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Transparent Conducting Oxide Thin-Films and Junctions for Semi-Transparent Photovoltaic Devices

Jeremiah S. McNatt1

NASA Glenn Research Center, Cleveland, OH, 44135

David W. Hoffman2, Ameena Fnu3, and Michael H.-C. Jin4

University of Texas at Arlington, Arlington, TX, 76019

Kulbinder K Banger5 and David A. Scheiman6

Ohio Aerospace Institute, Brookpark, OH, 44142

and

Lyndsey B. McMillon7

Miami University, Oxford, OH, 45056

The NASA Glenn Research Center (GRC) is collaborating with Alpha Micron, Inc. (AMI) to develop a transparent solar cell for smart window applications. This window uses liquid crystals powered transparent solar cells to vary the tinting of the window throughout the day. A p/n junction is formed between transparent conducting oxide (TCO) thin films to produce a solar cell with conversion efficiency high enough to meet the power requirements of the window. The TCO films are deposited by radio frequency (RF) sputtering. The deposited films are characterized for optimal optical and electronic properties. This paper presents the optimization of the experimental variables producing n-type aluminum-doped zinc oxide (ZnO:Al), p-type copper oxide (Cu2O), and indium tin oxide (ITO) thin-films along with first attempts at producing a working photovoltaic device. The analysis of this data will be used produce the framework for the transparent solar cell.

Nomenclature VALiD = Variable Attenuation Liquid Crystal Device TCO = transparent conducting oxide ITO = indium tin oxide ZnO:Al = aluminum doped zinc oxide Cu2O = copper oxide

1 Electrical Engineer, Photovoltaics and Power Technologies, MS 302-1. 2 Undergraduate Student, Electrical Engineering, 500 W. 1st St. Box 19031. 3 Graduate Student, Materials Science and Engineering, 500 W. 1st St. Box 19031. 4 Assistant Professor, Materials Science and Engineering, 500 W. 1st St. Box 19031. 5 Senior Research Associate, Photovoltaics and Power Technologies, MS 302-1. 6 Senior Research Associate, Photovoltaics and Power Technologies, MS 302-1. 7 Undergraduate Student, Mechanical Engineering, 501 East High Street.

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I. Introduction

HE NASA Glenn Research Center (GRC) in partner with Alpha Micron, Inc. (AMI) is developing a solar powered variable light transmitting window covering under the Ohio Third Frontier Research

Commercialization Program. This work combines the liquid crystal expertise of AMI with the photovoltaic expertise of NASA GRC. This paper is focused on the development of an innovative solar cell that can provide sufficient power to allow the liquid crystal film to transition while maintaining a high degree of transparency over the visible spectrum. Although the conversion efficiency of the solar cell we’ve designed is expected to be low, the power requirements for the liquid crystal film are so minimal that it’s likely there would be excess power which could be stored or put onto the grid. Alpha Micron, Inc. was founded by researchers from Kent State University to commercialize liquid crystalline technologies. Currently, AMI has product lines in ski goggles, racing helmets, and commercial eyewear that allow the user to lighten and darken their field of view. These devices are powered by a battery that supplies an electric field to the liquid crystals which changes their orientation to control the transmittance of the light. This same technology is being developed on a larger scale to produce an auto-adjusting “Adaptive Window.” The window uses the company’s VALiD™ liquid crystal technology to create a self-regulated tinted film that is powered by a solar cell and adjusts to the intensity of light hitting the window1. Initial work on this program focused on investigating commercial photovoltaic options to power the adaptive window. While many solar cell technologies would provide the necessary power requirements, none were sufficiently transparent. Also a future goal is to have the solar cell and liquid crystal in the same thin film packaging for ease of installation onto preexisting windows. This led to transparent conducting oxide junctions, which if properly deposited would fulfill all current and future requirements. And while there are many oxide junction studies in the academic community, there has been little effort to commercialize transparent oxide solar cells. Among this work, a 2% conversion efficiency device has been reported2. Optimizing the TCO solar cell technology to a point where it production is commercially viable presents an innovative research challenge.

II. Background Transparent conducting oxides are used in many applications where charge needs to be distributed over large

areas without impeding optical properties. Indium tin oxide is the most commonly used TCO for these applications. It is used to transmit charge to the pixels in computer monitors and even in the eyewear produced by Alpha Micron. From this, we plan to use a TCO device with the structure of a glass substrate, ITO, p-Cu2O, n-ZnO:Al, and ITO as shown in Figure 1. Additionally metallic top contacts maybe added to the structure. Similar structures reported in the literature have a power conversion efficiency of 2% which would provide sufficient power to control the VALiD window3. This cell will theoretically work, and can be produced using a roll-to-roll production process, a necessity for commercial production of the final product. When incorporated into the VALiD panel system, the glass will be replaced by a plastic similar to the substrate used in VALiD. The relevant oxide thin-film properties include transmittance, carrier density, mobility, work function, texture, and crystallinity.

The junction between n-type ZnO:Al and p-type Cu2O is chosen as an attempt to fabricate a high-voltage solar cell. While ZnO:Al, with a bandgap of 3.3 eV, is located on top of the device where light enters, Cu2O, with a lower bandgap of 2.5 eV, will serve as a primary absorber of the solar cell. The band diagram (Figure 2) indicates the built-in potential is over 1V, and with a nominal doping density for both layers, Analysis of Microelectronic and Photonic Structures (AMPS-1D) simulation predicts an open circuit voltage over 1V and the power conversion efficiency over 3% of the cell. A high voltage cell is desirable to drive the VALiD cell. The thickness of each layer was arbitrarily set to be 500 nm for the convenience of this model. The thicknesses will be optimized in future devices.

T

Figure 1. TCO Device Structure. The final device will likely be on a plastic substrate similar to that used on the VALiD film.

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The TCO materials were deposited and tested for solar cell development by sputtering, which provides a controlled method to deposit TCOs at a variety of conditions, such as pressure, argon flow rate, and power which all affect the optical and electrical properties of the oxide films. It is also easily scalable for future roll to roll depositions of the TCO solar cells for large area devices.

Once a sample has been sputtered, it can then be placed through a series of tests to check its resistivity, thickness, reflectance and absorbance. The difficulty in creating an ideal sample arises here. The preferred sample would be semi-transparent, thin, have a high absorbance, low reflectance, and low resistance. These attributes are difficult to engineer together –a series of trade-offs must occur. In order for the cell to be transparent, we need a thin film, however when the film gets thinner, the absorbance decreases. The electrical resistance can also change due to film thickness and uniformity. Therefore, throughout extensive testing the primary goal was to find a good combination where no requirement overly suffered and our cell still functions in an acceptable manner.

III. Experimental Standard glass microscope slides are ultrasonically cleaned in baths of acetone, followed by isopropyl alcohol,

and blown dry with nitrogen. A small piece of Kapton tape is adhered to the front side of the slide to measure the deposited TCO thickness. Slides are placed into the MRC 8667 radio-frequency (RF) sputter and the chamber is evacuated to approximately 5x10-7 torr. The chamber is backfilled with argon (and in some cases, argon/4% oxygen mixed gas) through a mass flow controller and allowed to equilibrate to the desired working pressure. The MRC 8667 has 3 independent 6.25” TCO targets that are indium bonded to water cooled copper backing plates. Variables such as working pressure, argon and argon/oxygen flows, power run through the sputter target, and sputter time are changed between depositions to optimize our TCO materials. Each TCO is optimized independently before being deposited in the stacked cell structure.

After deposition, the TCO materials are analyzed by a variety of optical and electrical measurements. The transmittance and absorbance of the deposited film is measured by a Lambda 950 UV/Vis spectrometer in a range of 2500 to 300 nm. Sample thickness is measured with a Dektak profilometer. The film electrical resistance, carrier type, and doping density are measured with a four point probe Van der Pauw configuration on a Hall system. The crystallographic structure of the film is measured with a Phillips PW3040 powder x-ray diffraction system.

Once each TCO material has been optimized, the full solar cell structure is deposited. To begin, one side of a cleaned glass slide is fully coated with indium tin oxide which serves as the back contact to the device. The sputter chamber is vented and a mask is attached to the ITO coated slide which defines twelve 0.5 cm2 regions. The sample is returned to the sputter, the chamber evacuated, and copper oxide is deposited. Again, the chamber is vented and with the mask still attached, the working distance of the device to the aluminum doped zinc oxide target is decreased from that of the Cu2O and ITO. Once the ZnO:Al is sputtered, the chamber is vented, the sample is returned to the standard working distance, and the ITO top contact is sputtered onto the mask defined regions. The mask is then removed and the twelve devices are tested by IV measurements.

IV. Results As previously stated, each of the transparent conducing oxides were optimized independently before being

deposited into photovoltaic devices. ITO and ZnO:Al have been optimized for optical and electrical properties. The thickness of these layers will likely need to be adjusted in the final cell design. Sputtering copper oxide can easily produce cupric oxide (CuO) and/or cuprous oxide (Cu2O) depending on deposition conditions. Either of these phases could produce a working photovoltaic device but Cu2O is more desirable for high voltage cell. Since the optimization of copper oxide is currently ongoing, both phases will be used in the cell structure. Early devices had shown issues with film stress that needed to be investigated before further optimization could occur. Current

Figure 2. Band Diagram of TCO Junction. This junction is expected to produce the necessary power to operate the liquid crystal film when fabricated into devices.

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devices are well adherent to the substrate but unfortunately show little light response. Details regarding the optimization processes along with the best case results for each TCO are presented.

A. Indium Tin Oxide The indium tin oxide serves as both the bottom and top contact to the TCO cell. As such, it must be high

transparent, highly conductive, and very uniform over a variety of thicknesses. Early work on depositing ITO produced dark films that were very infrared absorbing. The addition of oxygen to the argon gas decreased the light absorption over the whole spectrum, most significantly in the IR region. Figure 3 shows how light transmitting through the sample changes with the addition of oxygen to the sputter environment. The sputter time is also varied with these samples and the oxygen enhanced samples show little difference as the thickness increases. Although

very transparent, the films were still highly resistive and would not yet work as the solar cell contacts. Further adjustments to the working pressure and gas flow led to the current result of a <10 Ω/sq film. The uniformity of the film across the 3” length of the microscope slide was confirmed by measuring the resistivity and carrier doping levels in six locations. The resistivity varied less than 2 Ω/sq and the doping was only slightly higher towards the center of the sputter target.

B. Aluminum Doped Zinc Oxide A similar approach to that used on ITO was used to optimize ZnO:Al. Variations to working pressure,

argon/oxygen flows, sputter power, and time provided a wide range of thicknesses and levels of absorption but none provided a significant decrease in sheet resistance. The table here shows a selection of samples and their sheet resistance. Optimization of our standard parameters would only drop the sheet resistance into the 20,000 Ω/sq range. Eventually we began changing the distance from the ZnO:Al target to the glass slide. We are limited to how close the sample could be placed to the target but are pleased by the results of 244 Ω/sq at a distance of 2.3 cm. Even with such low resistance, the sample maintains a greater than 90% optical transmittance. X-ray diffraction analysis of this low resistance zinc oxide shows it to be highly (002) oriented in the c direction of the hexagonal structure.

C. Copper Oxide The first few batches of Cu2O thin films that were deposited exhibited the cubic Cu2O phase formation verified

by XRD and had a high transmittance when sputtered at RF power greater than 300 W. However, the sheet resistance of these films were over the sensitivity limit of the measurement tool. In this case, increasing the power produced a decrease in sheet resistance slightly but caused a detrimental effect of decreasing the light transmission through the film as shown in Figure 4. Since the Cu2O is the primary absorber for the photovoltaic device a tradeoff must be made between how much power can be produced versus how clear the solar cell should be to the human eye. Next, additional oxygen was added which increased the transmission slightly and reduced the resistivity of the film.

Target-to- substrate

distance (cm)

Sputter pressure (mtorr)

Ar/O2 flow rate

(sccm) Power

(W)

Sheet resistance (103 Ω/sq)

10 30 15/0 400 46.6

10 30 7/3 400 2011

10 10 7/0 400 710.9

10 4 7/0 200 2323

10 4 7/0 100 27.9

2.8 4 7/0 100 8.4

2.3 4 7/0 100 0.2

Figure 3. Transmission curves of ITO. ITO is sputtered with and without the addition of oxygen for 15, 30, and 45 minutes. ITO with added oxygen is highly transparent

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300 800 1300 1800 2300 2800

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Wavelength (nm)

0sccm O2 15 Minute Sputte

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0sccm O2 45 Minute Sputte

3sccm O2 15 Minute Sputte

3sccm O2 30 Minute Sputte

3sccm O2 45 Minute Sputte

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However, after this series of experiments to lower the sheet resistance of copper oxide thin films, x-ray and optical analysis indicated that the films with low sheet resistance have the CuO phase (cupric oxide) instead of the Cu2O phase (cuprous oxide). CuO has a lower bandgap (∼ 1.2 eV) than Cu2O (∼ 2.0 eV) and it may have provided greater carrier density than Cu2O which lowered the sheet resistance.

The balance between producing CuO and Cu2O occurs with the amount of oxygen flowing into the chamber during sputtering. Attempts are underway to decrease the resistivity of the Cu2O by altering other sputter parameters such as the working pressure and the deposition power. At this stage, both species will be deposited into the photovoltaic device structure until the Cu2O is fully optimized. Although not the original intent, if a sufficiently efficient device is produced with CuO, the device structure will be changed accordingly.

D. Solar Cell Design Modeling the full device was performed in conjunction with optimizing each transparent conducting oxide layer.

A one dimensional model, AMPS 1-D was used to determine an upper limit to the conversion efficiency and external quantum efficiency expected for our cell structure.

Obtaining the upper limit of the power conversion efficiency (PCE) of the device was targeted first. The thickness of both n- and p-type layers was fixed at 500 nm and the doping concentration of ZnO:Al layer is fixed at 1018 cm-3. The overall performance was successfully improved by increasing its doping density. The best PCE is about 3.7 % at the Cu2O doping concentration of 1019 cm-3. The primary cause of the improvement comes from the increase in built-in potential and Voc through the increase of doping concentration. The fill factor (FF) was also dramatically improved.

Reducing the thickness of Cu2O certainly decreased the PCE due to the large loss in optical absorption. The external quantum efficiency (EQE) calculation predicts that the EQE decreases from 0.9 to about 0.5 on average with the decrease in thickness. Short circuit current and fill factor are also expected to decrease to about half, which is consistent with the EQE results while Voc shows minimal change. A further evaluation is necessary to learn the optimized film thickness when the cell is coupled to the VALiD device.

Film

Sample-to-target

distance (cm)

Working pressure (mtorr)

Ar flow (sccm) O2 flow Power

(W)

Sputter time (min)

Doping (1020/cm3)

Resistivity (Ω/sq)

Thickness (nm)

ITO 10 6 20 0 200 20 4.04 7.7 300

ZnO:Al 2 4 7 0 100 40 3.42 55.4 441

Cu2O 10 5 10 5 500 15 3.00 2750 716

The sputtering parameters shown in the table above were used for the first round of TCO solar cell devices. However, a technical difficulty occurs in depositing ZnO and Cu2O without breaking vacuum because the deposition of ZnO was optimized for a working distance of ∼2.3 cm while Cu2O was at 10 cm. None of these junctions were rectifying, showing no photovoltage when measured under dark IV conditions. It is expect that these structures have the cupric oxide phase (CuO) based off previous experiments. The next series of devices were sputtered without additional oxygen in the copper oxide deposition which produces the higher resistivity cuprous oxide (Cu2O). These exhibited stress cracking during sputter but showed a minor rectifying behavior. The most recent devices have a thicker Cu2O layer (~1 μm) and show rectifying behavior. Light IV measurements of these devices is underway.

V. Conclusion Attempts at optimizing a series of transparent conducting oxides for use in a semi-transparent solar cell are

presented here. ITO and ZnO:Al have been optimized to produce highly transparent and highly conductive films. Sputtering copper oxide will produce cupric oxide or cuprous oxide depending on deposition conditions. Currently both phases are being used in our photovoltaic devices until it is determined that one phase clearly out performs the other. In future work more devices will be produced with varying resistivity and doping levels and tested for

Figure 4. Transmission spectra of low and high power sputtered Cu2O. Although the resistance of the film decreases with increased power, the amount of light transmitted decreased as well.

01020304050607080

250 750 1250 1750

high power low power

%T

nm

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performance. The final cell will be stacked as illustrated in Figure 1 with glass replaced by a polymer material similar to that currently used with VALiD. This arrangement will allow for the ITO to function as contact and the Cu2O and ZnO:Al to serve as the p-n junction respectively. The final stacked cell will be subjected to transmission, absorbance, and resistivity analysis via aforementioned methods. In addition to the traditional tests, dark and light IV tests will be run to determine the working performance of the solar cell.

Acknowledgments The authors greatly acknowledge the support for this work by Alpha Micron, Inc. and by the State of Ohio Third

Frontier Research Commercialization Program.

References

1Patt-McDaniel, Lisa, “Ohio Third Frontier 2009 Annual Report,” 2009. 2Mittiga, A., Salza, E., Sarto, F., Tucci, M., and Vasanthi, R., “Heterojuction solar cell with 2% efficiency based on a Cu2O

substrate,” Appl. Phys. Lett., Vol. 88, 2006, pp. 163502. 3Taheri, B., Palffy-Muhoray, P., Kosa, T., & Post, D. L.; “Technology for electronically varying helmet visor tint,”

Proceedings of SPIE, Vol. 4021, 2000, pp. 114-119.