Xuewen Yin

Cross-stacked superaligned carbon nanotube electrodes for efficient hole conductor-free perovskite solar cells

Cross-stacked superaligned carbon nanotube (CSCNT) sheets drawn from CNT arrays with excellent conductivity, porosity and flexibility are used as low-cost back contacts for hole conductor-free mesoscopic CH3NH3PbI3/TiO2 heterojunction perovskite solar cells (PSCs). PSCs integrated with CSCNTs achieve a power conversion efficiency of up to 8.65% and over 10.54% by further doping the CSCNTs with iodine. Moreover, PSCs encapsulated with a layer of polymer poly(methylmethacrylate) exhibit outstanding stability in the dark and under illumination. This work represents a significant step toward the commercialization of the low-cost, stable and high-efficiency hole conductor-free PSCs.

Hematite electron-transporting layers for environmentally stable planar perovskite solar cells with enhanced energy conversion and lower hysteresis

Non-obvious hysteresis and higher steady-state power conversion efficiency (PCE) were demonstrated by simply employing hematite (α-Fe2O3) as the electron transporting layer (ETL) to replace the conventional titania (TiO2) ETL in planar heterojunction perovskite solar cells. The achieved higher built-in potential across the perovskite layer for the devices using α-Fe2O3 ETLs led to more efficient charge extraction/transport and less charge recombination than using TiO2 ETLs. As a consequence, a significant reduction in the charge accumulation at the perovskite/α-Fe2O3 interface made the device much less sensitive to the scanning rate and direction, i.e., lower hysteresis. Furthermore, α-Fe2O3 based devices displayed good stability over 30 days of storage time with exposure to ambient air, owing to the higher crystalline quality and uniform grain size of the perovskite films deposited on α-Fe2O3 ETLs than on TiO2 ETLs.

High Efficiency Inverted Planar Perovskite Solar Cells with Solution-Processed NiOx Hole Contact

NiOx is a promising hole-transporting material for perovskite solar cells due to its high hole mobility, good stability, and easy processability. In this work, we employed a simple solution-processed NiOx film as the hole-transporting layer in perovskite solar cells. When the thickness of the perovskite layer increased from 270 to 380 nm, the light absorption and photogenerated carrier density were enhanced and the transporting distance of electron and hole would also increase at the same time, resulting in a large charge transfer resistance and a long hole-extracted process in the device, characterized by the UV-vis, photoluminescence, and electrochemical impedance spectroscopy spectra. Combining both of these factors, an optimal thickness of 334.2 nm was prepared with the perovskite precursor concentration of 1.35 M. Moreover, the optimal device fabrication conditions were further achieved by optimizing the thickness of NiOx hole-transporting layer and PCBM electron selective layer. As a result, the best power conversion efficiency of 15.71% was obtained with a Jsc of 20.51 mA·cm-2, a Voc of 988 mV, and a FF of 77.51% with almost no hysteresis. A stable efficiency of 15.10% was caught at the maximum power point. This work provides a promising route to achieve higher efficiency perovskite solar cells based on NiO or other inorganic hole-transporting materials.

CH3NH3PbI3 grain growth and interfacial properties in meso-structured perovskite solar cells fabricated by two-step deposition

Abstract Although the two-step deposition (TSD) method is widely adopted for the high performance perovskite solar cells (PSCs), the CH3NH3PbI3 perovskite crystal growth mechanism during the TSD process and the photo-generated charge recombination dynamics in the mesoporous-TiO2 (mp-TiO2)/CH3NH3PbI3/hole transporting material (HTM) system remains unexploited. Herein, we modified the concentration of PbI2 (C(PbI2)) solution to control the perovskite crystal properties, and observed an abnormal CH3NH3PbI3 grain growth phenomenon atop mesoporous TiO2 film. To illustrate this abnormal grain growth mechanism, we propose that a grain ripening process is taking place during the transformation from PbI2 to CH3NH3PbI3, and discuss the PbI2 nuclei morphology, perovskite grain growing stage, as well as Pb:I atomic ratio difference among CH3NH3PbI3 grains with different morphology. These C(PbI2)-dependent perovskite morphologies resulted in varied charge carrier transfer properties throughout the mp-TiO2/CH3NH3PbI3/HTM hybrid, as illustrated by photoluminescence measurement. Furthermore, the effect of CH3NH3PbI3 morphology on light absorption and interfacial properties is investigated and correlated with the photovoltaic performance of PSCs.

Rational Design of Solution-Processed Ti–Fe–O Ternary Oxides for Efficient Planar CH3NH3PbI3 Perovskite Solar Cells with Suppressed Hysteresis

Electron-extraction layer (EEL) plays a critical role in determining the charge extraction and the power conversion efficiencies of the organometal-halide perovskite solar cells (PSCs). In this work, Ti-Fe-O ternary oxides were first developed to work as an efficient EEL in planar PSC. Compared with the widely used TiOx and the pure FeOx, the ternary composites show superior properties in multiple aspects including the excellent stability of the precursor solution, good coverage on the substrates, outstanding electrical properties, and suitable energy levels. By varying the Fe content from 0 to 100% in the Ti-Fe-O composites, the conductivity of the resultant compact layer was markedly improved, confirmed by consistent results from the conductive atomic force microscopy and the linear sweep voltammetry measurements. Meanwhile, the compositional engineering tunes the energy level alignment of the Ti-Fe-O EEL/CH3NH3PbI3 interface to a region that is favorable for obtaining excellent charge-extraction property. The combinational advantages of the Ti-Fe-O composites significantly improved the photovoltaic performance of the as-prepared solar cells. An increase of over 20% in the short-circuit current (JSC) density has been achieved due to a modified EEL conductivity and energy alignment with the perovskite layer. The reduction in the surface recombination and enhancement of the charge collection efficiency also result in about 15% increase in the fill factor. Notably, the device also showed remarkably alleviated hysteresis behavior, revealing a prominently inhibited surface recombination.

Bifacial Modified Charge Transport Materials for Highly Efficient and Stable Inverted Perovskite Solar Cells

Significant efforts have been devoted to enhancing both the performance and long-term stability of lead halide perovskite solar cells (PSCs) to promote their practical application. In this context, a self-assembled monolayer composed of a dye molecule is demonstrated for the first time to be efficient in passivating the surface of the hole transport layer, NiO x, in the p-i-n PSCs through multiple functions, including the minimization of energy-level offset, reducing surface trap states, and enhancing wetting between NiO x and perovskite layers coupled with increasing perovskite crystallinity. Consequently, the dye monolayer has sufficiently improved the hole extraction efficiency and suppressed the charge recombination, validated by steady and transient photoluminescence measurements and the electrochemical impedance analysis. Concurrently, a mixed layer of BaSnO3 nanoparticles and [6,6]-phenyl-C61-butyric acid methyl (PCBM) (barium stannate (BSO)/PCBM) was exploited as an efficient electron transport layer, resulting in superior electron transport properties and correspondingly excellent device stability. By incorporating these bifacial modifications, the device performance of the inverted PSC was propelled to 16.2%, compared with 14.0% for that without any interfacial and compositional engineering. Benefiting from the excellent crystallinity of the perovskite through dye passivation and the blocking of moisture, oxygen, and ion migration by using the hybrid BSO/PCBM layer, over 90% of the initial power conversion efficiency has been preserved for the device after exposure to ambient air for 650 h.

Efficiently Improving the Stability of Inverted Perovskite Solar Cells by Employing Polyethylenimine-Modified Carbon Nanotubes as Electrodes

Inverted perovskite solar cells (PSCs) have been becoming more and more attractive, owing to their easy-fabrication and suppressed hysteresis, while the ion diffusion between metallic electrode and perovskite layer limit the long-term stability of devices. In this work, we employed a novel polyethylenimine (PEI) modified cross-stacked superaligned carbon nanotube (CSCNT) film in the inverted planar PSCs configurated FTO/NiO x/methylammonium lead tri-iodide (MAPbI3)/6, 6-phenyl C61-butyric acid methyl ester (PCBM)/CSCNT:PEI. By modifying CSCNT with a certain concentration of PEI (0.5 wt %), suitable energy level alignment and promoted interfacial charge transfer have been achieved, leading to a significant enhancement in the photovoltaic performance. As a result, a champion power conversion efficiency (PCE) of ∼11% was obtained with a Voc of 0.95 V, a Jsc of 18.7 mA cm-2, a FF of 0.61 as well as negligible hysteresis. Moreover, CSCNT:PEI based inverted PSCs show superior durability in comparison to the standard silver based devices, remaining over 85% of the initial PCE after 500 h aging under various conditions, including long-term air exposure, thermal, and humid treatment. This work opens up a new avenue of facile modified carbon electrodes for highly stable and hysteresis suppressed PSCs.

Vertically aligned ZnO/ZnTe core/shell heterostructures on an AZO substrate for improved photovoltaic performance

Vertically aligned ZnO/ZnTe core/shell heterostructures on an Al-doped ZnO substrate are developed for non-toxic semiconductor sensitized solar cells. Structural and morphological analysis serves as evidence of the successful synthesis of ZnO nanorods, ZnTe nanocrystals and ZnO/ZnTe heterostructures. The clearly observed quenching of photoluminescence (PL) from the heterostructure indicates efficient charge transfer occurring at the interface of ZnO and ZnTe, due to the type-II energy level alignment constructed by the two. The formation mechanism of the ZnO/ZnTe heterostructure is studied in depth via time-dependent reactions. It was found that the strain between ZnO and ZnTe modifies the band alignment at the interface of the heterostructure in a manner which depends on the growth time. Finally, sensitized solar cells based on the ZnO/ZnTe heterostructures with different ZnTe growth times were fabricated to evaluate the photovoltaic performance. By the careful control of the ZnTe growth time and as a result of the band alignment between ZnO and ZnTe, the power conversion efficiency (PCE) of the vertically aligned ZnO/ZnTe based solar cells could be improved to about 2%, along with a short-circuit photocurrent density of around 7.5 mA cm−2, a record efficiency for ZnO/ZnTe based sensitized solar cells. Notably, for the optimized system the internal quantum efficiency of the ZnO/ZnTe based solar cell approaches 100% in certain wavelengths, implying effective separation of photoexcited free carriers towards either the electrolyte or anode.

Highly efficient inverted perovskite solar cells based on self-assembled graphene derivatives

The performance of inverted perovskite solar cells (PSCs) based on graphene oxide hole transporting materials is still unsatisfactory due to the high degree of surface oxygen contents and the insulating properties. In this study, thickness-controlled and full-coverage graphene oxide films prepared using a layer-by-layer self-assembly technique are first developed as hole transporting layers for PSCs. Meanwhile, conductivity-tunable reduced graphene oxide films are in situ prepared using an environment-friendly and efficient reductant system. A superior PCE of 16.28% based on the as-prepared rGO is obtained, resulting in an increment by approximately 33% compared with 12.26% of the device based on GO-1 as mentioned. At the same time, this work reveals an anomalous charge-extraction behavior of PSCs based on GO or rGO HTLs. The competitive effect of interfacial recombination, charge transportation and radiation recombination in this process is proposed to analyze the internal mechanisms. This work provides a facile and novel method to prepare GO or rGO films, which can be used as efficient charge-extraction layers and even as electrodes in inverted PSCs.

Role of Alkyl Chain Length in Diaminoalkane Linked 2D Ruddlesden-Popper Halide Perovskites

Alkylammonium cations were widely applied in 2D halide perovskites as bulky spacers to improve their stability, while diaminoalkane molecules containing two –NH2 groups at the heads of the alkyl chains have been rarely investigated. Herein, we report on the synthesis and properties of a new series of 2D Ruddlesden–Popper halide perovskites with diaminoalkane as a bulky spacer, typically, with a compositional formula of (NH3(CH2)xNH3)(CH3NH3)2Pb3I10, and the spacer length effects were systematically studied. As the alkyl chain length increased (x = 3 to 8), the layered structure of the as-made 2D rectangular plates became more complete, and the differences in crystalline structure, light absorbance, band energy position and filming properties became evident. In particular, (DAT)(MA)2Pb3I10 (x = 8, abbr. as DAT) with a pure chemical composition was synthesized and showed excellent 2D characteristic properties compared with well-studied (CH3(CH2)3NH3)2(MA)2Pb3I10 (abbr. as BA). A crystal growth mechanism including “layered” and “stepped” growth was originally proposed to elucidate the orientation and morphology difference between 2D DAT and BA perovskites. Moreover, the deposited DAT film showed outstanding film characteristics, including a facile fabrication method and an ultralow surface roughness of 2.97 nm. Furthermore, we have successfully implemented this new 2D perovskite in solar cells, and obtained an inspiring power conversion efficiency.