**3.2. Lead-free perovskite-structured materials**

The development of lead-free perovskite-structured absorbers for solar cells is an important step toward commercializing this technology. For the perovskite-structured solar cells, lead is a critical component in the archetypical FA1-xMAx PbI1-γBrγ material. However, the presence of lead has raised questions as to whether toxicology issues will become problematic in the future for widespread deployment of PSCs [11]. Among various metal, tin is seen as the most viable metal to replace lead in perovskite-structure materials. The H. J. Snaith group reported the first completely lead-free CH3 NH3 SnI<sup>3</sup> perovskite solar cell prepared on a mesoporous TiO2 scaffold with a PCE of 6.4% [12]. Although Sn shows excellent carrier mobility, the instability state of Sn2+ also makes it prone to become metallic in ambient air. Furthermore, the significant difference between Sn-based perovskite and Pb-based perovskite is the crystallizing condition; Sn-based perovskite crystallizes without heating and can suppress the formation of the uniform film. All of the aforementioned factors have become obstacles for fabricating high-efficiency lead-free PSCs.

Sn-based lead-free perovskite-structured materials still have some problems that make it lag behind lead-contained PSCs. For instance, the Sn vacancies with low formation energy lead to high-doped holes, which will cause severe carrier recombination in PSCs. Zhao et al. reported

**Figure 5.** Schematic diagram of a perovskite device and crystal structure based on FA1-xMAx PbX3 as an active layer.

FAx MA1-xSnI<sup>3</sup> served as the light harvesting layer with SnF2 employed as an additive in DMSO [13]. Mixing cations such as MA and FA is a conventional method in the composition engineering to improve the PSCs performance. The addition of additives to perovskite precursor solution can reduce the doped-hole density and enhance the stability of Sn-base PSCs. Due to the fast crystallization of MASnI3 , which makes it difficult to control the film morphology, DMSO is further introduced to the perovskite precursor solution. Adding DMSO is seen as a critical step because it will react with SnI<sup>2</sup> and form an intermediate phase, SnI<sup>2</sup> ·3DMSO, that suppresses the fast crystallization and thus obtains a homogeneous film. Up to date, the leadfree perovskite-structured material, FA0.75MA0.25SnI<sup>3</sup> , with SnF<sup>2</sup> as an additive can achieve a PCE of 8.12% as shown in **Figure 6**.

#### **3.3. Lead-reduced perovskite-structured materials**

important factors for high-efficiency PSCs. The S. I. Seok group devoted themselves to over-

demonstrated a solution-based process to deposit a uniform and dense perovskite layer. The adoption of γ-butyrolactone and dimethyl sulfoxide (DMSO) mixed solvent followed by tolu-

formation of a uniform layer and significantly enhances the PCE to 16.2% with no hysteresis [8]. They also developed a two-step process based on an intramolecular exchange between

To date, the S. I. Seok group have combined the contributions mentioned earlier and modified

ion by a two-step process can decrease the deep-level defects that are seen at the nonradiative

The development of lead-free perovskite-structured absorbers for solar cells is an important step toward commercializing this technology. For the perovskite-structured solar cells, lead

of lead has raised questions as to whether toxicology issues will become problematic in the future for widespread deployment of PSCs [11]. Among various metal, tin is seen as the most viable metal to replace lead in perovskite-structure materials. The H. J. Snaith group reported

 scaffold with a PCE of 6.4% [12]. Although Sn shows excellent carrier mobility, the instability state of Sn2+ also makes it prone to become metallic in ambient air. Furthermore, the significant difference between Sn-based perovskite and Pb-based perovskite is the crystallizing condition; Sn-based perovskite crystallizes without heating and can suppress the formation of the uniform film. All of the aforementioned factors have become obstacles for fabricating

Sn-based lead-free perovskite-structured materials still have some problems that make it lag behind lead-contained PSCs. For instance, the Sn vacancies with low formation energy lead to high-doped holes, which will cause severe carrier recombination in PSCs. Zhao et al. reported

NH3 SnI<sup>3</sup>

**Figure 5.** Schematic diagram of a perovskite device and crystal structure based on FA1-xMAx

NH3

I–PbI2

for efficient PSCs as shown in **Figure 5**. The addition of iodide

NH3 PbI<sup>3</sup>

and achieved a PCE of 12% [7]. Then, they

–DMSO, which leads to the


PbI1-γBrγ material. However, the presence

PbX3

as an active layer.

perovskite solar cell prepared on a mesoporous

coming these obstacles. They proposed a sandwich-type architecture consisting of CH3

perovskite as a light harvester on mesoporous TiO2

ene drop-casting will form an intermediate phase, CH3

organic cations and DMSO molecules to fabricate FAPbI3

recombination centers and improve the PCE to 22.1% [10].

**3.2. Lead-free perovskite-structured materials**

is a critical component in the archetypical FA1-xMAx

the content of iodide in FAPbI3

82 Solar Panels and Photovoltaic Materials

the first completely lead-free CH3

high-efficiency lead-free PSCs.

TiO2

Although lead-free perovskite-structured photovoltaic materials solve the toxicity issues, efficiency is sacrificed for the replacement of lead. Partial substitution of lead in perovskitestructured materials is the alternative solution which can not only reduce the toxicity but also maintain the power conversion efficiency. Many literature indicates that owing to the facile oxidation of Sn2+ to Sn4+, lead-free CH3 NH3 SnI<sup>3</sup> perovskite-structured material usually exhibits reduced efficiency and lack of reproducibility [14]. Therefore, many scientists attempted to prepare perovskite film with partial replacement of lead. The M. G. Kanatzidis group fabricated a perovskite material with 50% of Sn doping concentration (CH3 NH3 Sn0.5Pb0.5I<sup>3</sup> ), and found that doping Sn into the perovskite active layer can efficiently regulate the band gap of the perovskite material from 1.55 to 1.17 eV [15]. With the tunable band gap, it is also observed that light absorption extends to the near-infrared region. In addition, CH3 NH3 Sn0.5Pb0.5I<sup>3</sup> shows superior film coverage and better film morphology, which ensures connectivity between grains and overcomes short-circuiting and charge leaking issues.

**Figure 6.** Schematic diagram of a perovskite device and crystal structure based on FAx MA1-xSnX3 as an active layer.

The J. Navas group reported a theoretical study on partially replaced Pb2+ in CH3 NH3 PbI<sup>3</sup> with Sn2+, Sr2+, Cd2+, and Ca2+ [16]. By doping the dopant into the perovskite-structured material, the different crystal structure and band gap will further affect the optical properties. In addition, the change in band gap mainly depends on the crystal structure of the perovskitestructured material. Undoped perovskite-structured material in company with the dopant of Sn2+, Sr2+, and Cd2+ is present in the tetragonal crystalline structure. In the case of Ca2+ doped perovskite, the predominant crystalline phase is a cubic phase. The values of the band gap with different dopants is in the trend of Sr2+ (1.50 eV) < Cd2+ (1.54 eV) < CH3 NH3 PbI<sup>3</sup> (1.57 eV) ≈ Sn2+ (1.57 eV) for the tetragonal structure and Ca2+ (1.52 eV) < CH3 NH3 PbI<sup>3</sup> (1.57 eV) for the cubic structure. With the greater ionic nature of the dopant–iodine interaction, a lower band gap can be obtained. By understanding the characteristic of the dopant, the growth of highly lead-substituted PSCs and high-efficiency PSCs will become a breakthrough for the commercialization in the near future.

material based on (BA)2

(MA)n-1PbnI

3n + 1

is facilitated by the near-perfect vertical orientation of the {(MA)*<sup>n</sup>* <sup>−</sup> <sup>1</sup>

efficiency. Based on these results, the S. Liu group further doped Cs<sup>+</sup>

**Figure 8.** Schematic diagram of a perovskite device and crystal structure based on Cs<sup>+</sup>

layer.

ion, opens a new trend for optoelectronic devices. Its long organic side group with hydrophobic property seems to be beneficial for improving moisture stability. The inorganic layer of the 2D structure stack with each other by intercalated bulky alkylammonium cations and maintain structural integrity by weak van der Waals forces [18]. The band gap and the thickness of the 2D layer can be tuned by increasing the *n* values. For *n* = 1 structure, the 2D perovskite is a simple structure with the thinnest layer. At *n* = ∞, the structure becomes the typical 3D perovskite structure. An attempt to introduce 2D perovskite into the solid-state solar cell as a light-absorbing material showed poor efficiency (~4.0%) [19]. The poor device efficiency can be explained by the difficult charge transport in the out-of-plane direction. The long-chain organic cation between the conducting inorganic layers is similar to the insulating spacing layers. The H. Tsai group overcame this obstacle by fabricating near-single-crystalline thin films with the hot-casting process; they reported a PCE of 12.52% with no hysteresis [20]. It also shows outstanding light-soaking stability and moisture resistance that retains over 60% of its initial PCE after 2250 h irradiation without encapsulation. Furthermore, when the 2D PSCs is encapsulated, it shows no degradation after irradiation or under humidity. The significant breakthrough of power conversion efficiency for 2D PSCs is attributed to the enhanced charge mobility, which

perovskite layer fabricated using the hot-casting technique can form a high-quality film, which is ideal for a photovoltaic device. Compared with conventional spin-coating film, the hot-casting technique provides a uniform and substantially large grain size film that is much more reflective. The film also presents a low density of pinhole, which is profitable for improving

replace MA2+ (**Figure 8**) [21]. The Cs doping enhances the optoelectronic properties attributed to the improvement of charge transfer kinetics, charge carrier mobility, and decreased of trap density. With this modification, the PCE can be successfully improved from 12.52% to 13.7%.

, where BA is a long-chain aromatic alkylammonium cat-

Perovskite-Structured Photovoltaic Materials http://dx.doi.org/10.5772/intechopen.74997 85

Pb*<sup>n</sup>* I3*<sup>n</sup>* <sup>+</sup> <sup>1</sup> }

2− layer. The 2D

into a 2D perovskite to

doped 2D perovskite as an active

The M.-C. Wu group adopted four kinds of alkaline–earth metal cations, including Mg2+, Ca2+, Sr2+, and Ba2+, to replace lead cations partially [17]. Among the four alkaline-earth metals, Ba2+ is most suitable for Pb2+ replacement in perovskite films; it also exhibits high power conversion efficiency. The Ba2+-doped perovskite films that can be processed in the environment containing moisture (1.0% relative humidity) are stable. At the optimal 3.0 mol% Ba2+ replacement, the PCE of the fabricated solar cell is increased from 11.8 to 14.0%, and the PCE of champion devices is as high as 14.9% with increased storage stability (**Figure 7**).

#### **3.4. Two-dimensional perovskite-structured materials**

The three-dimensional (3D) perovskite-structured material shows outstanding power conversion efficiency owing to its tremendous advantage, including long carrier diffusion lengths for electrons and holes, small exciton binding energy, appropriate band gap, and high extinction coefficient. However, moisture instability results from the hygroscopic nature of MA and suppresses the commercialization of 3D PSCs. The two-dimensional (2D) perovskite-structured

**Figure 7.** Schematic diagram of a perovskite device and crystal structure based on CH3 NH3 Pb1-xBax X3 an active layer.

material based on (BA)2 (MA)n-1PbnI 3n + 1 , where BA is a long-chain aromatic alkylammonium cation, opens a new trend for optoelectronic devices. Its long organic side group with hydrophobic property seems to be beneficial for improving moisture stability. The inorganic layer of the 2D structure stack with each other by intercalated bulky alkylammonium cations and maintain structural integrity by weak van der Waals forces [18]. The band gap and the thickness of the 2D layer can be tuned by increasing the *n* values. For *n* = 1 structure, the 2D perovskite is a simple structure with the thinnest layer. At *n* = ∞, the structure becomes the typical 3D perovskite structure. An attempt to introduce 2D perovskite into the solid-state solar cell as a light-absorbing material showed poor efficiency (~4.0%) [19]. The poor device efficiency can be explained by the difficult charge transport in the out-of-plane direction. The long-chain organic cation between the conducting inorganic layers is similar to the insulating spacing layers. The H. Tsai group overcame this obstacle by fabricating near-single-crystalline thin films with the hot-casting process; they reported a PCE of 12.52% with no hysteresis [20]. It also shows outstanding light-soaking stability and moisture resistance that retains over 60% of its initial PCE after 2250 h irradiation without encapsulation. Furthermore, when the 2D PSCs is encapsulated, it shows no degradation after irradiation or under humidity. The significant breakthrough of power conversion efficiency for 2D PSCs is attributed to the enhanced charge mobility, which is facilitated by the near-perfect vertical orientation of the {(MA)*<sup>n</sup>* <sup>−</sup> <sup>1</sup> Pb*<sup>n</sup>* I3*<sup>n</sup>* <sup>+</sup> <sup>1</sup> } 2− layer. The 2D perovskite layer fabricated using the hot-casting technique can form a high-quality film, which is ideal for a photovoltaic device. Compared with conventional spin-coating film, the hot-casting technique provides a uniform and substantially large grain size film that is much more reflective. The film also presents a low density of pinhole, which is profitable for improving efficiency. Based on these results, the S. Liu group further doped Cs<sup>+</sup> into a 2D perovskite to replace MA2+ (**Figure 8**) [21]. The Cs doping enhances the optoelectronic properties attributed to the improvement of charge transfer kinetics, charge carrier mobility, and decreased of trap density. With this modification, the PCE can be successfully improved from 12.52% to 13.7%.

The J. Navas group reported a theoretical study on partially replaced Pb2+ in CH3

gap with different dopants is in the trend of Sr2+ (1.50 eV) < Cd2+ (1.54 eV) < CH3

(1.57 eV) ≈ Sn2+ (1.57 eV) for the tetragonal structure and Ca2+ (1.52 eV) < CH3

champion devices is as high as 14.9% with increased storage stability (**Figure 7**).

**3.4. Two-dimensional perovskite-structured materials**

**Figure 7.** Schematic diagram of a perovskite device and crystal structure based on CH3

commercialization in the near future.

84 Solar Panels and Photovoltaic Materials

with Sn2+, Sr2+, Cd2+, and Ca2+ [16]. By doping the dopant into the perovskite-structured material, the different crystal structure and band gap will further affect the optical properties. In addition, the change in band gap mainly depends on the crystal structure of the perovskitestructured material. Undoped perovskite-structured material in company with the dopant of Sn2+, Sr2+, and Cd2+ is present in the tetragonal crystalline structure. In the case of Ca2+ doped perovskite, the predominant crystalline phase is a cubic phase. The values of the band

for the cubic structure. With the greater ionic nature of the dopant–iodine interaction, a lower band gap can be obtained. By understanding the characteristic of the dopant, the growth of highly lead-substituted PSCs and high-efficiency PSCs will become a breakthrough for the

The M.-C. Wu group adopted four kinds of alkaline–earth metal cations, including Mg2+, Ca2+, Sr2+, and Ba2+, to replace lead cations partially [17]. Among the four alkaline-earth metals, Ba2+ is most suitable for Pb2+ replacement in perovskite films; it also exhibits high power conversion efficiency. The Ba2+-doped perovskite films that can be processed in the environment containing moisture (1.0% relative humidity) are stable. At the optimal 3.0 mol% Ba2+ replacement, the PCE of the fabricated solar cell is increased from 11.8 to 14.0%, and the PCE of

The three-dimensional (3D) perovskite-structured material shows outstanding power conversion efficiency owing to its tremendous advantage, including long carrier diffusion lengths for electrons and holes, small exciton binding energy, appropriate band gap, and high extinction coefficient. However, moisture instability results from the hygroscopic nature of MA and suppresses the commercialization of 3D PSCs. The two-dimensional (2D) perovskite-structured

NH3 PbI<sup>3</sup>

NH3 PbI<sup>3</sup>

(1.57 eV)

NH3 PbI<sup>3</sup>

NH3 Pb1-xBax X3

an active layer.

**Figure 8.** Schematic diagram of a perovskite device and crystal structure based on Cs<sup>+</sup> doped 2D perovskite as an active layer.
