect the crystal growth of ZnO. The deposition involved the generation of primary nanocrystals with substantially high surface energy, followed by the preferred attachment of the primary nanocrystals along an energetically favorable orientation to form ZnO mesocrystals. The as-deposited ZnO mesocrystals exhibited remarkable optical properties at room temperature in terms of prominent near band-edge emission and substantially long exciton lifetime, attributable to the highly oriented crystallinity of mesocrystals that effectively suppressed the nonradiative charge recombination to extend the exciton decay dynamics. As compared to the structures prepared without the addition of surfactant or sc-CO2, ZnO mesocrystals from sc-CO2 emulsion displayed greatly improved photoactivity toward photoelectrochemical water oxidation, revealing their promising potential as photoanodes in relevant photoelectrochemical processes. The current study delivers the first demonstration of directly depositing ZnO mesocrystals on conductive substrates, which paves the way for utilization of mesocrystals as practical electrodes in technologically important energy conversion fields, such as electrochemical cells, photovoltaic devices, as well as photoelectrochemical water splitting, where the advantageous structural characteristics of mesocrystals, i.e., high crystallinity and abundant porosity, can be fully exploited.

Acetate in mixotrophic growth medium affects photosystem II in Chlamydomonas reinhardtii and protects against photoinhibition

Chlamydomonas reinhardtii is a photoautotrophic green alga, which can be grown mixotrophically in acetate-supplemented media (Tris–acetate–phosphate). We show that acetate has a direct effect on photosystem II (PSII). As a consequence, Tris–acetate–phosphate-grown mixotrophic C. reinhardtii cultures are less susceptible to photoinhibition than photoautotrophic cultures when subjected to high light. Spin-trapping electron paramagnetic resonance spectroscopy showed that thylakoids from mixotrophic C. reinhardtii produced less 1O2 than those from photoautotrophic cultures. The same was observed in vivo by measuring DanePy oxalate fluorescence quenching. Photoinhibition can be induced by the production of 1O2 originating from charge recombination events in photosystem II, which are governed by the midpoint potentials (Em) of the quinone electron acceptors. Thermoluminescence indicated that the Em of the primary quinone acceptor (QA/QA−) of mixotrophic cells was stabilised while the Em of the secondary quinone acceptor (QB/QB−) was destabilised, therefore favouring direct non-radiative charge recombination events that do not lead to 1O2 production. Acetate treatment of photosystem II-enriched membrane fragments from spinach led to the same thermoluminescence shifts as observed in C. reinhardtii, showing that acetate exhibits a direct effect on photosystem II independent from the metabolic state of a cell. A change in the environment of the non-heme iron of acetate-treated photosystem II particles was detected by low temperature electron paramagnetic resonance spectroscopy. We hypothesise that acetate replaces the bicarbonate associated to the non-heme iron and changes the environment of QA and QB affecting photosystem II charge recombination events and photoinhibition.

Characterization of singlet oxygen production and its involvement in photodamage of Photosystem II in the cyanobacterium Synechocystis PCC 6803 by histidine-mediated chemical trapping

Singlet oxygen production in intact cells of the cynobacterium Synechocystis 6803 was studied using chemical trapping by histidine, which leads to O2 uptake during illumination. The rate of O2 uptake, measured by a standard Clark-type electrode, is enhanced in the presence of D2O, which increases the lifetime of 1O2, and suppressed by the 1O2 quencher NaN3. Due to the limited mobility of 1O2 these data demonstrate that exogenous histidine reaches close vicinity of 1O2 production sites inside the cells. Flash induced chlorophyll fluorescence measurements showed that histidine does not inhibit Photosystem II activity up to 5mM concentration. By applying the histidine-mediated O2 uptake method we showed that 1O2 production linearly increases with light intensity even above the saturation of photosynthesis. We also studied 1O2 production in site directed mutants in which the Gln residue at the 130th position of the D1 reaction center subunit was changed to either Glu or Leu, which affect the efficiency of nonradiative charge recombination from the primary radical pair (Rappaport et al. 2002, Biochemistry 41: 8518–8527; Cser and Vass 2007, BBA 1767:233–243). We found that the D1-Gln130Glu mutant showed decreased 1O2 production concomitant with decreased rate of photodamage relative to the WT, whereas both 1O2 production and photodamage were enhanced in the D1-Gln130Leu mutant. The data are discussed in the framework of the model of photoinhibition in which 3P680 mediated 1O2 production plays a key role in PSII photodamage, and nonradiative charge recombination of the primary charge separated state provides a photoprotective pathway.

P‐115: Driving Voltage Reduction through Non‐radiative Charge Recombination Interfaces in Organic Light‐Emitting Diode

AbstractWe report a new way for the driving voltage reduction through non‐radiative charge recombination interfaces in organic light‐emitting devices (OLEDs). Non‐radiative charge recombination interfaces made between a hole transporting layer and a deep lowest unoccupied molecular orbital (LUMO) electron transporting layer results in significant increase of hole current conduction, which finally could give us voltage reduction in OLEDs. Such voltage reduction is attributed to strong columbic interaction at charge recombination interfaces. Voltage reduction from 5.2 to 4.3 V at 1000 cd/m2 in fluorescent blue devices is demonstrated with this our concept. Our suggested structure of non‐radiative charge recombination interfaces can be very useful for many practical organic semiconductor device applications.