Key factors in the development of low bandgap materials for organic solar cells

P. Verstappen, G. Pirotte, T. Vangerven, L. Lutsen, D. Vanderzande, J. Manca, and W. Maes
Hasselt University,
Belgium

Keywords: solar cells

Summary:

Organic photovoltaics (OPV) have emerged as an attractive solar cell technology, mainly aiming at portable and/or wearable consumer goods and building or automotive integration. During past years, OPV research has mainly focused on the development of new electron donor type materials, which upon combination with a fullerene acceptor have led to an impressive boost of the power conversion efficiency, currently up to 11.7% for solution-processed single junction devices. However, these top results have been achieved with relatively small size solar cells. Since the applied device preparation methodology is not compatible with the production of large area solar cells, a translation to printing techniques such as spray coating and roll-to-roll coating has to be made. For this purpose, material availability on a reasonable scale (i.e. multi-gram to kilogram), in high quality and with reproducible properties is of crucial importance, which is yet to be realized in the field. As an example, we have recently studied a range of commercial PTB7 polymers yielding device efficiencies ranging from 2.7 to 7%, illustrating the poor reproducibility of the synthesis protocols of these materials. Investigation of the samples by MALDI-TOF mass spectrometry revealed the presence of homo-coupled species in the poor-performing material batches. These species can be formed as impurities during the catalytic cycle of the Stille polycondensation reaction, typically used to prepare this type of conjugated polymers. In this contribution, the effect of these homo-coupled species on device performance will be highlighted. Furthermore, the implementation of continuous flow chemistry as a powerful tool to prepare high quality OPV polymers in a more reproducible and sustainable way will be discussed.