Eta
Methods
The expertise in diagnostics of the group is focused on the investigation of light induced charge separation by spectral and transient surface photovoltage techniques. Further, electrical transport is studied by photocurrent techniques and conventional methods. Solar cells can be characterized and light scattering of porous structures can be investigated. The SILAR wet chemical deposition is used and further developed for the deposition of ultra-thin sulfide layers.
Content
Surface photovoltage
Charge separation
The surface photovoltage (SPV) method is used to measure light induced charge separation near the surface. When positive and negative charges are separated a potential drop proportional to the separation length occurs. This voltage can be measured in the capacitor arrangement. Reasons for charge separation are built-in electrical field, preferential surface trapping, different diffusion of excess electrons and holes.
Kelvin probe and capacitor arrangement
In the Kelvin probe arrangement, the capacitor composed of the swinging electrode and the sample surface has an oscillating distance between the electrodes and therefore an oscillating capacitance C(t). When there is a potential difference between the electrode and the sample, this results in an oscillating current. The applied voltage V is adjusted so that the measured current vanishes: in this case there is no electric field between the electrode and the sample and the applied voltage is equal to the work function difference of electrode and sample. In the capacitor arrangement, the photo induced voltage drop near the surface is measured with a buffer with a high input resistance. The time resolution of this method is determined by the RC time constant of the capacitor and buffer arrangement, the time resolution of the scope, and the duration of the light pulse.
Charge separation
The surface photovoltage (SPV) setup allows to measure at controlled temperatures between -180°C and 300°C. When illuminated with chopped light from the monochromator, the illumination wavelength dependent photovoltage can be measured with the lock-in amplifier. At fixed wavelengths, the time-resolved signal can be measured after illumination with laser pulses. Available wavelengths are 337 nm with an N2 laser (pulse duration 5 ns) and 1064 (1st harmonic), 532 (2nd harmonic) and 355 (3rd harmonic) nm under illumination with a Nd:YAG laser (pulse duration 150 ps). Transients are measured over 8 orders of magnitude with one pulse by using an oscilloscope card (108 samples) with logarithmic increase of read-out time (by software) and of number of neighboured samples for averaging (by software).
Transient Photocurrent Measurement
Time of flight (TOF) and photocurrent transients of porous electrodes
Transient photocurrent measurements can be used to determine charge carrier mobility under drift condition in insulating layers as well as the effective diffusion constant of porous electrodes in electrolyte. Excess charge carriers are excited near the surface and drift (TOF, measurement of the displacement current) or diffuse to the back contact where they are extracted (measurement of the extracted charge current). The time dependence of the transients gives information about dispersive transport.
Current-Voltage Characterization, Quantum Efficiency, Light Scattering
Quantum efficiency setup
The apparatus "Quantum efficiency" provides the possibility for measuring the spectral response and quantum efficiency (in the range of 350-900 nm) as well as the current-voltage characteristics (several measurement places are available in the group).
Apparatus for measuring light scattering profiles
Another setup provides the possibility to investigate light scattering of diffuse reflectivity. Two independent working motors are controlled by a computer and therefore enable the detector to measure the scattered light depending on the angle relative to the incident beam.
SILAR
SILAR technique
The SILAR technique combines features of atomic layer epitaxy with chemical methods that are essential in the production of multi-layer thin film structures. This method is based on successive immersion of the substrate into separated cation and anion precursor solutions. It allows the conformal deposition of extremely thin sulfide layers with a control of the thickness in the order of 1 nm.
