Recent progress on reactively sputtered metal oxide PV interlayers is presented. A strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to pronounced device improvements. A new crystalline MoOx system employed for efficient hole extraction is shown to lead to prolonged OPV lifetimes1, and a new crystalline TiOx layer is shown to lead to efficient electron extraction. In order to meet the requirements on scalable OPV development, the up-scaling of the metal oxides from Roll-to-Roll (R2R) vacuum processing is discussed.
1 Ahmadpour et al, ACS Appl. Energy Mater. 2, 420 (2019)
Record efficiencies of OPV devices nowadays reach well above 10%. However, their organic nature makes them strongly sensitive to oxygen, light, heat and humidity.
We report on long-term stabilization by ternary blending the active layers with small amounts of stabilizing compounds of different classes of antioxidants[1,3], radical scavengers[1] and light stabilizers[2]. Lifetime testing was conducted under ISOS3-degradation conditions on bulk-heterojunction cells containing a wide selection of stabilizers. Microscopic and spectroscopic methods were applied to monitor chemical degradation over time, and the observed differences are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological and structural changes.
Both antioxidants and UV absorbers yielded an increase of the accumulated power generation by over a factor of 3 compared to the reference devices without additive. In both cases, stability improvement was caused by significant reduction of radicals within the photoactive layer, which in turn stabilizes the performance by decreasing exciton recombination. However, stabilization mechanisms of these two classes are quite different, as reflected in the burn-in. While antioxidant-stabilized cells manifested a simultaneous increase of the burn-in period and decrease of decay magnitude, UV-absorber-stabilized cells retained the same burn-in period as the reference.
[1] Turkovic V. et al, ACS Applied Materials & Interfaces 6, p 18525 (2014); http://dx.doi.org/10.1021/am5024989
[2] Turkovic V. et al, Journal of Physics D: Applied Physics 49, p 125604 (2016); http://dx.doi.org/10.1088/0022-3727/49/12/125604
[3] Turkovic V. et al, Applied Physics A 122:255 (2016); http://dx.doi.org/10.1007/s00339-016-9758-7
Although organic solar cells show intriguing features such as low-cost, mechanical flexibility and light weight, their efficiency is still low compared to their inorganic counterparts. One way of improving their efficiency is by the use of light-trapping mechanisms from nano- or microstructures, which makes it possible to improve the light absorption and charge extraction in the device’s active layer. Here, periodically arranged colloidal gold nanoparticles are demonstrated experimentally and theoretically to improve light absorption and thus enhance the efficiency of organic solar cells. Surface-ordered gold nanoparticle arrangements are integrated at the bottom electrode of organic solar cells. The resulting optical interference and absorption effects are numerically investigated in bulk hetero-junction solar cells based on the Finite-Difference Time-Domain (FDTD) and Transfer Matrix Method (TMM) and as a function of size and periodicity of the plasmonic arrangements. In addition, light absorption enhancement in the organic active layer is investigated experimentally following integration of the nanoparticle arrangements. The latter are fabricated using a lithography-free stamping technique, creating a centimeter scaled area with nanoparticles having a defined inter-particle spacing. Our study reveals the light harvesting ability of template-assisted nanoparticle assemblies in organic solar cells. As the approach is easily scalable, it is an efficient and transferable method for large-scale, low cost device fabrication.
A promising method for improving light-absorption in thin-film devices is demonstrated via electrode structuring
using Anodic Alumina Oxide (AAO) templates. We present nano-scale concave Al structures of controlled dimensions,
formed after anodic oxidation of evaporated high purity aluminum (Al) films and alumina etching. We investigate both
experimentally and theoretically the field-enhancement supported by these concave nanostructures as a function of their
dimensions. For the experimental investigations, a thin layer of organic polymer coating allows the application of a nondestructive
laser ablation technique that reveals field-enhancement at the ridges of the Al nanostructures. The
experimental results are complemented by finite-difference time-domain (FDTD) simulations, to support and explain the
outcome of the laser ablation experiments. Our method is easily up-scalable and lithography-free and allows one to
generate nanostructured electrodes that potentially support field-enhancement in organic thin-film devices, e.g., for the
use in future energy harvesting applications.
We report on development of flexible PCPDTBT:PCBM solar cells with integrated diffraction gratings on the bottom electrodes. The presented results address PCPDTBT:PCBM solar cells in an inverted geometry, which contains implemented grating structures whose pitch is tuned to match the absorption spectra of the active layer. This optimized solar cell structure leads to an enhanced absorption in the active layer and thus improved short-circuit currents and power conversion efficiencies in the fabricated devices. Fabrication of the solar cells on thin polyimide substrates which are compatible with the lithographically processed grating structures are done in order to obtain the efficiency enhancement in thin, flexible devices.
In-situ grown organic nanofibers have been prepared on metal electrodes patterned by electron beam lithography. A
systematic investigation shows that the light emission from these nanofibers driven by an AC gate voltage depends nonlinearly
on the amplitude of the AC gate voltage and linearly on the frequency of the gate voltage, which indicates that a
model involving thermally assisted charge-carrier tunneling can be applied. The photoluminescence spectra of parahexaphenylene
(p6P) and α-sexithiophene (6T) nanofibers illustrate that the emission color of the in-situ grown
nanofibers can be tuned by depositing two types of discontinuous organic layers on the same platform.
Electroluminescence from two nanofiber thin films suggests that the relative light emission contribution from the two
organic molecules can be varied by changing, e.g., the nominal thickness of the two materials.
We report an optimized inverted bulk-heterojunction (P3HT:PCBM) organic solar cell geometry in
order to both efficiently trap incident light within in the cell (increasing light absorption) and at the
same time provide efficient transport of the generated carriers to the electrodes (reducing the active
layer thicknesses). To address these issues, we have used two approaches. The first one consists of
including diffraction gratings that increase the light path length in the cell and thus enhance absorption
in wavelength intervals matching the absorption peak of the organic active layer on the bottomelectrode,
while the second approach includes Ag nanoparticles embedded on the solar cell topelectrode,
which scatter the incident light into the solar cell active layer.
The solar cells containing either gratings or nanoparticles exhibit a significant enhancement on the
power conversion efficiency. Furthermore, the solar cells do not contain the rare metal indium, but
employ a PEDOT:PSS based transparent electrode.
Para-hexaphenylene (p6P) molecules have the ability to self-assemble into organic nanofibers, which exhibit a
range of interesting optical and optoelectronic properties such as intense, polarized luminescence, waveguiding and
lasing. The nanofibers are typically grown on specific single-crystalline templates, such as muscovite mica, on which
mutually parallel nanofibers are self-assembled upon vapor deposition of the organic material under high vacuum
conditions. Besides such single-crystalline templates, the nanofibers can also be grown on non-crystalline gold
surfaces, on which the orientation of the nanofibers can be manipulated by structuring the gold surface prior to parahexaphenylene
(p6P) deposition. In this work it is demonstrated, how such organic nanofiber growth can be controlled
by modifying the design of the underlying gold structures prior to growth. Here, the investigated designs include
pinning lines and gratings. We demonstrate how gold gratings fabricated on an insulating substrate can enable
electrical contact to in-situ grown p6P nanofibers. Furthermore, the electrical characteristics of in-situ grown fibers are compared to that of transferred p6P nanofibers. The transferred nanofibers are initially grown on muscovite mica, and
subsequently transferred onto a target substrate by drop casting, and electrodes are applied on top by a special shadow
mask technique.
Nanofibers made from para-hexaphenylene (p6P) molecules hold unique optoelectronic properties, which make them
interesting candidates as elements in electronic and optoelectronic devices. Typically these nanofibers are grown on
specific single-crystalline substrates, on which long, mutually parallel nanofibers are formed. However, the lack of
ability to further process these substrates restrains their use in devices. In this work, a novel method for in-situ growth of
p6P nanofibers on nano- and micro-structured gold surfaces is presented. The substrates are prepared by conventional
microfabrication techniques such as lithography, etching and metal deposition, which increase their potential as device
platforms. The results presented here demonstrate, that both the growth direction and the nanofiber length can be
controlled by placement of nano- and micro-structured lines on the substrate. It is shown that the preferred growth
direction of the nanofibers is perpendicular to these structures whereas their length scales are limited by the size and
placement of the structures. This work therefore demonstrates a new technique, which can be useful within future
organic nanofiber based applications.
Forming structures similar to or smaller than the optical wavelength offers a wide range of possibilities to modify the
optical properties of materials. Tunable optical nanostructures can be applied as materials for surface-enhanced
spectroscopy, optical filters, plasmonic devices, and sensors. In this work we present experimental results on technology
and properties of periodical, polymer based optical structures modified by ordered adsorption of silver nanoparticles.
These structures were formed combining UV hardening and dip coating from colloidal solutions. We have investigated
the influence of silver nanoparticles assembly on the ambient conditions (deposition temperature and time) and surface
features (periodicities and shape) of the template micro structures. Optical absorbance as well as morphology of the
structures containing silver nanoparticles were investigated by UV-VIS spectroscopy, AFM, SEM and optical
microscopy. The influence of silver nanoparticles on the optical properties of the structures was investigated by polarized
light spectroscopy (Grating Light Reflection Spectroscopy - GLRS). From the results of this study we propose a low cost
procedure for fabricating structures that could be potentially new type of plasmonic sensors exploiting surface enhanced
plasmon resonance in silver nano structures.
Aligned ensembles of nanoscopic nanofibers from organic molecules such as para-phenylenes for photonic applications
can be fabricated by self-assembled molecular growth on a suited dielectric substrate. Epitaxy together
with alignment due to electric surface fields determines the growth directions. In this paper we demonstrate
how aligned growth along arbitrary directions can be realized by depositing the molecules on a micro-structured
and gold covered Silicon surface, consisting of channels and ridges. For the correct combination of ridge width
and deposition temperature fibers grow perpendicular to the ridge edge, emitting light polarized along the ridges
after UV excitation.
A new way of developing optical nanosensors is presented. Organic
nanofibers serve as key elements in these new types of devices,
which exploit both the smallness and brightness of the
nanoaggregates to make new compact and sensitive optical
nanosensors. On the basis of bottom up technology, we functionalize
individual molecules, which are then intrinsically sensitive to
specific agents. These molecules are used as building blocks for
controlled growth of larger nanoscaled aggregates. The aggregates in
turn can be used as sensing elements on the meso-scale in the size
range from hundred nanometers to a few hundred microns. The organic
nanofibers thereby might become a versatile tool within nanosensor
technology, allowing sensing on the basis of individual molecules
over small aggregates to large assemblies. First experiments of
Bovine Serum Albumin (BSA) coupling to para-hexaphenyl (p-6P)
nanofibers are presented, which could lead towards a new type of protein
sensors. Besides large versatility and sensitivity, the nanofibers
benefit from the fact that they can be integrated in devices,
either in liquids by the use of microfluidic cavities or all in
parallel.
Organic nanofibers from semiconducting conjugated molecules are well suited to meet refined demands for advanced
applications in future optoelectronics and nanophotonics. In contrast to their inorganic counterparts,
the properties of organic nanowires can be tailored at the molecular level by chemical synthesis. Recently we
have demonstrated the complete route from designing hyperpolarizabilities of individual molecules by chemically
functionalizing para-quaterphenylene building blocks to the growth and optical characterization of nonlinear,
optically active nanoaggregates. For that we have investigated nanofibers as grown via organic epitaxy. In the
present work we show how chemically changing the functionalizing end groups leads to a huge increase of second
order susceptibility, making the nanofibers technologically very interesting as efficient frequency doublers. For
that the nanofibers have to be transferred either as individual entities or as ordered arrays onto specific target
substrates. Here, we study the applicability of contact printing as a possible route to non-destructive nanofiber
transfer.
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