Improving Charge Transport via Intermediate‐Controlled Crystal Growth in 2D Perovskite Solar Cells

Abstract

Reduced-dimensional hybrid perovskite semiconductors have recently attracted significant attention due to their promising stability and optoelectronic properties. However, the issue of poor charge transport in 2D perovskites limits its application. Here, studies on intermediatecontrolled crystal growth are reported to improve charge carrier transport in 2D perovskite thin films. It is shown that the coordination strength of solvents with perovskite precursor affects the initial state of intermediate phase formation as well as the subsequent perovskite layer growth. Tuning the solvent composition with a mixture (5:5) of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) leads to the growth of highly orientated 2D perovskite films with much-improved optoelectronic properties (faster transport by ≈50x, longer carrier lifetime by ≈4x, and lower defect density by ≈30x) than the film prepared with pure DMF. Consequently, perovskite solar cells based on DMF/DMSO (5:5) show >80% efficiency improvement than the devices based on pure DMF.

Vertical Orientated Dion–Jacobson Quasi‐2D Perovskite Film with Improved Photovoltaic Performance and Stability

“…Recently, 2D perovskites have emerged to show some promise of addressing these issues due to their better thermodynamic and environmental stability . Due to different organic spacer groups, two main archetypes of 2D perovskites have been investigated, namely, Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) layered 2D perovskites with general formulas of A′ 2 A n −1 Pb n X 3 n +1 and A″A n −1 Pb n X 3 n +1 (A′ and A″ are mono‐ and diammonium organic cations respectively, A is CH(NH 2 ) 2 (FA), CH 3 NH 3 (MA) or Cs, X is halide, n = 1, 2, …).…”

“…The RP layered perovskites are more investigated 2D perovskites, which incorporate two layers of monovalent ammonium cations (butylamine, phenylamine, etc.) between the lead halide inorganic layers per unit cell, thus there is a van der Waals gap between the adjacent unit cells . In contrast, the unit cell of DJ layered perovskites contains only one single layer of diammonium organic cations, such as 1,4‐butanediammonium (BDA), inserted into lead halide inorganic layers, leading to a more rigid structure than that of RP phase .…”

“…Compared with the control film without NH 4 SCN, a redshift among PL peaks was observed in the PL spectra with 1 SCN − , 2 SCN − , and 3 SCN − . However, PL spectrum of the film with x SCN − shows nearly no peaks for the quantum wells of n = 1, 2, 3, and 4 no matter the use of front or back excitation (Figure S8, Supporting Information), unlike that from the quasi‐2D RP perovskite film . This implies that the film structure should be uniform from top to the bottom of the entire perovskite films.…”

Enhanced Charge Transport by Incorporating Formamidinium and Cesium Cations into Two‐Dimensional Perovskite Solar Cells

“…[10c, 12, 13, 20b,23] These results support our hypothesis that the triple cation 2D perovskite films exhibit reduced nonradiative recombination and longer carrier lifetime,w hich result from the improved film quality as discussed above for the triple-cation-based 2D perovskite thin films.S imilar results were shown in the Nyquist plot from the impedance measurement (see Figure S8). [24] We conducted time-resolved microwave conductivity (TRMC) measurements [25] of the carrier mobility ( Figure 3B) to compare perovskite thin films prepared from monocations and triple cations with 640 nm laser excitation. [24] We conducted time-resolved microwave conductivity (TRMC) measurements [25] of the carrier mobility ( Figure 3B) to compare perovskite thin films prepared from monocations and triple cations with 640 nm laser excitation.…”

“…Thetransfer resistance (R tr )at high frequency (the first small arc) shows that the transfer of electrons and holes is more efficient in PSCs based on triple cations than that of monocation-based PSCs when compared under the same conditions.T he large arc at low frequency is attributed to recombination resistance (R rec ), where PSCs based on triple cations show ah igher barrier for carrier recombination. [24] We conducted time-resolved microwave conductivity (TRMC) measurements [25] of the carrier mobility ( Figure 3B) to compare perovskite thin films prepared from monocations and triple cations with 640 nm laser excitation. The yield-mobility product fSm value increased from 0.41 cm 2 V À1 s À1 for BA 2 MA 4 Pb 5 I 16 to 2.8 cm 2 V À1 s À1 for BA 2 (Cs 0.02 MA 0.64 FA 0.34 ) 4 Pb 5 I 16 .F igure 3C shows the comparison of the fSm value over aw ide range of laser excitation intensities.T his dramatic improvement of the transport property is consistent with the much-reduced dark carrier density (n d = 1.94 10 17 cm À3 )f or the BA 2 (Cs 0.02 MA 0.64 FA 0.34 ) 4 Pb 5 I 16 sample compared to the BA 2 MA 4 Pb 5 I 16 sample (n d = 2.58 10 18 cm À3 ;F igure 3D).…”