Quantitative electrical characterisation of organic semiconductors by atomic force microscopy

Organic photovoltaics (OPVs) may be the future of energy production, one day possibly solving the energy crisis we currently face. However, OPVs still have lower efficiencies than conventional silicon technologies and suffer from low carrier mobilities and stability problems. To improve OPV performance and stability we need to characterise them, both on a device level and on a nanoscale level. The nanoscale is especially relevant to polymer:fullerene solar cells as their performance is heavily dependent on their morphologies. This thesis presents three techniques that use atomic force microscopy (AFM) to measure the nanoscale, electrical and mechanical properties of these systems, whilst maintaining access to the morphology of the sample. Force volume bias spectroscopy (FVBS) combines traditional force-volume measurements and current-voltage (IV) measurements using the Johnson-Kendal-Roberts model of contact mechanics to accurately determine the sample's Young's modulus and the tip-sample contact area. The calculated contact area is then used to calculate the current density from the associated IV curve. A modified Mott-Gurney model allows the extraction of mobility and charge carrier density dependence. The temperature dependence of P3HT between 70 and 130°C is investigated with FVBS, showing an increase in mobility with temperature. The properties of as-cast and annealed P3HT are also compared; with annealed P3HT showing a correlation between mechanical and electrical properties that is not present in the as-cast film. Time resolved EFM (Tr-EFM) is a non-contact technique that uses the change of phase of an oscillating cantilever in response to an applied pulse of light, to characterise the surface photovoltage (SPV) and timescale of charge accumulation and decay in photovoltaic materials. Two bulk heterojunction systems were studied; P3HT:PCBM and PTB7:PC70BM. The SPV and the dynamic response were studied as a function of illumination intensity and temperature. The dynamic response showed no clear trend with temperature or light intensity for either system. For P3HT:PCBM the SPV has complex temperature dependent behaviour, showing increased SPV with temperature and an SPV peak at 90°C. SPV decreased with temperature for PTB7:PC70BM. Both systems showed logarithmic behaviour of the SPV with light intensity, suggesting that the SPV is a measure of the open circuit voltage of the bulk heterojunction. Intensity modulated Kelvin probe force microscopy (IM-KPFM) uses a modulated light to dynamically modulate the SPV in the thin _lm devices. Changing the frequency of modulation allows the timescale of charge depletion to be measured. The temperature dependence of this is investigated for P3HT:PCBM and PTB7:PC70BM. Both P3HT:PCBM and PTB7:PC70BM showed decreasing timescales with temperature. The behaviour of P3HT:PCBM was more complicated than PTB7:PC70BM, showing peaks in the timescale measured. The results and bene_ts of IM-KPFM and Tr-EFM are then compared.