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6 Quantum Dot Solar Cells
Chemistry of Sustainable Energy
CdSe quantum dots
Energy (eV) vs. vacuum
Eg 2.19 eV
FIGURE 7.75 (See color insert.) The relationship between QD size and bandgap. (Adapted
with permission from Kamat, P. V. Boosting the efficiency of quantum dot sensitized solar
cells through modulation of interfacial charge transfer. Acc. Chem. Res. 45 (11):1906–1915.
Copyright 2012, American Chemical Society.)
energy >Eg. As noted above, the excess kinetic energy for these photons is usually
lost nearly instantaneously as heat—temperatures as high at 3000 K can exist in
a lattice at 300 K (Nozik 2002)! But what if that excess energy could be captured
and used? In that case, the theoretical solar-to-electric power efficiency could reach
as high as 66% (Ross and Nozik 1982)! Capture of this excess energy generates
hot carriers (hot electrons and hot holes), and transfer of this energy to electrons
and holes nearby can produce additional electron–hole pairs, aka electron–hole pair
multiplication (EHPM) [synonymous with carrier multiplication (CM) or multiple
exciton generation (MEG)]. Figure 7.76 depicts this process graphically, where the
hot carrier is designated as n1* . Transfer of the excess energy by EHPM generates an
additional hot carrier (n2* ) which can in turn generate yet another hot carrier, and so
on. Electron–hole pair multiplication must take place quickly—within a picosecond
or two—before cooling occurs as indicated by kcool in Figure 7.76. This is where
QDs enter in. Because QDs are unfathomably small, confined, and three-dimensional spaces, the hot electrons and hot holes generated therein cool at reduced rates
giving enough time for the EHPM process. The quantum yield (QY) is defined as
the number of electron–hole pairs (excitons) that are generated per absorbed photon.
The use of QDs and their propensity for EHPM means that the quantum yields can
be much greater than 1.
Given the possibility of enhanced electron–hole pair multiplication, QD PVs
hold great promise for improved efficiencies, but they have yet to best DSSC performance. In a recent example, a colloidal QD device prepared with lead sulfide QDs as
the p-type absorber atop n-TiO2 donor material on a fluorine-doped tin oxide (FTO)
FIGURE 7.76 The electron–hole pair multiplication process. (Reprinted with permission
from Beard, M.C., A.G. Midgett, M.C. Hanna et al. 2010. Comparing multiple exciton generation in quantum dots to impact ionization in bulk semiconductors: Implications for enhancement of solar energy conversion. Nano Lett. 10(8): 3019–3027. Copyright 2010 American
transparent conducting electrode gave a PCE of 8.5%. This much improved efficiency (for QD solar cells, see Figure 7.11) was attributed to a new design concept: a
donor supply electrode prepared by layering a very thin film of TiO2 onto a relatively
thick layer of the FTO. With this configuration the depletion zone in the PV was
increased and as a result more photocarriers could be extracted leading to improved
efficiency (Maraghechi et al. 2013). Despite this finding, the conversion efficiencies
for QD solar cells currently lag behind that of DSSC, although advancements will
undoubtedly continue to be seen.
7.6.2 Architecture and Materials
As one might expect, the architecture of the QD-sensitized solar cell is not unlike that
of a dye-sensitized solar cell. The anode is typically a TCO-coated glass substrate
upon which a mesoporous layer of a metal oxide is affixed. The key difference is, of
course, that it is now QDs—not dye molecules—that are layered atop of the semiconductor metal-oxide particles, as illustrated in Figure 7.77. Owing to the requirements
of the QD materials, the redox mediator is no longer I−/I3− and the electrode is not typically platinum. More detail regarding each of these materials follows.
Just as for the DSSC (and for the same reasons), the morphology of the semiconductor material is critical to the performance of the QDSSC. Zinc oxide and titanium oxide are the most often used, and, like DSSC, various morphologies have
been explored from the standard mesoporous film to nanotubes, nanowires, and the
inverse opal structure. It is the makeup and synthesis of the QD sensitizers, however,
that we will examine in more detail.
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FIGURE 7.77 Depiction of the anode–semiconductor–QD layers in a QDSSC. (Adapted
with permission from Santra, P.K. and P.V. Kamat. 2012. Tandem-layered quantum dot solar
cells: Tuning the photovoltaic response with luminescent ternary cadmium chalcogenides. J.
Am. Chem. Soc. 135 (2):877–885. Copyright 2012 American Chemical Society.)
188.8.131.52 Quantum Dots
The materials typically used for QDs in QDSSC are the metal chalcogenides, just
as for thin-film inorganic solar cells (Section 7.3.2). While cadmium sulfide (CdS) is
probably the most widely studied QD for PV applications, cadmium selenide (CdSe),
antimony sulfide (Sb2S3), indium phosphide (InP), indium arsenide (InAs), lead sulfide (PbS), and lead selenide (PbSe) are among the others that have been used. There
are two primary methods for connecting the dots to the metal oxide: (a) by direct
growth of the QDs onto the oxide surface or (b) by synthesis of the QDs first, then
linking them to the oxide surface. As with dye sensitization, the amount of coverage,
quality of the attachment, and overall morphology strongly impact the cell performance since it directly effects recombination and charge separation and transport.
Good coverage with good interfacial connectivity results in a high Jsc by reducing
recombination of the injected electrons from TiO2 to the electrolyte. However, a
thick layer of QDs makes electron transport more difficult and the porosity of the QD
layer is diminished, limiting the contact between the QD sensitizers and the redox
electrolyte and therefore diminishing the solar cell performance (Zhu et al. 2011).
Direct growth. The first method for preparing QDs—direct growth on the metaloxide electrode—can take place by either chemical bath deposition (CBD) or by
successive ionic layer adsorption and reaction (SILAR). In the CBD method,
the appropriate precursors are dissolved together and the metal-oxide electrode
immersed into this solution. For example, CdS QDs were grown on a TiO2 electrode by mixing together cadmium nitrate [Cd(NO3)2] and thiourea [NH2C(S)NH2]
in water, placing the electrode into the bath, then treating the assembly to microwave
irradiation whereupon the individual reagents presumably reacted on the TiO2 surface to make the CdS QDs (Zhu et al. 2011). X-ray diffraction was used to confirm
the desired outcome; the resultant XRD pattern (Figure 7.78) shows the expected
peaks for the cubic phase of cadmium and sulfur. The optimum concentration of the
bath components was found to be 0.5 M as the cell fabricated from this concentration
gave the best conversion efficiency (albeit only 1.8%).
The SILAR method of QD deposition takes place by cycling the electrode repeatedly into separate solutions of the cationic and anionic precursors. SILAR is considered the better method for deposition of QDs onto mesoporous metal oxides (Emin
et al. 2011) and can be illustrated by the work of Guijarro et al. Cadmium acetate
(0.5 M, aqueous) was used as the Cd2+ source and sodium selenosulfate (Na2SeSO3,
1 M aqueous) was the anionic precursor. As the transmission electron microscopy
images in Figure 7.79 illustrate, after twenty successive immersions of the TiO2 electrode in the cadmium and selenium solutions discrete CdS particles (≈10 nm in size)
are forming on the TiO2 surface (Guijarro et al. 2010).
The linker method. Attaching presynthesized nanocrystalline QDs to the metaloxide surface has also been used extensively, allowing more control of the diameter
and shape of the QDs—an important consideration given that the size tunability
of QDs is what makes their use in PVs attractive in the first place. However, the
performance of these QDSSCs is generally lower, likely due to a lower loading of
QDs on the electrode surface (Margraf et al. 2013). One of the most commonly used
class of linkers is a mercapto acid, for example, thioglycolic acid (HSCH2CO2H) or
FIGURE 7.78 X-ray diffraction pattern confirming the presence of the desired phases of
CdS quantum dots. (Reprinted with permission from Zhu, G., L. Pan, T. Xu et al. 2011.
One-step synthesis of Cds sensitized TiO2 photoanodes for quantum dot-sensitized solar
cells by microwave assisted chemical bath deposition method. ACS Appl. Mater. Interfaces 3
(5):1472–1478. Copyright 2011 American Chemical Society.)
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Cd: Se 1.23:1
After 20 SILAR cycles
FIGURE 7.79 TEM images for TiO2/CdSe QDs prepared by the SILAR method. (Adapted
with permission from Guijarro, N., T. Lana-Villarreal, Q. Shen et al. 2010. Sensitization
of titanium dioxide photoanodes with cadmium selenide quantum dots prepared by Silar:
Photoelectrochemical and carrier dynamics studies. J. Phys. Chem. C 114 (50):21928–21937.
Copyright 2010 American Chemical Society.)
mercaptopropionic acid (HSCH2CH2CO2H). Cysteine [HSCH2CH(NH2)CO2H], too,
has been found to be an effective linker, particularly with respect to efficiency of
charge injection to the TiO2 semiconductor that must proceed indirectly through the
linker. A cartoon illustrating the connection of CdSe QDs linked to a TiO2 surface
is shown in Figure 7.80. The TiO2 electrode surface is first modified by immersion
deposition of the linkers followed by immersion of the QD solution. With any of
these methods, careful optimization of the experimental variables (concentration of
solutions, time and temperature of immersion, even pH) is important to obtain the
best thickness of QD layer, connection between the QD and TiO2, and minimization
of aggregation. Needless to say, it is a work in progress.
184.108.40.206 Redox Mediator and Electrode Materials
While the iodide/triiodide couple is just about ideal for the DSSC, the reactivity
of this pair with metal chalcogenides (particularly CdS) make it an unacceptable
choice for the QDSSC. Although cobalt(II/III) and ferrocene/ferrocenium (Fe2+/
Fe3+) couples have been used in QDSSC, the polysulfide electrolyte S2 − /S2x − is the
most commonly chosen. In aqueous media, sodium sulfide exists in equilibrium with
the thiolate anion (HS–; Equation 7.19)
S2 − + H 2 O
HS− + HO −
Although the equilibrium lies to the right for this acid–base reaction, the sulfide ion
is the active reducing agent (Chakrapani et al. 2011).
Further, because of the use of sulfide in the electrolyte, platinum is an incompatible material for the counter electrode in a QDSSC. Carbon, gold, copper sulfide,
QDR = O2CCH2CH2S–QD
FIGURE 7.80 A depiction of the linker method for attaching QDs to TiO2 particles; relative
sizes are not to scale.
and even conducting polymers have been used in this role. For example, researchers
found that a porous matrix of PEDOT (see Figure 7.48) was “very suitable” for use
with the polysulfide electrolyte and performed slightly better than a gold-based electrode (Yeh et al. 2011). Similarly, 2-μm microspheres made up of copper, zinc, tin,
and sulfur (Cu2ZnSnS4) were prepared and used effectively as the counter electrode
in a QDSSC with the polysulfide electrolyte, showing improvement in both Jsc and
FF (Xu et al. 2012).
The mechanism for a QDSSC is, as expected, quite similar to that for a DSSC.
Figure 7.81 graphically summarizes both the architecture and the conduction process. Several postulated steps in the mechanism are given in Equations 7.20 through
7.22, where “h” stands for hole and with CdS as the QD sensitizer, TiO2 as the metaloxide semiconductor, and polysulfide as the electrolyte (Emin et al. 2011).
CdS + hn → CdS(h + e)(exciton generation)
CdS(h + e) + TiO2 → CdS(h) + TiO2 (e)(charge separation )
CdS(h) + S2 − → CdS + S− i (hole scavenging by redox mediator )
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Incident photons striking the device generate an exciton that undergoes dissociation at the QD/metal oxide (MOx) interface (Equation 7.20). Electron injection into
the conduction band of MOx results in charge separation and the electron forging its
way to the anode (Equation 7.21). The QD is oxidized and the holes are scavenged by
the redox mediator, reducing it (Equation 7.22) to give a radical anion, S−•, which in
turn complexes with S2− to make a polysulfide radical anion. Reduction of the QD to
its ground state takes place by the sulfide anion (S−), and the oxidized species S2−
reduced at the cathode to complete the electrochemical cycle, as indicated by the
dashed double-headed arrow in Figure 7.81.
As noted above, QDSSCs are not as yet very efficient PV devices. This poor performance is attributed to a multitude of recombination processes including some
based on “surface states” of the QD (for our purposes, a surface state is where the
material ends with dangling bonds at a surface, a defect much like a grain boundary
in multicrystalline silicon (Section 220.127.116.11). The various routes of recombination are
shown in Figure 7.82. Pathways (1) and (2) illustrate recombination of an electron
with a hole via the surface state, where (1) is recombination of the already injected
electron in the TiO2 with a trapped hole in the QD and (2) is recombination within
the QD. It is a short hop for these surface-trapped electrons to the electrolyte (3).
Alternatively, prior to injection into the TiO2, the separated electron in the QD (4) or
the TiO2 (5) can be captured by the electrolyte (Shalom et al. 2009). Back electron
transfer from the injected electron in TiO2 (pathway 5) has been shown to occur to S−•
much more rapidly than the analogous process in DSSC with the I−/I3− couple, making this a major hurdle to overcome for improved QDSSC performance (Chakrapani
et al. 2011). Some improvement can be obtained in limiting these paths of recombination by passivation of the surface states. We have already seen one example of
improving solar cell performance by passivating these dangling bonds, that being
FIGURE 7.81 The photoconduction process in a QD-sensitized solar cell.
FIGURE 7.82 Decay processes in a QD-sensitized solar cell. (Adapted with permission
from Shalom, M., S. Dor, S. Rühle et al. 2009. Core/CdS quantum dot/shell mesoporous solar
cells with improved stability and efficiency using an amorphous TiO2 coating. J. Phys. Chem.
C 113 (9):3895–3898. Copyright 2009 American Chemical Society.)
a-Si:H (Section 18.104.22.168). In QDs, surface states can be modified by coating the QD
with another material such as ZnS, TiO2, or even organic molecules (Emin et al.
2011). In so doing, this coating serves as a block between the QD and the electrolyte,
minimizing pathways (4) and (5) and thus improving the performance of the cell.
7.7 SUSTAINABILITY, PHOTOVOLTAICS, AND THE CZTS CELL
Solar is one of the fastest growing areas of renewable energy, but PV technology is
very much limited by resource availability. While silicon is one of the most abundant
materials on the Earth, silver—used for the electrodes in c-Si cells—will eventually
restrict their implementation (Feltrin and Freundlich 2008). In addition, several rare
elements are critical components in photovoltaics (recall Figure 1.8 in Chapter 1).
Gallium and selenium limit the installation of CIGS/Se solar cells—gallium and
selenium are both past their production peak, and the demand for gallium is projected
to outpace production within years (Knowledge Transfer Network 2010). Indium,
too, is a scarce element and indium tin oxide is used in a very wide variety of PV
devices. Germanium, cadmium, and tellurium also pose resource limitation issues.
A hypothetical situation in which CdTe cells supply 25 TW would require over 100
times more cadmium than available in the current world reserves (Peter 2011). These
constraints in material availability are one of the major driving forces behind thinner
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layer technology and increasing efficiencies, but there are limits to each (efficiency
is literally limited by the Shockley–Queisser limit). The alternative is to (a) use less,
(b) use material science and engineering concepts to utilize these materials more
effectively (e.g., nanotechnology), or (c) find replacements for resource-scarce materials. It is the latter approach that has led to the development of the “earth abundant”
photovoltaics based on Cu2Zn(Sn1−xGex)S,Se (CZTS or CZTSe).
CZTS and derivatives are members of the kesterite crystal family where kesterite refers to a relatively abundant mineral of formula Cu2 Zn 0.75 Fe 2 + 0.25Sn1.3S4 .
In effect, the CZTS/Se cells replace the rare elements of indium and gallium with
their neighbors zinc and tin on the periodic table. Like the chalcogenides covered
in Section 22.214.171.124, the kesterites have the same basic copper/sulfur structure, but the
M(III) ions have been replaced with an equal number of M(II) and M(IV) atoms
(Figure 7.83). The CZTS/Se materials are p-type absorbers in which modification
of the ratio of S to Se allows for tuning the band gap, as can be gleaned from comparing the values for CZTS (1.45 eV) and CZTSe (0.94 eV; Table 7.1). Band gaps
in the range of 1−1.5 eV can be attained (Mitzi et al. 2011). Recently, CZTS or
CZTSe cells substituted with germanium in varying ratios (replacing tin) have been
investigated to further tune the band gap and address device limitations. A 40%
Ge-doped CZTSe (Cu1.5ZnSn0.5Ge0.4Se4) shows a shift to a slightly larger band gap
(1.15 eV vs. 1.08 eV for pure CZTSe) but with essentially no improvement in PCE
(the Ge-substituted cell gives a PCE of 9.1% compared to the pure CZTSe cell’s
9.07%) (Bag et al. 2012).
Research effort into the use of CZTS/Se materials in solar cells is very active.
A comparison of a variety of CZTSSe cells with a CIGSSe standard (with a PCE
of 13.8%) was carried out to begin to understand the limitations of CZTSSe cell
efficiencies. The devices were fabricated on a molybdenum-coated glass substrate
followed by the absorber material (either CZTSSe or CIGSSe), then CdS/ZnO/ITO
FIGURE 7.83 The unit cells of (a) kesterite and (b) CZTSSe. (Reprinted with permission
from Fan, F.-J., L. Wu, M. Gong et al. 2013. Composition- and band-gap-tunable synthesis
of Wurtzite-derived Cu2ZnSn(S1–xSex)4 nanocrystals: Theoretical and experimental insights.
ACS Nano 7 (2):1454–1463. Copyright 2013 American Chemical Society.)