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Materials for energy

Grain boundaries give inorganic perovskites huge potential

20 Apr 2018 Robert Westbrook 
Topographical and photocurrent mapping of the samples
Topographical and photocurrent mapping of the samples. Courtesy of Nature Photonics

For nearly half a century, researchers have struggled to harness the curious property of above band-gap photovoltage in metal-oxide perovskites because of their poor bulk conductivity. But now, an international team led by researchers at INRS and ÉTS in Canada have paved the way for progress by identifying and manipulating nanoscopic conductive channels that lie between the grains of a BiMnO3/BiMn2O5 composite absorber. They say that the high voltage and long lifetime of this set of materials provides a serious competitor to the much-hyped lead-halide perovskites.

Charge carrier conduction plays a vital role in solar energy conversion, as a high photocurrent relies on charges being able to separate quickly before they recombine. Therefore, despite the possibility of giant voltages in metal oxide perovskites, their low conductivity has rendered them more of a scientific curiosity than a viable candidate for commercial solar cells.

It is for this reason that Joyprokash Chakrabartty and co-workers took to their conducting atomic force microscope (C-AFM) when trying to understand the remarkable efficiency of 4.2% obtained from their BiMnO3/BiMn2O5 composite. They found that although most of the material developed negligible photocurrent, there was a huge contribution coming from conducting channels along the boundaries between grains.

In these areas, the electronic bands that carry the charges are bent downwards, and the resulting local barrier potential enhances the conductivity. This finding flies in the face of much of the contemporary understanding of solar cells, in which grain boundaries are generally considered as a nuisance on account of their numerous defects that induce charge recombination.

Added benefits of knowing your boundaries

The band-bending effect is also responsible for the large voltage of 1.48V exhibited by the device. BiMnO3 and BiMn2O5 both have a band gap of around 1.25eV, and so the standard p-i-n model would limit the maximum voltage of this device to 1.25V. However, when charges accumulate at the grain boundaries, the warped electronic bands can work to enhance the voltage beyond the band gap of the composite’s constituents.

The researchers were also able to manipulate these conducting channels to improve device performance in a completely novel way. By applying voltage pulses to the device, they could switch the polarization of the ferroelectric BiMnO3, thus injecting more carriers into these channels and inducing a 20-fold increase in the photocurrent.

“Our findings are highly promising for the development of future solar technologies,” says Chakrabartty, “and also potentially useful in other optoelectronic devices.” Indeed, the insight gained in this work could break the deadlock in this field and allow for a host of next-generation inorganic perovskite devices to reach their potential.

Full details of the work can be found in Nature Photonics.

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