Harvesting solar energy using organic solar cells has been considered a viable alternative energy option mainly due to their light weight, low production costs and mechanical flexibility. The use of these organic photovoltaic devices is not wide-spread commercially due to their low efficiency. An organic solar cell is made up of a bulk heterojunction formed by donor/acceptor pairs of conjugated polymers. Common conjugated polymers are poly (2-methoxy-5-(3',7'-dimethyl-octyloxy))-p-phenylene vinylene designated as MDMO-PPV or poly-3(hexylthiophene) (P3HT) as the donor and for the acceptor, a soluble fullerene derivative such as [6,6]-phenyl C61 - butyric acid methyl ester or PCBM, a C60-derivative is employed.
A solution consisting of powders of the donor and acceptor dissolved in an organic solvent is spin cast onto a glass substrate coated with indium tin oxide (ITO). Then, aluminium electrodes are placed on top using a mask and a thermal evaporator. The ITO side active layer absorbs the light and creates excitons (bound electron-hole pairs), which are separated as charges at the junction. The structure of the heterojunction is determinant of the cell efficiency, hence it is important to do a detailed study of the structure.
Figure 1: (a) A common donor/acceptor pair used in organic solar cells: poly-3(hexylthiophene) (P3HT) as the donor (p-type) and [6,6]-phenyl C61 - butyric acid methyl ester (PCBM, a C60-derivative) as the acceptor (n-type). (b) HOMO and LUMO levels of P3HT and PCBM in comparison with the work functions of Au, PEDOT and ITO. (c) The stacking of an organic bulk heterojunction solar cell.
Organic Solar Cell Characterization
Conductive AFM techniques are capable of providing details at the nanoscale level of the heterojunction in organic solar cells. Contact mode and the point contact TUNA mode are not efficient in providing accurate results.
Thermal Annealing Effect on P3HT Thin Film
The P3HT (donor) was spin-coated on a glass substrate coated with ITO and a PEDOT layer. The P3HT layer was annealed at 120°C and spin-cast in a glove box. The AFM measured data showed that annealing procedures have an effect on the molecular ordering of the polymers. The cylindrical structures exhibited higher conductivity. Also poor conductors showed a layer having poor ordering of features. PeakForce TUNA data generated on a P3HT deposited on poly (3,4-ethylenedioxythiophene) (PEDOT) and ITO is illustrated in Figure 2.
Figure 2: Peak Force TUNA images of P3HT thin film spin-coated on glass/ITO/PEDOT substrate, and annealed at 120°C. Shown are (a) topography, scale 10nm; (b) peak current, scale 300pA; (c) DMT modulus, scale 15MPa (d) the overlay of conductivity map on topography. Image size is 2 µm × 2 µm, taken at 1 nN Peak Force, 3V DC bias, using Bruker’s PeakForce TUNA probe (Au coating, spring constant of 0.4N/m) on a MultiMode 8 AFM in a glove box with below 1 ppm O2 and H2O. Sample courtesy of Prof. Nguyen, UCSB.
P3HT:PCBM Organic Solar Cell
The P3HT and PCBM films were dissolved in a toluene solution and spin coated onto an ITO-coated glass substrate along with a thin layer of PEDOT. PeakForce TUNA was used to study this junction. The study revealed variations in conductivity and that a major part of the current was from holes along the P3HT. Hence, the regions rich in P3HT were high conductivity regions and regions rich in PCBM were poor conductivity regions.
The image also showed fiber-like features, which suggests that heterojunction had a lateral presence as well. PeakForce TUNA data measured on a P3HT and PCBM bulk heterojunction with the tip of the AFM probe used as the cathode is depicted in Figure 3.
Figure 3: PeakForce TUNA images of P3HT:PCBM solar cell with a PEDOT modified ITO/glass anode. Shown are (a) topography, scale 10nm; (b) Cycle-averaged Current, scale 5pA; (c) Adhesion, scale 8 ~10nN and (d) the overlay of conductivity map on topography. Image size is 2µm × 2µm, taken at 2.5V DC bias, a net-negative Peak Force of -1.5nN is shown in the force curve (e). Bruker’s Multimode 8 AFM is used with Bruker’s Peak Force TUNA probe (Au coating, spring constant 0.4N/m) in a glove box with below 1ppm O2 and H2O. Sample courtesy of Prof. Nguyen, UCSB.
Figure 3A shows granular structures that could possibly be polymer aggregates. Figure 3D is the adhesion map that displays features that are uniformly spread across the surface. The information from the adhesion map could lead to optimization of the active layer formation. The conversion efficiency of the organic solar cells was reflected in the imaging through the I-V curves.
The data provided by the PeakForce TUNA method gave a detailed insight into the structure of the heterojunction of the organic solar cells. Since the Peak Force used for imaging was a net-negative force, a single tip can be used for above 6 hours without any adverse effect on conductivity signal or resolution. This makes it better than other techniques such as the conductive-AFM based on the Contact Mode.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
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