Interface-Dependent Radiative and Nonradiative Recombination in

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C: Energy Conversion and Storage; Energy and Charge Transport

Interface Dependent Radiative and NonRadiative Recombination in Perovskite Solar Cells Ka Kan Wong, Azhar Fakharuddin, Philipp Ehrenreich, Thomas Deckert, Mojtaba Abdi-Jalebi, Richard H. Friend, and Lukas Schmidt-Mende J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00998 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

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Interface Dependent Radiative and Non-radiative Recombination in

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Perovskite Solar Cells

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Ka Kan Wong,1 Azhar Fakharuddin*,1 Philipp Ehrenreich,1 Thomas Deckert,1 Mojtaba Abdi-Jalebi,2

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Richard H. Friend,2 Lukas Schmidt-Mende1

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1

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2

Department of Physics, University of Konstanz, 78457 Konstanz, Germany

Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK

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Correspondence: [email protected]

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Abstract

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Interfacial engineering has shown to play an essential role to optimizing recombination losses in

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perovskite solar cells, however an in-depth understanding the various loss mechanisms are still

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underway. Herein, we study the charge transfer process and reveal the primary recombination

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mechanism at inorganic electron-transporting contact such as TiO2 and its modified organic rivals.

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The modifiers are chemically ([6,6]-Phenyl C61 butyric acid, PC60BA) or physically ([6,6]-Phenyl

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C61 butyric acid methyl ester, PC60BM and C60) attached fullerene to the TiO2 surface in order to

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passivate the density of surface states. We do not observe any change in morphology, crystallinity

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and bulk defect density of halide perovskite (CH3NH3PbI3 in this case) upon interface modification.

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However, we observe compelling evidences via photoluminescence and electroluminescence

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studies that the recombination dynamics at both time scales (slow and fast) are largely influenced

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by the choice of the selective contact. We note a strong correlation between the hysteresis and the

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so-called slow charge dynamics, both significantly influenced by the characteristics of the selective

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contact, e.g., a the presence of surface traps at the selective contact not only shows a larger

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hysteresis but also leads to higher charge accumulation at the interface and distinguishable slow

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dynamics (a slower stabilization of recombination dynamics at time scale of several minutes).

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Keywords: Interfacial charge transfer, coupled charge dynamics, radiative versus non-radiative recombination in perovskite solar cells, slow and fast charge dynamics, interfacial traps. 1

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Introduction

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Hybrid lead halide perovskites have emerged as a new class of absorber materials for photovoltaic

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devices. They show a strong absorption in the visible spectrum1-2 and a tunable band-gap via

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compositional engineering3-4, a high charge carrier mobility5-6, a low exciton binding energy7-8 and

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a low trap density9. Although the power conversion efficiency (PCE) in perovskite solar cell (PSC)

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has reached > 22%10, research is now focused to improve device stability11-15, control the

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microstructure morphology and crystallinity16-18, and to understand the various charge

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recombination phenomena19-22. In general, it is shown that the high performance is always coupled

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to an appropriate choice of electron and hole transport layers (ETL and HTL, respectively) that

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enable not only efficient charge transfer but also reduce non-radiative recombination losses of the

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photo generated charges23-24.

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Since the inception of PSCs, research has been focused on improving device performance and

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only recently, research activities are growing to understand the recombination mechanisms in

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PSCs22,

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recombination14, 28-31 and also degradation32-34 in the PSCs. This is often linked to the presence of

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surface defects and catalytic sites at the TiO2 surface,35 respectively, which is overcome by

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employing surface modifiers36-37. Recently, Olthof et al.38 showed that volatile bi-products exist on

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metal oxide surfaces due to a chemical decomposition at the metal oxide/perovskite interface which

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may change the energetic landscape and consequently induce an energetic barrier against effective

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charge injection. Extensive research reports have demonstrated the use of organic or inorganic

25-27

. It is reported that metal oxide selective contacts, e.g. TiO2, induce interfacial

2


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modifiers to alter interfacial properties in order to improve the PCE and also the operational

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stability as recently reviewed comprehensively by Fakharuddin et al23.

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Two notable features associated with the PSCs are (i) anomalous hysteresis39-42 i.e., a change

4

in current-voltage characteristics depending on the measurement parameters and (ii) ion migration43.

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Whereas, there is a consensus that hysteresis is largely determined by the selective contacts23, 40-41,

6

44-45

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recently being discussed46-48. What makes PSCs more interesting is the slow (from seconds to

8

minutes) and fast (milliseconds to sub-microseconds) charge dynamics, which are conceived to be

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due to ionic and electronic properties of halide perovskites49. However, recently, it has been shown

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that both the ionic and electronic processes are coupled48. Whereas various reports stated that the

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ionic environment dictates the non-radiative charge recombination in PSCs and also the energetics20,

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35, 49-51

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contacts and charge accumulation/transfer at the interface are relatively fresh46, 48. Until now, the

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impact of selective contacts has been characterized individually for organic and inorganic selective

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contacts, or to demonstrate the charge extraction capabilities of the two, often shown to influence

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hysteresis and stability23. The conceptual difference between the two material classes, however,

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offers a large potential to investigate interfacial charge accumulation/transfer processes in order to

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find a link between interfacial recombination dynamics to the bulk properties such as ion migration.

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In this study, we choose the most common inorganic selective contact, TiO2 and its modified

20

analogues with fullerene derivatives (C60, PCBA and PCBM) to investigate various charge transport

the origin of ion migration and to what extent it influences charge recombination kinetics is

, the understanding that the ionic movement is also influenced by the properties of selective

3



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mechanisms. These modifiers compare the effect of chemically anchored fullerene to TiO2 via a

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carboxyl group (PCBA), and physically attached fullerene (PCBM and C60). To mimic the influence

3

of the carboxyl group, a fullerene-free organic material, benzoic acid (which also anchors to the

4

TiO2 via a carboxyl-linker) is employed as an interface modifier. Our experiments note no change in

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the morphology, crystallinity, and bulk defect density of the perovskite deposited on all these

6

selective contacts, however, a clear difference is apparent in the recombination kinetics, notably

7

between the slow and fast charge carrier dynamics. We show compelling evidences via

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photoluminescence and electroluminescence studies that the recombination dynamics at both time

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scales are largely influenced by the choice of the selective contact. The fullerene modified

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interfaces showed significantly lower non-radiative (or higher radiative) recombination, and

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distinguishable slow dynamics (a faster stabilization of recombination dynamics at time scale of

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several minutes) compared to a pristine TiO2 or that modified with fullerene-free benzoic acid only.

13 14

Results and discussion

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To evaluate the impact of TiO2/MAPbI3 interface modification on the photovoltaic performance, we

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first examined J-V characteristics of the various PSCs made using pristine and surface modified

17

TiO2 (see supporting information for experimental details). Photovoltaic parameters of all samples

18

are shown in Fig. 1 and summarized in Table 1. It must be noted that J-V measurements may not

19

truly represent solar cells performance because the PCE is largely influenced by the measurement

20

protocol52-53. We therefore report both the average PCE and stabilized PCE values extracted from 4


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MPP tracking for a reliable comparison of our devices53. In general, the devices with

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fullerene-based modifiers showed a significant improvement in the PV parameters and higher

3

stabilized (and average) PCE. In order to mimic the effect of carboxylic groups only and to

4

investigate whether it has an effect on the TiO2 surface passivation, we compare the J-V curves of

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PSCs employing benzoic acid as a modifier with a pristine TiO2 analogue. The benzoic acid based

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PSCs showed a drop in the PCE indicating its deleterious effect on the TiO2 surface. An indication

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of the magnitude of the hysteretic effect (the so called hysteresis index, HI)54-55 by scanning in the

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forward and backward direction shows a larger hysteresis for benzoic acid modified TiO2 than a

9

pristine TiO2 rival (Fig SI 2). In contrary to the benzoic modifier, we observe a strong improvement

10

and a smaller HI for devices with fullerene-based modifiers, which is consistent with recent

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reports31, 56-59.

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Surprisingly, the PSCs employing the PCBA monolayer surpass in performance those employing a

13

spin-coated PCBM thin film and also the thermally evaporated C60, suggesting a mono passivation

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layer sufficiently inhibits recombination related to TiO2 surface defects or photocatalytic process at

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the TiO2-perovsktie interface31-32, 60. A champion PCBA device showed a stabilized PCE greater

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than 17% (Fig. 1d), probably due to an almost ideal surface coverage of the fullerene derivative on

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the TiO2 (PCBA is not highly soluble in chloroform and care has to be taken to get nicely dissolved

18

solutions that will lead to complete surface coverage). The EQE measurements in ambient condition

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at 1 sun systematically affirmed improved charge collection in the PSCs employing fullerene

20

derivatives and a lowest charge collection when benzoic acid is employed as TiO2/MAPbI3 interface 5



Page 6 of 25

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modifier. In the case of benzoic acid, the slightly lower EQE compared to pristine TiO2 suggests the

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formation of a resistive passivation layer at the TiO2-perovskite interface which eventually hinders

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charge injection. We also note the measured JSC values in JV curves are significantly lesser than

4

those calculated from EQE. This is due to the different measurement conditions of the two: The

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EQE is measured in ambient at 1 sun using a monochromatic source with background light, whereas

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the JV curves are measured at 82 mW/cm2 (in inert). With an increased light intensity, the JSC would

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increase as also reported in literature.61-63

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Table 1: TiO2/modifier and MAPbI3 film thickness, Urbach energy (calculated by fitting the Urbach tail in PDS spectra),

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The perovskite layer thickness is 300±20 nm in all the devices. Average J-V values measured at irradiation of ~82

12

mW.cm-2 with voltage stabilization for 5 s in N2 filled glove-box. The average is calculated for 20 samples each for

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pristine TiO2, PCBA and PCBM, 10 samples for benzoic acid and 8 samples for 8 nm C60. The value in brackets

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corresponds to the PV parameters of the champion cell. Stabilized PCE values are obtained from MPP protocol of a

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typical PSC for 180 s in N2 filled glove-box.

and average (champion) and stabilized photovoltaic parameters of PSCs made using pristine and surface modified TiO2.

16 17 Selective

TiO2/ modifier

Urbach

JSC

contact

thickness (nm)

energy

(mA.cm-2)

VOC (V)

FF

(meV) TiO2

50±10

16.9 ±

monolayer

13.6±2.0

8 nm

16.5 ±

11.6±1.9

--

16.4 ± 0.5

56.2±8.1

(best) PCE

PCE (%)

0.86±0.0

14.8±0.9

0.99±0.0

59.2±5.1

0.96±0.0

4.5

7.2±1.4

3.4

(8.9)

65.8±3.5

2

16.0±1.5

8.6±1.3 (11.4)

6

0.3

TiO2/PCBM

0.94±0.0 6

0.5

Acid

TiO2/C60

16.8 ±

Stabilized

(%)

0.4

TiO2/Ben.

Average

12.0±1.0

8.2

(13.2)

60.9±4.3

4

11.5±1.2

10.4

(13.4)

6


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TiO2/PCBA

monolayer

16.2 ± 0.3

17.0±1.0

1.00±0.0

66.6±3.8

2

13.9±1.2

12.0

(17.5)

1

2 3 4

Fig. 1: (a) J-V curves (not stabilized) of typical representative PSCs made using pristine and modified TiO2 measured at

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external quantum efficiency of the PSCs, and (d) stabilized PCE of a champion device employing TiO2/PCBA as an

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ETL. Inset of (d) shows the J-V curve in forward and reverse scans.

irradiation of 82 mW.cm-2, (b) stabilized PCE (or MPP tracking) of the PSCs (in a) for a reliable comparison, (c)

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To verify that the observed improvement is mainly based on the interface modification and is

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not influenced from, for instance, perovskite crystal growth after surface treatment as reported in

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literature15,

59, 64-65

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absorbance and crystallinity of the perovskite grown on the various selective layers. We found

, we further investigated any observable changes in the film morphology,

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similar surface coverage and grain distribution of the perovskite (Fig. S3a). All the MAPbI3 films

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show an absorption onset at 780 nm and there is no notable difference in the absorbance of

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perovskite films deposited on the various interfacial layers (Fig. S3b). X-ray diffraction patterns

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affirmed complete cubic phase transformation, and similar peak intensities further confirmed no

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differences in the crystal growth on TiO2 layers with and without modification (Fig. 2a). No traces

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of PbI2 are found in all the perovskite layers showing a complete transformation of initial precursors

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to perovskite. These findings prompt our assumption that the improved performance may not be

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attributed to a non-stoichiometry or a change of grain distribution and film quality due to the

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surface modification, but are dominantly based on the modified TiO2/MAPbI3 interface.

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Fig. 2: (a) XRD spectra of the MAPbI3 films deposited on the various charge selective contacts on glass, (b) PDS

4

absorption spectra of TiO2 only and its modified analogues.

absorption spectra of MAPbI3 films on TiO2 with different modifiers deposited on quartz glass. The inset shows PDS

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Another plausible explanation which has been proposed66-67 is that the fullerene interfacial

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modifiers may diffuse into the perovskite absorber layer and suppress recombination centres at the

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grain boundaries. However, we note a similar sub-bandgap level and crystallinity of the bulk

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CH3NH3PbI3 layers as observed from PDS and XRD spectra, respectively (Fig. 2 a & b). A similar

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peak to peak ratio of the various CH3NH3PbI3 phases in the XRD spectra which corresponds to the 9



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bulk crystallinity of the perovskite and the identical steepness of the PDS absorption spectra as well

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as similar sub-bandgap levels depict no change in the bulk properties of the CH3NH3PbI3. The

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similar Urbach energy, Eu, (Table 1, calculated from the slope of absorption onset; A ∝ exp( E /

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Eu ), where A is the absorbance and E is the excitation energy in electron-volts) that corresponds to

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the energetic disorder of a material further affirms a very similar defects density of the perovskite

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films68. Contrarily, the PDS spectra of modified TiO2 interfaces compared to the pristine analogue

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(Fig. 2b inset) shows a significant drop in the sub-bandgap states of TiO2 confirming passivation of

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surface traps. One may argue that the PDS spectra of the modified TiO2 contains footprints of C60 as

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reported in literature69. However, whereas the PCBM and C60 modified TiO2 may contain signature

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from fullerene absorbance, a more than two order of magnitude drop in the tail absorbance of TiO2

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for PCBA (which only forms a monolayer), or in the case of benzoic acid (an order of magnitude

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drop with no fullerene) clearly suggests passivation of surface states. Our findings suggest that

13

interfaces play an important role to achieve high performance devices which is in good agreement

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with the literature31, 56-59.

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In order to probe the interfacial charge extraction and the radiative/non-radiative

16

recombination, we measured photoluminescence (PL) and electroluminescence (EL) spectra of the

17

films and devices, respectively. As shown in Fig 3a & b, the steady-state PL spectra of all the

18

devices showed a reduced emission intensity due to charge quenching at the TiO2/perovskite

19

interface. In general, fullerene modified TiO2 interfaces showed slightly higher PL quenching than a

20

pristine TiO2 rival. The EL spectra (Fig. 3d) showed a highest intensity for C60 modified interface 10


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followed by PCBM and PCBA, whereas the benzoic acid modified perovskite device showed least

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EL intensity suggesting a poor charge injection in this device. A quantitative analysis of the

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time-resolved photoluminescence (TRPL) spectra of the perovskite films on the various interfaces

4

also affirmed a more efficient charge extraction for the fullerene derivatives compared to pristine or

5

benzoic acid modified TiO2 interface (Fig. 3c). As reported in the literature70-72, the observed PL

6

decay of a perovskite film changes with illumination time. To ensure reproducible results, we have

7

measured the TRPL after stabilizing the PL intensity by ~20 minutes of light exposure. Figure SI

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4a shows the PL intensity where the TRPL spectra of a reference perovskite films (on glass) before

9

and after light soaking are compared. We observe significantly different lifetimes for the charge

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carriers. Interestingly, as the total emission intensity remained unchanged (as indicated from the

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charge carrier density ‘n’ in the absolute PL spectra, Fig. SI 4a), it suggests a variation in the bulk

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properties of perovskite under light soaking. Given the time frame of PL saturation of minutes (Fig.

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SI 4b), it cannot be ascribed to trap filling by photoexcited electrons and holes (a process which

14

takes place in fractions of second)70 and rather needs to be assigned to halide redistribution or ion

15

migration which may change the bulk properties of the perovskite.

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Page 12 of 25

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Figure 3: (a) Steady-state PL spectra of the MAPbI3 deposited on the various selective contacts and a reference

4

time-resolved PL decay of the MAPbI3 films in measured in vacuum EL and (d) EL spectra of all devices employing the

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various selective contacts measured in vacuum at forward bias (1.5 V).

(MAPbI3 film on glass), (b) the same as (a) without a reference on glass for clarity and to compare charge quenching, (c)

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A comparison of EL emission intensity versus applied bias (Fig. 4 a) further revealed reduced

8

non-radiative recombination in PSCs employing fullerene based selective contacts. The TiO2/C60

9

modified interface yielded nearly an order of magnitude higher EL than a pristine TiO2, followed by

10

the PCBA and PCBM modified interfaces. For all the samples, a correlation between the EL intensity

11

and applied bias is observed which is attributed to higher injection currents (Jinj) in the device, i.e.

12

higher charge carrier density ‘n’, which leads to enhanced bimolecular recombination (Fig. SI 5a). 12


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The TiO2 modified with benzoic acid resulted in the lowest EL intensity and also the lowest Jinj.

2

Noteworthy is the comparison of Jinj and the EL intensity of the different samples. Firstly, the Jinj of

3

TiO2 and TiO2/Ben. acid samples cross at higher applied voltage (V > 1.4 V). Although initially

4

Jinj,TiO2 was the lowest, it becomes higher than Jinj,TiO2/Ben. Acid at V ~ 1.4V. Second notable finding is

5

that the Jinj does not always follow the EL intensity. Whereas the TiO2/C60 modified interface showed

6

the highest EL intensity and also the highest Jinj, the trend is not consistent for PCBA and PCBM

7

modified samples. Despite significantly higher EL intensity in PCBA, i.e., a higher bi-molecular

8

(radiative) recombination, a significantly lower Jinj (~8 mA at 1.5 V) is measured compared to PCBM

9

(Jinj ~12.5 mA at 1.5 V).

10

The Continuity Equation and boundary condition help correlate total current to interfacial defect

11

density at perovskite/TiO2 interface as

12

/

= − ∇ + − (1)

13

where n is the charge carrier density, q is elementary charge, is total current density of drift

14

and diffusion processes, denotes the charge generation rate, and represents

15

recombination rates in the bulk and at the interface. At steady state and non open-circuit conditions,

16

∇ at TiO2 interface equals − ≠ 0 where and recombination in bulk are

17

considered identical for all samples, i.e. similar absorption profile as well as bulk defects as

18

revealed by PDS spectra. Equation 1 suggests ‘n’ dependency on the current flowing through a

19

device as well as the charge generation and recombination rates. Assuming charge generation (similar 13



Page 14 of 25

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absorbance for all samples and similar bulk defect density as revealed by the PDS spectra), and

2

3

the cell, see Fig, SI 5a), decrease (increase) in current extracted out of the cell suggests a change in

4

interfacial trap density.

= 0, i.e., a constant amount of charge carriers in the device (for a constant forward bias applied to

5

14


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1 2

measured at a continuous forward applied bias (+1.5 V). At ~700 s, the applied bias is reversed (-1.5 V) and the devices

3

were measured again to track changes in EL.

4

A similar Jinj trend for the TiO2 and Benzoic acid is suggestive of similar recombination rates.

5

However, the lower EL signal for benzoic acid samples suggests that non-radiative (mostly

6

monomolecular) recombination is more dominant than bi-molecular compared to pristine TiO2

7

resulting in decreased radiative recombination. Regarding fullerene derivatives, the higher Jinj with

8

respect to TiO2 suggests an improved interfacial charge extraction owing to smaller interfacial defect

9

density and thus a lower recombination. The higher EL intensity suggests that bi-molecular (or

10

radiative) recombination dominates over non-radiative to still show higher EL signal than the pristine

11

TiO2. Given the bulk trap density of all the perovskite is similar, the increased Jinj and a higher EL

12

signal is attributed to inhibition of interfacial traps, whereas the trap density seems to be even

13

increased with benzoic acid as interfacial modifier. Regarding the interfacial traps, particularly, in the

14

case of TiO2, a possible chemical reaction could take place leading to decomposition of perovskite

15

crystals35,

16

formation of an insulating monolayer is expected which leads to charge accumulation at the interface

17

and thereby a higher non-radiative recombination and a lower Jinj. For a detailed understanding of EL

18

(transient) and the role of recombination we refer to works by Tress. et al.74-75

19

In order to investigate the temporal changes in interfacial charge accumulation and transfer, we

20

recorded EL of all the samples for a time scale much longer than that required for trap filling (even

21

deeper traps are reported to be filled in few seconds)70. As can be seen in Fig. 4b, a sudden drop in the

Figure 4: (a) Bias dependent EL of all the PSCs made using pristine and modified TiO2, and (b) EL of the same devices

73

. This suppresses the EL signal significantly. For benzoic acid modified interface,

15



Page 16 of 25

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EL intensity in the first 100s is evident for TiO2 and TiO2/Benzoic acid interfaces and a slower drop

2

for PCBM and PCBA modified TiO2. The C60 modified interface did not show such a significant drop

3

and seems to stabilize much earlier than the other samples. One must note that the C60 (8 nm) is

4

deposited via thermal evaporation, a method that leads to a compact fully covered layer deposition

5

(ensuring that the crack or pin-holes in the TiO2 film are patched) unlike solution procced PCBA and

6

PCBM which may contain uncovered TiO2 films areas. As the time scale of EL saturation is several

7

minutes, it cannot be attributed to trap filling. More likely it seems that the ion migration and halide

8

distribution, processes which are assumed to take place at such longer time scale dictates the EL

9

response70, 76.

10

To probe whether the monotonic decrease in the EL intensity is due to degradation, as reported in

11

literature77, we applied a negative bias (-1.5 V). We note that the drop of the EL intensity for TiO2 and

12

benzoic acid is reversible, whereas for fullerene derivative is not. One could argue that charge

13

accumulation at the former two interfaces hinder charge injection (which employs the Jinj to be also

14

consistent)78, the measured temporal current through the device (Fig. SI 6) remains stabilized and

15

unchanged before and after the applied bias. Referring back to the Continuity Equation (Equation 1),

16

this inconsistency could be attributed to a change in interfacial trap density over time and thus the

17

recombination ‘R’. Once again, as the time scale of this saturation (before and after applying negative

18

bias) is rather much longer than that expected for electronic trap filling, it suggests that the ionic

19

processes within the bulk are coupled with interfacial processes: The more defective the interface is,

20

the longer is the time required to reach a steady-state within the device. This could explain the faster 16


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1

stabilization of fullerene derivatives (C60 being the fastest) and a slower stabilization for TiO2 and

2

benzoic acid. Very recently, a clear link between ionic movement and interfacial recombination is

3

also reported by Pockett et al.46 using transient measurement. We also note that, although the negative

4

bias could eliminate the charges accumulated at the interfaces (TiO2 and benzoic acid), the ionic

5

processes remains less affected by the applied bias (the EL intensity stabilized much faster around the

6

negative bias region).

7

Conclusions

8

In this study, we investigate the charge transfer process and the primary recombination mechanism

9

at inorganic electron-transporting contact such as TiO2 and its modified organic rivals (with and

10

without a fullerene molecule). The modifiers are chemically ([6,6]-Phenyl C61 butyric acid,

11

PC60BA) or physically ([6,6]-Phenyl C61 butyric acid methyl ester , PC60BM and C60) attached

12

fullerene to the TiO2 surface in order to passivate the density of surface states. We do not observe

13

notable changes in morphology, composition, and sub-bandgap states in MAPbI3, suggesting no

14

change took place for the bulk properties due to interface modification. The devices with

15

fullerene-based modifiers exhibited great improvement in all photovoltaic parameters, and a lesser

16

hysteresis effect, compared to devices with pristine TiO2 and those modified with benzoic acid (a

17

carboxylic group with a benzene ring). An impressive stabilized PCE >17 % is achieved using only

18

a monolayer of PCBA despite the fact that the surface coverage by the PCBA molecules are not

19

perfect, indicating that such thin interfacial layers are essential to positively modulate surface

20

condition as well as to passivate surface traps on TiO2. We further show compelling evidences that 17



Page 18 of 25

1

the recombination dynamics at slow and fast time scales are largely influenced by the choice of the

2

selective contact (pristine or modified TiO2). We note a strong correlation between the hysteresis

3

and the so-called slow charge dynamics, both significantly influenced by the characteristics of the

4

selective contact, e.g., the presence of surface traps leading to a higher energetic disorder at the

5

selective contact not only shows a larger hysteresis but also leads to higher charge accumulation at

6

the interface and distinguishable slow dynamics (a slower stabilization of recombination dynamics

7

at time scale of several minutes). It appears that the metal oxide such as TiO2 in this case does not

8

form an ideal interface with MAPbI3 and in order to avoid the interfacial recombination and

9

stabilize the hysteresis due to interfaces, a fully covered thin mono-layer passivation is required.

10 11

Supporting information

12

The supporting information includes experimental methods, chemical structure of the interfacial modifiers, details of

13

PDS measurements, current-voltage curves of all the devices, SEM images and absorption spectra of perovskites,

14

additional details on photoluminescence and electroluminescence and currents of the films/devices.

15 16

Acknowledgements

17

We acknowledge funding by the BMBF in the frame of The ENARET-MED-ENERG-11-132 project “HYDROSOL”.

18

K.K.W would like to acknowledge DAAD for doctoral scholarship and thank Prof. Thomas Bein (LMU) and his group

19

members for perovskite synthesis demonstration. A.F. acknowledge Alexander von Humboldt Foundation for the

20

postdoctoral fellowship award. M.A.J. gratefully acknowledges Nava Technology Limited and Nyak Technology

21

Limited for a Ph.D. scholarship. M.A.J. and R.H.F. thank the EPSRC.

22 23

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M., Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor and Subgap Defect

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States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells. Energ.

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Environ. Sci.2018, 11, 151-165.

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Slow Dynamic Processes in Lead Halide Perovskite Solar Cells. Characteristic Times and Hysteresis. J. Phys.

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Chem. Lett. 2014, 5, 2357-2363.

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Mechanism of Perovskite CH3NH3PbI3 Diode Devices Studied by Electroluminescence and

(76) Sanchez, R. S.; Gonzalez-Pedro, V.; Lee, J. W.; Park, N. G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J.,

(77) Makoto, O.; Masaru, E.; Atsushi, W.; Masahiro, Y.; Hidefumi, A.; Yoshihiko, K., Degradation

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Photoluminescence Imaging Spectroscopy. Applied Physics Express 2015, 8, 102302.

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Processed Low Turn-on Voltage near Infrared Leds Based on Core-Shell Pbs-Cds Quantum Dots with

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Inverted Device Structure. Nanoscale 2014, 6, 8551-8555.

(78) Sanchez, R. S.; Binetti, E.; Torre, J. A.; Garcia-Belmonte, G.; Striccoli, M.; Mora-Sero, I., All Solution

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TOC Graphic

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Interface-Dependent Radiative and Nonradiative Recombination in

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