5 research outputs found
Correlating Structure and Photocurrent for Composite Semiconducting Nanoparticles with Contrast Variation Small-Angle Neutron Scattering and Photoconductive Atomic Force Microscopy
Aqueous dispersions of semiconducting nanoparticles have shown promise as a robust and scalable platform for the production of efficient polymer/fullerene active layers in organic photovoltaic applications. Semiconducting nanoparticles are a composite of both an n-type and p-type semiconductor contained within a single nanoparticle. In order to realize efficient organic solar cells from these materials, there is a need to understand how the size and internal distribution of materials within each nanoparticle contributes to photocurrent generation in a nanoparticle-derived device. Therefore, characterizing the internal distribution of conjugated polymer and fullerene within the dispersion is the first step to improving performance. To date, study of polymer/fullerene structure within these nanoparticles has been limited to microscopy techniques of deposited nanoparticles. In this work, we use contrast variation with small-angle neutron scattering to determine the internal distribution of poly(3-hexylÂthiophene) and [6,6]phenyl-C<sub>61</sub>-butyric acid methyl ester inside the composite nanoparticles as a function of formulation while in dispersion. On the basis of these measurements, we connect the formulation of these nanoparticles with their internal structure. Using electrostatic deposited monolayers of these nanoparticles, we characterize intrinsic charge generation using photoconductive atomic force microscopy and correlate this with structures determined from small-angle neutron scattering measurements. These techniques combined show that the best performing composite nanoparticles are those that have a uniform distribution of conjugated polymer and fullerene throughout the nanoparticle volume such that electrons and holes are easily transported out of the particle
Morphology-Dependent Trap Formation in Bulk Heterojunction Photodiodes
We show that local structural variation affects the rate
of aging
in nanostructured polymer solar cells by comparing time-resolved electrostatic
force microscopy (trEFM) and conventional device measurements on model
polymer blends. Specifically, we study photovoltaic devices made from
1:1 blends of the polyfluorene copolymers polyÂ(9,9′-dioctylfluorene-<i>co</i>-bis-<i>N,N</i>′-(4-butylphenyl)-bis-<i>N,N</i>′-phenyl-1,4-phenylene-diamine) (PFB) and polyÂ(9,9′-dioctylfluorene-<i>co</i>-benzothiadiazole) (F8BT). We photooxidize these films
in situ using 365, 405, and 455 nm illumination under ambient conditions,
with the wavelengths chosen to preferentially excite the different
components. During photooxidation, we observe a faster loss of photocurrent
generation from F8BT-rich domains, leaving the PFB-rich phases to
show higher photoresponse even at wavelengths absorbed predominantly
by F8BT. We propose that this effect is due to the more rapid degradation
of PFB hole-transport pathways in the F8BT-rich regions, resulting
in a loss of percolation pathways for hole transport in the F8BT-rich
phase
Intensity-Modulated Scanning Kelvin Probe Microscopy for Probing Recombination in Organic Photovoltaics
We study surface photovoltage decays on sub-millisecond time scales in organic solar cells using intensity-modulated scanning Kelvin probe microscopy (SKPM). Using polymer/fullerene (poly[<i>N</i>-9″-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]/[6,6]-phenyl C<sub>71</sub>-butyric acid methyl ester, PCDTBT/PC<sub>71</sub>BM) bulk heterojunction devices as a test case, we show that the decay lifetimes measured by SKPM depend on the intensity of the background illumination. We propose that this intensity dependence is related to the well-known carrier-density-dependent recombination kinetics in organic bulk heterojunction materials. We perform transient photovoltage (TPV) and charge extraction (CE) measurements on the PCDTBT/PC<sub>71</sub>BM blends to extract the carrier-density dependence of the recombination lifetime in our samples, and we find that the device TPV and CE data are in good agreement with the intensity and frequency dependence observed <i>via</i> SKPM. Finally, we demonstrate the capability of intensity-modulated SKPM to probe local recombination rates due to buried interfaces in organic photovoltaics (OPVs). We measure the differences in photovoltage decay lifetimes over regions of an OPV cell fabricated on an indium tin oxide electrode patterned with two different phosphonic acid monolayers known to affect carrier lifetime
Nanoscale Surface Potential Variation Correlates with Local S/Se Ratio in Solution-Processed CZTSSe Solar Cells
Thin film solar cells made from Cu,
Zn, Sn, and S/Se can be processed
from solution to yield high-performing kesterite (CZTS or CZTSSe)
photovoltaics. We present a microstructural study of solution-deposited
CZTSSe films prepared by nanocrystal-based ink approaches using scanning
probe microscopy (SPM) and scanning electron microscopy (SEM) coupled
with energy dispersive X-ray spectroscopy (EDS). We correlate scanning
Kelvin probe microscopy (SKPM) maps of local surface potential with
SEM/EDS images of the exact same regions of the film, allowing us
to relate observed variations in surface potential to local variations
in stoichiometry. Specifically, we find a correlation between surface
potential and the S/(S + Se) composition ratio. In particular, we
find that regions with high S/(S + Se) ratios are often associated
with regions of more negative surface potential and thus higher work
function. The change in work function is larger than the expected
change in the valence band position with these small changes in sulfur,
and thus the data suggest an increase in acceptor-like defects with
increasing sulfur. These findings provide new experimental insight
into the microscopic relationships between composition, structure,
and electronic properties in these promising photovoltaic materials
Submicrosecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics
We propose, simulate, and experimentally validate a new
mechanical
detection method to analyze atomic force microscopy (AFM) cantilever
motion that enables noncontact discrimination of transient events
with ∼100 ns temporal resolution without the need for custom
AFM probes, specialized instrumentation, or expensive add-on hardware.
As an example application, we use the method to screen thermally annealed
polyÂ(3-hexylthiophene):phenyl-C<sub>61</sub>-butyric acid methyl ester
photovoltaic devices under realistic testing conditions over a technologically
relevant performance window. We show that variations in device efficiency
and nanoscale transient charging behavior are correlated, thereby
linking local dynamics with device behavior. We anticipate that this
method will find application in scanning probe experiments of dynamic
local mechanical, electronic, magnetic, and biophysical phenomena