Improving quantum efficiency in organic solar cells with a small energetic driving force

Organic solar cells based on a polymer donor (PM7) and a non-fullerene acceptor (Y5) with a very small energetic difference between the local excited state and the charge transfer (CT) state are investigated. We find that the small energetic difference (Delta E-CT) leads to a low voltage loss (0.44 eV). However, the short-circuit current density (J(sc)) of the solar cell based on PM7:Y5 is very low, due to monomolecular recombination of the CT state excitons, limiting the internal quantum efficiency of the device. To solve the problem with the inefficient exciton dissociation, a polymer donor (PBDB-T) with a similar chemical structure to PM7 is employed as a second donor component for constructing ternary solar cells. We find that the frontier energy levels of the two donors are hybridized, allowing us to realize fine-tuning of the effective energy of the CT state and Delta E-CT of the ternary blend, by varying the PBDB-T content. As a result, a significantly improved CT state dissociation efficiency is achieved by adding a small amount of PBDB-T in the active layer. Meanwhile, the low voltage loss property of PM7:Y5 is very well maintained in the ternary solar cell, due to the energy level hybridization of the donor materials.

Automated summary

The study revealed that a small energetic difference between the local excited state and the charge transfer state leads to low voltage loss, but results in a low short-circuit current density. By adding a polymer donor (PBDB-T) with similar chemical structure to PM7 in the ternary solar cells, the efficiency of the charge transfer state dissociation can be significantly improved, while maintaining the low voltage loss property of PM7:Y5.