Photovoltage phenomena in nanoscaled materials Thomas Dittrich Hahn-Meitner-Institute Berlin 1
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Introduction From bulk to nanostructure: SPV on porous Si Retarded SPV response and its origin Photovoltage on ultrathin layers Summary and challenges 3
The reason for a photovoltage light absorption transport generation of excess charge carriers in solid matter separation of excess charge carriers in space 4
Th. Dittrich, unpublished Free and trapped excess charge carriers in c-si PL intensity (arb.un.) Photovoltage (V) 10 0 10-1 10-2 10-3 0.0-0.1-0.2 free excess charge carriers laser diode pulses 150 ns 10-7 10-6 10-5 10-4 10-3 10-2 Time (s) p-si HF treated 300 K N 2 laser pulses 15 MHz detector trapped excess charge carriers time dependent photoluminescence time dependent SPV Photovoltage induced by SCR 5
J. Rappich, Th. Dittrich, 2002 Dember-photovoltage in c-si 150 U D = kt q b b 1 ln 1+ + 1 b + 1 δp b n ambipolar diffusion U PV (mv) 100 50 measurement fit: δp max = 2 10 16 cm -3 b = 3.5 b mobility ratio b = µ n µ p 0 n-si(100) 5 kωcm δp excess carrier concentration n free carrier concentration 10-6 10-5 10-4 10-3 10-2 10-1 10 0 I / I 0 measurement at 300 K with a laser diode (902 nm, 150 ns, 100 W) 6
Sensitivity of photovoltage 10 0 10-1 10 14 cm -2 MOSFET photovoltaics Q - Q + 10-2 10 12 cm -2 U (V) 10-3 10-4 10 10 cm -2 scope 10-5 10-6 10 8 cm -2 d separation length 10-7 molecule 1 10 100 d (nm) 10 6 cm -2 lock-in SCR U = Q d ε ε 0 7
Time resolution of SPV (surface photovoltage) Kelvin probe Capacitor arrangement swinging electrode sample light V C(t) i(t) A electrode mica sample light C buffer R d i( t) = V = dt [ C( t) ( U )] 0 R = 1 GΩ, C = 100 pf given by lock-in given by light pulse, scope and RC t > 10 100 ms 1 ns < t < 10 100 ms 8
Th. Dittrich, I. Mora Sero, unpublished Example for SPV transients: porous TiO 2 with adsorbed dye molecules Photovoltage (V) 0.05 porous TiO 2 with N3 dye laser pulse 532 nm, 120 ps -120 C -20 C +80 C +180 C +270 C 0.00 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 high impedance buffer by S. Bönisch (HMI) Time (s) Time window for measurements: 1 ns 0.1 s 9
Nanoscale for SPV measurements Nanoscale in bulk Ultrathin layers 1 100 nm 1 100 nm porous semiconductors nanoparticle crystals polymers metal oxide layers dyes on metal oxides self assembled monolayers 10
Introduction From bulk to nanostructure: SPV on porous Si Retarded SPV response and its origin Photovoltage on ultrathin layers Summary and challenges 11
From bulk to nanostructure porous silicon c-si mesoporous Si nanoporous Si internal surface external surface 7...10 nm 20...50 nm 2...4 nm 3...5 nm E C E V Surface states Ondulating quantum wires space charge region (SCR) 12
Th. Dittrich,V. Duzhko, 2002 SPV in mesoporous Si Photovoltage (V) 0.02 0.01 0.00-0.01 laser pulse 337 nm 0.1 µj/cm² p + -Si -U PV (V) 10 2 10 1 10 0 10-4 10-3 10-2 10-1 10 0 10 1 10 2 Intensity (µj/cm²) very similar to p + -Si -0.02 meso-ps (as prepared) 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Time (s) much less free charge carriers in mesoporous Si 13
Surface conditioning of Si nanoparticles hydrogenated oxidized in air + + + + + + + + - - - - - - - - [H 3 O] + OH - c-si SiO x hydrogenated surface 14
Th. Dittrich,V. Duzhko, 2002 Oxidation and HF treatment of free-standing mesoporous Si 10-1 oxidized mesoporous Si 20 50 nm U PV /Φ photon (arb.un.) 10-2 10-3 10-3 10-2 10-1 10 0 HF treated mesoporous Si p + - Si pinning of E F in the bulk negative surface charge 0.8 1.0 1.2 1.4 1.6 Photon energy (ev) p-type in the bulk positive surface charge 15
Reducing size of nanoparticles in mesoporous Si oxidation HF treatment at 400 C for 20 min (in air) c-si SiO 2 hydrogenated surface 16
Th. Dittrich,V. Duzhko, 2002 Size reduction of free-standing mesoporous Si 30 20 1 3 oxidized mesoporous Si 5 p + -Si(100) 10 mωcm, 50 ma/cm 2, 20 min 50%HF:Eth (1:1) Photovoltage (mv) 10 0 0-10 laser pulse 2 4 1 2 as prepared 3 4 5-20 as prepared HF treated mesoporous Si 10-9 10-8 10-7 10-6 10-5 10-4 10-3 Time (s) 17
Strong retardation of SPV signal in nanoporous Si Photovoltage (V) 10 0 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Time (s) 18
Introduction From bulk to nanostructure: SPV on porous Si Retarded SPV response and its origin Photovoltage on ultrathin layers Summary and challenges 19
Retarded SPV transients for very different materials independent of thickness (por-tio 2 ) depend on morphology 12 laser pulse: 337 nm, 5 ns 20 nm depend on atmosphere depend on temperature Photovoltage (mv) 8 4 0-4 -8 PZT porous TiO 2 * 2 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Ti 4+ Pb 2+ O 2- Time (s) : 3 nanoporous Si PPV role of transport depend on surface treatment not simply trapping poly(phenylenevinylene) n 20
Relaxation time of a space charge Dielectric relaxation time (s) 10 0 10-4 10-8 porous semiconductors εε σ 0 τ M = water ε = 1 ε = 6 ε = 12 ε = 76 semiconductors 10-12 10-12 10-8 10-4 10 0 Conductivity (1/Ωcm) σ = q ( n µ + p µ p ) Porous semiconductors: low n and p Time-of-Flight on nanostructured bulks µ ~ 10-4 10-7 cm²/vs Ambipolar diffusion: n t > τ M Independent diffusion: t < τ M 21
V. Duzhko, Th. Dittrich, 2002 Correlation experiment between SPV and IV Photovoltage (V) 0.03 mesoporous Si oxidized in air t peak at 400 C, 5 min 0.02 0.01 0.00 25 C 75 C 125 C 175 C Conductivity (Ω -1 cm -1 ) 10-6 10-7 10-8 Au / mesoporous Si / p + - Si 175 C 125 C 75 C 25 C 10-7 10-6 10-5 10-4 10-3 Time (s) -3-2 -1 0 Voltage (V) 22
Correlation between retardation and dielectric relaxation time 10-4 ε = 6 ε = 12 10-5 τ M (s) 10-6 t peak = τ M 10-7 10-7 10-6 10-5 10-4 t peak (s) 23
Independent and ambipolar diffusion regions t peak ε ε σ 0 τ M = Photovoltage (V) 10 0 independent diffusion ambipolar diffusion 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Time (s) 24
Th. Dittrich, I. Mora Sero, unpublished Transport in the region of independent diffusion: porous TiO 2 with adsorbed dye molecules 10-1 0.17 * I 0 I 0 0.015 * I 0 Photovoltage (V) 10-2 10-3 10-4 -70 C +30 C +80 C +180 C +230 C +270 C SPV ~ t α (dispersive transport) fast initial charge separation SPV ~ I exc at beginning 10-10 10-7 10-4 100 ps 10-10 10-7 10-4 Time (s) 10-10 10-7 10-4 25
Introduction From bulk to nanostructure: SPV on porous Si Retarded SPV response and its origin Photovoltage on ultrathin layers Summary and challenges 26
Charge separation at ultrathin and thin layers thin layer dominant charge separation within the layer ultrathin layer dominant charge separation by injection d(t) ~ 10 200 nm d ~ 1 10 nm 27
Non-conventional thin film preparation ILGAR (Ion Layer GAs Reaction) (Patent by Ch.-H. Fischer and H. Muffler) conformal layer deposition sequential process dipping in precursor salt solution sulfurization or oxidation in oven CdS on glass / SnO 2 :F substrate 1 dip 3 dips 6 dips 10 dips 20 dips 1 dip ~ 1 2 nm 28
Th. Dittrich, unpublished Charge separation at CdS layers 10-1 N 2 laser pulse 20 dips CdS / SnO 2 :F distinguished transients different thickness dependence different intensity dependence Photovoltage (V) 10-2 10-3 10-4 10 dips 6 dips 3 dips 1 dip 0 dips thin layer ultrathin layer SPV amplitude (V) 10-1 10-2 10-3 CdS / SnO 2 :F SPV transients N 2 laser pulses slope 1 10-6 10-5 10-4 10-3 10-2 Time (s) 1 10 Number of dips 29
Th. Dittrich, V. Duzhko, V. Kytin, 2002 Photovoltage relaxation in ultrathin TiO 2 layers TiO 2 / Ti a wave function overlap of the trapped hole in TiO 2 and the electron in Ti Photovoltage (V) 0.04 0.02 351 K 296 K thermal activation higher temperatures lower temperatures tunneling controlled recombination 0.00 10-7 10-6 10-5 10-4 10-3 10-2 Time (s) control by thermal emission U PV ( t) q 2 = p d 1 ln 1 0 + 2 ε ε 0 a 2 d 2 t τ e t τ ( kt ) d 30
Charge injection from dye molecules used in DSSCs (Dye Sensitized Solar Cells) N3: Ru (dc bpy H 2 ) 2 (NCS) 2 HOOC COOH M. Grätzel, B. O Regan (1991) binding site to TiO 2 HOOC N N N Ru N COOH S* NCS NCS S/S + SnO 2 :F binding site to CuI electrolyte TiO 2 B. Mahrov, A. Hagfeldt, 2004 31
B. Mahrov, Th. Dittrich, 2004 Charge separation by electron and hole injection from dye molecules Photovoltage (mv) 2 1 0 0.00-0.02 in-phase SPV f mod = 8 Hz Xe-lamp electron injection N3 on porous TiO 2 hole injection E g (TiO 2 ) E g (CuI) -0.04 N3 on CuI 1 2 3 4 Photon energy (ev) 32
Th. Dittrich, I. Mora Sero, unpublished Adsorbed dye molecules for model experiments on TiO 2 layers PV normalized to maximum SnO 2 :F / TiO 2 (ILGAR) 1 10 dips 3 dips 0.1 1 dip 1 dip ~ 1 2 nm 0 dips initial charge separation length traps in TiO 2 model system for intraparticle transport tailoring of surface and retardation 10-8 10-7 10-6 10-5 10-4 10-3 Time (s) 33
Introduction From bulk to nanostructure: SPV on porous Si Retarded SPV response and its origin Photovoltage on ultrathin layers Summary and challenges 34
charge separation length SPV by capacitor arrangement: 1 ns 0.1 s porous Si, porous TiO 2 and ultrathin layers as model systems retarded SPV transients and independent diffusion electron and hole injection from dye molecules into ultrathin layers 35
Charge separation in membranes and SAMs Membranes in biological systems SAMs with receptors and charge transfer molecules charge selective membranes for photovoltaics tailoring of interfaces charge transfer in molecules in-situ monitoring (closing the pressure gap) local light-induced polarization (combining with KPFM, STM, ) 36
Acknowledgement V. Kytin V. Timoshenko S. Gavrilov F. Koch V. Duzhko M. Lux-Steiner T. Guminskaya S. Bönisch Ch.-H. Fischer H. Muffler J. Rappich P. Hartig A. Hagfeld B. Mahrov J. Bisquert I. Mora Seró DAAD TAU Y. Shapira A. Merson 37
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