A brief introduction to enter into the world of Perovskite Solar Cells, PSCs.

1. PEROVSKITE SOLAR CELLS
AN EMERGING PHOTOVOLTAIC TECHNOLOGY
Dawn John Mullassery
ECE
UBC

2. Abstract:
Perovskite solar cells (PSC) are the most "talked-about" renewable energy source. The rapid growth in
renewable energy and solar cell technology has made them a shining star in the photovoltaics industry.
Although this relatively new technology promises excellent energy future, it requires immense research. It
also holds huge potential for better engineering, more efficient solar cells which are expected to reach in
excess of 20% power conversion efficiency. Even though it is still in development stage , PSC has been
of such interest to scientists, that 'Science' magazine touted it as one of the top scientific breakthroughs of
2013.
This presentation will help you understand Perovskite solar cells, its chemical structure (lattice
arrangement), types of perovskites, basic fabrication process, drawbacks, possible solutions and its future.
Why Perovskites?
One of the major reasons for perovskites attracting such prominent responses is its high Power conversion
Efficiency. The following curve shows the PCEs of the perovskite based devices over recent years in
comparison to other existing technologies.
As observed in the below curve by NREL, the PCE of perovskite solar cells has been improved from
9.7% to 20.1% [2] within 4 years, and can be commented as the fastest advancement ever in the
photovoltaic industry. Such an improvement in photovoltaic performance can be attributed to optically
high absorption characteristics of the hybrid lead perovskite materials. Here, the different perovskite
materials are briefly discussed along with the fundamental details of the hybrid lead halide perovskite
materials. Although it could be argued that recent technologies have the luxury of more sophisticated
laboratory facilities and research needs, this exponential rise in efficiency is still incredibly significant
and impressive. This suggests that with more research, PCE of perovskite solar cells can continue to rise
at this rate over the coming years.
The fabrication techniques, stability, device structure and the chemistry of the perovskite structure are
also briefly described aiming for a better understanding of these materials and thus highly efficient
perovskite solar cell devices. The main focus of this research is to understand possible methods to reduce
toxicity due to lead and to improve Perovskite stability.

3. The advantages of Perovskite cells over CdTE, CIGS, c-Si are as given below. [3] This shows the
enormous potential of Perovskite Solar Cells.
CHARACTERISTICS
CdTe
Cadmium
telluride
CIGS
Copper Indium
Gallium
Selenide
c-Si
crystalline
Silicon
Perovskite
Raw Materials Cost Low Medium Low Low
Finished Materials
Cost
Low High High Low
Fabrication Cost Medium Medium High Low
Energy Payback
Period
Medium High High Low
Levelized cost of
energy (LCOE)
Medium High High Low
Efficiency Medium Medium High High
What are perovskites?
Perovskite is a type of mineral that is chemically found on earth's crust. It was first found in the Ural
Mountains and was named after a Russian nobleman and mineralogist, Lev Perovski (founder of the
Russian Geographical Society). The perovskite solar cells have the same structure of the perovskite
mineral, and hence named Perovskite Solar Cells. Perovskite (the mineral) is formed of calcium, titanium
and oxygen in the form CaTiO3. Perovskite structure is anything that has the generic form ABX3 and the
same crystallographic structure as perovskite (the mineral).
The standard perovskite lattice arrangement is as shown, but similar to many structures in
crystallography, this can also be represented in multiple ways. The simplest way to think about a
perovskite is as a large molecular cation (positively charged) of type 'A' in the centre of a cube. The
corners of the cube are filled by another set of cations 'B', and the faces of the cube with anions 'X'. [4]
Equivalent Perovskite Structure
Depending on the atoms/molecules used in the structure, perovskites may obtain a set of interesting
properties like superconductivity, spintronics and catalytic properties. Hence, scientists and researchers
are finding perovskites as exciting playground for physicists, chemists and material scientists.
In the case of perovskite solar cells, the most efficient devices so far have been produced with the
following combination of materials in the perovskite form ABX3 will have the following combination of
materials:
 A = A big inorganic cation - usually lead(II) (Pb2+)

4.  B = An organic cation - methylammonium (CH3NH3+)
 X3= A slightly smaller halogen anion – usually chloride (Cl-) or iodide (I-)
Since this is a relatively general structure, these perovskite based devices can also be given a number of
different names which can either refer to a more general class of materials or a specific combination. As
an example of this we’ve created the below table to highlight how many names can be formed from one
basic structure.
Metal Halide (or tri-halide)
Organo Lead Iodide (or tri-iodide)
Methylammonium Tin Chloride (or tri-chloride)
Fabrication and Measurement of Perovskite Solar Cells
The perovskite solar cells can be manufactured with simpler wet chemistry techniques in a traditional lab
environment, unlike silicon solar cells that need expensive, multi-step processes that need to happen
under extreme temperatures and in vacuum.
Methylammonium metal trihalides can be created using a variety of solvent techniques and vapor
deposition methods, and have the potential to be scaled up with relative feasibility.
In solution processing, the F-doped tin oxide (FTO) substrates were cleaned in an ultrasonic bath with a
combination of methanol and acetone, and finally dried with nitrogen gas. After the preperation of FTO
substartes, the 0.15 M and 0.3 M TiOx precursor solutions are prepared from titanium diisopropoxide
bis(acetylacetonate) (0.055 mL and 0.11 mL) with 1-butanol (1 mL).
The initial step is to spin-coat 0.15 M TiOx precursor solution on the FTO glass substrate at 3000 rpm for
30 s and then finely annealing the set-up at 125 °C for 5 min. Next step is to spin coat the 0.30 M
TiOx precursor on the initial TiOx layer. The spin coating is done at 3000 rpm for 30 s, and it is also
annealed at 125 °C for 5 min. This 0.30 M solution process was performed two times, after which the
FTO substrate was sintered at 500 °C for 30 min to form a compact TiO2 layer.

5. For the preparation of mesoporous TiO2 layer, a TiO2 paste is made using TiO2 powder (100 mg) and
polyethylene glycol (10 mg) in ultrapure water (0.5 mL). This solution is then mixed with acetylacetone
(10.0 μL) and Triton X-100 (5 μL) for 30 min and is left for 12 hours. This helps in suppressing the
formation of bubbles in the solution. The TiO2 paste obtained is coated on the substrate by spin-coating at
5000 rpm for 30 s. The cells are annealed at 120 °C for 5 min and at 500 °C for 30 min.
Methylammonium Iodide (CH3NH3I) was synthesized by reacting 23.2 mL of methylamine (CH3NH2)
and 25.0 mL of hydroiodic acid at 0 °C for 2 hours with stirring. The precipitate is collected by removing
the solvents at 50 °C for 1 hour.
The product we obtain is re-dissolved and stirred in diethyl ether for 30 min to remove any impurities and
dried using a rotary evaporator at 60 °C for 3 hours. The CH3NH3I thus produced is finally dried in
vacuum. To prepare methylammonium lead iodide with a perovskite structure, a solution of CH3NH3I
(98.8 mg) and PbI2 (289.3 mg) at a mole ratio of 1:1 inγ-butyrolactone (0.5 mL) is mixed at 60 °C. The
CH3NH3PbI3 solution is then introduced into the TiO2 mesoporous using a spin-coating method and
annealed at 100 °C for 15 min.
Next step is to prepare the hole-transport layer (HTM) by spin-coating. A solution of spiro-OMeTAD
(36.1 mg) in chlorobenzene (0.5 mL) is mixed with a solution of lithium bis(trifluoromethylsulfonyl)
imide (260 mg) in acetonitrile (0.5 mL) for 12 hours. This solution and 4-tert-butylpyridine (4 μL) is then
mixed with the Li-TFSI solution (8.8 μL) for 30 min at 70 °C. All procedures are carried out in air.
Finally, gold (Au) metal contacts is evaporated onto the sample as top electrodes.
Performance
The perovskites have excellent performance when considering the open-circuit voltage and short-circuit
current parameters. The given figure shows the current-density/voltage curves of the best- performing
solution-processed (blue lines, triangles) and vapour-deposited (red lines, circles) planar heterojunction
perovskite solar cells measured under simulated AM1.5 sunlight of 101 mW cm irradiance (solid lines)
and in the dark (dashed lines) [5].
Areas of Improvement
Although the perovskite solar cells have an upper hand over other solar cells in the field mentioned
earlier, they also face some serious problems when stability and some other factors are concerned. Below
given are the factors that are to be concerned
1) Stability - Perovskite solar cells degrade relatively quick when it gets in contact with moisture.
Different works are going on to determine possible solutions to improve the stability. Below given is a
diagram showing the effect of moisture on perovskites [6].

6. 2) Lead content : Many scientists and ecologists are considering the effect of Pb in perovskites to be
dangerous. The presence of Pb in easily degrading perovskite may cause a possibility of Pb entering the
environment, leading to health hazards.
Future Progress:
Although Perovskite solar cells have proven to be a field of much benefit, research and development has
to be made for the cells to be made available in the market. Recent works are being done on removing the
HTMs completely and using perovskites to transfer the holes to the electrode directly. This could bring
down the costs further, and make the fabrication process easier. Thin film perovskites, and foldable
perovskites are also gaining interests. If these thin layer perovskites can be put into multiple fields,
including aviation, perovskites will be the next big renewable energy source.
References:
[1] Retrieved from NREL NCPV Home Page, http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
[2] Hybrid Organic-Inorganic Perovskites Open a New Era for Low-Cost, High Efficiency Solar Cells, Guiming
Peng,1,2,3 Xueqing Xu,2 and Gang Xu2
[3] http://www.oxfordpv.com/sites/www.oxfordpv.com/files/media-downloads/general/oxford-pv-introduction-
152.pdf
[4] http://www.ossila.com/pages/perovskites-and-perovskite-solar-cells-an-introduction
[5] "Efficient planar heterojunction perovskite solar cells by vapour deposition" Mingzhen Liu, Michael B. Johnston
& Henry J. Snaith
[6] Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using
in Situ Techniques- Jinli Yang, Braden D. Siempelkamp, Dianyi Liu, and Timothy L. Kelly