Photo‐induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar Cells

Abstract

The photo‐induced charge transfer and the dynamics of persistent charge carriers in blends of semiconducting polymers and nanocrystals are investigated. Regioregular poly(3‐hexylthiophene) (P3HT) is used as the electron donor material, while the acceptor moiety is established by CdSe nanocrystals (nc‐CdSe) prepared via colloidal synthesis. As a reference system, organic blends of P3HT and [6,6]‐phenyl C61‐butyric acid methyl ester (PCBM) are studied as well. The light‐induced charge transfer between P3HT and the acceptor materials is studied by photoluminescence (PL), photo‐induced absorption (PIA) and light‐induced electron spin resonance spectroscopy (LESR). Compared to neat P3HT samples, both systems show an intensified formation of polarons in the polymer upon photo‐excitation, pointing out successful separation of photogenerated charge carriers. Additionally, relaxation of the persistent charge carriers is investigated, and significant differences are found between the hybrid composite and the purely organic system. While relaxation, reflected in the transient signal decay of the polaron signal, is fast in the organic system, the hybrid blends exhibit long‐term persistence. The appearance of a second, slow recombination channel indicates the existence of deep trap states in the hybrid system, which leads to the capture of a large fraction of charge carriers. A change of polymer conformation due to the presence of nc‐CdSe is revealed by low temperature LESR measurements and microwave saturation techniques. The impact of the different recombination behavior on the photovoltaic efficiency of both systems is discussed.

Light-induced electron spin resonance as a probe for charge separation and recombination in hybrid donor:acceptor systems

“…2, the polaron signals are visible with low intensity already in the spectra recorded in dark, and their intensity strongly increases under excitation with laser light (532 nm) [31]. Since photo-generated singlet excitons would not cause an ESR signal, the increase of the signal intensity under illumination provides a clear indication that the generated excitons must have been split at the donor:acceptor interface into separated charge carriers.…”

“…The charged molecule is called a radical anion, the electron which interacts with the carbon backbone is called a negative polaron. The second signal with a gfactor of 2.003 is attributed to a positive charge on the polymer chain, i.e., to a positive polaron [29,31,41].…”

“…3. The signal with a g-factor of 1.999 can be attributed to a PCBM molecule charged with an electron [29,31,41]. The charged molecule is called a radical anion, the electron which interacts with the carbon backbone is called a negative polaron.…”

“…It is limited to the inverse of the field modulation frequency. Usually a modulation frequency of 100 kHz is used which would result in a maximum time resolution of 10 ms [31].…”

“…Alternative methods are for example, photo-induced absorption (PIA) spectroscopy [11,[25][26][27][28] or light-induced electron spin resonance (L-ESR) [11,29,30]. Both, PIA and L-ESR are not only useful to study charge transfer, but enable also obtaining valuable information about the recombination of charge carriers [26,[30][31][32].…”