Substrate-Dependent Photoconductivity Dynamics in a High-Efficiency

Jan 17, 2019 - Film conductivity was interrogated by measuring the in-phase and out-of-phase forces acting between the film and a charged microcantile...
2 downloads 0 Views 11MB Size
Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE

C: Energy Conversion and Storage; Energy and Charge Transport

Substrate-Dependent Photoconductivity Dynamics in a High-Efficiency Hybrid Perovskite Alloy Ali Moeed Tirmzi, Jeffrey A. Christians, Ryan Patrick Dwyer, David T. Moore, and John A. Marohn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11783 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Substrate-Dependent Photoconductivity Dynamics in a High-Eciency Hybrid Perovskite Alloy Ali Moeed Tirmzi,† Jerey A. Christians,‡ Ryan P. Dwyer,† David T. Moore,‡ and John A. Marohn∗ † ,

†Dept. of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA ‡National Renewable Energy Laboratory, Golden, CO 80401, USA E-mail: [email protected]

Abstract

in a lead-halide perovskite crystal depends on the concentration (i.e., the chemical potential) of the relevant

Films

of

(FA0.79 MA0.16 Cs0.05 )0.97 Pb(I0.84 Br0.16 )2.97

chemical species present in the solution or vapor from

conductivity was interrogated by measuring the in-

13,5,9 Nonequi10 librium growth of the perovskite in the thin-lm form

phase

were grown over TiO2 , SnO2 , ITO, and NiO. and

out-of-phase

forces

acting

which the perovskite was precipitated.

the

should generate additional point- and grain-boundary

We followed the

defects. The concentration of defects in the crystal also

lms' conductivity vs. time, frequency, light intensity,

depends on the electron and hole chemical potential

and

con-

which  if the perovskite's background carrier concen-

ductivity was high and light-independent over ITO

tration is suciently low  could be strongly aected

and NiO.

by the substrate.

lm and a charged microcantilever. temperature

(233

to

312 K).

between

Film

Perovskite

Over TiO2 and SnO2 ,

the conductivity

Evidence that the substrate aects

was low in the dark, increased with light intensity,

band alignment and induces p- or n-type conductivity

and persisted for 10's of seconds after the light was

can be seen in XPS,

removed.

ments of lead-halide perovskite lms, in one example in

At elevated temperature over TiO2 ,

the

rate of conductivity recovery in the dark showed an activated temperature dependence (Ea

a lm as thick as

= 0.58 eV).

11 UPS, 12,13 and IPES 12 measure-

400 nm. 12

How the substrate changes

the near-surface and bulk conductivity of the perovskite

13 eects include the for-

Surprisingly, the light-induced conductivity over TiO2

is a topic of current research;

and SnO2 relaxed essentially instantaneously at low temperature. We use a transmission-line model for

mation of an interface dipole, the creation of a chem-

mixed ionic-electronic conductors to show that the

changes in perovskite lm morphology.

ically distinct passivation layer, and substrate-induced

measurements presented are sensitive to the sum of electronic and ionic conductivities.

Defects in halide perovskites are challenging to study

We rationalize

for a number of reasons.

Many of these defects are

the seemingly incongruous observations using the idea

mobile under the application of electric eld and/or

that

light, with iodine species and vacancies considered to

holes,

introduced

either

by

equilibration

with

1422 Moreover, recent reports

the substrate or via optical irradiation, create iodide

be most mobile species.

vacancies.

by Maier and coworkers show that the concentration of mobile iodine vacancies depends on illumination inten-

1

19 This eect, which is expected from defect-energy 1,3,9 needs to be considered in addition to calculations,

Introduction

sity.

the established eects of light on charge motion and

The extraordinary performance of solar cells made from

polarization when trying to understand light-related

solution-processed lead-halide perovskite semiconduc-

hysteresis phenomena.

tors is attributed to the material's remarkably high defect tolerance and low exciton binding energy.

15 The

Here rial

theoretically predicted ionic defect formation energy is

we with

15,20,2329

study

a

precursor

high-performing solution

relatively low and consequently the equilibrium defect

(FA0.79 MA0.16 Cs0.05 )0.97 Pb(I0.84 Br0.16 )2.97

solar cells to reach their Shockley-Queisser limit, it is

substrates  TiO2 , SnO2 , ITO, and NiO.

2,6 For perovskite concentration should be quite high.

(here-

after referred to as FAMACs) grown over four dierent tians and coworkers reported

necessary to understand how these defects form and

1000

hours

3032 Chris-

operational

stability for FAMACs devices prepared with an SnO2

identify which ones contribute to non-radiative recombination, loss of photovoltage, and device hysteresis.

mate-

stoichiometry

7,8

electron acceptor layer. based

At equilibrium, the concentration of a specic defect

stable.

devices,

SnO2

devices

were

much

more

While degradation of TiO2 devices has previ-

ACS Paragon Plus Environment 1

the

30 When compared to TiO 2

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ously solely been attributed to ultraviolet light induced degradation,

Page 2 of 20

dependent.

33,34 they revealed, using ToF-SIMS mea-

As in Ref. 35, here we follow conductivity dynamics

surements, dierent ionic distributions in TiO2 - and

using a charged microcantilever.

SnO2 -based devices after several hours of operation.

primarily used in scanning-probe microscope experi-

This observation demonstrates a clear dierence in the

ments to create images. However, they have also proven

light and/or electric eld induced ion/vacancy motion

useful in non-scanning experiments because of their

in SnO2 - and TiO2 -based devices. Motivated by these ndings, here we measure the

tremendous sensitivity as force sensors.

AC conductivity of the FAMACs lms in the kHz to

perovskite solar-cell materials have used Kelvin probe

MHz regime and study this conductivity as a func-

force microscopy to observe the dependence of the sur-

tion of light intensity, time, and temperature.

We

face potential and surface photovoltage on time, elec-

show that the light-on conductivity returns to its ini-

tric eld, and light intensity in order to draw con-

100

clusions about the spatial distribution of charges and

tial light-o value on two distinct timescales (sub ms and

10's

Microcantilevers are

Prior scan-

ning probe microscopy (SPM) studies of lead-halide

of seconds) in the material grown on the

ions.

5162 In studies of organic solar cell materials,

electron accepting substrates TiO2 and SnO2 . In con-

frequency-shift measurements have been used to follow

trast, material grown on the hole acceptor (NiO) and

the time evolution of photo-induced capacitance and

ITO substrates shows frequency-independent conduc-

correlate the photocapacitance risetime with device

tivity. We tentatively assign these distinct behaviors to

performance.

dierences in the perovskites' background carrier type

used

and concentration.

icon

monitor

local

dopant

concentration

in

sil-

44 and GaAs; 36 probing quantizied energy levels 38 examine photo-induced damage in in quantum dots; 41,42 quantify local dielectric organic solar cell materials;

We nd that the SnO2 -substrate

lms show higher dark conductivity and slower relaxation than the TiO2 -substrate lms.

to

6367 Sample-induced dissipation has been

We show that

43,4649,68 and probe

at room temperature and above, the relaxation of the

uctuations in insulating polymers;

conductivity is activated over TiO2 (and possibly over

dielectric uctuations and intra-carrier interactions in

SnO2 also).

semiconducting small molecules.

Surprisingly, the relaxation of conduc-

39 Here we make use of

tivity becomes considerably faster when the sample is

the tremendous sensitivity of a charged microcantilever

cooled to a low temperature of 233 K.

to passively observe the time evolution of a thin-lm

The simplest

35 through changes in cantilever

explanation we can devise for these diverse observa-

sample's conductivity

tions is that the measured conductivity changes arise

dissipation induced by conductivity-related electric-eld

from light-dependent electronic uctuations; at room

uctuations in the sample.

69

temperature, these electronic uctuations are coupled to slow, light-induced ionic/vacancy uctuations that are frozen out at low temperature.

2

Our observation

that the timescale of the conductivity recovery in the

2.1 Materials

SnO2 -substrate sample is much slower than in the TiO2 substrate sample supports the Christians

et al.

hypoth-

Methylammonium bromide (CH3 NH3 Br, MABr), and

esis of slower ionic motion in the SnO2 -substrate sample

formamidinium iodide (CH(NH2 )2 ,

30

compared to TiO2 -substrate sample. These experiments were motivated by our previ-

FAI),

were pur-

chased from Dyesol and used as received.

Lead (II)

iodide (99.9985% metals basis) and the SnO2 colloid

ously reported scanning probe microscopy study of

precursor (Tin(IV) oxide,

light- and time-dependent conductivity in a thin lm

35

Experimental Section

15%

in H2 O colloidal disper-

sion) were purchased from Alfa Aesar. All other chem-

3649

of CsPbBr3 . We used sample-induced dissipation 50 and broadband local dielectric spectroscopy (BLDS)

icals were purchased from Sigma-Aldrich and used as received.

to demonstrate for CsPbBr3 that conductivity shows o, with an activation energy and time-scale consis-

2.2 Oxide Layer Deposition

tent with ion motion.

a slow activated recovery when the light was switched We concluded that the sample

Indium tin oxide (ITO) glass was cleaned by soni-

conductivity dynamics were controlled by the coupled

cation in acetone and isopropanol, followed by UV-

motion of slow and fast charges. While CsPbBr3 served

ozone cleaning for

as a sample robust to temperature- and light-induced

additional thin oxide layer was deposited on the ITO

degradation, it has a relatively high band gap and is

glass (if necessary).

thus poorly suited for use in high eciency solar cells.

a previously reported low temperature TiO2 process.

Many high eciency devices reported to date rely on a

15 min.

Following cleaning,

an

TiO2 layers were deposited using

mixed cation/anion perovskite absorber layer (such as

Briey, TiO2 nanoparticles were synthesized as reported 70 and a 1.18 wt. previously by Wojciechowski et al.

FAMACs) to reach the desired bandgap and enhanced

%

stability needed for photovoltaic applications. The goal

nium diisopropoxide bis(acetylacetonate), was spin-cast

of the present study is to ascertain whether the con-

onto the ITO substrates with the following procedure:

ductivity dynamics observed for CsPbBr3 are evident

700

in FAMACs lms and to see whether they are substrate

Tin oxide electron transport layers were deposited on

ethanolic suspension,

rpm,

10 sec;

ACS Paragon Plus Environment 2

along with

1000 rpm,

10 sec;

20 mol %

2000 rpm,

tita-

30 sec.

Page 3 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry 71 The aqueous SnO colloid 2

charge is modulated by physically oscillating the can-

solution, obtained from Alfa Aesar, was diluted in water

tilever, by applying a time-dependent voltage to the

cleaned ITO substrates. with a ratio of

1 : 6.5

and spin-cast at 3000 rpm for

cantilever tip, or by doing both simultaneously. A sum-

30 sec. Both the TiO2 and SnO2 lms were then dried ◦ at 150 C for 30 min and cleaned for 15 min by UV-ozone

mary of the distinct measurements carried out below is given in Figure 1.

immediately before use. NiO lms were deposited from

Changes in the cantilever frequency and amplitude

H

a solution of nickel nitrate hexahydrate and ethylenedi-

may be expressed in terms of a transfer function

amine in ethylene glycol following a previously reported

which relates the voltage

procedure.

tip (the sample substrate is grounded) to the voltage

72

Vts

applied to the cantilever

Vt

dropped between the cantilever tip and the sample sur-

2.3 FAMACs Perovskite Film Deposition

operating in parallel with a capacitor

Deposition of the FAMACs perovskite layers was car-

The resulting transfer function is given in the frequency

ried out in a nitrogen glovebox following the method

domain by

face. The cantilever is modeled electrically as a capacitor

reported in Ref. 31. The precursor solution was made

172 mg

while the sample is modeled as resistor

FAI,

1000

for

20 sec.

10 sec,

rpm for

While the substrate was spinning,

1 + j g −1 ωτfast ˆ . H(ω) = 1 + j ωτfast

6000 rpm

0.1 mL

of

chlorobenzene was rapidly dripped onto the lm with approximately

6 sec

where

remaining in the spin-coating pro-

cedure, forming a transparent orange lm.

1

were then annealed for

hr at



100 C

Z.

The lms

to form highly

(5

× 10=6 mbar)

were in

a

performed

custom-built

(Rs−1 + jωCs )−1

(2)

denes the sample impedance

The transfer function

H

can be viewed as a lag

are given by, respectively,

τfast = Rs (Cs + Ctip ) = Rs Ctot

2.4 Scanning probe microscopy experiments

(1)

compensator whose time constant and gain parameter

specular FAMACs perovskite lms.

All

Rs

(Figure S2).

which simplies to

deposited by spin coating this precursor solution with the following procedure:

Cs

(jωCtip )−1 Vˆt (ω) ˆ = H(ω) = −1 (Rs + jωCs )−1 + (jωCtip )−1 Vˆts (ω)

507 mg PbI2 , 22.4 mg MABr, and 73.4 mg PbBr2 (1 : 1.1 : 0.2 : 0.2 mole ratio) and 40 µL of CsI stock solution (1.5 M in DMSO) in 627 mg DMF and 183 mg DMSO (4 : 1 v/v). The lms were by dissolving

Ctip

and

g = Ctot /Cs .

(3)

We give the time constant the subscript fast because

under scanning

vacuum

of the time constant's similarity to  τfast  measured

Kelvin

probe microscope described in detail elsewhere.

in impedance spectroscopy.

67,73

below that

Ctip  Cs ;

35 We show experimentally

consequently,

τfast ≈ Rs Ctip .

The cantilever used was a MikroMasch HQ:NSC18/Pt

This simplication allows us to associate photo-induced

conductive probe. The resonance frequency and quality

changes in cantilever frequency and amplitude to photo-

factor were obtained from ringdown measurements and

induced changes in sample resistance or, equivalently,

found to be

ωc /2π = fc = 70.350 kHz

and

Q = 24 000

sample conductivity.

respectively at room temperature. The manufacturer's

The complex-valued transfer function in Eq. 2 has

specied resonance frequency and spring constant were

a real part which determines the in-phase forces and

fc = 60

to

75 kHz

and

k = 3.5 N m−1 .

Cantilever

an imaginary part which determines the out-of-phase

motion was detected using a ber interferometer operating at

1490 nm

forces acting on the cantilever.

(Corning model SMF-28 ber). More

We show the equiv-

alent circuit and plot the shape of transfer function in

experimental details regarding the implementation of

Figure S2. The frequency shift measurements presented

broadband local dielectric spectroscopy and other mea-

in Figure 4b probe the real part of the transfer function,

surements can be found in the Supporting Information.

3

fc δF 0 2k A   2 fc  00 ˆ c) =− Cq + ∆C 00 Re H(ω V −φ 4k

∆f = −

Results

3.1 Theoretical background

where

Let us begin by summarizing the equations we will use

fc = ωc /2π

is the resonance frequency,

spring constant, and

to connect scanning-probe data to sample properties.

of the cantilever;

Interested readers are directed to Ref. 35 and Ref. 69

F0

A

k

(4)

is the

is the amplitude, respectively,

is the in-phase force;

Ct

is the

cantilever capacitance computed at rest with the can-

for a detailed derivation of the following equations. In

tilever at its equilibrium position;

our measurements we modulate the charge on the can-

∆C 00 ≡ 2(Ct0 )2 /Ct ,

with primes indicating derivatives with respect the tip-

tilever tip and the sample and observe the resulting

∆Cq00 ≡ Ct00 − ∆C 00 ; V is the voltage applied to the cantilever tip; and φ is the surface potensample distance;

change in the cantilever frequency or amplitude. This

ACS Paragon Plus Environment 3

The Journal of Physical Chemistry (a)

Broadband local dielectric spectroscopy Figs. 3, 7, 10 Parameters controlled: Vt = VAC Parameters probed: frequency shift ∆f BLDS(ωm) alpha α (ωm) Re (Ĥ (ωm))

(b)

(c)

Ring-down dissipation measurement Figs. 5, 6, 8, 9 Parameters controlled: drive Parameters probed: amplitude vs time

Frequency shift vs Voltage/ Amplitude vs Voltage Fig. 4 Parameters controlled: Vt = VDC Parameters probed: frequency shift ∆f (ωc) Re (Ĥ (ωc)) alpha α (ωc) normalized dissipation Υs amplitude change ∆A

quality factor Q

sample-induced dissipation Γs

Im (Ĥ (ωc))

Driving Force ON Amplitude [nm]

Im (Ĥ (ωc))

fam = ωam/2π = 45 Hz

{

VAC

FAMACs

FAMACs

FAMACs

VDC = Vt (-4 V to 4 V)

ITO

SnO2 or TiO2 or NiO

Driving Force OFF

Time [ms]

{

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 20

VDC = Vt (-4 V)

fm= ωm/2π = 200 Hz to 1.5 MHz

ITO

SnO2 or TiO2 or NiO

ITO

SnO2 or TiO2 or NiO

Figure 1: Schematic of the three scanning probe measurements used in this manuscript, highlighting the parameters that are controlled and probed. The measurements probe the complex frequency dependent tip-sample transfer function Hˆ (Eq. 2 and Figure S2). (a) In a broadband local dielectric spectroscopy measurement, the tip voltage Vt = VAC (ωm , ωam ) is amplitudemodulated at a xed frequency of fam = ωam /(2π) = 45 Hz and sinuodially modulated at a frequency fm = ωm /(2π) that is varied from 200 Hz to 1.5 MHz. At each fm the component of the cantilever frequency shift ∆fBLDS (ωm ) at frequency fam is measured using a frequency demodulator and lock-in amplier and α(ωm ) is calculated using Eq. 8. The quantity α(ωm ) primarily ˆ m )) (Eq. 7). (b) In a frequency shift ∆f vs. Vt measurement or an amplitude A vs. Vt measurement, the tip measures Re(H(ω voltage Vt = VDC is slowly varied from −4 V to 4 V while the cantilever frequency shift ∆f and the cantilever amplitude A are recorded. From ∆f a voltage-normalized frequency shift α(ω0 ) is calculated using Eq. S4; the quantity α(ω0 ) primarily measures ˆ c )). From A a voltage-normalized sample-induced dissipation γs is calculated using Eq. S9; the quantity γs primarily Re(H(ω ˆ c )) (Eqs. 6 and S8). (c) In a ring-down dissipation measurement, the cantilever drive is periodically switched measures Im(H(ω o, the cantilever amplitude is measured as a function of time, and the cantilever's mechanical quality factor Q is calculated from this ring-down transient. The observed change in Q is converted to an equivalent change in sample-induced dissipation Γs using ˆ c )) (Eq. 6). Eq. S10. The parameter Γs primarily measures Im(H(ω tial. The variable

α0

plotted in Figure 4a is a voltage-

have assumed the amplitude modulation frequency

∆f vs. ∆f = α0 (V − φ)2 and

ωm

normalized frequency shift, the curvature of the

is much smaller than

V

Supporting Information).

data dened by the equation

given by

transfer function

α0 = −

1/τfast

  fc  00 ˆ c) . Cq + ∆C 00 Re H(ω 4k

From Eq. 5 we can see that

α0

BLDS spectra obtained at low light intensity over SnO2 and TiO2 , Figure 3, where the majority of the response

ωc , with addi-

rolls o at

tional contributions from in-phase forces present at low frequency (ω/2π

< 0.1 Hz).

The sample-induced dissi-

where

F 00

is

the

out-of-phase

force

acting

on

We conclude that the BLDS

the modulation frequency. The voltage-normalized frequency shift

to the out-of-phase part of the transfer function,

   1 δF 00 ∆C 00 ˆ c) V − φ 2, = Im H(ω ωc A ωc

ωm  ωc .

measurement primarily measures the in-phase forces at

pation plotted in Figures 4a, 5, 6, 8, and 9 is sensitive

Γs = −

The term in Eq. 7 containing the factors

ˆ c −ωm ) and H(ω ˆ c +ωm ) is small as indicated by the H(ω

is sensitive to the real

part of the transfer function at frequency

The imaginary part of the

is signicant only at the frequency

where the real part of the transfer function starts

to roll-o.

(5)

ˆ H

∆fBLDS

α

plotted in Figures 3, 7, 10 is related to

by

α=

(6)

∆fBLDS (ωm ) . Vm2

measured quantity in each of the three dierent scan-

The voltage-normalized dissipation γs ∝ ˆ c )) plotted in Figure 4b is related to Γs through Im(H(ω 2 the equation Γs = γs (V −φ) . The BLDS measurements

ning probe measurements employed in this manuscript.

3.2 Experimental ndings

of Figures 3, 7, and 10 are frequency-shift measurements that probe the response of the sample to an oscillating

We now present data acquired on the FAMACs sam-

applied voltage,

ples prepared on a range of substrates. All of the substrates (TiO2 , SnO2 , NiO, ITO) are planar structures. The FAMACs thickness was ∼700 nm; the thickness of

 fc Vm2 h 00 ˆ m + ωc ) Cq + ∆C 00 Re H(ω ∆fBLDS (ωm ) = − 16k i  ˆ m − ωc ) |H(ω ˆ m )|2 (7) + H(ω

ETL/HTL layer was of

∼10 nm,

∼40 nm

and

Vm

illuminated from the top.

are the frequency and amplitude,

with average roughness

and the thickness of the ITO was

with an average roughness of

ωm

(8)

Figure 1 summarizes the experimental set-up and the

the

cantilever.

where

ωam

(see Experimental Section in

∼2 nm.

∼100 nm

The samples were

The high absorption coef-

cient of the perovskite lm means that electron and

respectively, of the oscillating applied voltage and we

ACS Paragon Plus Environment 4

Page 5 of 20

other words, a decrease in the time constant

a)

τfast

light. In Figure 3 we clearly see a roll-o of the

ωm

Ag

with

α

vs.

curves that depends on the light intensity in the

case of electron-acceptor substrates (TiO2 and SnO2 ),

spiro-OMeTAD

whereas in the case of the NiO-(hole acceptor) and ITO-

FAMACs Perovskite

substrate samples,

α

is independent of both

ωm

and

light intensity.

TiO2

For the rest of this section of the manuscript, we compare the light and frequency dependence of the conductivity in the TiO2 and SnO2 -substrate samples.

ITO

b)

glass

response. However some signicant dierences can also be seen: 1. In the dark, the SnO2 -substrate sample is more conductive than the TiO2 -substrate sample as seen by their dark BLDS response curves.

20

-2

)

20 15

Power Density (mW cm

)

-2

Both

samples show a light-dependent roll-o of the dielectric

25

Current Density (mA cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

10 5

18.7 mW/cm2

15

2. The conductivity of the SnO2 -substrate sample is more strongly aected by light than that of

10

the TiO2 -substrate sample; the roll-o moves to

5 0

higher frequencies for the same light intensity 0

400

Time (s)

800

for the SnO2 -substrate sample compared to the TiO2 -substrate sample.

1200

0 0.0

0.2

0.4

0.6 0.8 Potential (V)

1.0

These light-dependent conductivity eects can be

1.2

conrmed through quasi-steady-state measurements of the cantilever frequency shift (∆f ) vs. applied tip voltage (Vts ) and cantilever amplitude (A) vs. applied

Figure 2: (a) Control perovskite solar cells were fabricated with the TiO2 /FAMACs/spiro-OMeTAD/Ag architecture depicted in the schematic. (b) A reverse scan (scan taken from opencircuit voltage, VOC , to short-circuit current, JSC ) current density-voltage (J − V ) curve taken for a device with the architecture depicted in (a). From the reverse J − V scan, the VOC , JSC , ll factor, and power conversion eciency were measured to be 1.141 V, 22.49 mA/cm2 , 0.744, and 19.1%. The inset shows the power density of the device monitored at a constant bias of 0.94 V over the course of 1200 s which was found to stabilize at 18.7% eciency.

tip voltage (Figure 1b).

In these measurements the

cantilever is driven using constant-amplitude resonant excitation and the cantilever amplitude and frequency shift are recorded at each applied

Vts .

By tting the

measured frequency shift and amplitude data to Eq. S4 and Eq. S9 respectively  see Figure S3 for representative curves and Sec. S3 for calculation details  we can calculate the curvature (α0 ) change and a voltagenormalized sample-induced dissipation constant (γs ). These values are not aected by the tip voltage sweep width; the large wait time (500 ms) employed at each

hole generation was conned to the top

∼200 nm

applied tip voltage ensure that the measured response

of

is a steady-state response.

the sample, a distance signicantly smaller than the

700 nm

Figure 4a shows the sample-induced dissipation mea-

thickness of the FAMACs layer. Figure 2 shows

sured

device-performance data for a representative FAMACs lm prepared on a TiO2 substrate; this data demon-

a maximum and then decreases with light intensity can be explained by the existence of a time constant

τfast

α at large voltage-modulation

indicates qualitatively that not all of the

conductivity state. When

τfast

is less than

ωc−1 = 2 µs,

most of the sample charge responds instantaneously to

In our impedance model of the tip-sample interaction,

RC

that increased monotonically with light intensity.

At high light intensities, the sample reaches its high-

sample charge is able to follow the modulated tip charge. Figure S2, this decrease is attributed to the

In

sity. A non-linear increase in dissipation which reaches

50 Figure 1a) acquired of lms

the Experimental Section in Supporting Information).

ωm

method.

Figure S4 illustrates the predicted dependence of the

prepared on TiO2 , SnO2 , ITO, and NiO substrates (see

frequency

amplitude-voltage

curvature and sample-induced dissipation on light inten-

Figure 3 shows Broadband Local Dielectric Spec-

In Figure 3, a decrease in

the

response of the sample at the cantilever frequency.

strates the high quality of the lms used in this study.

troscopy data (BLDS,

through

this measurement we are sensitive to the out-of-phase

changes in the tip position, leading to a decrease in

roll-o

the out-of-phase force acting on the cantilever and a

of the tip-sample circuit. A light-dependent change in

reduction in sample-induced dissipation.

the roll-o frequency is consistent with sample conduc-

We see for

both the TiO2 -substrate sample and the SnO2 -substrate

tivity increasing with increasing light intensity or, in

sample that dissipation reached a maximum before

ACS Paragon Plus Environment 5

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a)

b)

c)

Page 6 of 20

d)

540 mW/cm2

1 mW/cm2

Figure 3: Distinct Broadband Local Dielectric Spectroscopy (BLDS) curves seen in FAMACs thin lms grown on (a) TiO2 , (b) SnO2 , (c) ITO, and (d) NiO. Curves are colored according to the applied light intensity (See right hand legend)and vertically opset by 0.05 for clarity. For reference, BLDS curve in the dark is shown in open black circles. Experimental parameters: modulation voltage Vm = 6 V, λ = 639 nm, tip-sample separation h = 200 nm except for the TiO2 -substrate sample where h = 150 nm. a)

a)

hν 0

b)

b)

Figure 5: Dissipation recovery in the dark is substrate dependent. Dissipation vs. time for FAMACs lm on (a) TiO2 (b) SnO2 -substrate. The indicated illumination intensity was turned o at t = 0 sec. Experimental parameters: Vts = −4 V, T = 292 K, (a) λ = 535 nm, h = 175 nm, (b) λ = 639 nm, h = 200 nm. applied tip voltage parabola (α0 ). The data of Figure 4,

which primarily measures

sample response at a single frequency (ωc ), corroborates the data of Figure 3 which shows the sample response at multiple frequencies. The solid lines in Figure 4 are a t to a one-time-constant impedance model described

Figure 4: Normalized (a) out-of-phase (dissipation) (b) inphase (curvature) force for FAMACs lms on dierent substrates. Shaded points indicate values in the dark. Solid lines show a t to the impedance model described in Ref 35. Experimental parameters: λ = 639 nm, h = 200 nm, A0 = 45 nm, Vts = −4 V to 4 V (bipolar sweeps).

in Ref. 35 and summarized in Sec. 3.1.

qualitatively

The model

explains the seemingly-anomalous peak in

sample-induced dissipation vs. light intensity data over both TiO2 and SnO2 . The one-time-constant model only qualitatively describes the charge dynamics in the SnO2 -substrate sample; adding further electrical components to the sample-impedance model, justied by the double roll-o seen in the Figure 3b data, would

decreasing when the light intensity was increased monotonically.

improve the SnO2 -substrate ts in Figure 4.

Figure 4b shows that, concomitant with a

We observed the dynamics of

dissipation peak, there is a non-linear change in the in-

two dierent methods.

phase response at the cantilever frequency, observed as

τfast in real time through

In Figure 5a, we show how

the dissipation changes for the TiO2 -substrate sample

a changes in the curvature of the frequency shift vs.

ACS Paragon Plus Environment 6

Page 7 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry a)

for dierent light intensities. Here we inferred sample-

b)

induced dissipation by measuring changes in the quality factor of the cantilever through a ring-down measurement (Figure 1c and Sec. S4).

The recovery of dis-

sipation clearly had two distinct timescales  a fast component and a slow component.

In Figures 5a

and b, when the light was switched on, there was a large and prompt (≤

100 ms)

increase in dissipa-

tion followed by a small and much slower increase that lasted for

10 s

or longer.

The presence of the

slow component was especially clear when the light intensity was greater than the light intensity giving the maximum dissipation.

Figure 6: Dissipation recovery over the TiO2 -substrate sample is temperature dependent. (a) Dissipation recovery transients in the dark after a period of continuous illumination. (b) Time constant for dissipation recovery Γs recovery vs. temperature with error bars calculated from the ts in (a). Error bars are small on the y-axis scale. The shaded region represents 2 standard deviations for a weighted least squares t to an exponential model τΓ (T ) = A−1 exp(Ea /kB T ). The best t parameters with two standard deviation error bars were Ea = 0.58±0.07 eV and A = 4.9±1.0 × 102 s−1 . Experimental parameters: Vts = −4 V, Ihν = 292 mW/cm2 , tsoak = 27 s, h = 150 nm, λ = 639 nm.

Whether the dissipation

increased or decreased when the light was switched o depended on the value that

τfast

(i.e. sample conduc-

tivity) reached during the light-on period. At low light

−2

15.6 mW cm−2 for the −2 TiO2 -substrate sample, Figure 5a, and 13 mW cm for intensities (2.06 mW cm

and

the SnO2 -substrate sample, Figure 5b), the dissipation

Γs

decreased when the light was switched o, indicating

that

τfast

was

tial rise in

Γs

≥ 2 µs.

On the other hand, the ini-

when the light was switched o for the

331 mW cm−2 dataset in Figure 5a and the 72 mW cm−2 dataset in Figure 5b is consistent with a light-on τfast being ≤ 2 µs. In such a case, when the light was switched o, the Γs promptly increased in ≤ 200 ms as τfast approached the value of 2 µs. Subsequently, Γs gradually decreased over 10's of seconds as τfast became ≥ 2 µs. In

Figure

induced

5b,

we

dissipation

show

a

time-resolved

measurement

dielectric response at a xed modulation frequency (ωm ). The response at each

light-

ωm

corresponds to the in-

phase force at that modulation frequency.

By doing

the measurement at dierent modulation frequencies,

substrate sample. The slow part of the recovery of dis-

we can visualize the time evolution of the full dielectric

500 sec)

the

we illuminate the sample and measure the time-resolved

SnO2 -

sipation was extremely slow (>

for

response for the TiO2 -substrate sample (Figure 7). Here

at room tem-

response curve in the dark after the light was turned

perature. This slow recovery indicates that the SnO2 -

o.

substrate sample retained its conductive state for a

curves for time just before switching o the light (open

much a longer time than did the TiO2 -substrate sample.

circles) and

Interestingly, the slow time constant for dissipation

circles) shows a fast (