Time-Resolved Photocurrent Generation

For typical semiconductors the bandgap is between 1 and 3 eV. The electronic population of the energy bands is determined by the Fermi-Dirac distribution function, i.e. 

In this expression Ep is the Fenni-energy level, which determines the population statistics. In a non-degenerate semiconductor - Ep and Ep - E are much larger than kT and the Fermi-Dirac distribution can be approximated by the Boltzmann distribution functions for the conduction and valence bands, i.e. 

The effective density of states of the conduction band and valence band are Nc and Nv, respectively. The concentrations of conduction band electrons, n, and valence band holes, p, are given by 

Thermal excitation of valence band electrons yields conduction band electrons, which can be written in the Kroger-Ymk notation, i.e. 

In this expression e is a conduction-band electron and h is a valence-band hole. If this reaction is in thermodynamic equilibrium, the following condition holds 

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The generation, separation, and recombination of electron-hole pairs, along with electronic transport can be studied in great detail with time-resolved photocurrent generation techniques. The examples given above are by no means limiting and do not cover aU possible fields of applications. Yet, they demonstrate the enormous potential that these techniqnes offer. In the smdy of dye-sensitized solar cells the use of IMPS have proven invalnable and has lead to fnndamental knowledge about... 

Oekermann, T., D. Schlettwein, and N.I. Jaeger (1999). Role of surface states and adsorbates in time-resolved photocurrent measurements and photovoltage generation at phthalocyaninatozinc(II)-photocathodes. J. Electroanal. Chem. 462, 222-234. 

Recently, Baumann et al.(43) have measured time-resolved photoconductivity in PDA-TS-6 crystals as well as polyacetylene excited by 25 ps pulses of a Nd YAG laser (ftU) = 2.3 eV). The response time of the detector was 200 ps. The transient signal shown in fig.5 reveals a fast initial peak with instrument-limited pulse-shape followed by a slower decaying tail. The field dependence of the peak height (fig.6) parallels that of the carrier generation process and is in accord with what Donovan and Wilson have found on a 20 ns time resolution. The quantum efficiency associated with the fast photocurrent peak is 1.5x10 times the dc-quantum efficiency measured at hu) = 2.7 eV. 

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