**4. Hole transport materials**

intervals by means of several events including absorption, diffusion, and emission together.

Among different atoms, Pb and Sn included in perovskite materials are fair due to that they yield higher efficiency levels due to their tunable band gap values and improved absorption. The main contribution to the tunable band gap comes from the orbitals of additive metals and halides [70–73]. Hence, the metals which will make the electronic contribution should be carefully selected from among numerous metals. Yet, the band gap harmony and proper valence arrangement requires watching the outermost orbital of the metal and one can see that there are highly less metals constituting mentioned configuration. With that, there is no specific procedure or sharp method to eliminate this hardness. Hence, these theoretical issues should

The intrinsic properties of OMH led them to be used in many diverse engineering applications till now, including thin film diode‐based devices, such as SCs, transistors, and light‐emitting diodes (LEDs). Moreover, it has also been reported that the band gap of OMH perovskite is decreased when the structure is shifted from 2‐D to 3‐D [9]. The less spaced band gap intervals are especially found applicative for constructing SC devices. Hence, the developed 3‐D structure of CH3NH3PbX3 is initially tested as an inorganic semiconducting sensitizer in 2009 in DSSCs. Nevertheless, the researchers got unsatisfactory amounts of PCE from the test results compared to those most efficient DSSCs performed till that time (∼3.5% versus ∼11%) [74].

The interest on the perovskite material remained poor until a research is exhibited in 2012. The corresponding research results reported 500 hours of stable lifetime of a perovskite thin film coated on TiO2. The distinguishing specialty of this kind perovskite was its 10 times enlarged absorbing coefficient compared to that of the widely known ruthenium‐based sensitizers. The new molecule has been recognized as a breakthrough discovery. In addition to all, the band gap intervals could be reduced more along with the processing of perovskite materials. And more, they can even be processed more to make them gain highly absorbing structure. Together with these improving arrangements, the optical adsorption can become tunable and recom‐ bination properties can also become enhanced [50, 75]. Consequently, as a cation for B, Pb is exclusively preferred for SC applications for the assumed reasons when they are put into perovskite molecule. Neatly, Pb including PSC are found to be theoretically ideal. They

Through the characterization work, the satisfaction of band gap values can be attained through ultraviolet photoelectron spectroscopy and UV‐Vis spectral measurements for any kind of material. Namely, the research [8] made for CH3NH3PbI3 has revealed that the minimum and maximum valance values are between –5.5 and –3.95 eV interval, giving out approximately 1.5 eV of band gap for this structure [8, 20]. Through looking at the given typical valence‐band and conduction‐band values, we infer that perovskite material satisfies enough hole and electron separation. The separation gives a band gap value of 1.5 eV that reveals us enough

be carried along with the observations and some experimental works.

With that, the testing groups reached a PCE efficiency of 6.5% [10, 11] later on.

practically constitute small spaced band gap intervals as well [8–11].

absorption is made using an onset wavelength of 830 nm.

By means of all, high Voc values are attained [69].

**3.3. Optical band gap**

286 Nanostructured Solar Cells

The HTMs for PSCs are similar to that of solid‐state DSSCs, in view of the general structure. The main distinction between them is that the HTM layers do not fill the pores of TiO2, and it is because they are currently coated by the perovskite material in a PSC. Fortunately, during the practical selection of the materials, this property eases the determination of HTM molecules for PSC devices for us. This is because it becomes more possible to choose them from a large range of materials—compared to the selections that are made for solid‐state DSSCs. Once decided to be used for PSC, the produced HTM layer from chosen molecules should meet the similar requirements as requested for solid‐state DSSCs, as follows:


Among them, spiro‐OMeTAD has been the most used HTM as small‐molecules in PSCs till now. Because it has already been tested and evaluated as a HTM layer in solid‐state DSSCs and OLED devices [67, 74, 82, 83]. Therefore, its interlayer functioning is well known by researchers. Namely, use of hybrid lead halide with spiro‐OMeTAD‐based HTM has given an efficiency of around 15% PCE [82, 84]. On the other hand, the biggest obstacle yielding spiro‐ OMeTAD in wide range of applications is its high cost that still remains high due to its difficult synthesis process. This obstacle has been a hindrance for further improvements of PSCs in terms of maintaining high efficiency and low cost production together [74]. At this point, new HTMs that are alternative to spiro‐OMeTAD have been an issue for improving cost‐effective designs in recent years. These new molecule designs are generally separated to organic and inorganic HTMs as for PSC applications. Organic molecules can either be polymers or other small molecules. Different types of HTM structures that are already used are given in **Figure 4** with their corresponding HOMO values within the cell.

**Figure 4.** (a) Recently used HTM materials used for PSCs and (b) their corresponding HOMO values.

Spiro‐OMeTAD and other small molecule type HTMs have more availability compared to the organic and inorganic HTMs [74] due to its ease of design where the triarylamine is an essential part of the molecule. Here, nitrogen is used as a doping site and can be stabilized through aromatic rings. The nonplanar triarylamine leads to have longer distances between molecules and thus reduces the hole mobility, here. Nevertheless, this makes the molecules more applicable as HTMs for SCs fortunately. Due to the advantages among different molecules, spiro‐OMeTAD has been a standard HTM for researchers, where the new small molecules are comparable to that of the standard HTM for this reason. During the application of new types of spiro‐OMeTAD and other small molecules some key issues for increasing the efficiency levels are (1) thickness optimization and (2) inclusion of additives, such as making composites [74]. There are also some works done for searching the device's physics on the interface between HTM layer and perovskite layer, as well.

#### **4.1. Early device materials as hole transporters**

A HTM layer is needed for sensitizing the TiO2 layer to satisfy the working of PSCs. The working principle of HTM‐based PSC originally depends on DSSCs developed in 1991 by O'Regan and Gratzel based on a liquid electrolyte layer. These cells are photoelectro‐chemical cells with a wide surface area of TiO2 film layer that is sensitized by molecular dyes [54]. The same principle is valid for solid‐state sensitized SCs and PSCs, as well. In Gratzel's works, a final efficiency of 12% has been reached. Nevertheless, a leakage is always possible within the corresponding device structure. Shortly, the liquid may leak from the bordered edges. Therefore, the same structure for that of DSSC has been protected in solid‐state SCs in principle, on the other side, the electrolyte is altered by a solid hole conductor material instead.

**Figure 5** represents the general structure of such solid‐state SC and its electron transfer properties. Here, the layered device structure benefits from the TiO2 layer for blocking the direct touch of TiO2 layer to HTM. The working principle of the solid‐state SC is the same as that of DSSC, where the only distinction stems from the difference in electron transmission processes occurred inside the liquid electrolyte and solid HTM. Namely, the electron is hopped through the solid HTM instead of reducing in the liquid electrolyte.

**Figure 5.** (a) General structure of solid‐state solar cells and (b) working principle.

designs in recent years. These new molecule designs are generally separated to organic and inorganic HTMs as for PSC applications. Organic molecules can either be polymers or other small molecules. Different types of HTM structures that are already used are given in **Figure 4**

**Figure 4.** (a) Recently used HTM materials used for PSCs and (b) their corresponding HOMO values.

Spiro‐OMeTAD and other small molecule type HTMs have more availability compared to the organic and inorganic HTMs [74] due to its ease of design where the triarylamine is an essential part of the molecule. Here, nitrogen is used as a doping site and can be stabilized through aromatic rings. The nonplanar triarylamine leads to have longer distances between molecules and thus reduces the hole mobility, here. Nevertheless, this makes the molecules more applicable as HTMs for SCs fortunately. Due to the advantages among different molecules, spiro‐OMeTAD has been a standard HTM for researchers, where the new small molecules are comparable to that of the standard HTM for this reason. During the application of new types of spiro‐OMeTAD and other small molecules some key issues for increasing the efficiency levels are (1) thickness optimization and (2) inclusion of additives, such as making composites [74]. There are also some works done for searching the device's physics on the interface between

A HTM layer is needed for sensitizing the TiO2 layer to satisfy the working of PSCs. The working principle of HTM‐based PSC originally depends on DSSCs developed in 1991 by O'Regan and Gratzel based on a liquid electrolyte layer. These cells are photoelectro‐chemical cells with a wide surface area of TiO2 film layer that is sensitized by molecular dyes [54]. The same principle is valid for solid‐state sensitized SCs and PSCs, as well. In Gratzel's works, a final efficiency of 12% has been reached. Nevertheless, a leakage is always possible within the corresponding device structure. Shortly, the liquid may leak from the bordered edges. Therefore, the same structure for that of DSSC has been protected in solid‐state SCs in principle,

on the other side, the electrolyte is altered by a solid hole conductor material instead.

**Figure 5** represents the general structure of such solid‐state SC and its electron transfer properties. Here, the layered device structure benefits from the TiO2 layer for blocking the

with their corresponding HOMO values within the cell.

288 Nanostructured Solar Cells

HTM layer and perovskite layer, as well.

**4.1. Early device materials as hole transporters**

In order to satisfy the HTM's working within the other film layers, the corresponding layer should fill the pores of the mesoporous TiO2 film layer. With that a heterojunction is produced. For filling process, a molecular HTM structure may be determined. This molecule may be selected from various types of structures, such as small molecule HTM (mostly spiro‐OMeTAD is used), and an inorganic or organic HTM. During the layer design, the hole diffusion length and conductivity of HTM is selected at the meantime, because these are parameters directly affecting the layer thickness of the mesoporous TiO2. Namely, the TiO2 film layer thickness is inversely proportional to the absorption coefficient. In other words, the less TiO2 layer thickness, the more the absorption coefficient value. Here, unfortunately, the mesoporous film thickness can only be stretched up to 2 µm, i.e., when spiro‐OMeTAD is used [85]. Hence, a breakthrough development in a solid‐state DSSC is not expected with typical molecular dyes that constitute ∼103 cm–1 of absorption coefficient. Neatly, in order to wholly collect the incident light arrays using those dyes, a TiO2 layer with a thickness of 10 µm is needed.

The main problem in the early HTMs leads researchers to think that they may achieve a high efficiency solid‐state sensitizer for SCs by means of a stronger light absorbing material. This becomes realized by means of the perovskites' property that has superiority against traditional dyes in terms of absorption. Namely, it has a stronger adsorption over a broad range and therefore full coverage absorption becomes realized within a very low thickness of 500 nm. This property of Perovskite becomes a real advantage for solid‐state SCs. Because the previous obstacle for them were that a mesoporous TiO2 film thickness restricted at around 2 µm length is needed in order to generate photocurrent with the given light absorption [30].

#### **4.2. New small molecules as hole conductors: spiro‐OMeTAD**

The perovskite basically yield high series and shunt resistances which results in lower FF for SC devices. This is mainly because of the perovskite layer with higher conductivity in contact with the thicker layer of HTM that has lower conductivity. Hence, reduced thickness and better conducting HTM layer is a must to develop more efficient PSC devices [50].

Recently, some efforts on spiro‐OMeTAD molecules used as HTM are under attention of researchers in view of their increased hole conductivities [86, 87]. For example, lithium bistrifluoromethanesulfonimide (Li‐TFSI) has been found out to increase the hole conductivity when doped to spiro‐OMeTAD, within some researches [87]. Using various techniques, spiro‐ OMeTAD has widely been used for enhancements in conductivity through increasing the hole mobility and charge density in HTM layers [88, 89].

Within the cell, the holes are injected from the perovskite into the spiro‐OMeTAD and electrons are passed from perovskite to the mesoporous TiO2 layer. Here, therefore, the TiO2 is an active counterpart of the photovoltaic conversion event which makes it a main element of the PSC. Lately, it was fortunately shown that TiO2 scaffold has leaded a PCE of up to almost 10% [20, 74]. In some studies, spiro‐OMeTAD has also been used while coupled with other scaffolds (i.e., Al2O3). Where it is mixed with perovskites and good PCE has been attained in [30, 74]. These are famed as meso‐super‐structured SCs where the CH3NH3PbX3 is served as an electron transport material (ETM) in addition to its basic sensitizer duty. Besides with all that, the metal oxide only acts as an insulator with a wide band gap and therefore do not take part in charge transport. Hence, the metal oxide becomes an inactive scaffold here. For that, these are referred as passive scaffolds in the PSC literature. It is good to say that a more uniform perovskite layer formation is applicable with a wide thickness in both of the scaffolds which improves the light absorption of PSCs, thus lead an increase in efficiency [74].

Besides all, new kinds of small molecules as for HTM are continuously introduced to the literature. For example, a Trux‐OMeTAD small molecule is reported where an excellent hole mobility is achieved [90]. Here, the constructed PSC has given efficiency as close as to 19%.
