Organic Semiconductors

Organic semiconductors have found application in a range of electronic devices due to the ease with which their properties can be tuned by synthesis. For example, organic light-emitting diodes are integrated into several technologies such as lighting arrays and displays. The field of organic photovoltaics has not yet fully been realised in industrial applications, but is currently experiencing a renaissance with remarkable advances in power conversion efficiencies recently reported for both single junction and double junction solar cells. Despite these major breakthroughs many aspects of device physics of organic solar cells, in particular those with low energy offsets, remain unknown. One of the most common aspects of the device is the energy level diagram (energetic landscape) of the solar cell, with such diagrams being ubiquitous in literature, appearing in almost every publication. Despite the importance of energy level diagrams in determining the elementary processes taking place in the device (e.g. charge generation, transport and extraction), accurately determining these diagrams is extremely challenging, especially for solution-processed systems. Currently, energy level diagrams are constructed by combining energy values for the individual components as obtained by different methods such as density functional theory, cyclic voltammetry, Kelvin probe and Ultra-violet photoemission spectroscopy (UPS), resulting in a large scatter in reported energy levels even for the same material systems. In addition, this approach neglects to account for interfacial effects such as formation of dipoles or band bending.

Recently, we have developed a new methodology to address this key challenge: a novel photoemission based technique that allows mapping of the energy level landscapes in various multi-layered devices with a superior vertical resolution of 1-2 nm. The technique is based on the combination of ultraviolet photoemission spectroscopy an essentially damage-free etching of materials in-situ using a gas cluster ion source. The new method has already been applied to a range of high performance photovoltaic systems, demonstrating both the efficacy and the versatility of the technique.

© Yana Vaynzof/IAP