Hot Carrier Dynamics and Charge Trapping in Surface Passivated β-CsPbI₃ Inorganic Perovskite

Abstract

Thermodynamically stable CsPbI3 inorganic perovskite has achieved high efficiency exceeding 20% with surface defect passivation, but a thorough understanding on the photophysics properties of surface passivated CsPbI3 inorganic perovskite is still lacking. Herein, we have used transient absorption spectroscopy to investigate the photophysical properties of β-CsPbI3 perovskites with and without passivation. The results indicate that the carrier trapping process has become slower because of the reduced deep defects that were varied to shallow defects due to surface passivation. The bimolecular recombination of β-CsPbI3 was also accelerated because of the improved carrier mobility after healing surface defects by passivation agents. Moreover, the efficient defect passivation can also elongate the hot carrier lifetime from 0.26 to 0.37 ps by impeding the charge trapping process. Our findings reveal that the defects passivation is beneficial to enhance defect tolerance, improve carrier transport, and slow down the hot carrier cooling for developing high-performance photovoltaics.

Enhancing the Hot Carrier Injection of Perovskite Solar Cells by Incorporating a Molecular Dipole Interlayer

“…Clearly, the TA plots of the control and PX-treated perovskite films show an obvious photobleaching (PB) band at ≈790 nm (at the bandgap of ≈1.52 eV), which is known as the band state filling between the valence band edge and the conduction band edge of perovskite or stimulated emission. [41,42] Scans at various delay times (Figure 4d-f) are extracted from pseudo-color plots. Note that the spectrum at the high energy tail gradually narrows from 1 to 10 ps, indicating the hot carrier thermalization process.…”

Direct Observation of Increased Free Carrier Generation Owing to Reduced Exciton Binding Energies in Polymerized Small-Molecule Acceptors

“…12 In the solar cells based on inorganic semiconductors, Wannier-type excitons are usually formed upon illumination, which are able to spontaneously dissociate into free electrons and holes, benefitting from the small E b that is usually smaller than the thermal energy k B T at room temperature. 8,13 On the contrary, due to the substantially large E b of organic semiconductors, tightly bound Frenkel-type excitons are predominantly generated in the organic solar cells (OSCs) upon photoexcitation, and they are difficult to separate into free charges to produce photocurrent. 14,15 Therefore, in actual OSC devices, two different organic semiconductor materials, featuring electron-donating (donor, D) and electron-accepting (acceptor, A) properties, were introduced in the active layer.…”

“…But the photophysical processes of these two types of materials are different, − mainly because the organic semiconductors have a much larger exciton binding energy ( E b ), even by orders of magnitude, than the inorganic semiconductors. − E b is defined as the energy difference between a bound electron–hole pair (exciton) and a free electron–hole pair . In the solar cells based on inorganic semiconductors, Wannier-type excitons are usually formed upon illumination, which are able to spontaneously dissociate into free electrons and holes, benefitting from the small E b that is usually smaller than the thermal energy k B T at room temperature. , On the contrary, due to the substantially large E b of organic semiconductors, tightly bound Frenkel-type excitons are predominantly generated in the organic solar cells (OSCs) upon photoexcitation, and they are difficult to separate into free charges to produce photocurrent. , Therefore, in actual OSC devices, two different organic semiconductor materials, featuring electron-donating (donor, D) and electron-accepting (acceptor, A) properties, were introduced in the active layer. They can form a heterojunction, and their energy-level difference at the interface serves as the driving force for charge transfer (CT) to overcome the exciton binding energies, generating free electrons (charge carriers) in the acceptor phase and holes (charge carriers) in the donor phase. − However, the introduction of this driving force inevitably leads to additional energy loss and lowers open-circuit voltage ( V OC ) in the OSCs. − Although the power conversion efficiency (PCE) of the single-junction OSCs with the highest performance has exceeded 19%, it still lags far behind the inorganic solar cells.…”

Modulating CsPbl3 crystallization by using diammonium agent for efficient solar cells

“…As depicted in Figure 5H, the energy loss rate of hot carriers is decreased after modulation of the perovskites with ammonium chlorides, and the HACl 2 ‐based perovskite possesses the slowest energy loss rate. Such a tardy energy loss rate has a high potential to take advantage of the excess hot carrier energy for further improving the photovoltaic performance of PSCs, realizing enhanced V oc of PSCs 58 . Bimolecular recombination and trap‐assisted monomolecular recombination would subsequently happen after hot carrier cooling.…”

“…Such a tardy energy loss rate has a high potential to take advantage of the excess hot carrier energy for further improving the photovoltaic performance of PSCs, realizing enhanced V oc of PSCs. 58 Bimolecular recombination and trap-assisted monomolecular recombination would subsequently happen after hot carrier cooling. Figure 5I shows the TA kinetics of control, PEACl-and HACl 2 -based perovskite films at 705 nm, and the details of the fitting parameters are listed in Table S3.…”