Multiple exciton generation in colloidal silicon nanocrystals

Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap identical with Eg = 1.20 eV) to be 2.4 +/- 0.1Eg and find an exciton-production quantum yield of 2.6 +/- 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth's crust, and poses no significant environmental problems regarding toxicity.     

Robust, functional nanocrystal solids by infilling with atomic layer deposition

Thin films of colloidal semiconductor nanocrystals (NCs) are inherently metatstable materials prone to oxidative and photothermal degradation driven by their large surface-to-volume ratios and high surface energies. (1) The fabrication of practical electronic devices based on NC solids hinges on preventing oxidation, surface diffusion, ripening, sintering, and other unwanted physicochemical changes that can plague these materials. Here we use low-temperature atomic layer deposition (ALD) to infill conductive PbSe NC solids with metal oxides to produce inorganic nanocomposites in which the NCs are locked in place and protected against oxidative and photothermal damage. Infilling NC field-effect transistors and solar cells with amorphous alumina yields devices that operate with enhanced and stable performance for at least months in air. Furthermore, ALD infilling with ZnO lowers the height of the inter-NC tunnel barrier for electron transport, yielding PbSe NC films with electron mobilities of 1 cm2 V(-1) s(-1). Our ALD technique is a versatile means to fabricate robust NC solids for optoelectronic devices.     

Low-temperature approach to high-yield and reproducible syntheses of high-quality small-sized PbSe colloidal nanocrystals for photovoltaic applications

Small-sized PbSe nanocrystals (NCs) were synthesized at low temperature such as 50-80 °C with high reaction yield (up to 100%), high quality, and high synthetic reproducibility, via a noninjection-based one-pot approach. These small-sized PbSe NCs with their first excitonic absorption in wavelength shorter than 1200 nm (corresponding to size < ～3.7 nm) were developed for photovoltaic applications requiring a large quantity of materials. These colloidal PbSe NCs, also called quantum dots, are high-quality, in terms of narrow size distribution with a typical standard deviation of ～7-9%, excellent optical properties with high quantum yield of ～50-90% and small full width at half-maximum of ～130-150 nm of their band-gap photoemission peaks, and high storage stability. Our synthetic design aimed at promotion of the formation of PbSe monomers for fast and sizable nucleation with the presence of a large number of nuclei at low temperature. For formation of the PbSe monomer, our low-temperature approach suggests the existence of two pathways of Pb-Se (route a) and Pb-P (route b) complexes. Either pathway may dominate, depending on the method used and its experimental conditions. Experimentally, a reducing/nucleation agent, diphenylphosphine, was added to enhance route b. The present study addresses two challenging issues in the NC community, the monomer formation mechanism and the reproducible syntheses of small-sized NCs with high yield and high quality and large-scale capability, bringing insight to the fundamental understanding of optimization of the NC yield and quality via control of the precursor complex reactivity and thus nucleation/growth. Such advances in colloidal science should, in turn, promote the development of next-generation low-cost and high-efficiency solar cells. Schottky-type solar cells using our PbSe NCs as the active material have achieved the highest power conversion efficiency of 2.82%, in comparison with the same type of solar cells using other PbSe NCs, under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm(2).     

Schottky solar cells based on colloidal nanocrystal films

We describe here a simple, all-inorganic metal/NC/metal sandwich photovoltaic (PV) cell that produces an exceptionally large short-circuit photocurrent (>21 mA cm(-2)) by way of a Schottky junction at the negative electrode. The PV cell consists of a PbSe NC film, deposited via layer-by-layer (LbL) dip coating that yields an EQE of 55-65% in the visible and up to 25% in the infrared region of the solar spectrum, with a spectrally corrected AM1.5G power conversion efficiency of 2.1%. This NC device produces one of the largest short-circuit currents of any nanostructured solar cell, without the need for sintering, superlattice order or separate phases for electron and hole transport.     

Efficiently harvesting excitons from electronic type-controlled semiconducting carbon nanotube films

We have employed thin films of highly purified semiconducting carbon nanotubes as near-infrared optical absorbers in heterojunction photovoltaic and photodetector devices with the electron acceptor C(60). In comparison with previous implementations of more electrically heterogeneous carbon nanotube/C(60) devices, we have realized a 10× gain in zero-bias quantum efficiency (QE) and even more substantial gains in power conversion efficiency (η(p)). The semiconducting nanotube/C(60) heterojunctions are highly rectifying with a peak external QE, internal QE, and η(p) of 12.9 ± 1.3, 91 ± 22, and 0.6%, respectively, in the near-infrared. We show that the device efficiency is determined by the effective length scale for exciton migration in the nanotube films, confirm the high internal QE via photoluminescence quenching, and demonstrate that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm. These results will guide the development of next-generation high-performance carbon nanotube-based solar cells and photosensitive devices.     

Charge photogeneration and recombination in poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene composite films studied by photocurrent response

The photocurrent spectral responses of poly[2-methoxy-5-(2′-ethyl-hexoxy-p-phenylene vinylene]:fullerene (C60) composites are measured as a function of C60 concentration. At low concentration, the relationship between the external quantum efficiency (EQE) and absorption spectra is exhibited as the strengthened antibatic effect, and the EQE of the composite devices declines with increasing concentration of C60. At higher concentration, however, the maximum EQE gradually coincides with the absorption peak (symbatic response) and the EQE of composite devices begins to increase with increasing C60 concentration. It is proposed that at low concentration, dopant C60 increases the self-absorption rate of composite films, and charge trapping by C60 molecules causes the loss of efficiency. At high C60 concentration, the large-scale aggregations in composite films build pathways for charge carrier transport to respective electrodes, inhibiting the self-absorption effect and charge recombination on C60.     

Multiple exciton generation in films of electronically coupled PbSe quantum dots

We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at approximately 4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.     

Low-temperature noninjection approach to homogeneously-alloyed PbSe(x)S(1-x) colloidal nanocrystals for photovoltaic applications

Homogeneously alloyed PbSe(x)S(1-x) nanocrystals (NCs) with their excitonic absorption peaks in wavelength shorter than 1200 nm were developed for photovoltaic (PV) applications. Schottky-type solar cells fabricated with our PbSe?.?S?.? NCs as their active materials reached a high power conversion efficiency (PCE) of 3.44%, with an open circuit voltage (V(oc)) of 0.49 V, short circuit photocurrent (J(sc)) of 13.09 mA/cm2, and fill factor (FF) of 0.54 under Air Mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm2. The syntheses of the small-sized colloidal PbSe(x)S(1-x) NCs were carried out at low temperature (60 °C) with long growth periods (such as 45 min) via a one-pot noninjection-based approach in 1-octadecene (ODE), featuring high reaction yield, high product quality, and high synthetic reproducibility. This low-temperature approach employed Pb(oleate)? as a Pb precursor and air-stable low-cost thioacetamide (TAA) as a S source instead of air-sensitive high-cost bis(trimethylsilyl)sulfide ((TMS)?S), with n-tributylphosphine selenide (TBPSe) as a Se precursor instead of n-trioctylphosphine selenide (TOPSe). The reactivity difference of TOPSe made from commercial TOP 90% and TBPSe made from commercial TBP 97% and TBP 99% was addressed with in situ observation of the temporal evolution of NC absorption and with 31P nuclear magnetic resonance (NMR). Furthermore, the addition of a strong reducing/nucleation agent diphenylphosphine (DPP) promoted the reactivity of the Pb precursor through the formation of a Pb-P complex, which is much more reactive than Pb(oleate)?. Thus, the reactivity of TBPSe was increased more than that of TAA. The larger the DPP-to-Pb feed molar ratio, the more the Pb-P complex, the higher the Se amount in the resulting homogeneously alloyed PbSe(x)S(1-x) NCs. Therefore, the use of DPP allowed reactivity match of the Se and S precursors and led to sizable nucleation at low temperature so that long growth periods became feasible. The present study brings insight into the formation mechanism of monomers, nucleation/growth of colloidal composition-tunable NCs, and materials design and synthesis for next-generation low-cost and high-efficiency solar cells.     

石墨烯量子点/CdS/CdSe共敏化太阳能电池

以三维锐钛矿TiO₂微球为上层光散射层材料, 以商业纳米TiO₂为下层连接材料, 采用刮刀法制备了一种新颖的双层TiO₂薄膜, 并应用于量子点敏化太阳能电池(QDSSC)。其中, 石墨烯量子点(GQDs)采用滴液法引入, CdS/CdSe量子点采用连续离子层吸附法(SILAR)制备。采用场发射扫描电镜、透射电镜、X射线衍射、紫外-可见漫反射光谱及荧光光谱对样品进行表征。实验还制备了CdS/CdSe量子点敏化及石墨烯量子点/CdS/CdSe共敏化太阳能电池, 并研究了石墨烯量子点及CdS不同敏化周期及对电池性能影响。研究结果表明, 石墨烯量子点及CdS不同敏化周期对薄膜的光学性质、电子传输及载流子复合均有较大影响。优选条件下, TiO₂/QGDs/CdS(4)/CdSe电池的光电转换效率为1.24%, 光电流密度为9.47 mA/cm², 显著高于TiO₂/CdS(4)/CdSe电池的这些参数(0.59%与6.22 mA/cm²)。这主要是由于TiO₂表层吸附石墨烯量子点后增强了电子的传输, 减少了载流子的复合。     

Enhanced UV photoresponse of KrF-laser-synthesized single-wall carbon nanotubes/n-silicon hybrid photovoltaic devices

We report on the KrF-laser ablation synthesis, purification and photocurrent generation properties of single-wall carbon nanotubes (SWCNTs). The thermally purified SWCNTs are integrated into hybrid photovoltaic (PV) devices by spin-coating them onto n-Si substrates. These novel SWCNTs/n-Si hybrid devices are shown to generate significant photocurrent (PC) over the entire 250-1050 nm light spectrum with external quantum efficiencies (EQE) reaching up to ~23%. Our SWCNTs/n-Si hybrid devices are not only photoactive in the traditional spectral range of Si solar cells, but generate also significant PC in the UV domain (below 400 nm). This wider spectral response is believed to be the result of PC generation from both the SWCNTs themselves and the tremendous number of local p-n junctions created at the nanotubes/Si interface. To assess the prevalence of these two contributions, the EQE spectra and J-V characteristics of these hybrid devices were investigated in both planar and top-down configurations, as a function of SWCNTs' film thickness. A sizable increase in EQE in the near UV with respect to the silicon is observed in both configurations, with a more pronounced UV photoresponse in the planar mode, confirming thereby the role of SWCNTs in the photogeneration process. The PC generation is found to reach its maximum for an optimal the SWCNT film thickness, which is shown to correspond to the best trade-off between lowest electrical resistance and highest optical transparency. Finally, by analyzing the J-V characteristics of our SWCNTs/n-Si devices with an equivalent circuit model, we were able to point out the contribution of the various electrical components involved in the photogeneration process. The SWCNTs-based devices demonstrated here open up the prospect for their use in highly effective photovoltaics and/or UV-light sensors.     

The potential of textured front ZnO and flat TCO/metal back contact to improve optical absorption in thin Cu(In,Ga)Se2 solar cells

The role of additionally textured front transparent conductive oxide − TCO (ZnO:Al) and flat TCO/metal contact on optical improvements in thin Cu(In,Ga)Se₂ (CIGS) solar cells are investigated by means of numerical simulations. A de-coupled analysis of two effects related to additional texturing of front surface of ZnO:Al TCO − (i) enhancement of light scattering and (ii) decreased total reflectance (antireflective effect) − reveals that the improvements in quantum efficiency, QE, and short-circuit current, J_SC, of the solar cell originate from an antireflective effect only. In order to improve optical properties of the back contact the introduction of a TCO layer (undoped ZnO) between CIGS and back metal contact is investigated from the optical point of view. In addition to ZnO/Mo, a highly reflective ZnO/Ag contact (ZnO is also assumed to work as a protection layer for Ag) is also included in simulations. Results show significant increase in reflectance related to introduced ZnO in front of Mo. Drastically increased reflectance is obtained if ZnO/Mo is substituted with ZnO/Ag. The improvements in QE and J_SC of a thin CIGS solar cell, related to ZnO/metal contacts are presented.     

Flexible top-emitting organic light-emitting diodes using semi-transparent multi-metal layers

In this paper, the improved device performance of top-emitting organic light-emitting diodes (TEOLEDs) with a thin multi-metal layer stack of nickel/silver/nickel (Ni/Ag/Ni) and aluminum/silver/aluminum (Al/Ag/Al) that were used as the anode and cathode on a flexible substrate is discussed. In particular, Indium-Tin-Oxide (ITO) as an anode electrode has been used recently even though it has some problems for flexible devices. Therefore we suggested that a thin multi-metal layer electrode as a new anode is fabricated instead of ITO anode. It was verified that the ITO-free TEOLEDs showed an enhanced probability of the recombination of the electrons and holes through an improved electron/hole charge balance. We also analyzed the optical and electrical characteristics using the current density, luminance, luminance efficiency, external quantum efficiency (EQE), CIE x, y coordinates, and EL spectra of flexible TEOLED devices were characterized. ITO-free, flexible, green-emitting OLEDs with a low cost and a simple process were demonstrated.     

Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.     

High carrier mobility in single ultrathin colloidal lead selenide nanowire field effect transistors

Ultrathin colloidal lead selenide (PbSe) nanowires with continuous charge transport channels and tunable bandgap provide potential building blocks for solar cells and photodetectors. Here, we demonstrate a room-temperature hole mobility as high as 490 cm(2)/(V s) in field effect transistors incorporating single colloidal PbSe nanowires with diameters of 6-15 nm, coated with ammonium thiocyanate and a thin SiO(2) layer. A long carrier diffusion length of 4.5 μm is obtained from scanning photocurrent microscopy (SPCM). The mobility is increased further at lower temperature, reaching 740 cm(2)/(V s) at 139 K.     

Band gap engineering in PbSe thin films from near-infrared to visible region by photochemical deposition method

Lead selenide (PbSe) thin films have been synthesized by the established photochemical deposition technique using lead nitrate and lead acetate as sources for the metal ions and sodium seleno sulphate as the selenium source along with triethanolamine, ammonia and hydrazine hydrate as complexing agents. A comprehensive study of the effect of substrate materials on physical properties of as deposited PbSe thin films is reported in this work. Two substrates were used in this investigation, namely soda lime glass slides and gold coin corning glass slides. The solution is irradiated with UV light and the photochemical reactions in the aqueous solution resulted in highly adherent metallic thin films. X-ray diffraction (XRD), scanning electron microscopy, optical and electrical measurement techniques were used for film characterization. The XRD analysis confirmed that all films were cubic, regardless of the cationic precursors and substrates used. The scanning electron microscope micrographs showed variations in morphology. The optical studies revealed that the films have good absorption in the visible region. The remarkable success of our effort was that we have been able to modify optical band gap of PbSe thin films over a wide spectral range by a cost effective route. The band gaps estimated from the transmission spectra were in the range 1.32–1.40 eV for films deposited on soda lime glass substrates and 1.46–1.55 eV for corning glass substrates. The room temperature conductivity of the PbSe films were in the range of 3.71 × 10^?7–513 × 10^?7 (Ω cm)^?1. The as deposited PbSe thin films with low transmittance in the visible region coupled with an appreciable reflectance in infrared region were found to satisfy the basic requirements for solar control coatings for window glazing applications in warm climates. Through this work we established that irrespective of metal salts, soda lime glass substrate was superior to corning glass substrate.     

Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films

The optical properties of the newly developed near‐infrared emitting formamidinium lead triiodide (FAPbI₃) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady‐state and time‐resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron–hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross‐section (5.2 × 10^?13 cm^?2) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap‐assisted recombination that turns to bimolecular at high pump fluences. From the temperature‐dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast to the strong PL quenching displayed by polycrystalline thin films. This difference is explained in terms of thermal activation of trapped carriers at the nanocrystal's surface, as opposed to the exciton thermal dissociation and trap‐mediated recombination, which occur in thin films at higher temperatures.     

Luminescent properties of ZnS:Mn2+/ZnS core/shell nanocrystals

High-quality ZnS:Mn2+/ZnS core/shell nanocrystals (NCs) with a core crystal diameter of 6.1 nm and 1.15 nm thick shells were synthesized via a high-boiling solvent process. The energy levels of the conduction band and valance band are estimated to be -3.2 eV and -6.8 eV by cyclic voltammetry and ultraviolet-visible (UV-vis) absorption spectra. The ZnS:Mn2+/ZnS NC emission peak is primarily located at 580 nm under 310 nm light excitation, originating from the charge transition from 4T1 to 6A1 within the 3d5 configuration of the Mn2+ ion. Based on ZnS:Mn2+/ZnS NCs as the active layer electroluminescent devices, the emission peak mainly locates at 460 nm with one shoulder emission peaking at 580 nm. The photoluminescence and electroluminescence properties of ZnS:Mn2+/ZnS NCs are investigated in the view of charge carrier injection and energy level alignment.     

锗掺杂二氧化钛薄膜的溶胶凝胶法制备和性能研究

二氧化钛是一种无毒、廉价、稳定的半导体材料,被广泛用作光电化学太阳能电池的电极材料,适当掺杂可以增强其光电性能.以钛酸丁酯和四正丁氧基锗烷为主要原料,采用溶胶-凝胶提拉涂膜法制备了Ge掺杂的TiO_2薄膜.通过X射线衍射、扫描电镜、紫外-可见吸收光谱、电流-电压曲线等测试手段研究了薄膜的结晶性能、微观结构和光电性能随Ge掺杂量的变化规律.结果表明,Ge掺杂量x=0.10时,形成Ti_(1-x)Ge_xO_2固溶体,x=0.15时,形成非晶态.掺锗后薄膜表面颗粒密度增大,薄膜比较致密.随着Ge掺杂量的增加,吸收光谱吸收边蓝移,光电化学性能也得到一定提高.在Ge掺杂量为0.05时,光电流达到最大值17A/m~2.同时,研究了锗掺杂对光电流的影响.     

Effect of Ascorbic Acid Additions on the Mechanism Underlying the Growth of Nanostructured PbSe Films via Hydrochemical Deposition

The effect of ascorbic acid additions on the growth mechanism of thin PbSe films produced by hydrochemical deposition has been studied by atomic force microscopy (AFM). A fractal analysis of AFM images of the lead selenide films has shown that their formation follows the “cluster–particle” aggregation mechanism. The fractal dimension assessed by the box-counting method is 2.26 at an average grain size of ~150 nm. From the transmission spectra of the films grown at deposition times from 20 to 120 min, we have evaluated their optical band gap, which has been shown to decrease from 0.595 to 0.53 eV with increasing deposition time. These values are attributable to the manifestation of quantum size effects in the grown PbSe films.     

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3?months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.