A one-step low temperature processing route for organolead halide perovskite solar cells

Article abstract of DOI:10.1039/c3cc44177f

Organolead trihalide perovskite solar cells based upon the co-deposition of a combined Al₂O₃-perovskite layer at T < 110 °C are presented. We report an average PCE = 7.2% on a non-sintered Al ₂O₃ scaffold in devices that have been manufactured from a perovskite precursor containing 5 wt% Al₂O₃ nanoparticles.

Full text of DOI:10.1039/c3cc44177f

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A One-Step Low Temperature Processing Route for Organolead Halide
Perovskite Solar Cells
Matthew J. Carnie,a Cecile Charbonneaua Matthew L. Daviesb Joel Troughtona Trystan M. Watsona
Konrad Wojciechowskic, Henry Snaithc and David A. Worsleya
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
50 devices, whereby a PCE of 9.7 % was achieved 13. In other work
Organolead trihalide perovskite solar cells based upon the co-
focussing on a mixed halide (CH3NH3PbI3ꢀxClx) perovskite it was
found that electron transport was faster when an alumina scaffold
was utilised than when using a TiO2 scaffold. Higher efficiencies
were achieved when using the insulating mesoporous Al2O3
deposition of a combined Al2O3/perovskite layer at T < 110 °C
are presented. We report an average PCE = 7.2 % on a non-
10 sintered Al2O3 scaffold in devices that have been
manufactured from a perovskite precursor containing 5 %
wt. Al2O3 nanoparticles.
55 scaffold than with
a
semiconducting mesoporous TiO2
scaffold/electronꢀacceptor. A PCE of 10.9 % was achieved with
VOC in excess of 1.1 V in a device utilising a sintered alumina
scaffold 14. Caesium tin iodide (CsSnI3) perovskite has also been
employed as solid hole conductor in a ssꢀDSSC with a PCE of
The spectral sensitization of nꢀtype TiO2 electrodes by
polypyridineruthenium (II) complexes was first described by
1
15 Clark and Sutin in 1977 . This combined with early work on
60 8.5 % 15
.
2
photoelectrochemical cells and dyeꢀsensitization to increase the
3
Organolead halide perovskite materials are a promising, low
cost, easily synthesised set of materials. They act as light
absorber and both electron and hole transporter. Indeed, efficient
devices have been made where no organic hole transporter is
65 utilised 16. All the technologies mentioned so far require a high
temperature heating step in order to sinter the TiO2 or Al2O3
nanoparticles. Perhaps from a manufacturing point of view, one
of the most promising aspects of the Al2O3 based devices is the
fact that, as Al2O3 is electrically insulating, there is no need to
70 sinter the nanoparticles, as particle interconnectivity will not be
as important as it is in TiO2 based devices where poor particle
efficiency of TiO2 photocatalysis , led to the realisation of dyeꢀ
sensitization for photovoltaic applications and eventually to the
work of O’ Regan and Grätzel on colloidal, high surface area
20 TiO2 films. The result was their seminal paper describing high
4
efficiency dyeꢀsensitized solar cells (DSSCs) in 1991 . Further
5
developments of screen printable polymerꢀorganic TiO2 pastes
and high efficiency cells built on metal substrates 6 has led to the
realisation that it may be possible to produce DSSCs utilising
25 rollꢀtoꢀroll processes. Sensitizer and electrolyte developments
have recently resulted in DSSCs reaching power conversion
efficiencies (PCE) of over 12 % 7.
interconnectivity results in slower electron transport and lower
So far the highest performing devices have utilized dye
sensitizers and liquid electrolytes, but alternative configurations
30 have been extensively investigated such as the solid state dyeꢀ
sensitized solar cell (ssꢀDSSC), with a solid organic hole
transporter , and alternative sensitizers such as quantum dots .
To date, ssꢀDSSC power conversion efficiency stands at 7.2 % 10
14
photocurrents 17. In the work outlined by Lee et al.
,
nanoparticles are sintered at 550 °C but this may not be necessary
75 and indeed, low temperature deposition of Al2O3 has now been
achieved with device performances better than that of devices
8
9
made with a sintered alumina film 18
.
In this work, we propose an alternative method of
Al2O3/perovskite deposition whereby the two step deposition
80 process can be performed in a single step. The alumina
nanoparticles are suspended in the perovskite precursor solution
and the Al2O3/perovskite layer is coꢀdeposited by spin coating in
a single deposition. This is followed by a low temperature heating
step at T < 110 °C. The mixed halide perovskite precursor and
and the highest recorded for quantum dot devices so far is PCE =
11
35 7.0%
In 2009, Kojima et al. published work on using
organometal halide perovskite as sensitizers in liquid electrolyte
based devices. They achieved
a PCE of 3.8 % with a
CH3NH3PbI3 perovskite and a VOC of 0.96 V with a CH3NH3Br3
perovskite 12. The realisation that these perovskite materials could
40 be utilised in ssꢀDSSCs as light harvesters lead to high efficiency
14
85 device manufacturing method outlined by Lee et al. have been
____________________________________________________
a SPECIFIC, College of Engineering, Swansea University, Baglan Bay
Innovation and Knowledge Centre, Central Avenue, Baglan SA12 7AX,
UK. E-mail: d.a.worsley@swansea.ac.uk
utilised. Our methodology differs in that instead of depositing a
lead halide perovskite precursor solution onto a preꢀsintered
Al2O3 scaffold, we have added Al2O3 nanoparticles directly to the
perovskite precursor solution. This is followed by a low
90 temperature heating step at T < 110 °C. This has the advantage of
combining two manufacturing steps into one and negates the need
for a high temperature (550 °C) sintering step. This results in
45 b School of Chemistry, Bangor University, Bangor, Gwynedd UK
cPhotovoltaic and Optoelectronic Device Group, Department of Physics,
Oxford University, UK
† Electronic Supplementary Information (ESI) available: See
DOI:10.1039/b000000x/
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DOI: 10.1039/C3CC44177F
devices that have a lower embodied energy with a relatively more
simple manufacturing process. This may prove to be of great
importance to the scale up and manufacture of this technology.
The alumina nanoparticle suspension is obtained from Sigmaꢀ
Aldrich (20 wt. % in isopropanol). Initial experiments involved
adding the alumina suspension directly to the perovskite
precursor solution. The resultant Al2O3/perovskite precursor
solution was nonꢀhomogeneous in appearance which we have
hypothesised as being caused by mixed solvent/solute
5
10 incompatibilities, as the perovskite precursor is poorly soluble in
isopropanol. Devices made with this Al2O3/perovskite solution
had low efficiencies. In order for high efficiency devices to be
made, it was found that the nanoparticles must be suspended in
DMF (N,NꢀDimethylformamide). This is achieved via solvent
15 exchange in a rotary evaporator whereby equal volumes of the
nanoparticle suspension and DMF were put into a round bottom
flask. A rotary evaporator is then used to remove the lower
boiling point isopropanol, thus leaving the nanoparticles
suspended in DMF. The nanoparticle suspension in DMF is then
20 added to the perovskite precursor solution which is also dissolved
in DMF. The first devices manufactured, did not perform well
and showed evidence of shunting in their IV curves. This was
solved by sonication of the nanoparticle suspension for one hour
prior to addition to the perovskite precursor solution. This
25 suggests that the shunting could have been caused by
nanoparticle aggregation leading to film imhomogeneity and
possible contact between the perovskite and the gold counter
electrode. Fig. 1 below shows the statistical IV data for all
manufactured devices, with varying nanoparticle weight
30 percentages dispersed in the perovskite precursor prior to
deposition.
40 Fig.2 Plan view SEM images of coꢀdeposited Al2O3/perovskite films. The
wt. % of alumina in the precursor solution prior to spin coating is shown
topꢀleft in each SEM micrograph. It can be seen that the coꢀdeposited
alumina/perovskite forms a dualꢀphase structure with alumina rich (lighter
shading) and alumina poor (darker shading) areas
45 It can be seen that device PCE improves as the wt. % is increased
from 0 % (a planar junction cell) to 5 %. At higher weight
percentages, PCE decreases. Fig. S1† shows the UVꢀVis
absorbance of coꢀdeposited Al2O3/perovskite films, spin coated
onto microscope slides, the average Al2O3/perovskite film
50 thickness was found to be 400 nm using a profilometer. A cross
sectional SEM of the best performing device in this study is
shown in Fig. S2†. The absorbance spectra show increasing
absorbance with increased particle loading. The increasing PCE
(Fig.1) could possibly be due to increased amounts of perovskite
55 crystallizing from the precursor solution with increased
nanoparticle wt. %. However, there is also expected to be an
increase in light scattering with an increase in nanoparticle wt. %
which could account for, at least a proportion of, the absorbance
increase. At the highest nanoparticle weight percentages (6 % and
60 8 % by wt. in the precursor solution), the UVꢀVIS absorbance
continues to increase despite the photovoltaic PCE decreasing at
wt. % > 5 %. This decrease in performance may occur due to the
dual phase nature that has been observed and is evident in Fig. 2
which shows planꢀview SEM images of the same films as in Fig.
65 S1†. EDX analyses of these films shows that there are alumina
rich areas (lighter shading, figure 2) and alumina poor areas
(darker shading) which could indicate poor wetting of the
alumina surface by the perovskite precursor solution. At higher
nanoparticle wt. %, electron transport through the perovskite film
70 could be restricted by the large alumina deposits. This is
supported by the IV data where devices made with the higher
weight percentage precursor solutions show lower photocurrents
and fill factors.
Fig. 1 Statistical IV data showing: a) efficiency (PCE), b) openꢀcircuit
voltage (VOC), c) shortꢀcircuit current density (JSC ) and d) fillꢀfactor (FF)
35 of devices vs. the nanoparticle wt. % in the perovskite precursor solution
before deposition
2
| Journal Name, [year], [vol], 00–00
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To date our most efficient organolead trihalide perovskite
devices, incorporating an alumina scaffold, have been
manufactured in the way described above can match the
performance of devices described in 15, especially when the dual
manufactured using the low temperature manufacturing method 45 phase nature of the coꢀdeposited layer is considered. If it is
described here. For comparison, a statistical analysis of the 8
most efficient devices made with a sintered (550 °C) Al2O3
scaffold has been conducted and the results are summarised in
Table S1† where it can be seen that the average PCE = 3.17 % (±
possible to encourage the perovskite to wet the surface of the
nanoparticles more effectively upon crystallization then it may be
possible to achieve large improvements in device light harvesting
efficiency, which may improve photocurrents. If fill factors can
5
0.88 %). The average device efficiency for the 8 most efficient 50 also be improved by HTM optimisation then it is conceivable that
low T, coꢀdeposited devices is PCE = 3.98 % (± 0.33 %). The
high efficiency devices can be manufactured using this low
temperature coꢀdeposition technique.
10 improvement in device PCE and improvement in device
consistency is in accordance with the recent report by Ball et al.
in which they have achieved their best device performances with
Conclusions
a low temperature Al2O3 deposition technique 15
.
55
Following the work presented so far to determine the optimum
15 nanoparticle wt. %, steps have been taken to improve device
efficiencies. We have observed that the performance of these
devices is sensitive to the relative humidity (RH) in the laboratory
We have shown that it is possible to obtain efficient lead halide
based perovskite devices by coꢀdepositing
a
single
Al2O3/perovskite layer at low temperatures. We believe this will
be of great interest to those working on the development and
at the point of manufacture. When devices are manufactured in an 60 scaleꢀup manufacture of these devices. Furthermore, we believe
atmosphere of RH < 35 %, improvements in PCE are significant.
20 Indeed, we have recently manufactured our highest efficiency
devices, using the low temperature method which is described in
the ESI† and by controlling the humidity during manufacture. The
average PCE of our ten most recent devices is 7.2 % (± 0.6 %).
Fig. 3 shows the IV curve of one such device with PCE = 7.2 %.
25
that with further optimisation, devices manufactured using this
low temperature method can be improved upon and leads to the
distinct realisation of highly efficient, low temperature, solidꢀ
state devices.
65 Notes and references
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30
It can be seen that the fill factor of this device and indeed, of all
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