Yongqian Gao

Resonance chemical imaging of polythiophene/fullerene photovoltaic thin films: mapping morphology-dependent aggregated and unaggregated C=C Species.

Resonance Raman spectroscopic imaging is introduced as a physical probe to identify and spatially map morphology-dependent variations of intra- and interchain interactions and order in poly-3-hexylthiophene (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) photovoltaic blend thin films. Absorption spectra and C=C symmetric stretching Raman modes of P3HT/PCBM blend films show contributions from two distinct species that are assigned as aggregated and unaggregated P3HT chains with characteristic Raman frequencies of approximately 1450 (I(C=C)(agg)) and approximately 1470 cm(-1) (I(C=C)(un)), respectively. Upon thermal annealing of blend films, the relative concentrations of I(C=C)(agg) and I(C=C)(un) species (R = I(C=C)(agg)/I(C=C)(un)) changes on average from 0.79 +/- 0.20 (as-cast) to 2.45 +/- 0.77 (annealed). It is proposed that R values report on the relative densities of states (DOS) of aggregated and unaggregated species, and resonance Raman imaging is then used to spatially map morphology-dependent variations of R values and uncover subclassifications of these species. From both R and frequency dispersion resonance Raman images of I(C=C)(agg) and I(C=C)(un) species, four distinct types of P3HT chains are identified and mapped in annealed P3HT/PCBM blend thin films: (i) highly aggregated/crystalline, (ii) partially aggregated, (iii) interfacial, and (iv) unaggregated/PCBM rich. The change in aggregation upon annealing is attributed to an increase in planarity of the P3HT chains that is determined from the ratios of C=C/C-C symmetric stretching mode intensities.

Competition between Auger Recombination and Hot-Carrier Trapping in PL Intensity Fluctuations of Type II Nanocrystals

Performing time-tagged, time-correlated, single-photon-counting studies on individual colloidal nanocrystal quantum dots (NQDs), the evolution of photoluminescence (PL) intensity-fluctuation behaviors in near-infrared (NIR) emitting type II, InP/CdS core-shell NQDs is investigated as a function of shell thickness. It is observed that Auger recombination and hot-carrier trapping compete in defining the PL intensity-fluctuation behavior for NQDs with thin shells, whereas the role of hot-carrier trapping dominates for NQDs with thick shells. These studies further reveal the distinct ramifications of altering either the excitation fluence or repetition rate. Specifically, an increase in laser pump fluence results in the creation of additional hot-carrier traps. Alternately, higher repetition rates cause a saturation in hot-carrier traps, thus activating Auger-related PL fluctuations. Furthermore, it is shown that Auger recombination of negatively charged excitons is suppressed more strongly than that of positively charged excitons because of the asymmetry in the electron-hole confinement in type II NQDs. Thus, this study provides new understanding of how both NQD structure (shell thickness and carrier-separation characteristics) and excitation conditions can be used to tune the PL stability, with important implications for room-temperature single-photon generation. Specifically, the first non-blinking NQD capable of single-photon emission in the near-infrared spectral regime is described.

Spectroscopic and electrical imaging of disordered polymeric solar cells: understanding aggregation effects on material performance

The manner in which polymer chains pack and organize in thin film structures is crucial to maximizing the efficiency of charge and energy transport processes in solar cell devices. We use new spectroscopic and electrical imaging tools to spatially map and correlate local structure (chain conformation, packing, morphology) to local photocurrent generation efficiency. Both Raman and photoluminescence approaches are used that provide unique insights into important structural attributes and how they vary with film morphology. Simultaneous electrical measurements are then used to establish the roles of specific structural features to photocurrent production.