In retrospect

Twenty-five years of low-cost solar cells

In 1991, an energy-efficient solar cell was reported that was both simple in design and relatively inexpensive. This invention has since inspired the development of solar cells that have even higher efficiencies.

About 85% of the world's energy requirements are currently satisfied by exhaustible fossil fuels that have detrimental consequences on human health and the environment1. Moreover, the global energy demand is predicted to double by 2050 (ref. 2). International action to achieve efficient and sustainable energy is therefore imperative (see www.se4all.org). Twenty-five years ago, O'Regan and Grätzel3 reported in Nature the landmark construction of a low-cost solar cell that could convert about 7% of the energy received from sunlight into electricity. In the past seven years, their work has inspired the production of solar cells that use compounds called perovskites4,5 and that can have conversion efficiencies of greater than 22% (see go.nature.com/2e3rq0e).

The basic concepts for O'Regan and Grätzel's technology were borrowed from photosynthesis, a process in which sunlight is absorbed by chlorophyll molecules and converted into chemical energy. In the authors' dye-sensitized solar cell (DSC), light is absorbed by ruthenium-based dye molecules that are deposited on the surface of titanium dioxide (TiO2) nanoparticles (Fig. 1a). At the interface between the dye and the nanoparticles, an excited electron and an associated hole (a conceptual particle formed by the absence of an electron) are produced. The electron is conducted by the TiO2 nanoparticles to an electrode (anode) and then transferred to a counter electrode (cathode). Finally, a liquid electrolyte — a mixture of a liquid solvent and ions — closes the circuit so that the electron recombines with the hole and is returned to the dye. Electrical energy is generated as the electron moves through the DSC.

Figure 1: The evolution of solar cells.
figure1

In 1991, O'Regan and Grätzel3 reported a low-cost 'dye-sensitized' solar cell (DSC), which has since led to the development of more-energy-efficient variations such as the perovskite solar cell4,5 (PSC). a, A conventional DSC consists of a film of titanium dioxide (TiO2) nanoparticles, on which dye molecules are deposited. When sunlight hits the dye, a negatively charged electron (e) and a positively charged hole (h+) are produced. The electron diffuses through the TiO2 film to an electrode and is subsequently transferred to a counter electrode. Finally, the electron enters a liquid electrolyte before recombining with the hole and being reabsorbed by the dye. b, In a modern solid-state PSC, the liquid electrolyte is replaced with a hole-transporting material, and chemical compounds called perovskites act as the light harvesters. In PSCs, gold is commonly used as the counter-electrode material to further enhance the efficiency.

The novelty of the DSC compared with previous solar cells was the extremely large surface area that was provided for the dye molecules by the TiO2 nanoparticles. The authors used a 10-micrometre-thick film of these nanoparticles, which had average diameters of about 15 nm. Because of the porous structure of the film, its surface area was 780 times larger than its geometric area, analogous to the stacks of thylakoid membranes in chloroplasts in which the electron-transporting reactions of photosynthesis take place.

After O'Regan and Grätzel's results were published, initial improvements in the performance of DSCs were made by the use of mononuclear rather than trinuclear ruthenium-based dye molecules6, which increased the conversion efficiency from about 7% to more than 11%. A molecularly engineered 'donor–chromophore–acceptor' dye7, which has a similar structure to that of chlorophyll, was shown to increase the efficiency further, to 13%.

The next step was to replace the liquid electrolyte with a solid hole-transporting material, to create a solid-state DSC8. This increased the stability of these solar cells and avoided problems associated with liquid leakage. However, the efficiencies of solid-state DSCs are about half those of their liquid counterparts9 because the hole-transporting materials do not permeate the TiO2 film as uniformly as liquid electrolytes do.

In the past seven years, the architecture of solid-state DSCs has been adapted for solar cells that, instead of dye, use perovskites — compounds that have the general formula ABX3, where A and B are two different positively charged ions and X is a negatively charged ion. Perovskite solar cells (PSCs; Fig. 1b) have created a tsunami effect among the photovoltaic community because of the excellent light-absorption properties of perovskites. The seminal work of the chemist Tsutomu Miyasaka and colleagues initiated this boom by producing PSCs that used liquid electrolytes4, and this has been followed by the transition to solid-state PSCs5. Several groups have demonstrated that the methylammonium lead triiodide (CH3NH3PbI3) perovskite can be used not only as the light-absorbing material, but also as the charge-transporting material10.

There are several PSC architectures in use today, but the highest reported conversion efficiency (greater than 22%) is based on a structure in which the perovskite is a light-absorbing semiconductor, TiO2 acts as the electron acceptor, and a poly(triarylamine) polymer is the hole-transporting material (see go.nature.com/2e3rq0e). PSCs have the potential to reach efficiencies of more than 25% and have been recognized by the World Economic Forum as one of the top ten emerging technologies of 2016 because of their potential to replace fossil fuels (see go.nature.com/2dpv26d). Nevertheless, PSCs have many drawbacks, such as poor material stability under excessive heat and light exposure, and toxicity because of the presence of lead.

Further advances in PSCs, through a combination of innovative steps in materials science, chemistry and device technology, could lead to a revolution in the renewable-energy sector. Solar cells have come a long way since O'Regan and Grätzel's landmark paper, and the future looks bright for PSCs as a potential means of obtaining truly renewable energy.Footnote 1

Notes

  1. 1.

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Correspondence to Mohammad K. Nazeeruddin.

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Nazeeruddin, M. Twenty-five years of low-cost solar cells. Nature 538, 463–464 (2016). https://doi.org/10.1038/538463a

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