Photoconductive polymer

Photoconductivity is an optical and electrical phenomenon, which material's electrical conductivity increase by absorption of electromagnetic radiation (e.g. visible light, ultraviolet light, infrared light). Photoconductive polymers can serve as good insulators when the electricity, free electrons and holes are absent.

In general, the polymers usually satisfy these two features.

1. Photoconductive polymers can absorb light to excite electrons from ground state to excited state. The photoexcited electron will form a pair of charge carriers, it can be separated by electric field.

2. Photoconductive polymers must allow migration of either photoexcited electrons or holes, or both, through the polymer in the electric field towards the appropriate electrodes.

Photoconductive polymers act merely as charge-transporting media, and it can be p-type or n-type, however most known photoconductive polymers are p-type (only transport holes). Photocurrents usually observed are very small in organic compounds. The mobilities μ are typically 10⁻¹²-10⁻¹⁸ m²V⁻¹s⁻¹. And photocurrents are usually effected by charge-carrier generation, injection and transport.

Photoconductive polymers have been developed into different types, there are two mainly types, one is negative photoconductivity, another one is magnetic photoconductivity. The photoconductive polymers have been greatly enriched the photoconductive material, and there are many applications (e.g. xerography, laser printers)

When light is absorbed by a material, the number of free electrons and electron holes increases and raises its electrical conductivity.[6] To cause excitation, the light that strikes to the materials must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. And this process will involve four kinds of processes: charge-carrier generation, charge injection, charge trapping, charge carrier transport.

Mechanism of intrinsic photogeneration of charge in amorphous photoconductors

The charge-carrier generation can be affected in different aspects: photons absorbed, polymer itself, photoexcitation of photosensitive material. The mechanism for intrinsic photogeneration is as illustrated.[7]

As Onsager originally developed this theory:[8]

The encounter complex will formed by photoexcitation with migration of the exciton to an acceptor site. The photogeneration efficiency is determined by the competition between carrier separation and geminate recombination. The photogeneration efficiency was defined by using the dissociation of ion pairs in weak electrolytes, which can be expressed as a function of electric field, temperature and the separation distance of the bound hole-electron pair.[9] The overall photogeneration efficiency ${\displaystyle \phi (E)}$ can be given by

${\displaystyle \phi (E)=\phi _{0}\int p(r,\Theta ,E)g(r,\Theta )d^{3}r}$

${\displaystyle d^{3}r}$ is a volume element, ${\displaystyle \phi }$ is the primary quantum yield, ${\displaystyle p(r,\Theta ,E)}$is the probability that a hole-electron pair separated by a distance ${\displaystyle r}$ at an angle ${\displaystyle \Theta }$ to the direction of electric field ${\displaystyle E}$, ${\displaystyle g(r,\Theta )}$ is the spatial distribution function between ions.

Efficient injection of charge into the layer plays an important role in the operation with a photogeneration layer.

Under quasi-steady state conditions, it can be written by the flowing equation:

${\textstyle {\frac {dn}{dt}}=\phi I-\gamma _{r}n^{2}-\gamma _{i}n\thickapprox 0}$

${\displaystyle \phi I}$ is the rate of the incident photons that are absorbed in the photogeneration, ${\displaystyle \gamma _{r}n^{2}}$ is the rate of the density of free carriers in the generation layer reduced by recombination, ${\displaystyle \gamma _{i}n}$ is the injection rate.

Assuming that the charges cross the interface will not return, the photoinjection efficiency ${\displaystyle \Upsilon }$ can be defined as[10]

${\displaystyle \Upsilon ={\frac {\gamma _{i}n}{I}}=\delta [(1+{\frac {2\phi }{\delta }})^{\frac {1}{2}}-1)]}$, when ${\displaystyle \delta =\gamma _{i}^{2}/2I\gamma _{r}}$

(i)For a large ${\displaystyle \delta }$ or low recombination rates, ${\displaystyle \Upsilon =0}$, in which case the photoinjection efficiency is determined by the generation efficiency.

(ii)For a small ${\displaystyle \delta }$ or high recombination rates, ${\displaystyle \Upsilon =(2\delta \phi )^{\frac {1}{2}}}$ and the photoinjection efficiency will depend on the injection rate, ${\displaystyle \gamma _{i}}$.

Charge transport can be defined as the process that photogenerated charge in photoconductors be injected into the transport material. When charges injected, charges will migrate through the medium and then reach to the opposite electrode. In this process, electrons or holes or both, involves 'hopping', for example, a sequence of transfers of charges among localized sites.[11] These localized sites are connected with individual functional groups or segments of the polymer chain.

Generally, hole injection, or transfer of holes into the transporting media. This process can be regarded as an oxidation step with cation radicals generate. Meanwhile, electron injection is a reduction process.

Based on the properties of charge transport, the photoconductive polymers usually satisfy one of the features:

(1)Photoconductive polymers are σ-conjugated.

(2)Photoconductive polymers have an extended π-electron system in the backbone pendant to the chain.

These features guarantee delocalization and stabilize the transport charge.

Charge trapping is an important processes, in which migrating charges can be immobilized in trap sites. If the traps are 'shallow', they may be referred to as 'transportinteractive'.[12] Hole trapping materials usually have lower oxidation potentials and work as host-transporting materials. Stronger electron acceptor have better ability to trap transport electron.

Charges can be immobilized by redox-irreversible side-reactions due to the geminate recombination and recombination of carriers in the circuit. In this process, charged moiety can be illustrated by the scheme:

(a)Charge immobilization due to a redox-irreversible side-reaction

(a) The redox steps to achieve trap-free migration of a hole involving neutral groups M and charged groups M+

(b)Charge immobilization due to a redox-irreversible side-reaction

(b) The intermittent species M_j⁺ can undergo two kinds of process:

(i) The migration of electron from M_k will result in forming M_j coming from M_j⁺

(ii) M_j⁺ undergo a side-reaction leading to a charged species X⁺ that won't further exchange the charge with the neighboring group M.[13]

The photoconductive polymer have been successfully applied in Xerography and laser printers. They used the layered organic photoconductive polymer with a polymeric charge-transport layer. The charge-transport layer is a solid solution compared with other printer that usually use liquid chemicals in printing process. The main advantages of organic photoconductive polymer are (i) near-IR sensitivity (ii) panchromaticity (iii)flexibility for application (iv) simple fabrication (v) low cost. Currently, the best organic photoconductive polymer are as sensitive as the inorganic devices based on selenium.

There is some potential application in photovoltaic cells. The limit of this application is that photoconductive polymer don't have high conversion efficiency.

Some possible applications are just reported by the literature but no commercial products. They are photothermoplastic imaging,[18] holographic recording[19] and optical switching devices.[20]

• Photodiode
• Photoresistor
• Infrared detector
• Lead selenide PbSe
• Indium antimonide (InSb)