Investigation of Electrical Properties of P3HT:PCPBM Organic Solar Cells

The electrical properties of organic solar cells (OSC) have been investigated to understand the charge carrier loss mechanisms impacting the over all efficiency. The power conversion efficiency of a solar cell is dependent upon the short-circuit current density (Jsc), open circuit voltage (Voc) and the fill factor (FF). This thesis is mainly about the investigation of these properties in two aspects: the current conducting mechanism is studied via electrical resistance measurements of the OSC devices and the origin of Voc is investigated through the fundamental studies using ultraviolet photoelectron spectroscopy (UPS).

In the electrical conducting mechanism study, the series resistance of poly (3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction (BHJ) organic solar cells (OSC) has been investigated. The bulk resistance of the active layer and the specific contact resistance between the active layer and the electrode are extracted from the measured series resistance using the vertical transmission line model (TLM) method. Results have shown that for an annealed P3HT:PCBM device which has an active layer thickness of 85 nm (optimum thickness for high efficiency), 17 % of the total series resistance is attributed to contact resistance and bulk resistance contributed the rest 83 %. Thermal annealing helps to reduce the contact resistance by a factor of 2 and the bulk resistance by a factor of 8. The resistances of pure PCBM devices increase linearly with thickness, while the increasing trend of the pure P3HT devices can be described by two different increasing regions, mainly due to the recombination effect.

The origin of Voc is investigated through UPS study. Voc of P3HT and PCBM organic solar cell devices is measured at temperatures ranging from 15 C to 145 C. The temperature dependence of the vacuum level shift and of the highest occupied molecular orbital (HOMO) energy level of P3HT and PCBM are measured by UPS in the same temperature range. The temperature dependence of the absorption edge is also studied in the same temperature range to obtain the temperature variation of the optical band gap energy (Eg). The measured Voc of the devices shows a clear decreasing trend with the increasing temperature and the total decrease is found to be about 0.1 V. However, by utilizing the measured values of the HOMO for the P3HT and LUMO for the PCBM, the calculated values of Voc and its temperature dependence shows an increasing trend with temperatures. The calculated values of Voc do not agree with the measured Voc values. This is a clear indication that the expression (HOMO - LUMO) - exciton binding energy does not hold, and that other factors are impacting the value of Voc.