슬라이드 1

Transcription

1 Chapter 8. Optoelectronic Devices ( 광전자소자 )

2 목차 8.1 광다이오드태양전지 8.2 발광다이오드 OLED 8.3 레이저 8.4 반도체레이저 Chap. 8. Optoelectronic Devices

3 8.1 광다이오드 반도체시료는광학적생성률에비례하는전도도의변화를줌으로써광전도체 (photoconductor) 로사용가능. 광학적또는고에너지방사검출기의응답속도와감도를개선하기위하여접합형소자를사용하여광자흡수에응답하도록설계된단일접합형광다이오드소자 (photo diode) EHP의광학적생성에대한 p-n접합의응답에대한광다이오드검출기 (photo detector) 구조흡수된광에너지를유용한전력으로바꾸어주는태양전지 (solar cell) Chap. 8. Optoelectronic Devices

4 8.1.1 조사된접합에서의전류와전압 광전류 (photo current) 공핍영역 W 내에서생성된캐리어들은접합전계에의해분리되어, 전자는 n 형영역에서정공은 p 형영역에서집속 접합양쪽의확산거리내에서열적으로생성된소수캐리어들은공핍영역으 로확산되어, 전계에의해다른쪽으로쓸려가게됨 접합에의해광학적으로생성된이들캐리어의집속으로인해생긴전류 (I op ) I op I qag op L p L I Lp Ln I qa pn n p n Chap. 8. Optoelectronic Devices I th e qv/ kt 1 op p n W A : 단면적, W : 공핍영역폭 g op : carrier 생성률 L p : n 형쪽전이영역의정공확산거리 L n : p 형쪽전이영역의전자확산거리 열적으로생성된전류를 I th 라하면, 광학적생성을더하여광조사에따른총 역방향전류를구할수있음. qv/ kt e 1 qag L L W op p n

5 8.1.1 조사된접합에서의전류와전압 p n Fig. 8-1 p-n 접합에서의광학적캐리어생성 : (a) 소자에의한빛의흡수 ; (b) n 형접합의확산거리내에서의 EHP 생성으로부터생기는전류 I op ; (c) 조사된접합의 I-V 특성 Chap. 8. Optoelectronic Devices

6 8.1.1 조사된접합에서의전류와전압 소자의양단을개방시켰을때의전압 Chap. 8. Optoelectronic Devices V oc kt q kt ln I q ln op / I th 1 L p L n W g 1 op L / / p p pn Ln n np 대칭적접합의특수한경우 (p n =n p, τ p = τ n ), W내에서의재결합을무시하면, kt gop g op g th 인경우 Voc ln q g g th = p n /τ n 은평형 (equilibrium) 상태에서의열적생성 - 재결합률을나타냄. 소수캐리어농도가 EHP 의광학적생성으로써증가함에따라, 수명 τ n 은짧아지 고 p n /τ n 은커짐. V oc 에대한한계는평형상태에서의접촉전위차 V 0. 광기전력효과 (photovoltaic effect) 조사된접합을가로질러서순방향전압이나타나는현상 th

7 8.1.1 조사된접합에서의전류와전압 Fig. 8-2 접합의개방회로전압에주는빛조사의영향 : (a) 평형상태에서의접합 ; (b) 빛조사시의전압 V oc 의출현 ( 역방향바이어스 ) ( 제 1 사분면 ) ( 제 3 사분면 ) ( 제 4 사분면 ) Fig. 8-3 I-V 특성곡선의여러사분면에서의빛이조사된접합의동작 : (a) 와 (b) 에서전력은외부회로에의해이소자로공급된다 ; (c) 에서는이소자가부하에전력을공급한다. Chap. 8. Optoelectronic Devices

8 태양전지 태양전지의원리 태양전지의필요성 태양전지의활용

9 태양전지란? 태양전지란무엇인가 태양광을받아서전력을발생시키는장치 기초과학 응용공학 태양전지가빛을받으면전류가흐르게된다 Introduction

10 태양전지가중요한이유 태양전지의중요성 화석연료의소모량이급증하고그매장량이한계에치닫게됨에따라새로운대체에너지의필요성이급증. 청정에너지이면서고갈되지않는에너지원인태양에너지를사용한태양전지에대한관심이고조. 화석연료소모량급증 대체에너지필요 태양에너지이용 Introduction

11 태양전지의활용 인공위성에전력공급을위핚태양전지이용 Introduction 태양전지를이용핚가정용발전기 전력공급이되지않는지역의태양전지를이용핚가정용발전기

12 태양전지의활용 Introduction 화성에서태양전지소자에서공급되는전력을사용하여작동하는탐지기

13 8.1.2 태양전지 화성탐사로봇패스파인더 태양전지의구조 Chap. 8. Optoelectronic Devices 태양전지패널

14 8.1.2 태양전지 플렉서블유기태양전지의응용분야 지난 2년여동안신규재료의개발과함께최근 8% 안팎의에너지변환효율을보여주고있으며 2015년안에 10% 대의소자가구현될수있다는예측이지배적이다. Chap. 8. Optoelectronic Devices

15 8.1.2 태양전지 태양전지의설계 얻을수있는광학적에너지의최대량을이용하기위해서는소자의표면가까이큰면적의접합을갖는태양전지를설계할필요가있다. 이평면형접합은확산이나이온주입으로써형성시키며, 표면에는반사를감소시키고표면재결합을적게하기위해적당한물질을도포한다. ( 금속접촉부 ) ( 무반사코팅 ) Fig. 8-5 태양전지의구성 : (a) 평면접합의확대도 ; (b) 손가락 모양의금속접촉부를보여주는평면도 Chap. 8. Optoelectronic Devices

16 채움지수 (fill factor) I m V m /I sc V oc 지상에서의태양전지응용 Chap. 8. Optoelectronic Devices 태양전지 태양전지의응용은외기권에한정되는것은아님. Fig. 8-6 빛이조사된태양전지의 I-V 특성. 최대전력직사각형이빗금그어져있다. 태양의세기가대기에의해감소된다고하더라도, 지상에서태양전지를사용하는응용에서는태양으로부터유용하게전력을얻을수있음. 비용절감과크기의축소 현재화석연료로전력을생산하는데는 KWh 당불과 3 센트밖에들지않지만, 비정질 Si 태양전지의경우는이의약 10 배가들고투자회수에는약 4 년이걸림. 효율향상을위한연구필요.

17 태양전지 Basic principle of solar cell Chap. 8. Optoelectronic Devices

18 태양전지 LED vs Photovoltaic Light-emitting diode (LED) Converts electrical input to light output: electron in photon out Light source with long life, low power, compact design. Applications: traffic and pp car lights, large displays, solid-state lighting. Photovoltaic (PV) Converts light input to electrical output: photon in electron out (generated electrons are swept away by E field of p-n junction). Renewable energy source. Chap. 8. Optoelectronic Devices

19 태양전지 Energy Band of p-n Junction Solar Cell Dark Flux & 0 V Short Circuit Current (Jsc) Open Circuit Voltage (Voc) Chap. 8. Optoelectronic Devices Maximum Power Point (Pmax)

20 태양전지 J-V Curve The photodiode is usually operated in reverse bias mode. Chap. 8. Optoelectronic Devices

21 태양전지 Equivalent circuit Equivalent circuit of a "real" solar cell showing both a shunt and series resistive loss. The series resistance, Rs, is a series loss due primarily to the ohmic loss in the surface of the solar cell. The shunt resistance, Rsh, is used to model leakage currents. A shunt resistance of a few hundred ohms does not reduce the output power of the solar cell appreciably. A series resistance of only 5 Ω can reduce output power by 30%. Chap. 8. Optoelectronic Devices

22 태양전지 Short circuit current (J sc ) & open circuit voltage (V oc ) J sat qv kt e J gen J 1 The short circuit current is found by setting V = 0 J SC J J sat qv( 0) kt e 1 J gen J gen J gen The open circuit voltage is found by setting J = 0 J J sat qv kt e 1 J 0 gen V OC kt J gen ln 1 q J sat by using the fact that J SC = J gen, J V OC V kt J sc V OC ln for J SC J sat q J sat J SC Chap. 8. Optoelectronic Devices

23 태양전지 Fill factor (FF) neglecting the effects of Rsh and Rs (Rsh=, Rs=0) FF P J V max sc oc J max pwr J sc V V max pwr oc J J m sc V V m oc The fill factor (FF), is thus defined as (VmJm)/(VocJsc), where Jm and Vm represent the current density and voltage at the maximum power point, this point being obtained by varying the resistance in the circuit until JxV is at its greatest value. The ratio (given as percent) of the actual maximum obtainable power, (Vmp x Jmp) to the theoretical (not actually obtainable) power, (Jsc x Voc) FF is a key parameter in evaluating the performance of solar cells. Chap. 8. Optoelectronic Devices

24 태양전지 Quantum efficiency IPCE (Incident Photon to electron Conversion Efficiency) EQE (External Quantum Efficiency) Short circuit 상태에서전체 photon이 electron으로변환되는효율 IQE (Internal Quantum Efficiency) Short circuit 상태에서흡수된 photon이 electron으로변환되는효율 PCE (Power Conversion Efficiency) or ECE (Energy Conversion Efficiency) Maximum FF Chap. 8. Optoelectronic Devices

25 Chap. 8. Optoelectronic Devices 태양전지

26 태양전지 Solar radiation Power reaching earth: 1.37 kw/m2 Chap. 8. Optoelectronic Devices

27 태양전지 Air mass coefficient Air mass (AM) coefficient characterizes the solar spectrum after the solar radiation has travelled through the atmosphere. AM1.5 is almost universal when characterizing power-generating panels. Ozone, water, nitrogen, oxygen, carbon dioxide, absorb certain wavelength. solar radiation ~ black body radiator at 5,800 K earth At the earth surface, the spectrum is strongly confined between the far infrared and near ultraviolet. Chap. 8. Optoelectronic Devices

28 태양전지 AM coefficient l0 l AM coefficien t cos 1 cos l l 0 l0 l l0 : thickness of the atmosphere earth l : path length through the atmosphere for solar radiation incident at angle θ AM0: the spectrum outside the atmosphere (5800 K black body) AM1: the spectrum to sea level with the sun directly overhead (θ ~ 0o) AM1.5 : almost universally used to characterize terrestrial solar panels (θ ~ 48.19o) Chap. 8. Optoelectronic Devices

29 태양전지 Third-generation photovoltaics 1st generation - c-si solar cells 2nd generation - thin film solar cells 3rd generation - high efficient, low cost, ecofriendly solar cells (DSSC, organic and nano solar cells) Chap. 8. Optoelectronic Devices

30 Crystalline Si solar cell 태양전지 Chap. 8. Optoelectronic Devices

31 태양전지 PERL Si solar cell Chap. 8. Optoelectronic Devices

32 Rear contact Si solar cell 태양전지 Chap. 8. Optoelectronic Devices

33 태양전지 Edge-defined film-fed growth Chap. 8. Optoelectronic Devices

34 a-si:h/c-si HIT Si solar cell 태양전지 Chap. 8. Optoelectronic Devices

35 태양전지 Thin film solar cell Chap. 8. Optoelectronic Devices

36 a-si thin film solar cell 태양전지 Chap. 8. Optoelectronic Devices

37 태양전지 Compound semiconductor thin film solar cell Chap. 8. Optoelectronic Devices

38 태양전지 High-efficiency: Tandem structures Chap. 8. Optoelectronic Devices

39 태양전지 Low-cost process: Roll-to-Roll CIGS thin film solar cell Chap. 8. Optoelectronic Devices

40 태양전지 DSSC ( Dye-Sensitized Solar Cell ) 햇빛을받으면전자를방출하는특정 염료와전해질을이용해전기를만듦. 염료감응전지를구성하는물질 : 태양광흡수용고분자 ( 염료분자 ) 넓은밴드갭을갖는반도체산화물 (N 형반도체역할, TiO2) 전해질 (P 형반도체역할 ) 촉매용상대전극 ( 양극 / 백금, 탄소나노튜브등 ) 태양광투과용투명전극 ( 음극 ) Chap. 8. Optoelectronic Devices

41 태양전지 DSSC ( Dye-Sensitized Solar Cell ) DSSC 염료가갖추어야할조건염료 감응전지를구성하는물질 : 가시광선전영역의빛을흡수 나노산화물표면과견고한화학결합 열및광화학적안정성 염료의 LUMO 가나노산화물의전도대 E 보다높아야함 현재가장효율이좋은염료분자 루테늄계유기금속화합물 Chap. 8. Optoelectronic Devices

42 태양전지 DSSC ( Dye-Sensitized Solar Cell ) DSSC 동작원리 태양빛 ( 가시광선 ) 이흡수되면염료분자는전자 - 정공쌍을생성하며, 전자는반도체산화물의전도대로주입 반도체산화물전극으로주입된전자는나노입자간계면을통하여투명전도성막으로전달되어전류를발생 염료분자에생성된홀은산화 - 환원전해질에의해전자를받아다시환원되어염료감응태양전지작동과정이완성 Chap. 8. Optoelectronic Devices

43 태양전지 DSSC ( Dye-Sensitized Solar Cell ) DSSC 작동메커니즘 D + light D* D2*+ TiO2 e-(tio2) + D+: 전자주입 e-(tio2) + C.E. TiO2+ e- (C.E.) + 전기에너지 D++ 3/2 I- D + ½I3- ½I3-+ e-(c.e.) 3/2 I-+ C.E. D: 염료 (dye) 분자 C.E.: 상대전극 (counter electrode) Chap. 8. Optoelectronic Devices

44 태양전지 DSSC ( Dye-Sensitized Solar Cell ) DSSC의단점 DSSC의최고변환효율 : 약 11% ( 실험적 ), 약 8% ( 상용화 ) 대면적화시효율감소 120도이상고온에서효율이급격하락 ( 유기물질을사용하는경우빛과열에불안정 ) DSSC의장점저가의제조설비및공정기술 발전단가를실리콘계의 1/5까지가능플렉서블, 투명, 다양한색상구현이가능 다양한응용성이기대됨투명제작으로 2~3장을겹치는다중제작이가능 같은면적에서효율을 2~3배늘릴수있는특징빛의조사각도가 10o로좁아도전기생산이가능 흐린날씨작동가능 Chap. 8. Optoelectronic Devices

45 태양전지 DSSC 제조과정 Chap. 8. Optoelectronic Devices

46 Organic & hybrid solar cells 태양전지 Chap. 8. Optoelectronic Devices

47 태양전지 Polymer-Fullerene solar cells TiOx layer connects the front cell and the back cell. Power conversion efficiency was 6% at illuminations of 200 mw/cm2. Chap. 8. Optoelectronic Devices

48 Overview Drawback with conventional solar cells Third generation solar cell concepts Implementing down conversion concepts in third generation solar cell concepts Third generation solar cell research at HYN

49 Sunlight Sunlight consists of a large number of photons distributed across a large wavelength and energy range The photon energy depends on the photon wavelength in the following manner Ephot= hc/λ The variation in photon energy makes efficient utilization of all sun light in one solar cell difficult. Conventional solar cells are made from one material exhibiting only electronic band gap Band gap energy Eg This makes it possible for such a solar cell to only utilize one photon energy optimally.

50 Drawback of conventional solar cell Conventional solar cells are made from one material exhibiting only electronic band gap Band gap energy Eg This makes it possible for such a solar cell to only utilize one ph oton energy optimally. A photon with sufficient energy can excite an electron across the b and gap creating a mobile electron and a mobile hole. If the photogenerated electron and hole is collected at the exter nal solar cell terminals, it will contribute to the current from the solar cell.

51 Drawback in conventional solar cell The voltage related to this process is determined by the energy difference at which the electron and hole can be extracted at the respective external terminals. Only photons with sufficient energy can excite e-across the band g ap Eg Insufficiently energetic photons with Ephot < Eg will not contribute to the photocurrent generation Photons with Ephot > Eg will initially generate energetic excited charge carriers Any energy in excess of Eg will be wasted heating up the solar cell through thermalization

52 Solar Spectrum

53 Consequences Optimum one sun Eg = 0.76 ev Conservation of energy dictates that a photon must have a mini mum energy Eph 2Eg in order to cause carrier multiplication Large current densities can be obtained by selecting a material with a low band gap energy Due to thermalization, the band gap puts an efficient upper limit to the extractable voltage Si bandgap:1.12 ev Good open circuit voltage:650 mv

54 The Shockley-Queisser limit The Shockley-Queisser limit is a measure of the upper obtai nable efficiency of a perfect solar cell based on only one sol ar cell material with only one electronic band gap. Perfect solar cell All photons incident on cell captured Complete absorption of all photons with E > Eg Complete thermalization occurs Loss less transport and collection of charge carriers Ideal materials: Only Auger or radiative recombination The efficiency limit of a perfect, conventional Si solar cel l is ~32%.

55 Third generation solar cell concepts

56 Third Generation Solar Cells Solar cells which use concepts that allow for a more efficient utilization of the sunlight than first generation and second generation solar cells Main approaches Modification of the photonic energy distribution prior to absorption in a solar cell Utilization of materials or cell structures incorporating several band gaps Reducing losses due to thermalisation Biggest challenge is reducing the cost/watt of delivered solar electricity Third generation solar cells pursue More efficiency More abundant materials Non-toxic material Durability

57 Efficiency and Cost Projections Third Generation Second Generation First Generation ARC Photovoltaics center of Excellence, University of New Soth Wales, Annual Report (2007)

58 Modifying the photon energy distribution Photon energy down-conversion One energetic photon creates two or more less energetic ones Nanocrystals/ Quantum dots Photon energy up-conversion Two or more low energetic photon s create one suffieciently energetic one Nanocrystals/quantum dots Phosphors Thermophotovoltaics The sunlight heats a thermally radiatin g object which again powers a suitabl e solar cell

59 c-si Solar cell operation

60 Loss mechanism of Photovoltaics (single p/n junction) Energy hν p- type e - heat loss e n- type η max = 32% 3 usable photovoltage (qv) Loss mechanisms 1. Thermalization loss 2. Junction voltage drop 3. Contact voltage drop 4. Recombination loss 5. Non absorbed photons heat loss 1 1 e - - h + pair/photon

61 Factors affecting the energy conversion efficiency of solar cell Thermodynamic efficiency li mit Quantum efficiency Maximum power point Fill factor I Dark and light I-V curves 0 P m =I m V m dark light V V open-circuit V open-circuit I short-circuit Maximum power P m Fill factor (squareness) FF=P m /(V open-circuit I short-circuit ) I short-circuit Increasing the R sh and decreasing R s = High FF give high

62 Motivation exist to increase the efficiency limit of solar cell Solar cell powered by Sun's unconcentrated black b ody radiation has theoretical maximum efficiency is 43% Solar cell powered by the Sun's full concentrated ra diation, the efficiency limit is up to 85%. Possibility of solar cell to attain high values of effici encies by utilizing Radiative recombination Carrier multiplication.

63 Proposed Device structure QD Metal contact Al

64 Concept of proposed work Introducing Core shell Quantum Dots (QDs) layers in p-n junction c -Si solar cell QD extending the band gap of solar cells for harvesting more of the light in the solar spectrum generating more charges from a single photon. Down conversion Harvesting full range solar energy by utilizing narrow band gap core and wide band gap shell material Core shell QD materials harvest UV and IR waves Using down conversion process, higher energy excitons are ex tracted out from the core shell QDs Device efficiency depends on the down conversation process

65 Implementing down conversion concepts in third generation solar cell concepts

66 Down Conversion Objective: transforming small wavelength photons into large wavelength photons Suitable materials must efficiently absorb high energy photons and reemit more than one photon with sufficient energies can be implemented by quantum dots and quantum wells Down conversion evidence: Multiple exciton generation E g 2 E g E gap E gap E g

67 Need for down conversion? The bandgap of a Si cell is close to the optimum bandgap for a Down Converter materials (eg: Quantum Dots) Modifying the front surface of a PV cell can increase carriers generated near the surface High energy (blue) light is absorbed very close to the surface, considerable recombination at the front surface will affect the blue portion of the quantum efficiency The Down converter (DC) is placed in front of a standard cell and can boost current by converting ultraviolet (UV) photons to a larger number of visible photons. DC does require that more visible photons are emitted than highenergy photons absorbed, i.e. its Quantum efficiency (QE) must be greater than unity

68 Why do we need QDs? Quantum dots can have size-tunable emission wavelength (λ = 400 n m to 620 nm) covering whole visible spectrum wavelength at which t hey will absorb or emit radiation can be adjusted larger the size, the l onger the wavelength of light absorbed and emitted greater the band gap of a solar cell semiconductor, the more energetic the photons ab sorbed, and the greater the output voltage. Lower band gap results in the capture of more photons including tho se in the red end of the solar spectrum resulting in a higher output of current but at a lower output voltage An optimum band gap that corresponds to the highest possible solarelectric energy conversion and this can also be achieved by using a mixture of quantum dots of different sizes for harvesting the maximu m proportion of the incident light.

69 Significance of QDs Quantum dots in three-dimensional array will have strong electronic coupling between them and excito ns will have a longer life Will facilitating the collection and transport of hot carriers to generate electricity at high voltage Additionally such an array makes it possible to generate multi ple excitons from the absorption of a single photon Will increase the operation of a photovoltaic(pv) cell requires 3 basic attributes 1. The absorption of light, generating either electron-hole p airs or excitons 2. The separation of various types of charge carriers 3. The separate extraction of those carriers to an external circuit

70 Advantage of Core Shell QD Over coating of a CdSe core with higher band gap compounds (e. g. ZnS) Nonradioactive transition is minimized More excitons are available Significantly enhances the quantum yield of luminescence Shell can alter the charge, functionality and reactivity of the surface Increases the absorption by reducing reflection, increasing absor ption process or increased light trapping Use quantum confinement to change or optimize band gap Practical improvements such as increased radiation resistance Increased open circuit voltage, e.g., Photon recycling

71 How Can Quantum Dots improve the device efficiency? Quantum dots can generate multiple exciton (electron-hole pairs) after collision with one photon.

72 How QD can increase the efficiency of c-si solar cell? A strong indication of QD absorption and down-conversion process Most of electron hole pairs generated in this UV regime are locat ed near surface of device The surface defects consume most of the photo-generated carri ers, which lead to inferior cell efficiency in UV wavelength range Addition of QDs layers on the c-si Nanostructured array solar cell c an produce photon down-conversion effect QD-originated photons with visible wavelengths can be absor bed in the depletion region and the power conversion efficien cy is improved. The incident photons wavelength is longer than 425nm, the Q Ds layer is served more like an AR coating which can also en hance the light harvesting. Proc. of SPIE Vol. 8256

73 Third generation solar cell research at HYN

74 Energy [W/(m.nm)] Solar spectrum W/O 595nm 365 nm 600nm 254 nm 602nm 0. 4 W/O 578nm 365 nm 254 nm 563nm 565nm UV-light source nm nm Wavelength (nm)

75 CIE 색좌표 x : y : x : y : x : y : x : y : x : y : x : y : nm 563nm 254 nm 565nm W/O 578nm W/O 595nm 254 nm 602nm 365 nm 600nm

76 Band Gap eV ZnTe 3.6eV 2.8eV ZnSe 6.7eV 3.4eV ZnS 7.2eV 4.5eV CdSe 6.3eV 3.6eV CdS 6.6eV 3.35eV CdTe 5.04eV 4.1eV PbS 5.0eV 4.2eV PbSe 4.93V 4.3eV GaAs 5.73eV 3.6eV GaN 7.0eV Conduction band Valence band 3.48eV InP 4.82eV 0.72eV InAs 1.07eV 0.5eV GaSb 1.2eV 0.35eV InSb 0.53eV 1.06eV AlAs 3.22eV 1.35eV AlSb 2.95eV 1.35eV SiC 2.95eV 2.2eV Si 3.31eV 0.79eV Ge 1.46eV eV P3H T 5.2eV 3.6eV PCPD TBT 5.5eV

77 8.1.3 광검출기 광다이오드의동작 광다이오드를다음의사분면에서동작시킬때, 그전류는본질적으로전압에는무관하나광학적생성률에는비례함. 광학적검출의응용에서는검출기의응답속도가결정적임. 예를들어광다이오드가 1ns 떨어져있는광펄스 (light pulse) 에감응한다면, 광학적으로생성된소수캐리어는 1ns보다훨씬적은시간에접합으로확산하고반대쪽으로넘어서쓸려가야함. 공핍영역의폭 W는충분히커서대부분의광자가중성인 p와 n형영역에서보다는 W내에서흡수되는것이바람직함. 캐리어가주로공핍층 W내에서생성될때공핍층형광다이오드 (depletion layer photodiode) 라함. Chap. 8. Optoelectronic Devices

78 8.1.3 광검출기 p-i-n 광검출기 (p-i-n photodetector) Fig. 8-7 p-i-n 광다이오드의개요도 공핍영역의폭을제어하는편리한방법중하나 i 영역은저항률이높기만하면참된진성반도 체일필요는없음. 이것은 n 형기판위에에피택셜방식을성장시킬 수있으며, p 형영역은확산으로써만들수있다. 이소자에역방향으로바이어스를가해주면인가전압은전부 i 영역을가로질러 서나타난다. i 영역내의캐리어수명이표동시간에비하여길다면, 광학적으로생성된캐리어 의대부분은 n 및 p 영역에모아질것이다. 광검출기를평가하는중요한지수중하나는, 검출기에입사되는광자하나당생 성되는캐리어의수로표현되는외부양자효율이다. Chap. 8. Optoelectronic Devices Q J P op op / q / h J op : 광전류밀도 P op : 입사된광전력

79 8.1.3 광검출기 애벌랜치광다이오드 (Avalanche Photodiode; APD) 빠른속도와내부이득 (internal gain) 때문에광섬유통신시스템에서유용 Fig. 8-8 광다이오드동작을향상시키기위해사용한다층이종접합 (multilayer heterojunction): (a) 좁은간극물질에서넓은간극물질을통과한 1.55μm 근처의빛을흡수하는애벌랜치광다이오드 : 정공은애벌랜치증식이일어나는 InAlAs 접합쪽으로쓸려간다. i 영역은저농도로도핑되어있다. (b) 광전류, 암전류, 이득은애번랜치증식때문에바이어스의함수로증가한다. (c) SACM APD 등에서볼수있는전형적인이득-대역폭특성 Chap. 8. Optoelectronic Devices

80 8.1.4 광검출기에서의이득, 대역폭및신호대잡음비 광통신시스템에서는광검출기의감도와응답시간 ( 대역폭 ) 이특히중요 이득을키우려고설계하면대역폭이작아지고그반대도성립함. 이득-대역폭곱 (gain-bandwidth product) 을광검출기에서특성지수로쓰는경우가많음. 검출기에서중요한또하나는신호대잡음비 (signal-to-noise ratio) 검출기에서사용할수있는정보의양과배경잡음의비잡음전류는온도와물질의암전도도에따라증가 광검출기의비교 p-i-n 다이오드 : 광전도기에비하여암전류가적고암저항은훨씬큼. 애벌랜치광다이오드 : 애벌랜치증식효과로광이득의장점이있지만, 잡음이 p-i-n에비하여증가하는단점이있음. 광전도기 : 여러가지잡음원의영향. 잡음등가전력 (noise-equivalent-power; NEP) 으로정량화광검출기의검출도 (detectivity) : D =1/NEP Chap. 8. Optoelectronic Devices

81 8.1.4 광검출기에서의이득, 대역폭및신호대잡음비 NEP 는광전도기의대역폭과면적에도관계 특이성검출도 (specific detectivity) D* 는단위면적과 1Hz의대역폭을갖는광검출기에대하여정의대역폭요구조건내에서가장큰 D* 를갖는광검출기가유리 광도파로 (waveguide) 구조 : 높은감도와대역폭을동시에성취 빛은전류의수송방향과수직방향으로광다이오드에입사 Fig. 8-9 광도파로형광다이오드. 광자는협에너지대역 InGaAs 영역 A 에서강하게흡수되고캐리어는영역 M 에서애벌랜치과정에의하여증식된다. 전하영역 C 는 A 와 M 사이의전계분포를최적화시키기위한것이다. Chap. 8. Optoelectronic Devices

82 8.2 발광다이오드 캐리어가순방향으로바이어스된접합을가로질러서주입될때, 그전류는보통전이영역에서와접합부근의중성영역에서는재결합에의한것으로간주. Si 나 Ge 와같은간접형재결합반도체 : 격자에열을방출. 직접적인재결합반도체 : 순방향으로바이어스된접합으로부터상당한빛이방출. 주입형전계발광이라하는이효과는광발생소자로서의다이오드의중요한응용을이룸 E c p-type V = 0 V < 0 V > 0 p-type p-type n-type n-type n-type E g E v hν Chap. 8. Optoelectronic Devices

83 8.2 발광다이오드 반도체레이저 (semiconductor laser) 순방향으로바이어스된 p-n 접합에서의발광재결합을이용 LED보다훨씬좁은파장대역에서간섭성이있고방향성이큰빛을방출 광섬유통신시스템에서유용 LED의파장과광효율파장 ( 또는주파수 ) h 1.24 Eg ( ev ) h, ( m) ( m) 외부광자효율 (external quantum efficiency) ext ( 내부복사효율) ( 방출효율) 1.24 E ( ev ) g LED가평탄형표면을갖고있다면, 반도체-외부계면에도달한광자중임계각보다큰각도를갖는광자는전반사되어궁극적으로는반도체내에서흡수에의해소멸됨. 이때문에전형적인 LED는돔모양으로캡슐화하여외부광효율을증대 Chap. 8. Optoelectronic Devices

84 8.2 발광다이오드 Fig LED 조명강도 (luminous intensity) 의시간적향상 Chap. 8. Optoelectronic Devices

85 8.2.1 LED 재료 가시광선과적외선파장을내는반도체레이저와 LED를요구하는넓은적용분야를볼때, 사용가능한많은종류의 Ⅲ- Ⅴ 물질들은대단히유용. AlGaAs와 GaAsP 시스템에덧붙여, InAlGaP 시스템은적색, 황색및오렌지색을, AlGaInN은청색과녹색을강하게냄. 많은응용에있어서 LED의발광이사람눈에꼭보일필요가있는것은아님. Fig GaAs 1-x P x 에서합금조성비의함수로주어진전도대역에너지 GaAs, InP 및이들화합물의혼합된합금 과같은적외선방출체는특히광섬유통 신시슽템이나 TV 리모콘에잘어울린다. Chap. 8. Optoelectronic Devices

86 8.2.2 광섬유통신 광섬유 (optical fiber) 광원과검출기사이의광학적신호의전송은광원과검출기사이에광섬유를놓음으로써크게증대시킬수있다. 광도관 (light pipe) 광섬유의한형태는비교적순수한용융실리카 ( 무수규산 : SiO 2 ) 의외층에, 보다큰굴절률을갖는도핑유리의중심체가들어있는것이다. 계단형굴절률의섬유는그표면에서의손실이거의없이주로중앙의핵심부에광속이유지빛은굴절률이급변하는계단부분에서의내부반사로인해섬유의길이방향으로빛이전송 Fig 다중모드섬유의두가지예 : (a) 약간큰굴절률 n 을갖는중심체가있는계단형굴절률의경우 ; (b) 중심체의 n 이포물선형으로경사진경사형굴절률의경우. 그림에서는섬유의단면 ( 왼쪽 ), 굴절률분포 ( 중앙 ) 및대표적인동작양식의모양 ( 오른쪽 ) 을보이고있다. Chap. 8. Optoelectronic Devices

87 광손실 (optical loss) 섬유의길이방향으로거리 x 에서의신호의 세기 I( x) 레일리산란 (Rayleigh scattering) 파장의증가에따른흡수의전체적인감소는파장과비교할수있는크기의작은임의 의불균일성이굴절률의변동을초래 적외선흡수 광섬유통신 I 0 e x 유리를이루는원소의진동여기 (vibrational excitation) 에기인 펄스분산 (pulse dispersion) 데이터펄스가섬유를따라전파되어가면 서퍼지게되는현상 굴절률의주파수의존성으로생길수있음. Fig 응용실리카광섬유에대한감쇠상수 α 대파장 λ 의대표적관계도. 피크는주로 OH - 불순물에의한것으로개선된섬유제조로써감소시킬수있다. Chap. 8. Optoelectronic Devices

88 8.2.2 광섬유통신 레이저광원 광원으로서레이저를사용하는것은기본적으로단일주파수로이루어져있고매우큰정보대역폭을갖기때문. 초기광전자시스템에서는레이저나 LED를제작하기위해이미잘발달된 GaAs-AlGaAs 시스템을사용최근시스템들은감쇠최소점 1.3 또는 1.55μm 부근에서동작 InP 에성장할수있는 InGaAs 나 InGaAsP 를이용하여제작가능 다중모드 (multi-mode) 의섬유는단일모드 (single-mode) 의섬유보다크나이것도간섭성의레이저빔을전송하는데사용될수있음. 섬유에서의손실량에따라중계기 (repeater station) 가일정한간격마다필요할수있음. Chap. 8. Optoelectronic Devices

89 8.2.2 광섬유통신 Fig 광섬유통신시스템의개략도. 전화나 TV에서와같은아날로그신호의전송을보이고있다. 신호가디지털화된후레이저광출력을변조하게되고이는광섬유를따라전송되는데, 이때섬유에서의손실을보상하기위해서중계기를사용하여주기적으로증폭된다. 스위칭회로가신호를적합한곳으로보낸다 (route). 광검출기와저잡음전치증폭기 (low-noise preamplifier; LNA) 에의해고아신호가전기신호로바뀐후에, 디지털신호로광섬유를진행하면서생긴왜곡 (distortion) 을재생기 (regenerator) 를이용하여교정한후에신호는아날로그신호로바뀐다. Chap. 8. Optoelectronic Devices

90 Introduction to Organic Light-Emitting Diodes Nano Quantum Electronics Laboratory Hanyang University

91 What is OLED? OLED : Organic Light-Emitting Diode 유기물합성에의한자발광디스플레이소자다이오드의전기적특성과유사전기적에너지가빛에너지로변환되어 OLED 빛이생성전계에의한빛의생성 (i.e. OELD) 음극 (Ca, Al:Li, Mg:Ag, etc.) 전자주입층 (EIL) 전자수송층 (ETL) + 발광층 (EML) 정공수송층 (HTL) 정공주입층 (HIL) 양극 ( 투명전극, ITO) Glass Light

92 What is OLED? 빛의발광과정 1. 캐리어주입 ( 전자, 정공주입 ) 2. 캐리어전송 ( 전자, 정공전송 ) 3. 전자-정공재결합그리고엑시톤의형성 4. 복사혹은비복사의엑시톤재결합그리고에너지전달 LUMO (Lowest Unoccupied Molecular Orbital) Anode + HIL HTL Emission EML + Exicton ETL EIL Cathode HOMO (Highest Occupied Molecular Orbital)

93 OLED Applications <OLED for a Mobile phone> < 3D OLED > <OLED Light> < flexible and rollable OLED >

94 Classification of OLEDs; Materials 정공주입층 정공의주입과전송 빠른정공의이동도가요구됨 정공주입층을이용해발광층설계의용의 Medium HOMO level (ITO-HTL) : 5.0~5.3 ev ITO의성능이좋은계면특성을요구함 대표하는물질 Cathode EIL ETL EML HTL HIL Anode (ITO) Substrate Metal phthalocyanine : CuPc, ZnPc Polyarylamine : mtdata, 1-NaphDATA, TDAPB, HI406, HT01 Acene-based : pentacene Doped polyarylamine-based : F4-TCNQ doped, SbCl 6 -doped, FeCl 3 doped Doped conductive polymers : PEDOT/PSS, PTT/PSS, PAni/PSS

95 Classification of OLEDs; Materials 정공수송층 HOMO level balancing 높은정공이동도, 낮은전자친화도 : 높은 LUMO 정공수송층으로인해엑시플렉스방지가용이함발광층으로부터엑시톤의확산을방지함발광층보다더큰에너지갭을가져야함. Cathode EIL ETL EML HTL HIL Anode (ITO) Substrate 대표물질 Polyarylamine-based : TPD, NPB, HT320, HT211

96 Classification of OLEDs; Materials 전자수송층 전자의주입과수송담당 전자의빠른이동도를요구함 Low LUMO level : ~3.0 ev 정공저지층역할 음극에서형성된엑시톤의거리를유지함 Cathode EIL ETL EML HTL HIL Anode (ITO) Substrate 대표물질 Metal complex : Al, Ga, In, Zn, Li, Na Oxadiazole, triazole or benzimidazolebased : PBD, TAZ, TPBI Quinoxaline, phenanthroline-based : TPQ, BCP, Bphen Perfluorinated C60

97 Types of Exciton Wannier exciton ( 전형적인무기물반도체 ) Excitons (bound electron-hole pairs) Frenkel exciton ( 전형적인유기물반도체 ) Semiconductor picture Conduction Band 엑시톤을확산이가능한무전하입자로취급할수있고, 여기상태에있는분자로도볼수있다 Molecular picture Valence Band Charge Transfer(CT) Exciton (typical of organic materials) Ground state Wannier Exciton Ground state Frenkel Exciton

98 Types of excitons Wannier excitons Gregory Wannier and Nevill Francis Mott에서명명반도체내에서보통유전상수는큰값을가짐 전계검사는전자와정공간의 Coulomb interaction을줄이려는경향이있음격자의공간보다반지름이큼 격자의위치에너지의영향에따라전자와정공의유효질량에포함됨 비슷한경우로, 낮은질량과 screened Coulomb interaction에의해서결합에너지가 0.01eV의단위로 hydrogen atom보다작다 보통반도체의결정체에서발견됨 낮은에너지간격과높은유전율을가지고있다액체상태에서도확인이가능 ( Ex. Liquid Xenon)

99 Types of excitons Frenkel excitons Yakov Frenkel 이발견 결합에너지는보통 0.1eV 에서 1eV 까지의값을가지고있음 낮은유전율을가진물질에서 단위격자가비슷한크기의전자와정공간에강한 Coulomb interaction에의해엑시톤은작아짐 Fullerene 같은분자의엑시톤은모두같은분자에위치함 전자와정공은같은단위격자에서발견된다 Alkalihalide 결정체와무기물의분자결정체는 aromatic 분자로구성되어있는것으로알려져있다

100 Types of Luminescence Photoluminescence and Electroluminescence (a) Photoluminescence (PL) Excited State (n) Excited State (-) Excited State (n) Light Light Ground State (n) Ground State (+) Ground State (n) (b) Electroluminescence (EL) Excited State (n) Excited State (-) Excited State (n) Electric Energy Cathode Cathode Cathode Light Anode Ground State (n) Anode Ground State (+) Anode Ground State (n)

101 Exciton Formation in OLEDs Polaron Lattice distortion으로전하이동음의폴라론 : 여기상태에서의전자형성양의폴라론 : 기저상태에서의정공형성엑시톤 : 양의폴라론 + 음의폴라론 Coulomb interaction에의해결합된전자정공쌍엑시톤의결합에너지는 Coulomb interaction의크기분자의여기된상태는엑시톤임음극으로부터주입된전자와양극으로부터주입된정공에의존하는엑시톤은얼마나잘형성되는가양전극에서의불균일한주입및전하이동때문에엑시톤형성이어려움

102 Singlet and Triplet Excitons

103 Fluorescence and Phosphorescence 형광 인광 대칭에의해허용된 siglet에의한감쇠빠르고 (~10 9 s -1 ) 효율적임 비대칭에의해허용되지않은 triplet에의한감쇠느리고 (>1 s -1 ) 비효율적임

104 Förster Energy Transfer Dipole-Dipole coupling 여기된도너분자와여기되지않은억셉터분자에서따른다. 보통 singlet-singlet 전이됨 에너지전이속도 매우빠름 < 10-9 s Acceptor (dye) Donor Donor * Acceptor Donor Acceptor *

105 Dexter Energy Transfer 분자간전자의교환에의함 도너에서억셉터까지엑시톤의확산 Singlet-singlet, triplet-triplet 전이모두가능에너지이동속도는분자궤도사이의겹치는정도에의존한다. Acceptor (ex. Phosphorescent dye) r DA : ~ 10A Donor Donor * Acceptor Donor Acceptor *

106 Förster and Dexter Energy Transfer Förster, Coulombic ( 긴거리 ~30 100A ) Dexter, e- exchange ( 짧은거리 ~6 20A ) D * A D * A SINGLET-SINLET TRANSFER SINGLET-SINLET & TRIPLET-TRIPLET TRANSFER D A *

107 Charge Injection Thermionic Emission Fowler-Nordheim Tunneling Ф J ev J s exp( ) 1 kbt J s E 2 b exp( ) E J s * 2 A T e exp( k T B bn ) 8 b * 2m ( q ) 3qh 3/ 2 Effective Richardson constant A * 4 em 3 h * n k B m* 120( ) A/ cm m 2 / K 2

108 Electrical Conduction; Ohm s Law Ohm s law 낮은전계 e : 전하량 ( 열에의해생성된캐리어 ) > ( 주입된캐리어 ) J Ohm en 0 V d n 0 : 열에의해생성된전하밀도 μ : 캐리어이동도 V : 인가전압 d : 유기층의두께 cf) Ohm s law V I R R 은전류에대해독립적이다 I : 전류, V : 전압, R : 저항, J : 전류밀도

109 Electrical Conduction; SCLC SCLC : Space Charge Limited Current 전계가증가함에따라, ( 열에의해생성된캐리어 ) < ( 주입된캐리어 ) 음극으로부터주입된총전류와공간전하의캐리어는공간전하에의해제한됨 전자전류 = 표동전류 - 확산전류 J SCLC dn ne F ( x) De( ) dx 9 V 8 d 2 3 F(x) : x에서의전계 n(x) : 단위부피당자유전자의수 J : 전류밀도 V : 인가전압 μ : 캐리어이동도 D : 확산계수

110 Electrical Conduction; Transition Voltage Transition voltage (V tr ) 옴의법칙에따르는전류에서 SCLC 전류로바뀌는전압 JOhm J SCLC J ohm en en 0 0 V d V d en 0 tr J SCLC 9 V 8 d 9 V 8 d 8 en0d V tr 9 2 tr 2 9 V 8 d 2 tr 2 2 3

111 Electrical Conduction; TCLC TCLC : Trap Charge Limited Current 캐리어가트랩에의해속박되었을때 9 8 V d 2 2 J SCLC J 3 TCLC V d Θ : 속박된캐리어와자유캐리어의비율 주입된전류가증가할때, 대부분의속박된캐리어는주입된캐리어에의하여채워진다. In organics without traps In organics with traps

112 Organic Light Emitting Devices (OLED) 1. Promising alternative anode materials (Al-doped ZnO) 2. High efficient cathode electrode (Mg:Ag) 3. Red OLEDs with HBLs 4. Blue and Green OLEDs with HBLs 5. Yellow OLEDs with multiple heterostructures 6. Green OLEDs with a stepwise doped HTL 7. Yellow OLEDs with a mixed layer 8. White OLED (R,G,B) 9. Polymer OLED (PLED) 10. OLEDs with combined inorganic nanocrystals and organic layers 11. OLEDs with combined inorganic nanocrystals and organic layers 12. OLEDs with a nanoscale Inorganic buffer layer

113 OLED Displays DC Power DC-DC Converter Video interface Controller Data Driver Circuit TFT Panel Scan Driver Circuit Pixel Our main research area (OLEDs) End of the Semester

114 OLED Pixel Structure e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

115 1. Promising alternative anode materials (Al-doped ZnO) e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

116 ZnO Anode Material Properties Electrical, Optical, and Electronic Properties of Aldoped Zinc-oxide Thin Films Acting as Anode Electrodes in Organic Light-emitting Devices

117 Surface, Electrical, and Optical Properties of AZO Thin Films Deposition of Al-doped ZnO films on glass substrates by using a radio-frequency sputtering system. Surface properties Electrical properties Film thickness 180 nm 220 nm 600 nm Resistivity ( cm) (a) (b) Hall mobility (cm 2 /Vs) Carrier concentration (cm -3 ) (c) Optical properties Film thicknesses ; (a) 180, (b) 220, and (c) 600 nm. Root-mean-square average of roughness : (a) 4 nm, (b) 2.8 nm, (c) 10.5 nm End of the Semester

118 Electronic Properties of Al-doped ZnO Thin Films Secondary electron emission coefficients of the Al-doped ZnO thin film with film thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the acceleration voltage for He, Ne, Ar, and Xe ions. Secondary electron emission coefficients of the Al-doped ZnO films with film thicknesses of (a) 180, (b) 220, and (c) 600 nm as functions of the ionization energy. Work functions (a) 180 nm: 4.62 ev (low barrier height) best anode (b) 220 nm: 3.67 ev (c) 600 nm: 3.38 ev Al-doped ZnO thin films an alternative anode in a high-efficiency and long lifetime OLEDs End of the Semester

119 2. High efficient cathode electrode (Mg:Ag) e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

120 Mg:Ag Thin Films Mg:Ag Thin Films with a Low Effective Barrier Height and a Low Work Function Acting as High- Efficiency Cathodes in Organic Light-emitting Devices 한국특허출원 유기발광소자에서 Mg-Ag 단일박막층을사용한음극전극형성방법 ( 출원번호 : , 2005/6/27)

121 Mg:Ag Thin Films with a Low Effective Barrier Height & a Low Work Function tunneling current thermionic current 0.22 ev 0.28 ev Total current = tunneling current + thermionic current (a) Mg:Ag/Alq 3 /ITO/Glass (b) Ag/Alq 3 /ITO/Glass End of the Semester Fowler-Nordheim Tunneling theory 2 I E exp E * Work function of the Mg:Ag thin film : 4.12 ev 2m 3qh Mg:Ag thin films with a low Mg concentration potential applications as cathodes in high-efficiency and long lifetime OLEDs

122 3. Red OLEDs with HBLs e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

123 Electroluminescence Mechanisms Electroluminescence Mechanisms for the Optical a nd Electrical Properties of Red Organic Light-emit ting Devices Utilizing a Hole-blocking Layers betw een an Electron Transport Layer and an Emission Layer

124 End of the Semester (a) with a 30-A BAlq HBL and (b) without a HBL.

125 CARRIER DISTRIBUTION (arb. units) Energy Band Diagrams and Carrier Distributions Hole Electron 2.3 ev 2.9 ev 2.3 ev 2.9 ev ITO 4.8 ev NPB Alq 3 : (HTL) DCJT BAlq B (HBL) Alq Al:Li 3 (ETL) (EML) 5.4 ev 5.4 ev 5.6 ev ITO 4.8 ev NPB (HTL) Alq 3: DCJT Alq 3 B (ETL) (EML) 5.4 ev 5.6 ev Al:Li End of the Semester DISTANCE DISTANCE Optical and electrical properties of the red OLEDs using an Alq 3 :DCJTB EML were affected by the existence of the HBL Color chromaticities were not significantly affected.

126 4. Blue and Green OLEDs with HBLs e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

127 Luminescence Mechanisms Luminescence Mechanisms of Green and Blue Organic Light-emitting Devices Utilizing Holeblocking Layers Solid State Communications Vol. 134, issue 5, pp (May, 2005)

128 EL spectra and energy band diagrams for blue OLEDs (a) Energy Band Diagram (b) 2.3eV 2.3eV (+) ITO 4.8eV NPB DPVB BAlq (HTL) i (HBL) (EML) 5.4eV 5.9eV 5.4eV 2.9eV Alq 3 (ETL) 5.6eV (-) Al:Li (+) ITO 4.8eV NPB (HTL) DPVBi (EML) 5.4eV 5.9eV 2.9eV Alq 3 (ETL) 5.6eV (-) Al:Li Schematic Diagram of Electron-hole Distribution Function under an applied Electric Field electro n electro n hole hole (a) ITO/α-NPD/DPVBi/BAlq/Alq 3 /Al:Li and (b) ITO/α-NPD/DPVBi/Alq 3 /Al:Li End of the Semester

129 Electroluminescence spectra of green OLEDs (+) NPB Alq 3 Alq BAlq 3 (HTL)(EML) (ETL) (HBL) (-) (-) electro n hole (+) Carrier density distributions; taking into account the electronic parameters and the layer thicknesses. Color chromaticities were significantly affected by the existence and the condition of the HBLs. End of the Semester

130 5. Yellow OLEDs with multiple heterostructures e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

131 Rubrene/NPB MQW Color-stabilized organic light-emitting devices with narrower spectral and color-stabilized emission by using N, N -bis-(1-naphthyl)-n, N - diphenyl-1,1-biphenyl-4,4 -diamine/5,6,11,12 - tetraphenylnaphthacene multiple heterostructures End of the Semester

132 ENERGY (ev) Schematic energy band diagram NPB Rubrene Liq ITO Alq 3 Al HTL and EML ETL and EML End of the Semester

133 EL INTENSITY (arb. units) EFFICIENCY (cd/a) EL INTENSTY (arb. units) EL INTENSITY (arb. units) Electroluminescence spectra Al/Liq/Alq 3 /NPB/ITO/glass Al/Liq/Alq 3 /3 periods of MQWs/NPB/ITO/glass WAVELENGTH (nm) Al/Liq/Alq 3 /5 periods of MQWs/NPB/ITO/glass WAVELENGTH (nm) End of the Semester WAVELENGTH (nm) with 5 periods of MQW with 3 periods of MQW without MQW CURRENT DENSITY(mA/cm 2 )

134 Commission Internationale de l Eclairage coordinates Al/Liq/Alq 3 /NPB/ITO/glass Al/Liq/Alq 3 /3 periods of MQW/NPB/ITO/glass Al/Liq/Alq 3 /5 periods of MQW/NPB/ITO/glass Color stabilized Yellow OLEDs The enhancement of the efficiency and the luminance of OLEDs with 5-periods of multiple heterostructures The CIE chromaticity coordinates stabilized with increasing the number of heterostructures End of the Semester

135 6. Green OLEDs with a stepwise doped HTL Luminescence mechanisms of highly efficient organic lightemitting devices fabricated utilizing stepwise doped hole transport layers End of the Semester

136 OLEDs fabricated utilizing stepwise doped hole transport layers Device I Al (100 nm) Device II Al (100 nm) Alq 3 (60 nm) Alq 3 (60 nm) 1.5% rubrene 1.0% rubrene 0.5% rubrene NPB (50 nm) 1.0% rubrene ITO ITO Current density vs. applied voltage Luminance vs. applied voltage The turn-on voltage of the OLEDs fabricated with a stepwise doped HTL smaller Luminance higher than that with an uniformly doped HTL. End of the Semester

137 7. Yellow OLEDs with a mixed layer Enhancement of efficiency and lifetime in organic light-emitting devices fabricated by using a mixed layer acting as a hole transport layer and an emitting/electron transport layer End of the Semester

138 OLEDs with various mixed layers DEVICE I DEVICE II DEVICE III DEVICE IV Al 100 nm/liq 2 nm Al 100 nm/liq 2 nm Al 100 nm/liq 2 nm Al 100 nm/liq 2 nm Alq 3 (60 nm) Alq 3 (50 nm) Alq 3 (50 nm) Alq 3 (50 nm) DCM1(3%)-doped Alq 3 (10 nm) DCM1(3%)-doped Alq 3 :NPB (1:1)(10 nm) DCM1(3%)-doped Alq 3 :NPB (1:1)(10 nm) Alq 3 :NPB (1:1)(10 nm) NPB (50 nm) NPB (50 nm) NPB (50 nm) NPB (40 nm) ITO 100nm ITO 100nm ITO 100nm ITO 100nm Glass substrate Glass substrate Glass substrate Glass substrate End of the Semester

139 High Efficiency and long lifetime OLEDs device IV Efficiencies as functions of current density Normalized intensities as functions of time OLEDs with a doped layer inserted into a mixed HTL and an EML/ETL the highest efficiency and the longest lifetime The emitting color of the OLEDs deeply pure yellow. End of the Semester

140 8. White OLED (R,G,B) e Cathode ETL(Electron Transport Layer) HBL(Hole Blocking Layer) EML(Emitting Layer) HTL(Hole Transport Layer) Anode Glass Substrate h Light End of the Semester

141 Optical Properties of White OLED Optical Properties of White Organic Light-emitting Devices Fabricated with Three Primary-color Emitters by Using Organic Molecular-beam Deposition Solid State Communications Vol. 135, issue 6, pp (August, 2005) End of the Semester

142 Unit Cells of White OLEDs Glass MgAg (150 nm) Alq3 (35nm) DPVBi:DCJTB (7nm, 3%) Alq 3 (12 nm) DPVBi (6 nm) NPB (40 nm) ITO Glass B G R End of the Semester

143 Schematic Energy Band Diagrams of White OLEDs Energy (ev) ITO ITO (a) (b) 2.45 NPB (HTL) 5.46 NPB (HTL) DPVBi Alq 5.26 Alq 3 3 (ETL) B Alq 3 + DCJTB (3%) DPVBi + DCJTB (3%) 3.11 DPVBi Alq 5.26 Alq 3 3 (ETL) B G G R R Mg:Ag Mg:Ag (a) Mg:Ag/Alq 3 /Alq 3 :DCJTB /Alq 3 /DPVBi/NPB/ITO (b) Mg:Ag/Alq 3 /DPVBi:DCJTB/Alq 3 /DPVBi/NPB/ITO End of the Semester

144 Current Efficiencies (a) Mg : Ag / Alq 3 / Alq 3 : DCJTB / Alq 3 / DPVBi / NPB / ITO (b) Mg : Ag / Alq 3 / DPVBi : DCJTB / Alq 3 / DPVBi / NPB / ITO WOLED with a red host DPVBi stable WOLEDs End of the Semester

145 Electroluminescence Spectra ITO NPB (HTL) 3.11 DPVBi Alq 5.26 Alq 3 3 (ETL) B DPVBi + DCJTB (3%) G R Mg:Ag Complete white OLEDs 3 nm 6 nm 9 nm End of the Semester

146 9. Polymer OLED (PLED) Electronic Structures of p-phenylene Biphenyltetracarboximide Polyimide/ Indium-tin Oxide Heterostructures Grown on Glass Substrates for Organic Light-emitting Diodes Solid State Communications Vol. 135, issue 1-2, pp (2005) BPDA-PDA (Polyimide) End of the Semester

147 Electronic Structure of BPDA-PDA/ITO Heterostructure ITO BPDA-PDA Vacuum Level LUMO 4.7 ev Eg 4.77 ev = 3.51 ev HOMO (a) BPDA-PDA polyamic acid layer (b) BPDA-PDA polyimide layer The ionization energy of the PI: 4.77 ev Optical energy gap : 3.51 ev. End of the Semester

148 High Efficiency OLEDs High efficiency OLEDs with hybrid nanoscale semiconductors and organic layers

149 10. OLEDs with combined inorganic nanocrystals and organic layers Effect of Thermal Annealing on the Interband Transitions and Activation Energies of CdTe/ZnTe Quantum Dots Journal of Applied Physics vol. 98, (July, 2005) (selected August 1, 2005 issue of Virtual Journal of Nanoscale Science & Technology) End of the Semester

150 1 m Effect of Thermal Annealing on the Interband Transitions and Activation Energies of CdTe/ZnTe Quantum Dots Annealed at AFM image of CdTe/ZnT e quantum dots Photoluminescence spectra at 14 K Photoluminescence spectra measured at several Integrated photoluminescence temperatures for CdTe/ZnTe intensities as functions of the annealed at 330 reciprocal temperature Highest activation energy and improvement of the size uniformity of the CdTe/ZnTe QDs : Ta = 330 End of the Semester

151 11. OLEDs with combined inorganic nanocrystals and organic layers Interband Transitions and Electronic Properties of InAs/GaAs Quantum Dots Embedded in AlxGa1-xAs/GaAs Modulation-doped Heterostructures End of the Semester

152 InAs/GaAs Quantum Dots Embedded in AlxGa1- xas/gaas Modulation-doped Heterostructures AFM image of InAs/GaAs quantum dots Photoluminescence spectra at several temperatures for the InAs quantum-dot arrays embedded in GaAs barriers End of the Semester The activation energy of the electrons confined in the InAs/GaAs QDs was 115 mev. Electronic subband structures for the InAs quantum-dot arrays embedded in GaAs barriers

153 Beautiful combination between inorganic nanocrystals and organic layers Organic layer Cathode Organic Layer Organic Layer Anode glass QD Monolayer ZnS shell QD CdTe core Structure of QD OLEDs Structure of QD workfunction anode Nanocrystal E 1 E C E V HH 1 (a) zero bias LUMO workfunction cathode HOMO electron hole Emission colors of QD OLEDs dependent on QD sizes End of the Semester (a) forward bias Electronic structure of combined nanocrystal semiconductors and organic layers

154 12. OLEDs with a nanoscale Inorganic buffer layer Highly efficient organic light-emitting diodes fabricated utilizing NiO buffer layers between anodes and hole transport layers End of the Semester

155 Surface, optical, and electrical properties of OLEDs with a NiO buffer layer Transmittance spectra for the NiO films oxidized at 500oC for (a) 0, (b) 1, (c) 2, (d) 3, and (e) 4 h. Atomic force microscopy images of the NiO films oxidized at 500 o C (a) 1, (b) 2, and (c) 3 h. Enhancement of the luminous efficiency and the power efficiency for the OLEDs with NiO buffer layers an increase of the thickness of the NiO buffer layer. End of the Semester (a) Current density vs. voltage (b) luminance vs. voltage

156 8.3 레이저 LASER (Light Amplification by Stimulated Emission of Radiation) 복사선의유도방출에의한빛의증폭특성 : 강한방향성, 단색성및간섭성발광 : 자연방출 (spontaneous emission) + 유도방출 (stimulated emission) 전하분포와볼츠만계수에대한검토로부터, 열적평형 (thermal equilibrium) 상태에서의상대적분포는 h 12 E2 E1 n n 2 1 e e ( E 2 h E )/ kt 12 1 / kt Fig 상위상태에서하위상태로광자방출을동반하는전자의유도천이 Chap. 8. Optoelectronic Devices

157 8.3 레이저 (Laser) 의기본개념 Laser: Light Amplification by Stimulated Emission of Radiation Laser 의특징레이저는단색빛이다. 모든파들이동일위상상태에있다. 레이저빔은멀리진행하여도거의퍼지지않는다. 에너지밀도가매우높다. 보통빛단색빛레이저

158 준안정상태 (metastable state) 상태수명이 10-3 초혹은그이상인들뜬상태.

159 두원자준위사이의천이 유도흡수 (induced or stimulated absorption): E 0 에서 hν의에너지를흡수해 E 1 으로올라가는과정. 자발방출 (spontaneous emission): 처음에 E 1 에있을때 hν를방출하고 E 0 로떨어지는과정. 유도방출 (Stimulated emission): hν인입사광자에의해 E 1 에서 E 0 로전이가유도되는방출.

160 3 준위레이저 (Three-level laser) Optical pumping ( 광펌핑 ) Spontaneous emission ( 자발방출 ) Population inversion( 밀도반전 ): Metastable state ( 준안정상태 ) 바닥상태보다들뜬상태에더많은원자들이있는경우. Stimulated emission ( 유도방출 )

161 2 level & 4 level laser 2 level laser 4 level laser(he-ne gas laser) excited level He Ne metastable state laser transition nm h h electron impact spontaneous emission radiationless transition ground level pumping stimulated rate emission rate p ~ 1Torr 10 :1 Population inversion 이되지않음. l n 2

162 8.3 레이저 E 2 2 Level System E 3 Meta Stable State E 1 E 4 E 3 Meta Stable State Optical Pumping E 2 Optical Pumping E 2 E 1 E 1 3 Level System 4 Level System Chap. 8. Optoelectronic Devices

163 8.3 레이저 B n1 ( 12) A21n2 B21n2 ( 12) 흡수 자연방출 유도방출 12 Fig 정상상태에서의흡수와방출의평형 : (a) 유도방출 ; (b) 흡수 ; (c) 자연방출 B 12,A 21,B 21 : Einstein Coefficient 열적평형상태 자연방출에대한 유도방출의비율은 일반적으로매우 작으며유도방출의기여는무시가능. 광자전계 (photon field) 가존재할때, 광학적공진공동 (optical resonant cavity) 를만들어줌으로써촉진 흡수이상유도방출을얻으려면, 밀도반전 (population inversion) : n 2 > n 1 Chap. 8. Optoelectronic Devices 유도방출율자연방출율 유도방출율흡수율 B B B21n2 ( 12) B21 ( A n A n2 ( 12) n ( ) B B n n )

164 Ruby laser Al 2O3 : Cr 0.003sec

165 8.3 레이저 광자의밀도가유도방출로증가되기위한조건 1. 광자전계가증진되는것을촉진하기위한광학적공진공동 2. 밀도반전을얻는방법을제공 유도방출이일어나기위한 공동의길이 L m 2 대기에서출력광의파장 λ 0 를 이용하고자할때는레이저 물질의굴절률 n 을고려 Fig 레이저공동내에서의공진양식 0 n Chap. 8. Optoelectronic Devices

166 8.4 반도체레이저 p-n junction LASER Hetero junction LASER The condition of Population inversion The nature of the coherent light Chap. 8. Optoelectronic Devices

167 반도체레이저 Ec Ev h Ga Asl Gal Asl Asl Gal Asl Ga e h h E E g E e 1atom 2 atoms many atoms

168 8.4.1 접합에서의분포반전 반전영역 (inversion region) 축퇴 (degenerate) 된물질사이에 p-n접합을형성순방향바이어스가충분히크면전자와정공은상당한농도로전이영역을넘어서주입 이영역은전도대에는고농도전자를, 가전자대에는고농도정공을포함이들분포의농도가충분히크면밀도반전의상태를형성 의사페르미준위 (quasi-fermi level) 의개념으로설명 n p N N c v e e ( E ( F c p F )/ kt n E v )/ kt nie n e i ( F E )/ kt n ( E F i i p )/ kt Chap. 8. Optoelectronic Devices Fig 순방향바이어스가인가되었을때 p-n 접합레이저의에너지대역도. 빗금친부분은접합에서의반전영역을나타낸다.

169 Chap. 8. Optoelectronic Devices 접합에서의분포반전밀도반전에대한조건반도체에서임의의주어진천이에너지 hν 에대하여분포반전 g p n g v c p n E F F E E E h h F F ) ( ) ( Fig 반전영역확대도 Fig 순방향바이어스에따른반전영역폭의변화 : V(a) < V(b)

170 8.4.2 p-n 접합레이저의방출스펙트럼 유도방출에관여하는광자의파장 인가해주는전류준위가커짐에따라자연방출에서유도방출로주도됨. 레이저동작에서는유도방출로중첩된거의단색광에가까운복사로형성 Fig 접합레이저에대한빛의세기대광자에너지 hν 의관계 : (a) 문턱값이하에서의비간섭성빛의방출 ; (b) 문턱값에서의레이저 ( 동작 ) 양식 ; (c) 문턱값이상에서의주된레이저 ( 동작 ) 양식. 세기의눈금은 (a), (b), (c) 의순으로크게압축되어있다. Chap. 8. Optoelectronic Devices

171 Chap. 8. Optoelectronic Devices p-n 접합레이저의방출스펙트럼 Fig. 8-21(b) 에서각양식들의분리 GaAs 의굴절률 n 이파장 λ 에의존한다는사실로말미암아복잡함. 0 n 2 m L n 2 n 2 m d d L L d d m n n 1 n d d L 만약 m(l 에서반파장의수 ) 이크면도함수를이용하여 λ 0 에대한 m 의변화율을구할수있다. 이제 m 과 λ 0 에서의불연속적변화형식으로바꾸면로쓸수있다. Δm = -1 로놓으면인접한양식사이에서의파장의변화 Δλ 0 를계산할수있다.

172 8.4.3 기본적인반도체레이저 Fig 간단한접합레이저의제작 : (a) 축퇴상태의 n 형시료 ; (b) p 형쪽확산 ; (c) 절단또는식각에의한접합의분리 ; (d) 개개의접합을소자로절단또는쪼갬 ; (e) 장착된레이저구조 Chap. 8. Optoelectronic Devices

173 8.4.4 이종접합레이저 Fig 레이저다이오드에서캐리어를전송하기위해이종접합을이용 : (a) 얇은 p 형 GaAs 층위에성장된 AlGaAs 이종접합 ; (b) 바이어스하에서얇은 p 형영역에전자가집속됨을보이는 (a) 의구조에대한에너지대역도. Chap. 8. Optoelectronic Devices

174 8.4.4 이종접합레이저 Fig 이중이종접합레이저구조 : (a) 주입된캐리어와발생된빛을집속하기위하여사용한다중층 ; (b) 레이저동작이발생되는방향에따라좁고가느다란부분으로전류주입이제한되도록설계된띠의기하학적구조. 이띠의기하학적구조를얻는여러방법중의하나로서이예는 (b) 의흐린영역을양자 (photon) 폭격하여얻는것이며, 이로써 GaAs 와 AlGaAs는반절연성으로바뀐다. Chap. 8. Optoelectronic Devices

175 8.4.4 이종접합레이저 Fig 캐리어집속과광도파집속의분리 : (a) 가장작은대역간극을갖는영역 (d) 에서캐리어를제한하고보다넓은영역에서굴절계수의계단을이용하여도파관 (w) 을얻기위해서 AlGaAs 합금조성의분리된변화를이용한다 ; (b) 보다좋은도파관과캐리어집속을얻기위한합금의조성비. 따라서굴절률을경사지게한다. Chap. 8. Optoelectronic Devices

176 8.4.4 이종접합레이저 Vertical Cavity Surface-Emitting Lasers (VCSELs) 공진기거울대신에 MBE나 OMVPE로성장시킨 Distributed Bragg Reflector (DBR) 반사기를포함. DBR 거울은두께가각물질에서의파장의 1/4 이되는 AlAs와 GaAs의여러교차층으로구성. Fig 수직공동을갖는표면방출형레이저다이오드의모식적인단면도 Chap. 8. Optoelectronic Devices

177 GaN blue LED 1993년니치아화학의나카무라슈지 ( 현재 UCSB 교수 ) 에의해 GaN 를기반으로한청색 LED 가개발됨. 1980년대적색은 AlGaAs, 녹색 ( 황녹 ) 은 GaP 기반으로개발.

178 Homework #8 고체전자공학제 6 판 Chapter 8. 연습문제 문제 2, 문제 8, 문제 16, 문제 18