Understanding the efficiency of solar cells

Organic solar cells are made up of an interface between an electron-donating material and an electron-accepting material, sandwiched between two metal electrodes. Incoming light absorbed by the cells induces the formation of excitons — tightly bound pairs of an electron and an electron hole — in the electron-donating material. When the excitons undergo charge separation at the interface, a free electron is transferred to the electron-accepting material, and this transfer results in the generation of electricity in the external circuit.

During the first phase of charge separation, excitons decay into an energetically intermediate state known as the charge transfer (CT) state. The CT state can exist as any of several energy states, depending on the modes of vibration of the particles. The most relaxed of these energy states, which has the least energy, is known as CT¹. To better understand charge transfer, a team including researchers from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia looked at the energy changes involved in charge generation.

Previously, it was thought that extra photon energy, which would produce higher-energy CT states than CT¹, would enhance the production of free electrons at the interface. However, by measuring light-to-charge conversion efficiency in a wide range of organic materials excited to higher CT states, the team showed that the efficiency of charge generation is independent of whether higher-energy states than CT¹ are formed. This is because excited CT states relax back to CT¹, losing their extra energy, much faster than charge separation can occur.

Understanding the process of charge generation and the possible energy losses involved could help researchers to develop more efficient solar cells in the future.

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