Showing papers by "Xiaoming Wen published in 2015"

Methylammonium Lead Bromide Perovskite-Based Solar Cells by Vapor-Assisted Deposition

Abstract: The past two years have seen the uniquely rapid emergence of a new class of solar cell based on organic–inorganic halide perovskite. Although less explored than its tri-iodide counterparts, CH3NH3PbBr3 has a larger bandgap of 2.3 eV with a higher voltage potential that is suitable for tandem solar cell applications. In this paper, we report a vapor-assisted method for depositing and fully crystallizing CH3NH3PbBr3 film on mesoporous TiO2 with good coverage. CH3NH3PbBr3 fabricated using this method has demonstrated long carrier diffusion length (>1 μm) as estimated by transient photoluminescence-quenching measurements. We demonstrate solar cells fabricated using such films and spiro-OMeTAD as the hole transport layer with an averaged (from forward and reverse scans) conversion efficiency of 8.7%, Voc of 1.45 V, Jsc of 9.75 mA/cm2, and fill factor of 61.5%.

Mobile Charge-Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite

Abstract: Organic–inorganic halide perovskite has emerged as a very promising material for solar cells due to its excellent photovoltaic enabling properties resulting in rapid increase in device efficiency over the last 3 years. Extensive knowledge and in-depth physical understanding in the excited state carrier dynamics are urgently required. Here we investigate the fluorescence intermittency (also known as blinking) in vapor-assisted fabricated CH3NH3PbBr3 perovskite. The evident fluorescence blinking is observed in a dense CH3NH3PbBr3 perovskite film that is composed of nanoparticles in close contact with each other. In the case of an isolated nanoparticle no fluorescence blinking is observed. The “ON” probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. As the vapor-assisted fabricated CH3NH3PbBr3 perovskite film is a cluster of nanoparticles forming a dense film, it facilitates mobile charge migratio...

Kesterite Cu2ZnSn(S,Se)4 Solar Cells with beyond 8% Efficiency by a Sol-Gel and Selenization Process.

Abstract: A facile sol–gel and selenization process has been demonstrated to fabricate high-quality single-phase earth abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic absorbers. The structure and band gap of the fabricated CZTSSe can be readily tuned by varying the [S]/([S] + [Se]) ratios via selenization condition control. The effects of [S]/([S] + [Se]) ratio on device performance have been presented. The best device shows 8.25% total area efficiency without antireflection coating. Low fill factor is the main limitation for the current device efficiency compared to record efficiency device due to high series resistance and interface recombination. By improving film uniformity, eliminating voids, and reducing the Mo(S,Se)2 interfacial layer, a further boost of the device efficiency is expected, enabling the proposed process for fabricating one of the most promising candidates for kesterite solar cells.

Fluorescent Metallic Nanoclusters: Electron Dynamics, Structure, and Applications

Abstract: Atomically precise Au nanoclusters (NCs) have emerged as fascinating fluorescent nanomaterials and attracted considerable research interest in both fundamental research and practical applications. Due to enhanced quantum confinement, they possess extraordinary optical, electronic, and magnetic properties and therefore are very promising for a wide range of applications, including biosensing, bioimaging, catalysis, photonics, and molecular electronics. Remarkable progress has been reported for the fundamental understanding, synthesis techniques, and applications. In this review, the updated advances are summarized in Au NCs, including synthesis techniques, optical properties, and applications. In particular, we focus on the optical properties and electron dynamic processes. In addition, the progress in other noble metallic NCs is included in this Review, such as Ag, Cu, Pt, and alloy, which have attracted much research interest recently. Finally, an outlook is presented for such fascinating nanomaterials in both aspects of future fundamental research and potential applications.

Tunability Limit of Photoluminescence in Colloidal Silicon Nanocrystals

Abstract: Luminescent silicon nanocrystals (Si NCs) have attracted tremendous research interest. Their size dependent photoluminescence (PL) shows great promise in various optoelectronic and biomedical applications and devices. However, it remains unclear why the exciton emission is limited to energy below 2.1 eV, no matter how small the nanocrystal is. Here we interpret a nanosecond transient yellow emission band at 590 nm (2.1 eV) as a critical limit of the wavelength tunability in colloidal silicon nanocrystals. In the “large size” regime (d > ~3 nm), quantum confinement dominantly determines the PL wavelength and thus the PL peak blue shifts upon decreasing the Si NC size. In the “small size” regime (d < ~2 nm) the effect of the yellow band overwhelms the effect of quantum confinement with distinctly increased nonradiative trapping. As a consequence, the photoluminescence peak does not exhibit any additional blue shift and the quantum yield drops abruptly with further decreasing the size of the Si NCs. This finding confirms that the PL originating from the quantum confined core states can only exist in the red/near infrared with energy below 2.1 eV; while the blue/green PL originates from surface related states and exhibits nanosecond transition.

Kesterite Cu2ZnSnS4 thin film solar cells by a facile DMF-based solution coating process

Abstract: Kesterite Cu2ZnSnS4 (CZTS) thin films were fabricated using a low-cost and environmentally friendly route from a dimethylformamide (DMF) solution of a metal–thiourea complex. Thermal gravimetric analysis (TGA) has been performed to reveal the thermal decomposition behavior of the CZTS precursors for drying and sulfurization process design. A facile solution method of in situ introducing sodium dopant by adding NaOH into the precursor solution is presented. The sodium dopant improves the open circuit voltage (Voc) and fill factor (FF) and thereby enhances the power conversion efficiency from 4.47% to 5.68%. The enhanced performance is related to the increased grain size and increased minority carrier lifetime. A large number of large voids observed in the bulk absorber and at the absorber/back contact interface are considered to be the main reason for the low short circuit current density (Jsc).

Theoretical and Experimental Investigation of the Electronic Structure and Quantum Confinement of Wet-Chemistry Synthesized Ag2S Nanocrystals

Abstract: The electronic and excitonic confinement of monoclinic silver sulfide Ag2S nanocrystals (NCs) has been investigated both theoretically and experimentally. Theoretically, the electronic band gaps and the excitonic ground-state energies of Ag2S NCs at different sizes are calculated using a hybrid method of density-functional theory (DFT) and effective mass approximation (EMA). The results have shown a good agreement with measured results, including the absorbance and photoluminescence spectra of drop-casted NC thin films. The Ag2S NCs, synthesized using wet-chemistry methods, have demonstrated a desirable size monodispersion, allowing the measurements to be sufficiently accurate. The results show that both the electronic band gap and the excitonic energy have significant blue shift only when the diameter of monodispersed NCs is reduced below 4 nm, and the excitonic Bohr radius of Ag2S is determined as being small at around 1 nm.

Introducing a protective interlayer of TiO2 in Cu2O–CuO heterojunction thin film as a highly stable visible light photocathode

Abstract: Visible light-induced photocurrent generation and photoelectrochemical stability of p-type Cu2O–CuO photocathodes are improved significantly upon incorporating an interlayer of TiO2 between Cu2O and CuO. The TiO2 layer hinders the electron conduction at the semiconductor–electrolyte interface (improved stability) as well as promoting electron transfer from Cu2O to CuO (increased photocurrent). Upon visible light illumination, the optimised multilayer Cu2O–TiO2–CuO heterojunction thin film yields a photocurrent of 2.4 mA cm−2 and retains 75% of its photoactivity over the measurement period. By comparison, the unmodified Cu2O–CuO generates a photocurrent of 1.3 mA cm−2 with photoactivity retention of only 32% after prolonged illumination. Wavelength-dependent incident photon-to-current efficiency (IPCE) reveals a considerable enhancement over the excitation region of Cu2O (400–560 nm). Transient fluorescence decay analysis suggests the promotion of electron transfer from Cu2O to CuO through TiO2. As a result, both photoactivity and photochemical stability of the photocathodes are improved.

Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics

Abstract: Air stability and efficient multiple exciton generation (MEG) are crucial for the application of PbSe quantum dot (QD) thin film in next generation photovoltaics. Recently it was reported that PbS QD thin films with solid-state halide atomic ligands exhibited superior performance in terms of stability and efficiency. There is great interest in applying these halide ligands to PbSe QD thin films to improve the stability, while their additional effects on the hot carrier dynamics and hence MEG efficiency remain unknown. Here, we demonstrate that proper halide treatments can modify both the stability and hot carrier thermalization of a PbSe QD thin film. This confirms that using proper halide ligands in the solid-state ligand exchange step for film fabrication can significantly improve the stability. The film subjected to an iodide treatment exhibited the best air stability, and additionally its hot carrier thermalization time can be three times longer than that with a chloride treatment. We suggest that str...

Improving Efficiency of Evaporated Cu2ZnSnS4 Thin Film Solar Cells by a Thin Ag Intermediate Layer between Absorber and Back Contact

Abstract: A 20 nm Ag coating on Mo back contact was adopted to improve the back contact of evaporated Cu2ZnSnS4 (CZTS) solar cells. The Ag layer helped reduce the thickness of MoS2 which improves fill factor (FF) significantly; additionally, it reduced secondary phases ZnS and SnS2−x, which may help carrier transport; it was also involved in the doping of the absorber layer, which compensated the intrinsic p-type doping and therefore drags down the doping level. The doping involvement may enlarge the depletion region and improve lifetime of the absorber, which led to enhancing open circuit voltage (), short circuit current density (), and efficiency significantly. However, it degrades the crystallinity of the material slightly.

Enhanced Visible Light-Induced Charge Separation and Charge Transport in Cu2O-Based Photocathodes by Urea Treatment

Abstract: Carrier density, photocharge transfer kinetics, and charge transfer resistance of the anodized Cu–Cu2O–CuO photocathode were greatly improved using thermal treatment with urea. Time-correlated single-photon counting (TCSPC) results revealed the faster electron transfer kinetics from Cu2O to CuO in the urea-treated Cu–Cu2O–CuO composite photoelectrodes. Preservation of the metallic copper component via the intermediate Cu3N during the treatment facilitated higher bulk conductance of the Cu–Cu2O–CuO photocathode for improved charge transport. Higher carrier density was also observed in the urea-treated photoelectrode, which was possibly attributed to the presence of nitrogen as a dopant. Furthermore, the compact outer layer of CuO protected the underlayer Cu2O from being in direct contact with the aqueous solution. This suppressed the photocorrosion of Cu2O and resulted in the higher photostability of the Cu–Cu2O–CuO film. When these advantages were combined, the urea-treated Cu–Cu2O–CuO film showed a highe...

Effects of blend composition on the morphology of Si-PCPDTBT:PC71BM bulk heterojunction organic solar cells

Abstract: This work investigates the effects of the solution blend composition on the morphology of binary bulk heterojunction organic solar cells composed of poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta2,1-b:3,4-b′]dithiophene-siloe 2,6-diyl]] (Si-PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The polymer–fullerene ratio was varied from 2:9 to 1:1 (PC71BM weight ratio from 82 to 50 wt%). The dependence of the self-assembly of the polymer phase on the blend composition was investigated using X-ray diffraction measurements. A high polymer loading is required to allow the formation of large crystalline polymer domains in this materials system. X-ray photoelectron spectroscopy measurements revealed that this blend compositional dependence on the crystalline domain size correlated with a gradual alteration of the vertical phase distribution of polymer throughout the active layer. Using photoluminescence and short-circuit current results, we show that the efficacy of exciton dissociation is highly dependent on the solution blend composition. Coupled with mobility measurements, which indicate the hole mobility is also heavily dependent upon the content of polymer in the film, we explain the optimum blend performance of Si-PCPDTBT:PC71BM organic solar cells which occurs at a blend ratio of 2:3.

Characterization of a Cu2ZnSnS4 solar cell fabricated by sulfurization of metallic precursor Mo/Zn/Cu/Sn

Abstract: Various characterization methods are implemented to investigate the fundamental properties of a Cu2ZnSnS4 (CZTS) solar cell. The chemical distribution across the CZTS grain boundaries, the surface potential of CZTS absorber, the minority lifetime, the carrier collection length, diode ideality factor, dark saturation current, and series resistance are revealed in the characterization measurement. The short minority lifetime, high defect density, and large series resistance are confirmed and need to be addressed in the future work for further efficiency improvement.

Effect of Blend Composition on Binary Organic Solar Cells Using a Low Band Gap Polymer.

Abstract: This report investigates the influence of the solution blend composition of binary bulk heterojunction organic solar cells composed of poly(2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H- cyclopenta[2,1-b:3,4-b'dithiophene-2,6-diy]] (PCPDTBT) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The blend polymer:fullerene composition was varied from 1:1 (50 wt% PC71 BM) to 2:9 (82 wt% PC71 BM). Increasing the amount of polymer in the blend results in the greatest overall absorption, as the donor material PCPDTBT is the main contributor to absorption. However, high polymer content leads to poor photovoltaic performance. For this material combination, the optimum blend polymer:fullerene composition was found to be 2:7. Increasing the fullerene content in the blend led to a significant improvement in the internal quantum efficiency of devices. This was correlated with an increase of the electron mobility, as the fullerene content was increased. Improved electron transport, leading to more balanced transport between electrons and holes, significantly improved the short circuit current density (Jsc) and fill factor (FF).

Illumination dependent carrier dynamics of CH3NH3PbBr3 perovskite

Abstract: The excellent light harvesting properties and potentially low cost fabrication of organometal halide perovskites have attracted great attention in their application as solar cell device. Apart from the general advantages of organic-inorganic perovskite, CH3NH3PbBr3 has a larger bandgap (~2.3eV) suitable to be the top cell in a tandem solar device. Here we use steady-state and time-resolved photoluminescence (PL) techniques to investigate the photophysical behaviour of CH3NH3PbBr3 perovskite including its carrier dynamics under continuous illumination. Samples were studied under different illumination conditions and the following observations were made: (1) defect assisted recombination is dominant under low excitation under nano-second scale measurement, (2) bimolecular and Auger recombinations dominate under high excitation under the minute timescale measurement, (3) the magnitude PL decay traces decrease over time under continuous excitation. We propose that both the density of photo-generated free carriers and the density of mobile ions have an impact on the carrier dynamic of CH3NH3PbBr3. This finding provides insights into the photophysical properties of perovskite materials.

Carrier dynamics and phonon properties of hafnium nitride: Potential hot carrier solar cell absorber

Abstract: Hot carrier solar cells aims to circumvent fundamental loss mechanisms by extracting carriers before thermalization of hot carriers occurs while also minimizing sub-bandgap losses. The absorber must be able to maintain a hot carrier for a sufficiently long time. Hafnium nitride owing to its phononic properties to slow carrier thermalization is a potential absorber for the hot carrier solar cell. The phonon properties have been measured by Raman spectroscopy. The carrier dynamics in hafnium nitride has been studied by ultrafast pump-probe transient absorption where nanosecond long lifetimes have been observed. The carrier temperature has been extracted from the transient absorption spectra to show the carrier temperature remained above 350 K for a long time with a decay time constant of 1.2 ns.

Ultrafast charge generation and relaxation dynamics in methylammonium lead bromide perovskites

Abstract: Methylammonium Lead Bromide (CH 3 NH 3 PbBr 3 ) is a promising material for tandem solar cell due to its high band gap. Ultrafast optical techniques on a time scale of femto- and picosecond are used to investigate the carrier dynamics in CH 3 NH 3 PbBr 3 . An ultrafast cooling of hot carriers occurs in sub-picoseconds in CH 3 NH 3 PbBr 3 by phonon scattering. Two ultrafast relaxation processes are attributed to optical phonon scattering and acoustic phonon scattering. The relaxation processes are evidently slower when CH 3 NH 3 PbBr 3 is in contact with compact TiO 2 (c-TiO 2 ) layer, suggesting better quality CH 3 NH 3 PbBr 3 . when deposited on c-TiO 2 . The nanosecond decay in CH 3 NH 3 PbBr 3 film is ascribed to electron-hole recombination. With the presence of c-TiO 2 layer, this process is accelerated due to electron transport across the CH 3 NH 3 PbBr 3 / c-TiO 2 interface.

Potential of transition metal nitrides and hydrides as hot carrier solar cell absorbers

Abstract: The hot carrier (HC) solar cell aims to minimize the major losses in single junction solar cells due to the non-absorbance of the sub-bandgap photons and thermalisation of above bandgap photons (‘hot carriers’) and hence can achieve very high efficiencies. One of the crucial requirements for this is to have an absorber which can sufficiently reduce the rate of hot carrier cooling so that they can be collected at higher energies. Bulk, abundant and non-toxic materials can facilitate cheap fabrication methods thereby significantly reduce generation cost of solar electricity. In this paper we will introduce a few bulk materials as potential candidates for HC solar cell absorber.

Ultrafast transient absorption study of hot carrier dynamics in hafnium nitride and zirconium nitride

Abstract: Hot carrier solar cell (HCSC) is a promising third generation photovoltaic device which can potentially able to overcome the Shockley-Queisser limit and achieve much higher power efficiency than present single junction solar cells. The theoretical efficiency for an ideal HCSC is predicted to be 65% under 1 sun solar radiation or 85% under maximal concentration. A critical requirement of the absorber is to slow down the cooling rate of photoexcited hot carriers, from few ps for typical semiconductors to hundreds of ps or longer. Transition nitrides HfN and ZrN theoretically have large phononic bandgaps and thus much slowed cooling time may be expected in these materials. In this investigation ultrafast transient absorption (TA) was used to investigate HfN and ZrN thin films. Experiments reveal three featured TA bands in the visible and near infrared, ascribed to the excited state absorption and bleaching. Cooling time of hot carriers as long as 3 ns in HfN is attributed to the significantly suppressed Klemens' decay. While hundreds of ps cooling time were observed in ZrN. The slowed thermalization process in HfN and ZrN suggests these bulk materials are a promising absorber candidate for hot carrier solar cells.

Fabrication and optical characterisation of InGaN/GaN nanorods

Abstract: We report the fabrication of densely packed InGaN/GaN nanorods with high hexagonal periodicity. Nanosphere lithography and reactive ion etching were adopted to fabricate the nanorods from planar multiple quantum wells (MQWs). Compared to the planar MQWs, the nanorods exhibit significant luminescence enhancement. This is mostly attributed to the increased radiative recombination and light extraction efficiency. Both photoluminescence and Raman measurements confirmed in-plane strain relaxation in the MQWs after nanofabrication. A reduction in strain-induced quantum confined Stark effect in the nanorods increased radiative recombination. This work is most crucial to the understanding of optical properties with respect to the carrier transport and recombination in InGaN/GaN nanorods.