Limiting factors of photon-to-current conversion in polymer/ nanocrystal bilayer hybrid solar cells: An analytical quantum efficiency model study

摘要

This paper introduces an analytical external quantum efficiency (EQE) model of planar hybrid solar cells (HSCs) based on photon-to-current conversion processes and uses this to investigate the factors that limit the maximum EQE (EQE_m) of devices; i.e., the photon absorption coefficient α, exciton diffusion coefficient D_z, exciton lifetime t_z, exciton dissociation rate k_(dis), electron diffusion coefficient D_e, electron lifetime τ_e, nanocrystals thickness d, and thickness of the polymer l. Our simulations indicate that relying solely on modifying k_(dis), D_e, or τ_e cannot achieve a breakthrough increase in the EQE_m of planar HSCs. However, increasing α, D_z, or τ_z could potentially lead to a large EQE_m (30-100%), especially in the context of high k_(dis) values. Moreover, the calculation results indicate that although both D_z and τ_z contribute to the exciton diffusion length (L_z) via the equation L_z~2 = D_zτ_z, the EQE_m has an asymmetric dependence on these variables. With a small k_(dis) (i.e., <10~4 cm/s), an increase in D_z results in an initial increase and then decrease in EQE_m, resulting in a peak value that increases with increasing k_(dis). When fcdis is sufficiently large (~105 cm/s), the EQE_m becomes saturated after the initial increase. Thus, although an increase in D_z can adversely affect device performance when the k_(dis) is lower than 10~4 cm/s, increasing τ_z always improves device performance, regardless of large k_(dis) becomes. This behavior can be attributed to the detrimental effect of excitons accumulating at the D/A interface, and can be used to optimize the material design and device engineering of planar HSCs and related solar cells for maximum photon-to-current conversion performance. In addition, we also demonstrate that the model can fit to the experimental data.