Abstract

This study focuses on the architecture modification of solution-processed methylammonium lead iodide ((CH₃NH₃)PbI₃)-based planar perovskite solar cells. The modification is obtained through a facile and cheap step formation at low temperature (<100 °C) using a simple inexpensive spin-coating technique.

(CH₃NH₃)PbI₃, as a light harvesting layer, was sandwiched between zinc oxide nanoparticle (ZnO-NP) thin film as a selective electron transport layer and organic polytriarylamine (PTAA) as a hole transport layer. The ZnO-NP was prepared by low-temperature sol-gel (<150 °C) of zinc acetate in methanol. This process produced a crystallite ZnO-NPs that were approximately 15 nm in diameter, as determined by transmission electron microscopy.

A one-step, two-step spin coating, and two-step dipping processes were used to grow the (CH₃NH₃)PbI₃ layer. The one-step process was the deposition of the PbI₂ and (CH₃NH₃)I mixture on the ZnO surface. During the two-step spin and two-step dipping, a layer of PbI₂ was spin coated on the ZnO surface. Afterward, the substrate was spin coated and immersed in a (CH₃NH₃)PbI₃ solution. Spin coated of the PTAA hole transport material and thermal evaporation of the silver top contact completed the device architecture glass-ITO/ZnO-NP/(CH₃NH₃)PbI₃/ PTAA/Ag.

Optical properties were investigated for each layer. Photoluminescence spectroscopy was used to determine the band gap energies of ZnO-NP (3.27 eV) and (CH₃NH₃)PbI₃ perovskite (1.62 eV). The field emission scanning electron microscope was employed to analyze the surface morphology of the different-step formation perovskite films. During two-step spin, the large grain structure was clearly observed. X-ray diffraction was utilized to obtain information about the crystallographic structure of the ZnO-NP and perovskite thin films. Results indicated the high crystallite tetragonal structure with a large grain size (70 nm). Energy-dispersive X-ray spectroscopy was used to determine the elemental composition of the deposited layers. The atomic force microscopy was used to determine the surface roughness of the ZnO-NPs and perovskite films which are 4.37 nm and 9.53 nm respectively; such property is crucial for layer interfaces during separation of excitons, which improves the current density of devices.

The electrical resistivity, carrier concentration, and mobility of ZnO-NP and PTAA were obtained. The low resistivity, high concentration, and excellent carrier mobility; according to the relative energy levels of various components, excitons generated in the (CH₃NH₃)PbI₃ layer can be dissociated by either transferring an electron to the underlying ZnO layer or hole transfer to the PTAA layer. Given the highly selective nature of both the ZnO and PTAA layers, highly efficient devices with good fill factors can be obtained using these layers. The current density–voltage (J–V) characterization of devices was investigated for different-step formation coating. Results indicated that the two-step spin possessed higher current density (12.76 mA/cm²) and larger open circuit voltage (0.83 V) with high power conversion efficiency (11.65%) compared with those of two other methods under the same illuminated global solar radiation AM 1.5G (100 mW/cm²). The series and shunt resistances obtained from the J–V curve of the two-step spin were 35.345 and 3333.3 ?, respectively.

The spectral response was performed for different-step formation (CH₃NH₃)PbI₃ perovskite solar cell devices. The photocurrent onset at 800 nm was consistent with the band gap of (CH₃NH₃) PbI₃. Furthermore, the efficiency obtained across almost the entire spectrum (350–760 nm) highlighted the excellent performance of the devices.

posted:19-6-2017