Quantitative insights into the phase behaviour and miscibility of organic photovoltaic active layers from the perspective of neutron spectroscopy

Mohamed Zbiri,Christian B Nielsen,Peter A Finn,Anne A Y Guilbert

doi:10.1039/d1tc01813b

Abstract

Neutron spectroscopy as a master microscopic probe of the composition-dependent phase behaviour and miscibility of organic solar cell active layers.

Highlights

We present a neutron spectroscopy based method to study quantitatively the partial miscibility and phase behaviour of an organic photovoltaic active layer made of conjugated polymer:small molecule blends, presently illustrated with the regio-random poly(3-hexylthiophene-2,5-diyl) and fullerene [6,6]phenyl C61 butyric acid methyl ester (RRa-P3HT:Phenyl C61 butyric acid methyl ester (PCBM)) system
Since the focus is on an energy range where intra-molecular vibrations are dominating, we suggest that the coupling between intra-molecular vibrations and the environment decreases in the hydrogenated PCBM (h-PCBM) rich phase and any possible effects are potentially averaged out
We presented a neutron spectroscopy based methodology to study phase behavior and morphology of the blend system P3HT:PCBM

Summary

Introduction

X-ray diffraction, the composition of the amorphous mixture of the blends[4,5] as well as changes in conformation with respect to the neat materials is more difficult to access.[6,7] crystallinity has been shown to improve charge transport[8] and potentially lead to extra driving force for charge separation by lowering the electronic energy levels,[2] a spinodal-type decomposition emerged as a new picture for phase separation at length scales directly relevant to the operation of the devices,[9,10,11] with the coarsening of this phase separation directly linked to burn in degradation mechanisms.[10]. MD simulations further revealed a conformational change of P3HT chain to accommodate PCBM with enhanced cofaciality between the polymer thiophene rings and the PCBM cage. This has further been supported by Zheng et al, whose MD simulations pointed towards the same cofaciality between P3HT and PCBM. With respect to previous studies,[17,19,20,21] we go a step further by using both inelastic neutron scattering (INS) and quasi-elastic neutron scattering (QENS) (Fig. 1a) to resolve simultaneously changes in microstructure, morphology and dynamics of both the polymer and fullerene upon blending as a function of temperature. The neutron spectroscopy based method presently described should be universal and relevant to study blends with new non-fullerene acceptors that are closely related in terms of chemical structures to the polymer, which otherwise lead to a poor contrast between the blend components when using conventional imaging and optical spectroscopy techniques.[24]

Results and discussion

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Conclusions