Charge Generation and Transport Phenomena in Disordered Organic Semiconductors and Photovoltaic Diodes

Abstract

Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability and mechanical flexibility. In contrast to traditional banded semiconductors, organic semiconductors are intrinsically disordered systems and exhibit therefore much lower charge carrier mobilities. They are also low dielectric constant systems -as such, their photo-excitations are excitonic at room temperature with the electron-hole pair remaining Coulombically-bound. Blending organic semiconductors with differing electron affinities, so-called electron acceptors and donors, creates molecular heterojunctions which deliver the required driving force for exciton separation. These so-called bulk heterojunctions (BHJs) are the dominant architecture for creating organic photovoltaic cells and photodetectors. However, not least because of an incomplete understanding of the underlying physical mechanism that control the conversion of photons to free charges, and the subsequent extraction of these free charges to the electrodes, these organic optoelectronic systems still lag behind their inorganic counterparts.Motivated by these factors, the work described in this thesis advances the fundamental understanding of the loss mechanisms associated with charge photogeneration and charge transport phenomena in BHJ organic solar cells and photodetectors, and presents new experimental methodologies to study these processes. The transport of photogenerated charge carriers in the percolated donor: acceptor pathways towards the extracting electrodes has been studied in-depth via existing and newly developed transient photovoltage and steady-state characterization techniques. A simple but conclusive understanding has been developed which allows for minimization of the detrimental recombination of free charge carriers for given device parameters such as film thickness, applied voltage and the mobility of the slower carrier type (either electrons or holes). The models' predictions have been experimentally validated for many different (> 25) BHJ solar cell systems. Inspired by the need to selectively optimize the processes which control the photocurrent output of the cell, such a charge photogeneration and extraction, a technique to quantify and disentangle both efficiencies has been developed as a next step. Corroborated by transient absorption spectroscopy, the dynamics of the photocarrier generation process was examined. The results suggest that the so-called charge-transfer state (CTS) separation limits the conversion of photons to free charges and the photocurrent output from short-circuit to the maximum power point, strongly depending on the slower carrier ii mobility. These findings were explained by the ability of the slower carriers to leave the donor: acceptor interface via 1) a high enough mobility, 2) a sufficiently large domain size, and 3) enough conduction pathways (entropy). This work also shows that the dynamics of charge extraction and CTS dissociation are simi...