Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the world’s renewable energy mix. The cumulative world PV installations reached around 100 GWp (gigawatts) (1) by the end of 2012. Some 85% use crystalline Si, with the rest being polycrystalline thin film cells, mostly cadmium telluride/cadmium sulfide ones. Thin-film cells tend to be cheaper to make with a shorter energy payback time. However, they do have the disadvantage, one that may become crucial when considering the terawatt range, that most of them contain rare elements like tellurium (as rare as gold), indium, and gallium. A newcomer to the PV field (2) has rapidly reached conversion efficiencies of more than 15% (see the figure).
Based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3), these perovskites tend to have high charge-carrier mobilities (3, 4). High mobility is important because, together with high charge carrier lifetimes, it means that the light-generated electrons and holes can move large enough distances to be extracted as current, instead of losing their energy as heat within the cell. On pages 344 and 341 of this issue, Xing et al. (5) and Stranks et al. (6) use time-resolved transient absorption and photoluminescence to show that the effective diffusion lengths are indeed relatively large in CH3NH3PbI3, about 100 nm for both electrons and holes—a high value for a semiconductor formed from solution at low temperature.
Another important consideration for these perovskites is that they are deposited by low-temperature solution methods (typically spin-coating). The low energy and ease of deposition is of obvious importance for eventual manufacturing of the cells. It also greatly emphasizes the importance of the diffusion lengths described in these two papers for CH3NH3PbI3. For those working on more conventional semiconductor films, the reported diffusion lengths of 100 nm may not appear to be special. However, low-temperature (below 100°C) solution-processed films tend to have considerably smaller diffusion lengths. Stranks et al. had previously described nanostructured cells using CH3NH3Pb(I,Cl)3 (essentially the iodide with a small amount of chloride) (7) and demonstrated a thin-film solar cell (not nanostructured) with an 11.4% conversion efficiency (8) [and, more recently, 15.4% using vacuum evaporation of the perovskite, which results in more uniform films (9)]. Because the perovskite film thickness was about 500 to 600 nm, this implies that the electron and hole diffusion lengths were at least of this order. Stranks et al. measured values of the diffusion length exceeding 1 µm for the mixed perovskite, an order of magnitude greater than the already impressively high value of around 100 nm for the pure iodide. Stranks et al. also showed that the electron and hole lifetimes in the mixed perovskite is longer than in the pure iodide, which can explain the longer diffusion length in the former. But they note that the reason the apparently small amount of chlorine has such a pronounced effect is an open question.
Although the large diffusion lengths can explain the high quantum efficiencies and photocurrents obtained with these materials, there is another characteristic of these cells that is no less exciting—the very high values of open-circuit voltages (VOC) typically obtained. For CH3NH3PbI3, VOC typically approaches 1 V, while for CH3NH3Pb(I,Cl), VOC > 1.1 V has been reported (7). Because the band gaps (Eg) of both these semiconductors are 1.55 eV, this results in much higher VOC-to-Eg ratios than usually observed for similar third-generation cells (10). With higher band-gap perovskites, VOC up to 1.3 V has been demonstrated (11).
There are three main considerations that will affect the outlook for these cells. First is the energy conversion efficiency. But with an efficiency of 15.4% after only several years work and no reason to believe that this is close to the limit, that aspect is in good shape.
Second is cost, which is more complicated because it includes energy cost (and energy payback time), as well as availability of the raw materials. The low temperature solution methods used translate into overall lower energy requirements in the cell manufacture. There are no rare elements involved (the gold back contact can be replaced with a much cheaper contact material). The use of lead will worry some. While there are other possible replacements (such as tin), it should not be a major issue even if lead is needed in commercial cells. To put things in perspective, for a production capacity of 1000 GW per year, less than 10,000 tons of lead would be needed. Compare this with the 4 million tons per year of lead used for lead-acid batteries.
The third aspect is stability. There are a few studies on storage lifetime but only one to date on an operating cell (under illumination at maximum power) for a sealed cell at 45°C (12). The study showed a decrease in efficiency of less than 20% after 500 hours. This is actually very encouraging. We just have to look at the advances made in organic cell stability over the years to realize that there is cause for optimism that these cells can be made commercially viable.
There is an enormous effort now under way in these cells. It is by no means unrealistic to expect development and production of these cells, if initially on a small scale, in a relatively small number of years. The fact that these semiconductors can be made so simply and yet with such good crystalline and electronic properties (something that was recognized previously, but clearly not widely known in the photovoltaic community) means that we can expect an increased effort from the materials community dedicated to the investigation of these materials as well as finding new ones.
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