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

The overall aim of the following experiment is to understand conductivity mechanisms in phase separated fullerene polymer blends through correlation of morphology with electrical performance. Morphology and electrical properties of polymer blends are two primary factors that control their performance inside organic solar cells. Correlation of morphology with electrical performance of the samples is achieved by concurrent measurements of mechanical and electrical properties of the sample using an atomic force microscope with a home-built controller and data acquisition system.

This is used to collect spatially resolved data about distance dependence of force between the A FM probe and the sample surface, as well as distance dependence of current between the A FM probe and the sample as a second step, perform automatic analysis of the force distance and current distance curves collected at each point of the scan. This produces high resolution maps of contact, stiffness, pull off force and current. Next, apply an approximate contact mechanics model to execute a mathematical conversion of contact, stiffness and current data in order to obtain young's modulus and resistance of the sample.

The results identify the chemical nature of domains within the sample based on mechanical signature, as well as quantitative differences in conductivity of polymer rich and following rich phases of the blend based on concurrent mechanical and electrical properties measurements. This method can help answer key questions in the field of organic solar cell development, such as efficiency and stability of those cells through understanding effects of active layer morphology on performance on source cell performance, and the correlation of the face composition of the active layer with electrical properties. Also, this method can be applied to other systems such as organic electronic materials and batteries.

The main advantage of this technique over other conductivity mapping methods is that the uncertainty in the tip sample contact area is virtually eliminated. This means you have a much clearer picture of the interfacial properties. Prepare the sample for signal acquisition.

Start with a P three H-T-P-C-B-M polymer solar cell sample without a top electrode. Mount this into a sample holder with external electrical connectors for the atomic force microscope. Next, connect the sample holder to a commercial multi-mode atomic force microscope, equipped with a nano scope five controller.

Install a conductive probe into the A FM probe holder and mount the holder in the microscope. Now connect the probe assembly to an external current amplifier. The output of the current amplifier is rooted into a digital acquisition card.

Just the probe to make an electrical connection between the A FM probe, the sample and the A FM voltage source. Be sure to connect the A FM deflection output, the force signal, the sample height output, and the distance signal to a digital acquisition card. Set the acquisition rate on the digital acquisition cards to 250, 000 samples per second and the acquisition time to one second.

Next, apply The desired bias between the A FM probe and the solar cell electrode samples. Were studied at both positive six volts and negative 10 volts in this experiment. Now set the a FM to run in peak force mode, collecting topography data with a peak force set point of 30 nano Newtons, a support oscillation amplitude of 300 nanometers, a support oscillation frequency of two kilohertz, a scan rate of one hertz, and a resolution of five 12 by five 12 pixels.

The noise level on current signal from the A FM probe can interfere with good signal acquisition. If this is a problem, try different wiring schemes for connecting the A FM probe current amplifier and the voltage source Collect force distance and current distance curves concurrently with acquisition of topography data. Here, this is done using lab view.

Matlab control of the experiment. Data analysis begins with reading the timestamped current force and distance signals into MATLAB. For the settings used create 2000 force distance and distance current curves.

For the first scan line, the number of curves is a function of the support oscillation frequency and scan rate. Shown here is a representative curve with the forced distance data shown in blue, the contact stiffness is given by the angle alpha defined in the diagram the value of the pull-off force. The first minimum of the force during reaction is also shown from each curve, determine the contact stiffness and the pull-off force.

The red curve in the diagram is for the force current data, the average value of the current as the support starts the retraction portion of its oscillation until the probe separates from the surface is referred to as the current its value is shown. For this data, determine this current for each curve to complete. The first scan line for contact stiffness, pull off force and current maps interpolate 2000 equally spaced data points for each of these quantities by 512 points to match the topography signal.

Repeat these steps for each of the 512 scan lines. Examples of the resulting images are shown in the top left are the results of topographic measurement. On the top right, a spatially resolved pull-off force measurements.

The bottom left shows the contact stiffness. The bottom right shows the current the sample was a P three HT PCBM polymer solar cell without top electrode at negative 10 volts, the image size is 10 micrometers by 10 micrometers. The correlations between the pull-off force contact stiffness and current images can be eliminated by taking into account the change in the contact area between the A FM probe and the surface.

During the experiment, use the data and the displayed equations to find E, the Young's modulus and row the resistivity. The variables are defined in the text protocol shown here as the calculated Young's modulus of the sample shown previously. The bias voltage is minus 10 volts.

Two types of domains with different young's ModuLite are evident. Those rich in polymer appear in blue domains, rich in fulling are dark red. The resistivity maps provide information on the electrical connectivity between the layers of the solar cell.

Here are the spatially resolved youngs modulus and resistivity from a different region of the same sample. This time with a bias voltage of six volts, the white arrows point towards regions of full enriched domains. Note that the resistivity switches as a function of the bias voltage polarity.

The regions have a low resistivity when there is a negative bias and a high resistivity when there is a positive bias Following this procedure. Other methods like power conversion, efficiency measurement of the complete solar cell can be performed in order to answer additional questions like correlating of correlation of morphology of the active layer in organic solar cells with device performance.