Howard M. Branz

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.

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%.

Nanoporous black silicon photocathode for H2 production by photoelectrochemical water splitting

Nanostructured Si eliminates several critical problems with Si photocathodes and dramatically improves a photoelectrochemical (PEC) reaction important to water-splitting. Our nanostructured black Si photocathodes improve the H2 production by providing (1) near-ideal anti-reflection that enables the absorption of most incident light and its conversion to photogenerated electrons and (2) extremely high surface area in direct contact with water that reduces the overpotential needed for the PEC hydrogen half-reaction. Application of these advances would significantly improve the solar H2 conversion efficiency of an ideal tandem PEC system. Finally, the nanostructured Si surface facilitates bubble evolution and therefore reduces the need for surfactants in the electrolyte.

Efficient heterojunction solar cells on p-type crystal silicon wafers

Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

Stand-alone photovoltaic-powered electrochromic smart window

Three different, innovative approaches have been taken to develop photovoltaic (PV) integrated electrochromic (EC) devices for smart-window applications. These are: (i) a stand-alone, side-by-side PV-powered EC window; (ii) a monolithically integrated PV-EC device; and (iii) a novel photoelectrochromic device based on a dye-sensitized TiO2 solar cell. The compatibility of PV-EC devices has been analyzed, and the potential for large energy savings for building applications has been suggested. The first monolithic, amorphous-silicon based, PV-powered electrochromic window is described in detail. The device employs a wide bandgap a-Si1−xCx/H n–i–p PV cell as a semitransparent power source, and a LiyWO3/LiAlF4/V2O5 EC device as an optical-transmittance modulator. The EC device is deposited directly on top of a PV cell that coats a glass substrate. The a-Si1−xCx/H PV cell has a gap of 2.5 eV and a transmittance of 60–80% over a large portion of the visible light spectrum. Our prototype 16-cm2 PV-EC device modulates the transmittance by more than 60% over a large portion of the visible spectrum. The coloring and bleaching times of the EC device are approximately 1 min under normal operating conditions (±1 V). A brief description of photoelectrochromic windows based on a combination of dye-sensitized TiO2 and WO3 EC-layer is also given.

Exciton splitting and carrier transport across the amorphous-silicon/polymer solar cell interface

The authors study exciton splitting at the interface of bilayer hybrid solar cells to better understand the physics controlling organic-inorganic device performance. Hydrogenated amorphous silicon (a-Si:H)∕poly(3-hexylthiophene) (P3HT) and a-Si:H∕poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) solar cells show photoresponse dominated by exciton production in the polymer. The a-Si:H∕P3HT devices are nearly as efficient as titania/P3HT cells. However, the a-Si:H∕MEH-PPV system has much lower photocurrent than a-Si:H∕P3HT, likely due to inefficient hole transfer back to the MEH-PPV after energy transfer from MEH-PPV to a-Si:H.

Hydrogen collision model of light-induced metastability in hydrogenated amorphous silicon

Abstract A new model of light-induced metastability (Staebler-Wronski effect) in hydrogenated amorphous silicon is proposed. When two mobile H atoms generated by photo-induced carriers collide, they form a metastable, immobile complex containing two SiH bonds. Excess metastable dangling bonds remain at the uncorrelated sites from which the colliding H were excited. The model accounts quantitatively for the kinetics of light-induced defect creation, both near room temperature and at 4.2 K. Other experimental results, including light-induced and thermal annealing kinetics, are also explained.

Area-Dependent Switching in Thin Film-Silicon Devices

We report on the area dependence of switching in both Cr/ p + a-Si:H/Ag(Al) and Cr/ p + μc-Si/Ag(Al) filament switches. The doped amorphous (a-Si:H) or microcrystalline (μc-Si) thin Si layers are made by hot-wire chemical vapor deposition. The device active region area (A) is varied over 5 orders of magnitude, from 10 -7 to 10 -2 cm 2 , using photolithographically defined Ag and Al top contacts. Before switching, the resistance of 100-μm 2 devices is normally about 100 kΩ for μc-Si and 10 GΩ for a-Si:H. After switching with applied current ramps, the resistance decreases to a few hundred ohms in all a-Si devices and to a few thousands ohms in μc-Si devices. In both μc-Si and a-Si:H devices, the switching voltage (V sw ) decreases with increasing device area according to V sw ~ V 0 -αln(A/A 0 ) with α=0.3V for a-Si:H and α=0.04V for μc-Si. For both materials, the switching current roughly obeys the power law I sw ∞ A β with β~1. A statistical model is proposed to explain the area scaling of the switching voltage and relate the parameters to the material properties.

Exciton harvesting, charge transfer, and charge-carrier transport in amorphous-silicon nanopillar/polymer hybrid solar cells

We report on the device physics of nanostructured amorphous-silicon (a-Si:H)/polymer hybrid solar cells. Using two different polymers, poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV), we study the exciton diffusion, charge transfer, and charge-carrier transport in bilayer and nanostructured a-Si:H/polymer systems. We find that strong energy transfer occurs in the a-Si:H/MEH-PPV system. However, inefficient hole transfer from the a-Si:H to the polymers renders negligible photocurrent contribution from the a-Si:H as well as very small currents in the a-Si:H/MEH-PPV devices. These results suggest that a-Si:H may be unsuitable for use in polymer-based hybrid cells. Nanosphere lithography and reactive ion etching were used to fabricate nanopillars in a-Si:H. The nanostructured a-Si:H/P3HT devices showed improved efficiency and almost perfect charge-carrier extraction under short-circuit conditions. By modeling these nanostructured devices, the loss mechan...