Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

Summary

Progress has been made in utilizing spin echo resolved grazing incidence scattering (SERGIS) as a neutron scattering technique to probe the length-scales in irregular samples. Crystallites of [6,6]-phenyl-C61-butyric acid methyl ester have been probed using the SERGIS technique and the results confirmed by optical and atomic force microscopy.

Abstract

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped crystallites. Neutrons are passed through two well defined regions of magnetic field; one before and one after the sample. The two magnetic field regions have opposite polarity and are tuned such that neutrons travelling through both regions, without being perturbed, will undergo the same number of precessions in opposing directions. In this case the neutron precession in the second arm is said to "echo" the first, and the original polarization of the beam is preserved. If the neutron interacts with a sample and scatters elastically the path through the second arm is not the same as the first and the original polarization is not recovered. Depolarization of the neutron beam is a highly sensitive probe at very small angles (<50 μrad) but still allows a high intensity, divergent beam to be used. The decrease in polarization of the beam reflected from the sample as compared to that from the reference sample can be directly related to structure within the sample.

In comparison to scattering observed in neutron reflection measurements the SERGIS signals are often weak and are unlikely to be observed if the in-plane structures within the sample under investigation are dilute, disordered, small in size and polydisperse or the neutron scattering contrast is low. Therefore, good results will most likely be obtained using the SERGIS technique if the sample being measured consist of thin films on a flat substrate and contain scattering features that contains a high density of moderately sized features (30 nm to 5 µm) which scatter neutrons strongly or the features are arranged on a lattice. An advantage of the SERGIS technique is that it can probe structures in the plane of the sample.

Introduction

The SERGIS technique aims to be able to yield unique structural information not accessible using other scattering or microscopy techniques from thin film samples. Microscopy techniques are typically surface limited or require significant alteration/sample preparation to view internal structures. Conventional scattering techniques such as reflectivity can provide detailed information about buried sample structures as a function of depth within the thin film but cannot probe structure in the plane of the thin film easily. Ultimately it is hoped that SERGIS will enable this lateral structure to be probed even when buried within the thin film sample. The representative re....

Protocol

1. Sample Preparation

1. Clean the silicon substrates by placing 2 in silicon wafers that are 4 mm thick in oxygen plasma for 10 min.
2. Spincoat the first layer on the substrates
   1. Filter the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) through a 0.45 µm PTFE filter (PALL).
   2. Use approximately 0.5 ml for each sample to spin coat a PEDOT:PSS thin film onto the two clean substrates at 5,000 rpm spinning for 60 sec.
   3. Dry each substrate for 10 min in an oven at 70 °C.
3. Prepare the blend solution for the second layer
   1. Dissolve some poly(3-hexylthiophene-2,5-diyl) (P3HT) ....

Results

The representative results from samples of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene-2,5-diyl) (P3HT) presented here are of significant interest because of their widespread application as bulk hetero-junction materials in organic photovoltaic cells^12,13. Typically during the fabrication of an organic photovoltaic device, a P3HT:PCBM blend solution is spin-cast from a blend solution to form a thin film on a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) coa.......

Discussion

The microscopy data in Figure 1 clearly shows that before annealing the P3HT:PCBM thin film is flat and smooth and after thermal annealing there are many large irregular PCBM crystallites present on the surface with lateral dimensions ranging between about 1-10 µm. This is attributed to PCBM migration towards the top surface of the film and subsequent aggregation to form large crystallites. A strong SERGIS signal associated with scattering from PCBM crystallites in the annealed sample is seen in

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Silicon 2 in silicon substrates	Prolog		4 mm thick polished one side
Oxygen plasma	Diener		Oxygen plasma cleaning system to clean substrates prior to coating
Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)	Ossila		PEDOT:PSS conductive polymer layer for organic photovoltaic samples
0.45 μm PTFE filter	Sigma Aldrich		Filer to remove aggregates from PEDOT:PSS and P3HT solutions
Chlorobenzene	Sigma Aldrich		Solvent for P3HT
Poly(3-hexylthiophene-2,5-diyl)	Ossila		P3HT - polymer used in polymer photovoltaics
Spin Coater	Laurell		Deposition system for making flat thin polymer films
Vacuum Oven	Binder		Oven fro annealing samples after preparation
Nikon Eclipse E600 optical microscope	Nikon		Microscope
Veeco Dimension 3100 AFM	Veeco		AFM
Tapping mode tips (~275 kHz)	Olympus		AFM tips
Quartz Disc			Refrence samples for SERGIS measurement
Spin Echo off-specular reflectometer	OffSpec at the ISIS Pulsed Neutron and Muon Source (Oxfordshire, UK)	Produces pulsed neutrons 2-14 Å
Neutron Detector	Offspec		vertically oriented linear scintillator detector
RF spin flippers	Offspec
Magnetic Field Guides	Offspec
Data Manipulation Software	Mantid		http://www.mantidproject.org/Main_Page