Opportunities and challenges in probing local composition of organic material blends for photovoltaics

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Natalie P. Holmes Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, Australia

Vanja Blazinic Department of Engineering and Physics, Karlstad University, Karlstad 651 88, Sweden

Paul Dastoor Centre for Organic Electronics, University of Newcastle, Callaghan, NSW 2308, Australia

Ellen Moons Department of Engineering and Physics, Karlstad University, Karlstad 651 88, Sweden (Received 1 November 2016; accepted 3 January 2017)

Probing the distribution of donor and acceptor molecules in the active layer of polymer solar cells requires high-resolution methods that provide chemical contrast. A combination of the synchrotronbased soft X-ray technique near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM) can map surface composition and local composition in lateral phase-separated domains, as well as identify molecular signatures of degradation. Here we illustrate, by way of selected results, the relevance of these complementary techniques to the field of organic photovoltaics. We demonstrate firstly that the determination of local composition from X-ray absorption spectra requires cautious use of fitting techniques. Furthermore, we show that drop-like clusters of PC70BM formed during the transfer of spin-coated polymer:PC70BM blend films onto Cu-grids lead to an underestimation of PC70BM/polymer concentration ratios. Finally, we show that the selective degradation of one of the components can impair the accurate determination of local blend composition.

I. INTRODUCTION

The active layer in an organic solar cell is a thin lightabsorbing and charge-generating film sandwiched between two electrical contacts that collect the photogenerated charges. In this layer at least two different organic molecules, acting as electron-donor and electronacceptor, are mixed. The degree of mixing, which strongly depends on the processing conditions, has a large impact on the power conversion efficiency of the solar cell.1–5 Therefore, it is of great importance to be able to image the active layer morphology. Microscopy techniques such as atomic force microscopy (AFM) and scanning electron microscopy (SEM) are routinely used to image the structure of the surface of the active layer. Soft X-ray spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, yield chemical information and can be used to determine the

Contributing Editor: Dean DeLongchamp a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.7

composition at the surface of the blend film.6–8 The rich fine structure of NEXAFS spectra at the C1s-edge is particularly useful in the challenging quest to distinguish the two organic molecules (donor and acceptor molecules) and provide chemical contrast with a variety of surface sensitivities. Even other absorption edges, such as O1s and N1s, in their respective photon energy ranges, are of interest for many common conjugated molecules. I