Photoelectrochemical cells (PECs) are solar cells based on a

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1 pubs.acs.org/jpcl Downloaded via on January 24, 2019 at 17:05:21 (UTC). See for options on how to legitimately share published articles. Photoelectrochemical Cell Measurements: Getting the Basics Right Photoelectrochemical cells (PECs) are solar cells based on a semiconductor electrolyte interface. The ease of formation of this interface and the expectation that an interface between a solid and a liquid will be physically perfect led to considerable investigation of these cells after their appearance in the early 1970s. However, this interest died down to a large extent until the appearance of the dye-sensitized solar cell (DSC) based on a very high surface area substrate, allowing good light absorption (and photocurrent generation) by a molecule-thick dye layer. 1,2 This idea was quickly adopted also for semiconductor sensitizers, 3 5 but there was only scattered interest and progress in these semiconductor-sensitized solar cells (SSSCs) for a long time. This situation has radically changed in the past 5 years, and the field is now rapidly developing, with parallel improvements in cell efficiencies. 6 9 (Some of the relatively high efficiency cells in the literature use methanol in the polysulfide electrolyte. As pointed out by Mora-Sero and Bisquert, 7 methanol is a sacrificial hole scavenger, and it is likely that cell efficiencies using this electrolyte are artificially increased. Also, such cells are unlikely to be stable, not only because of the loss of methanol but also because of the various oxidation species formed (all the way to (bi)carbonate).) Along with the welcome improvement in both efficiencies and understanding of semiconductor-sensitized solar cells, there is still often considerable lack of understanding of how these cells should be measured. However, along with this welcome improvement in both efficiencies and understanding of SSSCs, there is still often considerable lack of understanding of how these cells should be measured. Two particular measurement problems, use of an unsuitable counter electrode and misinterpretation of threeelectrode photoelectrode characterization, are still commonly seen. This lack of understanding probably reflects the different backgrounds of the early photoelectrochemists (who were invariably electrochemists, or at least possessed a strong electrochemistry background) and the SSSC community (more emphasis on materials chemistry). This can be deduced from two main observations. One is that, at least up to recently (the situation is beginning to change), Pt was almost universally used a counter electrode for these cells. This is presumably because Pt was the counter electrode of choice for the dyesensitized cells using polyiodide electrolytes. Indeed, Pt is a good counter electrode for polyiodide. However, polyiodide is not a good electrolyte for most SSSCs (the commonly used semiconductors in these cells are unstable in this electrolyte); polysulfide is the common electrolyte used in these cells, and Pt (also Au, which was more recently used for these cells) is a very poor electrode in this electrolyte, as was shown 3 decades ago. 10,11 Much better electrodes for polysulfide were shown in these same papers to be several metal sulfides (CoS, PbS, Cu 2 S), with CoS being the best for long-term stability. Additionally, high surface areas were important to obtain low polarization at these electrodes. These metal sulfide counter electrodes are now beginning to be used more commonly in SSSCs, (see, e.g., refs 12 15), but Pt is still commonly used and leads to underestimation of the cell efficiency. It is also interesting to note that, in many papers emphasizing these counter electrodes, the most important measurement, polarization curves showing the electrocatalytic activity of these counter electrodes, is not shown. More commonly used are impedance measurements, which, while related to the polarization curves and also certainly useful in their own right, are not so readily interpreted as are polarization curves. The second indication that some of the groups working with SSSCs are unfamiliar with the electrochemical principles that underlie these cells and, with even more serious consequences, can sometimes be seen when three-electrode measurements are used to characterize cells. Such three-electrode measurements have on not a few occasions been incorrectly interpretated, leading to overestimation, often very large indeed, of cell efficiencies. While nowadays most SSSCs are measured using a two-electrode cell, three-electrode measurements are sometimes used, using a potentiostat and reference electrode to maintain the potential of the photoelectrode at a well-defined value (described in more detail below). Three-electrode measurements do not give cell efficiencies but rather (if carried out correctly) photoelectrode efficiencies. Such measurements are perfectly valid (and even advisible in some cases; the poor counter electrode problem would not have lasted so long had three-electrode measurements been done as well as twoelectrode ones). However, it is critical to interpret the measurements correctly. The purpose of this is to clarify these measurement problems in the hope that they will no longer occur. A Short Comparison of Two- and Three- Electrode Measurements. It is important first to understand the information that three-electrode current voltage (I V) measurements provide and what the differences are with two-electrode measurements of the same photoelectrode. Although two-electrode measurements are obvious and do not require any elaboration here, the same cannot be said for three-electrode measurements to those who do not have some prior familiarity with them, and as is evident from the way that such measurements are sometimes misused (to be discussed in detail later). Figure 1 shows a schematic of the three- and two-electrode cells and the measurement leads. Received: February 23, 2012 Accepted: April 12, 2012 Published: May 3, American Chemical Society 1208

2 photoelectrode in isolation, a perfectly legitimate thing to do, as already noted (and even wise in some cases, particularly in novel systems). However, it should be stressed that efficiencies measured in this way are photoelectrode efficiencies and not cell efficiencies. Two-Electrode Measurements: The Role of the Counter Electrode. We begin with the simpler two-electrode system, that is, a cell made up of a photoelectrode and a counter electrode. The example that we use is shown for an n-type photoelectrode (the most common type). However, the same arguments apply for p-type photoelectrodes. To fully describe the operation of this system, we use a generic I V plot, as shown in Figure 2. (Note that all I V plots in this paper are Figure 1. Schematic diagram of cells for a three-electrode (left) and two-electrode (right) measurement of a SSSC. The cell components are the same in both cases. The two-electrode cell is normally thinner and sealed; the three-electrode cell requires space for a reference electrode, and sealing is less important in the short term. The threeelectrode cell is connected to a potentiostat, while the two-electrode cell is connected to a I V setup (or even directly to a multimeter). The counter electrode is shown on an FTO substrate but any stable, conducting substrate can be used. A three-electrode measurement uses a reference electrode to provide, as the name implies, a reference potential. Using a potentiostat, the working electrode (usually the photoelectrode; can also be the counter electrode if the purpose is to measure counter electrode polarization) is set at a fixed potential versus this reference electrode, and the current at this potential is measured. To obtain a current voltage plot of the photoelectrode, the potential versus the reference electrode is scanned and the current monitored as a function of this potential scan. In this type of measurement, the counter electrode performance is (in most cases) not important; the potentiostat maintains the potential of the photoelectrode at the desired value and reads out the current at that potential, and it supplies whatever voltage between the photoelectrode and counter electrode is necessary in order to maintain that current. For a very poor counter electrode (and particularly for relatively poorly conducting electrolytes), that voltage may be many volts. As long as the potentiostat can supply that voltage (and the limit for most potentiostats is at least some tens of volts if not more), the counter electrode will be able to pass the needed current. Three-electrode measurements do not give cell efficiencies but rather photoelectrode efficiencies. However, it is critical to interpret the measurements correctly. This is a good point to distinguish between the voltage limit that the potentiostat can supply between the working electrode and counter electrode (tens of volts or more) and the voltage limit between the working electrode and reference electrode (typically ±2 V). Using a three-electrode system therefore measures only the photoelectrode and ignores any polarization that occurs at the counter electrode. This is convenient if a good counter electrode is not available or if it is wished to measure the Figure 2. Typical photocurrent photovoltage plot of a well-behaved photoanode (red plot) showing the three main parameter points, short-circuit current (I SC ), open-circuit voltage (V OC ), and maximum power point (P MAX ). The I V curves of three counter electrodes (in the third quadrant): a good electrode (blue plot), a rather poor electrode (green plot), and a smooth Pt electrode (purple plot), the last measured in a 1 M S 2 /1 M s polysulfide electrolyte. The voltage scale is given as potential relative to the electrolyte potential. given according to electrochemical convention. In normal photovoltaic convention, the I V plot of the illuminated photoelectrode would be inverted and moved into the fourth quadrant; however, because in this paper we are describing the effect of additional electrodes, it is more useful to use electrochemical convention.) The red curve represents the I V plot of the photoelectrode itself, that is, ignoring the counter electrode or, put another way, assuming that the counter electrode behaved ideally (was totally nonpolarizable; in other words, its potential remained constant regardless of the current flowing through it). The three plots in the third quadrant represent the I V plots of three different counter electrodes. The blue line represents what would be a good, practical (and attainable) counter electrode that only polarizes a little when current is passed. The green curve represents what we will call here a medium electrode (in fact, it would be a rather poor one in reality), and the purple curve is a very poor electrode that polarizes greatly when current passes (this very poor electrode is an actual I V plot of a smooth Pt electrode in a 1 MNa 2 S + 1 M S electrolyte, a commonly used electrode/ electrolyte combination). The zero on the potential scale (xaxis) is the redox potential of the electrolyte. This should be a well-defined value for an actual PEC. This redox potential depends on the relative concentrations of the reduced and 1209

3 oxidized redox species in the electrolyte and is normally given by the well-known Nernst equation E = E 0 RT nf ln [red] [ox] where E is the (measured) redox potential, E 0 is the standard redox potential, n is the number of electrons transferred in the redox process, [red] and [ox] are the concentrations (more strictly, activities) of the reduced and oxidized species, respectively, and R, T, and F have their usual meanings. Thus, for example, if the reduced species concentration is greater than that of the oxidized species (commonly the case for n-type photoelectrodes), the redox potential will be more negative than the standard value. It should also be noted that the commonly used polysulfide electrolyte only follows this equation for solutions where the added free S concentration is no greater than the added sulfide concentration. For higher S concentrations, the potential becomes rapidly more positive. 11,16 This is due to the complicated equilibria present in these solutions. 16,17 Using a three-electrode system measures only the photoelectrode and ignores any polarization that occurs at the counter electrodem which is convenient if a good counter electrode is not available or one wishes to measure the photoelectrode in isolation. However, it should be stressed that efficiencies measured in this way are photoelectrode efficiencies and not cell efficiencies. The electrolyte should be a well-behaved redox system if any extrapolation to long-term stability is expected. As a practical example of an electrolyte that is not a well-behaved redox system, aqueous Na 2 SO 3 is a common case. Na 2 SO 3 is an effective hole scavenger (which is why it is commonly used). However, there is no defined electron acceptor in this electrolyte. One possibility for the electron acceptor in this solution is dissolved O 2. The point is that (a) the photoelectrode and counter electrodes may not be at the same potential in the dark (which imparts a bias to the cell already in the dark) and (b) such an electrolyte will not be regenerative (the composition of the electrolyte will change with time). Another common example is the use of S 2 (sulfide without any added S). Again, only one member of the redox couple is present. In this case, S (which will dissolve in S 2 to give polysulfide) will form as the cell operates. Initially some other species (possibly H 2 evolution because O 2 will be largely scavenged by the S 2 ) will be reduced at the counter electrode. Eventually, an equilibrium is formed where enough polysulfide has been formed to result in a well-behaved redox solution. However, this equilibrium situation is unlikely to behave in the same way as the initial pure S 2 electrolyte; therefore, cell parameters given in a pure S 2 solution are unlikely to be stable (we do not imply that they become worse, only that they will probably change; it is even possible that the cell improves after formation of polysulfide). We note that these considerations are important for stable regenerative PEC cells whose purpose is to generate electricity. For PECs whose purpose is to generate chemicals, they are not relevant. Keeping these points in mind, we now return to our (regenerative) PEC of Figure 2. There are three main data points in PV cells in general, the short-circuit current (I SC ), the open-circuit voltage (V OC ), and, most important for a functioning cell, the maximum power (P MAX ). Figure 3 shows Figure 3. Schematic I V curves of a photoelectrode (red plot) and counter electrode (green plot; the intermediate counter electrode from Figure 1) showing how the cell parameters, and specifically P MAX, are calculated, taking into account the counter electrode polarization. The purple vertical dashed line in the second quadrant shows how the voltage at maximum power (V Pmax ) is measured for no counter electrode polarization; the purple dashed horizontal line shows the current at P MAX (intersection with the current axis; not designated). The corresponding blue dashed lines show the same when the counter electrode polarization (shown by the intersection of the vertical green dotted line in the third quadrant with the potential axis) is taken into account. The potential scale is versus the electrolyte potential. how these data points are calculated taking the middle counter electrode plot in Figure 2 as an example. At any point on the I V curve of the photoelectrode, the same current that flows through the photoelectrode must also flow through the counter electrode. Therefore, the loss in voltage due to polarization of the counter electrode at a fixed current must be subtracted from the potential taken from the photoelectrode curve at the same current. If we consider first no counter electrode loss (P MAX designated in Figure 3 by purple text and dashed lines), P MAX is calculated to be at a current of 11 ma/cm 2, which is equivalent to a cell voltage of 0.43 V (i.e., the photoelectrode potential at 11 ma/cm 2 ). If we now take into account the voltage loss at the counter electrode, we calculate a maximum power point at 10 ma/cm 2 (note that the maximum power point changes depending on the counter electrode polarization). The equivalent photoelectrode potential is now 0.46 V (blue text and dashed lines); the polarization of the counter electrode (shown by the green dashed lines in the third quadrant) is V. Subtracting this from the 0.46 V given by the photoelectrode results in a cell voltage of V. The same method can be used at every point on the I V curve. Note that I SC will also decrease upon polarization of the counter electrode, although for the example in Figure 3, the decrease will be slight because the part of the photoelectrode I V curve 1210

4 near the I SC is nearly horizontal (had it been completely horizontal, there would be no difference between the two values of I SC ). V OC, on the other hand, does not change as a function of counter electrode quality because no net current flows at this point, and therefore, there will be no counter electrode polarization. Table 1 gives P MAX for the PEC in Figure 2 with an ideal (nonpolarized) counter electrode and for the PECs made using Table 1. Cell Parameters Calculated for Different Counter Electrodes a counter I I SC V(photo) V(counter) V(cell) P MAX (mw) ideal good medium bad a I (column 2) gives the operating current for which the other parameters are measured from Figure 2. I SC is for each counter electrode. V(photo) is the photovoltage based on the ideal photovoltage photocurrent characteristic, while V(counter) is the voltage loss (polarization) at the counter electrode at the chosen current. The difference between these two voltages gives the actual cell voltage (V(cell)). P MAX is the calculated maximum power for the cell for the different counter electrodes. the three different counter electrodes. The data points are also given for several different values of power near the P MAX for a broader picture of the effect of a polarized counter electrode on the overall two-electrode PEC. The P MAX for the ideal counter electrode is 4.73 mw. For the good counter electrode with feasible performance, it drops slightly to 4.50 mw. For the poorer counter electrodes, however, the loss in power is very considerable. These medium or poor electrodes are not worse than others actually used. Thus, Pt is a common electrode used in polysulfide electrolyte. However, it is a very poor electrode for this redox electrolyte, as we showed previously. 10,11 While the value of I SC will only be weakly dependent on the shape of the photoelectrode I V curve in most cases, P MAX will be much more dependent. However, the example given in Figure 3 provides a realistic example of these losses. We now describe an actual measurement using a Cd(Se,Te) photoanode measured by a three-electrode system. The photoelectrode was a painted Cd(Se,Te) solid solution film on Ti with a Se/Te ratio of 7:3 18 and an area of 1.4 cm 2. (As a secondary point of interest, this photoelectrode was made about 30 years ago.) The counter electrode was a sheet of smooth Pt with a total (both sides) area equal to that of the photoelectrode (1.4 cm 2 or 0.7 cm 2 /side). The electrolyte was 1MNa 2 S and 1 M S in water. Measurements were made using a saturated calomel electrode (SCE) in a three-electrode configuration, which measures the photoelectrode I V curve only (the counter electrode does not interfere), and using a two-electrode system (photo- and counter electrodes) with a Figure 4. Red plot: Photo I V of a Cd(Se,Te) (Se/Te 0.7:0.3) painted layer on Ti in polysulfide electrolyte (1 M S 2 /1 M S) under approximately full sun conditions measured against a reference electrode and with the potential corrected to the electrolyte potential. Purple plot: Photo I V of the cell using the above photoelectrode and a smooth Pt counter electrode of the same area (both sides) as the photoelectrode (1.4 cm 2 ). Dashed green plot: Cell photo I V calculated as described in Figure 3 using the measured I V characteristics of the Pt counter electrode (shown in Figure 2). variable resistance box as a load, which provides the I V plot of the entire cell. The results are shown in Figure 4. The threeelectrode measurement of the photoelectrode is the red curve (the potential axis in this case is the potential versus the polysulfide electrolyte, NOT versus SCE), while the twoelectrode measurement of the whole cell is shown by the purple curve (in which case, the x-axis gives the cell voltage). The green dashed line is the theoretical two-electrode measurement based on the measured I V plots of the two electrodes separately and analyzed as described in Figure 3 but over the whole I V curve of the photoelectrode and not just P MAX and I SC. The huge difference between the measured photoelectrode (ideal counter) and measured total cell is obvious. The measured total cell is also reasonably close to the calculated cell I V, showing that the analysis used above to estimate cell performance from separate photo- and counter electrode I V's is correct to a first approximation. The deviation between measured and theoretical I V curves of the total cell in the intermediate potential region is not understood but may be due to geometric factors (see below). We must note one important caveat regarding these conclusions. The Pt and good counter electrode polarization curves shown in Figure 2 are based on actual measurements using a three-electrode system in a beaker. In most nanoporous cells studied nowadays, the counter electrode is separated from the nanoporous photoelectrode by a spacer of some tens of micrometers. This reduces series resistance effects, which are probably not very major in the concentrated aqueous solutions most commonly used in SSCs (they could be more important in organic-based electrolytes as used in dye-sensitized cells). Probably more important, the separation of the photo- and counter electrodes is of the expected order of the diffusion layer thickness; quite possibly, sufficiently smaller that diffusion effects are substantially less in the closely spaced parallel geometry than those in the typical three-electrode geometry used by us to measure the counter electrode polarization curves. We are not aware of cell measurements made using varying distances between the relatively closely spaced parallel photo- and counter electrodes. Such measurements would be very useful. 1211

5 Three-Electrode Measurements. We now come to the (mis)- interpretation of data from three-electrode measurements that has resulted in claims of efficiencies sometimes many times higher than were actually the case. The author has encountered seven papers in the past 4 years in the literature (and a few more during refereeing) with reference electrode errors, some relatively minor and many very major, in various journals (mostly high profile ones). Figure 5 shows photocurrent photovoltage curves of two hypothetical photoelectrodes in polysulfide electrolyte, one a Figure 5. Photo I V curves of two hypothetical photoelectrodes, a typical good electrode (solid red plot) and a mediocre one (dashed blue plot). The lower axis shows the potential versus a SCE reference electrode and the top axis the cell voltage (assuming no counter electrode polarization). The vertical dashed purple line is the potential of a typical polysulfide electrolyte versus the SCE reference and is identified with zero cell voltage. fairly typical good sample (red plot) and the other (blue dashed plot) a considerably poorer one (although with the same photocurrent as the good electrode at zero potential versus SCE). The lower axis shows the potential measured against a reference electrode (SCE with a standard potential of V versus the standard hydrogen electrode, which, by electrochemical convention, is 0 V). The standard potential of the polysulfide electrolyte is 0.5 V versus SHE (i.e., 0.74 V versys the SCE reference electrode used to make the measurements; the exact potential depends on the dominant polysulfide species present). The counter electrode potential will be the same as the solution potential in the absence of net current flow (assuming, of course, a stable counter electrode that is not undergoing an electrochemical corrosion reaction). For the purpose of simplification, we assume an ideal counter electrode that does not polarize with current flow, in which case, the counter electrode will maintain the electrolyte potential of 0.74 V versus SCE. The cell voltage at any point on the I V curve will be the difference between the photoelectrode potential and the counter electrode potential at that point (as described earlier). For example, for the good cell, the potential at zero current is 1.34 V (versus SCE). In contrast to what has often erroneously appeared in published papers, this is not the V OC, which is = 0.6 V. The cell voltage for the same hypothetical photoelectrodes is given by the top axis. The vertical dashed line represents the electrolyte potential and is therefore the zero point of the cell voltage (i.e., I SC at its intersection with the I V plot). Table 2 gives the relevant parameters of the two photoelectrodes in Figure 5 measured against a SCE (and not Table 2. Comparison of Cell Parameters for Two Hypothetical Cells When Measured against SCE and the Actual Parameters That Would Be Obtained in a Two- Electrode Cell Measured Using an Ideal Counter Electrode good cell mediocre cell cell parameter SCE actual SCE actual V OC (V) I SC (ma/cm 2 ) fill factor P MAX (mw/cm 2 ) and efficiency (%) corrected for electrolyte potential) and what would be obtained from an actual cell using the same photoelectrodes and ideal counter electrodes (which is obtained by correcting the potential scale by, in the present case, 0.74 V). It is important to note that the difference between the SCE and actual measurements are not confined only to V OC but are seen in all of the parameters. This becomes obvious by considering the mediocre cell, which, measured against the SCE without correction, gives an apparently reasonable fill factor, but when the real I V curve (from the dashed vertical line and more negative) is considered, the fill factor is very poor. As with the effect of a poor counter electrode discussed previously, the poorer the real fill factor (and also V OC ) of the photoelectrode, the larger the error in not correcting for the electrolyte potential when using three-electrode measurements. Thus, the efficiency of the good cell in Table 2 (4.26%) increases three times when calculated against the SCE reference, while the mediocre cell at 2.00% increases almost five times (to 9.59%). It should be clear that if cell performance is measured with respect to a reference electrode, then use of different reference electrodes will result in different apparent values of cell efficiencies. It should be clear that if cell performance is measured with respect to a reference electrode, then use of different reference electrodes (without correction for electrolyte redox potential) will result in different apparent values of cell efficiencies. Thus, if the real V OC of a particular cell in a polysulfide electrolyte is 0.5 V, using a SCE reference, it will become V (0.74 V being the difference between the SCE potential and that of the electrolyte), that is 1.24 V. If we take a hypothetical reference electrode of +1 V (versus the standard hydrogen electrode), then the measured V OC now becomes 2 V! (0.5 V, the actual V OC V between the reference potential and electrolyte potential). These examples, based only on the V OC (and, as we have already noted, I SC and FF will also decrease to an extent depending on the shape of the I V curve) show clearly the importance of referring three-electrode measurements using a reference electrode back to the real voltages based on the redox potential of the electrolyte. Gary Hodes Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel 1212

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