Nanotechnology for Solar Cells:
A Study of the quantum dots attribute to high efficiency low cost solar cells
A proposal
for the
Application for Admission
to the
Doctor of Philosophy Programme
in the
Department of Chemistry,
The University of Hong Kong
by
Ye Zi
Table of Contents
Part I:
Introduction ………………………………………………………… 1
Background
Purposes
Research questions
Definitions
Outcomes and value
Part II: Literature review ……………………………………………………
8
Definition of the quantum dots solar cells
Properties of quantum dots
Development of quantum dots solar cells
Part III: Methodology
……………………………………………………
Systhesis of the quantum dot
Measurement of the quantum dot solar cells
References
11
Part I: Introduction
Background
Solar energy offers the opportunity for our society to make the inevitable
transition toward environmental sustainability.[1] This energy can be converted
directly to electricity by photovoltaics cells.[2] however, the low efficiency of the use
of the solar spectrum due to the energy threshold in the materials.
As a result, The researches on the solar cells mainly focus on the development of
the materials which can improve the efficiency of the light-electric conversion.
Scientists take efforts to design new nanostructured architectures and molecular
assemblies for the next generation of solar cells. While to develop the new generation
solar cells, nanotechnology is the key to the new solar-cell architecture, which
promises to transform everyday polymers into flexible energy producers.[3] Three
different types of solar cells based on the advances in nanotechnology have emerged:
(i) dye sensitized solar cells (DSSC), (ii) hybrid organic solar cells, and (iii) quantum
dot solar cells. [4]
Recently, there are many further studies conducted by many scientists. Professor
Gra ̈tzel directs the laboratory of photonics and interfaces at the E ́ cole Polytechnique
de Lausanne, where he has pioneered the use of mesoscopic thin films for
dye-sensitized solar cells and other energy applications, [5]Peter, Hodes, and Kamat
did the research on the dye-sensitized nanocrystalline solar cells (DSSC) and quantum
dot solar cells highlight recent progress including the processes that dictate the
photoconversion efficiency[6]. Through these studies, the two direction of nowadays
research on solar cells is to develop the nanostructures or particles and the quantum
dots to wide the range of the photoconversion and improve the efficiency of the
energy deliver or conversion.
Purpose
The major purpose for this study is to consider the nanomateirals especially
quantum dots applied in solar cells. The key to improve the efficiency of the solar
cells is to use the quantum dots to wide the range of the light absorb of the solar cells
or to improve the energy conversion efficiency between light and electric. Thus, the
mainly task is to analyze the optoelectric properties of different quantum dots. And
connect them to the solar cells.
Research question
Research question 1:
Why the quantum dots can applied in the solar cells?
The quantum dot sensitized solar cell (QDSSC) is considered to be a simple
analogue of dye-sensitized solar cell (DSSC).The quantum dots have been suggested
and tested as a next-generation sensitizer with advantages over molecular dyes.[7] Due
to the quantum confinement effect, the optical properties and the band gap of quantum
dot can be adjusted by changing their size.[8-11] And the high extinction coefficient of
the quantum dot can reduce the dark current and improve the efficiency of the solar
cells. Solar cells based on inorganic quantum dots that employ Forster-type resonance
energy transfer have already been proposed and demonstrated, which take advantage
of this mechanism to transfer the exciton generated in the quantum dot to a
high-mobility conducting channel. [12]
While different quantum dots have their own characteristic properties, they all
attributes to the improvement of the solar cells. The researches on the quantum dots
solar cells are to discover the properties or mechanism of all kinds of the quantum
dots and ultimately applied them to the sensitizer.
Research question 2:
How to synthesize the quantum dots and hybrid them to be a new material in solar
cell?
To deep the understanding of the quantum dots, synthesizing the quantum dots at
first is important. There is a wide variety of the synthesis to the quantum dots. Take
CdTe quantum dots as example. It can be synthesized in both organic phase and
aqueous phase. Based on the successful synthesis of quantum dots, the research on the
properties can be promoted fluently.
In addition, the hybrid materials show promising applications in optoelectronic
devices.[8]. For instance, CdSe-nanotube hybrids are capable of light-induced charge
transfer and can be used as an electron transport component in solar cells.[8-16] So to
hybrid the materials effectively and combine the properties of the nanomaterial to
enlarge the efficiency of the solar cells is also the main issues in this research.
Research question 3:
How to measure the optoelectric properties of the quantum dots or the hybrid new
material?
The measurement of the optoelectric properties of the quantum dots or the hybrid
new material is to learn its effect on the solar cells. It directly reflects the value of this
research. The measurement attributes to the guidelines of the application. Also, the
research on the mechanism can be conduct through the measurement of the
optoelectric properties.
Definition
Quantum dots: A quantum dot is a portion of matter (e.g., semiconductor) whose
excitons are confined in all three spatial dimensions. Quantum dots are
semiconductors whose electronic characteristics are closely related to the size and
shape of the individual crystal. Generally, the smaller the size of the crystal, the larger
the band gap, the greater the difference in energy between the highest valence band
and the lowest conduction band becomes, therefore more energy is needed to excite
the dot, and concurrently, more energy is released when the crystal returns to its
resting state.
Solar cell: A solar cell (also called a photovoltaic cell) is an electrical device that
converts the energy of light directly into electricity by the photovoltaic effect. It is a
form of photoelectric cells which, when exposed to light, can generate and support an
electric current without being attached to any external voltage source.
Outcomes and Value
Quantum dots may be able to increase the efficiency and reduce the cost of
today's typical silicon solar cells. The experimental proof from 2006 (controversial
results [13], quantum dots of lead selenide can produce as many as seven excitons from
one high energy photon of sunlight (7.8 times the bandgap energy). At the same time,
the relatively low cost of the quantum dots create the possibility of the wide
application of solar cells in the future.
Part II: Literature review
Definition of the quantum dot solar cells
Quantum dot solar cells: it’s a simple analogue of dye-sensitized solar cells, which
uses quantum dots as the photovoltaic material. Due to the bandgaps which are
tunable across a wide range of energy levels, the solar cells can extract more power
from the solar spectrum.
Properties of quantum dots
1. Different light can be obtained through the control of the sizes of Quantum dots.
Therefore, by changing the size of the quantum dots, and its chemical composition is
possible to make the emission spectrum covered the entire visible region. [14]
2. It can withstand a variety of special conditions and environment with good stability.
So it can guarantee normal function devices as well as the longer service life.
3. It has a wide excitation spectrum and a narrower emission spectrum, which means
that it can be used to emit different wavelengths of fluorescence, i.e. multicolor
display.
4. A large Stokes shift means a larger interval between the excitation and emission
spectra, which is conducive to the detection analysis.
Development of quantum dots solar cells
Since 1991, due to the high efficiency and relatively inexpensive fabrication
procedure compared with conventional inorganic solar cells, dye-sensitized solar cells
(DSSCs) became attractive.[15-18] The idea of using quantum dots as a path to high
efficiency was first noted by Burnham and Duggan in 1990.[19]There are also
researches on nanocrystals of other semiconductors, such as CdTe.
In 2005, the National Renewable Energy Laboratory in Golden, Colorado
reported a spectroscopic evidence that several excitons could be efficiently generated
upon absorption of a single, energetic photon in a quantum dot.[20-21]
In 2008, The quantum dots-PbSe reached external quantum efficiencies of 46% in
the infrared and 70% across the visible spectrum, which could be a promising step
towards the development of the future solar cells.[22]
Part III:
Methodology
Systhesis of the quantum dot
Preparation of quantum dots can be roughly divided into the synthesis of organic
phase and aqueous phase synthesis according to the solvent. Take CdTe quantum dots
as example.
1.organic phase synthesis
In 1993, Bawendi[23] came up with the preparation of CdTe quantum dots through
high temperature pyrolysis. It uses a ligand and higher boiling hydrocarbon solvent in
place of the precursor ion which used in the preparation of "wet" and water or other
polar solvents prior to the neutral organic metal. Chose Cd (CH3) and tri-octyl
tellurium phosphine (Tri-n-octylphosphine Tellurium, TOPTe) as precursors, using
the the TOPO ligand as an organic solvent, to prepare close monodispersed CdTe
quantum dots.
In 2001, Peng et al [24] on improved the traditional synthetic methods and
proposed a new method for the preparation of quantum dots. They chose CDO instead
of Cd(CH3)2 as Cd precursors, hexyl phosphonic acid (Hexylphosphine acid, HPA)
and tetradecyl phosphonic acid (n-Tetradecylphosphonic acid, TDPA) as a
coordinating solvent. Synthesized high-quality CdTe quantum dots in 90% TOPO.
Subsequently, Talapin, et al[25] used dodecylamine (Dodecylamine, DDA ) instead
of TOPO as the coordinating solvent to get CdTe quantum dots with quantum yield of
65%.
Thereafter, Schreder [26] prepared a core-shell structure of CdTe/CdS quantum
dots.Bailey et al[27] used Tri-n-octylphosphine the Selenium (TOPSe) and TOPTe as
Se, Te precursor, synthesized CdSe1-xTex ternary quantum dots of which the
maximum emission wavelength is 850 nm by changing the ratio of Se and Te. This
makes the fluorescence emission wavelength of the quantum dots of the group II-VI is
no longer confined to the visible light range, and reached the near-infrared region.
2. Aqueous phase synthesis
Aqueous phase synthesis is dispersed with the mercapto group-containing small
molecules, which cannot only deal with the low dispersibility in organic phase, but
also modified the quantum dots to improve the stability.
In 1996, Rogach[28] reported that the aqueous phase synthesis of CdTe quantum
dots obtained by selecting different thiol compounds as stabilizer.
So far, a mercapto group as a stabilizer to gain CdS, CdTe, CdSe quantum dots
have been successfully developed. And the emitting of these quantum dots almost
covering the entire visible light range (500 to 700 nm).
In addition, the development of the microwave synthesis and ultrasound synthesis
not only shorten the reaction time, but also increase the luminous efficiency of the
quantum dots. He et al[29-31] using microwave synthesis of CdTe quantum dots, which
the fluorescence quantum yield was up to 98%.
Recently reported that yang et al[32] used hydrazine hydrate as a buffer system,
and created CdTe quantum dots with no adjustment of pH value, nitrogen protection
is not required, and no heating under the conditions.
Through the investigation of the systhisis of the CdTe quatum dots, aqueous
phase synthesis of the quantum dots is favored with the mild reaction conditions, and
easy functionalization.
Measurement of the quantum dot solar cells
The following are the usual measurement of the quantum dot solar cells:
1. Luminescence spectrum.
2. Dependence of the thickness and porosity of the hybrid quantum dots films on
drop space.
3. Changes of efficiency (η), short-circuit current (Isc), open-circuit voltage (Voc),
and fill factor (FF) of solar cells after the printing of the quantum dots ink at the
solar cell surface.
4. EQE of solar cells before and after the inkjet printing of the quantum dots ink with
the drop space of specific range.
Reference
[1] Prashant V. Kamat , J. Phys. Chem. Lett., 2010, 1 (20), pp 3147–3148.
[2] Tim F. Schulze, Jens Czolk, Yuen-Yap Cheng, Burkhard Fückel, Rowan W. MacQueen, Tony
Khoury, Maxwell J. Crossley, Bernd Stannowski, Klaus Lips, Uli Lemmer, Alexander Colsmann,
and Timothy W. Schmidt, J. Phys. Chem. C, 2012, 116 (43), pp 22794–22801.
[3] Jeanne Trombly, Environ. Sci. Technol., 2004, 38 (19), pp 376A–376A.
[4] Prashan V. Kamat and George C. Schatz, J. Phys. Chem. C, 2009, 113 (35), pp 15473–15475.
[5] Gerald J. Meyer,ACS Nano, 2010, 4 (8), pp 4337–4343.
[6] Prashan V. Kamat and George C. Schatz, J. Phys. Chem. C, 2009, 113 (35), pp 15473–15475.
[7]Jun-Guo Song, Xin Song, Tao Ling, Xi-Wen Du, and Shi Zhang Qiao, Ind. Eng. Chem. Res., 2012,
51 (30), pp 10074–10078.
[8] Michalet, X.; Pinaud, F. F.; Bentolila, L. A.; Tsay, J. M.; Doose, S.; Li, J. J.; Sundaresan, G.; Wu, A.
M.; Gambhir, S. S.; Weiss, S., Science 2005, 28, 538.
[9] Yu, W. W.; Peng, X. G., J. Phys. Chem. C 2008, 112, 18737. Angew. Chem., Int. Ed. 2002,
41,
2368.
[10] Yu, W. W.; Qu, L. H.; Guo, W. Z.; Peng, X .G. Chem. Mater. 2003, 15, 2854.
[11] Yu, J. G.; Zhang, J.; Jaroniec, M., Green Chem. 2010, 12, 1611.
[12] ACS Nano, 2009, 3 (4), pp 755–755.
[13] Service, Rf. Science, 2004, 322 (5909): 1784.
[14] Kairdolf B. A., Smith A. M., Nie S. J. Am. Chem. Soc. 2008, 130(39): 12866-12867.
[15] O’Regan, B.; Gratzel, M., Nature 1991, 353, 737.
[16] Yu, J. G.; Fan, J. J.; Lv, K. L. Anatase, Nanoscale 2010, 2, 2144.
[17] Wu, M. K.; Ling, T.; Xie, Y.; Huang, X. G.; Du, X. W., Sci. Technol. 2011, 26, 105001.
[18] Mao, J.; Li, J. J.; Ling, T.; Liu, H.; Yang, J.; Du, X. W., Nano tech. 2011, 22, 245607.
[19] Barnham, K. W. J.; Duggan, G., J.of Applied Phys.1990, 67: 3490.
[20] Schaller, R.; Klimov, V.. Phys. Rev. Lett. 2004, 92.
[21] Ellingson, Randy J.; Beard, Matthew C.; Johnson, Justin C.; Yu, Pingrong; Micic, Olga I.; Nozik,
Arthur J.; Shabaev, Andrew; Efros, Alexander L., Nano Lett., 2005,5 (5): 865–71.
[22] ACS Nano, 2008, 2 (5), pp 813–813.
[23] Murray C. B., Norris D. J., Bawendi M. G. J. Am. Chem. Soc.1993, 115(19): 8706-8715.
[24] Peng Z. A., Peng X. G.,J. Am. Chem. Soc. 2001, 123(1): 183-184.
[25] Talapin D. V., Haubold S., Rogach A. L., J. Phys. Chem. B, 2001, 105: 2260-2263.
[26] Schreder B., Schmidt T., Ptatschek V., J. Phys. Chem. B, 2000, 104: 1677-1685.
[27] Bailey R. E., Nie S. J.,J. Am. Chem. Soc., 2003, 125: 7100-7106.
[28] Rogach A. L., Katsikas L., Kornowski A., J.Phys. Chem., 1996, 100.
[29] He Y., Lu H. T., Sai L. M., J. Phys. Chem. B, 2006, 110: 13370-13374.
[30] He Y., Lu H. T., Sai L. M., J. Phys. Chem. B, 2006, 110: 13352-13356.
[31] He Y., Sai L. M., Lu H. T., Chem. Mater., 2007, 19: 359-365.
[32] Ding Zhou, BaiYang, Chem. Mater. 2011, 23, 4857–4862.