Spectroscopic Characterization Of Charge Generation And Trapping In Third-Generation Solar Cell Materials Using Wavelength- And Time-Resolved Electric Force Microscopy

摘要

The mechanism of charge generation and charge trapping are a topic of intense research in many third-generation solar cell materials, such as thin films of organic small-molecules, organic polymers, and nanocrystal quantum dots. We present novel Electric Force Microscopy (EFM) techniques that are able to: (1) determine the chemical identity of charged trap species in polycrystalline pentacene, (2) correlate surface photopotential to the absorption of donor and acceptor materials, (3) detect electron trapping in bulk heterojunction polymer solar cells, (4) calculate exciplex density and charge trapping rates in hexabenzocoronene(HBC)-fullerene(C60) bilayer devices, and (5) confirm that charge generation occurs in the bulk of nanocrystal quantum dot thin films. In pentacene, the direct absorption of stable cations lead to the release of trapped charge. By varying the wavelength of illuminated light, we obtain the trap-clearing action spectrum, from which chemical information of the cation is determined. In addition to the work on pentacene, we report on the first image of photovoltage spectra in bulk heterojunction polymer solar cells, and find that charge trapping from donor-acceptor mixing creates an electric field that opposes geminate-pair splitting. Additionally, we measure fluctuations in the contact potential to provide quantitative information on the vacant trap density. In bilayer films of HBC-C60, we model the vertical distribution of photogenerated charges to determine exciplex density. We further note enhanced degradation in the active material in the absence of an electron-accepting fullerene. Lastly, we demonstrate the generation of free charge in the bulk of highly-coupled lead-salt nanocrystal solar cells in the absence of a chemical gradient or applied electric field.