• Phd Student involved: Giannis Antoniou

• Start date: September2019
• Current Status: Ongoing

• Phd Student involved: Fedros Galatopoulos

• Start date: September 2017
• Current Status: on going

• Phd Student involved: Apostolos Ioakimides

• Start date: January 2017
• Current Status: on going

• Phd Student involved: Sergey Pozov

• Start date: September 2016
• Current Status: on going

• Phd Student involved: Efthymios Georgiou

• Start date: September 2013
• Current Status: Completed in 2018

• Phd Student involved: Felix Hermerschmidt

Solution-processed organic and printed electronics, such as organic photovoltaic (OPV) and organic light emitting diode (OLED) technologies, are considered crucial for next generation electronics devices, since they can be fabricated cheaply and upscaled via roll-to-roll processes. OPVs fabricated in this way promise low-cost energy production with short payback times, while OLEDs are already on the market in the form of display applications, however as yet not solution-based. The two technologies are related to one another by the fact that they possess opposing underlying physical processes – OPVs produce electricity from light, whereas OLEDs generate light from electricity.
After an introduction to the field and to the fundamental device principles in chapters 1 and 2, this thesis deals with three key elements in solution-processed organic electronics. Firstly, arguably the most important component of an optoelectronic device, the active layer, is examined in chapter 3. The optical, electrochemical, morphological and transport properties of a series of new small molecules are investigated, which when blended with fullerene acceptors act as electron donors in OPV systems. The best power conversion efficiency is achieved by adding a polydimethylsiloxane additive to the small molecule:fullerene blend, which improves its morphology and doubles the hole mobility.
Secondly, having examined newly-synthesised small molecule active layer materials, in chapter 4 inkjet-printing is introduced as a roll-to-roll compatible technique with which to deposit high-efficiency polymer-based active layers for OPVs. The processing conditions for devices comprising polymer:fullerene blends with the low bandgap conjugated polymers Si-PCPDTBT as well as PCDTBT are presented. By controlling the inkjet printing processing conditions, a significant improvement in device power conversion efficiency is achieved compared to previously reported work using these materials.
And thirdly, having introduced inkjet-printing as a tool for active layer deposition, its use as a tool to generate ITO-free metal nanoparticle electrodes is presented in chapter 5. While previous work has focused on these types of grid structures in OPVs, their novel use in ITO-free OLEDs with a solution-processed architecture based on a conjugated polymer mixture of F8BT and PFO as light-emitting layers is presented. Fine grid lines are achieved and yield solution-processed OLEDs with similar device efficiencies to those based on ITO.

• Start date: 2011
• Current Status: completed in 2015

• Phd Student involved: Achilleas Savva

Solution based organic photovoltaics (OPVs) are considered one of the most promising future photovoltaic technologies for low-cost solar energy production. Attractive features of OPVs are the possibilities for thin flexible devices which can be fabricated using high throughput roll-to-roll process, resulting in a shorter energetic pay-back time. Most of the scientific literature in the field of OPVs is focused on the fundamental science, the materials properties as well as device physics. These disciplines are essential to continue driving the OPV technology towards high performance. This thesis initially addresses the basic state-of-the-art technologies around the OPVs science, going through the basic principles of OPVs, device architecture concepts, manufacturing technologies, material choices, electrical characterization methods and OPV device degradation mechanisms. In chapter 2, the origin of the improvement of the power conversion efficiency (PCE) of inverted OPVs when an interfacial insulating layer of PTE is introduced between ITO and TiOx bottom electrode, is studied. Using a series of measurements the interaction at the interfaces within the ITO/TiOx and ITO/PTE/TiOx is analyzed and correlated to observations based on device physics analysis. Based on our findings, we suggest that the altered morphology in combination with the difference in hydrophilicity of the PTE modified TiOx affects device performance either by increasing the carrier selectivity of the TiOx layer or by causing enhanced vertical phase segregation of the P3HT:PCBM layer. Within chapter 3, through detailed device physics analysis PEDOT:PSS PH is proposed as most suitable hole selective contact for inverted OPVs. By understanding the synergetic effects of PEDOT:PSS wetting agents on the hole selectivity of inverted OPVs we demonstrate a novel combination of PEDOT:PSS additives mixture, as an effective route to further increase the device performance and reliability of inverted OPVs. Finally, within chapter 4 it is shown that solution processed doped metal oxides charge carrier selective contacts, could be proven beneficial concerning product development targets of inverted OPVs. Using carefully designed solution process methods novel n- and p- doped metal oxides are introduced as charge carrier selective contacts in inverted OPVs exhibiting exceptional optoelectronic properties and functionality within the inverted structured PVs, as analyzed by a series of measurements and extensive device physics analysis.

• Start date: 2011
• Current Status: completed in 2014

• Phd Student involved: Marios Neophytou

attention over the last few years. New fields of applications are feasible as OPVs are
lightweight, flexible and have the potential for truly low fabrication cost. At present,
so-called bulk heterojunction structures based on blends of a conjugated polymer as
electron donor and a soluble fullerene derivative as acceptor represent the OPV
material system with the highest power conversion efficiency reported until now.
This thesis addresses several critical parameters in the processing issues of
polymer:fullerene based organic solar cells. Primarily, a sedulous investigation
concerning deposition parameters of an electron blocking-hole transporting interfacial
layer (Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) [PEDOT:PSS]) was
performed. The conductivity and thickness effect of the layer on overall device
performance was analyzed. Then the optically active layer consisting of the reference
poly-3-hexylthiophene and phenyl-C61-butyric acid methyl ester (P3HT:PCBM)
blend material system was investigated. Parameters such as composition ratio, layer
thickness and annealing conditions were examined in order to achieve the reference
organic solar cells for the newly founded Molecular Electronics and Photonics
research unit with optimized and highly reproducible power conversion efficiency
(PCE).

The processing steps described in this thesis yielded P3HT:PCBM devices
with repeatable and consistent PCE performance higher than 3 % and now the process
is used as a standard method to provide the reference solar cells in the Molecular
Electronics and Photonics laboratory. During the processing steps of P3HT:PCBM, a
21 kink in the shape of the current density/voltage (J/V) characteristics was observed.
Devices of different ratios of P3HT to PCBM were fabricated and sent for further
studies at the Jaume University in Spain. The s-shaped characteristics were explained
by capacitance measurements related to the presence of defect bands exhibiting
Gaussian shape located at E approximate to 0.38 eV above the highest occupied
molecular orbital level of the P3HT.
After establishing the reference process for P3HT:PCBM based solar cells new
synthesized conjugated polymers donors were investigated in terms of their potential
for organic solar cell applications. A perfluoro poly[(9,9-dioctylfluorenyl-2,7-diyl)-
alt-5,5- (4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (APFO-3) derivative and
different fluorescent boron-dipyrromethene conjugated polymers (BODIPY) were
characterized. Absorption and photo luminescent spectra as well as current density
characteristics of the fabricated photovoltaic devices were compared to commercially
available materials. Despite the exhibited low mobility and PCE values (in the range
of 0.2-1.2 %), some of the newly synthesized polymers, like PBT and PBTT, are
promising electron donors for further synthetic trials due to their high absorption
coefficient and high photovoltage (over 1 V).
Furthermore a systematic study of inkjet printing processing parameters for
organic solar cells applications was followed. Optimum parameters for inkjet printing
active layer were firstly identified, proving that the viscosity of the inkjet formulation,
substrate temperature, drop spacing and the height of the droplet in relation to the
surface are critical factors to achieving high quality inkjet-printed polymer-fullerene
22 based active layers. The last part of the chapter focuses on replacing the expensive
and brittle indium tin oxide (ITO) with an inkjet printed silver grid architecture.
Investigation of an inkjet-printed silver nanoparticle grid combined with different
conductivity PEDOT:PSS was performed. An ultimate control of the design
requirements of current collecting grid based on the proposed inkjet-printed process to
accurately control the uniformity and dimensions of the silver nanoparticle based grid
was achieved. The performed measurements revealed higher transparency of the
printed Ag grid when compared to different thicknesses of ITO. As a result a record
power conversion efficiency of 1.96 % is achieved for ITO-free P3HT:PCBM based
organic solar cells using the combination of PEDOT:PSS/inkjet printed nanoparticles
based current collecting grids.

• Start date: 2008
• Current Status: completed in 2012

• Phd Student involved: Christos Constantinou

• Start date: September 2013
• Current Status: On going

• Phd Student involved: Dimitris Pasias

• Start date: January 2017
• Current Status: On going