dr. M. Campoy Quiles: Is there a fundamental limit for light absorption in organic semiconductors?

The complex refractive index is the most fundamental and critical optical property of a material. For example in organic solar cells, higher light harvesting efficiency leads to higher potential efficiency without imposing strict constraints on the electrical transport properties across thick films. Moreover, knowledge of the refractive index enables the modelling and optimization of optoelectronic devices and can indirectly provide useful information regarding the solid state structure [1].

In this contribution I will focus on the fundamental understanding of the optical properties of semiconducting polymers. We have studied the extinction coefficient, k , as a function of chemical structure for more than 30 conjugated polymers and found an apparent absorption maximum, which appears at less than one third of the theoretical limit given by the sum rule, and has strong implications for light harvesting and emission properties of this class of materials [2]. In order to understand this, we investigate the origin of the optical absorption by looking into three directions: the chemical structure, molecular packing and the effect of inter- and intra-molecular interactions on the optical properties (via high hydrostatic pressure). We use our results to propose design rules for enhanced light harvesting in conjugated polymers.