DGIST Scholar: Color-Selective Organic Photodiodes: Junction Engineering

Department of Energy Science and Engineering Theses Ph.D.

Cited time in webofscience

Cited time in scopus

Color-Selective Organic Photodiodes: Junction Engineering

Title: Color-Selective Organic Photodiodes: Junction Engineering

Alternative Title: 색 선택성 유기 포토다이오드: 접합 공학

Author(s): Mingyun Kang

DGIST Authors: Mingyun Kang ; Youngu Lee ; Jongmin Choi

Advisor: 이윤구

Co-Advisor(s): Jongmin Choi

Issued Date: 2021

Awarded Date: 2021/02

Type: Thesis

Subject: organic photodiode, color-selective, junction engineering, polymer semiconductor, thin-film, reactive dedoping, chemical doping, photomultiplication

Abstract: Junction engineering methods, applied to color-selective organic photodiodes (OPDs), are demonstrated (1) to enhance the specific detectivity, (2) to improve the color selectivity, and (3) to boost the external quantum efficiency (EQE). (1) A facile and strategic junction tuning technology is reported to boost self-powered organic Schottky photodiode (OPD) performances by synergetic contributions of reactive dedoping effects. It is shown that dedoping poly(3-hexylthiophene-2,5-diyl) (P3HT) films with 1-propylamine (PA) solution significantly reduces not only acceptor-defect density but also intrinsic doping level, leading to dramatically enlarged depletion region width (DW) of metal/polymer Schottky junctions, as confirmed by ultraviolet photoelectron spectroscopy and Mott–Schottky junction analyses. As a result, whole penetration regions of photons corresponding to absorption bands of P3HT can be fully covered by the depletion region of Schottky junctions, even without the assistance of external electric fields. In addition, it is shown that non-solvent exposure effects of PA dedoping further enable lower paracrystalline disorder and, thus, higher charge carrier mobility, by means of two-dimensional grazing incidence X-ray diffraction (2D-GIXD), field-effect mobility, and space-charge-limited current analyses. As a result of such synergetic advantages of the PA dedoping method, non-power-driven green-selective OPDs were demonstrated with a high specific detectivity exceeding 6 × 10^12 Jones and a low noise-equivalent power of 5.05 × 10^(−14) W Hz^(−0.5). Together with a fast temporal response of 26.9 μs and a wide linear dynamic range of 201 dB, the possibility of realizing non-power-driven, near-ideal optimization of solution-processed OPDs with a facile dedoping method is demonstrated. (2) A precise and facile junction engineering of OPDs via chemical doping, while maintaining their thin-film nature to realize organic optical sensors with intrinsic ambient light cancellation, is demonstrated. It is shown that excitons with the desired wavelength range can be selectively separated by means of doping-induced fine engineering of the DW so that photons with unwanted wavelengths can avoid reaching the charge-separating depletion region. This method is different from previous narrowband detection strategies, in that only the DW, not the overall thickness of the active layer, is controlled to realize thin-film narrowband-selective OPDs. Optical absorption and exciton separation behaviors in each constituting layer of the OPDs are studied both theoretically and experimentally to explain the described mechanism of doping-induced spectral refining. Based on a carefully designed planar heterojunction architecture, thin-film (~500 nm) narrowband (FWHM < 60 nm) red-/NIRselective OPDs are realized without sacrificing a high specific detectivity over 10^12 Jones. To show the feasibility of the suggested doping-induced spectrally refined OPD, (i) an OPD-based optical communication platform with superior ambient noise-tolerance compared to Si-based platforms and (ii) - ii -Keywords: organic photodiode, color-selective, junction engineering, polymer semiconductor, thin-film, reactive dedoping, chemical doping, photomultiplication high performance ambient light pulse oximetry with two NIR-selective OPDs are demonstrated. (3) A new non-fullerene acceptor was desgined and synthesized by introducing thienylenevinylene (TV) groups into the conventional 2,2ʹ-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2ʹ,3ʹ-dʹ]-s-indaceno[1,2-b:5,6-bʹ]dithiophene-2,8-diyl]bis[methylidyne(3-oxo1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (ITIC) structure for its application in photomultiplication-type OPDs (PM-OPDs), which rely on acceptor molecules for both effective charge separation and efficient gain generation. The resulting TV–ITIC acceptor possesses not only extended π-conjugation length, which leads to lower energy bandgap as well as deeper lowest unoccupied molecular orbital level, but also enhanced hydrophobic characteristics, owing to the increased volumetric portion of the aliphatic chain, which improves the miscibility with the donor

Table Of Contents: Abstract ··························································································································i
List of contents·················································································································iv
List of tables···················································································································vii
List of figures ················································································································viii
Part 1. Color-Selective Organic Photodiodes ······················································1
I. Introduction ·················································································································· 2
II. Operational Mechanisms of OPDs······················································································· 4
2.1 Figures-of-Merit for OPDs ······················································································ 4
2.1.1 Dark Current and Noise Current ····································································· 4
2.1.2 External Quantum Efficiency (EQE) and Responsivity (R) ······································ 5
2.1.3 Noise-Equivalent Power (NEP) and Specific Detectivity (D*) ·································· 6
2.1.4 Photoactive Layer Thickness ········································································· 7
2.1.5 Full-Width at Half-Maximum (FWHM) ···························································· 9
2.1.6 −3 dB Cut-Off Frequency (f−3dB) ····································································10
2.1.7 Linear Dynamic Range (LDR) ······································································10
2.2 Realization of Color Selectivity ···············································································11
2.2.1 Bulk Heterojunction (BHJ) OPDs ··································································12
2.2.2 Schottky Junction OPDs ·············································································17
2.2.3 Optical Manipulation Methods······································································19
III. Conclusion ················································································································21
Part 2. Junction Engineering: Enhanced Specific Detectivity································22
I. Introduction ·················································································································23
II. Experimental Section ·····································································································25
2.1 Materials ··········································································································25
2.2 Device Fabrication ······························································································25
2.3 Thin Film Preparation and Characterization ·································································26
2.4 Device Characterization ························································································27
III. Results and Discussions ·································································································28
3.1 Optical and Junction Analyses ·················································································28
3.2 Microstructure Analyses ························································································31
3.3 Organic Photodiode Performances ············································································34
IV. Conclusion ················································································································39
Part 3. Junction Engineering: Improved Color Selectivity ···································40
I. Introduction ·················································································································41
II. Experimental Section ·····································································································44
2.1 Materials ··········································································································44
2.2 Photodiode Fabrication ·························································································44
2.3 Thin Film Preparation and Characterization ·································································45
2.4 Device Characterization ························································································45
2.5 Oximetry Calculation ···························································································46
III. Results and Discussions ·································································································48
3.1 Spectral Refining to NIR I (~780 nm) ········································································48
3.2 Spectral Refining to NIR II (~730 nm) ·······································································56
3.3 Spectral Refining to Red (~640 nm) ··········································································58
3.4 Noise-Tolerant Optical Sensors ················································································62
3.4.1 Optical Communication ··············································································62
3.4.2 Ambient Light Pulse Oximetry······································································63
IV. Conclusion ················································································································65
Part 4. Junction Engineering: Self-Amplification ··············································66
I. Introduction ·················································································································67
II. Experimental Section ·····································································································69
2.1 Materials ··········································································································69
2.2 Device Fabrication ······························································································69
2.3 Thin Film Preparation and Characterization ·································································70
2.4 Device Characterization ························································································71
III. Results and Discussions ·································································································72
3.1 Optical and Electrochemical Analyses········································································72
3.2 Microstructure Analyses ························································································74
3.3 PM-OPD Performances ·························································································75
IV. Conclusion ················································································································79
References ·······························································································80