Transient Photocurrent Measurements Research Articles

Nanostructured Ruddlesden–Popper-Layered Lead Bromide Perovskites with Stable and Selected Wavelength for Photodetection Applications

Success in photovoltaic power generation has raised a huge possibility in metal halide perovskites to exhibit excellent photodetection performance with the same physics of photocarrier generation, and their collection at terminal electrodes is employable with less defect-assisted carrier recombination loss. To test this, nanostructured Ruddlesden–Popper (RP)-layered perovskite series were synthesized based on a cyclic organic moiety and Br– halide, resulting in (CH)2(MA)n−1PbnBr3n+1 [where CH─2-(1-cyclohexenyl) ethylamine, MA─methylammonium, Br─bromine, and n = 1–4], to explore how best its mono-to-few layer (n) 2D and quasi 2D-structures can offer photodetection performance with maximum absorbance in the unexplored blue-to-green band gap RP perovskites with maximum stability. Thermogravimetric analysis revealed that the very stable nature of these RP perovskites is accompanied by a linear rise in the structural phase transition temperatures, which range from 270 °C (CH)2PbBr4 (n = 1) to 304 °C (CH)2(MA)3Pb4Br13. The RP perovskites (CH)2(MA)n−1PbnBr3n+1 (n = 1–4) demonstrated sharp exciton emission peaks from blue (∼408 nm) to green (∼528 nm). The X-ray diffraction results confirmed the highly crystalline nature of quasi-2D perovskite thin films. Layer-dependent photoluminescence studies exhibit strong room-temperature exciton photoluminescence and high carrier lifetime. The transient photocurrent measurements for (CH)2(MA)3Pb4Br13 (n = 4) RP perovskite thin films show nearly six times the bias voltage changed from 1 to 5 V at 405 nm laser light excitation. In addition, with respect to performance, the (CH)2(MA)3Pb4Br13 photodetector has shown the highest responsivity (67.2 mA/W), ultrafast transient photocurrent response (122 nA), and high EQE (20.6%) as compared to (CH)2(MA)2Pb3Br10 (n = 3) and (CH)2(MA)Pb2Br7 (n = 2) in ambient temperature.

Spike Current Induction by Photogenerated Charge Accumulation at the Surface Sites of Porous Porphyrinic Zirconium Metal-Organic Framework Electrodes in Photoelectrochemical Cells

Abstract Photoelectrochemical (PEC) capacitors have recently garnered increasing interest based on their charge accumulation and dissipation mechanisms, particularly with respect to spike and overshoot currents, and have therefore been investigated for biomedical applications, including nerve photostimulation and biomolecular sensing. Porous metal-organic frameworks (MOFs) are capable of accumulating large amounts of photogenerated charge at their surface sites, owing to their large surface areas, and therefore may have potential as a new material for use in PEC capacitors. To explore the PEC capacitor properties of MOFs, we performed transient photocurrent measurements using PEC cells comprising porphyrinic zirconium MOF (PZ-MOF) electrodes in a phosphate-buffered saline solution. We observed a clear growth and decay of the cathodic current during light irradiation and the generation of an anodic reverse current when the light was turned off, thus inducing spike and overshoot currents. However, no spike or overshoot currents were observed when excess oxygen was introduced into the electrolyte. These results indicate that PZ-MOFs have the ability for photogenerated charge accumulation at the surface pores near the interface between the PZ-MOF electrode and the electrolyte. Thus, we have confirmed that PZ-MOFs are a promising PEC capacitor material that may be used in future biomedical applications.

Accessing Deep Traps Distribution in FeS2‐Doped Organic Photovoltaics

Although doping has the potential to improve the performance of organic photovoltaic cells (OPVc), doping effects on charge transport, recombination, and energetic disorder are still obscure. Doping has two opposing effects: on the one hand, dopant ions create more trap centers, while free dopant‐induced charges fill deep states, potentially providing better performance. The optimum amount of dopants can considerably improve the performance of OPVc. Herein, the energetic distribution of trap states in P3HT: PC71BM‐based OPVc doped with iron pyrite nanocubes (NCs) is reported. Using the reverse bias transient photocurrent (TPC) measurement, the energetic trap distributions with different doping conditions are studied. The photovoltaic characteristics and TPC phenomena of the OPVc greatly improve through doping. Variations in trap distributions with doping levels are analyzed to interpret the obtained trap density of states profiles. The light‐dependent current–voltage characteristics help to identify the presence of a less trap‐assisted recombination process in the optimum device. This study highlights the mechanism for performance improvement in devices with optimal doping of iron pyrite NCs.

Facile and Effective Band Gap Engineering of 2D Ruddlesden–Popper Perovskites with Improved Structural and Optoelectronic Properties

Incorporation of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) into 3D metal-halide perovskite crystal structures has resulted in improved crystallinity and film morphology, reduced trap density, and enhanced charge carrier transport. However, due to intrinsic structural instability associated with 3D perovskites, their alkali metal ion-doped semiconductors are not a suitable option for real device optoelectronics. In recent years, Ruddlesden–Popper (RP)-phase quasi-two-dimensional (2D) metal halide perovskites have drawn considerable attention due to their tunable energy band gap, large electron and hole diffusion lengths (∼10 μm), and lower recombination rates compared to pristine 2D metal halide perovskites (n = 1). However, alkali metal ion-incorporated RP perovskite systems are not explored much; therefore, we demonstrate the incorporation of Rb+, using the RbI precursor, into the quasi-2D RP perovskite (BA)2(MA)Pb2Br7 (BA = C4H9NH3; MA = CH3NH3), which has also resulted in the successive replacement of Br– with I– in the PbBr6 network. Subsequent replacement of Br– with I– finely tuned the energy band gap from 2.92 to 2.43 eV, while keeping the phase of the pristine lead bromide RP perovskite intact, as confirmed by X-ray diffraction investigations. MA+ cations available at the outer edge sites got partially exchanged with Rb ions, which in turn has regulated the perovskite crystal growth, whereas the excess Rb+ alloyed with replaced Br– to form RbBr, which resulted in the formation of thinner grain boundaries and thus showed improved charge transport. Transient photocurrent measurements performed for RbI-doped perovskites (lateral configuration photodetector: fluorine-doped tin oxide (FTO)/RP-perovskite/FTO of channel width ∼150 μm), for 35 cycles under standard Xe-lamp (100 mW/cm2) illumination, have shown strong and fast photocurrent response with large photoresponsivity (3.92 × 10–4 A/W). Comparison of structural, optical, and optoelectronic properties of RbI-doped RP perovskites is carried out with the mixed RP perovskites and has further confirmed that Rb+ incorporation plays an important role in enhancing the crystal growth.

Photoelectrochemical Modelling of Semiconducting Electrodes for Neural Interfacing

Semiconducting electrodes are increasingly utilised for neural interfacing applications, such as neural recording, stimulation, and photomodulation. To characterize the performance of these electrodes, photoelectrochemical analysis is often undertaken in biologically relevant electrolytes. These include electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and for photomodulation applications, photocurrent (PC) measurements. From such measurements, it is possible to deduce key properties of semiconductor surfaces, such as electrochemical impedance and capacitance, as well as mechanisms of charge transfer. To extract these parameters from the experimental data, equivalent electrical circuit modelling is often employed, but usually only for a single technique at a time which often misses key insights about the processes occurring at the electrode-electrolyte interface. Here we present an equivalent circuit model that simultaneously describes the results from CV, EIS, and PC transient measurements. Using semiconducting nitrogen-doped ultrananocrystalline diamond (N-UNCD) electrodes in saline solution, we show that the model describes physical mechanisms that occur at the interface with electrolyte, encompassing the space charge region, the electrical double layer, and the electrolyte. Using the model we are able to optimize parameters relevant for neural interfacing and suggest that this framework may assist in the characterization of other semiconducting electrodes.

Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes.

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at ∼3.0 eV below the Fermi level, which decreased to ∼2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of ∼60 μA/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to ∼110 μA/cm2 for rGO/GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications.

Bionic opto-responsive fiber for directing neurite growth

Mounting evidence indicates that the nervous system plays a central function in pathogenesis. To study the association of the nervous system in diseases, tools that allow delicate control of neural behavior are crucial. Wireless electrical modulation of neural cells via opto-stimulation is an emerging approach to trigger and control neural behavior. Furthermore, to imitate the highly organized natural neural network, neurite outgrowth needs to be guided to achieve directionality. This work successfully combines bionic fibers with nano-groove topography and evident opto-response to guide neurite outgrowth. With excellent biocompatibility, the bionic opto-responsive fibers spatiotemporally controlled and guided PC12 neurite outgrowth. Transient photocurrent measurement revealed that this effect was attributed to anodic faradaic photocurrents. This bionic opto-responsive fiber has great potential to act as a tool to control and guide neurite outgrowth to study nervous system/disease interaction.

Photoinduced absorption spectroscopy (PIAS) study of water and chloride oxidation by a WO3 photoanode in acidic solution.

The mechanisms of water and chloride oxidation by a WO3 photoanode are probed by photoinduced absorption spectroscopy (PIAS) coupled with transient photocurrent (TC) measurements. Linear sweep voltammograms (LSVs) and incident photon to current efficiencies (IPCEs) are obtained, in the water oxidation electrolyte (1 M HClO4) and chloride oxidation electrolyte (3.5 M NaCl in 1 M HClO4). Other work shows that the faradaic efficiency of water oxidation to O2 in 1 M HClO4 is ca. 1.0, and that for chloride oxidation to Cl2 in 3.5 M NaCl plus 1 M HClO4 is ca. 0.62. The PIAS/TC data reveals a 0.4 order dependency of the rate of water oxidation on the steady state concentration of photogenerated surface holes, [hs+]ss, and an approximately first order dependency of the rate of chloride oxidation on [hs+]ss. Associated mechanisms and rate determining steps for water and chloride oxidation at the photoanode surface that account for these reaction orders are proposed.

Stability of a pyrimidine-based dye-sensitized TiO2 photoanode in sacrificial electrolytes

Dye sensitization of semiconductor metal oxides aims to extend the light absorption range into the visible region, of particular interest for their use as photoanodes in photoelectrochemical (PEC) water splitting. Organic dyes, however, suffer from limited chemical stability when they are exposed to constant potential and irradiation in aqueous media. Thus, in this work, we evaluate a novel metal-free donor-π-acceptor (D-π-A) dye (AT-Pyri), demonstrating significant sensitization of film photoanodes based on TiO2 nanoparticles. Among a series of tested sacrificial agents, the addition of triethanolamine (TEOA) to the sodium sulphate electrolyte prevents the degradation of AT-Pyri even after several hours of operation. Combining two sacrificial agents (methanol and TEOA) results in a synergetic improvement of the PEC water-splitting activity. Notably, the stabilization of the AT-Pyri photoanode is accompanied by a substantial increase in the photocurrent over the reference TiO2 photoanode, while the associated spikes in transient photocurrent measurements disappear.

Exploiting multiferroicity of TbFeO3 nanoparticles for hydrogen generation through photo/electro/photoelectro-catalytic water splitting

Hydrogen is a potential future energy source that could replace conventional fuel and provide the necessary energy. Multiferroic materials are the most likely choices for water splitting due to their ferroelectric characteristics and ability to function as magnetically recoverable catalysts. Multiferroic terbium orthoferrite nanoparticles were synthesized at low temperature by the polymeric citrate precursor route to study the photocatalytic, electrocatalytic and photoelectrochemical activity towards hydrogen production. Powder X-ray diffraction revealed successful formation of orthorhombic TbFeO3 nanoparticles. A larger aspect ratio of 3.9 and a bandgap of 2.13 eV were seen in the elongated TbFeO3 nanoparticles, which contributes to the photo/electro-catalytic activity. Magnetic and ferroelectric studies revealed weak ferromagnetism and ferroelectric polarization of 0.037 μC cm−2 in TbFeO3 nanoparticles, confirming multiferroicity. Visibly active and multiferroic TbFeO3 nanoparticles showed notable hydrogen evolution of 1.44 mmol h−1 g−1. In electrocatalytic and photoelectrochemical water splitting investigations, TbFeO3 nanoparticles demonstrated current densities of 30 and 60 mA cm−2, respectively. EIS, TRPL, transient photocurrent and Mott-schottky measurements were used to examine charge transfer kinetics. The high H2 evolution and good OER/HER tests were attributed to increased charger separation efficiency due to ferroelectricity induced band bending.

CdS Nanoparticles Decorated 1D CeO2 Nanorods for Enhanced Photocatalytic Desulfurization Performance

CdS nanoparticles were constructed onto one-dimensional (1D) CeO2 nanorods by a two-step hydrothermal method. The X-ray diffraction (XRD), transmission election microscopy (TEM), Raman spectra, X-ray photoelectron spectra (XPS) and UV-Vis diffuse reflection spectroscopy (DRS) techniques were used to characterize these CdS/CeO2 nanocomposites. It was concluded that when the molar ratio of CdS and CeO2 was 1:1, the nanocomposites exhibited the best photocatalytic desulfurization activity, reaching 92% in 3 h. Meanwhile, transient photocurrent (PT) measurement, photoluminescence (PL) spectra and electrochemical impedance spectroscopy (EIS) measurement indicated that the modification of CeO2 nanorods by CdS nanoparticles could significantly inhibit the recombination of photogenerated electrons and holes. In addition, the possible mechanism of photocatalytic oxidation desulfurization of the nanocomposites was proposed. This study may provide an effective CeO2-based photocatalyst for photocatalytic desulfurization applications.

Dynamics of Internal Electric Field Screening in Hybrid Perovskite Solar Cells Probed Using Electroabsorption

Electric fields arising from the distribution of charge in metal-halide perovskite solar cells are critical for understanding the many weird and wonderful optoelectronic properties displayed by these devices. Mobile ionic defects are thought to accumulate at interfaces to screen electric fields within the bulk of the perovskite semiconductor on application of external bias, but tools are needed to directly probe the dynamics of this process. Here, we show that electroabsorption measurements allow the electric field within the active layer to be tracked as a function of frequency or time. The magnitude of the electroabsorption signal, corresponding to the strength of the electric field in the perovskite layer, falls off for externally applied low-frequency voltages or at long times following voltage steps. Our observations are consistent with drift-diffusion simulations, impedance spectroscopy, and transient photocurrent measurements. They indicate charge redistribution on timescales ranging from 10 ms to 100 s, depending on the device interlayer material, perovskite composition, dominant charged defect, and illumination conditions. The method can be used on typical solar-cell structures and has the potential to become a routine characterization tool for optimizing hybrid perovskite devices.

Nickel Doped Zinc Oxide Thin Films for Visible Blind Ultraviolet Photodetection Applications

The current research aims to investigate the effect of nickel doping on the structural and opto-electrical characteristics of zinc oxide thin films. Sol-gel spin coating technique has been utilized to deposit Zn1-xNixO (x = 0, 0.005, 0.010, and 0.015) films on glass substrates. X-ray diffraction (XRD) analysis confirms the formation of crystalline zinc oxide thin films with hexagonal wurtzite structure. Williamson-Hall analysis has been performed to study the individual contribution of lattice strain and crystallite size to the peak broadening in the XRD pattern. Scanning electron microscopy (SEM), Photoluminescence spectroscopy, and UV–visible spectroscopic techniques have been used to examine the surface morphology and optical properties of the deposited films. Transient photocurrent measurements have been performed on all the films under the exposure of ultraviolet (UV) light of wavelengths 365 and 254 nm with on/off cycle of 100 s, and various device key parameters such as sensitivity, responsivity, and quantum efficiency, etc have been determined. Sensitivities of the fabricated photodetectors (PDs) are found to be 5463%, 3809%, 3100%, and 831% for pristine ZnO, Zn0.995Ni0.005O, Zn0.99Ni0.01O, and Zn0.985Ni0.015O, respectively. The UV photodetection mechanism, which is based on the interaction between chemisorbed oxygen on the surface of ZnO and photo-generated holes, has been thoroughly discussed.

On the origin of multihole oxygen evolution in haematite photoanodes

The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. We show here that the OER rate has a third-order dependence on the surface hole density. We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Acid-Modulated Synthesis of Novel π-Conjugated Microporous Polymers for Efficient Metal-Free Photocatalytic Hydrogen Evolution.

Simple synthetic modifications that tune the molecular structures, thereby the properties of the molecules, are of topical interest. Herein, we report the synthesis of two novel cationic rosaniline-based conjugated microporous polymers (CMPs) from identical monomers via simple acid modulation (Acetic acid and BF3 ⋅ Et2 O). The condensation reaction of rosaniline with 2,4,6-triformylphloroglucinol in acetic acid renders β-ketoenamine-linked CMP (CMP-A) while changing the acid to BF3 ⋅ Et2 O, the linkages transform to enol and undergoes BF2 -complexation, leading to boranil CMP (CMP-B). BF2 -functionalities in boranil CMP significantly modified the optical and functional properties compared to β-ketoenamine-linked CMP. The cationic-delocalization along with the extended π-delocalization supported by chromophoric BF2 -groups allow CMP-B to exhibit broad absorption spanning the visible to Near-Infrared region (NIR). The absorption red-edge of CMP-B appears around 1277 nm (optical band gap ∼1.58 eV) while CMP-A displays at 981 nm (optical band gap ∼1.83 eV). Most interestingly, as a photocatalyst, CMP-B catalyzes hydrogen evolution with a superior rate of 252 μmol g-1 over CMP-A (100 μmol g-1 ). It is about 2.5 times higher performance. The transient photocurrent measurements, electrochemical impedance data, and in-depth mott-Schottky analysis demonstrate that the BF2 -group in CMP-B generates photoinduced charge carriers and their migration towards the active sites for photocatalysis. These polymers show significant photocatalytic H2 generation without any supportive metal co-catalyst. The BF2 complexed building blocks are a unique class of metal-free photocatalysts for hydrogen evolution through green and cost-effective approach.

A wide solar spectrum utilization and fast charge carrier transport upon variable GaAs interlayer thickness in ZnPc/GaAs/ZnPc hybrid thin-film structures

The effects of inorganic layer thickness on the photophysical properties of the organic layer in an organic-inorganic hybrid system were investigated in this paper. Photosensitive Zinc Phthalocyanine (ZnPc) and GaAs layers were alternatively stacked through simple physical vapor deposition, and the impact of GaAs layer thickness was explored through structural, optical, and electrical characterization techniques. The strain-induced quantum size effect was evident in the hybrid structure, and it was confirmed in Raman analysis with broad vibration peaks of the GaAs phase, which resulted in a decrease in the GaAs thickness < Bohr radius (RB). In addition, a complementary absorption edge was obtained at 295 nm along with ZnPc absorption features, but GaAs failed to contribute to spectral absorption with its thickness > RB. The decreased photoluminescence intensity and fluorescence decay time of ZnPc after incorporation of GaAs imply that charge transfer from ZnPc to GaAs occurs quickly. The transient photocurrent measurement demonstrates that the current density of hybrid film with GaAs thickness < Bohr radius was significantly increased. As expected, the GaAs with thickness above the Bohr radius is ineffective for improving the photocurrent. Impedance spectroscopy analysis was applied to explore the electrical transport behavior in the hybrid system using a two-electrode system. An ultra-thin GaAs layer noticeably promoted mass transfer at the ZnPc-GaAs interface and increased the conductivity of the hybrid system with the least charge transfer resistance.

Probing the defects states in MAPbI3 perovskite thin films through photoluminescence and photoluminescence excitation spectroscopy studies

The luminescence properties of methylammonium lead iodide (MAPbI3) perovskite thin films deposited on corning 1737 glass substrates are studied using photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy. From the PL spectra analysis, it is realized that there are two emission peaks, a high-intensity peak at 1.58 eV (~783 nm) and a smaller shoulder peak at 1.64 eV (~754 nm) in the bandgap energy region. The peak at ~1.64 eV corresponds to the bandgap of MAPbI3, whereas the high-intensity peak at photon energy ~1.58 eV is due to the presence of shallow trap states. The calculated optical bandgap energy 1.58 ± 0.01 eV is close to the reported bandgap energy 1.60–1.61 eV and consistent with the PL peak position. The slight difference in bandgap value is due to shallow defect states in MAPbI3 films. The slow decay of photocurrent observed in the transient photocurrent measurements further confirms the presence of shallow trap states in MAPbI3 perovskite.

The effect of thermal annealing on defect states, light intensity and photocurrent of organic solar cells

Due to the low conversion efficiency and instability, organic solar cells (OSCs) still cannot be compared with conventional silicon-based solar cells that have been produced commercially. The low performance is attributed to the existence of defect states in solar cells. The defect states will prevent the carrier from transporting and then reduce the power conversion efficiency (PCE). In this work, the external quantum efficiency (EQE) measurement and current-voltage (I-V) measurement are used to characterize the performance of OSCs. The experimental results show that the power conversion efficiency of the cells increases from 0.56% to 3.52% after thermal annealing. The relationship between light intensity and photocurrent and the transient photocurrent (TPC) measurement is used to study the effect of annealing on the defect states of OSCs. The relationship between light intensity and photocurrent under different light intensities for cells before and after thermal annealing is compared. It is found that there is a linear relationship between light intensity and photocurrent of the cells after thermal annealing. Analysis of experimental results indicates that thermal annealing could reduce the density of defect states, effectively inhibits carrier recombination, promotes carrier transport, and ultimately improve the performance of cells.

Microscopic Structures, Dynamics, and Spin Configuration of the Charge Carriers in Organic Photovoltaic Solar Cells Studied by Advanced Time-Resolved Spectroscopic Methods.

Organic photovoltaics (OPVs) are promising solutions for renewable energy and sustainable technologies and have attracted much attention in recent years. Two types of organic semiconductors are used as donor materials to fabricate OPV cells. One type is a photoconductive polymer, and the other type is a small-molecule-based compound. The discovery of a bulk-heterojunction (BHJ) structure using a mixture of p- and n-type organic semiconductors has dramatically increased the power conversion efficiency (PCE) of OPV cells. In this feature article, we review our recent studies on organic BHJ thin films and OPVs by using advanced time-resolved spectroscopic techniques. Two topics regarding the microscopic behaviors of the charge carriers are discussed. The first topic is focused on how to quantify the local mobility of the charge carriers. Here, we discuss charge carrier dynamics in diketopyrrolopyrrole-linked tetrabenzoporphyrin (DPP-BP) BHJ thin films studied by time-resolved terahertz spectroscopy on a subpicosecond to several tens of picoseconds time scale and by transient photocurrent measurements on a microsecond time scale. The second topic concerns the spin configuration and interaction of the electron and hole of the polaron pairs in polymer-based BHJ thin films and OPV cells studied by the time-resolved electron paramagnetic resonance method, time-resolved simultaneous optical and electrical detection, and measurement of the magnetoconductance effect.