Organic Photovoltaics for Teaching and Learning

Get the thesis (in German): urn:nbn:de:hbz:468-20170824-112739-3

Abstract

The goal of this thesis, based on the current state of scientific research, was to develop and optimize an organic solar cell using the semiconductor components poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butanoic acid methyl ester (PCBM), which would allow experimental as well as conceptual access to the subject area 'organic photovoltaics' for teaching. The basis for this work was a form of a similar solar cell already developed in the context of an examination paper. The optimizations had to be carried out in such a way that the self-made solar cell could be produced quickly and with simple materials and methods on the one hand, and on the other hand that it provided reproducible performance and could thus supply a small load. In addition, the chemicals used had to be harmless for use in school experiments and thus comply with current regulations on hazardous substances. The results of our own experimental investigations can be summarised as follows.

1. The manufacturing process of the self-made solar cell could be simplified from the "adhesive tape variant" to an "Easy-OPV". This variation of the cell structure makes it possible to forgo the injection of the liquid alloy galinstan, which is used as an electron contact. This reduces the risk of damaging the photoactive layer with the injection needle, causing short circuits. In the Easy-OPV, the galinstan electrode is manufactured separately and the FTO glass, coated with the photoactive components, is placed on this electrode. This variant also offers the advantage that the dead weight of the galinstan drops no longer rests on the thin organic layer, which is also a possible source for short circuits. The cell's output could thus be stabilized and even increased. In particular, the short-circuit current strength has been increased, indicating improved contact between the photoactive layer and galinstan.
2. The solvents originally used for the photoactive components, chlorobenzene and chloroform in a ratio of 1:1, were successfully replaced by less hazardous or less toxic solvents. At first chloroform was not used and subsequently chlorobenzene was replaced by o-dichlorobenzene. By optimizing the aftertreatment conditions and the concentration of the P3HT:PCBM mixture in o-DCB, the short-circuit current strengths and efficiencies of the Easy OPV cells could be increased even more. The increase in performance is mainly due to increased crystallization of the P3HT molecules and the associated improved nanomorphology of the photoactive layer.
3. The cell performance could be increased further by using a combination of dichloromethane and o-dichlorobenzene as a solvent mixture for the P3HT:PCBL solution. Both the short-circuit current strengths and the efficiencies of the Easy-OPV could be increased by adding 5 - 10 % DCM to the main solvent o-DCB. This increase in performance is due to a more favourable vertical distribution of the components in the photoactive layer and the associated improvement in charge transport to the respective electrodes.
4. The goal of producing a flexible self-build OPV could not be achieved in this work. The simple replacement of the FTO glass substrate by an ITO-PET film substrate was not possible because the hole extraction layer of PEDOT:PSS could not be deposited on the film by conventional methods (coating with a sharp edge and spin coating). Direct electrochemical polymerization of PEDOT on the ITO film was investigated in two different variants, but did not lead to success in either of them. The first, in which PEDOT:PSS was electropolymerized from aqueous solution, did not allow the production of doped, conductive PEDOT layers, but only neutral, non-conductive ones. In the second variant, PEDOT was electrochemically deposited on the ITO film from an organic solvent (acetonitrile) with TBAP as the conductive salt. It was found that this method, which produces conductive layers on FTO glass, cannot be applied to ITO film because the ITO layer is removed or destroyed.

The scientific results obtained are the basis for the implementation and integration of the topic 'Organic Photovoltaics' into classroom teaching. The didactic use of the results can be summarized as follows:

1. The topic of organic photovoltaics offers the possibility of a multifaceted connection to school-relevant contents in chemistry lessons. Both in lower and upper secondary level, organic solar cells can be assigned to obligatory content fields under various basic concepts of a german chemistry curriculum. They enable the networking of chemical knowledge on the basis of a current and innovative subject.
2. The solar cell manufacturing process has been optimized to such an extent that it can be carried out quickly, easily and safely. The chemicals used allow the experiment to be used as a student experiment from grade 5 onwards.
3. The experiments and working materials were developed differentiated for the lower and upper secondary level. In order to describe the functional principle of organic solar cells, models were taken up which are already known in the respective grades and extended in such a way that they do not overwhelm the learner's technical knowledge but are still scientifically consistent.
4. The Flash animation on 'Organic Photovoltaics', conceived and programmed within the framework of this work, enables the dynamic representation of the processes in an organic solar cell on a submicroscopic level. Thus it represents an important medium for the support of the teacher and the learner. The functional principle of energy conversion in organic BHJ solar cells is shown in the animation in two different models - at particle level in a schematic cross-section of the solar cell and in an energy diagram. Thus, the flash animation can also be used in both secondary levels. In addition, the learning tool contains extensive explanations on the properties and functions of the materials used.
5. With the 'organic photo electronics' teaching and learning kit, an experimental set has been designed and developed that provides material equipment as well as extensive didactic materials for the construction and investigation of organic solar cells (and OLEDs) and for the conceptual development of their functionality. The didactic materials (experimental instructions, assignments and worksheets, flash learning tool) have been adapted to the level of the lower and upper secondary level and offer the possibility to propose and carry out further experiments based on the basic versions of individual experiments in a research-developing procedure for the verification of own hypotheses.
6. The experiments on organic photovoltaics and the flash animation were integrated as an experimental block into the unit "Innovative Kunststoffe" of the Wuppertal Chemistry Labothek. Thus the topic has already been integrated into the university didactic teaching and has been positively evaluated by pupils as well as teachers.

With the experiments and didactic materials developed in this work, which are freely available online at any time as a service for teachers (flash animation, experiment instructions and tasks of the Wuppertal Chemistry Labothek), as well as the presentation of the application possibilities in an experiment oriented chemistry lesson, a contribution to curricular innovation aims to be made.