The replacement of finite fossil fuels with renewable sources of energy is one of the largest challenges facing the next century of industrialized human activity and is a necessary transition to ensure global welfare. Not only does the transition to carbon free sources of energy have major implications in the impediment of climate change and keeping our planet habitable, but is critical to continue supporting the systems for generating clean water, maintaining clean cooking facilities, making the fertilizers that grow food, powering the hospitals that provide healthcare, reducing air pollution, and powering the schools and equipment that form the basis of our education system. Although of massive importance now, this will become even more important as the world population increases and the 1.2 billion people worldwide that currently do not have access to electricity will seek the most inexpensive routes to become electrified.1,2
To address this global energy challenge, we need to develop a vast infrastructure of renewable energy production, storage, and distribution. Although there are several promising avenues in shifting our society toward renewable sources, prerequisites in the replacement of existing technologies are scalable to terawatts, carbon-free, highly efficient, and low-cost. These types of technologies paired with continued research and investment, resilient energy policies, and effective grid integration are all critical factors that will play into the eventual success and continued prosperity of humankind.
Photovoltaic modules, which convert sunlight directly into electricity, provide a rather elegant solution to contribute to our growing energy demands as they have proven to exhibit long operational lifetimes due to their low maintenance requirements from no moving parts. Apart from this distinct advantage, the solar resource is a widely under-utilized source of energy. For example, by performing a simple calculation taking into account the power density at the surface of the sun (treated as a black body source), the angular range of the sun upon the earth, the attenuation of the radiation by the atmosphere, one can calculate the average integrated irradiance upon the earth’s surface. This is often defined as airmass 1.5 (AM1.5), which equates to an integrated irradiance of 1000 W/m2. To put this value in perspective, the average power consumed globally is 18.3 TW-year per annum, the total amount of power that hits emerged continents on earth is 23,000 TW-year per annum, whereas the total amount of known coal reserves on planet earth is estimated to be only 830 TWy.3 Using these values, it can be approximated that 13 days worth of energy from the sun is equivalent to the total energy reserves of coal that is currently known to exist.
Our ability to utilize this vast resource has been underwhelming in the last several decades. Currently solar energy contributes only 272 GW,4 a small fraction to global power production and consumption (18.3 TW). The hurdles in achieving TW production from solar lies largely in scalability and reducing the dependency on incentive programs and other project finance structures. For example, the success of photovoltaic installation in Germany and Japan is largely attributed to feed-in tariffs and associated policies,4 although more recently both countries have encountered financial as well as grid constraints which has slowed the continued expansion.
In order to achieve TW scale photovoltaics some of the future directions include 1) improving the performance of modules and 2) reducing the cost and time required to manufacture and install photovoltaics. Currently, silicon dominates ~93% of total PV production, largely due to the rapid drop in module prices over the past few years, and will continue to significantly contribute to energy production in the coming decades. One major drawback for silicon based photovoltaics that will eventually limit its growth and deployment is the high capital expenditure (capex). This term broadly encompasses the upfront cost to build a factory and to fill it with equipment.5 The capex for c-Si has been approximated to be $1/W-year, which equates to ~$1 billion to build a plant capable of producing 1 GW/year. On the other hand, a solution-processed material has potential to reduce capex to $0.06/W-year, which equates to ~$60 million to build a plant that can generate the same 1 GW/year.5 Therefore, capex innovation may be one of the most promising avenues, apart from increasing module efficiency, to achieve TW-scale photovoltaics.
From above, it appears basic scientific research will be critical in developing a material that enables both low-cost and high efficiency photovoltaic modules which may cleanly power the generations to come.