Research Proposal On Nanostructure Based PV Cell Optimization With Futuristic Trends

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Abstract
The paper introduces a brief review on solar cells and different techniques that have been used to increase the efficiency. The discussion is then directed towards the current and cutting edge challenges on the issues related to fabrication and the increase in the solar cell efficiency. Primary focus is on the use of nanostructures in the context of solar cell fabrication and performance. The paper specifically informs on the use of techniques which focus on light absorption and trapping . One such technique which is reported in the paper is the fabrication of transparent polymer templates for nanostructure amorphous silicon photovoltaic using low cost nano-imprint lithography of polydimethylsiloxane. The template contains a square two dimensional array of high aspect ratio nano-holes (300 nm diameter by 1 µm deep holes) on a 500 × 500 nm pitch. A 100 nm thick layer of a-Si:H is deposited on the template surface resulting in a periodically nano-structured film. Another technique used for increasing the absorption of light and optimizing the photo harvesting in the design is by the help of high-index nanostructures (Brongersma 2014). This is so as these nanostructures offer new ways of manipulating and harvesting light at a sub-wavelength scale. In this paper this technique is elaborated by illustrating the recent developments in nanoscale wires, particles and voids’ strong support of optical resonance which enhance/effectively control light absorption and scattering process.
In the futuristic development context a discussion about new material ( Perovskite solar cell ) is made an examination is done on solar cell fabrication using this material and the challenges thereof. A new technique or proposal of a new way and new idea is offered to synthesis Perovskite material to achieve high efficiency solar cells. It is shown that this is an easy fabricating method involving less cost and resulting into a high efficiency solar cell.
Keywords: Hydrogenated amorphous silicon, templated nanofabrication, nanoimprint lithography, nanoembossing, light trapping, grating coupling

Introduction

Compared to crystalline silicon based solar cells, thin-film hydrogenated amorphous silicon (a-Si:H) solar cells have the potential advantages of less raw material usage and lower fabrication costs as well as the benefits of high flexibility and light weight. However, the efficiency of single-layer, thin-film a-Si:H solar cells is relatively low. Several efforts have been made to increase single layer cell efficiency by improving the light trapping of devices (Wang 2012).
Thin-film a-Si:H solar cells display a very high refractive index property which makes it possible to reflect back a high proportion of the incident light. Specifically, thin-film a-Si:H Nanocone or thin-film a-Si:H NC arrays provide a wide band of wavelengths of light incident to be refracted. This reduces reflection of incident light having a wide range or broad-band wavelengths thereby increasing the percentage of light harvesting. This drastically improves the efficiency of solar cell using thin-film a-Si:H NC arrays. This is a clear improvement over the previously used quarter wavelength antireflection Coating (ARC) which suppresses the reflection of only restricted wavelengths and only for particular angle of incidence of light. (Zhu 2009).

Absorption Enhancements with Amorphous Silicon Nanowire & Nanocone Arrays

Hydrogenated Amorphous silicon (a-Si:H) has it crucial application in solar Cells. The thin silicon film could be deposited on materials like glass, plastic and even steel, thereby making this compound a very promising agent in fabricating most advanced and performance optimized solar cells. This material allows very thin 1 μm layer which brings most effective light and optimizes the cell effectiveness (Zhu 2009). This thin layer deposition is illustrated in the figure below:
The fabrication process used in the deposition was hot wire chemical vapour deposition (HWCVD) on an indium-tin-oxide (ITO) together with Langmuir-Blodgett method. (Zhu et. al.). If one wants to see that how delicate and fine these layers deposited appear, one may utilize the scanning electron microscope (SEM) images as shown in the below panel:
As an improvement to the thin film deposition, the a-Si:H NC arrays show the good anti-reflective properties yielding the best optimization in absorptions. However, the best results are obtained from the nanocone deposition arrays and the produce the best optimization. The comparative graphs are shown, along with the images in the below figure:

Cutting Edge Technologies

In this paper the cutting edge techniques of improving light absorption by the unique fabricating techniques are discussed; here the unique fabrication process is highlighted which makes it possible to deposit a vertical thin-film a-Si:H NC arrays. These array of nanocones or even nanowires are deposited by using a wafer scale Langmuir Blodgett assembly and etching technique (Karaagac 2013).
ZnO has a wide band gap of 3.37eV at room temperature, it also has a large excitation binding energy which is about 60eV. Besides, ZnO has a high breakdown strength and cohesion strength which gives a good physical ability. The mobility and the exaction stability of ZnO are also very high, so that ZnO nanowires solar cell has a good and stable performance (Wang 2009).
Besides the analysis of optical absorption, several other aspects have been considered in this nanostructure design: 1) The template can be cost-effectively incorporated into a single junction thin-film PV device using conventional PV film deposition techniques; 2) Fast, scalable production of the template using roll-to-roll techniques16 can be applied on inexpensive polymer substrates; 3) The patterned photovoltaic approach, when implemented at the appropriate dimensions, could have an impact on using PV materials characterized by high defect densities including organic, quantum dot, and oxide PV systems.
The sample is then loaded into STS Multiplex ICP-RIE with SF6 and C4F8 flow for directional etching. The exposed regions of the silicon surface are etched down 1 µm. After etching, the sample is put into the PE2 oxygen plasma etcher to remove most of the e-beam resist. The sample is then treated in piranha solution to thoroughly clean and to add hydroxyl groups on the surface.

Nanostructures Orientation: Deposition at the front surface of a cell

Best way to orient Nanostructures in order to optimize the coupling of light to the underlying semiconductor layer is the deposition at the front surface of the PV cell. Transparent conducting oxides like ZnO used as electrical conductive material is roughened in the process to make its refractive index higher (Wang 2009). Higher refractive index implies better light trapping and reduction of light reflection (Wang 2012). This property has been a key concept in increasing optimization, and the latest techniques of fabrication exploit this property in the process. Since this process is scalable, so by careful engineering of shape or size, the flow of light to the underlying semi-conductor layer can be further optimized. This is illustrated in the blow figure:
The above figure shows the experiment realizing anti reflection coating having resonance hi index nano-structure. Figure (a) above shows the SEM of the arrays having sub wavelength nanocone on the a-Si-PV Cell. Figure (b) above shows the SEM images of 250 nm Dia SiMie scatterer over Si-substrate fabricated by substrate conformal imprint lithography process. The scale bar used is 500nm. Figure (c) above displays the Full field em simulations of electric field intensity distributions of MieResonance produced over Si-Pillar as displayed in the figure (b) with plane dia.150nm & height 100nm. The outlines of Si-Pillar and the substrate are shown by white dashed line. In Figure (d), diagram of the ultra thin film Si PV cell which features 2 side nano grating designs having front & back surface of PV cell optimized separately to fabricate anti reflection & light trappings. In Figure (e) above, spectrum absorptions of optimal structures as per figure (d) above, having 1 corresponding to the incident photons which are absorbed; these optimal structures yield a PV current of 34.6mA cm–2 having absorptions improvement near to Yablonovitchlimit In Figure (f) above SEM cross section imagery of mono layer spherical-Si-nanoshells on the quartz substrate having scale bar up to 300 nm. Overall the figure shows the Full field em simulations for electric field distributions in Si-nano-shell which show the significance of excitations of whispering-gallery mode to improve absorbing of the sunlight(Brongersma 2014).
The figure (d) above shows the optimal structures which yield a PV current of 34.6mA cm–2 having absorptions improvement near to Yablonovitc_limit. Absorbing light from extremely thin layers need the broad band anti reflection coating. Different design considerations and design geometries help to optimize the light absorption. The techniques help to absorb broad band light and enable the PV current to approach the Yablonovitch_limit. Nanocones, Nanowires and other geometric patterns have been used an research to optimizes the absorption. In this endeavor, a specific fabrication technique uses nancones as the basic building blocks (Karaagac 2013). Below figure shows the silicon layer and the gratings at the different sides of the layer and even an ungrated layer:
This figure shows the 3-d imagery of the silicon in the air structure. The dimensions show the optimized structures – either 2 sided or 1 sided layers. The optimized thickness is of 2 μm. The four figures above have different absorption spectrums. This spectrum is shown in the below figure
The figure above also shows the comparisons. As shown clearly, the bar a figure s is very close to the Yablonovitch_limit of 35.5. This shows that the short circuit current obtained by a is very highly optimized. The PV currents generated by the 4 type of grating pattern is shown in the below figure. The line represents the optimal Yablonovitch_limit and the graph itself shows the relation between the thicknesses of the structure:

Photovoltaic material deposition

Patterned and un-patterned PDMS layer regions on a glass slide are simultaneously coated with 100 nm amorphous silicon by plasma enhanced chemical vapor deposition (PECVD). The deposition is done in an MVSystems PECVD reactor (MVSystems Inc.) at 200 °C from pure silane gas. The amorphous silicon produced in this study results in a typical hydrogen concentration of ~10%; the optical bandgap is of 1.7-1.8 eV. For planar amorphous silicon layers of 100 nm thickness, the largest gain in absorption is in the wavelength range of 400~540 nm as the absorption coefficient of the material becomes > 1E5 /cm in this wavelength range. For sample description, the area outside the nano-structured region is labeled “planar” sample, while the area inside nano-structured template is labeled as a “patterned” sample, both samples consisted of a glass/PDMS/a-Si:H stack. The 100 nm layer thickness is chosen because in amorphous silicon devices the Staebler-Wronski degradation limits the active layer thickness for highly efficient carrier extraction to approximately 100 nm.19 At this thickness, however, planar layers capture little of the light at wavelengths above 600 nm. The diameter of the nanoholes is reduced by around 100 nm on the surface by the amorphous silicon deposition. In the cross-section image processed by focused ion beam (FIB), 100 nm of amorphous silicon (which shows up as black) can be seen deposited mostly at the top of the posts with the coating continuing down the posts approximately 150 nm. (Donnelly 2014).

Optical measurement setup

Reflectance and transmission as a function of wavelength are measured on both patterned and planar samples. A tungsten lamp is used as the light source. The light beam is collimated with a lens tube. Then the light passed through an aperture to reduce the beam diameter to approximately 3 mm, which is small enough to easily align with the 5 mm × 5 mm nano-patterned area. A two-inch diameter integrating sphere with a 99% reflectance coating (Thorlabs, IS236A-4) is used for transmission, reflection, and incident beam intensity measurements. A Czerny-Turner CCD spectrometer (Science-Surplus, Compact Fiber Coupled CCD Spectrometer, 365 - 1100 nm) is fiber coupled to the detector port of the integrating sphere.
For the total beam intensity measurements, the entrance port of the integrating sphere is opened but the sample port at 180º is closed. The incident beam passed through the center of the entrance port striking the diffuse white reflective surface on the sample port scattering evenly around the sphere. The obtained spectrum is used as the “total” beam intensity to compare with the transmission and reflection spectra. For transmission measurements the sample is placed perpendicular to the incident beam in front of the entrance port. For reflection measurements the 180˚ port of the integrating sphere (opposite the entrance port) is opened and the sample, tilted at a 5˚ angle relative to the perpendicular, is placed up against this port. The tilt on the sample insures that light reflected specularly from the sample hits the side of the integrating sphere. Without it specular light could leave through the entrance port and not reach the detector.

Summary

In the domain of PV cell technology, efficiency is a critical parameter and most of the research endeavours focus on the efforts to increase the efficacy and effectiveness of the cell. In this paper initially a brief review on various state-of-art solar cell technologies is given, followed by discussing different methodologies and techniques that have been used to increase the efficiency of the cell. Those aspects are highlighted which are current and offer the cutting edge challenges in PV cell design.
Hydrogenated Amorphous silicon (a-Si:H) has it imporant application in PV Cells. The ultra thin silicon film can be deposited on the material like glass, plastic including stainless-steel. So a-Si:H comes up as a very promising compound to be used for fabricating most advanced and hi-performance optimized PV cell. This is so as this material can allow extremely thin 1 μm layer which provides the most effective light absorption and optimizes the cell effectiveness. The transmittance (relative to total beam) spectra show a significant difference between patterned and planar samples at longer wavelengths (600 nm – 850 nm). Especially around 700 nm - 750 nm, the patterned sample transmits 40 – 50% less light than the planar sample. Lower light reflection of the patterned sample is also observed at 400 nm – 650 nm wavelengths. There are many sharper optical features seen between 600 and 800 nm, and these features are attributed to complex optical resonances of our sub-wavelength periodic structures.
The absorbance spectra are calculated from transmittance and reflectance spectra. The spectra show that the patterned area has higher absorption in most of the visible spectrum. At longer wavelengths (650 nm – 800 nm), where the absorption of the planar area is very low, the patterned area has significantly more absorption. The weighted absorbance spectra are calculated from the absorbance spectra weighted by the spectral irradiance AM 1.5 spectra. The integrated weighted absorbance for patterned and planar samples shows that the patterned sample captured 55% of spectral irradiance energy, while the planar sample only got 35%. Moreover, this 20% improvement might be further increased by optimizing the amorphous silicon deposition profile.
Optimization is also increased by the use of transparent conducting oxides like ZnO used as electrical conductive material is roughened in the process to make its refractive index higher. Higher refractive index implies better light trapping and reduction of light reflection (Thitima 2009). This property has been a key concept in increasing optimization, and the latest techniques of fabrication exploit this property in the process.The increased absorption at longer wavelengths is not due to lower reflection; that is, the effect is not simply antireflection. In fact at longer wavelengths (600 nm - 800nm) the reflection of patterned samples is slightly higher than the reflection of planar samples. Nevertheless, at these long wavelengths the patterned sample absorption is much higher. Since the only absorbing material in the sample is amorphous silicon, this difference results from increased absorption in the silicon. Since there is the same amount of silicon per area as the planar sample, the effective light path through the silicon must be longer. This is consistent with coupling of the light into in-plane guided modes as proposed by other researchers at grating periods on this scale.
However in prior work, direct measurement of absorption by the semiconductor has not been possible due to the presence of nano-patterned metal layers. The results here indicate that the nano-patterned semiconductor itself acts as an effective coupling grating.
Absorption of light from extremely thin layers need the broad band anti reflection coating. Different design considerations and design geometries help to optimize the light absorption. The techniques help to absorb broad band light and enable the PV current to approach the Yablonovitch_limit. Nanocones, Nanowires and other geometric patterns have been used an research to optimizes the absorption. In this endeavor, a specific fabrication technique uses nancones as the basic building blocks. Light capture is critical at the long wavelength end of the visible spectrum because a-Si has such a very small absorption coefficient there. This is also in contrast to random textures that increase absorption non-resonantly resulting in a smaller absorption gain where it is most needed.

Perovskite based furutristic trends

The excellent advancement on inventions of the new material, for instance, Perovskite solar cell, is discussed along with its fabrication and challenges. The synthesis of Perovskite material to achieve high efficiency solar cells are explained in adequate details. Encouraging aspects for this material which leads to easy fabrication, lower costs and high efficiency solar cell are demonstrated. The method of fabrication which leads to this optimization is discussed at length. An idea, some analysis, as well as appropriate details are given with respect to a new way in which the PV cells can be fabricated.
The focus lies on the star performer which has revolutionized the fabrication of solar cells in terms of light harvesting and efficient conversion of the energy. Perovskite (CH3NH3PbI3) has optimized the light absorption and conversion to energy and it has revolutionized the fabrication of the thin-film solar cells. The unique characteristics of Perovskite cells which has led to integrate this material as well as technology in the mainstream solar cell design is demonstrated in this paper. An interesting as aspect of this technology is that Perovskite cells can be used in tandem with the other cells, as the former has unique property as a very high P V absorber. Whilst, Perovskite cells can act as light absorber, so these cell can also work in tandem with other cells which have more remarkable energy conversion. This creates a very high performance hybrid technology in solar cells and has the potential to evolve as a cost effective solar power. The cost effective solar power can lead to a wider adaptability of this renewable source of energy and thus makes a very interesting topic of research. (Chen 2015).
Another aspect of Perovskite cells which is demonstrated in this paper is the capability of Perovskite layer to integrate with the bulk-Hetero-Junction (BHJ) photovoltaic layer. This leads to a very efficient integration yielding a very high power conversion. This Power conversion efficiency is up to 20 %, which is regarded as indeed very high efficiency in the domain of solar cells. This means that at least 20 % of solar power is being successfully converted to energy. The below figure shows Efficiency close to 20%:
Another advantage of this integrated layer ( perovskite / BHJ ) is that it provides a very high short circuit density (JSC). The JSC is around 21.2 mA cm−2 and is much higher in comparison to traditional layer devices which can come up to only 19.3 mA cm−2. A histogram using 100 cells with hybrid layer is shown in the below figure. The JSC achieved is around 21.2 mA cm−2 whereas most of the cells have JSC around 20 mA cm−2 , which is quite an optimistic result. (Chen 2015).
The importance and application of very high short circuit density helps in drastically increasing the cell efficiency with the scope of further development and research in this area. A summary recommendation is proposed in the following section along with future road map to next level enhancement. However, JSc alone cannot provide the optimized energy harvesting. To produce the efficiency to the level of 25% or higher needs innovative ways of fabrication using Perovskite material.
I feel that the latest trend in innovation and harvesting maximum energy using Perovskite based cells is to use the hybrid type. A combination of the Perovskite along with silicon cell will give a number of benefits. For one, it will provide the best of the both worlds as Perovskite will help in absorption and the other best in class transformation for the purpose of harvesting. A new sandwich type of cell, where Perovskite can make an ideal sandwich with the Silicon cell can produce maximum optimization. The new research is expected to be drawn on similar lines or at least it is recommended that the new research can be undertaken to test this hypothesis.
So the hypothesis presented for the Perovskite based cell is that the best efficiency even up to the level beyond 25% can be achieved by using the sandwich cell. In effect 3 types of sandwiches should be compared and benchmarked – Perovskite- Perovskite, Perovskite-Si & Si-Si. It is felt that the best efficiency will be provided by an ideal combination of PerovskiteSi sandwich, as Perovskite- Perovskite alone can lose in terms of harvesting. So this should be the ideal topic of the next level research.
Another interesting aspect with Perovskite is its potential use in the lighting solutions. Perovskite material slab can be used to light the rooms, buildings and offices as required. The innovation required needs a slight deviation as the Perovskite slab has the potential to release light if the electron is passed through it. But it works best with the hybrid solution as I have been highlighting. Even the faint source of incidental electron passed through the slab can produce remarkable results for the lighting solution. However this source can provide by either a conventional cell or another Perovskite based cell. The overall solution can provide a an ideal solution with very high efficiency.
The development and research in those lines can result a dramatically cheap solution as Perovskite material in itself comes with a very low cost as compared to the conventional Si based solar cells. This cost effective solution can provide a very wide acceptability and feasibility for a widespread solar lighting solution for the domestic as well as commercial usage.

References

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