In-Situ Reflectance Spectroscopy and PLQY Measurement Systems: Comprehensive Characterization from Electronic-Structure Evolution to Luminescence Efficiency

Abstract

In-situ reflectance spectroscopy and photoluminescence quantum yield (PLQY) measurement are two important and highly complementary optical characterization techniques in materials science and optoelectronics research. The former reveals the evolution of electronic structure and optical properties during reactions by tracking the dynamic changes of a material's surface reflectance in real time; the latter focuses on a material's luminescence efficiency, quantitatively evaluating the competition between radiative and non-radiative recombination. This article systematically explains the principles, hardware composition and typical applications of the two measurement systems, and through comparison and case analysis, shows how they jointly build a complete picture of a material's optical behavior from the two dimensions of "process" and "outcome".

1. Introduction

In materials science research, understanding the evolution of a material's optical behavior during synthesis, growth or service is a key prerequisite for optimizing device performance and revealing new mechanisms. Modern spectroscopy provides two characterization strategies with their own advantages:

  • In-situ reflectance spectroscopy is a "process-oriented" characterization method that continuously collects reflectance signals in a real reaction environment to dynamically track the real-time changes of a material's electronic structure and interface states. Its core value lies in coupling spectral information with reaction progress, providing direct time-resolved evidence for understanding material formation and transformation mechanisms.

  • PLQY measurement is an "outcome-oriented" characterization method that, through precise measurement of a luminescent material's quantum efficiency, determines whether excited-state energy is effectively output in the form of photons or is lost through non-radiative pathways such as defects and thermal dissipation. Its value directly determines the practical potential of a material in applications such as light-emitting devices, displays and bioimaging.

The complementarity of the two techniques is reflected in the fact that in-situ reflectance spectroscopy answers the question of "how the electronic structure evolves during material change", while PLQY measurement answers "what level the material's luminescence efficiency has reached". Combined, they enable a comprehensive evaluation of optoelectronic material performance from the two dimensions of dynamic evolution and ultimate efficiency.

2. In-Situ Reflectance Measurement System

2.1 Measurement Principle

The basic measurand of in-situ reflectance spectroscopy is the reflectance $R$, defined as the ratio of the sample's reflected light intensity $I_r$ to the incident reference light intensity $I_0$ at a unit wavelength:

$$R(\lambda) = \frac{I_r(\lambda)}{I_0(\lambda)}$$

In practice, $I_0$ is usually obtained by measuring a standard reference mirror with known reflectance (such as an aluminized mirror or a Spectralon diffuse reflectance standard) under the same geometric conditions. By tracking the changes of $R(\lambda)$ with time or external stimuli (voltage, temperature, atmosphere, etc.), the dynamic evolution of the material's electronic structure can be correlated.

2.2 From Reflectance to Dielectric Function: The Kramers-Kronig Relations

Reflectance $R$ essentially only provides the intensity ratio information, with the phase lost. To obtain the complete optical constants of a material - the complex refractive index $\tilde{n} = n + i\kappa$ and the dielectric function $\tilde{\varepsilon} = \varepsilon_1 + i\varepsilon_2$ - the Kramers-Kronig (K-K) relations are needed.

The complex amplitude reflection coefficient of the reflected light can be written as:

$$r(\omega) = |r(\omega)| e^{i\theta(\omega)} = \sqrt{R(\omega)} e^{i\theta(\omega)}$$

where the phase $\theta(\omega)$ and reflectance $R(\omega)$ are related through the K-K integral relation:

$$\theta(\omega_0) = -\frac{\omega_0}{\pi} \mathcal{P} \int_0^\infty \frac{\ln R(\omega)}{\omega^2 - \omega_0^2} d\omega$$

where $\mathcal{P}$ denotes the Cauchy principal-value integral. After obtaining $\theta(\omega)$, $\tilde{n}$ and $\tilde{\varepsilon}$ can be solved:

$$\tilde{n} = \frac{1 - r}{1 + r}, \quad \tilde{\varepsilon} = \tilde{n}^2$$

Furthermore, information about the material's electronic band structure, including the band gap and interband transition energies, can be extracted from the peak position and lineshape of $\varepsilon_2(\omega)$. This makes in-situ reflectance spectroscopy a non-contact, vacuum-free probe of electronic structure.

2.3 System Composition

A complete in-situ reflectance spectroscopy measurement system consists of the following core components:

(1) Broadband Light Source

A stable broadband light source covering the ultraviolet to near-infrared band is the foundation of in-situ reflectance measurement. A typical scheme uses a combined deuterium lamp (190-400 nm) and tungsten-halogen lamp (360-2500 nm) source, coupled out through a fiber combiner, with a total coverage of up to 190-2500 nm, satisfying the wide-spectrum detection needs from high-energy interband transitions to low-energy sub-band transitions.

(2) Reflectance Accessories

Depending on the sample form and measurement requirements, the following accessory configurations can be selected:

  • Reflectance probe (Y-type fiber): Composed of a ring of emission fibers surrounding a single collection fiber (or a bifurcated Y-type fiber design), achieving quasi-normal-incidence geometry with 0° incidence and collection. Suitable for diffuse reflectance measurement of rough or non-specular surfaces, as well as space-constrained in-situ environments.

  • Specular reflectance accessory: Achieves specular reflectance measurement at fixed or variable incidence angles through precision optical elements (such as 8° near-normal incidence or variable-angle configuration), suitable for high-precision reflectance measurement of flat thin films or single-crystal surfaces, and can be combined with polarizers to obtain s/p polarization-resolved data.

(3) Detector and Spectrometer

  • UV-visible band (190-1100 nm): Uses a back-illuminated CCD array spectrometer with a high-sensitivity detector to achieve a good signal-to-noise ratio.
  • Near-infrared band (900-2500 nm): Uses an InGaAs linear-array detector spectrometer covering the near-infrared region.
  • Dual-channel or multi-grating switching configurations can achieve continuous full-band measurement.

(4) In-Situ Reaction Device

The in-situ reaction device is the key component coupling spectral measurement with the material preparation/service environment. Common types include:

  • In-situ electrochemical cell: Integrated with an optical window (quartz or sapphire), it can perform electrochemical cyclic voltammetry/potentiostatic testing and real-time reflectance spectral acquisition simultaneously, and is the core device for research on electrochromism, electrochemical deposition, etc.
  • Vacuum/atmosphere temperature-controlled chamber: Allows observation of reflectance changes during thin-film growth, annealing, catalytic reactions, etc. under a controlled atmosphere (inert gas, reactive gas) and programmed temperature control.
  • Temperature-controlled sample stage: Equipped with Peltier or resistive heating units, with a temperature range typically covering -40°C to above 300°C, meeting variable-temperature reflectance measurement needs.

2.4 Typical Applications

Electrochromic Materials

Electrochromic materials exhibit reversible color changes under different applied voltages, the essence of which is the change in band-filling states caused by electron injection/extraction. By recording reflectance spectra at different potentials in real time, in-situ reflectance spectroscopy can directly correlate the optical modulation amplitude $\Delta R$ with the amount of charge injection, evaluating the material's coloration efficiency, response speed and cycling stability.

Thin-Film Growth Thickness Control

During physical or chemical vapor deposition, the oscillation behavior of thin-film reflectance with thickness (caused by thin-film interference effects) can be precisely used for online thickness monitoring. The extreme points of the in-situ reflectance signal correspond to an optical thickness of $\lambda/4n$; by fitting the reflectance oscillation curves at multiple wavelengths, both the film thickness and optical constants can be determined simultaneously.

In-Situ Monitoring of Battery Material Charge/Discharge

The electrodes of lithium-ion or sodium-ion batteries undergo significant electronic-structure changes during charge/discharge (such as metal-insulator transitions and phase transitions). By modifying coin cells or Swagelok-type cells and embedding an optical window, the electrode reflectance spectra can be acquired at different charge/discharge states to track the real-time evolution of the active material's electronic states and identify irreversible side-reaction products.

Gas Sensing

After metal oxides, MOFs or porous thin films interact with target gas molecules, their surface electronic states and carrier concentrations change, manifesting as changes in the reflectance spectrum. By monitoring the time-response curve of reflectance at specific wavelengths, real-time sensing of gas concentration can be achieved, and spectral differential analysis can be used to improve selectivity.

Perovskite Crystallization Process

Organic-inorganic hybrid perovskite thin films undergo rapid crystallization from sol precursor to perovskite phase during solution-based film formation. In-situ reflectance spectroscopy tracks the dynamic characteristics of the band-edge absorption shift and interference fringe evolution during this process, providing a quantitative basis for optimizing annealing temperature, solvent ratio and additive usage.

3. PLQY Measurement System

3.1 Review of Basic Principles

Photoluminescence quantum yield (PLQY) is defined as the ratio of the number of emitted photons to the number of absorbed photons:

$$\Phi = \frac{N_{\text{em}}}{N_{\text{abs}}}$$

where $N_{\text{em}}$ is the total number of emitted photons and $N_{\text{abs}}$ is the total number of absorbed photons. PLQY directly reflects the fraction of excited-state energy used for radiative luminescence; the closer its value is to 1, the fewer non-radiative recombination channels exist in the material and the more excellent the luminescence performance. A decrease in $\Phi$ usually points to the presence of unfavorable factors such as lattice defects, surface dangling bonds, Auger recombination or impurity quenching.

3.2 Absolute and Relative Methods

Absolute Method (Two-Step Method)

The absolute PLQY method does not require any standard sample. It uses an integrating sphere to eliminate angular and scattering effects and directly determines the numbers of absorbed and emitted photons. The standard procedure is:

  1. Blank measurement ($E_a$): Place a blank substrate or solvent (without luminescent sample) in the integrating sphere and record the scattered light intensity spectrum $E_a$ at the excitation wavelength.

  2. Sample measurement ($E_b$ and $P$): Place the luminescent sample in the integrating sphere and record separately:
    - $E_b$: the scattered light intensity spectrum at the excitation wavelength (weakened by sample absorption).
    - $P$: the luminescence spectrum within the sample emission wavelength range.

  3. Calculation:

$$A = 1 - \frac{E_b}{E_a}$$

$$\Phi = \frac{P}{E_a - E_b}$$

where $A$ is the sample's absorptance at the excitation wavelength, and the denominator $E_a - E_b$ is equivalent to the number of excitation photons actually absorbed by the sample. This method has clear physical concepts and absolutely reliable results, and has become the internationally accepted standard PLQY measurement method.

Relative Method

The relative method estimates the PLQY by comparing the integrated luminescence intensity of the sample under test with that of a standard sample of known PLQY (such as Rhodamine 6G, fluorescein, quinine sulfate, etc.). Its advantage lies in lower requirements for instrument calibration, but it is limited by the reliability of the standard sample and the precise matching of measurement conditions. With the maturity and popularization of absolute-method measurement systems (integrating sphere and calibrated light source integrated), the use of the relative method in research-grade PLQY measurement is gradually decreasing.

3.3 System Composition

(1) Excitation Light Source

  • Monochromatic LED source: Commonly uses high-power monochromatic LEDs with central wavelengths of 365 nm, 405 nm, 450 nm, further purified through narrowband filters or a monochromator to ensure the monochromaticity of the excitation light.
  • Semiconductor laser: Provides monochromatic excitation with higher power density and narrower linewidth, suitable for measuring weakly luminescent materials.
  • The excitation wavelength should be selected within the sample's absorption band with sufficient spectral separation from the luminescence wavelength, to avoid interference of the excitation scattering peak with the luminescence spectrum.

(2) Integrating Sphere

The integrating sphere is the core optical element of the absolute PLQY method:

  • The inner wall is coated with Spectralon (polytetrafluoroethylene), a highly diffuse reflective material with >99% diffuse reflectance in the UV-visible-NIR band (250-2500 nm), ensuring sufficient homogenization of light within the sphere.
  • The standard configuration has a sample port (for placing the sample or blank), an excitation light entrance and a spectrometer collection exit.
  • The design of the integrating sphere effectively eliminates the angular dependence errors caused by emission anisotropy and scattering, and is the physical basis for realizing the absolute method.

(3) Spectrometer and Detector

  • A CCD array spectrometer is used, which captures the complete excitation scattering and luminescence spectra in a single exposure without mechanical scanning, significantly shortening measurement time and avoiding errors introduced by light source fluctuations.
  • A calibrated light source of known spectral irradiance (such as a tungsten-halogen calibration source) is needed to calibrate the system's wavelength-dependent sensitivity to obtain absolute spectral irradiance (unit: μW/nm or photons/nm); this step is crucial for absolute-method PLQY measurement.

(4) Sample Holder

  • Liquid samples: Standard quartz cuvettes (10 mm path length) placed at the integrating sphere sample port.
  • Solid thin films: Thin-film holders using back-side or front-side excitation configurations.
  • Powder samples: Placed in quartz sample cups with a flattened surface to ensure geometric consistency.

4. Comparison of the Two Measurement Systems

Comparison dimension In-situ reflectance spectroscopy PLQY measurement system
Measured quantity Surface reflectance $R(\lambda)$ and its change with external conditions Luminescence quantum efficiency $\Phi$ (emitted photons / absorbed photons)
Core information type Electronic-structure evolution (dielectric function, band gap, band transitions) Luminescence efficiency (ratio of radiative to non-radiative recombination)
Core hardware Broadband source (deuterium + tungsten-halogen lamps), reflectance probe or specular reflectance accessory, CCD/InGaAs spectrometer, in-situ reaction device Monochromatic LED or laser, integrating sphere (Spectralon inner wall), CCD array spectrometer, calibrated light source
Measurement mode Real-time, in-situ, time-resolved Steady-state, absolute or relative
Standard sample required Requires standard reference mirror ($I_0$ reference) Absolute method: not required; relative method: requires standard fluorescent sample
Main application fields Electrochromism, thin-film growth, battery in-situ monitoring, gas sensing, phase-transition kinetics LED/OLED material evaluation, quantum dot efficiency optimization, upconversion materials, photovoltaic materials, biological probes
Data post-processing Kramers-Kronig transform, thin-film interference fitting, differential spectral analysis Spectral sensitivity calibration, integrated area calculation, self-absorption correction
Characterization dimension "Process" - dynamic evolution "Outcome" - ultimate efficiency

5. Combined Application Cases

5.1 Perovskite Optoelectronic Materials

In perovskite solar cell and LED research, the two techniques form a natural complement:

  • In-situ reflectance spectroscopy monitors the band-gap shrinkage and film densification of perovskite thin films during annealing crystallization in real time, and analyzes the film thickness and refractive index changes through interference fringes, providing kinetic data for film-formation process optimization.
  • PLQY measurement subsequently evaluates the luminescence efficiency of the prepared perovskite thin film. A high PLQY value (>70%) is an important indicator of a high-quality perovskite film, indicating low trap-state density and little non-radiative recombination - precisely the direct results of the crystallization-process optimization revealed by in-situ reflectance spectroscopy.

The combination of the two achieves a closed-loop feedback of "process-structure-performance".

5.2 Electrochromic Light-Emitting Devices

Multifunctional electrochromic light-emitting devices can switch between a colored state and a light-emitting state:

  • Use in-situ reflectance spectroscopy to track the absorption modulation and electronic-structure evolution of the colored state at different potentials.
  • Use PLQY measurement to evaluate the luminescence efficiency in the light-emitting state and judge carrier injection balance and defect quenching.

The two characterizations jointly guide the optimal design of the device structure.

6. Summary and Outlook

In-situ reflectance spectroscopy and PLQY measurement systems provide indispensable characterization tools for optoelectronic materials research from the two complementary dimensions of dynamic evolution and ultimate efficiency. In-situ reflectance spectroscopy tracks reflectance changes in real time and, combined with the Kramers-Kronig transform, extracts dielectric-function and band-structure information, making it suitable for "process-oriented" research scenarios such as electrochromism, thin-film growth, battery materials, gas sensing and phase-transition kinetics. The PLQY measurement system, by means of the integrating sphere and the absolute method, precisely quantifies a material's luminescence efficiency, directly serving "efficiency-oriented" application scenarios such as LEDs, displays and bioimaging.

Looking ahead, the development trends of the two include:

  1. Multi-modal combination: Integrate in-situ reflectance and PLQY on the same measurement platform to simultaneously obtain real-time changes in reflectance and luminescence efficiency during reactions, achieving true "process + outcome" synchronous characterization.
  2. Expansion of spatial resolution: Combine microscopic optical systems to achieve micro-area in-situ reflectance and PLQY measurement, meeting the characterization needs of heterogeneous materials and multiphase systems.
  3. Intelligent data processing: Use machine learning methods to assist K-K transforms and spectral analysis, improving the efficiency and reliability of converting raw spectra into physical parameters.
  4. Extreme environment adaptation: Develop measurement accessories suitable for extreme conditions such as ultra-low temperature, strong magnetic fields and high pressure, expanding the application boundaries of the two techniques.

It is foreseeable that, with the deepening of optoelectronic material and device research, the synergistic application of in-situ reflectance spectroscopy and PLQY measurement will play an increasingly important role in revealing the essence of material optical behavior.

This article was compiled by Pynect.