Nikiforov, M. P., Darling, S. B. Concurrent Quantitative Conductivity and Mechanical Properties Measurements of Organic Photovoltaic Materials using AFM. J. Vis. Exp. (71), e50293, doi:10.3791/50293 (2013).
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.
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.
1. Signal Acquisition
- 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.
- Install conductive AFM probe into Multimode AFM probe holder.
- Create electrical connection between the AFM probe, sample and voltage source.
- 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.
- Apply 6V bias between AFM probe and ITO electrode.
- 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.
- 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
- Read time-stamped current, force and distance signals into MATLAB.
- 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.
- From each force - distance curve, determine contact stiffness and pull-off-force during withdrawal of the AFM probe (Figure 1).
- From each force - current curve, determine the average current while the AFM probe is in contact with the surface during withdraw (Figure 1).
- 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.
- 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
- Use equation (1) and (2) to obtain Young's Modulus (EMATERIAL) and resistance (ρ) of the material at each point of the scan:20
using FADH = FPULL - 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 = γPROBE + γMATERIAL - γPROBE-MATERIAL, where γPROBE - surface energy of probe material, γMATERIAL - surface energy of sample material, and γPROBE-MATERIAL - interfacial energy of sample material and probe material).20
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 moduli magnitude measured at different voltages demonstrates robustness of the measurement protocol described above with respect to electrostatic artifacts. Often variations in chemical composition within a material are associated with the local changes in Young's modulus. The sample used in this study is a solar cell device without the top electrode. The top layer (P3HT:PC61BM) in the stack is the solar cell active layer where conversion of light into electricity occurs. Solar cell performance strongly depends on the morphology and chemical composition of the active layer.
Contact stiffness and current measured using AFM are often correlated (Figure 4) because of the variations in contact area between the AFM probe and the surface. Such correlation often complicates quantitative determination of mechanical (Young's Modulus) and electrical (resistivity) properties of the material. The protocol, provided above, accounts for variations in contact area by direct measurements of the adhesion force between the AFM probe and the surface, which in turn allows for quantitative measurements of Young's modulus and resistivity. PC61BM-rich domains are stiffer than polymer-rich ones. Failure to account for contact area variability leads to domain misrepresentation. For example, stiff PC61BM-rich domain is visible on both contact stiffness and Young's modulus's line profiles (Figure 4A), while the other PC61BM-rich domain (Figure 4B) appears only on Young's modulus map.
The method described above allows for attribution of chemical composition on the surface of the P3HT:PC61BM layer. Two types of domains with different Young's moduli are evident in Figure 3(A) and 3(B). Knowledge about chemical composition of the active layer and literature data on mechanical properties of P3HT21-26 and PC61BM21 allows attribution of domains with Young's modulus around 0.01 GPa as P3HT-rich ones (appear blue on Figure 3(A) and (B)) and domains with Young's modulus around 0.1 GPa as PC61BM-rich ones (appear dark red on Figure 3(A) and (B)).27 Resistance maps (Figure 3(C) and (D)) provide information about electrical connectivity between the top surface of the P3HT:PC61BM layer and the ITO layer. In an operating solar cell, current travels from the bulk of the active layer toward current collectors (ITO and the electrode deposited on top of the P3HT:PC61BM layer, respectively), thus, resistance maps are vital pieces of information that allow for correlation of chemical composition and performance of solar cells. Figures 3(C) and 3(D) show that resistance of P3HT-rich and PC61BM-rich domains changes depending on the polarity of voltage applied to the AFM probe. P3HT-rich domains have lower resistance at positive voltage and higher resistance at negative voltage in comparison with PC61BM-rich domains. Possible injection of holes from the high-work function Pt probe, relatively high hole conductivity of P3HT28 and hole conductivity of PEDOT:PSS explain lower resistance of the P3HT-rich areas, as well as a higher barrier for electron injection and electron rejection properties of PEDOT:PSS were cited27 as reasons for higher resistance of the PC61BM-rich domains in comparison with P3HT-rich ones under positive bias of the AFM probe. At negative bias, resistance of P3HT-rich domains should increase and resistance of PC61BM domains should decrease because of a decrease in hole injection efficiency of PEDOT:PSS29 (resulting in decrease of electron rejection) and injection of electrons from the negatively biased Pt probe. Chemical attribution of domains based on mechanical properties measurements is valid only in proximity of the air-P3HT:PC61BM interface, while resistance measurements provide information about current pathways through the thickness of the film. In this regard, mechanical and electrical measurements provide complimentary information about the sample. Variation in resistance within P3HT-rich and PC61BM-rich surface domains reveals inhomogeneity of the domain structure throughout the active layer film thickness.
Summarizing, we described a protocol for quantitative measurements of Young's modulus and resistivity of soft materials by mitigating contact area uncertainty. Mechanical properties of PC61BM-rich and P3HT-rich domains are different, which allow for domain attribution on the surface of the film. Combination of 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.
Figure 1. Typical force - distance (blue) and current - distance (red) curves taken on ITO/PEDOT:PSS/P3HT:PC61BM with Pt probe.
Figure 2. Spatially resolved measurements of topography (A), pull-off force (B), contact stiffness (C), and conductivity at -10 V (D) on an ITO/PEDOT:PSS/P3HT:PCBM sample. Image size is 10 μm x 10 μm.
Figure 3. Spatially resolved variations of Young's Modulus (A, B) and resistivity (C, D) for two different locations on the surface measured at -10 V (A, C) and 6 V (B, D). Images (A) and (C) were calculated from the data presented in Figure 2. Image size is 10 μm x 10 μm. White arrows point toward PC61BM-rich domains, which demonstrate resistivity switch as a function of voltage polarity (low resistivity at negative bias and high resistivity at positive bias). Black dotted line (A, C) indicates areas used for line profiles on Figure 4.
Figure 4. Line profiles from the areas indicated with black dotted lines on Figures 2 and 3A, 3C. Strong correlation between contact stiffness and current due to contact radius variability is apparent. Elimination of contact radius variations reveals stiff PC61BM-rich domains, which are poorly visible otherwise (B). Click here to view larger figure.
No conflicts of interest declared.
MPN is grateful to the Director's Fellowship Program for financial support. MPN wants to thank Yu-Chih Tseng for help with development of the protocol for solar cell processing. This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences User Facility under Contract No. DE-AC02-06CH11357.