Photoluminescence Spectroscopy Uncovers Photovoltaic Properties

Introduction

Photovoltaic technology harnesses the power of the sun to generate electricity, offering a clean, renewable energy source that is crucial for sustainable development. Material characterization plays a vital role in optimizing the performance of photovoltaic devices, as it allows for the detailed analysis of materials and their properties. Photoluminescence spectroscopy is a powerful analytical tool used to investigate the quality of semiconductors and the efficiency of energy conversion, providing insights that can lead to the enhancement of photovoltaic cell performance.

Figure 1: Diagram of the layers of photovoltaic cells (solar cells).

Photoluminescence spectroscopy method.

Photoluminescence is a phenomenon where a material absorbs light energy (excitation) and then emits light of a different wavelength (emission). This process involves the interaction of light with the electronic structure of the material.

Basic Principles

• Excitation: When a material is exposed to light of a suitable wavelength, electrons within the material can absorb the energy and transition to higher energy states.
• Relaxation: The excited electrons eventually return to their lower energy states, releasing the absorbed energy in the form of light.
• Emission: The emitted light's wavelength depends on the energy difference between the excited and ground states. This emission is known as photoluminescence.

Figure 2: The principle of photoluminescence.

Types of Photoluminescence

• Fluorescence: Emission occurs promptly after excitation, typically within nanoseconds.
• Phosphorescence: Emission occurs over a longer period, often lasting seconds or even minutes.

Factors Affecting Photoluminescence

• Material Properties: The electronic structure, impurities, and defects within the material influence its photoluminescence properties.
• Excitation Wavelength: The wavelength of the excitation light affects the efficiency of excitation and the resulting emission spectrum.
• Temperature: Temperature can influence the intensity and spectral distribution of photoluminescence.
• External Fields: Electric or magnetic fields can affect the photoluminescence properties of certain materials.

Applications of Photoluminescence Spectroscopy

• Semiconductor Research: Analyzing the bandgap, impurity levels, and carrier dynamics in semiconductors.
• Biochemistry: Studying the properties of biological molecules and processes.
• Materials Science: Characterizing materials such as polymers, ceramics, and nanomaterials.
• Environmental Science: Detecting pollutants and contaminants.

Applications of Photoluminescence Spectroscopy in measuring Photovoltaic properties.

Photoluminescence spectroscopy is a powerful tool for analyzing the fundamental properties of photovoltaic materials. By studying the light emitted by a material when excited, researchers can gain insights into bandgap, impurities, recombination mechanisms, carrier dynamics, and overall material quality.

• Bandgap Determination: Measuring the emitted light's energy reveals the bandgap, crucial for understanding solar energy conversion.
• Impurity and Defect Analysis: Analyzing spectral features identifies impurities and defects affecting device efficiency.
• Recombination Mechanisms: Studying the spectral shape and intensity helps understand radiative and non-radiative recombination.
• Carrier Dynamics: Measuring photoluminescence decay and using time-resolved techniques reveals carrier lifetime, diffusion length, and mobility.
• Material Quality Assessment: Evaluating crystallinity, defect density, and homogeneity to identify and address material-related issues.

Result

Figure 3: Luminescence spectra of perovskit solar cell and FTO-TiO2 test structure.

In one study, the researchers compared the luminescence spectroscopy, the electroluminescence spectra of perovskite solar cells, and the luminescence spectra of solar cell specimens with FTO-TiO2 layers. Compares the photoluminescence (PL) and electroluminescence (EL) spectra of a perovskite solar cell. Both spectra exhibit a similar peak intensity at 783 nm. However, significant differences are observed in the short (less than 750 nm) and long (greater than 810 nm) wavelength ranges. These variations are attributed to the presence of FTO and TiO2 layers in the test sample, which emit luminescence only under illumination, not under forward bias voltage. To isolate the perovskite's luminescence, an optical filter set was used to restrict the detected signal to the 750-850 nm range.

The FTO (Fluorine-doped Tin Oxide) and TiO2 (Titanium Dioxide) layers play crucial roles in the structure and function of perovskite solar cells:

• FTO Layer: Acts as a transparent conductive electrode. It allows light to pass through while conducting electricity, enabling the collection of generated charge carriers.
• TiO2 Layer: Serves as a blocking layer and electron transport layer. It helps in the separation and transport of electrons, preventing recombination and enhancing the efficiency of the solar cell.

Measurement system

The Ocean QEPro is a high-sensitivity spectrometer, designed to deliver exceptional performance in applications such as excited photoluminescence in photovoltaic research. Its robust optical design ensures great spectral performance across a wide wavelength range of 185-1100 nm, with an optical resolution between 1.20-1.60 nm when using a 10 μm slit. The QEPro's high dynamic range of 85000:1 and signal to noise ratio of 1000:1 make it particularly sensitive, suitable for detecting even the most subtle photoluminescence signals. Additionally, its spectral buffering capability ensures data integrity during high-speed data collection, making it a reliable tool for advanced photovoltaic analysis.

Figure 4: QEPro spectrometer.

The Ocean LDC-1C LED light source is a state-of-the-art instrument designed for the precise excitation of photoluminescence in photovoltaic materials. It features a compact, single-channel driver and controller tailored for next-generation LED Light Source Modules (LSMs), ensuring optimal performance and protection against excessive current. With its user-friendly, menu-driven interface, the LDC-1C facilitates seamless operation and integration with LSMs, making it an essential tool for researchers and professionals in the field of photovoltaic technology.

Figure 5: LDC-C1 LED controler and LSM LED modules.

Conclusion

Photoluminescence spectroscopy is a powerful, non-destructive tool for analyzing photovoltaic materials. By providing insights into bandgap determination, impurity and defect analysis, recombination mechanisms, and carrier dynamics, it plays a crucial role in optimizing the performance of photovoltaic devices. The use of advanced instruments like the Ocean QEPro spectrometer and the Ocean LDC-1C LED light source enhances the precision and reliability of these measurements, paving the way for more efficient and sustainable solar energy solutions.