Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared to planar devices, but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of 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 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.
"In particular, NWs on graphene hybrid structures are of great interest due to the intriguing properties of NWs, including the capacity of dislocation-free growth in lattice-mismatched epitaxy [10-12], efficient light absorption and emission [13,14], freedom of composition integration and reduced materials consumption. NW devices on Si have been demonstrated such as lasers , light-emitting diodes  and photovoltaic solar cells [17-19]. Consequently, epitaxial NWs on mechanically flexible and electrically conductive graphene or graphite hold great potential in fabricating cost-effective and flexible devices. "
[Show abstract][Hide abstract] ABSTRACT: We report the self-catalysed growth of InAs nanowires (NWs) on graphite thin films using molecular beam epitaxy via a droplet-assisted technique. Through optimising metal droplets, we obtained vertically aligned InAs NWs with highly uniform diameter along their entire length. In comparison with conventional InAs NWs grown on Si (111), the graphite surface led to significant effects on the NWs geometry grown on it, i.e. larger diameter, shorter length with lower number density, which were ascribed to the absence of dangling bonds on the graphite surface. The axial growth rate of the NWs has a strong dependence on growth time, which increases quickly in the beginning then slows down after the NWs reach a length of approximately 0.8 μm. This is attributed to the combined axial growth contributions from the surface impingement and sidewall impingement together with the desorption of adatoms during the diffusion. The growth of InAs NWs on graphite was proposed following a vapour-solid mechanism. High-resolution transmission electron microscopy reveals that the NW has a mixture of pure zinc-blende and wurtzite insertions.
Nanoscale Research Letters 06/2014; 9(1):321. DOI:10.1186/1556-276X-9-321 · 2.78 Impact Factor
"Nano Res. 2014, 7(6): 816–823 an InP NW array with an axial pn-junction could convert more than 70% of the photons of the solar spectrum with energy above the band gap of InP into photocurrent, giving rise to a record 13.8% conversion efficiency in a cell of 1 mm 2 in area . However, a systematic experimental study of the absorption of light in III–V NW arrays has not been performed. "
[Show abstract][Hide abstract] ABSTRACT: 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.
Nano Research 06/2014; 7(6):816-823. DOI:10.1007/s12274-014-0442-y · 7.01 Impact Factor
"Processing of square millimeter (1 mm × 1 mm) detector elements was done by depositing insulating SiO 2 on the as-grown samples, followed by contacting the tip of the NWs using an indium tin oxide (ITO) layer (as described in Wallentin et al.)   . Figure 1(b) shows a schematic cross-section of the device layout. "
[Show abstract][Hide abstract] ABSTRACT: We report on electrical and optical properties of p(+)-i-n(+)photodetectors/solar cells based on square millimeter arrays of InP nanowires (NWs) grown on InP substrates. The study includes a sample series where the p(+)-segment length was varied between 0 and 250 nm, as well as solar cells with 9.3% efficiency with similar design. The electrical data for all devices display clear rectifying behavior with an ideality factor between 1.8 and 2.5 at 300 K. From spectrally resolved photocurrent measurements, we conclude that the photocurrent generation process depends strongly on the p(+)-segment length. Without a p(+)-segment, photogenerated carriers funneled from the substrate into the NWs contribute strongly to the photocurrent. Adding a p(+)-segment decouples the substrate and shifts the depletion region, and collection of photogenerated carriers, to the NWs, in agreement with theoretical modeling. In optimized solar cells, clear spectral signatures of interband transitions in the zinc blende and wurtzite InP layers of the mixed-phase i-segments are observed. Complementary electroluminescence, transmission electron microscopy (TEM), as well as measurements of the dependence of the photocurrent on angle of incidence and polarization, support our interpretations.
Nano Research 04/2014; 7(4-4):1-9. DOI:10.1007/s12274-014-0422-2 · 7.01 Impact Factor
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