**3.1 Vertical orientation**

*Solar Cells - Theory, Materials and Recent Advances*

organic cation similar to GA+

with high stability and PCE [37].

photovoltaic devices [40].

**2.2 Photophysical properties of 2D perovskite materials**

FA+

Cs+

without additives and any additional treatment [34]. The 2D perovskite formed by

layers, is called the ACI phase. Its chemical formula and crystal structure are shown in **Figure 2(c)** [35]. Due to its relatively small difference in ion size from MA+

, it has smaller exciton binding energy and weaker quantum confined effect. So, it is expected to achieve higher efficiency. The perovskite solar cells with BEA2+ ligand achieved high efficiency of 14.48 and 17.39% when doped with and without

 respectively [36]. Zhao's team achieved a high PCE of 18.48% by adding methyl ammonium chloride as an additive to effectively control the film quality of ACI 2D perovskite (GA)(MA)nPbnI3n + 1 (n = 3), showing great potential of ACI perovskite

Compared with 3D perovskites, 2D perovskite has a greater chemical and structural flexibility. The optical, electrical, and charge transfer properties can be regulated by controlling the width and composition of the potential well and barrier. In the 2D perovskite structure, a quantum well structure is formed between the insulating organic layer and the conductive inorganic layer, resulting in a quantum confinement effect [38]. The dielectric confinement effect is caused by the different dielectric constants of the potential well and the barrier, coupled with the quantum space limitation. The optical gap of 2D perovskites has a higher value than its 3D counterparts [39]. Zhang's work explored the inherent properties of 2D layered perovskites (PEA)2PbI4(N) and Cs2PbI4(N), and demonstrated that their structure and properties vary with N. The results reveal that both (PEA)2PbI4(N) and Cs2PbI4(N) are direct bandgap semiconductors. When N ≥ 3, their band/optical gap and exciton binding energy vary linearly by 1/N. This work shows that ultrathin 2D materials can become potential candidates for nano-optoelectronic devices, and nanoplates with N ≥ 3 can have similar properties to bulk materials in terms of carrier migration and exciton separation, so they can be effectively applied for

**Figure 3** shows the absorption and emission spectra of a series of ultra-thin (BA)2(MA)n-1PbnI3n + 1 (n = 1–5) crystal layers mechanically peeled off from the pure phase (fixed n) single crystal by Blancon et al. In the exfoliated crystal, as n decreases from 5 to 1 (quantum well thickness varies from 3.139 to 0.641 nm), the band edge absorption and emission peaks monotonically increase from 1.85 to 2.42 eV. Due to the quantum and dielectric confinement effect, exciton binding

*(Left) absorption and (right) PL Spectrum of the exfoliated (BA)2 (MA)n-1PbnI3n+1 crystals (from n = 1 to 5) [41]. Copyright 2017, American Association for the Advancement of Science (AAAS).*

, which can alternate interact with MA+

in the organic

and

**256**

**Figure 3.**

The precursor solution of 2D perovskite is prepared by mixing and dissolving ammonium salt of the organic cation spacers, ammonium salt of the A-site organic cations and metal halide according to a certain proportion (depending on n value). During the crystallization process, competition was confirmed to occur between the large organic cations and A-site organic cations [18]. To be more specific, the large organic spacers tend to form a low-dimensional perovskite while the A-site cations tend to form a 3D structure. The low-n phases are prone to have a horizontal orientation instead of vertical orientation, which is unfavorable to the charge transfer. Since the presence of insulating large-size organic cations will hinder the charge transport out-of-plane, Tsai, H. et al. used a hot-casting technology in 2016 to prepare high-quality films, which means that the substrate is preheated before spin-coating the perovskite precursor solution. With this method, a more beneficial crystal growth along (111) and (202) planes is observed instead of random orientation [42]. The application of the hot-casting method has promoted the efficiency of 2D PSCs and confirmed the importance of the crystal orientation perpendicular to the substrate for the performance improvement. At present, hot-casting method is widely used for better performance of the 2D PSCs. However, it is hard to keep temperature accurate and uniform when transferring the substrate from the hotplate to the spin-coater. To solve this problem, Li et.al partially replaced the BA+ with MA<sup>+</sup> in the BAMA quasi-2D perovskite, reducing the dependence on the preheating of the substrate. After being replaced, the quantum confinement effect of the perovskite film is weakened, the crystallization barrier is reduced, and higher quality perovskite film crystals and fewer defect states are obtained [43]. Beyond ion replacement, the morphology of the 2D DJ perovskite film with rigid piperidine ring was adjusted by MASCN additive at room temperature. By optimizing the amount of added MASCN, the perovskite film deposited on the substrate has good crystallinity, preferred orientation, reduced defects and better energy level alignment with the transport layer. The device had an inverted planar structure with a maximum PCE of 16.25%, which is the highest PCE of 2D DJ PVSCs without hot casting. After being exposed to air for 35 days, the unencapsulated device maintains about 80% of its initial efficiency (Hr 45 ± 5%). It provides a possiblely practical way for the development of high-performance 2D DJ PSCs [44].

Many other fabrication methods besides hot-casting have also been applied to obtain a vertical crystal orientation. Ke et al. used a two-step method of spincoating a stoichiometric precursor containing PEAI and PbI2, then performing FAI, and then dropping the solution and spin-coating it on the substrate to prepare 2D perovskite film [45]. Koh et al. also obtained preferential oriented growth of 2D perovskite perpendicular to the substrate through the immersion method, greatly improving the charge transport and extraction [46].

Adding additives was also proved to be an effective method to modify the crystal orientation. The role of additives in the 3D perovskite includes improving the morphology of the film, adjusting the energy level alignment, inhibiting the nonradiative recombination inside the film, eliminating the hysteresis of the device and so on [47–50]. Additives play the same roles in 2D PSCs. However, due to the different crystallization process, they have some other effects. Xinqian Zhang and his colleagues improved the PCE of (PEA)2(MA)n-1PbnI3n + 1 (n = 5) PSC from the initial 0.56% (without NH4SCN) to 11.01% through optimized NH4SCN addition. The performance improvement was attributed to the vertically oriented highly crystalline 2D perovskite film and balanced electron/hole transport [51]. NH4SCN additives was also proved to be a simple and effective method to induce the growth direction of 2D DJ phase perovskite crystals perpendicular to the substrate, and at the same time the phase distribution in the perovskite crystals can be concentrated near the phases of n = 3 and n = 4. The quasi 2D DJ phase (BDA)(MA))4Pb5I16 perovskite film based on NH4SCN treatment has a PCE of 14.53%. In addition, after storing under an environmental condition of 50 ± 5% humidity for 900 hours, the device retained 85% of its initial PCE [52]. Xu Zhang and his colleagues demonstrated in their work that a 2D BA2(MA)3Pb4I13 PSC doped with cesium cation (Cs+ ) had a PCE of up to 13.7%. The efficiency increased from 12.3% (without Cs+ ) to 13.7% (with 5% Cs+ ) due to perfectly controlled crystal orientation, increased grain size, excellent surface quality, reduced density of trap states, and enhanced carrier mobility [53].

### **3.2 Phase distribution**

The coexistence of 2D perovskite phase and 3D perovskite phase lead to a phase impurity inside the low-dimensional films. Though hot-casting method has become an effective way to improve the efficiency of 2D PSCs, the impurity and phase distribution in 2D perovskite films were ignored at the early stage of studying 2D PSCs. Jin's work also shows that multiple phases of n = 2, 3, 4, and n ≈ ∞ coexist in a 2D peovskite film with a nominal n value of 4. And they are naturally arranged perpendicular to the substrate. Through transient absorption spectroscopy analysis, they successfully observed continuous light-induced electron transfer from small n-phase to large n-phase driven by the band offset. And hole transfer in the opposite direction within hundreds of picoseconds was also observed. Exciton absorption peaks corresponding to different phases appear on the absorption spectrum, which is a strong evidence for the coexistence of mixed phases with different n values in the thin film. The strength of emission peaks corresponding to different n values was different when excited from the perovskite film side and the glass side, which confirmed the gradient distribution from small n value to large n value in the film [54]. Therefore, the mixed phases with multiple n values affect performance of 2D PSC from two aspects. One is phase purity, and the other is the arrangement of those phases. Besides gradient distribution in the vertical direction, other phase distributions may occur in low-dimensional perovskite films prepared by solution method. To realize high-performance low-dimensional perovskite cells, it is necessary to deliberately regulate the phase distribution inside the film.

In 2018, Liu's team used in situ time-resolved GIWAXS technology to track the transition process from precursor solution to solid film under different conditions of substrate temperatures and different solvents of precursor solutions. The results reveal that under lower temperature the intermediate phase formed by lead

**259**

*2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells*

iodide crystal and solvate complexes can cause multiple RP phases with random grain orientations. It is better for the disordered solvate to transform to perovskite directly [55]. The mixed phases in the low-dimensional perovskite thin film crystals are prone to energy transfer, resulting in large Voc loss. To get a more vertically phase distribution, later Liu's team transferred the device to a hot plate at a suitable temperature for several hours after the 2D perovskite device was prepared, which is called a slow post-annealing (SPA) method. Such a device obtains an open circuit voltage as high as 1.24 V, which proves that the quantum well effect in the perovskite film is reduced to greatly improve the charge transfer and extraction efficiency in the device. They compared the phase distribution of the SPA films with the film prepared by the hot casting method and at room temperature respectively. Inside the thin film prepared at room temperature, the phases with different n values are randomly distributed. There is a sudden and uneven phase gradient inside the thin film prepared by the hot-casting method, resulting in the presence of 2D phase and 3D phase at the bottom and top respectively. After SPA treatment, the phase distribution inside the thin film appears more orderly [56]. Tiefeng Liu and his colleagues reported that the phase distribution of different n values in a 2D perovskite film deposited on a hole transport layer is different from that on a glass substrate. Due to the colloidal characteristics of the perovskite precursor, the vertical distribution can be explained by the sedimentation equilibrium. The addition of acid changes the precursor from colloid to solution, thereby changing the phase distribution. The self-assembled layer was used to modify the acidic surface properties of the hole transport layer, which can cause the vertical distribution required for charge transport. The surface-modified 2D PSC had a higher open circuit voltage and a

Zhou and his colleagues proved that by controlling the crystal growth direction and growth rate, the phase distribution and carrier transport of quasi-2D perovskite films can be controlled. They found that using ethyl acetate as an anti-solvent can change the growth direction of quasi-2D perovskites by accelerating the formation of surface crystals. In addition, through the introduction of MACl and DMSO in the preparation process, the film with the phases of n = 3 and n = 4 was successfully obtained. With the addition of MACl and DMSO in the precursor solution, an intermediate phase is formed, which slows down the rate of crystallization in the solution. In addition, by correlating the phase distribution with the device characteristics, it was shown that the performance of the solar cell is sensitive to the phase

Zhang Jia et.al confirmed that the phase distribution obtained in the film prepared by the vacuum-poling method is different from the traditional films that the phases with n value from small to large are arranged in a gradient distribution from the bottom to the top. The research result showed that the phases with different n values show a uniform distribution inside the film. The uniform phase distribution was confirmed by the PL results. When excited from the perovskite film side and from the glass side, and after the film is peeled off by the tape, the measured PL spectrum showed negligible differences, proving the uniform distribution of the mixed phase. In this case, the excellent PCE up to 18% can be ascribed to the short-

The high stability and relatively poor capability to convert light to electricity of low-dimensional perovskite films are the two aspects that need to be balanced. Based on the above analysis, we conclude that crystal growth direction and phase

*DOI: http://dx.doi.org/10.5772/intechopen.93725*

higher efficiency than the control device [57].

purity and phase distribution [58].

est charge transfer path [59].

**4. Conclusions**

#### *2D Organic-Inorganic Hybrid Perovskite Light-Absorbing Layer in Solar Cells DOI: http://dx.doi.org/10.5772/intechopen.93725*

*Solar Cells - Theory, Materials and Recent Advances*

improving the charge transport and extraction [46].

that a 2D BA2(MA)3Pb4I13 PSC doped with cesium cation (Cs+

13.7%. The efficiency increased from 12.3% (without Cs+

**3.2 Phase distribution**

and then dropping the solution and spin-coating it on the substrate to prepare 2D perovskite film [45]. Koh et al. also obtained preferential oriented growth of 2D perovskite perpendicular to the substrate through the immersion method, greatly

orientation. The role of additives in the 3D perovskite includes improving the morphology of the film, adjusting the energy level alignment, inhibiting the nonradiative recombination inside the film, eliminating the hysteresis of the device and so on [47–50]. Additives play the same roles in 2D PSCs. However, due to the different crystallization process, they have some other effects. Xinqian Zhang and his colleagues improved the PCE of (PEA)2(MA)n-1PbnI3n + 1 (n = 5) PSC from the initial 0.56% (without NH4SCN) to 11.01% through optimized NH4SCN addition. The performance improvement was attributed to the vertically oriented highly crystalline 2D perovskite film and balanced electron/hole transport [51]. NH4SCN additives was also proved to be a simple and effective method to induce the growth direction of 2D DJ phase perovskite crystals perpendicular to the substrate, and at the same time the phase distribution in the perovskite crystals can be concentrated near the phases of n = 3 and n = 4. The quasi 2D DJ phase (BDA)(MA))4Pb5I16 perovskite film based on NH4SCN treatment has a PCE of 14.53%. In addition, after storing under an environmental condition of 50 ± 5% humidity for 900 hours, the device retained 85% of its initial PCE [52]. Xu Zhang and his colleagues demonstrated in their work

Adding additives was also proved to be an effective method to modify the crystal

due to perfectly controlled crystal orientation, increased grain size, excellent surface

The coexistence of 2D perovskite phase and 3D perovskite phase lead to a phase impurity inside the low-dimensional films. Though hot-casting method has become an effective way to improve the efficiency of 2D PSCs, the impurity and phase distribution in 2D perovskite films were ignored at the early stage of studying 2D PSCs. Jin's work also shows that multiple phases of n = 2, 3, 4, and n ≈ ∞ coexist in a 2D peovskite film with a nominal n value of 4. And they are naturally arranged perpendicular to the substrate. Through transient absorption spectroscopy analysis, they successfully observed continuous light-induced electron transfer from small n-phase to large n-phase driven by the band offset. And hole transfer in the opposite direction within hundreds of picoseconds was also observed. Exciton absorption peaks corresponding to different phases appear on the absorption spectrum, which is a strong evidence for the coexistence of mixed phases with different n values in the thin film. The strength of emission peaks corresponding to different n values was different when excited from the perovskite film side and the glass side, which confirmed the gradient distribution from small n value to large n value in the film [54]. Therefore, the mixed phases with multiple n values affect performance of 2D PSC from two aspects. One is phase purity, and the other is the arrangement of those phases. Besides gradient distribution in the vertical direction, other phase distributions may occur in low-dimensional perovskite films prepared by solution method. To realize high-performance low-dimensional perovskite cells, it is

quality, reduced density of trap states, and enhanced carrier mobility [53].

necessary to deliberately regulate the phase distribution inside the film.

In 2018, Liu's team used in situ time-resolved GIWAXS technology to track the transition process from precursor solution to solid film under different conditions of substrate temperatures and different solvents of precursor solutions. The results reveal that under lower temperature the intermediate phase formed by lead

) had a PCE of up to

)

) to 13.7% (with 5% Cs+

**258**

iodide crystal and solvate complexes can cause multiple RP phases with random grain orientations. It is better for the disordered solvate to transform to perovskite directly [55]. The mixed phases in the low-dimensional perovskite thin film crystals are prone to energy transfer, resulting in large Voc loss. To get a more vertically phase distribution, later Liu's team transferred the device to a hot plate at a suitable temperature for several hours after the 2D perovskite device was prepared, which is called a slow post-annealing (SPA) method. Such a device obtains an open circuit voltage as high as 1.24 V, which proves that the quantum well effect in the perovskite film is reduced to greatly improve the charge transfer and extraction efficiency in the device. They compared the phase distribution of the SPA films with the film prepared by the hot casting method and at room temperature respectively. Inside the thin film prepared at room temperature, the phases with different n values are randomly distributed. There is a sudden and uneven phase gradient inside the thin film prepared by the hot-casting method, resulting in the presence of 2D phase and 3D phase at the bottom and top respectively. After SPA treatment, the phase distribution inside the thin film appears more orderly [56]. Tiefeng Liu and his colleagues reported that the phase distribution of different n values in a 2D perovskite film deposited on a hole transport layer is different from that on a glass substrate. Due to the colloidal characteristics of the perovskite precursor, the vertical distribution can be explained by the sedimentation equilibrium. The addition of acid changes the precursor from colloid to solution, thereby changing the phase distribution. The self-assembled layer was used to modify the acidic surface properties of the hole transport layer, which can cause the vertical distribution required for charge transport. The surface-modified 2D PSC had a higher open circuit voltage and a higher efficiency than the control device [57].

Zhou and his colleagues proved that by controlling the crystal growth direction and growth rate, the phase distribution and carrier transport of quasi-2D perovskite films can be controlled. They found that using ethyl acetate as an anti-solvent can change the growth direction of quasi-2D perovskites by accelerating the formation of surface crystals. In addition, through the introduction of MACl and DMSO in the preparation process, the film with the phases of n = 3 and n = 4 was successfully obtained. With the addition of MACl and DMSO in the precursor solution, an intermediate phase is formed, which slows down the rate of crystallization in the solution. In addition, by correlating the phase distribution with the device characteristics, it was shown that the performance of the solar cell is sensitive to the phase purity and phase distribution [58].

Zhang Jia et.al confirmed that the phase distribution obtained in the film prepared by the vacuum-poling method is different from the traditional films that the phases with n value from small to large are arranged in a gradient distribution from the bottom to the top. The research result showed that the phases with different n values show a uniform distribution inside the film. The uniform phase distribution was confirmed by the PL results. When excited from the perovskite film side and from the glass side, and after the film is peeled off by the tape, the measured PL spectrum showed negligible differences, proving the uniform distribution of the mixed phase. In this case, the excellent PCE up to 18% can be ascribed to the shortest charge transfer path [59].
