World population growth has caused an increase in worldwide energy demand which has in turn led to an enhancement of energy production, still largely dependent on fossil fuels. While coal, oil and natural gas are still able to fulfil worldwide energy requirements, nowadays the natural resources are facing a decrease in availability and the generated pollution is no more a negligible matter. Indeed, in the early 2000s, high fossil fuel prices along with environmental concerns resulted in growing interest towards nuclear power and renewable energy sources. In fact, those are the world’s fastest-growing energy sources, each increasing by 2.5% per year. However, fossil fuel is still expected to supply up to 80% of world energy consumption until 2040.
Meanwhile, the world’s real gross domestic product (GDP) is presumed to rise by an average of 3.6% per year from 2010 to 2040. The fastest rates of growth are projected for the emerging non-OECD (Organisation for Economic Co-operation and Development) regions, where combined GDP is expected to increase by 4.7% per year compared to the OECD regions, where GDP will grow at a much slower rate of 2.1% per year, due to more mature economies and declining population growth trends. The strong growth in non-OECD GDP drives the very quick growth in future energy demand projected for these nations.
The International Energy Outlook 2013 (IEO2013) foresaw that world energy consumption will grow by 56 percent between 2010 and 2040 (Figure 1a). Also, according to the same source, world net electricity generation will increase by 93%, from 20.2 trillion kWh in 2010 to 39.0 trillion kWh in 2040.
In this scenario, with its average radiative power of 3×1024 J per year sent on the Earth surface, the Sun is the most powerful renewable energy source that mankind could exploit3. Although the contribute from photovoltaics to worldwide energy supply is currently negligible, compared to other sources (Figure 2), his impact could drastically change the market and help mankind to fulfil energy requirements4. This important event could only be achieved if efficiencies and costs reach marketable values.
Since the first generation of solar cells, mainly produced with crystalline Silicon, has been industrially implemented, outstanding improvements have been accomplished. Silicon-based cells allow now for high efficiencies to be achieved, however their high manufacturing cost, rigidity and dependence on the amount of crystalline Silicon, made it a non-scalable technology and led the research to cover different paths.
The second generation of solar cells, also called thin film solar cells, was able to solve a certain amount of problems concerning the previous one. A much lower amount of material was required to produce the devices, leading to a consequent reduction of production costs, moreover they showed lower temperature of usage and higher flexibilities. Polycrystalline and amorphous Silicon, along with CIGS (Copper Indium Gallium Diselenide) and CdTe, have been implemented for this generation. Nonetheless a lower energy efficiency and stability are still major obstacles to overcome.
Later on, a third generation of solar cells allowed for even lower production costs, thinner films and the use of organic materials along with the implementation of many other possible architectures. Among this generation, Dye sensitized solar cells (DSSC), Quantum Dots solar cells, Organic Solar cells and Perovskite solar cells (PSC) are the most relevant. Major attention is currently gained by the latter.
Since the first time that perovskite structured material were incorporated in a photovoltaic device in 20095, the efficiencies have already seen a rapid increase touching surprising power conversion efficiencies higher than 20%6. The very steep improvement curve, represented by red and yellow dots in figure 1-5, compared to the other emerging technologies, justify the excitement surrounding this very efficient device structure.
However, this project is focused on a promising approach aiming the improvement of organic solar cells efficiencies. In the figure below the modern printed high performance organic photovoltaic modules installed over the German Pavilion at EXPO 2015 are shown7.
The reported organic solar cells Power Conversion Efficiencies (PCE) started from values barely higher than 1% and achieved, in about two decades, values higher than 10%.
The appeal of this technology reside in some key points addressing very specific problems of the inorganic counterpart. OPV can be a solution for continuous and low cost manufacture by means of roll-to-roll technology; low weight and flexible modules can be created and also easily integrated in other products, thus opening paths to new market opportunities, such as wearable PV; last but not least low environmental impact.
Substantial improvements were achieved not only in the efficiencies but also in durability of the cells. The latter being strongly influenced by the material composition and structure. Figure 1-5 shows the actual highest efficiencies of different kinds of solar cells. Over the course of the four decades, a great number of different architectures and materials have been studied and some of them have seen a huge efficiency increase.