Rational molecular passivation for high-performance perovskite light-emitting diodes

TLDR: A major efficiency limit for solution-processed perovskite optoelectronic devices, for example light-emitting diodes, is trap-mediated non-radiative losses as mentioned in this paper.

Abstract: A major efficiency limit for solution-processed perovskite optoelectronic devices, for example light-emitting diodes, is trap-mediated non-radiative losses. Defect passivation using organic molecul ...

Figures

Figure 1│PeLED architecture, performance and perovskite film characteristics. a, The molecular structures of HMDA and EDEA. b, A HAADF cross-sectional image of an EDEA-treated device (left, scale bar 500 nm) and a zoom-in image (right, scale bar 100 nm). Represented characteristics for the optimized control, EDEA- and HMDA-treated devices: c, EL spectra at 2.5 V; d, EQE-J curves; e, Current densityvoltage-radiance (J-V-R) characteristics. f, TOF-SIMS for the 0.25 EDEA-treated perovskite film on the ITO/ZnO:PEIE substrate, showing depth profiles of unfragmented EDEA (M+= EDEA+) and two characteristic fragment ions ([M-NH3] + and [M-H]+). g, ATR-FT-IR (N-H stretching) for EDEA and PbI2:EDEA mixture. h, XRD patterns for the control, EDEA- (x = 0.25) and HMDA-treated (x = 0.25) films on the ITO/ZnO:PEIE substrates. α and # denote the diffraction peaks corresponding to α-FAPbI3 and ITO, respectively.

Figure 2│The evidence for the excellent passivation effects of EDEA treatment. a, Temperature dependence of capacitance-frequency plots for control, HMDA-treated and EDEA-treated devices (from 320 ~ 240 K). b, c, Trap density deduced from the room temperature C-f plots for the control (b) and the HMDA-treated (c) samples. d, Fluence-dependent PLQYs. e, Time-correlated single photon counting (TCSPC) probed PL lifetime. The excitation density for the TCSPC measurement is around 1015 cm-3.

Full text

Rational molecular passivation for high-
performance perovskite light-emitting diodes
Weidong Xu, Qi Hu, Sai Bai, Chunxiong Bao, Yanfeng Miao, Zhongcheng Yuan,
Tetiana Borzda, Alex J. Barker, Elizaveta Tyukalova, Zhang-Jun Hu, Maciej Kawecki,
Heyong Wang, Zhibo Yan, Xianjie Liu, Xiaobo Shi, Kajsa Uvdal, Mats Fahlman,
Wenjing Zhang, Martial Duchamp, Jun-Ming Liu, Annamaria Petrozza, Jianpu
Wang, Li-Min Liu, Wei Huang and Feng Gao
The self-archived postprint version of this journal article is available at Linköping
University Institutional Repository (DiVA):
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-157707
N.B.: When citing this work, cite the original publication.
The original publication is available at www.springerlink.com:
Xu, W., Hu, Qi, Bai, S., Bao, C., Miao, Y., Yuan, Z., Borzda, T., Barker, A. J.,
Tyukalova, E., Hu, Z., Kawecki, M., Wang, H., Yan, Z., Liu, X., Shi, X., Uvdal, K.,
Fahlman, M., Zhang, W., Duchamp, M., Liu, J., Petrozza, A., Wang, J., Liu, L., Huang,
W., Gao, F., (2019), Rational molecular passivation for high-performance perovskite
light-emitting diodes, Nature Photonics, 13(6), 418-424.
https://doi.org/10.1038/s41566-019-0390-x
Original publication available at:
https://doi.org/10.1038/s41566-019-0390-x
Copyright: Nature Research (part of Springer Nature)
http://www.nature.com/

1
Rational molecular passivation for high-performance perovskite light-emitting diodes
Weidong Xu
1,2
, Qi Hu
3
, Sai Bai
1
, Chunxiong Bao
1,4
, Yanfeng Miao
2
, Zhongcheng Yuan
1
, Tetiana
Borzda,
5
Alex J. Barker
5
, Elizaveta Tyukalova
6
, Zhangjun Hu
1
, Maciej Kawecki
7
, Heyong Wang
1
, Zhibo
Yan
1,8
, Xianjie Liu
1
, Xiaobo Shi
1
, Kajsa Uvdal
1
, Mats Fahlman
1
, Wenjing Zhang
4
, Martial Duchamp
6
,
Jun-Ming Liu
8
, Annamaria Petrozza
5
, Jianpu Wang
2
, Li-Min Liu
9,3
*, Wei Huang
2,10
*, and Feng Gao
1
*
1
Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping 58183, Sweden.
2
Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Jiangsu
National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University
(NanjingTech), 30 South Puzhu Road, Nanjing 211816, China.
3
Beijing Computational Science Research Center, Beijing 100084, China.
4
International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology,
Shenzhen University, Shenzhen 518060, China.
5
Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli
70/3, 20133 Milan, Italy.
6
School of Materials Science and Engineering, Nanyang Technological University (NTU), 50 Nanyang
Avenue, Singapore 639798, Singapore.
7
Laboratory for Nanoscale Materials Science, Empa, CH-8600 Dubendorf, Switzerland and Department
of Physics, University of Basel, CH-4056 Basel, Switzerland.
8
Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing
University, Nanjing 210093, P. R. China.
9
School of Physics, Beihang University, Beijing 1000834, China.
10
Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127
West Youyi Road, Xi’an 710072, China.
*Correspondence to: liminliu@buaa.edu.cn (L.M.L.), iamwhuang@nwpu.edu.cn (W.H.), feng.gao@liu.se
(F.G.)

2
Abstract
A major efficiency limit for solution-processed perovskite optoelectronic devices (e.g. light-emitting diodes,
LEDs) is trap-mediated non-radiative losses. Defect passivation using organic molecules has been
identified as an attractive approach to tackle this issue. However, implementation of this approach has been
hindered by a lack of deep understanding of how the molecular structures affect the passivation
effectiveness. We show that the so far largely ignored hydrogen bonds play a critical role. By weakening
the hydrogen bonding between the passivating functional moieties and the organic cation featuring the
perovskite, we significantly enhance the interaction with defects sites and minimize non-radiative
recombination losses. Consequently, we achieve exceptionally high-performance near infrared perovskite
LEDs (PeLEDs) with a record external quantum efficiency (EQE) of 21.6%. In addition, our passivated
PeLEDs maintain a high EQE of 20.1% and a wall-plug efficiency of 11.0% at a high current density of
200 mA cm
-2
, making them more attractive than the most efficient organic and quantum-dot LEDs at high
excitations.

3
Introduction
Solution-processed metal halide perovskites (MHPs) have received significant interest for cost-effective,
high-performance optoelectronic devices
1–4
. In addition to the great successes in photovoltaics (PVs), their
excellent luminescence and charge transport properties make them also promising for LEDs
5
. In order to
achieve high-efficiency PeLEDs, extensive efforts have been carried out to enhance radiative
recombination rates by confining the electrons and holes
6
. These confinement efforts include the use of
ultra-thin emissive layers
7
, the fabrication of nano-scaled polycrystalline features
8
, the design of low-
dimensional or multiple quantum well structures
9,10
, and the synthesis of perovskite quantum dots
11
. As a
result, the EQE values of PeLEDs have improved from less than 1% to ~ 14%
7–11
.
In addition to enhancing radiative recombination rates, equally important is to decrease the non-
radiative recombination for improving the device performance. Unfortunately, state-of-the-art solution-
processed perovskite semiconductors suffer from severe trap-mediated non-radiative losses
12-14
, which have
been identified as a major efficiency limiting factor for both PVs and LEDs
15,16
. The trap states are generally
believed to be associated with ionic defects, such as halide vacancies
17
. Defect passivation through a
molecular passivation agent (PA), which can chemically bond with the defects, is an attractive methodology
to tackle this issue
18
. A few function groups (e.g. –NH
2
, P=O) have been identified to passivate perovskite
semiconductors for photovoltaic applications
19–21
. It is found that these PAs show strong structure
dependent performance, even though they share identical functional groups to interact with the perovskite
defects
18–21
. A lack of deep understanding of how the PA chemical structures affect the passivation effects
prevents rational design of PAs to minimize the non-radiative recombination losses. These functional
groups have also been borrowed to improve the efficiency of LEDs, resulting in limited success so far. For
example, the use of trioctylphosphine oxide (TOPO) treatment in green PeLEDs can only result in moderate
EQE enhancement from 12% to 14%
22
.
Here, we demonstrate high efficiencies for PeLEDs through rational design of passivation molecules.
We demonstrate that the candidate amino-functionalized PAs which form stronger hydrogen bonds with
organic cations in perovskites are less effective in healing defects sites. Firmly based on our findings, we

4
design new passivation molecules with decreased hydrogen bonding ability and hence improve their
interaction with defects. In particular, we exploit O atoms within the PAs to polarize the passivating amino
groups through the inductive effect, reducing their electron-donating ability and hence relevant hydrogen
bonding ability. This results in enhanced coordination of the PA functional groups with the perovskite
defects sites and hence much improved passivation efficiency. As a result, we are able to substantially
decrease the trap-mediated non-radiative recombination and boost the electroluminescence (EL)
performance of PeLEDs, giving an average EQE of 19.0 ± 0.8% and a record value of 21.6%.
Results and discussions
PeLED architecture, films characterisations, and device performance
Amino groups have been frequently employed to passivate perovskite semiconductors due to their
coordination bonding to unsaturated PbI
6
-octahedral
20
. Here, we select two similar amino-functionalized
PAs, i.e. 2,2′-(ethylenedioxy)diethylamine (EDEA) and hexamethylenediamine (HMDA) (Fig. 1a), which
have identical length of alkyl chains; the difference is that EDEA has two additional O atoms within the
chain. We perform first-principles calculations to demonstrate that both of them can help to passivate the
surface iodide vacancy (V
I
) with Pb-N coordination bonding, and thus show the potential to improve the
EL performance (Supplementary Figs. 1 and 2). The formamidinium lead tri-iodide (FAPbI
3
) perovskite
layers are deposited by spin-casting the precursors with a molar ratio of PbI
2
: formamidinium iodide (FAI):
PA = 1: 2: x (x = 0~0.3), where FAI excess is used to eliminate the non-perovskite δ-phase (Supplementary
Fig. 3)
23
. We fabricate PeLEDs with the device architecture of indium tin oxide (ITO)/polyethylenimine
ethoxylated (PEIE): modified zinc oxide nano-crystals (ZnO:PEIE)/perovskite/poly(9,9-dioctyl-fluorene-
co-N-(4-butylphenyl)diphenyl-amine) (TFB)/molybdenum oxide (MoO
3
)/Au, as depicted in the high-angle
annular dark field cross-sectional scanning transmission electron microscope (HAADF-STEM) images in
Fig. 1b. Both HAADF-STEM and scanning electron microscope (SEM) images (Supplementary Fig. 4)
show the formation of separated nano-island features in the perovskite emissive layer. These nano-island
features have not resulted in strong leakage current (Fig. 1e), possibly due to different TFB thickness on
perovskite nano-islands and on ZnO:PEIE as well as unfavourable charge injection from ZnO to TFB

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Rational molecular passivation for high-performance perovskite light-emitting diodes" ?

N. B.: When citing this work, cite the original publication. 

Q2. What is the major efficiency limit for solution-processed perovskite?

A major efficiency limit for solution-processed perovskite optoelectronic devices (e.g. light-emitting diodes, LEDs) is trap-mediated non-radiative losses. 

Q3. What is the effect of the hydrogen bonding on the passivation effect?

Since both hydrogen bonds and passivating coordination bonds result from the lone pair electrons atthe N atoms in the amino groups, changes in the hydrogen bonding ability will affect the passivation effect. 

Q4. What is the effect of the inductive effects on the electrons at the N atoms?

Compared with HMDA, the electrons at the N atoms of EDEA polarize toward the O atoms due to the inductive effects, which hence reduce the electron-donating ability of the amino groups and the relevant hydrogen bonding ability33. 

Q5. What is the role of the hydrogen bonding between the passivating functional groups and the organic?

By weakening the hydrogen bonding between the passivating functional groups and the organic cations of perovskites, the authors significantly reduce the non-radiative recombination. 

Q6. What is the main efficiency limit for solution-processed perovskite?

state-of-the-art solutionprocessed perovskite semiconductors suffer from severe trap-mediated non-radiative losses12-14, which have been identified as a major efficiency limiting factor for both PVs and LEDs15,16. 

Q7. What is the characterization method used for the PeLED devices?

The PeLED devices are tested on top of the integration sphere and only forward light emission can be collected, consistent with the standard OLED characterization method. 

Q8. What is the effect of the electrons on the hydrogen bonds between the amino groups and FA+?

the hydrogen bonds between the amino groups and FA+ can be affected by the O atom because of the electron withdrawing inductive effect of O atoms. 

Q9. Where did the TEM measurements be performed?

The TEM measurements were performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in Nanyang Technological University, Singapore. 

Q10. What are the defects physics of the samples?

Assured about the presence of molecular interaction and a lack of structural changes in the thin films, the authors have investigated the defects physics of the samples. 

Q11. What is the simplest way to observe the depth distribution of EDEA in perovskit?

By ToF-SIMS the authors observe the depth distribution of EDEA across the perovskite film by monitoring the unfragmented positive molecular ion (C6H16N2O2+; m = 148.1 u) and by XPS the authors observe changes in line shape of C1s, O1s, N1s core level spectra in the resulting perovskite films compared to the control ones, confirming the adsorption of EDEA molecules in the perovskite films and thus providing the opportunities for passivation. 

Q12. What were the passivation agents used for the ODEA-treated devices?

The passivation agents (PAs), including hexamethylenediamine (HMDA), 2,2′- (ethylenedioxy)diethylamine (EDEA), 4,9-dioxa-1,12-dodecanediamine (DDDA), 2,2′- [oxybis(ethylenoxy)]diethylamine (ODEA), 4,7,10-trioxa-1,13-tridecanediamine (TTDDA), ethylene glycol diethyl ether (EGDE) were purchased from Sigma-Aldrich. 

Q13. What is the role of hydrogen bonds in the passivation of perovskite?

Their findings provide a broad avenue to explore the potential of molecular passivation for improving various perovskite applications where non-radiative losses should be minimized. 

Q14. What is the effect of the O atom on the passivation effect of the PA?

The authors designed three PAs (Fig. 4a) with different strength of inductive effects, which are expected to result in different hydrogen bonding abilities and hence different passivation effectiveness. 

Q15. What is the effect of PAs on the passivation efficiency of perovskite?

This results in enhanced coordination of the PA functional groups with the perovskite defects sites and hence much improved passivation efficiency.