Sample Preparation

For all of the substrates; ITO coated glass for devices and AFM, and quartz glass for absorption, the same cleaning procedure was used. The substrates were cleaned using the following process: sonication in (20:80) decon:water mix, rinse in water,

Chapter 2 Materials and Experimental Methods

56 sonication in water, rinse in propan-2-ol, sonication in propan-2-ol, dry blowing with nitrogen and 20 minutes UV-ozone treatment. UV-ozone treatment removes any carbon residues left after cleaning.120 All sonication was carried out for 15 minutes in an ultrasonic bath.

2.2.2.1 Organic Molecular Beam Deposition (OMBD)

OMBD is a thermal evaporation process of highly pure material in a background vacuum of typically below 10-6 mbar. It is a widely used deposition technique for small molecules that have low solubility. OMBD allows for sub-nanometre accurate control of molecular thin film thickness and deposition rates, measurable down to 0.1 Ås-1. With the high vacuum environment, OMBD provides a clean environment for the growth of films as the deposition rates are higher than the adsorption rate of contaminates.

The deposition of organic semiconductors occurs via thermal evaporation in a Knudsen cell based evaporation source, as shown in Figure 2.7. A purified material is placed in a boron nitride crucible and heated with a resistive coil up to 500oC, monitored by a thermocouple. A heat shield is used to reduce any thermal gradient within the crucible and a molecular beam is formed out of the small aperture in the evaporation source.

57 Figure 2.7: Schematic diagram of a evaporation source and formation of molecular beam.

The substrates are placed in the path of the beam and molecules adsorb onto the surface to create a thin film. The deposition rate is controlled by the evaporation source temperature and PID feedback monitoring using quartz crystal microbalances (QCM) placed in the beam path. The film thickness is controlled by an evaporation source shutter which acts as an “on/off” switch for the molecular beam. The film thickness monitored by the QCM has to be calibrated to a real film thickness due to the different densities of the organic materials and the geometry of the evaporation source to the substrate. Calibration using ex-situ AFM step edges is discussed in section 2.2.3.1.

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2.2.2.2 OMBD Chamber

A Kurt J. Lesker Spectros vacuum deposition system was used for growth of all thin films and devices. A schematic of the growth chamber is shown in Figure 2.8. The high vacuum environment is created and maintained by a two stage pump system. A scroll pump is used to create a modest vacuum of 10-1 mbar and then a cryogenic pump (CryoTorr) is used to further reduce the pressure to the base pressure of < 10-7 mbar. The growth chamber contains eight individually controlled organic deposition sources and three metal sources. These are monitored by four QCMs for the organic sources and one for the metal sources. The organic sources allow for single source growth and co-deposition with a main substrate shutter to control the layer thickness and individual source shutters to stop cross contamination. The source temperature is controlled by a Eurotherm Mini8® controller interfaced with a computer and the QCMs are interfaced with Sigma Instruments SQS-242 software to monitor the deposition rate and film thickness. The organic layers were deposited at rates between 0.5 to 1 Ås-1.

The samples sit in a sample holder which can accommodate up to 36 substrates that can be shadow masked for deposition of different layers. The sample holders and masks sit in a cassette holder, which holds up to three masks allowing for a complete OPV device to be fabricated without breaking vacuum. Between layer growths the masks are manipulated using a computer controlled transfer arm. The cassette shelf is rotated during the growth to allow for homogeneous deposition. All of the substrates and organic materials are handled under an inert atmosphere from a N2 glovebox attached to the vacuum chamber due to their sensitivity to oxygen and moisture. The glovebox was maintained at < 10 ppm O2 and H2O.

59 Figure 2.8: Schematic of the Kurt J. Lesker OMBD vacuum chamber.

2.2.2.3 Metal Deposition

Deposition of the MoOx layer, the top Al cathode and the Ag recombination layer was performed from a high temperature metal source. These are designed differently to organic sources and consist of a heat shielded tantalum crucible heater which is resistively heated by passing high currents at a low voltage. A metal (Al or Ag) pellet is placed in a crucible inside the heater and the current is increased until evaporation is seen, with the deposition rate controlled by the current flow. The first few nanometres of the metal layer are deposited at a low rate of < 1 Ås-1 to avoid

Chapter 2 Materials and Experimental Methods

60 damage of the underlying organic layers by the hot metal. The rate was later increased to 2 Ås-1 for the remaining deposition of the Al pellet, to a thickness of ~200 nm.

2.2.2.4 Device Design

The ITO substrates used for layer and device growth are supplied by Thin Films Devices Inc. with a sheet resistance of < 15 Ωsq-1

. The substrates are 1.2 x 1.2 cm2 with a 0.8 cm pre patterned strip of ITO, as displayed in Figure 2.9a. The organic layers and the metal cathode areas are defined by using different shadow masks. Figure 2.9b shows the organic layers deposited onto the ITO substrate, leaving a small strip to make the ITO anode contact. Figure 2.9c presents the OPV device configuration used throughout this thesis. Three cathodes are deposited onto each sample through 0.2 cm wide shadow masks with the device area of the OPV determined by the overlap of the ITO and the metal electrode, giving a pixel area of 0.16 cm2. Silver conductive paint was used to create the ITO contact and improve the connections from the electrodes to the J-V testing holder.

Figure 2.9: Schematic diagrams of a) ITO substrate, b) organic layer on ITO, and c) complete device with metal electrodes displaying active area.

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