Charge Transport and Recombination Measurements
Charge carrier mobility in non-(fully-) crystalline semiconductors is several orders of magnitude smaller than that of typical inorganic-banded semiconductors. Extremely small charge carrier mobility and density results in poor conductivity, making traditional methods for mobility evaluation (e.g., Hall effect) not applicable. To study charge carrier transport (e.g. mobility) and recombination (e.g. recombination rate constant) in (organic) thin film solar cells and similar devices such as photodetectors and LEDs, we use different types of transient electrical and electro-optical measurements in conjunction with numerical simulations.
Our home-built setup can operate in different modes:
Charge extraction with linearly increasing voltage (CELIV). This includes photo-CELIV using a nano-second pulsed laser, MIS-CELIV and MIM-CELIV. Electron and hole mobilities can be determined using these methods.
Resistance dependent photovoltage (RPV). Employing a very low fluence femto-second laser pulse allows us to determine the electron and hole mobility of thin film solar cells in a small perturbation regime.
Transient photovoltage (TPV). If carefully performed, this method may provide information about charge carrier recombination dynamics in thin film devices.
Transient photocurrent (TPC). Transient photocurrent measurements can be used to determine the rise and fall time of photodetector devices.
Double injection (DoI). These simple transient electrical measurements are useful to determine the recombination rate constant in thin film devices and effective mobilities.
Firgure 1 (Above): A 405 nm laser pulse is guided into an integrating sphere to excite charge carriers in the active layer of an organic thin film solar cell in order to study charge carrier transport and recombination properties of the device.
Firgure 2 (Below): In transient measurement techniques, such as CELIV or DoI, a function generator (second device from left, middle) is typically used to apply a (time-dependent) voltage pulse (e.g. square, or ramp function) on the device. An oscilloscope (device in the back, top, right side) is used to measure the transient photocurrent or photovoltage of the device as a function of time