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J. Phys. Chem. C 2008, 112, 17778–17787
FEATURE ARTICLE Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells Gary Hodes* Department of Materials and Interfaces, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: April 16, 2008; ReVised Manuscript ReceiVed: June 11, 2008
The liquid junction dye-sensitized solar cell (DSSC) has reached laboratory solar efficiencies of 11%. In contrast, the semiconductor-sensitized analogue (SSSC) has, up to now, exhibited a maximum efficiency of 2.8%. This begs the questions: is this difference fundamental? Will SSSCs always be inferior to DSSCs? We discuss the differences between the two types of cells, considering typical charge transfer times for the various current generating and recombination processes. Three main factors that could contribute to differences between the two types of cells are discussed: multiple layers of absorbing semiconductor on the oxide, the different electrolytes normally used for the two types of cell, and charge traps in the absorbing semiconductor. Entropic effects and the irreversible electron injecting nature of the normally used Ru dye to TiO2 are also briefly considered. We conclude that although the DSSC does possess some fundamental advantages, we can expect large improvements in efficiency of the SSSC, possibly reaching values comparable to the DSSC. Introduction The dye-sensitized solar cell (DSSC), which has reached 11% solar conversion efficiency,1 is being very heavily investigated, largely as a potential photovoltaic cell, but also because of the interesting science involved in the percolating nanoporous networks and because of other potential applications of these nanoporous systems (such as sensors or catalysts). Much less effort has been expended on the semiconductor-sensitized analogue (SSSC), where a light-absorbing semiconductor deposited on the porous transparent oxide takes the place of the dye. Semiconductor-sensitization of large bandgap oxides has been studied since Serpone et al. demonstrated that CdS adsorbed on TiO2 colloids injects electrons into the TiO2 upon photoexcitation.2 Since then, there has been increasing interest in this sensitization, earlier in the form of colloidal sols and later as sensitization of nanoporous oxide films (refs 3-6 show some early examples of this work). The use of a semiconductor instead of a dye to sensitize the nanoporous oxide film instead of a dye is often perceived to impart some advantages to the cells, mainly higher absorption (typically by a factor of 5 7) of the semiconductor coating compared with a single molecular layer of dye; greater stability of the semiconductor compared to organometallic or even pure organic dyes; and tailoring of optical absorption over a wider wavelength range than possible with dyes due both to the inherently wider bandgap range of semiconductors as well as the ability to tailor the bandgap by size quantization. More recently, the possibility of exploiting multiple exciton generation to obtain high efficiencies adds another potential advantage that could be exploited in the future.8,9 However, in spite of these potential advantages, the solar efficiency of liquid junction SSSCs has reached only 2.8% at present.10-12 * E-mail:
[email protected].
The SSSC has many features in common with conventional photoelectrochemical cells (PEC), where the semiconductor is deposited as a film on an electrically conducting substrate instead of onto a high surface area porous oxide. However, there are also some fundamental differences. One is that, in a SSSC, the photogenerated electron must be able to be injected into the oxide at an energy level normally taken to be at or above conduction band edge, although it could also be to somewhat lower-lying electron traps13 in/on the oxide. In other words, the semiconductor conduction band edge should be higher (having lower electron affinity) than that of the oxide. The relative positions of the semiconductor and oxide conduction bands is fixed not only by the material properties but can also be varied by interaction between the electrolyte and the solid phases. Another fundamental difference between the two types of cells is that, except for some special cases of hole-conducting porous oxides used in dye cells,14 the direction of current flow will be electrons into the oxide and holes to the electrolyte, even if the semiconductor is normally p-type in the bulk state (the semiconductor is normally in the form of small nanocrystals since the oxide is also in a (larger) nanocrystalline form). The direction of current flow is determined by relative energy levels rather than band bending in the semiconductor (which is assumed to be negligible in the small crystals used and surrounded by a screening electrolyte). Third, the main advantage of the SSSC over the PEC is that the local semiconductor thickness is small, meaning that electrons (holes) do not have far to travel before reaching the oxide (electrolyte). This means that semiconductors with very poor performance in a normal PEC, due either to fast electron-hole recombination or low charge mobilities, are more likely to operate well in the SSSC configuration, as long as fast removal from the semiconductor of at least one of the charges, and very preferably both, occurs. This is the same concept used in the ETA (extremely thin
10.1021/jp803310s CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
Feature Article
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17779
After completing his PhD at Queen’s University, Belfast, N. Ireland in 1971, Gary Hodes has been at the Weizmann Institute of Science, Israel, since 1972 at the Department of Materials and Interfaces. His interests over that time have focused on semiconductor film deposition by solution methods (electrodeposition and, later, chemical bath deposition) and use of the films in various types of solar cells. He then concentrated on nanocrystallinity and quantum size effects in these films. More recently, he has retured to investigating aspects of the deposition methods themselves as well as focusing on semiconductor-sensitized, porous solar cells.
absorber) cell, where a solid hole conductor is used instead of an electrolyte.15 In this paper, the various similarities and differences between the DSSC and liquid junction SSSC are compared, with the aim of revealing possible reasons why the SSSC is poorer than the DSSC and also whether there is good reason to anticipate much higher performance from SSSCs. In the first part, charge transfer times among the various components of the two types of cells are compared. Such charge transfer times have been extensively studied for the DSSC, and in spite of their often very strong dependence on a variety of factors, there is a generally accepted picture of these times for good cells. For SSSCs, much less information is available. However, as far as is possible, we make some comparisons of these times in the SSSC with those in the DSSC. The rest (and main part) of the paper then discusses the possible underlying reasons for differences between the two types of cells. Although many of the concepts discussed should be relevant to solid state cells, we confine this paper to liquid junction cells, since the difference in efficiencies between solid state DSSCs and SSSCs is less extreme than for the liquid junction cells and also to keep the paper clearly focused. Charge Transfer Times in the DSSC and SSSC: Experimental Data. Before discussing the various charge transfer times in the DSSC and SSSC, it is important to emphasize the principle, pointed out by Haque et al.,16 that what is important for efficient cell function is not so much the absolute rates of the desired (or “forward”) charge transfer and recombination processes, but the ratio between the two. As long as the forward charge transfer rate is considerably faster than the possible recombination reactions, then any further increase in the forward charge transfer rate does not improve the cell, and may even be undesirable if the forward and recombination rates are coupled. Another important caveat is that unless stated otherwise, recombination rates involving electrons in the oxide, which are strongly dependent on cell potential, are given for short circuit photocurrent conditions. For operating cells, the maximum power point would be more relevant; under these conditions, these recombination processes become orders of magnitude faster. At open circuit, the generation and recombination processes are balanced by definition. The various charge transfer times in the optimized DSSC have been well-studied and are shown in Figure 1 (the times given are typical order-of-magnitude values for good DSSCs operating at solar intensities). The first process is an ultrafast electron injection time from the dye to the TiO2, usually considered to be in the subpicoseconds regime, although there can be considerable dispersion to longer times. Also, Haque et al.16 have shown that the polyiodide electrolyte used in these cells causes a further order of magnitude reduction in electron injection rates. This injection competes directly with the electron-hole recombination rate in the dye, which is typically on the order of 10-7 s. Therefore, electron injection occurs with little loss (i.e., the electron injection efficiency is close to 100%).
Figure 1. charge transfer times for the DSSC (top) and SSSC (bottom). Solid arrows represent desired charge transfer processes; broken arrows, loss mechanisms. The electron trapping process in the absorbing semiconductor, shown by a red, broken arrow, may or may not lead to eventual recombination. The red rectangle at the semiconductor-oxide interface in the SSSC represents surface states on the semiconductor surface and the 10-12 to 10-10 s arrow represents typical trapping times for conduction band electrons into those surface states. The electron-hole recombination times in the semiconductor depend strongly on both the specific semiconductor as well as the nature of the semiconductor surface (for the small crystals normally used, surface recombination channels are likely to be more important than bulk ones for most semiconductors). The electron-hole recombination times given (10-11 to 10-6 s) cover both direct band-to-band recombination and trapmediated recombination.
The injected electrons diffuse across the porous TiO2 film to the substrate in the milliseconds regime; this transit is particularly dependent on the illumination intensity, being faster with stronger illumination. This is due to electron traps in the TiO2 which become progressively more filled, and therefore slow the transport less, at higher illumination intensity. It competes with injection of electrons in the oxide either to the polyiodide electrolyte or back to the oxidized dye cation. Fortunately, the polyiodide reduction is very slow at the TiO2 surface, which allows most of the slowly diffusing electrons to reach the substrate and generate current. Back transfer rate of electrons from the TiO2 to the dye cation varies over orders of magnitude depending on conditions,17 but the 10-4 s shown is typical for good cells at short circuit. This competes with the dye regeneration by the iodide, which is much faster (10-8 s). Thus, the forward charge transfer times are much faster than the competitive recombination pathways at and not too far from short circuit conditions. This means that, as long as the light harvesting (light absorbed by the dye) efficiency is high, the electron collection efficiency (the actual photocurrent measured) will be high, as observed. Recombination through the substrate has not been considered in Figure 1. However, although it is not a major loss mechanism in the liquid DSSC at solar illumination intensities (it becomes increasingly important at lower illumination intensities 18), it is very important in solid state cells using hole transport media. For the SSSC, there are much less data available for charge transfer times. In addition, we must be aware that, although the times shown in Figure 1 for the DSSC are for optimized cells, this is not the case for times for the SSSC. Thus, we use typical or “best” values, and in many cases, equivalent values with which to compare with the DSSC are as yet unknown. Within these constraints, we attempt to give some typical values in Figure 1. Among the limited information on electron injection times from an absorbing semiconductor into an oxide, many of the
17780 J. Phys. Chem. C, Vol. 112, No. 46, 2008 times given in older studies are upper limits rather than actual times. The times were measured using either dispersed colloidal systems or dry electrodes, and often under conditions far from those to which a solar cells would be exposed. Therefore, some caution should be used in extrapolating the results to porous electrodes used in the SSSC. The fastest time to date appears to be on the order of a picosecond for Cd3P219 and CdS20 on TiO2. More recently, times on the order of tens of picoseconds have been reported for CdS on TiO221 and 16 ps for CdSe on TiO222 (both in the absence of an electrolyte/hole conductor). For semiconductors covered with an organic capping layer, we can expect longer injection times than for bare semiconductors directly connected to the oxide. The same is expected for nanocrystals that are linked by molecules to the oxide. Thus, electron injection has been measured to occur over a wide range of times varying from a few picoseconds to 100 ps for ca. 3 nm CdSe nanocrystals molecularly linked to TiO2.23 Also, average electron lifetimes for the same system depended strongly on the CdSe crystal size, varying from
