Increasing the efficiency of a hybrid polymer photovoltaic cell with polymer nanofiber complexes of varied thickness

Introduction

The purpose of this investigation is to increase the power output of a specific polymer when utilized in a photovoltaic cell. Fossil fuels are an efficient and reliable source of energy, but they are detrimental to the environment, producing pollutants and greenhouse gases.((Solomon S, Plattner GK, Knutti R, Friedlingstein P. Irreversible climate change due to carbon dioxide emissions. Proc Natl Acad Sci U S A 2009;106:1704-9.))
Also, the supply of fossil fuels is quickly diminishing. According to Kaufman,((Kaufman A. Exploring solar energy. Prakken Publications; 1995.)) at the rate fossil fuels are currently consumed, the supply will be exhausted within fifty years. Assuming that this statement is true, there is significant need for a new source of energy that will last a long time, be safe, clean, and easy to produce.
Photovoltaic cells convert light energy into electrical energy using the photovoltaic effect.((Solar energy. In Microsoft Encarta Reference Library 2003 CD Rom]. Redmond, WA: Microsoft. 2003.)) The photovoltaic effect occurs when photons of light strike the active material, the semiconductor. The electrons of semiconductor atoms are excited from the valence band (valence electrons) into the conduction band, where they can move freely between atoms. The excited electron is known as a charge carrier, or an exciton. The electron moving from its original atom into other atoms is known as exciton dissociation. Exciton dissociation results in one free, excited electron, and an electron hole, or the lack of an electron where the excited electron previously occupied. The electrons are captured by a metal electrode (the negative contact), and the holes are captured by the positive contact, and used as electricity. ((Sun SS, Sariciftci NS. Organic photovoltaics: Mechanisms, materials, and devices. 2005.))Silicon is the most common semiconductor used. The photovoltaic effect will be applied in this investigation, but with a polymer in place of silicon.
This polymer, dissolved in a liquid, can be sprayed onto materials, painted onto materials, or even printed in rolls. Its only downside is its efficiency. Unlike conventional silicon solar panels, which have an average efficiency of 15-20%, the polymer solar panels have a typical efficiency of 2-3%. ((Winters J. Catching more rays. Mech Engg 2005;127:14A.)) Twenty percent efficiency for conventional solar panels would be perfectly acceptable if the solar panels were more practical. However, they are heavy, fragile and difficult to make. Conventional solar panels require extreme temperatures and pressures to produce, consuming large amounts of energy. ((Sun SS, Sariciftci NS. Organic photovoltaics: Mechanisms, materials, and devices. 2005.))Polymer solar cells require no extreme temperatures or pressures, and are thus easier and cheaper to produce. Polymer solar cells have an extreme advantage over silicon solar cells because of their special application properties. Polymer solar cells, if efficient enough, could be painted onto vehicles, buildings, or even clothing as a valuable energy source. Polymer solar cells have the added advantage of being flexible, allowing them to be painted onto curved surfaces such as cars or other vehicles. Finally, polymer solar cells have a significant weight advantage over silicon cells, weighing significantly less. This is especially advantageous for space applications. Current rocket technology relies on liquid fuel, which is very expensive. With silicon solar panel systems weighing hundreds of pounds, ((Sietsema R. Solar panel comparison. Available from: http://claymore.engineer.gvsu.edu/~solar/archives/development/solarpanel.html. [cited in 2009].)) the fuel costs add up very quickly. Lighter, cheaper solar power systems and reduced fuel costs could be as simple as using a different paint on space shuttles or unraveling rolled lightweight polymer cells into space. If the efficiency of the polymer solar panels could be increased, they could provide more than enough energy for sustainable development.
Currently, these polymer photovoltaic cells are very inefficient. The main reasons for this are suboptimal band alignment between electron donor and acceptor semiconductors, and low charge carrier mobilities stemming from polymer based semiconductors’ disorganized nature.((Ginley DS. Hybrid photovoltaic devices of polymer and ZnO nanofiber composites. Proceedings of the Fourth International Symposium on Transparent Oxide Thin Films for Electronics and Optics; vol. 426, 2005. p. 26-9.)) There are ultratech stepper refurbished parts that may also help improve the efficiency of these semiconductors.
Zinc oxide has been proven to increase efficiency by promoting exciton dissociation. This is due to the physics of the photovoltaic effect. The photovoltaic effect and exciton dissociation occur when there is a difference in potential (Fermi energy) between two areas of the solar cell, where a high potential area is in junction with a low potential  area. When the polymer is illuminated, its electrons are excited, making it high potential. Zinc oxide has non-excited electrons, making it low potential. Thus, Zinc oxide provides the potential difference required for the photovoltaic effect at the junction between it and the polymer [Figure 1]. This explains the result of previous studies (year three of this investigation), where zinc oxide was mixed homogenously with the polymer. Zinc oxide had an increased efficiency when mixed with the polymer because it increased the surface area of the high potential/low potential junction, and thus the efficiency on the one molecule thick layer between the polymer and the electrode. Because the zinc oxide only increases efficiency when it is at the interface between the polymer and the electrode, the goal of year 4 of this investigation was to increase the surface area of the zinc oxide/polymer junction by creating zinc oxide nanofibers in replacement of the homogenous mixture, and optimize nanofiber length for maximum efficiency. Year four of this investigation proved that increased nanofiber length resulted in increased surface area and increased efficiency. However, if nanofiber length (thickness of the nanofiber layer) surpassed polymer thickness, the nanofibers penetrated completely through the polymer and caused an electrical short, ruining the devices.
Figure 1
The polymer is applied through a process known as spin-coating. During this process, a substrate is spinning rapidly on a platform, and the liquid polymer is dropped onto the spinning substrate. The polymer spreads out to form a film. The thickness of the polymer is a function of polymer solvent (varying surface tension and density), spin rate (centripetal force), and polymer concentration (viscosity). Previous research has shown that polymer thickness has a significant effect on efficiency.((Chang C, Pai C, Chen W, Samson A. Spin coating of conjugated polymers for electronic and optoelectronic applications. Thin Solid Films 2004;479:254-60.
)) Increased polymer thickness results in higher resistance and thus lower efficiency.
Essentially, there are two conflicting variables when producing hybrid organic/inorganic devices with P3HT and ZnO nanofibers. As nanofiber length increases, efficiency increases. As polymer thickness increases, efficiency decreases. The two variables are linked in that the nanofibers must be as long as possible without surpassing polymer thickness. The goal of this research was to find the optimal thickness of these two layers, the “polymer nanofiber complex” that resulted in maximum efficiency.

Method

Before the production of solar devices it was necessary to develop procedures for reproducibly controlling the thickness of the polymer and nanofiber layers. The spin-coating process was the determining factor for polymer thickness, assuming constant polymer solvent and concentration. During spin coating, polymer solvent and concentration were kept constant at chloroform and 20g/L. Polymer films were produced with spin rates of 8210, 2620, 1110, 557, 313, and 190rpm. These rates were determined by previous research from Chang et al,((Chang C, Pai C, Chen W, Samson A. Spin coating of conjugated polymers for electronic and optoelectronic applications. Thin Solid Films 2004;479:254-60.)) to predict polymer thicknesses of 130, 180, 230, 280, and 330nm based on a regression equation matching known data. Profilometry was used to determine actual polymer thickness. These data were plotted, and a new regression function was calculated to relate spin rate and polymer thickness for given polymer solvents and concentrations [Figure 2]. To confirm the reproducibility of this regression, new slides were produced using the regression function. These thicknesses were measured, and a new regression function created. The difference between the first and second regression functions was less than the margin of error of the surface profiler equipment.
Figure 2
Figure 2: Polymer film thickness vs. spin rate for a given polymer solvent and concentration, using power regressions. The R-squared values for the first and second trials were 0.989 and 0.957, respectively
Figure 3
Figure 3: The reaction mechanism for the production of a zinc oxide nucleation layer from a Zinc Acetate precursor (adapted from Bahadur et al., 2007)
Figure 4

Figure 6
The zinc oxide nanofiber production procedures were also analyzed for nanofiber length. The nanofibers are produced using the sol gel process from zinc nitrate and zinc acetate precursors. Due to the surface properties of glass, zinc oxide nanofibers can not be grown directly onto glass. There must first be a flat layer of zinc oxide from which to grow the nanofibers. This is known as the nucleation layer. The nucleation layer is produced by spin coating at 2000rpm the following solution: 0.823g Zinc Acetate, 0.223mL Ethanolamine, 5mL 2-methoxyethanol. The slides were annealed at 300 degrees for 10 minutes, initiating the reaction seen in Figure 3, and resulting in a flat layer of zinc oxide. The unwanted ions were washed away by subsequently washing the slides in acetone and isopropanol. The nanofibers were produced in a solution of 1mM zinc nitrate at 70 degrees, according to the reaction seen in Figure 4. The pH was raised to 13 using sodium hydroxide. Previous research has shown that nanofiber length is a function of the reaction time in the zinc nitrate solution. Nanofiber slides were produced with reaction times of 5, 10, 15, 20, 25, and 30 minutes. Nanofiber length was measured using scanning electron microscopy [Figure 5]. Average fiber length was taken from three locations on each slide. The SEM images were taken from a 45 degree angle. Using the Pythagorean Theorem, the measured nanofiber length was multiplied by the square root of 2 to find actual nanofiber length. Reaction time was plotted against nanofiber length [Figure 6] and a regression function was produced to relate the two variables. This process was repeated to confirm the accuracy of the regression function.
Figure 7
Figure 8
Figure 9
Figure 10


The solar devices were produced using slides of patterned Indium Tin Oxide (ITO). Ten 1”x1” glass slides pre-coated with (ITO) underwent intense cleaning procedure using a micro weave cloth to remove dust and impurities [Figure 7]. It is important to note that the entire solar cell, when completed, is only ~1000nm thick, meaning that any dust or particle on the slide during production could be thicker than the entire solar cell, causing an electrical short and a ruined device, so the cleaning procedure is critical to the production process. The slides were placed into an ultrasonic vibrator with 100ml isopropyl alcohol for ten minutes. This was repeated with deionized water and acetone. The slides were placed into an ozone and ultraviolet light generator for thirty minutes. This was to ensure the absence of any impurity which could interfere with electrical conduction of the slide, and to enhance hydrophilic properties of the slides’ surface to facilitate spin-coating. The nanofiber layer was produced according to the procedure described above, in thicknesses (nanofiber lengths) of 100, 150, 200 and 250nm [Figure 8]. The ITO contact area was wiped with a Q-Tip soaked in ethanol prior to annealing to prevent fiber growth in this area.((Olson DC, Shaheen SE, Collins RT, Ginley DS. The effect of atmosphere and ZnO morphology on the performance of hybrid poly(3-hexylthiophene) / ZnO nanofiber photovoltaic devices. National Renewable Energy Labs; 2007.)) Two slides were produced with each nanofiber length, leaving two slides with no fibers as the control. The polymer layer was applied on top of the nanofiber layer using the process described above [Figure 9]. The polymer was applied such that it was always 30nm thicker than the nanofiber layer, to account for small variances in fiber length and minor residuals in the regression function. For example, to the 100nm nanofiber device, a polymer layer was added of 130nm. This process was completed in nitrogen to prevent the oxidation of the polymer. The slides were annealed at 200 degrees for 1 minute, to melt the polymer between the nanofibers, further increasing the surface area of the junction. A thermal evaporator was used to achieve zero humidity and a complete vacuum environment. Four slides at a time were placed into the thermal evaporator and each underwent application of both an 8nm BCP (bathocuproine) layer, and an aluminum electrode directly to the polymer surface of the slide  [Figures 10, 11]. The BCP effectively increased the thickness of the slide, reducing the chances of an electrical short due to dust or other impurities. It is also a hole-blocking layer. The use of the thermal evaporator assured intimate contact of the entire electrode with the entire surface of the polymer. The procedure of BCP layer and electrode application was repeated for all test slides. Each side of each test slide had the BCP and aluminum electrode layers applied, allowing each slide to serve as four different test trials. Each slide was inserted into a solar simulator, and attached to a semiconductor parameter analyzer, which was connected to a computer to graph and record power output [Figure 12]. Power output from the device was recorded in amperes using a sweep mode from minus to plus one volt. This was repeated for each of four test sites on each slide, and repeated for all ten test slides. The entire process was repeated, resulting in two slides, or 8 solar cells for each test group, and two slides for the control. The process of testing each solar cell for efficiency was repeated after 72 hours of air exposure to test the effects of air exposure on the devices. All slides, polymers, and used chemicals were disposed of as per safety protocol. All results were analyzed using Microsoft Excel.

Results and Discussion

Efficiency was calculated based on number taken from the J-V curve from the semiconductor parameter analyzer [Figure 13]. According to the data, nanofiber length, polymer thickness, and air exposure had an effect on solar cell efficiency. The addition of nanofibers initially decreased the efficiency compared to the control [Figure 14]. However, after 72 hours exposure to air, the nanofiber devices showed an increased efficiency, while the control decreased in efficiency [Figure 15]. This increase in efficiency may be due to the reduction of oxygen vacancies in the ZnO nanofiber crystals, and thus an increase in carrier concentrations. Some nanofiber devices had efficiencies greater than the control after 72 hours. The device with nanofiber length/polymer thickness of 100/130nm had an efficiency 29% greater than the control. The trend of air exposure over time indicates that further exposure would continue to increase efficiency in nanofiber devices, and would likely show asymptotic behavior after the polymer and nanofibers are completely oxidized. Maximum efficiency within nanofiber devices was seen in 100/130 and 250/280 devices. This indicates that both polymer thickness and nanofiber length have an effect on efficiency, with low polymer thickness increasing efficiency in the 100/130 devices, and long nanofiber length increasing efficiency in the 250/280 devices. After 72 hours, efficiency was measured using arbitrary units. This was due to the fact that the BCP layer degrades over time, decreasing efficiency, while maintaining the trend of efficiencies over all devices, so the efficiencies themselves are irrelevant. The solar devices initially have a low efficiency due to device conditions. Due to limited polymer availability, reduced polymer concentrations were used from the optimal 32g/L. As a result, the trend of efficiencies is more important than the efficiencies themselves. This trend indicates that after 72 hours, the solar device with nanofiber length/polymer thickness of 100/130nm increases efficiency 29% over the control, and continues to increase efficiency over time. Applying this trend to solar cells with optimal polymer concentrations would result in a final theoretical efficiency of 5.2%. After a 2 sample T-test, the p-value when comparing the 100/130 device and the control was calculated to be less than 0.0001, indicating that the results were statistically significant.
There were some minor errors in the procedures. First, the slides were intended to be removed from the annealing oven simultaneously. However, this was not possible because it took time to remove each device from the annealing oven, so there was a 10-15 second variation. Next, there was a slight technical problem with fiber growth. The fibers were grown in a 70 degree solution. However, the solution was heated from the bottom, resulting in a temperature gradient across the slides. This would have resulted in varying fiber diameters across the slide. However, this would have had a minimal effect as the gradient was the same on all slides. Finally, the polymer is laterally conductive. This means that energy is collected on the slide in an area slightly larger than the intended device area. However, since light was only being shined on the intended device area, and the effect was the same for all devices, the efficiency ratios are preserved and the effects are minimal. These problems had miniscule effects on the final efficiencies, and any effect would have been the same across all test groups, having minimal, if any effect on the data trend. Despite these small effects, this research generated new knowledge on polymer solar cells, in hopes of one day replacing fossil fuels with solar energy, reducing humanity’s effects on the environment and achieving true sustainable development.
Figure 13
Figure 14
Figure 15

Acknowledgments

I would firstly like to thank my parents Drs. Mark Monroe and Julie McKay for their never ending moral and financial support. I would also like to thank my teacher Mrs. Marion Zeiner for her guidance and help along the way. I would like to thank Dr. Jiangeng Xue for his selfless donation of time and materials. Finally, I would like to thank graduate students Ying Zheng, Jason Myers, and Bill Hammond for their time and help with equipment.

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