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II. Types of Organic Solar Cells

II. Types of Organic Solar Cells

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Types of Organic Solar Cells



161



A. Dye-Sensitized Solar Cells

The dye-sensitized nanostructured solar cell (DSSC) was rst developed

by Graătzels group in 1991.8,9 The operating principle of DSSCs is shown in

Figure 1. Light is harvested by an organic or organometallic sensitizer, which is

anchored to the surface of a wide band gap inorganic semiconductor via a

carboxylate, phosphonate, or hydroxamate group. Upon photosensitization and

the formation of excitons, electrons are injected from the sensitizing molecules

(the dye molecules) into the semiconducting metal oxides. Charge separation

then occurs at the interface of the inorganic semiconductor and sensitizer, and the

photo-excited electron is injected into the conduction band of the semiconductor

in the subpicosecond region. Electrons then migrate to the back contact electrode

via a network of semiconductor nanostructure. In the other end, the photosensitizers accept electrons from the redox mediator electrolyte. Hole transport is

aided by the redox cycles in the electrolyte. The iodide/triiodide redox couple

system is regarded as the best electrolyte in solvents containing DSSCs.

Polypyridyl ruthenium complexes have been extensively studied as the

candidates for photosensitizers due to their high stability toward sunlight,

excellent redox properties, and high versatility of turning the metal-to-ligand

charge transfer (MLCT; 4dπ2πL*) absorption over the visible spectrum by

judicious manipulation of ligand chelation.10,11 Metal oxides such as TiO2,

ZnO, SnO2, and Nb2O5 are commonly used as the electron accepting and

transport materials. The nanocrystalline structures provided huge surface area



Excited

state

CB



Ox

Mediator



Red



Dye

Ground

state



Nanocrystalline

semiconductor

photoelectrode



e-



Electrolyte



Load



e-



e-



FIGURE 1. The operating principles of dye-sensitized solar cells.



Counterelectrode



Transparent Conducting Glass



e-



162



Applications of Metal Containing Polymers in Organic Solar Cells



SCHEME 1.



for anchoring light-absorbing dye and enhance the absorption cross-section.

The charges are transported to the electrodes and the external circuit to generate electricity through the conduction band of the semiconductor nanostructure. Excellent and comprehensive discussions on the advances in structure

engineering of small molecule photosensitizers and inorganic semiconductor

nanostructure of DSSC have been published.8,9,12À17À20

The cell is on the verge of commercialization, offering a potential alternative for the currently used silicon-based photovoltaic devices.12 An unprecedented conversion efficiency of 11% could be manufactured from relatively

cheap materials.19 The most up-to-date efficient DSSCs are based on ruthenium

complexes. Several representative ruthenium sensitizers possess high extinction

coefficient and high photovoltaic performance are shown in Scheme 1.15À19

However, the presence of liquid electrolyte has the problems of leakage,

robust sealing, and device stability, thus results in limited commercialization.

Quasi-solid-state and solid-state DSSCs based on nonvolatile ionic liquid or

organic hole-conducting material/polymer as the electrolyte are, therefore,

developed to circumvent the sealing problem.



Types of Organic Solar Cells



163



B. Organic Thin Film Solar Cells

DSSCs are not considered to be pure organic PV cells because the active

layers are composed of organic-inorganic composite materials, which limit the

fabrication of flexible solar cells. Another type of organic solar cells is based on

pure organic materials in the active layers. The materials are introduced to the

electrode substrate either by vacuum deposition (for small molecules) or by spin

coating (for polymeric materials) techniques. Heterojunction and bulk-heterojunction cells are the two most popular device configuration. In a heterojunction

PV cells (Fig. 2), the active layer is composed of two separate layers of sensitizer

(the donor) and electron acceptor. Upon exciton generated by photoexcitation,

charge separation mainly occurs near the donor-acceptor interface. Holes and

electrons are then transported by the donor and accepting layers respectively, to

the corresponding electrodes. In a bulk-heterojunction photovoltaic cell, the

electron donors and acceptors are blended in the same layer. Due to the phase

separation, different types of materials in the same layer may aggregate and form

an interpenetrating network on a nanometer scale. The exciton generation and

separation processes also happen at the donor-acceptor interface, but the contact surface between them is greatly increased when compared to heterojunction

device. The holes and electrons formed are then transported via different percolated pathways to different electrodes separately.

Early examples of organic polymers used in PV devices include

poly(p-phenylenevinylene) (PPV) and its derivatives,21À25 while copper

phthalocyanine, α-sexithiophene, and fullerene are typical examples of low

molecular weight organic materials.5,26 Recently, the efficiency of organic solar

cells was improved by the design of new conjugated polymers with improved

structural, optical absorption, and charge transport properties. Examples of

these polymers are poly(3-hexylthiophene),27À32 poly(fluorene-co-thiophene),33



FIGURE 2. Device configuration for heterojunction (a) and bulk-heterojunction

(b) organic thin film solar cells.



164



Applications of Metal Containing Polymers in Organic Solar Cells



poly(phenylenevinylene)34,35 and their derivatives, and low band gap conjugated polymers based on different heterocyclic moieties on the polymer main

chain.36À40 Other than developing new materials, the efficiency could also

further enhanced by adopting new device architecture, such as insertion of an

optical spacer layer.41 In tandem polymer cells fabricated by solution processing, power conversion efficiency in excess of 6% was reported.42



III. SOLAR CELL CHARACTERIZATIONS

Under illumination with a light source, a typical solar cell exhibits the

current-voltage characteristics shown in Figure 3. The current drawn by the cell

when the terminals are connected to each other is the short circuit current (Isc,

in mA/cm2). In the presence of an external load with infinite resistance (open

circuit condition), the voltage developed is the open-circuit voltage (Voc, in V).

The performance of a photovoltaic cell is characterized by the power

conversion efficiency ηp, which is defined by:

ηp ¼



Voc 3 Isc 3 FF

Pin



ð1Þ



where Pin is the power of the incident light (mW/cm2) and FF is the fill factor,

which is defined as the ratio of the maximum power delivered by the cell



Current



Voc

Vmax

Voltage

Pmax = Imax x Vmax

I max



I sc



FIGURE 3. A typical current-voltage characteristic for a photovoltaic cell.



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