Pingli Qin

Low-Temperature Solution-Processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells

Lead halide perovskite solar cells with the high efficiencies typically use high-temperature processed TiO2 as the electron transporting layers (ETLs). Here, we demonstrate that low-temperature solution-processed nanocrystalline SnO2 can be an excellent alternative ETL material for efficient perovskite solar cells. Our best-performing planar cell using such a SnO2 ETL has achieved an average efficiency of 16.02%, obtained from efficiencies measured from both reverse and forward voltage scans. The outstanding performance of SnO2 ETLs is attributed to the excellent properties of nanocrystalline SnO2 films, such as good antireflection, suitable band edge positions, and high electron mobility. The simple low-temperature process is compatible with the roll-to-roll manufacturing of low-cost perovskite solar cells on flexible substrates.

Efficient hole-blocking layer-free planar halide perovskite thin-film solar cells

Efficient lead halide perovskite solar cells use hole-blocking layers to help collection of photogenerated electrons and to achieve high open-circuit voltages. Here, we report the realization of efficient perovskite solar cells grown directly on fluorine-doped tin oxide-coated substrates without using any hole-blocking layers. With ultraviolet-ozone treatment of the substrates, a planar Au/hole-transporting material/CH₃NH₃PbI₃-xClx/substrate cell processed by a solution method has achieved a power conversion efficiency of over 14% and an open-circuit voltage of 1.06 V measured under reverse voltage scan. The open-circuit voltage is as high as that of our best reference cell with a TiO₂ hole-blocking layer. Besides ultraviolet-ozone treatment, we find that involving Cl in the synthesis is another key for realizing high open-circuit voltage perovskite solar cells without hole-blocking layers. Our results suggest that TiO₂ may not be the ultimate interfacial material for achieving high-performance perovskite solar cells.

Recent progress in electron transport layers for efficient perovskite solar cells

Thin-film photovoltaics based on organic–inorganic hybrid perovskite light absorbers have recently emerged as a promising low-cost solar energy harvesting technology. Over the past several years, we have witnessed a great and unexpected progress in organic–inorganic perovskite solar cells (PSCs). The power conversion efficiency (PCE) increased from 3.8% to 20.1% and exceeded the highest efficiency of conventional dye-sensitized solar cells. Here, the focus is specifically on the recent developments of the electron transport layer (ETL) in PSCs, which is an important part for high performing PSCs. This review briefly discusses the development of the structure of PSCs, and we attempt to give a systematic introduction about the optimization of ETL and its related interfaces for efficient PSCs. Moreover, the introduction of appropriate interfacial materials is another important issue to improve PSC performance by optimizing the interfacial electronic properties between the perovskite layer and the charge-collecting electrode. Besides, some related issues such as device stability and hysteresis behavior are also discussed here.

Perovskite Solar Cell with an Efficient TiO2 Compact Film

A perovskite solar cell with a thin TiO2 compact film prepared by thermal oxidation of sputtered Ti film achieved a high efficiency of 15.07%. The thin TiO2 film prepared by thermal oxidation is very dense and inhibits the recombination process at the interface. The optimum thickness of the TiO2 compact film prepared by thermal oxidation is thinner than that prepared by spin-coating method. Also, the TiO2 compact film and the TiO2 porous film can be sintered at the same time. This one-step sintering process leads to a lower dark current density, a lower series resistance, and a higher recombination resistance than those of two-step sintering. Therefore, the perovskite solar cell with the TiO2 compact film prepared by thermal oxidation has a higher short-circuit current density and a higher fill factor.

Performance enhancement of perovskite solar cells with Mg-doped TiO2 compact film as the hole-blocking layer

In this letter, we report perovskite solar cells with thin dense Mg-doped TiO2 as hole-blocking layers (HBLs), which outperform cells using TiO2 HBLs in several ways: higher open-circuit voltage (Voc) (1.08 V), power conversion efficiency (12.28%), short-circuit current, and fill factor. These properties improvements are attributed to the better properties of Mg-modulated TiO2 as compared to TiO2 such as better optical transmission properties, upshifted conduction band minimum (CBM) and downshifted valence band maximum (VBM), better hole-blocking effect, and higher electron life time. The higher-lying CBM due to the modulation with wider band gap MgO and the formation of magnesium oxide and magnesium hydroxides together resulted in an increment of Voc. In addition, the Mg-modulated TiO2 with lower VBM played a better role in the hole-blocking. The HBL with modulated band position provided better electron transport and hole blocking effects within the device.

In Situ Synthesis of NiS Nanowall Networks on Ni Foam as a TCO-Free Counter Electrode for Dye-Sensitized Solar Cells

Nickel sulfide (NiS) nanowall networks have been prepared by a novel one-step hydrothermal method on a nickel (Ni) foam substrate. The Ni foam has a high conductivity and porous structure. To our knowledge, the Ni foam is used as a conductive substrate for the dye-sensitized solar cell (DSSC) for the first time. The Ni foam is used as not only the conductive substrate but also the Ni sources of the reaction. The Ni foam supported NiS prepared by this simple hydrothermal method shows high catalytic activity for reduction of triiodide ions. The DSSC with a transparent conductive oxide (TCO)-free NiS counter electrode (CE) was herein developed and showed a higher power conversion efficiency of 8.55% than that with a TCO supported NiS CE (7.47%) and a TCO supported platinum CE (7.99%).

In situ growth of double-layer MoO3/MoS2 film from MoS2 for hole-transport layers in organic solar cell

Efficient organic solar cells (OSCs) based on regioregular poly(3-hexylthiophene):fullerene derivative [6,6]-phenyl-C61butyric acid methyl ester composites have been fabricated on fluorine-doped tin oxide (FTO) coated glass substrates by a radio frequency (RF) sputtered and ultraviolet ozone (UVO) treated MoS2 film as the hole-transport layer (HTL). With the help of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, transmission spectra and the Hall-effect system, we find that the deposition temperature can modulate the contents of the various valence states of molybdenum, which can result in changes of the energy level, and the optical and electrical properties of the MoS2 films. MoS2 has been oxidized to a double-layered MoO3–MoS2 film by UVO treatment. Due to the presence of the molybdenum oxidation states Mo5+ and Mo6+, the MoS2 film shows p-type conductive behavior, and its smaller electron affinity can effectively block electron from exciton dissociation. By optimizing the HTL thickness and sputtering deposition temperature, a power conversion efficiency up to 4.15% has been achieved for an OSC that used a double-layered MoO3–MoS2 film as the HTL. Its JSC is bigger than that of the OSC with a pure MoO3 film as the HTL. This shows that this double layer MoO3–MoS2 interface is more favorable for hole-transfer.

Effects of annealing temperature of tin oxide electron selective layers on the performance of perovskite solar cells

Efficient lead halide perovskite solar cells have been realized using SnO2 as electron selective layers (ESLs). Here, we report on the effects of the annealing temperature of solution-processed SnO2 ESLs on the performance of perovskite solar cells. We find that the cells using low-temperature annealed SnO2 (LT-SnO2) ESLs outperform the cells using high-temperature annealed SnO2 (HT-SnO2) ESLs, exhibiting higher open circuit voltages and fill factors. Structural, electrical, optical, and electrochemical characterizations reveal the origin of the performance differences: LT-SnO2 produces better film coverage, wider band gap, and lower electron density than that of HT-SnO2. The confluence of these properties results in more effective transportation of electrons and blocking of holes, leading to lower interface recombination. Therefore, LT-SnO2 ESLs are preferred for manufacturing perovskite solar cells on flexible substrates.

Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement

We report a simple and effective interface engineering method for achieving highly efficient planar perovskite solar cells (PSCs) employing SnO2 electron selective layers (ESLs). Herein, a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) was used to modify the SnO2 ESL/perovskite layer interface. This APTES SAM demonstrates multiple functions: (1) it can increase the surface energy and enhance the affinity of the SnO2 ESL, which induce the formation of high quality perovskite films with a better morphology and enhanced crystallinity. (2) Its terminal functional groups form dipoles on the SnO2 surface, leading to a decreased work function of SnO2 and enlarged built-in potential of SnO2/perovskite heterojunctions. (3) The terminal groups can passivate the trap states at the perovskite surface via hydrogen bonding. (4) The thin insulating layer at the interface can hinder electron back transfer and reduce the recombination process at the interface effectively. With these desirable properties, the best-performing cell employing a APTES SAM modified-SnO2 ESL achieved a PCE over 18% and a steady-state efficiency of 17.54%. Impressively, to the best of our knowledge, the obtained VOC of 1.16 V is the highest value reported for the CH3NH3PbI3 (MAPbI3) system. Our results suggest that the ESL/perovskite interface engineering with a APTES SAM is a promising method for fabricating efficient and hysteresis-less PSCs.

Reducing Hysteresis and Enhancing Performance of Perovskite Solar Cells Using Low-Temperature Processed Y-Doped SnO2 Nanosheets as Electron Selective Layers

Despite the rapid increase of efficiency, perovskite solar cells (PSCs) still face some challenges, one of which is the current-voltage hysteresis. Herein, it is reported that yttrium-doped tin dioxide (Y-SnO2 ) electron selective layer (ESL) synthesized by an in situ hydrothermal growth process at 95 °C can significantly reduce the hysteresis and improve the performance of PSCs. Comparison studies reveal two main effects of Y doping of SnO2 ESLs: (1) it promotes the formation of well-aligned and more homogeneous distribution of SnO2 nanosheet arrays (NSAs), which allows better perovskite infiltration, better contacts of perovskite with SnO2 nanosheets, and improves electron transfer from perovskite to ESL; (2) it enlarges the band gap and upshifts the band energy levels, resulting in better energy level alignment with perovskite and reduced charge recombination at NSA/perovskite interfaces. As a result, PSCs using Y-SnO2 NSA ESLs exhibit much less hysteresis and better performance compared with the cells using pristine SnO2 NSA ESLs. The champion cell using Y-SnO2 NSA ESL achieves a photovoltaic conversion efficiency of 17.29% (16.97%) when measured under reverse (forward) voltage scanning and a steady-state efficiency of 16.25%. The results suggest that low-temperature hydrothermal-synthesized Y-SnO2 NSA is a promising ESL for fabricating efficient and hysteresis-less PSC.