Nicklas Anttu

InP Nanowire Array Solar Cells Achieving 13.8% Efficiency by Exceeding the Ray Optics Limit

Improving Nanowire Photovoltaics In principle, solar cells based on arrays of nanowires made from compound inorganic semiconductors, such as indium phosphide (InP), should decrease materials and fabrication costs compared with planar junctions. In practice, device efficiencies tend to be low because of poor light absorption and increased rates of unproductive charge recombination in the surface region. Wallentin et al. (p. 1057, published online 17 January) now report that arrays of p-i-n InP nanowires (that switch from positive to negative doping), grown to millimeter lengths, can be optimized by varying the nanowire diameter and length of the n-doped segment. Efficiencies as high as 13.8% were achieved, which are comparable to the best planar InP photovoltaics. Nanowire solar cells were fabricated that exhibit high photocurrents and low surface recombination. Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

Diameter-Dependent Photocurrent in InAsSb Nanowire Infrared Photodetectors

Photoconductors using vertical arrays of InAs/InAs(1-x)Sb(x) nanowires with varying Sb composition x have been fabricated and characterized. The spectrally resolved photocurrents are strongly diameter dependent with peaks, which are red-shifted with diameter, appearing for thicker wires. Results from numerical simulations are in good agreement with the experimental data and reveal that the peaks are due to resonant modes that enhance the coupling of light into the wires. Through proper selection of wire diameter, the absorptance can be increased by more than 1 order of magnitude at a specific wavelength compared to a thin planar film with the same amount of material. A maximum 20% cutoff wavelength of 5.7 μm is obtained at 5 K for a wire diameter of 717 nm at a Sb content of x = 0.62, but simulations predict that detection at longer wavelengths can be achieved by increasing the diameter. Furthermore, photodetection in InAsSb nanowire arrays integrated on Si substrates is also demonstrated.

Efficient light management in vertical nanowire arrays for photovoltaics

Vertical arrays of direct band gap III-V semiconductor nanowires (NWs) hold the prospect of cheap and efficient next-generation photovoltaics, and guidelines for successful light-management are needed. Here, we use InP NWs as a model system and find, through electrodynamic modeling, general design principles for efficient absorption of sun light in nanowire arrays by systematically varying the nanowire diameter, the nanowire length, and the array period. Most importantly, we discover the existence of specific band-gap dependent diameters, 170 nm and 410 nm for InP, for which the absorption of sun light in the array is optimal, irrespective of the nanowire length. At these diameters, the individual InP NWs of the array absorb light strongly for photon energies just above the band gap energy due to a diameter-tunable nanophotonic resonance, which shows up also for other semiconductor materials of the NWs. Furthermore, we find that for maximized absorption of sun light, the optimal period of the array increases with nanowire length, since this decreases the insertion reflection losses.

Colorful InAs Nanowire Arrays: From Strong to Weak Absorption with Geometrical Tuning.

One-dimensional nanostructure arrays can show fascinatingly different, tunable optical response compared to bulk systems. Here we study theoretically and demonstrate experimentally how to engineer the reflection and absorption of light in epitaxially grown vertical arrays of InAs nanowires (NWs). A striking observation is optically visible colors of the array, which we show can be tuned depending on the geometrical parameters of the array. Specifically, larger diameter NW arrays absorb light more effectively out to a longer wavelength compared to smaller diameter arrays. Thus, controlling the diameter provides a way to tune the optically observable color of an array. We also find that arrays with a larger amount of InAs material reflect less light (or absorb more light) than arrays with less material. On the basis of these two trends, InAs NW arrays can be designed to absorb light either much more or much less efficiently than a thin film of an effective medium containing the same amount of InAs as the NW array. The tunable absorption and low area filling factor of the NW arrays compared to thin film bode well for III-V photovoltaics and photodetection.

Photoemission electron microscopy using extreme ultraviolet attosecond pulse trains

We report the first experiments carried out on a new imaging setup, which combines the high spatial resolution of a photoemission electron microscope (PEEM) with the temporal resolution of extreme ultraviolet (XUV) attosecond pulse trains. The very short pulses were provided by high-harmonic generation and used to illuminate lithographic structures and Au nanoparticles, which, in turn, were imaged with a PEEM resolving features below 300 nm. We argue that the spatial resolution is limited by the lack of electron energy filtering in this particular demonstration experiment. Problems with extensive space charge effects, which can occur due to the low probe pulse repetition rate and extremely short duration, are solved by reducing peak intensity while maintaining a sufficient average intensity to allow imaging. Finally, a powerful femtosecond infrared (IR) beam was combined with the XUV beam in a pump-probe setup where delays could be varied from subfemtoseconds to picoseconds. The IR pump beam could induce multiphoton electron emission in resonant features on the surface. The interaction between the electrons emitted by the pump and probe pulses could be observed.

Absorption of light in InP nanowire arrays

An understanding of the absorption of light is essential for efficient photovoltaic and photodetection applications with III–V nanowire arrays. Here, we correlate experiments with modeling and verify experimentally the predicted absorption of light in InP nanowire arrays for varying nanowire diameter and length. We find that 2,000 nm long nanowires in a pitch of 400 nm can absorb 94% of the incident light with energy above the band gap and, as a consequence, light which in a simple ray-optics description would be travelling between the nanowires can be efficiently absorbed by the nanowires. Our measurements demonstrate that the absorption for long nanowires is limited by insertion reflection losses when light is coupled from the air top-region into the array. These reflection losses can be reduced by introducing a smaller diameter to the nanowire-part closest to the air top-region. For nanowire arrays with such a nanowire morphology modulation, we find that the absorptance increases monotonously with increasing diameter of the rest of the nanowire.

Geometrical optics, electrostatics, and nanophotonic resonances in absorbing nanowire arrays.

Semiconductor nanowire arrays have shown promise for next-generation photovoltaics and photodetection, but enhanced understanding of the light-nanowire interaction is still needed. Here, we study theoretically the absorption of light in an array of vertical InP nanowires by moving continuously, first from the electrostatic limit to the nanophotonic regime and then to the geometrical optics limit. We show how the absorption per volume of semiconductor material in the array can be varied by a factor of 200, ranging from 10 times weaker to 20 times stronger than in a bulk semiconductor sample.

A Comparative Study of Absorption in Vertically and Laterally Oriented InP Core-Shell Nanowire Photovoltaic Devices.

We have compared the absorption in InP core-shell nanowire p-i-n junctions in lateral and vertical orientation. Arrays of vertical core-shell nanowires with 400 nm pitch and 280 nm diameter, as well as corresponding lateral single core-shell nanowires, were configured as photovoltaic devices. The photovoltaic characteristics of the samples, measured under 1 sun illumination, showed a higher absorption in lateral single nanowires compared to that in individual vertical nanowires, arranged in arrays with 400 nm pitch. Electromagnetic modeling of the structures confirmed the experimental observations and showed that the absorption in a vertical nanowire in an array depends strongly on the array pitch. The modeling demonstrated that, depending on the array pitch, absorption in a vertical nanowire can be lower or higher than that in a lateral nanowire with equal absorption predicted at a pitch of 510 nm for our nanowire geometry. The technology described in this Letter facilitates quantitative comparison of absorption in laterally and vertically oriented core-shell nanowire p-i-n junctions and can aid in the design, optimization, and performance evaluation of nanowire-based core-shell photovoltaic devices.

GaAs nanopillar arrays with suppressed broadband reflectance and high optical quality for photovoltaic applications

We report on fabrication and optical characterization of GaAs nanopillar (NP) arrays, obtained using a combination of low-cost mask generation by self-assembled silica particles (nanosphere lithography) and dry etching. Tapered structures (conical and frustum NP arrays) are fabricated by appropriate optimization of process parameters. Significant suppression of surface reflectance is observed for both geometries over a broad wavelength range. Simulations, based on finite difference time domain (FDTD) method, show good agreement with reflectivity measurements and serve as a guideline for design of NPs and understanding their interaction with light. A combination of wet chemical etching and sulfur–based passivation of GaAs NPs, results in more than one order of magnitude enhancement in PL intensity and recovery of PL line-width, which is very promising for photovoltaic applications.

In Situ Characterization of Nanowire Dimensions and Growth Dynamics by Optical Reflectance

Optical reflectometry is commonly used as an accurate and noninvasive characterization tool when growing planar semiconductor layers. However, thin-film analysis schemes cannot be directly applied to nanowire systems due to their complex optical response. Here, we report on reliable in situ characterization of nanowire growth with high accuracy using optical reflectance spectra for analysis. The method makes it possible to determine the nanowire length, diameter, and growth rate in situ in real time with high resolution. We demonstrate the methods versatility by using the optical reflectance data for determining nanowire dimensions on both particle-assisted and selective-area grown nanowires. To indicate the full potential of in situ characterization of nanowire synthesis we evaluate the growth dynamics of InP nanowires in the presence of the p-type dopant precursor diethylzinc. We observe that the growth rate is strongly affected by the diethylzinc. At low diethylzinc flows, the growth rate decreases monotonously while higher flows lead to an initially increasing growth rate. From these in situ characterization data, we conclude that the surface migration length of adatom species is affected strongly by the addition of diethylzinc. We believe that this characterization method will become a standard tool for in situ growth monitoring and aid in elucidating the complex growth dynamics often exhibited during nanowire growth.