Transient Photovoltage Techniques Research Articles

Comprehensive Study of Carrier Recombination in High‐Efficiency CdTe Solar Cells Using Transient Photovoltage

Cadmium telluride (CdTe) solar cells represent a commercially successful photovoltaic technology, with an annual production capacity approaching 20 GW. However, improving the open‐circuit voltage (VOC) remains challenging. This study aims to deepen the understanding of charge carrier recombination in CdTe solar cells and to explore alternative dynamical characterization methods that address the limitations found in conventionally used time‐resolved photoluminescence for CdTe solar cells. Transient photovoltage and transient photocurrent techniques are utilized to investigate charge carrier dynamics under conditions resembling real‐world solar cell operation. The results reveal that an effective nonradiative recombination lifetime of 580 ns dominates the charge dynamics at VOC values below 850 mV. Above this threshold, radiative recombination becomes significant, with a radiative recombination coefficient of 1.1 × 10−9 cm3 s−1. Additionally, the stationary charge carrier density at 1 sun is determined to be around 1 × 1014 cm−3. By accurately determining both radiative and nonradiative recombination, this work provides a comprehensive understanding of carrier dynamics in high‐performing CdTe devices and paves the way for improving the VOC and performance of CdTe solar cells.

Stable MOF-In2S3/ZnFe layered dioxide heterostructures for improving photocatalytic degradation: Degradation pathways, kinetics, and charge migration

In this paper, MOF-In2S3 were modified by ZnFe-layered dioxides (LDO) with strong stability to obtain high catalytic performance composite. The successful preparation of ZnFe-LDO/MOF-In2S3 not only retains the advantages of large specific surface area of MOF-In2S3 and the stability of ZnFe-LDO, but also improves its charge separation efficiency and the light absorption region through the construction of heterojunctions. As expected, the experimental results show that ZnFe-LDO/MOF-In2S3 has superior photo-electrocatalytic properties. The photocatalytic degradation efficiency of ZnFe-LDO/MOF-In2S3 for TCH and OTC reach 92.92% and 80%, respectively, within 25 min. And the corresponding degradation rate constants are up to 1.41332 and 0.45451 L·mg−1·min−1, respectively, which are accord with the pseudo-second-order kinetic constants. The intermediate of 0.6LDO/M-I degradation of TCH was proposed by liquid mass spectrometry (LC-MS). Notably, a special transient surface photovoltage (TPV) technique was used for discussing the charge transfer kinetics and the lifetime of photogenerated carriers. Based on Mott-Schottky (M-S), Ultraviolet-visible absorption spectrum (UV), X-ray Photoelectron Spectroscopy (XPS) results, electron paramagnetic resonance (EPR) results, and active substance capture experiment results, a Z-scheme heterojunction system was proposed. The above research work proposed a new approach to enhance the stability of catalytic materials by using the aid of layered dioxides. In addition, the characterization of surface photovoltage provides a new way to study charge transfer.

Interface Modification for Energy Level Alignment and Charge Extraction in CsPbI3 Perovskite Solar Cells.

In perovskite solar cells (PSCs) energy level alignment and charge extraction at the interfaces are the essential factors directly affecting the device performance. In this work, we present a modified interface between all-inorganic CsPbI3 perovskite and its hole-selective contact (spiro-OMeTAD), realized by the dipole molecule trioctylphosphine oxide (TOPO), to align the energy levels. On a passivated perovskite film, with n-octylammonium iodide (OAI), we created an upward surface band-bending at the interface by TOPO treatment. This improved interface by the dipole molecule induces a better energy level alignment and enhances the charge extraction of holes from the perovskite layer to the hole transport material. Consequently, a Voc of 1.2 V and a high-power conversion efficiency (PCE) of over 19% were achieved for inorganic CsPbI3 perovskite solar cells. Further, to demonstrate the effect of the TOPO dipole molecule, we present a layer-by-layer charge extraction study by a transient surface photovoltage (trSPV) technique accomplished by a charge transport simulation.

Synergistic photocatalytic nanozymes to promote contaminant removal and hydrogen production

Photocatalytic nanozymes, which combine light-induced redox reactions and enzyme-like catalysis, have emerged as a promising class of nanomaterials. This alliance offers tremendous potential for light-enhanced enzyme-like activity and expands the applications of nanozymes in energy and environmental fields. In this study, a photocatalytic nanozyme OVs-TiO2/Pd hybrid nanosheets (HNSs) with abundant oxygen vacancies (OVs) and controllable Pd components was prepared by a one-step solid-phase reduction method. Benefiting from the synergistic effect and the strong metal-support interaction, OVs-TiO2/Pd HNSs exhibit remarkable light-enhanced oxidase-like and hydrogenase-like activity, they efficiently catalyze the degradation of antibiotics and the production of hydrogen, respectively. Electron spin resonance and transient photovoltage techniques elucidated that the enhanced enzyme-like activities are attributed to the increased generation of charge carriers and reactive oxygen species. This study builds a bridge between photocatalysis and enzyme-like catalysis, opening up a new avenue to explore the multiple functional nanomaterials for a myriad of applications.

Cooperation of carbon doping and carbon loading boosts photocatalytic activity by the optimum photo-induced electron trapping and interfacial charge transfer

Carbon-based materials with various advantages have gained growing research interests in photocatalytic field, in which, employing carbon materials as active catalysts presents new opportunities for low-cost, green and sustainable energy conversion strategic. The performance of these carbon-based photocatalyst highly depends on several factors, including photoexcitation, charge separation and transfer and surface catalytic reaction, and surface catalytic process for H2 production. Here, the graphitic carbon nitride (g-C3N4) system with doping carbon and loading carbon dots (CDs) was constructed via a direct thermal polymerization route to reveal the effect of trap states and displayed a 4-fold boosted H2 evolution efficiency relative to ordinary g-C3N4 during the photocatalysis. Transient absorption spectroscopy (TAS) results disclosed that the C-doping induced shallow trap states could capture photo-induced electrons to restrain deep trapping and direct recombination of photo-generated carriers. Density functional theory (DFT) calculation and transient photovoltage technique (TPV) test results confirmed that the fast holes’ transfer path established between CDs and C-doping g-C3N4 (CCN) make the CDs possess the extractive effect for the holes for the oxidation reaction with (TEOA), achieving spatial separation of electron-hole pairs. This study of interfacial charge transfer and transport dynamics provides a reference significance for the design and development of highly active carbon-based composite photocatalysts.

Selective Valorization of Glycerol to Formic Acid on a BiVO4 Photoanode through NiFe Phenolic Networks.

The integration of the glycerol oxidation reaction (GOR) with the hydrogen evolution reaction in photoelectrochemical (PEC) cells is a desirable alternative to PEC water splitting since a large quantity of glycerol is easily accessible as the byproduct from the biodiesel industry. However, the PEC valorization of glycerol to the value-added products suffers from low Faradaic efficiency and selectivity, especially in acidic conditions, which is beneficial for hydrogen production. Herein, by loading bismuth vanadate (BVO) with a robust catalyst composed of phenolic ligands (tannic acid) coordinated with Ni and Fe ions (TANF), we demonstrate a modified BVO/TANF photoanode for the GOR with a remarkable Faradaic efficiency of over 94% to value-added molecules in a 0.1 M Na2SO4/H2SO4 (pH = 2) electrolyte. The BVO/TANF photoanode achieved a high photocurrent of 5.26 mA·cm-2 at 1.23 V versus reversible hydrogen electrode under 100 mW/cm2 white light irradiation for formic acid production with 85% selectivity, equivalent to 573 mmol/(m2·h). Transient photocurrent and transient photovoltage techniques and electrochemical impedance spectroscopy along with intensity-modulated photocurrent spectroscopy indicated that the TANF catalyst could accelerate hole transfer kinetics and suppress charge recombination. Comprehensive mechanistic investigations reveal that the GOR is initiated by the photogenerated holes of BVO, while the high selectivity to formic acid is attributed to the selective adsorption of primary hydroxyl groups in glycerol on TANF. This study provides a promising avenue for highly efficient and selective formic acid generation from biomass in acid media via PEC cells.

Charge reaction kinetics on TiO2 nanotubes under photoelectrochemical water oxidation condition

Artificial photosynthesis relies on the photoreactions between the chemical molecules and photogenerated charges. The oxygen evolution reactions are regarded as the most difficult procedure, because they are multiple-step reactions with multiple charges transfer. Notwithstanding the extensive study on photoelectrochemical and photochemical water oxidation, understanding how the surface charge reacts with the chemical molecules is limited, especially the reaction kinetics of the surface charges. We prepared anatase TiO2 nanotubes array and dense polycrystalline TiO2 films on Ti substrates, where the photocurrent increased about 4–4.5 times on nanotubes compared with the dense films. Moreover, the surface charge reaction rate and the accumulated charge densities were analyzed by transient surface photovoltage and transient photocurrent techniques. It was found a 2nd-order charge reaction behavior could be found when the charge density was high, differently, a 1st-order charge reaction could be observed when the charge density was low. The nanotubes structures would in situ grow from the Ti substrates, which simultaneously enhanced charge separation and transfer kinetics to improve the PEC performance.

Influence of the Electron Selective Contact on the Interfacial Recombination in Fresh and Aged Perovskite Solar Cells

In this work, we have used TiO2 and SnO2 layers as electron selective contact (ESC) in n-i-p perovskite solar cells configuration. To study and compare the ion migration kinetics of these ESC, CsFAMAPbIBr and MAPbI3-based devices were fabricated and characterised in fresh (1 day) and aged (28 days) conditions. Depending on the ESC and perovskite composition, devices reveal a different progression over time in terms of hysteresis and performance. Using transient photovoltage (TPV) and transient photocurrent (TPC) techniques, we studied the kinetics of carrier extraction and recombination, which showed that aged devices present slower recombination kinetics compared to their fresh counterparts, revealing a positive effect of the aging process. Finally, transient of the transient, derived from the TPV technique, discloses that TiO2 accumulates more charges in the ESC/perovskite interface compared to SnO2 and that the ion migration kinetics are directly related to the perovskite composition.

Nongeminate Recombination in All-Polymer Solar Cells with Different Crystallinities

One of the most challenging issues facing the organic photovoltaic community is to realize a high fill factor (FF) even with thick active layers. This is because the thick active layer is beneficial for photon absorption but makes charge collection difficult, which is primarily restricted by nongeminate recombination in solar cells. In this work, we have studied nongeminate recombination in four kinds of polymer solar cells based on blends of donor-conjugated polymers with different crystallinities and acceptor-conjugated polymers with a naphthalene diimide unit by using transient photovoltage and photocurrent techniques. As a result, we find that nongeminate recombination is considerably suppressed with an increasing degree of crystallinity of donor polymers, leading to a high FF of more than 0.6 even with an active layer thickness of 300 nm. The origin of such a phenomenon is further discussed in terms of variations in the states of mixed phases with a cascaded energy structure between crystalline domains and amorphous domains evaluated by conductive atomic force microscopy.

Analysis of the Oxygen Passivation Effects on MAPbI3 and MAPbBr3 in Fresh and Aged Solar Cells by the Transient Photovoltage Technique.

Previous studies have revealed that for some perovskite compositions, power conversion efficiencies (PCEs) improved after storing the devices in different ambient conditions. With the aim of better understanding such improvements, we focus our attention on the carrier/ionic dynamic kinetics of fresh and aged PSCs with different perovskite compositions (MAPbI3 and MAPbBr3 ) and using spiro-OMeTAD as HTM. For that, we use transient photovoltage (TPV), a technique used to analyse the different recombination kinetics at equilibrium and at different illumination times. We observe that the aging treatment causes significant changes on the kinetics behaviour for bromide-based devices, resulting in a positive influence on the cell performance (from 3.5 % to 6.1 % PCE, in reverse scan). However, the kinetics for those iodide-based perovskite solar cells remains unchangeable (from 16.3 % to 15.0 % PCE, in reverse scan).

Carbon dots/Bi2WO6 composite with compensatory photo-electronic effect for overall water photo-splitting at normal pressure

Overall water photo-splitting is a prospective ideal pathway to produce ultra-clean H2 energy by semiconductors. However, the band structure of many semiconductors cannot satisfy the requirement of H2 and O2 production at the same time. Herein, we illustrate that carbon dots (CDs)/Bi2WO6 photocatalyst with compensatory photo-electronic effect has enhanced activity for overall water photo-splitting without any sacrificial agent. In this complex photocatalytic system, the photo-potential provided by CDs makes the CDs/Bi2WO6 (C-BWO) composite could satisfy the band structure conditions for overall water photo-splitting. The C-BWO composite (3 wt% CDs content) exhibits optimized hydrogen evolution (oxygen evolution) of 0.28 μmol/h (0.12 μmol/h) with an approximate 2:1 (H2:O2) stoichiometry at normal pressure. We further employed the in-situ transient photovoltage (TPV) technique to study the photoelectron extraction and the interface charge transfer kinetics of this composite catalyst.

Ultrathin nanoflake-assembled hierarchical BiOBr microflower with highly exposed {001} facets for efficient photocatalytic degradation of gaseous ortho-dichlorobenzene

A series of BiOBr catalysts with different microstructures were synthesized by tuning the solvothermal conditions. Assisted by the surface photovoltage, transient photovoltage and time-resolved photoluminescence techniques, the photoexcited charge carriers separation and transfer dynamics were comparatively investigated. The results indicated that the hierarchical BiOBr microflower (BiOBr MF) consisting of well-organized ultrathin nanoflakes were most efficient in promoting charge carrier separation and migration, resulting from the nearly 100 % {001} facets exposed with more oxygen defects. Furthermore, the photocatalytic performance was estimated through the degradation of gaseous ortho-dichlorobenzene (o-DCB) under visible light irradiation, and the BiOBr MF exhibited superior photoactivity. The enhanced mechanism underlying the charge transfer was disclosed by the energy band structures and the reactive oxygen species which were examined by room temperature electron spin resonance. In addition, the catalytic oxidation process of o-DCB over the BiOBr MF and the corresponding surface intermediates was revealed by in situ FTIR spectroscopy.

Photo-Induced Charge Transfer on Pt/Bi₂WO6 Composite Photocatalysts.

Pt/Bi₂WO6 composite photocatalysts were prepared by a facile photoreduction method. Pt nanoparticles with an average size of 5-8 nm were successfully deposited on the surface of Bi₂WO6 microspheres and the photocatalytic activity of Bi₂WO6 was greatly improved by Pt nanoparticles. The photo-induced charge transfer properties of samples were studied by means of surface photovoltage (SPV) and transient photovoltage (TPV) techniques, giving an insight into the intrinsic reasons of the improvement in photocatalytic activity. The SPV and TPV results revealed that the deposited Pt nanoparticles could trap photo-induced electrons and then largely enhance the separation efficiency of photo-induced charge carriers.

Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their 'working dynamic range' from which the recombination kinetics in thin-film solar cells can be determined.

Interfacial recombination kinetics in aged perovskite solar cells measured using transient photovoltage techniques.

The reduction of interfacial charge recombination kinetics in perovskite solar cells is key to increase device photovoltaic efficiencies. Thus, it is necessary to fully understand which are the major carrier losses and, thereafter, how they can be minimized. Transient Photovoltage (TPV) has been widely used to study carrier recombination in solar cells under operando conditions. Interestingly, a novel negative transient deflection appears in perovskite solar cells when carrying out TPV measurements and it has been related to the ionic accumulation at the perovskite interfaces, which is a process that requires great attention to fully understand how perovskite solar cells work. Herein, we moved one step further and continuously monitored the evolution of the negative trace with the aging of the perovskite solar cell. Importantly, we demonstrated that the negative signal changes with aging of the solar cells and such a change can be directly related to the enhancement of the open circuit voltage and fill factor of the devices and thus the solar cell efficiency. We postulate that this increase in efficiency is due to better/faster ion redistribution within the perovskite material.

Combining the post synthesis ligand-assisted technique and SILAR method to assemble the quantum dots onto the oxide semiconductor photoelectrodes and its applications for solar cells

The inverted type-I CdS-CdSe core-shell structure quantum dots (QDs) (denoted as CdS-CdSe QDs) applied in QDs sensitized solar cells (QDSCs) has attracted wide attention and researches due to its special photoelectric properties. But this post synthesis ligand-assisted (PSLA) technique need to assemble the previously prepared QDs onto the photoanode, leads to the low loadings of QDs and poor photoelectric performances of solar cells. Hence, we elaborately combine the PSLA technique and successive ionic layer absorption and reaction (SILAR) method to assemble the QDs onto the photoanode to improve the loadings of QDs and photoelectric properties of solar cells. The anatase TiO2 nanowires (NWs) electrode deposited with CdS-CdSe core-shell QDs and CdSe QDs (denoted as CdS-CdSe/CdSe QDs) was successfully prepared by combining the post synthesis ligand-assisted technique and SILAR method. The surface photovoltage (SPV), the transient photovoltage (TPV) and transient photocurrent (TPC) techniques were applied to systematically study the dynamics of charge separation and transfer. These measurements demonstrate that CdS-CdSe/CdSe QDs sensitized TiO2 NWs photoelectrodes exhibits favorable separation efficiency due to the increase of sunlight utilization. With the polysulfide electrolyte, the maximum photoelectric conversion efficiency (PCE) with the enhanced short circuit current (Jsc) of 17.76 mA/cm−2 for the CdS-CdSe/CdSe QDs and the Jsc of 11.4 mA/cm−2 for pure CdS-CdSe QDs were obtained under illumination at 100 mW/cm2. To enhance the PCE, the TiO2 NWs was optimized by N-doped and the optimized PCE of 3.13% with 18.22 mA/cm2 of Jsc was achieved for the QDSCs deposited with CdS-CdSe/CdSe QDs.

Distinguishing limits on the fill factor in organic solar cells processed from different solvents: Charge recombination kinetics vs. charge extraction

Abstract We use transient photovoltage (TPV) and transient photocurrent (TPC) techniques to understand charge recombination kinetics in a highly efficient organic solar cells (OSCs) system based on a blend of benzodithiophene terthiophene rhodanine (BTR) and [6,6]-phenyl C71-butyric acid methylester (PC71BM), processing different solvents. The OSCs fabricated from mixed solvents show the highest PCE of 10.2%, mainly due to an improved FF of 75.4% as compared to the devices from pure organic solvent, while JSC and VOC remain unchanged. Despite of difference in FF, non-geminate recombination is found to be the dominant photocurrent loss mechanism in all of the devices, regardless of processing solvents. Introducing a figure of merit, which is ratio between charge extraction rate and recombination rate by using the recombination kinetics data, we are able to correlate the competition between them to the fill factor of the devices.

Charge-carrier relaxation in sonochemically fabricated dendronized CaSiO3–SiO2–Si nanoheterostructures

We present systematic studies of charge-carrier relaxation processes in sonochemically nanostructured silicon wafers. Impedance spectroscopy and transient photovoltage techniques are employed. It is found that interface potential in Si wafers remarkably increases upon their exposure to sonochemical treatments in Ca-rich environments. In contrast, the density of fast interface electron states remains almost unchanged. It is found that the initial photovoltage decay, taken before ultrasonic treatments, exhibits the involvement of shorter- and longer time recombination and trapping centers. The decay speeds up remarkably due to cavitation treatments, which is accompanied by a substantial quenching of the photovoltage magnitude. It is also found that, before the treatments, the photovoltage magnitude is markedly non-uniform over the wafer surface, implying the existence of distributed sites affecting distribution of photoexcited carriers. The treatments cause an overall broadening of the photovoltage distribution. Furthermore, impedance measurements monitor the progress in surface structuring relevant to several relaxation processes. We believe that sonochemical nanostructuring of silicon wafers with dendronized CaSiO3 may enable new promising avenue towards low-cost solar energy efficiency multilayered solar cell device structures.

Impact of Electrodes on Recombination in Bulk Heterojunction Organic Solar Cells.

In recent years, the efficiency of organic solar cells (OSCs) has increased to more than 13%, although different barriers are on the way for reaching higher efficiencies. One crucial barrier is the recombination of charge carriers, which can either occur as the bulk recombination of photogenerated charges or the recombination of photogenerated charges and electrodic induced charges (EICs). This work studies the impact of EICs on the recombination lifetime in OSCs. To this end, the net recombination lifetime of photogenerated charge carriers in the presence of EICs is measured by means of conventional and newly developed transient photovoltage techniques. Moreover, a new approach has been introduced to exclusively measure the bulk recombination lifetime, i.e., in the absence of EICs; this approach was conducted by depositing transparent insulating layers on both sides of the OSC active layer. An examination of these approaches on OSCs with different active layer materials, thicknesses, and varying light intensities determined that the EICs can only reduce the recombination lifetime of the photogenerated charges in OSCs with very weak recombination strength. This work supports that for OSCs with highly reduced recombination strength, eliminating the recombination of photogenerated charges and EICs is critical for achieving better performance. Therefore, the use of a proper blocking layer suppresses EIC recombination in systems with very weak recombination.

Facile synthesis of bismuth oxyhalide nanosheet films with distinct conduction type and photo-induced charge carrier behavior

A modified successive ionic layer adsorption and reaction (SILAR) method was developed to fabricate 2D ordered BiOX (X = CI, Br, I) nanosheet array films on FTO substrates at room temperature. The formation of BiOX films were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), UV-vis absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS). The semiconductor surface states determine the type of semiconductor. Although BiOCI, BiOBr and BiOI belong to the bismuth oxyhalide semiconductor family and possess similar crystal and electronic structures, they show different conductivity types due to their respective surface states. Mott-Schottky curve results demonstrate that the BiOCl and BiOI nanosheet arrays display n-type semiconductor properties, while the BiOBr films exhibit p-type semiconductor properties. Assisted by surface photovoltage (SPV) and transient photovoltage (TPV) techniques, the photoinduced charge transfer dynamics on the surface/interface of the BiOX/FTO nanosheet films were systematically and comparatively investigated. As revealed by the results, both the separation and transfer dynamics of the photo-induced carrier are influenced by film thickness.