PerspectiveSOLAR CELLS

Lead halides join the top optoelectronic league

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Science  25 Mar 2016:
Vol. 351, Issue 6280, pp. 1401
DOI: 10.1126/science.aaf4603

In any solar cell that begins to approach the theoretical limits of performance, an intense internal luminescence photon gas must be present (see the figure) (1). On page 1430 of this is sue, Pazos-Outón et al. (2) provide evidence for such an internal photon gas in lead halide photovoltaic cells. These materials thus have properties similar to those of GaAs and have the potential to be among the topperforming solar cell materials. This is scientifically remarkable, because these compounds are the first high-quality halide semiconductors. The materials show promise for photovoltaics, light-emitting diodes (LEDs), laser refrigeration, thermophotonics, and a host of other major optoelectronic applications.

Many decades ago, Tom Mc-Gill of the California Institute of Technology predicted that the more ionic a semiconductor, the more tolerant it would be of dangling bond defects. In ionic materials, these dangling bond energy levels fall near the band edges, leaving the center of the bandgap relatively free of defect states. The lead halides appear to be a prime example of this effect. There are indications from recent laser cooling experiments (3) that single lead halide crystals can have ∼99% internal luminescence yield, a prerequisite for the buildup of an intense internal photon gas. Laser cooling relies on superb efficiency for luminescent extraction and is another strong indication of the potential performance of the lead halides. Laser cooling, LEDs, and solar cells all rely on >95% external extraction of photons from this internal photon gas.

It has been known for over 50 years that the Shockley-Queisser formula (4) for the open-circuit voltage Voc needs to be corrected from the ideal value if the external luminescence is less than 100%. The open-circuit voltage is penalized by Embedded Image (1) where ηext is the aforementioned external luminescence efficiency (5). The photon gas and the external luminescence are thus essential for achieving high voltage from a solar cell. This has led to the mantra that “a great solar cell needs to be a great LED” (2).

Record breakers.

In the conventional picture (top), a photon in a solar cell produces an electron-hole pair that is collected without need for external luminescence. Recent studies have shown that good luminescence extraction, assisted by photon recycling, is required for the highest open-circuit voltages in solar cells such as those made from GaAs. In this picture (bottom), photons are reabsorbed and re-emitted many times before an electron-hole pair is collected or a luminescent red photon escapes. Pazos-Outón at al. now show that lead halide materials have luminescence properties similar to those of GaAs and may, thus, also reach maximum efficiencies.


The idea that increasing light emission improves open-circuit voltage seems paradoxical, as it is tempting to equate light emission with loss. However, basic thermodynamics dictates that materials that absorb sunlight must emit in proportion to their absorptivity. At open circuit, an ideal solar cell would radiate out one photon for every photon that it absorbs. The external luminescence efficiency is a gauge of whether further loss mechanisms in addition to this photon emission are present at open circuit. At the optimum power point, the voltage is reduced by a few kT/q from open circuit, and fully 98% of the open-circuit photons are drawn out of the cell as real current. Good external extraction at open circuit comes at no penalty in current at the optimal operating bias point.

Photon recycling—that is, the reabsorption and reemission of photons—provides numerous opportunities for the photons to find the escape cone and contribute to external luminescence, but it is not the only such mechanism. Many years ago, Lush and Lundstrom (6) identified another mechanism, solar cell surface roughness. Breaking plane parallel symmetry tends to trap the incoming sunlight, boosting incoming current, but the random internal scattering also allows numerous opportunities for luminescence to escape. Photon recycling is a good option for plane parallel solar cells, but multiple elastic scattering events can also produce the same external luminescence and therefore the same voltage boost.

The report by Pazos-Outón et al. shows clearly the presence of the internal photon gas and the photon recycling events that are one route to a high—output voltage solar cell (see the figure). The next step will be to show that the superb external luminescent emission is compatible with heterogeneous electrical contacts. Generally, in every step of solar cell fabrication, it is profitable and experimentally straightforward to monitor the external luminescence efficiency, which predicts whether it will be an average or record cell. It is with external luminescence efficiency monitoring that solar cell efficiency records are broken today.

References and Notes

Acknowledgments: Supported by U.S. DOE “Light-Material Interactions in Energy Conversion” EFRC under grant DE-AC02-05CH11231.

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