Design of Crystalline Zinc-Tin-Oxide Semiconductor: Low Temperature-Solution Processed Flexible Photon Energy Conversion Devices

Abstract

Metal oxide semiconductors (MOS) with wide band gap promise high performance as an electron/hole transporting material in advanced optoelectronic applications as diverse as transparent displays and emerging solar cells due to their high carrier mobility, tunable high band gap, and high stability. Especially, solution processable MOSs on flexible substrate at low temperature promise to revolutionize the area of such emerging optoelectronics due to lower fabrication cost by high throughput and large area roll-to-roll process. Among various optoelectronics, solar cells which converts solar energy into electricity are expected to be the most promising technology due to their infinite and eco-friendly energy source. In particular, the solid state perovskite solar cells (PSCs) have received the most attention owing to their low fabrication cost and remarkably high power conversion efficiency of over 20 %. In contrast to rigid substrate based PSCs, however, the PSCs based on flexible substrate currently face significant challenges in improving energy conversing efficiency. The MOSs in PSCs are one of the biggest reasons for poor device performance due their high processing temperature and inferior optical/electrical properties. As a part of the solution for these problems, inverse-spinel zinc tin oxide (Zn2SnO4) has attracted significant attention in the solar cell area as an alternative to the conventional binary oxides such as TiO2 and ZnO because of its comparable energy levels, high chemical/photo-stability and superior electronic/optical properties. However, although it possesses attractive properties, it is actually difficult to replace the existing binary oxides because of its high processing temperature and complex fabrication process. In addition, the MOS in the PSCs possess many interfaces, e.g., transparent conducting oxide (TCO)/MOS interface and MOS/light absorbing material interface, which can operate as recombination center for photo-generated electrons. The loss of photo-generated electrons at interfaces leads to poor device performance especially in low temperature processed flexible PSCs. From this point of view, the thesis proposes two strategies for high performance flexible PSCs with simple low temperature process. First is development of low temperature process for obtaining high quality Zn2SnO4 (ZSO) film on flexible substrate below 100 oC. Second is interface design of low temperature processed ZSO film to minimize the photo-generated electron loss in flexible PSCs. First, the ZSO was explored to evaluate its potential to an electron collection materials (ECM) in solar cells, by employing dye sensitized solar cells (DSSCs) as a model device. For the purpose, ZSO dense/porous films were deposited as an ECM in DSSCs, and its device performance was evaluated via high temperature process above 500 oC. The ZSO based DSSC showed good device performance over 6 %, and the superior device properties were discussed on the view point of its light harvesting and charge collection (electron transfer/transport) capability with various electron dynamic analysis tools. These results demonstrated that the ZSO may be an attractive alternative to TiO2 for next generation solar cells, such as perovskite solar cells (PSCs). Second, a new synthesis strategy was proposed to fabricate ZSO nanoparticles (NPs) at much lower reaction temperature (< 100 °C) by controlling zinc complex precursor with hydrazine in aqueous solution. Their possible synthesis mechanism was discussed in detail. The new synthesis route yielded highly dispersed crystalline ZSO NPs, which facilitated a low temperature fabrication (~ 100 °C) of a compact ZSO film by simple spin coating method. Notably, the resultant film showed superior transmittance rather than bare ITO glass or polymer substrates in visible regions due to their anti-reflection (AR) effect attributable to low reflective index of ~1.4. Furthermore, its feasibility on flexible PSCs was examined by employing the low temperature processed ZSO film as an electron collection layer (ECL). The resultant ZSO based flexible PSC showed superior device performance over 15 %, which surpasses the performance of the existing TiO2 based flexible PSCs. Finally, general strategies were proposed to design a new type of ZSO ECL by using two ZSO nanoparticles (NPs) tuned in the energy level for improving the electron collection in flexible PSCs. The energy level of ZSO NPs was tuned by controlling particle size. The effect of their particle size and energy level on the electron collection in PSCs was discussed on the view point of difference in energy level at each interface, i.e., built in potential and schottky barrier. It was founded that the ZSO ECL designed with two features of ZSO QDs and NPs is highly beneficial for minimizing the electron loss originated from each interface, especially in flexible PSCs. Consequently, the designed ECL enabled the fabrication of a flexible PSC over 16.5 % with high reproducibility, which is the highest efficiency reported for the flexible PSCs. This thesis focused on development of low temperature, solution process for synthesizing Zn2SnO4 material for application to flexible devices and suggested a guideline of designing Zn2SnO4 ELC to improve the performance of flexible PSCs. Furthermore, this thesis will provide a new breakthrough to solve the faced problems in not only flexible PSCs but also other research area such as TFT and TCO.