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 results presented here demonstrate that it is possible to observe a SERGIS signal from irregular sample features and that the measured signal can be correlated with a characteristic length scale associated with the features present in the sample, as confirmed by conventional microscopy techniques.

Inelastic spin echo techniques were developed by Mezei et al.¹ in the 1970s. Since then the SERGIS technique (which is an extension of the ideas of Mezei et al.) has been successfully demonstrated experimentally using a variety of samples such as highly regular diffraction gratings^2-6 and circular de-wetted polymer droplets⁷. A dynamical theory has been developed by Pynn and coworkers to model the strong scattering from highly regular samples^3-6,8. This work has highlighted many practical aspects to be considered when performing this type of measurement and has led to a constant dialogue within a small multinational community.

Good results from SERGIS experiments will most likely be obtained if the sample being measured consists of a thin film on a flat substrate and contains scattering features with a high density of moderately sized features (30 nm to 5 µm) that scatter neutrons strongly, as demonstrated by the authors⁹. Unlike other established reflectivity techniques that probe the sample as a function of depth, the SERGIS technique has the advantage that it can probe structures in the plane of the sample surface. Furthermore, the use of spin-echo removes the requirement to tightly collimate the neutron beam in order to obtain either high spatial or energy resolution, consequently significant flux gains can be achieved. This is particularly relevant for grazing incidence geometries that are significantly flux limited because of the need to collimate the beam strongly in one direction. Using the OffSpec instrument it should therefore be possible to probe length scales from 30 nm to 5 µm in both bulk and surface structures.

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) in chlorobenzene at a concentration of 50 mg/ml.
   2. Prepare a solution of PCBM also in chlorobenzene at a concentration of 50 mg/ml.
   3. Mix the two solutions in a proportion of 1:0.7 P3HT:PCBM.
   4. Filter the mixed solution through a 0.45 µm PTFE filter.
4. Spincoat second layer by depositing approximately 100 µl of the P3HT:PCBM solution onto the PEDOT:PSS coated substrates and then spin at 2,000 rpm for 30 sec to form the second layer.
5. Leave one sample as cast and thermally anneal the other for 1 hr at 150 °C in an oven. This results in the growth of the large PCBM crystallites on the thin film surface.

2. Sample Characterization by Microscopy

1. Optical microscopy
   1. Take an optical microscopy image of both samples using a 40X microscope objective on an optical microscope operating in reflection mode, capturing the images using a CCD camera.
   2. Record a calibration image of a sample of known length at the same magnification used for the step 2.1.1
   3. Calculate the pixel size in microns for the images by determining the number of pixels for the sample of known size.
   4. Use this know pixel size to calibrate the images using any readily available microscopy software. An example of a calibrated optical microscopy image is shown in Figure 1.
2. Atomic Force Microscopy
   1. Take an atomic force microscope (AFM) image of the two samples.
   2. Analyze the data using any readily available scanning probe software to generate line profile figures like those presented in Figure 1.

3. SERGIS Experiment

1. Select a suitable reference sample to provide the reference polarization P₀, which enables the data acquired from the sample of interest data to be normalized.
2. Align the sample and reference sample
   1. Place all three samples on a positioning table; this can be translated across the neutron beam so each sample can be placed in the beam in turn.
   2. Position the P₀ reference sample in the beam by translating the sample table.
   3. Align the P₀ reference sample to an angular accuracy of <0.005° using standard reflection alignment methods.
   4. Place the sample of interest in the neutron beam by translating the sample table.
   5. Align both the sample of interest to an angular accuracy of <0.005° using standard reflection alignment methods.
   6. Repeat this alignment process for all samples of interest to be measured.
3. Tune the SERGIS instrument so it is in echo mode
   1. Set up the dedicated off-specular reflectometer OffSpec at the ISIS Pulsed Neutron and Muon Source (Oxfordshire, UK) to produce wavelengths from 2-14 Å. Further details of the set up used can be found here¹⁰.
   2. Tune the instrument to balance the total number of neutron precessions in each arm of the instrument by scanning the current in part of the guide field arrangement. This is achieved by setting the strength and inclination of the magnetic fields within the encoding arms of the instrument, which are defined by the distance between the RF spin flippers.
4. Set the angle of grazing incidence by tilting the sample table so the neutron beam is incident upon the P₀ sample (for this experiment at an angle of 0.3°).
5. Block the directly transmitted neutron beam from reaching the detector in order to prevent saturation problems.
6. Measure the samples
   1. Move the sample translation stage so the reference sample is once again in the neutron beam and measure the scattered neutron intensity as a function of position on a vertically oriented linear scintillator detector for the reference sample. Measure both spin up and spin down orientations by flipping the spin of the scattered beam immediately before the analyzer. Typically this is done for a period of about 1 hr. This enables the polarization to be determined as well as the scattered intensity for both settings.
   2. Translate the sample stage so as to measure the first of the samples of interest, again recording both spin up and spin down orientations as a function of position using a vertically oriented linear scintillator detector for a period of about 1 hr.
   3. Repeat steps 3.6.1 and 3.6.2 until sufficiently good counting statistics for this measurement have been obtained. Typically this is about 8 hr/sample in total.
   4. Repeat steps 3.6.1-3.6.3 for any further samples that need to be measured.
7. The collected data consists of both spin up and spin down 2D intensity maps for each sample. Calculate the polarization for each pixel in the 2D data sets by using the formula
   
   where P is the polarization and I_up and I_down are the measured spin up and spin down intensities respectively.
8. Normalize the data sets acquired for the samples of interest using the P₀ reference sample data collected to produce a normalized polarization intensity map according to the formula
   
   where P_Normalized is the determined polarization calculated and P_Sample is the sample polarization value and P₀ is the polarization measured using the P₀ reference sample.
9. Integrate the SERGIS data over a suitable range
   1. Select the area (i.e. the pixel range in the normalized polarization plot) for the SERGIS data integration. This area should be selected so as to avoid swamping the desired SERGIS signal by any potential polarization inhomogeneities resulting from imperfections in the field line-integrals. The available Q space that the SERGIS signal may be integrated over is effectively limited to a series of discrete Q values at any given spin-echo length configuration, where Q is the momentum transfer vector i.e. the change in momentum of a neutron after interacting with the sample
   2. Reduce the 2D data by integrating the normalized polarization to get the SERGIS correlation function G(y) which has been defined previously⁵. Strictly G(y) should be integrated to infinity over both Q vectors perpendicular to y, however, for experimental reasons the integration area is limited to selected detected intensity above the sample horizon.
10. Compensate for different scattering length densities at different wavelengths by treating the data in a similar manner to spin echo small angle neutron scattering data by plotting the data in the form:
    
    where λ is the spin echo length in nm and may be readily calculated using y = αλ² , where α is a constant determined using calibrated constants for the given instrument setup¹¹.

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