Cheap laser hope from light-emitting perovskite solar cell material

Commercial silicon-based solar cells operate at about 20 per cent efficiency for converting the Sun’s rays into electrical energy and it has taken over 20 years to achieve that rate of efficiency.

An Oxford research team led by Prof Henry Snaith recently pioneered a relatively new type of solar cell based on a perovskite material.

Perovskite solar cells already lie a fraction behind commercial silicon, having reached 17 per cent efficiency after two years of research, thereby transforming prospects for cheap large-area solar energy generation.

Now, researchers from Prof Sir Richard Friend’s group at Cambridge’s Cavendish Laboratory, working with Snaith’s Oxford group, have demonstrated that perovskite solar cells excel at emitting light.

The new findings, recently published online in the Journal of Physical Chemistry Letters, show that these cells can also produce cheap lasers.

By sandwiching a thin layer of the lead halide perovskite between two mirrors, the team is said to have produced an optically driven laser which proves these cells show efficient luminescence with up to 70 per cent of absorbed light re-emitted.

According to a statement, the researchers point to the fundamental relationship, first established by Shockley and Queisser in 1961, between the generation of electrical charges following light absorption and the process of recombination of these charges to emit light.

The lasing properties in these materials raise expectations for even higher solar cell efficiencies, said the Oxbridge team.

’This first demonstration of lasing in these cheap solution-processed semiconductors opens up a range of new applications,’ said lead author Dr Felix Deschler of the Cavendish Laboratory. ’Our findings demonstrate potential uses for this material in telecommunications and for light emitting devices.’

Most commercial solar cell materials need expensive processing to achieve a very low level of impurities before they show good luminescence and performance. According to Cambridge University, these new materials work well, even when very simply prepared as thin films using cheap scalable solution processing.

The researchers found that upon light absorption in the perovskite two charges (electron and hole) are formed very quickly - within one picosecond - but then take anywhere up to a few microseconds to recombine. This is long enough for chemical defects to have ceased the light emission in most other semiconductors, such as silicon or gallium arsenide.

‘These long carrier lifetimes together with exceptionally high luminescence are unprecedented in such simply prepared inorganic semiconductors,’ said Dr Sam Stranks, co-author from the Oxford University team.

‘We were surprised to find such high luminescence efficiency in such easily prepared materials. This has great implications for improvements in solar cell efficiency,’ said Michael Price, co-author from the group in Cambridge.

Snaith said: ‘This luminescent behaviour is an excellent test for solar cell performance – poorer luminescence (as in amorphous silicon solar cells) reduces both the quantum efficiency (current collected) and also the cell voltage.’

Solar cells are being scaled up for commercial deployment by the Oxford spin-out, Oxford PV. The efficient luminescence itself may lead to other applications with broader commercial prospects and a challenge identified by the Oxford and Cambridge teams is to construct an electrically driven laser.