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

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 P3HT^21-26 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.