The Interface Of Organic Small Molecule Solar Cells Research

The fundamental physics processes at the interfaces in organic solar cells and the effect of these interfaces on the performance of organic solar cells has been studied for small molecule system. Our effort focused on three parts:1. The effect of a buffer layer between acceptor and the cathodeBy introducing a new thin buffer layer of Alq₃ between acceptor(C₆₀)and the cathode to substitute the widely used BCP buffer, the lifetime of small molecule organic solar cells was improved about 150 times for unencapsulated devices. The power conversion efficiency was not decreased and was even a little improved. We have proposed that the thin buffer blocks metal atom diffusion into the organic film during fabrication of devices and blocks oxygen/water diffusion after the devices are removed from the vacuum. This process is quite different from the role of exciton blocking, and can be used to explain the results we obtained. Further experiments showed that our proposal is correct.2. Theoretical and experimental assessment of the interference effect in multiple organic thin layersOrganic solar cells are multiple thin layer optoelectronic devices, and the interference effect in multiple thin layers must be taken into account. Because of this interference effect, the incident light in organic thin films is redistributed. This redistribution of light will have a direct effect on the light absorption and the following exciton production and on the number of final free carriers that are collected by electrodes. By using the transfer matrix calculation of light propagation in layered organic thin films, the interference effect is taken into account, and the UV-Vis absorption spectrum of these thin films can be obtained. While there is a large discrepancy between the experimental results and the calculation based on exponential decay law, good agreement is found when comparing the transfer matrix-calculated results with those experimental ones, showing that the interference effect does play its role. Further calculation based on transfer matrix method showed that by appropriate selection of the thickness of the organic films and the structure of organic solar cells, the absorption at the interface, where the excitons are dissociated, can be enhanced several times.3. Exciton dissociation at interfaces studied by transient photovoltage techniqueThe photovoltage versus the time between two electrodes was measured by transient photovoltage technique, and an abnormal polarity change of transient photovoltage was observed. By using the interfacial dissociation at the metal\organic interface and the separation of free carriers under the force of the internal electric field, the obtained results could be explained. Thus we have experimentally verified the existence of interfacial dissociation of exciton. The effect of such interfacial dissociation on the measured photovoltage was estimated, and design rules for high power conversion efficiency organic devices were proposed.