Transient Photovoltage Measurements Research Articles

Influence of photoanode pre-heating treatment temperatures prior to dye immersion process on dye-sensitized solar cells photovoltaic performances

Several efforts of photoanode treatment, such as surface modification, doping introduction, defect engineering, and heating treatment, have been developed to improve the photovoltaic performances of dye-sensitized solar cells (DSSC). Amid these options, alternative routes that require less demanding conditions are highly sought. Here we introduce an effective and convenient, non-vacuum low-temperature procedure to enhance the DSSC’s photovoltaic performances by photoanode pre-heating treatment prior to the dye immersion process. The experimental result showed that pre-heating treatment at 120 °C improves the efficiency and short-circuit current by about 28.4% and 19.1%, respectively. The conducted transient photovoltage measurements revealed that the increase of pre-heating temperatures leads to a longer electron lifetime and higher charge collection efficiency.

An ultrahigh hydrogen production rate of photocatalyst Fe2S2(CO)6/MoS2/CdS integrated mimic hydrogenase and capacitor for H2 evolution with longer lifetime of photoexcited electrons

The hydrogenase mimics provide an innovative strategy for photocatalytic hydrogen production; however, the efficiency mismatch between electron production and utilization results in low hydrogen production rate. Herein, we report the integration of non-noble metal cocatalyst MoS2 decorated with hydrogenase model complex Fe2S2(CO)6 and CdS QDs to assemble Fe2S2(CO)6/MoS2/CdS QDs, which exhibited an excellent hydrogen production rate of 10,836 μmol/g/h. This rate is 127.4, 9.1, and 1.6 times higher than that of CdS QDs, Fe2S2(CO)6/CdS QDs, and MoS2/CdS QDs, respectively. In comparison to CdS or MoS2/CdS, transient photovoltage (TPV) measurements revealed that the photovoltage response of Fe2S2(CO)6/MoS2/CdS increased numerically by 3.3 times; meanwhile, Fe2S2(CO)6/MoS2/CdS significantly extended the lifetime of photo-generated electrons. The excellent performance was attributed to the distinct capacitance effect and functional active site of Fe2S2(CO)6/MoS2/CdS, which boosted electron storage and redistribution at the interface to alleviate recombination losses and facilitate hydrogen production.

Charge Carrier Dynamics at Carbon/Perovskite Interface: Implications on Carbon‐Based HTM‐Free Solar Cell Stability

Carbon‐based hole transport material (HTM)‐free perovskite solar cells (CPSCs) are an innovative device architecture that mitigates inherent challenges associated with record‐breaking perovskite solar cells (PSCs), which rely on metal/HTMs, including instability, manufacturing intricacy, and elevated costs. The photovoltaic efficiency and stability of CPSCs are profoundly influenced by the charge carrier dynamics at the interfaces. Herein, the charge carrier dynamics at the carbon(C)–perovskite interface in CPSCs and its implications on photovoltaic performances and stability, an aspect that has received limited exploration thus far are probed, are investigated using transient surface photovoltage (Tr‐SPV) and transient photoluminescence measurements. The study reveals that the C‐electrode effectively acts as a selective barrier, impeding electrons while facilitating the extraction of holes at the C–perovskite interface. This selective blocking mechanism holds significant implications for improving the performance and stability of CPSCs over HTM‐free PSCs with gold(Au) electrodes. The stability of CPSCs is evaluated by measuring shelf life, maximum power point tracking, Tr‐SPV, and X‐Ray diffraction measurements. By delving into these pivotal aspects, this work aims to contribute to the advancement and understanding of CPSCs for sustainable and efficient energy conversion.

Dynamic properties of photogenerated charge in BiOBr/Bi2WO6/GO ternary composites and its application for organic pollutants degradation.

Carbon-based Materials have been extensively researched for their prospect in the fields of environment and energy, especially for graphene oxide (GO). In this work, a novel sodium dodecyl sulfate (SDS)-assisted synthesis of BiOBr/Bi2WO6/GO ternary composite has been synthesized successfully by a handy hydrothermal method. Photoluminescence, Photocurrent, Electrochemical Impedance Spectroscopy, surface photovoltage and transient photovoltage measurements illustrate that construction of p-n BiOBr/Bi2WO6 heterojunction leads to the obviously enhancement of charge separation efficiency, and the photogenerated electrons trapped by GO can effectively inhibit the recombination process of photogenerated charge, resulting in the improvement of charge separation efficiency and the longer lifetime of photogenerated carriers for BiOBr/Bi2WO6/GO. The characterization of structure and morphology indicate that role of GO can also improve the visible light absorption range, and the SDS-assisted synthesis can reduce the size of particle in the composite and enhances the specific surface area of the composite by regulating the particle size and agglomeration. Under optimal conditions, BiOBr/Bi2WO6/GO (SDS) has the outstanding photocatalytic degradation performance and the degradation rate constants for oxytetracycline, tetracycline hydrochloride, methylene blue and rhodamine are 0.056, 0.057, 0.103 and 0.414min-1, respectively. Notably, the degradation rate constants obtained by BiOBr/Bi2WO6/GO (SDS) are more ten times higher than that of pure BiOBr and Bi2WO6. The possible mechanism of photocatalytic degradation was suggested for BiOBr/Bi2WO6/GO based on the dynamic properties of photogenerated charge and reactive oxidation species results. Surprisingly, the recyclability of the BiOBr/Bi2WO6/GO (SDS) composite obtained from the cyclic experiments has laid a foundation for the study of efficient and stable photocatalysts.

Quantifying Charge Extraction and Recombination Using the Rise and Decay of the Transient Photovoltage of Perovskite Solar Cells.

The extraction of photogenerated charge carriers and the generation of a photovoltage belong to the fundamental functionalities of any solar cell. These processes happen not instantaneously but rather come with finite time constants, e.g., a time constant related to the rise of the externally measured open circuit voltage following a short light pulse. Herein, a new method to analyze transient photovoltage measurements at different bias light intensities combining rise and decay times of the photovoltage. The approach uses a linearized version of a system of two coupled differential equations that are solved analytically by determining the eigenvalues of a 2×2 matrix. By comparison between the eigenvalues and the measured rise and decay times during a transient photovoltage measurement, the rates of carrier recombination and extraction as a function of bias voltage are determined, and establish a simple link between their ratio and the efficiency losses in the perovskite solar cell.

Surface passivation of sequentially deposited perovskite solar cells by octylammonium spacer cations

Effective passivation of defects is an important step toward achieving highly efficient and stable Perovskite Solar Cells (PSCs). In this work, we introduce the incorporation of two different octylammonium based spacer cations as 2D perovskite passivation layers, namely Octylammonium Bromide (OABr) and octylammonium iodide. PSCs with OABr as a 2D passivation layer demonstrated an enhanced Power Conversion Efficiency (PCE) of 21.40% (the control device has a PCE of 20.26%), resulting in a higher open circuit voltage of 40 mV. The 2D perovskite passivation layers lead to a smoother interface and a better contact with the hole transport layer, while transient photoluminescence and transient photovoltage measurements indicated reduced non-radiative recombination. Unencapsulated devices retained almost 90% of their initial PCE after 500 h of exposure under high ambient humidity conditions, confirming that the surface passivation treatment has led to improved device stability.

Thermal annealing effects on graphene/n-Si Schottky junction solar cell: removal of PMMA residues

Thermal annealing is one of the most effective ways to improve the efficiency of a graphene/n-Si Schottky junction solar cell. Here, its underlying mechanism has been investigated by comparative studies in terms of the removal of polymethyl methacrylate (PMMA) residues, using the J–V characteristics and transient photocurrent and photovoltage measurements. The obtained results have revealed that there are trap states that originate from the PMMA residues and cause the large photocurrent leakage as the intensity of the incident light increases. It is also found that the PMMA residues accelerate deterioration and rapidly invalidate hole doping effects. Such undesirable PMMA residues were effectively removed by the thermal annealing treatments, serving to reduce the photocurrent leakage and increase the stability.

Dual-suppression of bulk and surface charges recombination for hematite photoanode

Surface treatments are considered as one of the most effective strategies to overcome charges recombination of photoanodes. Herein, the E–Ti–Fe2O3 photoanode was prepared by hydrothermal, electrodepositon and subsequent KOH treatment methods on the surface of Ti–Fe2O3 photoanode. The photocurrent density of E–Ti–Fe2O3 reached 3.1 mA/cm2 at 1.23 V versus reversible hydrogen electrode (vs. RHE), which was about 6.5 times than that of Ti–Fe2O3 (0.48 mA/cm2 at 1.23 V vs. RHE). The photocurrent density of E–Ti–Fe2O3 was further improved by the modification of CoPi (4.1 mA/cm2 at 1.23 V vs. RHE). The excellent PEC activity can be attributed to the dual-suppression of bulk and surface charges recombination, which was proved by work function (WF) measurements, transient photovoltage (TPV) measurements and photoelectrochemistry (PEC) tests. This research provided a deep insight into the bulk and surface charge recombination of the photogenerated charges for photoanode.

Super-Branched PdCu Alloy for Efficiently Converting Carbon Dioxide to Carbon Monoxide.

The alloying of noble metals with Cu is one of the most effective strategies for improving catalytic performance and reducing cost in electrocatalytic carbon dioxide reduction reactions (CO2RR). Previous works usually focused on the influence of morphology and composition on the catalytic activity, but lacked the study of the valence state ratio of metals and the electron transfer behavior on alloys. In this work, PdCu-2 alloy (Pd/Cu molar ratio is 1:2) was obtained by a simple one-step solvothermal method, which can effectively convert CO2 to CO with a maximum Faradaic efficiency (FE) of 85% at -0.9 V (vs. RHE). Then, the effect of the chemical state of Pd and Cu on the catalytic performance was investigated. The X-ray photoelectron spectroscopy (XPS) shows that the binding energy of Pd in PdCu alloy has a negative shift, which has affected the adsorption of key intermediates. When the proportion of oxidized state and zero-valent metal in the alloy is about 1:2, the PdCu alloy shows the best catalytic activity. In addition, the transient photovoltage (TPV) measurements further demonstrate that due to the introduction of Cu, the electron transfer rate of PdCu-2 becomes the slowest, which helps the accumulation of electrons on PdCu-2 and leads to the improvement of catalytic performance for electrocatalytic CO2RR. This work can provide more insights into the alloy catalysts of electrocatalytic CO2RR.

2D/2D covalent organic framework/CdS Z-scheme heterojunction for enhanced photocatalytic H2 evolution: Insights into interfacial charge transfer mechanism

• The 2D/2D COF/CdS Z-scheme heterojunction was first reported. • The Z-scheme charge transfer pathways were first identified by Femtosecond transient absorption. • The excellent photocatalytic hydrogen evolution was achieved. • The improved charge separation and hydrogen-evolution kinetics were confirmed. Covalent organic frameworks (COFs) with high crystallinity and flexible designability have been considered as promising candidates for photocatalytic hydrogen evolution. However, the existence of unpropitious exciton effects in COFs leads to poor charge separation, and thus results in low photocatalytic efficiency. Herein, to improve the photoelectron migration efficiency, we designed a 2D/2D organic/inorganic direct Z-scheme COF-based heterojunction (TpTAP/CdS), by the in-situ growing of CdS nanosheets on the COF copolymerized via 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAP) and 1,3,5-triformylphloroglucinol (TP). The femtosecond transient absorption (fs-TAS) decay kinetics of TpTAP-COF and TpTAP/CdS further reveal the processes of shallow electron trapping and the recombination of the free photogenerated electron-hole pairs. In particular, the transient absorption traces for TpTAP-COF and TpTAP/CdS normalized to the photoinduced absorption peak can effectively verify the Z-scheme charge transfer between TpTAP-COF and CdS, which could enhance the charge mobility and separation, thus reducing the photocorrosion of CdS. Additionally, ultraviolet photoelectron spectroscopy (UPS), in-situ X-ray photoelectron spectroscopy (XPS), transient photovoltage measurements, and Electron spin resonance spectroscopy (ESR) further confirm the establishment of the internal electric field (IEF). This work demonstrates the important role of COFs in the construction of 2D/2D organic/inorganic direct Z-scheme heterojunctions and offers a new avenue to explain the criticality of dynamics of the photogenerated carriers for the construction of Z-scheme heterojunctions.

Accelerating defect analysis of solar cells via machine learning of the modulated transient photovoltage

Fast and non-destructive analysis of material defect is a crucial demand for semiconductor devices. Herein, we are devoted to exploring a solar-cell defect analysis method based on machine learning of the modulated transient photovoltage (m-TPV) measurement. The perturbation photovoltage generation and decay mechanism of the solar cell is firstly clarified for this study. High-throughput electrical transient simulations are further carried out to establish a database containing millions of m-TPV curves. This database is subsequently used to train an artificial neural network to correlate the m-TPV and defect properties of the perovskite solar cell. A Back Propagation neural network has been screened out and applied to provide a multiple parameter defect analysis of the cell. This analysis reveals that in a practical solar cell, compared to the defect density the charge capturing cross-section plays a more critical role in influencing the charge recombination properties. We believe this defect analysis approach will play a more important and diverse role for solar cell studies.

Unraveling photocatalytic electron transfer mechanism in polyoxometalate-encapsulated metal-organic frameworks for high-efficient CO2 reduction reaction

Unraveling the fundamental electron transfer mechanism is vital to deeply understand the heterogeneous photocatalysis process so as to develop new efficient photocatalysts. Herein, we present a deep investigation on the electron-transfer mechanism in heterogeneous photocatalytic CO2 reduction reaction by adopting a new type of structurally clear polyoxometalate-encapsulated metal-organic frameworks {Hn[Cd6L6Cl7]4[POM]8} (L=1,4-di(4H-1,2,4-triazol-4-yl)benzene, POM={SiW12}, {PW12} and {PMo12}, abbr.POM@CdMOF) as model photocatalysts. Transient photovoltage measurements indicate that various POM guests in photoactive CdMOF hosts show different electron transfer behaviors and charge separation pathways, inducing different photocatalytic activities. Thereinto, SiW12@CdMOF and PW12@CdMOF exhibit high photocatalytic CO2 reduction activities with CO yields of 4.35 mmolCO·molCd−1·h−1 and 3.60 mmolCO·molCd−1·h−1 comparably to most reported photocatalysts, whereas the performance of PMo12@CdMOF is poor. The in-situ transient photovoltage and DFT calculations revealed the key regulation role of different POM units on the interfacial electron transfer and the adsorption behavior of the important intermediates, thus describing a feasible route to efficiently perform the photocatalytic CO2 reduction. This work may provide a new and significant guideline for exploring high-performance photocatalysts.

Composition Engineering of Amorphous Nickel Boride Nanoarchitectures Enabling Highly Efficient Electrosynthesis of Hydrogen Peroxide.

Developing advanced electrocatalysts with exceptional two electron (2e- ) selectivity, activity, and stability is crucial for driving the oxygen reduction reaction (ORR) to produce hydrogen peroxide (H2 O2 ). Herein, a composition engineering strategy is proposed to flexibly regulate the intrinsic activity of amorphous nickel boride nanoarchitectures for efficient 2e- ORR by oriented reduction of Ni2+ with different amounts of BH4 - . Among borides, the amorphous NiB2 delivers the 2e- selectivity close to 99% at 0.4V and over 93% in a wide potential range, together with a negligible activity decay under prolonged time. Notably, an ultrahigh H2 O2 production rate of 4.753mol gcat -1 h-1 is achieved upon assembling NiB2 in the practical gas diffusion electrode. The combination of X-ray absorption and in situ Raman spectroscopy, as well as transient photovoltage measurements with density functional theory, unequivocally reveal that the atomic ratio between Ni and B induces the local electronic structure diversity, allowing optimization of the adsorption energy of Ni toward *OOH and reducing of the interfacial charge-transfer kinetics to preserve the OO bond.

Photoelectric Properties of Planar and Mesoporous Structured Perovskite Solar Cells.

The high efficiency of perovskite solar cells strongly depends on the quality of perovskite films and carrier extraction layers. Here, we present the results of an investigation of the photoelectric properties of solar cells based on perovskite films grown on compact and mesoporous titanium dioxide layers. Kinetics of charge carrier transport and their extraction in triple-cation perovskite solar cells were studied by using transient photovoltage and time-resolved photoluminescence decay measurements. X-ray diffraction analysis revealed that the crystallinity of the perovskite films grown on mesoporous titanium dioxide is better compared to the films grown on compact TiO2. Mesoporous structured perovskite solar cells are found to have higher power conversion efficiency mainly due to enlarged perovskite/mesoporous -TiO2 interfacial area and better crystallinity of their perovskite films.

Extension of in-gap electronic-state spectrum extraction method based on transient photo-voltage measurement

Herein, a general extraction technique for trap state density (DOST) of perovskite solar cells (PSCs) based on the transient photo-voltage (TPV) experimental data is presented, inspired by the extraction theory based on the Gauss type DOST distribution model presented by Lin. The method for extraction of exponential (Gauss) type DOST distribution is effective only when the TPV result satisfies the linear (quadratic function) relation, which is the one (two) order special case of the method reported in this paper. Our extended method is effective for arbitrary TPV result, which enhances the accuracy of DOST distribution extraction.

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

18.55% Efficiency Polymer Solar Cells Based on a Small Molecule Acceptor with Alkylthienyl Outer Side Chains and a Low-Cost Polymer Donor PTQ10

Enhancing the photovoltaic properties of SnS-Based solar cells by crystallographic orientation engineering

Tin monosulfide (SnS) is a promising light-harvesting material for solar cell applications, owing to its potential for large-scale production, cost-effectiveness, eco-friendly source materials, and long-term stability. However, SnS crystallizes in an orthorhombic structure, which results in a highly anisotropic charge transport behavior. Tailoring the crystallographic orientation of the SnS absorber layer plays a critical role in the enhancement of the transfer of charge carriers and the power conversion efficiency (PCE). By controlling the substrate tilting angle and temperature ramp rate in vapor transport deposition, the crystal growth orientation was tuned to a preferred direction which significantly suppressed the unfavorable (040) crystallographic plane. Through the combination of these two approaches, the PCE could be increased from 0.11% to 2%. The effect of the tilting angle was numerically simulated to investigate its role in controlling the film uniformity and directing the film growth. In addition, the correlation between the texture coefficient of the (040) plane and the charge transport properties was determined by a combination of analytical methods such as device performance studies, electrochemical impedance spectroscopy, along with transient photovoltage, space-charge-limited current, and dark current measurements. These techniques were blended together to prove that the marked improvement in PCE can be ascribed to a reduced charge recombination (in both SnS bulk and interfaces) and an enhanced hole mobility.

Wavelength dependent photoinduced charge carrier dynamics of heterojunction materials: The case of CuO/ZnO

Heterojunction materials composed of narrow and wide band-gap semiconductors possess different photoelectric responses under different wavelength incident light, and show distinct photogenerated charge carrier dynamics. In this work, taking the hydrothermal method fabricated CuO nanosheets / ZnO nanorods heterojunction materials as example, the wavelength dependent photoinduced charge carrier dynamics in this heterojunction material have been carefully studied. Transient surface photovoltage (TSPV) measurements reveal that the photogenerated holes gather at the surface of the fabricated CuO / ZnO with 355 nm light illumination, but photogenerated electrons gather at the surface under 532 nm light. Photocatalytic degradation experiments suggest that CuO / ZnO heterojunction material has better performance under ultraviolet light than that under visible light.

Solution-Processed, Inverted AgBiS2 Nanocrystal Solar Cells.

AgBiS2 nanocrystals are a promising nontoxic alternative to PbS, CsPbI3, and CdS quantum dots for solution-fabricated nanocrystal photovoltaics. In this work, we fabricated the first inverted (p-i-n) structure AgBiS2 nanocrystal solar cells. We selected spray-coated NiO as the hole-transporting material and used PCBM/BCP as the electron-transporting material. Combining transient photocurrent and photovoltage measurements with femtosecond transient absorption spectroscopy, we investigated the charge collection process on metal oxide/AgBiS2 interfaces and demonstrated that the NiO/AgBiS2 NC junction in the p-i-n configuration is more efficient for charge carrier collection. The fabricated p-i-n solar cells exhibited a 4.3% power conversion efficiency (PCE), which was higher than that of conventional n-i-p solar cells fabricated using the same sample. Additionally, inverted devices showed an ultrahigh short-circuit current (JSC) over 20.7 mA cm-2 and 0.38 V open-circuit voltage (VOC), suggesting their potential for further improvements in efficiency and, eventually, for large-scale production.

Charge carrier dynamics and photovoltaic properties of near-infrared absorbing squaraine incorporated solution-processed additive-free PTB7:PCBM based ternary solar cells

The introduction of a third component vehemently modifies the morphology and charge carrier dynamics in the blend of a donor–acceptor pair, thereby affecting the photovoltaic properties of organic solar cells (OSCs). Combining steady-state, impedance, and transient spectroscopic measurements, photovoltaic properties of 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine (SQ) incorporated additive-free PTB7:PC70BM OSCs are assessed. As observed from steady-state measurements, concomitant enhancement in open-circuit voltage (VOC) and short-circuit current density (JSC) has caused a 33% enhancement in power conversion efficiency (PCE) with good reliability and reproducibility. On introducing 25 wt.% SQ to the OSCs, V OC has increased from 0.74 to 0.80 V, while J SC has improved from 11.3 to 13.9 mA cm−2 with an increment in exciton dissociation probability rate from 81.5% to 94.9%. However, the fill factor values show inconsistent marginal variations with SQ addition. Equivalent circuit modeling of bias-voltage dependent impedance spectra along with transient photovoltage measurements reveal an improvement in effective charge carrier lifetime for the SQ incorporated OSCs, in comparison to the binary device. The addition of SQ also ensures better charge transport and extraction, as evidenced by photo-CELIV and transient photocurrent analysis. Atomic force microscopy (AFM) images confirm the effective tuning of the morphology of the active layer when SQ is introduced into the binary blend, favoring efficient charge dissociation and transport. The possible operation mechanism of SQ incorporated ternary OSCs is proposed based on photoluminescence and AFM measurements. Moreover, the un-encapsulated OSC with 25 wt.% SQ has retained 91% of the initial PCE, while for the binary device the PCE declined to ∼75% of the initial value after 200 h of continuous 1 sun illumination from a white LED in ambient atmosphere.