Investigation On The Influence Of Device Structure And Post-production Treatment On The Performance Of Small Molecular Solar Cells

Power generated by solar cells has recently gained special attention in the field of renewable energy resource. Regarded as one of the strategic and reserved technological products in the next generation, organic solar cells have their own special potentials in commercial markets. Therefore, the technology of developing organic solar cells has gradually been on the dominant position in industry, since the commercialization of silicon based solar cells is rather mature. Basically, small molecular solar cells have advantages in research and development due to the simplicity in molecular structure of small weight organic materials.In the beginning of this thesis, physical characteristics of the excitonic semiconductor materials was introduced, and comparison with conventional semiconductor materials was made, then the main difference of the two was showed in conclusion. Moreover, the basic structure and physical mechanics of organic solar cells were illustrated, including the historic review and the latest developments.Based on the Zinc Phthalocyanine/C₆₀ heterojunction structure, the thesis successively specified the design and fabrication of ITO anode, the pre-treatment process of the device substrate, vacuum evaporation technique and thermal annealing treatment in details. In this thesis, the device testing procedure and thermal annealing treatment were introduced, respectively.The goal of this study is to promote the charge extraction ability of small molecular solar cells and the power conversion efficiency. The work contains two aspects: the optimization of the device structure and the post-production treatment. Despite of the conventional layer optimization of films, the thesis imposed a great attention on the planar structure of the cathode, and drew brand new conclusions. In the study of layer optimization, the theory by adding restricted conditions according to the new experimental circumstances was modified and proper theoretical predictions in agreement with experimental results were presented. Since the experimental data could not meet the theoretical prediction in the thickness of C₆₀ layer, it was supposed that the conductivity of C₆₀ material has decreased because of long term exposure in the air during the fabrication process. As a result, a modified method has been suggested, and the thickness optimization issue has been solved successfully. The conclusion of this part defined the optimal layer structure: ITO (200nm)/ (25 nm)/C₆₀ (16 nm)/Al (100nm).In the planar structure analysis, the formula of the organic heterojunction by analytical method was derived, and the formula by varying testing conditions, i.e. effective applied voltage and light intensity, were verified. By simplifying the formula, the rule of short-circuited current density and open-circuited voltage with charge distribution on the surface of isolated conductors was explained. The theoretical and experimental concluded that N (Narrow) type device has higher open-circuited voltage while L (Long) type device has larger charge extraction ability, improving the short-circuited current density by 15%. Consequently, L type device was regarded as the best one in the following research in terms of current promotion.After the fabrication of solar cells, there are a series of treatments to stabilize or enhance the device performance, which are generally called as post-production treatment. In order to further improve the photo-current, thermal annealing treatment on small molecular solar cells was discussed in the last part of the main content of this thesis. The emphasis was focused on annealing temperature and time, and the optimal annealing process was as 100℃,20min. The optimal annealing process had respectively promoted the short-circuited current density and the open-circuited voltage of the devices by 46% and 103mV, lengthening its attenuation constant by 4 times (from 5.6h to 22.2h).As a result, the reasons were attributed to the improvement of interfacial morphology at the ITO/ZnPc interface and the decrease of series resistance due to better contact of the layers in the device.