Quantifying Local Thickness and Composition in Thin Films of Organic Photovoltaic Blends by Raman Scattering

Xabier Rodríguez-Martínez, Michelle S Vezie, Xingyuan Shi, Iain McCulloch, Jenny Nelson, Alejandro R Goni, Mariano Campoy-Quiles

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19 Scopus citations

Abstract

We report a methodology based on Raman spectroscopy that enables the non-invasive and fast quantitative determination of local thickness and composition in thin films (from few monolayers to hundreds of nm) of one or more components. We apply our methodology to blends of organic conjugated materials relevant in the field of organic photovoltaics. As a first step, we exploit the transfer-matrix formalism to describe the Raman process in thin films including reabsorption and interference effects of the incoming and scattered electric fields. This allows determining the effective solid-state Raman cross-section of each material by studying the dependence of the Raman intensity on film thickness. These effective cross sections are then used to estimate the local thickness and composition in a series of polymer:fullerene blends. We find that the model is accurate within ±10 nm in thickness and ±5 vol% in composition provided that (i) the film thickness is kept below the thickness corresponding to the first maximum of the calculated Raman intensity oscillation; (ii) the materials making up the blend show close enough effective Raman cross-sections; and (iii) the degree of order attained by the conjugated polymer in the blend is similar to that achieved when cast alone. Our methodology opens the possibility to make quantitative maps of composition and thickness over large areas (from microns to centimetres squared) with diffraction-limited resolution and in any multi-component system based thin film technology.
Original languageEnglish (US)
Pages (from-to)7270-7282
Number of pages13
JournalJ. Mater. Chem. C
Volume5
Issue number29
DOIs
StatePublished - 2017

Bibliographical note

KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: We are indebted to Bernhard Dörling from ICMAB for the development of the accelerated blade coating platform. We also acknowledge Antonio Sánchez-Díaz from ICMAB for the evaporation of MoO3/Ag electrodes. The Spanish Ministerio de Economía y Competitividad (MINECO) is gratefully acknowledged for its support through Grant No. SEV-2015-0496 in the framework of the Spanish Severo Ochoa Centre of Excellence program and through Grants No. MAT2012-37776, MAT2015-70850-P and No. CSD2010-00044 (Consolider NANOTHERM). We also acknowledge financial support from the European Research Council through project ERC CoG648901. M.S.V. acknowledges support from the Engineering and Physical Sciences Research Council (EPSRC) via a post-graduate research studentship. J.N. and X.S. thank the EPSRC for support via grants EP/M025020/1 and EP/K029843/1. M.S.V. and X.S. acknowledge additional supports from European COST Action MP1307 (StableNextSol) for inter-institutional research visits.

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