Photoluminescence Quantum Yield (PLQY) is a characterization of Perovskite Solar Cells (PSCs)Carrier recombination dynamicsandNon radiative lossesThe core indicator is directly related to the open circuit voltage (Voc) of the battery）Fill factor (FF) and photoelectric conversion efficiency (PCE).

Unlike traditional silicon-based batteries, perovskite materials such as methylamine lead iodide FAPbI₃Acrylamide cesium lead iodide FACsPbI₃The defect density of states and surface/interface recombination rate are extremely sensitive to PLQY, making PLQY analysis technology a key tool for "diagnosing" material quality and device performance in PSCs development.

This article will provide a deep analysis of basic concepts, measurement principles, key influencing factors, technical details, and application scenarios.

1、 The core definition of PLQY and its significance in PSCs

1. The essence of PLQY

PLQY refers to a material that, when excited at a specific wavelength,Number of emitted photoluminescence photons（Nemitted）Number of absorbed excitation photons（Nemitted）The ratio ofThe formula is as follows:

The value range of PLQY is 0~1 (or 0%~100%):

·高 PLQY（>80%）Indicating that carriers mainly undergo radiative recombination, and non radiative losses (such as defect state capture, interface recombination, Auger recombination) are extremely weak, resulting in excellent material/device quality;

·Low PLQY (<50%)Non radiative recombination is dominant, usually corresponding to multiple film defects, mismatched interface energy levels, or obstructed carrier transport, requiring optimization of preparation processes or passivation strategies.

2. The core impact of PLQY on the performance of PSCs

The photoelectric conversion process of PSCs can be summarized as "light absorption → carrier generation → carrier transport → carrier collection", which is directly reflected by PLQYThe degree of loss from carrier generation to collection：

·Compared to open circuit voltage (Voc)）The associationNon radiative recombination is VocThe main reason for deviating from the theoretical limit (Shockley Queeisser limit). According to the "Non radiative Voltage Loss (Δ Vₙᵣ）”Formula: For every order of magnitude increase in PLQY, ΔVₙᵣIt can reduce~60 mV (such as increasing PLQY from 1% to 100%, ΔVₙᵣCan be reduced from 200 mV to<50 mV);

·Correlation with Efficiency (PCE)High PLQY means that more charge carriers can be collected by the electrode, reducing "ineffective recombination" and thus increasing short-circuit current (Jₛc）Together with FF, drive PCE to break through 26% (current laboratory * high efficiency).

2、 The measurement principle of PLQY: absolute method vs relative method

The measurement of PLQY requires precise quantification of the "number of absorbed photons" and "number of emitted photons", with the core divided intoAbsolute lawandRelative methodThere are significant differences between the two in terms of principle, device complexity, and accuracy, among which the absolute method has become mainstream because it does not require standard samples and adapts to the characteristics of perovskite.

1. Absolute method (integrating sphere method): the first choice for PSCs measurement

Absolute law passedIntegrating SphereCapturing all photons emitted by the sample (including scattered light) and directly calculating PLQY is currently the gold standard for measuring PLQY in perovskite thin films/devices.

(1) Measurement principle

The integrating sphere is a hollow sphere with a high reflectivity material (such as polytetrafluoroethylene PTFE, reflectivity>99%) coated on its inner wall. Its core function is to convert "directionally emitted PL light" into "uniformly diffused light", ensuring that the detector can capture all emitted photons. The measurement is carried out in three steps:

1. Background correction (Blank Scan)When there is no sample, only the excitation light is introduced and the baseline signal of the excitation light inside the integrating sphere is recorded (eliminating interference from ambient light and detector dark current);

2. Reference Scan for Excitation LightPlace the "non absorbing blank substrate (such as quartz plate)" into an integrating sphere and record the signal of the excitation light reflected/scattered by the substrate (denoted as P)₀）, representing the number of excitation photons that have not been absorbed by the sample;

3. Sample ScanPlace the perovskite sample (film/device) into an integrating sphere and record two signals:

oThe excitation light signal not absorbed by the sample (Pₛ）；

oPL light signal emitted by the sample (Pₚₗ）.

Calculate PLQY using the following formula:

among which,P_pl,blankIt is the PL baseline signal recorded in background correction (usually negligible).

(2) Device composition

The core components of the absolute method PLQY testing system need to be adapted to the characteristics of perovskite:

·excitation light sourcePriority should be given to lasers with good monochromaticity and stable power (such as 488 nm and 532 nm semiconductor lasers) to avoid overlapping excitation wavelengths with the edge of the perovskite absorption band (to prevent insufficient carrier excitation);

·integrating sphereThe diameter is usually 10-20 cm (suitable for 1 × 1 cm perovskite film), and the PTFE coating on the inner wall needs to be uniform (to avoid errors caused by local reflectivity differences);

·detectorHigh sensitivity photomultiplier tubes (PMT) or spectrometers (such as CCD array spectrometers) should be used to cover the PL emission band of perovskite (such as FAPbI)₃The PL peak is between 850-880 nm;

·Temperature control/atmosphere control modulePerovskite is sensitive to water, oxygen, and temperature, and requires an inert atmosphere (N₂/Ar) chamber and variable temperature table (-196 ℃~300 ℃) to avoid sample degradation during testing.

2. Relative method: an auxiliary tool for rapid screening

Relative method through comparisonUnknown sampleandKnown standard samples of PLQYThe PL intensity is used to indirectly calculate the PLQY of the sample, which is suitable for rapid screening of a large number of samples (such as preliminary screening in process optimization).

(1) Measurement principle

Assuming the PLQY of the standard sample isPLQY_stdThe PL integral intensity isI_stdThe PL integrated intensity of the unknown perovskite sample isI_samIf the absorption coefficient, excitation light power density, and detector responsivity of the two are consistent, then:

（2）局限性

·Relying on the accuracy of standard samples (selecting standard samples that match the PL band of perovskite, such as rhodamine 6G and quantum dots, but with poor adaptability);

·The strong light scattering (high surface roughness) of perovskite thin films leads to significant measurement errors in PL intensity;

·It is impossible to exclude the influence of factors other than non radiative recombination, such as differences in absorption coefficients, on PL intensity, and the accuracy is much lower than the absolute method.

3、 Key influencing factors of perovskite PLQY measurement

Perovskite materialsinstabilityandcarrier dynamicsDue to factors such as long carrier lifetime and high defect sensitivity, PLQY measurements are susceptible to interference and require precise control of the following key parameters:

1. Sample characteristics: Control errors from the preparation end

·Uniformity of thin filmIf there are pinholes, agglomeration, or uneven composition in perovskite thin films, it will lead to local absorption/emission differences, and the representativeness of PLQY measurement results will decrease. It is necessary to optimize the spin coating/scraping process (such as anti solvent engineering, annealing temperature control) to ensure the uniformity of the film (roughness<5 nm);

·Surface/interface passivationThere is a large amount of Pb ² on the surface of non passivated perovskite⁺Defects and vacancies, strong non radiative recombination, low PLQY (usually<30%); And through PEAI and CsPbBr₃After passivation of quantum dots, PLQY can be increased to over 90%. Before testing, it is necessary to clarify whether the sample has been passivated to avoid misjudgment;

·Sample packagingUnpackaged perovskite will rapidly degrade when exposed to air (PbI caused by water oxygen)₂PLQY can decrease by more than 50% within 10 minutes. The sample needs to be temporarily sealed (such as cover glass+UV glue) or tested in an inert atmosphere.

2. Testing environment: Inhibit the degradation of perovskite

·Atmosphere controlIn a low water oxygen environment (H₂O<0.1 ppm，O₂<0.1 ppm) test, commonly used nitrogen glove box integrated with PLQY system;

·temperature controlTemperature has a significant impact on perovskite PLQY - at low temperatures (such as 77 K, liquid nitrogen temperature), non radiative recombination is inhibited, and PLQY significantly increases (such as from 60% at room temperature to 95% at low temperatures); At high temperatures (such as 85 ℃, the operating temperature of the device), PLQY decreases, reflecting thermal stability. Temperature conditions should be clearly defined during testing (usually labeled as "room temperature 25 ℃" or "working temperature 85 ℃");

·Excitation light damageHigh power excitation light (>100 mW/cm ²) can cause photocatalytic degradation of perovskite (such as ion migration and lattice distortion), and PLQY decreases with testing time. The linear response range (usually 0.1-10 mW/cm ²) needs to be determined through a "power dependence test" to ensure that the excitation light does not damage the sample.

3. Excitation conditions: Matching the absorption characteristics of perovskite

·excitation wavelength: It is necessary to choose a wavelength in the strong absorption band of perovskite (such as the absorption edge of methylamine lead iodide perovskite at 850 nm, and 488 nm or 532 nm excitation light can be used), to avoid excitation wavelengths that are too close to the absorption edge (resulting in low absorption efficiency and weak signal) or too short (resulting in local overheating of the sample);

·Excitation spot sizeThe excitation spot needs to cover a uniform area of the sample (diameter>1 mm) to avoid focusing on pinholes or defects, which may result in low PLQY. The position of the light spot can be observed through an optical microscope to ensure the representativeness of the testing area.

4、 Advanced Techniques for PLQY Analysis: From "Static" to "Dynamic+Multidimensional"

Traditional steady-state PLQY can only provide "average recombination characteristics", while carrier recombination in perovskite isdynamic processAdvanced technologies need to be combined to achieve in-depth analysis, such as carrier lifetime and interface extraction rate.

1. Time resolved PLQY (TR-PLKY): associated carrier lifetime

Time resolved PLQY combinationTime resolved photoluminescence spectroscopy (TR-PL)Not only can it measure the steady-state PLQY, but it can also obtain the carrier lifetime (τ) and analyze the recombination kinetics mechanism.

·principleBy exciting the sample with a pulsed laser (pulse width<1 ns), the decay curve of PL intensity over time is recorded, and the carrier lifetime (τ=1/(k)) is fittedᵣ+ kₙᵣ)）； Combining steady-state PLQY (=kᵣ/(kᵣ+ kₙᵣ））, can be calculated separatelyRadiation recombination rate (k)ᵣ）andNon radiative recombination rate (k)ₙᵣ）Identify the sources of non radiative losses (such as body defects, surface defects);

·applicationDistinguishing between "phase recombination" and "interface recombination" - if passivated, kₙᵣSignificant decrease in kᵣBasically unchanged, indicating that non radiative losses mainly come from surface defects, and passivation is effective; If kₙᵣThere is no significant change, indicating that the loss comes from bulk defects and the crystallization process needs to be optimized.

2. Variable temperature PLQY: Evaluating thermal stability and phase transition effects

Perovskites may undergo phase transitions during temperature changes (such as FAPbI)₃Below 150 ℃, it is easy to transition from the alpha phase (cubic phase, high PLQY) to the delta phase (orthogonal phase, low PLQY). Variable temperature PLQY can quantify the effect of temperature on PLQY

·test scopeUsually -196 ℃ (liquid nitrogen temperature) to 300 ℃ (high-temperature aging temperature);

·key information：

oLow temperature zone (<100 ℃): PLQY slowly decreases with increasing temperature, corresponding to enhanced non radiative recombination with thermal activation;

oHigh temperature zone (>150 ℃): If PLQY drops sharply (such as from 80% to 10%), it indicates a phase transition or thermal degradation, and the composition needs to be optimized (such as adding Cs)⁺Inhibit phase transition).

3. Spatial resolution PLQY (microscopic PLQY): locating defect enrichment areas

The defects of perovskite thin films, such as pinholes, grain boundaries, and ion aggregation, havespatial heterogeneityThe traditional integrating sphere PLQY reflects the "average level", while spatially resolved PLQY (based on confocal microscopy) can achieve μ m level spatial resolution and locate defect areas:

·deviceConfocal microscope+miniature integrating sphere+high-sensitivity detector, with a spot diameter that can be reduced to 1 μ m;

·applicationObserving the spatial distribution of PLQY - If the PLQY in a certain area is significantly lower than the surrounding area (such as<30% vs 80%), it indicates the presence of defect enrichment in that area (such as PbI)₂Precipitation requires optimization of anti solvent or annealing processes.

4. In situ PLQY: Real time monitoring of preparation/aging process

In situ PLQY testing with perovskitePreparation process (such as spin coating, annealing)orAging process (such as water oxygen, light aging)Combining real-time capture of changes in PLQY and revealing dynamic mechanisms:

·In situ preparation monitoringReal time measurement of PLQY during spin coating process, observing the jump of PLQY at the time of anti solvent dripping (reflecting the improvement of crystal quality), and optimizing the anti solvent dripping time;

·In situ aging monitoringDuring the process of water oxygen aging, if PLQY decreases linearly with time, it indicates that aging is a slow degradation process; If there is a sudden drop, it indicates the existence of a "turning point" (such as packaging failure), which guides the optimization of packaging processes.

5、 Typical Applications of PLQY Analysis in PSCs Development

PLQY analysis has been integrated into the entire process of PSCs from "material synthesis" to "device optimization", with the following core application scenarios:

1. Preparation process optimization: Find the "* * process window"

·Annealing temperature optimizationThe crystallinity of perovskite thin films increases with the increase of annealing temperature, while PLQY first increases and then decreases (for example, the PLQY * of methylamine lead iodide perovskite is high when annealed at 150 ℃, and decreases due to decomposition at 200 ℃). The annealing temperature can be quickly determined by PLQY;

·Anti solvent optimizationDifferent antisolvents (such as chlorobenzene, toluene, etc.)Diethyl ether）The impact on crystallization rate varies,Diethyl ether anti solvent can be used to prepare large grain thin films, and PLQY is 20%~30% higher than chlorobenzene. PLQY can be used as a key indicator for anti solvent screening.

2. Evaluation of defect passivation effect: Quantify the performance of passivation agents

Defect passivation is the core strategy for improving the efficiency of PSCs, and PLQY is the "gold standard" for evaluating passivation effects:

·Surface passivationAfter PEAI passivation, the surface Pb ² of perovskite⁺Defects were neutralized, and PLQY increased from 50% to over 90%, indicating effective passivation;

·Bulk passivationIncorporating guanidine salts (such as GuaI) into perovskite precursors can suppress bulk vacancy defects, increase PLQY by 15% to 25%, and extend carrier lifetime to over 1 μ s.

3. Interface engineering optimization: matching energy levels and reducing extraction losses

The interface of PSCs (such as perovskite/electron transport layer TiO ₂, perovskite/hole transport layer Spiro OMeTAD) is a critical region for carrier extraction. Mismatched interface energy levels can lead to carrier accumulation, increased non radiative recombination, and decreased PLQY

·If TiO₂The electron transport layer has not undergone surface modification (such as Al)₂O₃Coating), perovskite/TiO₂The interface has energy level imbalance and low PLQY; Through Al₂O₃After encapsulation, the interface energy level matching is improved, PLQY increases by 30%, and VocIncrease by 50 mV;

·The oxidation degree of the hole transport layer Spiro OMeTAD affects the conductivity, and insufficient oxidation leads to slow carrier extraction and a decrease in PLQY; The oxidation time can be determined by PLQY (e.g. air oxidation for 12 hours, PLQY * high).

4. Stability assessment: Predicting device lifespan

The decay rate of PLQY is positively correlated with the lifespan of PSCs:

·In the light aging test, if PLQY maintains above 80% within 1000 hours, it indicates excellent light stability of the device; If PLQY drops below 50% within 100 hours, it is necessary to optimize the anti photodegradation strategy (such as adding UV absorbers);

·In the thermal aging test, the thermal stability of PLQY can predict the lifespan of the device at operating temperature (for example, if PLQY remains above 70% after 500 hours of thermal aging at 85 ℃, the device lifespan may exceed 1000 hours).

6、 Current challenges and future development trends

Although PLQY analysis technology has been widely applied, there are still the following challenges regarding the specificity of perovskite, which also drives the development of technology towards higher accuracy and more comprehensive dimensions:

1. Existing challenges

·Difficulty in measuring large-area component PLQYThe current integrating sphere is only suitable for small area samples (<2 × 2 cm), while the PLQY measurement of large-area perovskite components (such as 10 × 10 cm) requires the development of a "surface light source excitation+large-area detector" system to avoid errors caused by edge effects;

·Difficulties in correcting light scatteringThe strong light scattering of perovskite thin films (reflectivity>20%) can lead to uneven light distribution within the integrating sphere, resulting in PLQY measurement errors (usually ± 5%). Therefore, a scattering correction algorithm based on Monte Carlo simulation needs to be developed;

·Real time capture of dynamic degradationThe photodegradation/thermal degradation of perovskite is a dynamic process ranging from milliseconds to hours. Traditional PLQY testing is slow (single test>1 minute) and difficult to capture the rapid degradation process. Therefore, a high-speed PLQY testing system (testing time<1 second) needs to be developed.

2. Future Development Trends

·Multi parameter coupling analysisCombining PLQY with other characterization techniques (such as in-situ XRD, XPS, KPFM) to simultaneously obtain PLQY, crystal structure, surface chemical state, and surface potential, comprehensively revealing the root cause of non radiative losses;

·Standardized measurement processAt present, there are significant differences in PLQY measurement results among different laboratories (the difference in PLQY of the same sample can reach 10%~20%). It is necessary to establish a "perovskite PLQY measurement standard" (such as sample preparation standard, excitation power standard, calibration method standard) to promote data comparability;

·Industrial application of in-situ online monitoringIntegrate in-situ PLQY monitoring modules in the perovskite module mass production line to screen for non-conforming components in real-time (such as components with PLQY below 70%) and improve mass production yield.

summary

The photoluminescence quantum yield (PLQY) analysis technology is the "eye" of perovskite solar cell development - from material defect diagnosis to device performance optimization, from static characteristic characterization to dynamic process monitoring, PLQY always runs through the core. With the development of technology towards "time resolution, spatial resolution, and in-situ online", PLQY will not only promote the efficiency of PSCs to exceed 27%, but also provide key support for quality control in industrialization, accelerating the transition of perovskite solar cells from the laboratory to the market.