Towards a Net-Zero Future: Key Breakthroughs and Challenges in Green Hydrogen Technology

The world is at a critical juncture in energy transition. To address climate change and reduce environmental pollution, finding clean, renewable energy sources has become an urgent task. The use of traditional fossil fuels not only brings serious environmental problems but also their limited resources compel us to develop sustainable alternatives. In this context, hydrogen, with its high energy density, water-only combustion products, storability, and transportability, is considered one of the most promising clean energy carriers of the 21st century. However, currently, over 95% of global hydrogen comes from fossil fuel reforming, a production process still accompanied by significant carbon emissions. Therefore, “green hydrogen” produced using renewable energy is the ideal pathway to achieve sustainable energy development. 

Among the various green hydrogen production technologies, using the inexhaustible solar energy to directly split water (H₂O) into hydrogen (H₂) and oxygen (O₂) is undoubtedly one of the most attractive solutions. Solar water splitting technologies, primarily including photocatalysis (PC) and photoelectrochemical (PEC), offer the core advantage of directly converting solar energy into chemical energy stored in hydrogen molecules, with only sunlight and water as raw materials, providing a clean, pollution-free process.

Compared to the two-step method of “solar photovoltaic power generation + water electrolysis” (PV-EC), PEC technology integrates light absorption and electrochemical catalysis in a single device, theoretically offering a more streamlined system design, lower energy losses, and more effective utilization of heat energy from sunlight. Particularly in PEC systems, the spatial separation of anodes and cathodes (or photoanodes and photocathodes) enables hydrogen and oxygen to be automatically collected separately, avoiding explosion risks and reverse reactions, a challenge faced by some distributed photocatalytic systems.

The concept of PEC water splitting was first proposed and verified by Fujishima and Honda in 1972, with a half-century of research history to date. Although scientists have made significant progress in material development and efficiency improvement, such as achieving solar-to-hydrogen (STH) conversion efficiencies approaching 20% using III-V semiconductors under laboratory conditions, the technology still faces severe challenges for large-scale commercial application. Material stability, quantum efficiency, production costs, and device scale-up are all key bottlenecks hindering its development. The dual promise of PEC technology—both generating energy and storing it in chemical form—gives it unique value in addressing the intermittency of renewable energy. However, the long development journey and obstacles yet to be overcome precisely illustrate the complexity of this technology, urgently requiring breakthroughs in fundamental materials science and precision measurement techniques.

Image source: Opportunities and Challenges of Industrialization of Hydrogen－The Photocatalyst effect (Fujishima et al., 1972)

Core Principles of Solar Water Splitting: The Journey from Photons to Hydrogen Molecules

The core of photoelectrochemical (PEC) water splitting is utilizing semiconductor materials to absorb solar energy and using photogenerated charge carriers to drive water’s oxidation-reduction reactions, ultimately producing hydrogen and oxygen.

A typical PEC cell consists of the following main components: at least one semiconductor photoelectrode (photoanode or photocathode, or both), a counter electrode (usually an inert conductor or another photoelectrode), an electrolyte solution for ion conduction, and a membrane for separating product gases (optional).

The working process can be broadly divided into the following three key steps:

Step One: Light Absorption

When photons with energy greater than or equal to the semiconductor’s band gap (E_g) strike the photoelectrode surface, they are absorbed by the material. To effectively utilize the solar spectrum, ideal semiconductor materials should have appropriate band gap values. From a thermodynamic perspective, the minimum energy required to split water is 1.23 eV, corresponding to a wavelength of approximately 1008 nm. However, considering the overpotential losses in reaction kinetics, effective water splitting typically requires a voltage of about 1.7-2.0 V. This means that a single semiconductor material’s band gap needs to be greater than this value to function without external bias, but too large a band gap would lead to insufficient utilization of the visible portion of the solar spectrum.

Step Two: Charge Carrier Generation & Separation

After photon absorption, electrons (e⁻) in the semiconductor’s valence band (VB) are excited to transition to the conduction band (CB), leaving positively charged holes (h⁺) in the valence band, forming electron-hole pairs. For these photogenerated carriers to drive water splitting reactions, they must effectively separate and transport to their respective reaction sites, avoiding recombination. This primarily depends on the space charge layer formed at the semiconductor-electrolyte interface, which generates an internal electric field. For n-type semiconductor photoanodes, the bands bend upward at the interface, and the internal electric field drives holes toward the semiconductor/electrolyte interface and electrons toward the bulk and external circuit. For p-type semiconductor photocathodes, the situation is reversed.

Step Three: Surface Electrochemical Reactions

After separation, the charge carriers reach the electrode surfaces and participate in water oxidation and reduction reactions:

• At the photoanode surface: Photogenerated holes have strong oxidizing properties, oxidizing water molecules to produce oxygen and protons (in acidic media) or hydroxide ions (in alkaline media). A typical reaction equation (in acidic media) is: 2H₂O + 4h⁺ → O₂ + 4H⁺ This is the oxygen evolution reaction (OER).
• At the photocathode or counter electrode surface: Electrons conducted through the external circuit have reducing properties, reducing protons (in acidic media) or water molecules (in neutral/alkaline media) to produce hydrogen. A typical reaction equation (in acidic media) is: 2H⁺ + 2e⁻ → H₂ This is the hydrogen evolution reaction (HER).

Image source: Perovskite Oxide-Based Electrodes for High-Performance Photoelectrochemical Water Splitting－PEC water splitting principles

Thermodynamic and Kinetic Requirements

For the entire water splitting reaction to proceed spontaneously (i.e., without external voltage bias), the semiconductor material’s band structure must meet specific thermodynamic requirements: the potential of its conduction band bottom must be more negative than the reduction potential of HER (E(H⁺/H₂)), while the potential of its valence band top must be more positive than the oxidation potential of OER (E(O₂/H₂O)).

In other words, the semiconductor’s band edges must “straddle” water’s redox potentials. However, satisfying thermodynamic conditions alone is insufficient. In actual reactions, both HER and OER need to overcome certain activation energies, or overpotentials, making the actual voltage required higher than 1.23V. To reduce overpotentials and increase reaction rates, co-catalysts are often loaded on the photoelectrode surface, such as noble metals (Pt for HER) or transition metal oxides/sulfides (for OER or HER).

PEC technology’s efficiency is fundamentally limited by the complex competition between photogenerated carrier generation, separation, transport, and surface reaction kinetics. Photogenerated electron-hole pairs are highly susceptible to loss through direct recombination or defect/surface state-assisted recombination before reaching reaction sites, releasing energy as heat or light. Unfortunately, carrier recombination typically occurs on nanosecond (10⁻⁹s) or even picosecond (10⁻¹²s) timescales, while surface electrochemical reaction rates are much slower, often on millisecond (10⁻³s) or longer timescales. This vast difference in timescales means that most photogenerated carriers recombine before having a chance to participate in reactions. Therefore, one core strategy for improving PEC efficiency is to accelerate charge separation, transport, and surface reaction rates through material design (such as nanostructuring, heterojunction construction) and loading efficient co-catalysts, while suppressing recombination processes.

Additionally, material selection itself involves inherent contradictions. Ideal solar absorption materials should have band gaps around 1.34 eV to maximize utilization of the solar spectrum, but as mentioned earlier, actual water splitting requires approximately 1.7-2.0 V voltage, and demands matching band positions. Typically, materials with smaller band gaps and strong light absorption (like Si) often have mismatched band positions or are unstable in electrolytes; while materials with suitable band positions and greater stability (like some wide-band-gap oxides such as TiO₂, WO₃) have poor utilization of solar light. This inherent trade-off between band gap, band position, and stability has forced researchers to explore more complex solutions, such as stacking two or more semiconductors with different band gaps to form tandem devices that absorb solar light over a wider spectral range and provide sufficiently high voltage, or applying external bias to assist the reaction. However, these strategies undoubtedly increase system complexity and cost.

Table 1: Brief Comparison of PEC and PC Water Splitting Technologies

Characteristic	Photoelectrochemical (PEC) Water Splitting	Photocatalytic (PC) Water Splitting
Basic Principle	Light absorption and electrochemical reactions occur at electrode/electrolyte interfaces, typically requiring external circuit connection	Semiconductor particles suspended or fixed on substrates, light absorption and catalytic reactions occur on the same particle surface
System Components	Photoelectrode, counter electrode, electrolyte, (membrane)	Photocatalyst powder/film, water
Charge Transport	Photogenerated charges transported through bulk and external circuit to reaction sites	Photogenerated charges transported within and on the surface of catalyst particles
Gas Separation	H₂ and O₂ produced at different electrodes, easy to separate	H₂ and O₂ produced on the same catalyst surface, easily mixed, difficult to separate with explosion risk
Efficiency Characterization	STH efficiency, photocurrent density (J-V), quantum efficiency (EQE/IPCE)	Hydrogen/oxygen production rate, quantum yield (QY)/apparent quantum efficiency (AQE)
Main Challenges	Electrode stability, efficiency, scale-up preparation	Gas separation, low quantum efficiency, catalyst recovery/stability
Typical Materials	III-V compounds, Si, Fe₂O₃, BiVO₄, WO₃, Cu₂O, etc.	TiO₂, CdS, g-C₃N₄, metal sulfides/oxides, etc.

Research Frontier Challenges: The Core Role of Precision Measurement in Breakthrough Efficiency, Stability, and Scale-up Bottlenecks

Despite the enormous potential of PEC water splitting technology, its path to commercialization still faces three core challenges: efficiency, stability, and scale-up costs. To effectively address these challenges, precise and reliable performance measurements, particularly IV curve and quantum efficiency (QE/IPCE) analysis, play a crucial role.

Efficiency

Solar-to-hydrogen conversion efficiency (STH) is the ultimate goal, but it still falls short of commercialization targets, especially for low-cost material systems. Low quantum efficiency (QE/IPCE) is one of the main bottlenecks limiting STH, primarily due to rapid recombination of photogenerated carriers and insufficient utilization of the solar spectrum by some materials.

Therefore, precise measurement of EQE/IPCE spectra is an important characterization technique for quantifying these photon-to-electron conversion losses, identifying wavelength ranges limiting efficiency, and evaluating materials’ intrinsic photoelectric conversion capabilities. Only through accurate QE analysis can researchers objectively evaluate the real effects of various efficiency enhancement strategies (such as nanostructuring, tandem devices, co-catalysts), and guide further material and device optimization.

Image source: A comprehensive machine learning strategy for designing high-performance photoanode catalysts-Figure 1

Stability

Photoelectrodes are susceptible to corrosion in harsh electrolyte environments and light conditions, leading to performance degradation. Long-term stable operation (thousands of hours) is a huge challenge. To assess materials’ true stability, long-term, continuous performance monitoring is essential. Periodically obtaining key parameter changes such as photocurrent density and onset potential through IV curves is a core means to quantify performance degradation rates, determine failure mechanisms, and evaluate the effectiveness of protection layers or new stable materials. Precise IV measurement data is the key basis for distinguishing real stability improvements from experimental contingencies. Meanwhile, there is often a trade-off between efficiency and stability, and reliable IV and QE data help researchers make informed decisions in seeking the optimal balance point.

Image source: Achieving 12.0% Solar-to-Hydrogen Efficiency with a Trimetallic-Layer-Protected and Catalyzed Silicon Photoanode Coupled with an Inexpensive Silicon Solar Cell－Fig. S8.

Scalability & Cost

Scaling up laboratory results to industrial scale faces numerous engineering challenges, such as uniformity in large-area fabrication, substrate resistance, reactor design, and the use of high-cost materials. In this process, accurate IV and QE characterization of large-area samples or modules is fundamental to verify scale-up process feasibility, evaluate performance uniformity, and compare different scale-up strategies. The lack of standardized, reliable large-area measurement techniques will seriously hinder the transition from laboratory to factory. Addressing the disconnect between “heavy materials, light engineering” urgently requires integrating precise measurements throughout the entire chain from material development to system integration, ensuring that laboratory optimizations can effectively translate into performance improvements in practical applications.

Addressing the challenges of efficiency, stability, and scalability in PEC water splitting cannot be separated from the “eyes” and “rulers” provided by precise measurements. Accurate IV and QE data are not only indicators for evaluating current performance but also key driving forces for guiding future research directions, verifying improvement effects, and ultimately pushing the technology toward practical applications.

Precision Metrology: The Key to Unlocking Water Splitting Performance – The Importance of IV and EQE Measurements

In PEC water splitting research, accurate, reliable, and standardized performance characterization is the cornerstone for evaluating material potential, guiding experimental directions, comparing different research results, and driving technological progress. The lack of unified testing standards and methods can make results from different laboratories or researchers difficult to compare or even contradictory, seriously hindering the healthy development of the field. Among numerous characterization methods, current-voltage (IV) characteristic curves and quantum efficiency (QE)/incident photon-to-current efficiency (IPCE) spectra are the most fundamental and important electrochemical and photoelectric performance measurement methods.

Current-Voltage (IV/JV) Measurement

IV curves describe the current response flowing through a PEC cell or photoelectrode as the applied voltage changes.

Measurement Method

Usually conducted in a three-electrode system, including the working electrode (WE, the photoelectrode sample), counter electrode (CE, typically platinum wire or graphite), and reference electrode (REF, such as Ag/AgCl or SCE), with voltage controlled and current recorded via potentiostat. Measurements need to be performed under precisely controlled light conditions (typically using calibrated solar simulators simulating AM1.5G spectrum) and in dark conditions, respectively. For evaluating complete device STH efficiency, a two-electrode system (photoelectrode directly connected to counter electrode) is used for measurement.

Key Parameters

A series of key performance parameters can be extracted from IV curves:

• Photocurrent Density (J_ph): The difference between current density under illumination and dark current density, directly reflecting the photoelectrode’s photoelectric activity.
• Onset Potential: The potential at which photocurrent begins to appear, related to the thermodynamic barrier and kinetic overpotential of water splitting reactions.
• Open-Circuit Voltage (V_oc)/Photovoltage: The voltage under zero current conditions, reflecting the degree of band bending in the semiconductor under illumination and the maximum voltage the device can generate (mainly used for PV-PEC or tandem devices).
• Short-Circuit Current Density (J_sc): The current density when the applied bias is zero, an important parameter for calculating STH efficiency (two-electrode system).
• Fill Factor (FF): FF=(J_max×V_max)/(J_sc×V_oc), describing the “squareness” of the IV curve, reflecting the device’s internal series resistance, parallel resistance, and recombination losses. Higher FF indicates better device quality.
• Energy Conversion Efficiency (PCE/STH): For two-electrode systems, STH efficiency can be calculated by dividing the maximum power point (P_max=J_max×V_max) by the incident light power (P_in): STH=(J_sc×V_oc×FF)/P_in (where V_oc refers to the open-circuit voltage in a two-electrode system, and the Faradaic efficiency for hydrogen production needs to be considered).

Importance

IV curves are the foundation for evaluating the overall performance of photoelectrodes or PEC devices. They not only provide macroscopic information such as efficiency and photoresponse activity but can also reveal device working characteristics, internal resistance, recombination losses, and other internal mechanisms, providing key basis for material optimization and device design.

Image source: Activating a Semiconductor–Liquid Junction via Laser-Derived Dual Interfacial Layers for Boosted Photoelectrochemical Water Splitting－Fig 3a

Quantum Efficiency (QE)/Incident Photon-to-Current Efficiency (IPCE) Measurement

QE or IPCE measures the efficiency at which a device converts incident photons to collected electrons at a specific wavelength.

Definition

• External Quantum Efficiency (EQE or IPCE): Defined as the ratio of collected electrons to photons incident on the device surface: EQE(λ)=number of collected electrons/number of incident photons×100%. EQE considers all optical losses (such as reflection, transmission, non-active layer absorption).
• Internal Quantum Efficiency (IQE): Defined as the ratio of collected electrons to photons absorbed by the device’s active layer. IQE excludes optical losses, better reflecting the material’s inherent photoelectric conversion ability and charge collection efficiency. Typically, EQE=IQE×(1−R), where R is reflectance.

Measurement Method

Basic equipment includes: broadband light source (typically xenon or halogen lamps), monochromator (for selecting monochromatic light of specific wavelengths), chopper (for modulating light signals to match lock-in amplifier detection), filters (for removing stray light and higher-order diffraction), PEC test cell, and calibrated photodetectors (such as silicon or InGaAs detectors) for precise measurement of incident monochromatic photon flux. By scanning the monochromator’s wavelength and using high-sensitivity current detection techniques such as lock-in amplifiers to record the corresponding photocurrent, the EQE spectrum as a function of wavelength can be obtained.

Parameter Information

• Spectral Response Range: Determining the wavelength range of solar light that the material can effectively utilize.
• Wavelength-Dependent Efficiency: Revealing the photoelectric conversion performance of the device at different wavelengths, identifying wavelength bands where performance bottlenecks exist.
• Band Gap Estimation: The optical band gap of materials can be estimated from the absorption edge of the EQE spectrum (where EQE begins to significantly decrease).
• Loss Mechanism Analysis: By measuring reflectance and calculating IQE, optical losses (reflection) and internal electrical losses (recombination, low charge collection efficiency) contributions to total efficiency can be distinguished.
• J_sc Calculation and Verification: By integrating the measured EQE spectrum with the standard solar spectrum (e.g., AM1.5G), the theoretical short-circuit current density J_sc(EQE) can be calculated. Comparing this value with J_sc(SS) obtained from IV measurements is an important means to verify measurement accuracy and device performance consistency.

Importance

EQE/IPCE measurements deeply reveal the fundamental physical processes in photoelectric conversion, including the relationship between light absorption, charge generation, separation, transport, and collection efficiency with wavelength. This microscopic information is important reference for understanding materials’ photoelectric properties, diagnosing performance limiting factors, and guiding material modification and device structure.

Table 2: Key Performance Indicators for Solar Water Splitting

In actual research, J_sc(EQE) calculated by integrating EQE is often found to differ from J_sc(SS) directly measured under solar simulators.

Ideally, the two should be equal, but various factors in experiments can lead to discrepancies. For example, spectral mismatch between solar simulators and the standard AM1.5G spectrum (even Class A allows ±25% deviation), temporal instability or spatial non-uniformity of light sources, nonlinear sample response to light intensity, slower charge transfer kinetics in PEC systems (leading to inaccurate steady-state current measurements), and sample degradation during measurement. Carefully analyzing the magnitude and cause of such mismatches is crucial for verifying experimental results’ reliability, diagnosing instrument issues, or revealing sample characteristics. Generally, a mismatch of less than 5% indicates that the measurement system and operation are relatively standardized and reliable.

Compared to solid-state photovoltaic (PV) devices, the measurement environment for PEC systems is inherently more complex. Samples immersed in liquid electrolytes face corrosive environments, solution resistance, and bubbles generated during reactions that may block light or cover electrode active sites. Additionally, ion diffusion and pH gradient formation at interfaces may affect electrode potential and reaction kinetics over time. Charge transfer processes at semiconductor/electrolyte interfaces may be slower, placing special demands on measurement time scales. These factors all present additional challenges for accurately and reproducibly obtaining IV and QE data, far exceeding conventional PV testing, thus requiring more specialized experimental apparatus and more rigorous operating procedures.

Beyond Standard Testing: “Non-Standard” Demands of Water-to-Hydrogen Research on Solar Simulators

Technical Challenges in PEC Water-to-Hydrogen Device Characterization

Complex Influences of Liquid-Phase Environment on Photoelectric Measurements

Compared to traditional photovoltaic devices, PEC water-to-hydrogen systems operate in liquid-phase electrolytes, leading to several unique measurement challenges:

• Complex Influences of Liquid-Phase Environment on Photoelectric Measurements Electrons conducted through the external circuit have reducing properties, reducing protons (in acidic media) or water molecules (in neutral/alkaline media) to produce hydrogen. A typical reaction equation (in acidic media) is: 2H⁺ + 2e⁻ → H₂ This is the hydrogen evolution reaction (HER).

Compared to traditional photovoltaic devices, PEC water-to-hydrogen systems operate in liquid-phase electrolytes, leading to several unique measurement challenges:

• Light Path Refraction and Scattering Issues: Incident light undergoes refraction when passing through gas-liquid interfaces, while electrolyte solutions may cause light scattering. These effects alter the actual light intensity distribution and spectral characteristics reaching the photoelectrode surface. According to Fresnel equations, at the water/air interface, approximately 2% of light flux is lost due to reflection, and this loss increases significantly as the incidence angle increases.
• Light Intensity Attenuation in Solution: Electrolyte solutions absorb light at specific wavelengths, particularly in the ultraviolet region, potentially causing non-linear light intensity attenuation. For example, a 1 cm thick water layer absorbs about 40% of 300 nm ultraviolet light but has almost no absorption for 800 nm infrared light.
• Dynamic Changes at Electrode/Electrolyte Interface: During measurement, phenomena such as bubble formation, local pH changes, surface adsorption/desorption may occur at the interface, leading to time-dependent fluctuations in photocurrent.
• Spatial Separation Between Reference and Working Electrodes: In three-electrode systems, the distance between reference and working electrodes leads to uncompensated resistance, especially in low-conductivity electrolytes, potentially significantly affecting voltage measurement accuracy.

Special Requirements for Vertical Immersion Electrode Configurations

Traditional photovoltaic measurement equipment is typically designed to illuminate flat-lying samples from above, however in PEC water-to-hydrogen research:

• Vertical Electrode Immersion Requirements: To allow bubbles to freely detach from electrode surfaces and maintain a stable liquid-solid interface, PEC electrodes typically need to be vertically immersed in electrolyte solution, requiring light sources that can provide horizontal illumination.
• Illumination Uniformity Challenges: For vertically placed samples, ensuring uniform illumination across the entire electrode surface becomes more difficult, especially for large-area electrodes.
• Back-side Electrode Illumination Needs: Some research designs, such as p-n photoelectrodes or structures with transparent conductive back contacts, may require illumination from the back side of the electrode, further increasing the complexity of light path design.
• Electrode Position Sensitivity: Due to light intensity attenuation with electrolyte depth, minor changes in electrode position may cause significant changes in actual received light intensity, affecting measurement reproducibility.

Solar simulators are the cornerstone of photovoltaic research, their importance needs no elaboration. However, when we turn our attention to solar water-to-hydrogen, especially testing PEC photoelectrodes or integrated PV-E microreactors, we find that traditional testing standards and equipment configurations designed for solid-state solar cells can no longer fully meet our needs.

Pain Point 1: New Challenges of Vertically Immersed Samples—Light Sources Must Provide Side Illumination

The greater challenge facing PEC experiments lies in sample placement. As mentioned earlier, to allow reaction-generated bubbles (H₂ and O₂) to smoothly detach from electrode surfaces, avoiding blockage of active sites affecting reaction continuity, photoelectrode samples are commonly placed vertically in reaction pools filled with electrolyte. This requires solar simulator light beams to illuminate samples from the side (horizontal direction). However, the vast majority of standard solar simulators on the market are designed for downward illumination, unable to meet this need. Finding a solar simulator that both meets IEC60904-9 A+ class spectral matching and A+ class temporal stability, while providing stable, calibrated side illumination capability, has been a major problem troubling PEC researchers. Completely different from standard PV testing where samples are placed horizontally with light sources illuminating vertically (top-down), the core scenario in PEC/PV-E experiments is: vertically or inclined immersion of photoelectrodes or integrated devices in specific electrolyte reaction pools. These reaction pools not only contain electrolytes but also need to accommodate counter electrodes, reference electrodes, and leave channels for gas escape and optical windows. This means traditional top-illumination solar simulators become extremely awkward:

• Light Path Blockage and Complexity: If forced to use top-illumination light sources, light must pass through the space above the electrolyte, electrode leads, bubbles, etc., causing serious light intensity loss, non-uniformity, and unpredictable scattering. Researchers are often forced to use mirror systems to change the light path, attempting to achieve side illumination, but this not only increases light path length, introducing additional optical losses and potential spectral distortion, but also makes light spot positioning and uniformity calibration extremely difficult and unreliable.
• Space Limitations and Operational Inconvenience: Top light sources often occupy valuable space above reaction pools, creating great inconvenience for electrode connections, electrolyte addition/replacement, and gas collection pipeline layout.

==>Core Need: Solar simulators with native support for high-quality side (horizontal) light beam output. This design can directly, unobstructedly illuminate the effective areas of vertically placed samples, maximizing simplification of light paths, ensuring effective utilization of light energy and standardization of test geometry.

Pain Point 2: Beyond Spectral Matching—The Ultimate Pursuit of Spectral Match, Temporal Stability, and Spatial Uniformity

The kinetic processes of water splitting reactions are exceptionally sensitive to instantaneous changes in illumination conditions. Any minor fluctuations in light intensity may directly manifest in photocurrent or hydrogen production rate readings, especially when conducting long-term stability tests (e.g., chronoamperometry/chronopotentiometry for hours or even hundreds of hours) to evaluate device lifespan and degradation mechanisms.

• Spectral Match: Why pursue A+ class, especially in spectral matching? Because many novel photoelectric materials, including perovskites and some PEC light-absorbing materials, have photoelectric responses highly sensitive to specific wavelengths (particularly near-ultraviolet and near-infrared absorption edges). Even simulators meeting Class A standards, with spectral deviations up to ±25% in some bands, may cause non-negligible errors in J_sc and efficiency evaluation for these materials. For example, research shows that testing perovskite cells with Class A spectral simulators can result in differences of over 2% between J_sc(SS) and J_sc(EQE), while using A+ class spectral simulators can reduce this difference to around 0.1%. In the race for efficiency limits, a difference of a few tenths of a percent in efficiency could mean the birth of a world record. Therefore, using A+ class solar simulators with higher spectral fidelity is crucial for obtaining more precise and reliable data.

Enlitech’s SS-X series meets the requirements of the IEC 60904-9:2020 standard in spectral quality, with output spectrum matching the standard AM1.5G solar spectrum at Class A or even A+ level, covering key wavelength bands from ultraviolet (UV) to near-infrared (300~1800nm), satisfying various solar materials’ light requirements.

• Temporal Instability: The A+ class level in IEC60904-9 Ed.3 standard (typically requiring short-term <0.5%, long-term <1%) is the minimum requirement rather than an ideal goal for water-to-hydrogen research. Any fluctuations beyond this range might mask real device degradation trends or be misinterpreted as performance fluctuations. Especially when studying slow photodegradation or catalyst deactivation processes, fluctuations from the light source itself are unacceptable noise sources.
• Spatial Non-uniformity: Similarly, Class A level is needed (typically requiring <2% non-uniformity). For larger photoelectrodes or research requiring local performance analysis (such as scanning photocurrent imaging), any significant intensity differences within the light spot will lead to calculation bias in current density, misleading judgments about material uniformity or device performance bottlenecks.

==>Core Need: Light sources with high precision (at least spectral match and light intensity stability reaching A+ class), especially temporal stability must be highly reliable, to support long-term, high-precision stability evaluations and kinetic studies.

Pain Point 3: Precisely Controllable Light Intensity Dependence Studies

Studying the relationship between photocurrent density (J_ph) or hydrogen production rate and incident light intensity (P_in) is a key method for revealing internal recombination mechanisms (such as trap-assisted recombination, Auger recombination), charge transport limitations, and reaction kinetics (such as surface states, mass transport). This requires solar simulators to:

• Wide Range, High-Precision Light Intensity Adjustment: Typically covering at least 0.1 sun to 1.0 sun (or even higher, like 1.2 suns for modeling concentration conditions or accelerated aging tests), with fine adjustment steps (such as 0.1% level resolution).
• Spectral Invariance: Most critically, throughout the entire light intensity adjustment process, the light source’s spectral distribution must remain highly stable (i.e., meeting AM1.5G spectral requirements). Otherwise, performance changes due to light intensity variations would be coupled with effects caused by spectral changes, making it impossible to isolate variables and leading to incorrect conclusions.

==>Core Need: Capability for fully automated, high-precision light intensity control over a wide range while maintaining constant spectral distribution.

Table 3: Comparison of IEC60904-9:2020 Class A+ and Class A Requirements

Addressing Water-to-Hydrogen Challenges: Enlitech’s SS-X Series Solar Simulators’ Innovative Solutions

After deeply understanding the unique and stringent light source requirements for water-to-hydrogen research mentioned above, Enlitech’s SS-X series solar simulators are specifically designed to solve these core pain points. They are not just light sources but precision instruments addressing frontier research needs:

• Native Four-Direction Illumination Flexibility: Addressing this pain point, Enlitech’s SS-X series solar simulators offer an ideal solution. This series not only achieves A+ class spectral match, A class spatial uniformity, and A+ class temporal stability according to IEC60904-9:2020 standards in core performance, but also features unique four-direction illumination capabilities. Besides standard downward illumination, it also offers upward illumination (convenient for glovebox integration) and left or right side horizontal illumination, perfectly suited for water splitting experiments with vertical samples. Enlitech has even launched specific optional configurations for water splitting measurements, further optimizing its applicability in this field. Additionally, its unique SS-IRIS technology allows precise, automated continuous adjustment of light intensity from 0.01 sun to a maximum of 1.2 sun (configured according to user needs, some models can reach higher intensities) without changing the spectrum, which is valuable for studying light intensity dependence effects and simulating device performance under different illumination conditions. Its good glovebox integration capability also meets research needs for air-sensitive materials.
  
• Excellent Performance Rating, High Spectral Match: The SS-X series fully complies with and exceeds the latest IEC 60904-9:2020 ED3 standards, with key indicators such as spectral match and temporal stability reaching the highest A+ class. Particularly, its excellent temporal stability provides a solid foundation for long-term stability monitoring and precise kinetic studies.
  
• Precise, Stable Spectrum-Invariant Light Intensity Control: The SS-X series incorporates advanced control systems that enable fully automated, high-resolution (<0.1% step) continuous light intensity adjustment from low intensity to as high as 1.2 suns (120mW/cm²) without altering AM1.5G spectral characteristics. This provides a powerful tool for in-depth studies of light intensity dependence.
• Water-to-Hydrogen Dedicated Optimization and Integration: We also offer specific optional configurations for water-to-hydrogen research, further optimizing horizontal beam quality and ensuring compatibility with common reaction pool dimensions and optical windows. Meanwhile, SS-X can be conveniently integrated with environmental control equipment such as gloveboxes, meeting testing needs for air-sensitive materials (such as perovskite photoelectrodes or even organic materials).

The SS-X series simulators, with their unique multi-directional illumination design and top-tier light source performance, directly address the core challenges in standard light source configuration for water-to-hydrogen research, providing researchers pursuing high-quality, highly reproducible data with a truly handy solution.

Quantum Efficiency Measurement Challenges in Water-to-Hydrogen Research

When evaluating water-to-hydrogen system performance, besides macroscopic IV characteristics and hydrogen production, understanding the entire process of photogenerated carrier generation, separation, transport, and injection into catalytic layers in devices requires quantum efficiency (QE) or more accurately, incident photon-to-current efficiency (IPCE) measurements.

Quantum Efficiency (QE) is typically an important indicator, just as solar cells measure external quantum efficiency (EQE) of current generation under different wavelength illumination, water-to-hydrogen researchers also want to understand their photoelectrochemical electrodes or photocatalysts’ utilization efficiency for photons of various wavelengths.

Similar to IV measurements, standard QE/IPCE measurement systems also typically adopt vertically downward light path designs. To measure quantum efficiency of samples vertically immersed in electrolyte, the monochromatic light beam from the monochromator must similarly be converted to precise horizontal output. This not only requires the beam to have good collimation, capable of precisely illuminating the active area of the sample through the reaction pool window and electrolyte, but also requires accompanying sample fixtures to position the sample in the light path, and reference photodetectors must accurately receive light beams in the same horizontal plane for light intensity calibration. The entire light path design, alignment, and calibration are much more complex than standard setups. Ordinary QE systems on the market obviously cannot meet this special side illumination requirement.

However, for QE measurements of water-to-hydrogen devices, the experimental design differs from traditional solar cell testing, thus facing several unique challenges:

• Device Structure and Environment: Photoelectrochemical water splitting typically involves photoelectrodes immersed in electrolyte. Unlike dry solar cells, these devices operate in liquid phase, where light must pass through transparent containers (like quartz glass windows) into liquid before reaching the photoelectrode surface. The presence of liquid media introduces refraction, reflection, and additional absorption in the light path. Traditional QE measurement systems may lead to inaccurate measured light intensity or efficiency if applied directly without correction for these situations. Additionally, photoelectrodes are often placed vertically, facing the light source. Using general QE testers designed for flat samples (typically illuminating horizontally placed samples from above) would not correctly illuminate vertical photoelectrodes, requiring light path adjustments.
• Light Source and Detection Sensitivity: QE measurements typically use monochromatic light scanning different wavelengths, meaning light source intensity may be very weak at specific wavelengths, generating very small photocurrents. For low-efficiency water-to-hydrogen materials, currents at certain wavelengths may even be below noise levels. Traditional QE equipment is mostly optimized for high photoelectric conversion efficiency solar cells, and when used directly for low-current photoelectrochemical cells, may have insufficient signal detection or high noise problems, requiring additional technology support like lock-in amplifiers. Furthermore, without proper reference calibration, such as first determining light intensity with standard solar cells in air, then switching to aqueous solution environments, changes in light flux due to media differences may cause result errors.

Solving Immersion Photoelectric Measurement Challenges: Enlitech QE-R Water-to-Hydrogen Dedicated Measurement Solution

Paradigm Shift in QE/IPCE Testing: From “Compromise” to “Breakthrough”

Enlitech’s QE-R quantum efficiency measurement system, an instrument renowned for high precision and reproducibility, has specially launched optional configurations for water splitting research (QE-R water-to-hydrogen measurement optional configuration), custom-tailored for precise spectral response testing tools for photoelectrochemical and photocatalytic experiments. A major feature of the QE-R solution is that its core involves precise redesign and modification of the light path (precise light path design modification), enabling stable, accurate horizontal side monochromatic light output (horizontal side illumination): Compared to traditional QE systems’ top-down illumination method, QE-R can project monochromatic light horizontally, conveniently directly illuminating photoelectrodes or reaction areas on the sides of electrolysis cells. This design perfectly adapts to typical water-to-hydrogen experimental configurations (such as vertically placed photoelectrodes), ensuring effective illumination of the active surface at each wavelength without needing to change sample placement to accommodate the light path. With specially designed sample fixtures and calibration procedures, researchers can conveniently perform EQE/IPCE spectral measurements on photoelectrode samples vertically immersed in electrolyte while maintaining QE-R system’s inherent high precision (guaranteeing its accuracy). This directly overcomes the geometric limitation challenges in PEC research QE measurements.

To ensure measurement precision, the QE-R solution also invests heavily in light source calibration and detection, equipping highly stable full-spectrum light sources and highly sensitive detectors, coupled with lock-in amplification technology to resolve extremely weak photocurrent signals.

Considering differences in light propagation characteristics between aqueous solutions and quartz media, QE-R includes specialized reference calibration steps in the correction process to ensure accurate calculation of actual incident light power at different wavelengths. In other words, researchers using QE-R can trust each set of quantum efficiency data measured, without worrying about hidden errors due to experimental setup geometry or media differences.

The QE-R solution also fully considers experimental convenience. Its design seamlessly connects with the aforementioned SS-X series solar simulators and holding systems, allowing researchers to complete spectral response testing and long-term illumination experiments on the same setup without repeatedly changing equipment.

Precision Stage: From “Making Do” to “Exacting Standards”

Excellent light sources alone are not enough; reliable sample placement and fixation are equally critical for obtaining reliable experimental data. Securely installing photoelectrode samples in PEC test pools, ensuring good electrical contact, precisely defining illuminated area, while preventing electrolyte leakage or contamination, are fundamental to obtaining accurate experimental results, yet challenging to accomplish. In water-to-hydrogen experiments, researchers often need to fix photoelectrodes at appropriate positions in electrolytic cells, such as facing the light source while immersed at specified depths in electrolyte. If not held securely, slight electrode position shifts during experiments could change the illuminated area or light path distance, affecting measurement results. Additionally, different experimental setups vary greatly in size and shape, from small three-electrode electrochemical cells to larger two-electrode water splitting devices, all requiring corresponding fixation methods.

Standard PV sample fixtures are completely unsuitable for liquid-phase environments. Improper mounting can lead to excessive contact resistance, unclear definition of illuminated area, electrolyte leakage, or even sample damage, seriously affecting experimental accuracy and reproducibility.

Therefore, well-designed, user-friendly dedicated test fixtures are key factors for PEC laboratories. They need to securely fix samples of different sizes and shapes, provide reliable electrical contacts, precisely define illumination windows, and seal well with reaction pools, while resisting corrosive electrolytes.

Enlitech deeply understands researchers’ difficulties in this area. Besides providing high-performance instruments, offering dedicated fixture solutions for water splitting research is even more important. More importantly, Enlitech can make minor customized adjustments according to specific customer needs, custom-designing sample stages and fixtures most suitable for researchers’ specific reaction pool designs and experimental procedures at reasonable prices, significantly enhancing experimental precision, efficiency, and convenience.

For various cells and reactors in water-to-hydrogen experiments, Enlitech provides dedicated fixtures and sample stage accessories to ensure samples maintain optimal positions and conditions during illumination.

Enlitech fully understands these needs and has developed modular fixture systems, providing water-to-hydrogen dedicated fixture solutions not only with standardized designs but also flexible local customization services that perfectly complement SS-X solar simulators with water-to-hydrogen modules and QE-R quantum efficiency systems, ensuring every link from light source to sample meets high standards.

Taking the application of SS-X four-direction illumination simulators as an example, their dedicated fixtures can precisely position samples at the center of side light source convergence, ensuring consistent sample illumination conditions in each experiment. Besides secure holding, fixtures also reserve space for electrode wiring and electrolyte containers, facilitating connection to test circuits and gas collection devices. For samples requiring special angle illumination or specific inclined placement, Enlitech also provides customized fine-tuning mechanisms, allowing researchers to fine-tune sample angles and heights to achieve ideal illumination geometries. Meanwhile, all fixture materials are selected for corrosion resistance, suitable for long-term contact with electrolytes, avoiding impurity contamination in experiments.

Through these thoughtful designs, Enlitech’s sample fixtures effectively reduce error risks from manual adjustments and temporary fixations. Each experiment can be conducted under highly reproducible conditions, enhancing data consistency. Whether conducting long-term durability tests or precise quantum efficiency scans, with stable holding systems, researchers can confidently trust that device positioning and status won’t become variables, allowing full focus on scientific questions themselves.

Conclusion: Platform Capability + Dedicated Modules = Research Accelerator

The emergence of these specialized equipment addressing PEC water splitting’s special needs (such as A+ class solar simulators with side illumination, QE systems with horizontal light output, and customized fixtures) marks that this research field has evolved from early basic material exploration to a stage requiring higher experimental precision and equipment professionalism. To deeply understand complex reaction mechanisms (such as interfacial dynamics, degradation processes), pursue higher efficiency and stability, researchers must minimize experimental errors and human interference, requiring specially optimized, high-quality measurement infrastructure. Additionally, many cutting-edge PEC materials (such as perovskites or specific catalysts/interface modification layers) are very sensitive to environmental factors like air and moisture. Therefore, seamlessly integrating precision measurement instruments like solar simulators and QE systems with controlled atmosphere environments like gloveboxes has become a necessary condition for conducting cutting-edge material research, not merely a special need. PEC research’s unique experimental environment—samples immersed in liquid, often requiring vertical placement, reactions accompanied by gas generation—poses severe challenges to traditional testing instruments, especially regarding high-fidelity side illumination light sources and quantum efficiency measurements of immersed samples. The existence of these experimental difficulties makes investing in high-performance characterization equipment specifically designed and optimized for PEC research no longer a luxury but a necessary guarantee for ensuring data quality, improving research efficiency, and accelerating technological breakthroughs.

Colleagues, the in-depth development of solar water-to-hydrogen research urgently needs more professional, more compatible characterization tools. Enlitech has recognized this need and, instead of developing entirely new, potentially single-function equipment, has adopted a more intelligent and forward-looking platform + modular strategy.

Enlitech, as a provider focused on photoelectric measurement solutions, has always been committed to close collaboration with the scientific community, deeply understanding the challenges and needs faced by frontier research. Our SS-X series A+ class solar simulators, with their excellent spectral fidelity, stability, and unique four-direction illumination (including side illumination) capability, provide ideal light source guarantees for PEC research. The water splitting measurement optional configuration of the QE-R quantum efficiency system precisely solves the QE testing challenges of vertically immersed samples through innovative horizontal light path design. Combined with customizable dedicated fixtures, Enlitech aims to provide a complete set of tailored, high-precision, high-efficiency measurement solutions for global solar water splitting researchers, helping scientists overcome challenges, the core of this strategy is empowerment. We believe that by empowering top platforms with the ability to precisely solve specific domain challenges, we can maximize your research efficiency, safeguard your data quality, and accelerate your exploration and breakthroughs in the solar water-to-hydrogen field.

If you are troubled by the challenges of water-to-hydrogen photoelectric characterization, or want to learn how to use SS-X or QE-R platform modular upgrades to enhance your research capabilities, we very much look forward to more in-depth technical discussions with you. Let us join hands to welcome the era of green hydrogen energy with smarter, more precise tools!