Consequence of thermal annealing on PCDTBT-based solar cells performance and composition profile (presentation video)
Author(s):
Olesia Synooka;
Kai R. Eberhardt;
Chetan Raj Singh;
Gernot Ecke;
Bernhard Ecker;
Elizabeth von Hauff;
Gerhard Gobsch;
Harald Hoppe
Show Abstract
As reported earlier, the photovoltaic performance of PCDTBT:PCBM polymer solar cells drastically decreases upon thermal annealing. It was demonstrated in the literature, that thermal annealing leads to increased trap formation and as a consequence disturb solar cell performance, especially via a reduced fill factor. This has been demonstrated by space-charge-limited-current analysis and ellipsometry, as well as, structural changes analyse of PCDTBT upon annealing. However, we decided in addition to investigate morphological changes occurring within PCDTBT:PCBM photoactive blend layers upon thermal annealing, as these must have an impact on charge transport. By application of several characterizations techniques, and especially supported by results of Impedance Spectroscopy and Auger Electron Spectroscopy (AES), indeed the existence of an unfavourable compositional gradient within the photoactive layer could be revealed. This compositional gradient may be in part accounted for harming the transport of electrons and holes in either direction.
Competing role of quantum localization and interfacial disorder in determining triplet exciton fission and recombination dynamics in polymer/fullerene photovoltaics
Author(s):
Eric R. Bittner
Show Abstract
Non-geminate charge recombination is a significant source of carrier loss in organic photovoltaic systems. Recent
experiments by Rao, et al. (Nature, 2013 500, 435-439) suggest that the recombination of triplet charge-transfer
(3CT) states can be suppressed by careful control of the molecular order in the vicinity of the phase boundary
between donor and acceptor materials polymer/fullerene bulk-heterojunction devices. In short, recombination
of 3CT states is effectively suppressed when the fullerene phase exhibits a high degree of local order near the
interface. Here we report upon our theoretical model that connects energetic disorder, dimensionality, and wave
function localization to show that inhomogeneous broadening introduces strong coupling between the interfacial
3CT and nearby fullerene triplet excitons and can enhance the decay of these states in systems with higher
degrees of energetic disorder.
Two-dimensional polaron coherence in Poly(3-hexylthiophene)
Author(s):
C. M Pochas;
Hajime Yamagata;
F. C. Spano
Show Abstract
The two-dimensional coherence function corresponding to positively charged polarons (“holes”) in poly(3-
hexylthiophene) π-stacks is calculated based on a Holstein-style Hamiltonian which treats electronic coupling,
vibronic coupling and disorder on equal footing. Assuming a model of isotropic site-energy disorder, the hole is
found to be delocalized between 1-2 nm along the polymer chain and between 0.5-1 nm along the stacking axis.
Charge separation in an n-p-n triad forming Lamellar structure (presentation video)
Author(s):
Damien Rolland;
Lucia Hartmann;
Natalie Banerji;
Martin Brinkmann;
Holger Frauenrath
Show Abstract
Since charge separation in organic photovoltaics takes place between n- and p-type semiconductors, their interface should be maximized within the active layer, while charge percolation pathways to the electrodes should be ensured. One way to obtain an ideal and thermodynamically stable morphology is to covalently link n- and p-type semiconductors, provided a sufficiently high internal order is achieved. To this end, we have synthesized a perylene-quaterthiophene-perylene triad substituted with poly(isobutene) segments that induce order at the nanoscale via microphase segregation. Charge separation under selective illumination on the quaterthiophene or perylene moiety was investigated by steady state and transient absorption spectroscopy, and related to the molecular packing and phase morphology deduced from electron and grazing incidence X-ray diffractions. Photoluminescence was quenched due to charge separation within the triad. In solution, these charges quickly recombined, but were shown to have a longer lifetime in films, which is beneficial for charge collection in photovoltaic devices. We have also investigated whether alignment of the supramolecular aggregates affects the photophysics.
X-ray photoemission spectroscopy study of vertical phase separation in F8BT:PDI/ITO films for photovoltaic applications
Author(s):
A. Brambilla;
A. Calloni;
E. Aluicio-Sardui;
G. Berti;
Z. Kan;
S. Beaupré;
M. Leclerc;
H.-J. Butt;
G. Floudas;
P. E. Keivanidis;
L. Duò
Show Abstract
We present a depth-resolved X-ray photoemission spectroscopy study of the Poly(9,9’-dioctylfluorene-cobenzothiadiazole):
Perylene tetracarboxylic diimide blend (briefly, F8BT:PDI), employed for the realization of the light
harvesting layer in organic photovoltaic devices. We address the problem of the vertical distribution of PDI molecules in
the blend, relevant for the optimization of the photo-generated charge collection in such devices. The depth resolution is
obtained by sputtering the organic layer with Ar+ ions. A thorough investigation of the effects of different sputtering
treatments on the F8BT:PDI film surface is presented. Changes in the stoichiometry of the organic layer, as well as the
cleavage of molecular bonds are detected, even after mild sputtering. In particular, we report about the formation of a
carbon-rich surface layer. Finally, a method is proposed for the calculation of the PDI concentration, which relies on the
detection of specific chemical markers and is robust against sputter-induced artifacts. As a case study, we evaluated the
PDI concentration in a 10 nm thick F8BT:PDI layer spin coated on indium tin oxide.
Exciton hopping in carbon nanotube solar cells studied with 2D white-light spectroscopy
Author(s):
Martin T. Zanni
Show Abstract
Semiconducting carbon nanotubes are an exciting new material for solar cells. Mesoscale films can now be assebled and
made into devices in which the semiconducting tubes are the photoactive layer, analogous to organic dyes or quantum
dots in dye-sensitized solar cells. In order to understand their exciton transport properties, we are studying the
photophysics of these films. Obtaining a comprehensive picture of the pathways, rates, and bottlenecks is challenging
because in many cases this relaxation spans a wide range of energies. The standard approach to study such a wide
frequency range is to use a tunable pump pulse to excite each electronic transition in turn, one after another. We have
developed two-dimensional white light spectroscopy (2D WL) which allows us to simultaneously examine a spectral
range spanning roughly 500-1400 nm. The spectra resolve energy transfer between all possible combinations of
excitonic states in the chirality-selected nanotubes, thereby providing an instantaneous and comprehensive snapshot of
the dynamical pathways. The new physics we uncover has important implications in the development of carbon
nanotube electronics and optoelectronics.
Direct measurement of energy transport in organic nanosystems
Author(s):
Katherine A. Koen;
Katie A. Clark;
David A. Vanden Bout
Show Abstract
Conjugated polymers are appealing materials for cost-effective production of flexible electronic devices, but the
efficiency of these devices may be compromised by exciton quenching by hole polarons. Exciton migration was
monitored directly in single poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) chains embedded
in a hole-injection device by applying super-resolution point-spread function fitting techniques to the fluorescence image
of each chain as holes were reversibly injected into the polymer. Correlated excitation polarization spectroscopy
techniques reported the orientation of the longitudinal axis of the rod-like polymers. The wide-field microscopy images
are diffraction-limited in one dimension but slightly elongated in the direction of the longitudinal axis of the rod-like
polymer in most cases and at every depth of fluorescence quenching explored.
The optical properties of conjugated materials and their aggregates: towards imaging of films and devices
Author(s):
Linda A. Peteanu;
Jiyun Hong;
SuKyung Jeon;
Janice Kim;
Diane Devi;
Jurjen Wildeman;
Matthew Y. Sfeir;
James H. Werner;
Andrew P. Shreve
Show Abstract
Applications of conjugated polymers in photovoltaics and displays drive the need to understand how morphology affects
emission and charge migration. Due to the inherent complexity of polymers, parallel studies of oligomer aggregates are
required to ‘build-up' an understanding of the polymer features. Fluorescence lifetime imaging microscopy (FLIM) is
used to probe variations in vibronic patterns and emission lifetime between individual aggregates and trends in these
properties as a function of aggregate size. This technique yields insight into the structure and packing properties of these
materials in the aggregated state.
Two-electron photo-oxidation of betanin on titanium dioxide and potential for improved dye-sensitized solar energy conversion
Author(s):
Fritz J. Knorr;
Deborah J. Malamen;
Jeanne L. McHale;
Arianna Marchioro;
Jacques E. Moser
Show Abstract
The plant pigment betanin is investigated as a dye-sensitizer on TiO2 with regard to its potential to undergo twoelectron
oxidation following one-photon excitation. Electrochemical, spectroelectrochemical and transient absorption
measurements provide evidence for two-electron proton-coupled photo-oxidation leading to a quinone methide intermediate
which rearranges to 2-decarboxy-2,3-dehydrobetanin. Time-resolved spectroscopy measurements of betanin on
nanocrystalline TiO2 and ZrO2 films were performed on femtosecond and nanosecond time-scales and provide evidence
for transient species with absorption bands in the blue and the red. The results shed light on previous reports of high
quantum efficiencies for electron injection and point the way to improved solar conversion efficiency of organic dyesensitized
solar cells.
Effect of atomic under-coordination on the properties of Ag and Cu nanoclusters
Author(s):
Shideh Ahmadi;
Xi Zhang;
Yinyan Gong;
Chang Q. Sun
Show Abstract
Density functional theory calculations have been carried out to investigate the effect of the atomic under-coordination on
the bond contraction, lattice strain, and electron configuration of Cuboctahedral and Marks decahedral structures of
silver and copper nanoclusters. Our calculated results are consistent in trend with experimental measurements including
extended X-ray-absorption fine structure (EXAFS), scanning tunneling microscope/spectroscopy (STM/S), X-ray
photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectra (UPS). This agreement approved the
prognostications made on the bond-order-length-strength (BOLS) correlation and nonbonding electron polarization
(NEP), suggesting that atomic under-coordination at the surface of nanoclusters cause bond contraction, which then leads
to lattice strain, charge densification, core electron entrapment, as well as polarization of valence charge. The results of
this work will contribute to the understanding of the intriguing properties of Ag and Cu nanoclusters.
Spectroelectrochemical photoluminescence of titanium dioxide nanosheets and nanoparticles in aqueous and nonaqueous environments
Author(s):
Riley E. Rex;
Fritz J. Knorr;
Jeanne L. McHale
Show Abstract
Spectroelectrochemical photoluminescence (SEPL) is used to investigate surface electron and hole traps on anatase
nanoparticles, anatase nanosheets and rutile nanowires in aqueous and nonaqueous environments. In aqueous
environment there is an overvoltage for occupying surface electron traps in rutile and anatase samples. For anatase,
this overvoltage is larger on (101) nanoparticles than on (001) nanosheets. The electrochemical energy levels of
electron traps determined by SEPL in acetonitrile are consistent with emitted photon energies as determined by the
photoluminescence spectrum. Our results show how the contacting solvent and particle morphology can influence
the redox potential of surface electron traps and thus guide further research on improving the performance of nano-
TiO2 in applications such as dye–sensitized solar cells and solar water splitting.
Ultrafast infrared spectroscopy reveals intragap states in methylammonium lead iodide perovskite materials
Author(s):
Artem A. Bakulin;
Huib J. Bakker;
Zhenhua Sun;
Zhuoying Chen
Show Abstract
Organic-inorganic hybrid materials have recently attracted a lot of attention for optoelectronic applications as they allow
combining the flexibility and processibility of organic molecular systems with the good stability and charge-transport
properties of inorganic crystalline materials. In this contribution, we report on the use of ultrafast vis-pump /
infrared(IR)-probe spectroscopy to investigate the optical signature of charge carrier dynamics in CH3NH3PbI3
perovskite films. We observed that this perovskite shows a complex response which involves strong ground-state bleach
contribution. This behaviour differs from most inorganic semiconductors and quantum dot materials that show strong
photoinduced absorption in IR upon carrier generation,. The bleaching probably results from a partial filling of intragap
states next to the conduction band edge. It may also indicate the existence of a Burstein–Moss type effect in perovskite
materials.
Ferroelectric domain walls as nanoscale pathways to novel functional properties (presentation video)
Author(s):
Patrycja Paruch
Show Abstract
In ferroelectrics, domain walls separate regions with different polarization. At domain walls, modified symmetry and electronic structure, or chemical segregation can lead to unusual, extremely localized functional properties, quite different from those of the parent phase.
Particularly exciting has been the discovery of domain-wall-specific electrical conductivity in multiferroic BiFeO3 [1]. Here, I will present our generalization of these observations to Pb(Zr,Ti)O3 [2], highlighting the key role of oxygen vacancies, whose distribution can be modulated to reversibly control domain wall transport. For these studies, we have also developed carbon-nanotube-based probes with exceptional performance [3].
[1] Seidel, Nat. Mat. 8, 229 (2009)
[2] Guyonnet, Adv. Mat. 23, 5377 (2011)
[3] Lisunova, Nano Lett. 13, 4527 (2013)
The photophysics of perovskite solar cells
Author(s):
Tze Chien Sum
Show Abstract
Solution-processed hybrid organic-inorganic perovskite solar cells, a newcomer to the photovoltaic arena, have taken the field by
storm with their extraordinary power conversion efficiencies exceeding 17%. In this paper, the photophysics and the latest findings on the carrier dynamics and charge transfer mechanisms in this new class of photovoltaic material will be examined and distilled. Some open photophysics questions will also be discussed.
Optical properties of low bandgap copolymer PTB7 for organic photovoltaic applications
Author(s):
Uyen Huynh;
Tek Basel;
Tao Xu;
Luyao Lu;
Tianyue Zheng;
Luping Yu;
Valy Vardeny
Show Abstract
We used both cw and transient spectroscopies for studying the optical properties and photoexcitations in the low bandgap
copolymer PTB7 that has been used in organic photovoltaic applications (OPV). Surprisingly we observed two
primary photoexcitations that are generated within ~150 fs (our time resolution); we identify them as singlet exciton (S1)
and triplet-pair (1TT). The singlet exciton has been considered to be the only primary photoexcitation in regular π-conjugated polymers and is related with a transient absorption band that peaks at an energy value close to the exciton
binding energy (~0.4 eV in PTB7). The TT pair is a novel photoexcitation species in low band-gap π-conjugated
copolymers. It has an absorption band close to that of isolated triplet exciton, and may readily dissociate at the donoracceptor
interfaces in the PTB7/fullerene blend. This finding may explain the underlying mechanism for the high
obtained power conversion efficiency in OPV devices based on the PTB7 copolymer.
Quantum dots: using the known as well as exploring the unknown
Author(s):
Ron Tenne;
Osip Schwartz;
Ayelet Teitelboim;
Pazit Rukenstien;
Jonathan M. Levitt;
Taleb Mokari;
Dan Oron
Show Abstract
Super-resolution microscopy, the imaging of features below the Abbe diffraction limit, has been achieved by a number
of methods in recent years. Each of these methods relies on breaking one of the assumptions made in the derivation of
the diffraction limit. While uniform spatial illumination, linearity and time independence have been the most common
cornerstones of the Abbe limit broken in super-resolution modalities, breaking the ‘classicality of light’ assumption as a
pathway to achieve super-resolution has not been shown. Here we demonstrate a method that utilizes the antibunching
characteristic of light emitted by Quantum Dots (QDs), a purely quantum feature of light, to obtain imaging beyond the
diffraction limit.
Measuring such high order correlations in the emission of a single QD necessitates stability at saturation conditions
while avoiding damage and enhanced blinking. This ability was facilitated through new understandings that arisen from
exploring the QD ‘blinking’ phenomena. We summarize here two studies that contributed to our current understanding
of QD stability.
Drug/protein interactions studied by time-resolved fluorescence spectroscopy
Author(s):
Thomas Gustavsson;
Dimitra Markovitsi;
Ignacio Vayá;
Paula Bonancía;
M. Consuelo Jiménez;
Miguel A. Miranda
Show Abstract
We report here on a recent time-resolved fluorescence study [1] of the interaction between flurbiprofen (FBP), a chiral
non-steroidal anti-inflammatory drug, and human serum albumin (HSA), the main transport protein in the human body.
We compare the results obtained for the drug-protein complex with those of various covalently linked flurbiprofentryptophan
dyads having well-defined geometries. In all cases stereoselective dynamic fluorescence quenching is
observed, varying greatly from one system to another. In addition, the fluorescence anisotropy decays also display a clear
stereoselectivity. For the drug-protein complexes, this can be interpreted in terms of the protein microenvironment
playing a significant role in the conformational relaxation of FBP, which is more restricted in the case of the (R)-
enantiomer.
PEGylated nanoparticles: protein corona and secondary structure
Author(s):
Sabiha Runa;
Alexandra Hill;
Victoria L. Cochran;
Christine K. Payne
Show Abstract
Nanoparticles have important biological and biomedical applications ranging from drug and gene delivery to biosensing.
In the presence of extracellular proteins, a “corona” of proteins adsorbs on the surface of the nanoparticles, altering their
interaction with cells, including immune cells. Nanoparticles are often functionalized with polyethylene glycol (PEG) to
reduce this non-specific adsorption of proteins. To understand the change in protein corona that occurs following
PEGylation, we first quantified the adsorption of blood serum proteins on bare and PEGylated gold nanoparticles using
gel electrophoresis. We find a threefold decrease in the amount of protein adsorbed on PEGylated gold nanoparticles
compared to the bare gold nanoparticles, showing that PEG reduces, but does not prevent, corona formation. To
determine if the secondary structure of corona proteins was altered upon adsorption onto the bare and PEGylated gold
nanoparticles, we use CD spectroscopy to characterize the secondary structure of bovine serum albumin following
incubation with the nanoparticles. Our results show no significant change in protein secondary structure following
incubation with bare or PEGylated nanoparticles. Further examination of the secondary structure of bovine serum
albumin, α2-macroglobulin, and transferrin in the presence of free PEG showed similar results. These findings provide
important insights for the use of PEGylated gold nanoparticles under physiological conditions.
Role of ZnS shell on stability, cytotoxicity, and photocytotoxicity of water-soluble CdSe semiconductor quantum dots surface modified with glutathione
Author(s):
Salwa Ali Ibrahim;
Wafaa Ahmed;
Tareq Youssef
Show Abstract
Biomedical applications of quantum dots (QDs) have become a subject of a considerable concern in the past few
decades. The present study examines the stability and cytotoxicity of two QDs systems in cell culture medium in the
presence and absence of a thin layer of ZnS shell. The two systems were built from core, CdSe QDs, surface modified
with glutathione (GSH), named CdSe∼GSH and CdSe/ZnS∼GSH. Our results demonstrated that 0.7 nm layer of ZnS
shell played a significant role in the stability of CdSe/ZnS~GSH QDs in supplemented cell culture medium (RPMI).
Also, a significant improvement in the physicochemical properties of the core CdSe QDs was shown by maintaining
their spectroscopic characteristics in RPMI medium due to the wide band gap of ZnS shell. Both systems showed
insignificant reduction in cell viability of HFB-4 or MCF-7 cell lines in the dark which was attributed to the effective
GSH coating. Following photoirradiation with low laser power (irradiance 10 mW cm-2), CdSe~GSH QDs showed a
significant decrease in cell viability after 60 min irradiation which may result from detachment of GSH molecules. Under
the same irradiation condition, CdSe/ZnS~GSH QDs showed insignificant decrease in cell viability or after 2 h
incubation from laser irradiation which was attributed to the strong binding between ZnS and GSH coatings. It can be
concluded that the stability of CdSe core QDs was significantly improved in cell culture medium by encapsulation with a
thin layer of ZnS shell whereas their cytotoxicity and photo-cytotoxicity are highly dependent on surface modification.
A striking mobility improvement of C60 OFET by inserting diindenoperylene layer between C60 and SiO2 gate insulator
Author(s):
Jin-peng Yang;
Keiichiro Yonezawa;
Alexander Hinderhofer;
Fabio Bussolotti;
Satoshi Kera;
Nobuo Ueno
Show Abstract
Gap states in organic semiconductors play a crucial role in determining Energy-Level Alignment and in many cases
they act as charge trapping centers to result in serious lowering of charge mobility. Thus origin of gap states has gained
increasing attention in order to realize higher mobility organic devises [1-4]. Bussolotti et al. have demonstrated recently
that gap states in a pentacene thin film increase even by exposing the film to inert gas and confirmed that the gas
exposure mediates structural defects in the film thus gap states [4]. The results have also indicated that preparation of
highly-ordered organic thin film is necessary to improve the device performance, namely to decrease trapping states. To
improve the ordering of molecule in the film, deposition of a template molecular underlayer is one of the simplest
methods to increase the domain size of overlayer film and its crystallinity, and thus we expect improvement of the charge
mobility [5]. Hinderhofer et al. reported recently that diindenoperylene (DIP; Figure 1a) could be used as a template
layer to grow highly ordered and oriented C60 film with its (111) plane parallel to the SiO2 substrate [6]. Considering the
hole mobility of DIP single crystal, which is quite low (~0.005 cm2 V-1S-1 at room temperature [7]), it is expected for the
DIP template C60 thin film system that lower drain current would be achieved to improve the on/off ratios based on n
type C60 transistor and its electron mobility (especially on the negative Vgs region, compared to PEN modified C60
transistors [8]).
Preparation of flexible TiO2 photoelectrodes for dye-sensitized solar cells
Author(s):
Wen-Ren Li;
Hsiu-Hsuan Wang;
Chia-Feng Lin;
Chaochin Su
Show Abstract
Dye-sensitized solar cells (DSSCs) based on nanocrystalline TiO2 photoelectrodes on indium tin oxide (ITO) coated
polymer substrates have drawn great attention due to its lightweight, flexibility and advantages in commercial
applications. However, the thermal instability of polymer substrates limits the process temperature to below 150 °C. In
order to assure high and firm interparticle connection between TiO2 nanocrystals (TiO2-NC) and polymer substrates, the
post-treatment of flexible TiO2 photoelectrodes (F-TiO2-PE) by mechanical compression was employed. In this work,
Degussa P25 TiO2-NC was mixed with tert-butyl alcohol and DI-water to form TiO2 paste. F-TiO2-PE was then prepared
by coating the TiO2 paste onto ITO coated polyethylene terephthalate (PET) substrate using doctor blade followed by
low temperature sintering at 120 °C for 2 hours. To study the effect of mechanical compression, we applied 50 and 100
kg/cm2 pressure on TiO2/PET to complete the fabrication of F-TiO2-PE. The surface morphology of F-TiO2-PE was
characterized using scanning electron microscopy. The resultant F-TiO2-PE sample exhibited a smooth, crack-free
structure indicating the great improvement in the interparticle connection of TiO2-NC. Increase of compression pressure
could lead to the increase of DSSC photoconversion efficiency. The best photoconversion efficiency of 4.19 % (open
circuit voltage (Voc) = 0.79 V, short-circuit photocurrent density (Jsc) = 7.75 mA/cm2, fill factor (FF) = 0.68) was
obtained for the F-TiO2-PE device, which showed great enhancement compared with the F-TiO2-PE cell without
compression treatment. The effect of compression in DSSC performance was vindicated by the electrochemical
impedance spectroscopy measurement.