DESC:
TECHNICAL FIELD
[0001] The present disclosure generally relates to photovoltaic and photo-detecting devices. In particular, the present disclosure pertains to transparent nano-structured substrate with an array of solid-dome shaped protrusions for its utilization in light harvesting devices for light trapping and spatial management of light absorption.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] A solar cell / photovoltaic cell is a device that converts solar energy directly into electricity by photovoltaic effect. This conversion of solar energy into electricity share great potential to make a large contribution for solving the problem of climate change. However, it requires highly efficient solar cells. They can be divided into three generations. First generation devices are made from crystalline semiconductor wafers, typically silicon with thicknesses of 200-300 micrometers. Second generation devices are based on thin film technology having thickness usually in the range of 1-2 micrometers, deposited on cheap substrates such as glass, plastic or stainless steel. Third generation solar cells are currently being researched with the goal to increase the efficiency of second generation solar cells. Structurally, the third generation photovoltaics consists of planar thin film electrodes, buffer layers and a photo-active layers stacked one above the other in an appropriate order. A photo-detector converts incident light into photocurrent and works with an externally applied reverse bias voltage. The principles of working of a photo-detector and a solar cell are similar.
[0004] When a photon is incident on such photovoltaic or photo-detecting devices, the photon get absorbed, energy is given to electron in valence band and it get excited to conduction band leaving a hole behind. Performance of such devices is limited by two important factors: first, light travels in the direction of thickness in a solar cell. Hence, a significant amount of incoming light gets reflected out from the device. This can be overcome by using device with thicker active layer. However, this comes at a cost of increased bulk recombination due to longer transit path for photo-generated charges, which effectively reduces photo-current and photo-voltages. Secondly, in most of the cases, maximum exciton generation occurs near middle of the active layer. This is acceptable for a semiconducting layer that has comparable electron and hole mobility. However, in many thin film semiconducting photovoltaic systems, there is a significant mismatch between the electron and hole mobilities. For example, in organic photovoltaic systems employing a distributed hetero-junction between a p-type polymer P3HT and an n-type organic molecule PCBM, the hole mobility is almost an order of magnitude smaller than the electron mobility. Thus, if the exciton generation is highest near the middle of the active layer, the charge collection delay is limited by transit time of the holes to the hole collecting electrode. If this transit time is comparable to the recombination life-time of the charge carriers, a significant fraction of the photo-generated carriers would be lost through recombination in the photo-active layer bulk during transit.
[0005] Thus, to improve the performance of these devices, it is required to employ different approaches that importantly employ the same or lesser photo-active volume. These approaches must additionally ensure enhanced absorption by light trapping, and better spatial management of exciton generation to reduce bulk recombination. Also, enhancing the charge transport behaviour of the device is a preferred outcome of the enhancement scheme. Importantly, the design should not introduce additional fabrication steps. The fabrication should be scalable to large areas.
[0006] Different approaches are used to solve these problems. A first approach is to employ dispersed metal nanoparticles. In this approach, absorption in the active layer by enhanced electric field caused by gold nanoparticles due to near field plasmonic effects or due to light trapping by scattering due to these particles which are dispersed in the active-layer or in the buffer layers. This improves absorption of light and increases the photocurrents. Also, careful positioning of dispersion of nanoparticles spatially can help in managing the exciton generation as desired. However, this approach is not cost-effective because of the usage of expensive metals such as gold and silver.
[0007] A second approach is to improve the performance of thin film solar cells by using nano-structured electrodes that first, scatters light to enable exciton generation in the vicinity of the desired interface leading to spatial management of light absorption and secondly, increases areal density of the electrode which subsequently increases the probability of charge collection. However, this scheme could occasionally lead to counteracting optical and electronic nano-scale effects. For example, it has been observed that in organic solar cells employing vertical nano-pillar array electrode structure, built-in electric field in the inter-electrode region in the thickness direction is significantly lesser than in the region above the nano-pillar. Thus, any exciton generated in the inter-pillar gap due to optical nano-scale scattering is not effectively separated, since transport of electrons and holes is diffusion controlled in the gaps between the pillars. Additionally, it has also been observed that there are undesired effects such as trapping of light within the nano-structure. Hence, it neither increases optical absorption, nor help in charge separation in the vicinity of the nano-structured, thus showing no true nano-scale enhanced performance.
[0008] A third approach is based on incorporation of nano-structured back electrodes using approaches such as nano-imprinting to prepare mould. This approach can exhibit enhancement due to plamonic effects and light scattering. However, nano-imprinting requires a setup to emboss the pattern on the master template onto the required surface by applying suitable pressure. This adds additional fabrication steps and additional fabrication facilities.
[0009] A fourth approach is based on bonding a nano-structured film to exterior interface of the transparent substrate which receives the light. The nano-structured films are prepared using pre-designed master templates. However, this method involves an additional step of bonding nano-structured film to the transparent substrate. Also, the nano-structure is on the exterior side of the device which faces the incoming light. Hence, there is no enhancement in the built-in electric fields, and the extent of charge separation would not be enhanced significantly.
[0010] A fifth approach is to use structured substrates that are made by UV-curing of resin coated on the transparent substrate, which is patterned by pressing a nano-structured master template on it. The nano-structured resin is an additional material layer which will remain on the substrate. This resulting structure is shown to improve the absorption and decrease the resistance of the cell leading to improved photocurrent. However this technique requires additional fabrication step to implement this design.
[0011] After careful scrutiny of these approaches/solutions, a person skilled in the art would immediately realize one or more drawbacks associated with such systems limiting their practical applications. There is therefore, a need to develop a transparent nano-structured substrate with an array of ellipsoidal protrusions to obtain synergistic optical and electronic nano-scale enhancements in light harvesting devices. Need is also felt to develop method of fabrication of transparent nano-structured substrate with an array of ellipsoidal protrusions and to manufacture light harvesting devices therefrom that does not necessitate additional recurring fabrication steps.
OBJECTS OF THE INVENTION
[0012] An object of the present disclosure is to overcome one or more disadvantages associated with conventional light harvesting devices.
[0013] Another object of the present disclosure is to provide a substantially transparent nano-structure substrate with dome shaped protrusions.
[0014] Another object of the present disclosure is to provide a substantially transparent nano-structure substrate with convex shaped ellipsoidal protrusions.
[0015] Another object of the present disclosure is to provide a substantially transparent nano-structure substrate that exhibits enhanced light trapping and better light management.
[0016] Another object of the present disclosure is to provide a method of fabrication of substantially transparent nano-structure substrate with convex shaped ellipsoidal protrusions.
[0017] Another object of the present disclosure is to provide a light harvesting device utilizing substantially transparent nano-structure substrate.
[0018] Another object of the present disclosure is to provide a method of fabrication of substantially transparent nano-structure substrate without introducing additional recurring fabrications steps or additional fabrication facility.
[0019] Another object of the present disclosure is to provide a substantially transparent substrate with nano-structures which is flexible and can bent over large angles for flexible optoelectronic device applications.
[0020] Another object of the present disclosure is to provide a substantially transparent substrate with a multiscale pattern which can be controlled and optimized, with the multiscale pattern consisting of two distinct regions – a nano-structured region and a planar region.
[0021] Another object of the present disclosure is to provide a nano-structured light harvesting device architecture which leads to improved light absorption across a broad band of wavelengths of interest which leads to improved photocurrent density
[0022] Another object of the present disclosure is to provide a nano-structured light harvesting device architecture on the substrate which carries the multiscale pattern which combines the optoelectronic advantages of both the planar and the nano-structured architectures.
[0023] Another object of the present disclosure is to provide a light harvesting device employing an architecture that enhances charge collection efficiency through improved electric field distribution within the active layer, which leads to improved fill factor.
[0024] Another object of the present disclosure is to provide a light harvesting device that exhibits enhanced photocurrent.
[0025] Another object of the present disclosure is to provide a light harvesting device that exhibits high fill factors.
[0026] Another object of the present disclosure is to provide a light harvesting device that exhibits improved External Quantum Efficiency (EQE).
[0027] Another object of the present disclosure is to provide a light harvesting device which exhibits improved light absorption due to simultaneous light trapping.
[0028] Another object of the present disclosure is to provide a light harvesting device with reduced weight.
[0029] Another object of the present disclosure is to provide a light harvesting device that is cost effective.
[0030] Another object of the present disclosure is to provide a light harvesting device that does not require utilization of any expensive metals such as silver and gold.
[0031] Another object of the present disclosure is to provide a substantially transparent nano-structure substrate that enables synergistic opto-electronic coupling at nano-scale to improve performance of light harvesting devices through coupled effects.
[0032] Another object of the present disclosure is to provide a light harvesting device built on the multiscale or nano-structured substrate which leads to improved capturing of light incident at all angles of incidence, compared to the planar device architecture.
[0033] Another object of the present disclosure is to provide a nano-moulded substrate two different periodicities which can be used for tuning the optical behaviour of the substrate.
[0034] Another object of the present disclosure is to provide the fabrication method which in mechanical strain is applied to the mould during the fabrication of the final substrate to tune the nano-structured periodicity on the moulded nano-structured substrate.
[0035] Various objects, features, aspects and advantages of the present invention will become more apparent from the detailed description of the invention herein below along with the accompanying drawing figures in which like numerals represent like components.

SUMMARY
[0036] The present disclosure generally relates to photovoltaic and photo-detecting devices. In particular, the present disclosure pertains to transparent nano-structured substrate with an array of dome shaped protrusions for its utilization in light harvesting devices for light trapping and spatial management of light absorption.
[0037] An aspect of the present disclosure provides a nano-structured substrate, substantially transparent to an incident light, for utilization in fabrication of light harvesting devices, the nano-structured substrate comprising: a first surface facing the incident light; and a second surface facing away from the first surface and substantially coplanar with the first surface; wherein, the second surface includes a plurality of dome shaped protrusions.
[0038] In an aspect, the plurality of dome shaped protrusions includes substantially convex shaped ellipsoidal protrusions, wherein each of the plurality of dome shaped protrusions defines a height h ranging from about 5 nanometers to about 150 nanometers, and wherein each of the plurality of dome shaped protrusions defines a width w ranging from about 50 nanometers to about 1000 nanometers. In an aspect, spacing s between any two dome shaped protrusions of the plurality of dome shaped protrusions ranges from about 0 nanometers to about 1000 nanometers.
[0039] In an aspect, in case when the nano-structured substrate is configured as a thin flexible substrate, the nano-structured substrate takes shape of any contour present in the thin flexible substrate, and size of nano-features and lateral periodicities of the nano-structured substrate are controlled by application of mechanical stress to an intermediate inverse mould during casting and curing of the substrate. In an aspect, a light sensing device is configured on the substrate whose patterns are tuned by application of mechanical stress on the intermediate inverse mould.
[0040] Another aspect of the present disclosure provides a light harvesting device that includes a nano-structured substrate defining a first surface facing an incident light and a second surface facing away from the incident light; a substantially transparent electrode; a back electrode; and an active layer; wherein, the nano-structured substrate includes a material substantially transparent to an incident light, and wherein the nano-structured substrate includes a plurality of dome shaped protrusions on the second surface.
[0041] Another aspect of the present disclosure provides a method of preparation of a nano-structured substrate, substantially transparent to an incident light, for its utility in fabrication of light harvesting device, the method comprising steps of: deposition of a self-assembled monolayer of polymeric nanospheres on surface of a first substrate using polystyrene solution to prepare a first mould; deposition of PDMS (polydimethylsiloxane) or a similar mouldable and curable polymer like material on the first mould followed by its curing prepare a second mould; casting of a substrate material, substantially transparent to the incident light, on the second mould; and solidification of the substrate material to realize the nano-structured substrate, substantially transparent to the incident light, comprising a plurality of dome shaped protrusions on its surface.
[0042] In an aspect, the self-assembled monolayer of polymeric nanospheres includes self-assembled monolayer of polystyrene nanospheres, the first substrate includes silicon, and the substrate material includes any or a combination of mouldable polymer like materials such as epoxy and glass.
[0043] Another aspect of the present disclosure provides a light harvesting device including: a nano-structured substrate defining a first surface facing an incident light and a second surface facing away from the incident light; a substantially transparent electrode; a back electrode; and an active layer; wherein, the nano-structured substrate includes a material substantially transparent to an incident light, and wherein the nano-structured substrate includes a plurality of dome shaped protrusions on the second surface.
[0044] Another aspect of the present disclosure relates to a method of preparation of a nano-structured substrate, substantially transparent to an incident light, for its utility in fabrication of light harvesting device, the method including steps of: deposition of a self-assembled monolayer of polymeric nanospheres on surface of a first substrate using polystyrene solution to prepare a first mould; deposition of PDMS (polydimethylsiloxane) or a similar mouldable and curable polymer like material on the first mould, followed by its curing, to prepare a second mould; casting of a substrate material, substantially transparent to the incident light, on the second mould; and solidification of the substrate material to realize the nano-structured substrate, substantially transparent to the incident light, including a plurality of dome shaped protrusions on one of its surface. In an embodiment, the self-assembled monolayer of polymeric nanospheres includes self-assembled monolayer of polystyrene nanospheres. In an embodiment, the first substrate includes silicon. In an embodiment, the substrate material includes any or a combination of mouldable and curable polymer like materials such as, but not limited to, epoxy and glass.
[0045] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0046] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0047] FIG. 1 illustrates an exemplary view of a section of a nano-structured substrate showing geometric design variables viz. width w, height h and spacing s in accordance with an embodiment of the present disclosure.
[0048] FIG. 2 illustrates an exemplary atomic force micrograph image of a nano-structured substrate with a plurality of dome shaped protrusions, realized utilizing epoxy polymer as substrate in accordance with an embodiment of the present disclosure.
[0049] FIG. 3A illustrates an exemplary representation of layer wise construction of previously known light harvesting device fabricated on substantially transparent planar nano-structure substrate in accordance with an embodiment of the present disclosure.
[0050] FIG. 3B illustrates an exemplary representation of layer wise construction of a light harvesting device realized using nano-structured substrate with plurality of dome shaped protrusions in accordance with an embodiment of the present disclosure.
[0051] FIG. 4 illustrates an exemplary representation of layer wise construction of an Indium Tin Oxide (ITO) free electrode light harvesting device incorporating dielectric layer in accordance with an embodiment of the present disclosure.
[0052] FIG. 5 illustrates an exemplary representation of layer wise construction of a light harvesting device with multiple active layers and multiple electrode layers stacked one above another in accordance with an embodiment of the present disclosure.
[0053] FIG. 6A illustrates an exemplary representation of layer wise construction of a light harvesting device where the layer immediately above the nano-structured substrate retains the shape of the dome shaped protrusion, in accordance with an embodiment of the present disclosure.
[0054] FIG. 6B illustrates an exemplary representation of layer wise construction of a light harvesting device where the layer immediately above the nano-structure substrate is flat, in accordance with an embodiment of the present disclosure.
[0055] FIG. 7 illustrates an exemplary representation of layer wise construction of a light harvesting device where shape of the dome shaped protrusion is not retained throughout the thickness of the device, in accordance with an embodiment of the present disclosure.
[0056] FIG. 8 illustrates an exemplary representation of layer wise construction of a light harvesting device realized utilizing nano-structured substrate with varying shaped protrusions in accordance with an embodiment of the present disclosure.
[0057] FIG. 9A illustrates bending test carried out on a flexible nano-structured substrate in accordance with an embodiment of the present disclosure.
[0058] FIG. 9B schematically illustrates the bending test on the flexible substrate in accordance with an embodiment of the present disclosure.
[0059] FIG. 10A illustrates an exemplary representation depicting preparation of first mould in accordance with an embodiment of the present disclosure.
[0060] FIG. 10B illustrates an exemplary representation depicting preparation of second mould by deposition of PDMS (polydimethylsiloxane) on the first mould in accordance with an embodiment of the present disclosure.
[0061] FIG. 10C illustrates an exemplary representation depicting preparation of second mould by peeling-off solidified PDMS from the first mould in accordance with an embodiment of the present disclosure.
[0062] FIG. 10D illustrates an exemplary representation depicting casting of the substrate on the second mould in accordance with an embodiment of the present disclosure.
[0063] FIG. 10E illustrates an exemplary representation of nano-structured substrate with a plurality of dome shaped protrusions in accordance with an embodiment of the present disclosure.
[0064] FIG. 11A illustrates an exemplary graphical representation depicting total percentage transmittance of planar and nano-structured epoxy substrates in accordance with embodiments of the present disclosure.
[0065] FIG. 11B illustrates an exemplary graphical representation depicting percentage specular reflectance of the planar and structured substrates in accordance with embodiments of the present disclosure.
[0066] FIG. 11C illustrates an exemplary graphical representation depicting percentage diffuse reflectance of the planar and the structured substrates in accordance with embodiments of the present disclosure.
[0067] FIG. 12A illustrates an exemplary graphical representation depicting percentage specular reflectance of the planar and structured device architectures in accordance with embodiments of the present disclosure.
[0068] FIG. 12B illustrates an exemplary graphical representation depicting percentage diffuse reflectance of the planar and structured device architectures in accordance with embodiments of the present disclosure.
[0069] FIG. 12C illustrates an exemplary graphical representation depicting a representative plot of measured current density (J) versus voltage (V) curve for the planar and structured devices in accordance with embodiments of the present disclosure.
[0070] FIG. 12D illustrates an exemplary graphical representation depicting external quantum efficiency (EQE) for the planar and structured device architectures in accordance with embodiments of the present disclosure.
[0071] FIG. 12E illustrates an exemplary graphical representation depicting a box plot of measured photocurrent densities at short circuit for the planar and structured devices in accordance with embodiments of the present disclosure.
[0072] FIG. 13A illustrates an exemplary representation depicting morphology of the nano-structured epoxy substrate obtained by atomic force microscopy in accordance with embodiments of the present disclosure.
[0073] FIG. 13B illustrates an exemplary representation depicting morphology of titanium dioxide film deposited on the nano-structured epoxy in accordance with embodiments of the present disclosure.
[0074] FIG. 13C illustrates an exemplary representation depicting morphology of the top surface of PH1000 anode coated on the nano-structured titanium dioxide in accordance with embodiments of the present disclosure.
[0075] FIG. 13D illustrates an exemplary representation depicting morphology of the top surface of the active layer which is coated on the PH1000 anode in accordance with embodiments of the present disclosure.
[0076] FIG. 14A illustrates an exemplary representation depicting simulated absorptance of the planar and structured device architectures in accordance to an embodiment of the present disclosure.
[0077] FIG. 14B illustrates exemplary graphical representation of current density versus voltage curve for the planar and structured device architectures in accordance to an embodiment of the present disclosure.
[0078] FIG. 14C illustrates exemplary graphical representation illustrating modulus of y-component of steady state electric field in the planar and structured architectures in accordance to an embodiment of the present disclosure.
[0079] FIG. 15 illustrates an exemplary representation of normalized power flux density in x-direction exhibiting occurrence of scattering in accordance with embodiments of the present disclosure.
[0080] FIG. 16A illustrates an exemplary graphical representation depicting variation in normalized power flux density transmitted to active layer in the x-direction (due to scattering) with varying width w of the dome shaped protrusions in accordance with embodiments of the present disclosure.
[0081] FIG. 16B illustrates an exemplary graphical representation depicting variation in normalized power flux density transmitted to the active layer in the x-direction (due to scattering) with varying height h of the dome shaped protrusions in accordance with embodiments of the present disclosure.
[0082] FIG. 17 illustrates exemplary graphical representation illustrating maximum photocurrent density obtainable from a planar and a nano-structured device in accordance to an embodiment of the present disclosure.
[0083] FIG. 18 illustrates exemplary representation of varying domain sizes of the planar and nano-structured domains on master template in accordance to an embodiment of the present disclosure.
[0084] FIG. 19A illustrates an schematic representation of application of mechanical strain to the intermediate PDMS mould during fabrication of final epoxy substrates in accordance to an embodiment of the present disclosure.
[0085] FIG. 19B illustrates an exemplary schematic top view of an unstretched PDMS mould in accordance to an embodiment of the present disclosure.
[0086] FIG. 19C illustrates an exemplary schematic top view of a stretched PDMS mould in accordance to an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0087] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0088] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the “invention” may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0089] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0090] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0091] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0092] The present disclosure generally relates to photovoltaic and photo-detecting devices. In particular, the present disclosure pertains to transparent nano-structured substrate with an array of ellipsoidal protrusions for its utilization in light harvesting devices for light trapping and spatial management of light absorption.
[0093] An aspect of the present disclosure provides a nano-structured substrate, substantially transparent to an incident light, for utilization in fabrication of light harvesting devices, said nano-structured substrate including: a first surface facing the incident light; and a second surface facing away from the first surface and substantially coplanar with said first surface; wherein, said second surface includes a plurality of dome shaped protrusions. In an embodiment, said plurality of dome shaped protrusions includes substantially convex shaped ellipsoidal protrusions. In an embodiment, each of said plurality of dome shaped protrusions defines a height h ranges from about 5 nanometers to about 150 nanometers, and wherein each of said plurality of dome shaped protrusions defines a width w ranges from about 50 nanometers to about 1000 nanometers. In an embodiment, spacing s between any two dome shaped protrusions of said plurality of dome shaped protrusions ranges from about 0 nanometers to about 1000 nanometers. In an embodiment, any two dome shaped protrusions of said plurality of dome shaped protrusions are located adjacent to each other so that their bases are without any spacing therebetween (i.e. spacing s=0 nanometers).
[0094] In an embodiment, substantially transparent nano-structure substrate can be made of any suitable material, as known to a person skilled in the art, to serve its intended purpose as laid down in various embodiments of the present disclosure. In a preferred embodiment, substantially transparent nano-structure substrate is made of any or a combination of glass and epoxy material. In an embodiment, the dome shaped protrusions can be of any possible geometry such as hemi-spherical, convex, ellipsoidal, convex shaped ellipsoidal and the like as known to a person skilled in the art to serve their intended purpose as laid down in various embodiments of the present disclosure. Most preferably, the dome shaped protrusions are convex shaped ellipsoidal in geometry.
[0095] FIG. 1 illustrates an exemplary view depicting a section of a nano-structure substrate 100 that is substantially transparent to an incident light. The nano-structure substrate includes a first surface facing the incident light and a second surface facing away from the first surface and substantially coplanar with said first surface. The second surface includes at least one dome shaped protrusion 102. In an embodiment, as illustrated in FIG. 2, dome shaped protrusion 102 can be of any appropriate height h and width w. Preferably, height of the dome shaped protrusion 102 ranges from about 5 nm to about 150 nm and width w of the dome shaped protrusion 102 ranges from about 50 nm to about 1000 nm. Most preferably, height of the dome shaped protrusion 102 is about 75 nm and width of the dome shaped protrusion 102 is about 500 nm. In an embodiment, the second surface includes a plurality of dome shaped protrusions 102 with spacing s between the two protrusions ranges from about 0 nm to about 1000 nm, preferably, the spacing is about 100 nm.
[0096] Another aspect of the present disclosure relates to a method of preparation of a nano-structured substrate 100, substantially transparent to an incident light, for its utility in fabrication of light harvesting device, the method including steps of: deposition of a self-assembled monolayer of polymeric nanospheres on surface of a first substrate using polystyrene solution to prepare a first mould; deposition of PDMS (polydimethylsiloxane) on the first mould followed by curing of PDMS to prepare a second mould; casting of a substrate material, substantially transparent to the incident light, on the second mould; and solidification of the substrate material to realize said nano-structured substrate 100, substantially transparent to the incident light, including a plurality of dome shaped protrusions 102 on one of its surface. In an embodiment, said self-assembled monolayer of polymeric nanospheres includes self-assembled monolayer of polystyrene nanospheres. In an embodiment, said first substrate includes silicon. In an embodiment, said substrate material includes any or a combination of epoxy polymer and glass. Alternatively, any other substrate material, as known to a person skilled in the art, can be utilized to subserve its intended purpose as fully laid down in the embodiments of the present disclosure. Diameter of the nanospheres can be chosen according to the periodicity (spacing s) required in final structure of the dome shaped protrusions 102 on the surface of the nano-structured substrate 100. FIG. 2 illustrates an exemplary Atomic force micrograph image of a nano-structured substrate 100 (utilizing epoxy polymer as the substrate) with a plurality of dome shaped protrusions 102 obtained by two-step moulding process as described hereinabove. Alternatively, any other methods as known to a person skilled in the art can be utilized to realize the nano-structured substrate 100 with a plurality of dome shaped protrusions 102.
[0097] Another aspect of the present disclosure provides a light harvesting device including: a nano-structured substrate defining a first surface facing an incident light and a second surface facing away from the incident light; a substantially transparent electrode; a back electrode; and an active layer; wherein, said nano-structured substrate includes a material substantially transparent to an incident light, and wherein said nano-structured substrate includes a plurality of dome shaped protrusions on said second surface.
[0098] In an embodiment, the light harvesting device can be any photovoltaic and photo-detector device. In a preferred embodiment, light harvesting device is a solar cell. In an embodiment, the substantially transparent electrode can be made of any material known to a person skilled in the art. In a preferred embodiment, the substantially transparent electrode is made of Indium Tin Oxide (ITO). In an embodiment, back electrode can be of any material known to a person skilled in the art. Preferably, back electrode is made of aluminium. In an embodiment, the active layer can be of any material or composition known to a person skilled in the art. In a preferred embodiment, the active layer is made of P3HT:PC60BM.
[0099] FIG. 3A illustrates an exemplary view depicting layer wise construction of previously known light harvesting device fabricated on substantially transparent planar nano-structure substrate (hereinafter, referred to as planar reference device) by coating with electrode layers and photoactive layers. FIG. 3B illustrates an exemplary view depicting layer wise construction of a light harvesting device, realized in accordance with embodiments of the present disclosure, utilizing nano-structured substrate with plurality of dome shaped protrusions. The light harvesting device, as illustrated in FIG. 3A and 3B, utilizes substantially transparent substrate and can be realized by coating electrode layers and photoactive layers thereon. The light harvesting device includes transparent electrode layer, hole transport layer, active layer and back electrode layer. Alternatively, light harvesting device can also include one or a combination of dielectric material layer and buffer layer in addition to transparent electrode layer, hole transport layer, active layer and back electrode layer. In an embodiment, a light or a photon enters the device through the transparent glass substrate along the boundary Bin (first surface) and then gets transmitted into the photoactive material layer across the boundary B0. The shape of Bin (first surface) remains the same as with planar reference devices. However, the shape of the boundary B0 differs as can be seen in the FIG. 3A and FIG. 3B. The radius of curvature of the interface between different layers in FIG. 3B can take on arbitrary values, and need not necessarily be equal to that of the nano-structure.
[00100] In an embodiment, a dielectric film with a dielectric constant and a sharp contrast between layers above and below it, is incorporated in the light harvesting device. This is particularly advantageous in case of replacement of ITO (Indium tin oxide) with any other material as material of construction for transparent electrode. In lab-scale, ITO is the most widely used electrode material. However, owing to the scarcity of Indium, alternate electrode materials need to be considered. The challenge(s) in utilization of these alternative electrode materials is insufficient light scattering at the interface. For example, refractive index of a polymer electrode material PEDOT:PSS is very close to that of the substrate (glass) in which case, scattering is insignificant. In such cases, one can use a suitably chosen high refractive index dielectric layer to separate nano-structured substrate and transparent electrode and to provide required contrast in optical properties. Materials suitable for the dielectric films can be selected from but not limited to inorganic oxide materials such as TiO2, Al2O3, ZrO¬2 and the like.
[00101] FIG. 4 illustrates an exemplary view depicting a layer wise construction for a light harvesting device incorporating dielectric layer to compensate for poor scattering properties of inorganic oxides while utilizing these inorganic oxides instead of ITO in fabrication of transparent electrode. In an embodiment, multiple light harvesting devices are stacked over each other with the transparent nano-structured substrate. FIG. 5 illustrates an exemplary view depicting a layer wise construction of a light harvesting device with multiple active layers and multiple electrode layers stacked one above another. This result in a tandem light harvesting device where each active layer can absorb different spectral ranges of light, thus, effectively covering the entire incident spectrum of light. The substantially transparent nano-structured substrate dimensions (w, h and s) can be appropriately chosen to obtain a desired scattering spectrum and scattering intensity. However, the order in which, transparent electrodes and buffer layers are stacked can be varied based on the particular device.
[00102] In an embodiment, the layer present immediately above the substantially transparent nano-structure substrate replicates the shape of the protrusion. FIG. 6A illustrates an exemplary view depicting a layer wise construction of a light harvesting device where the layer immediately above the nano-structured substrate does not retain the shape of the dome shaped protrusion. In an alternative embodiment, the layer immediately above the substantially transparent nano-structure substrate can be flat. FIG. 6B illustrates an exemplary view depicting a layer wise construction of a light harvesting device where the layer immediately above the nano-structure substrate is flat i.e. it does not retain shape of dome shaped protrusion. In this case, one or more of layers can be thick enough to result in a flat interface on the top after coating on the nano-structured substrate. This architecture can still have advantageous optical scattering effects with shifted spectral features and intensities and can lead to increased light absorption near the desired electrode. In an embodiment, one or more layers above the nano-structured substrate with protrusion can retain the shape of the protrusion. In an embodiment, one or more layers above the nano-structured substrate with protrusion can be flat. In an embodiment, all layers can retain shape of the dome shaped protrusion throughout the thickness of light harvesting device. In this case, both bottom interface and top interface of the active layer retain shape of the dome shaped protrusions. This results in additional advantages as back electrode have dome shaped structure that could help in more light trapping, and further increase in the built in fields.
[00103] FIG. 7 illustrates an exemplary view depicting a layer wise construction of a light harvesting device with shape of the dome shaped protrusion not being at the interface between the active layer and the back electrode. In an embodiment, a light harvesting device can be fabricated utilizing a substantially transparent nano-structured substrate with a plurality of dome shaped protrusions with randomly-varying height h, width w and spacing s. However, height h, width w and spacing s are within the acceptable ranges as laid down in other embodiments of the present disclosure. FIG. 8 illustrates an exemplary view depicting a layer wise construction of a light harvesting device with a plurality of dome shaped protrusions that vary in height h, width w and spacing s. As can be observed from FIG. 8, shapes of these protrusions may vary from each other. For example, some protrusions may be of convex shape, some protrusions may be of ellipsoidal shape, some protrusions may be of hemi-spherical shape and the like i.e. within the scope of dome shaped protrusions, as defined elsewhere in the present disclosure.
[00104] In an embodiment, the nano-structured substrate with a plurality of dome shaped protrusions can be utilized to fabricate a photo-detector where the input (light) is of a single wavelength or a narrow band of wavelengths, rather than broadband light. The advantageous effects of utilization of a nano-structured substrate with a plurality of dome shaped protrusions can be observed in terms of a faster transient response as effective transit time of the photo-generated carriers would be reduced by the nano-scale optoelectronic effect.
[00105] In an embodiment, the epoxy substrate can be a thin flexible substrate with nano-structures. In this case, the nano-structured thin flexible epoxy substrate can be useful for flexible optoelectronic platforms. This flexible nano-structured monolithic substrate can be fabricated by any process. As a particular example, a thin nano-structured substrate fabricated by manually blading the epoxy between controlled height-limiters has been carried out. The resulting thin epoxy was also tested for bending stability. FIG. 9A shows a snapshot of bending test, where the substrate is almost bent around 180 degrees. The substrate thickness is arbitrary. It can be rolled around practically by 360 degrees. The minimum radius of curvature can be very less, around a millimeter. Fig. 9B shows a schematic depiction of the bending test carried out on the flexible substrate with the nano-structures.
[00106] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

EXAMPLES
[00107] Fabrication of transparent nano-structured substrate with a plurality of dome shaped protrusions
[00108] Soft lithography based two-step moulding method is used to fabricate nano-structured substrate with a plurality of dome shaped protrusions that is substantially transparent to incident light. Details of the fabrication technique is fully outlined below, as illustrated in FIG. 10A through 10E:
[00109] Mould Preparation 1: A self-assembled monolayer of polystyrene nanospheres is deposited on surface of a silicon substrate using polystyrene solution to form a first mould M1 as shown in FIG. 10A. Diameter of the nanospheres is chosen according to required periodicity in structure of the final device.
[00110] Mould Preparation 2: A second mould M2 is prepared by depositing PDMS (polydimethylsiloxane) on the first mould M1 as illustrated in FIG. 10B, and after deposition of the PDMS on the first mould M1, the PDMS is cured and separated out from the first mould M1 to form a second mould M2. This mould M2 acts as an inverse of first mould M1 in terms of shape as illustrated in FIG. 10C.
[00111] Nano-structured substrate with plurality of dome shaped protrusions: Nano-structured substrate is prepared by casting of molten glass on second mould M2 as illustrated in FIG. 10D. Thereafter, solidified glass substrate is separated out from the second mould M2. This glass substrate resembles first mould M1 in structure as shown in FIG. 10E. The nano-structured glass substrate includes a plurality of dome shaped protrusions, wherein each protrusion has width w of about 250 nm, height h of about 50 nm and spacing s between two protrusions of about 100 nm.
[00112] Design, simulation and fabrication of a solar cell utilizing nano-structured substrate with a plurality of dome shaped protrusions
[00113] A solar cell (bulk hetero-junction polymer solar cell) is fabricated utilizing nano-structured glass substrate with a plurality of dome shaped protrusions by deposition of (coating) a thin layer of Indium Tin Oxide (ITO) electrode, a thin layer of PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) as a hole transport layer, a thin film of P3HT:PC60BM (blend of poly(3-hexylthiophene) and [6,6]-phenyl-C(61)-butyric acid methyl ester) as an active layer and Aluminium back electrode onto the nano-structured glass substrate, as illustrated in FIG. 3B. As illustrated in FIG. 3B, a light or a photon enters the device through the transparent glass substrate along the boundary Bin (first surface) and then gets transmitted into the photoactive material layer across the boundary B0. The shape of Bin (first surface) remains the same as with planar reference devices. However, the shape of the boundary B0 differs as can be seen in the FIG. 3A and FIG. 3B. The radius of curvature of the interface between different layers in FIG. 3B can take on arbitrary values, and need not necessarily be equal to that of the nano-structure.
[00114] Optical characterization of the epoxy substrates
[00115] Total optical transmittance (%T) of planar and structured epoxy substrates are seen to be almost the same for all wavelengths as shown in FIG. 11A. However, specular reflectance of the structured substrates are seen to reduce across a wide range of wavelengths of interest as seen in FIG. 11B, compared to the planar substrates. Also, diffuse reflectance of both the planar and structured substrates are observed to be almost the same from FIG. 11C. This shows that some light is trapped within the structured epoxy substrates due to optical scattering by the nano-structures.
[00116] Optical characterization of the devices
[00117] Specular and Diffuse reflectance of the complete device including all the constituent layers is shown in FIG. 12A and FIG. 12B. It is seen that the structured architecture results in a significantly reduced specular reflectance compared to the planar reference architecture. The diffuse reflectance shows little variation across the planar and structured architectures, although the structured device shows a slightly reduced value across the spectrum. This significant reduction in the specular reflection is a direct evidence of coupling of light which is normally incident on the device into trapped modes within the structure. It will be seen later that this trapping results in an increased photocurrent.
[00118] Optoelectronic characterization of devices
[00119] The structured device shows a significantly enhanced photocurrent at short circuit compared to the planar device, as seen from FIG. 12C. This increase is through a broadband improvement in external quantum efficiency (EQE) of the structured device as seen in FIG. 12D. The statistical data for the photocurrents for planar and structured devices is shown in FIG. 12E. It is seen that there is an improvement in average photocurrent in the structured architecture compared to the planar architecture.
[00120] Morphological characterization of devices
[00121] The line profiles of the top surface of various layers of the fabricated nano-structured solar cell, obtained by atomic force microscopy, are shown in sequence in FIG. 13A through FIG. 13D. FIG. 13A shows morphology of the nano-structured epoxy substrate. FIG. 13B shows morphology of TiO2 (titanium dioxide) layer, FIG. 13C shows the morphology of PH1000 anode layer, and FIG. 13D shows morphology of P3HT:PCBM active layer. It is seen that the nano-structure is retained upto the top of the active layer although there is a drop in the height of the nano-structure. This data is used in the simulations to understand the enhancement mechanism.
[00122] Optoelectronic simulation of the solar cell architectures
[00123] Optoelectronic coupled simulations are carried out to understand the mechanism of photocurrent enhancement in the nano-structured devices. It is seen from FIG. 14A, that the structured device exhibits an improved absorptance across a wide range of wavelengths (roughly 350-550 nm). At higher wavelengths, the planar architecture shows a higher absorptance. However, in the experiments we can see from FIG. 12D that the improvement in the photocurrent is across the entire spectrum considered. This discrepancy could be because of incomplete coverage of the nano-structures, leading to formation of planar patches between structured regions. This would lead to a resultant effect combining the effects of both planar patches and structured regions leading to observed improvements in the experiment. Further, the simulated J-V curves in FIG. 14B clearly show that the photocurrent in the structured device is greater. The simulated fill factors are also higher, because of the improved charge collection resulting from higher electric fields in the y-direction (as can be seen from FIG. 14C). The simulated results are tabulated in Table. 1. In addition, these simulations suggest that there is an overall improvement in the efficiency of the nano-structured light harvesting device, due to combined improvement in the open circuit voltage (Voc), photocurrent density at short circuit (Jsc), and fill factor (FF).

Table 1 – Performance parameters of planar reference device and the realized solar cell
Device type Voc (V) Jsc (mA/cm^2) FF (%) PCE (%)
Planar 0.5451 10.28 50.56 2.8346
Structured 0.5547 12.03 54.91 3.6642

[00124] Spectral tuning of the scattering behaviour
[00125] Due to presence of nano-structures, a part of incoming light in y-direction is scattered into x-direction, and this scattered electric field pattern results in higher absorption of light in the vicinity of lens i.e. near the anode (hole-collecting electrode). FIG. 15 illustrate net power flux density in x-direction for a free-space wavelength ? of 600 nm with width, w=200 nm and height, h=50 nm. From this, it is clearly observed that scattering at the nano-structure makes up to about 10% of the incident power density that is to be scattered into the x-direction.
[00126] A quantity Tx(?) that is a ratio of the power flux density scattered into the x-direction (calculated along the boundary B0¬) to the total input power density is a function of wavelength and design parameters as illustrated in FIG. 16A and FIG. 16B. A graph was plotted between Tx(?) and w to show the variation of Tx(?) with w. By varying width w of the lens, there is a red shift in the significant peaks of Tx(?) as illustrated in FIG. 16A. Thus, an appropriate w can be chosen depending on desired spectral position of the scattering peaks. Also, the intensity of the peak is also sensitive to w.
[00127] FIG. 16B illustrates that height of the protrusion h can be varied to effect a broadband enhancement in the scattering coefficient Tx(?). Dome shaped protrusions with larger height h exhibit more scattering across all wavelengths. However, height of the protrusion does not significantly affect the spectral nature of scattered power. The significant peak positions in FIG. 16B are seen to remain approximately unchanged with varying h.
[00128] Simulations are used to see effects of angle of incidence (?) of the incident broadband light on the planar and nano-structured photovoltaic architectures. It can be clearly seen from FIG. 17, that for all angles of incidence (varying from 0 to 80, shown in FIG. 17), that the structured architecture shows a higher extractable photocurrent density. Also for angles greater than 60°, planar architecture is seen to yield a negligible photocurrent, but the structured device exhibits a good photocurrent even at these large angles of incidence. This shows that the structured device is relatively better at capturing light from all directions, thus attractive for capturing diffused light in scarcely lit environments, indoor conditions etc.
[00129] FIG. 18 shows that domain sizes of structured and planar sub-domains on a master template (first mould M1) can be varied by varying solution processing parameters of the polystyrene bead suspension. The representative domain sizes wp and ws of the planar and the structured sub-domains can vary from small values such as 1 micrometer to large values such as 1 millimeter (both inclusive).
[00130] FIG. 19 illustrates a setup wherein axial mechanical stress is applied on the intermediate PDMS mould (second mould M2) 1906 that carries inverse nano-structure patterns relative to the first mould M1. Application of stress by using a stationary holder 1902 and a moving holder 1904 that are coupled with ends of the substrate, wherein the moving holder 1904 tends to move in a direction away from the stationary holder 1902, thus, exerting axial stress on the intermediate PDMS mould M2 of the substrate. Application of stress elongates the intermediate PDMS mould M2 in the direction of tensile stress and compresses it in the perpendicular axis. This means that the periodicity of nano-features increases along one axis and decreases along the other, relative to the unstretched mould.
[00131] Top view of the unstretched mould as seen from FIG. 19B shows a periodic arrangement of circles. However, top view of the stretched PDMS moulds in FIG. 19C shows a periodic arrangement of ellipses. The singular diameter ‘d’ in the unstretched mould in now converted into a major axis and minor axis d1 and d2 such that (d1>d and d2