Perovskite solar cells (PSCs) have become the core direction of next-generation photovoltaic technology due to their high theoretical photoelectric conversion efficiency (certified efficiency of single junction perovskite cells has reached 26.1%), low preparation cost, and adjustable bandgap (1.2-2.3 eV). Among them, the perovskite absorbing layer (such as MAPbI ∝, FAPbI ∝, Cs ₀) ₁₅FA₀. ₈₅Pb(I₀. ₈₅Br₀. The quality (crystallinity, defect density, interface passivation) of (₁₅) ∝ directly determines the carrier transport efficiency and device stability. ​Flash Evaporation Deposition (FED)By rapidly evaporating perovskite precursors (such as PbI ₂, MAI, FAI, etc.) at high temperatures, an initial film is formed on the substrate surface by condensation, and then transformed into perovskite phase through subsequent annealing or in-situ reaction, providing a new path for preparing high-quality absorbing layers. However, the complex composition (multi cation/halogen mixture), defect sensitivity, and dynamic non-equilibrium characteristics of the flash evaporation process of perovskite materials pose unique challenges to the optimization of process parameters and membrane quality control.

1Perovskite flash extraction film-forming instrumentCore process parameters and their influencing mechanisms

　　Perovskite flash extraction film-forming instrumentThe key lies in precise controlEvaporation rate, substrate temperature, atmospheric environment, multi-source co evaporation ratio, and time sequenceBy adjusting the nucleation density, grain growth direction, and phase transition process of the precursor, high crystallinity and low defect density perovskite thin films can be obtained. The following are the key parameters and their mechanisms of action:

1. Evaporation rate and film thickness uniformity

​DefinitionThe mass of precursor deposited on the substrate surface per unit time (usually expressed in Å/s or nm/s) is determined by the power of the evaporation source (resistance heating temperature), the design of the evaporation boat (such as the surface area and shape of the tungsten boat), and the vacuum degree of the chamber (affecting vapor diffusion).

​Impact​：

​Low speed(<0.1 nm/s): The precursor atoms/molecules have a long diffusion time on the substrate surface, which can easily form randomly nucleated large grains (but may introduce more defect sites due to slow condensation);

​The speed is too high(>10 nm/s): High supersaturation of gas-phase components leads to instantaneous nucleation density (>10 ¹ ²/cm ²), forming nanoscale grain aggregates (grain size<10 nm), increasing the proportion of grain boundaries (increasing carrier recombination centers), and uneven film thickness (edge to center difference>20%);

​Optimization scopeThe commonly used rate in the laboratory is 0.5~3 nm/s (for single component PbI ₂ or MAI), and when multiple components are co evaporated, the rate ratios of each source need to be coordinated (for example, the evaporation rate ratio corresponding to the PbI ₂: MAI=1:1 molar ratio needs to be adjusted according to the vapor pressure).

2. Matching of substrate temperature and lattice

​DefinitionReal time temperature (usually room temperature~150 ° C) of the substrate (such as FTO conductive glass, TiO ₂/ETL electron transport layer) during the deposition process.

​Impact​：

​Low temperature substrate(Room temperature~50 ° C): The precursor has a fast condensation rate and is prone to form amorphous or microcrystalline precursor films (requiring subsequent high-temperature annealing to transform the perovskite phase), but the amorphous film has many dangling bonds (high defect density,>10 ¹⁶/cm ³), which may cause cracking due to lattice stress during annealing;

​Medium high temperature substrate(80-120 ° C): Promotes surface diffusion and ordered arrangement of precursor molecules, forming metastable intermediate phases (such as PbI ₂ two-dimensional layered films or MAI-PbI ₂ pre reaction intermediates). During subsequent annealing, the nucleation energy barrier of the perovskite phase decreases (grain size increases to 100-500 nm), and grain boundary defects decrease;

​High temperature substrate(>150 ° C): May cause precursor decomposition (such as MAI deamination to generate MA ⁺ fragments) or excessive interface reactions (such as interface recombination enhancement between electron transport layer TiO ₂ and perovskite).

3. Atmosphere environment (inert/reactive gas)

​DefinitionThe type of protective gas (Ar, N ₂) and pressure (1-100 Pa) inside the chamber, and some studies introduce trace reactive gases (such as O ₂, CH ∝ NH ₂ vapor) to regulate phase transformation.

​Impact​：

​Inert atmosphere (Ar/N ₂, 1-10 Pa)Inhibit precursor oxidation (such as PbI ₂ being oxidized by O ₂ to PbO), but too low pressure (<1 Pa) may lead to rapid diffusion of gas-phase components and uneven nucleation density;

​Medium high pressure (10~50 Pa)Increase the probability of gas-phase collision, slow down the condensation rate, and promote orderly grain growth (the surface roughness of perovskite film decreases, and the root mean square roughness RMS decreases from 50 nm to 10-20 nm);

​Reactive regulationIntroducing trace amounts of CH ∝ NH ₂ vapor during MAI evaporation (or adjusting the ratio of iodomethamine by co evaporating MAI with a small amount of HI) can promote the in-situ reaction between PbI ₂ and MAI (reducing intermediate phase residue) and improve the purity of perovskite phase.

4. Multi source co evaporation ratio and timing (for multicomponent perovskites)

​DefinitionFor mixed cations (such as FA ⁺+Cs ⁺) or mixed halogens (such as I ⁻+Br ⁻) perovskites, synchronous or stepwise evaporation from multiple evaporation sources (such as PbI ₂, FAI, CsI, PbBr ₂) is required to precisely control the molar ratios of each component (such as FA ₀) ₈₅Cs₀. ₁₅Pb(I₀. ₈₅Br₀. ₁₅동).

​Impact​：

​Proportional imbalanceFor example, when MAI is insufficient, unreacted PbI ₂ remains (XRD detected PbI ₂ diffraction peak), leading to photo generated carrier traps (non radiative recombination centers); When FAI is excessive, it may form a rich FA phase (low bandgap region), which affects energy level matching;

​Timing controlStep by step evaporation (depositing PbI ₂ layer first, then covering MAI layer) can optimize crystal orientation through interface reactions (such as preferential growth of (110) crystal plane, arranged perpendicular to the substrate, which is conducive to lateral carrier transport); When co steaming, it is necessary to synchronously adjust the power of each source to maintain a stable molar ratio (deviation<± 2%).

2、 Key indicators and evaluation methods for membrane quality

The quality of perovskite thin films needs to be comprehensively evaluated through multidimensional parameters, mainly including:

1. Crystallinity and phase purity

​Characterization methodsX-ray diffraction (XRD) - determining crystallinity by the intensity and full width at half maximum (FWHM) of characteristic peaks (such as 12.6 ° for PbI ₂ and 14.1 °/28.4 ° for perovskite) (the smaller the FWHM, the larger the grain size, and the higher the crystallinity); UV Vis absorption spectroscopy - the strong light absorption edge of perovskite (~780-850 nm, depending on the halogen ratio); Raman spectroscopy - distinguishing PbI ₂ (~120 cm ⁻¹), MA ⁺ vibration modes (~900-1000 cm ⁻¹), and characteristic peaks of perovskite (~950-1050 cm ⁻¹).

​GoalPerovskite phase has high diffraction peak intensity (no impurity peaks), grain size>200 nm (calculated by Scherrer formula), and residual PbI ₂<5% (XRD peak area ratio).

2. Microscopic morphology and grain structure

​Characterization methodsScanning Electron Microscopy (SEM) - Observe the surface morphology of thin films (grain continuity, number of pores/pinholes), cross-sectional morphology (uniformity of film thickness, target deviation<10%); Atomic Force Microscopy (AFM) - Surface roughness (RMS<20 nm, favorable for subsequent electron/hole transport layer deposition); Transmission Electron Microscopy (TEM) - Grain Orientation and Grain Boundary Defect Density.

​Goal: Uniform grain size (100-500 nm), no obvious pores or cracks (coverage>95%), clear grain boundaries but few defects (high-resolution TEM shows continuous lattice stripes).

3. Defect density and carrier performance

​Characterization methodsSpace charge limited current (SCLC) method - measuring trap state density (target<10 ¹⁵/cm ³); Steady state/transient photoluminescence (PL) and time-resolved fluorescence (TRPL) - high PL intensity (few defects, weak non radiative recombination), long TRPL lifetime (>100 ns, compared to low-quality films<10 ns); Conductivity test - carrier mobility (>10 ⁻³ cm ²/V · s).

​GoalLow defect density (reducing carrier recombination), long fluorescence lifetime (reflecting a high proportion of radiative recombination), and high mobility (facilitating charge extraction).

3、 Typical cases and strategies for optimizing process parameters

Case 1: Optimization of flash evaporation film formation of MAPbI ∝ perovskite

​Initial problemThe MAPbI3 film prepared by traditional flash evaporation has a large amount of PbI ₂ residue (detected by XRD with a 12.6 ° peak), small grain size (<50 nm), and device efficiency of only 12% (lower than 20% by spin coating method).

​Optimization steps​：

​Evaporation rate adjustmentReduce the evaporation rate of PbI ₂ from 5 nm/s to 2 nm/s, synchronize the MAI rate to 1 nm/s (maintain a 1:1 molar ratio), decrease the nucleation density (from 10 ¹³/cm ² to 10 ¹²/cm ²), and promote grain growth;

​Base temperature increaseHeat the substrate to 100 ° C (original room temperature) to accelerate the surface diffusion of the precursor and form a metastable PbI ₂ - MA intermediate phase;

​Atmosphere controlAdjust the chamber pressure to 20 Pa Ar (originally 5 Pa), slow down the gas phase condensation rate, and reduce the number of pores;

​Subsequent annealingAfter deposition, annealing at 150 ° C for 30 minutes (the original annealing at 100 ° C for 10 minutes) promotes the transformation into perovskite phase (PbI ₂ residue disappears, and the peak intensity of perovskite (14.1 °) in XRD increases by 3 times).

​ResultAfter optimization, the grain size of the thin film reached 300-500 nm, with a coverage rate of>98%, and the device efficiency increased to 18.5% (Voc=1.05 V, Jsc=22.1 mA/cm ², FF=80%).

Case 2: FA ₀ ₈₅Cs₀. ₁₅Pb(I₀. ₈₅Br₀. Composition control of mixed perovskite with ₁₅) ∝

​ChallengeMixed halide perovskites are extremely sensitive to component ratios (Br ⁻ content affects bandgap and stability), and the multi-source co evaporation ratio in flash evaporation is prone to deviate from the design value.

​Optimization strategy​：

​Synchronous control of multiple evaporation sourcesUsing independent temperature controlled PbI ₂, FAI, CsI, PbBr ₂ evaporation boats, adjust the ratio of MAI to PbBr ₂ through a mass flow controller (Br ⁻/(I ⁻+Br ⁻)=15%);

​Real time monitoring and feedbackUsing an in-situ optical ellipsometer to monitor film thickness and refractive index (perovskite refractive index~2.3, PbI ₂~1.9), dynamically adjust evaporation rate;

​Stepwise sedimentationFirstly, deposit a mixed layer of PbI ₂/PbBr ₂ (controlling the ratio of I ⁻: Br ⁻), and then cover it with a mixed layer of FAI/CsI to promote uniform cation doping (Cs ⁺ entering the lattice to stabilize the perovskite structure and suppress phase separation).

​ResultThe bandgap of the thin film is stable at 1.6 eV (target value), and XRD shows no impurities (such as CsPbBr ∝ or FAI residue). After running for 1000 hours under 85 ° C light, the efficiency retention rate of the device is>80% (original unoptimized film<50%).

4、 Challenges and Future Directions

Despite the rapid and large-area advantages of flash evaporation film formation in the preparation of perovskite thin films (potentially compatible with roll to roll processes), the following key issues still need to be addressed:

​Non-equilibrium control of dynamic processesThe high-speed condensation of flash evaporation leads to complex nucleation and growth mechanisms (difficult to describe by traditional theoretical models), which require real-time monitoring of the phase transition process in combination with in-situ characterization (such as in-situ XRD, reflectance infrared spectroscopy);

​Uniformity of multiple componentsThe component segregation of mixed cation/halide perovskites (such as Br ⁻ enrichment regions) can easily lead to local bandgap differences, and it is necessary to optimize the synchronization accuracy of multi-source evaporation (deviation<± 1%);

​Large scale uniformity expansionOptimization parameters for small laboratory areas (<1 cm ²) may fail on large-sized (>10 × 10 cm ²) substrates (significant edge effects), requiring the development of linear evaporation sources or rotating substrate strategies;

​Collaboration with Interface EngineeringThe surface roughness or residual stress of the flash evaporation film may affect the contact of the electron transport layer (such as SnO ₂), and further passivation of defects is required in combination with atomic layer deposition (ALD) or solution post-treatment (such as anti solvent dropwise addition).

Conclusion

The perovskite flash evaporation film-forming instrument can accurately control the evaporation rate, substrate temperature, atmosphere environment, and multi-source co evaporation parameters to prepare perovskite absorbing layers with high crystallinity and low defect density, providing an important technical path for improving device efficiency and stability. In the future, by combining in-situ characterization technology with machine learning algorithms (optimizing parameter combinations), the flash evaporation process is expected to further break through the controllable preparation bottleneck of large-area, multi-component perovskite films, and promote the industrialization process of perovskite photovoltaic technology.