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

Abstract

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 (EQEm) of devices; i.e., the photon absorption coefficient α, exciton diffusion coefficient Dz, exciton lifetime τz, exciton dissociation rate kdis, electron diffusion coefficient De, electron lifetime τe, nanocrystals thickness d, and thickness of the polymer l. Our simulations indicate that relying solely on modifying kdis, De, or τe cannot achieve a breakthrough increase in the EQEm of planar HSCs. However, increasing α, Dz, or τz could potentially lead to a large EQEm (30–100%), especially in the context of high kdis values. Moreover, the calculation results indicate that although both Dz and τz contribute to the exciton diffusion length (Lz) via the equation Lz2 = Dzτz, the EQEm has an asymmetric dependence on these variables. With a small kdis (i.e., <104 cm/s), an increase in Dz results in an initial increase and then decrease in EQEm, resulting in a peak value that increases with increasing kdis. When kdis is sufficiently large (∼105 cm/s), the EQEm becomes saturated after the initial increase. Thus, although an increase in Dz can adversely affect device performance when the kdis is lower than 104 cm/s, increasing τz always improves device performance, regardless of large kdis 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.