SnO2 SnO2/ZnO ZnO-covered TiO2 ZnO
Titanium dioxide is so far the most studied nanostructured semiconductor material giving the highest solar-to-electric conversion efficiency. Work on other types of materials, however, is important to get a better understanding of the nanostructured oxide properties of the photoelectrode, as well as looking for other high efficiency systems .
In recent years considerable interest has been shown in developing thin semiconductor films from colloidal semiconductor suspensions as they exhibit interesting electrochemical and photoelectrochemical properties. These films possess highly porous structure and can be easily surface-modified with sensitizers, redox couples and short-bandgap semiconductor particles .
We have employed 30-50-Å diameter colloids to prepare photoelectrochemically active thin transparent SnO2 films on a conducting glass substrate. SnO2 is a stable large-bandgap semiconductor (Eg = 3.6 eV) and has been widely used in many electrooptical devices. Although it does not respond to visible light excitation, it can be sensitized with organic dyes which in their excited-state inject electrons into the conduction band of a large-bandgap semiconductor. Compared to TiO2 (or ZnO) SnO2 is a better electron acceptor since its conduction band (ECB = +0.45 V vs NHE at pH 1) lies -0.5 V more positive than that of TiO2. A wide variety of organic and organometallic molecules have already been employed to sensitize SnO2 electrodes. Some of these examples include chlorophyll, chlorophyllin, Ru(bpy)32+, rhodamine B. However, the power conversion efficiency in all these examples has remained very low (~0.1%) .
When SnO2 absorbs light, the electron-hole pairs are created:
If we sensitize SnO2 with (2,2’-bipyridine-4,4’-dicarboxylic acid)ruthenium2+, upon excitation with visible light, the excited sensitizer molecules inject electrons into the SnO2 particles. These electrons are then collected at the OTE surface to generate anodic photocurrent. The redox couple I3-/I- present in the electrolyte quickly regenerates the sensitizer .
Ru(II)* + SnO2 → SnO2(e) + Ru(III)
Ru(III) + I3- → Ru(II) + I* + I2
This transparent thin films of SnO2 nanocrystallites exhibit excellent semiconducting properties. These results highlight potential use of these materials in photoelectrochemical cells similar to those of TiO2 particulate films. Further experiments are currently in progress to employ these sensitizers and compare the photoelectrochemical sensitization behavior with other particulate films .
In dye sensitization, the photoexcited dye molecule injects an electron into the conduction band (CB) leaving the dye cation (D+) on the surface. The efficient removal of the positive charge on the D+ is an essential condition for obtaining high quantum efficiencies. Otherwise recombination of photogenerated electron and the D+ becomes more probable. Thus slow diffusion-limited transport of the ionic species that scavenge the positive charge on D+, increases the recombinations, reducing the short-circit (SC) photocurrent. Recently it has been found that dye-sensitized photoelectrochemical cells (DS PECs) made from porous films containing a mixture of tin(IV) and zinc oxides generates high photocurrents because of the efficient separation of the injected electron and the D+ in the composite system. In this work we compare the performance of DS PECs made from TiO2 and SnO2/ ZnO films that use the quasi-solid polymer electrolyte based on polyacrylonitrile (PAN). As expected, cells based on SnO2/ZnO films deliver conspicuously higher photocurrents with this electrolyte than those made from TiO2 films .
DS PECs using the electrolyte XI + I2 generate photocurrents via the following mechanism. The photoexcited dye molecule absorbed at the semiconductor surface injects an electron into the CB leaving a dye cation at the surface .
D* → D+ + e-
The positive charge on the D+ is scavenged by I- and the redox cycle is completed by formation of I3-, which accepts an electron from the counter electrode :
D+ + I- → D + I
I- + 2I → I3-
I3- + e- → I + 2I-
The factor that severely limits the photocurrent quantum efficiency of a DS cell is the recombination of D+ and e- or the back reaction :
D+ + e- → D
The system based on SnO2/ZnO suppresses the recombination because the ballistically injected “hot carrier” is transported across the wide zinc oxide particle suppressing the recombination .
Fig.1. (a) Schematic diagram showing excitation of a dye molecule adsorbed on a SnO2 particle and transfer of the electron into another SnO2 particle, traversing across few SnO2 particles to a ZnO particle. (b) Schematic energy level diagram depicting band positions of SnO2 and ZnO particles as shown in a and the ground (S) and excited (S*) energy levels of Ru dye.
The ZnO:SnO2 mixing ratio used (~53% ZnO), gives the optimum SC photocurrent and at this mixing ratio, the average distance between centers of two ZnO particles (estimated from the knowledge of densities of ZnO and porous SnO2) is ~1.1 μm. Thus a chain of 6-7 tin oxide nanoparticles could link two ZnO particles (Fig.1) and electron injected by dye sensitization should travel across 3-4 tin oxide particles in reaching a ZnO particle. Pumping of electrons into the ZnO particles build up the quasi-fermi level that drives electron to the back contact via the interconnection of SnO2 particles.
The SnO2/ZnO is unique as “hot electrons” are ballistically transported to a high band-gap (BG) material tunneling across few nanocrystallites of low BG material .
The SnO2/ZnO cell is more stable than the TiO2 cell. When the cells were exposed to direct sunlight for 20 h. TiO2-based cells showed nearly 25% drop in the open-circuit voltage and 12% drops in the short SC photocurrent. Whereas in the case of SnO2/ZnO cell, the drop in the open circuit voltage and the SC photocurrent ~8% and 2% respectively. The difference is perhaps due to higher oxidative catalytic activity of TiO2 compared to ZnO on absorption of BG radiation. Again dye photodegradation should be lesser in the SnO2/ZnO because dye is mostly adsorbed on SnO2, which is even a weaker oxidative photocatalyst than ZnO .
Besides TiO2, other semiconductors, such as SnO2, Fe2O3, ZrO2, Al2O3, ZnO, have also been studied for photoelectric conversion. Among all those materials studied, few other semiconductors could match TiO2 in solar energy conversion .
On the other hand, although ZnO possesses an energy band similar to that of TiO2, cis-dithiocyanato[N-bis-(4,4'-bipyridyl-2,2'-dicarboxylic acid)]ruthenium(II) (cis-Ru) sensitized nanocrystalline ZnO generated a much lower yield. In fact, nanostructured TiO2 is not perfect yet in that electron transport becomes more difficult with the increase of photocurrent in the absence of space charge layer. The efficiency of dye-sensitized nanocrystalline solar cells is limited in part by back-reaction of photoinjected electrons with triiodide ions in the electrolyte, and the presence of electron acceptors, such as oxygen and iodine, will lead to loss of photogenerated electrons over the nanostructured semiconductor electrolyte interface during the transport of the electrons to the back contact .
A rather unusual feature of these nanocrystalline films is lack of a depletion layer at the nanostructured semiconductor/electrolyte interface. As a result, the back electron transfer, the charge recombination between the electrons injected in the conduction band (CB) of the semiconductor and the oxidized sensitizer, still remains one of the major limiting factors to the efficiency of the dye sensitized solar cells .
If one can improve TiO2 film to further reduce the charge recombination, the overall power conversion efficiency would be increased. Doping of Zn2+ ion into TiO2 at pH 1.8 increases the quantum efficiency for photoelectric conversion under ultraviolet light excitation, especially at about 320 nm, because improves the property of electron transport for colloidal TiO2 film, resulting in a significant increase in short-circuit photocurrent, open-circuit photovoltage, and overall power conversion efficiency. ZnO covering leads to a positive shift of flat band potential and an increase in free electron concentration in the CB with respect to pure TiO2 film. The increase in free electron concentration after ZnO covering may be resulted from the blockade of surface states; the latter favors the charge transport and prevents loss of photogenerated electrons due to the presence of electron acceptors. As a consequence, electrons injected to the CB of TiO2-ZnO will transport to the back contact quickly. The covering of ZnO suppresses the dark current generation and, thus, improves both open-circuit photovoltage and short-circuit photocurrent. The overall power conversion yield is, therefore, increased remarkably .
Nanostructured ZnO electrodes with one order of magnitude larger particles (150 nm) than normally used in these type of systems (5–30 nm) are investigated. High quality ZnO films are prepared by a novel method for manufacturing nanostructured semiconductor layers on the conducting substrates. It is observed that the crucial parameters for optimizing the efficiency of the photoelectrochemical solar cells based on zinc oxide are the dye-sensitization process and the preparation of the nanostructured zinc oxide film .
This kind of ZnO films show suitable properties for solar energy conversion applications. The improvement in the overall solar cell efficiency from 2% to 5% originates from a controlled dye-sensitization procedure (avoiding dye aggregation) and from the new compression method (the exclusion of organic additives probably leads to better interfacial kinetics). ZnO films exhibit fast and efficient electron transport with small recombination losses. In comparison with TiO2, the ZnO electrodes showed actually better charge transport properties. Also, the light harvesting efficiency is high and comparable with TiO2 based solar cells. For further improvement of dye-sensitized ZnO solar cells a much better understanding of the electron transfer reactions at the ZnO/dye/electrolyte interface is required for systems prepared under optimal dye-sensitization conditions .