When light hits a perovskite, it excites negatively charged electrons and leaves behind positively-charged ‘gaps’ within the material’s crystalline structure. These electrons and gaps can then move through the perovskite to generate an electrical current. However, the charge carriers can also recombine instead, wasting the energy they carry.
“The efficiency of perovskite solar cells has been greatly improved in the past decade, but fundamental research on their photophysics is relatively backward,” said researcher Ming-Cong Wang from the KAUST Solar Centre. “One of the things that is not clear is how charge carriers behave before recombining.”
Some of the ions that make up the perovskite’s crystal lattice can help to localise electrons and gaps in different regions, which prevents them from recombining and prolongs their lives. However, this localisation also tends to make the charge carriers less mobile, which may adversely affect the solar cell’s performance. Understanding such effects could help researchers to fine-tune the composition of perovskites and boost their ability to generate electricity from sunlight.
The team studied two different aspects of charge-carrier behaviour in thin films of a promising perovskite known as CsFAMA (a caesium-containing triple-cation mixed halide perovskite). First, they used a series of brief laser pulses to excite the charge carriers and then examine them just a few picoseconds (trillionths of a second) later. They found that as the density of charge carriers increases, it linearly narrows the energy gap that electrons need to vault when they are excited by incoming light. According to Wang, this is different from the behaviour of conventional semiconductors.
The team then used a form of high-intensity microwaves called terahertz radiation to study how the charge carriers moved around. This showed that as the density of charge carriers increases, they are more likely to stick in a particular location.
The researchers believe that these two key observations have the same root cause. When light hits the perovskite, it can cause small rapid distortions in the lattice of ions that affect the charge carriers’ behaviour. Other solar researchers have very recently observed similar lattice fluctuations in perovskites as well, backing up the findings of the KAUST team. The work is published in Nature Communications.