WO2018010680A1

WO2018010680A1 - Rose petal textured haze film for photovoltaic cells

Info

Publication number: WO2018010680A1
Application number: PCT/CN2017/092847
Authority: WO
Inventors: Zijian Zheng; Kan Li; Yaokang ZHANG; Hongyu ZHEN
Original assignee: The Hong Kong Polytechnic University
Priority date: 2016-07-14
Filing date: 2017-07-13
Publication date: 2018-01-18

Abstract

Disclosed is an assembly of a solar cell and a haze film, comprising: a solar cell; and a biomimetic haze film having a first surface and an opposing second surface, the second surface of the biomimetic haze film comprises a plurality of closely packed microcraters, wherein each of the plurality of microcraters includes wrinkle-like sub-structures. Also disclosed are the use of a biomimetic haze film in a solar cell and the method of fabricating an assembly of a solar cell and a haze film. The haze film with rose petal texture enhances the light absorption of the photovoltaic cells.

Description

ROSE PETAL TEXTURED HAZE FILM FOR PHOTOVOLTAIC CELLS

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 62/362,056, entitled LOW-COST ROSE PETAL TEXTURED FILM WITH HIGH TRANSMISSION HAZE FOR PHOTOVOLTAIC CELLS, which was filed on July 14, 2016, and is hereby incorporated by reference in its entity.

TECHNICAL FIELD

The present disclosure generally relates to a haze film and, in particular to a biomimetic haze film for photovoltaic cells and, more particularly to a haze PDMS film with rose petal texture for enhancing the light absorption of the photovoltaic cells.

BACKGROND ART

Solution-processable photovoltaic devices such as organic solar cells (OSCs) and perovskite solar cells (PSCs) are promising low-cost technologies for solar energy harvesting. These thin-film photovoltaic cells have an active layer thickness lower than their saturation absorption thickness. As such, they are not able to trap enough incident photons, resulting in insufficient absorption of the incident light and thus low power conversion efficiency (PCE) or high optical power loss. In this regard, light absorption enhancement becomes a key issue to improve the power conversion efficiency (PCE) of these types of photovoltaic cells for commercial applications.

Numerous approaches to improve the PCE of OSCs and PSCs have been reported in the prior art, such as new material synthesis, bandgap engineering, interfacial engineering, and optical engineering. Among them, the optimizing of light trapping strategy plays a very important role. Trapping the incident light in the active layers of OSCs and PSCs can effectively improve the light absorption. Incident light trapping can be achieved by fabricating light trapping structures directly in the photovoltaic cells, such as fabricating an anti-reflection layer, designing V-shape or cone-shape device architecture, engineering light-scattering texture in the transparent electrode, or introducing plasmonic enhancement with metal nanostructures or metal nanoparticles. Although these methods can effectively improve the PCE of the solar cells, they introduce undesirable materials contamination that is difficult to eliminate, requiring complicated and specialized fabrication steps, or are limited to very specific light absorbers. Incident light trapping can also be achieved by fabricating light trapping structures separately and attaching them onto the photovoltaic cells. For example, nanostructured cellulose papers and thin films made of self-aggregated alumina nanowire bundles are available as haze films for increasing the PCE of OSCs. Wood composites-based haze film can also be attached onto a GaAs solar cell. However, the fabrication of these haze films still requires tedious procedures and the use of large amounts of hazardous solvents or etchants. In addition, both cellulose papers and alumina nanowires show potential instability issues to weathering such as acid rain.

DISCLOSURE OF INVENTION

A need therefore exists for a novel haze film for solar cell that eliminates or diminishes the disadvantages and problems described above.

Provided herein is an assembly of a solar cell and a haze film comprising a solar cell and a biomimetic haze film having a first surface and an opposing second surface, the second surface of the biomimetic haze film comprises a plurality of closely packed microcraters, wherein each of the plurality of microcraters includes wrinkle-like sub-structures.

In certain embodiments, an epidermis pattern on rose petals is transferred onto the second surface of the biomimetic haze film.

In certain embodiments, each of the plurality of microcraters is similar to a concave semi-sphere.

In certain embodiments, each of the plurality of microcraters has a pentagonal, hexagonal or heptagonal cross-sectional shape and a semi-circular, arc or parabolic longitudinal-sectional shape.

In certain embodiments, each of the plurality of microcraters is about 15 μm -35 μm in diameter and about 10 μm -20 μm in depth.

In certain embodiments, the wrinkle-like sub-structures within each of the plurality of microcraters has a size of 100 nm –1000 nm.

In certain embodiments, the biomimetic haze film is made from polydimethylsiloxane (PDMS) .

In certain embodiments, the solar cell is a thin-film solar cell, particularly a Perovskite solar cell or an organic solar cell.

In certain embodiments, the biomimetic haze film is pre-fabricated and attached to a transparent side of the solar cell permanently or reversibly.

Provided herein is a use of a biomimetic haze film in a solar cell, wherein the biomimetic haze film has a first surface and an opposing second surface, the second surface of the biomimetic haze film comprises a plurality of closely packed microcraters, wherein each of the plurality of microcraters includes wrinkle-like sub-structures.

In certain embodiments, each of the plurality of microcraters is about 15 μm -35 μm in diameter and about 10μm -20 μm in depth.

Provided herein is a method of fabricating an assembly of a solar cell and a haze film, the method comprises the steps of: i) providing a solar cell, ii) fabricating a biomimetic haze film, the fabrication comprising the steps of: a) mixing a PDMS prepolymer and a curing agent thereby obtaining a mixture； b) degassing the mixture thereby obtaining a degassed mixture； c) pouring the degassed mixture onto a surface of rose petal thereby obtaining an intermediate product； d) curing the intermediate product thereby obtaining a textured PDMS； and e) peeling off the textured PDMS from the rose petal, and iii) attaching the biomimetic haze film onto a transparent side of the solar cell.

In certain embodiments, in step a) the PDMS prepolymer and the curing agent is mixed at a ratio of about 10: 1 (w/w) .

In certain embodiments, in step d) the intermediate product is cured at about 70℃ for about 2 hours.

In certain embodiments, in step d) the intermediate product is cured at room temperature for around 48 hours.

In certain embodiments, the fabrication further comprises, after step e) , the steps of: f) treating the textured PDMS by ultrasonication in NaOH aqueous solution, acetone, thanol, and DI water in sequence, thereby obtaining a treated PDMS； and g) curing the treated PDMS at about 70℃ for about 2 hours.

In certain embodiments, step i) comprises providing a thin-film solar cell, particularly a Perovskite solar cell or an organic solar cell.

In certain embodiments, step iii) comprises attaching the biomimetic haze film to the solar cell permanently or reversibly.

In certain embodiments, step c) comprises pouring the degassed mixture onto a surface of a PDMS stamp thereby obtaining an intermediate product, the PDMS stamp comprises a first surface and an opposing second surface, the second comprises a plurality of closely packed micro-projections, wherein each of the plurality of micro-projections includes wrinkle-like sub-structures.

In certain embodiments, each of the plurality of micro-projections is similar to a convex semi-sphere.

In certain embodiments, each of the plurality of micro-projections has a pentagonal, hexagonal or heptagonal cross-sectional shape and a semi-circular, arc or parabolic longitudinal-sectional shape.

In certain embodiments, each of the plurality of micro-projections is about 15 μm -35 μm in diameter and about 10 μm -20 μm in height, the wrinkle-like sub-structures on each of the plurality of micro-projections has a size of 100 nm –1000 nm.

These and other aspects, features and advantages of the present disclosure will become more fully apparent from the following brief description of the drawings, the drawings, the detailed description of certain embodiments and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The appended drawings contain figures of certain embodiments to further illustrate and clarify the above and other aspects, advantages and features of the present disclosure. It will be appreciated that these drawings depict embodiments of the disclosure and are not intended to limit its scope. The disclosure will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1a is a perspective view of a biomimetic haze film according to certain embodiments of the present disclosure, FIGs 1b and 1c are top view and longitudinal sectional view of the biomimetic haze film shown in SEM images, the scale bar in FIG. 1b is 30 μm and the scale bar in FIG. 1c is 10μm；

FIG. 2 is an illustration of the light-scattering mechanism of the biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 3a and FIG. 3b are schematic drawings of a Perovskite solar cell with biomimetic haze film and an organic solar cell with biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 4 is an illustration of the haze effect of a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 5a and FIG. 5b are illustrations of optical measuring setups for diffusion transmittance and haze transmittance of a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 6 shows the diffusion transmittance and the haze transmittance of a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 7a and FIG. 7b show comparisons between a biomimetic haze film according to certain embodiments of the present disclosure and traditional haze materials in respect of diffusion transmittance and haze transmission；

FIG. 8a and FIG. 8b show scattering light power angular distribution of a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 9 shows absorption spectra of a bare Si wafer, Si wafers covered with flat PDMS film and covered with a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 10 show J-V curves of a Perovskite solar cell and an organic solar cell with and without a biomimetic haze film according to certain embodiments of the present disclosure；

FIG. 11 shows the normalized photocurrent values of Perovskite solar cells with and without a biomimetic haze film according to certain embodiments of the present disclosure measured at different angles of light incidence； and

FIG. 12 shows J-V curves of a Perovskite solar cell enhanced with a biomimetic haze film according to certain embodiments of the present disclosure before and after acid treatment of the biomimetic haze film.

MODES FOR CARRYING OUT THE INVENTION

The present disclosure is generally directed towards biomimetic haze films for thin film photovoltaic cells (e.g. amorphous silicon, CdTe, CIGS and organic or polymer cells) . The embodiments disclosed, however, are not limited to use in thin film photovoltaic cells. The biomimetic haze films can be equally used with other types of photovoltaic cells, such as wafer-based photovoltaic cells (e.g. polysilicon and monocrystalline silicone cells) . It will also be understood that, in light of the present disclosure, the biomimetic haze films disclosed herein can be successfully used in connection with other types of devices. Non-limiting examples of such applications include anti-reflection film for displays, additional layer on glass curtain walls and emission enhancement for light-emitting devices.

Additionally, where a numerical value or range is provided, unless specified otherwise, the value or range can be varied by ±5％without departing from the spirit of the present disclosure.

The present disclosure has realized that by attaching a pre-fabricated haze film layer on top of a transparent side of photovoltaic cells, the light absorption of the photovoltaic cells over a wide optical spectrum can be improved effectively. The haze film layer scatters the incident light that passes through and extends its optical pathway in the active layer of photovoltaic cells, which leads to more efficient light absorption of the photovoltaic cells. In addition, the haze film layer also enables significant enhancement of the absorption of small-angle incident light. Further, such pre-fabricated haze layer provides additional anti-reflection ability and does not affect the photovoltaic cell fabrication process.

A detailed description of exemplary embodiments of the transducer now follows.

Biomimetic Haze Film (BHF)

The BHF 1 provided herein is an independent accessory of a solar cell. It can be pre-fabricated and then attached, either permanently or reversibly to a transparent side (e.g. a glass layer) of an existing solar cell 4. The solar cell 4 can be of any type. In certain embodiments, the solar cell 4 is a thin film solar cell. In certain embodiments, the solar cell 4 is a Perovskite solar cell or an organic solar cell.

The BHF 1 can have a thin film structure. In certain embodiments, the total thickness of the BHF can be between 10 μm and 500 μm, for example between 20 μm and 450 μm, between 30 μm and 400 μm, between 40 μm and 350 μm, between 50μm and 300 μm, between 60 μm and 250 μm, between 70 μm and 200 μm, between 80 μm and 150 μm, and between 90 μm and 100 μm. The surface area of the BHF 1 can depend on the active area of the solar cell it is attached to. In certain embodiments, the surface area of the BHF can be between 0.001 cm² to 1 cm², for example, between 0.005 cm² and 0.9 cm², between 0.01 cm² and 0.8 cm², between 0.02 cm² and 0.7 cm², between 0.03 cm² and 0.6 cm², between 0.04 cm² and 0.5 cm², between 0.05 cm² and 0.4 cm², between 0.06 cm² and 0.3 cm², between 0.07 cm² and 0.2 cm², between 0.08 cm² and 0.1 cm², and 0.09 cm². In certain embodiments, the BHF is substantially unpatterned or flat on a first surface and has a topography on an opposing second surface. In certain embodiments, the first surface may be provided with certain structures to facilitate attachment to the solar cell. The topography of the second surface of the BHF 1 is replicated from the original rose petals as shown in Fig. 1a. As will be introduced in more detail below, an epidermis pattern on rose petals is transferred onto the BHF 1. In certain embodiments, an epidermis pattern on rose petals is uniformly transferred onto the BHF 1 meaning ～100％of the epidermis pattern on rose petals is transferred onto the BHF 1. In certain embodiments, ～98％, ～95％, ～93％, ～90％, ～88％, ～85％, ～83％, or ～80％of the epidermis pattern on rose petals is transferred onto the BHF 1.

In certain embodiments, the topography shows closely packed microscale craters 2 (i.e. microcraters) , which are similar to concave semi-spheres that are typically used in micro-lens arrays. In certain embodiments, the microcraters can have a generally polygonal cross-sectional shape. For example, the cross-sectional shape can be generally pentagon, hexagon or heptagon as shown in Fig. 1b. In certain embodiments, the microcraters can generally have a semi-circular, arc or parabolic longitudinal-sectional shape as shown in Fig. 1c. The average size of the microcraters 2 is around 15-35 μm in diameter and around 10-20 μm in depth. For example, the diameter can be between 16 μm and 34 μm, between 17 μm and 33 μm, between 18 μm and 32 μm, between 19 μm and 31 μm, between 20 μm and 30 μm, between 21 μm and 29 μm, between 22 μm and 28 μm, between 23 μm and 27 μm, between 24 μm and 26 μm, and 25 μm. For example, the depth can be between 11 μm and 19 μm, between 12 μm and 18 μm, between 13 μm and 17 μm, between 14 μm and 16 μm, and 15 μm. In each of the microcraters 2, there exist wrinkle-like sub-structures 3 in the size of several hundred nanometers as shown in Figs. 1b and 1c, which are also duplicated from the epidermis pattern on rose petals. the For example, the size of the wrinkle-like sub-structures 3 can be between 100 nm and 1000 nm, between 200 nm and 900 nm, between 300 nm and 800 nm, between 400 nm and 700 nm, and between 500 nm and 600 nm. In certain embodiments, wrinkle-like sub-structures 3 are near the center of the microcraters 2.

Fig. 2 illustrates the light-scattering mechanism of the BHF 1. In certain embodiments, the BHF 1 is made from polydimethylsiloxane (PDMS) or comprises PDMS. It is attached on a transparent side of a solar cell 4. Since the refractive index of air (n_air ＝ 1) is lower than that of PDMS (n_PDMS ～ 1.4) , the microcraters 2 on the textured surface of the BHF 1 can work as concave lens to diverge incident light into the BHF 1, and then into the solar cell 4 (n_GLASS ～ 1.5) . The wrinkle-like sub-structures 3 can also serve as anti-reflection covering for the underneath layers, i.e. layers of the solar cell 4.

BHF Fabrication

In the fabrication of the BHF and/or solar cells, the following materials are needed: PDMS prepolymer as a bulk material and Sylgard 184 as a curing agent are commercially available from e.g. Dow Corning； Titanium (IV) isopropoxide (～99.999％) , niobium (V) ethoxide (～99.95％) , YCl₃ (～99.99％) , PbCl₂ (～99.999％) , methylamine solution (MAI, ～33％in ethanol) , HI (～57％in water) , bis (trifluoromethane) sulfonimide lithium salt (Li-TFSI, ～98％) , 1, 8-diiodooctane (DIO, >～97％) and 4-tert-butylpyridine (tBP, ～96％) are commercially available from e.g. Sigma-aldrich； 2, 2’ , 7, 7’ -tetrakis (N, N-di-p-methoxyphenylamine) -9, 9’ -spirobifluorene (Spiro-OMeTAD, ～99.5％) is commercially available from e.g. Derthon Optoelectronic Materials Science Technology Co., Ltd； Poly [ {2, 5-bis (2-hexyldecyl) -2, 3, 5, 6-tetrahydro-3, 6-dioxopyrrolo [3, 4-c] pyrrole-1, 4-diyl} -alt- { [2, 2’ , 5’ , 2” -terthiophene] -5, -5” -diyl] (PDPP3T) is commercially available from e.g. Organtec Materials, Inc； 1- (3-methoxycarbonyl) propyl-1-phenyl [6, 6] C₇₁ (PC₇₁BM) is commercially available from American Dye Source, Inc； Poly (3, 4-ethylenedioxythiophene) : poly (styrenesulfonate) solution (PVP AI 4083) (PEDOT : PSS) is commercially available from e.g. Heraeus Clevios； and ZnO nanoparticles are prepared using an adapted procedure based on the work of Weller et al. The ZnO nanoparticles are in the size of about 5 nm, and dispersed in 2-methoxyethanol by a weight ratio of ～0.5 ％. The BHF formed from PDMS is referred to as a haze PDMS film (HPF) . A person skilled in the art will understand that, materials other than PDMS can be used as the bulk material in the fabrication of the BHF, provided the material has a refractive index higher than that of air (i.e. n>1) , and the material allows sufficient incident light to transmit therethrough. Likewise, other materials used in the fabrication can be replaced with equivalent alternatives without departing from the spirit of the present invention.

The BHF is fabricated by a one-step soft lithography replication process that can be independent of the solar cell fabrication. In certain embodiments, the process includes mixing the PDMS prepolymer and its curing agent at a ratio of ～10: 1 (w/w) , the mixture is degassed and poured onto the surface of rose petals. After being cured at ～70℃ for about 2 hours, the BHF is obtained by peeling off the textured PDMS from the rose petal surface.

In some other embodiments, PDMS prepolymer and the curing agent are first mixed in a ratio of ～10: 1 (w/w) and bubbles in the mixture are removed by degassing in a vacuum desiccator. The degassed mixture is then poured onto the surface of rose petals, which can for example be taped on a plastic petri dish. After being cured at room temperature for around 48 hours, the textured PDMS is peeled off from the rose petals. Then the textured PDMS is treated by ultrasonication in NaOH aqueous solution (～13 wt. ％) , acetone, ethanol, and DI water in sequence. Finally, the treated PDMS is cured at ～70℃for about 2 hours in an oven forming the BHF.

The soft lithography replication process does not require high cost or complicated equipment. It uniformly transfers the epidermis pattern on rose petal surfaces onto PDMS. The fabrication of the BHF can be made scalable in that PDMS stamp or stamp of other material can be used instead of rose petals as molds of the textured surface. The PDMS stamp or stamp of other material are made with the same aforementioned methods for fabrication of BHF. In certain embodiments, the transfer of pattern is made once, (i.e. transfer of epidermis pattern from the rose petals to a first PDMS stamp) so that the first PDMS stamp will have a negative pattern complimentary to the epidermis pattern of the rose petals. In certain embodiments, the transfer of pattern is made twice (i.e. a first transfer of epidermis pattern from the rose petals to a first PDMS stamp, and a second transfer of epidermis pattern from the first PDMS stamp to a second PDMS stamp) so that the second PDMS stamp will have the same pattern as rose petals.

As a result, a first PDMS stamp with a textured surface having a negative pattern that is complimentary to the epidermis pattern from the rose petals and/or a second PDMS stamp having a positive pattern that is the same as the epidermis pattern from the rose petals can be obtained. For the first PDMS stamp, the topography shows closely packed microscale craters (i.e. microcraters) , which are similar to concave semi-spheres that are typically used in micro-lens arrays. In certain embodiments, the microcraters of the first PDMS stamp can have a generally polygonal cross-sectional shape. For example, the cross-sectional shape can be generally pentagon, hexagon or heptagon (not shown) . In certain embodiments, the microcraters can generally have a semi-circular, arc or parabolic longitudinal-sectional shape (not shown) . The average size of the microcraters is around 15-35 μm in diameter and around 10-20 μm in depth. For example, the diameter can be between 16 μm and 34 μm, between 17 μm and 33 μm, between 18 μm and 32 μm, between 19 μm and 31 μm, between 20 μm and 30 μm, between 21 μm and 29 μm, between 22 μm and 28 μm, between 23 μm and 27 μm, between 24 μm and 26 μm, and 25 μm. For example, the depth can be between 11 μm and 19 μm, between 12 μm and 18 μm, between 13 μm and 17 μm, between 14 μm and 16 μm, and 15 μm. In each of the microcraters, there exist wrinkle-like sub-structures in the size of several hundred nanometers (not shown) . For example, the size of the wrinkle-like sub-structures can be between 100 nm and 1000 nm, between 200 nm and 900 nm, between 300 nm and 800 nm, between 400 nm and 700 nm, and between 500 nm and 600 nm. In certain embodiments, wrinkle-like sub-structures are near the center of the microcraters. For the second PDMS stamp, the topography is complimentary to that of the first PDMS stamp. In particular, the topography shows closely packed microscale projections (i.e. micro-projections) , which are similar to convex semi-spheres. In certain embodiments, the micro-projections of the second PDMS stamp can have a generally polygonal cross-sectional shape. For example, the cross-sectional shape can be generally pentagon, hexagon or heptagon (not shown) . In certain embodiments, the micro-projections can generally have a semi-circular, arc or parabolic longitudinal-sectional shape (not shown) . The average size of the micro-projections is around 15-35 μm in diameter and around 10-20 μm in height. For example, the diameter can be between 16 μm and 34 μm, between 17 μm and 33 μm, between 18 μm and 32 μm, between 19 μm and 31 μm, between 20 μm and 30 μm, between 21 μm and 29 μm, between 22 μm and 28 μm, between 23 μm and 27 μm, between 24 μm and 26 μm, and 25 μm. For example, the height can be between 11 μm and 19 μm, between 12 μm and 18 μm, between 13 μm and 17 μm, between 14 μm and 16 μm, and 15 μm. In each of the micro-projections, there exist wrinkle-like sub-structures in the size of several hundred nanometers (not shown) . For example, the size of the wrinkle-like sub-structures can be between 100 nm and 1000 nm, between 200 nm and 900 nm, between 300 nm and 800 nm, between 400 nm and 700 nm, and between 500 nm and 600 nm. In certain embodiments, wrinkle-like sub-structures are near the center of the micro-projections.

BHF can be fabricated using the same method discussed above using the first or second PDMS stamp instead of rose petals. As a result, a BHF with a textured surface having a positive pattern that is the same as the epidermis pattern from the rose petals or a negative pattern that is complimentary to the epidermis pattern from the rose petals can be fabricated.

Using PDMS stamp or stamp of other material, the fabrication can be industrialized easily. In addition, as the fabrication of the BHF is a separate process, it has no impact on the existing solar cell fabrication process.

Perovskite Solar Cell Fabrication

A schematic drawing of a Perovskite solar cell 4 with BHF 1 is shown in Fig. 3a. A fluorine-doped tin oxide (FTO) film or glass slide is commercially available from e.g. Zhuhai Kaivo Optoelectronic Technology Co., Ltd. In certain embodiments, it is cleaned in ultrasonic bath of acetone, isopropanol and water for around 30 min respectively prior to further processing. The FTO film or glass slide can then be treated with piranha solution (mixture of ～95％H₂SO₄ and ～30％H₂O₂, volume ratio ～3: 1) for around 10 minutes. Afterwards, a solution of titanium isopropoxide (～3％in ethanol, added with either ～5 wt. ％YCl₃ or ～5 wt. ％niobium (V) ethoxide) can be spin-coated on the rinsed and dried FTO film or glass slide at ～5000 rpm for ～30 seconds, and then annealed at ～100℃ for ～20 min in the air. The FTO film or glass slide can then treated with plasma for ～1 min to remove any organic residues. Afterwards, the FTO film or glass slide can be taken into a N₂-filled glove box, followed by spin-coating a perovskite precursor solution (～2.64M CH₃NH₃I and ～0.88M PbCl₂ in dimethylformamide) at ～2000 rpm for ～1 minute. After annealing at ～100℃ for ～90 minutes, a chlorobenzene solution of spiro-OMeTAD (doped with ～25 μL ～520 mg/mL Li-TFSI acetonitrile solution and ～36 μL tBP for each 1 mL solution) is spin-coated atop the perovskite layers. Finally, ～60 nm Au is deposited on top of the spiro-OMeTAD layers by vacuum deposition at a rate of ～0.3 nm/s.

Having prepared the Perovskite solar cell 4, the BHF 1 of the present disclosure can be attached thereon. The first surface of the BHF 1 is attached to the transparent side of the Perovskite solar cell, such that the second surface of the BHF 1 with biomimetic topography faces the external environment. Such attachment can be permanent or reversible. Methods of attachment include but are not limited to physical bonding, chemical bonding, heat bonding, mechanical fastening, glue bonding, welding, etc.

Organic Solar Cell Fabrication

A schematic drawing of an organic solar cell 4 with BHF 1 is shown in Fig. 3b. First, an active layer solution is prepared. PDPP3T: PC₇₁BM (～5 mg/ml : ～10 mg/ml) is dissolved into o-dichlorobenzene (oDCB) and stirred at ～70℃ for ～5 hours. Then, after the solution temperature cools down to room temperature (about 25℃) , chloroform (CF) is added and the solution is stirred for one hour. DIO is added and after ～30 min the solution is ready for device fabrication. In this ternary solvent system, oDCB : CF : DIO is mixed by a volume ratio of ～76 : 19 : 5. The active layer solution can then be used for device fabrication. An indium tin oxide (ITO) coated film or glass slide is cleaned using ultrasound in acetone, ethanol, and deionized water in sequence. Then PEDOT : PSS is spin-coated onto the ITO with ～1500 rpm for ～1 minute, and baked at ～120℃ for ～10 min to evaporate the solvent. The solution of PDPP3T: PC₇₁BM is spin coated at ～2500 rpm for ～1 minute and heated at ～80℃ for ～20 min to form the active layer. An electron transport layer ZnO is then spin-coated at ～1500 rpm for ～30 seconds and dried at ～80℃for ～30 minutes. Finally, ～100 nm aluminum electrode is thermal evaporated on the ZnO layer at a rate of ～0.1 nm/sfor the first 20 nm and ～1 nm/sfor the rest.

Having prepared the organic solar cell 4, the BHF 1 of the present disclosure can be attached thereon. The first surface of the BHF 1 is attached to the transparent side of the organic solar cell, such that the second surface of the BHF 1 with biomimetic topography faces the external environment. Such attachment can be permanent or reversible. Methods of attachment include but are not limited to physical bonding, chemical bonding, heat bonding, mechanical fastening, glue bonding, welding, etc.

BHF Characterization

The remarkable haze effect of the BHF is demonstrated in Fig. 4. In this particular example, the BHF is a HPF and the total thickness of the HPF is ～300 μm. A background paper printed with the word “Haze” is provided. The HPF of the present disclosure is placed atop the paper with a distance h between the paper surface and the HPF top surface. Observation is made from the top. When the HPF is attached closely to the background paper (i.e. the h is ～300 μm) , the haze effect is not obvious so the word “Haze” is clearly visible. However, as the distance becomes larger, e.g. when h＝1 cm, the word becomes too hazy to recognize.

Optical measurements are carried out to quantitatively evaluate the haze ability of the BHF, including a diffusion transmittance, a haze transmittance and a scattering light power angular distribution. The diffusion transmittance refers to the ratio of the light power passed through to the light power incident, while the haze transmittance is the percentage of the light scattered by the BHF among the total transmitted light. These two parameters demonstrate the haze ability from the perspective of optical power. The scattering light power angular distribution describes the directional distribution of scattered light quantitatively.

The optical measuring setup for diffusion transmittance and haze transmittance is illustrated in Fig. 5a. A spectrophotometer with integration sphere components is used to collect the transmitted light in every spatial direction to obtain the diffusion transmittance. Since only scattered light but not the direct light needs to be collected, the transmitted light that is parallel to the incident light is absorbed by an optical absorber. The result in Fig. 6 shows the BHF of the present disclosure presents an ultra-high diffusion transmittance of ～97％over the whole testing wavelength ranging from 400 nm to 800nm. Meanwhile, the BHF possesses a high haze transmittance of ～75％throughout the whole visible spectrum region. The results indicate the diffusion transmittance and haze transmittance are substantially independent of the wavelength of light in the tested wavelength range (400 nm to 800nm) .

Comparisons are made between the diffusion transmittance and haze transmission of the BHF of the present disclosure (HPF in particular) and traditional haze film structures.

The results are shown in Table S1 below and in Figs. 7a and 7b. The BHF of the present disclosure has a relatively high haze transmittance as well as a very high diffusion transmittance. By contrast, the thin film with alumina nanowire bundles A is able to increase its haze value to above 90％, but its optical transmission is reduced to 80～90％, which is undesirable for photovoltaic applications. As a result, the BHF of the present disclosure is ideal for photovoltaic applications due to its improved absorption efficiency of photovoltaic devices.

Table S1

Another advantage of the BHF of the present disclosure is the directional tuning effect for scattered light. As discussed earlier, the scattering light power angular distribution is measured to describe the directional distribution of scattered light quantitatively. The experimental setup is shown in Fig. 5b. By moving a photodetector along a circular path while locating the BHF in the center of the circle, the scattered light power at every single angle can be obtained. In certain embodiments, the angular interval can be set as ～3 degree, and the perpendicular direction to the BHF is defined as 0°. As shown in Fig. 8a where the light intensity received by the photodetector is plotted relative to the scattering angle and wavelength, the angular distribution varies very little throughout the whole visible spectrum region (400 ～ 800 nm) . Specifically, the angular distribution curves at 500, 600 and 700 nm are extracted from Fig. 8a and then plotted in Fig. 8b. These results indicate the angular distribution is almost irrelevant to the wavelength among the tested wavelength range. If the angular distribution edge (or the haze scattering angle) is defined as the angle at which the corresponding light power is 10％of the maximum value, the haze scattering angle is as large as about 56°.

A further advantage of the BHF of the present disclosure is the additional anti-reflection ability. A common solar cell made from a silicon wafer loses a substantial amount of light absorption as the result of surface reflection due to its high refractive indices (n ranges from ～3.7 to ～5.6 between 400 to 800 nm) , even if it is thick enough (～500 μm) to fully absorb photon that enters it. The BHF of the present disclosure provides a low-cost and facile approach to realize the anti-reflection function. As illustrated in Fig. 9, a flat PDMS film reduces the reflection (or increases the absorption) of Si wafer by ～10％at the optical range of 400 to 800 nm. With the additional biomimetic texture, the BHF of the present disclosure further reduces the surface reflection by 3％to 5％at the optical range of 400 to 800 nm, leading to a total anti-reflection ability of 13％～15％.

Solar Cell Characterization

The performance enhancement of solar cells with the BHF of the present disclosure is characterized as follows. PSCs based on methylammonium lead iodide (CH₃NH₃PbI₃) absorber and OSCs based on PDPP3T: PC₇₁BM active layer are fabricated in accordance with the aforementioned methods respectively. Various tests are run and measurements are made for the PSCs or OSCs with and without the BHF of the present disclosure. The data is listed in Table S2 below, wherein Voc refers to an open-circuit voltage, Jsc refers to a short-circuit current density, and FF refers to a fill factor. The Jsc-Voc characteristics of the PSCs and OSCs with and without the BHF are plotted in Fig. 10. The table and drawing show the PCE is improved dramatically with the help of BHF. Taking device No. 10 in Table S2 as an example, the Jsc is 24.5 mA/cm², which is ～18％larger than that of the corresponding bare cell. This strongly indicates a more thorough absorption of incident light. The PCE of the same solar cell also boosts to 19.2％, which is 15.6％higher than that of the bare cell. The other nine PSCs also show significant improvement on device performance with the aid of BHF, and the average enhancement in PCE value is 12.3％.

Table S2

Fig. 11 shows the angular dependence of photocurrent of PSCs from normal incident (denoted as 0°) to a series of oblique incident angles (10-80°) . It is a plot of normalized photocurrent values (normal incident as 100％current density) with and without BHF versus incident light angles. It shows the average photocurrent of bare PSCs dramatically drops from 100％to 27％when the angle is moved from 0° to 80°. With BHF, the photocurrent of PSCs still maintains ～90％at 40°, and ～65％at 80°. When taking the daily sunlight distribution into consideration, such effect approximately equals to >50％improvement of total solar energy harvesting in low latitude area, such as Hong Kong, and ～85％improvement of energy harvesting in high latitude area, such as Moscow. Put it another way, the photocurrent of PSCs can be enhanced by 2.7 folds at an incident angle of 80o with the BHF of the present disclosure. Therefore, the BHF of the present disclosure also provides a more stable device performance versus varied light incident angle.

Table S3 below also shows similar improvements for OSCs with BHF as for the PSCs with BHF. Overall, OSCs gains a PCE enhancement of ～5-10％.

Table S3

In addition, the BHF of the present disclosure shows remarkable resistance to strong acid, which is important for real solar panel application. In certain embodiments, the BHF is immersed in a HCl solution with a pH value of 1, which is far more extreme than that of acid rain (pH ≤ 5) , and performance of a PSC with the acid-treated BHF is tested. As shown in Fig. 12, the efficiency enhancement of PSC remains unchanged before and after the acid treatment of BHF, indicating a stable optical performance of the BHF in acidic environment. As such, the BHF can also be used to protect solar cells from the effects of acid rain.

The above characterizations demonstrate by duplicating the surface texture of rose petals with PDMS or other suitable material, the present disclosure is able to produce a low-cost biomimetic haze film. This film possesses an ultra-high haze transmittance of up to 75％and a high diffusion transmittance of up to 97％. Over 15％improvement of the power conversion efficiency of perovskite solar cell can be obtained by simply attaching the haze film onto the solar cell. The BHF of the present disclosure is capable of elongating the optical path of light absorber in solar cells and thus improving the light absorption efficiency of photovoltaic cells. As a result, the problem of unsaturated absorption caused by insufficient thickness of absorbers in thin-film solar cells can be solved. The BHF of the present disclosure is further capable of scattering the incident light while preserving 65％of the maximum photocurrent of solar cells at a small incident angle of 10°. Such property impairs the dependence of solar cell efficiency on incident angle, and thus eliminates the requirement of additional rotation system for solar cells, which is used to maintain perpendicular sunlight incidence on solar cells by rotating the solar panels according to the motion of the sun. In addition, the fabrication of the BHF of the present invention and its attachment onto existing solar cells are simple with low costs. The BHF is fabricated in a separate step. Therefore, no additional step is added in the solar cell fabrication. The attachment can be permanent or reversible. In addition, the BHF of the present disclosure shows remarkable resistance to strong acid, which is important for real solar panel application. It is believed that such advantages will make the BHF of the present disclosure a promising enhancing accessory for existing photovoltaic cells. In addition, the haze film enhanced solar cells can maintain a high power output even at a very small incident angle, leading to more efficient light harvesting, especially for those devices settled at high latitude areas.

Although the invention has been described in terms of certain embodiments, other embodiments apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims which follow.

Claims

An assembly of a solar cell and a haze film, comprising:

a solar cell； and

a biomimetic haze film having a first surface and an opposing second surface, the second surface of the biomimetic haze film comprises a plurality of closely packed microcraters, wherein each of the plurality of microcraters includes wrinkle-like sub-structures.
The assembly of claim 1, wherein an epidermis pattern on rose petals is transferred onto the second surface of the biomimetic haze film.
The assembly of claim 1, wherein each of the plurality of microcraters is similar to a concave semi-sphere.
The assembly of claim 1, wherein each of the plurality of microcraters has a pentagonal, hexagonal or heptagonal cross-sectional shape and a semi-circular, arc or parabolic longitudinal-sectional shape.
The assembly of claim 1, wherein each of the plurality of microcraters is about 15 μm -35 μm in diameter and about 10 μm -20 μm in depth.
The assembly of claim 1, wherein the wrinkle-like sub-structures within each of the plurality of microcraters has a size of 100 nm –1000 nm.
The assembly of claim 1, wherein the biomimetic haze film is made from polydimethylsiloxane (PDMS) .
The assembly of claim 1, wherein the solar cell is a thin-film solar cell, particularly a Perovskite solar cell or an organic solar cell.
The assembly of claim 1, wherein the biomimetic haze film is pre-fabricated and attached to a transparent side of the solar cell permanently or reversibly.
Use of a biomimetic haze film in a solar cell, wherein the biomimetic haze film has a first surface and an opposing second surface, the second surface of the biomimetic haze film comprises a plurality of closely packed microcraters, wherein each of the plurality of microcraters includes wrinkle-like sub-structures.
The use of claim 10, wherein an epidermis pattern on rose petals is transferred onto the second surface of the biomimetic haze film.
The use of claim 10, wherein each of the plurality of microcraters is similar to a concave semi-sphere.
The use of claim 10, wherein each of the plurality of microcraters has a pentagonal, hexagonal or heptagonal cross-sectional shape and a semi-circular, arc or parabolic longitudinal-sectional shape.
The use of claim 10, wherein each of the plurality of microcraters is about 15 μm -35 μm in diameter and about 10μm -20 μm in depth.
The use of claim 10, wherein the wrinkle-like sub-structures within each of the plurality of microcraters has a size of 100 nm –1000 nm.
The use of claim 10, wherein the biomimetic haze film is made from polydimethylsiloxane (PDMS) .
The use of claim 10, wherein the solar cell is a thin-film solar cell, particularly a Perovskite solar cell or an organic solar cell.
The use of claim 10, wherein the biomimetic haze film is pre-fabricated and attached to a transparent side of the solar cell permanently or reversibly.
A method of fabricating an assembly of a solar cell and a haze film, comprising the steps of:

i) providing a solar cell,

ii) fabricating a biomimetic haze film, the fabrication comprising the steps of:

a) mixing a PDMS prepolymer and a curing agent thereby obtaining a mixture；

b) degassing the mixture thereby obtaining a degassed mixture；

c) pouring the degassed mixture onto a surface of rose petal thereby obtaining an intermediate product；

d) curing the intermediate product thereby obtaining a textured PDMS； and

e) peeling off the textured PDMS from the rose petal, and

iii) attaching the biomimetic haze film onto a transparent side of the solar cell.
The method of claim 19, wherein in step a) the PDMS prepolymer and the curing agent is mixed at a ratio of about 10: 1 (w/w) .
The method of claim 19, wherein in step d) the intermediate product is cured at about 70℃ for about 2 hours.
The method of claim 19, wherein in step d) the intermediate product is cured at room temperature for around 48 hours.
The method of claim 22, wherein the fabrication further comprising, after step e) , the steps of:

f) treating the textured PDMS by ultrasonication in NaOH aqueous solution, acetone, ethanol, and DI water in sequence, thereby obtaining a treated PDMS； and

g) curing the treated PDMS at about 70℃ for about 2 hours.
The method of claim 19, wherein step i) comprises providing a thin-film solar cell, particularly a Perovskite solar cell or an organic solar cell.
The method of claim 19, wherein step iii) comprises attaching the biomimetic haze film to the solar cell permanently or reversibly.
The method of claim 19, wherein step c) comprises pouring the degassed mixture onto a surface of a PDMS stamp thereby obtaining an intermediate product, the PDMS stamp comprises a first surface and an opposing second surface, the second comprises a plurality of closely packed micro-projections, wherein each of the plurality of micro-projections includes wrinkle-like sub-structures.
The method of claim 26, wherein each of the plurality of micro-projections is similar to a convex semi-sphere.
The method of claim 26, wherein each of the plurality of micro-projections has a pentagonal, hexagonal or heptagonal cross-sectional shape and a semi-circular, arc or parabolic longitudinal-sectional shape.
The method of claim 26, wherein each of the plurality of micro-projections is about 15 μm -35 μm in diameter and about 10 μm -20 μm in height, the wrinkle-like sub-structures on each of the plurality of micro-projections has a size of 100 nm –1000 nm.