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This article reviews the various hole transporting materials (HTMs) used in perovskite solar cells (PSCs) in achieving high photo-conversion efficiency (PCE) and operational stability. The PSCs are the latest development in solution processable solar cells offering PCE (~22%) on a par with that of practically deployed silicon and thin film solar cells. HTMs and electron transporting materials (ETMs) are important constituents in PSCs as they selectively transport charges within the device, influence photovoltaic parameters, determine device stability and also influence its cost. This article critically approaches role of structure, electrochemistry, and physical properties of varied of choice of HTMs categorized diversely as small and long polymers, organometallic, and inorganic on the photovoltaic parameters of PSCs conceived in various device configurations. Achievements in tailoring the properties of HTMs to best fit for PSCs are detailed; a well-designed HTM suppresses carrier recombination by facilitating the passage of holes but blocking electrons at the HTM/perovskite interface. Moreover, in many PSCs the HTM acts as the first line of defense to external degrading factors such as humidity, oxygen and photon dose, the extent of which depends on its hydrophobicity, permeability, and density.

Nanostructure thin films of bisbenzimidazo[2,1-a:2′,1′-a′]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-10,21-dione7 (BIdiisoQ)
were prepared by using vacuum thermal evaporating procedure under vacuum of 2.45 ×10−5
mbar with thickness
of 150 nm. Structural and optical properties for the as-deposited and the annealed BI-diisoQ thin films were carried out in
the temperature range of (373–623 K). The X-ray diffraction (XRD) examination of BI-diisoQ confirmed that the annealed
films were a mixture of amorphous and crystalline structure, whilst the as-deposited films are utterly amorphous. Scanning
Electron Microscopy (SEM) images showed a clear shape of nanorods of BI-diisoQ with diameter of 40 nm at 632 K. The
optical characteristics of nanostructure thin films of BI-diisoQ were investigated by measuring transmittance and reflectance
spectrum versus the incidence of visible light in the range of 190–2500 nm. Our conception dissected that the optical transition
type of BI-diisoQ nanorod films is indirect allowed transition with optical and fundamental energy gaps equal to1.54 eV
and 3.54 eV respectively, which decreased to 1.36 eV and 3.45 eV at the annealed temperature of 623 K. The oscillation
energy, Eo, and the dispersion energy, Ed, were investigated by using the ideation of single oscillator model (SOM). Moreover,
the non-linear optical susceptibility, χ(3), and non-linear refractive index, n2, were found to be temperature annealing dependence.
The optical investigation of BI-diisoQ nanorod films indicated that these films have excellent optical characteristics,
and thus can be recommended as a potential material for integrated highly optical applications.

Nanostructure thin films of bisbenzimidazo[2,1-a:2′,1′-a′]anthra[2,1,9-def:6,5,10-d′e′f′]diisoquinoline-10,21-dione7 (BIdiisoQ)
were prepared by using vacuum thermal evaporating procedure under vacuum of 2.45 ×10−5
mbar with thickness
of 150 nm. Structural and optical properties for the as-deposited and the annealed BI-diisoQ thin films were carried out in
the temperature range of (373–623 K). The X-ray diffraction (XRD) examination of BI-diisoQ confirmed that the annealed
films were a mixture of amorphous and crystalline structure, whilst the as-deposited films are utterly amorphous. Scanning
Electron Microscopy (SEM) images showed a clear shape of nanorods of BI-diisoQ with diameter of 40 nm at 632 K. The
optical characteristics of nanostructure thin films of BI-diisoQ were investigated by measuring transmittance and reflectance
spectrum versus the incidence of visible light in the range of 190–2500 nm. Our conception dissected that the optical transition
type of BI-diisoQ nanorod films is indirect allowed transition with optical and fundamental energy gaps equal to1.54 eV
and 3.54 eV respectively, which decreased to 1.36 eV and 3.45 eV at the annealed temperature of 623 K. The oscillation
energy, Eo, and the dispersion energy, Ed, were investigated by using the ideation of single oscillator model (SOM). Moreover,
the non-linear optical susceptibility, χ(3), and non-linear refractive index, n2, were found to be temperature annealing dependence.
The optical investigation of BI-diisoQ nanorod films indicated that these films have excellent optical characteristics,
and thus can be recommended as a potential material for integrated highly optical applications.

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