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US lab offers insights into new class of semiconductors

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Hybrid perovskites are 'best of both worlds' for light harvesting


A paper published in the August 10th edition of the journal Nature Photonics by researchers at the University of Notre Dame in Indiana, describes their investigations into the fundamental optical properties of a new class of semiconducting materials known as organic-inorganic 'hybrid' perovskites. They conclude that the materials offer the best compromise between cost and performance for light harvesting.

'Perovskites' refers to the structural order these materials adopt upon drying and assembling in the solid state. In solid-state thin film solar cells, hybrid perovskites have recently shown light-to-electricity conversion efficiencies approaching 20 percent, rivaling that of commercial solar cells based on polycrystalline silicon. More importantly, these materials are easy and cheap to process using coating and or printing in contrast to solar technologies that typically require high purity materials, especially for silicon solar cells, and high-temperature processing.

However, the scientific community does not yet fully know how these unique materials interact with light on a fundamental level.

In this study, Joseph Manser under the direction of Prashant Kamat, present new insights into the excited-state properties of hybrid methylammonium lead iodide (CH3NH3PbI3) thin films through a technique known as 'transient absorption pump-probe spectroscopy' . This approach was used to  examine the events that occur trillions of a second after light absorption in the hybrid methylammonium lead iodide. They analysed both the relaxation pathway and spectral broadening in photoexcited hybrid methylammonium lead iodide and found that the excited state is primarily composed of separate and distinct electrons and holes known as free carriers.

"The fact that these separated species are present intrinsically in photoexcited hybrid methylammonium lead iodide provides a vital insight into the basic operation of perovskite solar cells," Manser said. "Since the electron and hole are equal and opposite in charge, they often exist in a bound or unseparated form known as an 'exciton.' Most next-generation photovoltaics based on low-temperature, solution-processable materials are unable to perform the function of separating these bound species without intimate contact with another material that can extract one of the charges."

This separation process siphons energy within the light-absorbing layer and restricts the device architecture to one of highly interfacial surface area. As a result, the overall effectiveness of the solar cell is reduced. "However, from our study, we now know that the photoexcited charges in hybrid perovskites exist in an inherently unbound state, thereby eliminating the additional energy loss associated with interfacial change separation," Manser said. "These results indicate that hybrid perovskites represent a 'best of both worlds' scenario, and have the potential to mitigate the compromise between low-cost and high-performance in light-harvesting devices."

Although the research was on the fundamental optical and electronic properties of hybrid perovskites, it does have direct implications for device applications. Understanding how these materials behave under irradiation is necessary if they are to be fully optimised in light-harvesting assemblies.

Manser and Kamat's research was supported by the Department of Energy's Office of Basic Energy Science.

The paper 'Band filling with free charge carriers in organometal halide perovskites' by J Manser et al, appears in Nature Photonics (2014) doi:10.1038/nphoton.2014.171

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