Method of preparing perovskite material and solar cell containing it as a light absorber

Abstract

Methods and systems of producing perovskite material, in which the nucleation and crystallization processes are decoupled and, hence, independently controlled resulting in highly uniform nucleation sites for subsequent crystallization of perovskites. Methods and systems for using perovskite material and mixed perovskite films for solar cells.

Claims

1. A cryo-controlled nucleation method of producing perovskite material comprising: spin-coating a precursor onto a substrate to obtain a uniform precursor film on the substrate, whereby the precursor spin coating deposition time is optimized to prevent premature crystallization due to excess evaporation of precursor solvents; subsequently treating the precursor film in a coolant bath, thereby sharply freezing the solvent containing solutes of the precursor; increasing temperature of the substrate with the precursor film to a temperature above the melting point of the solvent of the precursor; successively subjecting the substrate to a blow-dry process to remove residual solvents in the precursor film; and subjecting the substrate to thermal annealing to completely convert the precursor film into perovskite.

2. The method of claim 1, wherein the precursor solvent consists of a mixture of dimethylformamide and dimethyl sulfoxide.

3. The method of claim 1, wherein the precursor consists of CH.sub.3NH.sub.3I, CH (NH.sub.2).sub.2I, CH.sub.3NH.sub.3Br, CsI, PbI.sub.2, PbBr.sub.2, and PbCl.sub.2.

4. The method of claim 3, wherein the concentration/amount of the precursor ranges from 0 M to 1.2 M.

5. The method of claim 1, wherein the temperature of the substrate with the film increases about 50° C. from the step of treating the precursor film in the coolant bath to the step of subjecting the substrate to the blow-dry process.

6. The method of claim 1, wherein the solvent of the precursor is sharply frozen by decreasing temperature to around −180° C. or below.

7. The method of claim 1, wherein the temperature of the substrate with the film is increased slowly by blow dry process from around −180° C. or below to the room temperature.

8. The method of claim 1, wherein the substrate is a glass/fluorine-doped tin oxide (FTO)/SnO.sub.2 substrate.

9. The method of claim 1, wherein the coolant bath is a liquid nitrogen (LN.sub.2) bath.

10. The method of claim 1, wherein the rapid solidification of the solvent at cryogenic temperature prevented chemical reaction and the coalescence of the precursors.

11. The method of claim 1, wherein the perovskite is precursors/perovskite film (CH.sub.3NH.sub.3).sub.x (CH (NH.sub.2).sub.2).sub.1-xPbI.sub.3, where x is in the range higher than 0 but smaller than 1.

12. The method of claim 1, wherein the perovskite is precursors/perovskite film —Cs.sub.0.05 ((CH.sub.3NH.sub.3).sub.0.17(CH (NH.sub.2).sub.2).sub.0.83).sub.0.95 Pb(I.sub.0.84Br.sub.0.16).sub.3.

13. The method of claim 1, wherein the perovskite is precursors/perovskite film having Cl incorporated to Cs.sub.0.05((CH.sub.3NH.sub.3).sub.0.17(CH (NH.sub.2).sub.2).sub.0.83).sub.0.95 Pb(I.sub.0.84Br.sub.0.16).sub.3-y Cl.sub.y, where y is in the range higher than 0 but smaller than 1.

14. The method of claim 1, wherein the nucleation and crystallization phases are effectively decoupled during the perovskite growth.

15. The method of claim 14, wherein the step of increasing temperature of the substrate with the precursor film prior to successively subjecting the substrate to a blow-dry process to remove residual solvents in the precursor film enhances the he decoupling of the nucleation and crystallization phases for the as-cast precursor films, yielding higher quality perovskite thin films after thermal annealing.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1—The schematic diagrams showing: a) the conventional solution process and b) cryo-controlled nucleation technique for perovskite synthesis.

(2) FIG. 2 a,b)—The results of dynamic absorption of the precursor film without (a) or with (b) the cryogenic treatment followed by the blow-dry process.

(3) FIG. 3—The morphology of the precursor/perovskite films prepared by the solution processes with or without cryo-controlled technique.

(4) FIG. 4 are images of the precursor solution during the temperature reduction process.

(5) FIG. 5 is appearance of the Type B and Type C precursor films prepared by the conventional method and the cryo-controlled nucleation technique at different time duration after spin-coating.

(6) FIG. 6 shows results of dynamic absorption of the Type A precursor film.

(7) FIG. 7 is SEM image of the morphology of the Type A (a, b, c), Type B (d, e, f) and Type C (g, h, i) perovskite films prepared by different methods.

DETAILED DESCRIPTION OF THE INVENTION

(8) In the FIGS. 1-3 shown the method of producing perovskite material in which a precursor (1) is applied by spin-coating (6) onto a substrate (2) and a uniform precursor film (3) is obtained on the substrate (2), and the precursor deposition time is optimized to prevent premature crystallization due to excess evaporation of precursor solvents, and substrate (2) with the precursor film (3) is subsequently treated in a coolant bath (4) and the solvent of the precursor is sharply frozen, then slowly increasing temperature of the substrate with a film to a temperature above the melting point of the solvent of the precursor, after which the substrate is successively subjected to a blow-dry process (5) to remove residual solvents in the precursor film and finally the substrate is subjected to the thermal annealing (7) to completely convert the precursor film into perovskite.

(9) In particular embodiments of the invention:

(10) The precursor solvent consists of a mixture of dimethylformamide and dimethyl sulfoxide, the precursor consists of CH.sub.3NH.sub.3I, CH (NH.sub.2).sub.2I, CH.sub.3NH.sub.3Br, CsI, PbI.sub.2 and PbBr.sub.2, the temperature of the substrate with the film increases about −50°.

(11) The claimed invention also relates to a solar cell panel consisting of a substrate with an electron transport layer, on which a layer of perovskite material produced by abovementioned method is sequentially placed, a hole transport layer (HTL) and an electrode layer.

(12) In particular embodiments of the solar cell panel: the substrate with an electron transport layer composed of SnO.sub.2 deposited on a glass coating with-fluorine doped Tin Oxide (FTO)—using a sol-gel process, the hole transport layer (HTL) was precipitated by spin-coating a solution consisting of spiro-MeOTAD 80 mg/ml in chlorobenzene, with the addition of Li-TFSI 17.5 μl of the initial solution at a concentration of 520 mg/ml in the dissolved state in acetonitrile and 29 μl of t-BP, the electrode layer consists of gold (Au), applied by thermal evaporation through a shadow mask with a working active area of 0.03 cm.sup.2.

(13) In this invention developed a novel cryo-controlled nucleation technique, which effectively decouples the nucleation and crystallization phases and ensures the formation of a uniform seed layer for subsequent perovskite growth. The schematic diagrams shown in FIG. 1a,b indicate the differences between the conventional spin-coating process and the proposed perovskite growth technique. The conventional 1-step process for a mixed perovskite precursor solution can result in non-uniform films with poor morphology due to coalescence of perovskite crystallites resulting from rapid solvent evaporation from a complex solution of the constituents with different solubilities. FIG. 1a shows typical optical images of a mixed halide precursor and perovskite films (Cs.sub.0.05(MA.sub.0.17FA.sub.0.83).sub.0.95Pb(I.sub.0.84Br.sub.0.16).sub.3) that we used in this work. The inhomogeneous formation of large crystallites is observed in the precursor film during the spinning process and a very non-uniform perovskite film is obtained after thermal annealing. FIG. 1b illustrates our proposed material growth method, which is a 4-step process as detailed below: (i) spin-coating of the precursors; (ii) cryogenic treatment; (iii) blow-dry process for the removal of the solvent; and (iv) thermal annealing. First, the precursors were spin-coated onto a glass/fluorine-doped tin oxide (FTO)/SnO.sub.2 substrate. The spin-coating time was optimized to provide a uniform precursor film while preventing premature crystallization caused by excessive spinning due to the evaporation of a large amount of solvent. Second, the as-cast precursor film was dipped into a liquid nitrogen (LN.sub.2) bath immediately. This treatment had two significant impacts on the precursor film. The rapid reduction in temperature induced abrupt freezing of the solvent consisting of a mixture of DMF (m.p. −61° C.) and DMSO (m.p. 19° C.). The rapid solidification of the solvent at cryogenic temperature prevented chemical reaction and the coalescence of the precursors and, thereby, ensured the uniform distribution of the solutes. This phenomenon is demonstrated in FIG. 4, which shows the images of the precursor solution during the temperature reduction process. The rate of chemical reaction is described by the Arrhenius equation k=Ae−Ea/(kBT), where k is the rate constant, T is the absolute temperature, A is a constant for chemical reaction that defines the rate due to the frequency of collisions in the correct orientation, Ea is the activation energy for the reaction, and k.sub.B is the Boltzmann constant. The Arrhenius equation illustrates strong temperature dependence of the chemical reaction rate which was substantially reduced by immersing the substrate in the LN.sub.2 bath. If premature chemical reaction is not properly inhibited, large perovskite crystallites may be formed in the early stage and results in poor film morphology, which is irreversible even by introducing additional post-deposition treatments (e.g., thermal annealing). Third, upon removal from the LN.sub.2, the temperature of the substrate increased slowly to above the melting point of the solvent, which is estimated to be roughly in the range of −50° C. in our case and the sample is subjected to a blow-dry process, using dry N.sub.2, to facilitate the removal of the residual solvents in the precursor film before thermal annealing. A supersaturated condition was induced in the solution resulting in the uniform precipitation of the precursors over the entire substrate. It is important to point out that at this point the temperature of the substrate is still well below the temperature needed for chemical reactions, which prevents the coalescence of large pre-maturely formed perovskite crystallites. In addition, the blow-dry process is essential for rapid removal of residual solvents. If the films containing precursors with large amounts of residual solvents were directly annealed, the precursors would have re-dissolved in the solvent and will nullify all the effects of the cryogenic treatment. The resultant perovskite films will become very rough with a high density of pinholes as in the case of films processed using the conventional technique. Another important attribute of the proposed cryo-controlled technique is the ease of scalability of the cryogenic dipping and the blow-drying steps to large area substrates with high uniformity. Finally, the sample was subjected to a thermal annealing treatment to completely convert the precursors into the perovskite material. Using this 4-step process, we were able to obtain very smooth precursor/perovskite films with excellent coverage over the entire substrate as observed from the insets in FIG. 1b, which is very different from the films deposited using the conventional method as indicated in the insets of FIG. 1a.

(14) FIG. 1. The schematic diagrams showing: a) the conventional solution process and b) cryo-controlled nucleation technique for perovskite synthesis. The inset figures are the images of the precursor and the perovskite films under an optical microscope. c) The appearance of the type A precursor films prepared by the conventional method and the cryo-controlled nucleation technique at different time duration after spin-coating.

(15) To investigate the applicability of the proposed cryogenic process to a variety of different compositions of mixed perovskites, in this invention conducted detailed characterizations on the following three types of perovskite films: (1) Type A precursor/perovskite films-MA.sub.xFA.sub.1-xPbI.sub.3, where x is greater than 0 and less than 1, with the precursor recipe from Ding et al.; (2) Type B precursor/perovskite films-Cs.sub.0.05(MA.sub.0.17FA.sub.0.83).sub.0.95 Pb(I.sub.0.84Br.sub.0.16).sub.3 in which Cs and Br were incorporated into the precursor solution in addition to the basic elements in Type A films. It is known that the introduction of tertiary (FA/MA/Cs) cations and binary (I/Br) halide anions in Type B perovskite can significantly enhance its thermal stability and suppress the formation of photoinactive nonperovskite phase (yellow phase) in the resultant film, leading to enhanced performances for the PSCs; and (3) Type C precursor/perovskite films-Cl is incorporated to the Type B precursor.

(16) Both the conventional and cryo-controlled growth processes were applied to all three types of films. For Type A films grown using the conventional process, it was observed that the crystallization of the perovskite began in the early stage of spin-coating process resulting in a fast and short time window for the crystallization step. The as-cast precursor film was gray in color, which yields a perovskite film with high roughness and poor coverage after the thermal annealing process. From FIG. 1c, it is observed that there was a rapid change in the color of the Type A as-cast precursor film and, after thermal annealing, the resultant perovskite film had a very rough appearance. As for Type B and Type C precursor films, the time window was relatively longer compared to Type A precursor film. The as-cast precursor films were shiny at the beginning, but became rough and gray after 180 s. Hence, the conventional perovskite growth method was not optimized for preparing Type B and C perovskites as it yielded grey films with very poor coverage as shown in the photos in FIG. 5. With the proposed cryogenic process, the crystallization rate was substantially lowered and the time window for the film formation was lengthened for all three perovskite compositions. From FIG. 1c it is shown that the time window for film formation was significantly increased for Type A precursors. The cryogenic treatment was able to preserve the Type B and Type C precursor films in a shiny and smooth appearance for a longer time (>15 min) before carrying out the next processing step.

(17) An in situ investigation of the 1-step spin-coating process of MAPbX.sub.3 (X═I, Br, Cl) showed that the composition of the precursor solution has strong impact on the interaction between solvents and the solutes and thus alters the film casting dynamics during the spinning process, leading to the formation of ordered or disordered precursor solute phases. For most commonly used deposition recipes, the solvents tend to retain in the films and disrupt the precursor orders so that the quality of the resultant perovskite films may be influenced by a number of random factors, particularly for the processing conditions (e.g., concentration of solvents trapped in the glove box), resulting in diverse qualities for the perovskite materials even based on the same precursor formulation. Our proposed 4-step process is an effective strategy to reduce the impact caused by the solvent-solute interactions on the quality of the as-cast precursor films, which is particularly useful for mixed perovskites with high complexities in the composition. This is because the nucleation and crystallization phases are effectively decoupled during the perovskite growth.

(18) The blow-dry process, similar to VASP and single gas quenching technique, relies on rapid residual solvent removal to control the film crystallization and improve uniformity. Adding the cryogenic treatment prior to blow-drying further enhances the decoupling of the nucleation and crystallization phases for the as-cast precursor films, yielding higher quality perovskite thin films after thermal annealing. To investigate the role of these two steps in the film formation, in this invention conducted a dynamic absorption measurement on the as-cast precursor films treated by the blow-dry process alone and one prepared by the cryogenic technique followed by the blow-dry process. FIG. 2 shows the comparison of the experimental results between these two types of samples based on Type B precursor films while the experimental results on Type A and Type C films are shown in FIG. 6. As observed in FIG. 2, the unreacted pale-yellow precursor films absorbed photons between the range of 350 to ≈450 nm. The increase in absorption beyond 450 nm indicates that chemical reactions had taken place between the precursors within the films leading to the formation of perovskite crystallites. It is clearly observed that the rate of conversion of the perovskite layer is more gradual for the sample treated with cryogenic process. Similar phenomena are also observed for the Type A and Type C samples (FIG. 6). The data show that chemical reactions between precursors and the evaporation of the residual solvents took place simultaneously. The low temperature ambient slows down the rate of precursor reaction and in the meantime the blow-dry process can effectively extract most of the residual solvent from the precursor films when the temperature increased, preventing the disordering of the precursor due to the melting of the residual solvents.

(19) FIG. 2. a,b) The results of dynamic absorption of the precursor film without (a) or with (b) the cryogenic treatment followed by the blow-dry process.

(20) Retardation of the rate of chemical reactions by the cryogenic step in the process is also clear from the film morphology. FIG. 3 shows the typical SEM images for the Type B films after each processing step: (i) as-cast precursor films with or without cryogenic treatment (FIG. 3d,a, respectively); (ii) the precursor films after the blow-dry process (FIG. 3b,e); and (iii) the final perovskite films after thermal annealing (FIG. 3c,f). As observed in FIG. 3a, substantial aggregation was observed for the as-cast precursor film without cryogenic treatment whereas a highly uniform precursor film was obtained for the cryogenically treated sample as shown in FIG. 3d. After the blow-dry process, the cryogenically treated film exhibited excellent coverage (FIG. 3e) compared to the control film which showed significant roughness and contained high concentration of pinholes (FIG. 3b). Due to the substantial differences in the morphology of precursor films, it is not surprising that the resultant perovskite films exhibited tremendous differences in quality after thermal annealing. The control perovskite film indicated poor coverage and high concentration of pinholes, as shown in FIG. 3c, whereas the perovskite film grown by the proposed 4-step technique indicated excellent film morphology (FIG. 3f).

(21) FIG. 3. The morphology of the precursor/perovskite films prepared by the solution processes with or without cryo-controlled technique. a) The as-cast precursor film without cryogenic treatment, b) the precursor film without cryogenic treatment and after blow-dry process, c) the perovskite film obtained from thermally annealed precursor film in (b), d) the as-cast precursor film with cryogenic treatment, e) the precursor film with cryogenic treatment and after blow-dry process, and f) the perovskite film obtained from thermally annealed precursor film in (e).

(22) It is known that the constituents used in the precursors greatly affect the dynamics of perovskite conversion. It is found that the average grain size of Type A perovskite film is substantially smaller compared to that of Type B films (FIG. 3f) since the rate of crystallization of Type A film is much faster than the Type B film during the thermal annealing stage leading to significantly shorter crystallization time. However, large grain size is desirable for high-performance PSCs, since films with larger grain size have fewer grain boundaries and lower trap density. In our previous work, in this invention shown that the crystallization time is substantially lengthened when the concentration of Cl in the precursor solution increases and the perovskite grain size is proportional to the crystallization time. Furthermore, Bouchard et al. utilized synchrotron X-ray diffraction microscopy to obtain direct evidence for chlorine-induced preferential crystalline orientation leading to the growth of larger perovskite crystallites on TiO.sub.2. Thus, we considered modification of the precursor solution composition by adding Cl to further increase the grain size of the perovskite (Type C). Our experimental data show that the crystallization rates for the three types of perovskites in this work are: Type A>Type B>Type C, with a strong impact on the average grain size as observed in the corresponding films: Type A<Type B<Type C, as clearly shown in the SEM images in FIG. 3f, and FIG. 7,f,I.

(23) A novel 4-step cryo-controlled method for the deposition of high quality mixed perovskite films without the use of antisolvents has been demonstrated. This method results in increased grain size and crystallinity and lower defect density for all three different perovskite compositions investigated. Thus, the method has universal applicability and it is readily scalable to larger device areas. The improved film quality and consequently increased PCE result from decoupling of the nucleation and crystallization phases during the conversion of perovskite from precursors. Processing at cryogenic temperature suppresses premature reactions of the precursors and prevents premature coalescence of nuclei into large crystallites, enabling uniform film formation following the blow-drying and annealing steps. The method is of particular interest for Cs-containing perovskite formulations, since it is shown to successfully suppress the formation of the undesirable yellow phase δ-CsPbI.sub.3. Using this method, a champion PCE of 21.4% with a V.sub.OC=1.14 V, J.sub.SC=23.5 mA cm.sup.−2, and FF=0.80 can be achieved for optimized perovskite composition.