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6  Quantum Dot Solar Cells

6  Quantum Dot Solar Cells

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Chemistry of Sustainable Energy

CdSe quantum dots



2.8 nm



3.3 nm



4.0 nm



4.2 nm



Energy (eV) vs. vacuum



–3.28 eV

–3.41 eV

Eg

2.51 eV



Eg

2.37 eV



–3.51 eV

Eg

2.25 eV



–3.57 eV

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)



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Solar Photovoltaics

n*1



kcool



n1



(1)



kEHPM

n*2



(2)



n2

hv



kEHPM

n3



n*3

(m–1)

kEHPM



n*m



nm



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

Chemical Society.)



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.

7.6.2.1 Semiconductor

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.



272



Glass/TCO substrate



Chemistry of Sustainable Energy



TiO2



Quantum dot



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.)



7.6.2.2  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,



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Solar Photovoltaics



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



CdS

(220)



CdS

(111)



CdS

(311)



Intensity



TiO2/CdS electrode



TiO2 electrode

20



30



40



50

2 theta/degree



60



70



80



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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Chemistry of Sustainable Energy

Cd: Se 1.23:1



20 nm



20 nm

TiO2



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.

7.6.2.3  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 −



(7.19)



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,



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Solar Photovoltaics

S

S

QDR



O



Quantum

dot



O



O



O

O



QDR

QDR

TiO2



QDR



S

O

O



QDR

O



S

O



QDR

O



O



O



S



O

O



S



S

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).



7.6.3 Mechanism

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)



(7.20)







CdS(h + e) + TiO2 → CdS(h) + TiO2 (e)(charge separation )



(7.21)







CdS(h) + S2 − → CdS + S− i (hole scavenging by redox mediator )







(7.22)



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Chemistry of Sustainable Energy



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−

x is

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 7.3.1.2). 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



CB



Hole

scavenging



Anode



Fermi level

+ hv



VOC



S–/Sx2–

Reduction



Oxidation



Quantum

dot

Load



FIGURE 7.81  The photoconduction process in a QD-sensitized solar cell.



Cathode



Energy



MOxCB



Electron

injection



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Solar Photovoltaics



Electron

CB



CB



4

2



1

Energy



5



3



Surface

state

1



2



Hole



Metal

oxide



Quantum

dot



S–/Sx2–



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 7.3.1.2). 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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Chemistry of Sustainable Energy



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 7.3.2.2, 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

(a)



(b)



Cu

Cu



Zn

Se



Sn

Zn

Zn



Cu

Cu



Se



Se

Sn



Se



Sn

Sn



Se

Zn



Cu

Cu

Se



Cu

Se



Zn

Zn

Se



Sn

Se



Cu

Cu

Se



Cu



Cu



Zn



Se



Se



Se



Se

Zn

Zn



Cu



Cu

Cu

Cu



Cu

Cu



Zn

Se



Se



Sn

Se



Cu

Se

Cu

Cu



Cu



Cu



Zn



Sn



Cu



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.)



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