How to analyze the defects in perovskite devices and measure diode ideality factor?

Understanding the I-V characteristic of the internal structure of device is fundamental to develop a highly efficient and highly stable solar cell. 
When measuring I-V characteristic in a p-n diode one needs to consider the diffusion current, recombination current and series resistance effects, which could result in the device’s performance. Therefore, researchers could know whether the device is parallel to an ideal diode by calculating the ideality factor η.

The ideality factor η is a number between 1 and 2. An ideal diode has an ideality factor of 1, indicating the structure of the p-n device is perfect with no defects, while an ideal diode is impossible to produce. Scientists aim to fabricate a diode which diode characteristics curve could approaching the ideal diode the most. Herein, we introduce a method to fine out the defects of devices and then improving the fabrication techniques by analyzing the ideality factor of the diode.

But how to exactly analyze the ideality factor of the diode?
The target journals indicate that the most effective method to analyze the ideality factor is through the adjustment of light intensity measurement.

High precision light intensity adjustment function
Researchers could do an experiment to measure the ideality factor, and find the relationship between J-V curve and other photoelectric parameters such as Jsc, Voc, FF, etc., through the adjustment of light intensity measurement. And it is easy for researchers to use these raw data to fit a graph and analyzing the devices’ structure.
Take these researchers’ works adopting the adjustment of light intensity measurement to analyze the ideality factor of the devices for examples.

   《Nature Photonics》” Surface passivation of perovskite film for efficient solar cells.”
In this study led by Prof. Jinbi You, the Institute of Semiconductors at CAS, the team tested the device response under different light intensities and found that both the control and PEAI-treated devices show a linear Jsc versus light intensity relationship. This indicates that carriers can transport smoothly in the device and that there is no obvious charge barrier induced at the interface even though insulating PEAI was introduced. The relationship between Voc and light intensity is also plotted. It can be seen that the control device shows a slope of 1.88kBT/q, while PEAI-treated devices show a much smaller slope (1.27kBT/q,), where kB is the Boltzmann constant, T is temperature and q is the electric charge, It is known that deviation of slope from kBT/q reflects defect-assisted recombination in the devices. These results further confirmed that the recombination has largely been suppressed in the perovskite layer.


   《Nature Communications》"Strain engineering in perovskite solar cells and its impacts on carrier dynamics."
Prof. Qi Chen, Beijing Institute of Technology, and Prof. Huanping Zhou, Peking University, and Prof. Lijun Zhang, Jilin University, and other materials scientists have developed the high efficiency perovskite films with 20.7 % PCEs, which is certified by NIM, China, by modulating the status of residual strains in controllable manner via rational strain engineering.
To investigate how the carrier recombination process of devices is affected by the residual strain, the team conducted the combined measurement of light-intensity-dependent Voc, electrochemical impedance spectroscopy (EIS) and transient photovoltage decay (TPV). The light-intensity-dependent Voc provides critical insights into the mechanism of recombination processes in solar cells. The corresponding charge carrier recombination process is reflected by the ideality factor of n as determined by the slope of the Voc versus incident light-intensity according to the equation Voc= Eg/qnkBT/qlnJ0/J; where q is the elementary charge, kB is the Boltzmann constant, and T us the temperature. When the ideality factor n approaches 2, Shockley-Read-Hall (SRH) type, trap-assisted recombination dominates. As shown the figure bellow, from the relationship between Voc~ln(I), the ideality factor n are 1.01, 1.55 for the devices with/without tensile strain, respectively. It indicated that trap-assisted SRH recombination is effectively suppressed by reducing the tensile strain that is mainly located at the absorber surface. The alleviated SRH recombination may be attributed to the reduced trap density in strain-free devices, wherein crystal structure homogeneity is achieved.

Are you still using the conventional filter to adjust light intensity manually? Generally, the conventional measurement of manual light intensity adjustment function has difficulty in calibrating the light intensity, and it takes lots of effort to position the light intensity and to measure at least 15 different light intensities to fit a graph and analyze it. 

Enli Technology develops a total solution integrating AAA steady state solar simulators with automatic light intensity adjustment function and KA5000 IV tracer software, specially designed for analyzing defects of photovoltaic devices and diode ideality factor.

Through the high-precise light intensity adjustment function, Enli Tech SS-X series solar simulators can do the measurement of different light intensities from 0.02 sun to 1 sun and obtain enough data points. It’s easy for researchers to measure characteristics of devices under low light intensity. In this way, researchers can analyze the ideality factor and defects in perovskite devices through the adjustment of light intensity measurement.