Rose Petals for Enhanced Solar Cells

Summary

I grew up in the Chinese province of Xinjiang. When I last visited Xinjiang three years ago in the summer,
I had realized a dramatic change: the removal of a solar cell field nearby my home.

This led me to wonder: if the solar panels had been more efficient or cost-effective, would they still be here
today?  I began researching various ways to improve solar cell, and came across a very interesting studies such as using the anti-reflective properties of corneal moth eyes to decrease reflectance. 

One day, on a science tour at Botanical gardens, a guide told me that flowers have tiny structures which allow them to shimmer. I then learned that rose petals have nanostructures which enhance their light-capturing abilities. 

I speculated that solar cells could borrow from nature's evolutionary processes. Through the use of soft lithography and polydimethylsiloxane, I was able to create molds of the tiny structures on rose petals and transfer them onto solar cells. 

The results yielded a staggering 34.9-36.5% (amp/watts) improvement from current c-Si cells’ responsivities. This supported my hypothesis that rose petal nanostructures not only reduced reflection but also redirected photons, suggesting  light-focusing  properties  and  the  potential  for optimizing  light  absorption. 

This study provides a springboard for future applications regarding the utilization of rose nanostructures, adding to a growing body of literature in photovoltaic nanocoatings.

The next step of my project might include exploring other flowers for solar cell optimization. I am intrigued by the ability for nature to inspire technology. 

 

Question / Proposal

Fossil fuels are non-renewable, that is, they draw on finite resources that will eventually dwindle, becoming too expensive or too environmentally damaging to retrieve. In contrast, solar energy is remarkably renewable and clean.

Given its importance, individuals have conducted significant research in photovoltaic materials, and, currently, c-Si solar cells (≈ 22% efficiency) dominate the market. However, some laboratory- made nanostructures have met efficiencies of 25.6% and have further potential to reach 33.7% (Shockley-Queisser Limit.)

For this reason, this project investigates the possibility of bio-inspired coatings. When choosing a biomaterial to create a photovoltaic cell, rose-petal surfaces were selected for several reasons. The primary criterion that was considered was that past studies extensively imitate moth eyes and butterfly wings to form antireflection layers. Yet, when compared with animal counterparts, limited research on microfabrication of plant-surfaces exists. In addition, although moth corneal arrays have antireflection properties, these nanostructures fail to change the photons’ propagation.

It was observed in past literature that rose petals’ nanostructures not only reduced reflection but also redirected photons, suggesting  light-focusing  properties  and  the  potential  for optimizing  light  absorption.

On that account, the primary objective of this study was to analyze nature-based nanostructures in further detail. My hypothesis for this project predicted that an anti-reflective structure, such as rose petal nanostructures, would improve the solar cell efficiency in a two-fold manner through the reduction of reflectivity and the redirection of photons. 


 

Research

For years, scientists have been exploring ways to improve the efficiency of solar cells. The intrinsic qualities of photovoltaics afford it great utility for the following reasons: [1] most developing countries are in a remote region with optimal access to the sun’s rays, and not much else; [2] other energy sources available result in exploitation of the ecosystem; and [3] solar designs are applicable and affordable to both homes and villages.2 Data trends expect photovoltaics to be the fastest growing energy sector, accounting for 16% of global energy production by the year 2050.

Scientists have made the most progress in composition, but recent research investigates nanostructure coatings. Scholars have attempted to improve light capture, albeit compromising either light redirection or anti-reflection properties in the interest of the other– resulting in less enhancement. Another shortcoming is that existing research of inorganic constituents, such as oxide coatings, produces unmitigable reflections.6 Furthermore, anti-soiling microstructures can extend efficiency but do little to improve light management. Biological nanostructures are sorely missing from this group.

For this reason, my project investigated the possibility of bio-inspired coatings. When choosing a biomaterial to create a photovoltaic cell, rose-petal surfaces were selected for several reasons. The primary criterion that was considered was that past studies extensively imitate moth eyes and butterfly wings to form antireflection layers. Yet, when compared with animal counterparts, limited research on microfabrication of plant-surfaces exists. In addition, although moth corneal arrays have antireflection properties, these nanostructures fail to change the photons’ propagation. While this was not an issue in the past, the advent of thin cells requires light redirection.

My research provides a springboard for future applications regarding the utilization of rose petal nanostructures. These findings add to a growing body of literature in photovoltaic nanocoatings. Moreover, this research can be utilized in many technical applications due to the fact that these nanostructures exhibit a plethora of qualities beneficial to today’s technology. The strength of this contribution lies in Rosoideae rosa’s reusable, weather-resistant, abundant, low-cost, light management and superhydrophobic characteristics.

Method / Testing and Redesign

Part 1: Rose Petals Preparation

Achieving a sustainable rose-based photovoltaic system is vital to light management and practical utility. Ideally, the system should improve photon redirection. In search of such a method, various colored roses that lacked pre-treatment were collected, as to avoid dehydration and potential collapse of surface structures. The samples were prepared by cutting the petals’ midsections into squares with side lengths of 3 centimeters each.

Part 2: Polydimethylsiloxane (PDMS) Negative Replicas

To replicate the rose hierarchical structures, polydimethylsiloxane was used to prepare a negative replica mold (Figure 5). First, the PDMS solution was synthesized from the silicone pre-polymer (Momentive RTV 615A) and crosslinking agent (Momentive RTV 615B) by mixing them in a glass petri dish in a 9:1 ratio. 

I observed that the solution of PDMS possessed one undesirable trait: the unwanted presence of air bubbles. To solve this problem, the glass dish was placed inside a vacuum chamber for one hour (or three 20-minute intervals) to evacuate the air bubbles that may have potentially caused defects in replication. Afterwards, three rose samples were positioned in a petri-dish (front-side facing up) and then submerged in polydimethylsiloxane solution. As a precaution, the petri-dish was returned back to the vacuum chamber for 20 mins – assuring that no air bubbles remained.

The PDMS/rose assembly was heated for one hour at 75C using a heat plate. Once again, this interval of time and temperature was selected through trial-and-error, as to facilitate the PDMS curing process while reserving the shape and structures of the rose petals. Lastly, the PDMS/rose assembly was soaked in a heated piranha solution (mixture of sulfuric acid and hydrogen peroxide in 7:1 volume ratio) at 110C for 10 mins, so that the organic rose petal could oxidize – leaving behind its contour’s imprint.

Part 3: Photoresist Positive Replicas

Positive (fabricated) replicas of rose nanostructures were synthesized upon the perfection of the negative PDMS molds. This process was accomplished by casting my PDMS molds onto photoresist films that solidified under UV-light. To evaluate effectiveness, the glass substrate was observed under a Scanning Electron Microscope (SEM) to see if the nanostructures from the polydimethylsiloxane (PDMS) imprint had transferred over to the photoresistive film.

Part 4: Surface Characterization

To further investigate surface morphology of rose petals, samples were sputtered with a 10-nm gold layer by Hummer VII sputter system, a physical vapor deposition (PVD) method of thin film deposition, prior to SEM characterization. The contact angles of the negative replicated PDMS and flat pristine PDMS were measured using a ramé-hart Contact Angle Goniometer with a 20 μL deionized water droplet on the same surface. From this experiment, I report that the fabricated nanostructure film had exhibited hydrophobic properties. 

Part 5: Optoelectronic Characterization

To confirm optoelectronic efficiency, one set of Si photodiodes (coated with photoresist films of positive rose replica) were compared to another set (coated with flat photoresist films). A Keithley 4200 SCS and Cascade probe station recorded the I-V measurements.

 

Results

Observation of Rose Hierarchal Structures

Using SEM, the surface morphology of various rose structures was investigated and it was observed that the rose hierarchical structures were capable of reducing light reflectionand increasing light absorption. The nanostructures show the uniform and closely packed micro-convex cells, else known as micropapillaes. After analyzing the SEM images, it was predicted that the two different rose morphologies represented the diversity of functional surfaces that were developed during evolution. Furthermore, it was concluded that a higher aspect ratio (represented as the ratio between vertical depth to base diameter) benefitted light management in both types of rose nanostructures, as the micro-convex shaped cells trapped light and reduced specular reflections. I concluded that all rose nanostructures redirected photons by absorbing greater intensities of light. From these observations, it was inferred that pigmented exteriors and surface micropapillae are analogous structures: exhibiting similar functions yet disparate features.

Optoelectronic Characterization of SU-8 2002 Photoresist Film

To test optoelectronic efficiency of rose-nanostructure films, total of 22 voltage sweep tests were conducted at -10V bias – each producing 112 data points. A Semiconductor Parameter Analyzer was used to record the data under normal light condition (2.24 𝜇𝑤/𝑐𝑚2). To assess accuracy, eight tests divided into two sets were conducted: four tested photodetectors attached with the nanostructure imprint and four tested photodetectors attached without the nanostructure imprint. The experiment was then repeated under the conditions of low-light (0.05 𝜇𝑤/𝑐𝑚2) and no-light (0.00 𝜇𝑤/𝑐𝑚2). Note that the no-light condition is not displayed, as there was no current generated (Table 1).

During testing for photovoltaic characterization, a current was produced from -10V to 0V that remained constant until breakdown voltage at 0V. Breakdown voltage reverses the current produced and makes the diode conduct appreciably in the opposite direction. This allowed for a direct comparison between unmodified and modified surfaces.

Upon gathering data, relative responsivity was analyzed as the ratio of responsivity of rose structure to that with referenced device of flat surface.27 As hypothesized, these tests revealed that under ordinary illumination (2.24 𝜇𝑤/𝑐𝑚2), the photoresist with nanostructures had a groundbreaking improvement of 34.9%. In detail, the nanostructures increased responsivity from 0.670 amps/watt to 0.904 amps/watt. At -10V bias, the current averaged 1.59 × 10−7 amps with nanostructures while the control averaged a mere 1.136 × 10−7. Furthermore, under low-light (0.05𝜇𝑤/𝑐𝑚 2), the photodetector responsivity was enhanced by 36.5% – from 0.577 amps/watt to 0.787 amps/watt. At -10V, the photoresist was measured with nanostructures to be 3.57 × 10−9 amps while the control averaged 2.73 × 10−9 amps. Graph 1 on the following page compares the two.

 

Conclusion

The data justify the hypothesis that SU8 photoresist solution can effectively replicate the structure and function of Rosoideae rosa nanostructures. By doing so, I have created a low-cost solar-cell coating that can improve energy efficiency. This study is the first step towards superhydrophobic plant-based enhancement of optical and light management. 

Although successful, there are a few shortcomings that need to be addressed and appropriate caution exercised. First, I noticed a procedural limitation: my laboratory’s Spin Coater did not function appropriately. This resulted in a slightly uneven distribution of structures across the photoresist surface. Second, I noted that the rose samples were somewhat compressed before testing. Although I detected no major deformities, there could be potential for minor error. Third, due to machine failure, I utilized piranha solution (7-parts H2SO4 and 1-part H2O2) instead of ultrasonic cleaning. The problem with piranha solution is that it hydroxylates PDMS, a phenomenon that compromises the photoresist curing by introducing excess water.

However, despite few possible faults, the photoresist solution and PDMS mold effectively replicated the nanostructures. These findings suggest the following opportunities for future research: [1] Incorporating rose petals’ superhydrophobic properties for application in optical devices in high-precipitation regions; [2] implementing this experiment’s low-cost solar technology in developing states; [3] and studying other – perchance similarly superhydrophobic – biological nanostructures for photovoltaic application. Specifically, examining varied species of flowers or organisms, as to observe if their structures provide any supplementary benefits. In point of fact, I am currently in the process of further analyzing rose nanostructures.

In this experiment, I achieved all engineering objectives. Specifically, the photodetector responsivity was enhanced by 34.9% under ordinary illumination (2.24 𝜇𝑤/𝑐𝑚 2). To recap results, the nanostructures increased responsivity from 0.670 amps/watt to 0.904 amps/watt. At -10V bias the current averaged 1.59 × 10−7 amps with nanostructures while the control averaged 1.136 × 10−7. Under low-light conditions (0.05 𝜇𝑤/𝑐𝑚 2), I  enhanced photodetector responsivity to 36.5% from 0.577 amps/watt to 0.787 amps/watt. At -10V, I measured 3.57 × 10−9 amps with nanostructures while the control averaged 2.73 × 10−9 amps. My results indicate statistical significance between the mean responsivities of the flat and imprinted photoresistst.

This study provides a springboard for future applications regarding the utilization of Rosoideae rosa nanostructures. These findings add to a growing body of literature in photovoltaic nanocoatings. Moreover, this research can be utilized in many technical applications due to the fact that these nanostructures exhibit a plethora of qualities beneficial to today’s technology. The strength of this contribution lies in Rosoideae rosa’s reusable, weather-resistant, abundant, low-cost, light management and superhydrophobic characteristics.

About me

Howdy! I'm Benjamin Li, a senior at Plano West Senior High School. 

Little bit about me:

- Favorite wavelength is 600nm
- Enjoys swimming
- Passionately a monkey

Passionately a monkey?! That's right-- I'm more curious that Curious George could ever be. It unnerves me to not know how something works, and I keep seeking ways to improve things. My curiosity has led me to the intersection between STEM and passion for discovery. 

I've recently taken a deep interest into learning semiconductor physics and how to optimize solar cells. In the hometown I grew up in, Beijing, solar panels have seen exponential growth-- until the residents deemed them useless and a waste of space. 

I've always wondered what life would be like if solar cells were optimal. Would Beijing not have so much air pollution? Would residents not need to buy oxygen smoothies? I'm not sure, but I do know that solar cells and renewable energy holds the key to the future. 

Along my journey, I've been inspired by many scientists. First off, I believe that a scientist is one who solves problems and inspires others along the way. While my parents may not be nearly as accomplished as Einstein, Feynman, or Newton, they have taught me that science is about enjoying the process of learning and fixing mistakes. Like the many great scientists, they have inspired me tremendously. 

In college, I want to pursue a degree in engineering. But right now, winning at Google is what I'm excited about. 





 

Health & Safety

This research project was conducted at the University of Texas at Dallas. There were no major health/safety concerns to be addressed in this project. The lab manager that I worked under is Wenchuang Hu. His contact information is as follows:

Email Address: wenchuang.hu@utdallas.com
Phone Number: (972) 883-5725

Experimentation was carried out using standard laboratory procedures. Gloves, goggles, and a lab coat were worn at all times. 

Special machinery such as the Scanning Electron Microscope were operated by a graduate student due to lab policies (must be 18 years or older to operate special equiptment). Hazardous material (such as Piranha solution, a mixture of hydrochloric acid and sulfuric acid) was also handled by the graduate student inside of the clean room. The graduate student whom I worked with was Honglei Wang. Her contact information is as follows:

Email Address: hxw11302@utdallas.edu
Phone Number: (972) 883-5725

 

 

Bibliography, references, and acknowledgements

Works Cited: 

[1]              Catchpole, K. R. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 3493-3503.

[2]              W. Barthlott and C. Neinhuis, Planta 202 (1), 1-8 (1997).

[3]              K. Koch, B. Bhushan and W. Barthlott, Soft Matter 4 (10), 1943-1963 (2008).

[4]              L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang and D. Zhu, Advanced Materials 14 (24), 1857-1860 (2002).

[5]              Q. Chen, G. Hubbard, P. A. Shields, C. Liu, D. W. E. Allsopp, W. N. Wang and S. Abbott, Applied Physics Letters 94 (26), 263118 (2009).

[6]              Hersch, P.; Zweibel, K. Basic Photovoltaic Principles and Methods; Solar Energy Research Institute: Golden, CO, 1982.

[7]              Y.-F. Huang, S. Chattopadhyay, Y.-J. Jen, C.-Y. Peng, T.-A. Liu, Y.-K. Hsu, C.L. Pan, H.-C. Lo, C.-H. Hsu, Y.-H. Chang, C.-S. Lee, K.-H. Chen and L.-C. Chen, Nat Nano 2 (12), 770-774 (2007).

[8]      P. Vukusic and J. R. Sambles, Nature 424 (6950), 852-855 (2003).

[9]              S. Niu, B. Li, Z. Mu, M. Yang, J. Zhang, Z. Han and L. Ren, Journal of Bionic Engineering 12 (2), 170-189 (2015).

[10]           R. Hünig, A. Mertens, M. Stephan, A. Schulz, B. Richter, M. Hetterich, M. Powalla,

U. Lemmer, A. Colsmann and G. Gomard, Advanced Optical Materials 4 (10), 1487-1493 (2016).

[11]           A J. Schulte, D. M. Droste, K. Koch and W. Barthlott, Beilstein Journal of Nanotechnology 2, 228-236 (2011).

[12]           L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia and L. Jiang, Langmuir 24 (8), 4114-4119 (2008).

[13]           K. Koch, B. Bhushan, Y. C. Jung and W. Barthlott, Soft Matter 5 (7), 1386-1393 (2009).

[14]           H. L. Gorton and T. C. Vogelmann, Plant Physiology 112 (3), 879-888 (1996).

[15]           H. Bargel, K. Koch, Z. Cerman and C. Neinhuis, Functional Plant Biology 33 (10), 893-910 (2006).

[16]           K. Koch, B. Bhushan and W. Barthlott, in Springer Handbook of Nanotechnology, edited by B. Bhushan (Springer Berlin Heidelberg, Berlin, Heidelberg, 2010), pp. 1399-1436.

[17]           S. H. Tan, N.-T. Nguyen, Y. C. Chua and T. G. Kang, Biomicrofluidics 4 (3), 032204 (2010).

[18]           J. D. Myers, W. Cao, V. Cassidy, S.-H. Eom, R. Zhou, L. Yang, W. You and J. Xue, Energy & Environmental Science 5 (5), 6900-6904 (2012).

[19]           Baghzouz, Yahia. "Basic Photovoltaic Theory." Handbook of Clean Energy Systems.

[20]           Reimer, L., 1998. Scanning Electron Microscopy, second ed, Springer, Berlin.

[21]           Zhiyuan Huang, Xin Li, Melika Mahboub, Kerry M. Hanson, Valerie M. Nichols, Hoang Le, Ming L. Tang, and Christopher J. Bardeen Nano Letters 2015 15 (8), 5552-5557


There are many people that I would like to thank for assisting me on this journey:

1) Parents:
My parents have endlessly supported me in my scientific endeavors. Whether it be providing transportation to and from the research laboratory, allowing me to share my creative thoughts, or simply giving me words of encouragement, my parents have always been there for me. Their love, inspiration, and support are the fuel behind my creative machine. Without them, I would have never had the opportunity to explore my passions in the scientific world. 

2) University of Texas at Dallas:
I extend my gratitude to the University of Texas at Dallas for graciously accepting me as a affliate researcher over the summer. They have allowed me to explore my research passions and provided resources to delve into my curiosity. I vividly remember presenting my project proposal and final results to the UTD community and the amount of enthusiasm that reciprocated. The professors, graduate students, and learning environment at the University of Texas at Dallas have inspired me to pursue my scientific passions to the fullest. 

3) Dr. Wenchuang Hu
Dr. Hu was the principal investigator in my laboratory that I worked under. On the first day that we met, he warmly welcomed me to his group, shared his passions about his research, and challenged me to think outside the box. His stellar personality coupled with his brilliant mind have inspired me to stand at the intersection between global impact and innovation. Because of him, I want the world to know that scientists, like celebrities or politicians, can make a positive change in this world. Dr. Hu has endlessly supported me on this journey, and I cannot thank him enough. 

4) Honglei Wang
Honglei was the graduate student that I primarily worked under during my time at UTD. During my first days of research, she was the one who held my hand and encouraged me to think creatively. Besides passing on her extensive knowledge of semiconductor physics, chemistry, and laboratory techniques, Honglei has also passed on to me her passion for solving problems. She has helped me tremendously during the collection of my data, as she collected all the SEM images (I was not able to operate the scanning electron microscope images due to lab policy). Honglei has also taught me how to analyze data from I-V curve characteristics. Her patience and passion for sharing her knowledge and research have inspired me as a scientist.