People have exploited the sun's immense power for millennia. The total amount of solar power received on the ground at any time is approx. 10 17 watts, while global demand for electricity is approx. 12 W (see go.nature.com/2leb8sk). Although solar cells may appear ubiquitous, only 1.3% of the electricity comes from the United States to these devices (see go.nature.com/2leb8sk). The solar cell industry therefore still has a huge opportunity for growth, to provide clean and renewable energy to the world's population. Economic factors are driving decision making in the energy sector, and for larger solar cells to be adopted, scientists are continually driven to make these devices more durable and more efficient in converting sunlight into electricity. In a paper of Nature Einzinger et al. 1
Improving the efficiency of highly optimized solar cells without introducing significant complexity or cost is difficult to achieve, but is a potentially transformative target. Silicon solar cells currently dominate the market, but have well-documented efficiency restrictions. The most sharp deficiency described in 1961 is that high energy photons produce unwanted heat 2 . Conventional silicon solar cells are therefore limited to converting only 30% of the sold solar energy into electricity in an otherwise perfect unit 2 . Strategies to overcome this barrier include stacking different solar cells in series, or finding ways to split the energy of photons before assembly. Einzinger and colleagues demonstrate a method in which a molecular layer splits a high energy excitation generated by the absorption of a high energy photon into two lower energy excitations (Figure 1). These excitations are then transmitted across a carefully designed interface to a silicon solar cell where they are converted into an electrical current. This strategy involves no additional electrical contacts or modification of the solar cell's function. It has long been considered a potentially simple but effective way to avoid losing energy as heat and to design silicon solar cells that could have an efficiency of up to 1.4 times as high tech devices 3 . 4 .
The process by which a few low-energy excitations are formed by an absorbed photon is called singlet fission. This mechanism has been revived by interest to a great extent because of its potential to increase solar harvest 5 – 7 . The archetypal material for activating singlet fission is called crystalline tetracene. This material divides (high energy) blue or green light into two excitations that have energy equivalent to the lowest energetic excitation of silicon. Such energy matching makes tetracene and silicon ideal potential partners for the transfer of singlet fission products. The combination of tetracene and a silicon solar cell was previously used to search for evidence of efficiency gains caused by singlet fission, but the decisive step for transmitting low energy excitations in the solar cell could not be verified 8 9 . In singlet fission, the high-energy excitation is a singlet exciton, a bounded state of an electron, and an electron solution (a hole) having zero magnetic moment (i.e., zero spin). The low-energy excitons are triplet excitons – electron-hole pairs that have a spin of 1. In contrast to their singlet counterparts, triplet excitons cannot return to the ground state by emitting radiation due to quantum-mechanical spin-selection rules. This limitation prolongs their lifetime, but inhibits a common energy transfer mechanism that would otherwise allow long-distance transfer of the excitations through, for example, a thick passivation layer (an inert material which protects the surface of a solar cell from contamination).  In 1979, physicist David Dexter recognized the potential use of a tetracene layer on top of a silicon solar cell 3 . He predicted that two triplet excitons born from the singlet fission could be efficiently transferred to the solar cell by a separate mechanism common to these excitations, thereby doubling the electric current produced by the unit into blue and green light absorbed by the sun. However, this mechanism has a short range and can only be done efficiently through ultra-thin barriers. Such barriers are often insufficient to passivate the surfaces of semiconductors, such as silicon, resulting in power losses that could overcome the beneficial effects of singlet fission. Although the optical properties of tetracene are ideal for this energy transfer process, its poor electrical properties complicate its direct integration into high efficiency solar cells.
The remarkable demonstration presented by Einzinger et al. is based on the discovery that an unusually thin passivation layer can be made from hafnium oxynitride. This material can be deposited in such a way that its thickness is controlled with high precision and can protect the silicon surface with a thickness of only 0.8 nanometers. Conventional solar cells use opposing electrical contacts on the top and bottom of a light absorbing semiconductor to extract electrons and holes, giving rise to electrical current. In contrast, some highly efficient solar cells are designed with both types of contact separate, but on the back of the unit. Such a back-contact cell design combined with the passivation layer is the key to the author's singlet fission solar cell.
Einzinger and colleagues show that when the tetracene layer on top of their solar cell is thrilled with blue or green light, triplet excitons are formed (Figure 1). These excitons are transmitted via the hafnium oxynitride to the silicon solar cell without the need for additional electrical contacts. The exciton yield, which is defined as the average number of excitons transmitted to the solar cell per day. Photos, is approx. 1.3. In addition to the yield of more than 1, further evidence of triplet exciton transfer is in magnetic field effects having a distinctive signature for triplet excitons born from singlet fission.
Following excitation of tetracene, collisions between electrons and holes in silicon layers lead to light emission. Modeling of this light shows that 76% of triplet excitons are transmitted through the passivation layer compared to 56% of singlet excitons. The singlet excitons are not beneficial because they represent only one electron hole pair per unit. Foton. Therefore, reducing their impact on the solar cell by increasing the number of singlet fission in the tetracene and thus the proportion of triplets at the interface is an important goal for the near future.
At this stage, Einzinger and colleagues have & # 39; the solar cell is relatively inefficient and the design is ripe for optimization. The hafnium oxynitride passivates the silicon surface, but the first few injected electrons and holes have been found to initially fill the gaps at the silicon surface before moving into the solar cell. This finding shows that the interface is still imperfect, yet shows the potential of this strategy in a functioning solar cell. Moreover, the mechanism of triplet exciton transfer and how it can be accelerated is currently not well described by theory. Despite the need for significant improvement, the field is now heading for effective triplet exciton transfer to silicon, which one day could make Dexter's dream come true.