Bulk heterojunction morphology of polymer:fullerene blends revealed by ultrafast spectroscopy.

Almis Serbenta,Oleg V Kozlov,Paul H M Van Loosdrecht,Maxim S Pshenichnikov,Giuseppe Portale

doi:10.1038/srep36236

Almis Serbenta, Oleg V Kozlov + Show 3 more

Open Access

https://doi.org/10.1038/srep36236

Copy DOI

Abstract

Morphology of organic photovoltaic bulk heterojunctions (BHJs) – a nanoscale texture of the donor and acceptor phases – is one of the key factors influencing efficiency of organic solar cells. Detailed knowledge of the morphology is hampered by the fact that it is notoriously difficult to investigate by microscopic methods. Here we all-optically track the exciton harvesting dynamics in the fullerene acceptor phase from which subdivision of the fullerene domain sizes into the mixed phase (2–15 nm) and large (>50 nm) domains is readily obtained via the Monte-Carlo simulations. These results were independently confirmed by a combination of X-ray scattering, electron and atomic-force microscopies, and time-resolved photoluminescence spectroscopy. In the large domains, the excitons are lost due to the high energy disorder while in the ordered materials the excitons are harvested with high efficiency even from the domains as large as 100 nm due to the absence of low-energy traps. Therefore, optimizing of blend nanomorphology together with increasing the material order are deemed as winning strategies in the exciton harvesting optimization.

Highlights

We show for three polymers, regiorandom (RRa) P3HT, regioregular (RRe) P3HT and MDMO-PPV, selected as benchmark materials for exemplary cases of bulk heterojunction (BHJ) morphologies, that their blends with PC71BM contain the mixed phase with PC71BM domains typically up to 15 nm in size
The exciton harvesting dynamics from the PC71BM phase have been successfully obtained by a photo-induced absorption (PIA) technique and modeled by the Monte-Carlo simulations to yield valuable information on the BHJ morphology
The BHJ blends studied contain mixed-phase PC71BM domains of the size of several nanometers as well as the large PC71BM domains with sizes exceeding 50 nm. These findings are fully consistent with the paradigm of a hierarchical BHJ morphology[56,63,64] and were independently confirmed by GISAXS/GIWAXS, AFM, TEM/SEM and time-resolved PL measurements

Summary

Introduction

AFM provides information only about the surface topography, which is not necessarily representative for the bulk morphology[17] Aiming to overcome these limitations, complementary methods to control and optimize the morphology have been developed based on spectroscopic approaches, e.g. monitoring photoluminescence (PL) of interfacial charge-transfer states[18], or measuring exciton diffusion by PL quenching[19] or pump-probe spectroscopy[20,21,22]. The information about blend morphology can be extracted by excitation of the fullerene acceptor (e.g. soluble C70 derivative PC71BM), and detecting the time that is taken for the exciton to diffuse to the fullerene-polymer interface where it dissociates into separated charges[35,36] (Fig. 1). Unique spectroscopic signatures of subtle changes in BHJ morphology observed hold great promise for applications of the proposed technique for on-the-fly characterization of fully functional devices

Methods

Results

Conclusion