1、有机电子学系列讲座 (2): 有机半导体的基础知识 Zhu Furong ( 朱福荣) Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University 2015 年7 月21 日, 吉林大学 2 Outline: 1. Introduction ( 基础科学和新兴技术) 2. Fundamentals ( 有机半导体的基础知识) 3. Transparent electrode ( 透明电极) 4. Organic light-emitting diodes ( 有机电致发光二极管)
2、 5. Organic photovoltaic devices ( 有机光伏) 6. Organic thin film transistors ( 有机薄膜晶体管) 7. Applications ( 有机光电器件的应用) 8. OLED application - Opportunities PCBM contributed very little for light absorption New low bandgap semiconductors and new n-type materials with strong light absorption are desirable P
3、CBM p-type n-type 29 30 N-type materials for OSCs O OCH 3 PCBM CN OC 8 H 17 H 17 C 8 O CN O n PCNEPV S S H 17 C 8 C 8 H 17 OCH 3 H 21 C 10 O CN CN n PF1CVTP Primarily limited to fullerene-based materials Low absorption in visible range Energy loss due to energy levels mismatch between donor and acce
4、ptor31 Efficiency potential of OSCs Ben Minnaert, et al. Prog. Photovolt: Res. Appl. 2007, 15,741748 S C 6 H 13 * * n P3HT PCBM Potential PCE = 15 20% Current highest PCE = 10% Glass ITO PEDOT:PSS P3HT PCBM Al -+ Glass ITO PEDOT:PSS P3HT PCBM Al -+ Inefficient utilization of the solar energy by curr
5、ent available materials. 500 1000 1500 2000 2500 3000 3500 0 2 4 6Electrons Out Photons In (x 10 -18 ) Wavenlength (nm) 500 1000 1500 2000 2500 3000 3500 0.00 0.05 0.10 0.15 0.20Power Out Power In Wavelength (nm) FF 0.7 V oc 0.65 V 14.3% 5 % P3HT PCBM Me O O S nLimitations in OSCs Possible approache
6、s: Low band gap photoactive materials Tandem cells Light harvesting (scattering, SP and waveguide) 32V DS V GS + _ + _ + + + + + + + + + + _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ + + + + Au P3HT SiO 2 n-type Si 2- 2th GS sat i DS V V L WC I For the case that (saturation region): GS DS V V G S D Field-effect
7、 mobility e n E v V d GS) ( + + + + OTFT measures single carrier mobility (either hole or electron) Organic thin film transistors (OTFTs) 33 34 Poly(3-hexylthiophene) (P3HT) 1996 (Bao et al., APL, 1996, 69, 4108) Very good solubility in organic solvents Well-ordered layers High OTFT-mobility 0.1 cm
8、2 /Vs S C 6 H 13 S C 6 H 13 S C 6 H 13 n tail head HT-HT coupling Orientation of P3HT plays a crucial role in determining OTFT mobility35 Hopping conduction: Electrons hop between the transporting (hopping) sites by thermal activation, L. D. Landau, J. Phys. (USSR) 3, 664 (1933). Charge transport in
9、 organic semiconductor nq Quantify the hopping rate “Conductivity” evaluation: Organic semiconductor is usually amorphous with a disordered structure Band conduction in organic semi- conductor is invalid Energy HOMO LUMO Distance Hopping conduction +ve -v e Mobility measurements Understand charge tr
10、ansport mechanism for materials design and device optimization Charge mobility ( ) is defined as the ratio of charge carrier velocity ( ) and electric field (E) Polymer Mobility Polythiophene (P3HT) 0.1 cm 2 /Vs (hole) 10 -3 cm 2 /Vs Poly fluorene 10 -4 cm 2 /Vs (hole) PPV and PPP derivatives 10 -7
11、-10 -3 cm 2 /Vs (hole) Chemical or structural defects Chemical purity Variations in film morphology Device response time Device efficiency Operational stability Fundamental understanding of charge transport mechanism Charge mobility = /E , cm 2 /Vs Charge mobility 3637 Space charge limited current (
12、SCLC) technique: I-V measurement Field effect transistor Time of flight (TOF) photoconductivity Photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) Measurement techniques 38 TOF photoconductivity measurement TOF experiment provides direct information on the microscopic origin
13、of charge transport in disordered materials. ITO transparent electrode Polymer film Metal electrode (Al/Au) Oscilloscope N 2 laser 337 nm Pulse width: 5 ns R V - + Variable Resistor =d 2 /U.t T =charge carrier mobility d=sample thickness U=applied voltage t T =transit time =d 2 /U.t T =charge carrie
14、r mobility d=sample thickness U=applied voltage t T =transit time39 Non-dispersive transport Time (s) Photocurrent t T Dispersive transport Indicates the excellent purity and chemical regularity of the polymer Indicates the presence of traps and structural disorder in the polymer Photocurrent Time (
15、s) The current stays constant while the carriers drift across the device Transit time can readily be obtained from the linear TOF transient signal No constant current plateau and current drops continuously Double logarithmic plot provides the transit time 40 Limitations of the TOF measurements Limit
16、ed to thick films (micron) Organic electronics, e.g., OLED requires less than 0.1 micron Q 0 CV (the amount of photo-charge should be smaller than the charge stored on the contacts) RC of the set-up transit time41 ) 0 ( 36 . 0 1 3 2 2 max 2 j j At d Oscilloscope Trig ITO Polymer film (Al/Au) Functio
17、n Generator Pulsed laser Pulse Generator R Variable Oscilloscope Oscilloscope Trig ITO Polymer film (Al/Au) Function Generator Pulsed laser Pulse Generator R Variable R Variable ) 0 ( 2 3 max 0 j t j t is the transit time A is the rate of voltage change is the conductivity of the film is the dielect
18、ric constant d is the film thickness Schematic of photo-CELIV method PhotoCELIV Phys. Rev. Letts., 84 4946 (2000). 42 0 5 10 15 20 25 0 1 2 3 4Current Density mA cm -2 Time s Voltage4.5V4V3.5V3V2.5V2V1.5V1V Transit Time 1V dark Figure (a) PhotoCELIV transients for pure P3HTfilm at various applied vo
19、ltages; (b) variation of charge mobility with applied electric field 80 90 100 110 120 130 140 -8.5 -8.0 -7.5 -7.0E 1/2V 1/2 cm -1/2 ln( ) cm 2 /Vs 4.5 x10 -4 cm 2 /Vs PhotoCELIV transients for pure P3HT film shows only the extraction of holes. Charge mobility in P3HT film43 Charge transport models
20、Pool-Frenkel model 1.Based on the carrier transport across the trapping barrier-generated by ionic impurities or by the traps generated by the twisting or bending of the main polymer chains. 2.The Coulomb potential is an effective pair potential that describes the interaction between two point charg
21、es. Where PF =(e 3 / 0 ) 1/2 E = electric field, e=electronic charge =dielectric constant 0 =permittivity of free space l n ( ) E 1/2 carrier transport under a uniform field and isotropic media The mobility of PAT12 and PAT18 against E 1/2 based on pool-frenkel model Jpn.J.Appl.Phys., vol.39 (2000),
22、6309 ) / exp( 2 / 1 0 kT E PF Gaussian disorder (Bassler) model, GDM 22 21 / 2 2 exp exp , 3 GDM CE o r kT kT H. Bassler, Phys.Stat.Sol.175, (1993) 15 the high temperature limit of the mobility the energetic disorder parameter the positional disorder parameter C an empirical constant HOMO LUMO Hoppi
23、ng process The charge transport in conjugated polymers originates from hopping between localized states induced by the Disorder The localized states are distributed (Gaussian) due to positional and energetic disorders in the polymer chain Reduction in width of manifold results in an increase of the
24、mobility X Long range spatial correlation is neglected 2 1 2 2 exp exp 3 GDM E kT 45 Field & temp dependent mobility = width of Gaussian DOS Temperature dependence The energetic disorder parameter can be calculated from the slope of the temperature dependence The positional disorder parameter can be
25、 obtained by plotting against ( /kT) 2 2 1 2 2 0 2 0 2 0 2 exp exp 3 0, 2 exp 3 2 ln ln 3 1000 ln ( ) GDM E kT when E kT kT plot vs T Zero-field mobility calculate zero-field mobility 46 Correlated Gaussian disorder model eaE kT kT CDM 2 78 . 0 exp 5 3 exp 2 / 3 2 S.V Novikov,et al, Phys. Rev.B, 81 (1998) 4472. Spatial correlations in site energy due to long range charge dipole interaction ln( ) E 1/2 can be extended to lower electric fields Intersite distance a can be estimated
