Organic and Hybrid Photovoltaics Group

About us

The Friedel Research Group on Organic and Hybrid Photovoltaics is based at the Institute of Solid State Physics, Graz University of Technology, Graz, Austria.
Our research is focusing on three major topics:


Phase-separation, colloids, interfaces and their effects in organic/hybrid optoelectronics

Solution-processable semiconductors offer an attractive way towards easy low-cost and large area electronic devices. Materials qualifying here are soluble conjugated organic molecules and suspensions of inorganic semiconductor colloids. Are such materials used for donor and acceptor bulk- heterojunctions in photovoltaics, their distribution, size and morphology in donor and acceptor phases throughout the blend and the nature of their interface have considerable impact on photophysics and charge-transport. In our group, we emphasize on the synergies between nanoscale morphology, interface properties and their effects on optoelectronic devices, in particular solar cells.
Further, various colloids are synthesized and utilized in this group, such as wide-band gap semiconductors (SiC, TiO2, SnO2, MoO3), polymers (MF), silica, carbon and metals. Thereby, their application is not limited to the active layer of devices, but also for intergration into electrode and photonic structures.


Sol-gel synthesis and tuning of silicon carbide materials for energy applications

Silicon carbide (SiC) is a durable, high-temperature and corrosion resistant wide-band gap semiconductor. In electronic applications, often expensive single crystals of the material are used, where a minimum density of defects is vital for device operation. But there are other applications, like hybrid organic-inorganic photovoltaics or electrochemical applications, where less costly non-perfect or polycrystalline material might be satisfactory or even more suitable. These are produced in our group via sol-gel processing and carbothermal reduction. The material of the cubic 3C-SiC polytype can be derived in form of nano-/microcrystals, porous structures or fibers and due to the liquid precursor also infiltrated and or deposited as thin film e.g. via spin-coating. Dopants are introduced in-situ from soluble compounds to achieve n-type of p-type material.


Alternative natural and nature-inspired materials in optoelectronics

Environmentally friendly biodegradable cellulosic materials, as natural wood pulp fibers, artificial viscose or regenerated cellulose are integrated into organic/hybrid optoelectronic devices, such as photodiodes and sensors. Advantageous interaction of cellulose’s surface chemistry with various conductive colloids is utilized to develop sophisticated novel electrode materials for optoelectronic devices with little consumption of energy, rare or poisonous materials as possible. A nature-inspired material with special optical characteristics are synthetic opals. Made from dielectrics or metals and organized in a periodic array, they affect the propagation of light. These materials (also known as photonic crystals) are used for capture, filtering and guiding of light, and via core-shell species for introduction of encapsulated emitters into device ordered structures.

Microscopic Inhomogeneities in Solution-Processed Organic Solar Cells Caused by Colloidal Interlayers

Organic semiconductors play an important role in modern technology and society. Their comparatively easy processability at low-cost and flexible molecular design to meet required physical properties make them highly attractive. First realizations, e.g. in mobile phone displays, are already on the market. However, especially in photovoltaics, a couple of problems are to tackle before reliable, large-area application becomes likely. One problem is the considerable batch-to-batch and device-to-device variation of organic photovoltaic devices. It has been shown that devices prepared by the same experimentalist under identical conditions and materials show performance variations of up to 10% and so do published research results scatter for identical systems. From microscale investigations of organic solar cells it has been found that there are considerable spatial variations in device performance. As active layer systems are usually engineered to nanoscale perfection, it has been suspected that rather the charge selective interlayers, like PEDOT:PSS, could be responsible. However, there is still a gap in understanding of the microscale physical processes caused by PEDOT:PSS in these devices. The question is if this could be an effect of PEDOT:PSS's colloidal nature. A combination of various spatially resolved characterization techniques allows us to correlate local layer morphology to the according local device physics. An integration of the latter in consequence must be able to explain the performance of the entire device. This is done with focus on the conditions of the PEDOT:PSS interlayer and compared to other material interlayers and their morphology.
FWF-funded project
Collaborator

Two solar cell pixels - comparison of visual morphology and laser-beam induced current landscape.


Photophysics and Charge Transport in Hybrid Blends of P3AT and beta-SiC Nanocrystals

Our hybrid solar cells join two very different semiconductor material classes:
The donor, poly(3-alkylthiophenes) (P3ATs), one of the most studied class of polymeric semiconductors with reasonable charge mobilities and suitable absorption profile.
The acceptor, cubic silicon carbide (3C-SiC), an expensive high-temperature and corrosion-resistant inorganic wide-band gap semiconductor with high charge mobilities, which is rather found in high-power transistors or... as an abrasive.
The positions of e.g. P3HT's HOMO and LUMO energies and the SiC's valence and conduction band indicate that they are a suitable donor-acceptor couple. Still, silicon carbide as acceptor in hybrid cells has been widely neglected in the past, probably for its indirect band gap or missing absorption contributions in the visible, but mostly expensive production of suitable nanoporous or nanocrystalline material. We prepare SiC submicron-sized crystals via sol-gel processing at high purity and only at a tiny fraction of the costs, which the aggressive etching of commercial SiC wafers causes to obtain similar material. In first fast-laser spectroscopy results on P3HT:SiC blends, we confirmed the presence of potential charge transfer state emission, one indicator for a functional donor/acceptor system. The Potential of P3HT:3C-SiC Composite Structures for Hybrid Photovoltaics

Photophysics on P3HT:SiC-nanocrystal blends


Goal of our present research is the understanding of the polymer/SiC interface, which is greatly affected by surface states of the SiC particles (e.g. dangling bonds). Their effect on the heterojunction charge transfer and on the transport processes within the SiC phase across grain boundaries is our major concern. With the tuning of the silicon carbide surface during synthesis and choice of appropriate surface functionalization we have two suitable tools at hand leading towards functional SiC-hybrid devices. Thereby we are not limited to the common nanoparticle blend approach, but also infiltrated nanoporous SiC films are in focus.
FWF-funded project
Collaborator


Cellulose in Optoelectronics

Cellulose, a sustainable and bio-compatible material, available in various appearances: Fibers, e.g. from wood-pulp fibers a.k.a. paper, or artificially derived a.k.a. viscose, but also thin films from nanofibrillated cellulose or regenerated cellulose. Together with the Schennach research group, we combine cellulose materials with suitable conductors and integrate them into organic optoelectronic devices.

Photodiode made from wood pulp cellulose fibers.


Unmodified paper (i.e. without the use of fillers or surface coatings) has been used to generate functioning photo-responsive diodes. This has been realized by wrapping silver nanowires around the paper fiber structure, forming a translucent electrode network, which was used as the substrate for a common organic P3HT:PCBM photovoltaic device. Photodiodes based on wood-pulp fiber networks
Collaborator


Equipment and Techniques

In our research group we presently do/have:

- photovoltaic characterization: dark-IV, photo-IV monochrom. and solar (ABB-rated solar simulator), spectral response (EQE)

- photocurrent mapping (for wavelength 530nm, theoretical resolution 1.7µm)

- photocurrent transients (for wavelengths 405, 530, 625 and 850nm, resolution 50ns)

- electroluminescence

- chemical laboratory for sol-gel chemistry and polymer processing (including large volume centrifuge, micro-volume syringe pump)

- inert-gas device fabrication (Argon gloveboxes) with spin-coater and thermal evaporator

- UV-VIS spectrophotometry

- Dektak stylus profiler (thickness measurements)

- heat-treatment (furnaces for 250, 1000, 1900 degC)

(Parts of this equipment may be available for collaboration or against charge)

Publications

Google scholar


Patents

Photovoltaic Device

Method for Producing an Object at Least Partly with a Silicon Carbide Structure from a Blank of a Carbon-Containing Material

Winter Term

Topics in Semiconductors: Organic and Hybrid Photovoltaic Materials and Devices

course no. 513.131

Lecture notes & Co.

This lecture covers the principles and physics of organic and hybrid photovoltaics and related systems. It will further give an overview on characterization methods, optimization approaches, novel & alternative concepts, and also on suitable application areas and their realistic potential in “real- world-use”.

- Photovoltaic donor and acceptor materials and their synthesis (conjugated polymers & small molecules, inorganic nanocrystals & nanostructures)
- Excitonic organic/inorganic hybrid photovoltaic devices
- Steady-state & time-resolved characterization methods
- Photophysics & device physics of excitonic photovoltaic cells (exciton generation, dissociation, and charge extraction)
- Impact of interfaces & morphology on recombination and charge transport
- Advanced device structures (light-trapping, photonic crystals, plasmonics, tandem-cells)
- New types of transparent electrodes
- Bio-photovoltaics
- Dye-sensitized solar cells
- Environmental & operational stability
- Application areas & large-area production

OPV in Experimentelles Praktikum / Forschungslabor 1,2

course no. 513.010, 513.011, 513.014

Guidelines & Co.

In this practical lab course, a set of organic solar cells is prepared (1st day) and characterized (2nd day) regarding the device phyiscs (diode behaviour, photoresponse, efficiency) and correlated to the optical properties of the organic semiconductor. Further, the devices are checked for microscopic defects via imaging.

Vacuum and Thin Films in Fortgeschrittenenpraktikum Technische Physik

course no. PHY.M40

Guidelines & Co.

In this practical lab course, thin films are prepared via traditional vacuum deposition and modern solution-processing techniques. These films are characterized for technique-specific morphological properties and depending on the processing parameters. The vacuum system of the thermal evaporator is further investigated regarding its vacuum characteristics.


Summer Term

Übungen Physik 2 für USW (Elektrodynamik, Optik)

course no. UNT.075_1

Problem Sheets & Co.

This exercise accompanies the lecture of the same title (no. UNT.074). This course is meant for practicing the application of the lecture knowledge on real problems.

OPV in Experimentelles Praktikum / Forschungslabor 1,2

course no. 513.010, 513.011, 513.014

Guidelines & Co.

In this practical lab course, a set of organic solar cells is prepared (1st day) and characterized (2nd day) regarding the device phyiscs (diode behaviour, photoresponse, efficiency) and correlated to the optical properties of the organic semiconductor. Further, the devices are checked for microscopic defects via imaging.

Vacuum and Thin Films in Fortgeschrittenenpraktikum Technische Physik

course no. PHY.M50

Guidelines & Co.

In this practical lab course, thin films are prepared via traditional vacuum deposition and modern solution-processing techniques. These films are characterized for technique-specific morphological properties and depending on the processing parameters. The vacuum system of the thermal evaporator is further investigated regarding its vacuum characteristics.

Group Members

No UV-light for healthier solar cells! Heribert, Tina, myself & Olivia, in the yellow device lab.

Dr. Bettina Friedel

Head of the group

• Room:PH03 142
• Email:bfriedel@tugraz.at
• Tel.:+43 316 873 8978

I studied physics at the University of Paderborn. In 2007 I gained my PhD with Prof. S. Greulich-Weber on Sol-gel based Cubic Silicon Carbide. From 2007-2011 I was a research associate at the Cavendish Laboratory (University of Cambridge), Cambridge, United Kingdom with Prof. N. Greenham. In June 2011, I joined the Institute of Solid State Physics at Graz University of Technology in my current position as assistant professor. Current research interests are photophysics and charge transport in organic and hybrid solar cells in correlation to morphological structures, colloids, sol-gel silicon carbide and cellulose electronics.


Students

Tina (Huei-Ting) Chien
Olivia Kettner
Jacqueline Köhler
Florian Pilat
Madlen Pramstrahler


Alumni

Heribert Kopeinik
Markus Pölzl

News

03/12/2015 - Most recent addition to our group: Carlo

We are enormously grateful to the FWF for agreeing to this purchase and to the company TOPCAST Engineering for building this fine machine for us. So far, Carlo is behaving well and producing lots of high quality samples.


-->