New ‘annihilation’ molecule enhances efficiency of solar energy upconversion

Generating solar energy from low energy photons via photochemical upconversion could become more effective by using perylene monoimide as the emitter or ‘annihilation’ molecule. 

Conventional silicon solar cells are limited to approximately 29% efficiency because they are only able to capture a limited part of the light spectrum.   

To beat this limit, researchers are exploring upconversion, which involves combining low energy photons of light, which would otherwise be lost, to produce higher energy photons able to be captured by photovoltaics devices. 

Efficiencies of 43%, or even 50% with the aid of a solar concentrator, have been predicted by devices employing effective upconversion. 

Some new and emerging solar energy technologies, including those made from thin films and with lead halide perovskites, have high band gaps and are well matched to photochemical upconversion because it targets a suitable part of the light spectrum. 

During the process of photochemical upconversion, photons are harvested by a ‘sensitiser’ molecule and the subsequent energy is transferred to an ‘annihilator’ molecule.  

The energy is stored in the annihilator molecules for hundreds of microseconds, which is long enough for two excited annihilators to interact and reach an excited state, producing the desired high-energy photon. This process is governed by electron spin dynamics and understood through quantum mechanics. 

One common annihilator molecule, diphenylanthracene, is useful for generating blue light from green, but isn’t well suited to thin film or perovskite solar cells, which absorb red and near-infrared light. 

Another candidate, rubrene, is reliable at generating high energy photons, but its efficiency is limited by a slow diffusion rate.  

In the search for new and effective annihilators, researchers at the ARC Centre of Excellence in Exciton Science have identified perylene monoimide (PMI) as a promising option. 

PMI, when coupled with the same sensitiser as rubrene, produced high energy photons with over 12% efficiency under ‘one sun’ conditions. 

But perhaps most promisingly, it proved to be five times more effective than rubrene under the equivalent of 0.1 suns, demonstrating its likely suitability for low intensity, ‘sub solar’ conditions.   

The researchers, based at UNSW Sydney and Monash University, in collaboration with North Carolina State University, have published their results in the journal Energy and Environmental Science. They are available here.