Harvesting Light with Nanostructures: New Crops for Silicon Devices

It seems that cat-like reflexes pay off for solar cell technology… Nanowhiskers deposited on traditional silicon solar cells help in the harvesting of bountiful deep-red solar rays.

Researchers are steadily making progress towards improved solar cell technologies, like Combine harvesters barreling down a field of tall golden crop. The benefits are huge; the problem is getting there without spending a fortune. A major limitation in the harvesting of solar energy is a common problem in almost every process ever invented, whether it be a mechanism for energy conversion or reaping, threshing, and winnowing: Efficiency.

Silicon photovoltaics are currently the most viable form of solar cell for carbon-neutral generation of terawatt (TW) levels of renewable power (Garnett NanoLetters 2010, Lewis Proc. Natl. Acad. Sci. 2007). And yet commercial silicon solar cells typically convert sunlight into electricity at efficiencies only around 20%. Some alternative high efficiency devices are beginning to approach DARPA’s goal of developing solar cells that demonstrate at least 50 percent efficiency (the Very High Efficiency Solar Cell (VHESC) program), although not quite in the affordable, manufacturable range as desired by the agency, which would provide soldiers portable power in the field. So, how do get there? What makes traditional solar cells so expensively inefficient?

The basis of a solar cell (or photovoltaic cell) is the conversion of solar energy into electricity, and a requirement for efficient conversion, i.e. with minimal waste of available solar power, is the strong absorption of photons by the semiconductor of the solar cell, for example the silicon layer. When a photon (the unit particle of light) of sunlight incident upon a silicon substrate is reflected or back-scattered off of the material instead of absorbed, these photons are essentially wasted, not being converted into useful electricity via donation of photon energy to electrons of the semiconductor (silicon) crystal. The problem with many flat or polished silicon-based semiconductor materials is that they display a naturally high reflectivity (20-40%), which leads to optical losses that are significant limiting factors in achieving the maximum theoretical efficiency of a solar cell (Kang 2011, Manea 2005).

Traditional mechanisms for reducing the reflection of photons incident upon a solar panel include surface texturing (adding micrometer-scale tilted pyramid structures to the silicon surface) and antireflection coatings based on transparent material layers that rely on wave interference mechanisms to reduce reflection of incident light. Tilted micro-scale pyramid structures reduce reflection losses by bouncing light from structure to structure, increasing the chance that light will be absorbed by the semiconductor substrate (Baraona 1975 V-grooved silicon solar cells). The phenomenon of light bouncing from pyramid to pyramid increases the length that the incident light travels, i.e. the optical path length. Increased optical path length, which is paramount especially in thin silicon photovoltaic cells, enhances absorption of visible light by the silicon semiconductor, thus increasing solar cell efficiency. Anti-reflection optical coatings serve to reduce unwanted Fresnel reflections, i.e. reflections that occur as light travels from a medium of one refractive index to another (ex. from air to semiconductor).

Although surface texturing and antireflection coatings can extensively reduce reflection losses and thus increase the efficiency of solar cells, these methodologies may not be sufficient for the more economical and portable thin-wafer silicon cells (which require further increases in optical path length to maximize light absorption in the thin silicon layer). Traditional micro-groove textures and interference-based coatings also suffer from spectral dependence, in other words they may reduce reflection effectively at low wavelengths in the visible spectrum (~400-800nm light), but are less efficient at harvesting light in the near infrared spectrum (800+ nanometer light). With the composition of solar radiation that penetrates the atmosphere consisting significantly of wavelengths greater than 800nm (near infrared (NIR) and infrared light – see diagram below), solar cells with traditional antireflection coatings and surface textures leave a huge amount of potential energy out of the picture.

So how do we harvest the bountiful solar radiation that exists in the long-wavelength spectrum? A research group in Taiwan came up with an ingenious solution. You might say Chia-Hua Chang and coworkers had a cat-like intuition on NIR solar cell technology… the researchers found that depositing indium-tin-oxide (ITO) Nanowhiskers onto the micro-structured surface of textured silicon (combining micro- and nano- textures) improved optical transmission of near-infrared light, thus improving the efficiency of a conventional silicon solar cell to over 90% external quantum efficiency for wavelengths between 460 and 980nm.

The basis of these nanowhiskers in reducing reflectance and increasing absorption/transmission of light is a property of their very small size. The nanowhiskers, columns of ITO around 720nm in height and slanted at an angle of ~54.7°, help light to diffuse into the silicon layer (diffused transmission – represented by yellow haze around ‘whiskers’ in figure below) by forward scattering incident photons and thus increasing the length that light travels in the solar cell. The result of the increase light path length is enhancement of optical absorption in the semiconductor layer. The forward scattering reduces reflection losses for visible light, mostly for wavelengths less than 600nm. So far, we observe the combined effect of micro-gooves and Nanowhiskers in reducing reflection and improving solar cell efficiency in the visible light spectrum. But how about the role of Nanowhiskers in harvesting near-infrared light?

Nanowhiskers function as ‘impedance matching layers’ between air and the semiconductor substrate at high wavelengths. In other words, the whiskers help to ‘soften’ the transition of near-infrared light from the air (which has a refractive index that differs from that of silicon) into the silicon layer, thus improving optical transmission of near-infrared (red arrow in diagram above) light into the crystalline silicon solar cells. Chia-Hua Chang and coworkers demonstrated that reflectance for ITO nanowhiskers is reduced to less than 5% for wavelengths between 350nm and 1200nm, a significant improvement over commercially available reference silicon solar cells (see graph below).

“Antireflective nanostructures can be tailored for optimal light harvesting.” – Chia-Hua Chang et al. The antireflective nanostructure layer should be efficient at reducing reflectance of incident photons regardless of the wavelength of the incident light or the angle that the light makes with the surface. The combination of micro-scale (10-6 meters) structures and nanoscale (10-9 meters) ‘whiskers’ reduces reflection of light and improves solar cell efficiency over a wide range of wavelengths. Nanowhiskers improve optical absorption of silicon substrates to a greater extent than do micro-textured surfaces alone or textured surfaces coated with traditional antireflective material. It seems that cat-like reflexes pay off for solar cell technology… Nanowhiskers deposited on traditional silicon solar cells help in the harvesting of bountiful deep-red solar rays.

Solar Cell Semiconductor Materials:
1. Silicon – dominant material for commercial high-efficiency solar cells (Kang et al)
2. Bulk Silicon carbide (wide-bandgap and superior thermal properties)
3. Bulk copper indium gallium selenide – thin film solar cells
4. Cadmium telluride (CdTe) – thin film solar cell material, less efficient than polysilicon
5. Quantum dots – toxicity issues, currently low efficiencies

References
1. Garnett, Yang. Light Trapping in Silicon Nanowire Solar Cells. Nano Letters 2010, 10, 1082-1087
2. Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical challenges in solar energy utilization, Proc. Natl. Acad. Sci. U.S.A. 2007, 104: 20142-20152.
3. C.H. Chang et al. Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics. Nanotechnology 2011, 22: 095201
4. Kang et al. Anti-reflective nano- and micro-structures on 4H-SiC for photodiodes. Nanoscale Research Letters 2011, 6:236
5. Manea et al. Silicon solar cells technology using honeycombtextured front surface. Sol Energy Mater Sol Cells 2005, 87:423
6. Baraona, Brandhorst. V-grooved silicon solar cells. 11th Photovoltaic Specialists Conf., Phoenix, Ariz., 1975
7. Solar radiation diagram – Wiki Commons
8. Surface Texture and Nanowhiskers diagram – Created by Paige Brown
9. Efficiency and reflectance percentage graph – modified from Wiki Commons (Data from C.H. Chang et al.)

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Chang CH, Yu P, Hsu MH, Tseng PC, Chang WL, Sun WC, Hsu WC, Hsu SH, & Chang YC (2011). Combined micro- and nano-scale surface textures for enhanced near-infrared light harvesting in silicon photovoltaics. Nanotechnology, 22 (9) PMID: 21258142