EL测试技术简介

电致发光（Electroluminescence, EL）是半导体器件在电场激发下通过载流子注入与复合发光的物理现象，广泛用于光伏器件（如太阳能电池）、LED及光电子器件的性能表征。

Electroluminescence (EL) is a physical phenomenon where semiconductor devices emit light through carrier injection and recombination under electric field excitation. It is widely used for the performance characterization of photovoltaic devices (such as solar cells), LEDs, and optoelectronic devices.

一、EL的物理机制

1. 载流子注入与复合发光

当器件（如太阳能电池或LED）处于正向偏压时，载流子（电子与空穴）通过电极注入并在有源区（如pn结、钙钛矿层）复合。复合过程中，部分能量以光子形式释放（辐射复合），形成EL信号。

关键参数：

辐射复合系数（B）：材料本征特性，决定理论**发光效率。

载流子浓度(n,p）：与注入电流密度及复合损失相关。

1. Carrier Injection and Recombination Luminescence

When a device (such as a solar cell or LED) is under forward bias, carriers (electrons and holes) are injected through the electrodes and recombine in the active region (such as the pn junction or perovskite layer). During the recombination process, a portion of the energy is released in the form of photons (radiative recombination), forming the EL signal.

Key parameters:

Radiative recombination coefficient (B): An intrinsic material property that determines the theoretical maximum luminescence efficiency.

Carrier concentration (n, p): Related to the injection current density and recombination losses.

2. 非辐射复合的影响

若器件存在缺陷态（如晶体缺陷、界面陷阱），部分载流子通过俄歇复合或SRH复合损失能量（发热），导致EL强度衰减。

2. The Impact of Non-Radiative Recombination

If a device has defect states (such as crystal defects or interface traps), some carriers lose energy through Auger recombination or SRH recombination (generating heat), leading to a decrease in EL intensity.

二、实验方法与系统搭建

1. 核心组件

2. EL成像流程参数设定：

校准电流注入条件（如太阳能电池的或LED的额定电流）。

调整积分时间避免信号过饱和（例如：硅电池EL通常需10–60秒积分）。

信号采集：

重复多次采集取平均（降低随机噪声）。

标定暗电流噪声（关闭偏压，记录本底信号）。

2. EL Imaging Process

Parameter Setting:

Calibrate the current injection conditions (such as the rated current of the solar cell or LED).

Adjust the integration time to avoid signal saturation (e.g., EL of silicon cells typically requires an integration time of 10–60 seconds).

Signal Acquisition:

Repeat multiple acquisitions and take the average (to reduce random noise).

Calibrate dark current noise (turn off the bias and record the background signal).

三、EL成像的应用场景

1. 器件缺陷诊断

隐裂检测（太阳能电池）：EL图像中暗线对应裂纹路径（因载流子无法通过裂纹区域复合），分辨率可达微米级。

接触失效分析（LED/光伏电池）： 电极边缘或栅线断裂导致的局部暗区 → 接触电阻升高或电流传输中断。

2. Application Scenarios of EL Imaging

1. Device Defect Diagnosis

Hidden crack detection (solar cells): Dark lines in the EL image correspond to the crack paths (since carriers cannot recombine in the crack region), with a resolution that can reach the micrometer level.

Contact failure analysis (LEDs/photovoltaic cells): Local dark areas caused by electrode edge or grid line fractures → Increased contact resistance or interrupted current transmission.

2. 材料质量评估

多晶硅晶界复合：晶界表现为暗线（复合中心聚集载流子），钝化处理后EL亮度提升如：钝化方法晶界对比度（GB）EL强度提升未钝化35% - 氢等离子体处理8% 200%

钙钛矿薄膜均匀性：EL亮度与晶粒尺寸正相关（大晶粒区域非辐射复合降低）。

2. Material Quality Assessment

Polycrystalline silicon grain boundary recombination: Grain boundaries appear as dark lines (recombination centers accumulate carriers), and EL brightness increases after passivation treatment. For example, passivation method grain boundary contrast (GB) EL intensity increase: Unpassivated 35% - Hydrogen plasma treatment 8% 200%

Perovskite film uniformity: EL brightness is positively correlated with grain size (non-radiative recombination is reduced in large-grain regions).

3. 载流子输运效率分析

通过EL强度分布与电流密度（J）的映射关系，反推测横向串联电阻（Rseries）和分流电阻（Rshunt）：

并联电阻评估：EL图像中异常亮斑（电流局部集中） → Rshunt ↓。

串联电阻诊断：边缘亮度衰减→ 电极接触电阻或横向导电层（如TCO）性能劣化。

3. Carrier Transport Efficiency Analysis

By mapping the relationship between EL intensity distribution and current density (J), the lateral series resistance (Rseries) and shunt resistance (Rshunt) can be inferred:

Shunt resistance evaluation: Abnormal bright spots in the EL image (local current concentration) → Rshunt ↓.

Series resistance diagnosis: Edge brightness attenuation → Deterioration of electrode contact resistance or lateral conductive layer (such as TCO) performance.

四、数据处理与定量分析

1. 全局参数提取

平均灰度（Iavg）：表征整体辐射复合效率（理想效率器件应接近理论值）。

均匀性指数（HU）：HU ≥ 95%。

2. Data Processing and Quantitative Analysis

1. Global Parameter Extraction

Average grayscale (Iavg): Characterizes the overall radiative recombination efficiency (an ideal efficiency device should be close to the theoretical value).

Uniformity index (HU): HU ≥ 95%.

2. 故障定位与分级

阈值分割算法：根据阈值（如平均强度的30%）识别暗斑/暗线 → 输出缺陷密度（单位面积缺陷数）。

缺陷分类示例（太阳能电池）：缺陷类型EL特征对效率的影响微裂纹细长暗线（宽度≈10 μm）Δη≈−2%焊点脱落圆形暗斑（直径>1 mm）Δη≈−5%边缘分流边缘亮带（异常电流集中）Δη≈−3%

2. Fault Location and Grading

Threshold segmentation algorithm: Identify dark spots/dark lines based on a threshold (such as 30% of the average intensity) → Output defect density (number of defects per unit area).

Defect classification example (solar cells): Defect type EL characteristics Impact on efficiency

Micro-cracks Fine dark lines (width ≈ 10 μm) Δη ≈ -2%

Solder joint detachment Circular dark spots (diameter > 1 mm) Δη ≈ -5%

Edge shunting Bright bands at the edge (abnormal current concentration) Δη ≈ -3%

五、联用技术与进阶分析

1. EL + IV曲线

效率损失分解：结合EL强度与IV参数（如FF、Rseries）的关联，量化复合损失与传输损失的权重比。

2. Combined Techniques and Advanced Analysis

1. EL + IV Curve

Efficiency loss decomposition: Quantify the weight ratio of recombination losses and transport losses by correlating EL intensity with IV parameters (such as FF, Rseries).

2. 动态EL成像（时间分辨）

脉冲调制分析：对Si电池施加脉冲电流（纳秒级），观测载流子扩散长度→对应于EL亮区传播速率。

寿命成像：使用时间分辨EL技术（TREL）测量局域载流子寿命τ(x,y) → 区分SRH与俄歇复合。

2. Dynamic EL Imaging (Time-Resolved)

Pulse modulation analysis: Apply a pulsed current (nanosecond level) to Si cells to observe the carrier diffusion length → Corresponding to the propagation rate of the EL bright region.

Lifetime imaging: Use time-resolved EL technology (TREL) to measure local carrier lifetime τ(x,y) → Distinguish between SRH and Auger recombination.

3. EL +热成像（红外）

焦耳热定位：EL暗区对应的温度异常升高 → 识别高串联电阻区域（如烧结不良的银栅线）。

3. EL + Thermal Imaging (Infrared)

Joule heat localization: Abnormal temperature increase corresponding to EL dark areas → Identify high series resistance regions (such as poorly sintered silver grid lines).

六、挑战与解决方案

6. Challenges and Solutions

总结

电致发光成像（EL）通过载流子注入条件下的复合发光，直观反映了光电器件的缺陷分布、材料质量与载流子输运特性。其核心优势包括：

非破坏性：无需切割或特殊处理样品。

高灵敏度：可检测微米级缺陷。

定量分析能力：与效率损失参数（Rseries、Rshunt）结合，指导精准工艺优化。

通过与其他技术（PL、热成像、IV测试）的协同，EL可提供多维度的器件诊断，成为提升太阳能电池效率、优化LED发光均匀性及保障器件可靠性的关键工具。

Summary

Electroluminescence imaging (EL) intuitively reflects the defect distribution, material quality, and carrier transport characteristics of optoelectronic devices through luminescence under carrier injection conditions. Its core advantages include:

Non-destructive: No need to cut or specially treat samples.

High sensitivity: Capable of detecting defects at the micrometer level.

Quantitative analysis capability: Combined with efficiency loss parameters (Rseries, Rshunt) to guide precise process optimization.

By working in conjunction with other technologies (PL, thermal imaging, IV testing), EL can provide multi-dimensional device diagnostics, becoming a key tool for improving solar cell efficiency, optimizing LED emission uniformity, and ensuring device reliability.