Scientific Column
Scientists Qi Chen et. al. in BIT Reported 1000hr Solvent-free Low-temperature Encapsulation Technology for Perovskite Solar Cells on Advanced Energy Materials
Highlights
- This work paves the way for an economically feasible, scalable and robust encapsulation strategy for hybrid perovskite optoelectronics.
- The article describes 4 important requirements for encapsulation materials of the perovskite solar cells.
- By using low-cost paraffin as an encapsulant, compatible with the perovskite absorber, a solvent-free low-temperature melting encapsulation technology was demonstrated, enabling the complete encapsulation operation to be carried out in the surrounding environment.
- This strategy can not only remove residual oxygen and moisture to prevent phase separation of the perovskite, but also inhibit material volatilization to prevent the decomposition of the absorbent. So that the perovskite solar cells device has good thermal and moisture stability.
- The packaged devices achieves a 1000-hour working lifetime at the continuous maximum power point output in the ambient environment.
Summary
In January 2020, Advanced Energy Materials magazine published a solvent-free low-temperature packaging technology for perovskite solar cells developed by Professor Chen Qi from Beijing Institute of Technology. The article mentioned the advantages of solvent-free and low-temperature processing packaging strategies, namely the advantages of low-cost packaging materials paraffin. Its unique solvent-free low-temperature processing capability is compatible with perovskite absorbents, can be processed under ambient environments, and has scalability.
In addition, non-polar paraffins can effectively remove residual oxygen and moisture during the encapsulation process, prevent the perovskite phase separation, inhibit the volatilization of the perovskite, and hinder the decomposition of the absorbent. With these advantages, the packaged device still maintains more than 80% of the original efficiency after 1000 hours of continuous testing under the maximum power point (MPP) under ambient environments, thus showing excellent stability.
Background
For commercial solar cells, efficiency, lifetime and cost are the three most critical parameters. With extremely competitive efficiency, the cost of perovskite solar cells is only about half of that of silicon solar cells, and their lifetime is the main issue. Therefore, improving the long-term stability to extend the life span is the primary task for the development of perovskite solar cells. Recently, many chemical strategies, including doping, composition engineering, size engineering, grain boundary modification, and functional transport material design have been used to solve the inherent instability problems of perovskite, such as ion migration, thermal decomposition/phase change and Hygroscopic material.
However, the improvement effect is still limited, especially when the perovskite solar cell is exposed to the surrounding environment and continuous operating conditions. Ionic crystal behavior and composition characteristics are the main species that make perovskite materials sensitive to environmental conditions (especially the two main factors that are sensitive to oxygen and moisture) leading to rapid material degradation and device performance. Separating perovskite solar cells from these species is essential to protect them from environmentally-induced degradation.
The encapsulation has been widely used in commercial electronic devices, and has also been used in perovskite solar cells which has shown a significant increase in device stability. So far, several packaging strategies have been used for perovskite solar cells, but a key problem that still exists is that the commonly used packaging materials cannot meet the long-term stability goal. For packaging materials used in perovskite solar cells devices, in addition to their inherent long-term stability, four requirements must be met:
1) It is chemically inert when exposed to the materials used in solar cell devices and can directly contact the battery to avoid species volatilization.
2) Because perovskite materials and organic transport materials are sensitive to most organic solvents, they cannot contain solvents during the packaging process, or at least no destructive solvents.
3) Low temperature processability, because of the poor thermal stability of perovskite, the packaging process is required to be no higher than 150-170°C.
4) Low water vapor transmission rate (WVTR), effectively preventing moisture intrusion. In addition, the cost of packaging and environmental processability are equally important for mass production.
This article demonstrates the advantages of low-cost packaging material paraffin as a solvent-free and low-temperature processing packaging strategy for perovskite solar cells. Its unique solvent-free low-temperature processing ability is compatible with perovskite absorbents and can be processed under ambient conditions at room temperature, which has deep industrial scalability.
Key Results
Figure 1. Top view SEM image and PL spectra of perovskite film.
a) Fresh samples, b) Unencapsulated samples, c) N2 and d) UVCA encapsulation film in the ambient environment. (b)-(d) Aging for 1 hour under the ambient light. e) XRD patterns and f) steady-state PL spectra of perovskite film samples prepared under different conditions.
Figure 2. Encapsulation structure diagram.
a) UVCA with paraffin wax and b) Schematic diagram of the encapsulation structure of UVCA without paraffin wax. Photographs of both sides of the device encapsulated by UVCA with paraffin wax of c) and e). d) and f) paraffin-free UVCA.
Figure 3. Voc-light-dependent curve and ideality factor n.
Using Enlitech’s solar simulator can automatically adjust the incident light intensity from 2.6 to 100 mW cm-2 (from 0.26 to 1 sun) and cooperate with the IVS-KA6000 software to automatically measure the Voc related to the light intensity. The corresponding parameters are drawn in the figure above. The light intensity-related Voc data graph (Sun-Voc) of the packaged device after 6 hours of light, and the ideal factor n obtained by fitting the Sun-Voc relationship. “Reference” is a fresh device.
In addition, the article points out that defects can be studied by studying the energy loss of each device. The light-intensity-dependent Voc can provide important insights into the mechanism of the recombination process in PV devices. At Voc, there is no net current (J = 0 mA cm−2) through the device, so all photo-generated charge carriers should be recombined in the perovskite film. The corresponding charge carrier recombination process is reflected by the ideal factor n, which is determined by the slope of Voc and the incident light intensity, as shown in the formula:
Where q is the basic charge, k is Boltzmann’s constant, T is the temperature, and Φ is the light intensity. Using KA-Viewer software can accurately fit the ideal factor n. When the ideal factor n is close to 2, Shockley-Read-Hall (SRH) type, trap-assisted compound dominates. On the contrary, in the case of recombination of free electrons and holes, the ideality factor should be 1. (Refer to Link) The ideality factor n calculated from the slope of the fitted curve is 1.54, 1.58 and 1.84 for the fresh device, UVCA with/without paraffin encapsulated device. The packaged device is aged for 6 hours under light before testing. It can be concluded from the change of the ideal factor that the trap assisted recombination in the aging UVCA of the paraffin encapsulation is close to the fresh device, which is because the UVCA of the paraffin encapsulation inhibits the generation of defects.
However, UVCA without a paraffin encapsulation device showed a significant change from 1.54 to 1.84, indicating a higher trap-assisted recombination rate. The above results indicate that in addition to inhibiting phase separation and degradation, UVCA with paraffin encapsulation also shows superior advantages in inhibiting defects. Non-polarity enables paraffin wax to completely remove residual oxygen and moisture, thereby achieving excellent material stability.
Figure 4.
a) J-V curve of the device before and after packaging. b) The external quantum efficiency of UVCA and the corresponding integrated photocurrent Jsc (EQE) of the perovskite solar cells device with paraffin encapsulation. c) thermal stability and d) humidity stability trajectory of the packaged device. e) Continuous MPP tracking under different packaging conditions.
UVCA with paraffin encapsulation device shows excellent stability. Even after 1000 hours of MPP measurement, the device still retains more than 80% of its initial PCE. This work shows that the perovskite solar cells encapsulated by the environment can also survive the MPP tracking under the environment for more than 1000 hours. As far as we know, this is also one of the few works that complete the packaging process in the ambient environment. Therefore, we believe that this low-cost and direct packaging method will shorten the gap between basic research and commercialization of perovskite solar cells.
Conclusion
In this paper, a low-temperature (below 100°C) encapsulation technology compatible with perovskite solar cells is developed by using non-polar low-cost paraffin as an encapsulant. It was found that low melting point paraffin can remove residual oxygen and moisture during the encapsulation process and prevent the escape of volatile substances when the perovskite decomposes. Through this encapsulation, the phase separation of the perovskite absorption layer and the generation of vacancy defects are much less, thereby inhibiting the decomposition of the film, and significantly improving the thermal stability and moisture stability of the device.
In the end, the resulting device achieved excellent long-term stability, and the efficiency remained 80% of the initial value under MPP tracking for more than 1000 hours. More importantly, this paraffin-based solvent-free encapsulation method can be operated at room temperature and is easier to be adopted in large-scale production. Therefore, it provides new insights for the further development of packaging technology for the commercial utilization of perovskite optoelectronics.
Article Information
1000 h Operational Lifetime Perovskite Solar Cells by Ambient Melting Encapsulation
Sai Ma, Yang Bai, Hao Wang, Huachao Zai, Jiafeng Wu, Liang Li, Sisi Xiang, Na Liu, Lang Liu, Cheng Zhu, Guilin Liu, Xiuxiu Niu, Haining Chen, Huanping Zhou, Yujing Li, Qi Chen
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