Surface Photovoltage (SPV)
A surface photovoltage (SPV) is the light-induced change of the contact potential difference at the surface of a photoactive material. SPV signals can arise, for example, due to separation of photo-generated charge carriers in a space charge region of a semiconductor or due to injection of charge carriers excited by light into an insulator. SPV signals depend on processes of light absorption, photo-generation, charge separation, charge transport, recombination and trapping and emission of charge carriers. Therefore, SPV techniques can be applied in order to study related processes and properties of photoactive materials for photovoltaics and photocatalysis.
Details of the measurement principle
SPV signals are detected with electrodes in the Kelvin-probe or fixed capacitor arrangements. Parameters of the exciting light (cw or pulsed or modulated excitation, variation of the photon energy from UV to NIR, variation of the intensity over many orders of magnitude) and of the sample (ambient from vacuum over gas atmospheres to electrolytes, temperature from about -180°C to 300°C, external bias potential, front or back illumination) can be varied. Under excitation with short laser pulses in the fixed capacitor arrangement, time dependent SPV signals can be measured from the ns to the min range and even longer. SPV measurements allow for dedicated experiments on light-induced processes in photoactive materials as well as for monitoring of indicative parameters in technology development.
Indicative parameters for SPV analysis are, for example,
- the sign of SPV signals (gives the direction of charge separation),
- onset energies of SPV signals (give information about transitions and electronic states from which charge separation is possible; such as band gaps or defect states)
- amplitudes of SPV transients (SPVmax) excited in a semiconductor at high intensity (large signal case, information about band bending - j0)
- dependence of SPVmax on a bias potential (information about the density of interface states - Dit, application for c-Si surfaces)
- dependence of the light intensity on the absorption length of a semiconductor at constant SPV (information about the diffusion or transport length)
- rise and decay times (information about generation, recombination, trapping and transport processes)
Measurement of surfacestate density
The set-up is dedicated to the measurement of SPV transients in the large signal case and at varied bias potential (UF). The density of surface states is obtained from the analysis of the dependence of j0 on UF. The temperature of the sample can be decreased down to 80 K.
Modulated spectral dependent measurements
Modulated spectral dependent SPV measurements can be performed over a wide range of photon energies with a quartz prism monochromator. The sensitivity is up to about 100 nV. The set-up can be applied for the measurement of the diffusion or transport length in photovoltaic absorbers. The set-up is suitable for routine measurements.
Temperature dependant modulated spectral dependent measurements
The set-up is equipped with a tunable Nd:YAG laser, a quartz prism monochromator, halogen and Xe lamps, a home-made measurement chamber with a liquid nitrogen cryostat and a heater, a vacuum system and a logarithmic read-out of SPV transients.