This document summarizes research into depositing electron transport layers (ETLs) for perovskite solar cells using atomic layer deposition (ALD) at low temperatures. TiO2 and SnO2 were deposited as ETLs using thermal ALD at 185°C with and without a plasma pre-clean treatment. XRD analysis found the films were amorphous. XPS showed doping TiO2 with Al2O3 did not change the Ti oxidation state. A plasma pre-clean improved conductivity for TiO2 and SnO2. Wetting of the perovskite layer differed between ETLs, with O2 plasma SnO2 displaying the best wettability despite not having the highest conductivity. Further

1. ALD of Electron Transport
Layers (ETL) for Planar
Perovskite Solar Cells
M. McCarthy1, A. Walsh1, M. E. Pemble1,2, S. O'Brien1 and I. M. Povey1
1Tyndall National Ιnstitute, Lee Maltings Complex, Dyke Parade, Cork, Ireland
2University College Cork, Werstern Road, Cork, Ireland
Email: ian.povey@tyndall.ie
1. Introduction
2. ALD TiO2
• The 1st metal oxide to be deposited using ALD
was TiO2. This is quite a common ETL to be used
in planar perovskite solar cells.
• Doping TiO2 with Al2O3 using sol-gel has been
reported to increase device efficiencies,
enhance stability and even increase
conductivity.
• Doping using ALD is unlike traditional doping as
it forms a laminate.
Plasma pre-clean, thermal
TiO2
• Precursors:
Tetrakis(dimethylamino)titanium
(TDMAT),
H2O
• Reaction Conditions:
Temperature 185 °C
Pre-clean treatment O2 Ar Plasma
400 cycles
Growth rate 0.05 nm per cycle
Thermal TiO2 doped with
Al2O3
• Precursors:
Tetrakis(dimethylamino)titanium
(TDMAT),
Trimethylaluminium (TMA)
H2O
• Reaction Conditions:
Temperature 185 °C
Supercycle growth
Growth rate 0.05 nm per cycle – TiO2
Growth rate 0.1 nm per cycle – Al2O3
Growth Electrical measurements
2.5 cm x 2.5 cm sample
150 nm Katya mask, 5 nm Ti adhesion layer
100 nm x 100 nm pad measurement
3. ALD SnO2
Growth Electrical measurements
XPS
XPS 2.5 cm x 2.5 cm sample
150 nm Katya mask, 5 nm Ti adhesion layer
100 nm x 100 nm pad measurement
Plasma pre-clean, thermal
SnO2
• Precursors:
Tetrakis(dimethylamino)tin (IV)
(TDMASn),
H2O
• Reaction Conditions:
Temperature 185 °C
Pre-clean treatment O2 Plasma
326 cycles
Growth rate 0.06 nm per cycle
Thermal SnO2
• Precursors:
Tetrakis(dimethylamino)tin (IV)
(TDMASn),
H2O
• Reaction Conditions:
Temperature 185 °C
326 cycles
Growth rate 0.06 nm per cycle
O2 Plasma SnO2
• Precursors:
Tetrakis(dimethylamino)tin(IV)
(TDMASn),
O2 Plasma
• Reaction Conditions:
Temperature 185 °C
326 cycles
Growth rate 0.06 nm per cycle
• Planar perovskite solar cells using TiO2 are
limited due to a conduction band misalignment
with the perovskite layer.
• e- injection is energetically hindered. This can
lead to the accumulation of photogenerated
charges which can hamper the performance of
the device.
• SnO2 has a deep conduction band.
• When TiO2 is replaced with this there is a
barrier-free band alignment between the
perovskite light-harvester and the ETL. This is
important for an efficient perovskite solar cell as
it gives improved e- extraction with improved
hysteric behaviour.
4. CH3NH3PbI3-xClx
Plasma pre-clean, thermal TiO2 Thermal TiO2 doped with Al2O3 Plasma pre-clean, thermal SnO2 Thermal SnO2 O2 Plasma SnO2
5. Summary
ALD growth by both thermal and plasma processes were demonstrated for TiO2 and SnO2 metal oxides at
185 °C on FTO coated glass. One growth for each metal oxide included a 5 minute oxygen plasma pre-clean
before being deposited on. This surface cleaning method was implemented to tune the blocking layer and
investigate if it would allow for better wetting of the perovskite. XRD characterisation of all thin films showed
to be amorphous. XPS revealed no difference in the Ti4+ to Ti3+ ratio between undoped and Al2O3 doped TiO2.
This is not unexpected for laminate doping. Plasma pre-clean, thermal TiO2 displays an improved conductivity
compared to that of Al2O3 doped TiO2.
Only Sn4+ was observed in the XPS valence spectra for both O2 plasma SnO2 and plasma pre-clean, thermal
SnO2. Plasma pre-clean, thermal SnO2 has improved conductivity compared to other SnO2 growths deposited
on FTO coated glass. A mixed halide perovskite solution was synthesised and spin coated on each of the
ETLs. A noticeable difference in wettability could be observed. A plasma pre-treatment is clearly important
electrically but does not effect wettability. Although not electrically the most conductive, O2 plasma SnO2
has the best wettability. Further investigation of differing the ratio of Al2O3 doped TiO2 as well as a post-
growth plasma exposure are currently underway.
Lee, M. et al. (2012). Efficient Hybrid Solar Cells
Based on Meso-Superstructured Organometal
Halide Perovskites. Science 338:643-647
This schematic shows a planar perovskite solar cell. The perovskite
layer absorbs UV light and there is subsequent charge generation. Both
positive and negative charge carriers are transported through the
perovskite to charge selective contacts → ETL and HTL
Research over the last few years has allowed efficiencies of perovskite
based solar cells to dramatically increase to up to 22% this year while
still keeping cost low.
One drawback of this type of solar cell however, is it’s lifetime.
This Tandem cell consists of a perovskite solar top-cell over a silicon heterojunction
bottom-cell. The perovskite top cell absorbs visible light but transmits infra-red and
near infra-red light to the bottom cell.
This research focuses on a low-temperature ETL using the ALD technique. Most
methods for depositing ETLs use other processes such as spin-coating, but these
require annealing afterward of up to 600 °C. For this process the ALD reactor is set
at 185 °C for the deposition of all the ETLs and there is no annealing afterward.