CELL

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 photovoltaic modules.

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

inexpensive materials, possibly in very small quantities. These photovoltaic 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 market 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 energy conversion efficiency and longer term stability .

Summary of the Invention

In accordance with the first aspect, the present invention provides a photovoltaic device comprising: a photon receiving surface; a first photon absorbing layer comprising a material that has a Perovskite structure and having a first bandgap; and a second photon absorbing layer comprising an inorganic material that has a Chalcogenide structure and having a second bandgap; the first photon absorbing layer being positioned at least partially between the photon receiving surface and the second photon absorbing layer; wherein the second photon absorbing layer is arranged to provide encapsulation for at least a portion of the first photon absorbing layer and the photovoltaic device is arranged such that received photons are absorbed by at least one of the first and the second photon absorbing layers

In advantageous embodiments the second photon absorbing layer is arranged to provide complete high quality encapsulation of the photovoltaic device and no further encapsulation is required.

In embodiments, the first photon absorbing layer comprises a Methylammonium Lead Bromide (CH ₃ NH ₃ PbBr ₃ ) Perovskite and the second photon absorbing layer comprises antimony selenide (Sb ₂ Se ₃ ) . The thickness of the Sb ₂ Se ₃ may be between 200nm and Ιμιη.

In embodiments, the device allows for an energy conversion efficiency that is at least 8% relative higher than the efficiency of a corresponding device with a single

Perovskite absorbing. Further, the device may provide an improved resilience to degradation. For example,

degradation of the efficiency figure may be lower than 2% for a period of at least 60 days.

In embodiments, the second photon absorbing layer is arranged to reduce the amount of moisture reaching the first photon absorbing layer by 50% relative when compared to a Perovskite photovoltaic device with a single photon absorbing layer similar to the first photon absorbing layer. The second photon absorbing layer may be arranged to reduce the amount of moisture reaching the first photon absorbing layer by 95% when compared to a Perovskite photovoltaic device with a single photon absorbing layer similar to the first photon absorbing layer. In some embodiments, the device further comprises a silver or gold interconnecting layer with a thickness comprised between 2nm and 5nm disposed between the first and the second photon absorbing layers. The interconnecting layer may be arranged to reflect a portion of high energy photons, that have entered the device, towards the second photon absorbing layer.

In some embodiments, the device further comprises a wide bandgap n-type metal oxide layer comprising Ti0 ₂ or ZnO nanoparticles disposed between the photon receiving surface and the first photon absorbing layer; the layer being arranged as an electron transport layer which facilitates the transport of electrons from the

photovoltaic device to a contact structure. In addition, the device may comprise a W0 ₃ layer, disposed between the second photon absorbing layer and a rear contact of the photovoltaic device, arranged as an hole transport layer which facilitates the transport of holes from the

photovoltaic device to a contact structure.

In embodiments, the first bandgap is larger than the second bandgap and the Perovskite material is a self- assembled material comprising an inorganic-organic compound .

In accordance with the second aspect, the present

invention provides, a method of manufacturing a

photovoltaic device comprising the steps of: forming a first solar cell structure on a substrate, the first solar cell structure comprising a first photon absorbing layer that comprises a material that has a Perovskite structure and having a first bandgap; and forming a second solar cell structure comprising a second photon absorbing layer comprising an inorganic material that has a Chalcogenide structure and having a second bandgap; the second solar cell being positioned at least partially onto the first solar cell; wherein the second solar cell is in electrical contact with the first solar cell and is arranged to provide encapsulation of at least a portion of the first solar cell; and the photovoltaic device is arranged such that received photons are absorbed by at least one of the first and the second solar cells.

In embodiments, the method further comprises the step of depositing a silver or gold interconnecting layer between the steps of forming the first and the second solar cells. The interconnecting layer has a thickness comprised between 2nm and 5nm and is arranged to reflect a portion of high energy photons that have entered the device towards the second photon absorbing layer.

The method may further comprise the step of selecting at least one property of the silver or gold layer to tune the optical field distribution within the photovoltaic device. The electrical contact of the gold or silver

interconnection layer can be further increased by

introducing a thin layer (10-20nm) of water-free

Pedot : PSS/C ₆ o interlayers .

In some embodiments the method comprises the step of evaporating Sb ₂ Se ₃ a a vacuum pressure between 6 and 10 mTorr from a Sb ₂ Se ₃ powder source or spin coating a solution containing Sb ₂ Se ₃ .

Further, the method may comprise the step of forming a layer comprising Ti0 ₂ or ZnO nanoparticles disposed between the photon receiving surface and the first photon

absorbing layer; the layer being arranged as an electron transport layer which facilitates the transport of electrons from the photovoltaic device to a contact structure. In addition, W0 ₃ layer may be formed between the second photon absorbing layer and a rear contact of the photovoltaic device; the W0 ₃ layer comprising W0 ₃ being arranged as an hole transport layer which facilitates the transport of holes from the photovoltaic device to a contact structure.

The encapsulation of perovskite based PV devices consists of a protective layer, disposed onto the exposed surface of the device, that protects the device from the external environment, in particular from moisture, to prevent degradation of the perovskites. The encapsulation layers can just be disposed onto the portion exposed to the environment and may not cover the back surface or sides of the devices. Encapsulation techniques based composite carbon nanotubes and an inert polymer matrixes have been used in the art, but did not achieve long term good performance .

Advantageous embodiments of the invention use an antimony selenide (Sb ₂ Se ₃ ) photon absorbing layer in conjunction with an organic-inorganic perovskite photon absorbing layer in a tandem structure. This allows improving the energy conversion efficiency of the PV devices while providing high quality encapsulation and therefore improving stability.

Antimony selenide exhibits suitable optoelectronic properties — it has an acceptable bandgap of 1.1 eV and strong absorption (≥ 105 cm ^-1 ) — and can be deposited by thermal evaporation at low temperature (350 °C) and under low vacuum (~8 mTorr) .

Furthermore, antimony selenide, as used in these

embodiments, provides high quality encapsulation for the PV device and no further encapsulation is required.

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 schematic illustration of a cross-section of a photovoltaic device comprising of two solar cell structures, in accordance with an embodiment;

Figure 2 is an energy band representation of the

photovoltaic device, in accordance with an embodiment;

Figure 3 is a schematic illustration of the optical field distribution in the photovoltaic device of figure 1;

Figure 4 shows SEM micrographs of Sb ₂ Se ₃ thin films used to encapsulate devices in accordance with embodiments;

Figure 5 shows 2D and 3D AFM images of Sb ₂ Se ₃ thin films used to encapsulate devices in accordance with

embodiments ; Figure 6 shows the I-V curve of a single junction Sb ₂ Se ₃ solar cell fabricated with 300°C annealed films (a) and the I-V curve of a single junction wide band gap

perovskite ( FAPbI ₃ -MAPbBr ₃ ) solar cell (b) .

Figure 7 shows simulated current-voltage curves for a Chalcogenide solar cell, a Perovskite solar cell, and the Chalcogenide-Perovskite tandem device of figure 1;

Figure 8 shows efficiency-lifetime curves for a

conventional Perovskite device and the Chalcogenide- Perovskite tandem device of figure 1; and

Figure 9 is a flow-diagram outlining a series of method steps for manufacturing a photovoltaic device comprising of two photon absorbing layers of different materials, in accordance with an embodiment .

Detailed Description of Embodiments

Embodiments described herein are directed to a

photovoltaic device which has photon receiving surface, a first photon absorbing layer comprising a material that has a Perovskite with a first bandgap; and a second photon absorbing layer comprising an inorganic material that has a Chalcogenide structure with a second bandgap. The PV device is configured as a tandem cell and the second photon absorbing layer provides encapsulation of the device while, at the same time, absorbing photons and contributing to the generation of the device photo- current .

The perovskite photon absorbing layer is positioned between a glass substrate and the Chalcogenide photon absorbing layer. The perovskite photon absorbing layer receives photons, through the transparent substrate, before the Chalcogenide photon absorbing layer. Photons not absorbed by the perovskite layer can be absorbed by the Chalcogenide layer. This device design provides improved power conversion efficiency (PCE) and stability of the device.

A contribution to the improved PCE is provided by the increased open circuit voltage (VOC) of the photovoltaic device. Another contribution to the improved PCE is provided by the increased photocurrent due to better absorption of the solar spectrum over the first and the second absorbing layers.

In the embodiment described, a Methylammonium Lead Bromide (CH ₃ NH ₃ PbBr ₃ ) is used to form the first photon absorbing layer. CH ₃ NH ₃ PbBr ₃ has a perovskite structures and is deposited by spin-coating. The low energy and ease of deposition provides a substantial advantage in the manufacturing process of the photovoltaic device.

Referring to figure 1, there is shown a schematic

representation of a cross-section of a photovoltaic device 10 realised on a glass substrate 2. The glass substrate 2 2 is coated with a conductive transparent oxide (TCO) , such as ITO. Device 10 further comprises a first photon absorbing layer 6 comprising of a material that has

Perovskite structure and a second photon absorbing layer 16 comprising of an inorganic material that has a

Chalcogenide structure.

The Perovskite layer 6 is made of Methylammonium Lead Bromide (CH ₃ NH ₃ PbBr ₃ ) material which is a p-type

semiconducting material. The CH ₃ NH ₃ PbBr ₃ layer has a direct band gap of 1.93 to 2.3 eV that corresponds to a light absorption onset of less than or equal to 550 nm. This makes this material an excellent light harvester with significantly improved stability in humid atmosphere.

The Chalcogenide layer 16 is made of Antimony Selenide (Sb ₂ Se ₃ ) , which is a simple binary compound with a fixed phase and stoichiometry . The thickness of the Sb ₂ Se ₃ layer 16 is approximately 200 nm to 1 μιη. It has a very strong absorption coefficient (>105 cm ^-1 at short wavelengths) and its bandgap is approximately 1.1 eV. The constituent of Sb ₂ Se ₃ is non-toxic and low in cost. The Sb ₂ Se ₃ material can be deposited by thermal evaporation at a vacuum pressure between 6 and 10 mTorr from a Sb ₂ Se ₃ powder source or by spin coating a solution containing Sb ₂ Se ₃ .

Chalcogenide layer 16 is disposed above the Perovskite layer 6 in device 10 and works as an encapsulation layer to improve the lifetime of the unstable Perovskite material. Therefore, device 10 exhibits an improved stability in comparison to a PV device comprising a single photon absorbing layer of Perovskite material.

In addition to this, Chalcogenide layer 16 has a different range of absorption spectrum in comparison to the

Perovskite absorption spectrum. Therefore, tandem device 10 absorbs photons from a wider spectrum of wavelengths. Device 10 allows absorbing a broader range of wavelengths of solar spectrum to develop high efficiency and stable thin film PV devices. In the embodiment described, the photovoltaic device comprises further layers as

illustrated in figure 1. These layers are arranged in a way to form two separate solar cell structures; a front solar cell 21 (comprising Perovskite layer 6) and a back solar cell 22 (comprising Chalcogenide layer 16) . The front solar cell 21 is disposed on the glass substrate 2. A wide bandgap n-type metal oxide layer 4 is disposed between the photon receiving surface 2 and the Perovskite layer 6. This layer comprises either Ti0 ₂ or ZnO

nanoparticles . Metal oxide layer 4 is an electron

transport layer that facilitates the transport of

electrons from the photovoltaic device 10 to a contact electrode 3. An electron barrier layer 8, comprising M0O ₃ , is placed on top of the Perovskite layer 6. A semi- transference silver (Ag) layer 10 is deposited on M0O ₃ layer 8. Silver layer 10 establishes a series connection between the front solar cell 21 and back solar call 22. The Ag layer 10 also helps in reflecting back a portion of high energy photons leaking out of the Perovskite layer 6 without getting absorbed. This layer has a thickness of approximately 2 nm to 5 nm. In alternative embodiments, this layer 6 can be replaced by a thin gold layer. One or more properties (such as thickness) of the silver or gold layers can be modified to tune the optical field

distribution within the photovoltaic device.

PFN layer 12 is added adjacent to the gold or silver interconnection layer which functions as a dipole layer reducing the work function of the metal interconnection layer to facilitate the charge recombination enabling efficient electrical contact between the top and bottom cells .

The back cell 22 comprises an electron transport layer 14 to facilitate the transport of electrons from the back cell 22 to the front cell 21. The electron transport layer is realised using a metal oxide such as ZnO or T1O ₃ . The metal oxide layer 14 is disposed between the PFN layer 12 and the Chalcogenide layer 16. A hole transport layer 18 is disposed between the Chalcogenide layer 16 and a rear metal contacting structure 17 which is made of silver (Ag) . Hole transport layer 18 comprises WO ₃ and facilitates the transport of holes from the PV device to the metal contacting structure 17.

The metallic contacting structure 17, comprises of a pattern of silver contacts or a planar silver layer, is in electrical contact with the front solar cell and is arranged to extract electrons from the photovoltaic device 10.

Incoming light photons reach the glass substrate 2 and get transmitted to the metal oxide layer 4 of the front solar cell 21. A portion of these photons is absorbed by the Perovskite layer 6 and converted in electron-hole pairs in the front solar cell 21. The majority of the photons absorbed by the Perovskite material have an energy which is higher than the bandgap of the Perovskite layer 6. A further portion of photons, generally with energy lower than the bandgap of Perovskite layer 6, is transmitted through the front solar cell to the back solar cell. A portion of these transmitted photons is absorbed in the Chalcogenide layer 16 and converted to electron-hole pairs in the back solar cell 22. The majority of these photons, converted in the second solar cell, have an energy which is higher than the bandgap of Chalcogenide layer 16. The Perovskite layer 6 absorbs high energy photons from the solar spectrum and the Chalcogenide layer 16 absorbs the transmitted low energy photons. This is because the band gap of Perovskite material is larger than the band gap of Chalcogenide material . Photons having an energy smaller than the band gap of the Perovskite layer 6 and the Chalcogenide layer 16 may be transmitted through both the front and the back solar cells .

In order to further avoid degradation of the Perovskite light absorbing material, in some embodiments, perovskite layer 6 may be infiltrated in a 2D and 3D mix-structure. For example, a scaffold oxide, such as a Zr02 structure, may be formed and the Perovskite material may be

infiltrated in such structure to improve stability.

The mixture of 2D/3D perovskite structure forms an exceptional gradually-organized multi-dimensional

interface. The interface engineering a built-in 2D/3D perovskite will form a peculiar bottom-up phase-segregated graded structure. The unique combination of the 2D layer acting as a protective window against moisture, preserving the 3D perovskite and of the efficient 3D one provides the stable perovskite solar cells for widespread deployment.

Referring now to figure 2, there is shown an energy band diagram 20 of device 10. Reference alphabets ^A c' and ^Λ ν' in this figure are used to refer to conduction and valance band edges of the layers of the device 10. Energy levels in figure 2 are referred to the vacuum level . Referring to the front cell in figure 2, it is evident that the conduction band edge 6c of the Perovskite material is higher in energy than the conduction bands of Ti0 ₂ (4c) and ITO (2c) layers. This configuration of the conduction band edges allows electrons to flow towards the glass substrate 2 of the front cell. Electrons generated in the front solar cell 21 are prevented from travelling towards the back solar cell 22 due to very high conduction band edges (not shown in figure 2) of the Mo0 ₃ layer. The valence band edge 6v is lies between the valence band edges of the Ti0 ₂ layer, 4v, and the Mo0 ₃ layer, 8v. This configuration of the valence band edges allows holes to flow towards the dipole layer 12 and recombine with electrons coming from the back solar cell 22. Holes generated in the front solar cell 21 are prevented from travelling towards the glass substrate 2.

The energy bands of the second solar cell in figure 2 show the same trend as discussed with reference to the first solar cell. W0 ₃ layer 18 has a high energy conduction band edge (not shown in figure 2) that prevents electrons from reaching the Ag contact 17. The valence band edge 14v prevents holes from reaching the dipole layer 12.

Electrons generated in the back cell 22 recombine with holes coming from the front solar cell 21 in the dipole layer 12. Holes generated in the back solar cell 22 can be extracted through the Ag contact 17.

The interfacial dipole layer reduces the work function of the metal (Gold or silver) interconnection layer,

facilitating better charge recombination across the interface, thereby enabling efficient electrical contact between the top and bottom cells.

Figure 3 is a schematic illustration of the optical field distribution for the front (reference numeral 31) and back (reference numeral 33) cells having an ultra-thin interconnection Ag layer (reference numeral 32) in-between. The sun-light incidenting on the photon receiving surface of the device comprises of a wide range of photons with different energies. Each active layer of device is responsible for absorbing certain intensities and wavelengths of light from visible to near infrared wavelengths of the solar spectrum. The front cell 31 absorbs high energy photons whereas the low energy photons are transmitted through the front cell 31 and they get absorbed at the back cell 33. Most of the high energy photons leaking out of the front cell 31 and moving towards the back cell 33 are reflected back to the front cell by the intermediate semi-transference Ag layer 32. As a result a micro-cavity is formed in the back cell 33 providing an effective optical manipulation between the front and back cells. The reflectivity of the ultra-thin Ag layer 32 can manipulate the optical field distribution within the front and the back cells.

Figure 4 shows SEM micrographs of the Sb ₂ Se ₃ thin films: (a) as deposited with substrate heating; (b) annealed at

100 ^° C; (c) annealed at 200 ^° C; and (d) annealed at 300 ^° C.

The as-deposited and annealed films have shown almost similar topography with roughness values falling between the ranges of 13.8 nm to 18 nm. All the films have showed smoother and tightly packed morphology. The roughness values are reduced slightly after the annealing. The films with smoother surface are more suitable for achieving better quality interface between the adjacent layers for fabricating high efficiency solar cell. Figure 5 shows 2D and 3D AFM images of the Sb ₂ Se ₃ thin films. Figure 5(a) shows the film as deposited with substrate heating. Figure 5(b) shows a film annealed at 100 ^° C. Figure 5(c) shows a film annealed at 200 ^° C and figure 5(d) shows a film annealed at 300 ^° C. The as-deposited and annealed films show almost similar topography with roughness values falling between the ranges of 13.8 nm to 18 nm. All the films show smoother and tightly packed morphology. The roughness values are found to reduce slightly after the annealing. It can be observed from the SEM images that all the films exhibit smaller uneven grains with almost identical morphologies. The films with smoother surface are more suitable for achieving better quality interface between the adjacent layers while fabricating a solar cell.

Referring now to figure 6, there are shown two measured IV curves for a single junction Sb ₂ Se ₃ solar cell fabricated with 300°C annealed films (a) and the I-V curve of a single junction wide band gap perovskite ( FAPbI3-MAPbBr3 ) solar cell (b) . The efficiency of Sb2Se3 based solar cell was around 5% and the efficiency of FAPbI3-MAPbBr3 based solar cell was around 12.4%. The parameters of two devices are given in following table from the respective two I-V curves .

Referring now to figure 7, there is shown a comparison of current-voltage characteristics of three different PV devices. Curve 72 is for a single layer Chalcogenide cell, curve 74 is for a single layer Perovskite cell, and curve 76 is for the Perovskite-Chalcogenide device 10 (as shown in figure 1) comprising both the Perovskite and

Chalcogenide layers . The open circuit voltage for the Perovskite-Chalcogenide device 10 is V _oc = 1-5 V, which is approximately equal to sum of the open circuit voltages for the single layer Chalcogenide cell (0.4 V) and the single layer Perovskite cell (1.1 V) . The short circuit current for the device 10 is J _sc = 21 mA/cm ² , which is smaller than the short circuit current for the single layer Chalcogenide cell (24.1 mA/cm ² ) and single layer Perovskite cell(21.8 mA/cm ² ).

Table 1: Performance characteristics of Chalcogenide, Perovskite, and Perovskite-Chalcogenide device 10, where Jsc _? V _oc , FF and E _ff correspond to short-circuit current density, open circuit voltage, fill factor and efficiency respectively .

Figure 8 shows simulated results for lifetime/stability characteristics of the conventional single layer

Perovskite solar cell and Chalcogenide-Perovskite solar cell 10 as shown in figure 1. In figure 8, data sets 82 is for the conventional single layer Perovskite solar cell and it is evident from this data that the efficiency of these solar cells decreases with the lifetime/stability of Perovskite material. Data set 84 is for the Chalcogenide- Perovskite solar cell device 10 under high humidity conditions (85% humidity) . Data set 84 shows that

normalised efficiency is not affected for a lifetime of 60 days under extreme humid conditions. This stability in the efficiency of the Chalcogenide-Perovskite solar cell is a result of encapsulation of the Perovskite material using the Sb ₂ Se ₃ . Antimony Selenide (Sb ₂ Se ₃ ) is an inorganic semiconductor material exhibiting intrinsically benign grain boundaries (GBs) and possess strong covalent Sb-Se bonds. Furthermore the crystal growth along the (100) and (010) surfaces in Sb ₂ Se ₃ require no breaking of the Sb-Se bonds and thus produce no dangling bonds. Absence of dangling bonds results in less-reactive sites for oxygen and water absorption. Hence the material is intrinsically highly stable as it does not allow water or oxygen molecules to pass through it and reach the adjacent perovskite layers.

Referring now to figure 9, there is shown a flow diagram 90 outlining a series of method steps to manufacture a PV device in accordance with embodiments. The first step 92 of the method consists in forming a first solar cell structure 21 on a substrate. The first solar cell

structure comprises of a first photon absorbing layer 6 that comprises a material that has a Perovskite structure and having a first band gap. The substrate is typically coated with a conductive transparent oxide, such as ITO.

At step 94, a second solar cell structure 22 is formed. The second solar cell structure 22 comprises of a second photon absorbing layer comprising an inorganic material that has a Chalcogenide structure and having a second band gap. The second solar cell 22 is positioned at least partially onto the first solar cell 21.

At step 96, the second solar cell 22 is arranged to be in electrical contact with the first solar cell 21 and to provide encapsulation of the first solar cell. The photovoltaic device 10 is arranged such that received photons are absorbed by at least one of the first and the second solar cells. 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.

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".