We begin our journey at the advent of the Stone Age. Human-like species, or hominids, aren’t able to run as fast as their prey, nor do they stand an earthly chance of surviving brawls with other predators. They can, however, yield a much more powerful weapon – their intelligence. They learn to utilise stone tools in order to gain a technological advantage, and they learn to yield stone weapons in order to exercise their superiority over other species. Here commences the birth of civilisation.
Fast-forward three and a half million years and we arrive at the Bronze Age. The Bronze Age is considered to have begun during the fourth millennium BC with the onset of the production of bronze – an alloy consisting of a copper majority with a supplement of tin. By combining the attributes of two metals, the homo sapiens concocts a metal with strength and durability unmatched by any other material at the time. Stone certainly cannot contend.
Fast-forward another mere three millennia and we arrive at the Iron Age. The Iron Age is considered to have begun during the early first millennium BC and to have ended by the Middle Ages. Iron, although simpler in structure than bronze, is a more difficult metal to extract from its ore. Nonetheless, the reward is immense. The discovery of steel, one of the strongest common materials on the planet, provides human civilisation with another rung in technological advancement.
This is the three-age system. Some say that we currently live in a fourth age – the Silicon Age. Nowadays we can scarcely step in any direction without being in close proximity to a silicon transistor – an essential component of every electronic circuit and which exists in masses of a few billions at a time in a smartphone.
Silicon extends its range of applications to photovoltaic technology. Silicon is by far the most prevalent material in solar cells, the building blocks of solar panels. However, a newer material, perovskite, has captivated the interest of many researchers in the field of semiconductor electronics. Its promisingly high efficiency could put an end to the days of silicon solar cells. For solar cells, could this be the start of the Perovskite Age?
Perovskite was originally discovered in the Ural Mountains of Russia in 1839 by German mineralogist Gustav Rose, and which he named after the Russian mineralogist Lev Perovski. Perovskite was found to be a mineral composed of calcium titanate, CaTiO3. Nowadays perovskite also lends its name to a class of materials which have the same crystal structure as calcium titanate and have the general chemical formula ABX3 – sometimes referred to as the perovskite structure to avoid confusion.
The limelight is on a particular type of perovskites called organometal halide perovskites. Their structure is broadcasted by the name: A is an organic (carbon-containing) cation (positively charged ion); B is a metal cation;
X is a halogen (a group of chemical elements including fluorine and chlorine) anion (negatively charged ion). Even within this one type of perovskites, it’s clear that there are a countless number of possible combinations. One example of particular interest due to its high efficiency is methylammonium lead iodide (CH3NH3PbI3). We can match up the groups to confirm that this is a perovskite: A is the methylammonium cation ([CH3NH3]+); B is the lead cation (Pb2+); X is the iodide anion (I3–).
Where do I even begin with the benefits of perovskite solar cells? They have an excellent power-to-weight ratio, making them attractive to the aerospace and space industries. They can be made incredibly thin – perovskite films can achieve a reasonable efficiency at only 300 nanometres thick (200 times thinner than the diameter of a typical human hair) compared to approximately 1000 nanometres in gallium arsenide and approximately 2000 nanometres in silicon. They not only have a low manufacturing cost, but also a favourable levelised cost of electricity (the total costs over its lifetime divided by the total electricity generated over its lifetime), which demonstrates their commercial potential.
There are sadly a few considerations which mean that perovskite solar cells are still a thing of the future and a not a thing of the present. High-efficiency perovskites, such as the methylammonium lead iodide we identified earlier, tend to have a high lead content, and if there’s anything we know about lead is that it’s toxic (hence the development of unleaded petrol). We can of course choose perovskites which don’t contain lead, but sacrifices in efficiency would have to be made.
Perovskites are also relatively unstable and can degrade not only in water but in plain air. This is a major limiting factor in the use of perovskite solar cells, since what use are they if you can’t place them outside? Luckily considerable progress has been made in extending the lifetimes of perovskites – in 2009, when perovskites were first discovered to have photovoltaic properties, lifetimes of only a mere few minutes were recorded. In 2016, a lifetime in the vicinity of a thousand hours was achieved, and in 2017, ten thousand hours. Although these are ballpark figures, it’s clear that considerable improvements are being made in perovskite technology.
Fortunately for silicon, it’s likely we won’t be entirely eradicating silicon solar cells in the next few decades or so; rather, we’ll incorporate the strengths of both silicon and perovskites to construct a superior solar cell, much like the ancient humans did five thousand years ago with copper and tin to form bronze. Perovskite films are anticipated to be able to be attached onto present silicon solar cells without difficulty in order to achieve a higher efficiency. Although this doesn’t seem like much in the grand scheme of things, it’s certainly a step in the right direction towards a greener world.