CNT-based photovoltaic and light emitting diodes

Transcription

1 Brazil-China Scientific Symposium: PKU & FAPESP, CNT-based photovoltaic and light emitting diodes Lian-Mao Peng Department of Electronics Peking University

2 Why Carbon Nanotube? Carbon nanotube films are the blackest known material, absorbing close to 1% of incident visible light Semiconducting carbon nanotubes are direct bandgap materials with extremely high carrier mobility CNT band gap can be tuned by controlling diameter to match the solar spectrum Semiconducting CNT can be ohmically contacted (for both electrons and holes) with appropriate metals asymmetrically contacted CNT light emitting diodes (LEDs) and photodiodes (IR detectors) Possible carrier multiplication (CM) effect possible convention efficiency h>31% (i.e. optimal for a single band gap solar conversion device ) Virtual contacts cascade cells efficient photovoltage multiplication room temperature IR detection, signal amplification

3 CNT film: extremely dark matter t~3mm P.M. Ajayan et al., Nano Letters 8 (28) 446 Theoretical calculation predicted that CNT film has an extremely low index of refraction (n= ) and absorbs light strongly a effective =.12 mm -1 and an absorption length 1/ a effective = 8.3 mm.

4 Optical Reflection of CNT Film The measured reflectance of the Au mirror is R total = 94.5%, which agrees well with the calculated value of R total = 94.1%. A glassy carbon is conventionally regarded as a black object and has a measured reflectance of R total = 8.5% (the blue dots). The measured reflectance of the reference sample is R total = 1.6%, which also agrees with its NIST-certified value of R total = 1.4%. But the reflectance of the carbon nanotube array is R total =.45%.

5 Nanotube Electronic Structure E g,1 ~ 1/d tuning d distribution to match the solar spectrum Spectroscopy of CNT excitons. a, Valence (bottom) and conduction (top) bands with different angular momentum for a (19, ) semiconducting CNT. Red: k = ±k min ; blue: k = ±2k min ; green: k = ±4k min. kψ is the wave vector along the axis. b, Single-particle density of states.

6 Sc-contacted CNT Z.Y. Zhang/L.-M. Peng et al., Nano Letters 7:363 (27) b G (4e 2 /h) L = 3 nm d = 2.nm 25 K V gs (V) T = 4.3 K 2 K Linear I ds -V ds characteristics, down to 4.3K Ohmic contact, metallic conductance d I ds (ma) T = 4.3 K V ds (V) V gs = - 7 V V gs = - 6 V V gs = - 5 V V gs = - 4 V The on-state conductance increases with decreasing T (reduced phonon scattering) At 4.3K, G=.62*G (with G = 4e 2 /h being the quantum conductance) No Coulomb blockage no potential barriers at the CNT/Sc interface

7 Sc-contacted CNT: n-type FET affording perfect electron conductance in CNT, but not holes... Z.Y. Zhang/L.-M. Peng et al., Nano Letters 7:363 (27) Sc CNT n-fet SiO2 (1nm) Sc N + Si (back gate with V G ) E F Vg=, V ds= Ec Vg=V th, V ds> e- Ev CNT V ds =.1V Vg<, V ds> Vg>V th, V ds> e - I ds (A) I on /I off ~ V gs (V) off-state on-state The as-grown SWCNTs are not p-type, they are basically intrinsic.

8 Pd-contacted CNT: p-type FET Pd affording perfect hole conductance in CNT, but not electrons... A. Javey/H. Dai et al., Nature, 424(23)654 CNT p-fet SiO2 (1nm) Pd Vg=, V ds= Ec Vg=V th, V ds< N + Si (back gate with V G ) E F Ev CNT h + I ds (A) T = 3 K Vds=.1V V g (V) Vg>, V ds< off-state Vg<Vth, V ds< h + on-state

9 Symmetrically contacted CNT Z.Y. Zhang/L.-M. Peng et al., Nano Letters 7:363 (27) Pd: valence band Hole conduction Sc: conduction band Electron conduction I ds (A) 1x1-6 1x1-7 1x1-8 1x1-9 1x Ohmic contacts for both n- and p-fets linear I-V I ds (ma) 1-1 V gs =-2V V (V) ds V ds =.1V L=4mm Pd-contacted V gs (V) p-cnt FET V OUT (V) Gain ~ 1.3 I ds (A) 1x1-6 1x1-7 1x1-8 1x1-9 1x1-1 I ds (ma) V gs =1V V (V) ds 1 V DD =5V L=4mm V IN (V) Doping-free CMOS inverter V ds =.1V L=4mm Sc-contacted V gs (V) n-cnt FET

10 More than CMOS:Multifunctional Device Barrier-free bipolar diode LED + detector ABS[I ds (A)] n=1.3, I s =.8nA, d ~ 3.5nm n=1.8, I s =8pA, d ~1.7nm V ds (V) Rsh Rs I ds (ma) D V gs =V L=4mm C V ds (V) almost perfect diode with n~1.8 I=(V-IR s )/R sh + I s exp[(q(v-ir s )/nkt)-1] S. Wang et al., Adv. Mater. 2 (28) 3258 B A (c) e - Sc e - Sc Sc h + Sc h + A B C D Pd h + Pd h + e - Pd e - 无掺杂碳纳米管多功能器件单元 : 1. 无势垒 电流更大, 2. 同时注入电子和空穴 双极性 3. 低的阈值电压 < E g 4. 高效的 LED, 光二极管 Pd

11 Carbon Nanotube Light Emitting Devices 碳纳米管三极管

12 1 st Carbon Nanotube Light Emitter Advantage: direct band-gap material Two modes: Ambipolar and unipolar Optical emission from an ambipolar CNTFET Large bias (~1V) More terminals (e.g. FET) Ph. Avouris et al. IBM Science 3 (23) 783

13 CNT ambipolar light emitter Thermally assisted electron and hole tunneling through the barrier Symmetric injection of both electron and hole simultaneously at room temperature Three terminal field-effect transistor geometry IBM Ph. Avouris et al.: Science 3 (23) 783

14 Optical emission from a CNTFET Polarization The CNT is a linearly polarized dipole radiation source cos 2 q d~1.4nm Eg~.75eV 165nm

15 Effects of carrier relaxation M. Freitag/P. Avorius, Nano Letters 4 (24) 163 DE~25meV, long channel device carrier thermalization DE=18meV, short channel device Red line: d=1.8nm E=.58eV Blue line: d=1.5nm E=.68eV The width of the emission peak is strongly device-structure dependent. Long devices (5 um) show narrow spectral peaks that we attribute to relaxed carrier recombination, while short devices (5 nm) show broad peaks due to hot carrier recombination. The hot carrier distribution is limited to energies below the energies of the optical/zone boundary phonons near 18 mev.

16 Carbon Nanotube Diodes 碳纳米管二极管

17 Carbon Nanotube Diodes Most optoelectronic devices behave like diodes, admitting a much larger current under forward bias (V>) than under reverse bias (V<). This rectifying behavior is a feature of optoelectronic devices, since an asymmetric junction is needed to achieve charge separation. Carbon Nanotube Diodes Chemically doped p-n junction diode Split gate p-n junction Asymmetrically contacted CNT..

18 Chemically doping CNTs C. Zhou et al. Science 29 (2) 1552 Not stable in air, hard to control, and not a very good diode Vg=-9.5V

19 The Split Gate Diode Large bias, typically > 1V, multiple (>4) electrodes [J.U. Lee, APL 87 (25) 7311] Almost perfect diode! Vg1>>, n-type Vg1<<, p-type

20 Photovoltaic Effect V bs > Vg1=-Vg2=+1V n=1., Rs=18MW small current Increasing illumination power J.U. Lee, APL 87 (25) 7311

21 Multiple Electron-Hole Pair Generation N.M. Gabor/P.L. McEuen et al., Science 325 (29) 1367 Three independent gates, V 1, V 2, and global back gate V G, allow selective electrostatic doping along the length of the nanotube. By applying voltages of opposite polarities on V 1 and V 2, a p-n junction is realized, which yields a built-in electric field E along the length of the nanotube (Fig. 1B). Photo-generated e-h pairs created in the junction are separated by the built in potential and accelerated to the device contacts, leading to photocurrent at zero bias.

22 Multiple Electron-Hole Pair Generation DE 22 =2DE 11

23 CNT Exitonic LED: channel length ~ 2mm (a) Electroluminescence intensity (a.u.) (12,4) SWCNT with a diameter of 1.14nm DE ~ 3meV V gs =1V 11.mA 8.mA 7.mA 5.5mA Energy (ev) (b) Electroluminescence intensity (a.u.) Device 1 Device Current (ma) S.Wang/L.-M. Peng et al., Nano Letters 11 (211) 23-29

24 Strong Excitonic Effects In a (6,5) SWNT the exciton binding energy is about.43 ev. This implies an exciton Coulomb interaction on the order of.8 ev, assuming a normal correlated exciton structure where the net binding energy is half the Coulomb energy. In this tube the (renormalized) band gap is only 1.7 ev, so the exciton Coulomb energy is half the band gaps, quite remarkable! This (6,5) tube has a diameter of.8 nm, and the exciton extends for about 2.4 nm along the length as shown. Thus the electronic geometry is approaching 1D.

25 CNT Exitonic LED: channel length ~ 2mm (a) Electroluminescence intensity (a.u.) (12,4) SWCNT with a diameter of 1.14nm DE ~ 3meV V gs =1V 11.mA 8.mA 7.mA 5.5mA Energy (ev) (b) Electroluminescence intensity (a.u.) Device 1 Device Current (ma) S.Wang/L.-M. Peng et al., Nano Letters 11 (211) 23-29

26 LED based on arrays of CNT larger current higher emission intensity Mixture of metallic and semiconducting CNTs, I-V is basically determined by m-cnts (a) 15 (b) 5 I ( ma) Number (b) EL intensity(a.u.) mA 9mA 1mA 11mA energy(ev) Emission spectra from arrays of CNT V (V) integrated intensity serpentine CNT identical semi-tubes Diameter (nm) CNT film mixed m- and s-tubes I(mA)

27 LED based on serpentine CNT Identical CNT + larger current high emission intensity + narrow peak 6 4 I(mA) V (V) (b) I (ma) Y.G. Yao/J. Zhang, Adv. Mater. 29, 21, V (V) Metallic, not suitable for LED 沟道 ~2μm

28 LED based on serpentine CNT Identical CNT + larger current high emission intensity + narrow peak e - Sc Channel ~2μm (d) I(mA) EL Intensity(a.u.) I=52mA,V=3.2V Pd h V (V) -5 For a single channel LED, current ~ a few ma energy(ev)

29 Serpentine vs arrays of CNT D.M. Yu et al., Small (213) DOI: 1.12/smll Well defined emission peaks from serpentine CNT, with DE < 1meV Emission spectra from arrays of CNT (b) EL intensity(a.u.) 8mA 4 9mA 1mA 3 11mA 2 1 integrated intensity energy(ev) serpentine CNT identical semi-tubes CNT film mixed m- and s-tubes I(mA)

30 Substrate-induced exciton in CNT L.H. Ye et al., Appl. Phys. Lett. 13 (213) 2315 (14,), a semiconducting CNT E 11 E 11

31 Potential for Carbon Nanotube Infrared Detectors higher operating temperature, smaller pixels Image taken at 11K with T2SL camera (32 by 256)

32 Photovoltaic Effects S. Wang et al., JPCC 29, 113:6891

33 V oc and I sc : length dependence I ds (na) I sc (na) L-.6mm L-1.5mm L-2.5mm L-3.5mm Isc Voc V ds (V) L(mm) Voc is typically less than.2v Voc (V) Pd Pd P out (nw) Sc Pd p-i-n L(mm) p-n Sc Sc optimal length ~1.5mm, BUT V oc and I sc are not high

34 CNT + virtual contacts Tandem Cells L.J.Yang/L.-M. Peng et al., Nature Photonics 5 (211) Similar to Tandem cell but different! Doping-free high and low T applications Two parallel channels Better performance Different chemical potential Along the channel virtual contact Two tubes in the channel Virtual contact V oc doubling V oc =

35 Photovoltage Multiplication 在 1 微米长的碳纳米管上引入 4 个虚电极实现了光电压的 5 倍增 Three virtual contacts quadruple cells module L~1mm V oc =1.4V L.J.Yang/L.-M. Peng et al., Nature Photonics 5 (211)

36 Diodes based on arrays of CNTs 直接生长样品 : 缺陷小 电流较大 效率高 Q.S. Zeng et al., Optical Materials Express, 2, , 212

37 CNT thin film Diodes I (A) I (A) V g (V) W = 2 μm, L = 1 μm V g (V) W = 2 μm, L = 1 μm I (ma) I (ma) V (V) V (V)

38 CNT array based IR detector 基于碳纳米管平行阵列的红外光探测器 I sc (na) Detectivity D* ~ 1 7 cmhz 1/2 /W 的探测率 typically ~ cmhz 1/2 /W for HgGdTe or InSb (d) I (ma) (c).3 V oc (V).2.1. I (A) V (V) V (V) V oc I sc Power density (W/cm 2 ) (b) I (na) (d) I SC (na) V (V) dark 785 W/cm W/cm time (s) 157 W/cm W/cm 2 Q.S. Zeng et al., Optical Materials Express, 2, , 212

39 Performance improvement via cascading cells 采用级联技术提高探测效率 Cascading cells can significantly improve the performance of the detector Typically hundreds of cells are needed to reach ~ cmhz 1/2 /W R V (V/W) D * Rv v n A R v A v n Power density (W/cm 2 ) A N R D N v n = (4k B TR D ) 1/2 N 1/2 Signal/Noise = V OC / v n N 1/2 HgGdTe or InSb detector Complicated to fabricate, especially to make them smaller 制作复杂, 尺寸难以做小, 有效区比例小 D* 基本都是 ~ cmhz 1/2 /W ( 中红外 )

40 I sc (pa) I sc (na) I sc (na) Cascade CNT thin film cells Single diode Five diodes in series V oc (V) Voc Isc Power density (mw/cm 2 ) V oc (V).2.1. V oc I sc Power density (W/cm 2 ) 8 4 V oc (V) V oc I sc Power density (W/cm 2 ) I (na) V (V) 1572 W/cm 2 W = 2 μm, L = 1 μm

41 I sc (pa) Performance improvement via cascading cells L.J. Yang/L.-M. Peng et al., Small 9 (213) 1225 Cascade cells can significantly improve the performance of the detector V oc (V) Voc Isc 探测极限 Detection Limit (mw/cm 2 ) Power density (mw/cm 2 ) 碳纳米管级联器件相比单极探测器的性能改善 电压响应率 Voltage Sensitivity (V/W) 探测率 Detectivity (cmhz 1/2 /W) 单级探测器 级探测器 13.3 Significantly improved >1times Increased ~ N 1/2 More than 2 times Increased ~ N 1/2 More than 2 times

42 Integrated CNT electronic and optoelectronic circuit FETs,CMOS inverter Symmetric contacts CMOS inverter Source Sc Gate Pd Source Drain Source FET Drain Pd(p) Sc(n) Photovoltage light signal Virtual contacts: V oc =.2 1.V p-n diode Asymmetric contacts detector

43 Light signal DV G Electric signal.12-.2v is not sufficient to make a clear switch, but.6-1.v is sufficient V out (V).6.3 dark V out (V) illumination V in (V) V in (V) Optical signal electric signal V out (V) V high -dark High dark Low with light.2 V low -light Timing

44 Integrated CNT electronic and optoelectronic circuit V out (V) V high -dark V low -light High dark; Low with light 数字光信号 数字电信号 对称电极 CMOS 电路 Sc Source Source Timing Gate Pd Source FET Pd(p) Sc(n) Symmetric contacts CMOS inverter Drain Drain Photovoltage light signal p-n diode asymmetric contacts detector

45 Conclusions Electron and holes can be injected into semiconducting carbon nanotube with high efficiency Perfect diodes can be constructed using asymmetric contacts: Pd for hole and Sc (or Y) for electrons leading to high performance light emitting diodes (LED) and detectors Highly efficient photovoltage multiplication can be realized by introducing virtual contacts to CNT infrared detectors with higher detectivity Electronic and optoelectronic devices can be most conveniently integrated on the same CNT

46 Acknowledgements Device fabrication and measurements: Prof. X.L. Liang Dr. Z.Y. Zhang Dr. S. Wang Dr. L. Ding Dr. H.L. Xu Miss L.J. Yang Mr. Q.S. Zeng Ultra long SWCNTs: Prof. Y. Li Prof. J. Zhang Prof. J. Liu Dr. W.W. Zhou $$$: MOST, NSF-China