Spectroscopic study of charge-separation dynamics in [pi]-conjugated systems
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In this dissertation, we investigate the dynamics of excited states involved in the operation of organic photovoltaic devices. Using spectroscopic and numerical tools, we study the photophysical properties of push-pull polymers, donoracceptor interfaces, and solution-processed small-molecule organic photovoltaic devices. In the first place, we develop new numerical methods to improve the quality of the acquired data as well as that of the analysis. These allow for a reliable extraction of excited-state populations from transient absorption measurements. Then, we investigate the properties of long-lived species in F8BT films through time-resolved photoluminescence (PL) and ultrafast transient absorption (TA) measurements. We show that singlet-singlet annihilation results in long-lived geminate electron-hole pairs which recombine over distributed timescales, ranging from femtoseconds to milliseconds. We follow with a study of the well-characterized PFB:F8BT interface. There, we show that the separated electron-hole pairs are degenerate in energy with the singlet exciton, placing their energy ;::::j 300 me V above that of interfacial bound electron-hole pairs. We also show that, when optically pushing these into a hot state, the properties of the excited state accessed are identical to those of a charge-transfer state initially formed by a singlet exciton. Finally, we investigated the kinetics of photogenerated excited states in pDTS (FBTTh2)2 :PC70BM photovoltaic devices to monitor the populations of free and trapped charges under different illumination and bias conditions. We show that the charge separation mechanism is driven entirely by the local energetic landscape, pulling the charge pair 9 nm apart in 30 ps, with no effect of applied bias. On longer timescales, we show that charges separate across the device in less than 1 ?s but get blocked at the electrode interface for up to 10 ?s. We also detangled the signatures of charges trapped on the PC70BM and the pDTS( FBTTh2)2 and show that most traps are located on the latter.
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