Introduction
Perovskite solar cells (PSCs) have attracted attention because of their low cost and high conversion efficiency. The high performance of PSCs is closely related to the quality of the light-absorbing layer, which in turn depends on the crystallization conditions. During formation of high-quality crystals, annealing is an essential step that evaporates solvents and drives film crystallization.
Heating a material to a set temperature for a period of time is a widely used thermal treatment across many types of solar cells. Thermal processing relieves residual stress and internal defects while adjusting the material's microstructure. In silicon cell manufacturing, sintering via annealing reduces the electrical resistivity of surface electrodes and produces low-resistance contacts. During perovskite crystallization, the annealing method, duration, and temperature strongly influence film crystallinity and crystal structure.
The formation of perovskite films involves two relatively independent stages: nucleation and growth. By adjusting processing parameters, these two stages can be controlled separately to manage film growth. Film uniformity and coverage are directly related to the behavior in these two stages.
Goals of Annealing
Annealing of perovskite films is a multiobject process intended to improve overall device performance and lifetime by removing residual solvent, stabilizing crystal structure, increasing carrier mobility, eliminating impurities, tuning crystallinity and morphology, enhancing stability and durability, and promoting film formation and growth. The main objectives are:
- Remove residual solvent: Solvents are often used during perovskite preparation to improve solubility and crystallization. Annealing removes these residual solvents, improving device performance and stability.
- Stabilize crystal structure: Perovskite materials can adopt multiple crystal structures, which affect device performance. High-temperature annealing promotes crystal rearrangement and stabilization.
- Increase carrier mobility: Carrier mobility affects photovoltaic efficiency. During annealing, increased temperature can raise carrier mobility and thus device efficiency.
- Eliminate impurities and improve crystal quality: Annealing can remove impurities in perovskite films, improving crystal quality.
- Tune crystallinity and morphology: Adjusting crystallinity and crystal morphology by annealing can improve photoconversion efficiency.
- Enhance stability and durability: Annealing helps improve device stability and durability, extending operational lifetime.
- Promote film formation and growth: Annealing supports perovskite film formation through controlled nucleation and growth stages.
Thermal Annealing
Direct thermal annealing of the photoactive layer is a simple and widely used crystallization method. During thermal treatment, there is often an annealing window that significantly improves film crystallinity. Choosing the annealing window is critical to crystal growth. For example, for a mixed-halide perovskite Csx(FA0.83MA0.17)1-xPb(I0.83Br0.17)3, film growth proceeds through three stages with different crystal phases.
The first stage is precursor dissolution; the second stage is the dominant δ phase (2H); the third stage includes multiple intermediate phases, including two hexagonal polymorphs (4H, 6H), an MAI–PbI2–DMSO intermediate, and the α phase. Annealing in the second stage leads to stronger diffraction intensity and preferred orientation, indicating a more internally stable and symmetric structure. Annealing within the proper window can avoid formation of undesirable hexagonal polymorphs during crystallization. The annealing time and temperature determine crystal quality. However, because the perovskite structure can be fragile, prolonged thermal annealing may cause film degradation and compromise device stability.
Annealing Time and Temperature
In fact, annealing time and temperature determine crystal quality. By tuning these parameters, perovskite layers with different crystallization behaviors can be obtained. Because perovskite structures are fragile, long-term thermal annealing can lead to film decomposition and reduced device stability. Some researchers have used high-temperature rapid crystallization to shorten annealing time. Short, high-temperature annealing can drive solvent removal, create supersaturation, accelerate nucleation, and increase nucleation density, which improves carrier transport. This rapid annealing approach is attractive for production but may sacrifice crystal quality. Rapid solvent evaporation can cause abrupt grain contraction. Films can become rough and appear turbid under scattered light, especially at elevated temperatures. Immediate annealing after spin coating causes rapid volume shrinkage, leaving insufficient time for solute diffusion and resulting in rough surfaces and continuous cracking at grain boundaries. Considering that slower solvent evaporation can both reduce crystallization speed and improve crystal quality, gradient thermal annealing is an effective method to slow solvent evaporation and the film crystallization process. Compared with direct annealing, gradient annealing produces a slower color change from yellow to black, indicating a slower crystallization rate. Therefore, control of annealing time and temperature is critical to perovskite film quality; annealing equipment that can precisely control temperature and provide uniform heating is important for successful research and production.
Challenges in Industrial Production
In industrial manufacturing of perovskite solar cells, the post-coating annealing and crystallization step after slot-die coating is among the most challenging processes. During annealing, differences in thermal conductivity among device materials can cause uneven surface temperatures, which affects yield during the crystallization step.
