Sensitization of Single Crystal Substrates - ACS Symposium Series

Chapter 1

Sensitization of Single Crystal Substrates

Downloaded by MIDDLE EAST TECHL UNIV on April 5, 2016 | http://pubs.acs.org Publication Date (Web): October 16, 2015 | doi: 10.1021/bk-2015-1196.ch001

Laurie A. King,1 Meghan E. Kern,1 and B. A. Parkinson* Department of Chemistry and School of Energy Resources, University of Wyoming, Laramie, Wyoming 82071 *E-mail: [email protected]. 1These authors contributed equally to this work.

Sensitizing large band gap semiconductors to subbandgap light began with the development of photography and has recently been applied to solar energy conversion. In this chapter we review work that employs single crystal semiconductor substrates, such as oxides, chalcogenides and pnictides, to investigate this important fundamental photoinduced interfacial electron transfer processes using sensitizers from dyes to polymers to quantum dots.

Early History of Dye Sensitization In the 1830s artists began capturing the world around them using the newly developed photographic techniques. However, photography did not reach the masses until films containing grains of silver halides were developed. Silver halides are ionic semiconductors with band gaps ranging from 2.7 – 3.2 eV making them mostly insensitive to visible light (1). The optical absorption of visible light by dyes adsorbed on the multifaceted micron-sized grains of silver halides enables “sensitization” to visible light where photon absorption by the dye promotes an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) that subsequently is injected into the silver halide producing a latent image or a reduced silver cluster. The reduced silver center in the grain is a nucleation site for subsequent development of the whole grain by chemical reduction by developers to produce a photographic negative. This photon multiplication process resulted in films sensitive enough to capture images with very little light using either fast shutter speeds or long exposures such as in astronomical applications (2). © 2015 American Chemical Society Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Photosensitization of semiconductor electrodes began in the 1960s when Gerischer and Tributsch studied the ability of organic dyes to sensitize ZnO single crystals (3, 4). Unlike photography, where the electron is immediately trapped and reduces a Ag+ center to Ag0, the photoexcited electron of the dye molecule is injected into the ZnO conduction band and can be drawn off as current. The oxidized dye can be regenerated by a reducing agent in an electrolyte to enable a continuous flow of photocurrent (Figure 1) whereas in photography the reducing agent (developer) further reduces Ag+ around the Ag0 nucleation sites to produce a dark area forming a negative image. Sensitizing semiconductor electrodes enabled continuous monitoring of the electron transfer reaction (current) with the additional advantage of being able to change the driving force for this interfacial reaction by adjusting the bias, semiconductor band positions and redox electrolyte. Subsequent work led to the establishment of the general principles of electron transfer processes in photosensitized systems. In particular, the alignment of the electron energy levels between sensitizer and the energy bands of the semiconductor were shown to control electron injection. For example, photocurrent was not measureable for dye sensitization of a metal (5). Examples of early sensitization studies include zinc oxide (n-type), copper oxide (p-type) (6), and gallium phosphide (GaP) (p-type) (7). These early studies led to work by Tributsch and Calvin on the photosensitization of ZnO single crystals with chlorophyll; the first direct link between solar energy conversion in nature and photoelectrochemistry (8, 9). Throughout early dye sensitization experiments, the dyes were weakly bound to the single crystal surface by physisorption. To alleviate the problem of dye desorption experiments were typically performed with dye in solution and hence adsorbed dye is in dynamic equilibrium with dissolved dye. Spectral sensitization was demonstrated with dyes adsorbed again to ZnO (11) but also TiO2 (12) and SrTiO3 (13) single crystal surfaces. Further investigations with single crystal semiconductors led to developing greater insight into the fundamental process that dictate electron transfer at these interfaces. For example, the conduction band of a semiconductor can be shifted with respect to the dye LUMO by controlling the pH of solution (59 mV per unit of pH). To probe the effect this has on dye sensitized photoanodes, two dyes were selected such that the conduction band edge of SrTiO3 at pH 4 approximately matched the LUMO level of the cyanine dyes. Indeed, a threshold for electron transfer was reached by increasing the pH of the electrolyte above 4 such that the conduction band of SrTiO3 was moved above that of the dye LUMO (13). Such early work with single crystals also led to the need for quantification of photosensitization driven electron transfer. There are two frequently reported efficiencies for photocurrent generation – incident photon current efficiency (IPCE), also known as the external quantum yield and absorbed photon current efficiency (APCE), also known as the internal quantum yield. Such spectral measurements enable the comparison between sensitizer absorbance and photocurrent generation. The early measurements of these efficiencies produced very low IPCEs and APCEs (10). The high pH is necessary to deprotonate the thiol to promote thiolate-QD binding that stabilizes the suspension. Once a stable suspension is prepared, sensitizing the substrate with QDs can be straightforward with the carboxylate binding to the metal oxide substrate with suitable electronic coupling between the QD and substrate. TOP/TOPO-capped CdSe QDs were ex situ ligand exchanged utilizing MPA as the bifunctional linker molecule (92). The obtained MPA-capped CdSe QDs required this time consuming procedure and yielded suspensions that were stable for only a few hours. In order to circumvent this stability issue, commercially synthesized MPA-capped CdSe QDs prepared utilizing the ex situ ligand exchange method were obtained (ON-MPA/CdSe). MPA-capped CdSe QDs were also synthesized in water following a synthesis reported by Chen et al. (104) (aq-MPA/CdSe) but this produced a suspension with a broad absorption spectrum, indicative of a large size distribution. Despite these challenges, the attachment and performance of the two methods to prepare MPA-capped CdSe QDs on single crystal substrates were compared. 28 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Rutile (001) and anatase (001) single crystal substrates were exposed to aq-MPA/CdSe and ON-MPA/CdSe QD samples. Sensitized photocurrents mimicking the absorption spectra of the suspensions were obtained for both samples of QDs, however, the aq-MPA/CdSe QDs yielded no photocurrent response until further purification with isopropyl alcohol and resuspension in water (pH 10.5) presumably removing unreacted precursors. Interestingly, the photocurrent stability of the two samples was different despite the similar surface chemistry of the samples, as they both in principle should be capped completely with MPA. The photocurrent response from aq-MPA/CdSe was reproducible and stable over long periods of illumination in both aqueous sulfide/polysulfide electrolyte, a typical electrolyte for QDSSCs, and the bipyridyl cobalt mediator (Co(dtb)3) in acetonitrile, a common electrolyte for DSSCs. The photocurrent response over the course of 15 h of the ON-MPA/CdSe on anatase (001) TiO2 single crystals was stable for many hours with aqueous sulfide/polysulfide electrolyte (Figure 20), consistent with the results obtained for the aq-MPA/CdSe. In contrast the sensitized photocurrent for the ON-MPA/CdSe decayed immediately upon illumination in the cobalt mediator solution in acetonitrile. The stability of the photocurrent response for this sample could be extended by purging the cobalt mediator with nitrogen prior to measurements but quickly decayed upon exposure to air. Because the stability of both the ON-MPA/CdSe and aq-MPA/CdSe suspensions is the same prior to exposure to the TiO2 crystal, the variance between the photocurrent stability in the presence of oxygen is presumably due to slight differences between the surface chemistry making one more prone to oxidation than the other.

Figure 20. (A) IPCE spectra of (a) N3 on an unmodified anatase (001) TiO2 single crystal measured at short circuit in an acetonitrile electrolyte with a Co(dtb)32+ mediator. The solution absorbance spectrum in water (b) is compared with the IPCE spectrum of (c) ON-MPA/CdSe initially and (d) after 15 h illumination measured at short circuit in a sulfide/polysulfide electrode. Panel (B) shows the photocurrent versus time for ON-MPA/CdSe adsorbed on the same crystal in (a) sulfide/polysulfide electrolyte, and (b) unpurged and (c) purged Co(dtb)32+. Reproduced with permission from Sambur, J.; Riha, S. C.; Choi, D.; Parkinson, B. A.; Langmuir 2010 26(7) 4839-4847. Copyright (2010) American Chemical Society (92). 29 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Electronic Coupling The collection of photoexcited electrons in solar energy conversion devices such as the QDSSC, relies on the efficient transfer of electrons from the QDs into the metal oxide substrate. The efficiency of this process is strongly influenced by both the delocalization of photogenerated excitons within the QDs and the strength of electronic coupling between the QDs and substrate that is influenced by the nature of the capping/bridging ligands (94). The exciton wave function confined in the QD can be extended from the QD core to the ligand molecule if the QD and capping ligands are electronically coupled by exchanging the insulating long-chain surfactant capping groups with smaller molecules (105). The importance of electronic coupling in QD sensitization was evident when the effect of capping ligands on the electron transfer properties of QD-interfaces was studied (59). To systematically investigate this effect, rutile (110) TiO2 and ZnO (0001) single crystal substrates were sensitized with CdSe QDs capped with oleic acid/TOPO (OA/TOPO-CdSe), 11-mercaptoundecanoic acid (MUA-CdSe), and MPA (MPA-CdSe) where MPA- and MUA-CdSe QDs were prepared via ex situ ligand exchange of OA/TOPO-CdSe. The insulating effect of long chain surfactants was apparent as no photocurrent was generated from OA/TOPO-CdSe on either substrate whereas both MPA- and MUA-CdSe yielded photocurrent response. MPA-CdSe exhibited IPCE values much larger than those obtained from MUA-CdSe on either substrate demonstrating that shorter chain bifunctional linker molecules have better electronic coupling between QDs and substrates than longer chain molecules and the rate of electron transfer through alkyl chains is known to increase with decreasing chain length (106), it was therefore likely that the differences observed in IPCE values between MPA-CdSe and MUA-CdSe were due to the length of the alkyl chain. This led to the systematic study of the effect the length of the bifunctional linker molecule on photocurrent yields (60). Photocurrent spectra were obtained for ZnO single crystal substrates sensitized with CdSe QDs capped with 4-mercaptobenzoic acid (MBzA), 2-mercaptoacetic acid (MAA), MPA, 8-mercaptooctanoic acid (MOA), and MUA. The results demonstrated the distance dependence of electron transfer through alkyl chains. As shown in Figure 21, IPCE values measured at the exciton peak wavelength increased as the length of the alkyl chain of the linker molecule was decreased demonstrating that longer chain linkers like MUA are not as effective at transporting charge as short chain linkers such as MPA and MAA that reduce carrier recombination. These results are consistent with an optical study of electron transfer from CdS QDs to TiO2 nanoparticles (96).

Quantum Dot Stability The low efficiencies of QDSSCs in comparison to DSSCs have led to the exploration of a variety of strategies to increase power conversion (81). One attractive method involves increasing the open circuit potential of the device by tuning the redox potential of the electrolyte; however, the stability of QDs in the presence of electrolyte is a major problem. A prime example of this is when 30 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


employing the iodide/triiodide (I-/I3-) redox electrolyte commonly used for DSSCs as the regenerator for QDSSCs. Stability studies of QDSSC performance revealed that photocorrosion of metal chalcogenide QDs occurs in the presence of I-/I3- as evident by the rapid decay of the photocurrent (107, 108).

Figure 21. Dependence of the IPCE values at the exciton peak on the (-CH2 -) units of molecular linkers on ZnO (0001). The molecular structures of MPA and MBzA are also shown. A solid line (left) connecting the highest values of IPCE for each MXA molecule is shown, the dashed line (right) corresponds to calculated light harvesting efficiency (LHE) values estimated for close-packed monolayers of CdSe QDs (2.62 nm diameter) with different MXA linkers. Reproduced with permission from Liang, Y.; Thorne, J. E.; Kern, M. E.; Parkinson, B. A., Langmuir 2014 30(42) 12551-12558. Copyright (2014) American Chemical Society (60).

The corrosion process was investigated by continuous 532 nm illumination of MPA-capped CdSe QDs attached to TiO2 single crystal substrate in aqueous KI electrolyte (109). IPCE spectra obtained before and after long-term illumination revealed not only a significant loss of photocurrent but also a substantial blue shift of the first exciton peak indicative of a decrease in the QD diameter (Figure 22a). Interestingly, the onset of photocurrent after extended illumination occurred at 530 nm, just above the incident photon energy. This effect was further probed using monochromatic illumination at different photon energies. For all incident photon energies examined, a decrease and blue shift of the IPCE spectra was observed and the onset of photocurrent after extended illumination was shifted to just above the energy of the incident photons (Figure 22b). These results revealed that photoelectrochemical corrosion of QDs occurred when the incident photon energy was greater than the band gap of the QDs but stopped when the photon energy was less than that of the QD band gap signifying a size-selective etching process. 31 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 22. (A) IPCE spectra of MPA-capped CdSe QDs adsorbed on an anatase (001) measured before (black circles) and after (green triangles) 20 hours of 47.7 mW cm−2 (532 nm) in an aqueous iodide (0.25 M KI) electrolyte at short circuit vs Pt. The inset shows the short circuit photocurrent vs time (red dots) for the first 2 hours. The dashed black line represents a double exponential fit to the data and the individual components are represented as pink and blue lines. (B) IPCE spectra normalized to the maxima of the excitonic features measured on anatase (001) before (black circles) and after 12 hrs of continuous 30 μW cm−2 600 nm illumination (red squares) and after an additional 3 hrs of continuous 30 μW cm−2 580 nm illumination (blue triangles). Reproduced with permission from Sambur J. and Parkinson, B. A., ACS Appl. Mater. Interfaces 2014 ASAP, DOI: 10.1021/am507974h. Copyright (2014) American Chemical Society (109).

Core/shell QDs have been shown to prevent photocorrosion and enhance the stability of QDs in the presence of usually corrosive electrolytes. Photocurrent stability tests of MPA-capped CdSe and CdSe/ZnS QDs attached to TiO2 crystals was performed in aqueous KI electrolyte (91). The photocurrent from the CdSe QDs decreased by half in minutes and 90% of the initial photocurrent signal was gone after only a few hours. Conversely, ~21% of the initial photocurrent signal remained for the CdSe/ZnS QDs after 20 hours of continuous illumination. Normalized IPCE spectra of CdSe/ZnS QDs before and after illumination exhibited similar shapes where a significant blue shift was observed for CdSe QDs consistent with photocorrosion by the electrolyte. Evidently the use of a stable, wide band gap shell material on the QD core can shield the core from photocorrosion and in turn improve device stability. It also suggests that the thin ZnS core layer may not be continuous on ~80% of the QDs as is usually suggested in the cartoons drawn to depict these core/shell structures. 32 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Quantum Dots Absorbing in the near-IR The use of lower band gap and or larger QDs can extend absorption of a device into the infrared region of the solar spectrum. The potential for device efficiencies that exceed the Shockley-Quiesser limit is another attractive feature of employing near-infrared (n-IR) QDs as sensitizers (110). The small band gaps allow for the production of multiple excitons at the higher photon energies in the visible range of the solar spectrum. Therefore, the realization of multiple exciton generation (MEG) and collection (MEC) as well as hot carrier injection is possible. The extension of sensitized photocurrent into the n-IR was shown when ZnO single crystals were sensitized with PbS QDs (58). For electron transfer to occur, the energetic alignment between the sensitizer and the substrate must be thermodynamically favorable. Mott-Schottky analysis of ZnO single crystals revealed that the conduction band edge is at -4.77 eV. Because the intrinsic Fermi level of PbS is known to be approximately -4.7 eV with a nearly symmetric distribution of the conduction and valence band (111, 112), the energetic alignment of the ZnO/PbS interface should allow for the injection of electrons from all sizes of PbS QDs (Figure 23A).

Figure 23. (A) Band alignment between ZnO single crystal electrodes, PbS QDs, and redox couples. The positions of the energy level are only shown relatively, not to scale. The dashed lines indicate back transfer or recombination of injected electrons. (B) UV–Vis spectra of various size PbS QDs in hexane solution. The diameters of the four different batches of PbS QDs are estimated to be 3.04 nm (blue curve), 3.62 nm (green curve), 4.16 nm (red curve), and 5.50 nm (magenta curve). Reproduced from Phys. Status Solidi A, 211/9, Liang, Y.; Novet, T.; Thorne, J.; Parkinson, B. A.; Photosensitization of ZnO single crystal electrodes with PbS quantum dots, 1954-1959, Copyright (2014), with permission from Wiley (58).

IPCE spectra measured for ZnO single crystals sensitized with PbS QDs ranging in size from 3.2-5.5 nm in diameter are shown in Figure 23B. Sensitized photocurrents extending into the n-IR region were obtained for all PbS QD 33 Kilin; Photoinduced Processes at Surfaces and in Nanomaterials ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

samples on ZnO confirming that electron injection from a wide distribution of PbS QD sizes into ZnO is indeed possible. IPCE spectra for each size of PbS QDs on ZnO exhibited slight red shifts from the absorption spectra of the suspensions suggesting enhanced electronic coupling between QDs and substrate or also between neighboring QDs. The largest size of PbS QDs tested (5.5 nm) resulted in sensitized photocurrent out to 1580 nm that may currently be the record for the longest wavelength sensitization achieved for dyes or QDs on oxide substrates.


Multiple Exciton Generation and Collection The formation of multiple photogenerated carriers from the excess energy of absorbed high energy photons has been observed via indirect optical methods such as transient absorption and time-resolved photoluminescence for many suspensions of QD such as PbS, PbSe, InAs, nanocrystalline Si, as well as carbon nanotubes (77, 110). These methods rely on the detection of signatures of MEG such as Auger recombination and enhanced luminescence or decay rates. The minimum threshold energy necessary to induce MEG has been shown to be close to twice the band gap; however, the minimum energy measured to induce MEG within such materials has often been reported to be higher than this (110). While indirect measurements have proven MEG is possible in QDs, the collection of multiple carriers within QD-devices has only recently been shown (93, 113). The first direct measurement of the production and collection of photocurrent from MEG was measured in a model single crystal sensitized photoanode (93). Four distinct sizes of PbS QDs (2.5, 3.1, 4.5, 9.9 nm corresponding to band gap energies of 1.39, 1.27, 0.96, and 0.85 eV, respectively) were attached to single crystal anatase (001) TiO2. To efficiently extract multiple carriers generated within QDSSCs, the energetics of the interface must be such that the conduction band of the QD is more negative on the electrochemical scale than that of the conduction band of the semiconductor for dissociation of excitons into free carriers. Features consistent with the first excitonic band present in the absorption spectrum of the suspension were observed in the IPCE spectra for QDs 4.5 nm or smaller in diameter (Figure 24). No excitonic features were present in the IPCE spectrum of the 9.9 nm QD sample coinciding with the energetics of the conduction band of the QDs being more positive than that of the TiO2. However, photocurrent response was detected for this sample at wavelengths

Sensitization of Single Crystal Substrates - ACS Symposium Series

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