Yuzhuan Xu

Hole-conductor-free perovskite organic lead iodide heterojunction thin-film solar cells: High efficiency and junction property

Efficient hole-conductor-free organic lead iodide thin film solar cells have been fabricated with a sequential deposition method, and a highest efficiency of 10.49% has been achieved. Meanwhile, the ideal current-voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model confirms that the TiO2/CH3NH3PbI3/Au cell is a typical heterojunction cell and the intrinsic parameters of the cell are comparable to that of the high-efficiency thin-film solar cells.

Modified Two-Step Deposition Method for High-Efficiency TiO2/CH3NH3PbI3 Heterojunction Solar Cells

Hybrid organic-inorganic perovskites (e.g., CH3NH3PbI3) are promising light absorbers for the third-generation photovoltaics. Herein we demonstrate a modified two-step deposition method to fabricate a uniform CH3NH3PbI3 capping layer with high-coverage and thickness of 300 nm on top of the mesoporous TiO2. The CH3NH3PbI3 layer shows high light-harvesting efficiency and long carrier lifetime over 50 ns. On the basis of the as-prepared film, TiO2/CH3NH3PbI3 heterojunction solar cells achieve a power conversion efficiency of 10.47% with a high open-circuit voltage of 948 mV, the highest recorded to date for hole-transport-material-free (HTM-free) perovskite-based heterojunction cells. The efficiency exceeding 10% shows promising prospects for the HTM-free solar cells based on organic lead halides.

Efficient hybrid mesoscopic solar cells with morphology-controlled CH3NH3PbI3-xClx derived from two-step spin coating method.

A morphology-controlled CH3NH3PbI3-xClx film is synthesized via two-step solution deposition by spin-coating a mixture solution of CH3NH3Cl and CH3NH3I onto the TiO2/PbI2 film for the first time. It is revealed that the existence of CH3NH3Cl is supposed to result in a preferential growth along the [110] direction of perovskite, which can improve both the crystallinity and surface coverage of perovskite and reduce the pinholes. Furthermore, the formation process of CH3NH3PbI3-xClx perovskite is explored, in which intermediates containing chlorine are suggested to exist. 13.12% of power conversion efficiency has been achieved for the mesoscopic cell, higher than 12.08% of power conversion efficiency of the devices fabricated without CH3NH3Cl via the same process. The improvement mainly lies in the increasing open-circuit photovoltage which is ascribed to the reduction of reverse saturation current density.

Efficient CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP hole-transporting layers

CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP (TPA = 4,4′-((1E, 1′E,3E,3′E)-[1,1′-biphenyl]-4,4′-diylbis(buta-1,3-diene-4,1-diyl)); DP = bis(N,N-di-p-tolylaniline); n = 1, 2, 3, 4) as hole-transporting materials (HTMs) have been fabricated. After optimization of the mesoporous TiO2 film thickness, devices based on 2TPA-2-DP with power conversion efficiencies (PCEs) of up to 12.96% have been achieved, comparable to those of devices with (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) (spiro-OMeTAD) as HTM under similar conditions. Further time-resolved photoluminescence (PL) measurements showed a fast charge transfer process at the perovskite/2TPA-2-DP interface. With the aid of electrochemical impedance spectra, a study of the electron blocking ability of 2TPA-2-DP in the device reveals that the presence of 2TPA-2-DP can greatly increase charge transfer resistance at the HTM/Au interface in the device, thus reducing the recombination. Furthermore, the perovskite solar cells based on these four HTMs exhibit good stability after testing for one month.

GaAs-based room-temperature continuous-wave 1.59 {mu}m GaInNAsSb single-quantum-well laser diode grown by molecular-beam epitaxy

Starting from the growth of high-quality 1.3μmGaInNAs∕GaAs quantum well (QW), the QW emission wavelength has been extended up to 1.55μm by a combination of lowering growth rate, using GaNAs barriers and incorporating some amount of Sb. The photoluminescence properties of 1.5μm range GaInNAsSb∕GaNAs QWs are quite comparable to the 1.3μm QWs, revealing positive effect of Sb on improving the optical quality of the QWs. A 1.59μm lasing of a GaInNAsSb∕GaNAs single-QW laser diode is obtained under continuous current injection at room temperature. The threshold current density is 2.6kA∕cm2 with as-cleaved facet mirrors.

A thin pristine non-triarylamine hole-transporting material layer for efficient CH3NH3PbI3 perovskite solar cells

A new non-traditional organic hole-transporting material (HTM), 4-(4-phenyl-4-α-naphthylbutadienyl)-N,N-bis(4-benzyl)-aniline (PNBA), has been employed in CH3NH3PbI3 perovskite solar cells for the first time. The pore filling of PNBA into mesoporous TiO2/CH3NH3PbI3 scaffold is investigated in detail. As high as 11.4% of light-to-electricity conversion efficiency has been achieved, comparable to corresponding spiro-OMeTAD-based devices under the same conditions. It is revealed that the uniform and thin PNBA film is sufficient as a HTM for perovskite solar cells, and can facilitate hole transport to the metal cathode and also block electron transfer from the perovskite to the metal cathode.

Predication of an ultrahigh filling fraction for K in CoSb3

The filling fraction limit (FFL) for potassium in skutterudite CoSb3 has been investigated by first-principles methods. The FFL is derived by minimizing the formation energy of a reaction that includes the formation of both the filled skutterudite KyCo4Sb12 and the potential secondary phases KSb and CoSb2 when doping K into CoSb3. Our calculations show that K has an ultrahigh filling fraction up to more than 60% in CoSb3, as compared with those of previously reported fillers such as Sr, Ba, Ca, La, Ce, Eu, and Yb [X. Shi et al., Phys. Rev. Lett. 95, 185503 (2005)]. The extent to which this affects the thermoelectric properties of K-filled skutterudites is discussed.

High-indium-content InxGa1−xAs/GaAs quantum wells with emission wavelengths above 1.25 μm at room temperature

High-indium-content InxGa1-xAs/GaAs single/multi-quantum well (SQW/MQW) structures have been systematically investigated. By optimizing the molecular-beam epitaxy growth conditions, the critical thickness of the strained In0.475Ga0.525As/GaAs QWs is raised to 7 nm, which is much higher than the value given by the Matthews and Blakeslee model. The good crystalline quality of the strained InGaAs/GaAs MQWs is proved by x-ray rocking curves. Photoluminescence measurements show that an emission wavelength of 1.25 mum at room temperatures with narrower full width at half maximum less than 30 meV can be obtained. The strain relaxation mechanism is discussed using the Matthews-Blakeslee model

1.55 {mu}m GaInNAs resonant-cavity-enhanced photodetector grown on GaAs

We report the design, growth, fabrication, and characterization of a GaAs-based resonant-cavity-enhanced (RCE) GaInNAs photodetector operating at 1.55μm. The structure of the device was designed using a transfer-matrix method (TMM). By optimizing the molecular-beam epitaxy growth conditions, six GaInNAs quantum wells were used as the absorption layers. Twenty-five (25)- and 9-pair GaAs∕AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. At 1.55μm, a quantum efficiency of 33% with a full width at half maximum of 10nm was obtained. The dark current density was 3×10−7A∕cm2 at a bias of 0V and 4.3×10−5A∕cm2 at a reverse bias of 5V. The primary time response measurement shows that the device has a rise time of less than 800ps.

The Effect of Humidity upon the Crystallization Process of Two-Step Spin-Coated Organic–Inorganic Perovskites

Moisture is shown to activate the reaction between PbI2 and methylammonium halides. In addition, two activating mechanisms are proposed for the formation of CH3 NH3 PbI3 and CH3 NH3 PbI3-x Clx films from a series of carefully controlled experiments. When these rapidly formed perovskite films are directly fabricated into the devices, poor photovoltaic properties are found, due to heavy surface charge recombination. However, the cell performance can be significantly enhanced to 13.63 % and to over 12 % in the steady state for CH3 NH3 PbI3 and to 15.50 % and over 14 % in the steady state for CH3 NH3 PbI3-x Clx , if the rapidly formed perovskite film is annealed. Thus, it is believed that moisture (below 60 % RH) is not a problem for the fabrication of highly efficient perovskite solar cells.