Luis G. Gerling

A prototype reactor for highly selective solar-driven CO2 reduction to synthesis gas using nanosized earth-abundant catalysts and silicon photovoltaics

The conversion of carbon dioxide (CO2) into value-added chemicals and fuels, preferably using renewable energy and earth-abundant materials, is considered a key priority for future energy research. In this work, a bias-free reactor device for the solar-driven conversion of CO2 to synthesis gas (syngas) has been developed. The integrated fluidic device consists of a cathode made of copper foam coated with low-cost nanosized zinc flakes as catalyst to perform the CO2 reduction reaction (CO2RR) to syngas, an adapted silicon heterojunction solar cell structure as photoanode with nickel foam as catalyst to facilitate the oxygen evolution reaction (OER), and a bipolar membrane separating the respective catholyte and anolyte compartments. The membrane allows for the operation of the catholyte and anolyte at different pH values. Stable and tunable hydrogen-to-carbon monoxide (H2 : CO) ratios between 5 and 0.5 along with high CO Faradaic efficiencies of up to 85% and CO current densities of 39.4 mA cm−2 have been demonstrated. Under photoelectrolysis conditions, the photovoltage of the photoanode was varied between 0.6 V and 2.4 V by connecting up to four heterojunction solar cells in series, and thus reducing the overall cell voltage solely by solar energy utilization. Bias-free operation of the integrated device has been achieved under ambient conditions with active areas for CO2RR and OER, respectively, of 10 cm2. An operation current density of 5.0 mA cm−2 was measured under 100 mW cm−2 illumination of the complete device, which corresponds to a solar-to-syngas conversion efficiency of 4.3%.

V2Ox-based hole-selective contacts for c-Si interdigitated back-contacted solar cells

Over the last few years, transition metal oxide layers have been proposed as selective contacts both for electrons and holes and successfully applied to silicon solar cells. However, better published results need the use of both a thin and high quality intrinsic amorphous Si layer and TCO (Transparent Conductive Oxide) films. In this work, we explore the use of vanadium suboxide (V2Ox) capped with a thin Ni layer as a hole transport layer trying to avoid both the intrinsic amorphous silicon layer and the TCO contact layer. Obtained figures of merit for Ni/V2Ox/c-Si(n) test samples are saturation current densities of 175 fA cm−2 and specific contact resistance below 115 mΩ cm2 on 40 nm thick V2Ox layers. Finally, the Ni/V2Ox stack is used with an interdigitated back-contacted c-Si(n) solar cell architecture fully fabricated at low temperatures. An open circuit voltage, a short circuit current and a fill factor of 656 mV, 40.7 mA cm−2 and 74.0% are achieved, respectively, leading to a power conversion efficiency of 19.7%. These results confirm the high potential of Ni/V2Ox stacks as hole-selective contacts on crystalline silicon photovoltaics.

Intermittent Chaos for Ergodic Light Trapping in a Photonic Fiber Plate

Extracting the light trapped in a waveguide, or the opposite effect of trapping light in a thin region and guiding it perpendicular to its incident propagation direction, is essential for optimal energetic performance in illumination, display or light harvesting devices. Here we demonstrate that the paradoxical goal of letting as much light in or out while maintaining the wave effectively trapped can be achieved with a periodic array of interpenetrated fibers forming a photonic fiber plate. Photons entering perpendicular to that plate may be trapped in an intermittent chaotic trajectory, leading to an optically ergodic system. We fabricated such a photonic fiber plate and showed that for a solar cell incorporated on one of the plate surfaces, light absorption is greatly enhanced. Confirming this, we found the unexpected result that a more chaotic photon trajectory reduces the production of photon scattering entropy.

PEDOT:PSS as an Alternative Hole Selective Contact for ITO-Free Hybrid Crystalline Silicon Solar Cell

Organic-inorganic hybrid solar cells were fabricated by spin coating of conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole selective contact upon textured n-type crystalline silicon wafers. Two different PEDOT:PSS commercial products (Clevios HTL Solar and PH1000) were compared, enhancing their conductivity and wettability by addition of dimethyl sulfoxide cosolvent and Capstone FS-31 surfactant. Transmission line measurements (TLM) revealed that the sheet resistance of the PEDOT:PSS layer was comparable with that of conventional Indium-Tin oxide (ITO) electrodes, allowing its use as a conductive transparent layer in an ITO-free solar cell. Quasi-steady-state photoconductance measurements revealed an implied open-circuit voltage of 644 mV for HTL Solar, which is equivalent to a saturation current density of 255 fA/cm2. In general, the performance of HTL Solar was better than PH1000, exhibiting power conversion efficiency (1 × 1 cm2 cell area) of 11.6%, as compared with 8.5%.

Analysis of temperature dependent current-voltage and capacitance-voltage characteristics of an Au/V2O5/n-Si Schottky diode

Electronic properties of Au/V2O5/n-Si Schottky device have been investigated by temperature dependent current–voltage (I–V) and capacitance–voltage (C–V) measurements ranging from 300 K to 150 K. Ideality factor (n) and barrier height (ϕ) for the Schottky device were obtained from I–V characteristics as 2.04 and 0.83 eV at 300 K and 6.95 and 0.39 eV at 150 K respectively. It was observed that in presence of inhomogeneity at metal–semiconductor interface, the ideality factor increases and barrier height decreases with the decrease of temperature. The Richardson constant value was estimated as 137 A–cm−2–K−2 from modified Richardson plot, which is closer to the known theoretical value of n-Si where mean value of barrier height (ϕb0¯), and its standard deviation (σ0) were estimated using double Gaussian distribution (DGD) analysis. Different device parameters, namely, built-in potential, carrier concentration, image force lowering and depletion width were also obtained from the C–V–T measurements. First time...

Cost-effective cleaning solutions based on H 2 O/NH 3 /H 2 O 2 mixtures for ALD Al 2 O 3 passivated IBC c-Si solar cells

In this work we study cost-effective cleaning solutions applied to interdigitated back-contacted solar cells (IBC), which are passivated by means of atomic layer deposited Al<inf>2</inf>O<inf>3</inf> films. The cleaning baths must guarantee very clean surfaces as well as relatively low etching Al<inf>2</inf>O<inf>3</inf> rates to avoid excessive undercutting at the edges of strip-like regions. We compare the standard high-cost cleaning procedure used in the microelectronic industry (RCA1/2) with simpler cleaning baths based on H<inf>2</inf>O/NH<inf>3</inf>/H<inf>2</inf>O<inf>2</inf> mixtures considering different temperatures. The best option is the RCA1/2 sequence yielding surface recombination velocities below 4 cm/s but with a total Al<inf>2</inf>O<inf>3</inf> etch around 500 nm after the cleaning stage. Nevertheless very simple and less aggressive cleaning baths performed at only 45 °C obtain a relatively good surface passivation quality, achieving S<inf>eff</inf> values of 20 ± 5 cm/s reducing the under etch to only 80 nm.

Straightforward determination of the effective mobility-lifetime product of small molecule organic solar cells

The effective mobility-lifetime product, a figure of merit for charge carrier transport in a photovoltaic device, was determined for small-molecule bulk heterojunction organic solar cells by means of a simplified methodology comprising variable light intensity measurements. The recombination losses within the intrinsic active layer, accounted by a recombination current that reduces the generated photocurrent, were correlated to lower mobility-lifetime values that reduce device performance.