Band-gap narrowing together with carrier-lifetime prolongation in organic-inorganic trihalide perovskite – Drs Lingping Kong and Gang Liu

JULY 25, 2016

A team of HPSTAR scientists led by Dr. Gang Liu utilized high-pressure technique to tune the electrical and photovoltaic performance in organic-inorganic hybride perovskites. 70%-100% carrier-lifetime increasing was found in mildly compressed organic-inorganic trihalide perovskite together with bad-gap narrowing. The story is published on the July 21th edition of PNAS

Hybrid perovskites are a family with generic perovskite chemical formula (ABO3) materials that hold great promise in the clean energy world. It has taken the solar cell field by it’s first discovered since 2009.

Hybrid perovskites was believed to be the next generation of solar cells. The solar cells convert solar energy into electrical energy so it's a sustainable and environmentally friendly energy source, giving high performance at a low cost. In addition to solar cell technologies, the hybrid perovskites have potential to be used in a variety of optoelectronic applications. Its power-conversion efficiency has been increased to 22.1% at present.

High absorption for solar energy and photovoltage for longing the carrier-lifetime in hybride perovskites are key factors for their actual usage.

Various methods were tried by scientist of improve the two aspects of performances. However, it’s hard to gain the two ends. Increasing in absorption always comes at the cost of shorter carrier lifetime.

“Pressure was proved to be a powerful and clean tool for tuning the crystal and electronic properties of energy materials (eg. 2D materials, metallic glass)”, said Gang. “So we are trying to apply pressure to the hybride perovskite system to hopefully improve the performance of organic-inorganic trihalide perovskites”, Lingping added.

Unexpectedly, both the band-gap narrowing and carrier-lifetime prolongation was found at ~0.3 GPa from optical absorption as well as time-resolved photoluminescence measurements, respectively.

“We are really excited that the carrier-lifetime was prolonged up to 70% to 100% compared to the uncompressed sample”, said Gang.

While when they further compress the sample to higher pressures, a jump happed on band-gap as well as carrier lifetime.

They also carried out x-ray diffraction on the sample to find possible structure change. Almost at the same pressure, a structural phase transition happened on the sample from single crystal x-ray diffraction.

So the band-gap narrowing as well as photovoltage increasing ws attributed to the electron-density redistribution and trap states near the edges of conduction band and valence band induced from the lattice varying from pressure.

“Our findings have clearly demonstrated the usefulness of pressure-driven modulation on crystal structures that leads to a desirable improvement in material properties”, Lingping said.

Caption: Pressure-driven band-gap evolution of MAPbI3 and  In situ high-pressure TRPL measurements on a MAPbI3 single crystal at 0.5 GPa, courtesy of Gang Liu.