Patent Application
20170084400
The present invention relates to techniques for producing perovskite films, which may be used in perovskite-based solar cells.
Harvesting energy from the sun is regarded as one of the most promising ways to solve the energy issues on the earth. Developing solar cells that can efficiently covert solar energy into electricity is therefore highly desired. Although the present dominant silicon-based photovoltaic technology can achieve power conversion efficiencies (PCEs) of ˜25%, large scale applications are still limited by the high material and manufacturing costs. Great efforts have been made to develop efficient, low-cost photovoltaic technologies, including dye-sensitized solar cells, organic photovoltaic, and colloidal nanocrystal solar cells.
Recently, perovskites (such as alkylammonium trihalolead(II)) have been demonstrated to be efficient photovoltaic materials due to their excellent light harvesting capabilities, fast carrier mobility and tolerance of defects. Perovskite nanoparticles have been used as replacement for dyes in liquid dye-sensitized solar cells and have been shown to achieve PCEs of 3.8%. Rapid development in this field has occurred in the last two years after the liquid environment was replaced with a solid hole transporting material (HTM) and the thickness of mesoporous scaffold layer was reduced from several microns, as in typical DSSCs, to a few hundred nanometers.
The PCE of perovskite-based solar cells has been improved in a number of ways, for example utilizing a mesoporous Al2O3 scaffold, a perovskite light absorber and an organic HTM of 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-bifluorene (spiro-OMeTAD) achieved a PCE of 10%. Perovskite solar cells with PCEs of over 12% were reported by modification of the mesoporous scaffold layer and using different HTM materials. The PCEs of these organic-inorganic hybrid solar cells were further improved to 15% by controlling the loading of perovskites inside the mesoporous scaffold using a two-step sequential deposition technique. Furthermore, the perovskite sensitizer functions efficiently in a planar heterojunction system to deliver a PCE of 15%, which largely simplified the solar cell structure by eliminating the need for a mesoporous scaffold. Solution-processed planar heterojunction solar cells with a PCE of 15.7% were also reported using the two-step sequential deposition method and a thin film of ZnO nanoparticles as electron transporting material.
Thus, thin-film photovoltaics based on organic-inorganic perovskite absorbers have emerged as a promising avenue for low-cost solar technology. To date, the perovskite layer in highly efficient solar cells has generally been fabricated by either vapour deposition or a two-step sequential deposition process. The sequential deposition method requires a mesoporous scaffold to limit the growth of perovskite crystals otherwise it produces highly roughened perovskite films due to unconstrained crystal growth and uncovered pin-holes in the perovskite layer for charge recombination. High quality perovskite thin films with controlled morphology, film thickness and surface roughness are mainly achieved by vapour deposition. However, the vapour deposition process is significantly more expensive both in terms of production cost and energy consumption.
The perovskite solar cell normally has three different configurations as shown in
For industry purposes, a solution-based process is more feasible due to the low manufacturing cost. However, when such a method was used for the planar structure, the perovskite solar cell gave a quite low performance. For example, the CH3NH3PbI3 planar solar cell only gave a PCE less than 5%.
In other configurations (such as the mesostructures), the perovskite is deposited by spin-coating. While this results in a mesoporous film that provides a device with reasonable efficiency, when this method is used for the planar structure, the resultant perovskite solar cell exhibits poor performance. For example, a CH3NH3PbI3 planar solar cell formed by spin coating typically result in a PCE of less than 5%.
Very recently, a vapour-assisted two-step solution process was developed that produces high quality polycrystalline perovskite thin films. However, this process is extremely time consuming, as it takes more than 2 hours for the vapour (in this case CH3NH3I) to fully react with the deposited film (in this case PbI2) to form the perovskite film.
What is desired is a faster, facile solution processing technique that can produce high quality perovskite films with controlled morphology for the construction of planar structured thin film devices with superior performance.
The present invention is directed towards addressing at least some of the aforementioned problems of the prior art.
Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.
The present invention relates to methods for the preparation of a perovskite layer on a substrate, and in particular, a method for forming a solar cell that includes a perovskite layer formed by the method of the invention. Specifically, the present invention relates to a fast crystallisation deposition (FCD) for the formation of the perovskite layer.
In one aspect of the invention, there is provided a method for the preparation of a cohesive non-porous perovskite layer on a substrate comprising: forming a thin film of a solution containing a perovskite material dissolved in a solvent onto the substrate to form a liquid film of the solution on the substrate, applying a crystallisation agent to a surface of the film to precipitate perovskite crystals from the solution to form the cohesive non-porous perovskite layer on the substrate.
In an embodiment, the method accelerates the precipitation of perovskite crystals in comparison to the case where perovskite is allowed to crystallise through the slow evaporation of the first solvent. Preferably, the step of applying the crystallisation agent results in the precipitation of the perovskite crystals within about 3 seconds of the application of the crystallisation agent. More preferably, within about 2 seconds. Even more preferably, within about 1 second. In the absence of the crystallisation agent, precipitation of perovskite from solution takes an extended period of time. This increased time for precipitation of perovskite from solution has a negative impact on the formation of the perovskite films and the efficiency of the solar cells prepared from them.
In an embodiment, the step of forming the perovskite layer includes forming a perovskite layer including perovskite grains that have a number average diameter of about 1 μm or less. Preferably, the grains have a number average diameter of about 800 nm or less. More preferably, the grains have a number average diameter of 600 nm or less. Most preferably, the grains have a number average diameter of 400 nm or less.
In an embodiment, the step of forming the perovskite crystals includes forming perovskite crystals in the cubic or tetragonal phase.
In an embodiment, the precipitation of perovskite crystals happens on contact between the dissolved perovskite material and the crystallisation agent. Preferably, contact between the dissolved perovskite material and the crystallisation agent results in super-saturation of the perovskite material within the solvent. This super-saturation results in the precipitation of a number of perovskite nuclei which grow to form perovskite grains in the perovskite layer.
In an embodiment the method of forming a thin film of the solution on the substrate includes coating the solution containing the perovskite material dissolved in the solvent onto the substrate to form the film of the solution on the substrate. Suitable coating techniques include roll-to-roll printing, screen printing, dip-coating, doctor blading and spin coating. Preferably the coating technique is spin coating. Spin coating is particularly advantageous as it allows the rapid formation of an evenly distributed thin film of solution over the surface of the substrate.
In an embodiment the crystallisation agent is a liquid or gas which causes the nucleation of perovskite crystals.
In certain embodiments, the crystallisation agent is a liquid in which the perovskite material is poorly soluble, very poorly soluble or insoluble. Preferably the crystallisation agent is an organic liquid. It is preferred that the organic liquid is one in which the perovskite material is poorly or very poorly soluble or insoluble, so that its addition induces precipitation of the perovskite.
Preferably, the organic liquid is selected from the group consisting of: chlorobenzene, 1,2-dichlorobenzene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene, 1,3,5-trichlorobenzene, 1,2,3-trichlorobenzene, benzene, toluene and xylene. More preferably the organic liquid is chlorobenzene. Chlorobenzene is particularly advantageous as perovskites are insoluble in chlorobenzene. When chlorobenzene comes in touch with the solvent with the perovskites dissolved therein during spin-coating, the perovskites crystallize and precipitate out rapidly and form thin films of good quality.
In an embodiment, the step of applying the crystallisation agent includes coating the crystallisation agent on to the film. Suitable coating techniques include techniques which can quickly apply this crystallisation agent to the film of the perovskite solution such as atomised vapour spraying or spin coating. Spin-coating is particularly advantageous for depositing the crystallisation agent as it allows the rapid formation of an evenly distribution of a film of this crystallisation agent over the thin film of solution containing the perovskite material.
In an alternative embodiment, the crystallisation agent is a dry gas and/or an inert gas. By dry gas it is meant a gas that is free of moisture. In this case, the step of applying the crystallisation agent to the surface of the film includes blowing the gas so that it contacts the surface of the film to evaporate the solvent from the perovskite solution, leading to faster super-saturation and crystallization of the perovskite.
In an embodiment the crystallisation agent is applied to the surface of the film from about 2 seconds to about 10 seconds after formation of the film. Preferably the crystallisation agent is applied to the surface of the film from about 2 seconds to about 8 seconds after formation of the film. More preferably, the crystallisation agent is applied to the surface of the film from about 4 seconds to about 6 seconds after formation of the film. The inventors have found that application of the crystallisation agent within this time period is conducive to forming a cohesive layer with the desired morphology, as well as providing good surface coverage of the substrate.
In the case where the film of the solution containing the perovskite material is formed via a spin coating process, the film is typically taken to have formed after any excess solution has been spun off. That is, an amount of solution is deposited onto a spin-coating apparatus for the purpose of spin coating a substrate. Excess solution is spun off the substrate as a result of the rotary motion of the substrate. The remaining solution forms a thin film over the surface of the substrate. Once this thin film has been formed, the crystallisation agent is then applied. This is preferably within the time frames discussed above.
In an embodiment, the method further includes the step of heat-treating the substrate to evaporate residual solution and crystallisation agent. This is advantageous as a heat treatment step may also assist in the promotion of nucleation of perovskite crystals. Preferably, the heat treatment step is conducted at a temperature of from about 60° C. to about 120° C. More preferably from about 80° C. to about 110° C. Most preferably the heat treatment step is conducted at about 100° C. The heat treatment step can assist in the formation of a perovskite layer having the desired morphology.
In an embodiment, the concentration of the perovskite material in the solution is from about 10 wt % to about 80 wt %. More preferably, the concentration is from about 20 wt % to about 60 wt %. The inventors have found that when the concentration of the perovskite material is in this range within the solution, films of the desired thickness can be formed. The inventors have also found that by adjusting the concentration, the thickness of the resulting film can be controlled. Therefore, the concentration of the perovskite material in solution can be used to tune the film thickness to a desired thickness.
In an embodiment, the thickness of the film layer is from about 50 nm to about 1000 nm. Preferably, the thickness of the film layer is from about 100 nm to 600 nm. More preferably, the thickness of the film layer is from about 200 nm to 500 nm. Most preferably, the thickness of the film layer is from about 300 to 400 nm.
In an alternative arrangement, it is preferred that the thickness of the film is less than 1000 nm. More preferably, the thickness of the film is less than 800 nm. Even more preferably the thickness of the film is less than 600 nm. Most preferably the thickness of the film is less than 400 nm. It is advantageous to provide a thinner film as this allows for a lighter coating which reduces material usage and cost.
In an embodiment the perovskite material is an organo-metallic compound. A wide range of materials can be used. In particular, the perovskite material is a polarizable metal ion. It is preferred that the perovskite has the general formula ABX(n)Y(3-n), wherein A is an organic cation having a +1 oxidation state, B is a metal cation having a +2 oxidation state, X and Y are anions that are different to each other having a −1 oxidation state, and n ranges from 0 to 3 and can be a non-integer. Preferably, the organic cation is a cation that includes an amine group. More preferably, the organic cation is an alkyl amine. Preferably, the metal cation is selected from the group consisting of: Ba2+, zn2+, Ca2+, sr2+, Cd2+, Cu2+, Ni2+, Mn2+, Fe2+, Co2+, Pd2+, Ge2+, Sn2+, Pb2+, Sn2+, Yb2+, and Eu2+. More preferably the metal cation is selected from the group consisting of Pb2+or Sn2+. Preferably X and Y are independently selected from the group consisting of halide ions, such as fluoride (F−), chloride (Cl−), bromide (Br−), iodide (I−) and astatide (At−) ions. More preferably, the compound is a compound selected from: CH3NH3PbClxI3-x CH3NH3PbBrxI3-x, NH2CH=NH2PbClxI3-x and NH2CH═NH2PbBrxI3-x where x ranges from 0-3 and can be a non-integer. Most preferably the compound is CH3NH3PbI3.
In an embodiment, the solvent is an organic solvent, such as an organic solvent that is able to dissolve the perovskite material at the required concentrations. Preferably, the organic solvent is selected from the group consisting of: formamides, lactones, sulfoxides, and ketones. More preferably, the organic solvent is selected from N,N-dimethylformamide (DMF), gamma-butyrolactone, dimethyl sulfoxide, or methylethyl ketone.
In another aspect of the invention, there is provided a method of forming an optoelectronic device, the device including: an anode, a substrate layer having a cohesive non-porous perovskite layer formed by the method described previously, and a cathode.
In another aspect of the invention, there is provided an optoelectronic device including: an anode, a substrate layer having a cohesive non-porous perovskite layer formed thereon, and a cathode.
Perovskite solar cells have become one of the most promising up-and-coming photovoltaic technologies. Currently, the record efficiency for a perovskite solar cell is about 15%. However, the method of the present invention is able to produce a perovskite layer on a substrate which provides a solar cell having an efficiency of about 16.2% under standard AM 1.5 conditions.
In an embodiment, the substrate is a semiconductor layer. Preferably, the semiconductor layer is an n-type semiconductor. More preferably the semiconductor layer is formed from a material selected from semi-conductive metal oxides or sulphides. Preferably, the metal oxides are selected from the group consisting of titanium, tin, zinc, gallium, niobium, tantalum, indium, neodymium, palladium, cadmium, nickel, vanadium or copper, molybdenum, or tungsten; and the metal sulfides are selected from the group consisting of sulfides of zinc or cadmium. Most preferably, the substrate is TiO2.
In an embodiment, the anode is a transparent conducting substrate. Suitable materials include metal oxides, metal nanowires such as silver nanowire, and graphene. Preferably, the anode is a metal oxide selected from tin oxide, indium oxide, or zinc oxide.
In an embodiment the anode is doped with a dopant. Preferably the dopant is selected from the group consisting of: fluorine, tin, or aluminium. More preferably, the anode is selected from the group consisting of indium tin oxide (ITO), fluorine doped tin oxide (FTO), or aluminium doped zinc oxide (AZO). Most preferably, the anode is FTO.
In an embodiment, the cathode is carbon, a metal, or a metal oxide. Preferably, the cathode is selected from the group consisting of Ag, Au, Pt, ITO, graphene, or carbon.
In an embodiment, the device further includes a charge transporting material. Preferably the charge transporting material is a p-type or hole-transporting, semiconducting material. More preferably, the charge transporting material is a spiro compound. Preferably, the spiro compound is spiro-OMeTAD.
Alternatively the charge transporting material may be selected from polymeric hole-transporting materials such as: poly-3-hexylthiophene (P3HT), poly-[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl]](PCPDTBT), (poly-[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]) (PCDTBT) and poly-triarylamine (PTAA).
Or, in another alternative, the charge transporting material may be an inorganic semiconductor hole-transporting material, such as CuI, doped CuI and CuSCN.
In another aspect of the invention, there is provided a solar cell that includes a perovskite layer formed as defined previously.
Preferably the solar cell has an efficiency that is around 10% under standard AM 1.5 conditions. More preferably, greater than 13% under standard AM 1.5 conditions. Even more preferably, the solar cell has an efficiency that is greater than 15% under standard AM 1.5 conditions. Most preferably, the solar cell has an efficiency that is greater than 16% under standard AM 1.5 conditions.
As used herein, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised”, are not intended to exclude further additives, components, integers or steps.
Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.
The present invention relates to a method of producing a perovskite layer on a substrate.
Typically, perovskite layers are produced by forming a perovskite solution comprising a solvent with the perovskite material dissolved therein, and applying the solution to a substrate to form a perovskite solution layer on the substrate. The solvent is then evaporated slowly via traditional drying mechanisms to form a perovskite layer. There are only a few solvents that can dissolve a high concentration of perovskite, such as N, N-dimethylformamide (DMF). These solvents generally have high boiling points (153° C. and a vapor pressure of 3.5 hPa at 20° C. for DMF). But the solvent can only be dried at relatively low temperature to prevent decomposition of organometal halide perovskites. For example the MA lead tri-iodide perovskite decomposes above 100° C. As such, the precipitation of perovskite crystals from the solution takes a relative long time due to the slow evaporation rate even at a high spin speeds in the situation where the perovskite layer is spin coated on to the substrate.
The slow precipitation of the perovskite crystals provides for slow crystal growth and allows Ostwald grain ripening to occur. The effect of this is a thin film that includes large dendritic perovskite crystals and large voids as shown in
In a planar solar cell structure, each layer should be well covered without pin-holes; otherwise, the top layer will be in contact with the bottom layer, causing current leakage. For example, as shown in
The method of the present invention relates to two fast crystallisation deposition processes which can be realised by adopting either a solution based approach or a gas assisted approach. This method results in the formation of a cohesive non-porous perovskite layer.
In the solution-based approach, fast crystallisation is achieved through the application of a crystallisation solution to a layer of a perovskite solution containing a solvent within which is dissolved perovskite material. The crystallisation solution interacts with the perovskite solution to decrease the solubility of the perovskite material in the solvent, causing rapid precipitation and crystallisation of the perovskite material to form a crystallised perovskite layer.
In the gas assisted approach, fast crystallisation is achieved by applying a stream of dry gas to the layer of perovskite solution. The stream of dry and/or gas causes solvent evaporation which results in super-saturation of the perovskite material in the perovskite solution inducing fast crystallisation of the perovskite material from solution as a crystallised perovskite layer.
In the standard process, the coated substrate 108 is allowed to dry by standard means known to the skilled addressee, which may include drying in an ambient environment or drying in an oven of some type. An illustration of the morphology 110 shows an uneven surface which provides incomplete coverage of the substrate.
In the FCD process, once excess solvent has spun off, a crystallisation agent 112 is applied, and is spun coat over the perovskite solution layer. The crystallisation agent induces rapid crystallisation of the perovskite material. Excess crystallisation agent is spun off, leaving a coated substrate 114. An illustration of the morphology 116 shows an evenly distributed layer which provides coverage of the substrate surface.
The substrates were prepared by depositing a dense TiO2 layer (30 nm thick) on fluorine-doped tin oxide (FTO) coated glass using spray pyrolysis. A DMF solution of CH3NH3PbI3 (˜45 wt %) was then spin-coated on the FTO substrate at 5000 rpm. After a short period of time (ca. 6 seconds), a solution of chlorobenzene was rapidly added and allowed to spread on the surface of the substrate. Due to the insolubility of CH3NH3PbI3, and also the two components that make up this material, in chlorobenzene, rapid nucleation and growth of the perovskite crystals occurs. An instant color change of the film from light yellow to dark brown was observed. In contrast, if no chlorobenzene was introduced, the liquid film dried more slowly during spin-coating and a shiny-grey film was obtained. The films were then subjected to annealing at 100° C. for 10 min to evaporate any residual solvent and to further promote crystallization.
Analysis of the two films via scanning electron microscopy (SEM) revealed strikingly different morphologies. The film obtained by FCD exhibits full surface coverage of the substrate and is uniform over very large area (
To investigate the kinetics during film formation using FCD, chlorobenzene solution was introduced onto the spinning substrate at different time to initiate the nucleation of perovskite crystal.
To understand the observable differences in morphology, the spin-coating process can be divided into three stages. In the first stage, the spin-off of excess solvent is a dominant process while the solution concentration remains little changed. If the chlorobenzene solution is introduced at this stage, rapid nucleation and growth of perovskite crystals happens together with solvent spin-off. Because the nucleation occurred firstly at the interface between perovskite solution and the dropped chlorobenzene solution, the mass diffusion can decrease the local concentration of perovskites near the substrate surface, leading to insufficient perovskite coverage after film formation (
Solar cells were constructed with perovskite films produced by FCD using the optimized protocol.
Solar cells utilizing perovskite films prepared by normal spin-coating exhibited a PCE of only 1.5%, which mainly resulted from the low-resistance shunting and loss of light absorption due to the incomplete surface coverage. In comparison, solar cells utilizing a thin perovskite layer of 150 nm produced by FCD yield a high Jsc of 17 mA cm−2 and a PCE of 8%, suggesting the importance of full coverage of perovskite on substrate. The increase of film thickness from 150 nm to 350 nm generally leads to higher Jsc and higher PCEs, which is mainly ascribed to the enhanced light absorption (see for example
FTO-coated glass substrates (TEC8, Dyesol) were patterned by laser cutting and washed by ultrasonication with soap (5% Hellmanex in water), deionized water, and ethanol. A 30-nm-thick dense layer of TiO2 was then coated on the substrates by spray pyrolytic deposition of a bis(isopropoxide)bis(acetylacetonato)titanium(IV) solution (75% in 2-propanol, Sigma-Aldrich) diluted in 2-propanol (1:9, volume ration) at 450° C. FTO glasses with dense TiO2 layers were used within 2 weeks of their preparation.
CH3NH3I (0.200 g) was mixed with PbI2 (0.578 g) in anhydrous N,N-dimethylformamide (1 mL) by shaking at room temperature for 20 min to produce a clear CH3NH3PbI3 solution with concentration of 45 wt %. CH3NH3PbI3 solutions with concentrations of 25, 35 and 55 wt % were prepared in similar manner. To deposit perovskite films, the CH3NH3PbI3 solution (50 μL) was first dropped onto a TiO2 coated FTO substrate (substrate area ˜1 cm×1 cm). The substrate was then spun at 5000 rpm for 30 s and after six seconds anhydrous chlorobenzene (150 μL) was quickly dropped onto the center of the substrate. This instantly changed the color of the substrate from transparent to light brown. For comparison, the effect of adding chlorobenzene after 2, 4 and 8 seconds on film crystallization was also tested. The obtained films were then dried at 100° C. for 10 min.
SEM images of CH3NH3PbI3 perovskite films prepared by FCD using different concentrations of perovskite solutions. (a,d) 25 wt %, (b,e) 35 wt %, (c,f) 55 wt % are shown in
UV-Visble spectra of perovskite films prepared by FCD using different concentrations of perovskite solutions in DMF are shown in
The hole-transporting material was deposited by spin coating at 2200 rpm for 30 s. The spin coating solution was prepared by dissolving 52.8 mg (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) (spiro-MeOTAD), 10 μL of a stock solution of 500 mg mL-1 lithium bis(trifluoromethylsulphonyl)imide in acetonitrile and 14.4 μL 4-tert-butylpyridine in 640 μL chlorobenzene. Device fabrication was finally completed by thermal evaporation of a 70-nm-thick film of silver as the cathode. Devices were left in a desiccator overnight and tested in next day. Note that the champion cell was made using spiro-MeOTAD from Luminescence Technology Corp., while other cells were made using spiro-MeOTAD from Merck KGaA. All the device fabrication process was done in the N2-filled glove box.
A sun simulator (Oriel) fitted with a filtered 1,000 W xenon lamp was used to provide simulated solar irradiation (AM1.5, 100 mW cm−2). Current-voltage characteristics were measured using a Keithley 2400 source meter. The output of the light source was adjusted using a calibrated silicon photodiode (Peccell Technologies). The photodiode was fitted with a color filter provided by the supplier to minimize the optical mismatch between the calibration diode and the solar cells. The solar cells were masked with a non-reflective metal aperture of 0.16 cm2 to define the active area of the device and avoid light scattering through the edges. IPCE spectra were recorded using a 150 W xenon lamp (Oriel) fitted with a monochromator (Cornerstone 260) as a monochromatic light source. The illumination spot size was chosen to be slightly smaller than the active area of the test cells. IPCE photocurrents were recorded under short-circuit conditions using a Keithley 2400 source meter. The monochromatic photon flux was quantified by means of a calibrated silicon photodiode (Peccell Technologies). The surface morphology of perovskite films was investigated using an FEI Nova NanoSEM 450 microscope operating at 5 kV. The cross section images were performed with a FEI Nova dual beam, focussed ion beam system, combined SEM and gallium ion beam instrument. Prior to performing the cross section, two Pt protecting layers were deposited in situ with an electron beam source at 6.3 nA and ion beam source at 0.30 nA. The milling of the cross sections was obtained with a gallium ion source at a 52° tilting angle. The absorption spectra of the perovskite films were measured on a PerkinElmer Lambda 950 UV/VIS/NIR spectrometer. X-ray diffraction (XRD) experiments were conducted by a Philips X-ray diffractometer with Cu Kα radiation. The samples were scanned from 10° to 60° with a step-size of 0.02°.
A perovskite layer was formed on a substrate by spreading 25 μL 45 wt % CH3NH3PbI3 DMF solution on to a TiO2 compacted layer coated FTO substrate on a spin-coater. A 60 psi dry gas stream was blown to the film during the spun at 6500 rpm for 10 second from the third second of the spin-coating. The films then annealed at different temperatures on a hotplate for 5 min, and then cooled down to room temperature on a steel substrate. Finally, Spiro-OMeTAD and a metal conductor were deposited in sequence.
Here a simple accelerated precipitation method based on a dry gas blowing method was applied during the spin-coating process and achieved a much higher solar cell performance of PCE of 13.9%, as shown in Table 3. All the photovoltaic parameters were improved, especially the photocurrent which was doubled and the efficiency tripled. During the spin-coating, the dry gas blowing technique accelerated the drying process of the solvent, and induced a rapid precipitation of the perovskite crystals.
After the perovskite was deposited by the blowing-gas method, a low temperature annealing process (˜100° C.) was required to achieve a high performance solar cell. A detailed study was carried out to investigate the effect of the annealing process. Different annealing temperatures (25° C., 70° C. and 100° C.) were applied after the perovskite films were formed.
The performance of the perovskite films annealed at different temperatures was evaluated when made into devices.
Performance results for a device fabricated using a normal spin-coating method are shown in Table 3. As can be seen, the device exhibits poor photovoltaic performance in comparison with a method according to an embodiment of the invention.
Deposition of CuI on perovskite films, for example for devices made by using CuI as a hole conductor for the layer structured perovskite based solar cells.
30.4 mg of CuI was dissolved in 1.6 ml of a mixture of 1:39 propyl sulphide and chlorobenzene. A few microliters of this solution was taken to the tip of a glass pasture pipette and 2-3 drops were placed on the section of the FTO glass that was not coated by the perovskite film. Next the drop was spread over the perovskite film maintaining a constant distance between the film and the pipette. The spreading was done at a rate of ˜0.2 cm s−1. A full solution bead was maintained between the pipette and the film to form a uniform CuI film. The perovskite film was kept at 850° C. throughout CuI coating and extra 2-3 minutes were required to evaporate the propyl sulphide. The optimum film thickness was achieved with 22 repeats of the CuI spreading. However, comparable results were also observed from the films with 18 to 40 CuI solution spreading repeats.
Three drops of 10% w/w dispersion of graphite in chlorobenzene was placed and spread over CuI film using glass pipette. Then it was heated on the hot plate at 80° C. for 1 minute.
A copper tape was pasted on the graphite film. The positive and negative contacts were soldered to copper plate and FTO glass respectively.
The thickness of a 16-time spread CuI film was around 340 nm.
The reduced overall thickness of the device support to avoid the energy loss of excited charges. This is further exemplifies by the improved Voc of the device by 30% than previous devices, see
According to the results of two batches that have been produced using the two novel deposition methods employing a thin layer of CuI have shown a higher reproducibility (Table 4 and 5). The average efficiency of the two batches shows a significant improvement (Table 4 and 5) over results in previously published literature. The cell area of the devices kept constant at 1 cm2 and the active area was set at 0.16 cm2 by placing a mask on it.
It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014900910 | Mar 2014 | AU | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AU2015/050108 | 3/17/2015 | WO | 00 |
