That is, the change in recombination rates from fast to slow is due to changes in the recombination processes, from surface to bulk recombination [24].
Figure 3a, b shows the EQE spectra and photocurrent density of the bulk recombination spectra under different doping concentrations of the Si substrate (i.e., 1 × 1014, 1 × 1015, 1 × 1016, and 1 × 1017 cm–3).
In addition, the study shows that the bulk recombination of carriers has an influence on the conversion efficiency of the cell, the conversion efficiency of the actual solar cell with carrier recombination is reduced by 20.0% of the ideal cell without carrier recombination.
The bulk recombination of Frenkel pairs increases with decreasing temperature resulting in suppression of the production of freely migrating vacancies (the driving force of the void growth).
On the other hand, at the moderate level of carrier's density, bulk recombination lifetime can also increase.
With the increase of doping concentrations, the bulk recombination increases sharply according to the following reasons: (1) the reduced bulk lifetime results in SRH recombination increasing synchronously and (2) the increased excess minority carrier concentration (i.e., δp) leads to the increase in bulk recombination.
While the surface recombination is dealt with by depositing a passivation layer of SiO2 or SiNx, the bulk recombination is minimized by use of nearly defect-free monocrystalline substrate.
Therefore, we can determine the bulk recombination properties of the wafer from the effective minority carrier lifetime measurements.
Hence, after the pulse, the balance between the surface and bulk recombination processes is shifted in favor of the bulk recombination.
The TRPL decay curves, shown in Fig. 3f, are exponentially fitted, where τ1and τ2 represent the bulk recombination in perovskite bulk films and the delayed recombination of trapped charges, respectively [31].
