Researchers at Stanford University and the Department of Energy's SLAC National Accelerator Laboratory have gained new understanding of the happenings inside a hybrid perovskite material in the first few trillionths of a second after it's hit with simulated sunlight.
The blue and green spheres are atoms. When light hits, electrons start to separate from positively charged 'holes,' the first step in creating an electrical current (yellow streaks). Meanwhile, atoms begin to vibrate within the perovskite's structure.
The research, conducted at the atomic scale, could help explain how electric charges move efficiently through hybrid perovskites following the absorption of light, the crucial first step in generating an electric current. The study used laser pulses that match the intensity of solar radiation, and thus mimic natural sunlight. The authors say their discovery could lead to improvements in the performance of perovskite solar cells and a new way to probe their functionality.
It is easy to understand how silicon, with its highly ordered atomic structure, provides a direct path for electrons and holes to travel through the solar cell and provide the efficiency that it does in common commercial solar panels. Perovskites, however, usually don't enjoy the same level of structural order. They are, in fact, commonly filled with defects and grain boundaries, so the question of how they are able to provide such impressive efficiency remains puzzling for scientists.
In the study, the research team used laser pulses to simulate waves of sunlight from both ends of the visible light spectrum ' high-energy violet light and low-energy infrared light. The results were measured at the picosecond timescale. One picosecond is one trillionth of a second.
'In the first picoseconds after sunlight hits the perovskite, the electrons and holes in the crystalline lattice start to split,' the team explained. 'The separation was uncovered by measuring the emission of high-frequency terahertz light pulses oscillating a trillion times per second from the perovskite thin film. This is the first time anyone has observed terahertz emission from hybrid perovskites.'
The terahertz emission also revealed that electrons and holes closely interact with lattice vibrations in the crystalline material. This interaction, which occurs on a femtosecond timescale, could help explain how electric currents navigate through the patchwork of crystal grains in hybrid perovskites.
"As the electric charges separate, we observe a sharp spike in the terahertz emission, matching a vibrational mode of the material,' the researchers said. 'That gives us clear evidence that the electrons and holes are strongly coupling with the atomic vibrations in the material.'
This finding raises the possibility that coupling to the lattice vibration could protect the electrons and holes from charged defects in the perovskite, shielding the electric current as it travels through the solar cell. Similar scenarios have been proposed by other research teams.
'Our technique could open up a new way of probing a solar cell right when the photon is absorbed, which is really important if you want to understand and build better materials. The conventional way is to put electrodes on the device and measure the current, but that essentially blurs out all of the microscopic processes that are key. Our all-optical, electrode-less approach with femtosecond time resolution avoids that problem.'
The researchers also found that terahertz light fields are much stronger when perovskite is hit with high-energy light waves. 'We found that radiated terahertz light is orders of magnitudes more intense when you excite the electrons with violet light versus low-energy infrared light,' the team said. 'That was an unexpected result'. This discovery could provide new insights on high-energy 'hot' electrons, according to the team.
'Violet light imparts electrons with excess kinetic energy, creating hot electrons that move much faster than other electrons'. 'However, these hot electrons lose their excess energy very rapidly'. Harnessing the energy from hot electrons could lead to a new generation of high-efficiency solar cells, added the team.
'One of the grand challenges is finding a way to capture the excess energy from a hot electron before it relaxes'. 'The idea is that if you could extract the current associated with hot electrons before the energy dissipates, you could increase the efficiency of the solar cell. People have argued that it's possible to create hot electrons in perovskites that live much longer than they do in silicon. That's part of the excitement around perovskites.'
The study revealed that in hybrid perovskites, hot electrons separate from holes faster and more efficiently than electrons excited by infrared light.
The ability to measure terahertz emissions could also lead to new research on non-toxic alternatives to conventional lead-based perovskites. The researchers explained that most of the alternative materials being considered are not as efficient at generating electricity as lead. 'Our findings might allow us to understand why lead composition works so well while other materials don't, and to investigate the degradation of these devices by looking directly at the atomic structure and how it changes.'
