Open-circuit Photovoltage Decay Research Articles

Enhancing electrical and optical properties of anatase TiO2 nanotubes through electrochemical lithiation

In presented paper, anatase TiO2 nanotubes (NTs) were obtained by anodic oxidation of Ti-foil followed by subsequent annealing in air at 400 °C. After thermal treatment, TiO2 nanotubes (NTs) were electrochemically lithiated by means of galvanostatic (GS) discharge, as a part of GS cycling, at 25°C and 55 °C. Microstructural properties of the as-prepared material were observed by scanning and transmission electron microscopy (SEM, TEM), while changes in chemical bonding with lithiation were examined by X-ray photoelectron spectroscopy (XPS). Specific electrical conductivity, obtained by 4-point probe method, showed multifold increase after Li-ion insertion in TiO2 NTs, at both temperatures. By using diffuse reflectance spectroscopy (DRS), the decrease in energy gap, from 3.04 eV for the as-prepared TiO2 NTs to 2.81 eV for lithiated TiO2 NTs, was observed. The photoluminescence (PL) measurements suggest that Li-ion intercalation led to suppression of deep-level trap states within the bandgap, of TiO2 NTs, and promoted shallow defects associated to F-centers. Open-circuit photovoltage decay (OCVD) measurements confirm the promotion of shallow defect states. Furthermore, HER measurements indicate that the electrochemical lithiation is a promising strategy for enhancing the catalytic performance of TiO2.

Charge transfer mechanism of AZO-ZnO photoanode based on impedance study for solar cell application

• Studied the effect of growth temperature on the length and defect of ZnO NR. • Charge transfer mechanism of NRs was investigated using electrochemical analysis. • Obtained current density of 0.82 mA/cm 2 and photoconversion efficiency of 0.53 %. • Working mechanism of the NR-based cell was explained using a band diagram. Charge transfer mechanism in ZnO nanorods grown on aluminium doped zinc oxide substrates using a single step hydrothermal method was investigated as a function of ZnO nanorod (NR) growth temperature/nanorod’s length. Band-to-band transition and the defect state transition in the prepared nanorods was confirmed using photoluminescence spectroscopy. The increase in active sites up to an optimum growth temperature/nanorod’s length results in an increase of current density and photoconversion efficiency of the prepared NR. But with the further increase in the latter, the current density decreases due to the recombination of electrons. Nanorod grown at the optimized condition exhibited a current density of 0.82 mA/cm 2 and a photoconversion efficiency η of 0.53 %. Electrochemical impedance spectroscopy and open-circuit photovoltage decay (OCPVD) measurement were employed to determine the charge transfer resistance R ct , double layer capacitance ( C dl ) and the recombination time τ of zinc oxide nanorods of various length. The nanorod prepared at the optimized condition of 140 °C having an average length of 2.4 μm had superior current density and the better charge transfer mechanism. The study confirmed that this was due to the low charge transfer resistance R ct = 1300 Ω value and better recombination time τ = 2.38 s for the sample, which makes ZnO nanorods a promising candidate to be used as a photoanode in quantum dot sensitized solar cells.

Photoelectrochemical global approach to the behaviour of nanostructured anatase under different irradiation conditions

An anatase film electrode on FTO glass has been produced by a mix of commercial nanostructured anatase with water in a dispersion using a glass rod followed by calcination at 400 °C for one hour with the ramp of 20 °C per minute. The structural morphology of the resulting surface has been analysed by scanning electron microscopy (SEM) and its photoelectrochemical behaviour in the dark, at laboratory environment and under UVA light has been examined using different electrochemical techniques as open circuit photovoltage decay (OCP), linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Mott-Schottky plot measurements allowing a global electrochemical approach. Analysis of the results has allowed a better understanding about its photoelectrochemical properties and charge transfer abilities in photocatalytic applications.

Revisiting open-circuit photovoltage decay in organic solar cells for the determination of bimolecular recombination constants

Open-circuit photovoltage decay from the steady state for the determination of bimolecular recombination constants has been studied in organic solar cells (OSCs) with three prototypical bulk heterojunctions including fullerene and non-fullerene acceptors. A simple theory for the determination of recombination constants from the initial decay rate of the open-circuit photovoltage was shown. The effective lifetimes were experimentally determined from the initial decay rates and were proportional to the −0.5 power of the excitation light intensity, indicating that the recombination process under the open-circuit condition in the OSCs is bimolecular. The experimental setup was also simple, and hence the experiment and the analysis are applicable to a variety of OSCs under operation. Information on the transport properties including bimolecular recombination constants is useful for the design and the optimization of the device architecture of OSCs.

Facile sputtering enables double-layered ZnO electron transport layer for PbS quantum dot solar cells

PbS colloidal quantum dot solar cells (CQDSCs) employ ZnO electron transport layer have achieved high efficiency. However, there is nearly no efficient and batch production method to balance the charge separation and recombination within the device, which is one of the most obviously barrier to a satisfactory conversion efficiency. Here, a n+-n double-layered ZnO electron transport layer (DETL) is prepared by a facile one-step magnetron sputtering under different Ar pressure, and employed in heterojunction PbS colloidal quantum dot solar cells (CQDSCs) for the purpose of increasing charge separation at heterojunction interface via energy-band alignment modulation. The ZnO DETL, composed of a 100-nm-thick n+-ZnO bottom layer (n = 8 × 1019 cm−3) and a 20-nm-thick n-ZnO top layer (n = 3 × 1016 cm−3) significantly improve the power conversion efficiency (PCE) of the CQDSCs by a factor of ~35% compared to the device with single-layered n- ZnO. Open-circuit photovoltage decay (OCVD) measurements prove that the graded energy alignment of ZnO DETL effectively reduces both interfacial and trapping-assisted charge recombination, relative to the single-layered ZnO. The facile Ar-pressure tuning method makes the energy-band alignment process more convenient and sheds a light on the application of DETL electrons transport layer, fabricated by the universal technique of magnetron sputtering.

Diffusion Dynamics of Mobile Ions Hidden in Transient Optoelectronic Measurement in Planar Perovskite Solar Cells

As for the dynamics of a photovoltaic device, persistent photovoltage, generally implying long-lived charge carriers, is expected. Although in the observation of open-circuit photovoltage decay in ...

Mo5+ doping induced interface polarization for improving performance of planar perovskite solar cells

In this paper, we investigate how interface-induced polarization affects the photovoltaic performance of hybrid perovskite solar cell (PSC) devices. The polarization of the hole transport layer (HTL) is regulated through incorporating metallic-like MoOx into PEDOT:PSS. The common MoO3 doped into PEDOT:PSS is used as a reference, and the device that used PEDOT:PSS-MoOx as the HTL shows an enhanced Jsc and FF compared to the reference device. The open-circuit photovoltage decay and impedance spectroscopy measurements indicate that trap-assisted recombination is effectively suppressed at the interface between the hybrid perovskite and the PEDOT:PSS-MoOx HTL, while severe trap assisted recombination takes place at the perovskite/PEDOT:PSS and perovskite/PEDOT:PSS-MoO3 interface. We attribute these experimental findings to the fact that the incorporation of metallic-like Mo5+ into PEDOT:PSS enhances the conductivity of HTL and the interface polarization between PEDTOT:PSS layer and perovskite, which helps to induce an interface polarization electric field to facilitate separation of charges and screen the recombination between the traps and free charges.

Influence of Charge Transport Layers on Capacitance Measured in Halide Perovskite Solar Cells

Summary Capacitance-based techniques have been used to measure the electrical properties of halide perovskite solar cells (PSCs) such as defect activation energy and density, carrier concentration, and dielectric constant, which provide key information for evaluating the device performance. Here, we show that capacitance-based techniques cannot be used to reliably analyze the properties of defects in the perovskite layer or at its interface, because the high-frequency capacitance signature is due to the response of charge carriers in the hole-transport layer (HTL). For HTL-free PSCs, high-frequency capacitance can be considered as the geometric capacitance for analyzing the dielectric constant of the perovskite layer because there is no trapping and de-trapping of charge carriers in the perovskite layer. We further find that the low-frequency capacitance signature can be used to calculate the activation energy of the ionic conductivity of the perovskite layer, but the overlapping effects with charge transport materials must be avoided.

Insight on Shallow Trap States-Introduced Photocathodic Performance in n-Type Polymer Photocatalysts.

Graphitic carbon nitride (g-C3N4) is a robust organic semiconductor photocatalyst with proven H2 evolution ability. However, its application in a photoelectrochemical system as a photocathode for H2 production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C3N4 photocathodes in recent years, factors affecting an n-type semiconductor’s properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C3N4 photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C–OH terminal groups. As compared to the g-C3N4 photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C3N4 is 5 times longer than that observed for g-C3N4. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around −10 μA/cm2 at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott–Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C3N4, thus further demonstrating g-C3N4 can be both a photoanode and a photocathode candidate.

Role of carbon nanodots in defect passivation and photo-sensitization of mesoscopic-TiO2 for perovskite solar cells

In this work, a recently developed carbon nanodots (CnDs) were introduced in the mesoscopic TiO2 to form a TiO2: CnDs composite, to be used as an electrode in CH3NH3PbI3 based perovskite devices. It resulted the improvement in morphology of perovskite film and induced a carbon-doping passivating the oxygen defects. In addition, the presence of carbon dots enhanced the charge carrier mobility of TiO2 and provided extra photo-generation ability to the perovskite device due to its absorbing nature. The improved morphology, carbon-doping, photogeneration ability and the improved charge transfer characteristics with the introduction of carbon dots was verified by the scanning electron microscopy (SEM), atomic force microscopy (AFM), X-Ray photoelectron spectroscopy (XPS), UV-visible absorption, X-ray diffraction, space charge limited current (SCLC) measurement, photoluminescence (PL), electrochemical impedance spectroscopy (EIS), current leakage and open circuit photovoltage decay (OCPD) measurements. An optimized concentration of CnDs (3 wt%) within the TiO2 resulted in a maximum power conversion efficiency of 19.45% with negligible hysteresis, which is 16% higher than the device with pristine TiO2.

A Phosphonic Acid Anchoring Analogue of the Sensitizer P1 for p-Type Dye-Sensitized Solar Cells

We report the synthesis and characterization of the first example of an organic dye, PP1, for p-type dye-sensitized solar cells (DSCs) bearing a phosphonic acid anchoring group. PP1 is structurally related to the benchmarking dye, P1, which possesses a carboxylic acid anchor. The solution absorption spectra of PP1 and P1 are similar (PP1 has λmax = 478 nm and εmax = 62,800 dm3 mol−1 cm−1), as are the solid-state absorption spectra of the dyes adsorbed on FTO/NiO electrodes. p-Type DSCs with NiO as semiconductor and sensitized with P1 or PP1 perform comparably. For PP1, short-circuit current densities (JSC) and open-circuit voltages (VOC) for five DSCs lie between 1.11 and 1.45 mA cm−2, and 119 and 143 mV, respectively, compared to ranges of 1.55–1.80 mA cm−2 and 117–130 mV for P1. Photoconversion efficiencies with PP1 are in the range 0.054–0.069%, compared to 0.065–0.079% for P1. Electrochemical impedance spectroscopy, open-circuit photovoltage decay and intensity-modulated photocurrent spectroscopy have been used to compare DSCs with P1 and PP1 in detail.

Charge Recombination with Fractional Reaction Orders in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

In water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs), charge recombination competes with catalytic water oxidation to determine the overall efficiency of the system. The kinetics of these processes have been difficult to understand because transient absorbance (TA) experiments typically show nearly complete charge recombination on the submillisecond time scale; in contrast, electrochemical measurements such as open circuit photovoltage decay suggest a charge recombination time scale that is 2-3 orders of magnitude longer. Here we explore these processes with dye-sensitized nanocrystalline TiO2 and TiO2/Ta2O5 core-shell photoanodes in aqueous electrolytes using TA spectroscopy, intensity-modulated photovoltage spectroscopy (IMVS), and photoelectrochemical impedance spectroscopy (PEIS). The fast recombination rates measured by TA result from strong laser excitation that leads to high electron occupancy in TiO2, whereas IMVS modulates the concentration of charge-separated states near solar irradiance levels. The recombination processes measured by electrochemical methods such as IMVS, PEIS, and transient photovoltage are the discharging of injected electrons in TiO2, as evidenced by the close agreement between the nearly first-order recombination rates probed by IMVS and the RC time constants derived from PEIS data. However, IMVS measurements at variable probe light intensity reveal that the reaction orders for the recombination of injected electrons with oxidized sensitizer molecules are far from unity. This kinetic analysis is relevant to understanding steady-state recombination rates in full WS-DSPECs in which molecular and nanoparticle catalysts are used to oxidize water.

Characterization of Rh:SrTiO3 photoelectrodes surface-modified with a cobalt clathrochelate and their application to the hydrogen evolution reaction

Water is a promising source of clean hydrogen. Besides water electrolysis, the direct photoelectrochemical dissociation of water that combines light absorption and electrochemical water splitting into a single device is actively investigated by the scientific community. We report results on the p-type rhodium-doped strontium titanate (Rh:SrTiO3) surface-modified by adsorption of a 3d-transition metal complex which is known to be a good catalyst of the hydrogen evolution reaction (HER) from water. The tris-dioximate cobalt hexachloroclathrochelate with an encapsulated cobalt(II) ion was selected as a surface HER co-catalyst for this study. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The kinetics of the photoelectrochemical water dissociation was studied using the open circuit photovoltage decay (OCPD) and photoelectrochemical impedance spectroscopy (PEIS) methods. Under open circuit conditions, the cobalt cage complex did not substantially affect the HER kinetics compared to the bare doped Rh:SrTiO3 semiconductor. But when a reverse bias was applied, a significant difference caused by presence of the clathrochelate was observed. The rate constants of the charge transfer and the recombination processes were both affected, leading to the conclusion that the macrobicyclic cobalt-encapsulated species not only increased the rate of charge transfer, but also played a role as recombination centers for photoexcited minority carriers. The space charge capacitance was determined under the inversion conditions. Only under strong reverse bias it was found that, whereas bulk Rh:SrTiO3 has a p-type conductivity, the surface of both bare Rh:SrTiO3 and the cobalt clathrochelate-covered Rh:SrTiO3 show the properties of a n-type semiconductor.

Efficient Bulk Heterojunction CH3NH3PbI3-TiO2 Solar Cells with TiO2 Nanoparticles at Grain Boundaries of Perovskite by Multi-Cycle-Coating Strategy.

A novel bulk heterojunction (BHJ) perovskite solar cell (PSC), where the perovskite grains act as donor and the TiO2 nanoparticles act as acceptor, is reported. This efficient BHJ PSC was simply solution processed from a mixed precursor of CH3NH3PbI3 (MAPbI3) and TiO2 nanoparticles. With dissolution and recrystallization by multi-cycle-coating, a unique composite structure ranging from a MAPbI3-TiO2-dominated layer on the substrate side to a pure perovskite layer on the top side is formed, which is beneficial for the blocking of possible contact between TiO2 and the hole transport material at the interface. Scanning electron microscopy clearly shows that TiO2 nanoparticles accumulate along the grain boundaries (GBs) of perovskite. The TiO2 nanoparticles at the GBs quickly extract and reserve photogenerated electrons before they transport into the perovskite phase, as described in the multitrapping model, retarding the electron-hole recombination and reducing the energy loss, resulting in increased VOC and fill factor. Moreover, the pinning effect of the TiO2 nanoparticles at the GBs from the strong bindings between TiO2 and MAPbI3 suppresses massive ion migration along the GBs, leading to improved operational stability and diminished hysteresis. Photoluminescence (PL) quenching and PL decay confirm the efficient exciton dissociation on the heterointerface. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements show the reduced recombination loss and improved carrier lifetime of the BHJ PSCs. This novel strategy of device design effectively combines the benefits of both planar and mesostructured architectures whilst avoiding their shortcomings, eventually leading to a high PCE of 17.42% under 1 Sun illumination. The newly proposed approach also provides a new way to fabricate a TiO2-containing perovskite active layer at a low temperature.

Consequences of changes in the ZnO trap distribution on the performance of dye-sensitized solar cells.

The well-established indoline dye D149 and three indoline dyes with improved binding stability (DN91, DN216 and DN285) were used as sensitizers for electrodeposited porous ZnO and studied in dye-sensitized solar cells and showed very similar cell characteristics. After storage of the cells in the dark for four weeks, an increase in short-circuit current density was observed. Electrochemical impedance spectroscopy served to detect a shift of the conduction band edge energy to lower energies and, hence, an increased injection efficiency of electrons from the excited state of the sensitizer to the ZnO conduction band is detected as the main cause of this change. Additional photoelectrochemical experiments at different illumination intensities, intensity modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS), transient photocurrent measurements, as well as measurements of the open-circuit photovoltage decay (OCVD) were performed to confirm this conclusion and, further, to prove a reduced rate of recombination for these injected electrons which prevented a corresponding decrease in the open-circuit voltage for many cells, frequently observed earlier. It is therefore shown how the long-term stability of ZnO-based DSSCs can be improved.

Impact of drying procedure on the morphology and structure of TiO2 xerogels and the performance of dye sensitized solar cells

Different morphologies of TiO2 photoelectrodes for dye sensitized solar cells were obtained by using TiO2 gels dried in normal conditions (TiO2 amb) and in CO2 atmosphere at high pressure (TiO2 press). After a subsequent calcination, the powders were processed as pastes and drop-casted on conductive glass in order to prepare photoanodes for dye sensitized solar cells. N719 commercial dye was used as sensitizer in all the experiments. The structure and morphology of the processed TiO2 materials were investigated via X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 adsorption/desorption measurements. The dye adsorption capacity of the photoanodes was tested using ultraviolet-visible absorption spectroscopy. The photovoltaic performances of the dye sensitized solar cells were investigated using current/voltage curves (I/V), open circuit photovoltage decay measurements and electrochemical impedance spectroscopy. The conversion efficiency (η) and short circuit density (Jsc) for TiO2 amb were 1.90 % and 5 mA/cm2 respectively, while the TiO2 press cell exhibited a 40 and 50 % increase in Jsc and η values respectively. This was correlated with increased dye loading capacities due to a broader distribution of pore size towards the mesopore region.

Controllable growth of vertically aligned Bi-doped TiO2 nanorod arrays for all-oxide solid-state DSSCs

In this study, vertically aligned Bi-doped TiO2 nanorod arrays as photoanodes were successfully grown on the fluorine-doped tin oxide by hydrothermal method. Structural analysis showed that bismuth was successfully incorporated into the TiO2 lattice at low concentration, but at higher concentration, phase segregation of Bi2O3 in the TiO2 matrix was occurred. TiO2 nanorods with 3 % bismuth concentration had minimum electrical resistivity. As the solid-state electrolyte, Mg-doped CuCrO2 nanoparticles with p-type conductivity were synthesized by sol–gel method. The fabricated all-oxide solid-state dye-sensitized solar cells with Bi-doped TiO2 nanorods displayed better photovoltaic performance due to the presence of Bi. The improved cell performance was correlated with the higher dye loading, slower charge recombination rate and the higher electrical conductivity of the photoanodes. After mechanical pressing, the all-oxide solid-state DSSC exhibited enhanced photovoltaic performance due to the formation of the large neck between adjacent nanoparticles by mechanical sintering. The open-circuit photovoltage decay measurement of the devices and electrical conductivity of the nanoparticles before and after pressing revealed that the mechanical pressing technique reduces charge recombination rate and facilitates electron transport through the interconnected nanoparticles.

Electrospun Mo-BiVO4 for Efficient Photoelectrochemical Water Oxidation: Direct Evidence of Improved Hole Diffusion Length and Charge separation

Bismuth vanadate (BiVO4) is a promising visible light absorbing material for use as a photoanode in photoelectrochemical water splitting. The present study reports fabrication of nanoporous Molybdenum (Mo) doped BiVO4photoanodes by a simple and scalable electrospinning technique. Water oxidation photocurrent increases by up to four times, and a ten-fold increase in incident photon-to-current conversion efficiency (IPCE) was achieved by Mo doping. Significant enhancement in the photocurrent under front side illumination conditions compared to back side illumination was observed for the Mo-doped samples. Estimation of the hole diffusion lengths of pristine and Mo-doped BiVO4using the Gartner modelshowed an increase in hole diffusion lengths from 47nm to 183nm after Mo doping. This was supported further by open circuit photovoltage decay measurements where the better charge separation was revealed for Mo:BiVO4.Overall, the enhanced photoelectrochemical water oxidation with Mo doping can be attributed to increased charge separation and enhanced hole diffusion length, which reduced charge transfer resistance for water oxidation.

Enhanced photovoltaic performance of dye-sensitized solar cells using a new photoelectrode material: upconversion YbF3-Ho/TiO2 nanoheterostructures.

New up-conversion YbF3-Ho/TiO2 (UC/TiO2) nanoheterostructures are designed and explored as an efficient photoelectrode material to yield dye-sensitized solar cells (DSSCs) with enhanced performance. In this study, we analyze the photogenerated charge transfer properties of the UC/TiO2 nanoheterostructures via surface photovoltage (SPV) and transient photovoltage (TPV) techniques, and the interfacial dynamics of charge transfer and recombination processes in DSSCs using electrochemical impedance spectroscopy (EIS) and open circuit photovoltage decay (OCVD) techniques. It is found that these UC/TiO2 nanoheterostructures combine the upconversion function of YbF3-Ho and the semiconductive merits from TiO2. More importantly, the hetero-junction interface in the UC/TiO2 nanoheterostructures not only induces direct electron-injection from YbF3-Ho to TiO2 by utilizing near-infrared light, but also further improves the existing merits of TiO2 through facilitating the interfacial photoinduced charge separation, suppressing the photoinduced charge recombination and prolonging the lifetimes of excited electrons, which can give further improvement of the photovoltaic performances. When integrating the UC/TiO2 nanoheterostructures into DSSCs, an overall energy conversion efficiency of 8.0% is achieved. There is a 23% enhancement in the overall conversion efficiency and a 19% improvement in the photocurrent, compared to the pristine devices.

Heterotriangulene-based unsymmetrical squaraine dyes: synergistic effects of donor moieties and out-of-plane branched alkyl chains on dye cell performance

Unsymmetrical squaraine sensitizers with two different donor moieties, triphenylamine (NSQR) and heterotriangulene (NSQ1–3), for dye-sensitized solar cells (DSSCs) have been designed and synthesized. These dyes utilize the indolium moiety to control charge recombination dynamics at the TiO2-dye-electrolyte interface by connecting linear and branched alkyl functionalities. In the present study, an efficient heterotriangulene (HT) donor and a branched alkyl group at sp3-C atoms were strategically incorporated to increase the power conversion efficiency (PCE) of zwitterionic dyes by improving photo-current density (Jsc) and open-circuit potential (Voc) of the cell. Among these four dyes, NSQ3 exhibited the highest efficiency of 6.73% with a Jsc of 18.74 mA cm−2, Voc of 0.53 V, and fill factor (ff) of 68.3%, without any co-adsorbent under an irradiance of 100 mW cm−2 (simulated AM 1.5G sunlight). In the presence of 3α,7α-dihydroxy-5β-cholanic acid (CDCA), NSQ1, NSQ2 and NSQ3 showed an efficiency of 7.07%, 7.38% and 7.17%, respectively. Despite the low Voc, far red light harvesting efficiency, reduced dye aggregation, long lifetime (τ) of injected electrons and high quantum efficiency of NSQ1–3 played constructive roles in achieving high PCE efficiency. Deceleration of charge recombination for NSQ dye cells was further studied by electrochemical impedance spectroscopy (EIS) and open-circuit photo-voltage decay (OCVD) measurements.