Photoelectroconversion by Semiconductors A Physical Chemistry Experiment Qinbai Fan, Debra Munro, and L. M. N ~ ' Cleveland State University, Cleveland, OH 4411 5 S u n l i g h t i s a clean a n d inexhaustible . source of energy, so solar energy conversion is Electron i m p o r t a n t i n modern society. Solar cells Energy n-type rcmisondactor p-type remicanducto. change solar energy to electricity and are CB CB widely used in everyday life. Solar cells are - - - - - - .- - . EF used a s power sources in diverse applications, from a small calculator to the space shuttle. Undergraduate physical chemistry laboratory courses generally do not include experiments on photoelectrochemistry. We propose here a n experiment that will give students some experience with photochemistry, electrochemistry, and basic theories about semiconductors. I n this experiment, a liquid-junction solar cell is used, a s opposed to the more common Electron Energy p n solid-junction solar cell, which is dinicult and complicated to make. The process by which semiconductor liquid-junction cells convert sunlight into electricitv resembles the one orrurriniin solid p n jun&on solar cells. The main d~ffcri.ncc!is that the rircuit Involves redox reactions a t the photoelectrode and a t the counter electrode. These redox reactions are equal, but opposite in direction, so there is no net chemical change ( I ) . The con- Figure 1. Schematic diagrams representing (A) the n-type and p-type semiconductors version efficiency of the electrode can he en- and (B) a p-n junction. CB is the conduction band; VB is thevalence band; 6 is the Fermi hanced (1-5) by modifying the surface of the level; and EBis the energy of the barrier. photoelectrode. The following proposed experiment consists of constructing a photochemical cell, determining the cell parameters, then modifying the surface of the electrode, and determining the cell parameters once again. This experiment illustrates some fundamental physical and chemical principles related to light and electricity interconversion a s well a s the properties of semiconductors. Background Semiconductors are characterized by the difference in energy between the valence-band and the conduction-band electrons. The valence band is the overlap of filled orbitals, and the conduction band is the overlap of untilled orbitals. The bandgap is the difference between the lowest energy electrons in the conduction band and the highest energy electrons in the valence band. Electrons can be promoted fmm the valence band to the conduction band by the absorption of a photon of light that has energy equal to or greater than the handgap energy. When the electron moves into the conduction hand, it leaves behind a vacancy or '%ole" in the
Figure 2. Schematic diagrams of the energy levels involved in a photovoltaic cell: (A) befare the n-type semiconductor is placed in the electrolyte; (6)when the n-type semi'Author to whom correspondence should be ad- conductor is immersed in the electrolyte;(C) when the semiconductor is illuminated. E,, is the redox potential; V, is the open-circuit voltage. dressed.
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Figure 3. Arrangement of the experimental apparatus with enlarged detail of the photoelectrode. valance band. Both the negatively charged electrons in the condudion band and the positively charged holes in the valence band are mobile and can serve as charge carriers. Equilibrium Potential Gradient In the Dresence of a ~ o t e n t i aaadient l (or electric field) the elect~:onsand holes'tend to move in opposite directions, resulting in an electrical current. Without the ~otential gradient; the electron-hole pairs produced will recombine without producinc the electrical current. For this reason. photwoltaic devices require en equilibrium potential gradient in the illumlnatcd area of the semiconductor. A potential gradient can be created by forming an interfaceior junction, with another semiconducting material or an electrolyte. Formation of a Solid p-n Junction In the solid p-n junction solar cell, the interface is formed by "doping" the surface of a p- or n-type semiconductor with atoms that will invert the semiconductor type. An n-type semiconductor is formed by doping a lattice having N valence electrons with a material having N + 1valence electrons. The extra electron will move into the conduction band. As the number of electrons in the conduction band increases, the chemical potential of the electron, or the Fermi level, moves closer to the conduction band, and the material becomes n-type. Similarly, if a lattice having N valence electrons is doped with a material having N - 1 valence electrons, a vacancy is created in the valence band. As the number of vacancies increases, the Fermi level moves closer to the valence hand and a p-type material is created (Fig. 1A). When either the surface of a p-type semiconductor is made into n-tvoe or an n-tvoe is made into D-tvoe.. a w n junction is for;ded and the ~ i mlevels i of the p and n s:des become eaual. In order for this to haDDen. the bands must bend. When the bands bend, this creates &I energy barrier because the electrons, or holes, must now move uphill to migrate from the n to p, or p to n regions. This energy barrier is the upper limit to the photovoltage created when the semiconductor is illuminated (Fig. 1B).
Figure 4. I-V curves obtained with (a) polished'photoelectrode. (b) polished and etched photoelectrode, and (c) polished, etched electrode with chemically deposited nickel. The chemical potential of the electron in a liquid is called its redox ~otentialinstead of its Fermi level. The two are equivalent. When an n-type semiconductor is placed in a redox solution, the conduction band and valence bend bend upward so that the Fermi level of the electrons in the semiconductor equals the redox potential of the electrons in the solution. The bands bend downward if a p-type semiconductor is placed in a redox solution. Once again, a n energy barrier is created that must be overcome by the electrons or holes (Figs. 2Aand B). When sunlight is absorbed by the semiconductor, this creates electron-hole pairs that are separated by the potential gradient. In an n-type semiconductor, the holes travel to the solution interface, and electrons move through an external circuit to the counter electrode, producing electrical power (Fig. 2C). In a p-type semiconductor, the current is reverse. Maximum Power Output of the Cell Theoretically, the maximum power output of the cell is a function of three things: the open-circuit voltage, the short-circuit photocurrent, and the fill factor. As stated previously, the upper limit to the photovoltage created by illumination, or open-circuit voltage (Voc),is represented by the barrier height. The short-circuit photocurrent (I,,), is proportional to the yield of the reactions at the photoelectrode. The fill factor (FF) is defined as (~,,II,,V,,. The efficiency of the solar cell can be determined by
. -.
Formation of a Liquid p-n Junction In a liquid-junction cell, a junction is formed when the semiconductor is immersed in a redox-couple solution. The principal elements of the liquid-junction photovoltaic cell are the photoelectrode, the counter electrode, the electrolytic solution, and the semiconductor-electrolyte interface.
The practical maximum output is governed by the recombination of holes and electrons that occurs in surface and near-surface states. Just as the valence band is formed from the hondinr orbitals and the conduction band from the antibonding&bitals ofthe lattice atoms, weaker bondinp at defect sites on thc surface lceds to less s~littinr! between the bonding and antibonding orbitalsand introduces states in the bandgap between the valence and conduction bands ( I ) . When the semiconductor is illuminated, the photogenerated electrons and holes are trapped by these surface states and recombine. Such recombination results in losses of photovoltage, photocurrent, and power output, thus reducing cell effkiency From a practical stand point, the open-circuit voltage (V,,) is the potenVolume 72 Number 9 Se~tember1995
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Values of Cell Parameters
tial a t which the rate of photogeneration of electrons or holes equals the rate of their recombination. Recombination results in lower V,,. The fill factor is also a measurement of loss mechanisms in photovoltaic cells. For conventional cells, F F is typically less than 0.5.
Polished (a)
Exoerimental Apparatus
Photocurrent L (mAIcrn2)
A PAR M273 potentiostat was used with a Hewlett Packard 7035b X-Y recorder to obtain the current-voltage (I-V) curves. Typical scan rate was 20 mV/s for optimum results. Other commercially available instruments can be used. The I-V curve can also be obtained with a variable power supply (-2-V to +2-V range) or using a dry battery connected in series with a n ammeter and in parallel to a voltmeter. The electrochemical cell was irradiated using a halogen ~, a t the lamp with a light intensity of 30 m ~ l c mmeasured same position a s the working electrode without correction for soiution absorbance. hii intensity was measured by a radiometer rEdmund1. The short-circuit current and the open-circuit voltages were measured using a Keithley 197A autoranging digital voltmeter. The n-type silicon can be purchased from semiconductor suppliers. The one we used was a single-crystal wafer, a discard from a silicon research laboratory. This wafer has a resistivity of 8-10 ohm cm. Any commercially available n-silicon with low resistivity can be used. A single crystal is not necessary. Chemicals Caution: Use
aeid-resistant gloves to handle etching
solution. All chemicals were reagent&ade. The electrolyte used was 0.1 M &Fe(CN)6 and 0.1 M &Fe(CN)e. The etching solution contained 5 parts concentrated HN03,2 parts HF, and 2 parts concentrated CH3COOH. For chemical deposition of nickel, a solution containing 7.5 g NiClz, 12.5 g NH4C1, 16.2 g sodium citrate, and 2.5 g sodium hypophosphite dissolved in Hz0 to produce 250 mL was used. Concentrated NH40H solution is also needed. The epoxy used was a commercially available superfast epoxy purchased from a n office supply store. The counter electrode was a 1-cm2piece of platinum foil with thickness of 0.05 in. Procedure The phototoelectrode was made using a piece of silicon approximately 1 x 1 cm2. Nickel was chemically deposited on the silicon by immersing the silicon electrode in a small amount of ~ i solution, ~ + heating the solution to a gentle boil, and adding a few drops of NH40H until the color of the solution changed to green. The silicon electrode was then taken out of the solution with a pair of forceps and rinsed with deionized water. Acopper wire was soldered to the nickel layer for electrical connection. Exposed copper and all except one face of the plated silicon was covered with epoxy for electrical insulation (see Fig. 3). The uncovered face of the electrode was polished gently with 600-grit sandpaper until the nickel was removed. The photoelectrochemical cell was assembled a s shown in Figure 3. The electrode was placed in approximately 25 mL of electrolyte solution contained in a 50-mL beaker with the polished face a s close to the cell wall a s possible. The counter electrode was placed parallel to the silicon near the other side of the cell. The short-circuit current (Iee)and the open-circuit voltage ( V d were measured by simply connecting the digital voltmeter across the cell with the silicon electrode under illumination from the halogen 844
Journal of Chemical Education
Polished and Polished, then etched etched, dipped in ~ i ~ ~ s o l u t i o n (b) (c)
2.0
8.7
13.1
Fill Factor
0.18
0.25
0.38
Efficiency(%)
0.22
1.77
6.3
Photovoltage Voc (V)
lamp. An I-V curve was obtained from 0 to -0.4 V with the potentiostat, also with the photoelectrode illuminated. The electrode was then taken out of the electrolvte. rinsed with deionized water and pollshed. It was dipp&l in the etchinc! solution for 5 s a n d rinsed w11h disrilled water. A second get of I,, and V,, and a second I-V curve were obtained. The electrode was then polished and etched a s before. The electrode was then dipped in the ~ i ' +solution for 10 s, rinsed with water, and returned to the cell. Afinal set of Isc and V, and a final I-V curve was obtained. The I-V curves are shown in Figure 4. Results From the I-V curves, the cell parameters were deteris given by the maximum area of a rectanmined. (Nmaa gle subscribed under the I-V curve.
x 100% cell efficiency = . .(Mmax lncldent power
Values of I.,, V,, ( N,, listed in the table.
fill factor, and cell efficiency are
Conclusions The I-V curves and cell parameters show that the etchine-. Drocess and nickel deoosition result in hiehcr cell efficiency. Similar efficiency enhancement on other semiconductors were observed previously when t h e metals deposited were platinum (6) and ruthenium (1-5). The etching process increases the surface area of the electrode. More reactive sites are available for t h e oxidation of F ~ ( c N ) ~The . nickel deposited on the electrode forms tiny "islands" that may act a s catalysts, increasing the reaction velocity on the electrode surface and leading to higher currents. The chemisorbed nickel may also interact with the surface states and move them away from the center of the bandgap. When the surface states are moved closer to either the valance hand or the conduction band, they cannot t r a p electrons or holes a s efficiently, and the probability of recombination is greatly diminished. Details of these theories can be found in the references (1-7). This proposed experiment can be carried out by the students after a brief introduction to electrochemical theory and surface chemistry in physical chemistry. The theory of semiconductors i n fundamental uhvsics and cvclic voltammetry in analytical chemistry are prerequisites for this ex~ e r i m e n tThe . students gain the experience of working on "research project" and are encouraged to discuss findings
-
."
a
and interpretation of the data as the surface of the electrode is modified. The experiment can also be developed into an undergraduate research or independent study project to study the modification of the electrode surface by different metals or to study different electrolytic solutions. The possibilities are unlimited. The same electrode can be reused in different experiments as long as the epoxy is still insulating. All that is needed is a quick polish of the uncovered surface.
Literature Cited 1. Heller,AAe C k m . Re& 1381.14, 154. 2. Morrison. S. R. Ektrofhemistv at S~mLmnduetorand Oxidized Metal E l e e t d e $ ; Plenum: New York, 1980. 3. Bard. A J.; F a u h e l L.R. Eleetmekrnlrnl Methods. Fundamnfclls and Applicotlm8; Wiley: New York. 1980. 4. Lewia,N. %Ann. Re". afPhys. Cham. 1331,42,543. 5. Lewis,N. S A n n Re". ofMatm Sci. 1384.14.95.
6 . Fan, H.; Fan, Q.: Den&X.AcfoEnsrgioa Soloris Wnico 1990,11i4J. 429.
7 . Nakato,Y,Egi,T.;HiramoM,M.;iRubomura. H. J. Phys. C k m . 1384.88.4218.
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