Data Cleaning

Checking the quality of the data and the associated correction and pruning is usually called data cleaning. The data in this work consists of two main com-ponents: the production parameters and the measurement results. Whereas for the production parameters human errors, i.e. wrong entries in the ELN, need to be corrected, the measured data has additionally to be checked for artefacts from the measurements. Data cleaning is an essential cornerstone for a reliable data analysis and this section is intended to describe how the data cleaning is carried out in order to obtain a consistent and reliable base of data for the analysis.

Manufacturing Data

The webinterface, through which the details of production for each substrate are entered, already provides important measures for assuring data consis-tency, completeness and quality. The use of mandatory elds and syntax checking ensures that e.g. if a temperature is required, it has to entered nu-merically. Similarly, pull-down menus limit the options to sensible choices for certain parameters, e.g. oering only valid spin-coating programs. However, these mechanisms cannot safeguard against all mistakes and these errors need to be found and corrected before the data is used for the analysis. If a param-eter in question is found, e.g. unreasonable long or negative time intervals between production steps, one must resolve the issue with the person who entered the data. If in doubt, the parameter is either set to a value indicating unknown or the whole corresponding data set for the substrate is exempt from the analysis requiring the parameter.

Measured Data

To ensure the quality of the OSCs, all are already visually inspected for dam-ages and defects in the photovoltaic active region during production. Often a particular behaviour in the measured data is not caused by variations in the production process, but by such defects. This measurement data, together with measurement data, which is obviously awed due other reasons, has to

5.3. DATA PREPARATION 105 be treated separately in order to obtain a useful set of data for the analysis.

To describe and quantify the cases, in which an OSC is not considered for the analysis, they have been categorised as follows:

• OSCs with only partial coverage of absorber layer

• Defects due to bubbles during spin-coating of the absorber layer

• Scratches in the absorber layer and Al-cathode caused by handling

• Particles in the absorber layer from dust or agglomerates

• Contact Problems

• Unspecied

Figure 5.7 shows the eect of these defects on the illuminated IV-curves and absorption measurement as well as corresponding pictures. The degree to which they aect the measurements varies and generally depends on the extent of the defect. The primary causes of the above mentioned defects are briey discussed here.

The PEDOT:PSS layer is very homogeneous and covers the whole substrate area. Hence, if the area of an OSC is only partially covered during the sub-sequent spin-coating with the absorber layer, the aluminium cathode has direct contact to the PEDOT:PSS layer. The extent of the S-shaped IV-curve depends on the overlap area of the PEDOT:PSS|aluminium contact.

The PEDOT:PSS|aluminium is creating a Schottky-like contact, while the area covered with absorber layer (if any) still generates photocurrent (see gure5.7A). An only partially covered OSC also aects the absorption mea-surement. In the region without absorber layer most of the light is reected, leading to a dierent spectrum due to a missing layer (see gure 5.7 B).

Whereas bubbles have never been observed in the PEDOT:PSS layer, bubbles in the absorber layer can occasionally be created during the spin-coating of the absorber solution. While spinning the substrate, the bubble distorts the otherwise smooth lm and can lead to a very inhomogeneous absorber layer. This inhomogeneity in thickness often aects more than one OSC per substrate, because of the comparably large size of bubbles. Bubbles cause S-shaped IV-curves as shown in gure 5.7 A and appear in the absorption

106 CHAPTER 5. DATA ANALYSIS METHODS AND ENVIRONMENT spectrum, where they equal in their signature those of partially covered OSCs (gure5.7 B).

Scratches in the organic layers can arise during handling the substrates while moving them from one process step to the next. Due to the small size, a scratch usually does not appear in the absorption spectrum, but can dete-riorate the IV-curve substantially with multifaceted behaviour. A scratch through absorber and PEDOT:PSS layer prior to the aluminium evapora-tion will contribute a short circuit between ITO and Al electrodes as seen in gure5.7 A. A scratch after Al-deposition can exhibit a wide range of char-acteristics. It can e.g. cause an ohmic behaviour if ITO and aluminium es-tablish a good contact or an instable behaviour of the IV-curve, if an existing small scale short circuit is burned out during the voltage sweep. Furthermore, scratches after cathode evaporation can destroy an aluminium interconnect and eectively render the OSCs on the same bus (see gure4.8) useless.

Despite spin-coating the PEDOT:PSS layer under cleanroom conditions and ltering the solutions used for the absorber layer during preparation, particles on the substrate surface or from the solutions occasionally are seen in the layers. The eect of these particles on the measurement results depends on their size and quantity. Small particles in low quantities (1-2 per OSC) do not seem to aect the IV properties. More or larger particles however can cause anything from a Schottky-like contact to ohmic behaviour by introducing changes in the absorber layer (see gure5.7 C). Due to their small size with respect to sampled area for absorption, these particles are generally not seen in absorption spectrum unless they cause large distortions with variations in layer thickness (see gure5.7 D 4).

The OSCs are contacted in a four point measurement conguration. However, if the contact probes do not have a good contact to the aluminium or ITO interconnects, the IV measurement data is getting distorted. This manifests itself in spikes in the IV-curve, as seen in gure5.7 C.

Finally, if an IV measurement is obviously awed, e.g. showing a Schottky-like behaviour, but no damage to the OSC can be found (even when looking at the photovoltaic active area with a microscope), it is categorised as un-specied for excluding it from the analysis.

Once the measurement data set is cleaned, it can be analysed and parameters can be extracted.

5.3. DATA PREPARATION 107 S 413 p6 (not covered) S 423 p8 (scratch) S 452 p1 (with bubble)

A

illuminated IV-curves

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300 400 500 600 700 800

Wavelength / nm

S 413 p6 (only partially covered) S 413 all without p9

B

C

illuminated IV-curves

Figure 5.7: The categorisation of measurement data upon pruning prior to the data analysis. (A) shows the eects on IV-curves when the OSC is not com-pletely covered with the absorber layer, a bubble during spin-coating causes strong inhomogeneities in the absorber layer or a scratch creates a short cir-cuit. Signatures in the absorption spectrum of a bubble and a partially covered OSC are similar and the example of a non-covered OSC is shown in (B). (C) shows an S-shaped IV-curve causes by several particles in the photovoltaic active region and contacting problems. In (D) pictures of the four cases il-lustrated above are shown: no coverage with absorber (1), scratches in the cathode (2), inhomogeneities caused by a bubble during spin-coating (3) and a large particle (4).