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Probing morphology-dependent aggregation and photocurrent generation in polymer/fullerene photovoltaic devices

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Please use this identifier to cite or link to this item: http://hdl.handle.net/1928/12839

Probing morphology-dependent aggregation and photocurrent generation in polymer/fullerene photovoltaic devices

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Title: Probing morphology-dependent aggregation and photocurrent generation in polymer/fullerene photovoltaic devices
Author: Gao, Yongqian
Advisor(s): Grey, John Jr
Committee Member(s): Guo, Hua Jr
Qin, Yang Jr
Dunlap, David
Grey, John Jr
Department: University of New Mexico. Dept. of Chemistry
Subject: Materials and Spectroscopy
LC Subject(s): Solar cells--Testing.
Raman spectroscopy.
Charge transfer.
Degree Level: Doctoral
Abstract: In this dissertation, new spectroscopic and electrical imaging approaches were developed to map morphology-dependent aggregation properties of polymer chains in model solar cell devices. These techniques reveal new correlations between local structure and material performance on sub-micron size scales which are not accessible by other techniques. Resonance Raman spectroscopic imaging was developed 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-C61-butyric acid methyl ester (PCBM) photovoltaic blend thin films. The C=C band of P3HT backbone was decomposed into aggregated and unaggregated component contribution at ~1450 cm-1 and ~1470 cm-1, respectively. The ratio, R, is used to report on the relative densities of states (DOS) of aggregated and unaggregated species. From both R and frequency dispersion resonance Raman images of these individual 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. Secondly, the effect of aggregated and unaggregated species of P3HT on photocurrent is explored by a combined resonance Raman-photocurrent imaging (RRPI) approach. Maps of R values and photocurrents are generated for both as-cast and annealed P3HT/PCBM devices that permit direct spatial correlations between the P3HT aggregation state and local photocurrent generation efficiency. Regions of increased P3HT aggregation are observed at both P3HT/PCBM interfaces and in P3HT-rich areas that result in decreased photocurrent generation. Voltage-dependent RRPI studies are also performed at several applied bias levels that reveal distinct changes in photocurrents due to morphology-dependent charge mobility characteristics. Thirdly, the effect of composition of P3HT: PCBM on aggregation and of P3HT and corresponding solar cells is studied. P3HT: PCBM thin film solar cells of variable weight/weight (w/w) compositions (i.e., 1:1 to 1:4) were fabricated to systematically perturb polymer packing (aggregation) properties. On average, increasing the PCBM weight fraction, Raman spectra in the dominant P3HT C=C stretching mode region (~1450—1470 cm-1) whereas symmetric stretching C-C modes show decreased intensities and red shifts. Raman bands of P3HT C=C modes can likewise be decomposed into contributions from both aggregated and unaggregated chains and, R values decrease with increased PCBM content. Most aggregated (ordered) P3HT chains reside primarily outside PCBM-rich regions but, reverses for >1:1 PCBM w/w loadings where all aggregated P3HT chains reside within PCBM-rich regions. This effect is attributed to a change in the type of P3HT aggregation from inter- to primarily intra-chain. The results reveal that the polymer aggregation state and its spatial location in the film that together have a large impact on charge transport properties and material performance. Lastly, intensity modulated photocurrent spectroscopy (IMPS) and imaging is used to study the aggregation effect on charge transport and recombination processes in P3HT/PCBM devices. This frequency-domain technique provides access to both bulk and interfacial charge transport and correlations between frequency-dependent photocurrent and local structure are revealed. Maps of the photocurrent and phase shift were recorded at several modulation frequencies spanning ~100 Hz up to 10 KHz. It was found that recombination processes involving trapped charge dominates the IMPS profiles. Temperature- and color-dependent IMPS are now being performed to better understand charge transport mechanisms.
Graduation Date: May 2011
URI: http://hdl.handle.net/1928/12839


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