Vikram L. Dalal

A photonic-plasmonic structure for enhancing light absorption in thin film solar cells

We describe a photonic-plasmonic nanostructure, for significantly enhancing the absorption of long-wavelength photons in thin-film silicon solar cells, with the promise of exceeding the classical 4n2 limit for enhancement. We compare identical solar cells deposited on the photonic-plasmonic structure, randomly textured back reflectors and silver-coated flat reflectors. The state-of-the-art back reflectors, using annealed Ag or etched ZnO, had high diffuse and total reflectance. For nano-crystalline Si absorbers with comparable thickness, the highest absorption and photo-current of 21.5 mA/cm2 was obtained for photonic-plasmonic back-reflectors. The periodic photonic plasmonic structures scatter and reradiate light more effectively than a randomly roughened surface.

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH3NH3PbIxCl1−x perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 1011/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

Simple Model for Internal Photoemission

We have studied the influence of film thickness and collision processes upon the internal photoemission process in metal‐semiconductor systems. A simple, one‐dimensional random walk model has been used. The analysis shows that significantly larger yields may be expected for film thicknesses of the order of or less than electron‐phonon mean free paths. The influence of energy dependent mean free paths has also been studied. The results are in qualitative agreement with the results of Fermi‐age theory, but are somewhat different quantitatively.

Photonic crystal based back reflectors for light management and enhanced absorption in amorphous silicon solar cells

Photonic crystal back-reflectors offer enhanced optical absorption in thin-film solar cells, without undesirable losses. We fabricated metallic photonic crystal back-reflectors using photolithography and reactive-ion etching and deposited a-Si:H solar cells. The photonic crystal has triangular lattice symmetry, a pitch of 760 nm, and was designed with rigorous simulations. Scanning electron microscopy demonstrates excellent long range periodicity and conformal a-Si:H growth. The average light absorption increases by 7%, relative to a flat reference device, with an enhancement factor approaching 6 at near-infrared wavelengths. The photonic crystal back reflector strongly diffracts light and increases optical path lengths of solar photons.

Design considerations for high‐intensity solar cells

The factors affecting the efficiency of a solar cell change when the solar cell is subjected to concentrated sunlight. In this paper, we examine the effects of high solar intensities on Si and GaAs solar cells. It is shown that the current‐collection efficiency in Si solar cells increases at intermediate levels but may be reduced at very high solar intensities due to plasma recombination. Methods to avoid the degradation of efficiency are suggested. The open‐circuit voltage increases with concentration, and the rate of increase is faster in Si than in GaAs. The fill factor also increases with concentration, again at a faster rate in Si than in GaAs. Consequently, the efficiency in a Si solar cell under concentration may increase faster than in a GaAs solar cell. The effect of increased temperature is also examined. It is shown that the increase in temperature degrades the efficiency of Si faster than in GaAs. Thermal analysis shows that it is possible to limit the temperature rise to a low value (25 °C) e...

Temperature Dependence of Hole Velocity in p‐GaAs

The temperature dependence of the saturation drift velocity of holes in p‐GaAs has been experimentally determined. The material used was grown epitaxially and had a low field mobility μ≈400 cm2/V sec and carrier concentration p0≈1×1016/cm3, both at T=300°K. The saturation velocity was 1.0×107 cm/sec at T=300°K, and decreased to 7×106 cm/sec at T=469°K. The temperature dependence of the low‐field mobility was also measured and was found to be in good agreement with the recent theory of Wiley and DiDomenico.

HOLE VELOCITY IN p‐GaAs

The drift velocity‐electric field curve for holes in p‐GaAs has been experimentally determined at 300 °K on oriented samples. The velocity becomes a sublinear function of field around E=1.5×104 V/cm, but no saturation was observed up to the highest fields reached (Emax=6×104 V/cm, vmax=7.8×106 cm/sec). This result is in disagreement with the recent predictions of Kim based on analysis of GaAs avalanche diode oscillators.

Low temperature epitaxial silicon film growth using high vacuum electron‐cyclotron‐resonance plasma deposition

We report on the growth technique and electrical properties of epitaxial Si films grown at low temperatures using an electron‐cyclotron‐resonance plasma deposition technique. We have used standard high vacuum apparatus to grow high quality films at 450–525 °C. A critical step in achieving high quality films is an in situ hydrogen plasma cleaning of the wafer before growth. We have systematically studied the influence of ion bombardment during growth by biasing the substrate, and find that the films are crystalline for substrate bias voltages less negative than about −15 V, but become polycrystalline as the magnitude of the negative bias is increased. The crystallinity of the film was measured using Raman spectroscopy. The undoped films are n type with carrier concentrations in the 1016–1017 cm−3 range. The Hall mobilities measured for the films are comparable to values obtained in bulk Si crystals. We can achieve abrupt profiles in carrier concentrations between the heavy doped substrate and the epilayer,...

Influence of pressure and ion bombardment on the growth and properties of nanocrystalline silicon materials

We report on the growth and properties of nanocrystalline silicon:H films deposited using plasma discharge at 45MHz under varying pressure regimes from 50 to 500mTorr. X-ray diffraction data revealed that the primary orientation in these films was ⟨111⟩. The amount of hydrogen dilution needed to crystallize the films was found to be a strong function of deposition pressure, with a significantly higher hydrogen dilution needed to crystallize films at higher pressures. Langmuir probe data showed that these results could be attributed to the increase in density of low-energy hydrogen ions impinging on the substrate at lower pressures.

Growth and properties of microcrystalline germanium–carbide alloys grown using electron cyclotron resonance plasma processing☆

Abstract We report on the growth and properties of a new material, microcrystalline (Ge, C), which has potentially important optical, electrical and structural properties. The material was grown using a remote, low pressure electron cyclotron resosnance (ECR) plasma process on glass, stainless steel and c-Si substrates. The growth was done with hydrogen dilution and ion bombardment at temperatures of 350–400°C. We discovered that the optical absorption curve parallels that of c-Ge, with increased bandgaps as C is incorporated. We obtained up to 2% C incorporation, which increased the gap to 1.1 eV. At comparable bandgaps, the absorption coefficient of the (Ge, C) material is much larger than that of c-Si. Raman and X-ray measurements detected microcrystalline structure, and a dependence of grain size on the substrate used. The lattice constant contracted with C incorporation, approximately obeying Vegard’s law. Both undoped and n-doped materials were grown.