Zonglong Zhu

Efficiency Enhancement of Perovskite Solar Cells through Fast Electron Extraction: The Role of Graphene Quantum Dots

We report on a significant power conversion efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin graphene quantum dots (GQDs) layer between perovskite and TiO2. A strong quenching of perovskite photoluminescence was observed at ∼760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells. From the transient absorption measurements, the improved cell efficiency can be attributed to the much faster electron extraction with the presence of GQDs (90-106 ps) than without their presence (260-307 ps). This work highlights that GQDs can act as a superfast electron tunnel for optoelectronic devices.

High-Performance Hole-Extraction Layer of Sol-Gel-Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells**

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH3 NH3 PbI3 in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH3 NH3 PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer.

Enhanced Efficiency and Stability of Inverted Perovskite Solar Cells Using Highly Crystalline SnO2 Nanocrystals as the Robust Electron-Transporting Layer

Highly crystalline SnO2 is demonstrated to serve as a stable and robust electron-transporting layer for high-performance perovskite solar cells. Benefiting from its high crystallinity, the relatively thick SnO2 electron-transporting layer (≈120 nm) provides a respectable electron-transporting property to yield a promising power conversion efficiency (PCE)(18.8%) Over 90% of the initial PCE can be retained after 30 d storage in ambient with ≈70% relative humidity.

High performance flexible solid-state asymmetric supercapacitors from MnO2/ZnO core–shell nanorods//specially reduced graphene oxide

With a view to developing flexible solid-state asymmetric supercapacitors, we have specially designed and nanoscopically engineered two types of electrodes: a MnO2/ZnO core–shell nanorod array and a HI-reduced graphene oxide assembly, both deposited in situ on a carbon cloth. These materials were thoroughly characterized by structural and spectroscopic techniques. The flexible solid-state asymmetric supercapacitors with cathodes and anodes made of these materials have demonstrated superior performance characteristics. They can be cycled in a wide potential window of 0–1.8 V for 5000 cycles with only 1.5% capacitance loss. The demonstrated volumetric energy density of 0.234 mW h cm−3 and volumetric power density of 0.133 W cm−3 are much higher than those of similar devices reported previously in the literature.

High performance inverted structure perovskite solar cells based on a PCBM:polystyrene blend electron transport layer

Hybrid organic/inorganic perovskite solar cells are among the most competitive emerging photovoltaic technologies. Here, we report on NiO-based inverted structure perovskite solar cells with a high power conversion efficiency of 10.68%, which is achieved by adding a small percentage (1.5 wt%) of high molecular weight polystyrene (PS) into the PCBM electron transport layer (ETL). The addition of PS facilitates the formation of a highly smooth and uniform PCBM ETL that is more effective in preventing undesirable electron–hole recombination between the perovskite layer and the top electrode. As a result, the VOC of the PCBM:PS-based cells is increased from 0.97 V to 1.07 V, which leads to significantly enhanced power conversion efficiencies of the solar cells. Our study provides a simple and low-cost approach to improving the ETL film quality and the performance of inverted perovskite solar cells.

A Quasi-Quantum Well Sensitized Solar Cell with Accelerated Charge Separation and Collection

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed.

Cobalt-Embedded Nitrogen Doped Carbon Nanotubes: A Bifunctional Catalyst for Oxygen Electrode Reactions in a Wide pH Range

Electrocatalysts for the oxygen reduction and evolution reactions (ORR/OER) are often functionally separated, meaning that they are only proficient at one of the tasks. Here we report a high-performance bifunctional catalyst for both ORR and OER in both alkaline and neutral media, which is made of cobalt-embedded nitrogen doped carbon nanotubes. In OER, it shows an overpotential of 200 mV in 0.1 M KOH and 300 mV in neutral media, while the current density reaches 50 mA cm(-2) in alkaline media and 10 mA cm(-2) in neutral media at overpotential of 300 mV. In ORR, it is on par with Pt/C in both alkaline and neutral media in terms of overpotential, but its stability is superior. Further study demonstrated that the high performance can be attributed to the coordination of N to Co and the concomitant structural defects arising from the transformation of cobalt-phthalocyanine precursor.

Rational Design of Dipolar Chromophore as an Efficient Dopant-Free Hole-Transporting Material for Perovskite Solar Cells

In this paper, an electron donor-acceptor (D-A) substituted dipolar chromophore (BTPA-TCNE) is developed to serve as an efficient dopant-free hole-transporting material (HTM) for perovskite solar cells (PVSCs). BTPA-TCNE is synthesized via a simple reaction between a triphenylamine-based Michlers base and tetracyanoethylene. This chromophore possesses a zwitterionic resonance structure in the ground state, as evidenced by X-ray crystallography and transient absorption spectroscopies. Moreover, BTPA-TCNE shows an antiparallel molecular packing (i.e., centrosymmetric dimers) in its crystalline state, which cancels out its overall molecular dipole moment to facilitate charge transport. As a result, BTPA-TCNE can be employed as an effective dopant-free HTM to realize an efficient (PCE ≈ 17.0%) PVSC in the conventional n-i-p configuration, outperforming the control device with doped spiro-OMeTAD HTM.

Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells

The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

Building High-Efficiency CdS/CdSe-Sensitized Solar Cells with a Hierarchically Branched Double-Layer Architecture

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO2. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (Voc) and short-circuit current density (Jsc). Also, when the double layer was subjected to further branching, a large increase in Jsc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance.