5 research outputs found
Revealing the Chemistry and Morphology of Buried Donor/Acceptor Interfaces in Organic Photovoltaics
With
power conversion efficiencies (PCEs) of <13% and plagued
by stability issues, organic photovoltaics (OPVs) still lack wide
adoption, despite significant recent advances. Currently, the most
progress in OPV device performance is achieved by âtrial-and-errorâ
preparation procedures that lead to complex and largely unknownîždespite
tremendous analytical effortsîžmorphologies. Here, we demonstrate
a proof-of-principle, chemical imaging methodology that combines experimental
high spatial sensitivity and chemical selectivity with theoretical
modeling, capable of analyzing the three-dimensional composition and
morphology of virtually any device. Allowing the precise measurement
of composition and direct visualization of film morphology with depth,
our approach reveals the intricate buried donor/acceptor (D/A) interface
of a model polymer/fullerene system, polyÂ(3-hexylthiphene-2,5-diyl)/[6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (P3HT/PCBM). In particular, our
technique is able to identify and quantify the D/A interface length,
that is, the extent of molecular mixing at the D/A interface, a parameter
crucial for device performance, yet never measured. Extracting this
parameter allows demonstrating that, contrary to the general understanding,
when starting with a fully mixed D/A phase in our model system, thermal
annealing, which is known to substantially (however limited) increase
the device performance by phase segregation, does not create but small
amounts of pure phases, leaving the device mostly mixed, which limits
the performance improvement
Understanding the Interface Dipole of Copper Phthalocyanine (CuPc)/C<sub>60</sub>: Theory and Experiment
Interface dipole determines the electronic energy alignment
in
donor/acceptor interfaces and plays an important role in organic photovoltaics.
Here we present a study combining first principles density functional
theory (DFT) with ultraviolet photoemission spectroscopy (UPS) and
time-of-flight secondary ion mass spectrometry (TOF-SIMS) to investigate
the interface dipole, energy level alignment, and structural properties
at the interface between CuPc and C<sub>60</sub>. DFT finds a sizable
interface dipole for the face-on orientation, in quantitative agreement
with the UPS measurement, and rules out charge transfer as the origin
of the interface dipole. Using TOF-SIMS, we show that the interfacial
morphology for the bilayer CuPc/C<sub>60</sub> film is characterized
by molecular intermixing, containing both the face-on and the edge-on
orientation. The complementary experimental and theoretical results
provide both insight into the origin of the interface dipole and direct
evidence for the effect of interfacial morphology on the interface
dipole
Anomalously Large Polarization Effect Responsible for Excitonic Red Shifts in PbSe Quantum Dot Solids
The formation of solid thin films from colloidal semiconductor quantum dots (QDs) is often accompanied by red shifts in excitonic transitions, but the mechanisms responsible for the red shifts are under debate. We quantitatively address this issue using optical absorption spectroscopy of two-dimensional (2D) and three-dimensional (3D) arrays of PbSe QDs with controlled inter-QD distance, which was determined by the length of alkanedithiol linking molecules. With decreasing inter-QD distance, the first and second exciton absorption peaks show increasing red shifts. Using thin films consisting of large and isolated QDs embedded in a matrix of small QDs, we determine that a dominant contribution to the observed red shift is due to changes in polarization of the dielectric environment surrounding each QD (âŒ88%), while electronic or transition dipole coupling plays a lesser role. However, the observed red shifts are more than 1 order of magnitude larger than theoretical predictions based on the dielectric polarization effect for spherical QDs. We attribute this anomalously large polarization effect to deviations of the exciton wave functions from eigenfunctions of the idealized spherical quantum well model
Reversible and Irreversible Electric Field Induced Morphological and Interfacial Transformations of Hybrid Lead Iodide Perovskites
We report reversible and irreversible
strain effects and interfacial atomic mixing in MAPbI<sub>3</sub>/ITO
under influence of external electric bias and photoillumination. Using
conductive-probe atomic force microscopy, we locally applied a bias
voltage between the MAPbI<sub>3</sub>/ITO and the conductive tip and
observed local dynamic strain effects and current under conditions
of forward bias. We found that the reversible part of the strain is
associated with a current spike at the current onset stage and can
therefore be related to an electrochemical process accompanied by
local molar volume change. Similar partly reversible surface deformation
was observed when the tipâsample contact was illuminated by
light. Time-of-flight secondary ion mass spectrometry of electrically
biased regions revealed massive atomic mixing at the buried MAPbI<sub>3</sub>/ITO interface, while the top MAPbI<sub>3</sub> surface, subjected
to strong morphological damage at the tipâsurface contact,
revealed less significant chemical decomposition
Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics
It has been proposed that interface morphology affects the recombination rate for electrons and holes at donorâacceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an âinvertedâ quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C<sub>60</sub> cap layer. We call this apparently unprecedented growth process from a buried interface âinverse quasi-epitaxyâ where the crystallites of these âsoftâ van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C<sub>60</sub> acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of η<sub>P</sub> = 5.4 ± 0.3% for single-junction and η<sub>P</sub> = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donorâacceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells