WO2019157438A1 - Immersion lens array for the patterning of photoresist by maskless lithography - Google Patents

Abstract

Self-aligned maskless photolithography using a contiguous immersion lens array is provided. This process is especially suitable for use in fabrication of perovskite solar cells. In these devices, the lens array can 1) lithographically define a mechanical support structure for the perovskite active material and 2) provide light concentration to the perovskite active material in operation.

Description

Immersion lens array for the patterning of photoresist by maskless lithography

FIELD OF THE INVENTION

This invention relates to self-aligned

photolithography, especially in connection with

photovoltaic devices such as solar cells.

BACKGROUND

Some materials considered for use in solar cells have attractive optoelectronic properties, but are sufficiently mechanically fragile that special measures must be employed to mitigate the fragility issue. However, it remains a subject of active research to determine how best to perform such mitigation. One approach is to add mechanical support structures to the solar cell. Such mechanical support structures tend to be optically inert, which can

undesirably reduce solar cell efficiency. Accordingly, it would be an advance in the art to provide improved

mechanically supported solar cells.

SUMMARY

In this work, we describe the utilization of a space filling array of lenses to lithographically pattern arrays of arbitrary shapes without the assistance of a photomask. One application of this work is the fabrication of a mechanically reinforcing-scaffold into which an array of perovskite microcells are deposited. The scaffold enhances the resilience of the fragile perovskite material, which is brittle and extremely susceptible to fracture under applied loads. In this application, the lenses also behave as self- aligned and self-tracking solar concentrators, thereby solving the problem of guiding both normal and off-normal light away from the inert scaffold and into the photoactive microcells .

BRIEF DESCRIPTION OF THE DRAWINGS

FIGs. 1A-B show exemplary embodiments of the

invention .

FIGs. 2A-G show an exemplary fabrication sequence.

FIGs. 3A-B show fabrication and the resulting

structure of experimental devices.

FIGs. 4A-D show the effect of different angles of incidence on a contiguous lens array.

FIG. 5 shows images of several scaffolds having various geometrical parameters.

FIGs. 6A-B show images of a fabricated lens array and scaffold.

FIG. 7A shows simulated (left) and experimental

(right) light transmission through a lens array + scaffold.

FIG. 7B show definitions of the geometrical parameters immersion (I), spacing (d) and radius of curvature (R) .

FIG. 7C shows a profile map of part of a fabricated lens array. FIG. 7D is a photograph of a completed self-aligned mechanically supported solar cell structure.

FIG. 7E is a photograph of a structure similar to that of FIG. 7D, except without the lenses.

FIGs. 8A-C show J-V curves of representative

experimental devices.

FIG. 9A-C show examples of the effect of different illumination conditions on the illumination pattern

provided by the immersion lens array.

DETAILED DESCRIPTION

A) General principles

FIGs. 1A-B show exemplary embodiments of the

invention. The example of FIG. 1A is a photovoltaic device including a contiguous lateral 2-D array of optical

immersion lenses 102 having no lateral gaps between the lenses in the 2-D array. For simplicity of illustration, a simple cross section is shown here. In the examples below, the 2D lens array is shown more explicitly. Here an immersion lens is defined as any lens having media with different refractive indices at its input and output, by analogy with liquid immersion lenses as used in microscopy. In this work, immersion lens concepts are used to provide a relatively high index optical path (i.e., having no

intervening air gap) between the lenses of the lens array and the cells of the mechanical support structure, as described below. Thus the intervening layer (s) of solid transparent media play the role of the immersion medium.

Such concentrators comprising a close-packed array of spherical lenslets (i.e., there is no gap between lenses) with an immersion backing can define lithography patterns as described below. This array was utilized to fabricate a repeating pattern with a different shape than the lenses through maskless lithography. By maskless lithography, we mean the visible or ultraviolet light passes directly through the array of lenses and is focused into a pattern of repeating shapes (e.g., circles or hexagons) . This process could be used to make a reinforcing scaffold for an array of perovskite solar microcells and concentrate incident light away from the scaffold and into the

microcells in optical contact with the immersion backing. The optical and mechanical properties of the lenslet array can be readily tuned by materials selection. Since the lens array can be fabricated in a number of ways, including (but not limited to) embossing, micro-milling, printing (inkjet or 3D), injection molding, and solution-based deposition, the material library from which to select is nearly

limitless .

The example of FIG. 1A also includes a mechanical support structure 106, where the mechanical support

structure has cells corresponding to lenses of the

contiguous lateral 2-D array of optical immersion lenses 102. Here also the 2-D array of cells of the mechanical support structure is shown more explicitly below. A photovoltaic material (or materials) 104 is/are disposed in the cells of the mechanical support structure. The

contiguous lateral 2-D array of optical immersion lenses 102 is configured to provide light concentration to the photovoltaic material in operation, as shown by the block arrows on FIG. 1A. Here the contiguous lateral 2-D array of optical immersion lenses 102 is configured as a

structure having a lensed first surface and a flat second surface opposite the lensed first surface. The mechanical support structure 106 is disposed on the second surface of the contiguous lateral 2-D array of optical immersion lenses 1 02.

The example of FIG. IB is similar to the example of FIG. 1A, except that a transparent substrate 1 08 having opposite first and second sides is included. Here the contiguous lateral 2-D array of optical immersion lenses 1 02 is disposed on the first side of the transparent substrate and the mechanical support structure 1 0 6 is disposed on the second side of the transparent substrate 1 08.

In preferred embodiments, the photovoltaic device is a solar cell. Any solar cell material can be used, but this approach is particularly appropriate for mechanically fragile solar cell materials like perovskite solar cell materials. Here a perovskite is defined as any material having a chemical formula ABX₃ where A is a monovalent cation, B is a divalent metal cation, and X is a monovalent (typically halide) anion. All three components (i.e., A, B, or X) can comprise either a homogeneous or heterogeneous mixture of elements and molecules (e.g., A can be some composition of methylammonium (CH₃NH₃ ⁺) , formamidinium

(CH_{3 (}NH_{2) 2} ⁺) , Cs⁺, Rb⁺, etc., B some composition Pb²⁺, Sn²⁺, Ge²⁺, EU²⁺, and/or other divalent metal cations, and X is typically some composition of Cl-, BP, I-, BFA, PF₆-, SCFT, etc.) . Specific perovskites of interest for solar cell application include, but are not limited to: CH₃NH₃Pbl₃,

C S ₀.05 [ (CH₃NH₃₎ 0.17 (CH_{3 (}NH_{2) 2) 0}.8₃] 0.95Pb ( Io.8₃Bro.₁₇₎ 3,

Cso._{25 (}CH_{3 (}NH_{2 ) 2 ) 0}.75Pb (I₀.8₀Br₀._{20) 3}, and Rbo. osCso.₁₀FA₀. ssPbl3. Practice of the invention does not depend critically on the composition of the mechanical support structure.

Practice of the invention also does not depend

critically on the lateral shapes or arrangement of the lenses of the lens array. Any arrangement of these

features can provide self-alignment as described below. A presently preferred configuration for the lens array is as a hexagonal array of lenses having equal size, as in the examples below.

FIGs. 2A-G show an exemplary fabrication sequence for the structure of FIG. IB. The starting point of FIG. 2A shows lens array 102 disposed on transparent substrate 108. FIG. 2B shows the result of depositing photoresist 202 on the opposite side of substrate 108 from the lens array, and then exposing it (e.g., with visible or ultraviolet

radiation) through the lens array as shown by the block arrows. FIG. 2C shows the result of developing the

photoresist and removing the unexposed parts of the

photoresist pattern. FIG. 2D shows the result of

depositing mechanical support structure 106. As can be seen, the mechanical support structure ends up located at the non-exposed locations of the photoresist pattern, which provides the self-alignment of this process.

FIG. 2E shows the result of removing the exposed photoresist 202. FIG. 2F shows the result of depositing photovoltaic material 104, and is the same structure as the structure of FIG. IB. By replacing the starting point of FIG. 2A with the starting point of FIG. 2G and then

carrying out the steps of FIGs. 2B-F, the structure of FIG. 1A can be fabricated.

Accordingly, an embodiment of the invention is a method of performing photolithography, where the method includes :

a) providing a contiguous lateral 2-D array of optical immersion lenses having no lateral gaps between the lenses in the 2-D array; b) disposing the contiguous lateral 2-D array of optical immersion lenses in proximity to a photosensitive layer;

c) exposing the photosensitive layer to visible or ultraviolet radiation through the contiguous lateral 2-D array of optical immersion lenses, where a pattern of radiation formed by the contiguous lateral 2-D array of optical lenses defines a pattern in the photosensitive layer; and

d) processing the photosensitive layer to transform the pattern in the photosensitive layer into a

corresponding structural pattern. See FIGs. 2A-B.

A method of fabricating a photovoltaic device can include performing photolithography as described above, where the contiguous lateral 2-D array of optical immersion lenses also provides light gathering for the photovoltaic device in operation. The photovoltaic device can be a perovskite solar cell, and the corresponding structural pattern defined by the self-aligned lithography can be a mechanical support structure for the solar cell.

The contiguous lateral 2-D array of optical immersion lenses can be disposed on a first side of a transparent substrate and the photosensitive layer can be disposed on a second side of the transparent substrate opposite the first side. Alternatively, the contiguous lateral 2-D array of optical immersion lenses can be formed by a lensed first surface of a transparent structure and the photosensitive layer can be disposed on a flat second surface of the transparent structure opposite the lensed first surface.

In both of these two cases, processing the

photosensitive layer to transform the pattern in the photosensitive layer into a corresponding structural pattern can include the steps of:

a) developing exposed portions of the photosensitive layer;

b) selectively removing unexposed portions of the photosensitive layer while not removing the exposed

portions of the photosensitive layer to create a scaffold pattern in the photosensitive layer;

c) infiltrating a mechanical support material in the scaffold pattern to provide a mechanical support structure having cells; and

d) removing the exposed portions of the photosensitive layer. See FIGs. 2C-E.

After that, an active solar cell material can be disposed in the cells of the mechanical support structure, as shown on FIG. 2F.

There are several significant advantages of the above- described structures and methods.

1) Self-alignment of light concentrators: In our demonstrated example, the lenslet array design was

optimized so that it fabricated a repeating hexagonally- packed structure in which both normal incident light and off-normal light could be concentrated into the active area of perovskite microcells deposited into the structure.

2) Contiguous nature of lenses: In some prior art examples, microlens arrays that have been used in

patterning have included gaps between each lens, allowing uncontrolled/unfocused light to pass directly through. Our lens array is instead contiguous and allows all light incident on the sample to be controllably focused. 3) Readily tuned size and shapes of patterns: By changing the focusing distance (i.e., changing the

immersion length) or the angle of the light source relative to the lens array, the size and shape of the patterned features could be tuned. Both these effects are seen in the ray simulations. Arbitrary shapes based on some combination of the base shapes from these aberrated effects could then be defined.

Possible variations include but are not limited to the following :

1) The lenslet array can be integrated into the fabrication process of many types of devices other that perovskite solar cells, such as other thin-film and

multij unction/tandem solar cells (e.g., organic

photovoltaics , copper indium gallium diselenide, III-V solar cells), light-emitting diodes, transistors,

batteries, electrodes, anti-reflection coatings, etc.

2) The lens geometry may be changed to tune the optical properties of the lenses. The lenslets' dimensions (e.g., radius, immersion distance), shape (e.g., spherical, aspherical), packing (e.g., hexagonal or cubic packing), and periodicity are some parameters that can be adjusted readily .

3) The lenses have been demonstrated with a

photocurable polymer, but materials including, but not limited to epoxies, ceramics, polymers, and composites comprising two or more classes of materials (i.e.,

composites) can be utilized as applications demand. The usage of soft polymeric materials for the lens (e.g.- PDMS) enables these arrays to be flexible and compatible with curved surfaces. 4) The lenslet arrays were fabricated on both rigid (FIG. 5) and flexible (FIGs. 6A-B) substrates in the initial demonstrations. The lenses can be printed onto any arbitrary substrate. An additional example would be

stretchable substrates such as PDMS and polyurethane.

5) While the initial demonstration of the technique used molding to pattern the lenslet array, many other techniques such as embossing, nano-imprint lithography (NIL), inkjet, or 3D printing can be used to fabricate the lenslets .

6) Longitudinal index grading of the lenslet array can be employed. In this approach, the index of refraction is continuously and longitudinally graded within the lenslet array. Typically the index of refraction would increase as light gets deeper into the lenslet array (i.e., closer to the underlying photovoltaic device) . This variation can improve the performance of the lenslet array as a solar concentrator and/or reduce optical loss at the interface between the lenslet array and the underlying optoelectronic device. Further details relating to longitudinal index grading can be found in US 9,329,308, hereby incorporated by reference in its entirety.

This work is the first time that a space-filling

(i.e., completely contiguous) lenslet array has been utilized to pattern photoresist without a mask. The shape of the lenses themselves can be tuned to form arbitrary repeating structures in a photoresist. In the demonstrated example of scaffold-reinforced perovskite microcells, the lenslet array not only fabricates the reinforcing scaffold, but also directs light into the photoactive microcells, resulting in boosted efficiency. In applications where light focusing is required into or out of the device, this invention allows for the facile fabrication of arrayed patterns into which the device can be built, which are then automatically well-aligned with the lenslets. For example, this lens array could fabricate a reinforcing scaffold for fragile solar cells and

concentrate the sunlight into those microcells so that there is no device efficiency loss due to light incident on the photoinactive scaffold. Furthermore, fabricating the lens array out of a UV-absorbing material would provide improved device stability for perovskite solar cells, which have been characterized to degrade in the presence of UV irradiation. In practice, the resulting devices are typically configured as multiple small solar cell elements, each element being disposed in a corresponding cell of the support scaffold. It is convenient to refer to such devices as being 'a solar cell' in the singular, even though multiple solar cell elements may be present,

especially since the elements are often electrically connected in parallel to act like a single device.

In general terms, the structural pattern formed by this photolithography process corresponds to the lens array. However, this correspondence can take various forms. One way this correspondence can vary is that the structural pattern can be defined by exposed areas on a photoresist or by unexposed areas on a photoresist. In the first case, the resulting structural pattern corresponds to the optical pattern, and in the second case the resulting structural pattern corresponds to the complement of the optical pattern.

Another way this correspondence can vary is that the pattern of the light from the lens array can correspond more or less closely to the lens array pattern. For example, the optical pattern from a hexagonal lens array can be an array of round shapes, even though each lens has a hexagonal boundary and the lens array is space filling. The resulting individual cells are discrete and round. This round cell shape is suitable for making the individual solar cells, they don't have to have a more complex shape. They are likely more mechanically stable, have thicker scaffold walls (simply due to the geometry of smaller round shapes in larger hexagonal arrays), and don't have sharp corners that would produce stress concentration. Round cells will probably also be easier to encapsulate which will lead to improved temperature stability. When operating, light incident on the lenses is all tunneled into the round active cell and there is even some tracking inherent in the arrangement.

Examples of varying the relation between the lens array and the resulting optical pattern are shown on

FIGs. 9A-C. The result of FIG. 9A is achieved by

integrating over multiple angles of incident plane waves, which is effectively the same as moving the array around during fabrication or using multiple exposures at

once. The result of FIG. 9B is achieved by increasing the immersion distance (this would result in much worse non normal incident light collection, however) . The result of FIG. 9C is achieved by adding an aspheric coefficient, i.e. changing the profile of the lens.

In general the structural pattern formed in this lithography process (e.g., perovskite active areas) depends on the surface profile of the lenses (spherical, aspheric, some other arbitrary surface) , the aperture of the lens (shape and size), the immersion distance, and the source light. The pattern of the tessellation of the lenses will effectively dictate the aperture of the lens, therefore affecting the active area as mentioned above. All of these factors are simulatable and designable. Also note the tessellation pattern of the lenses will determine the tessellation of the devices (hexagonally packed lenses will result in hexagonally packed devices, no matter their shape) .

B) Examples

FIGs. 3A-B show an example of lenslet array

fabrication of scaffold-reinforced solar cells with

integrated light concentration. The lenslet array serves to both fabricate the scaffold and to focus sunlight onto the microcells to mitigate parasitic absorbance and shading by the reinforcing scaffold, which shields the fragile

perovskite cells from mechanical stress and moisture.

FIG. 3A shows a space-filling lens array 302 fabricated on the glass side of an ITO-glass substrate 304. Photoresist 306 is then deposited onto the ITO side. UV light

(l = 365 nm) exposure then occurs through the lenses, which concentrates the light into the photoresist, forming a patterned scaffold array 308 as described in greater detail above. FIG. 3B shows the result after the scaffold array is then subsequently filled with the individual layers of the perovskite microcells. Upon exposure to sunlight, the lenses focus light away from the scaffolds and

preferentially into the perovskite microcells. Here 310 is PTAA (polytriarylamine) , 312 is Cso.25FAo.75Pb ( Io.soBro.20) 3, 314 is C60/BCP (bathocuproine) , and 316 is a silver electrode.

FIG. 4A shows a modeled lenslet array optimized for patterning a hexagonal scaffold and concentrating light into the cells from multiple angles. FIGs. 4B, 4C and 4D show the modeled irradiance patterns of the structure of FIG. 4A at normal, 30°, and 60° incidence, respectively. These results show self-tracking of the microcells.

FIG. 5 shows micrographs of some of the scaffolds fabricated by various lens arrays on glass. As the

immersion depth I increased, the scaffold wall widths increased. Furthermore, with an increase in the lens radius of curvature R (i.e., weakening the lens), the scaffold wall widths decreased. Here the scale bar is 1 mm.

FIGs. 6A-B show scaffolds with a 1 mm spacing

fabricated on flexible ITO-coated polyethylene

terephthalate substrates. In the photograph of FIG. 6A, both the scaffolds (left side) and lenses (right side) can be observed, while the micrograph of FIG. 6B shows the relatively sharp features of the scaffold.

FIGs. 7A-E show results from scaffold-reinforced solar cells with integrated photon management lens arrays.

FIG. 7A shows a comparison between simulated ray-tracing light transmission (left) and the light transmission of the actual fabricated scaffold on ITO/glass substrate (right) .

FIG. 7B is a schematic showing relevant geometric factors for the lenses: immersion I, spacing d, and radius of curvature R. Note that in the immersion calculation, total distance to the active material is what matters. If the substrate is an equal index of refraction to the lens material, the substrate will provide an increase in

immersion distance equal to substrate thickness. If the substrate index is different than that of the lens

material, the increased immersion distance will be modified by a refractive interface. In general, the immersion depth is the integral of index of refraction times distance on the relevant path. All devices in this study have a spacing d equal to 1 mm. FIG. 7C is a 3D profile map of a 2 mm x 2 mm region of the lens array with R = 1.25 mm, I = 140 (jm.

FIGs. 7D-E are photographs of scaffold-reinforced solar cells partitioned by mechanically reinforcing

scaffolds held at an orientation with the lenses facing toward a lamp behind the devices. FIG. 7D is a photograph of a lensed device as described above, while FIG. 7E is a photograph of a similar device that had the lenses removed after they were used to define the scaffold. In the device with lenses (FIG. 7D) the light is focused into the

perovskites (bright points) and away from the scaffold (dark lines), while in the device without lenses (FIG. 7E) the light is uniformly distributed and thus shines through the scaffolds as well.

FIGs. 8A-C show J-V curves of representative devices both with and without lens arrays, where R = 1.25 mm and I is 150 mpi (FIG. 8A) , 300 mpi (FIG. 8B) , and 500 mpi (FIG. 8C) . Here the insets are micrographs of the scaffolds fabricated using these lens arrays (scale bar: 1 mm) .

Increasing the immersion distance resulted in an increased scaffold thickness.

FIGs. 9A-C show irradiance patterns for the lens array of FIG. 4A with variations of: integrating multiple source light angles (FIG. 9A) , increasing immersion distance

(FIG. 9B) , or adding an aspheric coefficient (FIG. 9C) .

Claims

1. A photovoltaic device comprising:

a contiguous lateral 2-D array of optical immersion lenses having no lateral gaps between the lenses in the 2-D array;

a mechanical support structure, wherein the mechanical support structure has cells corresponding to lenses of the contiguous lateral 2-D array of optical immersion lenses; photovoltaic material disposed in the cells of the mechanical support structure;

wherein the contiguous lateral 2-D array of optical immersion lenses is configured to provide light

concentration to the photovoltaic material in operation.

2. The photovoltaic device of claim 1, wherein the

photovoltaic device is a solar cell.

3. The solar cell of claim 2, wherein the photovoltaic material is a perovskite solar cell material.

4. The photovoltaic device of claim 1:

further comprising a transparent substrate having opposite first and second sides;

wherein the contiguous lateral 2-D array of optical immersion lenses is disposed on the first side of the transparent substrate; and

wherein the mechanical support structure is disposed on the second side of the transparent substrate.

5. The photovoltaic device of claim 1: wherein the contiguous lateral 2-D array of optical immersion lenses is configured as a structure having a lensed first surface and a flat second surface opposite the lensed first surface; wherein the mechanical support structure is disposed on the second surface of the contiguous lateral 2-D array of optical immersion lenses.

6. The photovoltaic device of claim 1, wherein the

contiguous lateral 2-D array of optical immersion lenses is configured as a hexagonal array of lenses having equal size .

7. A method of performing photolithography, the method comprising : providing a contiguous lateral 2-D array of optical immersion lenses having no lateral gaps between the lenses in the 2-D array; disposing the contiguous lateral 2-D array of optical immersion lenses in proximity to a photosensitive layer; exposing the photosensitive layer to visible or ultraviolet radiation through the contiguous lateral 2-D array of optical immersion lenses, wherein a pattern of radiation formed by the contiguous lateral 2-D array of optical lenses defines a pattern in the photosensitive layer; processing the photosensitive layer to transform the pattern in the photosensitive layer into a corresponding structural pattern.

8. A method of fabricating a photovoltaic device comprising performing the method of claim 7, wherein the contiguous lateral 2-D array of optical immersion lenses provides light gathering for the photovoltaic device in operation.

9. The method of claim 8, wherein the photovoltaic device is a perovskite solar cell, and wherein the corresponding structural pattern is a mechanical support structure for the solar cell.

10. The method of claim 8, wherein the contiguous lateral 2-D array of optical immersion lenses is disposed on a first side of a transparent substrate and wherein the photosensitive layer is disposed on a second side of the transparent substrate opposite the first side.

11. The method of claim 10, wherein the processing the photosensitive layer to transform the pattern in the photosensitive layer into a corresponding structural pattern comprises, in sequence:

developing exposed portions of the photosensitive layer;

selectively removing unexposed portions of the

photosensitive layer while not removing the exposed

portions of the photosensitive layer to create a scaffold pattern in the photosensitive layer; infiltrating a mechanical support material in the scaffold pattern to provide a mechanical support structure having cells; and removing the exposed portions of the photosensitive layer .

12. The method of claim 11, further comprising disposing an active solar cell material in the cells of the mechanical support structure.

13. The method of claim 8, wherein the contiguous lateral 2-D array of optical immersion lenses is formed by a lensed first surface of a transparent structure and wherein the photosensitive layer is disposed on a flat second surface of the transparent structure opposite the lensed first surface .

14. The method of claim 13, wherein the processing the photosensitive layer to transform the pattern in the photosensitive layer into a corresponding structural pattern comprises, in sequence: developing exposed portions of the photosensitive layer; selectively removing unexposed portions of the photosensitive layer while not removing the exposed

portions of the photosensitive layer to create a scaffold pattern in the photosensitive layer; infiltrating a mechanical support material in the scaffold pattern to provide a mechanical support structure having cells; and removing the exposed portions of the photosensitive layer .

15. The method of claim 14, further comprising disposing an active solar cell material in the cells of the mechanical support structure.