Charge Injection, Carriers Recombination and HOMO Energy Level Relationship in Perovskite Solar Cells

Abstract

We present a comparative study between a series of well-known semiconductor polymers, used in efficient organic solar cells as hole transport materials (HTM), and the state-of-the art material used as hole transport material in perovskite solar cells: the spiro-OMeTAD. The observed differences in solar cell efficiencies are studied in depth using advanced photoinduced spectroscopic techniques under working illumination conditions. We have observed that there is no correlation between the highest occupied molecular orbital (HOMO) energy levels of the organic semiconductors and the measured open-circuit voltage (VOC). For instance, spiro-OMeTAD and P3HT have a comparable HOMO level of ~5.2 eV vs vacuum even though a difference in VOC of around 200 mV is recorded. This difference is in good agreement with the shift observed for the charge vs voltage measurements. Moreover, hole transfer from the perovskite to the HTM, estimated qualitatively from fluorescence quenching and emission lifetime, seems less efficient for the polymeric HTMs. Finally, the recombination currents from all devices were estimated by using the measured charge (calculated using photoinduced differential charging) and the carriers’ lifetime and their value resulted in accordance with the registered short-circuit currents (JSC) at 1 sun.

Highly Efficient Perovskite Solar Cells Enabled by Multiple Ligand Passivation

“…The corresponding WF and VBM of control perovskite is 4.22 and 1.55 eV, leading to an IE of 5.77 eV. The HOMO of spiro‐OMeTAD is around 5.2 eV . Thus, the energy barrier between the perovskite and spiro‐OMeTAD is reduced from 0.57 to 0.17 eV after multiple ligand passivation.…”

“…The HOMO of spiro-OMeTAD is around 5.2 eV. [14] Thus, the energy barrier between the perovskite and spiro-OMeTAD is reduced from 0.57 to 0.17 eV after multiple ligand passivation. To check whether the reduced energy barrier between IE of perovskite and HOMO of spiro-OMeTAD helps improve the hole extraction efficiency, the lifetime of perovskite coated with spiro-OMeTAD in the presence/absence of ligand is measured.…”

Interface Engineering for Highly Efficient and Stable Planar p‐i‐n Perovskite Solar Cells

“…[83] Compared with the charge extraction process, charge transfer and recombination at the interface have a much stronger impact on the device performance like V oc and J sc . [87][88][89] Interfacial structural and electronic mismatches usually act as the energy barriers for charge transport and charge recombination. Interface engineering is needed to eliminate this effect.…”

Fullerene‐Based Materials as Hole‐Transporting/Electron‐Blocking Layers: Applications in Perovskite Solar Cells

“…This point is confirmed by TRPL measurements (Figure c). The pattern obtained with TRPL has been fitted using a biexponential decay, as it has been previously reported in the literature . The value of the obtained characteristic times is depicted in Table S3.…”

“…Two characteristic times τ 1 and τ 2 have been obtained from the fitting. The fastest one, τ 1 , is associated with the charge carrier injection into HTM, whereas the slowest one, τ 2 , is associated with recombination . Deposition of FU7 layer on top of perovskite reduces τ 1 significantly indicating an efficient hole transfer.…”