Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM

Summary

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution.

Abstract

Organic photovoltaic (OPV) materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials affects performance of photovoltaic devices. Thus, understanding of spatial variations in composition as well as electrical properties of OPV materials is of paramount importance for moving PV technology forward.^1,2 In this paper, we describe a protocol for quantitative measurements of electrical and mechanical properties of OPV materials with sub-100 nm resolution. Currently, materials properties measurements performed using commercially available AFM-based techniques (PeakForce, conductive AFM) generally provide only qualitative information. The values for resistance as well as Young's modulus measured using our method on the prototypical ITO/PEDOT:PSS/P3HT:PC₆₁BM system correspond well with literature data. The P3HT:PC₆₁BM blend separates onto PC₆₁BM-rich and P3HT-rich domains. Mechanical properties of PC₆₁BM-rich and P3HT-rich domains are different, which allows for domain attribution on the surface of the film. Importantly, combining mechanical and electrical data allows for correlation of the domain structure on the surface of the film with electrical properties variation measured through the thickness of the film.

Introduction

Recent breakthroughs in power conversion efficiency (PCE) of organic photovoltaic (OPV) cells (pushing 10% at the cell level)³ in concert with compliance with high-throughput and low-cost manufacturing processes⁴ have brought a spotlight onto OPV technology as a possible solution for the challenge of inexpensive manufacturing of large-area solar cells. OPV materials are inherently inhomogeneous at the nanometer scale. Nanoscale inhomogeneity of OPV materials and performance of photovoltaic devices are intimately connected. Thus, understanding inhomogeneity in composition as well as electrical properties of OPV materials is of paramount importance for moving OPV technology forward. Atomic force microscopy (AFM) has been developed as a tool for high-resolution measurements of surface topography since 1986.⁵ Nowadays, techniques for materials properties (Young's modulus,^6-10 work function,¹¹ conductivity,¹² electromechanics,^13-15 etc.) measurements are attracting increasing attention. In the case of OPV materials, correlation of local phase composition and electrical properties holds promise for revealing better understanding of the inner workings of organic solar cells.^{1, 16-17} AFM-based techniques are capable of high-resolution phase attribution⁸ as well as electrical properties mapping in polymeric materials. Thus, in principle, correlation of polymer phase composition (through mechanical measurements)¹⁸ and electrical properties is possible using AFM-based techniques. Many AFM-based techniques for measurements of mechanical and electrical properties of materials use the assumption of constant area of contact between the AFM probe and the surface. This assumption often fails, which results in strong correlation among surface topography and mechanical/electrical properties. Recently, a new AFM-based technique for high-throughput measurements of mechanical properties (PeakForce)¹⁹ was introduced. PeakForce TUNA (variation of the PeakForce method) provides a platform for concurrent measurements of mechanical and electrical properties of the sample. However, the PeakForce TUNA method produces mechanical and electrical property maps, which usually are strongly correlated because of unaccounted variability of contact during measurements. In this paper, we present an experimental protocol for removing correlations associated with varying contact radius while maintaining accurate measurements of the mechanical and electrical properties using AFM. Implementation of the protocol results in quantitative measurements of materials' resistance and Young's Modulus.

Protocol

1. Signal Acquisition

1. Install sample (polymer solar cell without cathode (ITO/PEDOT:PSS/P3HT:PC₆₁BM)) into a commercial Multimode AFM (Veeco, Santa Barbara, CA) equipped with Nanoscope-V controller.
2. Install conductive AFM probe into Multimode AFM probe holder.
3. Create electrical connection between the AFM probe, sample and voltage source.
4. Route current amplifier output (current signal), Multimode AFM deflection output (force signal), Multimode AFM sample height output (distance signal) into a digital acquisition card (NI-PCI-6115 DAQ). The gain on Femto DLPCA-200 current amplifier is 1 nA/V at 50 kHz bandwidth.
5. Apply 6V bias between AFM probe and ITO electrode.
6. Run Multimode AFM in PeakForceTM mode collecting topography signal: peak force set point of 30 nN, a support oscillation amplitude of 300 nm, a support oscillation frequency of 2 kHz, a scan rate of 1 Hz, and a resolution of 512 by 512 pixels.
7. Collect signals listed in section d by LabView/MATLAB control concurrently with acquisition of topography signal (step e).

2. Data Analysis Step 1: Generation of Pull-off Force, Contact Stiffness, and Current Maps

1. Read time-stamped current, force and distance signals into MATLAB.
2. Create 2,000 force - distance, and force - current curves for the first scan line. Number of curves is a function of support oscillation frequency and scan rate.
3. From each force - distance curve, determine contact stiffness and pull-off-force during withdrawal of the AFM probe (Figure 1).
4. From each force - current curve, determine the average current while the AFM probe is in contact with the surface during withdraw (Figure 1).
5. Interpolate 2,000 equally spaced contact stiffness, pull-off-force, and current points by 512 points to match resolution of topography signal. The first scan line for contact stiffness, pull-off-force, and current maps is done.
6. Create contact stiffness, pull-off-force, and current maps by repeating steps b through e 512 times. Results are shown in Figure 2.

3. Data Analysis Step 2: Elimination of Contact-area Artifacts

1. Use equation (1) and (2) to obtain Young's Modulus (E_MATERIAL) and resistance (ρ) of the material at each point of the scan:²⁰
   
   using F_ADH = F_PULL - 8 nN (adhesion due to water meniscus between the AFM and the surface),²⁰ contact stiffness (k), and current (I) maps; probing voltage (V), film thickness (L), and adhesion energy (w = γ_PROBE + γ_MATERIAL - γ_PROBE-_MATERIAL, where γ_PROBE - surface energy of probe material, γ_MATERIAL - surface energy of sample material, and γ_{PROBE-MATERIAL} - interfacial energy of sample material and probe material).²⁰

Results

Young's modulus and resistivity maps (Figure 3) present typical results of the measurements described above. Mechanical and electrical properties of the ITO/PEDOT:PSS/P3HT:PC₆₁BM stack were measured at negative (-10 V) and positive (+6 V) voltages applied to the AFM probe. Imaging artifacts, associated with electrostatic interaction between the AFM probe and the sample, are a common problem for quantitative measurements of functional properties using AFM. The similarity of Young's modul...

Materials

Name	Company	Catalog Number	Comments
Plextronics inks	Plexcore	PV 1000
ITO-coated glass substrates	Delta Technologies, Inc		25 Ohms/sq
30 MHz synthesized function generator	Stanfor Research Systems	DS345
Current amplifier	Femto	DLPCA-200
Multimode AFM	Veeco, Santa Barbara, CA		equipped with Nanoscope-V controller
DAQ card	National Instruments	NI-PCI-6115
Metal Pt probes	RMNano	12Pt3008
MATLAB software	Mathworks
LabView software	National Instruments