Relation Between Fabrication Method and Photovoltaic Performance of Planar Heterojunction Perovskite Solar Cells

Abstract

Although one-step deposition method has intensively been studied because of simple and easy fabrication of perovskite films, uncontrolled crystallization of perovskite during one-step deposition often results in films with small crystallites and low surface coverage, leading to low photovoltaic performance. In this study, we have proposed the optimum processing condition to afford favorable film morphology and crystal orientation for achieving high power conversion efficiency of perovskite solar cells. First, two different morphologies, tree-like and flower-like morphologies, are developed depending upon the spin-coating time and post-heat treatment temperature. When the perovskite is crystallized from the liquid film after short spin-coating time, the flower-like morphology is developed, whereas the tree-like morphology is developed when the perovskite is crystallized for long spin-coating time. When the morphology evolution is monitored using in-situ optical microscopy and X-ray diffraction to investigate the origin of the difference between tree-like and flower-like morphologies, it reveals that CH3NH3I–PbI2–solvent complex is formed to develop the tree-like morphology before CH3NH3PbI3 crystals are formed whereas the flower-like morphology is developed when the CH3NH3PbI3 crystals are formed directly from the liquid film without formation of CH3NH3I–PbI2–solvent complex. The film with flower-like morphology, as prepared from DMSO solution, has large-sized crystallites, and the crystallites are highly orientated along (112) and (200) directions, resulting in a high PCE of 13.85%, whereas the film with tree-like morphology has small-sized crystallites with random crystal orientation, exhibiting very low PCEs. Second, we successfully fabricated CH3NH3PbI3 crystals with two different orientations to the substrate using two different organic precursors (CH3NH3I and CH3NH3Cl), and then investigated the effect of crystal orientation on the photovoltaic performance of CH3NH3PbI3 solar cells with planar heterojunction structure. The power conversion efficiency (PCE) of inverted cell (13.60%) prepared with I3 film (made from precursor CH3NH3I) is higher than that of the inverted cell (11.26%) prepared with I2Cl film (made from precursor CH3NH3Cl) mainly due to higher short circuit current (JSC) and higher fill factor (FF) of I3-based cell, whereas the PCE of normal cell (12.88%) prepared with I2Cl film is higher than that of the inverted cell (3.97%) made of I3 film. Considering that I3 and I2Cl films exhibit different crystal orientations, we realize that the crystal orientation of perovskite is directly related to PCE. The PCE difference due to different crystal orientation is interpreted by the charge carrier lifetime extracted from transient photoluminescence spectrum: Shorter lifetime affords faster charge transfer from perovskite layer to charge transport layer, thus the device with shorter lifetime exhibits higher JSC and PCE. Strong hysteresis of J–V curves of normal cells is also interpreted by slow and imbalanced charge transfer from perovskite to CTL.