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2…Dye-Sensitised Solar Cells Dye-Sensitised Solar Cells

2…Dye-Sensitised Solar Cells Dye-Sensitised Solar Cells

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Fig. 7.1 a Schematic diagram illustrating the architecture and main electronic transition steps

occurring in a typical DSSC. White circles represent semiconductor particles and the black circles

the dye molecules; charge flow is represented by arrows. b Energy level diagram illustrating the

relative energy position of DSSC main components: semiconductor, sensitiser dye and

electrolyte. Electron injection from hot states (kinj*) and relaxed states (kinj) of the excited

dye, and charge recombination (krec) between the electron in the semiconductor conduction band

and the dye cation, are represented by arrows



The injection process is in kinetic competition with the relaxation processes of the

dye in its excited state (Eq. 7.3). Efficient electron transport within the semiconductor allows the collection of electrons on the back contact of the electrode,

typically a thin layer of fluorine doped tin oxide (F:SnO2, or FTO) or indium doped

tin oxide (In:SnO2, or ITO) deposited on a transparent substrate, like glass. This

electrode is known as photoanode because it promotes the photo-oxidation of the

sensitiser and moves electrons to the external circuit. The redox electrolyte in

contact with the sensitised semiconductor reduces the oxidised dye, regenerating

the sensitiser, and transports the resulting positive charge to the counter electrode

(Eq. 7.4). This electrode (the cathode, as it collects positive charges) is usually a

conducting glass covered with a transparent platinum or carbon thin coating. An

external circuit connecting this sandwich-like structure allows the transport of the

collected electrons from the anode to the cathode. These electrons will promote the

reduction of the redox mediator (Eq. 7.6), closing the circuit. Well-known sources

of efficiency loss in DSSC involve transfer of electrons across the semiconductor/

electrolyte interface, either to oxidised dye molecules (charge recombination,

Eq. 7.5) or to the oxidised component of the redox couple (charge interception,

Eq. 7.7).

Excitation:

TiO2 jjS þ hm ! TiO2 jjSÃ :



ð7:1Þ



þ

TiO2 jjSÃ ! eÀ

cb À TiO2 jjS :



ð7:2Þ



Injection:



7 Solar Energy Conversion



271



Relaxation:

TiO2 jjSÃ ! TiO2 jjS þ light or heat:



ð7:3Þ



TiO2 jjSþ þ 3=2 IÀ ! TiO2 jjS ỵ 1=2 I

3:



7:4ị



Dye regeneration:



Charge recombination:



e

cb TiO2 jjS ! TiO2 jjS:



7:5ị







1=2 I

3 ỵ e Pt ! 3=2 I :



7:6ị







2e

cb ỵ I3 ! 3 I :



7:7ị



Charge transport:



Charge interception:



Developments in DSSCs depend on our understanding and control of the

fundamental kinetic and thermochemical nanoscale phenomena that govern the

‘conversion of photon energy into electron energy’, namely on the interfacial

electron transfer and charge transport dynamics. Efficient dye-sensitised solar cells

depend on the fine-tuning of the energies of the states implicated and of the rates of

the processes involved.

In an electric insulating material the energy gap between the highest occupied

energy state, the valence band and the conduction band (that in a molecular

analogy would be an excited state) is of tens of eV. For semiconductors this gap

can be as small as 1 eV, which enables, even at ambient temperature, some excited

electrons to partially fill the conduction band, giving the material electrical conduction properties. The particular energetic position of the conduction band in

some wide band gap semiconductors (for example rutile TiO2, Eg = 3.2 eV)

allows molecular excited states of suitable sensitiser molecules to efficiently

populate the semiconductor conduction band. For efficient electron injection to

occur in a DSSC, the energy of the excited dye should be higher than the conduction band of the semiconductor. The importance of energy matching between

the dye excited state and the semiconductor conduction band can be demonstrated

by comparing the sensitisation of distinct semiconductors by Ru(dcbpy)2(NCS)2

(N3) (structure shown in Chap. 4: compound 4.3) [6–8]. The conduction band

edge of TiO2 is below the LUMO of N3, whereas the conduction band edges of

niobium pentoxide (Nb2O5) and zirconium dioxide (ZrO2) are above it (0.2–0.3

and 0.9–1.0 eV, respectively). As expected, spectroscopic probing reveals the

absence of conduction band electron population for excited N3-ZrO2 (550 nm

excitation), as the conduction band is too negative to allow electron injection from

N3. Excitation of N3 at 650 nm excites the dye to the lowest vibrational level of the

sensitiser’s excited state. In these conditions the appearance of N3+, as a probe of

electron injection, was only observed for N3-TiO2, consistent with the fact that the



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energy of the lowest vibrational level of N3 excited state lies below the bottom of

the other semiconductor conduction bands. But, for 450 nm excitation, high

injection yields were observed for both TiO2 and Nb2O5-sensitised semiconductors .

This observation of excitation wavelength dependence of the electron injection

yields, is consistent with the observation of extremely fast photoexcited electrons

transfer even from vibrational ‘hot’ states of the sensitiser into the semiconductor

conduction band. The observed electron injection has a dispersive kinetics regime

with components on the femto- and pico-second timescale, competing with nuclear

relaxation (kT *1012 s-1), and thus allowing non-thermalised electron transfer. It

could be expected that the electron injection yield would not decrease when shifting

the conduction band edge more negative (enhancing the cell potential), but the

observation of dispersive kinetics for electron injection, including slow components, forecasts that this is not entirely the case [9].

The optimal working conditions of DSSCs are a result of quite favourable

differential kinetics. It is energetically possible for the injected electron to

recombine back into the dye, but the rate at which this occurs in an optimised

device is quite slow compared to the rate that the dye recaptures an electron from

the electrolyte. Why is such slow recombination observed after such a fast electron

injection? Transport of photoinjected electrons through the semiconductor particle

network occurs with a rate of micro- to milliseconds, so the reason is not the fast

electron removal through the semiconductor. Are there also energy, geometrical or

spatial constraints? The answer to these questions offers a good opportunity to

briefly explore the next pair of equations:

2p 2

H FC ðaDGÞ:

h

"



ð7:8Þ



H = H0 expẵbr r0



7:9ị



kET ẳ



Following Fermis Golden Rule (Eq. 7.8), the interfacial electron transfer rates

depend on the overlap of the sensitiser dye electronic orbital distribution function

with the wide continuum of energy levels of the semiconductor, the electronic

coupling factor H2, and on the overlap between the vibrational levels involved in

the transition [10]. This nuclear vibration factor (Franck-Condon factor, FC) is

explicitly dependent on the relative energetics of these states. We can assume that

solvent dynamics are not important in interfacial electron transfer since solvent

molecules immediately adjust their positions to the newly formed charges. Under

these conditions, and if the electronic coupling is high enough, the rate becomes

solely controlled by the nuclear vibration of the electron donor and acceptor and

thus dependent on the Gibbs energy change resulting from the electron transfer,

DG. However, if the electronic coupling is low enough, the rate will depend both

on DG and the H value. As a consequence, when the electronic coupling is weak,

geometry and distance will also affect the electron transfer rates, as shown in

Eq. 7.9.

Considering electron injection, a sensitiser directly bound to the semiconductor

allows a strong electronic coupling between the vibronic levels of the excited



7 Solar Energy Conversion



273



sensitizer and the high density of acceptor states, thus a high value of H. Under

these conditions the injection rate should follow the FC factor Gibbs energy

dependence, and this indeed was observed [11]. It was also observed that the

existence of (non-conjugated) spacers between the sensitiser and the TiO2 nanoparticles lead to an exponential reduction of the electron injection rate (see

Eq. 7.9) [11]. These observations are consistent with the understanding of the fast

electron injection as described by Eqs. 7.8 and 7.9.

What about the much slower electron recombination? The nuclear FC factor

shows an almost quadratic dependence on DG, leading to an inverted region, where

the reaction rates become slower as the reactions become more exergonic. Such

energy constraints may be the reason behind the slow recombination. Although

such behaviour was observed for particular systems [12], it was found that this

could not be the general explanation. In fact, DG for electron recombination for

some of the more efficient polypyridyl sensitisers is not exergonic enough to be in

the inverted region. So, only diminished electron coupling can explain the slow

recombination observed for such dyes. How can a high electronic coupling for

direct electron transfer (injection) become a low coupling for recombination?

Equation 7.9 suggests that if the distance for electron recombination is higher than

the distance for electron injection, the recombination rate would decrease exponentially. A quite interesting example is given by the molecular modelling of the

N3 dye relevant orbitals for electron transfer. The appropriate orbital for electron

injection (the LUMO) is located on the bypyridyl rings, closely bound to the

semiconductor, whereas the relevant orbital for charge recombination (the HOMO)

is located on the NCS groups, away from the semiconductor surface. In this particular case, distance constraints lead to optimum conditions for fast electron

injection and slow recombination, as favourable molecular electronic distribution

takes the ‘hole’ away from the semiconductor [13].

TiO2 electron interception by species in the electrolyte is also in kinetic

competition with electron transport through the semiconductor. Again, for optimised devices this reaction is rather slow. For this particular reaction the I3-/Iredox pair has the advantage that the loss of photoinjected electrons to I3- is

relatively slow, as a consequence of the fact that the reaction involves the transfer

of two electrons and the breaking of the I–I bond [14]. The fact that the interception reaction is slow is probably the main reason why the majority of the high

efficiency DSSCs reported to date use the I3-/I- redox pair mediator.

Naturally the rate of electron transport through the semiconductor to the anode,

in direct competition with recombination and interception, is pertinent in the

overall performance of a DSSC. Photo-injected electrons are transported by diffusion in a free random walk [15] as a result of an electron concentration gradient.

The diffusion time will depend on the distance to the anode (the semiconductor

film thickness) and the diffusion coefficient of electrons. But electrons also seem to

become trapped within the semiconductor particles for some time (picoseconds to

nanoseconds). This trapping and thermal release (detrapping) mechanism requires

energy, and is important in retarding charge recombination in DSSCs. The process



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L. G. Arnaut et al.



is particularly dependent on the position of the Fermi level in the semiconductor

and the outcome is a small apparent diffusion coefficient.

On the other side of the sandwich-like DSSC, electron transfer from the counter

electrode to species in the electrolyte should be very fast. For the iodide redox pair,

since platinum at the cathode acts as a catalyst, breaking the I–I bond, the

regeneration of the chemisorbed iodide ions is fast.

If the above kinetic considerations are fundamental for the DSSC performance,

we should emphasise that energetic considerations are also essential. For example,

tuning the photophysical properties (absorption, HOMO and LUMO energies) and

redox characteristics of the sensitiser dye with the semiconductor band gap and

redox pair potential is crucial. Efficient regeneration by the electrolyte needs the

energy level of redox potential to be equal to or higher than the ground state of dye

(HOMO). There is a quest for dyes that improve the solar spectrum harvest

coverage to lower energy photons, so that the LUMO energy level become closer

to the HOMO level, as in the diagram in Fig. 7.2. This should not be made at the

expense of the dye redox potential, because in that case both the HOMO and

LUMO energy levels would shift up in that same diagram. An example is the

comparison between N3 and Ru(dcb)2+

3 dyes. N3 shows better photon collection in

the red part of the spectrum, but E8(RuIII/II) is lower [16]. This means a relative

loss of driving force for the regeneration of the N3 cation (although in this particular case this loss is not sufficiently high to compromise regeneration).



Fig. 7.2 Illustration of the spectroelectrochemical characterisation of a laboratory scale DSSC

with a nanocrystalline TiO2 film sensitised with the RuN3 dye, measured under simulated AM

1.5G solar irradiation (100 mW cm-2); electrolyte composition: methoxyproprionitrile with

0.6 M propylmethylimidazolium iodide, 0.1 M LiI, 0.1 M tert butylpyridine, and 0.1 M iodine or

guanidium thiocyanate; 5 lm thick TiO2 film (9 nm particles) and scattering layer. a J–V curve,

b IPCE spectrum (Courtesy of Patricia Jesus, Coimbra Chemistry Centre). In an optimised RuN3

DSSC the JSC reaches a value 20 mV and the maximum incident photon to current efficiency

reaches 85 % [25]



7 Solar Energy Conversion



275



7.2.2 Device Construction and Characterisation

Although other semiconductors have been investigated, the use of the wide band

gap metal oxide semiconductor titanium dioxide (TiO2) nanocrystals was revealed

to be crucial to present day state-of-the-art solar cell overall performance. The size

and morphology of the particles used are of extreme importance. For 15 nm

diameter particles in a 10 lm transparent thick film there is a 1,000-fold increase

in surface area, compared with a single crystal of equal projected area. This fact

enables the adsorption of large quantities of dye, resulting in efficient light harvesting. Typically, the TiO2 particles are present only in the form of anatase or as a

mixture of 80 % anatase and 20 % rutile. A key point on the performance of the

anode semiconductor film is the quality and characteristics of the colloidal TiO2

paste used. Preparation of this paste may be a complex procedure [17, 18]; high

performance DSSCs are obtained with homemade or commercially available

nanoparticles mixed with additives such as a-terpineol and ethyl cellulose in acidic

water/alcohol media [17]. Alternatively, commercially available pastes can be

used. The best-performing nanocrystalline TiO2 films are typically fabricated by

squeeze-printing (doctor blading) or screen-printing of the paste, followed by high

temperature sintering (400–500 °C) in order to enable electron conduction

between the nanoparticles. The counter electrode can also be deposited by a

printing method. Typical cells consist of a transparent layer of 10–20 lm thickness

made from 10–20 nm semiconductor particles. Improvements in light harvesting

can be achieved by the introduction of a layer of larger TiO2 particles

(100–400 nm). This scattering layer is particularly important when using dyes with

low absorbance in the red and near infrared, as the light reflections inside the film

enhance substantially the optical path length.

Iodide/iodine is the most commonly used redox couple, but the electrolyte is

typically a chemically complex mixture that includes not only the redox couple but

also supplementary additives. A common electrolyte composition consists of

methoxyproprionitrile with 0.6 M propylmethylimidazolium iodide, 0.1 M LiI,

0.1 M tert butylpyridine, and 0.1 M iodine or guanidium thiocyanate. It was

empirically observed that the presence of additives enhances device performance.

It is known that additives shift the conduction band edge, but probably it is the

decrease in the electron interception rate (Eq. 7.7), due to the adsorption of

additives at the TiO2 surface, that is most active in the device performance

enhancement. This is possibly due to the blocking of active reduction sites or

inhibition of close approach of electron acceptors to the TiO2 surface.

The sensitiser dye constitutes the heart of the DSSC, allowing the use of the

solar spectrum to drive electrons from a lower to a higher energy level, and

ultimately generating the cell electric potential difference. Several efforts have

been focused on the development of organic and organometallic dye molecules.

Among them, polypyridylruthenium complexes, yielding solar-to-electric conversion efficiencies of 11 % with simulated sunlight proved to be among the most

efficient sensitisers [19, 20]. In general, sensitiser dye molecules can be regarded



276



L. G. Arnaut et al.



as being constituted by a light antenna part, a spacer and an anchor group. The

anchor group provides proper attachment of the dye to the semiconductor. The

number [21], nature [8] and position [22] of this anchor groups may vary. Carboxylic acid groups are the most commonly used.

The introduction of the electrolyte and the sealing of the cell are critical steps

to assemble a durable DSSC. Although highly efficient and durable dye-sensitised

solar cells need pure materials, complex and controlled procedures [23], reasonable cells can be constructed using commercial available lower cost materials and

following simplified processing steps [24].

The overall DSSC performance can be accessed by measuring the incident

photon-to-current conversion efficiency (IPCE) profile over the solar spectrum and

the current–voltage (J–V) curves under illumination. The IPCE can be defined as

the ratio between the number of electrons collected (the current density measured

in the external circuit) and the number of photons with a given energy that reaches

the cell. It is a collective measure of the cell performance, depending on electron

injection (ginj) and collection (gcoll) efficiencies, as well as on the light harvesting

efficiency (LHE), Eq. 7.10. The IPCE can be assessed at each wavelength by

measuring the short circuit photocurrent density (JSC/mA cm-2) under monochromatic illumination (k/nm) with a given intensity (Iinc/mW cm-2), Eq. 7.11.

(hc/e is 1,240 nm, the wavelength of a photon of 1 eV energy). Due to sensitivity

issues this is typically done by scanning the wavelength range of interest with

chopped light while illuminating the cell with a bias light, and distinguishing the

alternating current output signal from the total output current using a lock-in

technique [26].

IPCEkị ẳ LHEkị/inj gcoll

IPCEkị ẳ



hc JSC kị

1240JSC kị





:

e kIinc ðkÞ

kIinc ðkÞ



ð7:10Þ

ð7:11Þ



Figure 7.2a presents the IPCE as a function of the wavelength measured for a

DSSC made with the N3 dye and Fig. 7.2b shows the J–V curve for the same cell.

Important information about the microscopic cell performance can be obtained

from the current density at short circuit (JSC), the open circuit photovoltage (VOC)

and the cell’s fill factor (FF) defined as,

FF ¼



Pmax

JSC VOC



ð7:12Þ



where Pmax is the product of the photocurrent and photovoltage at the voltage

where the power output of the cell is maximised.

Charge flow in a DSSC involves the conduction of electrons in the semiconductor nanoparticles and of ions in the electrolyte. The current achieved at short

circuit, JSC, depends on the electron injection yield and on electron conduction

losses by recombination or interception. The cell voltage is associated with the

buildup of electron density in the TiO2. The maximum potential produced under



7 Solar Energy Conversion



277



illumination (VOC) corresponds to the difference between the chemical potential of

the electrons at the semiconductor and the chemical potential of the holes at the

hole conductor. So, the device photovoltage depends on the redox couple, as it sets

the electrochemical potential at the counter electrode and the semiconductor’s

electrons potential, determined by its Fermi level.

Ultimately the conversion efficiency of a DSSC, gDSSC, is determined by the

photocurrent density measured at short-circuit, the open-circuit photovoltage and

the cell’s fill factor, corrected by the intensity of the incident light (Iinc).

gDSSC ẳ



JSC VOC FF

:

Iinc



7:13ị



The characterisation of a DSSC device or the study of partial processes that

occurs at such cells uses a series of optical and electrochemical techniques, either

stationary or time-resolved. The studies cover a wide range of timescales,

accompanying the wide time span of phenomena occurring in a DSSC (from fs/ps

for electron injection to ms for electron transport). Optical transient absorption

techniques (see Chaps. 8, 14, 15) are used in combination with transient electrical

measurements to follow the appearance and disappearance of chemical species and

charges on a DSSC [27].



7.2.3 Novel Approaches

The gap between the full potential of solar energy andour and our energy needs

can only be met by raising the efficiency of the conversion processes. In DSSCs

these efficiencies are still well below the theoretical limit. Reaching this limit is

both a scientific and an engineering endeavour. Thermodynamic analysis established the efficiency limit of 27–31 % for photovoltaic conversion efficiency [28],

based on several premises that new developments may overcome. These

assumptions are: a single layer cell, one electron–hole pair formation per absorbed

photon, thermal relaxation of the electron–hole pair energy into semiconductor

band gap energy, illumination with non-concentrated light.

A DSSC is a complex device with complex interactions between the device

components. Innovation in a particular component is intrinsically related to

changes in the performance of other components. For example: chromophores that

can collect light more efficiently would make it possible to reduce the semiconductor thickness, with the implication of reducing electron interception by oxidised redox species and consequently increasing electron concentration, but also

allowing further exploration of redox couples, including solid-state hole conductors. In the following paragraphs we will focus on recent and forecasted advances

in (i) sensitiser dyes, (ii) electron conduction and (iii) hole conduction.

In a perfect world the absorption spectrum of a dye-sensitised solar cell would be

optimum for the solar spectrum, which means high molar absorption coefficients



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L. G. Arnaut et al.



below 900 nm. Research on dyes that effectively harvest the red and near-IR part of

sunlight had significant progress over the last decade [5].

Due to its chemical instability, the weakest part of the Ru dyes is the SCNligand [29]. Probably the most promising thiocyanate-free Ru complexes are

cyclometalated complexes, showing IPCE with a maximum of 80 % and an

absorption window that extends beyond 800 nm [5]. The presence of fluoride

substitution in these ligands adjusts the electron density of the dye and allows the

sharing of the positive charge of the oxidised dye between the central metal and

the ligand, contributing to a rapid charge transfer to the iodide ions in the electrolyte [5]. Other recent optimisation efforts include the development of heteroleptic ruthenium complexes, with thiophene moieties attached to the bpy-ligand

enhancing both the absorption coefficient and shifting the spectral response to the

red [30].

More recently, research efforts have been focused on the design of new aromatic molecules , which can cover the whole visible solar light spectrum. Among

them, interest has been focused on perylenes [31], phthalocyanines [32] and

especially tetrapyrrolic macrocycles, which play an important role in natural

photosynthetic pigments, like porphyrins [33], chlorins [33] and bacteriochlorins

[30]. The long list of sensitisers even includes dyes from fruit extracts [34].

Examples of many of these structural classes can be found in Chap. 4.

The adsorption of dyes with complementary absorption characteristics can be

used as a strategy for improving light harvesting. This panchromatic approach can

be based on the adsorption of a dye mixture or a more sophisticated multi-dye

layer sequence [35]. The IPCE spectral shape for the panchromatic cell results in

the superimposition of the IPCE spectrum for single dyed cells, and the J–V curves

yield a higher value for the JSC than for the corresponding single dyed cells.

Antenna structures, which use distinct chromophores for light absorption and

electron injection have also been tried. Efficient energy transfer from the antenna

dye to the injection dye is required [36, 37]. Another appealing idea for the

improvement of DSSC efficiency is the use of dyes with a high ground-state S0–S2

excited state transition probability or capable of multiphotonic absorption. In

conjugation with the appropriate conduction band energy semiconductors, these

dyes would allow the injection of very energetic electrons and a consequent

increase in the cell potential.

Quantum dots, with high absorbing coefficients in the visible, have also been

used as sensitisers in DSSCs. Despite the high theoretical maximum efficiency the

energy conversions obtained are still low. Recently, an energy conversion efficiency as high as 4.22 % was achieved with CdS and CdSe as co-sensitisers of

TiO2, taking advantage of the two materials in light harvesting and electron

injection [38].

New architectures for DSSCs include new photoanodes [9]. The expectation is

that the transport of photogenerated electrons along new nanostructured materials,

such as nanowires or nanotubes, will be faster than in a network of sintered

titanium oxide particles. The random walk of electrons through the nanoparticle

particles limits the collection of charges to the millisecond timescale. Reducing the



7 Solar Energy Conversion



279



number of grain boundaries, which are obstacles to fast electron transport, would

reduce the loss of charge carriers by recombination and interception and therefore

increase electron collection. It has been demonstrated that the morphology of

semiconductor films has an effect on the electron transport losses [39]. Changing

the usual granular TiO2 morphology into a columnar morphology leads to an

increase in the short-circuit current, assigned to differences in electron transport

with an increase in the electron lifetime in the TiO2 columnar film. Those nanowire, nanorod or nanotube (and even some more unusual nanoplant-like [40])

semiconductor morphologies should have high surface area, otherwise this would

require the design of new dyes with higher absorption coefficients in order to

obtain efficient light harvesting.

Although the most commonly used redox couple to act as a hole transport

medium is the I3-/I- this does not mean the couple is necessarily unique. Actually

the space for improvement for DSSCs that use this redox mediator is mostly

limited to improvements in better light harvesting dyes [41]. Corrosion, light

absorption and diffusion limitations had been identified for the I3-/I- pair and it

has been replaced successfully by cobalt-based redox systems [14], as well as by

organic hole conductors [42]. Difficulties in sealing to prevent evaporation and

water diffusion into the cell led to research into the substitution of liquid redox pair

electrolyte, replacing the liquid for solid or quasi-solid hole-conduction media,

such as polymeric, gel [43], or solid electrolytes [44].

Solid-state redox mediators or hole conductor materials would make it possible

to construct completely solid-state DSSCs that will probably have considerable

added commercial value. One of the main difficulties in substituting liquid electrolytes is the need for an ‘interpenetration’ of the sensitised metal oxide by the

electrolyte, in order to have efficient contact between the sensitiser cation (the

hole) and the mediator. Additionally, prospective solid hole collectors should have

the following properties: the valence band of the hole collector material must be

located above the bottom of the sensitiser dye ground state; it must be transparent

throughout the visible spectrum, where the dye absorbs light; and the deposition of

the solid material should be done without degrading the monolayer of sensitiser

dye adsorbed on TiO2.

In terms of stability, inorganic oxide semiconductors of Ni and Cu are among

the very few oxides which have been shown to possess a suitable band gap and

band-position, although the efficiency of the cells constructed with those compounds is still low [45, 46]. Both low intrinsic hole mobility and hindered penetration of the hole collector into the dyed mesoporous TiO2 film (due to the bigger

particle sizes compared to that of TiO2 pores), seem to be the reason for the poor

performance. A promising material is CuBO2, which is transparent over a wide

spectral range with suitable band gap and exhibiting high conductivity and hole

mobility [47].

Polymer materials as a hole-transporting layer, in an all solid-state polymer-based

DSSC, have also been devised. Cells with polymers such as poly(N-vinylcarbazole)

forming a solid junction comprising nanocrystalline TiO2/dye monolayer and hole

transporter at which photo-induced charge separation proceeds, obtained a relatively



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high conversion efficiency with a simple composition and a cheap polymer material

[48]. In order to avoid the difficulty of polymer penetration into the porous TiO2

in situ polymerisation of pre-penetrated monomers has been developed. Polymers

prepared as carboxylated diacetylenes, were found to be efficient hole conductors

[49]. Conjugation of dye and interfacial engineering with in situ polymerisation of

poly(3,4 ethylenedioxythiophene (PEDOT, see Chap. 4: compound 4.10) yielded an

average efficiency of 6.1 %, which represents a remarkable improvement for

polymer-based DSSC [50].

We can expect that the joint efforts of molecular science and nanotechnology

will tackle the problems that still persist so that efficient, low cost and environmental friendly DSSCs can be widely available in the near future.



7.3 Organic Solar Cells

7.3.1 Exciton Diffusion and Charge-Carrier Mobilities

Another very appealing concept for the conversion of light into electric power is

the fabrication of organic photovoltaic cells with materials simply processed from

solution. Such solar cells would combine low cost, large area, lightweight, flexibility and versatility.

A first step in making efficient organic solar cells was reported by Tang in 1986,

who reported a cell with a power conversion efficiency of 1 % under AM2 illumination [51]. These cells were made of one 30 nm thick layer of copper phthalocyanine deposited on ITO glass and, on the top of it, a 50 nm layer of a perylene

tetracarboxylic derivative was deposited, followed by an opaque Ag layer. Charge

separation was driven by the differences of electron affinities (and/or of ionisation

potentials) between the two organic layers. Donor/acceptor bilayer devices have an

intrinsic limitation. Organic molecules rarely exceed molar absorption coefficients

of 105 mol-1 dm3 cm-1 in the most intense regions of the solar spectrum. Harvesting 90 % of the photons with such absorptivities requires a layer thickness of

100 nm. When one photon is absorbed by a layer of solid material made of such

molecules, it originates a Coulombically bound electron–hole pair, or exciton, that

must migrate to the donor/acceptor interface with the other layer before the exciton

dissociates in a charge-separated state. This is an essentially diffusive process of an

uncharged species, which must compete with its decay to the ground state. The

exciton diffusion length, L = (D)1/2, determined by the exciton diffusion coefficient D and lifetime, must be of the same magnitude as the layer thickness to

promote the efficient formation of charge-separated states. Singlet exciton

migration can be understood in terms of long-range electrostatic coupling between

the transition dipoles of initial and final states (Förster mechanism), whereas triplet

excitons migrate via short-range exchange due to orbital overlap (Dexter mechanism see Chap. 1). The diffusion of singlet excitons is faster but this may be offset



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