When a semiconductor absorbs light, a particle-like entity called an exciton can be produced. Excitons comprise an electron and a hole (the absence of an electron), and have two possible states: singlet and triplet. Triplet states were thought to be poor emitters of light, but, on page 189, Becker et al.1 report that semiconductors known as lead halide perovskites have bright triplet excitons. The results could signify a breakthrough in optoelectronics because triplet states are three times more abundant than singlet states2 and currently limit the efficiency of organic light-emitting diodes3.
Conventional wisdom holds that triplet states are dark because of the spin selection rule4, which forbids electrons from changing their intrinsic angular momentum (spin) during an optical transition — the process in which an atom or molecule switches from one energy state to another by emitting or absorbing light. The rule is taught in quantum-mechanics classes when atomic transitions are first introduced, and is so general that one might think that it is written in stone. Fortunately, there are loopholes that can be exploited.
The search for emissive triplet states has focused on a certain principle of quantum mechanics: if an electron’s spin is coupled to another form of angular momentum (namely, orbital momentum), the sum of the two momenta needs to be conserved, rather than the spin alone. The effect is known as spin–orbit coupling in atomic physics and as intersystem crossing in the study of organic semiconductors. It is responsible for weak emission from triplet states in atoms and organic molecules, especially when heavy elements are involved. However, until now, the strength of triplet emission was thought always to be inferior to that of singlet emission.
Lead halide perovskites seem to dispose of all conventional wisdom in materials science. Like organic semiconductors, they are relatively easy to fabricate, and their bandgap (a property that determines their conductivity and optical properties) can be tuned by varying their composition. Yet, like thin-layer (epitaxial) inorganic semiconductors, they are highly crystalline and exhibit efficient charge transport. It is as if their properties were selected from a materials scientist’s wish list, combining the best aspects of organic molecules, nanocrystals and epitaxial inorganic semiconductors.
Becker and colleagues’ study suggests that there is another feature of lead halide perovskites to be added to this list. The authors used a combination of theoretical and experimental work to show that nanocrystals of caesium lead halide perovskites (CsPbX3, where X is chlorine, bromine or iodine) have bright triplet excitons (Fig. 1). This property results in an emission rate surpassing that of other known nanocrystals5.
The energy difference between the triplet and singlet states in CsPbX3 nanocrystals is relatively small (of the order of 1 millielectronvolt). Becker et al. therefore explored the material’s emission at cryogenic temperatures (a few kelvin), to prevent transitions between triplet and singlet states. It is unclear to what extent bright triplet states affect the material’s emission efficiency at room temperature — when thermal energy greatly exceeds the singlet–triplet splitting energy and all states are equally populated. Nevertheless, the authors’ findings are of fundamental relevance.
Future work will certainly investigate whether bright triplet states exist in other types of perovskite, such as hybrid perovskites that have organic, positively charged ions (cations). Such materials include the archetypal methylammonium lead iodide (CH3NH3PbI3), and are typically prepared not as nanocrystals, but as solid-state films6. Unlike CsPbX3 nanocrystals, these films comprise micrometre- or millimetre-sized crystalline domains, in which excitons dissociate into pairs of free electrons and holes at room temperature. More generally, Becker and colleagues’ theoretical analysis might help scientists to identify other semiconducting materials (either organic or inorganic) that have bright triplet excitons.
Research into hybrid perovskites has been fuelled in the past few years by the successful incorporation of these materials into solar cells. Such devices can now convert more than 22% of the energy received from sunlight into electricity7, which is a record for perovskite solar cells. However, because of a concept known as quantum-mechanical reciprocity, there is an unavoidable energy loss in solar cells: that due to photoluminescence, which is the reverse of the absorption process8. As a consequence, the best solar cells are also the best light emitters — an idea reinforced by Becker and colleagues’ work.
Perovskite solar cells are now leaving academic labs and entering the market, thanks to substantial industrial efforts. The competition is mainly silicon solar cells, which have become so cheap that they negate some of the advantages of perovskite fabrication. For this reason, tandem solar cells (consisting of two sub-cells) and innovative architectures involving perovskites are being developed that can outperform commercial silicon devices in terms of efficiency, if not cost9.
Light emission is an application in which organic semiconductors and nanocrystals have already found commercial success, because of their ability to produce vivid colours and to be incorporated into thin panels. And yet the electric-current densities in organic light-emitting diodes are much lower than in their inorganic counterparts as a result of poor electrical conductivity. Perovskites could allow high current densities and efficiencies to be realized on large-area, thin panels10.
Becker and colleagues’ study highlights the potential of perovskite materials as efficient light emitters. Although the findings might seem surprising at first sight, they should be seen as a natural consequence of quantum-mechanical reciprocity — that the class of material brought to the forefront by solar-cell technology could find applications in light emission.
Nature 553, 163-164 (2018)