Kim M. Jones

40.8% efficient inverted triple-junction solar cell with two independently metamorphic junctions

A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III–V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga.51In.49P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In.04Ga.96As middle junction, and a metamorphic 0.89 eV In.37Ga.63As bottom junction. The two metamorphic junctions contained approximately 1×105 cm−2 and 2–3×106 cm−2 threading dislocations, respectively.

Nanostructured black silicon and the optical reflectance of graded-density surfaces

We fabricate and measure graded-index “black silicon” surfaces and find the underlying scaling law governing reflectance. Wet etching (100) silicon in HAuCl4, HF, and H2O2 produces Au nanoparticles that catalyze formation of a network of [100]-oriented nanopores. This network grades the near-surface optical constants and reduces reflectance to below 2% at wavelengths from 300 to 1000 nm. As the density-grade depth increases, reflectance decreases exponentially with a characteristic grade depth of about 1/8 the vacuum wavelength or half the wavelength in Si. Observation of Au nanoparticles at the ends of cylindrical nanopores confirms local catalytic action of moving Au nanoparticles.

Multi-scale surface texture to improve blue response of nanoporous black silicon solar cells

We characterize the optical and carrier-collection physics of multi-scale textured p-type black Si solar cells with conversion efficiency of 17.1%. The multi-scale texture is achieved by combining density-graded nanoporous layer made by metal-assisted etching with micron-scale pyramid texture. We found that (1) reducing the thickness of nanostructured Si layer improves the short-wavelength spectral response and (2) multi-scale texture permits thinning of the nanostructured layer while maintaining low surface reflection. We have reduced the nanostructured layer thickness by 60% while retaining a solar-spectrum-averaged black Si reflectance of less than 2%. Spectral response at 450 nm has improved from 57% to 71%.

The first true inorganic fullerenes

Boron nitride and materials of composition MX2, where M is molybdenum or tungsten and X is sulphur or selenium, can form fullerene-like structures such as nested polyhedra or nanotubes. However, the analogy to the carbon fullerene family falls short because no small preferred structure akin to C60(ref. 5) has been found. We have discovered nano-octahedra of MoS2of discrete sizes in soots that we prepared by laser ablation of pressed MoS2targets. These nano-octahedra are much larger than C60structures, having edge lengths of about 4.0 and 5.0 nanometres, and may represent the first ‘inorganic fullerenes’.

Carbon nanotube network electrodes enabling efficient organic solar cells without a hole transport layer

We report on the effects of replacing both In2O3:Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA/cm2. Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices.

ZnO nanocoral structures for photoelectrochemical cells

We report on synthesis of a uniform and large area of a new form of ZnO nanocorals. These nanostructures can provide suitable electrical pathways for efficient carrier collection as well as large surface areas for the photoelectrochemical (PEC) cells. PEC devices made from these ZnO nanocoral structures demonstrate significantly enhanced photoresponse as compared to ZnO compact and nanorod films. Our results suggest that the nanocoral structures could be an excellent choice for nanomaterial-based applications such as dye-sensitized solar cells, electrochromic windows, and batteries.

Dislocation density reduction through annihilation in lattice‐mismatched semiconductors grown by molecular‐beam epitaxy

Epitaxial InAs/GaAs, GaAs/Ge/Si, GaAs/InP, and InAs/InP heterostructures are grown by molecular‐beam epitaxy. Transmission electron microscopy studies reveal that, for these heteroepitaxial systems, the threading dislocation density is inversely proportional to the epilayer thickness. At a given thickness, the threading dislocation density is relatively insensitive to lattice mismatch (3.2%<‖Δa‖/a<7.2%), to differences in thermal expansion coefficients (6.9×10−7<‖Δα‖<3.4×10−6 K−1), to interfacial surface chemistry, and to epilayer morphology. Epitaxial layers incorporating growth interrupts produce lower overall defect densities, yet they maintain defect‐reduction profiles similar to those observed in layers without the growth interrupt.

Optimization of open circuit voltage in amorphous silicon solar cells with mixed-phase "amorphous+nanocrystalline… p-type contacts of low nanocrystalline content

Both the origins of the high open circuit voltages VOC in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy TEM and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i/p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase amorphous+nanocrystalline p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC .© 2007 American Institute of Physics. DOI: 10.1063/1.2714507

NANOPARTICLE PRECURSOR ROUTE TO LOW-TEMPERATURE SPRAY DEPOSITION OF CDTE THIN FILMS

In this letter we report a nanoparticle‐derived route to CdTe thin films. CdTe nanoparticles 39±8 A in diameter, prepared by an organometallic route, were characterized by x‐ray diffraction, UV‐Vis spectroscopy, transmission electron microscopy, and energy dispersive x‐ray spectroscopy. CdTe thin‐film deposition was realized by spraying a nanoparticle/butanol colloid onto SnO2‐coated glass substrates at variable susceptor temperatures. The resultant CdTe films were characterized by atomic force microscopy, x‐ray diffraction, and UV‐Vis spectroscopy. Smooth and dense CdTe thin films were obtained using growth temperatures ∼200 °C less than conventional spray pyrolysis. A growth temperature dependence upon CdTe grain size formation and crystallinity was observed by atomic force microscopy and x‐ray diffraction. UV‐Vis characterization revealed a transformation in the optical properties of the CdTe thin films as a function of growth temperature.

Chemical fluctuation-induced nanodomains in Cu(In,Ga)Se2 films

The microstructure and chemistry of CuInSe2 single-crystals and Cu(In,Ga)Se2 thin films from high-efficiency devices are investigated by transmission electron microscopy and x-ray energy-dispersive spectroscopy. We find strong chemical fluctuations at the nanoscale, which result in a lattice comprising a mixture of relatively Cu-poor and Cu-rich nanodomains in both cases. These nanodomains are crystallographically coherent, and no structural lattice defects are found at the interfaces between them. These nanodomains may interconnect, forming three-dimensional, interpenetrating Cu-poor and Cu-rich percolation networks. Such interconnected structures may play a role in the high device performance of Cu(In,Ga)Se2 thin-film photovoltaics.