Anti-reflection packaging film for flexible perovskite solar cell and preparation method of anti-reflection packaging film
By employing a MgF2, H2, and Al2O3 stacked film structure in flexible perovskite solar cells, the problems of anti-reflection and water-oxygen barrier were solved, thereby improving photoelectric conversion efficiency and cell stability.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SICHUAN UNIV
- Filing Date
- 2026-01-26
- Publication Date
- 2026-04-21
AI Technical Summary
Existing antireflective coatings are not ideal in terms of antireflection effect and water and oxygen barrier properties in flexible perovskite solar cells, which affects the photoelectric conversion efficiency and stability of the cells.
A stacked structure consisting of a low-refractive-index MgF2 film, a high-refractive-index H2 film, and a medium-refractive-index Al2O3 film is adopted. By controlling the thickness and refractive index difference of each layer, anti-reflection and encapsulation functions are achieved, preventing water and oxygen from penetrating.
It significantly reduces light reflection loss, increases short-circuit current density, enhances battery mechanical strength, and effectively inhibits the decomposition of perovskite materials in a water-oxygen environment, thus extending battery life.
Smart Images
Figure CN121908733A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of solar cell technology, and in particular to an anti-reflection encapsulation film for flexible perovskite solar cells and its preparation method. Background Technology
[0002] Perovskite solar cells have become a research hotspot in photoelectric conversion materials due to their simple fabrication process, easily adjustable bandgap, and low cost. Currently, the efficiency of perovskite cells has exceeded 27%. In just sixteen years, perovskite solar cells have achieved such rapid development, surpassing polycrystalline silicon cells, which have the largest market share in crystalline silicon photovoltaics. How to further improve the photoelectric conversion efficiency of perovskite solar cells has become a major concern. One of the main factors affecting the photoelectric conversion efficiency of solar cells is the reflection loss of incident light. Adding a suitable thin film to the surface of the perovskite solar cell and utilizing the interference principle can significantly reduce light reflection, increase the short-circuit current density of the perovskite solar cell, and thus improve the photoelectric conversion efficiency.
[0003] Common antireflective coating materials include TiO2, SiO2, MgO, MgF2, Al2O3, and SiN. x These materials possess certain wear resistance properties. SiO2 has a low refractive index, a wide transparency region, and low absorption, resulting in a strong, wear-resistant, and corrosion-resistant film. TiO2 has low hardness but very low porosity, making it resistant to chemical reactions and corrosion by most acids, salts, and other solvents, making it an important corrosion-resistant and wear-resistant coating material. Therefore, antireflective coating systems composed of inorganic thin films can also offer advantages such as improved strength, wear resistance, and corrosion resistance.
[0004] Furthermore, the materials of the functional layers in perovskite solar cells are sensitive to water vapor and oxygen in the air, and are prone to decomposition in the presence of water and oxygen, thus greatly shortening their lifespan. Encapsulation technology can effectively isolate the working components from the external environment, further improving the lifespan of the cells. The literature "Gas chromatography–massspectrometry analyses of encapsulated stable perovskite solar cells" (DOI:10.1126 / science.aba2412) proposes a new polymer-glass "blanket cover" encapsulation technology to prevent perovskite decomposition and further enhance the system's airtightness. However, solar cell encapsulation is performed on the top layer of the device in contact with the atmosphere (conventional encapsulation structures only consider protecting the functional layers on the non-substrate side of the cell from water and oxygen corrosion). For flexible perovskite cells, the substrate used is a flexible polymer organic substrate with poor water and oxygen barrier performance, and the problem of water and oxygen permeation on the substrate side still needs to be considered.
[0005] Therefore, in response to the problems faced by flexible perovskite solar cells, how to provide an anti-reflection encapsulation film that can simultaneously reduce reflection and block water and oxygen, thereby improving cell stability, is a pressing issue that needs to be addressed in this field. Summary of the Invention
[0006] In view of this, the present invention provides an anti-reflection encapsulation film for flexible perovskite solar cells and a method for preparing the same, in order to solve the problems of unsatisfactory anti-reflection effect, water and oxygen barrier properties, and overall performance of existing anti-reflection films.
[0007] To achieve the above objectives, the present invention adopts the following technical solution: An anti-reflection encapsulation film for flexible perovskite solar cells, the anti-reflection encapsulation film comprising a MgF2 low-refractive-index film layer, an H2 high-refractive-index film layer, and an Al2O3 medium-refractive-index film layer arranged in sequence.
[0008] Preferably, the thickness of the MgF2 low-refractive-index film is 100~130 nm; The thickness of the H2 high refractive index film is 50~70 nm; The thickness of the Al2O3 medium refractive index film is 70~90 nm.
[0009] Another object of the present invention is to provide a method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells, comprising the following steps: An anti-reflection encapsulation film for flexible perovskite solar cells is obtained by sequentially depositing an Al2O3 medium refractive index film, an H2 high refractive index film, and a MgF2 low refractive index film on the surface of a flexible substrate.
[0010] Preferably, the flexible substrate includes polyethylene terephthalate, polyethylene naphthalate, or ultrathin flexible willow glass.
[0011] Preferably, the deposition conditions for the Al2O3 medium refractive index film include: a substrate temperature of 25°C and a vacuum degree of 4×10⁻⁶ during the deposition process. -4 ~3×10 -3 Pa, deposition rate 0.5~2 nm / s.
[0012] Preferably, the deposition conditions for the H2 high refractive index film include: a substrate temperature of 25°C and a vacuum degree of 5×10⁻⁶ during the deposition process. -4 ~2×10 -3 Pa, deposition rate is 0.05~0.2 nm / s.
[0013] Preferably, the deposition conditions for the MgF2 low-refractive-index film include: a substrate temperature of 25°C and a vacuum degree of 5 × 10⁻⁶ during the deposition process. -4 ~2×10-3 Pa, deposition rate is 0.5~1 nm / s.
[0014] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: The anti-reflection encapsulation film prepared by this invention can reduce the reflection loss of sunlight incident on the battery, increase the short-circuit current density of the perovskite solar cell, and further improve the photoelectric conversion efficiency of the cell. The constructed anti-reflection encapsulation film possesses properties such as wear resistance and corrosion resistance, which can improve the mechanical strength of the flexible perovskite solar cell. Furthermore, the metal oxide and fluoride film layers located on the outer side of the flexible organic substrate of the flexible perovskite solar cell can block the penetration of water and oxygen from the environment, preventing the perovskite cell from decomposing under water and oxygen conditions and affecting the battery's lifespan.
[0015] The anti-reflection encapsulation film disclosed in this invention not only achieves significant anti-reflection effects across a wide spectral range of 300–900 nm, but also endows devices with excellent water and oxygen isolation encapsulation capabilities. This contributes to increasing the short-circuit current density (J / L) of perovskite solar cells. SC While improving photoelectric conversion efficiency (PCE), it can also effectively suppress the decomposition of perovskite materials induced by water and oxygen environment, thereby significantly extending the device life.
[0016] This invention achieves precise control over the constructive and destructive interference of incident light by adjusting the refractive index differences and optical thicknesses of each layer. On one hand, the outermost MgF2 low-refractive-index film reduces reflection at the air-film interface; on the other hand, the refractive index gradient between the middle H2 high-refractive-index film and the Al2O3 medium-refractive-index film can create multiple controlled optical path differences within the film, causing destructive interference of reflected light at the interface, thereby further reducing the overall reflectivity of the device. The alternating stacking of high and low-refractive-index films helps to broaden the effective wavelength band of the anti-reflection effect, achieving broadband anti-reflection; at the same time, the gradual change in refractive index between the layers can also reduce interfacial light loss, increase the photon flux transmitted to the perovskite absorption layer, and ultimately effectively improve the overall photoelectric conversion efficiency of the device.
[0017] Furthermore, both the Al2O3 medium-refractive-index film and the H2 high-refractive-index film exhibit excellent density, forming a multi-level barrier against water and oxygen in the composite structure, effectively preventing external moisture and oxygen from penetrating the perovskite layer. This integrated anti-reflection and encapsulation design can significantly delay the degradation of perovskite materials under humid, hot, and oxidizing environments, and is an important technical approach for the long-term stable operation of flexible perovskite photovoltaic devices. Attached Figure Description
[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.
[0019] Figure 1 This is a schematic diagram of the flexible perovskite solar cell prepared in Example 1 of the present invention; Among them, 0-antireflection encapsulation film layer, 1-flexible substrate, 2-transparent conductive film, 3-charge transport layer 1, 4-perovskite absorber layer, 5-charge transport layer 2, 6-metal electrode layer; Figure 2 The JV curves of the batteries in Comparative Example 1 and Comparative Example 2 of this invention are shown. Figure 3 The graphs show the battery efficiency changes over time for Comparative Examples 1 and 2 of this invention. Figure 4 The JV curves are for the batteries of Embodiment 1 and Comparative Example 1 of this invention; Figure 5 J is the battery of Embodiment 1 and Comparative Example 1 of the present invention. SC Box plot; Figure 6 The graph shows the change of battery efficiency over time for Embodiment 1 and Comparative Example 1 of the present invention. Detailed Implementation
[0020] The present invention provides an anti-reflection encapsulation film for flexible perovskite solar cells, the anti-reflection encapsulation film comprising a MgF2 low refractive index film layer, an H2 high refractive index film layer, and an Al2O3 medium refractive index film layer arranged in sequence.
[0021] In this invention, the thickness of the MgF2 low refractive index film is 100~130 nm, specifically 105 nm, 110 nm, 115 nm, 120 nm, or 125 nm.
[0022] In this invention, the thickness of the H2 high refractive index film is 50~70 nm, specifically 52 nm, 55 nm, 58 nm, 60 nm, 62 nm, 65 nm, or 68 nm.
[0023] In this invention, the thickness of the Al2O3 medium refractive index film is 70~90 nm, specifically 72 nm, 75 nm, 78 nm, 80 nm, 82 nm, 85 nm, or 88 nm.
[0024] This invention also provides a method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells, comprising the following steps: An anti-reflection encapsulation film for flexible perovskite solar cells is obtained by sequentially depositing an Al2O3 medium refractive index film, an H2 high refractive index film, and a MgF2 low refractive index film on the surface of a flexible substrate.
[0025] In this invention, the flexible substrate includes polyethylene terephthalate, polyethylene naphthalate, or ultrathin flexible willow glass.
[0026] In this invention, the deposition conditions for the Al2O3 medium refractive index film include: a substrate temperature of 25°C and a vacuum degree of 4 × 10⁻⁶ during the deposition process. -4 ~3×10 -3 Pa, specifically 5 × 10 -4 Pa, 6×10 -4 Pa, 8×10 -4 Pa, 1×10 -3 Pa, 1.5×10 -3 Pa, 2×10 -3 Pa; deposition rate 0.5~2 nm / s, specifically 0.8 nm / s, 1 nm / s, 1.2 nm / s, 1.4 nm / s, 1.5 nm / s, 1.8 nm / s.
[0027] In this invention, the deposition conditions for the H2 high refractive index film include: a substrate temperature of 25°C and a vacuum degree of 5 × 10⁻⁶ during the deposition process. -4 ~2×10 -3 Pa, specifically 6 × 10 -4 Pa, 7×10 -4 Pa, 8×10 -4 Pa, 9×10 -4 Pa, 1×10 -3 Pa, 1.5×10 -3 Pa; the deposition rate is 0.05~0.2 nm / s, specifically 0.06 nm / s, 0.08 nm / s, 0.1 nm / s, 0.12 nm / s, 0.15 nm / s, and 0.18 nm / s.
[0028] In this invention, H2 is an antireflective material manufactured by Merck AG, Germany.
[0029] In this invention, the deposition conditions for the MgF2 low-refractive-index film include: a substrate temperature of 25°C and a vacuum degree of 5 × 10⁻⁶ during the deposition process. -4 ~2×10 -3 Pa, specifically 6 × 10-4 Pa, 7×10 -4 Pa, 8×10 -4 Pa, 9×10 -4 Pa, 1×10 -3 Pa, 1.5×10 -3 Pa; the deposition rate is 0.5~1 nm / s; specifically, it can be 0.6 nm / s, 0.7 nm / s, 0.8 nm / s, or 0.9 nm / s.
[0030] A flexible perovskite solar cell comprising the above-mentioned anti-reflection encapsulation film, the flexible perovskite solar cell comprising an anti-reflection encapsulation film layer, a flexible substrate, a transparent conductive film, a charge transport layer 1, a perovskite absorber layer, a charge transport layer 2, and a metal electrode layer arranged in sequence.
[0031] In this invention, the Al2O3 medium refractive index film layer of the antireflective encapsulation film is in contact with the flexible substrate.
[0032] In this invention, the anti-reflective encapsulation film layer serves both encapsulation and anti-reflection purposes.
[0033] In this invention, the flexible substrate is preferably polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or ultrathin flexible willow glass; the transparent conductive film is preferably a metal oxide conductive material such as indium tin oxide; charge transport layer 1 and charge transport layer 2 are respectively an electron transport layer and a hole transport layer, the electron transport layer is preferably a SnO2 electron transport layer, a TiO2 electron transport layer, or a ZnO electron transport layer; the hole transport layer is preferably Spiro-MeOTAD, PEDOT:PSS, or PTAA. The perovskite absorber layer is preferably a perovskite-structured semiconductor material, and its bandgap width can be adjusted in the range of 1.25~2.2 eV.
[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0035] Example 1
[0036] Fabrication of antireflection encapsulation film on flexible substrate
[0037] An antireflection encapsulation film for flexible perovskite solar cells was obtained by sequentially depositing an Al2O3 medium refractive index film, an H2 high refractive index film, and a MgF2 low refractive index film on the surface of a flexible substrate. The deposition conditions for the Al2O3 medium refractive index film were as follows: Al2O3 particles were used as the evaporation source, the substrate temperature was 25℃, and the vacuum degree during the deposition process was 1.5 × 10⁻⁶. -3 Pa, deposition rate 1.4 nm / s; H2 high refractive index film deposition conditions: H2 particles as evaporation source (purchased from Merck, Germany), substrate temperature 25℃, vacuum degree of the deposition process 1.5 × 10⁻⁶ -3 Pa, deposition rate of 0.2 nm / s; MgF2 low refractive index film deposition conditions: MgF2 particles as evaporation source, substrate temperature 25℃, vacuum degree of 1.5×10⁻⁶ during coating process. -3 Pa, deposition rate is 1 nm / s.
[0038] Fabrication of flexible perovskite solar cells
[0039] like Figure 1 As shown, a flexible perovskite solar cell is assembled in the following order: anti-reflection encapsulation film (0), flexible substrate (1), transparent conductive film (2), charge transport layer 1 (3), perovskite absorber layer (4), charge transport layer 2 (5), and metal electrode layer (6); the Al2O3 medium refractive index film in the anti-reflection encapsulation film (0) is in contact with the flexible substrate (1).
[0040] The flexible substrate is PET with an indium tin oxide transparent conductive film deposited on it; the transparent conductive film is an indium tin oxide transparent conductive film; charge transport layer 1 is a SnO2 electron transport layer; the perovskite absorber layer is an organic-inorganic hybrid perovskite with a band gap of 1.5 eV; charge transport layer 2 is a Spiro-OMeTAD hole transport layer; and the metal electrode layer is a metal Au electrode. The specific fabrication process includes: A. Drug preparation Precursor solutions were prepared for the charge transport layer 1 SnO2 electron transport layer, 1.5 eV organic-inorganic hybrid perovskite, and the charge transport layer 2 Spiro-OMeTAD hole transport layer.
[0041] The process of preparing the precursor solution for the charge transport layer 1 SnO2 electron transport layer is as follows: SnO2 colloidal dispersion is mixed with deionized water at a volume ratio of 1:3, and the mixed solution is filtered through a nylon filter with a pore size of 0.2 nm to obtain the ETL precursor solution.
[0042] The preparation of the 1.5 eV organic-inorganic hybrid perovskite precursor solution involves weighing 0.27 mmol CsI, 0.726 mmol MABr, 0.624 mmol FABr, 2.925 mmol FAI, and 4.50 mmol PbI2. The weighed reagents are dissolved in a mixture of 2400 mL DMF and 600 mL DMSO, and then thoroughly dissolved in a mixer to form the perovskite precursor solution.
[0043] The process of preparing the charge transport layer 2 Spiro-OMeTAD hole transport layer is as follows: 520 mg Li TFSI and 399 mg FK209 Co(III) TFSI are dissolved in 1 mL of acetonitrile to prepare Li salt and Co salt, respectively. 73 mg HTL, 18 mg Li salt, 29 μL Co salt and 30 μL tBP are dissolved in 1 mL of CB to prepare HTL precursor solution.
[0044] B. Cleaning the substrate
[0045] The PET substrate with indium tin oxide deposited was ultrasonically cleaned with deionized water and ethanol and then treated in a UV ozone device for 15 minutes.
[0046] C. Preparation of charge transport layer 1SnO2 electron transport layer
[0047] Tin dioxide precursor solution was spin-coated onto the ITO conductive layer on the substrate in air using a spin-coating method. The spin-coating speed was 4000 rpm, the spin-coating acceleration was 2000 rpm / s, and the spin-coating time was 20 s. After spin-coating, the substrate was annealed at 110°C for 40 minutes.
[0048] D. Preparation of the perovskite light-absorbing layer
[0049] The perovskite precursor solution was spin-coated onto the charge transport layer 1 SnO2 electron transport layer using a one-step spin-coating method in an N2 glove box. The spin-coating speed was 3000 rpm, the spin-coating acceleration was 1000 rpm / s, the spin-coating time was 30 s, and 450 μL of CB antisolvent was added at 26 s. Then, the mixture was annealed at 100 °C for 40 minutes.
[0050] E. Fabrication of the charge transport layer 2 Spiro-OMeTAD hole transport layer
[0051] A hole transport layer of approximately 200 nm thickness was prepared on the perovskite absorber layer by spin coating at a rotation speed of 4000 rpm and a spin coating acceleration of 4000 rpm / s for 30 s.
[0052] F. Preparation of gold electrodes
[0053] A gold electrode of approximately 100 nm thickness was deposited on a substrate with a charge transport layer 2 Spiro-OMeTAD hole transport layer by vacuum evaporation to form a complete battery structure.
[0054] Battery test
[0055] A steady-state solar simulator manufactured by ABET Technology was used as the AM1.5 light source, and the light intensity was calibrated using calibrated GaAs solar cells. A Keithley 2634B source meter was used to test the device and collect data.
[0056] Comparative Example 1
[0057] The only difference between this comparative example and Example 1 is that it does not include an anti-reflective encapsulation film.
[0058] Comparative Example 2
[0059] The only difference between this comparative example and Example 1 is that a 123 nm MgF2 low refractive index film layer is used instead of the anti-reflection encapsulation film.
[0060] The JV curves of Comparative Example 1 (PET) without antireflective coating and Comparative Example 2 (MgF2 / PET) with MgF2 monolayer antireflective coating are shown below. Figure 2 As shown, through Figure 2 It can be seen that the short-circuit current density J of both is... SC Open circuit voltage V OC The fill factor (FF) and photoelectric conversion efficiency (PCE) are 22.41 and 23.5 mA / cm, respectively. 2 1.13 / 1.12V, 0.750 / 0.771V, and 18.98% / 20.28% (passed) Figure 2 It can be seen that MgF2 can significantly improve the short-circuit current density of flexible perovskite solar cells (from 22.41 mA / cm² before introduction to 23.5 mA / cm²). 2 This resulted in an efficiency improvement from 18.98% to 20.28%; the efficiency changes over time for Comparative Example 1 and Comparative Example 2 are as follows: Figure 3 As shown, through Figure 3 Stability monitoring results showed that the introduction of MgF2 did not contribute much to improving the stability of flexible perovskite solar cells. After 2000 hours, the cell efficiency of the device with MgF2 monolayer antireflection film was 71.4% of the initial value (68.1% for the unprepared one).
[0061] The JV curves of the batteries in Example 1 and Comparative Example 1 are as follows: Figure 4 As shown, the battery J of Example 1 and Comparative Example 1 SC Box plot as shown Figure 5As shown; the battery efficiency versus time curves of Example 1 and Comparative Example 1 are as follows. Figure 6 As shown. (Through) Figure 4 It can be seen that the introduction of the anti-reflection encapsulation film significantly improves the short-circuit current density J of the flexible perovskite solar cell. SC 22.54→23.61mA / cm 2 The improvement is 4.75%, which can increase battery efficiency from 19.15% to 20.18%; Figure 5 The short-circuit current density J of 16 flexible perovskite solar cell devices fabricated in the same batch was demonstrated. SC The box-type statistical chart demonstrates the high repeatability of the improvement effect, with the batch average short-circuit current density increasing from 22.01 mA / cm². 2 Increased to 23.05 mA / cm 2 And according to Figure 6 It can be seen that multilayer antireflection encapsulation films improve the short-circuit current density J of flexible perovskite solar cells. SC At the same time, its stability can be significantly improved: after 2000 hours, the efficiency of the flexible perovskite solar cell with multilayer anti-reflection encapsulation film remains at 94.5% of the initial value, while that without multilayer anti-reflection encapsulation film decreases to 67.9% of the initial value.
[0062] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.
[0063] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. An anti-reflection encapsulation film for flexible perovskite solar cells, characterized in that, The anti-reflective encapsulation film comprises a MgF2 low-refractive-index film layer, an H2 high-refractive-index film layer, and an Al2O3 medium-refractive-index film layer arranged in sequence.
2. The anti-reflection encapsulation film for flexible perovskite solar cells according to claim 1, characterized in that, The thickness of the MgF2 low-refractive-index film is 100~130 nm; The thickness of the H2 high refractive index film is 50~70 nm; The thickness of the Al2O3 medium refractive index film is 70~90 nm.
3. A method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells as described in claim 1 or 2, characterized in that, Includes the following steps: An anti-reflection encapsulation film for flexible perovskite solar cells is obtained by sequentially depositing an Al2O3 medium refractive index film, an H2 high refractive index film, and a MgF2 low refractive index film on the surface of a flexible substrate.
4. The method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells according to claim 3, characterized in that, The flexible substrate includes polyethylene terephthalate, polyethylene naphthalate, or ultrathin flexible willow glass.
5. The method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells according to claim 4, characterized in that, The deposition conditions for the Al2O3 medium refractive index film include: substrate temperature 25℃, and a vacuum degree of 4×10⁻⁶ during the coating process. -4 ~3×10 -3 Pa, deposition rate 0.5~2 nm / s.
6. The method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells according to claim 5, characterized in that, The deposition conditions for the H2 high refractive index film include: substrate temperature 25°C, and a vacuum degree of 5×10⁻⁶ during the deposition process. -4 ~2×10 -3 Pa, deposition rate is 0.05~0.2 nm / s.
7. A method for preparing an anti-reflection encapsulation film for flexible perovskite solar cells according to any one of claims 4 to 6, characterized in that, The deposition conditions for the MgF2 low-refractive-index film include: substrate temperature 25°C, and a vacuum degree of 5×10⁻⁶ during the deposition process. -4 ~2×10 -3 Pa, deposition rate is 0.5~1 nm / s.