Lower Carrier Recombination Rate Research Articles

Accurate and efficient band gap predictions of metal halide perovskites using the DFT-1/2 method: GW accuracy with DFT expense

The outstanding optoelectronics and photovoltaic properties of metal halide perovskites, including high carrier motilities, low carrier recombination rates, and the tunable spectral absorption range are attributed to the unique electronic properties of these materials. While DFT provides reliable structures and stabilities of perovskites, it performs poorly in electronic structure prediction. The relativistic GW approximation has been demonstrated to be able to capture electronic structure accurately, but at an extremely high computational cost. Here we report efficient and accurate band gap calculations of halide metal perovskites by using the approximate quasiparticle DFT-1/2 method. Using AMX3 (A = CH3NH3, CH2NHCH2, Cs; M = Pb, Sn, X = I, Br, Cl) as demonstration, the influence of the crystal structure (cubic, tetragonal or orthorhombic), variation of ions (different A, M and X) and relativistic effects on the electronic structure are systematically studied and compared with experimental results. Our results show that the DFT-1/2 method yields accurate band gaps with the precision of the GW method with no more computational cost than standard DFT. This opens the possibility of accurate electronic structure prediction of sophisticated halide perovskite structures and new materials design for lead-free materials.

PEG-assisted synthesis of crystal TiO2 nanowires with high specific surface area for enhanced photocatalytic degradation of atrazine

High-specific-surface-area nanowire material synthesized by a facile way is of great significance and greatly desired as it has higher charge carrier mobility and lower carrier recombination rate. TiO2 nanowires with an average diameter of ca. 50nm, length of ca. 5μm and high specific surface area up to 152 m2 g−1 were synthesized using titanium sulfate as a Ti precursor by a one-step poly(ethylene glycol) (PEG)-assisted hydrothermal route. The growth of TiO2 nanowires follows the mechanism of ‘oriented interconnection-bundled nanowires separation-crystallization’ under the action of PEG-400. The as-synthesized TiO2 nanowires calcined at 700°C exhibited both anatase and TiO2 (B) phase, and grew along the [100] direction, which not only inhibits the transformation of anatase to rutile and reduces the recombination of electron–hole pairs, but also has more preferentially exposed {001} facets. TiO2 nanowires calcined at 800°C exhibited anatase phase and demonstrated the highest photocatalytic performance comparable to Degussa P-25 for atrazine degradation under UV irradiation. The degradation of atrazine was mainly driven by OH during the photocatalytic process. The quantity of reactive species such OH and O2− generated by TiO2 nanowires under UV irradiation was much higher than that of TiO2 nanoparticles.

Development of high efficient visible light-driven N, S-codoped TiO2 nanowires photocatalysts

One-dimensional (1D) nanowire material (especially nonmetal doped 1D nanowires) synthesized by a facile way is of great significance and greatly desired as it has higher charge carrier mobility and lower carrier recombination rate. N, S-codoped TiO2 nanowires were synthesized using titanium sulfate as a precursor and isopropanol as a protective capping agent by a hydrothermal route. The obtained doped nanowires were characterized by XRD, SEM, HRTEM, SAED, XPS, BET and UV–vis absorption spectrum. The incorporation of N and S into TiO2 NWs can lead to the expansion of its lattice and remarkably lower its electron-transfer resistance. Photocatalytic activity measurement showed that the N, S-codoped TiO2 nanowires with high quantum efficiency revealed the best photocatalytic performance for atrazine degradation under visible light irradiation compared to N, S-codoped TiO2 nanoparticles and S-doped TiO2 nanowires, which was attributed to (i) the synergistic effect of N and S doping in narrowing the band gap, separating electron–hole pairs and increasing the photoinduced electrons, and (ii) extending the anatase-to-rutile transformation temperature above 600°C.

Determination of defect density of state distribution of amorphous silicon solar cells by temperature derivative capacitance-frequency measurement

In this contribution, we demonstrate the application temperature dependent capacitance-frequency measurements (C-f) to n-i-p hydrogenated amorphous silicon (a-Si:H) solar cells that are forward-biased. By using a forward bias, the C-f measurement can detect the density of defect states in a particular energy range of the interface region. For this contribution, we have carried out this measurement method on n-i-p a-Si:H solar cells of which the intrinsic layer has been exposed to a H2-plasma before p-type layer deposition. After this treatment, the open-circuit voltage and fill factor increased significantly, as well as the blue response of the solar cells as is concluded from external quantum efficiency. For single junction, n-i-p a-Si:H solar cells initial efficiency increased from 6.34% to 8.41%. This performance enhancement is believed to be mainly due to a reduction of the defect density in the i-p interface region after the H2-plasma treatment. These results are confirmed by the C-f measurements. After H2-plasma treatment, the defect density in the intrinsic layer near the i-p interface region is lower and peaks at an energy level deeper in the band gap. These C-f measurements therefore enable us to monitor changes in the defect density in the interface region as a result of a hydrogen plasma. The lower defect density at the i-p interface as detected by the C-f measurements is supported by dark current-voltage measurements, which indicate a lower carrier recombination rate.

Synergetic effect of CdS quantum dots and TiO2 nanofibers for photoelectrochemical hydrogen generation

In this work, we developed a new type of nanostructured photoanodes for photoelectrochemical water splitting. They are based on CdS–TiO2 nanocomposite films, supported on conductive Ti sheets, prepared by an easy-to-achieve three-step method. It involves the production of TiO2 nanofibers (NFs) using a controlled corrosion route of polished Ti sheets, the preparation of size-controlled CdS quantum dots (QDs) by the polyol process and the direct impregnation of TiO2/Ti sheets by QDs in suspension. The photoelectrochemical (PEC) properties of the resulting nanostructures were measured, using a homemade electrochemical cell illuminated with a standard Xenon lamp, and compared to those of bare TiO2 NFs. A net enhancement of the photocurrent was observed after CdS impregnation, suggesting a low carrier recombination rate and a higher efficiency of the PEC device for solar water splitting, as the induced photocurrent is related to the electrons needed to reduce H+ ions into H2 at the cathode electrode (Pt wire).

Improvement of the photocatalytic properties of TiO2 by (Fe+Mo) co-doping—A possible way to retard the recombination process

Low visible light absorption and high charge carrier recombination rate are two main disadvantages of TiO2 as a photocatalyst which severely limit its practical applications. To overcome the problems, Fe mono-doped and (Fe+Mo) co-doped TiO2 were synthesized and studied. It was found that (Fe+Mo) co-doping can further increase the visible absorption and improve the photocatalytic property of TiO2 compared with Fe mono-doping; Fe mono-doping improves the photocatalytic property of TiO2 only at very low doping level (Fe concentration less than 1.0%), while by co-doping a small amount of Mo with Fe, the effective doping concentration of Fe can be pushed to a higher level and the photocatalytic property of TiO2 can be further improved. Photoluminescence spectra indicated that Mo dopant may play a role in retarding the recombination process when co-doped into TiO2 with Fe. The mechanism behind was discussed. It was suggested that doping a small amount of Mo into Fe-TiO2 might be an efficient way to further improve the photocatalytic property of Fe-TiO2 without losing its photocatalytic specificity.

P‐Type InP Nanopillar Photocathodes for Efficient Solar‐Driven Hydrogen Production

Water splitting by using sunlight for the production of hydrogen yields a storable product, which can be used as a fuel. There is considerable research into H2 generation, namely the reduction of protons to H2 in aqueous solution using semiconductor photocathodes. To maximize the photoelectrochemical (PEC) performance, the selection of the active materials and device configurations should be carefully considered. First, the short-circuit current density (Jsc) should be maximized by choosing materials with high optical absorption coefficients and low carrier recombination rates, both in the bulk and at the surface. The reflectance should be minimized by using surface nanotexturing to further improve light absorption. The onset potential (Eos) of the PEC device versus the reversible H /H2 redox potential should be maximized. Finally, the surface energy needs to be controlled to minimize the accumulation of gas bubbles on the surface of the photoelectrode. Light absorbers with band gaps in the range of 1.1–1.7 eV provide both a good match to the terrestrial solar spectrum and a significant fraction of the 1.23 eV free energy required to split water. Overpotentials associated with the electron transfer to (solvated) protons in aqueous solution should be minimized by improving carrier transport from semiconductor to electrolyte by decorating the semiconductor with cocatalysts, tuning band edges, and decreasing contact resistance. p-Type Si has been extensively investigated as a photocathode for photochemical hydrogen production. Planar Si has relatively low short-circuit current densities under AM1.5 G illumination, approximately 10 mAcm 2 (reference [9]), compared to what can be achieved in a pn junction solar cell (> 35 mAcm ). Nanostructuring and incorporation of cocatalysts have been used to raise the short-circuit current density to over 30 mAcm . A recent study using np Si radial junction microwires reported an Eos value of 0.54 V and an Jsc value of 15 mA, leading to an overall efficiency near 6%. The onset potential observed to date for p-Si photocathodes is less than half of the value required for overall water splitting (1.23 V). This low onset potential limits the performance of tandem or “Z-scheme” approaches, which would function without external bias, as it limits the potential overlap required for spontaneous water splitting. An ideal photocathode for use in a solar-driven hydrogen production system without bias should have both a high current density and a favorable open-circuit potential versus the reversible H/H2 redox couple. Herein, we employ nanotextured p-InP photocathodes in conjunction with a TiO2 passivation layer and a Ru cocatalyst to increase both Jsc and Eos values under H2 evolution conditions. InP has a number of attractive attributes as a photocathode: 1) Its band gap of 1.3 eV is well-matched to the solar spectrum; InP-based solar cells have achieved AM1.5 G efficiencies of up to 22%. 2) The conduction band edge of InP is slightly above the water reduction potential, thus electron transfer is favorable in this system. 3) The surface-recombination velocity of untreated InP is low (ca. 10 cms 1 for n-type and 10 cms 1 for p-type), which is particularly important for nonplanar devices with high surface areas, such as those explored in this study. For these reasons, InP has been studied previously as a photocathode for both water splitting and CO2 reduction. [18–20] Specifically, Heller and Vadimsky reported attractive PEC performances with current densities up to 28 mAcm 2 and conversion efficiencies of approximately 12% in InP photocathodes. Motivated by these results, we use InP as a model material system to elucidate the role of surface nanotexturing on the PEC device performance. We find that nanotextured InP photocathodes exhibit drastically enhanced performances compared to our planar cells that were processed using identical conditions. We examine the various effects of nanotexturing [*] M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, J. W. Ager, Prof. A. Javey Material Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) E-mail: jwager@lbl.gov ajavey@berkeley.edu M. H. Lee, K. Takei, J. Zhang, R. Kapadia, M. Zheng, J. Nah, Prof. A. Javey Electrical Engineering and Computer Sciences University of California, Berkeley, CA 94720 (USA) M. H. Lee, T. S. Matthews, J. W. Ager, Prof. A. Javey Joint Center for Artificial Photosynthesis Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)

Quasi-single Crystal Semiconductors on Glass Substrates Through Biaxially Oriented Buffer Layers

AbstractHigh efficiency photovoltaic devices are normally fabricated on single crystalline substrates. These single crystalline substrates are expensive and volume production for widespread usage has not been realistic. To date, large volume production of solar cells is on less expensive non-crystalline substrates such as glass. Typically the films grown on glass are polycrystalline with less than ideal efficiency. It was proposed that a dramatic gain in the efficiency may be achieved if one uses a biaxially oriented buffer layer on glass to grow biaxial semiconductor films to fabricate solar devices compared to that of films grown directly on glass. Biaxial films are not exactly single crystal but have strongly preferred crystallographic orientations in both the out-of-plane and in-plane directions. Typically the misorientation between grains can be small (within a few degrees) and may possess low carrier recombination rate. In this paper we shall discuss growth techniques that would allow one to produce biaxial buffer layers on glass. A specific strategy using an atomic shadowing mechanism in an oblique angle deposition configuration that allows one to grow biaxial buffer layers such as CaF2on glass substrate will be discussed in detail. Results of heteroepitaxy of semiconductor materials such as CdTe and Ge on these biaxial buffer/glass substrates characterized by x-ray pole figure, reflection high energy electron diffraction (RHEED) pole figure and transmission electron microscopy (TEM) will be presented.

Metamorphic In 0.7Al 0.3As/In 0.69Ga 0.31As thermophotovoltaic devices grown on graded InAs yP 1−y buffers by molecular beam epitaxy

The application of an In 0.7Al 0.3As window layer material on the properties of lattice-mismatched (LMM) single-junction (SJ) In 0.69Ga 0.31As thermophotovoltaic (TPV) cells grown by solid source molecular beam epitaxy (MBE) was studied as an alternative to conventional InAsP window layers. For these 0.60 eV bandgap SJ devices, high performance was achieved, displaying open-circuit voltage of 355 mV, power density of 0.532 W/cm 2 and a fill factor of 66.5% measured at a current density value of 2.26 A/cm 2. The measured internal quantum efficiencies were close to ∼100% at wavelengths between approximately 1.2–1.4 mm, suggestive that low carrier recombination rates in the vicinity of this alternative In 0.70Al 0.30As/In 0.69Ga 0.31As window/emitter interface has been achieved. The quantum efficiency data coupled with the device parameters show that InAlAs can be successfully substituted for conventional InAsP window layers in metamorphic TPV devices. By using an interface consisting solely of III-As alloys for the window/emitter material pairing as opposed to the conventional InAsP/InGaAs window/emitter design, this approach avoids the problematic III-P/III-As interface transition for MBE growth of TPV devices that has been shown to impact carrier collection efficiency [Hudait MK, Lin Y, Palmisiano MN, Ringel SA. 0.6-eV band gap In 0.69Ga 0.31As thermophotovoltaic devices grown on InAs y P 1− y step-graded buffers by molecular beam epitaxy. IEEE Electron Dev Lett 2003;24(9):538–40].