Exploiting Perovskites Multidimensionality for High Performance Photodiodes

Quasi-2D perovskites (q2D PVKs) have emerged as attractive materials for optoelectronic devices, such as solar cells, LEDs, or photodiodes, because of their structural diversity and higher stability compared to the 3D counterpart. To develop high-performance perovskite photodiodes (PPDs), efforts have been put into minimizing their dark-current density (J_D), as this figure of merit impacts the noise (i_n) and thereby the detectivity of the device. Our previous work has shown that, in absence of charge injection, a thermal charge generation mechanism occurring at the interface between the electron-blocking layer (EBL) and the perovskite is responsible for the experimental J_D. This mechanism consists of a thermal excitation of an electron from the EBL HOMO to the conduction band (CB) of the perovskite and depends on the energy offset (Φ) between the abovementioned energy levels. A higher barrier directly translates into lower J_D, hence to photodiodes with higher detectivity. In this regard, q2D PVKs are an excellent candidate material, as they usually consist of a 2D-3D gradient, where high-bandgap 2D or quasi-2D phases are located at the interface with the EBL (in a p-i-n device), resulting in an upshift of the CB and an increase of Φ.

By means of solvent engineering and varying the organic spacers, we finely tuned this 2D-3D gradient and fabricated PPDs based on q2D PVK active layers. All 2D-3D graded perovskites exhibited a significant decrease of the average JD to about 5×10^-8 mA cm^-2, confirming the positive effect of introducing 2D phases at the EBL interface. Moreover, by performing temperature-dependent J_D measurements, we determined the activation energy (Ea) of the thermal charge generation process and we found that indeed E_a increased from 0.87 eV to 1.16 eV, for 3D FAMA and quasi-2D perovskite BA₂MA₃Pb₄I₁₃, respectively. To confirm the importance of gradient direction, we fabricated devices with a 3D-2D gradient by using cyclohexylmethylammonium iodide as organic spacer and characterized the resulting films in depth. In this case, the 2D phases are located at the top of the film, whereas 3D ones are at the bottom, meaning that in such devices the EBL interface is by all means similar to when using a bulk 3D perovskite as active layer. Both J_D and E_a were in fact found to be similar to the ones obtained for pure 3D perovskites, further confirming the importance of having a 2D-3D gradient to achieve low dark-current density.

Despite several 2D-3D graded photodiodes have shown decreased J_D, which is strictly related to their performance in dark conditions, other figures of merit related to PPDs behavior under illumination are equally important, such as EQE and response time. By measuring grazing-incidence wide-angle X-ray scattering (GIWAXS), we introduced a distinction between parallel- and perpendicular- oriented films, and we linked such crystal orientation to their light behavior. This study allowed us to select perpendicular-oriented q2D PVKs as optimal devices with low J_D, high EQE (> 80%), and fast response speed.

Finally, as we proved that increasing Φ is crucial for obtaining low dark-current PPDs, we fabricated devices using the optimized q2D PVK films and a deep-HOMO EBL, namely PTAA:polyTPD. The combination of these materials allowed us to further increase Φ and minimize JD to 5×10⁻⁹ mA cm⁻², which is the lowest J_D reported in the literature to the best of our knowledge, leading to ultralow noise of 6 fA Hz^−1/2 and to an outstanding real specific detectivity of 7×10¹² Jones. Furthermore, such devices displayed exceptional light sensitivity, with a linear response over more than 8 orders of magnitude of light intensity.

Overall, in our work we outlined several key requirements to exploit multidimensional perovskite films for high-performance perovskite photodiodes, paving the way for further developments in the field of PPDs.