您将学到什么：
- The relationship between RF energy and assessing potential photovoltaic-material performance.
- 这两种重叠的方式激发和测量PV材料（例如钙钛矿）。
- 如何解决冲突的测试结果。
It may seem from a first glance that there’s little in common between the dc performance of solar cells and their constituent photovoltaic (PV) materials and the RF world of microwaves and terahertz (THz) energy. Of course, a close connection exists at the atomic-physics level, as the mobility and lifetime of electrons and holes in the PV material translate into light-induced current flow in PV systems when energized by electromagnetic radiation.
这两个参数（运动和寿命）是半导体最重要的特性之一，因为需要大的电荷运输迁移率来实现有效的电荷注入和提取。
Both parameters can be measured without contacts via spectroscopic methods using terahertz or microwave radiation. However, the resultant measurement data found in literature often differs by orders of magnitude. This has made it difficult to use the results for reliable assessments of material quality.
Higher-Accuracy PV-Material Performance Determination
近年来，钙钛矿半导体特别引起了人们的关注，因为它们相对便宜，易于处理并且能够高效。现在，研究人员Helmholtz-Zentrum柏林（HZB）在更准确地确定PV材料（例如钙钛矿）的潜在性能方面迈出了重要一步，而无需制造太阳能电池或进行物理接触。他们结合了15个实验室的专业知识，以定量建模并分析此类测量值的太阳能电池的电流 - 电压特性。
笔记：If you’re not familiar with HZB, it’s the result of the merger of two research institutions—the Hahn-Meitner-Institut (established in 1959) and BESSY GmbH (1979). With approximately 1,100 employees, it’s one of the largest non-university research centers in Berlin.
TRMC和OPTP
Both charge-carrier mobilities and lifetimes can be probed without physical contact by Time-Resolved Microwave Conductivity (TRMC) and Optical-Pump Terahertz-Probe (OPTP) spectroscopy, making them excellent tools for the characterization of photovoltaic materials（图。1）。OPTP can monitor fast processes like trapping into defect states and exciton formation, while TRMC adds the slower processes such as long-living trapped carriers or weak recombination in advanced materials, e.g., halide perovskites.
非接触式的测量是一个major advantage of OPTP and TRMC over more conventional techniques such as dc-conductivity and Hall-effect measurements. That’s because it overcomes interface issues such as contact resistance, unintentional doping, and bandwidth limitation, and it allows for the study of bare films and even powders. However, these measurements of an electrical potential corresponding to solar-cell operating conditions can’t be applied directly due to the lack of those same contacts.
新的Measurement Process
Therefore, the OPTP and TRMC measurement conditions must be translated to the specifics of the thin-film device, which results in different interpretations due to subtleties and difference in the models used by the different institutions. To overcome this obstacle, the HZB team worked with the other institutions to develop a consensus on the interpretation of TRMC and OPTP measurements, using a halide perovskite (Cs,FA,MA)Pb(I,Br)3thin film as a case study.
This measurement process is a complex multistep sequence:
- 第一步是表征载体诱捕和激子形成（激子是电子和电子孔的结合状态，它们被静电库仑力彼此吸引），因为相应的量子产率对于分配相关的寿命和相关的寿命至关重要电子和孔流动性（图2）。
- Second, material is exposed to continuous “1 sun” illumination while OPTP and TRMC spectroscopy are measured after pulsed excitation.
- 第三，OPTP光谱法（图3）or TRMC spectroscopy（图4）用于测量分别以THZ或GHz频率交替的电场中电荷载体的迁移率，必须通过薄膜太阳能电池将其转换为DC-Transport。
- 最后,结果的精度as determined by an inter-laboratory comparison. Resolving the differences and providing final data with confidence was a primary goal of the project.
两种技术都通过测量光导不传导性瞬变以及迁移率来探测光激发电子，孔和激子的形成和动力学。来自光泵的脉冲诱导（钙钛矿）样品中电荷载体的光生成。根据它们的迁移率，这些载体在Terahertz探针脉冲的电场或腔中的站立微波炉中横向加速，分别在Terahertz或Gigahertz频率上交替。
The photogenerated carriers then acquire an oscillating drift velocity, and thereby a part of the terahertz or microwave radiation is absorbed. The corresponding pump-induced change in the transmitted terahertz electric field (ΔE/E) or in the reflected microwave power (ΔP/P) is measured and analyzed in a first step for the sheet photoconductivity or photoconductance. TRMC and OPTP measurements yield mobilities for charge carriers oscillating at GHz and THz frequencies.
为了表征DC设备的性能，必须估算与直接传输有关的有效移动性，这是通过其他数据分析和转换来完成的，以确定所需的参数。
通过结合15个实验室的专业知识，HZB工作确定了错误来源。因此，它为光伏材料的解释测量提供了一个“最佳实践”模型，以更好地预测其在太阳能电池中的潜在性能（图5）。
The work is detailed in their lengthy and intense, yet informative and readable, 16-page paper “Predicting Solar Cell Performance from Terahertz and Microwave Spectroscopy“ 出版于Advanced Energy Materials（来自15个机构的29位作者！）。这是23页的支持Supporting Information文档。
