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Photovoltaics Beyond the Shockley-Queisser Limit

We are developing strategies to dramatically increase the efficiency and lower the cost of solar energy.


The limits of current photovoltaics: All single junction PV cells developed to date share a fundamental energy loss mechanism, thermalization. This refers to the fact that the absorption of a high-energy photon generates one electron-hole pair just as the absorption of a low- energy photon does. The extra energy of photons above the bandgap is lost via thermalization. This is the dominant loss in solar cells as shown in the Figure below, leading to the so-called Shockley– Queisser limit on efficiency, which is 29% for an idea silicon cell (bandgap 1.1eV). In fact, the best Si solar cells achieve efficiencies around 26%, a number that has not improved by more than 1% in the last twenty years. To improve single junction devices, we need to develop strategies that can overcome thermalisation. This calls for new physics! 


Figure 1: Losses in an ideal silicon cell leading to the Shockley–Queisser limit. Figure credit NREL


Overcoming thermalisation losses with singlet fission (SF): SF offers the possibility of overcoming losses due to thermalisation, as every photon absorbed above the bandgap (Eg) leads to the formation of two electron-hole pairs. If a SF material were combined with a material of half the bandgap, and the energy of the triplet excitons formed via fission could be utilised it would allow for improved harvesting of solar energy. In the ideal case this could lift the maximum efficiency of a silicon solar cell from 29% to 38%.


Coupling triplets to Si PVs: We have recently made a breakthrough that opens a new route to combining SF materials with conventional inorganic solar cells. We demonstrated that efficient resonant energy transfer of triplet excitons from organic semiconductors to inorganic semiconductors is possible. Triplet excitons generated via SF in pentacene were transferred to PbSe nanocrystals with 95% efficiency9. The efficiency of this process comes as a surprise, as triplet excitons cannot be transferred via the usual coulomb-mediated Förster transfer route since they are not optically coupled to the ground state. Instead triplet transfer is mediated via the exchange interaction, which requires direct wavefunction overlap. That such overlap is possible even in the presence of nanocrystal ligands is unexpected.


A “photon multiplier” based on singlet fission: Our demonstration of efficient triplet transfer across the organic/inorganic interface opens the door to an all-optical method of utilising the triplet excitons generated via fission, effectively converting a carrier multiplication process into a photon multiplication process. The basic scheme of such a “Photon-Multiplier” is shown in Figure 3 and consists of a thin film of a SF material of appropriate band gap and triplet energy doped with a small amount of inorganic nanocrystals. The device function as follows: light absorption creates singlet excitons (1) that rapidly undergo SF to form two triplet excitons (2). The triplets then diffuse and undergo efficient triplet transfer into the nanocrystals (3). The electron-hole pairs in the nanocrystals can then recombine radiatively, emitting two low-energy photons for each high-energy photon absorbed (4). The emitted photons are then absorbed in a conventional solar cell onto which the photon multiplier is coated or laminated.




 Rao et al, Nature Reviews Materials (2017), Read more

 Tabachnyk et al. Nature Materials (2014), Read More


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