Morphology visualization of P3HT:Fullerene blends by using subsurface atomic layer deposition

The heart of every organic solar cell (OSC) is the organic donor:acceptor bulk heterojunction (BHJ). The complex morphology of the BHJ, composed of both ordered and disordered domains, dictates device performance and efficiency. In addition to challenges in controlling the morphology, its characterization is also complicated, mainly because the disordered domains are silent to most analytical tools. Recently we showed that, diffusion and in-situ conversion of organometallic Atomic Layer Deposition (ALD) metal oxide precursors, can be used to spatially map the amorphous domains in BHJs. Here, we harness this approach to study the morphology evolution of a series of poly(3-hexylthiophene-2,5-diyl) (P3HT):fullerene blends with various fullerenes: [6,6]-phenyl-C61 butyric acid methyl ester (PC₆₀BM), [6,6]-phenyl-C71 butyric acid methyl ester (PC₇₀BM), Bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6,6]C62 (BIS-PCBM) and 1′,1″,4′,4″-tetrahydro-di[1,4]methanonaphthaleno[5,6]fullerene-C60 (ICBA). Optical absorption, electron microscopy and EDS quantification allow us to selectively probe the distribution, amount and composition of the amorphous intermixed phase in the different BHJs and to study the effect of fullerene type on morphology. Interestingly, we find that PC₆₀BM, PC₇₀BM and BIS-PCBM efficiently occupy the amorphous domains of P3HT significantly reducing the free volume available for precursor diffusion. In contrast, thermodynamic compatibility between P3HT and ICBA and strong film vitrification by ICBA stalls phase separation during BHJ processing. Moreover, this methodology allows us to visually image the vertical distribution of ICBA molecules inside the organic blend and to correlate reported device performance with morphological P3HT:ICBA evolution due to thermal annealing. This gained insight is unattainable in conventional characterization tools and provides directive tools towards the processing of new OSC systems.