Showing papers by "Weijian Chen published in 2018"

Universal passivation strategy to slot-die printed SnO 2 for hysteresis-free efficient flexible perovskite solar module

Abstract: Perovskite solar cells (PSCs) have reached an impressive efficiency over 23%. One of its promising characteristics is the low-cost solution printability, especially for flexible solar cells. However, printing large area uniform electron transport layers on rough and soft plastic substrates without hysteresis is still a great challenge. Herein, we demonstrate slot-die printed high quality tin oxide films for high efficiency flexible PSCs. The inherent hysteresis induced by the tin oxide layer is suppressed using a universal potassium interfacial passivation strategy regardless of fabricating methods. Results show that the potassium cations, not the anions, facilitate the growth of perovskite grains, passivate the interface, and contribute to the enhanced efficiency and stability. The small size flexible PSCs achieve a high efficiency of 17.18% and large size (5 × 6 cm2) flexible modules obtain an efficiency over 15%. This passivation strategy has shown great promise for pursuing high performance large area flexible PSCs.

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption.

Abstract: Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 μm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with pr...

Slow Response of Carrier Dynamics in Perovskite Interface upon Illumination

Abstract: The current–voltage hysteresis, as well as the performance instability of perovskite solar cells (PSCs) under a working condition, is serving as the major obstacle toward their commercialization while the exact fundamental mechanisms to these issues are still in debate In this study, we investigated the slow variation of photogenerated carrier dynamics in a (FAPbI3)085(MAPbBr3)015 perovskite interface under continuous illumination Different response behaviors of carrier dynamics in the perovskite interfaces with and without the hole transport layer, Spiro-OMeTAD (Spiro), were systematically studied by time-dependent, steady-state, and time-resolved photoluminescence It was demonstrated that a light-induced defect curing process is dominantly responsible for the carrier dynamics evolution for the perovskite interface without Spiro, whereas both defect curing process and mobile ion migration should be accounted for the dynamic response of the perovskite interface contact with Spiro When contacted with

Role of Surface Recombination in Halide Perovskite Nanoplatelets.

Abstract: Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Oxygen-deficient bismuth tungstate and bismuth oxide composite photoanode with improved photostability

Abstract: A homogeneous layer of Bi2O3-Bi14WO24 composite (BWO/Bi2O3) thin film was fabricated using a combination of electrodeposition and thermal treatment. The evenly distributed Bi14WO24 component within the Bi2O3 layer was found to be important in stabilising the photoelectrochemical performances of Bi2O3 photoanode by promoting the photoelectron transport. The unmodified Bi2O3 suffered from severe photocorrosion as proven by X-ray diffraction (XRD) and inductively coupled plasma (ICP) analyses while the composite thin film was active without noticeable activity decay for at least 3 h of illumination. This strategy might be applicable to other photocatalysts with stability issues.

Improving carrier extraction in a PbSe quantum dot solar cell by introducing a solution-processed antimony-doped SnO2 buffer layer

Abstract: Solution-processable lead selenide (PbSe) colloidal quantum dots (QDs) are promising candidates for photovoltaics due to their efficient multiple exciton generation and carrier transport. However, despite these advantages, currently the best PbSe QD solar cells (QDSCs) still have short-circuit current densities (JSC) of about 25 mA cm−2. Here, we report the introduction a solution-processed trivalent antimony-doped tin oxide buffer layer at the interfaces in the device, which led to significant improvement in the JSC. Consistent with the optical simulations, the external quantum efficiency of the devices was improved in a region corresponding to the PbSe QD/buffer layer interfaces (400–600 nm), implying enhanced electron extraction. The improved performance is attributed to optimized gradient energy level alignment and shunt blocking at the interfaces. With this simple interfacial treatment, the JSC of the champion device was increased significantly to 26.7 mA cm−2, a more than 8% improvement compared to the control device. A further increase in the fill factor was also observed, leading to an over 11% improvement in the champion power conversion efficiency, from 7.1% to 7.9%. This work offers a simple method for interfacial engineering that led to PbSe QDSCs with efficiencies that are among the highest reported in the literature.

Improving hole extraction for PbS quantum dot solar cells

Abstract: Colloidal quantum dots (CQDs) are promising candidates for low-cost and high efficiency light harvesting materials owing to their solution processability and bandgap tunability. The most efficient lead chalcogenide QD solar cell reported to date is based on PbS, with a power conversion efficiency of 11.3%. However, in a typical QD cell, the hole transport layer has a much lower mobility than that of the ilayer, which hinders the hole extraction. In this work, we use Ag doped PbS QDs to form the p-type layer to increase its mobility and conductivity and significantly improve the device performance from 9.1% to 10.6% of power conversion efficiency

Ultrafast carrier dynamics in GaN/InGaN multiple quantum wells nanorods

Abstract: GaN/InGaN multiple quantum wells (MQW) is a promising material for high-efficiency solid-state lighting. Ultrafast optical pump-probe spectroscopy is an important characterization technique for examining fundamental phenomena in semiconductor nanostructure with sub-picosecond resolution. In this study, ultrafast exciton and charge carrier dynamics in GaN/InGaN MQW planar layer and nanorod are investigated using femtosecond transient absorption (TA) techniques at room temperature. Here nanorods are fabricated by etching the GaN/InGaN MQW planar layers using nanosphere lithography and reactive ion etching. Photoluminescence efficiency of the nanorods have been proved to be much higher than that of the planar layers, but the mechanism of the nanorod structure improvement of PL efficiency is not adequately studied. By comparing the TA profile of the GaN/InGaN MQW planar layers and nanorods, the impact of surface states and nanorods lateral confinement in the ultrafast carrier dynamics of GaN/InGaN MQW is revealed. The nanorod sidewall surface states have a strong influence on the InGaN quantum well carrier dynamics. The ultrafast relaxation processes studied in this GaN/InGaN MQW nanostructure is essential for further optimization of device application.

Ab initio calculation of transport properties between PbSe quantum dots facets with iodide ligands

Abstract: The transport properties between Lead Selenide (PbSe) quantum dots decorated with iodide ligands has been studied using density functional theory (DFT). Quantum conductance at each selected energy levels has been calculated along with total density of states and projected density of states. The DFT calculation is carried on using a grid-based planar augmented wave (GPAW) code incorporated with the linear combination of atomic orbital (LCAO) mode and Perdew Burke Ernzerhof (PBE) exchange-correlation functional. Three iodide ligand attached low index facets including (001), (011), (111) are investigated in this work. P-orbital of iodide ligand majorly contributes to density of state (DOS) at near top valence band resulting a significant quantum conductance, whereas DOS of Pb p-orbital shows minor influence. Various values of quantum conductance observed along different planes are possibly reasoned from a combined effect electrical field over topmost surface and total distance between adjacent facets. Ligands attached to (001) and (011) planes possess similar bond length whereas it is significantly shortened in (111) plane, whereas transport between (011) has an overall low value due to newly formed electric field. On the other hand, (111) plane with a net surface dipole perpendicular to surface layers leading to stronger electron coupling suggests an apparent increase of transport probability. Apart from previously mentioned, the maximum transport energy levels located several eVs (1 ~2 eVs) from the edge of valence band top.