Nicky Evans1,Philipp Weitkamp2,Olivia Gough1,Benjamin Putland1,Raghunath Dasari3,Seth Marder4,Klaus Meerholz2,Selina Olthof2,Moritz Riede1,Henry Snaith1
University of Oxford1,University of Cologne2,Georgia Institute of Technology3,University of Colorado Boulder4
Nicky Evans1,Philipp Weitkamp2,Olivia Gough1,Benjamin Putland1,Raghunath Dasari3,Seth Marder4,Klaus Meerholz2,Selina Olthof2,Moritz Riede1,Henry Snaith1
University of Oxford1,University of Cologne2,Georgia Institute of Technology3,University of Colorado Boulder4
Investigations into the charge transport layers used within organic photovoltaics (OPV) can help us in finding ways to improve device efficiency and stability. In particular, dedicated charge transport layers have the purpose of aiding in selective charge transport to the respective electrodes. Care has to be taken to ensure advantageous energy level alignment, resulting in minimised losses in solar cell device performance. Though the more commonly used electron transport layer materials in OPV, such as metal oxides, exhibit satisfactory optical and electronic properties, there is limited control over their energy-level matching within a device. Additionally, the stability of many of these materials, particularly under ultraviolet irradiation, is a limiting factor.<br/>This study investigates the use of an electronically and energetically tuneable, fullerene-derived electron transport layer for OPV, namely PCBCB. These films exhibit a reduction in solubility once they are treated by specific annealing conditions, which intriguingly allows for non-orthogonal-solvent based films to be processed on top of these layers. We investigated these films with respect to their conductivity as well as energetic properties and were able to tune these properties via the use of an electron-donating additive. It was observed that with an increased additive concentration, the conductivity increased while at the same time the physical film quality improved. Furthermore, the use of the additive led to a shift in the position of the Fermi level toward the lowest unoccupied molecular orbital, ultimately improving the energy level matching of the layer within the device stack.<br/>Device stacks comprised of a PM6:Y6 donor-acceptor blend active layer were fabricated atop a series of such electron transport layers in an n-i-p architecture. Intriguingly, this resulted in variations in active layer morphology. The changes were explored using GIWAXS and significant differences in the crystalline stacking were found for the active layer deposited on top of doped fullerene-derivative ETL, in contrast to a metal-oxide transport layer. This is thought to be a product of different levels of phase segregration throughout the active layer, relating to differences in contact angle between the active layer and ETL when deposited. These morphological changes appear to result in significant differences in device performance characteristics, particularly in J<sub>SC </sub>and consequently fill factor and power conversion efficiency. Thus, understanding the impact of prior layers on active layer formation could be integral to further understanding and optimising device performance characteristics.<br/>Furthermore, a series of stability studies are currently being conducted to test the lifetime of OPV devices comprised of a fullerene-derivative ETL, in comparison to those using a typical zinc oxide (ZnO) electron transport layer. From intial results, we see that a highly ‘doped’ organic ETL and a bilayer of doped and nominally intrinsic transport layers, both lead to device performance that is close to competing with ZnO based devices, and an increased lifetime as compared to devices comprising of the metal oxide.