Tailoring the electrical-physical properties of colloidal quantum dot optoelectronics via material engineering

Abstract

Colloidal quantum dots (CQDs) are promising nanomaterials for optoelectronic applications owing to their unique electronic properties and facile processability. The lead sulfide (PbS) CQDs are mainly used as photoactive materials in CQD optoelectronics due to their infrared (IR) absorption capability with easily tunable absorption range and high absorption coefficient. Realizing the efficient CQD optoelectronic devices demand the harmonious development of CQD surface chemistry and device architecture. In particular, the development of CQD surface passivation strategies based on molecular lead halide (PbX2, X=I, Br) and 1,2 ethanedithiol (EDT) provided a foundation to achieve the recent air-stable and efficient CQD optoelectronoic devices. However, despite these recent advancements, there are several challenges remaining in CQD optoelectronics hinder further improvements. In the CQD photovoltaics, the Fermi level (EF) mismatch between the materials used devices causes undesirable energy band alignment for charge extraction. Additionally, the surface cracks of hole transporting CQD layer results from solid-state ligand exchange process induces interfacial traps by permitting the metal electrode penetration. Also, in the short wave infrared (SWIR) photodetector applications, the non-polar surface facet characteristics of CQDs which band gaps are within SWIR region makes them vulnerable from oxidation and aggregations resulting low device performances. In this study, these issues are addressed by diverse material engineerings. The band alignment and interface issues of CQD photovoltaics are tailored by developing the polycatechol functionalized MXene (PCA-MXene). The functionalization of MXene surfaces with polycatechol enables the homogeneous dispersion of MXene in diverse organic solvents that used in the fabrication of CQD photovoltaic process, resulting the effective combination of MXene and CQDs. This inducing the modification of work function of CQDs through the charge redistribution by surface dipole of MXene, ultimately promoting hole extraction in CQD photovoltaics. Moreover, the surface crack issue is addressed by the 2D nano structure of MXene. The PCA-MXene employed as an interlayer inhibits metal electrode penetration into photoactive layer by covers the surface cracks present in the hole transporting CQD layer. Owing to these advantages, the CQD photovoltaics incorporated with PCA-MXene achieve a power conversion efficiency (PCE) of 13.6%, accompanied by enhanced thermal stability, compared to the control device with a PCE of 12.8%. Also, by developing the proper SWIR CQD purification and surface manipulation processes by adjusting the alkali metal and halide ligands, the CQD photodetector external quantum efficiency (EQE) of 80% and responsivity of 1.0 (A/W) at 1550 nm wavelength is achieved. . Keywords: quantum dot, optoelectronic, Work function, interface, surface chemistry 유 형 렬. Hyung Ryul You. Tailoring the electrical-physical properties of colloidal quantum dot optoelectronics via material engineering. Department of Energy Science and Engineering. 2025. 47p. Advisors Prof. Jongmin Choi. Co-Advisors Dae-Hyun Nam. Ph.D./ES 201924011 List of Contents |양자점은 물질의 화학 조성 변화 없이 결정 크기변화를 통해 전자구조를 제어할 수 는 특징을 가지고, 저비용 공정이 가능한 물질로, 차세대 나노소재로 유망한 물질이다. 그중 황화납 (PbS) 양자점은 근적외선 및 단파 적외선 영역을 타겟으로 밴드갭 조절이 용이하며, 우수한 흡광 특성을 가져 다양한 양자점 광전소자로 응용 가능하다. 양자점 광전소자는 근 십수년 간 급격한 발전을 이룸에도 불구하고 효율 상승을 억제하는 페르미레벨 불일치, 계면 크랙, 불안정한 표면화학 문제가 존재한다. 본 논문에서는 신소재인 폴리카테콜 화 맥신 물질 (PCA-MXene) 을 양자점 태양전지에 도입하였다. 양자점 박막에 균일하게 도핑된 맥신 소재의 쌍극자는 양자점의 일함수 조절을 통해 정공전달에 유리한 에너지밴드를 형성 하게 한다. 또한 MXene 소재의 2D 구조 도입을 통해 정공전달층 크랙 사이로 금속 전극이 침투하는 문제를 해결하였다. 해당 전략을 통해 양자점 태양전지의 효율을 기존 12.8% 대비 13.6%로 향상시켰고, 맥신 표면의 소수성인 폴리카테콜에 의해 소자내 수분침투를 억제하여 열안정성 또한 30% 수준 향상시켰다. 또한 1550 nm 적외선 양자점 기반 포토디텍터 구현을 위해 양자점 분야에서 문제로 대두되던 표면화학 문제를 해결했다. 완전 불활성 기체 공정 도입을 통한 양자점 정제과정 제어 및 기존대비 증가된 Br 기반 리간드를 통해 1550 nm 파장대에서 응답도 1.0 A/W 및 양자효율 80%을 가지는 양자점 포토디텍터를 개발하였다.