WO2011156567A2

WO2011156567A2 - Rough surfaces for organic-based solar cells

Info

Publication number: WO2011156567A2
Application number: PCT/US2011/039747
Authority: WO
Inventors: Michael E. Mackay
Original assignee: University of Delaware
Current assignee: University of Delaware
Priority date: 2010-06-09
Filing date: 2011-06-09
Publication date: 2011-12-15
Anticipated expiration: 2012-12-09
Also published as: WO2011156567A3

Abstract

A solar cell includes in sequence a transparent anode layer, a hole-conducting layer including one or more polymers and particles insoluble in the polymers, a bulk heterojunction layer including an electron donor and an electron acceptor, and a conductive cathode layer.

Description

ROUGH SURFACES FOR ORGANIC-BASED SOLAR CELLS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional patent application No. 61/352,969, filed 9 June 2010, the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

Solar cells, also known as photovoltaic devices, are of increasing interest as electrical energy sources. Solar cells employing an active layer made of organic materials are of particular interest, in view of the fact that they are based on potentially plentiful and inexpensive carbon-based materials rather than the various materials used in earlier devices. One class of organic-based solar cells employs a blend of poly(3-hexylthiophene), also known as P3HT, and [6,6]-phenyl C₆rbutyric acid methyl ester (PCBM), a fullerene-based nanoparticle having a diameter of about 0.7 nm. PCBM plays the role of electron acceptor and P3HT, a member of the polythiophene family of conducting polymers, serves as the electron donor.

The P3HT: PCBM layer may for example be deposited on a

poly(3,4-ethylenedioxythiophene) : polystyrene sulfonate (PEDOT: PSS) hole conducting layer residing on an indium tin oxide (ITO) anode, in which the PEDOT: PSS layer aids in preventing efficiency-destroying charge recombination.

The internal morphology of P3HT:PCBM solar cells affects performance.

Unfortunately, the vertical PCBM concentration profile formed directly after spin coating the P3HT: PCBM blend in typical devices is nearly opposite of that desired for an ideal device, inasmuch as the dense PCBM layer present at the interface with the hole- conducting layer results in charge transport problems. It is difficult to make the morphological changes needed to overcome this problem using traditional methods, however, because the close proximity of the crystallization and degradation

temperatures of P3HT make it difficult to effect such changes by melting the polymer. Thus, devices annealed near or above the melting temperature of P3HT have exceptionally poor performance. Alternatively, efforts to effect the desired

morphological changes by adjusting solvent casting conditions are complicated by the high volume fractions of PCBM needed to make a device, such that solubility limitations of both the P3HT and the PCBM dominate the structures formed upon film casting. Proper selection of processing parameters such as solvent choice, annealing methods and polymer molecular weight can enhance device performance, but, due to the nature of solution casting the degree of control offered by standard techniques is severely limited. Thus, methods and compositions providing control over P3HT: PCBM solar cell morphology would be of benefit in the solar cell industry-

SUMMARY OF THE INVENTION

The invention provides a solar cell including in sequence a transparent anode layer, a hole-conducting layer including one or more polymers and particles insoluble in the polymers, a bulk heterojunction layer including an electron donor and an electron acceptor, and a conductive cathode layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures la through If show top-down and 3D atomic force microscopy images of PEDOT: PSS layers containing from zero to 333 parts of Si0₂ per 100 parts of

PEDOT: PSS by weight.

Figures 2a and 2b show top down and 3D AFM images, respectively, of a film such as that of Figure 1 containing 333 parts of Si0₂ per 100 parts of PEDOT: PSS by weight, after coating with a 1 : 1 by weight film of P3HT and PCBM and annealing for 20 minutes at 140°C.

Figure 3 is a plot of current density (J) versus voltage (V) for solar cells using P3HT: PCBM spin coated on top of a PEDOT: PSS layer spin coated from solutions containing 0, 83, 167 and 333 parts by weight of 120 nm Si0₂ spheres per 100 parts of PEDOT: PSS, respectively.

Figure 4 is a plot of absorbance versus wavelength for a pure PEDOT: PSS film, for PEDOT.- PSS films containing Si0₂ at three concentrations, and for a pure glass substrate.

Figure 5a is a diagram illustrating the fate of incident light hitting a PEDOT: PSS film containing no Si0₂ particles. Figure 5b is an analogous diagram for a PEDOT: PSS films containing Si0₂ particles.

Figure 6 shows absorption spectra of 1 : 1 by weight P3HT: PCBM films residing on PEDOT: PSS films containing various concentrations of Si0₂ particles.

Figure 7 shows photoluminescence data obtained from 1 : 1 by weight

P3HT: PCBM films on top of PEDOT: PSS films with various concentrations of dispersed Si0₂ particles.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that roughening the surface of a hole conducting layer comprising one or more polymers (e.g., PEDOT:PSS) by including particles such as Si0₂ in it, followed by forming a bulk heterojunction donor: acceptor active layer such as P3HT: PCBM on the roughened hole-conducting layer, results in significantly improved cell performance. Accordingly, the invention provides a solar cell comprising in sequence a transparent anode layer, a hole-conducting layer, a polymer-based active layer and a conductive cathode layer, wherein the hole-conducting layer has dispersed therein particles that produce a rough-surfaced hole-conducting layer. This in turn results in a rough-surfaced interface between the hole-conducting layer and the active layer. Typically each of the layers contacts the one following it in the above sequence, although in some embodiments additional layers may be included either within the sequence or on the outer surface of the anode layer and/or the outer surface of the cathode layer.

For simplicity, the invention will be described with respect to an exemplary embodiment in which the transparent anode layer is ITO, the hole-conducting layer is a PEDOT: PSS layer, the particles in the hole-conducting layer are Si0₂ particles, the heteroj unction donor: acceptor active layer is P3HT: PCBM, and the conductive cathode layer is aluminum. However, the invention is not limited to those specific

embodiments.

It is believed that the structures provided by the invention provide, to at least some degree, donor-acceptor feature sizes approaching the roughly 1-10 nm diffusion length scale of a polymer solar cell exciton and therefore may result in increased interfacial area between the donor and acceptor phases and increased exciton separation, as desired for optimal performance.

The transparent anode layer may for example be an ITO layer, and may be made by methods known in the art. Other transparent anode layers may be used instead, for example graphene or carbon nanotubes, and the anode layer will typically be in the form of a coating on a transparent support such as glass. It will typically have a thickness in a range from 100 to 200 nm, although thicker or thinner layers may be used.

The hole-conducting layer comprises one or more polymers, and particles insoluble in those polymers. A typical hole-conducting layer includes PEDOT and PSS, and typically has a thickness in a range from 25 to 50 nm according to the invention, although thicker or thinner layers may be used. The weight ratio of PEDOT to PSS will typically be at least 1 : 10 (i.e., at least one part of PEDOT per 10 parts of PSS), or at least 1 :8. Typically the weight ratio will be at most 1 :4, or at most 1 : 5. In most cases, the ratio is about 1 : 6. Materials other than PEDOT: PSS may be used for the layer, preferably ones that absorb less than 10% of the light impinging on the cell, or less than 5%, or less than 2%, or less than 1%.

Particles for inclusion in the hole-conducting layer may include any of a variety of materials insoluble in the polymers and of a size to provide a suitably rough su rface at the interface with the active layer. Colloidal metal oxides may be used, for example colloidal alumina, ceria, tin oxide, yttria, zinc oxide and zirconia. Advantageously, colloidal silica particles may be used.

The particles may have an approximately monodisperse size distribution, or they may be polydisperse. In either case, they typically have an average largest dimension (or an average smallest dimension) of at least 30 nm, or at least 60 nm, or at least 90 nm. Typically, the average largest (or smallest) dimension of the particles will be at most 210 nm, or at most 180 nm, or at most 150 nm. In some embodiments, the particles are approximately spherical, in which case the above figures refer to the diameter. Materials other than Si0₂ may be used for the particles, and preferably absorb less than 10% of the light impinging on the cell, or less than 5%, or less than 2%, or less than 1%. In some embodiments, the refractive index of the particles is in a range from 1.40 to 1.80. The content of particles in the hole-conducting layer is typically at least 50 parts, or at least 80 parts, or at least 150 parts per hundred parts by weight of polymer in the hole-conducting layer. It is typically at most 700 parts, or at most 500 parts, or at most 350 parts per hundred parts of polymer in the hole- conducting layer.

The active layer typically has a thickness in a range from 90 to 250 nm, although thicker or thinner layers may be used. The layer may comprise any bulk heterojunction donor: acceptor material known in the art. Typical donor materials include polythiophene derivatives (e.g., P3HT) and copolymers comprising fluorene, carbazole or cyclopentadithiophene moieties. Typical acceptor materials are fullerene derivatives, for example PCBM.

The heterojunction donor: acceptor layer is in the form of a phase separated structure including a donor-rich phase and an acceptor-rich phase. Preferably, the volume percentages of these phases are approximately equal at about 50% each, but either phase may constitute as little as 30% or as much as 70% of the layer. The relative volumes of the phases will depend, inter alia, on the relative amounts of donor and acceptor incorporated in the layer. In the case of a P3HT: PCBM layer, the weight ratio of P3HT to PCBM will typically be at least 25:75, or at least 40:60. Typically the weight ratio will be at most 75:25, or at most 60:40. In most cases, the ratio is about 1 : 1.

The conductive cathode layer may be composed of any material commonly used in the art for solar cell construction, with aluminum being used in most cases. General methods for producing the above layers are known in the art, and exemplary suitable methods are described in the Examples.

Figure 1 is a compilation of AFM images of PEDOT: PSS layers on a silicon substrate, in which increasing amounts of 120 nm Si0₂ particles were dispersed in the PEDOT: PSS. Figures la and lc are top down and 3D AFM images of a pure PEDOT: PSS film, respectively. Figures lb and Id show a PEDOT: PSS film made with 10 mg/mL of Si0₂ in the coating solution, along with 12 mg/mL of PEDOT: PSS. Figures le and lg and Figures If and lh show films made with 20 and 40 mg/mL of Si0₂ in the coating solution, respectively. The resulting loadings of Si0₂ were 83, 167 and 333 parts by weight per 100 parts of PEDOT: PSS, respectively.

It can be seen that increasing the solution concentration of Si0₂ increased the overall surface roughness of the PEDOT: PSS films.

Figures 2a and 2b show top down and 3D AFM images, respectively, of a film such as that of Figure 1 containing 333 parts of Si0₂ per 100 parts of PEDOT: PSS by weight, after coating with a 1 : 1 by weight film of P3HT and PCBM and annealing for 20 minutes at 140°C. As can be seen, the presence of Si0₂ resulted in substantial roughness even in the upper surface of the P3HT: PCBM coating.

Solar Cells Employing Rough PEDOT: PSS Surfaces

As seen above, adding Si0₂ particles to the PEDOT: PSS layer added a substantial amount of roughness and surface area to the interface of that layer with the active P3HT: PCBM layer. PEDOT: PSS layers as described above were then tested in solar cells and, as shown in Figure 3, a substantial increase in device performance was seen with PEDOT: PSS layers having rough surfaces compared with devices employing pure (smooth) PEDOT: PSS films. Figure 3 is a plot of current density (J) versus voltage (V) for solar cells using P3HT: PCBM spin -coated on top of a PEDOT.-PSS layer spin coated from solutions containing 0, 10, 20 and 40 mg/mL of 120 nm Si0₂ spheres, respectively, along with 12 mg/mL of PEDOT: PSS. The resulting loadings of Si0₂ were 0, 83, 167 and 333 parts by weight per 100 parts of PEDOT.-PSS, respectively.

Cell properties of the devices indicated in Figure 3 are shown in Table 1, where Jsc is the short circuit current, Voc is the open circuit voltage, and FF is the fill factor. As can be seen, the pure (smooth) PEDOT: PSS device produced the worst cell but upon addition of Si0₂ the efficiency, short circuit current and fill factor all increased.

Table 1

Spectroscopic Measurements

To determine the effect of the Si0₂ particles on light scattering and absorption, absorption spectroscopy measurements were performed on the PEDOT: PSS layers. Figure 4 is a plot of absorbance versus wavelength for a pure PEDOT: PSS film for all three Si0₂ concentrations studied and for a pure glass substrate for comparison. The absorbance of pure glass was subtracted, via a reference sample, to produce the PEDOT: PSS absorption curves shown. As can be seen in the figure, pure PEDOT: PSS began to absorb light around 350 nm but did not absorb substantially until below 300 nm. Addition of the Si0₂ particles, however, resulted in absorbance beginning at 600 nm, a substantially lower energy wavelength than for pure PEDOT: PSS. The 10 and 20 mg/mL samples showed nearly identical absorption but the 40 mg/mL sample had significantly enhanced absorption compared with the others.

The inset in Figure 4 shows scattering efficiency versus wavelength for 120 nm spheres according to Mie theory calculations, which describe the scattering expected from particles that are of the same size order as the wavelength of the incident light. The scattering increase according to Mie theory directly correlates with the increase in observed absorbance in the rough PEDOT: PSS layers, indicating that Mie scattering from the 120 nm spheres was the cause of the observed absorbance increase and that the films containing Si0₂ particles did not actually absorb significantly more light. The reason can be seen with reference to Figure 5.

As shown pictorially in Figure 5, incident light hits the PEDOT:PSS film (Figure 5a) and some light that is above the band gap energy is absorbed by the film, while the rest passes through. For the samples containing Si0₂ (Figure 5b) the same amount of light is absorbed by the film, but some of the light is scattered as it contacts the particles and never reaches the detector. Hence, an apparent increase in absorption is seen when in reality the rough films are merely scattering the light away from the detector. Increasing the number of particles on the surface increases the number of scatterers and the total light scattering.

The intensity of light as a function of distance within a material can be determined by the equation

I = I₀ e -^az ( l ) where I is the light intensity at a distance z into the material, I₀ is the incident light intensity and a is the absorption coefficient. The active layers of the solar cell films studied here were about 90 nm thick and roughly 50% by volume P3HT. Using a value of 1.75 xlO⁵ cm ¹ for a, as reported in the technical literature, and assuming the absorption properties do not change substantially for P3HT when it is in a mixture with PCBM, only about 60% of the incident light within the absorption spectrum of P3HT will be absorbed. Even if the entire 90 nm film had consisted of P3HT, only 80% of the incident light would have been absorbed, so regardless of the assumptions made on the absorption properties of P3HT in a mixture, all of the light was not harvested by the 5 active layer in the cells studied here.

Figure 6 shows absorption spectra of 1 : 1 by weight P3HT: PCBM films residing on PEDOT: PSS films containing various concentrations of Si0₂ particles. Despite the additional scattering provided by the Si0₂ particles and the less than 100% absorption by the active layer, P3HT: PCBM films coated on rough PEDOT: PSS layers showed l o minimal absorption increases compared to a pure PEDOT: PSS device. The pure

PEDOT:PSS device showed a 25% increase in short circuit current but the increase in absorption was only 8% over the entire absorption spectrum, not nearly enough to account for the Jsc increase. It was not possible to determine whether even the relatively small observed absorption increase was in fact the result of actual absorption, i s rather than merely an apparent increase due to scattering light away from the detector, as discussed above. In any case, the increase in observed absorption due to the presence of Si0₂ was small compared to the increases seen in the device properties.

Figure 7 shows photoluminescence data obtained from 1 : 1 by weight P3HT: PCBM films on top of PEDOT: PSS films with various concentrations of dispersed0 Si0₂ particles. In contrast to the very small observed absorption increases due to Si0₂ inclusion discussed above, photoluminescence increased dramatically. Whereas the absorption for 10, 20 and 40 mg/mL samples increased 3%, 5% and 8%, respectively, the photoluminescence increased by 40%, 67% and 80% for these same samples. The increase in photoluminescence is consistent with production of excitons farther from5 PCBM molecules when rough PEDOT: PSS films were used rather than smooth ones.

EXAMPLES

PEDOT: PSS films containing dispersed Si0₂ particles were prepared as follows. The Si0₂ particles (120 nm, Bangs Laboratories, Inc., Fishers, IN) were added to a stock solution of PEDOT: PSS (H.C. Stark) such that the final solution contained 10, 200 or 40 mg of Si0₂ particles per total mL of solution, along with 12 mg/mL of PEDOT: PSS.

The resulting mixtures were sonicated for 30 minutes to break up any aggregates, filtered through a 200 nm PTFE filter and then spin coated at 3000 RPM for 60 seconds. The films were then dried in an oven for 30 minutes at 130°C and either characterized or transferred to a glove box to be coated with a 1 : 1 by weight solution of P3HT: PCBM.5 Photoluminescence measurements were performed with a Horiba Jobin Yvon

Fluoromax-4 spectrofluorometer with an incident wavelength of 550 nm. UV-Vis absorption data were obtained with a Shimadzu UV-3600 UV-Vis spectrometer without an integrating sphere.

Patterned Slide Preparation

All cells used in this work were created on 1" glass slides coated with indium tin oxide (ITO). The slides were purchased from Delta Technologies, Ltd of Loveland, CO. The ITO anode layers were pattered by etching with aqueous HCI after protecting the areas not to be etched with Scotch® brand transparent tape, or by protecting those areas with a positive working photoresist available from Rohm and Haas (Spring House, PA) under the trade name S1818.

Etching was performed by placing the slides in a solution of 60% HCI (40% H ₂0) at 60°C for 60 seconds, and checked with a voltage meter to assure proper etching. The slides were then cleaned by sonicating first in acetone for 15 minutes and then in isopropanol for 15 minutes.

PEDOT: PSS Application

PEDOT used in the Examples was a medium-conductivity grade available from Clevios under the trade name Clevios™ P AI 4083. The product contains PEDOT and PSS in a 1 :6 weight ratio at a total solids content of 1.3-1.7 wt% in aqueous medium. The cleaned slides were briefly dried with nitrogen and then directly coated with PEDOT: PSS containing the indicated amount of Si0₂. The PEDOT: PSS was kept refrigerated until just before use, and was warmed to ambient temperature before use for spin-coating the slides.

All solutions were first diluted with water at a ratio of 4 ml. of Clevios™ P AI 4083 to 1 mL of Millipore water, i.e., water that had been purified using a Milli -Q Millipore water purification system, resulting in a total PEDOT:PSS concentration of 12 mg/mL. The solution was deposited on the ITO slides from a 3 mL syringe while being filtered through a 200 nm nylon filter. All slides were completely flooded with the PEDOT: PSS solution and then spin coated for 60 seconds at 3000 RPM. A second coating was then deposited through the syringe filter and again spun under the same conditions. This procedure produced a roughly 25 nm thick layer of PEDOT: PSS.

After spin coating, a small strip of the PEDOT: PSS film was removed from the entire edge of the wafer, exposing the contact areas of the slides. This was done by wetting a cotton applicator and wiping the edges of each slide. After wiping, the slides were placed in an oven at 130°C and baked for 20 minutes. Then, a vacuum was applied to the oven and the slides were baked for another 20 minutes under reduced pressure. The slides were then removed, placed in holders and transferred directly to a glove box. Active Layer Application

The active P3HT: PCBM layer was applied generally according to the following procedure. To make a solution, both components were weighed and placed in a vial outside the glove box. Once the appropriate amounts were added, the vials were returned to the glove box, with caps loose so as to remove any oxygen during transfer, and the desired solvent was added. Anhydrous solvents were used and the solutions were stirred for a sufficiently long time before spin coating. Chlorobenzene was used as the solvent, typically requiring 2 or 3 days of stirring to achieve full dissolution. This choice of solvent avoided any significant dissolution of the underlying PEDOT: PSS layer when the P3HT: PCBM layer was applied, as desired for optimal cell performance.

The most common concentration used in this work was a 30 mg/mL solution (15 mg of P3HT and 15 mg of PCBM to 1 mL of solvent), but many concentrations can be used depending on the P3HT: PCBM ratio and the desired film thickness. Spin speeds from 500 RPM to 2500 RPM were typically used.

Once the active layer was created, the film was wiped off of the contacts. This was done by wetting a cotton applicator in either chloroform or chlorobenzene and wiping in the same manner as was used with the PEDOT: PSS films. This operation was performed in a glove box.

Cathode Deposition

The slides were inserted into a thermal evaporator, a mask was provided according to the ITO pattern on the slides, and an aluminum layer of about 80 nm thickness was applied. Due to the intense heat of the filament, a sufficiently large distance between the devices and filament is desired and a relatively small deposition rate is also desired. Most devices in this work used an aluminum deposition rate of approximately 0.2 nm/sec. The metal electrode covered part of one patterned ITO strip and extended outward to cover a patterned ITO island near the edge of the cell. This island was used to assure that when a probe contacted the metal electrode extension it could still make good contact even if the metal were scratched fully away directly around the probe. In this arrangement, because the ITO is in direct contact with the metal, the probe need only touch the ITO in order to contact the metal electrode.

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

Claims

What is Claimed :

1. A solar cell comprising in sequence a transparent anode layer, a hole- conducting layer comprising one or more polymers and particles insoluble in the polymers, a bulk heteroj unction layer comprising an electron donor and an electron acceptor, and a conductive cathode layer.

2. The solar cell of any preceding claim, wherein the particles comprise Si0₂ particles.

3. The solar cell of any preceding claim, wherein the electron acceptor comprises a fullerene derivative.

4. The solar cell of any preceding claim, wherein the electron acceptor comprises PCBM.

5. The solar cell of any preceding claim, wherein the electron donor comprises a polythiophene derivative.

6. The solar cell of any preceding claim, wherein the electron donor comprises P3HT.

7. The solar cell of any preceding claim, wherein the one or more polymers of the hole-conducting layer comprise PEDOT and PSS.

8. The solar cell of any preceding claim, wherein the particles comprise Si0₂, the polymers of the hole-conducting layer comprise PEDOT and PSS, and the Si0₂ particles are present in an amount of at least 50 parts per hundred by weight of the PEDOT and PSS.

9. The solar cell of any preceding claim, wherein the particles are spherical.

10. The solar cell of any preceding claim, wherein the particles have an average diameter of at least 30 nm.

11. The solar cell of any preceding claim, wherein the hole-conducting layer has a thickness in a range from 25 nm to 50 nm.

12. The solar cell of any preceding claim, wherein the bulk heterojunction layer has a thickness in a range from 150nm to 250 nm.

13. The solar cell of any preceding claim, wherein the conductive cathode layer comprises aluminum.

14. The solar cell of any preceding claim, wherein the anode layer comprises

ITO.