Perovskite for Photovoltaic applications chemical composition affect

Perovskite for Photovoltaic Applications
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Fig (1)
The spatial map of topography was performed by AFM. PEA+ and NMA+- based quasi-2D perovskite have small grains (10nm) unlike the EDA+-based 3D perovskite (grain size around100nm). The chemical composition of the samples would be responsible for the surface morphology. Notably, 2D perovskite will have a smoother surface than 3D perovskite. Since PEA+ and NMA+- based quasi-2D perovskite contain aromatic alkylammonium, such as Phenyl ethyl ammonium and Nephtyl methylammonium as organic spacer respectively, the aromatic species in organic spacer chemical structure play a crucial role of maintaining the orientation of film growth and hence improve the crystallinity. Zhang et al. (2018) argued that, the presence of rigid benzene ring in PEA+ as an organic spacer in quasi-2D perovskite might confine the structural freedom, which in turn boost the film crystallinity, bigger grain size, and enhanced photovoltaic performance with out of plane orientation of the resulting perovskite material [1]. However, it should be noted that, EDA+-based 3D perovskite material also has large grain size and high surface roughness (RMS=26nm) due to random orientation growth of film and the absence of organic spacer formed under relatively higher temperatures in a bid to enhance performance. In addition, several studies have confirmed that, unlike 2D, 3D perovskite has detrimental features for PV applications due to the fact that 3D perovskite are relatively unstable compared with 2D perovskite materials, which have a smoother surface. [2-4]
Fig (2)
Imaging surface potential, permanent charge distribution, and photo-generated charge carriers with relatively high spatial resolution can be achieved by Kelvin probe force microscopy (KPFM). In this work, a sensitive and non-destructive KPFM was performed to investigate a complex interplay between morphology, the chemical composition and the structural defects and optoelectronic properties of 3D and quasi-2D based perovskite films. Through minimizing the electrostatic force between tip and the surface of the sample by applying an appropriate DC-voltage, surface potential or contact potential difference (CPD) can be imaged. Grevin (2018) defined CPD as the difference between the vacuum level at the tip, and the sample surface. Vacuum level in this definition refers to the energy of an electron at rest just outside the surface of the solid [5].
Based on the histogram distribution data, the average CPD in the dark was +160 mV, -17 mV, and -73mV for NMA+-based quasi 2D, PEA+-based 2D quasi and EDA+-based 3D perovskite, respectively. This means that the surface of NMA+-based quasi 2D perovskite would be more p-type characteristic than PEA+-based quasi-2D and EDA+-based 3D perovskite. Once illumination at 490nm, ( )mW leaser intensity, the average CPD of all the samples shifted to a positive value (redshift). This well-known behaviour of perovskite film under illumination could attribute to generate charge carriers and fermi-level splitting. Moreover, the spatial CPD maps of PEA+-based 2D quasi and EDA+-based 3D perovskite in dark show significant contrast between grain interiors (GIs) and grain boundaries (GBs), unlike NMA+-based quasi-2D one. It is worth to mention that the GBs in PEA+-based 2D quasi and EDA+-based 3D perovskite would act as recombination sites or trapped sites owing to lower work function than GI [6].
The reason behind the variation of CPD map before illumination still need more investigation to gain in-depth insight about intrinsic defects of the surface. The absence of photo-generated charge carriers in dark condition supposed to cancel the energy band bending. Whereas, many works have been argued that in dark condition, the electrostatic contrast in surface potential image result from permanently trapped charge carriers or ch...