Paul Stradins

Efficient black silicon solar cell with a density-graded nanoporous surface: Optical properties, performance limitations, and design rules

We study optical effects and factors limiting performance of our confirmed 16.8% efficiency “black silicon” solar cells. The cells incorporate density-graded nanoporous surface layers made by a one-step nanoparticle-catalyzed etch and reflect less than 3% of the solar spectrum, with no conventional antireflection coating. The cells are limited by recombination in the nanoporous layer which decreases short-wavelength spectral response. The optimum density-graded layer depth is then a compromise between reflectance reduction and recombination loss. Finally, we propose universal design rules for high-efficiency solar cells based on density-graded surfaces.

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.

Effects of embedded crystallites in amorphous silicon on light-induced defect creation

We investigate effects of embedded crystallites in hydrogenated amorphous silicon on light-induced metastable dangling-bond defect creation in a systematic manner. Inclusion of a small volume fraction of crystallites into the amorphous matrix significantly suppresses defect creation against moderate light illumination. Excess carriers generated in the amorphous matrix tend to recombine in the embedded crystallites, which suppresses nonradiative recombination within the amorphous matrix and the subsequent defect creation. The presence of a small volume fraction of crystallites, however, is no longer effective to improve the stability against strong light exposure such as pulsed laser irradiation. In this case, the higher carrier concentration favors bimolecular direct carrier recombination within the amorphous matrix.

Van der Waals metal-semiconductor junction: Weak Fermi level pinning enables effective tuning of Schottky barrier

The Schottky barrier for carrier injection into 2D semiconductors can be effectively tuned by using 2D metals. Two-dimensional (2D) semiconductors have shown great potential for electronic and optoelectronic applications. However, their development is limited by a large Schottky barrier (SB) at the metal-semiconductor junction (MSJ), which is difficult to tune by using conventional metals because of the effect of strong Fermi level pinning (FLP). We show that this problem can be overcome by using 2D metals, which are bounded with 2D semiconductors through van der Waals (vdW) interactions. This success relies on a weak FLP at the vdW MSJ, which is attributed to the suppression of metal-induced gap states. Consequently, the SB becomes tunable and can vanish with proper 2D metals (for example, H-NbS2). This work not only offers new insights into the fundamental properties of heterojunctions but also uncovers the great potential of 2D metals for device applications.

Realization of GaInP/Si Dual-Junction Solar Cells With 29.8% 1-Sun Efficiency

Combining a Si solar cell with a high-bandgap top cell reduces the thermalization losses in the short wavelength and enables theoretical 1-sun efficiencies far over 30%. We have investigated the fabrication and optimization of Si-based tandem solar cells with 1.8-eV rear-heterojunction GaInP top cells. The III–V and Si heterojunction subcells were fabricated separately and joined by mechanical stacking using electrically insulating optically transparent interlayers. Our GaInP/Si dual-junction solar cells have achieved a certified cumulative 1-sun efficiency of 29.8% ± 0.6% (AM1.5g) in four-terminal operation conditions, which exceeds the record 1-sun efficiencies achieved with both III–V and Si single-junction solar cells. The effect of luminescent coupling between the subcells has been investigated, and optical losses in the solar cell structure have been addressed.

Material quality requirements for efficient epitaxial film silicon solar cells

The performance of 2-μm-thick crystal silicon (c-Si) solar cells grown epitaxially on heavily doped wafer substrates is quantitatively linked to absorber dislocation density. We find that such thin devices have a high tolerance to bulk impurities compared to wafer-based cells. The minority carrier diffusion length is about half the dislocation spacing and must be roughly three times the absorber thickness for efficient carrier extraction. Together, modeling and experimental results provide design guidelines for film c-Si photovoltaic cells.

Angle-resolved XPS analysis and characterization of monolayer and multilayer silane films for DNA coupling to silica.

We measure silane density and Sulfo-EMCS cross-linker coupling efficiency on aminosilane films by high-resolution X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. We then characterize DNA immobilization and hybridization on these films by (32)P-radiometry. We find that the silane film structure controls the efficiency of the subsequent steps toward DNA hybridization. A self-limited silane monolayer produced from 3-aminopropyldimethylethoxysilane (APDMES) provides a silane surface density of ~3 nm(-2). Thin (1 h deposition) and thick (19 h deposition) multilayer films are generated from 3-aminopropyltriethoxysilane (APTES), resulting in surfaces with increased roughness compared to the APDMES monolayer. Increased silane surface density is estimated for the 19 h APTES film, due to a ∼32% increase in surface area compared to the APDMES monolayer. High cross-linker coupling efficiencies are measured for all three silane films. DNA immobilization densities are similar for the APDMES monolayer and 1 h APTES. However, the DNA immobilization density is double for the 19 h APTES, suggesting that increased surface area allows for a higher probe attachment. The APDMES monolayer has the lowest DNA target density and hybridization efficiency. This is attributed to the steric hindrance as the random packing limit is approached for DNA double helices (dsDNA, diameter ≥ 2 nm) on a plane. The heterogeneity and roughness of the APTES films reduce this steric hindrance and allow for tighter packing of DNA double helices, resulting in higher hybridization densities and efficiencies. The low steric hindrance of the thin, one to two layer APTES film provides the highest hybridization efficiency of nearly 88%, with 0.21 dsDNA/nm(2). The XPS data also reveal water on the cross-linker-treated surface that is implicated in device aging.

Matrix-embedded silicon quantum dots for photovoltaic applications: a theoretical study of critical factors

Si Quantum dots (QDs) are offering the possibilities for improving the efficiency and lowering the cost of solar cells. In this paper we study the PV-related critical factors that may affect design of Si QDs solar cell by performing atomistic calculation including many-body interaction. First, we find that the weak absorption in bulk Si is significantly enhanced in Si QDs, specially in small dot size, due to quantum-confinement induced mixing of Γ-character into the X-like conduction band states. We demonstrate that the atomic symmetry of Si QD also plays an important role on its bandgap and absorption spectrum. Second, quantum confinement has a detrimental effect on another PV property – it significantly enhances the exciton binding energy in Si QDs, leading to difficulty in charge separation. We observe universal linear dependence of exciton binding energy versus excitonic gap for all Si QDs. Knowledge of this universal linear function will be helpful to obtain experimentally the exciton binding energy by just measuring the optical gap without requiring knowledge on dot shape, size, and surface treatment. Third, we evaluate the possibility of resonant charge transport in an array of Si QDs via miniband channels created by dot-dot coupling. We show that for such charge transport the Si QDs embedded into a matrix should have tight size tolerances and be very closely spaced. Fourth, we find that the loss of quantum confinement effect induced by dot-dot coupling is negligible – smaller than 70 meV even for two dots at intimate contact.

In Situ Gas-Phase Hydrosilylation of Plasma-Synthesized Silicon Nanocrystals

Surface passivation of semiconductor nanocrystals (NCs) is critical in enabling their utilization in novel optoelectronic devices, solar cells, and biological and chemical sensors. Compared to the extensively used liquid-phase NC synthesis and passivation techniques, gas-phase routes provide the unique opportunity for in situ passivation of semiconductor NCs. Herein, we present a method for in situ gas-phase organic functionalization of plasma-synthesized, H-terminated silicon (Si) NCs. Using real-time in situ attenuated total reflection Fourier transform IR spectroscopy, we have studied the surface reactions during hydrosilylation of Si NCs at 160 °C. First, we show that, during gas-phase hydrosilylation of Si NCs using styrene (1-alkene) and acetylene (alkyne), the reaction pathways of the alkenes and alkynes chemisorbing onto surface SiH(x) (x = 1-3) species are different. Second, utilizing this difference in reactivity, we demonstrate a novel pathway to enhance the surface ligand passivation of Si NCs via in situ gas-phase hydrosilylation using the combination of a short-chain alkyne (acetylene) and a long-chain 1-alkene (styrene). The quality of surface passivation is further validated through IR and photoluminescence measurements of Si NCs exposed to air.

Light trapping by a dielectric nanoparticle back reflector in film silicon solar cells

Drop-coated high-refractive-index nanoparticles used as a back reflector for thin-film solar cells are non-absorbing Mie-scatterers that enhance light trapping. We present optical measurements and theory for this approach. A 40% enhancement of the photocurrent and efficiency of a 2.5 μm thick single-crystal Si solar cell on display glass is achieved by adding a back reflector of 270 nm rutile TiO2 nanoparticles.