Improving the
efficiency of
Solar cells
using
nanotechnology
Jimmel Stewart
PH751
Submitted 11/05/2012
Jimmel Stewart (js617)
PH751
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Table of Contents
Abstract ...............................................................................................................................................................3
Introduction ................................................................................................................................................. 4-5
Review of Paper 1 ...................................................................................................................................... 6-7
Review of Paper 2 .................................................................................................................................... 8-13
Review of Paper 3 ................................................................................................................................. 14-16
Review of Paper 4 ................................................................................................................................. 17-20
Review of Paper 5 ................................................................................................................................. 21-24
Discussion/Conclusion .............................................................................................................................. 25
References ...................................................................................................................................................... 26
Jimmel Stewart (js617)
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Abstract
Solar cells are of an increasing interest not only to scientists and researchers but also the general
public. It is becoming increasingly familiar to see solar cells on top of houses. Solar cells supply a
renewable, clean energy resource which is currently being reviewed to improve their efficiency. The
play a crucial role to future of earth due to the increasing demand we have for fuel, fossil fuels in
particular. This poses a problem as they will eventually run out, and if they do before we have
modified and improved the current renewable energy resources we could be turning back to the age
of no electricity. We have become a society in which electricity is has become central, we use it for
entertainment, power for lighting and for every day appliances which make our lives much easier.
The purpose of this review is to outline the current understanding of Solar cell efficiency. This is
done by looking at how light energy is captured and converted to electricity. The review of these
papers looks at some of the disadvantages and how they can be solved, as well as looking at what
has been successful for improving them. This review gives an insight into the recent developments of
the solar cell. The review of one paper explores the limits for photovoltaic (PV) energy conversion by
looking at new cell designs and what limits they present, with the intent of trying to overcome these.
They look at scale-up limits, limits of current efficiency and energy Loss which are some of the
current disadvantages faced by the newer solar cells. Another review looks at increasing the
efficiency of polymer solar cells by silicon nanowires. They have been successfully used to produce
hybrid organic/inorganic solar cells with a high efficiency being achieved from them. This opens
doors for improvement of not only solar cell efficiency but of nanowire technology. If nanowires
could be improved further then could efficiency of the solar cell also be increased? It is a new
avenue to approach. A third review also looks at nanowires but also look at using not only silicon but
ZnO nanotubes. It appears that ZnO has a greater effect on the cell than silicon does. The paper
focuses on hybrid nanostructure heterojunction solar cells fabricated using vertically aligned ZnO
nanotubes grown on reduced grapheme oxide. The larger interfacial area was responsible for this
improving the power output. They also have the advantage that they use lower temperatures than
other methods resulting in low cost of materials. A fourth paper looks deeper into using silicon
nanowires assembling them with vertical and zigzag structures. This work not only improved solar
cells but also the fields of opto-electrics, photonics and photovoltaic’s. The final paper to be
reviewed looks at temperature as a limit by looking into using active cooling systems for photovolvic
modules. This cooling technique proved useful as it did improve the efficiency. And so it is hoped
that by reading and researching scientists can create a clean, renewable and highly efficient solar cell
for future generations.
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Introduction
The conversion of sunlight into electricity is an abundant, clean and renewable source of
energy. It has the means and capability to improve and provide humankind with a solution
to the increasing problem of dependence upon and limited amounts of natural fossil fuel
resources, which is having an effect on the levels of CO2 emissions.
The solar cell began with Alexander Edmond Becquerel, who was the first person to report
the photovoltaic effect in 1839. Then Charles Fritts developed the first actual Solar cell using
selenium in 1883. Unfortunately Fritts solar cells were unsuccessful, as they had a very low
efficiency rate of converting solar power to electricity. Decades later in the 1950s Gerald
Pearson, Calvin Fuller and Daryl Chapin discovered the use of silicon as a semiconductor,
which led on to the first silicon solar cells.
This review will concentrate on the photovoltaic effect of solar cells with an emphasis on
the role of efficiency. The conversion of sunlight to energy is a straight forward process, it is
made up of packets of energy called photons; it is the power that makes almost everything
happen on earth, it brightens up the darkest parts of space, powers our body and helps
plants to grow.
The reason why solar cells are of interest to us today is due to the increase of humankinds
demands and need for a better life style, that has resulted in climate change.
Nanotechnology is of great interest has it successfully allowed us to improve solar cells. The
questions surrounding the topic include things such as looking at advantages of using
nanotechnology including the possibilities and limitations of solar cells in general, as well
which solar cells have the greatest potential, i.e. which of the solar cells has reported the
greatest efficiency to date and how to develop nanostructures to further improve efficiency.
I have chosen the following papers, ‘Increasing the efficiency of polymer solar cells by silicon
Nanowires’, ‘Nanostructed Organic and Hybrid Solar cells,’ ‘Porphyrin-sensitized Solar cells
with Cobalt (II/III) – Based Redox Electrolyte Exceed 12 Percent Efficiency’ and the paper
‘Assessing Possibilities and Limits for Solar Cells.’
The papers that are being reviewed, attempt to address the key questions that arise when
explaining the potential of solar cells. Choosing papers that have been written by different
authors allow the reader the opportunity to analyse the different issues and perspectives
addressed by different authors. There are many questions that need to be answered to
solve the problem of the solar cell; for example how can one control the band width to
access the full potential of a given solar cell? How can one improve the efficiency of
photoinduced charge separation and transport of charge carriers across the
nanoassemblies? How can we increase their efficiency and lower their cost to make them
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more accessible to all? These are few of the questions tackled in the review in order to
establish a clearer picture on some of the ways that could improve the awareness and
importance of solar cells with the aid of nanotechnology to reach the goals required.
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Review of Paper 1
Assessing Possibilities and Limits for Solar Cells [1]
Pabitra K. Nayak, Juan Bisquert and David Cahen
Received 2011 March 7; accepted 2011 May 5
This paper looks at the solar cell efficiencies and their future. The authors of the paper
suggest that there are several different criteria such as the type of cell and the performance
of the module data. Therefore if the efficiency of today’s solar cells is to improve we should
consider looking into the cell and analysing its performance. From analysing the data we can
see any possible limitations to progress. Why this is important is because we still use the
Shockley and Queisser (SQ) limits that are based on single absorber solar cell calculation and
if researchers are to go beyond these SQ limits this can only be done by avoiding these limits
with the use of smart optics or optoelectronics.
The authors explain that coming up with the best criteria and reasons for failures will give
rise to better types of cells (Organic, DSSC), thus paving the way for future predictions. The
main question that is addressed in this paper is what are the limits for photovoltaic (PV)
energy conversions? The paper mentions that SQ obtained there limits with the use of one
absorber, absorbing the radiation in or by analysing the radiation that left the absorber.
What was remarkable about this experiment is that the incident solar spectrum, wave
shifting techniques, the need for non linear optical or electronic effects was not needed.
Therefore the SQ limit provides researchers today with the physics and thermodynamic
understanding of the solar cells. There have been significant amounts of research into
finding practical ways to manipulate the light captured and improve the SQ limit application.
Therefore by finding limits that go beyond the SQ limit it should provide new parameters to
help different solar cell type’s progress further. The authors then goes onto explain possible
situations that have arisen from different solar cell experimental data.
The first step when looking at the solar cells experimental data was to determine which the
best efficient cell on the commercial market is. The second step involved investigating the
short circuit current (JSC) and the maximum theoretical current (JSCMAX) of the highest
reported laboratory cell. The final step looks at the open circuit voltage (VOC) which has
given the highest recorded value of the band gap or the optical absorption edge energy so
far.
The experimental data for the organic solar cells show that the absorption edge has a higher
energy that can be determined by the quasi Fermi levels. However the researchers
attempted to use black dye which proved to be unsuccessful due to its poor efficiency.
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What the result showed is that there are discrepancies involved with the crystalline, the
CIGS cells, the Si:H and the CdTe. This is due to the low efficiencies that are produced. The
problem with the last two is that for the Si:H the mobility of the amorphous material is low.
The main problem with the CdTe, are the grain boundaries.
The organic and dye sensitive solar cell (DSSC) produced efficiencies lower than some of the
cells that they were competing against. Therefore the question asked is that do these types
of cells have a flaw in there development, or is there a key factor that is missing in the way
that they are produced. What was found is that the carriers’ mobility was significantly low in
organic materials. However in a DSSC the carrier mobility for the nanocrystalline TiO 2 is
higher and shows that it is favoured over other types of organic cells.
When looking at the electronic charge of the carriers, the authors noticed that the disorders
in the absorber need to be filter through, so that the recombination could occur, this
method has its disadvantages due to the absorbers width being limited. Therefore because
of this disadvantage the carriers’ thickness and the power efficiency are lowered.
The tails gate section of the absorption edge uses the least amount of photon absorption,
due to different materials. The energies cross section is lower than the absorption range.
Research has gone into many cells, in particular the organic cells. the key issue with the
organic cell is the problems with the transportation, and this could be due to impurities.
This paper has demonstrated how important it is to look at the limitations to help identify
how the efficiency of the solar cell could be improved. The authors explain that for the new
generation of solar cells to be successful the need to move beyond the SQ limitation
equation is a priory. When looking at organic molecular cells what was found in these types
of cells is that the dielectric constant was low and the use of non crystalline or molecular
systems showed a energy loss of 500 – 750 meV compared with the conventional PV cell.
The author’s solution to improving the energy loss is to not use low energy EG absorbers.
The use of a 1.4 eV absorber may be beneficial as it can be placed onto pure SQ for more
investigation. It has also been noted that the development of the PV cells are misguided
when considering molecular cells. Therefore considering the types of material being used, is
a key factor to future choices and the direction that researchers take.
Jimmel Stewart (js617)
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Review of Paper 2
Increasing the Efficiency of Polymer Solar Cells by Silicon Nanowires
[2]
B Eisenhawer, S Sensfuss, V Sivakov, M Pietsch, G Andrä and F Falk
Received 2011 May 9; accepted 2011 July 5
Can mixing nanowires into a polymer blend and subsequently coating the
polymer/nanowires onto a substrate; produce nanowire enhanced polymer solar cells in a
reel to reel process? This is the question that the paper tries to answer.
The authors begin by explaining that the polymer based organic solar cells; have caught the
attention of researchers due to the low cost and potential to be prepared on flexible
substrates. Although bulk heterojunction cell (BCJ), that was introduced in 1995 is the
leading technique that can be applied to different substrates, it has produced low
efficiencies due to its poor carrier mobility in the materials for the electrons and holes. This
disadvantage affects the way that the light is absorbed in the cell and enhances the
resistance allowing the efficiency to decrease.
Many have attempted to solve these issues by introducing inorganic nanostructured
conductor material into the cells, with the aim to adjust the carriers distance. Therefore
improving the time taken for the electrons to travel and increase the conductance
properties. Therefore the idea of nanowires had potential as it provides a large surface area
that can be connected to the polymer and a low volume which improves the conducting
pathway.
In this paper P3HT: [60]-PCBM (poly (3-hexylthio-phene): [6, 6]-phenyl-C61-butanoic acid
methyl ester) are the main types of materials being used. The paper goes on to explain that
the introduction of silicon nanowires (SiNW) increased the short circuit current density from
7.17 to 11.6 Ma cm-1 while the device efficiency improved from 1.2% to 1.9%. This showed
that the efficiency of the device was low, and could have been due to two reasons. The first
is that a low open circuit voltage was detected in the cell and the second could have been
due to the thickness of the polymer layer.
The author mentions that even though there has been a small increase in the short current
density and the device efficiency. The potential for the SiNW in polymer solar cells shows
great promise as a conducting material. Other researchers have gone on to use different
materials; these include the use of ZnO nanofibers which produced an efficiency of 2.03%
and TiO2 NWs which increased the efficiency to 3.2%. Both materials were used as an
electrode, where the latter increased its relative improvement by 15%. The author discusses
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another paper that focused on Indium tin oxide (ITO) nanopillar electrodes in
polymer/fullerene solar cells. It explained that the nanopillar ITO electrode mixed with the
BHJ layer increased the devices properties to 2.5% without the microstructured electrode.
Researchers working on the P3HT: [60]-PCBM decided to add SiNWs and replace the PCBM.
The efficiency decreased and researchers realised that pressing SiNWs into the polymer
would not be suitable when used on solar cells that would be applied to flexible substrates.
Therefore the authors of this paper look to find a route that will produce reel to reel hybrid
polymer solar cells by mixing and applying the nanowires on to a substrate. This showed an
improvement in the efficiency of the solar cells due to the doping of the SiNWs.
In this paper the authors give a brief description of the key points of the experiment. These
include realisation of SiNWs and realisation of solar cells. They begin by explaining how
SiNWs were grown in a homebuilt cold wall ultra high vacuum (UHV) chemical vapour
deposition (CVD) reactor and etched with the use of an electroless silver catalyst. This
process was done with the use of a gold collides immobilised onto the Si (111) wafer using
3- aminopropyltriethoxysilane (APTES).Once this has been accomplished the wafer is placed
into the 5% hydrogen fluoride, to remove the organic material and the oxide that is on the
substrate.
The authors then explain that once this method is done, the wafer is placed into the growth
chamber, where it will undergo a heat of 600 ˚C and put under a 2 mbar pressure. The
precursors that are used are as follows; 5 sccm Ar, 5 sccm SiH4 and 0.05 sccm PH3 (2% in He).
The SiNWs are grown through a VLS (Vapour Liquid Solid) method that is done over a period
of 20 minutes and results in the SiNWs growing to a length of approximately 5 µm. The next
step that the authors undertake is to remove the gold (Au) droplets using a solution known
as aqua regia.
The authors give a brief description of the electroless etching process, and describe the
method by stating that the NW is cleaned heavily and phosphorous doped Si(100). The
authors also mention that the preparation of the NWs is achieved by two processes; the first
is by placing the wafer into a silver nitrate and HF (0.02 M AgNO3 AND 5M HF in water) at
room temperature for 30 seconds.
The next step is to place the wafer into a solution of 10 ml 5 M HF and 1ml 30% H2O2 for
approximately 4 minutes. This will allow the wafer to be etched, while producing 3 µm
SiNWs in length and a diameter of 50 µm. The wafers are then immersed into a deionized
water solution and the silver nanoparticles are removed with a solution known as nitric acid.
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This method that has been used can be seen in the image below labelled figure 1.
Fig 1 Top-down electroless etching
The investigation into the realization of the solar cell uses bulk heterojunction with NWs
incorporated into the P3HT: [60] PCBM polymer layer. The author explains that the
techniques used have been performed inside a glovebox. The method used to prepare the
cells are as follows, uses a 50-80 nm thick PEDOT:PSS (poly(ethylene-dioxythiophene)poly(styrenesulfonate)). The PEDOTT:PSS covers the ITO glass, this is done by a method
called spin coating that is used in conjunction with Clevios AI 4083. Once this has been done
the next method is to dry the Ar under a temperature of 150˚C. The photoactive material
was produced using the P3HT :[60] – PCBM solution mixed with SiNW that contains
chlorobenzene. The author explains that this method has been used on a vast amount of
samples, and has improved the performance of the cell due to the crystallization in the
structure of the polymer.
After discussing the experimental procedures the authors went onto discuss how different
solar cells were investigated. The authors explained that they didn’t use an Al for the top
contact and they used the optical microscopy and scanning electron microscope (SEM)
technique. The first technique used is the Optical microscopy; showed dark spots that were
several micrometers apart, which is shown in fig 2.
Fig 2 the dark spots in the image show the SiNW agglomeration.
Although when investigating with the second method used known as the SEM, the authors
noticed that the dark spots consisted of agglomerated NWs, which is shown in fig 3.
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Fig 3 the image above shows how the agglomeration of NW looks at 2µm.
What was found is that the layout of the NWs is inhomogeneous; leading the authors to
note that for the solar cells efficiency to improve, this area of research will need some
improvement. However the investigation undertaken using the SEM showed that the
photoactive layer increased its charge due to the incident electron beam from the SEM.
Although this did not show up in the NWs, the researchers understood that this was due to
the conductive electron pathway that was provided by the polymer layer. The conductive
electron pathway improved the transportation and resulted in an improvement in the solar
cell.
It was found that when solar cells are heavily doped the spin coating of the SiNW at 200 rpm
onto the PEDOT:PSS, resulted in a cell improvement. However when applying the dropcasting method before using the spin coating technique the researchers found that the cell
started to move apart. Therefore when using the drop casting method to apply the NW onto
the glass, the authors found that the surface distribution of the NW on the glass was
consistent (fig 4) This has prompted researchers to spend more time investigating drop cast
materials.
Fig 4 is the NW that are drop casted with the SiNW, it also show an agglomeration process
happening.
The authors carried out a second experiment and altered concentration of the NW in the
P3HT:PCBM. The results obtained (fig 5) showed that the cells current increased when the
NWs were applied to the glass substrate and the resistance decreased.
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Fig 8 shows the current density increasing and the resistance decreasing when SiNWs are
introduced.
The third experiment in this paper looked at solar cells that were moderately doped with
SiNWs. The method that the authors used in this section was the CVD and electroless
etching. What the researchers found was that the results (fig9) that were obtained from the
final experiment showed that both the heavy and the moderated doped SiNWs had
corresponding results.
Fig 9 from looking at the graph what can be seen is that the current density increases and
the resistance decreases when the SiNWs are introduced.
It was also found that when adding SiNWs to the PCBM there was an increase in the amount
of current produced. However when investigating the CVD method the current observed
showed a lower current density, due to the thickness of the polymer layer. Although when
using 50% SiNW or 50% NW the researchers found that the results for the current and the
resistance gave the best results.
From the results in the paper it can concluded that when applying the SiNW suspension into
the P3HT:PCBM blend, the solar cell has shown great promise. The authors explain that
even when the methods chosen have been put in conditions that have exhibited low density
levels the solar cell has increased its efficiency by 10%. This efficiency increase is due to the
combining of the short circuit current increase and the reduction in the resistance that has
paved the way and open avenues to improve the fill factor.
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The uses of the SEM in the experiment have also shown that by adding the n-doped SiNWs
into the PV cell it improved the transportation of the electron in the cell. However when
using the external quantum efficiency test, with the spin coated heavily doped SiNWs there
is a spectral change when comparing it to the cells that did not contain the NWs.
Therefore the performance of the cell is not due to the absorption effects of the SiNWs;
however the result obtained is a significant improvement. More research must be
undertaken to improve the distribution of the polymer in the NW and refrain from causing
the NW to agglomerate. If the improvement in both the distribution and the agglomeration
can be meet, this will improve NWs placed on the solar cell.
Also the improvement shown in the transportation of the polymer solar cells has lead the
authors to believe that by considering thicker cells in the SiNW, it could be a beneficial and
should be looked into.
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Review of Paper 3
Hybrid nanostructured heterojunction solar cells fabricated using
vertically aligned ZnO nanotubes grown on reduced graphene
oxide[3]
Kaikun Yang, Congkang Xu, Liwei Huang, Lianfeng Zou and Howard Wang
Received 2011 July 15; accepted 2011 September 12
The researchers of this paper show that by coating a solar cell with reduced graphene oxide
(rGO) it will improve the conductivity of the material. The authors explain that by
constructing inorganic and organic hybrid photovoltaic cells on the rGO, through a
hydrothermal process and by using spin casting, it will improve the separation of exciton
states.
The authors of this paper focus on ways to produce cheaper and more affordable solar cells
by using lower temperatures and cheaper materials to manufacture the product. The
authors explain that by separating the organic polymers from the inorganic nonmaterial it
could provide society with a relatively low cost solar cell. The authors mention that the
research that has gone into the heterojunction solar cell showed that the morphology and
the orientation are not the only factors of the heterojunction solar cell to be considered.
What is important is the type of materials selected, due to the dependence of the charge
separation and how the electrons are transported. This work is novel as it uses reduced
graphene oxide (rGO) as the electrode and the zinc oxide (ZnO) as the acceptor in the
photovoltaic material (PV).
The authors go on to explain that a large amount of research has gone into the
heterojunction ZnO nanorod (ZnO-NRs)/P3HT solar cells on indium tin oxide (ITO) coated
glass. The ITO is an expensive material due to the price of indium and how it is created.
Other alternatives have been explored; these include solution casting of polymeric and the
use of nanoparticulate materials.
Graphene is a type of material which has the potential to improve the solar cell, which the
paper goes on to explain. Graphene is a one atom thick material that has exceptional
physical properties which include high intrinsic carrier mobility, quantum electronic
transport and limited optical absorption. There have been different types of solar cells that
have been placed onto graphene electrodes; these include Schottky junction, organic PV
devices and the Grätzel cell. However no research on the ZnO nanotubes (ZnO-NT)/P3HT)
has been done previously, making it a good area to work on.
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The advantage of the ZnO is that it has high electron mobility, there is high transparency in
visible wavelength range and the band energies are compatible with the organic donor
materials due to the heterojunction.
The authors discuss that if the ZnO-NRs are aligned vertically it can improve the efficiency of
the photovoltaic cell. This improvement in efficiency is due to the special crystal properties
in the material. With the aid of the hydrothermal method, the ZnO can form nanorods (NRs)
and nanotubes (NTs) that are grown anisotropically at an affordable manufacturing cost.
The papers main focus is on the use of hydrothermal methods to form vertically aligned
ZnO-NTs and ZnO-NRs on rGO films. This approach showed researchers that the hollow
structures of the ZnO-NTs allow more contact with the P3HT and produces exceptional
levels of PV performance.
In this paper the authors describe how they performed the experiment. They started by
explaining that the solar cells were produced on glass substrates, that was assembled using
graphene oxide (GO). The next step is known has the Hummers method that was adjusted
to fit this experiment. The authors explain that the GO was mixed with three substances e.g.
graphite powders (1-2µm), KMnO4, NaNO3 AND H2SO4. The following step involved spin
casting the GO film from 1mg ml-1 at speed of 1500 RPM. For 12 hour duration at 60 ̊C the
GO film was placed in the hydrazine, at which point they were annealed for 1 hour at a
temperature of 400 ̊C.
The authors moved on to explain that the ZnO-NRs were grown vertically from 10 Mm Zn
(CHCOOH)2 and the use of the hydrothermal method at a temperature of 90˚C was used
with hexamethylenetetramine (HMTA) aqueous solution. Once this had been accomplished
the ZnO-NRs were placed under a temperature of 95˚C in a 5 M KCl aqueous solution. This
helped to produce tubular structured ZnO nanotubes (ZnO-NTs).
The machinery that was used to show the performance of the PV cell and the current
density versus voltage are known as the Agilent 4156C device analyzer. The purpose of this
equipment is to measure the performance and the current density versus voltage through
the air. However the composition and the morphology of the material was investigated
using a Zeiss Supra 55 field emission scanning electron microscope (FESEM) that is used in
conjunction with an energy dispersive x-ray spectroscopy (EDS) at a voltage of 5 kV.
The optical absorption properties were investigated using a Cary 50 Bio ultraviolet-visible
(UV-vis) spectrophotometer. To find the measurements for the photoluminescence (PL) the
researchers used a HeCd laser excitation source, these were not the only pieces of
equipment used but the ones that were focused on in more detail. Other instruments that
were included were the Renishaw Invia confocal Raman spectrometer with a laser source of
excitation 488 nm, the Thermoggravimetric analyses (TGA) etc.
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The results of the GO suspension show they are exceptionally stable, after being stored in
storage for over 6 months at room temperature. The results showed no visible solid
sedimentation. This is due to oxidation process producing polar functional groups on the GO
sheets. The use of dimethylformamide (DMF), tetrahydrofuran (THF), N-methylpyrrolidone
(NMP) and ethylene glycol is also used in polar solvents.
The results also showed that the thickness of the individual GO sheets is larger than the
pristine graphene. This is due to the functional groups that are produced by the graphene
which are bonded to the surface, and the distortion of the carbon lattices.
The images that can be seen from the AFM topography images (fig 15.) show the different
thicknesses and root mean square (RMS). Due to the consistency of the GO film, what was
found is that in the there was high crystallinity with visible diffraction peaks. This suggests
that the structure showed a strong dominance of Zn and O.
What was found is that when the light radiation was absorbed in the P3HT, it excited the
electrons
The paper has shown that, the study of water processable GO sheets has been applied as a
precursor. The use of the rGO transparent film that has electrode capabilities and a
conductive film, were used to grow vertical aligned ZnO-NRs and NTs. This was done
through a process that used a hydrothermal at low temperatures. What was found is that
P3HT, PEDOT:PSS on the PV devices showed excellent efficiency, due to the amount of area
covered by the ZnO-NTs. The investigation of this paper has opened avenues to producing
low cost materials, which can contribute to the way that flexible solar panels are produced.
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Review of Paper 4
Realization of Vertical and Zigzag Single Crystalline Silicon
Nanowire Architectures [4]
V.A. Sivakov, G.Brönstrup, B.Pecz, A.Berger, G.Z. Radnoczi, M. Krause, and S.H.
Christiansen
Received 2009 October 16; accepted 2010 January 25
In this paper the authors give a brief description on the processes involved in producing
Silicon nanowires (SiNWs). They then focus on the way that the SiNW is placed onto wet
chemical etching, while using the redox reaction processes of Ag nanoparticles and the use
of single crystalline silicon wafers.
The authors then cast their attention on the development and research of the silicon
nanowires (SiNWs) due to the abundance of the materials needed and its ability to be
nontoxic. The authors explain that it will have a large impact on the industrial infrastructure,
opto-electronics, photonics and photovoltaic’s. This will allow manufacturers to produce
low cost alternatives and high production turnouts.
The authors discuss how the SiNWs are fabricated. The method used is known has the
vapour liquid solid growth (VLS). This process forms metal nanoparticles at low
temperatures that are liquefied to produce nanoscale droplets that can be saturated to the
maximum from a gas phase e.g. chemical vapor deposition (CVD), molecular beam epitaxy
(MBE), electron beam evaporation (EBE).
Therefore when working with SiNWs, attention to detail must be of importance for the
devices to function as a unit. The key factors to be considered include the crystal structure,
geometry, interfacial properties, Si core and the SiO2, dopant concentrations and the
impurity of the material. The key question that is addressed from the point of view of the
authors is what is the amount of metal (gold (Au)) from the catalyst and where is it occupied
in the SiNW to initiate VLS growth.
In different areas of research it has been seen that there is an advantage of having even a
small amount of Au as a dopant to impact on the way that the opto-electronic properties of
the SiNWs perform. This has led the authors to find ways to understand how the Au atoms
propagate, incorporate and attach itself on the SiNWs and Si (111) substrate. The authors
explain that the Au from the catalyst is versatile and is distributed over and in between the
surface of the SiNWs. This occurs when the temperature is as low has 600 ˚C, and produces
a phenomenon known as the Ostwald ripening at the top and side of the NW.
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When comparing with alternative wet chemical methods, the authors explain that the
process used in this paper can be used at room temperature unlike any other. Leading to
SiNWs that are more than 50 µm in length and attempting to make the SiNW catalyst
impurity free.
The experimental procedure is as follows, Boron-doped Silicon wafers were cleaned by
placing them in acetone for 2 mins. Once this had been done the wafers were then rinsed
with ethanol for a further 2 mins. The removal of the SiO2 was then distributed in 40%
hydrofluoric acid (HF) solution for a short time period that was then followed by a short 2%
rinse of HF. Once these methods were achieved the use of deionized water was used to
rinse the wafers, and then dried using nitrogen.
The method used showed that for a few minutes there was no hydrogen on the surface of
the silicon. Therefore allowing the deposition of silver onto the silicon wafers due to the
being potentially oxide free. The authors explain that the chemical etching method used to
investigate SiNWs was researched using two procedures. The first step involved placing Ag
nanoparticles onto the surface of the silicon wafer; this was done by placing the silicon
nanowires into a silver nitrate (AgNO3) for a period of 15 to 60s.
The second step involved placing the silicon wafers into a Teflon vessel at room
temperature for approximately 1hr. However the different morphology is formed by the Ag
nanoparticles that were put into a 50ml solution containing 5 M HF and 30% of H 2O2. Once
the etching procedures had been accomplished, the wafers were then rinsed in deionized
water.
The Ag nanoparticles contained on the surface of the SiNWs is removed by washing it for 15
mins in concentrated (65%) nitric acid and HNO3. The types of equipment used to
investigate the experiments consisted of using the SEM and TEM to analyse the structure of
the SiNWs that were etched.
Direction of the growth and the structure of the crystal in the SiNWs that was etched was
investigated by using electron back scatter diffraction (EBSD). The EBSD and the SEM were
mixed together to focus on the individual grain orientation, the texture and to determine
the orientation of the correlation on the surface of the material of the polycrystalline.
The results of this paper explained that the use of wet chemical etching was performed to
appreciate the different types of morphologies that can occur when using different
processes. Therefore having the means to control the morphologies and the different
optical properties can be fabricated to your chosen specification.
The morphologies for the SiNW can be seen in fig 10. What is shown here are the cross
sections investigated by using the SEM.
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Fig 10 shows the different images of the SEM cross sectional micrographs of the SiNW arrays
formed due to the wet chemical.
The researchers noticed that from the images in fig 10 that the etching process involved in
the silicon depended on the Ag nanoparticles morphology. To understand and prove that
this is true the authors used the TEM to investigate. The reason why this method was
chosen was due to how the Ag nanoparticles existed on the surface and its crystallographic
structure of the silicon wafer.
What the researchers also found is that the Ag nanoparticles were not composed of regular
shapes; they were spaced out and connected to the Silicon wafer. This led the researchers
to investigate further with the TEM and what they found was that the surfaces of the silicon
exhibited elongated grains of Ag which was connected to the surface of the silicon.
Therefore leading to a shorter time period for the solution treatment due to the Ag
becoming disordered (fig 10 a).
When the time period was extended, it was found that the Ag began to agglomerate causing
it to form quasicontinous multicrystalline layers (fig 11). The agglomeration process has
caused the authors to believe that this is the reason for an increase in the homogenous
silicon etching profile.
Fig 11 the image above show the cross sectional area of the Ag nanoparticles using a TEM
cross-sectional micrographs.
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The SiNW arrays also depend on the orientation of the silicon wafer. When comparing the
results of this paper with other papers what is found is that most results resembled that of
the results in this paper. How this paper is different from others is that it found that etching
of Si (111) wafers which were assumed to be straight can in fact be assembled in a zig zag
fashion.
To prove the different etching directions existed the authors used the EBSD with the SEM.
This helped to prove the theory by showing evidence of the orientation of the SiNW at the
same time focusing on the size and distribution of the grains. This method enabled the
researchers to look at the crystal orientation and its direction.
To support this investigation and to be able to understand the findings of this research, the
use of thermodynamic process was used to distinguish the differences between the etching
morphologies.
What can be concluded from this paper is that the construction of the SiNW can be acquired
by changing the orientation and the processing methods. What has come from this
experiment are SiNWs that can be straight or perpendicular to the surface of the Si wafer.
The process used to etch is extremely acidic; this allows the temperature of the etching
method to increase dramatically.
The increase in temperature causes the etching direction to change over a period of time
producing the zig zag formation of the SiNWs. When comparing the formation of the zig
zag/ straight SiNWs with conventional single crystalline Si wafers, the data shows that the
absorption levels have increased and the reflectance level has decreased. This research has
allowed more investigation into the way that optical devices are produced.
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Review of Paper 5
An active cooling system for photovoltaic modules [5]
H.G. Teo, P.S. Lee and M.N.A Hawlader
Received 2010 September23; accepted 2011 January 5
This paper investigates the electrical efficiency of photovoltaic (PV) cell. The authors explain
that when the PV cell absorb large amounts of solar radiation, the temperature in the cell is
affected causing it to overheat and reduce the efficiency of the cell. This paper has
researched the hybrid photovoltaic/thermal (PV/T) solar cell and constructed a system that
is designed to reduce the temperature of the PV cell. To do so the authors have used
parallel ducts, which have inlet/outlet manifolds to allow the airflow to be distributed.
The authors also mentioned that tests have been conducted on the PV panel to determine
what the experiment would show with and without the manifolds. What the authors found
was that the efficiency and the temperature showed a linear trend. Therefore when the
experiment was conducted without the manifold the temperature began to raise causing
the module to overheat and limiting the solar cell to 8-9% efficiency rating. By then applying
the manifold to the experiment, the temperature began to drop, causing the efficiency of
the solar cell to increase to 12-14%. Therefore to understand the experimental results, the
authors developed a heat transfer simulation model to compare the temperature achieved
by The PV panel.
Many researchers believe that the PV cell is becoming more and more popular due to the
demand for renewable energy. The main benefit of the PV cell is that it can convert the
Sun’s radiation directly into electricity and supply households with current needed to run
the appliances. Although this is a benefit, it is important to point out that solar radiation
converts only 15%of its power to electricity; the rest is lost through heat and the efficiency
of the current decreases, due to the temperature of the PV panel increasing.
If the temperature of the PV panel is decreased, the electrical efficiency would be increased.
Different types of techniques have been attempted theses include air cooling and water
cooling, that have been used to control the temperature. There has also been a vast amount
of numerical and experimental studies to understand the efficiency and affordable hybrid
PV/T system.
The authors go onto discuss why they have focused their attention on adjusting the
configuration of the PV panel. By altering the structure of the panel the performance of the
system can be monitored. The author has some suggested considerations including the one
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with the highest efficiency, which is an experiment that uses the glass to glass PV that
contains a cooling duct.
However to improve the transfer of heat from the PV panel and reduce the temperature
effectively, the absorbers wall and plate should be rough. This roughness produces a drop in
pressure due to the increased pumping power. Different types of studies have gone into
improving the PV/T air hybrid system. These range from including a plane booster and a flat
plat collector to the module, optimization study of the absorber geometry and applying a
double pass solar air heater with longitudinal fins. The latter showed that there is an
improvement in the flow rate that resulted in the coefficient of the heat transfer increasing.
The papers main focus pays close attention to the efficiency of the electrical output, with
and without the cooling system, which will be done by controlling the air flow rate and the
temperature, monitored by the simulation model.
The authors explain the setup of this experiment by explaining that the first step is to
investigate how the PV/T air system would perform. This was attempted by placing the PV
module onto the roof of the National University of Singapore. The benefit of doing this
experiment on the top of the EA building was that it gave the best opportunity to
investigate the amount of output produced by the PV module and saw how the
temperature could affect the efficiency.
The setup of the PV module is as follows. The use of a four 55 watt polycrystalline solar
module was installed to produce the current. The current that was produced was then
diverted to four deep cycle gel batteries. The air ducts on the PV module were arranged so
that the air could pass through underneath the panel; however the fins were placed inside
the duct, to increase the transfer of the heat to the fluid.
To cool the module and stop it from overheating, a direct current blower was attached to
the batteries to ensure that the system stayed cool. Added to the PV module was a piece of
equipment known as the maximum point tracker (MPPT), this was used to balance the
amount of power that was produced by the solar panel and allow the maximum electrical
power to be obtained.
To produce the best outcome of the PV module, the researchers designed manifolds that
could benefit from the surrounding air flow. These different types of manifolds were
designed using a computational fluid dynamics (CFD) package. The results showed that the
distribution between the air ducts stop the recirculation of air in the PV module. This
resulted in an improvement of reduction of temperature of the module.
It was shown (fig 12) that the function of the electrical efficiency is a linear curve when
measured against the temperature of the PV module. The experiment also included the
cooling and non cooling methods used. When looking at the graph in fig 12 the electrical
efficiency begins to fall as the temperature increases.
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Fig 12 Electrical efficiency vs PV Temperature
The temperature in fig 13 also shows that the temperature is linearly proportional to the
irradiation. When the cooling method is put into place what is seen is that the temperature
increases to 1.4˚C in increments of 100 W/m2. If this cooling method is not used then the
temperature of the PV module will increase to 1.8 ˚C.
Fig 13 PV temperature of the solar irradation
To find the theoretical efficiency the use of the Equation below can be used. From using
this results obtained can be used to find the deduction.
When comparing the theoretical electrical efficiency to the experimental data fig 14. What
was found is that the efficiency of the theoretical data showed that it was 1-2% higher than
that of the experimental data. This was due to the theoretical data only taking into account
one panel. The experiment data investigated four panels that were connected in series and
parallel.
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Fig 14 Theoretical vs Experimental data
The investigation of the flow rate showed that when it reached 0.055 kg/s the value of the
PV module stayed constant. However when the flow rate increased above the value of 0.055
kg/s, what was found is that the value also stayed constant. This was due to the thermal
efficiency collector. What the thermal efficiency collector did is extract the heat due to the
heat reaching its maximum level and saturating at this level. This maintained a constant
value shown above the 0.055kg/s.
In the final section of the results, the authors explain that the efficiency of the systems
results have shown that the electrical efficiency has shown more promise than the thermal
efficiencies due to its stability. The results showed that the electrical efficiency is
approximately 10.9, whereas the efficiency of the thermal data showed that it is around
40% . This higher figure was put down to solar irradiation being converted to heat.
Significant improvements have been shown in the electrical and thermal energy of the
hybrid PV/T module. The results that were obtained showed that the addition of the active
cooling system lowered the temperature to 38˚C from 68˚C and the efficiency rose to 12.5%.
The authors mentioned that the PV module could absorb 0.055 kg/s intake of the
surrounding air, which is enough to cool the module.
However if the air intake is increased, the energies of the thermal and electrical properties
are not affected, which is due to the PV module having the option to change the settings of
the blower.
The key factor of this paper is the temperature gradient as it can affect the performance of
the electrical part connected to the PV module. Therefore the aim is to increase the
electrical efficiency by looking at the temperature and the temperature gradient over the PV
module.
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Discussion/Conclusion
To conclude we can see that the limitations of the solar cell can be improved by moving
beyond the SQ limitation, the disadvantage that was found in the first review is that the loss
of energy is far greater than that of the conventional solar cells. However this can be
improved if the absorbers of the EG band gap are left out. The advantage of this is that the
1.4 eV absorbers due to the absorbers having the ability to be placed on Pure SQ.
When looking at the different density that the SiNW suspension can be placed under, the
temperature increased its efficiency and reduced the reduction in the resistance. Therefore
improving the transportation of the electron, and showing that the performance is not due
to any absorption effects. The disadvantage of the SiNW is that is that the NW begins to
agglomerate. One way of overcoming this problem is to produce thicker SiNW.
The orientation and the processing of the SiNW is one way of improving the efficiency, this
is due to the increased levels of absorption and the reduction in the reflectance. The
improvements in many of the solar cells that have been investigated in the review show that
the electrical and thermal energy need to be fitted to the solar cells.
This could insure that the overheating problem can be overcome if the intake of the
surrounding air is used to cool than the system of the PV module. Therefore increasing the
efficiency is the key factor in the review and a way of doing this can be solved by controlling
the temperature of the PV module itself.
The research into solar cells so far has show much improvement over the years and a lot has
been done to improve their efficiency. What is clear from this research is that with new
ideas there will always be new problems to face and limitations to overcome. It is an area of
research that due to its importance should be constantly revised. From all the research that
is out there they all have the common goal of increasing efficiency of solar cells but they all
approach it from different angles which are good as it increases the chances of success.
Solar cell research is definitely a good area to be involved with.
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Reference
[1] P. K. Nayak , J. Bisquert , and D. Cahen, Assessing Possibilities and Limits for Solar Cells,
Advanced materials, 2011 pg 1-7
[2] B. Eisenhawer1, S. Sensfuss2, V. Sivakov, M Pietsch, G Andr¨a
and F Falk, Increasing the efficiency of polymer solar
cells by silicon nanowires, Nanotechnology, 2011, pg 1-8
[3] K. Yang, C. Xu, L. Huang, L. Zou
and H.Wang , Hybrid nanostructure heterojunction solar
cells fabricated using vertically aligned
ZnO nanotubes grown on reduced
graphene oxide, 2011, pg 2-9
[4] V. A. Sivakov, G. Bro¨nstrup, B. Pecz, A. Berger, G. Z. Radnoczi, M. Krause,| and
S. H. Christiansen, Realization of Vertical and Zigzag Single Crystalline Silicon Nanowire
Architectures, J. Phys. Chem, 2010, pg 3768-3803
[5] H.G. Teo, P.S. Lee, M.N.A. Hawlader, An active cooling system for photovoltaic modules,
Elsevier, 2012, pg 309-315
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