Solution Preparation

The second main preparation step is the preparation of the solutions to be used for spin-coating. Two dierent kinds of solutions are the basis for the organic layers of an OSC: the PEDOT:PSS solution¹ for the polymeric an-ode and a solution, consisting of a mixture of polymer and PCBM, for the photovoltaic active blend layer.

1Technically, PEDOT:PSS is a colloidal suspension, but the term solution will be used

as in coating solution.

50 CHAPTER 3. OSC PRODUCTION

Table 3.2: The parameters recorded during substrate cleaning for each of the i cleaning steps. The cleaning procedure usually starts with manually wiping the ITO-substrates using glass detergent, followed by a series of ultrasonic baths in solvents. The last bath is in DI-water, before the substrates are quickly blow dried and exposed to an UV-ozone treatment.

Parameter Symbol Standard

Method manual wiping, then wet cleaning

Detergent (wiping) standard glass detergent

Solvents (wet cleaning) 2x acetone, 2x isopropanol,

1x DI water

Duration ofi^th bath t_bath(i) 600s

Temperature ofi^th bath T_bath(i) Room Temperature (RT)

Use of sonication yes

Duration of UV-Ozone treatment t_{U V} 600s

PEDOT:PSS

PEDOT:PSS is bought from H.C. Starck and is available in several dierent formulations. PEDOT:PSS is used as organic anode on top of the ITO fa-cilitating the hole extraction out of the device [26]. The formulations dier in their suspending solvent, in the size of the dispersed droplets and their degree of PEDOT's doping with PSS.

For the standard OSCs, PEDOT:PSS formulation AI 4083 is used as received and only ltered through a 0.45µm lter prior to spin-coating.

Absorber Solution

The ingredients, i.e. solution material and solvent, for the absorber solu-tions are bought separately and stored in dark in the glovebox, where the preparation takes place as well. The absorber solution is made by mixing an electron donating polymer and the electron acceptor PCBM in the desired amounts into an organic solvent. The choice of solvent and the concentra-tions of donor and acceptor have a large inuence on the absorber properties and hence on the OSC properties [30, 58]. For this work, the standard poly-mer is P3HT 4002E (Rieke Metals), the standard electron acceptor material is PCBM (Solenne BV), dissolved and mixed together in o-dichlorobenzene.

The three most common ways of preparing absorber solutions are shown in gure 3.3. To keep track of the way a solution is prepared and used, each vial containing a solution receives an unique ID and the solution preparation

3.2. OSC PRODUCTION PROCESS 51 process is recorded in a similar way to the process steps of the substrate.

The standard procedure (1) begins with adding the amounts of PCBM, poly-mer and solvent, which are desired for the nal solution, into a vial. Subse-quently the absorber solution is ltered and continuously stirred on a mag-netic stirrer. If necessary the solution is heated while stirring to facilitate dissolving of its ingredients. An absorber solution is characterised by the pa-rameters given in table3.3and by its absorption spectrum, which is recorded for every solution.

2 3

1 Solution Material

Mixing

Solution

Solvent

Extra Solvent

Other Solution

Spin−coating

Figure 3.3: Process overview of the absorber solution preparation. For the standard process (1), the solution materials and solvents are combined into one solution, which is continuously mixed until being used in the spin-coating process. Two alternatives are preparing one highly concentrated master solution at a xed polymer:PCBM ratio (1) and diluting it with extra solvent (2) or making two separate master solutions (1) with one component only and mixing them in the desired ratios and extra solvent (2,3). Solvents and solutions can additionally be ltered at any stage.

Whereas the absorber solution in the standard process is already prepared in the concentrations and ratios desired for spin-coating, the other two ap-proaches oer more exibility: a highly concentrated master solution with a xed ratio of polymer:PCBM is made (1) and can be later diluted to the desired concentration by adding extra solvent (2), making the preparation of series of concentrations simple. The third approach is to mix separate highly concentrated solutions of polymer and PCBM (1). These master solutions, together with extra solvent (2), then can be easily combined (3) to give new

52 CHAPTER 3. OSC PRODUCTION

Table 3.3: The parameters characterising an absorber solution. An absorber solution usually consists of two components: an electron donor and an elec-tron acceptor. The standard materials are P3HT (4002E, Rieke Metals) and PCBM (Solenne BV) respectively.

Parameter Symbol Standard

Mixing method Magnetic Stirrer

Mixing temperature Tmix ambient

Electron donor P3HT (4002E, Rieke Metals)

Electron acceptor PCBM (Solenne BV)

Concentration of donor cD 10-30mg per ml

Concentration of acceptor cA 10-30mg per ml

Solvent o-dichlorobenzene (DCB)

Solution lter 0.45µm PTFE Membrane

solutions at the desired concentration and ratios. The latter method is the most exible, yet requires the most preparation steps. These three ways are not mutually exclusive, but simply oer dierent degrees of exibility.

Filtering the solution after preparation is a necessary step to reduce the impu-rities contained in the solution, which would otherwise create inhomogeneities in the absorber lm during spin-coating (see section 3.2.3). However, when a solution is ltered, a certain amount of the polymer and the PCBM are re-tained in the lter. Thus the concentration of the solution after ltering will have changed slightly, making the comparison between absorber solutions, which were ltered separately, dicult.