This document summarizes research on improving the stability of perovskite solar cells. It discusses using tin instead of lead in perovskite materials to reduce toxicity. A carbon and epoxy electrode is also proposed to improve moisture stability. Test results show the carbon-epoxy electrode maintains solar cell efficiency for over 20 days in humid conditions, while a carbon-only device degrades after 16 days. Coating the carbon-epoxy with silver further enhances moisture resistance and electrical conductivity.

1. Perovskite Solar Cell Degradation Solutions
Abstract—
Use of conventional sources of energy to generate electricity is
increasing rapidly due to growing energy demands in every sector
which is the major cause for pollution as well and also is an
environmental concern for future. Considering this, there is lot of
R&D going on in the field of alternate energy sources with recent
advancements in technology. One of the recent advancement is the
perovskite solar technology in the photovoltaics industry. The
power conversion efficiency of perovskite solar cells has been
improved from 9.7 to 20.1% within 4 years which is the fastest
advancement ever in the photovoltaic industry. Such a high
photovoltaic performance can be attributed to optically high
absorption characteristics of the hybrid lead perovskite materials.
In this review, different perovskite materials are discussed along
with the fundamental details of the hybrid lead halide perovskite
materials. The fabrication techniques, stability, device structure
and the chemistry of the perovskite structure are also described
aiming for a better understanding of these materials and thus
highly efficient perovskite solar cell devices. In addition some
advantages and drawbacks are also discussed here to outline the
prospects and challenges of using the perovskites in commercial PV
devices.
Key words - Perovskite Solar Cells, Degradation Solution,
Perovskite Instability, Deterioration Factor
I. INTRODUCTION
Solar energy is the most abundant energy resource in Earth, where
every minute enough sunlight reaches the Earth’s surface to meet the
world’s energy demands for one year. So the question is why this is
not happening, well because so far the conversion of sunlight into
energy has not been very effective
Currently the conversion of sunlight into energy can be attained
through solar photovoltaic (PV) cells which are further divided into
so many different categories, concentrated PVs and solar thermal
technologies. As per references made, today, solar energy provides
only a small portion of net global electricity generation which is
approx. 1%, but the use of solar PVs is getting popular and it is
growing briskly due to the annual reduction in the cost of such
technologies [1]. Solar cells are categorized into different categories
such as first generation cells made of crystalline silicon and they have
dominated the PV market over the past half century. Then we have
‘Third generation’ PV technologies which have been developed to
pursue high power conversion efficiency (PCE) and low cost, these
include organic PVs (OPVs), light condensed cells, organic–
inorganic hybrid solar cells, dye-sensitized solar cells (DSSCs), and
so on. The third generation of solar cells got so much popularity
because of the large flexibility in the shape, color and transparency
particularly in case of DSSCs, which makes them one of the most
promising technologies for PV conversion applications. Similarly
there is another technology which is fairly recent, the perovskite
technology for solar applications. The amazing fact about this
technology is that it is fastest growing technology in terms of the
power conversion efficiency and in just under four years, the
efficiency boosted from 3.9 % to 20.1 %. The technology further
enlightens the hope for the future and many big companies are
showing active interest in the technology. Perovskite is a mineral
which was first discovered in the Ural mountains of Russia by Gustav
Rose in 1839. It was named after the Russian mineralogist Lev
Perovski. It is found in the Earth’s mantle. Perovskite is any mineral
which has ABX3 crystal structure, A and B are 2 cations of very
different sizes and X is an anion that bonds to both. Most Common
type is crystal structure for CaTiO3 which is also known as
Perovskite structure. Synthetic Perovskites are now recognized as
possible inexpensive base materials for high-efficiency of up to 20%.
Perovskite technology has high potential of becoming the market
leader in the PV industry because in just under 4 years. The
technology has so many advantages including the manufacturing
process, the perovskite solar cells can easily be manufactured in the
labs. They are so inexpensive to manufacture which makes them
highly sought after for the energy conversion but they have certain
disadvantages as well under the belt. The review discusses the
disadvantages in details which is the toxicity of lead and the
degradation of the perovskite solar cells when they are exposed to the
damp conditions which is probably the major obstacle in their
commercialization.
II. BACKGROUND REVIEW
Organo-metal halide Perovskite Solar Cells (PSC) have recently
emerged as a transformative photovoltaic (PV) technology. Power
conversion efficiency attained with the use of the hybrid
organic−inorganic perovskiteCH3NH3PbI3 has now exceeded 15%,
making it more economical compared with thin-film PV technology.
Development of PSCs took decades-long researches on dye-
sensitized solar cells and quantum dot solar cells. Recent perspective
articles that highlight the evolution of PSCs have identified areas of
future research for achieving efficiencies greater than 20% [2]. In
this paper we present different types of perovskite solar cells, the
manufacturing, market potential, and challenges in commercialization
and future improvements. Following the discovery of these organo
metallic halide materials by Mitzi’s group in the 1990s, Miyasaka’s
group uncovered their photo electrochemical properties in 2009.
Because of the instability of CH3NH3PbI3 in a solvent medium,
research on this material remained inactive until solid-state solar cells
were designed in 2012. CH3NH3PbI3 film with absorptions up to
800 nm (bandgap1.5 eV). There is also an important and useful
Dawn John Mullassery, Electrical and Computer Engineering, The University of British Columbia
Report submitted on 14/04/2016.
Dawn John Mullassery is a student at The University of British Columbia, Vancouver, BC, CANADA.
(e-mail: dawn.john@alumni.ubc.ca).

2. band edge at 1.23 eV.
Fig. 1X-ray Diffraction (XRD) and crystal structure. (a) XRD
patternderived from a ground powder of CH3NH3SnI3. (b)
Simulated crystalstructure of CH3NH3SnI3 obtained from the
diffraction pattern given in(a) showing the tetragonal
conformation of the perovskite lattice.
feature of these organometallic halide perovskite solar cells which is
the relatively high open-circuit voltage (VOC ≈ 1 V). Recent
researches are now focusing on boosting the open-circuit voltage
even higher using CH3NH3PbBr. For example, inclusion of chloride
ions in CH3NH3PbBr3 films yields VOC as high as 1.5 V. The
halide part could be iodide, bromide, chloride etc. depending upon
the required open circuit voltage. While fabrication of the solar
perovskites, spiro-OMeTAD material is used as the hole conductor.
Research is underway to explore alternate organic and inorganic hole
conductors. Higher hole conductivity reported for inorganic hole
conductors has been shown to be possible using cheaper and readily
available materials such as CuI [2]. A basic understanding of the hole
transport properties is crucial for further development of perovskite
solar cells. Because the technology is fairly new so there is still lots
of R&D required to encounter the issues faced while employing the
cells in the damp conditions. It is found that In the presence of
moisture, the perovskite undergoes rapid decomposition which results
in significant decline in device performance. Test results reveal that
un-encapsulated perovskite solar cells reported in 80% drop in PCE
over a 24h period. Even more concerning is the decomposition to
PbI2 because it is sparingly soluble in water and this would result in
extreme toxicity.
III. RESULTS
This paper focused on the different kinds of Methyl Ammonium
Lead Perovskites. But the usage of lead in perovskite products may
actually bring in a concern for the toxicity issues. The toxicity should
be considered as an issue due to perovskite instability in moist
conditions or damp atmosphere. Experiments have shown that
perovskites degrade relatively fast in damp conditions, and this
property hinders the growth of perovskite industry.
In this section we will try to discuss some possible methods to
reduce the perovskite degradation. It will also throw light on the use of
Tin as a suitable substitute to Lead.
A. Tin replacing Lead
It is understood that the main problem with a Lead perovskite is
its toxic nature. The possible replacement for Lead will be Tin (Sn)
and Germanium (Ge) members of the group 14 metals. But the
problem with these metals are its instability issues. As we move up the
group 14 elements, we can see that there is a chemical instability in
the group 14 metals, in the required oxidation state. This problem
exists with Tin too. Sn2+ ion is oxidized to Sn4+, and it acts as a P-
type dopant. This is known as 'self-doping'.
Due to this effect the experiments were done in stable
environments, and the instability can be derived from the Sn2+ ions
instability under oxygen and moisture. All the measurements were
done under inert conditions.
To know about the purity and structure, an X- Ray diffraction was
performed. The X-Ray diffraction pattern is shown in Fig1. This
pattern is in well agreement with the simulated data, and confirms the
tetragonal structure of the perovskite structure. The lattice parameters
derived from the X-ray diffractogram were a = 8.7912 °A and c =
4.4770 °A. [3]
The optical characterization can be observed in Fig. 2a. We can
see that there is a broad absorption edge at 1000 nm (approx.) and a
wide photoluminescence peak at approximately 980 nm. Alongside is
the lead perovskite, CH3NH3PbI3_xClx, shows a sharper absorption
edge at 770 nm, and a narrower emission spectrum. Photo thermal
Detection Spectroscopy (PDS) was also done for better bandgap
estimation of CH3NH3SnI3 perovskite. Fig. 2b shows the PDS
measurements.
Fig. 2 (a) Normalized steady state photoluminescence (PL) with photo excitation at 500
nm, and absorption taken with reflectance and transmission employing an integrating
sphere of the tin-based and lead-based perovskites CH3NH3SnI3 and CH3NH3PbI3_xClx
respectively.(b) The absorption profile of CH3NH3SnI3 as determined through photo-
thermal deflection spectroscopy (PDS), with the band gap of the material determined
using the Tauc plot (shown in inset).21 We note that since there may be strong exciton
absorption at the band edge, the Tauc plot determined band gap can only be considered
an estimate.
For further comparisons, the perovskite sensitized solar cell was
fabricated. It composed of an FTO coated glass with compact or
mesoporous TiO2 solution and further coated with Methyl
Ammonium Tin Iodide and Spiro-OMeTAD. The whole setup was
made in a nitrogen filled chamber, for avoiding exposure to air.
Encapsulation was done by using hot melt polymer laminate with a
glass cover slip, and also epoxy resin to seal the edges. It is done to
expose the device to negligible amounts of oxygen and moisture
during fabrication. After the fabrication, the cells were removed from
the chamber and measurements were made. The Lead perovskites
were also measured under similar conditions, except for the

3. Fig 4.(A) a pure carbon-based device, where moisture can permeate through
the carbon layer and cause perovskite decomposition. (B) C + epoxy-based
device, where moisture is rejected by the hydrophobic C + epoxy thin film. (C)
C + epoxy/Ag paint-based device, where the boosting compact hydrophobic Ag
paint layer makes the device waterproof. Note that the electrical contacts are
also increasingly improved from (A) to (C).
encapsulation. The different performance parameters were monitored
including the V-I characteristics. The results are given in Fig 3.
Fig 3(a) The solar cell performance parameters extracted from
measuring current–voltage curves under AM1.5 simulated sun light of 100 mW cm_2 for
TiO2-based perovskite sensitized solar cells employing CH3NH3SnI3 (Sn) and
CH3NH3PbI3_xClx (Pb) absorbers.
Fig 3(b) Current–voltages curves of the best Sn-based and Pb-based devices for the
batch of devices shown in (a). Light J–V curves are denoted with solid symbols and the
dark I–V curves with hollow symbols. The Pb-based perovskite is shown both on TiO2
(blue curve) and Al2O3 (red curve) giving short circuit currents (Jsc) of 19.6 mA cm_2
and 21.9 mA cm_2, open circuit voltages(Voc) of 0.98 V and 1.04 V, fill factors (FF) of
0.60 and 0.66, and power conversion efficiencies (h) of 11.5% and 15.0% respectively.
While the Sn based perovskite showed negligible photovoltaic properties on Al2O3, on
TiO2 a maximum h of 6.4% was obtained, corresponding to a Voc of0.88 V, Jsc of 16.8
mA cm_2 and FF of 0.42. It is important to note that the Sn-based devices were
fabricated, metal electrodes evaporated and devices sealed in a nitrogen filled glove box
prior to exposing to air.
B. Improving Moisture Stability Carbon + Epoxy method
As read in the previous section, all perovskites face the problem of
stability. In this section we would like to discuss about a method
which can improve the moisture stability to a major extent. Here we
discuss about the new method of introducing a hole-transporter-free
Perovskite Solar Cell based on a 'Carbon and epoxy' electrode that
will act as both hole-selective medium and mainly, water-repellent
membrane. The application of Silver coating over this would further
improve the perovskite stability and improve the hole conduction.
The efficiency is marked at approximately 11%. Even when the
whole setup was immersed in water, the performance remained stable
for 80 minutes. When the cell stability was investigated under
extreme environments, like high humidity and 50 °C temperature
conditions, the performance degradation was observed to be stable
within the testing time period.
Here the usual Hole Transport Material is changed by carbon based
interface. This will provide enough stability during the dry ambient
conditions. But under damp or moist conditions, the stability reduces.
This is due to the porous nature of carbon. Moisture will penetrate
through the moist carbon layer, and will degrade the perovskite
completely. This can be seen in Fig 4(a). So, to avoid this problem,
the carbon electrode is made into a carbon and epoxy mix electrode.
This will improve the electrical conductivity and also will reduce the
penetration of water into the structure. This can be observed in Fig
4(B). To further increase the moisture resistance, a thin layer of
liquid silver is coated over the surface of the material. This not only
reduced the water penetration, but also improved the overall electrode
conductivity as shown in Fig.4(C).
The test for moisture stability was performed under a moist condition
of 60 % to 80% RH. The contact angle test results determine the
efficiency of using such a system. Fig 5.A shows the contact angle
photographs of the three conditions (Carbon coating, Carbon and
Epoxy coating and Carbon Epoxy with Silver coating.) The different
contact angles are also mentioned in the figure. The Fig 5(B) shows
the normalized efficiency curve as a function of time. The Carbon-
based perovskite shows a decline in efficiency up to 40% of the
initial efficiency, after a time span of 16 days [4]. In contrast, 'Carbon
and epoxy'-based and 'Carbon and epoxy/Ag' systems show steady
efficiency even after 20 days. No performance decline is observed for
the Carbon based epoxy/Ag paint-based device.
Fig. 5 (A) Contact angle photographs and SEM images of the three films: C, C + epoxy
and C + epoxy/Ag paint.
Fig 5(B) Normalized efficiency as a function of storage time in the ambient atmosphere
(RH ∼ 60 to 80%).
The cross-sectional scanning electron microscopy (SEM) image is
shown in Fig 6(A). It shows an approx. 350 nm meso-TiO2 thin film,
which is filled with perovskite. Another Approx. 350nm capping
layer is also observed, which is further covered by Carbon and epoxy

4. a)
b)
c)
Fig 6(A) cross-sectional SEM image of a C + epoxy-based PSC device. (B)
Energy level diagram of C + epoxy-based PSCs. (C) J–V curves under 1 sun AM
1.5 illumination
paint layer. Here the carbon particles are closely and thickly packed,
and is within the epoxy layer.
The energy level diagram is shown in Fig 6(B). It shows both Carbon
and epoxy and Carbon and epoxy/Ag paint-based systems. Fig 6(C)
shows the Carbon and epoxy-based system with high performance
with Voc, Jsc, and FF of 0.99 V, 18.17 mA cm−2
and 0.51,
respectively. This will give a high Power Conversion Efficiency of
9.17%. After coating silver paint on C + epoxy film, the Fill Factor
value is improved to about 0.6, which further improves the PCE
value to 10.70%. Thus, this method can be found as a much
beneficial moisture stability method to protect perovskites from being
degraded.
C. Ambient stability improvement using Iodide reduced
graphene oxide with Dopant free spiro-OMeTAD.
Here, an ultra-thin reduced graphene oxide (RGO) is combined with
dopant-free spiro-OMeTAD and used as a Hole Transport Material.
The Hole Transport Material was fabricated by spin-coating a
Reduced Graphene Oxide on the perovskite film and followed by
proper casting of dopant-free spiro-OMeTAD. By using this
structure, the PCE of PSCs reached up to 10.6%, much higher than
the efficiencies of cells having Germanium Oxide/dopant-free spiro-
OMeTAD, which has 6.1%, and also the dopant-free spiro-OMeTAD
structure that has 6.5%. Apart from this, the PSC devices with RGO
has a better stability.
The first step of the process is to produce reduced GO, and it is done
by adding FeI2 aqueous solution to 100 mL 10 mg mL−2
GO
dispersion. The pH is altered by adding conc. HCl acid. RGOs with
different pH were designated as RGO-1, RGO-1.5, RGO-2, and
RGO-3 etc.
For the fabrication of RGO, it is first deposited dynamically on the
prepared perovskite surface by spin-coating the RGO, followed
immediately by the deposition of spiro-OMeTAD, again by spin-
coating. It is left to dry for 12 hours. Finally, Gold was deposited by
thermal evaporation method to form the top electrode.
To evaluate the results, the RGO was fabricated on top of Methyl
Ammonium Lead Iodide perovskite. The electron microscope picture
is as shown below Fig 7.
Fig. 7Typical top view SEM images of RGO (a) and RGO/spiro-OMeTAD (b) on the
surface of the perovskite/TiO2 layer; (c) typical cross-sectional SEM image of the
RGO/spiro-OMeTAD coated perovskite layer.
The VI characteristics obtained after the experimentation is given
below Fig 8.
Fig 8(a) Forward bias to short-circuit (FB-SC, solid line) and SC-FB (dashed line) J–V
curves of perovskite solar cells with different hole transport layers. Devices were
scanned at 0.022 V/s (b) The incident photon to current efficiency spectra of perovskite
solar cells with different hole transport layers and integrated current density. (c) The
photocurrent density as a function of time for the cells held at a forward bias of the
maximum output power point (0.60, 0.56, 0.51, 0.51, 0.46 and 0.77 V for the devices
based on RGO-1, RGO-2, RGO-3, GO, dopant-free spiro-OMeTAD and doped spiro-OMeTAD,
respectively). The black, red, blue, magenta, dark cyan and dark yellow squares on the
J–V curves in (a) represent the value of the stabilized photocurrent density measured in
(c). The cells were measured under simulated AM 1.5, 100 mW cm−2
solar irradiation with
a cell area of 0.06 cm2
determined by using a metal mask and was placed in the dark
prior to measurements
Here, the devices with RGO-1 and RGO-2 were found to have
improved photovoltaic performance when compared to the RGO-3.
RGO-shows better photovoltaic performance than RGO-2-
perovskites. The RGO-1-perovskites exhibited an average PCE of
9.31%,with a Jsc of 16.73 mA/cm2
, a Voc of 910 mV and a Fill Factor
of 0.61.

5. The stability of the different RGOs/GO is as shown above. RGO1
shows the best performance in stability without dropping its efficiency
much, even till 500 hours.
IV. MARKET POTENTIAL
By 2020, the world's renewable energy source will take a
significant share of the energy market. The renewable energy
contribution will become close to 50% by 2050. And a major
share of this will be held by solar technology. Perovskites as a
new innovation in solar technology, has far surpasses all existing
technologies in its ease of construction and cost. Almost all solar
research companies have focused their research into perovskite
technology and it throws light on the huge market available for
perovskites. Further regulations on coal and petroleum will drive
energy interests into perovskite technology, as it is a cheap and
efficient source of energy. To conclude, the perovskite
technology is one of the fastest growing market segment in solar
technology. We expect that the perovskite energy will hold a
major share of the renewable energy sources, which will surpass
conventional energy sources in the near future.
V. CONCLUSION
Although perovskite technology has bloomed as one of the fastest
growing technologies of all time. But they still need to overcome so
many barriers amongst the major hurdle is its moisture stability. As
suggested in the results, the perovskite stability can be improved by
C+epoxy/Ag method or RGO with defined pH level HTMs. Another
problem is the usage of Pb in perovskites that may cause health
hazards. By replacing Pb with Sn, the problem can be solved. And to
solve the problem of Sn instability issues, the above HTM methods
may be used. Here we propose the possibility of using Sn perovskites
with improved stability using the above HTM methods. But the
technology still needs to improve its PCE and other efficiencies,
which more R & D can assure. There is no doubt that the solar
perovskite cells are the upcoming market leaders in the PV industry
not forgetting this is the technology with the fastest advancement in
the PCE from 3.9 to 20.1 in just under four years whereas other solar
techniques have been evolved over the period over last 20 – 30 years.
Many big companies are anticipating the solar perovskite technology
going commercial over the next few years because the demand for
clean energy is rapidly rising considering the environmental problems
and this technology can come out as an outbreak in the history of PV
industry.
VI. ACKNOWLEDGEMENT
We would like to thank Dr. Peyman Servati, University of British
Columbia, for all his help and guidance.
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