Probing Perovskite Carrier Dynamics under Sunlight

Understanding the nature of photogenerated carriers and their subsequent dynamics in 2 perovskites is important for the development of related materials and devices. Most 3 ultrafast dynamic measurements on the perovskite materials were conducted under high 4 carrier densities, which likely obscures the genuine dynamics at low carrier densities 5 under solar illumination conditions. In this study, we presented a detailed experimental 6 study of the carrier density-dependent dynamics in hybrid lead iodide perovskites using 7 a highly sensitive transient absorption spectrometer. We found that the carrier lifetime 8 was about a hundred nanosecond in the linear response range, representing sunlight 9 excitation, which was much longer than under high carrier densities. We also elucidated 10 that the fast carrier decay (<1 ps) and the medium decay processes (tens of ps) occurred 11 via the defect state trapping, and we determined its effects on the utilization percentage 12 of photogenerated carriers through quantitative analysis. Furthermore, we obtained the 13 Shockley-Queisser limit that took into account the carrier trapping effect, which 14 directly reflected the material performance. 15

Solar cell technology is considered one of the best energy shortage solutions for 1 reducing carbon emissions. Over the last decade, perovskite-based solar cells have 2 attracted significant attention and developed rapidly because of its high power 3 conversion efficiency and low fabrication cost, making them one of the most promising 4 solar cell materials. 1,2 The extraordinary photovoltaic performance of perovskite-based 5 solar cells can be attributed to their strong light absorption, 3 high carrier mobility 4, 5 and 6 long charge diffusion length. 6, 7 Recent progress has been made to improve photovoltaic 7 efficiency in fabrication protocols, 8, 9 chemical compositions 10, 11 and phase 8 stabilization methods. 12,13 Meanwhile, intense research efforts have also been made to 9 understand these fundamental photophysical mechanisms. 14,15,16,17 10 Charge and energy transfer processes occur in solar energy harvesting systems 11 from femtosecond to nanosecond time scales, and understanding these processes is the 12 key to determining the design principle for photovoltaic materials and devices. 17

13
Ultrafast spectroscopy is a powerful tool that can be used to assess the dynamics of 14 photocarriers in semicondutors. 18 Transient absorption (TA) spectroscopy is the most 15 commonly used method for studying the ultrafast charge and energy transfer processes 16 in perovskites. 18 This technique has been widely used in perovskites to discover the 17 slow hot carrier cooling 7 and reveal the carrier dynamic mechanisms, such as the hot 18 phonon bottleneck, 19 Auger heating, 20 and band filling effects. 21 However, the 19 sensitivity of TA spectrometer is limited; therefore, most reported TA measurements 20 were conducted at much higher carrier densities, such as 10 17 cm -3 , than that under air 21 mass (AM) 1.5G solar illumination conditions. 16 For high-quality methylammonium 22 lead iodide (MAPbI3) films, the calculated carrier density reaches 4×10 14  Here, we report the ultrafast dynamics of photogenerated carriers in hybrid 1 perovskite cesium formamidinium lead iodide (Cs0.1FA0.9PbI3) thin film. It was 2 measured by highly sensitive TA spectrometer, that we recently developed, which 3 enabled us to investigate the carrier dynamics under very low carrier densities. The TA 4 experimental results revealed that carrier dynamics is highly carrier density-dependent 5 with changes of pump intensity. The dynamics in the linear response range showed two 6 fast carrier decays from the trapping process and one slow decay process, which was 7 attributed to the trap-assisted recombination. Additional studies on the thin film with 8 PbCl2 in precursor indicated effective passivation of the trap state density. By 9 quantitative analyzing of the correlation between the TA curve and the carrier capture 10 percentage, we obtained a Shockley-Queisser limit including carrier trapping effects, 11 which quantitatively reflected the performance of solar materials.

13
Pump Intensity-dependent Carrier Dynamics 14 The highly sensitive TA spectrometer was developed to study the carrier dynamics 15 of solar energy materials under very low carrier densities. 23 A sensitivity level (ΔT/T) 16 of 10 -7 was achieved by a novel technique of combining 1 kHz macro-pulse and 200 17 kHz micro-pulse divided down from a fiber laser with 1 MHz repetition rate and using 18 a balanced detector scheme. The more details of TA spectrometer were described in 19 supplementary information (SI). Cs0.1FA0.9PbI3 thin films was chosen to demonstrate 20 the perovskite carrier dynamics under solar illumination since a partial substitution of 21 Cs + for HC(NH2)2 + (FA + )in FAPbI3 perovskite was proved to substantially improve 22 photo-and moisture stability along with photovoltaic performance. 24 Cs0.1FA0.9PbI3 23 thin films were prepared using a one-step method.(see SI) 24 First, we conducted ensemble TA spectral measurements of the Cs0.1FA0.9PbI3 thin 25 film under high intensity pumping to obtain the basic TA spectral features. The TA 26 spectra pumped at 50 nJ and 515 nm, as well as 690 nm, are shown in Figures 1 and S7.

27
The carrier density was 2.6 ×10 15 cm −3 , which corresponded to 1 nJ pump light at 515 28 nm with a light spot diameter of 3 mm, while 6.7 ×10 14 cm −3 corresponded to 1 nJ pump 1 light at 690 nm with a diameter of 3.5 mm (see SI). We observed that both TA dynamics 2 were very similar. Immediately after photoexcitation, a ground-state bleaching (GSB, 3 positive change of transmission ΔT/T) band centered at about 790 nm was observed, 4 which was consistent with the band gap obtained from the static absorption spectrum 5 shown in Figure S3. In addition, a photo-induced absorption (PIA, negative ΔT/T) band 6 centered at around 815 nm was observed. The GSB feature was attributed to the band 7 filling, that is the presence of photogenerated band gap carriers blocking the optical 8 absorption of the probe pulse. The PIA was due to the excited state absorption of the 9 hot carriers. After 1 ps, as the hot carriers cooled down the band filling dominated and 10 the PIA disappeared. The GSB signal was longer than 1 ns.  To elucidate the carrier recombination mechanism over a range of excitation 6 intensities, the dynamic curve can be modeled by the simple rate equation in the reported TA studies, owing to high carrier densities. Figure 2 shows the 2 normalized dynamic traces of the GSB signals at 790 nm for various pump intensities 3 by two orders of magnitude, and some obvious features were clearly observed. First, 4 the rising edge of the GSB signal, which was carrier cooling process, was slower at 5 high pump intensities. This is caused by hot phonon bottleneck effect, which reduces 6 the hot carrier cooling rate when the high density of carriers are excited. 16, 19 (see Figure   7 3A) Second, at < 1 ps, the quick decay process only appeared at low pump intensities, 8 and the other fast decay process appeared in the tens of picoseconds range. This feature 9 was attributed to the trapping process (see Figure 3B), whose assignment will be 10 discussed in the following section. Third, at a delay range of 100-3500 ps, the decay 11 rate of the GSB signal slowed down with decreasing pump intensity. This was caused 12 by the low bimolecular and Auger decay rates at low carrier densities. 16,21,26 Fourth,13 when the pump energy was below 3 nJ, the normalized TA curves indicated the same 14 dynamics, independent on the pump intensity. Hence, only monomolecular process was 15 observed, without any nonlinear process. Figure S8 shows that the pump energy of the 16 10 nJ condition at 690 nm reached the linear response range, which was consistent with 17 the carrier density due to the low absorption coefficient at 690 nm. Furthermore, we 18 inferred that under very low pump intensity conditions in the linear response range, the 19 carrier dynamics were the same as that under AM 1.5G. The carrier densities generated 20 by the 3 nJ pump light at 515 nm and 10 nJ at 690 nm were 7.8 ×10 15 and 6.7 ×10 15 cm -21 3 , respectively, which indicated the upper limit of carrier density in the linear response.

22
This was the first time that the carrier dynamics of three-dimensional perovskites under 23 solar illumination were obtained. proportional to the trap state density. Hole trapping was not indicated in the schematic. only trap-assisted recombination was possible. This decay process is dependent on the 1 trap cross-section, energy depth, density, and distribution, which were subject to sample 2 processing and handling conditions. 16 However, this process usually lasts tens or even 3 hundreds of nanoseconds. 16 Therefore, we attributed this fast decay process to carries 4 trapping by the defect states, as trapping reduces the densities of the band gap carriers. 5 This has also been widely observed in oxides, 27 perovskite nanocrystals, 14, 28 carbon 6 films 29 and two dimensional materials. 30 And the long decay process was ascribed to 7 trap-assisted recombination. To further investigate the characteristics of the trap state in perovskite, additional 9 TA dynamics in the linear response range was measured at a pump wavelength of 690 10 nm. Figure 3 shows a comparison of the normalzied TA curves at the same probe 11 wavelength of 790 nm with the pump energy of 3 nJ at 515 nm and 10 nJ at 690 nm.

12
The fitting results in Figure 3 shows both the fast decay and the medium rate decay 13 percentages at the pump wavelength of 690 nm were noticeably smaller than at 515 nm. PbCl2 has been added to precursor solution to form mixed lead iodide perovskites 2 to provide higher solar cell efficiencies, 5,6 which has been correlated to larger crystal 3 sizes 49 and coherent long-range packing of the crystals in films. 50 It has also been 4 attributed to the passivation of defects and a recudction in the trap state density. 14 5 Therefore, the passivation effect of Cs0.1FA0.9PbI3 perovskite with the PbCl2 (5%) 6 precursor was investigated by TA dynamics in the linear response range. Figure S9   7 shows that the GSB peak of the PbCl2-passivated perovskites was 784 nm, a small blue 8 shift from 790 nm. When the pump energy was below 3 nJ, the carrier dynamics was 9 not dependent on the pump intensity ( Figure S10). Carrier trapping and trap-assisted 10 recombination were highly dependent on material synthesis. 16 To exclude any 11 randomness, five pristine and five PbCl2 doped samples were tested together, and these 12 curves were consistent, as shown in Figure S11 and Table S1, and Figure 6 shows two In conclusion, we obtained ultrafast carrier dynamics of Cs0.1FA0.9PbI3 perovskite 8 thin films under different carrier densities. At a very low carrier density, the highly 9 sensitive TA spectrometer allowed us to determine the genuine carrier dynamics under 10 sunlight illumnaiton. According to the carrier dynamic curves in the linear response 11 range, we found that the fast trapping process occured in less than 1 ps, and defect-12 assisted recombination occurred in about a hundred nanosecond. We also extracted the 13 utilization percentages of the photogenerated carriers over the solar spectrum, which 14 were limited by carrier trapping and could be raised by PbCl2 passivation. We obtained 15 an estimation of the Shockley-Queisser limit, accounting for influences from carrier 16 trapping. These results not only provide a fundamental understanding of the intrinsic 1 photophysical behavior of perovskites under solar illumination conditions, but also 2 provide a suitable method for assessing the performance of perovskite solar cell 3 materials to expedite the process of selecting high-quality materials. Yamada, Y., Nakamura, T., Endo, M., Wakamiya, A., Kanemitsu, Y. Photocarrier recombination 2 dynamics in perovskite CH 3 NH 3 PbI 3 for solar cell applications. J. Am. Chem. Soc. 2014, 3 136 (