Photoinduced Absorption Measurements Research Articles

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate.

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

AbstractIonic carbon nitrides based on poly(heptazine imides) (PHI) represent a vigorously studied class of materials with possible applications in photocatalysis and energy storage. Herein, for the first time, the photogenerated charge dynamics in highly stable and binder‐free PHI photoanodes using in operando transient photocurrents and spectroelectrochemical photoinduced absorption measurements is studied. It is discovered that light‐induced accumulation of long‐lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping is previously reported for various semiconductors, it has not been shown before for carbon nitride materials. Furthermore, it is found that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors. These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to operate under zero‐bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes establish a strong rationale for the use of PHI films in various applications, such as bias‐free photoelectrochemistry or photobatteries.

Photo-induced effects on amorphous and liquid selenium by pulsed laser illumination

Abstract Amorphous and liquid selenium exhibit interesting photo-induced changes due to photo-flexibility of lone-pair semiconductors. This study conducts a detailed investigation of the photo-structural changes in amorphous and liquid selenium. We performed photo-induced optical absorption measurement of amorphous and liquid selenium using a nanosecond pulsed laser with a weak intensity of 1.0 mJ/pulse. The measurement indicated photo-induced absorption in the time region from 10−7 to 10−2 s in both amorphous and liquid selenium, which is referred to as fast photodarkening. This suggests a local structural change around a Se atom, such as a formation of three-fold coordination, which produces branched chains in the network, and deviations in the bond angle and dihedral angles. Durable photodarkening, which is semi-permanently preserved after prolonged light illumination, can be attributed to the local structure; however, it also requires an extended network structure. Our findings yield a profound insight into a variety of photo-induced structural changes in a network of selenium chains.

Efficiency-limiting processes in cyclopentadithiophene-bridged donor-acceptor-type dyes for solid-state dye-sensitized solar cells.

The charge generation and recombination processes in three novel push-pull photosensitizers for dye-sensitized solar cells (DSSCs) are studied by ps-μs transient absorption (TA) and quasi-steady-state photoinduced absorption (PIA) spectroscopy. The three cyclopentadithiophene-based photosensitizer dye molecules exhibit comparably low power conversion efficiencies ranging from 0.8% to 1.7% in solid-state DSSCs. We find that the photocurrents increase in the presence of Li-salt additives. Both TA and PIA measurements observe long-lived dye cations created by electron injection from the dyes' excited state for two dyes from the series. However, the third dye shows significantly lower performance as a consequence of the less efficient electron injection even after the addition of Li-salts and faster electron-hole recombination on the ns-μs time scale. In essence, the prerequisites for this class of donor-π bridge-acceptor photosensitizers to reach higher charge generation efficiencies are a combination of strong dipole moments and fine tuning of the electronic landscape at the titania-dye interface by Li-salt addition.

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...

Generation of Photoexcitations and Trap-Assisted Recombination in TQ1:PC71BM Blends

The generation and recombination of long-lived photoexcitations is clarified in TQ1 films and TQ1:PC71BM 1:1 and 1:3, by weight, blends using photoinduced absorption measurements. At 80 K triplets are formed in TQ1 films, while both triplets and polarons are formed in the 1:1 and 1:3 blends. We suggest that the triplet state acts as a loss mechanism for generation of free charges in these blends and suggest an energy diagram for the photoexcitations in the blends. We estimate the triplet polaron annihilation (γTPA) constant to be 1.12 × 10–14 and 3.10 × 10–13 cm3 s–1 for the TQ1:PC71BM 1:1 and 1:3 blends, respectively. At 300 K triplets are mainly formed in the TQ1 films, while only polarons are present in the TQ1:PC71BM blends. Using frequency measurements we show that the TQ1:PC71BM 1:1 blend shows nondispersive kinetics (ζ = 1), while the 1:3 blend exhibits dispersive kinetics (ζ = 0.87). Using intensity-dependent measurements, we show that trap-assisted recombination is the dominating recombination me...

Method for characterizing bulk recombination using photoinduced absorption

The influence of reaction order and trap-assisted recombination on continuous-wave photoinduced absorption measurements is clarified through analytical calculations and numerical simulations. The results reveal the characteristic influence of different trap distributions and enable distinguishing between shallow exponential and Gaussian distributions and systems dominated by direct recombination by analyzing the temperature dependence of the in-phase and quadrature signals. The identifying features are the intensity dependence of the in-phase at high intensity, PAI∝IγHI, and the frequency dependence of the quadrature at low frequency, PAQ∝ωγLF. For direct recombination, γHI and γLF are temperature independent, and for an exponential distribution, they depend on the characteristic energy Ech as γHI=1/(1+Ech/kT) and γLF=kT/Ech, while a Gaussian distribution shows γHI and γLF as functions of I and ω, respectively.

Probing polaron excitation spectra in organic semiconductors by photoinduced-absorption-detected two-dimensional coherent spectroscopy

We report a theoretical description and experimental implementation of a novel two-dimensional coherent excitation spectroscopy based on quasi-steady-state photoinduced absorption measurement of a long-lived nonlinear population. We have studied a semiconductor-polymer:fullerene-derivative distributed heterostructure by measuring the 2D excitation spectrum by means of photoluminescence, photocurrent and photoinduced absorption from metastable polaronic products. We conclude that the photoinduced absorption probe is a viable and valuable probe in this family of 2D coherent spectroscopies.

On the influence of dislocation walls in CdTe:Cl

Studies were performed on two types of chlorine-compensated cadmium telluride crystals with a different density of native dislocations walls. The crystals were investigated by current–voltage measurements, photo-induced current transient spectroscopy and absorption measurements, in the view of investigating the influence of the density of dislocation walls on their charge transport properties and electronic levels scheme. It results that a higher density of dislocation walls increases the dark current in CdTe. To the contrary, the optical absorption properties do not seem to be influenced by the presence of dislocation walls. The PICTS measurements demonstrated that a lower density of dislocation walls provides a higher concentration of compensation-related defects and a lower concentration of the defects responsible for the peaks observed at high temperature, possibly associated to donor-pair complexes.

Charge separation dynamics at bulk heterojunctions between poly(3-hexylthiophene) and PbS quantum dots

Photo-induced electron transfer between poly-(3-hexylthiophene) (P3HT) and small (2.4 nm) PbS quantum dots (QDs), capped by different ligands, was studied by picosecond and femtosecond time-resolved fluorescence and by photo-induced absorption (PIA) measurements. In line with previous experiments, we observed that the efficiency of the quenching of P3HT by PbS QDs increased upon decreasing the average thickness of the ligand shell. This trend was also observed in the PIA spectra and in prior work on the performance of photovoltaic devices where the active layer was a blend of P3HT with PbS QDs capped by different ligands. Combining the pico- and femtosecond fluorescence decays showed that the quenching in blend films of P3HT and PbS QDs treated with 1,4-benzenedithiol occurred over a broad time scale ranging from tens of femtoseconds to hundreds of picoseconds. This complex kinetics was attributed to exciton hopping followed by electron transfer to the conduction band of the QDs. We also compared the wavelength dependence of the internal quantum efficiency (IQE) in the hybrid photovoltaic devices to those devices where the photoactive layer consists of PbS QDs only. Although excitation in the first excitonic transition of the PbS QDs yielded a similar IQE in both devices, the IQE of the hybrid devices tripled at wavelengths where also P3HT started to absorb. This suggests that upon excitation of P3HT in the latter devices, charge generation occurs by photo-induced electron transfer from P3HT to the QDs rather than by energy transfer to the QDs followed by exciton dissociation in the QDs.

Plasmon-Polaron Coupling in Conjugated Polymer on Infrared Nanoantennas.

We propose and demonstrate a novel type of coupling between polarons in a conjugated polymer and localized surface plasmons in infrared (IR) nanoantennas. The near-field interaction between plasmons and polarons is revealed by polarized photoinduced absorption measurements, probing mid-IR polaron transitions, and infrared-active vibrational modes of the polymer, which directly gauge the density of photogenerated charge carriers. This work proves the possibility of tuning the polaronic properties of organic semiconductors with plasmonic nanostructures.

Determination of photocarrier density under continuous photoirradiation using spectroscopic techniques as applied to polymer: Fullerene blend films

We propose a method to determine the density of photocarrier under continuous photoirradiation in conjugated polymers using spectroscopic signals obtained by photoinduced absorption (PIA) measurements. The bleaching signals in the PIA measurements of polymer films and the steady-state absorption signals of oxidized polymer solution are employed to determine the photocarrier density. The method is applied to photocarriers of poly (3-hexylthiophene) (P3HT) in a blended film consisting of P3HT and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The photocarrier density under continuous photoirradiation of 580 mW/cm2 is determined to be 3.5 × 1016 cm−3. Using a trend of the carrier density increasing in proportion to the square root of photo-excitation intensity, we provide a general formula to estimate the photocarrier density under simulated 1 sun solar irradiation for the P3HT: PCBM film of an arbitrary thickness. We emphasize that the method proposed in this study enables an estimate of carrier density without measuring a current and can be applied to films with no electrodes as well as to devices.

Origin of Stark Signals Induced by Continuous Photoirradiation for Working Dye-Sensitized Solar Cells Revealed by Photoinduced Absorption Measurements

It has recently been reported that characteristic signals resulting from a Stark effect are included in the spectra of photoinduced absorption (PIA) measurements for dye-sensitized solar cells (DSCs), and the origin of the Stark effect has been discussed in several papers. Here, we explore the origin of the Stark signal observed under continuous photoirradiation from results of continuous wave (cw)-PIA experiments for several control DSCs of ruthenium-based sensitizers (N719). Typical line shape resulting from the Stark effect was observed in the quadrature phase of cw-PIA spectra under short-circuit conditions. The Stark signals are found to arise in relation to stable photocurrent and be independent of the presence of particular electrolyte ions. Time-resolved PIA measurements demonstrate that the Stark signals rise and decay in time scales much slower than the response time of injected electrons in TiO2. We conclude that redox actions of a pair of I–/I3– are responsible for the Stark effect under continuous photoirradiation and conjecture that the observed Stark signals are due to local electric fields acting on neutral dyes given by complexes consisting of dyes and iodine molecules or iodine-based ions. We propose to use Stark signals to monitor electrostatic environments around neutral dyes in working DSCs.

Nongeminate recombination in neat P3HT and P3HT:PCBM blend films

The slow decay of charge carriers in polymer–fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.

Correlation between Temperature Activation of Charge‐Carrier Generation Efficiency and Hole Mobility in Small‐Molecule Donor Materials

In organic solar cells, free charge carriers are generated at the interface between an electron-donating and an electron-accepting material. The detailed mechanisms of the generation of free charge carriers are still under discussion. In this work, we investigate the influence of temperature on the generation efficiency of free charge carriers in blends of dicyanovinyl substituted oligothiophene (DCVnT) molecules and C60 by quasi-steady-state photoinduced absorption (PIA) measurements. The observed positive temperature dependence of charge-carrier generation can be directly correlated to the charge-transport behavior. The determined activation energy scales inversely with the hole mobility for all investigated DCVnT derivatives, suggesting higher dissociation probability of bound interfacial charge pairs at high mobility. Furthermore, the energetic disorder parameter, σ, determined by CELIV (charge extraction by linearly increasing voltage) measurements for a DCV6T derivative, matches the activation energy from the PIA measurements. In conclusion, these results underline the need for high-mobility donor materials for optimal charge-pair dissociation in organic solar cells.

Intramolecular Excimer Formation between 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione Chromophoric Groups Linked by a Flexible Alkyl Spacer

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (ΦF) and fluorescence lifetime (τF) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor-acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.

Spectroscopic investigations on Stark components observed in photoinduced absorption measurements for dye-sensitized solar cells

Spectral features of the Stark signals that are observed in photoinduced optical spectra for dye-sensitized solar cell systems are examined using continuous-wave- photoinduced absorption techniques combined with bias application. The Stark signals are found to almost disappear with increasing the bias voltage. The disappearance of Stark signal indicates that there exists a unidirectional built-in electric field acting on neutral states of dye molecules. Time-dependent measurements demonstrate that the Stark signal slowly rises, whereas the photocurrent exhibits a rapid rise and then decays slowly. This finding indicates that, although the screening effect occurs at the beginning of the cell operation, it is gradually lost during the operation. We conclude that the occurrence of the Stark effect is attributed to the electrolyte ions that gradually become deactivated or transfer away from the TiO2 layer.

Continuous lateral gradients in film morphology for position sensitive detection and organic solar cell optimization

We present a method to fabricate binary organic donor and acceptor blends exhibiting a controlled lateral gradient in morphology. Upon combining photometry, ellipsometry and Xray maps together with photoinduced absorption measurements, we show how the gradual exposure to solvent vapor results in a varying degree of polymer crystallinity for the polythiophene/soluble fullerene system along one direction. These morphologically graded samples are characterized by a spectral photoresponse that depends on the specific location in the area of the device where the light beam impinges, a property that stands as proof-of-concept for position sensitive detection. Moreover, we demonstrate that the development of graded morphologies is an eective one-step method which allows for fast performance optimization of organic solar cells. Finally, the appropriateness of eight dierent solvents for morphology control via vapor annealing is evaluated in a time-eective way using the advanced method, which helps to identify boiling point and solubility as the key processing parameters.

Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%

We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH3NH3)PbI3 as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI2 and deposited onto a submicron-thick mesoscopic TiO2 film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (JSC) exceeding 17 mA/cm2, an open circuit photovoltage (VOC) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH3NH3)PbI3 NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO2 film. The use of a solid hole conductor dramatically improved the device stability compared to (CH3NH3)PbI3 -sensitized liquid junction cells.

Formation of Aluminum Oxide Films on Silicon Surface by Aluminum Evaporation in Oxygen Gas Atmosphere

ABSTRACTWe report formation of thin aluminum oxide AlOx films on the silicon surface by a simple method of Al metal evaporation in oxygen gas atmosphere. 520 μm thick 30-Ωcm p-type-silicon substrates with a top bare surface and a rear surface coated with 100 nm thick thermally grown SiO2 layers were prepared. AlOx films were formed on the top surfaces by Al metal evaporation up to 20 s in oxygen gas atmosphere at 0.8 Pa with a flow rate of 3 sccm. Samples were subsequently annealed with 9.0x105 Pa H2O vapor at 260°C for 3 h. Measurement of capacitance response to a modulation voltage at 500 kHz as a function of bias gate voltages C-V revealed that AlOx films had the effective oxide thickness ranging from 2.0 and 2.6 nm were formed. C-V measurements also revealed that negative fixed charges were accumulated with a density of 5x1012 cm-2 in AlOx films. Photo-induced carrier microwave absorption measurement resulted in a high minority carrier effective lifetime τeff of 3.6x10-4 s comparable to that of 4.1x10-4 s for thermally grown SiO2 passivation. Field effect passivation was probably caused by negative charges in AlOx so that the surface recombination velocity decreased to 70 cm/s. X-ray reflectivity analysis indicated that the interfacial layer like SiOx was formed between AlOx and Si substrate. High pressure H2O vapor heat annealing caused increase in the density and decrease in the thickness of AlOx layers, although it increased the density and thickness of the interfacial SiOx layer thickness. H2O vapor treatment is effective to improve the quality of nanometer thick AlOxlayer.