Efficient and Environmentally Stable Perovskite Solar Cells Based on ZnO Electron Collection Layer

Abstract

ZnO thin films prepared by spin-coating of nanoparticles at low temperature were utilized as the electron collection layer in CH 3 NH 3 PbI 3 -based perovskite solar cells having a planar heterojunction structure. The thickness of ZnO layer as a key parameter to determine the photovoltaic performance was optimized. The highest power conversion efficiency of up to 13.9% with small hysteretic behavior of currentvoltage curves was achieved under AM 1.5 illumination (100 mW cm ¹2). The ZnO-based devices exhibited significantly improved durability as compared to the control device with exposure to the ambient air environment.Since the initial report in 2009, 1 organo-lead-halide-based perovskite solar cells (PSCs) have been attracting attention, and there has been great progress in this field.2 Intensive developments have elevated the power conversion efficiency (PCE) of PSCs to 19.6% within half a decade.3c Although a majority of PSCs are based on mesoporous TiO 2 requiring sintering at 500°C, parallel studies into low-temperature-processed PSCs for realizing a wearable/stretchable energy supplier are also in focus.3 In particular, ZnO-based PSCs are attractive owing to the following characteristics of ZnO materials: (1) ZnO is a wideband semiconductor having the band gaps and band energies similar to those of TiO 2 . (2) ZnO has higher electron mobility than TiO 2 .4 (3) Importantly, high crystallinity of ZnO could be attained without sintering processes. 5a Although some efficient PSCs based on ZnO have been reported so far, 5 their doubtable performance as indicated by the large hysteresis 5b from the photovoltaic behaviors and poor stability against ambient air suggest that further investigations are necessary. 6 In this study, we fabricated PSCs with ZnO electron collection layers that were processed at temperature of as low as 150°C. The effect of ZnO layer thickness on the device performance was investigated. Our results indicated that the ZnO-based PSCs can show high PCE with weak hysteretic behavior and excellent stability against ambient air without encapsulation.The ZnO nanoparticles used in this investigation were prepared according to procedures in the literature. 5a The ZnO nanoparticles were dispersed in butanol with a concentration of 6 mg mL ¹1 . The solution was spin-coated on ITO substrates to form a relatively compact ZnO layer and then heated at 150°for 5 min. This procedure could be repeated several times to obtain a certain thickness according to experimental requirements. A 460 mg mL ¹1 solution of PbI 2 in DMF was then spin-coated on top of the ZnO layer at 4000 rpm. After a short baking process at 70°C for 30 min, the substrate was dipped into a CH 3 NH 3 I solution (10 mg mL ¹1 in 2-propanol) for 50 s followed by heating at 70°C for 20 min to realize the desired crystallite formation. The details of fabrication and characterization of PSCs are described in the Supporting Information. Figure 1a schematically shows the device configuration of the PSCs studied in this investigation, and Fig...

The Effect of Post-Baking Temperature and Thickness of ZnO Electron Transport Layers for Efficient Planar Heterojunction Organometal-Trihalide Perovskite Solar Cells

“…This reveals that the optimization of the ZnO electron transport layer works to cover on the surface of the FTO glass well. Thus, it is suitable for the planar perovskite-structure photovoltaic device [32]. Here, the ZnO electron transport layer is also known as the hole blocking layer that can avoid the intimate contact between the MAPbI 3 and the FTO, so the layer can suppress the charge recombination inside the cells.…”

Stable Perovskite Solar Cells Based on WO3 Nanocrystals as Hole Transport Layer

“…Currentvoltage measurements (Keithley 2440 source meter) were performed by using a solar simulator (Newport Inc.) with an AM1.5G filter under an irradiation intensity of 100 mW cm ¹2 . The typical XRD patterns of the resultant WO 3 NCs are shown in Figure 1a. All the diffraction peaks can be readily indexed to the orthorhombic phase WO 3 (JCPDS card No.…”

“…The high-resolution transmission electron microscopy (HRTEM) is shown in Figure 2b. The selected-area electron diffraction (SAED) pattern obtained by focusing the electron beam on a single WO 3 NC is displayed in the inset of Figure 2b, which shows diffused rings, indicating that the WO 3 NCs are constructed of polycrystalline. Figure 2c shows the AFM image of the WO 3 NCs after spin-coating on a quartz substrate.…”

“…Herein, we report a simple solvothermal method using WCl 6 and NH 3 ¢H 2 O as precursors in ethanol solvent at 200°C for 24 h to fabricate WO 3 nanocrystals (NCs) as the hole transport layer in an inverted perovskite solar cell. The thin WO 3 NC film with a smooth and compact surface enabled the formation of a continuous and compact layer of well-crystallized CH 3 NH 3 PbI 3 in a two-step solution process. A cell efficiency of 7.68% was obtained, which is a promising planar perovskite solar cell based on an inorganic hole extraction layer.…”

“…In this study, for the first time, we fabricated WO 3 nanocrystals by a simple solvothermal method using WCl 6 and NH 3 ¢H 2 O as precursors in ethanol solvent at 200°C for 24 h. Further, considering the high work function and deep energy level of WO 3 , the obtained WO 3 NCs were applied as HTLs in perovskite solar cells to obtain an efficiency of 7.68%.…”