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Conjugated polymers: New materials for photovoltaics.

Polymers can be made to act like semiconductors, giving a new meaning to “catching some rays”.

Gordon G. Wallace
Paul C. Dastoor
David L. Officer
Chee O. Too

Polymeric photovoltaics present the tantalizing possibility of producing coatings that function as sunlight-harvesting paints on roofs or even as an integral part of fabrics to produce electricity from sunlight. MacDiarmid, Shirakawa, and Heeger (1) brought the unique properties of conjugated polymers to the fore in 1977 when they discovered that chemical doping of these materials resulted in increases in electronic conductivity over several orders of magnitude. Since then, electronically conducting materials based on conjugated polymers have been applied in diverse items such as sensors, biomaterials, light-emitting diodes, polymer actuators, and corrosion protection agents. In this article, we review the principles behind generating the photovoltaic effect in conjugated polymers, consider progress to date, and discuss the exciting possibilities that lie before us.

Conjugated polymers
Conjugated polymers have a framework of alternating single and double carbon–carbon (sometimes carbon–nitrogen) bonds (Figure 1). Single bonds are referred to as sigma-bonds, and double bonds contain a -bond and a pi-bond. All conjugated polymers have a sigma-bond backbone of overlapping sp2 hybrid orbitals. The remaining out-of-plane pz orbitals on the carbon (or nitrogen) atoms overlap with neighboring pz orbitals to give pi-bonds

 

Although the chemical structures of these materials are represented by alternating single and double bonds, in reality, the electrons that constitute the pi-bonds are delocalized over the entire molecule. For this reason, polyaniline (PAn) and poly(N-vinylcarbazole) (PVCZ) are considered to be conjugated polymers, with the nitrogen pz orbital assisting the delocalization of the pi-electrons. In some conjugated polymers such as polyacetylene (PA) and PAn, delocalization results in a single (degenerate) ground state, whereas in other polymers the alternating single and double bonds lead to electronic structures of varying energy levels.

The behavior of conjugated polymers is dramatically altered with chemical doping. Generally, polymers such as polypyrrole (PPy) are partially oxidized to produce p-doped materials:

Oxidation/reduction of polypyrrole

p-Doped polymers have wide application—for example, electrochromic devices, rechargeable batteries, capacitors, membranes, charge dissipation, and electromagnetic shielding. Less effort has gone into synthesizing and characterizing n-doped materials.

Photovoltaic effect in conjugated polymers
Photovoltaic semiconductors.

For inorganic semiconductors, the mechanism of charge generation from incident photons is well established. Because these materials are typically crystalline solids, their electronic structure can be described in
terms of energy bands. For an idealized semiconductor,the electronic
structure consists of a conduction band and a valence band separated by an energy gap, the size of which depends upon the material. In the case of silicon, for example, the band gap is 1.12 eV, whereas for gallium arsenide, it is 1.4 eV. Although there are different types of band gaps (direct and indirect), for simplicity, we are restricting our discussion to materials with direct band gaps (optical transitions between free electrons and holes are allowed).

The band gaps of most semiconducting materials are typically between 0.1 eV and 2.2 eV, and as such are comparable to the energies of photons whose frequencies lie within (or just outside) the visible spectrum. It is energetically feasible, therefore, that an incident visible-light photon has sufficient energy to excite an electron from the valence band into the conduction band of the material. As a consequence of this single event, two charge carriers are produced—an electron in the conduction band and a so-called “hole” in the valence band. Although the hole that is produced is simply an empty electronic state, which can be occupied by other electrons in the valence band, it behaves as though it is an independent carrier of positive charge.

Photovoltaic polymers. Conducting polymers also act as semiconductors, and their electronic properties appear to be analogous to those of inorganic semiconductors. How can this be, when conducting polymers would appear, at first glance, to lack the crystallinity required for the occurrence of energy bands in the solid state?

The characteristics of the pi-bonds are the source of the semiconducting properties of these polymers. First, the pi-bonds are delocalized over the entire molecule; and then, the quantum mechanical overlap of pz orbitals actually produces two orbitals, a bonding (pi) orbital and an antibonding (pi*) orbital. The lower energy pi-orbital produces the valence band, and the higher energy pi*-orbital forms the conduction band. The difference in energy between the two levels produces the band gap that determines the optical properties of the material. Most semiconducting polymers appear to have a band gap that lies in the range 1.5–3 eV, which makes them ideally suited as optoelectronic devices working in the optical light range.

Putting induced charges to work
The charge conduction mechanism appears to be more complex for conducting polymers than for inorganic semiconductors. Although the action of an incident photon on a conducting polymer excites an electron from the valence band into the conduction band, the resulting electron and hole are bound, and their motion through the material is coupled. These coupled moieties are known as excitons and are responsible for many of the electronic properties found in the most common and efficient polymer-based electronic devices.

Terms and Defiinitions

The characteristics of the photovoltaic effect of a device are shown in Figure 2. The short circuit current (Isc) is the current when the voltage is zero. The open-circuit voltage (Voc) is the voltage when the current is zero. The maximum output of the cell is given by the product IppVpp, where Ipp is the current and Vpp is the voltage at peak power. The values of Ipp and Vpp are obtained from the I vs V curve for the device where the product IV is maximum at a point along the curve. The fill factor (FF) is also important and is defined as

FF = IppVpp/IscVoc

In addition, there are two definitions of the conversion efficiency.

The energy conversion efficiency (Y%) is given by

Y% = (Output energy/Total incident energy) × 100%
= (IppVpp/Total incident energy) × 100%
= (IscVocFF/Total incident energy) × 100%

The quantum efficiency (Q) is given by

Q =
Number of photons effectively used

Number of photons absorbed
= Output energy/Absorbed energy

In practice, Q is obtained as shown here (2):

Q =
[1.24 × 103 × Photocurrent density (µA/cm2)]

[Wavelength (nm) × Photon flux (W/m2)]

Monochromatic light is used to determine Q, and tungsten, tungsten halogen, or xenon lamps are used to determine Y.

 

In conventional semiconductors, the excited electron and the resulting hole migrate freely to opposite electrodes, where they can do useful work in an electrical device (Figure 3). In a conducting polymer, however, the electron and hole that are generated by the incident photon are bound into an exciton (Figure 4).

How then, can we obtain any useful work from a conducting polymer if the electron and hole are not separated? It turns out that the bound exciton can be split at interfaces (Figure 5). The simplest interface is created at the junction between the electrode and the conducting polymer. Under open-circuit conditions, holes are collected at the high work function electrode (indium tin oxide, ITO), and electrons are collected at the low work function electrode (aluminum). Indeed, the Voc generated by these devices depends upon the work function difference between the two electrodes. Unfortunately, the exciton-splitting process that occurs at a conducting polymer–electrode interface is not very efficient and is one of the causes of the low quality of early polymer photovoltaics. Another cause of the very low efficiencies of early devices is the effect of impurities, such as oxygen, which act as traps to the migrating excitons.

Attempts to improve the efficiency of the exciton-splitting process led to the development of new conducting- polymer species that contained electron-donating and electron-accepting species. By creating interfaces among conducting polymer molecules of differing electron affinities, it is possible to enhance the probability of electron transfer between molecules. This process (photoexcited charge transfer) causes the bound charges to separate, and the junction formed at the donor–acceptor interface is analogous to a semiconductor heterojunction.

These heterojunctions work very well at separating excitons that arrive at the junction. Unfortunately, the lifetime of excitons is short, and only excitons that are formed within ~10 nm of the junction will ever reach it. This short exciton range clearly limits the efficiency of these photovoltaic devices. In an attempt to develop a more efficient photovoltaic structure, interpenetrating networks of electron-accepting and electron-donating polymers have been produced (3). With these materials, the number of heterojunctions within the polymer blend is greatly increased, and thus the probability that an exciton will encounter a junction and be separated.