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.

Photoinduced Charge Transfer in Poly(3-hexylthiophene)/TiO2 Hybrid Inverse Opals: Photonic vs Interfacial Effects

“…Pump modulation dependence studies (Figure b) show that the polaron lifetimes for P3HT/meso-TiO 2 and i-P3HT-TiO 2 -o are 20 and 18 ms, respectively, while that for the P3HT:PCBM film is 0.3 ms. The longer lifetime with an inorganic acceptor is in line with previous works that reported lifetimes of 3–13, 2, and 2–12 ms for P3HT combined with CdSe quantum dots, nanoporous ZnO, and TiO 2 films, respectively. ,, …”

Optical Properties of Organic Semiconductor Blends with Near‐Infrared Quantum‐Dot Sensitizers for Light Harvesting Applications

“…Overall larger quenching ratios are observed in the P3HT emission region than in the QD region on the spectra of the same sample which is indicative of a reduced efficiency of the charge transfer processes originating in the QDs. The results are in general agreement with other studies of blends employing oleic acid capped QDs that report inhibition of the charge transfer process by the insulating QD capping layers or by the presence of phase separation due to aggregation of the nanocrystals27.…”

Electron Paramagnetic Resonance Tracing of Electronic Transfers in Push–Pull Copolymers/PCBM or Nanocrystal Composites

“…This leads us to assume that line 6 is due to impurities interacting with PCBM to give PCBM – ° and not to the “spike” found in the EPR spectrum of fullerene and the fullerene derivative anion radical. Actually none of the hypotheses evoked to explain the presence of this spike in ref and are in agreement with our results (all hypothesis should lead to a ratio of PCBM – °/P2 + ° < 1)…”