표면 개질을 통한 고성능의 유기태양전지 제작 및 특성 연구

Abstract

Organic solar cells (OSCs) have attracted much attention as energy source for future mobile electronics because of their advantages such as low manufacturing cost, lightweight, flexibility, and transparency, etc. The development of new organic materials and more efficient device structures has enabled rapid progress of OSCs in recent years. The power conversion efficiency (PCE) of OSCs is now exceeded over 17%. Although the low PCE which was one of the serious problem has been overcome, and considered to be one-step loser to commercialization, further improvement in performance is still needed to compete with other solar cells based on inorganic semiconductor which was already commercialized. The charges generated by incident light from the photoactive layer should pass through the interface after separation and transfer to reach the electrode. However, even a few tens meV of the energy barrier formed at the interface between the photoactive layer and charge transport layer can cause significant charge accumulation, thereby leading to a decrease in photovoltaic characteristics. Therefore, an appropriate interface engineering is essentially required to achieve effective charge transport by preventing charge recombination at the interface. In this study, the various interface engineering methods were introduced to maximize the performance of OSCs and analyzed their effects on real devices. In the first work, the interface problem between PEDOT:PSS and photoactive layer was investigated. Since the PEDOT:PSS hole transport layer has a metallic property intrinsically, it can form an energy barrier by means of metal-semiconductor contact at its interface with the photoactive layer. In this study, hydroquinone (HQ) was applied on PEDOT:PSS surface to remove this contact barrier at the interface between metallic PEDOT:PSS HTL and semiconducting photoactive layer. HQ treatment of the PEDOT:PSS surface lowered the hole transport barrier and reduced sheet resistance of PEDOT:PSS via the secondary doping effect of HQ. Thus, the device fabricated with the HQ-modified PEDOT:PSS showed a 28% increase in PCE compared to the device without HQ treatment. In the second work, the interface problem of tunnel-junction intermediate connection layer (ICL) was investigated. In tunnel-junction type tandem solar cells, the ICL generally consists of a hole conducting layer and a polyethyleneimine (PEI) polyelectrolyte layer. However, because pristine PEI is intrinsically an insulator, the photocurrent is limited even if it is slightly thick in the intermediate connection layer. In this study, high-efficiency homo-tandem solar cells are demonstrated with enhanced efficiency by introducing carbon quantum dot (CQD)-doped PEI on tunnel-junction ICL. The tandem solar cells with CQD-doped PEI layer in ICL leads to a better series connection in tandem solar cells and it yields a maximum PCE of 12.13%, representing a 15% increase compared with tandem solar cells with a pristine PEI layer. In the third work, the effect of interface engineering on the charge dynamics of the photoactive layer was explored. The exciton dissociation probability is also affected by the internal field generated by the work function difference between cathode and anode. In this study, The CQD having NH2 ligands added to the PEI work-function modifying layer to improve both work-function modification effect and charge transport property. A CQD-doped PEI layer induced a lower work-function of ITO than that of ITO with a pristine PEI, which induced a stronger internal field. This strengthened internal field induced better exciton dissociation efficiency, thereby improving performance of OSCs. In the last work, the interface problem between PEDOT:PSS and ITO electrode was investigated. The low-conductivity PEDOT:PSS such as AI4083 is mainly utilized in current OSCs. Thus, the contact between PEDOT:PSS and ITO are not ohmic. Despite the high possibility that there are serious interface problems, little attention has been paid to the interface between PEDOT:PSS and ITO. In this study, a conjugated polyelectrolyte WPFSCz- employed between ITO and low-conductivity PEDOT:PSS to overcome complicate organic-inorganic interfacial problems. The inserted WPFSCz- layer modifies the work-function of the ITO, thereby forming effective cascading energy alignment, which is favorable for good hole transport. Also, the introduction of WPFSCz- layer reduces recombination losses at the interface by eliminating the interfacial trap sites, resulting in an improvement in fill factor. These effects result in a significant increase in the PCE from 15.86 to 17.34%. Through these researches, the interface problems caused by contact between different types of materials can be overcome by introducing various interfacial engineering methods in OSCs. These methods enhanced PCE by improving charge transfer characteristics and charge extraction characteristics, thereby achieving high-performance OSCs successfully.