Team Information

First Author: Sai Ma 
Corresponding Authors: Yujing Li, Qi Chen
DOI: 10.1002/aenm.201902472

Key Highlights

• Scalable and Robust Encapsulation Strategy: This work paves the way for an economically viable, scalable, and robust encapsulation strategy for hybrid perovskite optoelectronics.
• Encapsulation Material Requirements: The paper introduces four critical requirements for encapsulation materials used in perovskite solar cell devices.
• Solvent-Free Low-Temperature Melting Encapsulation: Demonstrates a solvent-free, low-temperature melting encapsulation technique using low-cost paraffin as an encapsulant compatible with perovskite absorbers, enabling complete encapsulation operations under ambient conditions.
• Prevention of Phase Separation and Decomposition: The strategy not only removes residual oxygen and moisture to prevent perovskite phase separation but also suppresses material volatilization to inhibit absorber decomposition, resulting in PSCs with excellent thermal and moisture stability.
• Device Lifetime: The encapsulated PSCs achieved a 1000-hour operational lifetime under continuous maximum power point output at room temperature conditions.

Introduction

In January 2020, Advanced Energy Materials published research on solvent-free low-temperature encapsulation technology for perovskite solar cells by Professor Qi Chen of Beijing Institute of Technology and colleagues. The paper discusses the advantages of solvent-free and low-temperature processing encapsulation strategies, particularly highlighting the benefits of low-cost paraffin as an encapsulation material. Its unique solvent-free, low-temperature processing capability is compatible with perovskite absorbers and can be processed under ambient conditions, showing great scalability potential.

Furthermore, non-polar paraffin effectively removes residual oxygen and moisture during encapsulation, preventing perovskite phase separation and suppressing perovskite volatilization, thereby inhibiting absorber decomposition. With these advantages, the encapsulated devices maintained over 80% of their initial efficiency after 1000 hours of continuous testing under maximum power point (MPP) in ambient conditions, demonstrating exceptional stability.

Background

For commercial solar cells, efficiency, lifetime, and cost are the three most critical parameters. While perovskite solar cells offer highly competitive efficiency at approximately half the cost of silicon solar cells, their lifetime remains the primary concern. Therefore, improving long-term stability to extend device lifetime is the top priority in perovskite solar cell development. Recently, various chemical strategies, including doping, compositional engineering, dimensional engineering, grain boundary modification, and functional transport material design, have been employed to address the inherent instability issues of perovskites, such as ion migration, thermal decomposition/phase changes, and hygroscopic materials.

However, improvements remain limited, particularly when perovskite solar cells are exposed to ambient conditions and continuous operation. The ionic crystal behavior and compositional characteristics make perovskite materials sensitive to key environmental elements, especially oxygen and moisture, leading to rapid material degradation and device performance decline. Isolating perovskite solar cells from these elements is crucial for protecting them against environmentally induced degradation.

Encapsulation, widely used in commercial electronic devices, has been applied to perovskite solar cells and shows significant enhancement in device stability. To date, several encapsulation strategies have been used for PSCs, but a key remaining issue is that commonly used encapsulation materials cannot meet long-term stability goals. For encapsulation materials used in PSC devices, four requirements must be met in addition to their inherent long-term stability:

• Chemical Inertness: Must be chemically inert when exposed to materials used in solar cell devices, capable of direct contact with the cell while preventing material volatilization;
• Solvent-Free or Non-Destructive Solvents: Must be solvent-free or at least free of destructive solvents during the encapsulation process, as perovskite materials and organic transport materials are sensitive to most organic solvents;
• Low-Temperature Processing: Requires encapsulation processing temperatures not exceeding 150-170°C due to poor thermal stability of perovskites;
• Low Water Vapor Transmission Rate: Must have low water vapor transmission rate (WVTR) to effectively prevent moisture infiltration. Additionally, encapsulation costs and environmental processability are equally important for large-scale production.

This paper demonstrates the advantages of low-cost paraffin encapsulation material as a solvent-free and low-temperature processing encapsulation strategy for perovskite solar cells. Its unique solvent-free, low-temperature processing capability is compatible with perovskite absorbers and can be processed under room temperature ambient conditions, showing great potential for industrial scalability.

Figure Analysis

Figure 1. Top-view SEM images and PL spectra of perovskite films.

a) Fresh sample, b) unencapsulated sample, c) N₂ and d) UVCA films in ambient environment.
(b)-(d) were aged under illumination in ambient environment for 1 hour. e) XRD patterns and f) steady-state PL spectra of perovskite film samples prepared under different conditions.

Figure 2. Encapsulation structure diagram.

Schematic illustration of encapsulation structure with a) UVCA with paraffin and b) UVCA without paraffin. Side-view photographic images of devices encapsulated by UVCA with paraffin c) and e). d) and f) UVCA without paraffin.

Figure 3. Voc light dependence and ideality factor n.

Using Enlitech’s solar simulator, which can automatically adjust light intensities from 2.6 to 100 mW cm⁻² (0.26 to 1 sun), combined with IVS-KA6000 software for automatic measurement of light-intensity-dependent Voc, the corresponding parameters are plotted in the figure above. The light-intensity-dependent Voc data plot (Sun-Voc) for encapsulated devices after 6 hours of illumination, and the ideality factor n obtained from fitting the Sun-Voc relationship. “Reference” represents the fresh device.

Furthermore, the study indicates that defects can be investigated by studying energy losses in each device. Light-intensity-dependent Voc can provide important insights into recombination process mechanisms in PV devices. Under Voc conditions, there is no net current (J = 0 mA cm⁻²) flowing through the device, so all photogenerated charge carriers should recombine within the perovskite film. The corresponding charge carrier recombination process is reflected by the ideality factor n, which is determined by the slope of Voc versus incident light intensity, as shown in the formula:

where q is the elementary charge, k is the Boltzmann constant, T is temperature, and Φ is light intensity. The KA-Viewer software can accurately fit the ideality factor n. When the ideality factor n approaches 2, Shockley-Read-Hall (SRH) type, trap-assisted recombination dominates. Conversely, in the case of free electron and hole recombination, the ideality factor should be 1. (Reference link)

The ideality factors n calculated from the fitting curves are 1.54, 1.58, and 1.84 for fresh devices, UVCA with/without paraffin encapsulation devices respectively, with encapsulated devices aged under illumination for 6 hours before testing. From the changes in ideality factor, it can be concluded that the trap-assisted recombination in aged UVCA with paraffin encapsulation is close to that of fresh devices, due to the suppression of defect generation by paraffin encapsulation. However, UVCA without paraffin encapsulation shows a significant change from 1.54 to 1.84, indicating a higher trap-assisted recombination rate. The above results suggest that besides suppressing phase separation and degradation, UVCA with paraffin encapsulation also shows superior advantages in suppressing defect generation. The non-polar nature enables paraffin to completely remove residual oxygen and moisture, thereby achieving excellent material stability.

Figure 4.

a) J-V curves of devices before and after encapsulation. b) External quantum efficiency and corresponding integrated photocurrent Jsc(EQE) of PSC devices with paraffin encapsulation UVCA. c) Thermal stability and d) humidity stability traces of encapsulated devices. e) Continuous MPP tracking under ambient environment with different encapsulation conditions.

UVCA devices with paraffin encapsulation demonstrate excellent stability. Even after 1000 hours of MPP measurement, the device retains over 80% of its initial PCE. This work demonstrates that PSCs with ambient encapsulation can also survive over 1000 hours in MPP tracking under ambient conditions. To our knowledge, this is also one of the few works where the encapsulation process was completed under ambient conditions. Therefore, we believe this low-cost and direct encapsulation method will bridge the gap between fundamental research and commercialization of PSCs.

Summary

This work developed a low-temperature (below 100°C) encapsulation technology compatible with perovskite solar cells by using non-polar, low-cost paraffin as an encapsulant. The low-melting-point paraffin was found to remove residual oxygen and moisture during encapsulation and prevent the escape of volatile substances during perovskite decomposition. Through this encapsulation, the perovskite absorption layer showed significantly less phase separation and vacancy defect generation, thereby suppressing film decomposition and substantially improving the device’s thermal and moisture stability. Ultimately, the resulting devices achieved excellent long-term stability, maintaining over 80% of their initial efficiency for more than 1000 hours under MPP tracking. More importantly, this paraffin-based solvent-free encapsulation method can be operated under ambient temperature conditions, making it more amenable to large-scale production. Therefore, it provides new insights for further development of encapsulation technologies for commercial perovskite optoelectronic applications.

Publication 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, and Qi Chen
DOI: 10.1002/aenm.201902472