Study on improving of electron extraction of organic-inorganic hybrid perovskite solar cell by interface engineering

Abstract

Recently, the remarkable development of organic-inorganic perovskite solar cells (PSCs) has led many scientists to study various inorganic and organic materials of PSC as well as interface charge transport mechanism of PSC. In addition, as the device structure becomes simpler, the role of electron transport layer (ETL) becomes more important, and suitable design and engineering of ETL and its interface is one of the important to improve the electron extraction and device efficiency. According to the charge selection interface, the device has different interfacial properties (electron injection driving force, contact, ion movement, or trap density), so the electron extraction ability of the device can be changed. The conventional TiO2 ETL based PSCs have showed several problems such as high temperature process, low mobility, and hysteresis behavior. Therefore, research is needed to understand the ETL/perovskite interface and develop a new ETL to improve electronic extraction in devices. Desirable ETLs should have appropriate electronic and morphological characteristics. Here, I try to understand the electronic extraction and recombination at the ETL/perovskite interface and to improve the electron extraction by engineering the ETL/perovskite interface. In chapter 2, I study perovskite solar cells which are devoid of a discrete n-type electron collection layer to understand the electron accumulation at perovskite interface. I employ ethoxylated polyethylenimine (PEIE) to modify the interface between the perovskite absorber layer and the metallic transparent fluorine-doped SnO2 (FTO) electrode. Surprisingly, the PEIE interlayer obviates the requirement for the conventional dense-TiO2 (d-TiO2) compact layer (or organic fullerene layer), usually required to selectively extract electrons from the perovskite film. The self-organized PEIE interlayer produced a strong induced dipole moment at the perovskite-FTO interface, this results indicating that electrons accumulate within the perovskite film at this interface. The resultant “n-type” contact region within the perovskite absorber layer, progressing to an intrinsic (i) region within the bulk of the perovskite layer, represents an n-i homojunction and favorably enables selective electron extraction at the FTO electrode. Resulting solar cells deliver current-voltage measured power conversion efficiencies (η) of over 15.0 % and a substantial stabilized efficiency (η) of 9.1 %. Although the solar cell performance remains lower than the highest reported efficiencies for perovskite solar cells employing discrete charge selective extraction layers, it indicates the role of electron transport layer (ETL) and show significant potential for “homo-junction” perovskite solar cells, once the metallic-to-perovskite contact is fully controlled. Additionally, this work identifies the potential impact of modifying the interface between the perovskite absorber and the subsequent contact materials with dipolar organic compounds, which may be applicable to optimizing contact at perovskite-semiconductor heterojunctions. In chapter 3, I introduce the use of a fullerene (C60)–poly(allylamine) (PAA) linked electron-transport layer (ETL) in planar perovskite solar cells. By heating a spin-coated layer of C60 with a thin layer of PAA, a covalently linked C60-PAA layer was obtained. The concentration of the PAA was optimized to 0.08 wt% for a C60-PAA ETL that displays good solvent resistance while maintaining reasonable electron mobility and electron-quenching ability. The optimized planar device yielded PCEs of 16.9% and 15.6% at reverse and forward scans, respectively. In maximum power output (MPP) efficiency measurements, the device yielded a power conversion efficiency (PCE) of 15.7%. The small measured capacitance and rapid current stabilized time reveals there is efficient charge extraction in the C60-PAA ETL, which may contribute to the observed reduction in hysteresis relative to cells with planar TiO2 ETLs. In chapter 4, I study the parameters that can influence the electron extraction in planar perovskite solar cells (P-PSCs) using spin-coated SnO2 and TiO2, anodized-TiO2, and bilayered electron transport layers (ETL) composed of SnO2 and TiO2 or SnO2 on a-TiO2 (SnO2@a-TiO2). These varied free energy difference (∆G) values between the ETL and perovskites, electron mobility (μe) of the ETL, and quality of physical contact between the ETL and fluorine-doped tin oxide (FTO). Among the various ETLs, the bilayered ETL (SnO2@a-TiO2) consisting of SnO2 (~30 nm) and a-TiO2 (10 nm) gives a large ∆G as well as defect-free physical contact. The resulting P-PSC employing the SnO2@a-TiO2 ETL exhibits a 21.1% J–V scan efficiency and a 20.2% maximum power point (MPP) efficiency with reduced hysteresis. This result emphasizes that a large free energy difference (∆G) value between the electron transport layer (ETL) and perovskites plays an important role in electron extraction. More importantly, the defect-free physical contact is also crucial for achieving improved electron extraction.