Authors: Iftikhar Ahmed Channa
Source: OPUS Repository (Kooperativer Bibliotheksverbund Berlin-Brandenburg) (article, 2019-01-01)
Field: Materials Science — Polymers and Plastics

Development of Solution Processed Thin Film Barriers for Encapsulating Thin Film Electronics Recently, organic solar cells (OSCs) with efficiencies of 17% have been demonstrated, which brings organic photovoltaics in the same league as inorganic thin film technologies.OSCs require encapsulation by transparent and high quality barrier materials to achieve decent lifetimes without compromising performance.The most common practice for encapsulation of the OSCs is the lamination between barrier sheets using adhesives.This lamination process adds extra processing steps and thus increases overall processing cost and limits the throughput.Many attempts have thus been made to create coated barriers with quality comparable to those processed from vacuum assisted techniques.Direct application of coated barriers on top of OSCs will not only minimize cost but also maximize throughput as direct coating processes can be performed with roll-to-roll methods.Therefore, the goal of this work is the development of materials and processes for the encapsulation of organic thin films electronics by direct coating.The thesis is subdivided into six chapters.The main objective of Chapter 1 is to express the motivation towards the research direction.In this chapter, also the challenges and hurdles faced by coated barriers are discussed.Chapter 2 describes the theoretical background of diffusion and permeability and introduces industrial units for measuring the barrier quality.Various factors are also defined briefly which influence the barrier quality.This chapter gives theoretical details of the barriers based on filler platelets and describes various theoretical models for predicting the barrier quality from the platelet properties such as size, shape, concentration and orientation.Finally basics about organic solar cells and the working principle along with degradation mechanism are described in this chapter.Chapter 3 describes the state of the art of coated barriers.Silica layers processed from the polymer class of polysilazanes are also described in detail, along with the methods of processing.Finally, miscellaneous materials like ORMOCERS and fluoropolymers are also discussed in this chapter.Chapter 4 is devoted to experimental details describing all of the raw materials and processing methods used in the work.In Chapter 5 results on experimental data discussed.This chapter is further divided into three parts.Part I provides results obtained from the investigations on filler based barriers using clay as filler.Clay based barriers were prepared as a well characterized reference system.In Part II, the novel concept of barriers based on PVB films filled with glass flakes was investigated.To this end, barriers were prepared from glass flakes of different aspect ratios and different loading v concentrations to systematically study the effect of aspect ratio and loading concentration on barrier quality and optical transmission.It was found that the glass flakes are distributed homogeneously in the PVB film, with an almost perfect orientation of the platelets' long axes parallel to the film surface.In this way, barrier films with optical transmission values of > 85% and moisture permeation values of ~0.14 g.m -2 .day - were obtained with glass flakes having an aspect ratio of 2000 at a loading concentration of 25 vol%.The barrier properties persisted even after 20,000 cycles of bending at a radius of 3 cm.The WVTR values measured for different aspect ratios and different loadings were shown to be in reasonable accordance with the predictions of the Bharadwaj model.The haze of the glass flake filled PVB films, which, according to optical simulations, is mainly due to surface roughness of the films, was reduced by coating a smoothing layer on top.The lifetime of organic solar cells (OSCs) increased from few hours to beyond 150 h under damp heat conditions without any loss in efficiency, when the devices were encapsulated with the glass flake based barrier films.Part III describes the results obtained on barrier films based on perhydropolysilazane (PHPS).Two methods were used to cure PHPS, namely curing by exposure to damp heat and curing by irradiation with deep UV.Curing with deep UV in addition with heat is found to be the quickest way to cure PHPS completely.FTIR has been used to find the end point of curing which can subsequently be used to predict the barrier properties of cured PHPS layers.Prepared barrier films show water vapor transmission rates (WVTR) of <10 -2 g m -2 day -1 (40 o C / 85%RH) and oxygen transmission rates (OTR) of <10 -2 cm 3 m -2 day -1 bar -1 at ambient conditions maintaining optical transmission of >90% in visible region.Flexibility of the resulting barrier films is improved by coating a barrier stack of several thin PHPS layers alternating with organic polymer interlayers.These stacks show an increase of WVTR values by less than 10% after 3000 bending cycles.Direct coating of the PHPS films on top of organic solar cells enhances the device lifetime in damp heat conditions from few hours to around 700 hours.Chapter 6 gives the conclusions and provides an outlook on the possible impact of the developments of this thesis on the rollto-roll production of printed opto-electronics. Organic solar cell ; Materials science ; Nanotechnology ; Coating ; Organic electronics ; Electronics ; Roll-to-roll processing ; PEDOT:PSS ; Encapsulation (networking) ; Thin film ; Photovoltaics ; Lamination ; Polymer ; Photovoltaic system ; Computer science ; Engineering ; Composite material ; Electrical engineering ; Transistor Synthesis and properties of polymers ; Semiconductor Lasers and Optical Devices