35. 311-316 Trends in Fotovoltaic Cells Nanotechnology.pdf - incdmtm

International Conference 

2ND International Conference on Innovations, Recent Trends and Challenges 

in Mechatronics, Mechanical Engineering and New High-Tech Products 

Development 

MECAHITECH‘10 

Bucharest, 23-24 September 2010 

Trends in Fotovoltaic Cells Nanotechnology 

Gheorghe Popan, Anton Vieru, Iulian Sorin Munteanu 

National Institute Of Research And Development For Mecatronics And Measurement Technique – INCDMTM 

Bucharest 

ABSTRACT 

This work describes several methods of increasing efficiency and cost reduction of the 

photovoltaic elements, based on the latest scientific research, like: using thin metal films and 

organic substances in the construction of photovoltaic elements, improving efficiency 

photovoltaic cells by changing the surface nano-morphology, implementation techniques of 

the aerospace industry to manufacture photovoltaic cells. The work also emphasizes on new 

innovative trends in use of new sources of solar energy. 

INTRODUCTION 

Various techniques to improve the efficiency of silicon solar cells is included such as 

solar concentrators, improving the efficiency of dirty silicon, surface polarization, and 

multiband technology. Polymer and plastic solar cell technology is also discussed as is the 

generation of hydrogen from solar cells. 

These include: organic dye sensitized solar materials, silicon nanorods for solar cells, silicon 

ink solar technology, electron carrier multiplication in nanocrystals, the use of nanoscale 

structures for solar absorption and collection, quantum dot solar technology, and carbon 

nanotube technology in solar energy. Thin film solar cell technology is discussed including 

CIS/CIGS films, inorganic film, amorphous silicon film, and flexible substrates. 

NEED TO USE SOLAR ENERGY 

The sun offers much more energy than what we could consume. In order to cover the 

world-wide requirements of electric power with the fotovoltaic is sufficient only 1,5% of the 

European continent. 

The sun offers good energy and more than we need. Also in times of "energetic hunger" 

like these, the energy that the sun continuously irradiates on the earth is equal to 10.000 

times the worldwide energetic requirements. 

Between all the energetic sources available to us, the sun represents the more longlasting 

energetic source by far, with a life expectation of billions of years. 

In central Europe the sun is an energy source on which trust can be made. In order to 

cover worldwide energetic requirements, by using photovoltaics a surface area of 145000 

Km 2 would be a sufficient. This corresponds to a square having 380 km sides. 

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METHODS OF INCREASING EFFICIENCY AND COST REDUCTION OF THE 

PHOTOVOLTAIC ELEMENTS 

Using thin metal films and organic substances in the construction of photovoltaic 

elements 

Because of optical microcavity effects, using thin nonpatterned metal films instead of 

indium tin oxide in organic solar cells can result in similar efficiencies. Large-area transparent 

conductors are essential in many important applications, such as thin-film solar cells, 

traditional LCDs, and organic LEDs (OLEDs). The widely used transparent conducting oxides 

(TCOs), such as indium tin oxide (ITO), are typically deposited using plasma sputtering or 

sol-gel methods. There is a natural tradeoff between transparency and conductivity, with the 

best films exceeding 90% transparency in the visible part of the spectrum at sheet 

resistances below 15Ω/square. This level of performance is suitable for thin-film solar-cell 

applications, where a sparse metal grid can be added to the TCO film as an auxiliary 

conductor to minimize ohmic losses during charge collection. 

However, TCOs typically exhibit a combination of shortcomings (e.g., brittleness, 

expensive source materials, processing problems, or availability of suitable flexible 

substrates). 

They are particularly problematic in reel-to-reel processing of thin-film, flexible devices 

because they are susceptible to cracking, which raises the film's electrical resistance and 

makes it permeable to oxygen and moisture that accelerate device degradation. An acute 

need exists for transparent conductors that are fundamentally different from TCOs in their 

mechanical, processing, and cost characteristics. 

Figure 1. Metal-organic-metal photovoltaic (PV) cells with thin nonpatterned metal films achieve the 

same power-conversion efficiency as those with conventional indium tin oxide (ITO) electrodes. hν: 

Light energy. [1] 

The search for TCO replacements for organic photovoltaic (OPV) devices has focused 

on carbon nanotubes, graphene, highly conductive polymers, and metallic microgrids 

combined with conducting polymers. But few of these approaches have yielded devices that 

perform as well as those using ITO, and fewer can be scaled up cost-effectively. 

Instead, we considered using a very thin, unpatterned metal film. Metals are malleable 

and can be deposited relatively cheaply and rapidly onto continuously spooled substrate. In 

organic optoelectronics, thin metal films have been investigated as stand-alone transparent 

electrodes and in conjunction with conducting oxides. Generally, the transparency of a metal 

film drops exponentially with increasing thickness, while the sheet resistance rises rapidly. 

This tradeoff between transparency and electrical conductivity limits the range of feasible 

metal thicknesses to 10–20nm. At the low extreme, it can be difficult to maintain film 

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continuity because the metal tends to aggregate into droplets on glass and plastic, while at 

the high end transparency suffers. As a result, OPV cells using continuous metal films as 

transparent electrodes have not achieved parity with ITO-based cells. 

Improving efficiency photovoltaic cells by changing the surface nano-morphology 

Aluminum-doped zinc oxide provides a promising alternative to both fluorine-doped tin 

oxide and indium tin oxide for microcrystalline and amorphous thin-film photovoltaics. 

Satisfying the universal goal of improving solar-energy conversion 

efficiency will require modulating the path taken by light on the surface of 

solar cells. 

Greater surface roughness is one way to make light scattering more 

efficient. A rough surface both increases the length of a photon's light path 

and also enhances absorption of light by reducing reflection. 

Typical surface-texturing methods include wet etching the front electrode using a 

chemical solution. However, repeated wet etching increases processing costs and thickens 

the transparent conductive oxide film. Chemical-vapor deposition represents an alternative 

method of creating a rough surface. This so-called self-texturing technique also does away 

with the need for additional chemical-texturing steps. 

Figure 2 shows schematically the difference in Light scatting between nontextured and 

textured surfaces. 

Figure 2. Schematic diagram showing light absorption as a function of surface morphology of (a) 

nontextured and (b) textured surfaces [2]. 

An early example of self-textured transparent conductive oxide for solar-cell fabrication 

was reported by the group of Fay using boron as a doping source. High surface roughness 

and low resistivity were achieved. However, this self-texturing approach has rarely been 

studied in conjunction with other dopant materials. Because Al is more cost-effective than 

boron, in the work we investigated self-texturing of Al-doped zinc oxide (ZnO) using lowpressure 

chemical-vapor deposition. 

Figure 3 shows the total and diffused transmittance (TT and DT) of undoped ZnO and 

Al-doped ZnO films as a function of Al content. For undoped ZnO, the TT is above 83% for 

the wide range between 600 and 1100nm. As Al content increases, however, the TT value 

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reduces to 73% at 600nm. TT values in the wavelength range from 600 to 1100nm are 

similar, indicating that Al-doped ZnO transmits light uniformly in the visible and IR range for 

solar-cell applications. In particular, the introduction of trimethylaluminum doping is very 

effective in raising the DT value to 29% at 600nm (see Figure 3). 

The haze factor, defined as the DT/TT ratio, quantifies the light-scattering capability in 

air of a ZnO thin film. The calculated maximum haze factor was 43% at a wavelength of 

600nm for an Al content of 43mTorr. This result is 2.5 times higher than the haze factor of 

undoped ZnO and 2.8 times higher than for mass-produced fluorine-doped tin oxide. 

Figure 3. Total- and diffused-transmittance spectroscopy of Al-doped ZnO films as a function of Al 

content. (a) 0 wt%, (b) 5 wt%, (c) 7 wt%, and (d) 8 wt%. 

The Yonsei University Institute of TMS (Telecommunications, Multimedia, State-ofcharge) 

Information Technology, a Brain Korea have achieved a self-textured Al-doped ZnO 

surface without chemical etching. The surface roughness of the resulting films proved to be a 

function of Al content.They obtained a maximum roughness of 82.1nm at an Al content of 

7wt%. 

Based on the surface optical characteristics, we calculated a haze factor of Al-doped 

ZnO as large as 43% at a wavelength of 600nm. 

These findings indicate that Al-doped ZnO has a higher light-scattering capacity than 

fluorine-doped tin oxide, which makes it a plausible alternative in the mass production of 

transparent conductive oxide for use in solar cells. In the future we will focus on preparing Al- 

ZnO films with low resistivity and testing their practical application in solar cells. 

Implementation techniques of the aerospace industry to manufacture photovoltaic 

cells 

Lens arrays couple sunlight into a common slab waveguide to create large, 

inexpensive optics for solar photovoltaics. 

Silicon-based photovoltaics (PVs) convert less than 20% of incident sunlight into 

electrical energy, yet account for almost all solar-power generation. High-efficiency solar 

cells developed for the space industry have demonstrated more than 41% conversion 

efficiency by layering multiple semiconductor junctions that capture large portions of the solar 

spectrum. Fabrication and material costs limit these cells to only a few square centimeters, 

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making them impractical for flat-panel installations. Concentrator photovoltaics (CPV) 

incorporate large-area optics that collect and deposit energy onto small, efficient solar cells 

with the promise of reducing electricity-generation costs compared to silicon-based PVs. 

Optics for solar concentration typically consist of lenses or mirrors focusing onto 

secondary elements that eliminate intensity variations at the PV cell. A common approach 

places dozens of lenses into a shared tracking platform, each focusing onto independent 

secondary optics and solar cells. The large quantity of components increases mounting, 

alignment, and electrical-connection costs. Our proposed concentrator design replaces 

discrete optics with a 2D lens array and a common slab waveguide. Sunlight collected by the 

array focuses onto localized mirrors positioned to reflect light at angles that exceed the 

critical angle for total internal reflection and, therefore, couple into the waveguide. Coupled 

light is homogenized as it propagates towards the exit aperture at the slab edge(s) (see 

Figure 4). The PV cell and heatsink mount directly to the output edge. The coupling mirrors 

are fabricated using simple lithography techniques that make the design compatible with 

large-scale manufacturing, including roll processing. 

Figure 4. Micro-optic concentrators combine a lens array and slab waveguide. At each focus, 120° 

mirrored prisms couple light into the waveguide (inset). PV: Photovoltaic. [3] 

The amount of focusing provided by solar concentrators is defined by the geometricconcentration 

ratio, which describes the ratio of input to output apertures. Optical efficiency is 

the fraction of light reaching the PV cell. It accounts for surface reflections, material 

absorption, and losses associated with propagation within the waveguide. 

The micro-optic concentrator uses 120°-apex prisms placed at each focus that 

symmetrically reflect and couple sunlight into the waveguide. 

Since the thickness remains uniform, guided light may strike a subsequent coupler and 

reflect out of the system as loss. The geometric-concentration ratio becomes the waveguide 

length divided by twice its thickness, with no dependence on width. The lens and acceptance 

angle determine the size of each coupling prism as well as impact losses within the 

waveguide. Propagation losses scale with waveguide length, leading to a tradeoff between 

geometric-concentration ratio and optical efficiency. 

TRENDS IN USE OF NEW SOURCES OF SOLAR ENERGY 

It seeks the implementation of solar cells: 

- Component materials used in exterior cladding of buildings; 

- Achieving external windows of buildings; 

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- Make clothing items and accessories for solar cells; 

- Making the surface of solar cell roof tiles; 

- Implementation of solar cells personal items (sunglasses, bags, backpacks, caps) 

- Using solar cell component materials for the realization of streets, sidewalks and highways 

The idea of asphalt for solar power isn’t particularly new – the concept was first 

conceived a decade ago, although its only been fully realized in the last couple of years. In 

2007, a Dutch engineering company began siphoning the heat from asphalt to heat several 

homes and offices, as well as an aircraft hanger. The system used a network of plastic pipes 

through which the asphalt heated the cold water and channeled it into underground 

containers where it was kept hot until needed. 

More recently, researchers at Worcester Polytechnic Institute (WPI) have conducted 

tests using actual pieces of asphalt as well as computer models. The scientists found that hot 

water created by an asphalt energy system could be used to generate electricity by being 

passed through a thermoelectric generator. Heated asphalt can also heat buildings by the 

method of passing the water through pipes under the asphalt – just as the Dutch team had 

discovered. 

Asphalt has several advantages as a source of solar power. It’s a huge infrastructure 

that is already in place – in the United States for example, there are an estimated four million 

miles of asphalt road surface. 

In general, asphalt is removed and the roads are resurfaced every decade or so; this 

would provide the opportunity to put in the necessary equipment with minimal expense. 

Asphalt also retains its heat after the sun has gone down – giving it an advantage over 

solar panels. Removing the heat from asphalt can actually lower the temperature of the road 

surface too – making towns and cities cooler during hot weather. And unlike solar panels, 

which are all too visible, virtually all the asphalt collection equipment would be hidden under 

the ground. 

Intriguing though these findings are, don’t expect to see the widespread use of asphalt 

solar power any time soon. It may be many years before a network of pipes under the roads 

is able to provide energy – but at least it’s a step in the right direction. 

REFERENCES 

[1] Max Shtein – Thin metal films as simple transparent conductors 28 December 2009, 

SPIE Newsroom. DOI: 10.1117/2.1200912.1848 

[2] Doyoung Kim and Hyungjun Kim – Self-textured transparent conductive oxide film 

improves efficiency of solar cells February 2010, SPIE Newsroom. DOI: 

10.1117/2.1201002.002596 

[3] Jason H. Karp and Joseph E. Ford – A micro-optic solar concentrator 26 April 2010, SPIE 

Newsroom. DOI: 10.1117/2.1201004.002673 

[4] Prachi Patel-Predd – Efficient Thin-Film Solar Cells Thursday, December 04, 2008 

[5] Jason H. Karp, Joseph E. Ford – Trends in Solar Cells and Photovoltaic Technology 

(2004 - 2012), University of California at San Diego La Jolla, CA (updated September 2008) 

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35. 311-316 Trends in Fotovoltaic Cells Nanotechnology.pdf - incdmtm

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