**4. Current issues and future challenges**

To date, perovskite-structured photovoltaic devices have presented considerable progress to achieve high efficiency. However, many issues have to be overcome before the commercialization. The fast degradation phenomenon under humidity or constant irradiation bears the brunt. The other crucial factor is the presence of J − V hysteresis. With the existence of the hysteresis phenomena, the accuracy of power conversion efficiency will fall under suspicion. The fundamental mechanisms of the efficient carrier transport in perovskite should also be considered. The complication of the synthesis methods and the construction of PSCs make it much more difficult to overcome these obstacles. Recently, many researchers have devoted lots of effort to address these issues and adapt the properties of corresponding materials.

#### **4.1. Instability under humidity**

The stability of PSCs is dominated by lots of factors, such as high humidity, intense illumination, and increased temperature [22]. Exposure to such a severe condition leads to the rapid degradation of efficiency and restricts PSCs from outdoor application. Many studies have revealed that when the MAPbI3 is exposed to moisture, it will immediately degrade into methylammonium (MA), hydrogen iodide (HI), and lead iodide (PbI2 ) owing to its solubility in water [23]. Hence, how to effectively prevent the invasion of moisture in the environment should be considered thoroughly. Prior to developing the encapsulations material, it is important to understand what happens during the degradation. CH3 NH3 PbI<sup>3</sup> taken as an example of the degradation mechanism of perovskite is shown as follows:

$$\text{CH}\_3\text{NH}\_3\text{PbI}\_{3\text{(s)}} \leftrightarrow \text{PbI}\_{2\text{(s)}} \star \text{CH}\_3\text{NH}\_3\text{I}\_{\text{(aq)}}\tag{4}$$

$$\text{CH}\_3\text{NH}\_3\text{I}\_{\text{(aq)}} \leftrightarrow \text{CH}\_3\text{NH}\_{2\text{(aq)}} \star \text{HI}\_{\text{(aq)}}\tag{5}$$

$$\text{4H}\_{\text{(aq)}} \star \text{O}\_{\text{(g)}} \leftrightarrow \text{2I}\_{\text{(aq)}} \star \text{H}\_{\text{2}} \text{O}\_{\text{(l)}} \tag{6}$$

$$\text{HI}\_{\text{(aq)}} \xleftarrow{\text{hv}} \text{H}\_{\text{2}\_{\text{(6)}}} + \text{I}\_{\text{2(s)}}\tag{7}$$

conducive to PSCs. In the case of inorganic material, it provides a barrier toward permeant penetration. Defect-free inorganic thin films as encapsulant material show the characteristic of brittleness, which will not be break resistant to moisture conditions [25]. Therefore, constructing a material consisting of both organic and inorganic materials can combine both

Perovskite-Structured Photovoltaic Materials http://dx.doi.org/10.5772/intechopen.74997 87

In the characterization of photovoltaic performance, a hysteretic response appears with different scanning conditions; thus the accuracy of the cell efficiency is doubted. The photocurrent density-voltage (J-V) hysteresis behavior will be affected by scanning direction, scanning rate, and applied voltage [26]. The hysteresis J-V behavior remains a challenge for the advancement of PSCs. Here are some situations that result in erroneous device efficiencies. For operating in different scanning directions, the forward scan that holds at short-circuit conditions to open circuit voltage conditions tends to have a lower efficiency than holding at the reverse scan. Hence, the forward scan is inclined to underestimate the power conversion efficiency of PSCs. In contrast, the reverse scan tends to overestimate the power conversion efficiency. If the scan rate of measuring the device is faster than its response time, hysteretic behavior will also be deteriorated. While holding at the reverse scan, the efficiency increases with the increasing scan rate. On the other hand, the efficiency measured at the forward scan decreases with the increasing scan rate. Moreover, the J-V hysteresis phenomenon becomes more intense under the faster scan rate in both scanning directions. Another important factor that can also affect the J-V hysteresis is the starting biases. The tendency is similar to the previous condition. The more positive the starting voltage holding at the reverse scan, the higher the efficiency can be measured; whereas, while holding at the forward scan, the more negative the starting voltage, the lower the efficiency of PSCs can be recorded. The existence of J - V hysteresis plays a crucial role in the accuracy of power conversion efficiency; hence, many scientists have dedicated research to searching for the solution. One of the possible methods of J - V hysteresis elimination is to measure the device with a slow scan rate. That hysteresis influenced by applied voltage can be solved by light-soaking at different biases was reported by the M. D. McGehee group [27]. Prior to the characterization, the precondition for perovskite-absorber devices with different biases presents a significant effect on the photovoltaic performance. The short-circuit current density, fill factor, and power conversion efficiency under reverse scan can be suppressed by light-soaking with a more negative bias. In contrast, the photovoltaic performance under the forward scan will be enhanced by lightsoaking with a more positive bias. Hence, the declined performance under the reverse scan and the increased performance under the forward scan prevent PSCs from the J-V hysteresis

Although a perovskite-structured active layer presents high efficiency of light-harvesting behavior, finding the suitable electron transport layer (ETL) and hole transport layer (HTL) is a critical issue for improving the performance of the photovoltaic device. After solar irradiation, an electron-hole pair will be generated at the perovskite-structured active layer. How to

advantages and meet the flexibility and gas barrier conditions.

**4.2. J-V hysteresis phenomena**

phenomenon.

**4.3. Carrier transport behavior**

First, the CH3 NH3 PbI<sup>3</sup> will degrade into PbI<sup>2</sup> and CH3 NH3 I owing to the sensitivity to water. The formation of CH3 NH3 I results in the co-existence of CH3 NH3 I, CH3 NH2 , and HI toward the progress of equilibrium reaction. After that, the HI tends to decompose in two ways, redox reaction and photochemical reaction. Based on the aforementioned equilibrium reactions, it is easy to understand that not only the moisture in the environment but also the oxygen and UV radiation affect the stability severely [24]. The design of the encapsulant should consider the permeation mechanisms of the organic and inorganic materials. For an organic polymer encapsulant, gas or vapor permeates into the PSCs by the typical solution—the diffusion model. Although an organic polymer encapsulant possesses lots of advantages including flexibility, conformability, and processability, the permeability of gas is too high, which is not conducive to PSCs. In the case of inorganic material, it provides a barrier toward permeant penetration. Defect-free inorganic thin films as encapsulant material show the characteristic of brittleness, which will not be break resistant to moisture conditions [25]. Therefore, constructing a material consisting of both organic and inorganic materials can combine both advantages and meet the flexibility and gas barrier conditions.

#### **4.2. J-V hysteresis phenomena**

**4. Current issues and future challenges**

**4.1. Instability under humidity**

86 Solar Panels and Photovoltaic Materials

have revealed that when the MAPbI3

CH3 NH3 I

4 HI(aq)

NH3 PbI<sup>3</sup>

First, the CH3

The formation of CH3

HI(aq)

NH3

To date, perovskite-structured photovoltaic devices have presented considerable progress to achieve high efficiency. However, many issues have to be overcome before the commercialization. The fast degradation phenomenon under humidity or constant irradiation bears the brunt. The other crucial factor is the presence of J − V hysteresis. With the existence of the hysteresis phenomena, the accuracy of power conversion efficiency will fall under suspicion. The fundamental mechanisms of the efficient carrier transport in perovskite should also be considered. The complication of the synthesis methods and the construction of PSCs make it much more difficult to overcome these obstacles. Recently, many researchers have devoted lots of effort to address these issues and adapt the properties of corresponding materials.

The stability of PSCs is dominated by lots of factors, such as high humidity, intense illumination, and increased temperature [22]. Exposure to such a severe condition leads to the rapid degradation of efficiency and restricts PSCs from outdoor application. Many studies

ity in water [23]. Hence, how to effectively prevent the invasion of moisture in the environment should be considered thoroughly. Prior to developing the encapsulations material, it

(aq) ↔ CH3 NH2(aq)

and CH3

NH3

NH3

+ O2(<sup>g</sup>) ↔ 2 I2(aq)

↔ <sup>h</sup>*<sup>ν</sup>* H2(<sup>g</sup>)

I results in the co-existence of CH3

the progress of equilibrium reaction. After that, the HI tends to decompose in two ways, redox reaction and photochemical reaction. Based on the aforementioned equilibrium reactions, it is easy to understand that not only the moisture in the environment but also the oxygen and UV radiation affect the stability severely [24]. The design of the encapsulant should consider the permeation mechanisms of the organic and inorganic materials. For an organic polymer encapsulant, gas or vapor permeates into the PSCs by the typical solution—the diffusion model. Although an organic polymer encapsulant possesses lots of advantages including flexibility, conformability, and processability, the permeability of gas is too high, which is not

methylammonium (MA), hydrogen iodide (HI), and lead iodide (PbI2

is important to understand what happens during the degradation. CH3

CH3 NH3 PbI3(s) ↔ PbI2(s) +CH3 NH3 I

will degrade into PbI<sup>2</sup>

example of the degradation mechanism of perovskite is shown as follows:

is exposed to moisture, it will immediately degrade into

) owing to its solubil-

taken as an

NH3 PbI<sup>3</sup>

+ HI(aq) (5)

+ H2 O(l) (6)

+ I2(s) (7)

I, CH3

I owing to the sensitivity to water.

, and HI toward

NH2

(aq) (4)

In the characterization of photovoltaic performance, a hysteretic response appears with different scanning conditions; thus the accuracy of the cell efficiency is doubted. The photocurrent density-voltage (J-V) hysteresis behavior will be affected by scanning direction, scanning rate, and applied voltage [26]. The hysteresis J-V behavior remains a challenge for the advancement of PSCs. Here are some situations that result in erroneous device efficiencies. For operating in different scanning directions, the forward scan that holds at short-circuit conditions to open circuit voltage conditions tends to have a lower efficiency than holding at the reverse scan. Hence, the forward scan is inclined to underestimate the power conversion efficiency of PSCs. In contrast, the reverse scan tends to overestimate the power conversion efficiency. If the scan rate of measuring the device is faster than its response time, hysteretic behavior will also be deteriorated. While holding at the reverse scan, the efficiency increases with the increasing scan rate. On the other hand, the efficiency measured at the forward scan decreases with the increasing scan rate. Moreover, the J-V hysteresis phenomenon becomes more intense under the faster scan rate in both scanning directions. Another important factor that can also affect the J-V hysteresis is the starting biases. The tendency is similar to the previous condition. The more positive the starting voltage holding at the reverse scan, the higher the efficiency can be measured; whereas, while holding at the forward scan, the more negative the starting voltage, the lower the efficiency of PSCs can be recorded. The existence of J - V hysteresis plays a crucial role in the accuracy of power conversion efficiency; hence, many scientists have dedicated research to searching for the solution. One of the possible methods of J - V hysteresis elimination is to measure the device with a slow scan rate. That hysteresis influenced by applied voltage can be solved by light-soaking at different biases was reported by the M. D. McGehee group [27]. Prior to the characterization, the precondition for perovskite-absorber devices with different biases presents a significant effect on the photovoltaic performance. The short-circuit current density, fill factor, and power conversion efficiency under reverse scan can be suppressed by light-soaking with a more negative bias. In contrast, the photovoltaic performance under the forward scan will be enhanced by lightsoaking with a more positive bias. Hence, the declined performance under the reverse scan and the increased performance under the forward scan prevent PSCs from the J-V hysteresis phenomenon.

#### **4.3. Carrier transport behavior**

Although a perovskite-structured active layer presents high efficiency of light-harvesting behavior, finding the suitable electron transport layer (ETL) and hole transport layer (HTL) is a critical issue for improving the performance of the photovoltaic device. After solar irradiation, an electron-hole pair will be generated at the perovskite-structured active layer. How to

photovoltaic materials with high power conversion efficiency, including (1) organolead halide perovskite-structured materials, (2) lead-free perovskite-structured materials, (3) lead-reduced perovskite-structured materials, and (4) two-dimensional perovskite-structured materials, are reported herein in detail. Many problems that occur with these materials must be overcome before commercialization. It has been proved that the element in perovskite-structured material plays an important role in enhancing stability under humidity or constant irradiation and achieving high power conversion efficiency. Numerous alternative materials for ETL and HTL have been discovered to facilitate carrier transport at perovskite-structured material interface/ ETL and perovskite-structured material/HTL interface. If there is a breakthrough development in fabrication methods, enhancement of stability, and optimized device structure can be realized, the commercialization of environmental-friendly high-efficiency PSC will be in the near future.

Perovskite-Structured Photovoltaic Materials http://dx.doi.org/10.5772/intechopen.74997 89

The authors appreciate Prof. Wei-Fang Su, Prof. Yang-Fang Chen and Mr. Meng-Huan Jao at National Taiwan University for useful discussion and suggestions. The authors acknowledge the financial support from the Ministry of Science and Technology, Taiwan (MOST 106-2221-E-182-057-MY3, MOST 106-2119-M-002-030 and MOST 106-2632-E-182-001) and

Department of Chemical and Materials Engineering, College of Engineering, Chang Gung

[1] Research Cell Record Efficiency Chart [Internet]. 2017. Available from: https://www.nrel.

[2] Kojima A, Teshima K, Shirai Y, Miyasaka T. Organometal halide perovskites as visiblelight sensitizers for photovoltaic cells. Journal of the American Chemical Society. 2009;**131**:

gov/pv/assets/images/efficiency-chart.png [Accessed: January 27, 2018]

Chang Gung Memorial Hospital, Linkou (CMRPD2F0161 and BMRPC74).

**Acknowledgements**

**Conflict of interest**

**Author details**

**References**

There are no conflicts of interest to declare.

Ming-Chung Wu\* and Yin-Hsuan Chang

6050-6051. DOI: 10.1021/ja809598r

University, Taoyuan, Taiwan

\*Address all correspondence to: mingchungwu@cgu.edu.tw

**Figure 9.** Energy band diagram of different electron and hole transport layers with respect to perovskite-structured photovoltaic devices.

transport the electron and hole efficiently to ETL and HTL, respectively, has been discussed thoroughly. Therefore, selecting the appropriate materials for the carrier transport layer is considered as a primary issue to prevent charge recombination and diminish energy loss at the interface between the perovskite active layer and ETL or HTL. In recent studies, the electron transport materials and the hole transport materials have been explored to construct an optimal band alignment for achieving high efficiency [28–30].

A good electron-transporting material should exhibit (1) good electron mobility to facilitate electron collection, (2) wide band gap for not hindering the absorption behavior of the perovskite active layer, and (3) both the valence band/conduction band should be lower than that of perovskite-structured material to promote electron migrate to ETL and block the hole to transport. Metal oxides, such as TiO2 , ZnO, and SnO2 , are widely applied as ETLs, especially TiO2, owing to their electrical and optical properties [31]. A good hole-transporting material should exhibit (1) efficient hole mobility to promote hole collection, (2) the valence band/conduction band both should be higher than the perovskite-structured active layer to promote hole migration to HTL and further transport to the electrode, and (3) there should be photochemical stability. The most widely-explored materials for HTL has been divided into 3 groups: (1) small molecules (e.g., spiro-OMeTAD), (2) inorganic materials (e.g., CuI, CuSCN, NiO, etc.), and (3) conducting polymers (e.g., P3HT, PEDOT, PTAA, etc.) [32, 33]. **Figure 9** examines the proposed band alignment for the commonly used materials for ETL and HTL which is helpful for understanding interface properties and constructing high-efficiency perovskite-structured photovoltaic devices.
