WO2018068101A1

WO2018068101A1 - A method of forming a light absorbing perovskite layer for a photovoltaic cell and a photovoltaic cell comprising the light absorbing perovskite layer

Info

Publication number: WO2018068101A1
Application number: PCT/AU2017/051114
Authority: WO
Inventors: Ashraf Uddin; Naveen Kumar ELUMALAI
Original assignee: NewSouth Innovations Pty Ltd
Current assignee: NewSouth Innovations Pty Ltd
Priority date: 2016-10-13
Filing date: 2017-10-13
Publication date: 2018-04-19
Anticipated expiration: 2019-04-13

Abstract

The present disclosure is directed to perovskite materials of the formula ABX₃ formed by simultaneous ion hybridisation via A-site cation hybridisation (metal-organic) and X-site anion hybridisation (halide-pseudohalide) and their use in stable hybrid perovskite based photovoltaic devices. In preferred embodiments the A-site cation hybridisation is done by hybridising formamidinium ion (FA⁺) with either caesium ion (Cs⁺) or rubidium ion (Rb⁺), the X-site anion hybridisation is done by hybridising one of iodide ion (I''), bromide ion (Br") or chloride ion (CI") with one of thiocyanate ion (SCN"), selenocyanate ion (SeCN") or cyanate ion (OCN"). In embodiments the cation B is selected to be Pb²⁺. Additionally contemplated are photovoltaic devices comprising: a substrate; an electron transport layer; a light-absorbing layer comprising the perovskite material of the present disclosure; a hole transport layer; and a contact arrangement.

Description

A METHOD OF FORMING A LIGHT ABSORBING PEROVSKITE LAYER FOR A PHOTOVOLTAIC CELL AND A

PHOTOVOLTAIC CELL COMPRISING THE LIGHT ABSORBING PEROVSKITE LAYER

- 1 -

Field of the Invention

The present invention generally relates to photovoltaic thin film devices comprising a perovskite based light absorbing material.

Background of the Invention

The majority of commercial photovoltaic (PV) modules based on silicon solar cells utilise solar cells fabricated using high quality silicon wafer. Despite the constant drop of silicon prices over the last few years, the cost of silicon still represents a large portion of the final price of these PV modules.

Substantial investments have been made in the last 10 years to develop PV devices which use inexpensive

materials, possibly in very small quantities. These PV devices are often referred to as thin film solar cells.

Cadmium telluride cells are an example of a thin film solar cell technology which had a major commercial success and competes on the marked with conventional wafer-based silicon cells.

Recently, PV devices that use perovskite based light absorbing materials have shown very good performance with a low production cost. A record efficiency of 22% has been demonstrated for a perovskite based solar cell in late 2016. Moreover, perovskite PVs has the lowest energy payback time (EPBT) compared to any PV technology ever developed .

One of the main problems of perovskite based PV devices is related to operational stability. In other words, the time it takes before the performance of the PV device start degrading. The stability of current perovskite PV devices is in the range of ~4000 hours. This figure is not compatible with the majority of commercial PV

applications . There is a need in the art for perovskite based PV devices with improved stability that are compatible with today' s commercial solar applications .

Summary of the Invention

In accordance with the first aspect, the present invention provides a method for manufacturing a stable perovskite photovoltaic device comprising the steps of: providing a substrate; forming an electron transport layer which facilitates the transport of electrons from the

photovoltaic device to a contact structure; forming a photon absorbing layer that comprises a material that has an ABX₃ perovskite structure; forming a hole transport layer which facilitates the transport of holes from the photovoltaic device to a contact structure; and forming a contact arrangement for extraction of charge carriers from the photovoltaic device; wherein the step of forming a photon absorbing layer comprises the steps of: providing a first monovalent organic cation; providing a divalent metal cation; providing a monovalent halide anion; hybridising the first monovalent organic cation with a second metal divalent cation; and hybridising the monovalent halide anion with a pseudohalide compound; and the steps of hybridising the first monovalent organic cation with a second metal divalent cation and hybridising the monovalent halide anion with a

pseudohalide compound are performed in a manner such that the stability of the material that has a perovskite structure is improved.

In embodiments, the steps of hybridising the first monovalent organic cation with a second metal divalent cation and hybridising the monovalent halide anion with a pseudohalide compound are performed simultaneously in a dual-ion hybridisation process. These steps may be performed as fractional hybridisation processes.

The first monovalent organic cation is generally an A-site of the compound and may be a Formamidinium ion (FA⁺) . The first monovalent organic cation may be hybridised with alkali ions, such as a Caesium ion (Cs⁺) or/and Rubidium (Rb⁺) . The halide anion is generally an X-site I^~, Br^~ or Cl^~ ion and may be hybridised with a thiocynate compound, such as SCN-, SeCN- or OC . The perovskite absorbing layer formed by following steps defined above provides an improved stability in comparison to CH₃NH₃Pbl₃ perovskite absorbers used in current thin films perovskite solar cells. The stability may be improved more than 30 times.

In embodiments, the method further comprises the step of forming a charge selective interlayer adjacent the photon absorbing layer to further improve stability of the perovskite material. The charge selective interlayer may comprise Sn0₂ or Spiro-OMeTAD .

In accordance with the second aspect, the present

invention provides a photovoltaic device manufactured in accordance with the method of the first aspect.

In accordance with the third aspect, the present invention provides a composition for an absorber material of a solar cell, the composition comprising a perovskite material of formula ( I ) :

ABX, wherein

A is a hybridised monovalent cation comprising a Formamidinium ion (FA⁺) and a Caesium ion (Cs⁺) ;

B is a divalent metal cation selected from the group consisting of Ge²⁺, Sn²⁺, or Pb²⁺, and

X is a hybridised ion comprising a monovalent halide selected from the group consisting of I^~, Br^~ or Cl^~ and a pseudohalide compound selected from the group consisting of SCN^", SeCN^" or OCN. In accordance with the fourth aspect, the present

invention provides photovoltaic device comprising: a substrate; an electron transport layer; a light-absorbing layer comprising a perovskite material based on the composition of the third aspect; and a hole transport layer; a contact arrangement; wherein photons absorbed in the light-absorbing layer induce generation of charge carriers that are extractable via the contact arrangement .

Advantageous embodiments of the present invention allow to substantially improve the chemical and thermodynamic stability of the absorber layer of perovskite solar cells without compromising their high performance. Embodiments use a new approach to harness the integrated benefits of simultaneous ion hybridization in perovskite materials via A-site Cation hybridization (Metal - Organic) and X-site Anion hybridization (Halide - Pseudohalide ) within the highly successful framework (in terms of power conversion efficiency) of the archetypal organic-inorganic hybrid lead halide perovskite (ABX₃) configuration. The invention is unprecedented and first of the kind to inter-combine the phase/entropic stabilization characteristics of concurrent metal and pseudohalide ion incorporation within the perovskite crystal lattice.

Embodiments of the invention provide a stable hybrid perovskite material, for the fabrication of PV devices, via dual-ion hybridization that is highly photo-active as well as intrinsically robust with excellent moisture, thermal and phase stability.

Brief Description of the Drawings

Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a flow diagram outlining a series of method steps to manufacture a photovoltaic device comprising a stable hybrid perovskite material in accordance with an embodiment of the invention;

Figure 2 shows an illustration of the dual-ion

hybridisation process in accordance with embodiments;

Figure 3 is a schematic illustration of a photovoltaic device comprising of a stable hybrid perovskite material in accordance with an embodiment;

Figure 4 shows a simulated current-voltage curve for a photovoltaic device comprising of a stable hybrid perovskite material in accordance with an embodiment;

Figure 5 shows simulated efficiencies as a function of time for a photovoltaic device comprising of a stable hybrid perovskite material and a conventional perovskite based PV device;

Figure 6 is a schematic view of the doping process of the Electron Transport Layer and the perovskite device structure employing triple cation based perovskite as light absorbing layer with a J-V curve; and

Figure 7 is a schematic representation of fabricated PSCs (CA device and CC device) (a) and J-V curves (both FB-SC and SC-FB direction at 0.05 V/s) of the best performing CC device (b) and CA device (c) .

Detailed Description of Embodiments

Embodiments described herein are directed to a PV device which has a photon absorbing layer that comprises a material that has an ABX₃ perovskite structure. The absorber material is formed by dual-ion hybridisation of the A-site and the X-site of the perovskite. In the main embodiment, the absorber material is formed by hybridising a Formamidinium ion (FA+) with a Caesium ion (Cs+) or/and Rb⁺ and hybridising a I-, Br- or CI- ion with a thiocynate compound, such as SCN-, SeCN- or OCN. The two

hybridisation steps are performed simultaneously to provide a perovskite material with a better chemical and thermodynamic stability.

The degradation of perovskite PV devices is predominantly due to the chemical and structural instability of the light absorbing perovskite materials. Specifically, the ions at the A-site and the X-site are found to be the most vulnerable components leading to the destruction of the perovskite crystal structure, when exposed to ambient conditions or heat, moisture and light (UV) . The

Applicants realised that these issues require a

modification of the perovskite material in addition to innovative encapsulation techniques. PV devices based on the hybrid inorganic-organic lead halide perovskite materials consisting of the ABX₃ crystal structure, where A is an organic cation, B is a metallic cation (typically Pb²⁺) and X = is a halide anion

(typically Cl^~, I^~ and Br^~) have most of the requisites for the implementation of a low cost PV device with a good energy conversion efficiency. The limitation, however, is that the as-formed hybrid lead-halide perovskite thin films lack chemical and structural stability, undergoing rapid retrograde degradation in the presence of moisture, heat or light.

Chemical degradation in a humid environment and thermal degradation of the organic component within hybrid perovskites at relatively low temperatures (~80-90°C for CH₃NH₃Pbl₃), are the most recognized degradation pathways for perovskite materials. In addition to the moisture and heat-related degradation, other factors or potential degradation mechanisms such as oxygen, light-induced trap state formation, and electric bias-induced ion migration are also important.

The method for forming a stable perovskite device

described herein targets the thin film PV device stability problem where it originates, i.e. the chemical structure of the light absorbing perovskite layer. The Applicants are of the opinion that such a technique, used in synergy with effective encapsulation, provides a perovskite PV device with long term stability. Mere encapsulation of solar modules is insufficient to guarantee very long-term stability .

The method provides a feasible strategy to solve the intrinsic stability issue prevalent in HPSCs without compromising performance efficiency, by means of dual-ion hybridization and compositional engineering of hybrid perovskite materials. The resulting perovskite addresses the degradation issues arising out of moisture, heat and light simultaneously. These simultaneous benefit have never been achieved in the art.

Further, the method will also greatly benefit the

auxiliary research established on hybrid perovskite materials which include optoelectronic devices such as LEDs, Photodetectors , Laser, and FETs etc.

Referring now to figure 1, there is shown a flow diagram 10 with steps to realise a PV device with a photon absorbing layer that comprises a material that has an ABX₃ perovskite structure using the dual-ion hybridisation process .

At step 12, a substrate upon which the PV device is manufactured is provided. The substrate can be a polymer or a glass and be flexible or hard.

At step 14, an electron transport layer which facilitates the transport of electrons from the photovoltaic device to a contact structure is formed. Thin film perovskite cells do not have a semiconductor junction like traditional silicon solar cells . Therefore an electron and a hole selective layers allow to split carriers in the device.

At step 16, the photon absorbing layer that comprises the material with an ABX₃ perovskite structure is formed. Step 16 include a number of different steps that allow for the dual-hybridisation process to be performed. At step 16a first monovalent organic cation is provided. At step 16b, a divalent metal cation is provided and, at step 16c, a monovalent halide anion is provided. The first monovalent organic cation and the second metal divalent cation are hybridised at step 16d and simultaneously the monovalent halide anion the pseudohalide compound are hybridised at step 16e.

Subsequently, at step 18, a hole transport layer which facilitates the transport of holes from the photovoltaic device to a contact structure is formed before forming a contact arrangement for extraction of charge carriers .

Referring now to figure 2, there is shown a schematic diagram illustrating an embodiment of the dual-ion hybridisation process .

The A-site of the perovskite is hybridised with FA⁺

(HC (NH₂) ₂) and Cs⁺ or/and Rb⁺.

Cs⁺ or/and Rb⁺ allows for increased formation of the lead-X halide complex providing a good moisture stability.

Further Cs⁺ provides high inter-ionic interaction with A site ions, enhancing thermal and light stability. Cs⁺ also allows removing halide segregation under light soaking and helps stabilising the physical structure of the

perovskite .

FA⁺ provides an enhanced thermal stability due to the organic cation and good photoactive and bandgap

properties, enhancing the energy efficiency of the device.

The X-site of the perovskite is hybridised with a I^~, Br^~ or Cl^~ ion and a SCN^~ ion. SCN increases the entropic stability of the perovskite and enhances thermal

stability. Further, it increases interaction between the organic cation (X hydrogen bonding) improving light stability. The hybridisation process is a fractional process, the fractional substitution allows for better phase stability and reduces the moisture solvation of FA⁺ ions, improving moisture stability. SCN^~ also allows for improved thermal stability.

Referring now to figure 3 there is shown a schematic illustration 30 of a photovoltaic device comprising of a stable hybrid perovskite material in accordance with an embodiment. The device has a dual-ion hybridised

perovskite layer Cs_xFAi-_xPB ( SCN) _xIi__x 32 formed in accordance with the method described above. Layer 32 is positioned between a Spiro-OMeTAD electron blocking layer 34 and a Sn0₂/PCBM hole blocking layer 36. The operation of the electrons and holes blocking layers have been discussed above .

Device 30 is realised on a glass substrate coated with a conductive FTO layer 38. The front contact of device 30 is realised using a silver layer 40.

The type of device shown in figure 3 offers four degrees of freedom serving as the template for developing the most stable perovskite film developed till date. Furthermore, the stability of the dual-ion hybridized perovskite absorber films can be further enhanced during fabrication by employing suitable and stable charge selective

interlayers adjacent to the perovskite films thereby opening up extensive possibilities for overall stability improvement .

The rationale behind the concept of simultaneous A- and X- site ion hybridization is explained in the following section. Among the various factors that could affect the stability of perovskite materials, crystal-structure transition (or phase transition) is an important one that has received less attention. Materials with ABX₃

composition can adopt different crystal structures depending on the size and interaction of the A cation and the corner-sharing BX₆ octahedra. Goldschmidt tolerance factor (t) is a reliable empirical index to predict which structure is preferentially formed. The Goldschmidt tolerance factor is calculated from the ionic radius of the atoms using the following expression:

where r_A is the radius of the A cation, r_B is the radius o the B cation, and r_x is the radius of the anion. Any Substitution of ABX components should satisfy the requirement of Goldsmith tolerance factor, 0.9<t<l, to maintain the stable perovskite structure; where t=l depicts ideal high-symmetry cubic structure.

Light, heat and moisture induced phase transition results in deviation of the tolerance factor from the ideal value thereby resulting in the non-photoactive yellow phase of the perovskite. The method designed by the inventors uses dual-ion hybridization to tune the tolerance factor as a compositional design strategy which not only stabilizes the perovskite structure, but also passivates the vulnerability of the A- and X-sites. The method employs Formamidinium iodide (FAPb∑3) serving as the archetypal 01 base material for ion hybridization to start with. Tuning of the tolerance factor by alloying the large-tolerance- factor FAPbI₃ and small-tolerance-factor CsPbI₃ in a precise stoichiometry proportion results in a stabilized α-phase (black phase) in the mixed perovskite. Such mixed Cs_xFAi-xPBI₃ alloys (where x is 0.1 - 0.2) exhibits lower δ- to-α phase-transition temperature compared to pure FAPbI₃ and CsPbI₃. The Cs alloying appears to not only stabilize the

perovskite phase of the FAPbI₃ compound, but also suppress the decomposition to Pbl₂. Incorporation of partial Cs ion in FA-site results in contraction of cubo-octahedral volume and thereby enhances (FA-I) interaction. Such metal hybridization with the organic cation also increases the interaction between the organic cation -X hydrogen bonding thereby resulting in enhancement of the stability of the perovskite crystal lattice and hindering the retrograde degradation to Pb∑2. Moreover, Cs incorporation in FA site with a fraction of x (0.1 - 0.2) results in an optimal bandgap of Eg ~1.5-1.6 eV, much favourable for light absorption similar to that of MAPbI₃. The stability of CsxFAx-xPBIs alloys could be further enhanced by replacing two of the halide ions (I^~) with pseudohalides or

thiocyanate (SCN^~) ions. Pseudo-halides have similar chemical behaviours and properties to true halides. The incorporation of SCN^" as a dopant opens up the fundamental band gap versus MAPI by 8 meV, and has a remarkable effect on the photoluminescence response concluding that SCN^" incorporation is a valuable addition to the hybrid halide family. The SCN incorporated films exhibited a band gap of 1.53 eV which is similar to the values calculated by DFT studies as the SCN incorporation matches the perovskite structure of CH₃NH₃PbI₃.

Typically, the first step of degradation in moisture involves the formation of hydrated intermediate containing isolated PbX₆ ^4~ octahedra. The formation constant of the lead halide complex is essentially its equilibrium

constant, which reflects the binding tightness between the halides and the central Pb²⁺ ion. The calculated formation constant is only 3.5 for Pb±4^2~ which is in the range of the weak interactions between I^~ and Pb²⁺. In the case of CH₃NH₃Pb (SCN) ₂I, the interaction between Pb²⁺ and SCN^" is much stronger, and the formation constant is up to 7 for Pb(SCN)₄. Moreover, comparing the spherical shape of I^~ ions with the linear shape of SCN^" ions as indicated by their Lewis structures, the lone pairs of electrons from the S and N atoms in SCN^" can interact strongly with the Pb ion, which in turn stabilizes the frame structure of

CH₃NH₃Pb ( SCN) ₂I · Recent theoretical analysis suggests that the perovskite material is intrinsically thermodynamically unstable with respect to phase separation into Pbl₂ and CH₃NH₃I. The incorporation of SCN significantly enhances the energetics of resistance to decomposition when compared with conventional MAPbI₃. Based on the theoretical calculations, the decomposition energy of SCN incorporated perovskites is positive (A_RH=+1.97eV) whereas that of conventional MAPbI₃ is negative (Δ_κΗ=-0.09 eV) . The positive value indicates that SCN based films does not spontaneously decompose, unlike MAPbI₃.

Conventions!: CH_sHH₃?b!₃ C 3NH5! + PM2 &_SH = -0.09 eV ( 1)

X-siie hybridized: (€Η ΝΗ^)₂ΡΜ5€Ν)₂ί₂ - 2CH₃NH₃{SCN) + P&!₂ &_SH = +1.97 eV i2)

A-sits hybridized: Cs^FA_{0 i}PblZ → Cs^FA^i + Pbl₂ &_XH - +0.89 e 3)

Combining (2) and (3) would result in A_RH ¾2 .8-3 eV resulting in high resistance to spontaneous decomposition of perovskite structure.

A_RH is the energetics required for the decomposition of perovskite structure. The negative sign indicates

spontaneous decomposition. Moreover, DFT studies have also shown that SCN incorporated perovskites exhibit low effective masses for both holes and electrons, and improved chemical stability against phase separation when compared to MAPI .

Hence, simultaneous hybridization of Cs⁺ (metal) ion and FA⁺ (organic) ion at the A-site and hybridization of the halide ions with SCN (pseudohalide ) ions at the X-site combines the advantages of both strategies resulting in a highly stable perovskite structure with an optimal band gap of 1.5-1.6 eV with the cubic or pseudo-cubic structure (near ideal tolerance factor) , thereby achieving high PCE as well as high stability simultaneously.

The step of forming the photon absorbing layer 32 require the preparation of a perovskite precursor solution mixture. In this embodiment, a perovskite with Iodide and SCN ions hybridised at the X-site and Cs⁺ and FA⁺ ion hybridised at A-site is used.

Synthesis of Pb(SCN)₂.

An aqueous solution of Pb(BF₄) (10 mL, Aldrich) and NaSCN (3.5 g, Aldrich) is mixed under stirring in deionized (DI) water (10 mL) . The precipitate is filtered off and washed 6 times with DI water to obtain Pb(SCN)₂- A Lewis Base adduct approach is employed for the preparation of

FAi__xCs_xPb ( SCN) ₂I perovskite precursor solution. To begin with the precursor is primed by dissolving (200 mg) of Lead thiocyanate Pb(SCN)2, Formamidinium iodide FAI (172 mg) , and DMSO (78 mg) in 600 mg of DMF under stirring at 60 °C for 3 h. A-site hybridization (i.e. mixing Cs⁺ and FA⁺ ions) to form the desired FAi__xCs_xPb ( SCN) ₂I perovskite would be achieved by substitution of corresponding amount of Csl (13, 26, 39, and 52 mg of Csl for X = 0.05, 0.10, 0.15, and 0.20 respectively) instead of FAI . The solution is filtered by syringe filter having 0.45 μιη pore size.

Tuning of precursor stoichiometry by compositional engineering (i.e. modifying the concentration of FAI,

Pb(SCN)₂, and Csl) allows tailoring of the crystallization kinetics resulting in improved thin film homogeneity.

Optimized precursor solution composition would be

identified by successive characterization of the deposited perovskite film (via spin coating) and also by device characterization of fabricated perovskite solar cells with the as-prepared precursor involving additional post processing techniques as described in the device

fabrication section. In alternative embodiments, the precursor preparation methodology may be varied

hybridization at X-site using alternative but analogous material compositions, i.e. replacing I with CI and Br, and even SCN with other pseudohalides such as SeCN and OCN. Device fabrication .

Device 30 is fabricated on fluorine-doped tin oxide (FTO) 38 coated glass (Pilkington, 7Q/D^-1) . Initially, patterned FTO Substrates are cleaned sequentially in Hellmanex detergent, acetone, isopropyl alcohol. FTO is then cleaned for 10 minutes using oxygen plasma. The hole-blocking layer 36 is formed by immersing the cleaned FTO substrate in a bath of 40 mM SnCl₄ in aqueous solution for 30 minutes at 80 °C. The substrates are then immediately rinsed with two consecutive deionized water baths and then sonicated for 10 s in an ethanol bath followed by drying in

nitrogen. Subsequently, a 7.5 mg/ml of Phenyl-C60-butyric acid methyl ester (PCBM) solution in chlorobenzene is spin coated inside a N₂ glovebox onto the Sn0₂ compact layer at 2000 rpm for 45 seconds and annealed at 70°C for 10 minutes . The Sn0₂ compact layer reduces UV induced degradation. The as-prepared precursor perovskite solution is then be spin-coated in a nitrogen-filled glovebox at 2000 rpm for 45 s, on the substrate pre-heated at 70°C. Additional processing methods such as inert gas assist, solvent annealing and heat treatment are employed to improve the film quality. Since degradation of the as- formed perovskite film is in many ways analogous to its initial formation, the additional film processing methods is employed to enhance the phase stability of the as- synthesized perovskite films. Hence, 500 of diethyl ether are dropped on the spinning substrate after 10 s to improve the film morphology. The films are dried inside a N₂ glovebox on a hot plate at a temperature of 70°C for 1 minute. The films are then annealed in an oven in an air atmosphere at 185°C for 90 minutes. The electron-blocking layer 34 is with a 96mg/ml of spiro-OMeTAD solution in chlorobenzene with additives of 32 μΐ of Li-TFSi (170 mg/ml 1-butanol solution) and 10 μΐ of 4-tertbutylpyridine per 1 ml of spiro-OMeTAD solution. Spin-coating is carried out in a nitrogen-filled glovebox at 2000 rpm for 60 s. A 120 nm silver electrode is thermally evaporated under vacuum of ¾ 10^_1 Torr, at a rate of ¾ 0.2 nm-s^-1.

Characterization .

Hybrid perovskites are intrinsically complex materials, where the presence of various types of interactions and structural disorder may play an important role in the material properties. Ex-situ and in-situ measurements, such as UV vis, X-ray diffraction, vibrational

spectroscopy, spectroscopic ellipsometry and X-ray absorption spectroscopy (XAS) are essential techniques for studying optical properties, structure and phase

transformations in materials.

The carrier lifetime of as-prepared perovskite materials is studied by monitoring time-resolved photoluminescence (TRPL) measurements. The influence of ion hybridization on carrier lifetime and ion migration in the fabricated devices are studied using impedance spectroscopy and novel charge transport measurement techniques, such as

Resistance dependent photo-voltage (RPV) and High

Intensity-RPV, incident photo-current (IPC). As charge accumulation within the bulk and at the interfaces influences the degradation stability of the devices, it is important to understand the impact of ion hybridization on these aspects. The essence of this approach is simply to measure the steady state photocurrent over the broad range of illumination intensity (9 orders of magnitude) as it reveals vital information with respect to device

performance and stability under illuminated conditions. It is very important to account for these effects to address the stability issues in perovskite PV devices.

Referring now to figure 4, there is shown a simulated J-V plot for the device described above and shown in figure 3 at room temperature .

The current-voltage (I-V) graph is simulated using the standard single diode equation for the solar cell. The curves are extrapolated with theoretically calculated short circuit current density and open circuit voltage with the low series resistance and high shunt resistance values of the typical perovskite solar cell under 1 sun at AMI .5 condition and room temperature ~25°C. - I n ^¬ stability testing.

The Applicants have used a careful combination of stress factors and thorough analysis of PV parameter decaying curves, to understand the underlying degradation pathways of the device described above. Standardized and

accelerated stress tests, as described in the standard ISOS-protocols have also been used for stability

evaluations. The accelerated moisture-tolerance tests for perovskite materials is performed by monitoring the reflection of the corresponding perovskite films in air with 95% relative humidity at room temperature using an experimental tool developed by the Applicants.

Referring now to figure 5 there is shown a simulated aging trend for 500 h of a high performance dual-ion hybrid perovskite device at room temperature under constant illumination and maximum power point tracking. A trend for the existing conventional perovskite is also shown for comparison .

The simulated lifetime graphs for the dual-ion hybridized material as shown in figure 5 is extrapolated combining the calculated energy values shown in equation (1-3) and the trends obtained in the previous studies in perovskite solar cells with conventional and A-site ion hybridization under continuous illumination. Referring now to figure 6, there is shown a schematic view of the doping process of the Electron Transport Layer and the perovskite device structure employing triple cation based perovskite as light absorbing layer. Figure 6(c) shows the J-V curve for the device with a ZnO layer. In the embodiment of figure 6, Rb is used as alternative to Cs . Rb has shown promising stability figures.

The Applicants have performed Lithium (Li) doping of a low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation- based MAo.₅₇FA₀.₃₈Rbo.o₅Pb±3 (MA: methylammonium, FA:

formamidinium, Rb: rubidium) perovskite solar cells

(PSCs) . Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of Li doped ETL by 30 meV, which

contributes to the reduction of the electron injection barrier from the photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating lithium-doped ZnO (Li-doped) is raised from 14.07 to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics, and mitigated trap-assisted recombination phenomena in Li-doped devices, thoroughly investigated by means of electrochemical impedance

spectroscopy (EIS) analysis. Li-doped PSCs also exhibit lower photocurrent hysteresis than ZnO devices, which is investigated with regard to the electrode polarization phenomena of the fabricated devices.

Figure 6(c) represents the J-V characteristics of PSCs at the scan rate of 0.05 V/s employing triple-cation perovskite with Li-Doped ZnO as electron transport layer. The PCE of the best device in the FB-SC scan (forward bias to short circuit) is 16.14% with a Voc of 1026.26 mV, Jsc of 22.22 mA/cm², and fill factor of 70.77%. Relatively, the J-V scan performed in the reverse direction SC-FB (short circuit to forward bias) has slightly lower PCE of about 13.15% with a with a Voc of 940.94 mV, Jsc of 22.04 mA/cm², and fill factor of 63.38%. The difference in the PCE values arising from the scan direction is attributed to the hysteresis phenomenon.

Figure 7 is a schematic representation of fabricated PSCs (CA device and CC device) (a) and J-V curves (both FB-SC and SC-FB direction at 0.05 V/s) of the best performing CC device (b) and CA device (c) . Here, CA ETL demonstrates a 50 meV upshift in Fermi level position with respect to CC ETL, contributing to higher n- type conductivity and lower electron injection barrier at the interface.

The PCE of the best device CA in the FB-SC scan (forward bias to short circuit) is 16.45% with a Voc of 1012.96 mV, Jsc of 23.61 mA/cm², and fill factor of 68.79%. Relatively, the J-V scan performed in the reverse direction SC-FB (short circuit to forward bias) has slightly lower PCE of about 12.58% with a with a Voc of 930.47 mV, Jsc of 23.76 mA/cm², and fill factor of 56.89%. The difference in the measured photovoltaic parameters is attributed to hysteresis phenomenon arising due to the interfacial polarization issues existing at the perovskite/ZnO layer interface . On the other hand, the PCE of the best device CC in the FB-SC scan is only 10.01% with a Voc of 804.02 mV, Jsc of 22.25 mA/cm², and fill factor of 55.84%. Overall, the CA devices demonstrate about 82% higher average PCE (CA PSC: 15.14%, CC PSCs: 8.33%) compared to CC devices . Apart from the higher photovoltaic performance, CA devices also show mitigated photo-current hysteresis phenomena compared to CC devices, which have been found to be correlated with suppressed electrode polarization phenomena in larger grain sized perovskite atop CA ETL. The term "comprising" (and its grammatical variations) as used herein are used in the inclusive sense of "having" or "including" and not in the sense of "consisting only of".

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are,

therefore, to be considered in all respects as

illustrative and not restrictive.

Claims

The Claims as defined in the invention are as follows :

1. A method for manufacturing a stable perovskite

photovoltaic device comprising the steps of: providing a substrate; forming an electron transport layer which facilitates the transport of electrons from the

2. The method of claim 1 wherein the steps of hybridising the first monovalent organic cation with a second metal divalent cation and hybridising the monovalent halide anion with a pseudohalide compound are performed

simultaneously in a dual-ion hybridisation process.

3. The method of claim 1 or claim 2 wherein the steps of hybridising the first monovalent organic cation with a second metal divalent cation and hybridising the

monovalent halide anion with a pseudohalide compound are fractional hybridisation processes. . The method of any one of the preceding claims wherein the first monovalent organic cation is an A-site

Formamidinium ion (FA⁺) .

5. The method of any one of the preceding claims wherein the second metal divalent cation is alkali ions.

6. The method of any one of the preceding claims wherein the second metal divalent cation is a Caesium ion or/and Rubidium ions .

7. The method of any one of the preceding claims wherein the halide anion is an X-site I^~, Br^~ or Cl^~ ion.

8. The method of any one of the preceding claims wherein the pseudohalide compound is a thiocynate compound.

9. The method of any one of the preceding claims wherein the pseudohalide compound is SCN^~, SeCN^~ or OCN.

10. The method of any one of the preceding claims wherein the stability of the material that has a perovskite structure is improved at least 30 times when compared to the stability of CH₃NH₃PbI₃.

11. The method of any one of the preceding claims wherein the method further comprises the step of forming a charge selective interlayer adjacent the photon absorbing layer to further improve stability of the material that has a perovskite structure.

12. The method of claim 11 wherein the charge selective interlayer comprises Sn0₂ or Spiro-OMeTAD .

13. A photovoltaic device manufactured in accordance with the method of any one of claims 1 to 12.

1 . A composition for an absorber material of a solar cell, the composition comprising a perovskite material of formula ( I ) :

ABX₃ ( I ) wherein

X is a hybridised ion comprising a monovalent halide selected from the group consisting of I^~, Br^~ or CI and a pseudohalide compound selected from the group consisting of SCN^", SeCN^" or OCN.

15. A photovoltaic device comprising: a substrate; an electron transport layer; a light-absorbing layer comprising a perovskite material based on the composition of claim 14; and a hole transport layer; a contact arrangement; wherein photons absorbed in the light-absorbing layer induce generation of charge carriers that are extractable via the contact arrangement .