Low-Temperature Processes for Next-Generation Photovoltaics : Organic Solar Cells and Perovskite Solar Cells

다국어 초록 (Multilingual Abstract)

The fabrication of semiconductors with excellent electrical and optical properties using solution processes has made a significant contribution to the field of optoelectronic devices such as displays and next-generation solar cells. In particular, the technology of forming a semiconductor on a flexible substrate through a solution process at a low temperature can reduce the manufacturing cost by enabling a large-area and mass production through a roll-to-roll process and can form products on various curved surfaces. Solar cells can provide sustainable energy supply based on infinite solar energy, which can also be a solution to global environmental problems. Because solar cells can be fabricated based on such low-temperature-solution processes, developing related technologies is one of the ways to commercialize solar cells. As an example, organic solar cells (OSCs) have received considerable attention recently due to their low manufacturing costs and high-power conversion efficiency (PCE) of more than 10%. However, unlike the case of using a high-temperature process, the PCE is lower when OSCs are fabricated on a flexible substrate through a low-temperature process.
For this reason, zinc oxide (ZnO) sol-gel, which is mainly used in organic solar cells, requires a high temperature of 200 ̊C or more to have excellent electrical characteristics. Therefore, to solve this problem, it is essential to use a conjugated polymer electrolyte that replaces zinc oxide without a heat-treatment process or use a non-thermal processing method as a nanomaterial in which zinc oxide is formed. Besides, researchers are using silver nanowires (AgNWs) to replace indium tin oxide (ITO), which is an expensive transparent conducting electrode (TCE), formed directly on a low-cost, flexible substrate. However, this transparent electrode also requires a temperature of 100 ̊C or more for superior electrical properties. To manufacture a flexible solar cell using a low-cost plastic substrate, development of a low-temperature process technology that can solve such a problem is required.
In this study, we have developed flexible solar cell by introducing poly [(9,9-bis(3'-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)] (PFN), which is one of the complex polymer electrolytes to replace ZnO. To realize this, we evaluated the possibility of using solar cells based on flexible substrates with AgNWs. First, since PFN was applied as an electron transport layer (ETL) instead of ZnO, it could reduce the processing time, and the process temperature could be drastically lowered simultaneously. In particular, a flexible substrate using AgNWs as a TCE causes a phenomenon in which AgNWs are lifted from the substrate due to the event of plastic from warping during the heat-treatment process. Because of this phenomenon, AgNWs contact the metal electrode on the opposite side, which increases the possibility of short-circuiting in the solar cell structure. However, using PFN can reduce the likelihood of this happening. TO investigate the role of the ETL, we studied the optical properties, electron transport and collection ability using scanning electron microscopy (SEM), ultraviolet-visible-near infrared (UV-VIS-NIR) spectroscopy and ultraviolet photoelectron spectroscopy (UPS). For fabrication of flexible OSCs, the organic photoactive layer was fabricated on PFN using {poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)car-bonyl]thieno[3,4-b]thiophenediyl] (PTB7) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM)}. This flexible solar cell not only achieved a much-improved PCE compared to the previously reported solar cell of AgNWs electrode on PET, but it also maintained a retained efficiency of less than 10% at 1000 cycles repeated bending in a radius of R  4.8 mm.
Second, a heat-treatment process is necessary for the formation of NW junctions when fabricating AgNW electrodes optimized for the flexibility characteristics. However, a solar cell manufactured using heat causes instability of the device itself. Therefore, in this study, cold isostatic pressing (CIP), which does not require heat-treatment, is used to realize intimate contact between AgNWs. Also, non-treated zinc oxide nanoparticles were applied to the ETL to conduct all the processes of fabricating flexible OSCs at room temperature. Transmission electron microscopy (TEM) and SEM were used to investigate the shape of AgNWs fabricated by CIP. poly[[2,6’-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-th) and PC71BM were used as a photoactive layer for flexible OSCs and showed stable flexibility, continuous efficiency, and extreme bending properties. Besides, OSCs based on CIP treated AgNW electrodes achieved 100% reproducibility by overcoming the production yield of 75% of the conventional heat-treated AgNW electrodes based OSCs. Compared with the existing heat-treated AgNW electrodes formation method, it was confirmed that the AgNW electrodes using the CIP method contribute to the improvement of the performance of the flexible OSCs by forming a uniform film shape with excellent electron collecting ability.
Finally, Organic-inorganic hybrid perovskite solar cells, which can be manufactured simply by solution process technology, have attracted growing attention in terms of high efficiency and low fabrication cost. In particular, the production of high-quality perovskite films without pinholes is essential for high efficiency solar cells. Here, we have investigated the effect of solvent additive on perovskite film formation according to Gutmann’s donor number (DN) in PbI2 precursor solution based on two-step spin-process. Among the various solvent additives, dimethyl sulfide (DMS) has a high DN of 23.5 kcal·mol-1. The PbI2 precursor solution with DMS lead to the formation of perovskite thin film with a large grain size of a mean of and excellent electrical properties. The power conversion efficiency of our mixture of perovskite layers (FAPbI3)x(MAPbBr3)1-x based solar cell achieves a maximum of 21.2% and an average of 20.8% under reverse voltage scan under AM 1.5G with an irradiance of 100mW·cm-2. Our work suggests promising potential for better performance improvement of solvent engineering based planar perovskite solar cells.