Perovskite – A Better Alternative
What makes a solar cell absorb energy? What is the primary element it isn’t possible without? Most of you would agree that it is ‘silicon’. Have you ever imagined a replacement for silicon; perhaps with something that has better efficiency and availability?
Let me introduce you to a compound called ‘perovskite’. It has a possibility of taking the efficiency of solar cells to the next level and will soon evolve to be the preferred primary component of solar cells.
Originally, it was a compound of calcium, titanium and oxygen but over the years, it has become a common term for most compounds that have a similar (ABX3) crystal structure. Among perovskites that are commonly researched, one that stands out is methylammonium lead trihalide (CH3NH3PbX3, where X could be any halogen atom e.g. iodine, chlorine etc.). Its band gap varies from 1.5 and 2.3 eV, depending on its halide content.
We can thus define a perovskite solar cell as a solar cell which contains a perovskite-structured compound made out of a mineral such as hybrid lead or tin halide-based material as the active layer that absorbs light energy.
‘Perovskite‘ is named after the Russian mineralogist, Count Lev Aleksevich von Perovski. In the year 1839, a German mineralogist called Gustav Rose discovered perovskite from samples found in Ural mountains. It was first incorporated into a solar cell in Toin University by a Japanese researcher called Tsutomu Miyasaka in 2009. The efficiency at this stage was about 3.8%.
In 2012, Henry Snaith and Mike Lee from the University of Oxford discovered that perovskite was stable when it came in contact with a solid-state hole transporter such as spiro-OMeTAD, negating the requirement of a mesoporous TiO2 layer to transport electrons. The efficiency at this stage rose to about 10%. In 2013, a 2-step solution processing deposition technique was developed by Burschka which improved the efficiency to 15%.
On the other hand, Olga Malinkiewicz and Liu demonstrated the possibility of fabricating planar solar cells by co-evaporation and achieved similar efficiency. In 2014, Yang Yang from UCLA showed a reverse-scan efficiency of 19.3% using a planar thin-film architecture and later in November that year, researchers from KRICT, South Korea achieved a breakthrough record of 20.1% unstabilized efficiency.
In the following year, researchers at EPFL, Switzerland broke the record with an efficiency of 21.0% and in March 2016, researchers from KRICT and UNIST in South Korea achieved the highest recorded efficiency of 22.1% for a single-junction perovskite solar cell.
Silicon cells have dominated the solar panel manufacturing arena since the beginning. Considering its abundance, it is likely to be a popular choice for manufacturers. However, the daunting process and related expenses that one has to go through to make it usable on a solar cell may provoke them to try something that can produced and tested in a laboratory.
Silicon cells are rigid and made out of sliced silicon wafers exposed to high temperature, as opposed to perovskite cells that can be made out of commonly available industrial materials with far less energy expenditure.
Over a period of 60 years, silicon cells have gradually matured to produce an efficiency of 22.7%. It is probably as efficient as a silicon based solar cell can be, unless a breakthrough proves it otherwise.
Since manufacturers are already producing silicon-based solar panels on a commercial scale, it has become irrelevant for scientists to further experiment on. The hope of creating a solar cell that is efficient beyond a silicon cell can ever be is driving their impulse towards experimentation on perovskite cells.
A study by NREL compares the power conversion efficiencies of current photovoltaic technology (including traditional thin-film photovoltaics) with that of perovskites.
From the graph above, we can observe the impressive rise in efficiency of perovskite solar cells within 3 years. It is now comparable to cadmium telluride which took about 40 years to reach that far.
Here’s a comparison between open-circuit voltage of perovskites pitted against competing photovoltaic technologies:
The graph above shows the amount of loss of a photon’s energy in the process of converting light to electricity.
In case of conventional solar cells, the loss of absorbed energy can be as high as 50%. In case of perovskite-based solar cells, the loss is significantly lesser and closely similar to the amount of photon energy utilization of leading monolithic crystalline technologies, albeit at a much lower price.
When combined with a bottom cell such as silicon (Si) or copper indium gallium selenide (CIGS), a perovskite cell can suppress cell disruptions and complement each other to increase its efficiency.
In the current market scenario, solar power costs about 75 cents per watt. Since perovskite is much cheaper to produce than silicon, it has a potential of bringing down costs to 10 or 20 cents per watt in the near future.
We won’t require inflexible structures like conventional solar panels any more since they hardly play any role in energy conversion. Experiments have been carried out to apply techniques such as spraying, dipping and printing perovskites on solid surfaces.
Scientists from the University of Sheffield, England produced the first perovskite solar cells in 2014 using a spray-painting process.
We haven’t observed the production of perovskite cells on a commercial scale, though their popularity has risen constantly, putting it on a much better position than older photovoltaic technologies. There are still many unanswered questions on their durability and toxicity.
However, the renewable energy sector as well as researchers are on their toes, excited about how this technology will change the face of solar photovoltaic technology forever.