Photoinduced Absorption Signal Research Articles

Photoexcited Polaron Relaxation as a Structurally Sensitive Reporter of Charge Trapping in a Conducting Polymer

AbstractConjugated polymers (CPs) play a central role in electronic applications due to their easily tuned electronic and ionic conductivities via chemical or electrochemical doping. Although doping improves charge conduction by introducing high densities of carriers into the CP, the accompanying structural changes and their impact on carrier mobility remain elusive. Methods capable of probing carrier distributions and their dependence on polymer morphology are needed to better understand how to improve conductivity. Here, a transient absorption (TA) spectroscopy approach is demonstrated, capable of directly probing mobile and trapped carriers in doped CPs and that is also sensitive to polymer nanostructure by using a model polythiophene system with tuned crystallinity. Exciting polarons in the polymer films produces distinct photoinduced absorption signals in the near‐infrared spectrum that decay during the picosecond timescale in the form of biphasic, stretched exponential kinetics, which reflect a distribution of mobile (free) and trapped polarons. The kinetic analysis provides evidence for mobile polarons irrespective of polymer film crystallinity, whereas polarons located in impure amorphous phases with reduced chain ordering exist within a deeper distribution of trap states. Altogether, these observations suggest a stronger correlation of carrier trapping with local chain ordering (planarity or aggregation) rather than polymer crystallinity.

Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Thickness-dependent electronic relaxation dynamics in solution-phase redox-exfoliated MoS2 heterostructures.

Electronic relaxation dynamics of solution-phase redox-exfoliated molybdenum disulfide (MoS2) monolayer and multilayer ensembles are described. MoS2 was exfoliated using polyoxometalate (POM) reductants. This process yields a colloidal heterostructure consisting of MoS2 2D sheet multilayers with surface-bound POM complexes. Using two-dimensional electronic spectroscopy, transient bleaching and photoinduced absorption signals were detected at excitation/detection energies of 1.82/1.87 and 1.82/1.80eV, respectively. Approximate 100-fs bandgap renormalization (BGR) and subsequent defect- and phonon-mediated relaxation on the picosecond timescale were resolved for several MoS2 thicknesses spanning from 1 to 2 L to ∼20 L. BGR rates were independent of sample thickness and slightly slower than observations for chemical vapor deposition-grown MoS2 monolayers. However, defect-mediated relaxation accelerated ∼10-fold with increased sample thicknesses. The relaxation rates increased from 0.33 ± 0.05 to 1.2 ± 0.1 and 3.1 ± 0.4ps-1 for 1-2 L, 3-4 L, and 20 L fractions. The thicknesses-dependent relaxation rates for POM-MoS2 heterostructures were modeled using a saturating exponential function that showed saturation at thirteen MoS2 layers. The results suggest that the increased POM surface coverage leads to larger defect density in the POM-MoS2 heterostructure. These are the first descriptions of the influence of sample thickness on electronic relaxation rates in solution-phase redox-exfoliated POM-MoS2 heterostructures. Outcomes of this work are expected to impact the development of solution-phase exfoliation of 2D metal-chalcogenide heterostructures.

Exploring the Early Time Behavior of the Excited States of an Archetype Thermally Activated Delayed Fluorescence Molecule.

Optical pump-probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter DMAC-TRZ. The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump-probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (3LE → 3LEn) is observed, which is quenched in higher polarity solvents as 3CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of DMAC-TRZ: (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation.

Enhanced photoluminescence of double perovskite Cs2SnI6 nanocrystals via Na+ doping.

Double perovskites without lead element have attracted great attention in recent years. Further increasing the photoluminescence quantum yield of lead-free double perovskites is necessary for their potential applications. In this work, Na+ doped Cs2SnI6 nanocrystals were synthesized by hot injection method. It was displayed that all the NCs have uniform hexagonal shape with good crystallization. Energy dispersing spectroscopy and X-ray photoelectron spectroscopy proves the Na+ ions were doped in the lattice of perovskite structure. The photoluminescence intensity of doped NCs is increased by 2.7-fold than that of pure NCs. A maximum photoluminescence quantum yield of 72% is obtained. The luminous mechanism was investigated by femtosecond transient absorption spectrum and a self-trap emission was proved by the observation of ground state bleaching and photo-induced absorption signals.

Thickness-Dependent Ultrafast Photocarrier Dynamics in Selenizing Platinum Thin Films

The atomically thin layered transition metal dichalcogenide (TMDCs) PtSe2 is a new emerging two-dimension (2D) material, which has a transition from indirect-gap semiconductor to semimetal with increasing thickness. Defects in 2D TMDCs are very ubiquitous and play a crucial role in understanding many electronics and optoelectronics in TMDCs. In this article, PtSe2 films with different thickness are obtained by selenizing the variously thick Pt films. Extrapolation of the onset absorption from the visible-infrared spectrum demonstrates that the selenization of 1 and 3 nm Pt films shows semiconductor-like character, while those of thick Pt films (with thickness of 10 and 15 nm) show metallic behavior. The transient absorption (TA) spectroscopy reveals that all films show immediately photobleaching signals after photoexcitation at 800 nm, and the subsequent relaxation process shows strongly thickness dependence. The thinnest film shows biexponential relaxation with typical time constants of 1.28 and 101.2 ps. In contrast, the photobleaching signals are developed into photoinduced absorption signals in thicker films, and the absorption magnitude increases with the thickness of the films. The optical pump and terahertz (THz) probe spectroscopy reveals that all samples show positive photoconductivity after photoexcitation of 800 nm, the subsequent recovery is completed within 2.0 ps, and the recovery time constant decreases with the increase of the films' thickness. Our TA and THz spectroscopy results reveal that the defect states of the films play dominated role in the relaxation of photocarrier after photoexcitation, and edgesite states are inferred to make dominated contributions to the defect states in the selenization of platinum films.

Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Simultaneous Monitoring of Photoinduced Absorption Signals and Short-Circuit Photocurrent during Photoexcitation in Dye-Sensitized Solar Cells

Continuous-wave photoinduced absorption (PIA) measurements were performed simultaneously with photovoltage or photocurrent (PC) measurements to monitor slow transfer processes of electron carriers in operating dye-sensitized solar cells (DSCs). Time-resolved simultaneous experiments demonstrate that the rise curve of PC during photoexcitation is delayed against that of PIA signals. The delay is analyzed by a model that considers effects of electron transfer on PIA signals and shown to be determined by transfer time of electrons over the TiO2 film. On the contrary, no delay is found between the decay curves at excitation–off of PIA signals and PC, suggesting that the transfer kinetics of TiO2 electron varies after the electrons reach the transparent electrode. These spectral features indicate that the time-resolved PIA signals of TiO2 electron under short-circuit conditions are primarily determined by its slow transfer process and that the simultaneous PIA experiments are effective to study the slow electr...

Intraexciton Transitions Observed in High Stability Doped Single-Wall Carbon Nanotube Films and Solutions

Normally hidden excited state transitions were observed in large diameter single-walled carbon nanotubes for the first time by controlling the degree of doping. With increased doping, the normally observed photoinduced bleach (PB) signal associated with the creation of excitons switched to a photoinduced absorption (PA). A delay in the onset of the PA signal made it clear that the absorption was due to an intraexcitonic transition. The study was afforded by the development of novel methods that allowed samples to be reproducibly and stably prepared in specific states. Our results further the understanding of the photophysics of large diameter single-walled carbon nanotube materials, which are promising for a variety of optoelectronic applications.

The influence of molecular interface modification on the charge dynamics of polymeric semiconductor:ZnO heterostructure

We demonstrate an enhancement of photocurrent in hybrid photovoltaic cells based on nanoparticles of zinc oxide (ZnO) and poly(3-hexylthiophene) (P3HT), through molecular interface modification with and without cis-bis(4,4′-dicarboxy-2,2′bipyridine) ruthenium (II) (N3-dye) and α-Sexithiophen-2-yl-phosphonic Acid (6TP) as interface modifiers. We identify the formation of long-lived polarons at P3HT:ZnO interface by means of quasi-steady-state photoinduced absorption (PIA) spectroscopy. Furthermore, by probing the pump-modulation-frequency-dependent PIA signal, we find that P3HT:ZnO-N3 and P3HT:ZnO-6TP exhibit more sharply decaying density with increasing modulation frequency, which is indicative of a longer average lifetime, approximating 1 ms as opposed to ∼0.2 ms without ZnO surface modification. This highlights the importance of the molecular interface modification in the steady-state polaron dynamics in hybrid solar cells.

Size-Dependent Charge Transfer Yields in Conjugated Polymer/Quantum Dot Blends

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b′)dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude that the long-lived charge transfer yield is diminished at larger dot sizes as the energy level offset at the polymer/quantum dot interface is changed through decreasing quantum confinement.

Photo-induced absorption in the pump probe spectroscopy of single-walled carbon nanotubes

Femtosecond pump probe spectroscopy is employed to study the photo-induced absorption feature in the single-walled carbon nanotube transient spectrum. The two advantages of the experiment, a chirality enriched sample and tuning the pump wavelength to the resonance of a specific nanotube species, greatly facilitate the identification of the photo-induced absorption signal of one tube species. It is found that a photo-induced absorption feature is located at one radial breathing mode to the blue side of the E11 state. This finding prompts a new explanation for the origin of the photo-induced absorption: the transition from the ground state to a phonon coupled state near the Eii state. The explanation suggests a superposition mechanism of the photo-bleach and photo-induced absorption signals, which may serve as a key to the interpretation of the complex pump probe transient spectrum of carbon nanotubes. The finding sheds some light on the understanding of the complex non-radiative relaxation process and the electronic structure of single-walled carbon nanotubes.

Broadband and Picosecond Intraband Absorption in Lead-Based Colloidal Quantum Dots

Using femtosecond transient absorption spectroscopy, we demonstrate that lead chalcogenide nanocrystals show considerable photoinduced absorption (PA) in a broad wavelength range just below the band gap. The time-dependent decay of the PA signal correlates with the recovery of the band gap absorption, indicating that the same carriers are involved. On this basis, we assign this PA signal to intraband absorption, that is, the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. We confirm our experiments with tight-binding calculations. This broadband response in the commercially interesting near- to mid-infrared range is very relevant for ultra-high-speed all-optical signal processing. We benchmark the performance with bulk Si and Si nanocrystals.

Spectroscopic investigation of excitons, photocarriers, and bias-induced carriers in regioregular poly(3-alkylthiophene)

Excitons, photocarriers, and bias-induced carriers in regioregular poly(3-alkylthiophene) (P3AT) films were investigated using a variety of spectroscopic techniques, such as continuous-wave- (cw-) and femtosecond (fs-) transient photoinduced absorption (PIA) and device modulation (DM) spectroscopies. Comparison between the cw PIA and DM spectra of the poly(3-hexylthiophene) (P3HT) film reveals that photocarriers generated by cw photoexcitation are dominated by localized polarons, whereas charge injection primarily creates delocalized polarons. The photoexcitation intensity dependence of the cw PIA signals demonstrates that both localized and delocalized photogenerated polarons obey a bimolecular recombination process under cw photoexcitation. The proportion of polarons generated by cw photoexcitation in the P3HT film is estimated to be 1/(1$+$0.137 ${I}^{1/2}$) for $I$ (mW/cm${}^{2}$) by comparison of the spectral intensities between the DM and cw PIA spectra. DM spectroscopy reveals that both localized and delocalized polarons rapidly increase at a rise point of the bias current. The cw PIA bleaching spectra of P3AT films are shown to vary with the length of the alkyl sidechains of P3AT and be almost reproduced using a single Huang-Rhys factor. The Huang-Rhys factors obtained reveal a trend that the electron-phonon couplings of final exciton migration sites become strong with the increase in the interlamella distance in the P3AT film, which indicates that interlamella couplings affect the electron-phonon couplings of the migration sites located at two-dimensional lamellas.

Effects of air and light exposure on the opto-electronic properties of polymer:fullerene solar cells

ABSTRACTIn this study we investigate how exposure to ambient air and light during device processing affects the opto-electronic properties of poly-3-hexylthiophene (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells (BHJ). The properties of pure P3HT layers prepared in an inert atmosphere, under ambient conditions, and additionally degraded under light in ambient conditions were investigated using photoluminescence (PL) and photoinduced absorption (PIA). It was observed that exposure to air during processing leads to oxygen doping of the polymer. Exposure to air combined with light was found to significantly decrease the PL and PIA signals. The current-voltage (I-V) and external quantum efficiency (EQE) characteristics of solar cells fabricated in an inert atmosphere were compared to solar cells processed under ambient conditions. It was observed that processing in air leads to a reduction in the photocurrent in the devices which is attributed to electron trapping by oxygen in the active layer.

Characterization of the Pore Filling of Solid State Dye Sensitized Solar Cells with Photoinduced Absorption Spectroscopy

Near steady-state photoinduced absorption (PIA) and UV-Vis absorption spectroscopy are used to characterize the pore filling of spiro-MeOTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) into the nanoparticulate TiO2electrode of a solid-state dye-sensitized solar cell (ssDSC). The volumetric ratio of filled to unfilled pore volumes, as well as the optical signature of interacting chemical species, that is, the hole-transfer yield (HTY), are investigated. PIA spectroscopy is used to measure the HTY, relative to the amount of spiro-MeOTAD present, without needing to determine the extinction coefficients of the dye and spiro-MeOTAD cation species. The Beer-Lambert law is used to relate the relative PIA signal to the penetration length of the hole-conductor in the TiO2film. For the sample thickness range of1.4–5 μm investigated here, the optimum characteristic penetration length is determined to be3.1+0.46 μm, which is compared to 1.4 μm for the 200 mgmL−1concentration of spiro-MeOTAD conventionally used. Therefore, doubling the effective penetration of spiro-MeOTAD is necessary to functionalize all the dye molecules in a ssDSC.

Photoinduced absorption spectroscopy on MDMO-PPV:PCBM solar cells under operation

Abstract Near-steady-state photoinduced absorption (PIA) spectroscopy is employed to study the charge carrier dynamics in polymer:fullerene (poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene]: 1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C 61 ; MDMO-PPV:PCBM) solar cells under operating conditions. Artifacts in the PIA spectra originating from thermal heating of the aluminum electrode and from interference of incoming and reflected light waves are analyzed. PIA signals from functioning devices are studied as a function of applied bias and temperature and complemented with photocurrent measurements under identical conditions. Comparison between photocurrent and PIA measurements shows that PIA mainly probes trapped cationic charge carriers while the photocurrent samples mobile carriers. The number density of trap sites is estimated at 10 17 cm −3 and trapped carriers can be stabilized by applying a reverse bias potential over the device.

Long-Lived Photoinduced Charges in Donor−Acceptor Anthraquinone-Substituted Thiophene Copolymers

The photoinduced charge-transfer properties of a series of polyalkylthiophene copolymers, carrying anthraquinone substituents covalently linked to the conjugated backbone, have been studied in the solid state by photoinduced absorption (PA) and light-induced electron spin resonance (LESR) spectroscopy. The measurements indicate the formation of metastable charges arising from the photoinduced electron transfer from the polythiophene backbone to the anthraquinone moieties. At low temperatures (below 200 K), long-lived persistent charges are formed, exhibiting lifetimes that extend for several minutes; their recombination kinetics has been studied by following the formation and decay of the PA and LESR signals. The results are rationalized using a model originally proposed to describe the low-temperature recombination kinetics of long-lived photoexcited carriers in amorphous inorganic semiconductors. It is clearly evidenced that, in these polymers, the number of acceptor substituents in the chain, easily tuned by chemical tailoring, plays a key role in the photoexcitation scenario.

Recombination studies in a polyfluorene copolymer for photovoltaic applications

We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in weight ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temperature dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from ∼1 eV to 2 eV as well as a low energy band peaking at ∼0.35 eV. We attribute the low energy band to the P 1 transition of polarons and part of the high energy band to the correlated P 2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive.

Photophysics of charge transfer in a polyfluorene/violanthrone blend

We present a study of the photophysical and photovoltaic properties of blends of violanthrone in poly[9,$9\text{\ensuremath{-}}\mathrm{bis}$(2-ethylhexyl)-fluorene-2,$7\text{\ensuremath{-}}\mathrm{diyl}$] $({\mathrm{PF}}_{2∕6})$. Photoluminescence quenching and photocurrent measurements show moderate efficiencies for charge generation, characteristic of such polymer/dye blends. Pump-probe measurements on blend films suggest that while $\ensuremath{\sim}47%$ of the total exciton population dissociates within $4\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ of photoexcitation, only $\ensuremath{\sim}32%$ subsequently results in the formation of dye anions. We attribute the discrepancy to the likely formation of complex species with long lifetimes, such as stabilized interface charge pairs or exciplexes. This conclusion is supported by the appearance of a long lifetime component of $2.4\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ in the dynamics of the photoinduced absorption signal associated to polarons in photoinduced absorption bands centered at $560\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.