Thin Films Solar Cells on Flexible Substrates

Carbon Nanotube


Thin film silicon solar cells are classified into p-i-n and n-i-p configurations which refer to their deposition sequence; n-i-p processing starts with the n-layer which is normally grown on a metallic back contact. Historically this configuration is connected to flexible substrates because it was used on opaque substrates or poorly transparent substrates like steel foils or high temperature polymers. However, the configuration is not limited to this choice, it is in fact compatible with any kind of substrates, such as rigid or flexible, transparent or opaque. Nevertheless, flexible substrates have remained the main application of n-i-p cells because roll-to-roll processing makes them very interesting to reduce the production costs as well as the energy payback time, particularly when low cost substrates like poly-ethylene are used.

The activities of the n-i-p group combine general aspects of thin film silicon solar cells with special requirements that are imposed by the deposition sequence and the desired compatibility with low temperature substrates. Two main lines of work can be distinguished: .    Substrate texturing .    Light scattering and absorption enhancement

These two combined lines result in, for example, high efficiency triple junctions cell on innovative flat light scattering substrate presented in the last section.

Towards a more fundamental understanding of absorption enhancement in solar cells, we fabricate cells on periodic gratings that permit the study of coupling into guided modes [1].

We obtained fully flexible solar cells on a low cost poly-ethylene substrate with a stabilized efficiency of 9.8% for 0.25cm2 laboratory cells [2].

Research highlights

Substrate texturing

Amorphous and microcrystalline silicon are poor absorbers, particularly for light with energies just above their respective band gaps. Some means of absorption enhancement is required which is commonly called light trapping. It can be achieved by texturing of the interfaces. A common approach for n-i-p cells are back contacts made from so called “hot silver”, which is the texture that silver develops by partial recrystallization during growth on a heated substrate. Unfortunately this is too hot for poly-ethylene, we have to devise other ways. We investigate the incorporation of texture into the substrate itself, during cell fabrication this texture is carried into the other interfaces because of conformal coverage.

Periodic substrate textures

We obtained promising results on low cost poly-ethylene substrates like the one shown below. The substrate texture has been manufactured by a commercial manufacturer.


The used texture is periodic with a simple sinusoidal wave pattern, thus the incoming light is diffracted into the rainbow colours. For use in solar cells the substrate is coated with back reflector consisting of silver and zinc-oxide. The right figure shows that the back reflector reproduces the substrate texture, but silicon with a thickness comparable to the period already modifies significantly the sinusoidal wave into something that resembles an inverse cycloid.
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In addition to the substrate shown above, we also investigate embossing processes in order to manufacture novel substrates textures.


The figure above illustrates the embossing process; starting from a master substrate, a negative mold is formed in a polymer (PDMS), then the mold is brought into contact with a UV-sensible lacquer on a substrate. After curing by UV expose it is demolded and the initial texture is reproduced on the substrate.


The process permits, among other options, to reproduce textures that require high temperature processes like hot silver on any other substrate, for example poly-ethylene substrate.     This process reproduces the initial texture with high fidelity. The image above compares AFM surface morphologies of a ZnO master (left) and a replica (right). Features with size below 100 nm are well reproduced.
More information: articles K. Söderström and J. Escarré

Light scattering and absorption enhancement

Absorption enhancement in silicon by light scattering at textured interfaces has been proposed as early as 1982 by E. Yablonovitch. The idea is the following; take a slab of silicon with textured surface and shine weakly absorbed light on it. The transmitted light will be scattered at the surface roughness, some of it into angles above the Brewster angle. This part will bounce back and forth within the slab by total internal reflection. It would thus be trapped until its complete absorption, except that each bounce scatters a certain part out of the slab. The amount of light trapping thus depends on the angular width of this so-called escape cone. This can be related to an average light path enhancement of 4 times the square of the refractive index which is about 60 for silicon. There are a few underlying assumptions that are quite difficult to realize. Despite a significant amount of research over the years, the path enhancement in current cells is more likely to be between 20 and 30, and it is still an open question how Yablonovitch’s limit can be reached. Part of the work in the n-i-p group is devoted to the investigation of such fundamental questions, but always keeping in mind the application in real devices.

Plasmons and guided modes

A light beam that bounces back and forth within a slab by total internal reflection is not an unknown concept in optics, in fact this a simplified description of waveguides. An alternate view on light trapping is thus simply the question of how efficient we can couple an incoming plane wave to guided modes in the silicon layer. So far there appears little relation with plasmons, but remember that plasmon polaritons (as they should be called in this context) are just waves that propagate parallel to the interface in a multilayer structure; therefore the title of this section.


Waveguides are often discussed in terms of dispersion diagrams where the photon energy is plotted against the momentum p (or the wave vector k). For photons these two quantities are related by the speed of light, thus they are represented by straight lines in such a diagram. Note that perpendicular incidence would mean a line that falls on the energy axis. The indicated light lines represent grazing propagation parallel to the interfaces; there is a slight curvature because the refractive index depends on energy.
The diagram to the left shows the modal structure of a 200 nm thick a-Si “waveguide” between air and a zinc-oxide substrate. Such an asymmetric structure is known to have a cut-off, i.e. no guided wave can propagate at energies below 0.25 eV. Between 0.25 and 0.75 eV, only the fundamental mode of s-polarization (s0) can be guided, between 0.75 and 1.25 it can guide two modes (s0 and p0), and for higher energies more and more modes appear. All of these modes are confined between the light lines of silicon and zinc-oxide.
The diagram to the right shows a yet more unusual configuration consisting of a 200 nm thick silicon “waveguide” between a silver “cladding” and air. Most of the modes resemble the waveguide modes of the left image, only that they extend a little further to the left, going as far as the light line of air. Only the lowest energy mode behaves a little strange; it is p-polarized, it has no cut-off, and it runs below the light line of silicon. This particular p0 mode is called plasmon polariton. As mentioned above, it does not propagate in a guiding medium but on the interface between two media.
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Light trapping and guided modes

More evidence for the correspondence between light trapping and guided modes was produced in the following experiment: Solar cells were fabricated on a substrate textured with a 1D sinusoidal grating with known period. Such a periodicity folds the above diagrams into Brillouin zones and perpendicular incidence can be represented by vertical lines emerging from the centre of each Brillouin zone. Whenever the characteristics of guided modes and such a vertical line intersects, coupling becomes possible. The excitation of guided modes should thus be visible for specific energies in the form of sharp resonance phenomena. For the 1D grating there should be an additional dependence on the polarization of the incident light.


The figure shows the external quantum efficiencies of cells on the grating and a flat reference substrate. Note that there are sharp resonances between 600 and 750 nm. The observed polarization dependence and their variation with changes of the angle of incidence further support the idea of guided mode excitation.
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High efficiency triple junctions cell on innovative flat light scattering substrate

To reconcile the opposing requirements of layer growth and light scattering which need flat and rough interfaces, respectively, the separation of the light-scattering interface from the growth interface would be of high interest. With this new approach, light scattering is promoted by a textured layer with a low index of refraction filled with a material with a higher refractive index. This stack is then polished to obtain a flat substrate onto which the cell is grown.

We first fabricated this type of substrate as shown in the figure above and experimentally studied them in single-junction, thick µc-Si:H solar cells [Söderström Solmat 2012]. In second we have been able to fully exploit the potential of these substrates to lead to high efficiency solar cells by growing triple-junctions a-Si:H/µc-Si:H/µc-Si:H in nip configuration. This solar cell exhibits efficiencies of 13.7% in the initial state and 12.5% after degradation as shown in the below. The efficiency after degradation is among the highest reported to this date for purely silicon based n-i-p thin film solar cells [Söderström accepted for publication in JAP].


Key publications

[1] K. Söderström, G. Bugnon, F.-J. Haug, S. Nicolay and C. Ballif, Experimental study of flat light-scattering substrates in thin-film silicon solar cells, Solar Energy Materials and Solar Cells, vol. 101, p. 193-199, Elsevier, 2012
[2] K. Söderström, F.-J. Haug, J. Escarré, O. Cubero, C. Ballif, Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler, Applied Physics Letters 96, 213508, 2010
[3] T. Söderström, F.-J. Haug, X. Niquille, V. Terrazzoni, and C. Ballif, Asymmetric intermediate reflector for tandem micromorph thin film silicon solar cells, Appl. Phys. Lett., Vol 94 , pp. -063501, 2009
[4] F.-J. Haug, T. Söderström, O. Cubero, V. Terrazzoni-Daudrix, X. Niquille, S. Perregeaux, and C. Ballif, Periodic textures for enhanced current in thin film silicon solar cells, Presented at the MRS Spring Meeting, San Francisco, 2008
[5] T. Söderström, F.-J. Haug, V. Terrazzoni-Daudrix, X. Niquille, M. Python and C. Ballif, N/I buffer layer for substrate microcrystalline thin film silicon solar cell, Journal of Applied Physics, Vol 104, pp. -104505, 2008


New flexible solar cell technology in development

QDOTS imagesCAKXSY1K 8US-based Natcore Technology with research partner Rice University has developed what it describes as an inorganic flexible thin film solar cell by solution processes.

The production process for the cells has the potential to move to a roll-to-roll manufacturing lineThe device was made using Natcore’s liquid phase deposition (LPD) process. A cadmium/selenium (CdSe) absorber layer was grown onto a back contact substrate based on single-walled carbon nanotubes (SWNT). LPD was also used to grow a copper/selenium (CuSe) window layer onto which silver contacts were deposited. The resulting solar device shows potential for this process to make a flexible solar cell, free of high temperature semiconductor processing.

With further work the process has potential for roll-to-roll (R2R) production. The company’s R&D centre is situated near a former Kodak R2R photo film plant in Rochester, New York state.

Black silicon

A few years ago Natcore Technologies began attracting interest for its LPD technology in an application for improving the light absorption properties of multi-crystalline silicon cells, known as black silicon solar cells. LPD, developed at Rice University, makes it possible to grow a wide range of inorganic materials on a range of substrates using a room-temperature, environmentally friendly chemical bath.

In the flexible solar cell work nanotubes were used for a back contact embedded into the absorber layer, reducing the diffusion length to the back contact, to potentially lead to higher efficiency, because of a lower percentage of hole electron recombination.

There is potential to make the development compatible with the company’s multi-junction tandem solar cell technology to enable high efficiency extremely thin and flexible solar cells.

Other companies bringing to market high efficiency flexible thin film solar cells include Alta Devices, which has developed a process of growing very thin layers of solar cell materials on gallium arsenide (GaAs) wafers. The California-based company has been working on a GaAs solar cell technology for military and other applications, targeting the commercial unmanned aerial vehicle (UAE) market where very lightweight and efficient solar cells on the wings of craft can extend flight times without adding extra weight.

Dots, rods and tetrapods: CdSe gets in shape

Jan 31, 2011

QDOTS imagesCAKXSY1K 8Researchers from the South China University of Technology have presented a surfactant-free recipe for fabricating high-quality CdSe nanocrystals (NCs). The morphology, which includes irregular dots, rods, tetrapods and sphere-shapes, can be controlled easily by varying the experimental conditions. More importantly, the preparation techniques involved are simple, low-cost and can be used to fabricate other II-VI group semiconductor NCs.

CdSe Nanocrystals

CdSe nanocrystals: dots, nanorods and tetrapods

The CdSe NCs were produced with a fixed Cd/Se molar ratio of 2:1 and using 2.32 g of trioctylphosphine oxide (TOPO); at the same time, all the trioctylphosphine selenide (TOPSe) injections were kept at 1 ml, but with different concentrations. No other ligands were used in the case study.

Homogeneous CdSe NCs with different morphology were obtained under such experimental conditions. The sample quality (size distribution, optical properties, tetrapod selectivity) is as good as that of the best CdSe NCs synthesized by using extra ligands. As for the growth mechanism, we believed that the decomposition of TOPSe and cadmium myristate at a temperature of 240 or 300 °C would also supply in situ-generated TOP and myristic acid in the reaction mixture, which affected the anisotropic growth of CdSe NCs.

To further investigate the application of this surfactant-free recipe, the group is now optimizing the experimental conditions and has found that well controlled morphology of CdTe and/or CdSexTe1–x NCs can also be successfully fabricated.

Thanks to the easily controllable NC-growth kinetics, such a synthesis route is very promising for low cost, large-scale preparation of CdSe and CdTe NCs for application in solution-processed thin-film solar cells.

More information can be found in the journal Nanotechnology.

About the author

The study was funded by the National Natural Science Foundation of China (nos. 50703012, 50773023 and 50990065), the National Basic Research Program of China (973 program no. 2009CB623600) and SCUT grant (no. 2009ZZ0003). The experiments were performed at the Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional Materials group. Hongmei Liu is a PhD student in materials science and holds a bachelors degree in chemistry. Currently she is exploring the fabrication of high-quality semiconductor nanostructures, together with the measurement and application of the resulting nanostructures in the field of solution processed thin-film solar cell systems and other nano-electronic devices.

Major Breakthroughs in Solar Technology for 2013

QDOTS imagesCAKXSY1K 8Despite a tough market leading to widespread cost reductions and negative returns for many operators in the photovoltaic sector in 2012, solar technology nonetheless took major strides and achieved a number of landmark breakthroughs in key research areas.


In materials research, the North Carolina State University (NCSU) in Raleigh, North Carolina used cutting-edge nanotechnology to develop slimmer and more affordable solar cells.

The cells are comprised of sandwiched nanostructures which not only cut down on material usage and expenditures but also improve solar absorption and raise conversion efficiency.

As an added bonus, the manufacturing processes for the new technology are compatible with techniques currently employed throughout the industry for the production of thin-film solar cells.

In terms of government-funded initiatives, the National Renewable Energy Laboratory (NREL), a research arm of the US Department of Energy, teamed up with Natcore Technology to create the most absorbent solar cell ever devised, capable of capturing some 99.7 per cent of available sunlight.

 The new technology resulting from this collaborative effort between the government and private sectors could reduce the cost of solar cells by around two to three per cent while lifting energy output by up to 10 per cent. The black silicon used for the cells is also far cheaper than standard anti-reflection technologies.
nanotechnology-solar-cells-1A key area of research for 2012 was improved storage techniques for renewable energies, with scientists from Houston’s Rice University in Texas developing a remarkable “paintable” battery which can be applied to any tractable surface. The rechargeable battery opens a new vista of possibilities for the convenient storage of solar energy.

In the field of flexible thin-film cells, a joint undertaking between scientists from Canada and Saudi Arabia smashed the world record for solar efficiency, surpassing the ousted place holder by a staggering 37 per cent. The colloidal quantum dot (CQD) thin-film solar cell, developed by scientists from Canada’s University of Toronto and the King Abdullah University of Science & Technology in Saudi Arabia, achieved a world-record efficiency level of seven per cent via the application of a “hybrid passivation scheme.”

The new technology could potentially be applied to the cheap, mass manufacture of thin-film solar cells by using flexible substrates to “print” the devices in a process akin to that traditionally employed for the production of newspapers.paintable-battery-rice-university