Photoexcited carrier lifetimes, hole injection and electron injection dynamics as well as charge recombination at each interface in MASn0.5Pb0.5I3 perovskite solar cells. 
Due to the unique characteristics such as simple low-temperature preparation method and high efficiency with a record of over 20%, organometal trihalide perovskite (CH3NH3PbI3)-based solid-state hybrid solar cells have attracted an increasing interest since 2012 when it was reported. During the last several years, some of the fundamental photophysical properties of perovskite related to the high photovoltaic performance have been investigated. Optical absorption, charge separation and recombination are very important factors influencing the perovskite solar cell performance. In this chapter, our recent results of optical absorption, charge separation (electron and hole injection) and charge recombination dynamics at each interface in perovskite solar cells, and their relationships to photovoltaic properties will be introduced. Our results suggest that charge recombination is a key factor in improving the performance of the perovskite solar cells.
- Urbach energy
- charge separation
- charge recombination
- optical absorption
- Sn/Pb cocktail perovskite
Organolead halide perovskites in the format of AMX3 (A=organic molecule, e.g., CH3NH3(MA), B=Pb, X=Cl, Br, and I) can be easily crystallized from solution at relatively low temperature (i.e., ≤100 °C), which makes it possible to use them as light absorbing materials in different kinds of solar cells. Following the recently reported certified high power conversion efficiency (PCE) of over 20%, [1-12] the interest in organolead halide perovskite-based organic-inorganic hybrid solid-state solar cells has increased dramatically over the past several years. The higher PCEs of organolead halide perovskites (especially MAPbI3) result from their unique properties that are key for achieving high photovoltaic performance, which are (1) a direct band gap and a high optical absorption coefficient; [13, 14] (2) large dielectric coefficient leading to smaller exciton binding energy;  (3) long photoexcited carrier lifetimes (>100 ns) and long diffusion lengths (100 – 1000 nm or even longer); [16, 17] (4) no deep state defects and very small Urbach energy. 
It is reasonable to expect that further improvements in photovoltaic performance can be achieved by increasing the light harvesting up to NIR wavelengths of 1000 nm, since MAPbI3 perovskite only absorbs light at wavelengths below 800 nm constrained by its optical band gap of 1.5 eV. In addition, practically, Pb-free organometal halide perovskites are preferred due to the potential toxicology issue of Pb. Replacing Pb with Sn or mixing Pb and Sn in organometal halide perovskites can result in increased light harvesting in the NIR region up to 1000 nm [19, 20] and, at the same time, reduce the toxicity issue related to Pb. Several research groups have reported, very recently, Sn-based or Pb/Sn cocktail MASnxPb1-xI3 (0≦x≦1) perovskite solar cells. [21-24] However, it is found that the PCE of Sn/Pb cocktail MASnxPb1-xI3 perovskite solar cells is far inferior to that of MAPbI3 perovskite solar cells.
In order to improve the photovoltaic performance of Pb and Sn/Pb cocktail perovskite solar cells, it is critical to gain a thorough understanding of the optical absorption properties, the photoexcited carrier lifetimes, as well as the charge separation and recombination dynamics at each interface. In this chapter, we will focus on recent studies of the optical absorption, photoexcited carrier lifetime, the charge separation and charge recombination dynamics at each interface in perovskite solar cells, including Pb-based and Sn/Pb cocktail perovskite solar cells. The relationships of each of these physical properties to the photovoltaic performance of the solar cells will be discussed and the methodologies for improving the photovoltaic performance of perovskite solar cells will be proposed.
2.1. Sample preparation
Samples of Pb-based perovskite hybrid solar cells were prepared by the following method.  F-doped SnO2 layered glass (FTO glass, Nippon Sheet Glass Co. Ltd) was patterned using Zn powder and 6 N HCl aqueous solution. Titanium diisopropoxide bis(acetylacetonate) solution in ethanol was sprayed onto this patterned FTO glass at 300 °C to prepare compact TiO2 layers. Porous TiO2 layers were fabricated by spin-coating TiO2 pastes of different nanoparticle sizes (18 nm: PST-18NR or 30 nm: PST-30NRD, JGC Catalysts and Chemicals Ltd.) in ethanol (TiO2 paste : ethanol = 1:2.5 weight ratio for PST-18NR or TiO2 paste : ethanol = 2:7 weight ratio for PST-30NRD), followed by heating the substrates at 550 °C for 30 min. For some TA measurements, glass, instead of FTO was used as the substrate and a porous Y2O3 layer was fabricated on the glass substrates. Next, CH3NH3I and PbCl2 were mixed with a 3:1 molar ratio for preparing a 40 % solution of perovskite in N,N-dimethylformamide and the mixture was spin-coated on the TiO2 and Y2O3 porous substrates. After heating at 100 °C for 45 minutes, the substrates were spin-coated with a mixture of 55 mM of tert-butylpyridine, 9 mM of lithium bis(trifluoromethylsyfonyl)imide salt, and 68 mM of
The Sn/Pb cocktail perovskite samples were prepared using the following method.  PbI2 (Purity: 99.999 %), SnI2, and regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) were purchased from Sigma Aldrich and used as received. F-doped SnO2 coated glass ((FTO glass), Nippon Sheet Glass Co. Ltd) was patterned using Zn and a 6 N HCl aqueous solution. A compact TiO2 layer was produced by spraying titanium diisopropoxide bis(acetylacetonate) solution in ethanol onto this patterned glass at 300 °C. The substrate was then dipped in a 40 mM solution of TiCl4 in water for 30 min. Then the substrate was baked at 500 °C for 20 min. Next, a porous TiO2 layer was produced by spin-coating with a TiO2 paste (PST-18NR, JGC Catalysts and Chemicals Ltd.) in ethanol (TiO2 paste : ethanol = 2:7 weight ratio). The substrate was then heated at 550 °C for 30 min. For certain TA measurements, glass substrates other than FTO ones, with a porous Al2O3 layer deposited on them, were used. The substrates were then spin-coated with a mixture of SnI2, PbI2, and CH3NH3I (0.5 : 0.5 : 1.0 (molar ratio)) in dimethylformamide (DMF) (40 wt %), and they were baked at 70 °C for 30 min. Next, P3HT in chlorobenzene solution (15 mg/ml) was spin-coated on the prepared perovskite layer and the substrate was put in nitrogen at ambient temperature for 1 h. For conducting TA measurements, samples of Sn/Pb cocktail perovskite deposited on both Al2O3 and TiO2 substrates with and without P3HT as a hole transport material (HTM) were employed. Ag and Au electrodes were finally deposited by vacuum deposition for characterizing the photovoltaic performance, which was evaluated using an AM 1.5G 100 mW/cm2 irradiance solar simulator (CEP-2000SRR, Bunkoukeiki Inc) with a 0.4 cm x 0.4 cm mask.
2.2. Characterization methods
2.2.1. Optical absorption measurements
A gas-microphone photoacoustic (PA) technique  was used to study the optical absorption properties of the samples. The light source was a 300 W xenon arc lamp. By passing the light through a monochromator, a monochromatic light beam was obtained. This beam was modulated with a mechanical chopper and focused onto the surface of the sample placed in a sealed PA cell filled with N2 gas. The measurements of PA spectrum were carried out in the wavelength range of 500–1200 nm with a modulation frequency of 33 Hz at room temperature. The PA signal was measured by first passing the microphone output through a preamplifier and then to a lock-in amplifier. The PA spectra were normalized with the PA spectrum obtained from a carbon black sheet.
2.2.2. Transient Absorption (TA) Measurements
Two different TA setups were used: the charge separation (electron injection and hole injection) dynamics was characterized by a femtosecond TA technique (fs-TA) [27, 28] and charge recombination dynamics was characterized by a nanosecond TA technique (ns-TA) [28-31]. In the fs-TA setup for characterizing the charge separation, [27, 28, 31] the laser source used was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The light was separated into two parts. One part was used as a probe pulse. The other part was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm. This was used as a pump light to excite the sample. In this study, a pump light wavelength of 470 nm and a probe beam wavelength of 775 nm were used. Deschler and co-workers  reported very recently that the TA response of ground state bleaching (GSB) for CH3NH3PbClI2 on glass peaked at 1.65 eV and was spectrally unchanged up to times beyond 200 ns. Therefore, the probe light wavelength of 775 nm (i.e., the photon energy of 1.64 eV) used in this study is appropriate to monitor the GSB of CH3NH3PbClI2 on Y2O3 and TiO2 substrates with or without
In the ns-TA setup for characterizing the charge recombination [28-31], the pump light source was an OPO (Surelite II – 10FP) output excited by a Nd:YAG nanosecond pulse laser (Panther, Continuum, Electro-Optics Inc.), with a pulse width of 5 ns and a repetition rate of 0.5 Hz. A pulse light with a wavelength of 470 nm was used as the pump light to excite the sample. The probe light was produced from a fiber coupled CW semiconductor. Three different probe wavelengths, of 785 nm, 658 nm and 1310 nm, were used. Specifically, the probe beam of 785 nm was employed to measure the TA responses of GSB for MAPbClI2 on Y2O3 substrates, i.e., the recombination of electrons and holes in MAPbClI2.  The probe beam of 658 nm was used to measure the trapped electrons in TiO2  based on the research of Yoshihara and co-workers, which was used to investigate charge recombination between the electrons in TiO2 and the holes in the perovskite. The probe beam of 1310 nm was used to monitor the holes in
3. Optical Absorption Study: Bandgap and Urbach Energy of Pb and Sn/Pb cocktail perovskite
Figure 1 shows the optical absorption spectra of the Pb (MAPbI2) and Sn/Pb cocktail (MASn0.5Pb0.5I3) perovskite samples on a porous TiO2 substrate measured using the PA technique (we refer to this as the PA spectrum in the following) at room temperature. From the position of the shoulder in each PA spectrum,  the bandgap energies of MAPbI2 and MASn0.5Pb0.5I3 are determined to be 1.52 and 1.21 eV, respectively, which are almost the same as those given in our earlier reports. [23, 25] We find that, below the shoulder, the trend of the absorption coefficient is exponential. The slope of this exponential tail, known as the Urbach tail, corresponds to the absorption tail states, and is usually quantitatively expressed by the Urbach Energy
4. Dynamics of Photoexcited Carrier Recombination and Charge Transfer in Pb-based Perovskite Solar Cells and their Relationships to the Photovoltaic Properties [25, 39, 40]
To perform systematic investigations on the charge separation and recombination dynamics in perovskite solar cells, TA measurements were conducted for MAPbI3 on either Y2O3 or TiO2 substrates, with and without a
The TA response of MAPbI3/Y2O3 with
Figure 4 shows the TA response of MAPbI3/Y2O3 with a layer of
The photoexcited electron injection and the recombination dynamics were measured using the TA techniques for MAPbI3 deposited on TiO2 substrates. Figure 6(a) shows the normalized TA responses of MAPbI3/TiO2 for 400 ps measured with different pump light intensities. The decay processes in the normalized TA responses overlapped very well with each other when the pump light intensity was lower. However, when the pump light intensity became larger than 3.75 μJ/cm2, a faster decay process, resulting from Auger recombination, appeared in the TA response.  To determine the electron injection dynamics from MAPbI3 to TiO2, the TA response of MAPbI3/TiO2 measured under a lower pump light intensity (0.9 μJ/cm2) was used to make a comparison with that of MAPbI3/Y2O3 as shown in Figure 6(b). The TA response of MAPbI3/TiO2 decayed much faster than that of MAPbI3/Y2O3 and can be fitted to a single exponential decay function very well, with a time constant determined to be 1.8±0.1 ns, which is approximately 2 - 3 orders smaller than the photoexcited charge carrier lifetimes (∼μs) of MAPbI3 as shown above. Then the electron injection time
Next, the recombination dynamics in MAPbI3/TiO2 without the layer of
Then, the recombination dynamics in MAPbI3/TiO2 with
Then another kind of cell with 30 nm sized TiO2 nanoparticles were also studied, which we call cell B. The energy conversion efficiency of cell B was typically 8-10%. It was found that the recombination dynamics in the Perovskite solar cells has a great influence on the IPCE, Jsc and the energy conversion efficiency.  Two kinds of TiO2/MAPbI3/
5. Dynamics of Photoexcited Carrier Recombination and Charge Transfer in Sn/Pb Cocktail Perovskite Solar Cells and their Relationships to the Photovoltaic Properties 
To find the mechanism responsible for the low PCE, especially the low
Secondly, the photoexcited charge separation and recombination dynamics of Sn/Pb cocktail perovskite deposited on a TiO2 substrate with P3HT as a hole transport material were evaluated, which is shown in Figure 12. The charge separation at the perovskite/TiO2 interface occurs in as fast as 1 ps. It is worth noting that the recombination time at the Sn/Pb cocktail perovskite/TiO2 interface is 880 µs, which is about two to three orders of magnitude slower compared to that occurring at the MAPbI3/TiO2 interface as shown in Figure 10. On the other hand, the recombination time at the TiO2/P3HT interface is 190 µs, which is much faster compared to that without P3HT. This indicates that the recombination of electrons in the TiO2 becomes faster when P3HT is used, which originates from the recombination at the TiO2/P3HT interface. This recombination is one of the main reasons for the lower
It is important to understand how the optical absorption, charge separation and recombination dynamics relate to the incident photon to current conversion efficiency (IPCE) spectrum and the current-voltage (I-V) characteristics of the solar cell. Figure 13 shows a typical IPCE spectrum of a Sn/Pb cocktail perovskite solar cell. Based on the photoexcited carrier relaxation dynamics of Sn/Pb perovskite on an Al2O3 substrate, and the charge transfer dynamics at TiO2/perovskite/P3HT interfaces, which are summarized in Table 1, it is possible to calculate the charge separation efficiency.  First, it is determined that the hole injection efficiency
|N/A||Hole injection||Electron injection||Electron/hole injections|
|1 ps||1 ps||1 ps/1 ps|
|4 ps (55%)|
650 ps (25%)
>>3 ns (20%
|N/A||16 ps||880 μs||190 μs|
As shown in Figure 13(b),
In summary, by conducting a systematic study on the optical absorption, photoexcited charge carrier lifetimes, and charge separation and charge recombination dynamics, we have explored the ways through which the photovoltaic performance of Pb and Sn/Pb cocktail perovskite solar cells can be improved.
For MAPbI3 solar cells, we find that the great differences in the IPCE and
For the Sn/Pb cocktail perovskite solar cells, it is determined that the bandgap is 1.21 eV and light harvesting can be extended to a wider wavelength over 1000 nm. The Urbach energy is calculated to be 34 meV, which is more than twice that of the MAPbI3 perovskite. Three recombination processes for the photoexcited carriers were found with lifetimes being 4 ps (55%), 650 ps (25%) and one much larger than 3 ns (20%). The larger Urbach energy and the faster recombination suggest that there are defects in the prepared Sn/Pb cocktail perovskite. These results are significantly different from those of the MAPbI3 perovskite, in which case the photocarrier lifetimes are larger than 100 ns and almost no defects were observed. Moreover, the charge separation and charge recombination dynamics were explored. We find that both the electron injection into TiO2 and hole transfer to the P3HT layer occurred on a timescale of 1 ps. It was surprising to find that the charge recombination lifetime at the Sn/Pb cocktail perovskite/TiO2 interface was as long as 880 µs, which is 2-3 orders of magnitude greater than that occurring at the MAPbI3/TiO2 interface. However, the charge recombination lifetime became shorter, i.e., 190 µs, when P3HT was employed as a HTM. This implies that the charges can be more effectively collected without the HTM. On the basis of the above results, we find that the loss in the charge separation efficiency originates from fast photoexcited carrier recombination with a lifetime of 4 ps, and the loss in charge collection efficiency was due to the charge recombination occurring at the TiO2/P3HT interface, thus leading to lower IPCE values. Also, the low Voc and FF were found to result from the large Urbach energy and the recombination occurring at the TiO2/P3HT interface. These findings indicate that the photovoltaic performance of Sn/Pb cocktail perovskite solar cells can be further improved by reducing the defects in the material and the recombination occurring at the TiO2/HTM interface through proper interfacial engineering.
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