Principles of surface photovoltage (SPV) techniques and applications on solar cell materials

Transcription

1 Principles of surface photovoltage (SPV) techniques and applications on solar cell materials an extended lecture at the HZB Thomas Dittrich SPV (surface photovoltgae) tradition at HZB starting in the 6ies at the former Academy of Sciences of the former GDR (K. Heilig, H. Flietner, ): Investigation of surface state distribution on chemically treated csi surfaces after 1991 joined with the former HahnMeitnerInstitute managed by Tributsch and Flietner: further development of the equipment, Hterminated csi surfaces, insitu SPV for electrochmical treatments of csi surfaces (Th. Dittrich, H. Angermann, K. Kliefoth, J. Rappich ) setup for time, spectral and temperature dependent SPV over wide ranges of parameters for very different inorganic, organic and hybrid materials (Th. Dittrich) perspective: extended use of the great potential of SPV techniques just closing the pressure gap in surface science

2 Some research areas at HZB where SPV is involved csi surfaces (interface formation and conditioning, ) basic characteristics of materials (chalcopyrites, CuAlO, TiO, Cu 3 BiS 3, C 3 N 4,defects ) charge transport in nanoscale materials (nanoporous semiconductors, nanoparticles ) charge separation in hybrid systems (organicinorganic interfaces ) SPV very useful for research and development of materials and materials combinations for solar cells no contact preparation, can be applied in very different ambience (UHVgaseselectrolyte )

3 1 Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces Work functions of two metal electrodes energy E vac open contacts E F,s W s W ref E F,ref sample electrode reference electrode work functions

4 Contact potential difference between two metal electrodes energy CPD W ref shunted contacts E F W s electrons move from the electrode with the lower work function to the electrode with the higher work function until E F,s = E F,ref = E F W CPD = S q Q = CPD C W ref contact potential difference charge on the electrodes capacitor dq dc( t) I = = CPD dt dt AC current generation with C(t) for example, with a vibrating reference electrode Measurement of the CPD with an external potential energy W s W ref external bias periodic variation of the capacitance (vibration) I = [ CPD V ] b dc( t) dt V b q V b V b is adjusted to get a zero AC current I = : CPD = V b

5 Change of the CPD by a surface dipole energy W s ΔCPD W ref Presence of a surface dipole (ΔCPD<) I = [ CPD ΔCPD V ] b dc( t) ' dt V b is adjusted to get a zero AC current I = : CPD ΔCPD = Vb ' energy W s q V b W ref V b Role of the surface dipole for the sign of the change of the CPD CPD positive ΔCPD negative ΔCPD time without molecular dipole with molecular dipole

6 SPV: caused by light induced change of a surface dipole CPD positive ΔCPD negative ΔCPD time in the dark under illumination The sign of the surface photovoltage energy W s CPD W ref in the dark R m measurement resistance C m measurement capacitance a common E F is formed R m buffer under illumination energy W s CPD ΔCPD W ref light on at t = measurement interval: Δt << R m C m CPD is preserved q U ph positive voltage corresponds to the lower electron potential therefore: U ph ΔCPD R m buffer for Δt >> R m C m voltage at the external contact drops to zero due to discharging the capacitor with respect to ΔCPD

7 Surface space charge region of a depleted ntype semiconductor in the dark energy surface space charge region compensating charge on acceptor like surface states E F E C q ϕ E V Surface space charge region of a depleted ntype semiconductor under illumination energy q ϕ ill E C E Fp ΔCPD E V ntype under illumination: negative ΔCPD since excess electrons (holes) are separated towards the bulk (surface)

8 Surface space charge region of a depleted ptype semiconductor in the dark surface space charge region compensating charge on donor like surface states energy E C E F E V q ϕ Surface space charge region of a depleted ptype semiconductor under illumination energy E C E V E Fn q ϕ ill ΔCPD ptype under illumination: positive ΔCPD since excess electrons (holes) are separated towards the surface (bulk)

9 SPV in the picture of the parallel plate capacitor SPV d = Q ε ε Q Q Q charge being separated in space d distance between centers of positive and negative charge carriers ε = As/Vcm ε relative dielectric constant d separation length Center of charge from Poisson equation SPV e εε LS x () t = dx [ n( y, t) p( y, t)] dy total number of electrons per unit area L S N( t) = n( x, t) dx mean position (center of charge) of electrons 1 L S xn ( t) = x n( x, t) dx N( t) charge separation length d( t) = xn ( t) xp ( t) SPV e () t N( t) ( x ( t) x ( t) ) = εε n p

10 Sensitivity of surface photovoltage cm MOSFET photovoltaics cm U (V) cm scope molecule 1 8 cm 1 6 cm lockin d (nm) SCR d W d D x ntype space charge region W bulk semiconductor electrical field x d trapping x diffusion d δ δ some processes leading to charge separation d δ δ D x δ moleculeδ injection x polarization d d δ δ δ δ δ δ restructuring x D x internal photoemission

11 1 Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces SPV measurement in the Kelvinprobe arrangement light vibrating electrode sample V C(t) i(t) A d i( t) = V = dt [ C( t) ( ΔU )] resolution time given by lockin and feedback (usually > 1 ms) Kelvinprobe used in laboratory K8: Besocke DELTAPHI

12 Installation of SPV with Kelvinprobe at CISSY flange with Kelvinprobe Prof C.H. Fischer, Dr I. Lauermann, A. Grimm E. Zillner sputter chamber analysis chamber (XES, PES) central chamber manipulator transfer SPV load lock transfer glovebox sample holder in the UHV chamber (p < 1 8 mbar) controlled atmosphere quasi insitu sample preparation combination of different measurement techniques LED, spectral dependent measurements planned SPV measurement in the fixed capacitor arrangement light electrode mica sample C buffer R R = 1GΩ, C 1 pf SnO :F electrode on front side of a quartz cylinder for measurements between ~1 ns and ~1 ms (R = 1 GΩ) electrode pressed gently on the sample surface via a cardanic spring (Dittrich 1998)

13 buffer for standard measurement Electronic components minimum resolution time given by light pulse, buffer and scope maximum measurement time given by discharge (RC) sample connectors FET with buffer scope > 1 1 Ω pf R S C mica C sample ~1 pf/m C BNC R out 5 Ω R in SPV R m 5 Ω / 1 M Ω Logarithmic radout of transients: from 1 8 to 1 3 data points and averaging Number of the data point read out avaraging window around the data point first 1 MP without averaging variable averaging around each MP regarding a function log read out log increment for averaging int(((exp(mp/6)*1)48)/1) int(((exp(mp/6)*1)48)/1) int(((exp(mp/6)*1)48)/1) MP data point in the saved transient

14 Sophisticated transient measurements GAGE, CompuScope 14 in use 1 8 samples, resolution used 1 Ms/s logarithmic readout of transients over 8 orders of magnitudes logarithmic increment of samples for averaging differential transient measurement with two identical buffers and electrodes with the same light pulse to avoid correlated noise Photovoltage (mv).6.3. SnO :F / CdS shifted baselines channel A channel B A: illum. electrode laser pulse B: dark electrode Time (s) See for more details Th. Dittrich, S. Bönisch, P. Zabel, S. Dube, Rev. Scient. Instr. 79 (8) About the problem of SPV measurements over wide ranges buffer R signal sample I ph I D R p R s C m C i Rc C i for example, the capacitance of the surface space charge region R c contact resistance to the surface space charge region R parasitic resistance R m Photo voltage (V) CdS, ILGAR(alt) dips MehrbereichsLED: 5M light on example for a measurement with variable illumination time R m = 15 GΩ 1 ms 1 ms.1 s 1 s 15 s 1 s. Th. Dittrich, M. Franke, R. Subbiah, unpublished Time (s)

15 cryostate 18 C 3 C Tcontroller high vacuum system vacuum gauge gas supply SiO window buffer scope lockin SiO lenses pulse laser attenuator SiO prism monochromator with lamp reference signal trigger pulse 1 ns.1 s 1 3 s Kelvin probe fixed capacitor Nd:YAG, tunable, 5 ns Nd:YAG, 164, 53, 355 nm, 15 ps N laser, 337 nm, 5 ns Xe lamp, halogen lamp ev cw 3 khz ultrabright LEDs cw MHz measurement of spectra and transients (all materials) Lab K8 at HZB (Dittrich)

16 Mobile SPV setups testing samples, for example, in a glove box intensity dependent measurements box for Kelvinprobe with compact controller planned LED high impedance buffer measurement box SPV oscilloscope electrode sample battery constant current source for LED Example for biasdependent SPV (Sisurfaces) pulsed laser diode U F buffer trigger insulator N flow light pulse pulse on scope buffer, on/off voltage source U F, on/off SPV transient time PC / time control ϕ (U F ) surface state distribution of csi surfaces (D it ) Lab AHA 16 at HZB (Angermann)

17 Example for combined insitu SPV and PL in electrolytes detector laser diode Si filter N laser potentiostat, scangenerator RE CE WE sample holder screening SCOPE for SPV SCOPE for PL change of band bending and surface recombination (csi) Lab AHA 14 at HZB (Rappich) SPV with high spatial resolution (KPFM) Kelvin Probe Force Microscopy Kelvin probe between sample surface and AFM tip electrostatic force adjusted to zero by bias potential sophisticated method for very high spatial resolution Lab H 14 at HZB (Sadewasser)

18 Advantages of SPV techniques can be applied in very different ambience gas atmospheres vacuum, UHV electrolytes large experimental variability huge time range large spectral range high sensitivity dependence on any slow and fast processes leading to charge separation makes interpretation usually difficult importance of parameter control need for model systems SPV = Q( x, t, λ, T, Φ, R, L, D it d( x, t, λ, T, Φ, R, L, Dit, τ, Δn, N, τ...) ε ε ( x, t,...) T s ( E)...) 1 Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces

19 energy E C E V H α 1 W H thickness of the wafer W width of the space charge region α 1 absorption length L diffusion length Assumptions 1 no influence of drift no influence of absorber thickness low signal case L, α L, α 1 1 >> W << H Δp << n SPV = f ( Δn) = QE( λ) I( λ) (1 R( λ)) α( λ) L f D s( Δn ) 1 α( λ) L L x= QE Quantum efficiency I incident light intensity R reflectivity D diffusion coefficient s surface recombination velocity λ wavelength

20 E V E C energy SPV W α 1 H problem: surface recombination velocity depends on E Fp solution: surface recombination velocity remains constant by keeping the SPV constant The procedure (Goodman 1961) The light intensity is adjusted to keep the SPV constant at different wavelengths, i.e. at different absorption lengths. L L s L D R I QE L L s L D R I QE = ) ( 1 ) ( )) ( (1 ) ( ) ( ) ( 1 ) ( )) ( (1 ) ( ) ( λ α λ α λ λ λ λ α λ α λ λ λ ) ( ) ( 1 λ λ SPV SPV =

21 Assumptions QE( λ 1) (1 R( λ 1)) QE( λ ) (1 R( λ )) QE( λ1) I( λ 1) (1 R( λ1)) QE( λ ) I( λ ) (1 R( λ )) = 1 1 α( λ ) L α( λ ) L 1 I( λ I 1) ( λ ) = = const 1 1 α( λ ) L α( λ ) L 1 1 ( α( ) L) I ( λ) = const λ Determination of L light intensity 1 ( α( ) L) I ( α( λ)) = const λ L absorption length

22 Experimental setup monochromator with lamp beam splitter shielded box with sample, SPV electrode and high impedance buffer optical chopper photo detector feedback unit for SPV = const Lockin 1 Lockin reference signal Advantage: direct measurement of L set as a standard Disadvantages: α(λ) should be well known (o.k. for conventional semiconductors) not applicable for very short or very long L due to assumptions 1

23 1 Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces Donor and acceptor states donor acceptor occupied neutral negative nonoccupied positve neutral Surface and interface states are crucial for semiconductor devices such as solar cells. What is the distribution of surface states? What is the influence of chemical surface treatments? What limit can be reached by wet chemical surface passivation? SPV can be applied to study the density of surface states.

24 Dependence of charge in surface states on the surface Fermilevel energy E C E FS E C E FS E V E V shift of E FS leads to change in occupation of donor and acceptor surface states in depletion: charge in surface states is compensated by charge in the space charge region in inversion and strong inversion: appearance of additional mobile inversion charge Example: danglingbond model of the Si/SiO interface intrinsic DBs extrinsic DBs donor like / acceptor like / donor like / donor like / density of states Si O density of states E V E C energy E V E C energy

25 Free and trapped excess charge carriers in psi PL intensity (arb.un.) free carrier lifetime psi HF treated 3 K N laser pulses 15 MHz detector time dependent photoluminescence Photovoltage (V)..1. laser diode pulses 15 ns trapped excess charge carriers time dependent SPV Time (s) Th. Dittrich (1), unpublished Large signal SPV for measurement of surface band bending excitation of SPV by strong light pulses (Δn > n,p ): SPV close to its saturation value ϕ excitation of SPV by short light pulses: avoid light induced change of charge in surface states consideration of charge separation due to carrier diffusion (DemberSPV): strong dependence on light intensity and doping caused by different diffusion coefficients for electrons and holes ambipolar diffusion corresponds to DemberSPV

26 Large signal SPV for constant quasi Fermilevels β Doping factor b Mobility ratio ϕ surface band bending in the dark ϕ surface band bending under illumination Δp excess carrier concentration β = ni / n b = μ / μ n p u = ϕ q / kt u u U D kt b 1 δp b 1 = ln(1 ) q b 1 n β b / β u u Δp β ( e e ) β ( e e ) ( β β )( u u) = u u n e e i U ph = ϕ ϕ U D i 1 PV amplitude Dembervoltage determination of the surface band bending (δp should be measured independently) E.O. Johnson, Phys. Rev. 111 (1958) 153 experiment: analysis of the amplitude of SPV transients Independent measurement of the maximum excess carrier concentration and of the mobility ratio 15 measurement at 3 K with a laser diode (9 nm, 15 ns, 1 W) U D = kt q b 1 b 1 δp ln 1 b 1 b n U PV (mv) 1 5 measurement fit: δp max = 1 16 cm 3 b = 3.5 nsi(1) 5 kωcm I / I Th. Dittrich,, unpublished

27 MIS capacitor (metalinsulatorsemiconductor) energy E C E FS E V U F distribution of the voltage drop across the insulator and the space charge region charge neutrality condition du F = dui dϕ = dq dq dq ref SC it Determination of the surface state distribution distribution of the voltage drop du F = dui dϕ charge neutrality condition = dq dq dq ref SC charge on the gateelectrode dq ref = C du change of charge in surface states dq it i = q D it i dϕ it q D du F it = Ci ( 1) dϕ dq SC ( ϕ, n dϕ Y. W. Lam, J. Phys. D: Appl. Phys. 4 (1971) 137 surface band bending measured as a function of the applied field voltage C i from independent measurement Q SC calculated from ϕ and doping level )

28 Procedures for hydrogenation of nsi(111) S. Rauscher, Th. Dittrich, Appl. Phys. Lett 66 (1995) 318 Dependencies of U ph on U F for nsi(111) surfaces measurement cycle with increasing and decreasing U F values steepest dependence for lowest D it shift of U ph U F dependencies on U F axis corresponds to fixed surface charge hysteresis due to slow surfaces states (no full discharge during one cycle) S. Rauscher, Th. Dittrich, Appl. Phys. Lett 66 (1995) 318

29 D it analysis for hydrogenated nsi(111) surfaces D midgap it Ci du F = ( q dϕ U ph =.3V dq 1) dϕ SC U ph =.3V preparation D it midgap (ev 1 cm ) treatment includes anodic oxidation in the oscillating regime S. Rauscher, Th. Dittrich, Appl. Phys. Lett 66 (1995) Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces

30 From bulk to nanostructure porous silicon csi mesoporous Si nanoporous Si internal surface nm...4 nm external surface...5 nm nm E C surface states ondulating quantum wires E V space charge region (SCR) Size reduction of freestanding mesoporous Si oxidized mesoporous Si 5 p Si(1) 1 mωcm, 5 ma/cm, min 5%HF:Eth (1:1) Photovoltage (mv) 1 1 laser pulse 4 1 as prepared as prepared HF treated mesoporous Si oxidation HF treatment Time (s) csi at 4 C for min (in air) SiO hydrogenated surface Th. Dittrich,V. Duzhko, phys.stat.sol.(a) 197 (3) 17

31 Retarded SPV transients 1 independent of thickness (portio ) laser pulse: 337 nm, 5 ns depend on morphology nm depend on atmosphere depend on temperature Photovoltage (mv) PZT porous TiO * Ti 4 Pb O Time (s) : 3 nanoporous Si PPV role of transport depend on surface treatment not simply trapping poly(phenylenevinylene) n V. Duzhko, Th. Dittrich, Phys. Rev. B, 1, J. Appl. Phys., Dielectric relaxation time (s) porous semiconductors εε σ τ M = water ε = 1 ε = 6 ε = 1 ε = 76 semiconductors Conductivity (1/Ωcm) σ = q ( n μ p μ p ) Porous semiconductors: low n and p TimeofFlight on nanostructured bulks µ ~ cm²/vs Ambipolar diffusion: n t > τ M Independent diffusion: t < τ M

32 Diffusion photovoltage independent diffusion of excess electrons and holes until τ M is reached At longer times there will be the socalled Demberphotovoltage V. Timoshenko, V. Duzhko, Th. Dittrich, phys. stat. sol.(a), Approximation for electron diffusion: Analytical solution of diffusion equation e n i ( ) = λs SPV t Dt 1 exp( ) π εε 4Dt Dielectric relaxation time and t peak ε ε t S peak M = = λ τ σ D 1.5 Photovoltage (mv) λ (nm) S D(cm /s) Time (s) I. MoraSero, Th. Dittrich, G. GarcíaBelmonte, J. Bisquert, J. Appl. Phys., 6 Diffusion photovoltage in mesoporous Si correlation of conductivity and SPV(t) Conductivity (Ω 1 cm 1 ) Au / mesoporous Si / p Si 175 C 15 C 75 C 5 C Photovoltage (V).3 mesoporous Si oxidized in air at 4 C, 5 min C 75 C 15 C 175 C τ M (s) 1 4 τ M ε ε t peak = σ 1.5 ( ε = 3.5) 3 1 Voltage (V) Time (s) t peak (1.5 s) Adopted from V. Duzhko, F. Koch, Th. Dittrich, J. Appl. Phys.

33 Anomalous diffusion in porous TiO 1 I / 6 t > t peak : slope ½, as for bulk limited diffusion Photovoltage (V) C 3 C 18 C 7 C I / 67 t < t peak : slope different from ½, anomalous diffusion in the independent diffusion region one common intercept at the duration time of the laser pulse Time (s) anomalous diffusion and PV < x α PV > ~ t α = α / Th. Dittrich, I. MoraSeró, G. GarcíaBelmonte, J. Bisquert, Phys. Rev. B, 6 Retarded SPV and anomalous diffusion energy screening length x λ S low mobility, for example, due to trapping λ S determines detection limit for charge separation diffusion controls charge separation Debye length λ = D ε ε k B T e n

34 3D CTRW simulation under trap limitation (Continuous Time Random Walk) initial charge separation at t=t Δx(t ) = a L ongoing diffusion and recombination _ Δx(t) increasing charge separation length neutralization of electrons at x > λ S λ S / _ trap distribution N t ~ 1 cm 3 waiting time r random number 1/f = 1 ps dispersion parameter α Nt g(e) = kt SPV E exp kt 1 ti = ln( r) e f T =.5 T E / kt i (following J. Nelson, Phys. Rev. B, 1999) J. A. Anta, I. MoraSeró, Th. Dittrich, J. Bisquert, J. Phys. Chem. C 111 (7) Measured and simulated transients for portio repetition rate of laser pulses: 1 Hz for comparison: λ S less but of the same order in TOF measurements under SCLC conditions V. Kytin, Th. Dittrich, Phys. Rev. B, 3 J. A. Anta, I. MoraSeró, Th. Dittrich, J. Bisquert, J. Phys. Chem. C 111 (7) 13997

35 Tunneling controlled recombination in ultrathin TiO TiO deposition by ILGAR electron injection from adsorbed N3 dye molecules measurement at 7 C logarithmic decay a tunneling length b initial charge distribution parameter rate equation ( Δn( x, t)) Δn( x, t) x = exp t a Δ n( x, t) = n τ x t x exp exp exp b τ a qna t a U ( t) = εε τ b d dx Γ Γ b t, exp b τ x a I. MoraSeró, Th. Dittrich, A. Belaidi, G. GarciaBelmonte, J. Bisquert, J. Phys. Chem. B 19 (5) Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces

36 Pdcentred porphyrin 1 5 M Pdporphyrin in DCM optical length 1 cm N N Pd N N O OH Optical density Photon energy (ev) well pronounced and narrow Q and Sbands Very thin nptio (anatase) layer nm SnO :F substrate spin coating from suspension increase of internal surface about 1 times

37 Internal surface coverage with soaking time variation PdP on nptio (cm ) Normalized OD M PdP in DCM on ultrathin nptio dipped for 3 min dipped for 1 min Soaking time (min) Photon energy (ev) adsorption induced decrease of concentration in solution from submonolayer coverage clustering for higher coverage mv 8" 5 mv CPD (relative scale) 5 mv 1 mv mv 1' 1' 5' 1' SPV spectra of PdP / TiO (Kelvin probe) 5 mv 3' Photon energy (ev) P. Zabel, Th. Dittrich, M. Funes, E. N. Durantini, L. Otero, J. Phys. Chem. C 113 (9) 19.

38 mv drift A Q B S 8" A 5 mv CPD (relative scale) 5 mv 1 mv mv TiO 1' 1' 5' 1' Q A B A S 5 mv 3' Photon energy (ev) TiO mv 8" 5 mv CPD (relative scale) 5 mv 1 mv mv Q S 1' 1' 5' 1' Q S 5 mv 3' Photon energy (ev)

39 mv 8" CPD (relative scale) 5 mv 5 mv 1 mv mv 5 mv Q S Photon energy (ev) 1' 1' 5' 1' Q 3' C C C S C C Dominating mechanisms of charge separation ph O d HO d Pd p Pd n TiO shift of protons on physisorbed water (A) n TiO injection from isolated PdP (Q, S) into TiO d TiO p Pd ' d Pd p TiO n TiO charge separation in TiO n TiO injection from interacting PdP (Q, S ) into TiO

40 Effective medium model SPV is a superposition of all 4 processes positive charge separated in space SPV d = εε Pd p Pd d εε Pd ' p Pd ' d εε TiO p TiO d εε H O p H O effective charge separation lengths Rate equations (neglecting transport) dp dp dp dt TiO dt H O dt Pd dp dt Pd ' = G = G = G Pd = G TiO H O Pd ' B B B B TiO H O Pd Pd ' p p p p TiO Pd Pd ' n H O n n TiO n TiO TiO TiO assumption: all separated negative charge insight TiO, i.e. there is one common n TiO with one center of charge dn TiO dt = G TiO G G G B H O Pd Pd ' TiO B B B H O Pd p Pd ' p TiO Pd p p Pd ' n H O n TiO TiO n n TiO TiO generation rates effective recombination constants

41 Generation rates G G G G TiO Pd Pd ' H O = Φ OD = Φ OD = Φ OD = G Pd ' TiO Pd Pd ' = Φ OD Pd ' OD TiO hω 3.4 =.875 exp Et optical thickness of TiO : 1 nm E t.8 ev, uncertainty due to defects Photon flux (arb. un.).5. measured with pyroelectric detector Photon energy (ev) OD = OD Pd OD exp Pd ' relation between OD Pd and OD Pd as a free quasifitting parameter Φ photon flux calibrated with a Siphotodiode at 9 nm: cm Example for simulation ΔCPD (V) measurement simulation soaking time 1 min injection from interacting molecules d/ε Pd = 1 8 cm injection from noninteracting molecules d/ε Pd = cm Optical density.4. Q, S 1 x Q', S' Photon energy (ev) photogeneration in TiO d/ε TiO = cm B Pd = 1 1 cm³/s B Pd = cm³/s B TiO = 1 1 cm³/s P. Zabel, Th. Dittrich, M. Funes, E. N. Durantini, L. Otero, J. Phys. Chem. C 113 (9) 19.

42 1 7 strong differences for different processes of charge separation d ε rel 1 (cm) ε rel unclear for the different processes ( 3 for TiO, 1 for vacuum) B (cm 3 s 1 ) (PdP / TiO)' PdP / TiO TiO HO clustering of molecules leads to strong increase of B (role of intermolecular transport) Soaking time (min) P. Zabel, Th. Dittrich, M. Funes, E. N. Durantini, L. Otero, J. Phys. Chem. C 113 (9) Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces

43 Internal photoemission and exciton dissociation energy x H TPP Au energy x 1 _ 3 5 _ 4 TPP: tetraphenylporphyrin internal photoemission 1 absorption exciton formation 3 exciton diffusion 4 exciton dissociation 5 escape of holes Charge separation at H TPP / Au Absorption (arb.un.) 3. H TPP (5 nm) / glass Q bands H TPP / Au S band nm negative surface charging of H TPP / Au ΔCPD (V).1. 1 nm 5 nm photovoltage proportional to the layer thickness for H TPP / Au.1. 1 ML H TPP / TiO no direct correlation with absorption for H TPP / Au Photon energy (ev) Y. Zidon, Y. Shapira, L. Otero, Th. Dittrich, Phys. Rev. B 75 (7) 19537

44 Internal PE of electrons from Au into H TPP 1 H TPP(5 nm) / Au ΔCPD / Φ ph (arb.un.) 1 1 ΔCPD α α shifted by.81 ev α shifted and integrated Photon energy (ev) correlation of the photovoltage with the superposition of the shifted absorption and integrated absorption spectra evidence for charge separation by internal photoemission Y. Zidon, Y. Shapira, L. Otero, Th. Dittrich, Phys. Rev. B 75 (7) Slow transients on thin and thick H TPP / Au H TPP / Au ΔCPD mv light on light off light on 5 nm abs. photovoltage (mv) 1 nm 1 nm 5 nm E A =.4 ev.5 Hz 1 s Time (s) 75 nm /T (1/K) slow relaxation process: negative surface charge faster relaxation process: positive surface charge activation energy of the negative polaron Y. Zidon, Y. Shapira, L. Otero, Th. Dittrich, Phys. Rev. B 75 (7) 19537

45 1 Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces Electron injection from CdSeQDs into TiO I. MoraSeró, J. Bisquert, Th. Dittrich, A. Belaidi, A. S. Susha, A. L. Rogach, J. Phys. Chem. C 111 (7) CdSe f mod =.5 Hz CdSeQDs / TiO TiO deposition by dipping in the aqueous solution TiO d Photovoltage (µv) 6 3 d = nm d = 1 nm Photon energy (ev)

46 Transport of injected electrons in TiO Photovoltage (µv) 1 1 slope.73 power law intensity dependence Φ photon flux SPV β Φ Light intensity (arb.un.) I. MoraSeró, J. Bisquert, Th. Dittrich, A. Belaidi, A. S. Susha, A. L. Rogach, J. Phys. Chem. C 111 (7) Charge separation from CdSe QDs into ultrathin TiO normalized OD OD: CdSe QD1 CdSe QD CdSe QD3 CdSe QD4 PV / Φ: CdSe QD1 CdSe QD CdSe QD3 CdSe QD normalized PV / Φ Photon energy (ev) I. MoraSeró, J. Bisquert, Th. Dittrich, A. Belaidi, A. S. Susha, A. L. Rogach, J. Phys. Chem. C 111 (7) 14889

47 Layerbylayer assembly of watersoluble nanocrystals thickness control with a quartz microbalance (exchange of solution) control of optical absorption, up to cycles min for one monolayer of nanoparticles 8 min for saturation within one cycle 8 min 1 min 1 min 1 min TGA H O PDDA H O solution with negatively charged nanoparticles (ph 1) solution with positively charged PDDA A. Rogach, Chem. Mater. 1 () 156 Direct observation of charge separation in type II tunneling multilayered structures of CdTeQDs / CdSeQDs Energy CdTe CdSe diffusion of excitons disruption of excitons charge separation charge transport recombination 3. nm.9 nm ~1 () nm nanocrystals coated with thioglycolic acid deposition controlled by PDDA N layers variation of number of layers (N) variation of spacing between layers D.Gross, I.MoraSeró, Th.Dittrich, A.Belaidi, C.Mauser, A.J. Houtepen, E.da Como, A.L.Rogach, J.Feldmann, JACS 13 (1) 5981

48 Dependence of the direction of charge separation on the order of QDlayer depostion Photovoltage (mv) 5 SnO :F PDDA () CdSe (TGA) CdTe (MGA) in phase phaseshifted by Photon energy (ev) Photovoltage (mv) 5 SnO :F PDDA () CdSe (TGA) CdTe (MGA) in phase phaseshifted by Photon energy (ev) A: spacing between QD layers 1 nm 1xCdTe / 6x CdSe B: spacing between QD layers 1 nm 1xCdSe / 6x CdTe S: spacing between QD layers nm 1xCdSe / 4x CdTe R: spacing between QD layers 1 nm 1xCdSe / 4xCdTe D.Gross, I.MoraSeró, Th.Dittrich, A.Belaidi, C.Mauser, A.J. Houtepen, E.da Como, A.L.Rogach, J.Feldmann, JACS 13 (1) 5981

49 D.Gross, I.MoraSeró, Th.Dittrich, A.Belaidi, C.Mauser, A.J. Houtepen, E.da Como, A.L.Rogach, J.Feldmann, JACS 13 (1) Contact potential difference and surface photovoltage (SPV) Measurement configurations and instrumentation examples 3 SPV after Goodman: determination of the diffusion length in semiconductors 4 Measurement of the surface state density by large signal SPV on csi 5 SPV transients: recombination processes and dielectric relaxation time 6 On the effective charge separation length: model experiments with clustered molecules 7 Thin organic layers: internal photoemission and exciton dissociation in H TPP 8 Charge separation in quantum dot layers 9 SPV on chalcopyrite surfaces

50 Influence of surface treatments.3 Mo / CIGSe ΔCPD (V)..3.6 / KCN etched / CdS / In S 3 / CdS / ZnO / In S 3 / ZnO Photon energy (ev) E. Zillner, A. Gonzales, S. Sadewasser, Th. Dittrich, unpublished Work function differences and SPV maximum ΔCPD for CdS / ZnO system decrease of ΔCPD for In S 3 system after deposition of ZnO no direct correlation between ΔCPD ad SPV CPD dark (V)..5.4 SPV (V) KCN etched KCN / CdS KCN / In S 3 KCN / CdS / ZnO KCN / In S 3 / ZnO E. Zillner, A. Gonzales, S. Sadewasser, Th. Dittrich, unpublished

51 Different behaviour of CdS and In S 3 buffer layers depletion due to fixed positive (projected) surface charge Q in general: the larger Q the larger the band bending and therefore SPV Origin of Q unoccupied donor states in the CdS and In S 3 surface layers CIGSe / In S 3 and CIGSe / CdS systems SPV(In S 3 ) > SPV(CdS) Q (In S 3 ) > Q (CdS) CIGSe / In S 3 / ZnO and CIGSe / CdS / ZnO systems SPV(In S 3 ) > SPV(In S 3 /ZnO): Q decreased after ZnO deposition SPV(CdS) < SPV(CdS/ZnO): Q increased after ZnO deposition E. Zillner, A. Gonzales, S. Sadewasser, Th. Dittrich, unpublished Intensity dependent work function differences monitoring of light on light off systuration values SPV max =.537 V (CIGSe / CdS / ZnO system) very abrupt (.3 V in one decade) onset of the small signal region (SPV intensity) role of shunt resistance or trapping of electrons and holes and trap saturation ideality factor (n) 1.5. CIGSe / CdS / ZnO.1 green LED Photovoltage (V) ΔCPD (V).6 NG3 filter LED current variation on 1% 3% 6%.9 off off off Time (s) 1% RGB Intensity (arb. un.) E. Zillner, A. Gonzales, S. Sadewasser, Th. Dittrich, unpublished

52 Contact free measurement of transport parameters in solar cells chargeselective contacts can be characterized without completing the solar cell detailed analysis of losses at different interfaces becomes possible q VOC V I SC = I exp n kb T 1 R OC p n I (arb.un.) R p (arb. un.) In S > 1 4 CdS / ZnO In S 3 / ZnO I = I ( ϕ, p, Δn, n, μ, μ, trapping, surface recombination)? i n p E. Zillner, A. Gonzales, S. Sadewasser, Th. Dittrich, unpublished d W d D x ntype space charge region W bulk semiconductor electrical field x d trapping x diffusion d δ δ some processes leading to charge separation d δ δ D x δ moleculeδ injection x polarization d d δ δ δ δ δ δ restructuring x D x internal photoemission

53 References including SPV 1: C. Sahin, Th. Dittrich, C. Varlikli, S. Icli, M. Ch. LuxSteiner, Role of side groups in pyridine and bipyridine ruthenium dye complexes for modulated surface photovoltage in nanoporous TiO, Sol. En. Mats. & Sol. Cells 94 (1) 686. F. Mesa, G. Gordillo, T. Dittrich, K. Ellmer, R. Baier, S. Sadewasser, Transient surface photovoltage of ptype Cu 3 BiS 3, Appl. Phys. Lett. 96 (1) L. Sheppard, T. Dittrich, J. Nowotny, T. Bak, Photovoltage studies of nonstoichiometric rutile titanium dioxide, Appl. Phys. Lett. 96 (1) 715. M. Funes, P. Zabel, Th. Dittrich, E. N. Durantini, L. Otero, Interaction induced transition in the nanoporous TiO / Pdporphyrin system, phys.stat.sol.(c) 7 (1) 8. I.MoraSeró, D. Gross, T. Mittereder, A. A. Lutich, A. S. Susha, T. Dittrich, A. Belaidi, R. Caballero, F. Langa, J. Bisquert, A. L. Rogach, Nanoscale interaction between CdSe or CdTe nanocrystals and molecular dyes fostering or hindering directional charge separation, small 6 (1) 1. P. Zabel, Th. Dittrich, M. Funes, E. N. Durantini, L. Otero, Charge separation at Pdporphyrin / TiO interfaces, J. Phys. Chem. C 113 (9) 19. P. Zabel, Th. Dittrich, Y.L. Liao, C.Y. Lin, K.T. Wong, F. Fungo, L. Fernandez, L. Otero Engineering of gold surface work function by electrodeposition of spirobifluorene donoracceptor bipolar systems, Organic Electronics 1 (9) 137. Th. Dittrich, S. Bönisch, P. Zabel, S. Dube, High precision measurement of surface photovoltage transients, Rev. Scient. Instr. 79 (8) Y. Zidon, Y. Shapira, H. Shaim, Th. Dittrich, Interactions at tetraphenylporphyrin / InP interfaces observed by surface photovoltage spectrocopy, Appl. Surf. Sci. 54 (8) 355. I. MoraSeró, Th. Dittrich, A. S. Susha, A. L. Rogach, J. Bisquert, Large increase of electron extraction from CdSe quantum dots into TiO by N3 dye coadsorption, Thin Solid Films 516 (8) References including SPV : J. Mwabora, K. Ellmer, A. Belaidi, J. Rappich, W. Bohne, J. Röhrich, Th. Dittrich, Reactively sputtered TiO layers on SnO :F substrates: a Raman and surface photovoltage study, Thin Solid Films 516 (8) R. Beranek, B. Neumann, S. Sakthivel, M. Janczarek, Th. Dittrich, H. Tributsch, H. Kisch, Exploring the electronic structure of nitrogenmodified TiO photocatalysts through photocurrent and surface photovoltage studies, Chem. Phys. 339 (7) 11. MoraSeró, J. Bisquert, Th. Dittrich, A. Belaidi, A. S. Susha, A. L. Rogach, Photosensitization of TiO layers with CdSe quantum dots: Correlation between light absorption and photoinjection, J. Chem. Phys. C 111 (7) Y. Zidon, Y. Shapira, Th. Dittrich, Modulated charge separation at tetraphenylporphyrin / Au interfaces, Appl. Phys. Lett. 9 (7) 1413 Y. Zidon, Y. Shapira, Th. Dittrich, L. Otero, Light induced charge separation in thin tetraphenylporphyrin layers deposited on Au, Phys. Rev. B 75 (7) Th. Dittrich, B. Neumann, H. Tributsch, Sensitization via reversibly inducible Ru(dcbpyH ) (NCS) TiO charge transfer complex, J. Phys. Chem. C 111 (7) 65. MoraSeró, Th. Dittrich, G. GarciaBelmonte, J. Bisquert, Determination of spatial charge separation of diffusing electrons by transient photovoltage measurements, J. Appl. Phys. 1 (6) Th. Dittrich, I. MoraSeró, G. GarciaBelmonte, J. Bisquert, Temperature dependent normal and anomalous diffusion in porous TiO studied by transient surface photovoltage, Phys. Rev. B 73 (6) I. MoraSeró, Th. Dittrich, A. Belaidi, G. GarciaBelmonte, J. Bisquert, Observation of diffusion and tunneling recombination of dyephotoinjected electrons in ultrathin TiO layers by surface photovoltage transients, J. Phys. Chem. B 19 (5) C. LévyClement, S. Lust, M. Mamor, J. Rappich, Th. Dittrich, Investigation of ptype macroporous silicon formation, phys. stat. sol. (a) (5)

54 References including SPV 3: B. R. Sankapal, A. Ennaoui, T. Guminskaya, Th. Dittrich, W. Bohne, J. Röhrich, E. Strub, M. Ch. LuxSteiner, Characterization of pcui prepared by the SILAR technique on Cutape/nCuInS for solar cells, Thin Solid Films (5) 14. Th. Dittrich, H.J. Muffler, M. Vogel, T. Guminskaya, A. Ogacho, A. Belaidi, E. Strub, W. Bohne, J. Röhrich, O. Hilt, M. Ch. LuxSteiner, Passivation of TiO by ultrathin alumina, Appl. Surf. Sci. 4 (5) 36. Th. Dittrich, L. Dloczik, T. Guminskaya, M. Ch. LuxSteiner, N. Grigorieva, I. Urban, Photovoltage characterization of CuAlO crystallites, Appl. Phys. Lett. 85 (4) 74. B. Mahrov, Th. Dittrich, L. Dloczik, G. Boschloo, A. Hagfeldt, Photovoltage study of charge injection from dye molecules into transparent hole and electron conductors, Appl. Phys. Lett. 84 (4) Th. Dittrich, V. Duzhko, Photovoltage in freestanding mesoporous silicon layers, phys. stat. sol. (a) 197 (3) 17. V. Duzhko, F. Koch, Th. Dittrich, Transient photovoltage and dielectric relaxation time in porous silicon, J. Appl. Phys. 91 () 943. Th. Dittrich, V. Duzhko, V. Kytin, J. Rappich, F. Koch, Trap limited photovoltage in ultrathin metal oxide layers, Phys. Rev. B 65 () P. Hartig, Th. Dittrich, J. Rappich, Engineering of Si surfaces by electrochemical grafting of pnitrobenzene molecules, Appl. Phys. Lett. 8 () 67. P. Hartig, Th. Dittrich, J. Rappich, Surface dipole formation and nonradiative recombination at psi(111) Si surfaces during electrochemical deposition of organic layers, J. Electroanal. Chem. 545 () 1. V. Duzhko, V. Yu. Timoshenko, F. Koch, Th. Dittrich, Photovoltage in nanocrystalline porous TiO, Phys. Rev. B 64 (1) V. Duzhko, Th. Dittrich, B. Kamenev, V. Yu. Timoshenko, W. Brütting, Diffusion photovoltage in poly(pphenylenevinylene, J. Appl. Phys. 89 (1) 441. References including SPV 4: Th. Dittrich, Th. Burke, F. Koch, J. Rappich, Passivation of an anodic oxide / psi interface stimulated by electron injection, J. Appl. Phys. 89 (1) V. Yu. Timoshenko, V. Duzhko, Th. Dittrich, Diffusion photovoltage in porous semiconductors and dielectrics, phys. stat. sol. (a) 18 () 7. Th. Dittrich, M. Schwartzkopff, E. Hartmann, J. Rappich, On the origin of the positive charge on hydrogenated Si surfaces and their dependence on the surface morphology, Surface Science 437 (1999) 154. V. Yu. Timoshenko, E. A. Konstantinova, Th. Dittrich, Investigation of photovoltage in porsi/psi structures by pulsed photovoltage method, Fizika i tekhnika poluprovodnikov 3 (1998) 613. V. Yu. Timoshenko, P. K. Kashkarov, A. B. Matveeva, E. A. Konstantinova, H. Flietner, Th. Dittrich, Influence of photoluminescence and trapping on the photovoltage at the porsi/psi structure, Thin Solid Films 76 (1996) 16. S. Rauscher, Th. Dittrich, M. Aggour, J. Rappich, H. Flietner, H. J. Lewerenz, Reduced interface state density after photocurrent oscillations and electrochemical hydrogenation of nsi(111): a SPV investigation, Appl. Phys. Lett. 66 (1995) 318. Th. Dittrich, S. Rauscher, Th. Bitzer, M. Aggour, H. Flietner, H. J. Lewerenz, Electronic properties of nsi(111) during electrochemical surface transformation towards Htermination, J. Electrochem. Soc. 14 (1995) 411. Th. Dittrich, H. Angermann, H. Flietner, Th. Bitzer, H. J. Lewerenz, Surface electronic properties of electrolytically hydrogen terminated Si (111), J. Electrochem. Soc. 141 (1994) Th. Dittrich, H. Angermann, W. Füssel, H. Flietner, Electronic properties of the HFpassivated Si (111) surface during the initial oxidation in air, phys. stat. sol. (a) 14 (1993) 463. Th. Dittrich, M. Brauer, L. Elstner, Simultaneous determination of surface potential and excess carrier concentration with the pulsed surface photovoltage method, phys. stat. sol. (a) 137 (1993) K9.