**3.3. Optical band gap**

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 be carried along with the observations and some experimental works.

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

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

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 absorption is made using an onset wavelength of 830 nm.

Shortly, the fundamental request for a SC is its strong and wide absorption range. In these terms, OMH perovskite is suitable for its tunable crystal sizes. Namely, with that condition, the band gap value of the absorber material may be arranged by differently utilized cations and anions [23, 28, 76, 77] within the crystal arrangement, as well. As an example, PbIyBrI3‐y crystal structure can be modified to derive different absorption levels through varying the "*y*" value here. Derivation of varied absorption levels is generally referred as "tuning band gap." This is essential for perovskite's future, because the development of various types of advanced molecules is allowed by means of such procedure that makes perovskite attain a tunable band gap [23, 78, 79]. Here, the usability of this strategy is shown for CH3NH3PbBr3‐xClx SCs, where Voc of 1.5 V is derived. This shows the availability of this idea as a method. One more parameter for getting high power values is reaching high photocurrent values. Namely, this can be made through an engineering on *n*‐type and *p*‐type materials in order to match wider band gap perovskites. Also, the widened band gap of perovskite may require synthesizing new hole transport materials since valence band is shifted [78–81]. Therefore, engineering for deriving appropriate hole transporters is especially important for PSCs, too.
