Interfacial Charge Recombination Research Articles

All-Inorganic CsPbBr3 Perovskite Solar Cells with 10.45% Efficiency by Evaporation-Assisted Deposition and Setting Intermediate Energy Levels.

Nowadays, inorganic CsPbBr3 perovskite is emerging as a promising candidate as a light-absorbing layer in photovoltaic devices due to its excellent photoelectric property and superior stability under humidity and thermal attacks in comparison with organic cation-based hybrid perovskites. However, the impure perovskite phase and severe interfacial charge recombination have limited the further improvement of device performance. In this work, a vapor-assisted solution technique was introduced to prepare a high-purity CsPbBr3 film in a perovskite solar cell (PSC). To further reduce the electron-hole recombination and enhance charge extraction, we introduced the novel intermediate energy level of manganese sulfide (MnS) as a hole transport layer in CsPbBr3 PSC. The as-optimized CsPbBr3 PSC based on all-inorganic transport layers delivers a power conversion efficiency (PCE) of 10.45% in comparison with 8.16% for the device free of an intermediate layer, which is one of the highest PCEs achieved among the CsPbBr3-based PSCs to date. Moreover, the optimized device retained 80% PCE of its initial efficiency over 90 days under 80% relative humidity at 85 °C, indicating an excellent environmental tolerance to boost the commercial application of low-cost, efficient, and stable all-inorganic PSCs.

Pyridine-Terminated Conjugated Organic Molecules as an Interfacial Hole Transfer Bridge for NiOx-Based Perovskite Solar Cells.

To engineer the NiOx/perovskite interface and promote interfacial hole transfer, two pyridine-terminated conjugated small organic molecules (PTZ-1 and PTZ-2) are synthesized to link the NiOx and perovskite layers for NiOx-based perovskite solar cells (PSCs). One terminal pyridine group interacts with the NiOx layer, while the other one coordinates with the Pb atoms of the perovskite layer, erecting an interfacial hole transfer bridge between NiOx and perovskite. Surface modification of the NiOx film with the PTZ molecules is able to enhance hole extraction, increase hole mobility and conductivity of NiOx, reduce defect density, and retard interfacial charge recombination. As a consequence, power conversion efficiency is improved from 12.53 to 16.25 and 17.00% upon surface modifications of NiOx with PTZ-1 and PTZ-2, respectively. Furthermore, the modified PSCs exhibit almost no hysteresis and show good stability after storage in air (relative humidity of 30-40%) for 500 h without encapsulation.

LiTFSI‐Free Spiro‐OMeTAD‐Based Perovskite Solar Cells with Power Conversion Efficiencies Exceeding 19%

AbstractTo date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)2, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm2) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]+ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)2‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)2. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.

Bifunctional Dye Molecule in All‐Inorganic CsPbIBr2 Perovskite Solar Cells with Efficiency Exceeding 10%

Inorganic lead halide perovskites are attracting increasing attention due to their much better thermal stability than the organic–inorganic hybrid perovskite materials. Thus, the low power conversion efficiency (PCE) is a key issue for the inorganic lead halide perovskite solar cells (PSCs). This is mainly due to their wider bandgap and larger energy loss (Eloss) in the devices. Herein, for solving this issue, a dye molecule‐assisted engineering using the dye of 5,15‐bis(2,6‐dioctoxyphenyl)‐10‐(bis(4‐hexylphenyl)‐amino‐20‐4‐carboxyphenylethynyl)porphyrinato]zinc(II) (YD2‐o‐C8) is demonstrated. Results indicate that this molecule has a bifunctional effect, not only as a co‐sensitization layer for CsPbIBr2 with broader absorption spectrum but also reduces the Eloss by interface passivation. Specifically, the light absorption range of the photoactive layer is broadened from 600 to nearly 680 nm. At the same time, the interfacial charge recombination is highly reduced. After optimizing, the champion PCE is enhanced from 7.02% to 10.13%, and record‐high open‐circuit voltage (VOC) of 1.37 V and short‐circuit currents (JSC) of 12.05 mA cm−2 are achieved. This study opens a simple and efficient way to improve the efficiency of inorganic PSCs.

Influence of Hole Mobility on Charge Separation and Recombination Dynamics at Lead Halide Perovskite and Spiro-OMeTAD Interface

High efficiency lead halide perovskite solar cells employ spiro-OMeTAD or PTAA as a hole transporting material. This type of hole conductor requires dopants mainly to improve hole mobility. Although such doping has improved solar cell performance, in particular open circuit photovoltage and fill factor, the mechanism of the improvement has rarely been elucidated. Here, we demonstrate influence of dopants in spiro-OMeTAD on interfacial charge separation and recombination processes for a MAPbI3 perovskite film sandwiched by an m-TiO2 film and a spiro-OMeTAD layer by employing a series of transient absorption spectroscopies. The interfacial charge recombination time was significantly retarded by the doping. We propose that the retardation of the charge recombination originates from relatively longer distance of holes from the perovskite/spiro-OMeTAD interface owing to the increased hole mobility by the doping, potentially increasing Voc of the solar cell.

Achieving High Open-Circuit Voltage on Planar Perovskite Solar Cells via Chlorine-Doped Tin Oxide Electron Transport Layers.

The open-circuit voltage deficit is one of the main limiting factors for the further performance improvement in planar structured perovskite solar cells. In this work, we elaborately develop chlorine binding on the surface of tin oxide electron transport layer for a high open-circuit voltage device (1.195 V). The chlorine passivation on SnO2 not only effectively mitigates the interfacial charge recombination between SnO2 and perovskite but also enhances the binding of chlorine with lead at the SnO2/perovskite interface. The chlorine-passivated SnO2 electron transport layer exhibits a better energy alignment with the perovskite layer and an improved electron mobility, which will promote efficient electron transfer at the interface. In addition, the elevated Fermi level of SnO2 electron transport layer increases carrier extraction and suppresses interfacial recombination, which is responsible for the open-circuit voltage enhancement. Planar perovskite solar cells with chlorine-passivated SnO2 exhibit a higher open-circuit voltage of 1.195 V than that of reference ones (1.135 V) for a lower band gap of 1.58 eV perovskite absorbers, which achieve a power conversion efficiency of 20% with negligible hysteresis.

(Invited) Designing Novel Organic Small Molecular As Electron Transport Layers for Planar Perovskite Solar Cells

Compared to the traditional-architecture perovskite photovoltaics (n-i-p type), which use metal oxide as electron transport layers(ETLs) and organic semiconducting materials as hole transport layers, the fabrication of metal-oxide-free, solution-processed inverted perovskite solar cells (PSCs) is more desired because of low-temperatures and all-solution-based applications in future commercial PSC modules. In a typical configuration of inverted PSCs, the widely used ETL compound is the fullerene-based phenyl-C61-butyric acid methyl ester (PCBM), which currently is the best organic ETL material. The cost of this compound is very high, and the morphology and electrical properties are very sensitive to experimental conditions. In this talk, I will talk the recent progress of new organic ETL materials in my group for the replacement of PCBM in inverted PSCs. We believe that easily-accessible simple n-type small molecules are promising ETL candidates to further propel inverted PSCs to practical applications. References Ning Wang, Kexiang Zhao, Tao Ding, Wenbo Liu, Ali Said Ahmed, Zongrui Wang, Miaomiao Tian, Xiao Wei Sun,* Qichun Zhang*, “Improving Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer”, Adv. Energy Mater. 2017, DOI: 10.1002/aenm.201700522.Pei-Yang Gu, Ning Wang, Chengyuan Wang, Yecheng Zhou, Guankui Long, Miaomiao Tian, Wangqiao Chen, Xiao Wei Sun,* Mercouri G. Kanatzidis*, Qichun Zhang,* “Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecular as electron transport layer”, J. Mater. Chem. A. 2017, 5, 7339 – 7344.Pei-Yang Gu, Ning Wang, Anyang Wu, Zilong Wang, Miaomiao Tian, Zhisheng Fu,* Xiao Wei Sun,* Qichun Zhang* “Azaacene Derivative as Promising Electron Transport Layer for Inverted Perovskite Solar Cells”, Chem. Asian J. 2016, 11(15), 2135–2138.

Open atmospheric processed perovskite solar cells using dopant-free, highly hydrophobic hole-transporting materials: Influence of thiophene and selenophene π-spacers on charge transport and recombination properties

We have synthesized three donor-π-acceptor polymers by crosslinking 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b'] and pyrrolo[3,4-c]pyrrole-1,3-dione with thiophene, thieno[3,2-b]thiophene, selenophene as π-spacer. These polymers have been integrated successfully as dopant-free hole-transporting materials in fully open atmosphere solution-processed lead halide perovskite solar cells. Due to the synergistic impact of linear and long alkyl chains with different π-spacers, these hole transporters retard interfacial charge recombination and afford rich solubility and morphology resulting long carrier lifetime with uniform film coverage. In addition, these hole transporters possess a deeper highest occupied molecular orbitals (−5.40, −5.37 and −5.42 eV) and suitable over potential with perovskite valence band (−5.43 eV), favorable for efficient hole extraction. Further, the large number of S-based heterocycles helps to strengthen the interaction of the perovskite and hole transporting layer interface and high hydrophobicity (∼104°) facilitate enhanced stability with insignificant hysteresis. As a result, the power conversion efficiency has reached 14% under AM1.5 G without any additives in fully open atmospheric conditions. Our findings highlight the better energy transfer capabilities of additive-free hole transporters as a promising candidate for developing stable PSCs in the open air.

NaNbO3/MoS2 and NaNbO3/BiVO4 Core−Shell Nanostructures for Photoelectrochemical Hydrogen Generation

NaNbO3/MoS2 and NaNbO3/BiVO4 core–shell heterostructures show absorption range extending to the visible region and high charge transfer rate at the interface and lower charge recombination which re...

Quaternary quantum dots with gradient valence band for all-inorganic perovskite solar cells

The severe interface charge recombination caused by the large energy difference between perovskite material and carbon electrode significantly limits the further performance improvement of the all-inorganic perovskite solar cells (PSCs). We apply innovatively multilayer of quaternary Ag-In-Ga-S (AIGS) quantum dots (QDs) with cascade-like valence bands as hole-transport materials to assemble all-inorganic PSCs, and the resultant all-inorganic PSCs exhibit a power conversion efficiency (PCE) of 8.46%, which is enhanced by 20.9% in comparison with 7% for the pristine device. The high performance of the PSCs indicates that sequential layers of AIGS QDs with cascade-like energy levels can facilitate the charge separation, reduce the barrier the holes crossing and suppress the charge recombination. Stack of QDs with cascade-like energy levels provide solution-processed PSCs with a new method to enhance device performance.

Polymer/Fullerene Blend Solar Cells with Cadmium Sulfide Thin Film as an Alternative Hole-Blocking Layer.

In this work, chemical bath-deposited cadmium sulfide (CdS) thin films were employed as an alternative hole-blocking layer for inverted poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. CdS films were deposited by chemical bath deposition and their thicknesses were successfully controlled by tailoring the deposition time. The influence of the CdS layer thickness on the performance of P3HT:PCBM solar cells was systematically studied. The short circuit current densities and power conversion efficiencies of P3HT:PCBM solar cells strongly increased until the thickness of the CdS layer was increased to ~70 nm. This was attributed to the suppression of the interfacial charge recombination by the CdS layer, which is consistent with the lower dark current found with the increased CdS layer thickness. A further increase of the CdS layer thickness resulted in a lower short circuit current density due to strong absorption of the CdS layer as evidenced by UV-Vis optical studies. Both the fill factor and open circuit voltage of the solar cells with a CdS layer thickness less than ~50 nm were comparatively lower, and this could be attributed to the effect of pin holes in the CdS film, which reduces the series resistance and increases the charge recombination. Under AM 1.5 illumination (100 mW/cm2) conditions, the optimized PCBM:P3HT solar cells with a chemical bath deposited a CdS layer of thickness 70 nm and showed 50% power conversion efficiency enhancement, in comparison with similar solar cells with optimized dense TiO2 of 50 nm thickness prepared by spray pyrolysis.

Controlling Interfacial Charge Transfer and Fill Factors in CuO-based Tandem Dye-Sensitized Solar Cells.

We designed and synthesized a series of novel electron-accepting zinc(II)phthalocyanines (ZnPc) and probed them in p-type dye sensitized solar cells (p-DSSCs) by using CuO as photocathodes. By realizing the right balance between interfacial charge separation and charge recombination, optimized fill factors (FFs) of 0.43 were obtained. With a control over fill factors in p-DSSCs in hand we turned our attemtion to t-DSSCs, in which we combined for the first time CuO-based p-DSSCs with TiO2 -based n-DSSCs using ZnPc and N719. In the resulting t-DSSCs, the VOC of 0.86 V is the sum of those found in p- and n-DSSCs, while the FF remains around 0.63. It is only the smaller Jsc s in t-DSSCs that limits the efficiency to 0.69 %.

SrCO3-modified brookite/anatase TiO2 heterophase junctions with enhanced activity and selectivity of CO2 photoreduction to CH4

Utilizing the abundant solar energy to chemically convert CO2 into high-energy chemical fuels such as CO, CH4 or CH3OH offers a way to address the serious concerns about the increasing atmospheric CO2 levels and the depletion of fossil fuels. Herein, a series of SrCO3-modified TiO2 composites have been synthesized by adding SrCl2 into a TiCl4 hydrothermal reaction solution. It is found that brookite TiO2quasi nanocubes with high phase purity (denoted as pristine TiO2) can be derived from the TiCl4 reaction solution, and the additive Sr2+ ions lead to the formation of SrCO3-modified TiO2 heterophase junctions (HPJs) containing brookite nanorods and anatase nanoparticles. When used the SrCO3-modified TiO2 HPJs (hereafter denoted as SrCO3/HPJs) as photocatalyst for CO2 photoreduction, significantly enhanced activity and selectivity for CO2 photoreduction to CH4 can be achieved as compared to the pristine TiO2. Especially, 1.0% SrCO3/HPJs composite shows CH4/CO production activities of 19.66/2.64 μmol g−1 h−1 with an overall photoactivity of 162.6 μmol g−1 h−1, which is 12.3 times of the pristine TiO2 that shows an overall photoactivity of 13.2 μmol g−1 h−1 with CH4/CO production activities of 0.79/3.46 μmol g−1 h−1. The brookite/anatase TiO2 HPJs can effectively promote the photogenerated charge separation and restrain the interfacial charge recombination, and the SrCO3 species on the TiO2 HPJs can act as an efficient synergistic catalyst to improve the CO2 adsorption and activation abilities, and thus cause the enhanced activity and selectivity for CO2 photoreduction to CH4. This work represents the first example of coupling TiO2 HPJs with alkaline earth metal carbonates for efficient CO2 photoreduction.

Enhancing photocatalytic activity of tantalum nitride by rational suppression of bulk, interface and surface charge recombination

Rational design of photocatalysts is essential to achieve efficient solar energy conversion. For narrow bandgap Ta3N5 photocatalyst, various charge recombination occurring in the bulk, interface and on the surface significantly impairs its activity for solar hydrogen (H2) generation. Herein, a synergistic engineering approach is designed to solve this critical recombination challenge. First, hollow spherical structure of Ta3N5 with Mg doping is prepared to not only reduce the charge migration distance and increase the surface area, but also increase the electron mobility for facilitated charge transfer. Second, an MgO nano-layer covers the surface of hollow Ta3N5 structure to passivate surface defects, thus promoting the interfacial charge transfer between Ta3N5 and co-catalysts. Finally, dual co-catalysts (Pt/CoOx) for redox reactions are loaded onto the hollow Ta3N5 structure to reduce the surface recombination and overcome the sluggish surface reaction. Remarkably, the combination of hollow structure, Mg2+ doping, MgO interfacial layer, and dual co-catalysts effectively improves the charge separation and transfer in Ta3N5 photocatalyst. This newly designed photocatalyst exhibits a considerably improved H2 generation performance of 56.3 μmol h−1 under simulated sunlight, compared to that of reference Pt/Ta3N5 hollow spheres.

Enhancing electron transport via graphene quantum dot/SnO2 composites for efficient and durable flexible perovskite photovoltaics

Low-temperature processed GQDs and SnO2 nanoparticles composites (G@SnO2) have been prepared through a facile synthetic path. Facilitated electron transfer and suppressed interfacial charge recombination enable flexible perovskite solar cells with superb efficiency and excellent durability.

Interfacial recombination kinetics in aged perovskite solar cells measured using transient photovoltage techniques.

The reduction of interfacial charge recombination kinetics in perovskite solar cells is key to increase device photovoltaic efficiencies. Thus, it is necessary to fully understand which are the major carrier losses and, thereafter, how they can be minimized. Transient Photovoltage (TPV) has been widely used to study carrier recombination in solar cells under operando conditions. Interestingly, a novel negative transient deflection appears in perovskite solar cells when carrying out TPV measurements and it has been related to the ionic accumulation at the perovskite interfaces, which is a process that requires great attention to fully understand how perovskite solar cells work. Herein, we moved one step further and continuously monitored the evolution of the negative trace with the aging of the perovskite solar cell. Importantly, we demonstrated that the negative signal changes with aging of the solar cells and such a change can be directly related to the enhancement of the open circuit voltage and fill factor of the devices and thus the solar cell efficiency. We postulate that this increase in efficiency is due to better/faster ion redistribution within the perovskite material.

Achieving 20% Efficiency for Low‐Temperature‐Processed Inverted Perovskite Solar Cells

AbstractLow‐temperature‐processed inverted perovskite solar cells (PVSCs) attract increasing attention because they can be fabricated on both rigid and flexible substrates. For these devices, hole‐transporting layers (HTLs) play an important role in achieving efficient and stable inverted PVSCs by adjusting the anodic work function, hole extraction, and interfacial charge recombination. Here, the use of a low‐temperature (≤150 °C) solution‐processed ultrathin film of poly[(9,9‐dioctyl‐fluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐secbutylphenyl) diphenylamine)] (TFB) is reported as an HTL in one‐step‐processed CH3NH3PbI3 (MAPbI3)‐based inverted PVSCs. The fabricated device exhibits power conversion efficiency (PCE) as high as 20.2% when measured under AM 1.5 G illumination. This PCE makes them one of the MAPbI3‐based inverted PVSCs that have the highest efficiency reported to date. Moreover, this inverted PVSC also shows good stability, which can retain 90% of its original efficiency after 30 days of storage in ambient air.

A BiVO4 film photoanode with re-annealing treatment and 2D thin Ti3C2TX flakes decoration for enhanced photoelectrochemical water oxidation

Bismuth vanadate (BiVO4) has been envisioned as a promising photoanode material for photoelectrochemical (PEC) water oxidation. However, the PEC performance of BiVO4 based photoanodes is usually dragged down by the poor charge transport and slow kinetics for an oxygen evolution reaction (OER). Here, a novel composite photoanode of Ti3C2TX/BiVO4 was fabricated for the first time by spin coating thin Ti3C2TX (MXene) flakes onto the surface of a BiVO4 film grown on the FTO substrate. A facile re-annealing treatment of bare BiVO4/FTO photoanode in argon can impressively boost the photocurrent density from 2.1 mA cm−2 up to 2.95 mA cm−2. Experimentally, we clearly elucidated that by re-annealing, the interfacial (BiVO4/FTO) charge recombination could be effectively suppressed due to the better contact between the BiVO4 film and FTO. The coating of thin Ti3C2TX flakes onto the BiVO4 film would further increase the photocurrent density to 3.45 mA cm−2 (measured at 1.23 V vs. reversible hydrogen electrode (RHE) under 1 sun illumination), exhibiting a photoconversion efficiency of 0.78% and a surface charge separation efficiency of 73%. The outstanding PEC performance can be mainly ascribed to the coating of the thin Ti3C2TX (MXene) flakes which function as co-catalysts, facilitating the charge transfer of the photogenerated carriers and reducing the charge recombination on the BiVO4 surface. Our findings provide a facile re-annealing strategy and a novel two-dimensional (2D) material of Ti3C2TX (MXene) as co-catalysts to improve the performance of the BiVO4 photoanode for PEC water oxidation.

Schottky/p‐n Cascade Heterojunction Constructed by Intentional n‐Type Doping Perovskite Toward Efficient Electron Layer‐Free Perovskite Solar Cells

Electron transport layer (ETL)‐free perovskite solar cells (PSCs) are getting much more attention with their simpler structure and potentially low cost as well as higher stability. However, the elimination of ETL (such as TiO2) with intrinsically deep valance band level leads to the absence of the hole blocking mechanism and thus serious charge recombination at the FTO/perovskite interface compared with ETL‐based devices. An interface band bending associated built‐in electric field is an essential driving force of charge separation. Here, by intentional polarity tailoring of perovskite via incorporation of Sb as a shallow donor, ETL‐free PSCs with optimized energy level alignment both at the front and rear interfaces are constructed, resulting in an enhanced built‐in electric field and thus efficient majority carrier collection and minority blocking as well as reduced interfacial charge recombination at both interfaces. Device simulation calculations also confirm the importance of polarity control for device performance improvement. The effect of doping on the perovskite films properties and device performance are systematically demonstrated. Correspondingly, ETL‐free PSCs with a champion power conversion efficiency of 12.62% is achieved.

High-Efficiency, Hysteresis-Less, UV-Stable Perovskite Solar Cells with Cascade ZnO-ZnS Electron Transport Layer.

Perovskite solar cells (PSCs) have reached certified efficiencies of up to 23.7% but suffered from frailness and instability when exposed to ambient atmosphere. Zinc oxide (ZnO), when used as electron transport layer (ETL) on PSCs, gives rise to excellent electronic, optic, and photonic properties, yet the Lewis basic nature of ZnO surface leads to deprotonation of the perovskite layer, resulting in serious degradation of PSCs using ZnO as ETL. Here, we report a simple but effective strategy to convert ZnO surface into ZnS at the ZnO/perovskite interface by sulfidation. The sulfide on ZnO-ZnS surface binds strongly with Pb2+ and creates a novel pathway of electron transport to accelerate electron transfer and reduce interfacial charge recombination, yielding a champion efficiency of 20.7% with improved stability and no appreciable hysteresis. The model devices modified with sulfide maintained 88% of their initial performance for 1000 h under storage condition and 87% for 500 h under UV radiation. ZnS is demonstrated to act as both a cascade ETL and a passivating layer for enhancing the performance of PSCs.