LETTER pubs.acs.org/NanoLett
Electrolyte-Induced Inversion Layer Schottky Junction Solar Cells Pooja Wadhwa,† Gyungseon Seol,‡ Maureen K. Petterson,† Jing Guo,‡ and Andrew G. Rinzler*,† †
Department of Physics and ‡Department of Electrical and Computer Engineering, University of Florida, Gainesville, Florida 32611, United States
bS Supporting Information ABSTRACT: A new type of crystalline silicon solar cell is described. Superficially similar to a photoelectrochemical cell a liquid electrolyte creates a depletion (inversion) layer in an n-type silicon wafer, however no regenerative redox couple is present to ferry charge between the silicon and a counter electrode. Instead holes trapped in the electrolyte-induced inversion layer diffuse along the layer until they come to widely spaced grid lines, where they are extracted. The grid lines consist of a single-walled carbon nanotube film etched to cover only a fraction of the n-Si surface. Modeling and simulation shows the inversion layer to be a natural consequence of the device electrostatics. With electronic gating, recently demonstrated to boost the efficiency in related devices, the cell achieves a power conversion efficiency of 12%, exceeding the efficiency of dye sensitized solar cells. KEYWORDS: Solar cell, carbon nanotubes, electrolyte, nonphotoelectrochemical
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ecently we described electronic modulation of the fundamental characteristics (open circuit voltage, fill factor, power conversion efficiency) of a nanotube/n-Si Schottky junction solar cell.1 The native device exhibited a power conversion efficiency (PCE) of 8.5%. By exploiting an electrolyte gate, having access to the junction through the thin, porous single wall carbon nanotube (SWNT) film, a continuous, reversible modulation of the PCE from 3.6 to 10.9% was demonstrated (the gate circuit drawing negligible power in the steady state). The presence of an electrolyte at the semiconductor surface, we have come to realize, affords new opportunities in solar cell design. This is demonstrated in a device construction similar to the previous gated, SWNT/n-Si devices however, rather than use a continuous nanotube film to form the junction, the film is etched in a grid pattern to cover only a fraction of the n-Si surface. Figure 1a shows the schematic of such a device. A gold contact with a 2 4 mm2 rectangular window was evaporated onto a 1 μm thick oxide layer on an n-Si wafer. The gold metallization was used as an etch mask to etch the oxide within the window down to the bare Si surface. A 45 nm thick, 6 8 mm2 rectangular area SWNT film was transferred across the window contacting the gold and forming the junction with the exposed n-Si. This SWNT film, previously used as a continuous layer, was here defined by standard photolithography and etching in an oxygen plasma to create the grid pattern shown in Figure 1b (device fabrication details are provided in the Supporting Information). The grid lines shown are 100 μm wide with 300 μm spacing between adjacent lines. As was done previously, a second gold contact and a second SWNT film were deposited on the r 2011 American Chemical Society
oxide layer near the junction. This second SWNT film and its gold contact comprised the gate electrode once the electrolyte was added. Figure 2 shows the cell current density versus voltage (JV) characteristic in the dark and under illumination (AM1.5G, 100 mW/cm2) for two distinct devices in the absence of electrolyte. One device had a continuous SWNT film across the entire window and the other had been etched into a grid pattern limiting the Si coverage with the nanotubes to 27% of the window area. The nanotube film/n-Si contact forms a “conventional” metalsemiconductor Schottky junction solar cell. Fermi level equilibration of the n-Si with the nanotubes transfers electrons from the n-Si to the SWNTs generating a depletion layer and band bending in the Si, in the vicinity of the nanotubes. Photons absorbed in the Si generate electronhole pairs that are separated by the built-in potential, enabling power generation from the device. For our doping density of ∼1015 donors/cm3, this depletion layer extends e1 μm into the Si from the contact with the nanotubes. Given the relatively small extent of this depletion layer the reduced junction area of the grid device yields a reduced short circuit photocurrent. This reduction in the photocurrent does not scale in direct proportion to the reduced junction area because high-quality single crystal silicon has long diffusion lengths, allowing photocarriers generated far from the junction to diffuse there and contribute to the photocurrent. Nevertheless, the photocurrent in the grid film is reduced by more than a factor of 2 over that Received: March 10, 2011 Revised: May 3, 2011 Published: May 20, 2011 2419
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Figure 2. JV curves in both the light (AM 1.5G, 100 mW/cm2) and dark for a continuous SWNT film covering the n-Si window and for an etched film (as in Figure 1b). The etched film in this case covers 27% of the n-Si window. The current densities are the measured currents normalized to the full window area surrounded by the Au contact.
Figure 1. (a) Schematic of the device. Not shown is the EMI-BTI ionic liquid electrolyte that extends across both the gate electrode SWNT film and the n-Si junction. (b) Photograph of a SWNT film across the exposed n-Si within the gold electrode window in which the SWNT film was etched to form the grid pattern shown. The SWNT film grid lines are 100 μm wide with 300 μm between them. The seeming break in the grid lines at the bottom edge of the window is an illusion. The lines run continuously up onto the gold electrode.
of the continuous film, yielding a corresponding decrease in the full window-area-normalized power conversion efficiency. The situation becomes substantially more interesting on the simple addition of the 1-ethyl-3-methylimidazolium bistrifluoromethylsulfonyl)imide (EMI-BTI) ionic liquid electrolyte. Figure 3 compares the illuminated JV curves of a grid SWNT film device before and after addition of the electrolyte with the gate electrode electrically floating. Also shown in Figure 3 is the case with 0.75 V applied to the gate electrode relative to the SWNTgrid junction electrode (Table SI1 in the Supporting Information provides the corresponding cell characteristics). The simple addition of the electrolyte (gate floating or not) more than recovers the short circuit photocurrent lost to the reduced areal coverage of the n-Si by the nanotubes. To explain this behavior, we conclude that the electrolyte induces its own depletion (or inversion) layer in the Si across the large gaps between the nanotube grid lines. Electrolyte-induced depletion layers are well-known from photoelectrochemistry studies of electrolytesemiconductor interfaces.2 By incorporating a counter-electrode and a suitable regenerative redox couple in the supporting electrolyte such junctions form the basis of the liquid-junction regenerative solar cell, the best known example of which is the Gratzel cell.3 In such
Figure 3. JV curves of the grid cell before and after addition of the EMI-BTI ionic liquid electrolyte (gate electrode floating) and with 0.75 V applied to the gate electrode.
cells, the counter electrode forms one output terminal of the cell and the redox couple serves as a shuttle necessary to ferry charge between the semiconductor surface and the counter electrode, effectively completing the internal circuit of the cell. Our nanotubeelectrolyte/n-Si cell must be distinguished from such photoelectrochemical cells in that there is no redox couple and the EMI-BTI electrolyte was selected for its very broad electrochemical window ranging from 2.6 to þ2.0 V (vs Fc/Fcþ, i.e., centered at 5.1 V relative to the vacuum level).4 Hence our electrolyte does not participate in the charge transport. Instead photogenerated holes that make it to the electrolyte-induced inversion layer in the Si are trapped by the electric field within the layer and diffuse along it until they encounter a nanotube grid line where they are collected. Because the electric field, which accumulates holes at the surface also repels electrons, deleterious surface recombination is largely avoided. Consideration of how the depletion layer develops in photoelectrochemical cells further demonstrates that the inversion layer appearing in our devices must be of distinct origin. In the electrolyte of a photoelectrochemical cell, the electrochemical (Nernst) potential of the incorporated redox couple sets the equilibrium distribution of the couple between its reduced and 2420
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Figure 4. (a) Experimental JV curves at the specified gate voltages. (b) Simulation geometry and parameters. (ch) Simulation results at VGate = 0.75, 0, þ0.75 V and VBias = 0, 0.3 V.
oxidized states. When the electrolyte comes into contact with the semiconductor, the two exchange charge, simultaneously shifting the electrochemical potential of the redox couple and the Fermi level of the semiconductor until they are in equilibrium (thus establishing the depletion layer in the semiconductor). But in our device, the large electrochemical window over which the EMIBTI electrolyte undergoes no redox precludes such charge exchange. Accordingly, we must look elsewhere for the cause of the inversion layer required by the JV curves. The situation is reminiscent of so-called “grating” metal insulatorsemiconductor (MIS) cells first described by Godfrey and Green in the late 1970s.5 In those devices, narrow metal lines (Al or Mg) on the front surface of p-type silicon collected
electrons trapped by an inversion layer formed at the p-Si surface in the regions between the widely spaced metal lines. The inversion layer in those devices was induced by positive charge trapped in an SiO layer grown on the Si. In the present case, the gate voltage is certainly capable of inducing charge (of either sign depending on the polarity of the gate) adjacent to the surface of the n-Si but interestingly the high short circuit current (VBias = 0 V) seen in the grid film of Figure 3 occurs immediately on introduction of the ionic liquid. This implies that negative ions accumulate at the n-Si surface upon simple introduction of the electrolyte. To determine if such charge separation can be explained by the native electrostatics, we have modeled the system using the 2421
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Nano Letters device simulation package Synopsys TCAD Sentaurus.6 To simulate the effect of the electrolyte, we exploit the program’s ability to simulate dielectric coatings, using as proxy for the electrolyte a dielectric coating with the very large dielectric constant (ε = 5000), that is, the mobile free ions of the electrolyte are replaced by a “dielectric” layer having a bound charge possessing an extreme polarizability (the free charge of electrolytes precludes definition of a real, DC dielectric constant for them so that the AC dielectric constants available in the literature are not relevant). The value of ε = 5000 comes from the ratio of a characteristic dielectric layer length (∼100 μm) relative to that of the characteristic Debye layer dimension in the electrolyte (
