WO2001049475A1

WO2001049475A1 - Photovoltaic device with optical concentrator and method of making the same

Info

Publication number: WO2001049475A1
Application number: PCT/US2001/000336
Authority: WO
Inventors: Peter Peumans; Stephen R. Forrest; Vladimir Bulovic
Original assignee: The Trustees Of Princeton University
Priority date: 2000-01-05
Filing date: 2001-01-05
Publication date: 2001-07-12
Also published as: WO2001049475A8; AU2763601A

Abstract

A method of fabricating a low-cost, flexible and highly efficient photovoltaic device. The method includes providing a layer of thin film, shaping the thin film into a plurality of parabolic shaped concentrators (5B01), forming an aperture (5B02) in the bottom of each of the parabolic shaped concentrators (5B01), coating the concentrators (5B01) with a reflective material (503), encapsulating the concentrators (5B01) with a transparent insulating layer (505), and depositing an anti-reflective coating (511) on the top of the parabolic shaped concentrators (5B01).

Description

PHOTOVOLTAIC DEVICE WITH OPTICAL CONCENTRATOR AND METHOD OF MAKING THE SAME

RELATED U.S. APPLICATION DATA

This application claims the benefit of U S Provisional Application 60/174,455

filed January 5, 2000, and is a continuation-in-part of U S Patent Application Serial No

09/449,800 filed November 26, 1999

FIELD OF INVENTION

The present invention generally relates to thin-film photovoltaic devices (PVs)

More specifically, it is directed to photovoltaic devices, e g , solar cells, with structural

designs to enhance photoconversion efficiency by optimizing optical geometry for use

with optical concentrators Further, the present invention is directed to a low cost

fabrication process for making such photovoltaic devices

BACKGROUND OF THE INVENTION

Optoelectronic devices rely on the optical and electronic properties of materials to

either produce or detect electromagnetic radiation electronically or to generate electricity

from ambient electromagnetic radiation Photovoltaic devices convert electromagnetic

radiation into electricity Solar cells, also known as photovoltaic devices, are specifically

used to generate electrical power PV devices are used to drive power consuming loads to

provide, for example, lighting, heating, or to operate electronic equipment such as

computers or remote monitoring or communications equipment These power generation

applications also often involve the charging of batteries or other energy storage devices so that equipment operation m; y continue when direct illumination from the sun or other

ambient light sources is not available.

The falloff in intensity of an incident flux of electromagnetic radiation through a

homogenous absorbing medium is generally given by:

I = IX^ax ( 1 )

wheie I₀ is the intensity at an initial position (at x-0), u is the absorption constant and x is

the depth from x=0 Thus, the intensity decreases exponentially as the flux progresses through the medium Accordingly, more light is absorbed with a greater thickness of

absorbent media or if the absorption constant can be increased. Generally, the absorption

constant for a given photoconductive medium is not adjustable. For certain

photoconductive materials, e.g., 3,4,9,10-perylenetetracarboxylic-bιs-benzιmιdazole

(PTCBI), or copper phthalocyanme (CuPc), very thick layers are undesirable due to high

bulk resistivities By suitably re-reflecting or recycling light several times through a given

thin film of photoconductive material the optical path through a given photoconductive

material can be substantially increased without incurring substantial additional bulk

resistance A solution is needed, therefore, which efficiently permits electromagnetic flux

to be collected and delivered to the cavity containing the photoconductive material while

also confining the delivered flux to the cavity so that it can absorbed.

Less expensive and more efficient devices for photogeneration of power have been

sought to make solar power competitive with presently cheaper fossil fuels Therefore

organic photoconductors, such as CuPc and PTCBI, have been sought as materials for

organic photovoltaic devices (OPVs) due to potential cost savings. The high bulk

resistivities noted above make it desirable to utilize relatively thin films of these materials However, the use of very thin organic photosensitive layers presents other obstacles to

production of an efficient device. As explained above, very thin photosensitive layers

absorb a small fraction of incident radiation thus keeping down external quantum

efficiency. Another problem is that very thin films are more subject to defects such as

shorts from incursion of the electrode material Co-pendmg U.S. Patent Application Serial

No. 09/449,800 entitled "Organic Photosensitive Optoelectronic Device With an Exciton

Blocking Layer ' to Forrest et al. incorporated herein by reference describes photosensitive

heterostructures incorporating one or more exciton blocking layers which address some of

the problems with very thin film OPVs Howevei, other solutions are needed to address the problem of low photoabsorption by very thin films, whether the films are organic or

inorganic photoconductors

The use of optical concentrators, as known as Winston collectors, is common in the

fields of solar energy conversion. Such concentrators have been used primarily in thermal

solar collection devices wherein a high thermal giadient is desired. To a lesser extent, they

have been used with photovoltaic solar conversion devices However, it is thought that

such applications have been directed to devices wherein photoabsorption was expected to

occur upon initial incidence of light upon the active photoconductive medium If very thin

photoconductor layers are used, it is likely that much of the concentrated radiation will not

be absorbed It may be reflected back into the device environment, absorbed by the

substrate or merely pass through if the substrate is transparent. Thus, the use of

concentrators alone does not address the problem of low photoabsorption by thin

photoconductive layers

Optical concentrators for radiation detection have also been used for the detection of Cerenkov or other radiation with photomultiplier ("PM") tubes. PM tubes operate on an

entirely different principle, i.e., the photoelectric effect, from solid state detectors such as

the OPVs of the present invention. In a PM tube, low photoabsorption in the

photoabsorbing medium, i.e., a metallic electrode, is not a concern, but PM tubes require

high operating voltages unlike the OPVs disclosed herein.

The cross-sectional profile of an exemplary non-imaging concentrator is depicted

in Fig. 1. This cross-section applies to both a conical concentrator, such as a truncated paraboloid, and a trough-shaped concentrator. With respect to the conical shape, the device

collects radiation entering the circular entrance opening of diameter d, within ± θ_max (the

half angle of acceptance) and directs the radiation to the smaller exit opening of diameter

d₂ with negligible losses and can approach the so-called thermodynamic limit. This limit is

the maximum permissible concentration for a given angular field of view. A trough-

shaped concentrator having the cross-section of Fig. 1 aligned with its y axis in the east-

west direction has an acceptance field of view well suited to solar motion and achieves

moderate concentration with no diurnal tracking. Vertical reflecting walls at the trough

ends can effectively recover shading and end losses. Conical concentrators provide higher

concentration ratios than trough-shaped concentrators but require diurnal solar tracking

due to the smaller acceptance angle. (After High Collection Nonimaging Optics by W.T.

Welford and R. Winston, (hereinafter "Welford and Winston") pp 172-175, Academic

Press, 1989, incorporated herein by reference).

SUMMARY AND OBJECTS OF INVENTION

The present invention discloses photovoltaic device structures which trap admitted light and recycle it through the contained photosensitive materials to maximize

photoabsorption The device structures are particularly suited for use in combination with

optical concentrators

It is an object of this invention to provide a high efficiency photoconversion

structure for trapping and converting incident light to electrical energy

It is a further object to provide a high efficiency photoconversion structure

incorporating an optical concentrator to increase the collection of light

It is a further object to provide a high efficiency photoconversion structure in which the incident light is admitted generally perpendicular to the planes of the

photosensitive material layers

It is a further object to provide a high efficiency photoconversion structure in

which the incident light is admitted generally parallel to the planes of the photosensitive

material layers

It is a further object to provide a high efficiency photoconversion structure utilizing

generally conical parabolic optical concentrators

It is a further to provide a high efficiency photoconversion structure utilizing

generally trough-shaped parabolic optical concentrators

It is a further object to provide a high efficiency photoconversion structure having

an array of optical concentrators and waveguide structures with interior and exterior

surfaces of the concentrators serving to concentrate then recycle captured radiation

It is a still further object to provide a low cost method for making such highly

efficient photovoltaic devices BRIEF DESCRIPTION C F THE DRA WINGS

The foregoing and other features of the present invention will be more readily

apparent from the following detailed description of exemplary embodiments taken in

conjunction with the attached drawings.

Figure 1 is a cross-sectional profile of a prior art radiation concentrator for use in conjunction with the present invention.

Figures 2A-2E depict embodiments of device structures in accord with the present invention in which light is accepted in a direction generally perpendicular to the planes of

the photosensitive layers.

Figure 2A is side view of a perpendicular type embodiment with a concentrator

attached.

Figure 2B is top down view of Figure 2A along line A-A having a circular aperture

for use with a conical concentrator.

Figure 2C is a top down view of Figure 2A along line A-A having a rectangular

aperture for use with a trough-shaped concentrator.

Figure 2D is a perspective representation of a collection of perpendicular type PVs with conical concentrators.

Figure 2E is a perspective representation of a collection of perpendicular type PVs

with trough-shaped concentrators.

Figure 3 is a cross-sectional view of a portion of an array of perpendicular type

PVs with concentrators wherein the concentrators reflect light on their interior and

exterior surfaces.

Figures 4A-4E depict embodiments of device structures in accord with the present invention in which light is accepted a direction generally parallel to the planes of the

photosensitive layers.

Figure 4A is side view of a parallel type embodiment with a concentrator attached

Figure 4B is end-on view of Figure 4A along line B-B having a circular aperture

for use with a conical concentrator.

Figure 4C is a end-on view of Figure 4A along line B-B having a rectangular

aperture for use with a trough-shaped concentrator

Figure 4D is a perspective representation of a parallel type PV with a conical

concentrator

Figure 4E is a perspective representation of a parallel type PV with a trough-shaped

concentrator.

Figures 5A-D depicts the steps of fabπcating a light trapping, photovoltaic cell of

the present invention

DETAILED DESCRIPTION

In Fig. 2A, a cross-sectional view which can correspond to two different device

structures is depicted Both structures permit light to be introduced into a reflective cavity,

or waveguide, containing photosensitive layers such that the light is initially incident in a

direction generally perpendicular to the planes of the photosensitive layers so this type of

structure is generally referred to herein as a "perpendicular type structure". A

perpendicular type structure can have two types of preferably parabolic cross-section

concentrators as described above-"conιcal" and "trough-shaped"- Figs. 2B-2E provide

different views on conical versus trough-shaped structures whose common cross-section is shown in Fig. 2 A The same numerals are used for corresponding structure in each of

figures 2A-2E

Accordingly, light incident from the top of these embodiments enters into one or

more concentrator structures 2B01 (conical) or 2C01 (trough-shaped) The light admitted

to each concentrator is then reflected into an aperture 2B02 or 2C02 in top reflective layer

203 As shown in Figs 2B and 2C, aperture 2B02 is a generally circular shaped opening for use with a conical concentrator, and aperture 2C02 is a generally rectangular shaped

opening for use with a trough-shaped concentrator Only the bottom surface of layer 203

need be reflective so the top surface may be non-reflective and/or be optionally coated

with, for example, a protective layer to enhance weather resistance Passivated oxides or

polymer coatings, for example, may be suitable protective coatings After passing through

the aperture, the admitted radiation is trapped in a wa\ eguιde structure formed between

top layer 203 and bottom reflective layer 204 The space between the two layers is

occupied by several layers compπsing a thin film photovoltaic device of the type such as

those disclosed in co-pending U S Patent Application Serial Nos 09/136,342, 09/136,166,

09/136,377, 09/136,165, 09/136,164, 09/449,800 to Forrest et al (the "Forrest

Applications"), which are herein incorporated by reference in their entirety

More specifically, in an exemplary embodiment of a thin film PV cell with an

optical concentrator geometry, and with reference particularly to Fig 2A, below top layer

203 is a transparent, insulating layer 205 of, for example, glass or plastic, through which

the light admitted by aperture 2B02 or 2C02 initially traverses On its initial pass, the light

then traverses a transparent electrode 206 of, for example, degenerately doped indium tin

oxide (ITO) On its initial pass, the light then traverses one or more active layers 207 which may include one or more rectifying junctions, or exciton blocking layers for

efficient conversion of optical energy to electπcal energy Any light which is not absorbed

on this pass is reflected by layer 204 back through active layers 207, transparent electrode

206, and transparent insulating layer 205 to be reflected off of top layer 203 to repeat the

cycle again until the light is completely absorbed Layer 203 may be comprised of a

metallic material or a 1/4 wavelength dielectric stack of the type known in the art Layer

204 is typically a metallic film such as silver or aluminum which also can serve as the

lower electrode Alternatively, the lower electrode could be in whole or part a transparent

conductive material such as degenerately doped ITO in conjunction with a reflective

metallic film which in turn could optionally be deposited upon a substrate such as glass,

metal or plastic Fig 2A depicts two typical incident light rays Those of ordinary skill in

the art will appreciate that there are numerous other possible trajectories for incident

radiation and that the ray depicted is merely for illustration

The process of trapping the admitted light until it is absorbed enhances the

efficiency of the photoconversion and may be referred to as "optical recycling" or "photon

recycling" A structure designed to trap light within may generally be called a waveguide

structure, or also an optical cavity or reflective cavity The optical recycling possible

within such optical cavities or waveguide structures is particularly advantageous in devices

utilizing relatively high resistance organic photosensitive materials since much thinner

photoactive layers may be used without sacrificing conversion efficiency

In Fig 2D and 2E, a plurality of PV cells with concentrators are shown in one

integrated structure Those of ordinary skill in the art would appreciate that the number of

PV cells in such integrated structures may be increased as desired In Fig 2E it should be appreciated that the trough- shaped concentrator are shown having open ends. Optionally,

the ends of a trough are closed with a structure having a reflecting surface facing the

interior of the trough to help capture additional light into the apertures. Vertical or sloped

planar surfaces may be used. Also, each end of the trough may be closed with a shape

generally resembling half of a parabolic cone. Such structures permit the trough interior

surface to be smoothly curved in its full extent.

It should be appreciated in the array of perpendicular-type PVs depicted in Figs. 2D and 2E with reference to Fig. 2A, that after the admitted light has entered an aperture 2B02

or 2C02, the light will not be reflected back across the plane defined by the top surface of

the top reflective layer 203. Therefore, the space between the exterior of the concentrator

and top layer 203 may be empty or filled with a non-transparent material. For mechanical

stability, it is preferable that at least part of this volume should be filled with material to

provide support for the concentrator. Also, it should be appreciated that the Fig. 2 A, 2D,

2E structures as described above utilize three separate reflective surfaces for, respectively,

the interior of the concentrator, the upper reflective surface of the waveguide structure and

the lower reflective surface of the waveguide structure. In Fig. 3, an alternative array

structure is depicted in cross-section which can utilize a single reflective film to provide

both the concentrator reflections and the "upper" waveguide reflections. The

concentrator/reflector 301 is a reflective layer, typically metal such as silver or aluminum,

deposited on a layer 302 of molded or cast transparent insulating material such as plastic

or glass. Layer 302 is made with the shape of the concentrator array formed into it. The

transparent upper electrode, one or more photosensitive layers, labeled collectively as 303,

and lower reflective layer (optionally also the lower electrode) 304 complete the waveguide structure This arrangement allows the manufacture of a PV concentrator array

to begin with a preformed bare concentrator array structure Thereafter, a double-duty

reflective coating can be deposited on the concentrator side of the array structure and the

photoactive and conductive layers for extraction of photogenerated current can all be

deposited on the lower surface using masking and photolithographic techniques Since

physical support is provided by layer 302, reflective layer 301 can be made much thinner

than would be possible if layer 301 were needed to be a partly or completely self- supporting concentrator/reflector

It should be appreciated that as just described, layer 301 has reflecting surfaces on

both its interior and exterior parabolic surfaces Optionally, layer 301 could be two

separate coatings on the inteπor and exterior of a generally conical or trough-shaped base

mateπal such as molded or cast plastic or glass This implementation more easily permits

the concentrator interior and exterior surface shapes to be slightly different thus permitting

independent optimization of the concentrator reflections and the waveguide structure

reflections

In Figs 4A - 4E, different versions of structures which peπmt light to be

introduced into a reflective cavity, or waveguide, containing photosensitive layers such

that the light is initially incident in a direction generally parallel to the planes of the

photosensitive layers so that this type of structure is generally referred to herein as a

"parallel type structure" As with perpendicular type structures, parallel type structures can

have both generally "conical" and "trough-shaped" type concentrators Figs 4B-4E

provide different views on conical versus trough-shaped structures whose common cross

section is shown in Fig 4A The same numerals are used for corresponding structure in each of figures 4A-4E

more concentrator structures 4B01 (conical) or 4C01 (trough-shaped) The light admitted

to each concentrator is reflected into an aperture 4B02 or 4C02 at the base of each

concentrator As shown in Figs 4B and 4C, aperture 4B02 is a generally circular shaped

opening for use with a generally conical concentrator, and aperture 4C02 is a generally

rectangular shaped opening for use with a trough-shaped concentrator The remaining structure is now descnbed with respect to a typical incident light ray but those of ordinary

skill in the art will appreciate that there are numerous other possible trajectories for

incident radiation and that the ray depicted is merely for illustration The typical ray enters

a transparent, insulating layer 403 of, for example, glass or plastic The typical ray then

reflects off reflective layer 404, which is typically a metallic film of, for example, silver or

aluminum The reflected ray then retraverses part of transparent layer 403 and then

traverses transparent conductive layer 405 which serves as one electrode of the device and

is typically a conductive oxide such as degenerately doped ITO The typical ray then

traverses the photoactive layers 406 which are photosensitive rectifying structures such as

those descnbed in the Foπest Applications or inorganic photosensitive optoelectronic

structures made from, for example, silicon Any optical intensity in the typical ray that has

not been absorbed reflects off upper reflective layer 407 which may be a metallic reflective

film of, for example, silver or aluminum and typically serves as an electrode layer

Optionally, the electrode function may be served in part or whole by a second transparent

electrode with the reflective function provided by a separate layer

In attaching the concentrator structure to the PV, care should be taken to avoid shorting of the electronically active layers This can be accomplished by providing a thm

insulating protective coating around the edges of the photoconductive layers It should be

further appreciated that a reflective coating may be optionally located around the edges of

the device to reflect light back into the device More specifically, in Fig 4A, a reflective

layer (not shown) electπcally insulated from the electronically active layers would

optionally be placed at the right end of Fig 4A so that (as illustrated) the typical ray could

reflect back toward the concentrator Again the reflective layer may be compπsed of a

reflective mateπal or it may be comprised of a 1/4 wavelength dielectric stack It should be appreciated that the proportions of the device depicted in the figures are merely

illustrative The device may be made generally thinner vertically and longer horizontally

with the result that most light is absorbed before it ever reaches the edges of the device

opposed to the concentrator Only light which is truly normal to the plane of the aperture

and thus truly parallel to the planes of the photoactive layers would have a substantial

probability of reaching the edge opposite the concentrator This should represent a small

fraction of the incident light

In Fig 4B, aperture 4B02 is illustrated as coveπng a section of only transparent

insulating layer 403 Provided the instructions above relating to preventing electrical shorts

between electrically active layers is heeded, the concentrator 4B01 and aperture 4B02 may

be disposed to allow direct illumination of transparent electrode 405 or photoactive layers

406 In Fig 4C, it should be appreciated that transparent insulating layer 403 is not

depicted since generally rectangular aperture 4C02 is shown as completely overlapping

layer 403 in the view shown As with aperture 4B02, aperture 4C02 may be varied m size

along with concentrator 4C01 to provide direct illumination oi more or less of the interior of the PV.

Figs. 4D and 4E are perspe tive illustrations of exemplary embodiments of the

present invention in parallel type structures. In Fig. 4D, a generally conical concentrator is

illustrated delivering incident light to a parallel type structure having an upper reflective

layer 408, one or more photosensitive layers and rectifying structures including one or

more transparent electrodes labeled collectively as 409, and a transparent insulating layer

and bottom reflective layer labeled collectively as 410. Fig. 4E depicts a generally trough- shaped concentrator 4C01 delivering light to a similar PV structure in which the layers are

labeled accordingly. It should be appreciated that the trough-shaped concentrator is shown

having open ends which may optionally be closed off with a reflecting surface as described

above with regard to Fig. 2E.

The concentrator may be formed of only metal or of molded or cast glass or plastic

which is then coated with a thin metallic film. With the parallel type structure, the

waveguide photoabsorbing structures are more readily manufactured separately from the

concentrator structures with the pieces being attached subsequently with suitable adhesive

bonding materials. An advantage of the peφendicular type structure, as described above, is

that its manufacture can begin with preformed concentrator structures which are used as

the substrate for further build up of the device.

It should be appreciated that the terms "conical" and "trough-shaped" are generally

descriptive but are meant to embrace a number of possible structures. "Conical" is not

meant to be limiting to an shape having a vertical axis of symmetry and whose vertical

cross-section would have only straight lines. Rather, as described above with reference to

Welford and Winston, "conical" is meant to embrace, among other things, a structure having a vertical axis of symmetry and a generally parabolic vertical cross section as

depicted in Fig 1 Further, the present invention is not limited to concentrators, either

"conical" or "trough-shaped", having only smoothly curved surfaces Rather, the general

conical or trough shape may be approximated by some number of planar facets which

serve to direct incident light to the exit aperture

Further, the generally circular and generally rectangular apertures are preferred for

use with optical concentrators but are exemplary Other shapes for apertures are possible

particularly in the peφendicular type structure Concentrators having generally parabolic sloped sides may be fitted to a number of aperture shapes However, the 3D parabolic and

trough-shaped concentrator with their respective circular and rectangular apertures are

preferred

It should also be appreciated that the transparent insulating layer, e g , 205 or 403,

is present to prevent optical microcavity interference effects Therefore, the layer should be

longer than the optical coherence length of the incident light in all dimensions Also, the

transparent insulating layer can be placed on either side of the photoactive layers For

example, in Fig 2A, the aperture could be in layer 204 and layer 203 could be just a

reflecting layer Accordingly, any concentratoi 2B02 or 2C02 would be placed over the

aperture wherever it may he This would have the effect of permitting the admitted light to

reach the photoactive layers initially before reaching the transparent insulating layer For

many possible photoactive materials, however, the embodiments specifically disclosed

herein, e g , Fig 2A, are preferred since they allow the transparent insulating layer to

protect the underlying photoactive layers from the environment Exposure to atmospheric

moisture and oxygen may be detrimental to certain materials Nonetheless, those of ordinary skill in the art would understand this alternate version of the device with the

benefit of this disclosure

Figures 5 A-D depicts the steps of fabricating a light trapping, photovoltaic device

Such method is relevant to all thin film photovoltaic technologies (e g , organic, thm film

silicon, etc ) The following is a description of a preferred method of fabπcating a light trapping, photovoltaic cell First, a layer of thin film is shaped into a plurality of small,

Wmston-type collectors or concentrators 5B01 having a parabolic profile normal to the longitudinal direction Figs 5A, 5B The layer of thin film is preferably comprised of a

polymeπc mateπal Some prefeπed polymeric mateπals include, by way of non-limiting

example, PET, polystyrene, polyimide This is preferably accomplished by placing the

polymeπc film atop a support 520, heating the same and then forming the film over a mold

530 The layer of thin film may be comprised of other mateπals including metal The

support preferably includes a plurality of V-shaped recesses 525 that are adapted to mate

with pointed ends 535 that extend from the mold 530 Accordingly, the pointed ends 535

of the mold pierce the layer of thin film during the molding step in order to form the

requisite apertures 5B02 in the concentrators Fig 5B The thus formed concentrators

preferably have a width of about 100 to about 200 μm at the open upper end thereof and a

height of from about 100 to about 200 μm Fig 5B Note, while the concentrator is

depicted as conical in shape in Fig 5B, a trough shaped concentrator may also be formed

by utilizing an appropnately shaped mold

The inteπor and exterior parabolic surfaces of the collectors or

concentrators are then coated on the top and bottom with a reflective coating 503 Fig 5C

The coating may be compnsed of a metallic mateπal such as silver or aluminum or a dielectric material. The reflectivity on the bottom side is particularly important as light will

reflect numerous times off this surface. It should be apparent that if the concentrator itself

was formed from a reflective material, e.g. a metallic material, a reflective coating need

not be applied. A protective layer may be applied to the reflective metal coating. Preferred

materials for the protective layer include passivated oxides or polymer coatings. Thereafter, the structure is preferably completely encapsulated in a transparent material

505 such as glass or a transparent polymer. Fig. 5D. The encapsulating or filler polymer

must be sufficiently stable under solar illumination, especially near the aperture where

high intensities (approximately 10 suns) are encountered. Thereafter, the structure is

completed by the deposition of the photovoltaic cell 514 from the bottom, and an anti-

reflection coating 511 from the top. Fig. 5D. A preferred material for the anti-reflective

coating is silicon dioxide. The photovoltaic cell may be of the type disclosed in the Forrest

Applications. For example, the photovoltaic cell may include an anode 506, a layer of

organic material 507, and a cathode 504 (preferably a silver cathode). Further, the device

may be encapsulated with a layer of encapsulating film 512. The encapsulating film may

be comprised of, by way of non-limiting example, polyimide, SiO₂, or SiN_x. The layer of

organic material may comprise a hole transporting layer such as CuPC, an electron

transporting layer such as PTCBI and an exciton blocking layer such as bathocuporine

(BCP). The thus formed photovoltaic device preferably has a height of from about 200 to

about 400 μm.

While the particular examples disclosed herein refer preferably to organic

photosensitive heterostructures the waveguide and waveguide with concentrator device

geometries described herein suitable as well for other photosensitive heterostructures such as those using inorganic mεteπals and both crystalline and non-crystalline photosensitive

materials The term "photosensit e heterostructure" refers herein to any device structure

of one or more photosensitive mateπals which serves to convert optical energy into

electrical energy whether such conversion is done with a net production or net

consumption of electπcal energy Preferably organic heterostructures such as those

descnbed in the Forrest Applications are used

Also, where a reflective electrode layer is called for herein, such electrode could also be a composite electrode comprised of a metallic layer with an transparent conductive

oxide layer, for example, an ITO layer with a Mg Ag layer These are described further in

the Foπest Applications

It should be appreciated that the terms "opening" and "aperture" are generally

synonymous and may be used herein somewhat interchangeably to refer to the optical

entrance or exit of a concentrator as well as a transparent hole or window which allows

radiation to reach the intenor of a PV Where it is necessary to draw a distinction between

the two types of openings or apertures, for example, m the claims, antecedent context will

provide the suitable distinction

Thus, there has been described and illustrated herein waveguide structures for PVs

and use particularly in conjunction with optical concentrators Those skilled in the art,

however, will recognize that many modifications and variations besides those specifically

mentioned may be made in the apparatus and techniques described herein without

departing substantially from the concept of the present invention Accordingly, it should

be clearly understood that the form of the present invention as described herein is

exemplary only and is not intended as a limitation on the scope of the present invention

Claims

What is claimed is

1 A method of fabricating a photovoltaic device comprising the steps of

providing a layer of thin film

shaping the thin film layer into a plurality of parabolic shaped

concentrators,

forming an aperture in the bottom of each of the parabolic shaped

concentrators,

coating the concentrators with a reflective material, encapsulating the concentrators with a transparent insulating layer,

depositing a thin film photovoltaic cell on the bottom of each of the

parabolic shaped concentrators, and

depositing an anti-reflection coating on the upper end of the parabolic

shaped concentrators

2 The method of claim 1 wherein the thm film layer is comprised of a

polymeric material

3 The method of claim 1 wherein a protective layer is applied to the bottom

surface of the reflective material

4 The method of claim 1 wherein the photovoltaic cell includes an anode, an

organic layer, and a cathode

5. The method of claim 1 wherein the concentrators each have a height of

about 100 to about 200 μm.

6. The method of claim 1 wherein the concentrators each have a width of

about 100 to about 200 μm.

7. A method of fabricating a photovoltaic device comprising the steps of: providing a layer of a reflective thin film:

shaping the reflective thin film layer into a plurality of parabolic shaped

concentrators;

forming an aperture in the bottom of each of the parabolic shaped

concentrators;

encapsulating the concentrators with a transparent insulating layer;

depositing a thin film photovoltaic cell on the bottom of each of the

parabolic shaped concentrators; and depositing an anti-reflection coating on the upper end of the parabolic

shaped concentrators.

8. The method of claim 7 wherein the reflective thin film layer is comprised of

a metallic material.

9. The method of claim 7 wherein a protective layer is applied to the bottom

surface of the reflective thin film layer. 10 The method of claim 7 wherein the photovoltaic cell includes an anode, an

organic layer, and a cathode

1 1 The method of claim 7 wherein the concentrators each have a height of

about 100 to about 200 μm

12 The method of claim 7 wherein the concentrators each have a width of

about 100 to about 200 μm

13 A method of fabπcating a plurality of parabolic shaped concentrators for

use with a photovoltaic device, the method comprising the steps of

providing a layer of thin film

shaping the thm film layer into a plurality of parabolic shaped

concentrators,

forming an aperture in the bottom of each of the parabolic shaped

concentrators,

coating the concentrators with a reflective material, and

encapsulating the concentrators with a transparent insulating layer

14 The method of claim 13 wherein the thin film layer is comprised of a

polymeric matenal

15 The method of claim 1 wherein the concentrators each have a height of about 100 to about 200 μm

16 The method of claim 13 wherein the concentrators each have a width of

about 100 to about 200 μm

17 A method of fabricating a plurality of parabolic shaped concentrators for use with a photovoltaic device, the method comprising the steps of

providing a layer of a reflective thin film shaping the thm film layer into a plurality of parabolic shaped

concentrators,

forming an aperture in the bottom of each of the parabolic shaped

concentrators, and

encapsulating the concentrators with a transparent insulating layer

18 The method of claim 17 wherein the reflective thin film layer is comprised of a metallic material

19 The method of claim 17 wherein the concentrators each have a height of about 100 to about 200 μm

20 The method of claim 17 wherein the concentrators each have a width of

about 100 to about 200 μm