Revival of Solar Paint Concept: Air-Processable Solar Paints for the

Article pubs.acs.org/JPCC

Revival of Solar Paint Concept: Air-Processable Solar Paints for the Fabrication of Quantum Dot-Sensitized Solar Cells Muhammad A. Abbas,† Muhammad A. Basit,‡,§ Seog Joon Yoon,∥,⊥ Geun Jun Lee,◊ Moo Dong Lee,◊ Tae Joo Park,*,†,‡ Prashant V. Kamat,∥,⊥ and Jin Ho Bang*,†,◊,#

J. Phys. Chem. C 2017.121:17658-17670. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 09/27/18. For personal use only.

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Department of Advanced Materials Engineering, ‡Department of Materials Science and Chemical Engineering, ◊Department of Bionano Technology, and #Department of Chemical and Molecular Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ∥ Department of Chemistry and Biochemistry and ⊥Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: One way to revolutionize solar energy production and expand it to a large scale is to reduce the manufacturing cost and complexity of the fabrication process. The ability to make solar cells on the surface of any shape would further transform this technology. Quantum dot-sensitized solar cells (QDSSCs) are an ideal candidate to push solar cell technology in this direction. In this regard, making a paint that can be applied by a paint brush to any transparent conductive surface to turn it into the photoanode of QDSSCs is the ultimate goal. We herein demonstrate the feasibility of one-coat fabrication of QDSSCs from a lead sulfide (PbS)-based solar paint. This is possible because of its unique ability to regenerate after oxidation occurred during heat treatment in air. Hence, the whole fabrication process can be carried out in air unlike a first-generation solar paint based on cadmium sulfide (CdS) and cadmium selenide (CdSe). Two solar paints using a commercially available titanium dioxide (TiO2) and a p-type TiO2 powder were synthesized and evaluated. Also, the performance-limiting parameters are thoroughly investigated using various spectroscopic and electrochemical characterization methods. The implication of new insights into the PbS-based solar paint for further development of paint-on solar cells is discussed.

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appropriate choice of materials to develop solar paint.22,23 To avail a high surface area for sensitization, nanoparticles of TiO2 or zinc oxide (ZnO) have been used, which require annealing at a high temperature (400−550 °C) to form a well-connected film with a high electrical conductivity and good mechanical stability. However, almost all semiconducting QDs used for the sensitization decompose at high temperatures in air. Hence, one of the main challenges of the solar paint is to develop a procedure for the low-temperature fabrication of a TiO2 film in air that is well connected and mechanically stable while keeping the sensitizer intact. Compared to conventional fabrication of photoanode of QDSSCs, solar paint provides an opportunity to reduce the time for the fabrication of photoanode from more than 24 h to just 2 h.22 Despite the lower PCE of solar paint-based QDSSCs, it provides the prospect of transforming the way to fabricate solar cells. With further research there is a potential to improve the PCE of solar paint. At a quick glance, QDSSCs based on solar paint might look similar to colloidal quantum dot solar cells (CQDSCs) based on colloidal quantum dot (CQD) inks.

INTRODUCTION Houses require repainting every once in a while, and this task is often a heavy burden. However, this task may become exciting if the paint applied had the potential to turn the outer surface of the home into solar cells that could generate electricity yearround. The concept of solar paint has been around for quite some time now. To date, however, there has been very limited progress to make it a reality due to various technological challenges. The advent of inorganic nanocrystal (quantum dots, QDs)-sensitized solar cells provides unique opportunities to engineer these solar cells as a solar paint. The field of QDSSCs took a leap forward with the introduction of CdS and CdSe cosensitization, which increased the power conversion efficiency (PCE) of QDSSCs to 4.22%.1 Since then, many inorganic QDs have been explored as efficient light harvesters as part of efforts to further improve the PCE of QDSSCs.1−13 During the past few years, progress in the development of QDSSCs has been very slow and no major improvements of the PCE of QDSSCs have been reported. In very recent years, however, the interest in QDSSCs has been renewed by a series of research articles demonstrating significantly enhanced PCEs and solar cell stability.3,14−21 The availability of a wide variety of QDs as sensitizers on a wide band gap semiconductor opens the possibility to make an © 2017 American Chemical Society

Received: May 29, 2017 Revised: July 24, 2017 Published: August 2, 2017 17658

DOI: 10.1021/acs.jpcc.7b05207 J. Phys. Chem. C 2017, 121, 17658−17670

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The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of One p-SILAR Cycle for Solar Paint Synthesis

solar paint has still attracted the interest of people, including Bill Gates, to further develop this technology.34,35 Among all of the inorganic QDs utilized as light harvesters, PbS possesses a distinctive property; oxidized species (i.e., oxides and oxysulfides) formed after a high-temperature heat treatment in air can be regenerated back to PbS when exposed to a polysulfide environment.36 Furthermore, the oxysulfide phases left on the surface after regeneration could serve as a surface defect passivating layer for a higher PCE.37 This exclusive heat treatment and regeneration process of PbS inspired us to develop the PbS-based solar paint. This report demonstrates a totally air-processable solar cell fabrication based on the PbSbased solar paint. Two solar paints utilizing a commercially available TiO2 and a p-type TiO2 (p-TiO2) powder were prepared and explored for solar cell fabrication. Our in-depth characterization determined that the controlled growth of PbS on TiO2 is very important to obtain PbS QDs with an optimum size to achieve a higher PCE and better stability. X-ray photoelectron spectroscopy (XPS) was employed after every step to analyze the changes on the surface of TiO2 and identify the culprits that may limit the performance of the resulting QDSSCs. Electrochemical impedance spectroscopy (EIS) and open-circuit voltage decay (OCVD) analysis were also performed to elucidate the working mechanism in these solar paint-based QDSSCs to identify the factors that contribute to the higher PCE of solar paint based on p-TiO2.

However, there are several drawbacks to adopt CQDSCs technology as a solar paint: (i) The formation of photoactive film from the CQD ink requires spin coating to form thin films (less than 200 nm), because a thicker film leads to a decrease in performance due to inefficient charge extraction.24−26 This limits the application of the CQD ink only into a small area substrate with a flat surface. A rough, immobile solid surface cannot be used for a substrate, and moreover, the implementation of CQD ink at large scale would be nearly impossible. On the other hand, a simple paint brush can readily be used to make 10−20 μm thick film with a solar paint that is ideal for the high-performance QDSSCs. (ii) Fabrication of CQD film requires an inert environment and specialized protocols.27−30 For example, a rigorous, time-consuming surface functionalization of colloidal QDs, mostly through a ligand exchange process, is required to yield well-defined films. (iii) Counter electrodes in CQDSCs are made of high-cost precious metals (e.g., Au and Ag) which are deposited only by high-vacuum techniques (less than 10−6 mbar) such as thermal evaporation. In addition to the cost issue, they are opaque in nature;29,30 therefore, a transparent substrate is a must to make the photoactive film. For QDSSCs, however, an opaque substrate can be used to make the photoactive film because full or semitransparent counter electrodes are already available.31−33 Given such restrictions on CQDSCs, conventional QDSSCs are likely to be the best candidate to develop the solar paint technology at present. The Kamat group made a breakthrough a few years ago and achieved a PCE of 1.08% with a mixture of TiO2−CdS and TiO2−CdSe nanoparticles-based solar paint.22 However, the deposition of CdSe onto TiO2 requires an oxygen-free environment. Furthermore, heat treatment was performed at 200 °C in their process, which also has to be carried out under a nitrogen atmosphere to prevent undesired decomposition of the QD sensitizers. Such requirements of these controlled environments diminish the attractiveness of the solar paint concept as a one-coat method to fabricate the photoanode of QDSSCs. Despite the current limitations, the unique idea of

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EXPERIMENTAL SECTION

Synthesis of Solar Paints. The synthesis of p-type TiO2 (Ti0.905O2) was carried out using a procedure reported earlier.38 The sensitization of TiO2 was carried out by a pseudosuccessive ionic layer adsorption and reaction (p-SILAR). The summary of the one p-SILAR cycle is shown in Scheme 1. Briefly, an appropriate amount of TiO2 powder was added to a 50 mL plastic centrifuge tube. Then 20 mL of a 20 mM PbNO3 solution in methanol was added, and the mixture was stirred vigorously for a few seconds. To remove the unreacted solution, it was centrifuged at 6000 rpm for 4 min and the supernatant was decanted. For further cleaning, 20 mL of methanol was added to the tube, stirred, and centrifuged again. To complete the reaction, 20 mL of a 20 mM solution of Na2S in water/methanol (1:1) was added to 17659


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The Journal of Physical Chemistry C the cleaned precipitates. Centrifugation was carried out once more to remove the unreacted Na2S solution. Another cleaning step was performed with the addition of 20 mL of methanol and subsequent centrifugation. These four steps constitute a single p-SILAR cycle. Three p-SILAR cycles were carried out for the sensitization of the TiO2 surface with PbS. To passivate the PbS with ZnS, two p-SILAR cycles were carried out using the above-mentioned procedure. Instead of the Pb precursor, however, a 20 mM solution of Zn(CH3COO)2 in methanol and a 20 mM solution of Na2S in water/methanol (1:1) were used as the Zn and S precursors, respectively. The as-prepared powder was then dried under air at 60 °C for 48 h. Fabrication of Solar Cells from Solar Paint. A paste was made from the PbS-sensitized TiO2 by a process reported previously,39 which was adopted for our experimental conditions. Briefly, 0.125 g of ethyl cellulose (30−70 mPa·s, 5% in toluene/ethanol 80:20 (25 °C), Sigma-Aldrich 46080) and 0.125 g of ethyl cellulose (5−15 mPa·s, 5% in toluene/ethanol 80:20 (25 °C), Sigma-Aldrich 46070) was added to a mortar grinder and dissolved in an appropriate amount of ethanol. Subsequently, 0.5 g of PbS-sensitized TiO2 powder and 1.7 mL of αterpineol was added and mixed in a mortar grinder for more than 15 min until a paste with an appropriate viscosity was obtained. To make a film, the solar paints were applied onto TiCl4-treated F-doped SnO2 (FTO) glass by the doctor blade technique, and this film was dried and subsequently heated at 250 °C for 10 min in air. Note that this annealing temperature is significantly lower than a typical annealing temperature (more than 450 °C). While such low-temperature annealing cannot remove the organic binder completely, we intentionally limited the annealing temperature to below 250 °C, which can readily be accessible by a hand-held heat gun, because the conventionally performed high temperature would not be appealing for the practical exercise of solar paint concept. CuS counter electrodes were prepared according to a previously reported procedure.40,41 A sandwich-type cell was assembled using the solar paint film as the photoanode, CuS as the counter electrode, polypropylene tape as the separator, and a freshly prepared polysulfide solution (0.125 M S/1.0 M Na2S in 3:2 methanol:deionized water) as the electrolyte. To compare with conventional QDSSCs, QDSSCs were made using P25 TiO2 films according to the methods published earlier and evaluated.5,42 Characterization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were captured on a Hitachi S-4800 electron microscope and a JEOL 2010F electron microscope equipped with energy-dispersive X-ray spectroscopy (EDS), respectively. X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (D/Max-2500/PC, Rigaku). A UV−vis spectrophotometer (S-3100, SCINCO) assembled with a diffuse reflector was used to measure the diffuse reflectance spectra of various films. The Brunauer−Emmett−Teller (BET) surface area of TiO2 was determined by using a BELSORP-mini II (BEL Japan). To obtain the XPS spectra, a PHI Versa Probe system with a 100 W Al K Alpha Xray source was used. Transient absorption spectroscopy was performed using a 2010 Ti:sapphire laser system (Clark-MXR) equipped with a CCD spectrograph (Ocean Optics, S2000-U-UV-vis). Semitransparent PbS films placed in a vacuum cell were excited by pump pulses (387 nm), and the following optical events in the films were monitored by probe pulses. The current−voltage (J−V) response, photocurrent stability, and OCVD measurements were measured using a Keithley 2400 source meter when the cell was illuminated by a solar simulator (HAL-320, Asahi Spectra) which was calibrated to 1 sun using a standardized silicon diode (CS-20, Asahi Spectra). The incident photon-to-charge carrier conversion efficiency (IPCE) was measured on a custom-assembled apparatus equipped with a xenon lamp and a monochromator from Newport and the Keithley source meter. EIS and Mott−Schottky plots were obtained using a potentiostat (REF600-25049, Gamry Instruments).

the photoanode of QDSSCs. Degussa P25 TiO2 powder (P25 TiO2) is well known to have a fairly large surface area (BET surface area ≈ 60 m2/g), which is also a prerequisite to deposit a large number of QDs to achieve higher performance of the fabricated QDSSCs. It consists of very small individual nanoparticles with sizes of a few tens of nanometers (Figure 1A). XRD analysis (Figure 2A) showed that P25 TiO2 contains

Figure 1. SEM images of (A) P25 TiO2 and (B) p-TiO2.

Figure 2. XRD patterns of the (A) TiO2 powders used to make solar paints: (⧫) anatase peaks and (○) rutile peaks. (B) TiO2 powders after depositing PbS by p-SILAR (insets: photographs of PbS-sensitized TiO2 powders). Plus (+) signs represent peaks from the PbS phase.

a mixture of anatase and rutile phases, with anatase being the dominant phase. It has been suggested that the presence of a small amount of rutile phase in combination with the anatase phase in this TiO2 is beneficial to enhance the performance of nanostructured dye-sensitized solar cells and QDSSCs.43 The deposition of PbS QDs passivated by ZnS was carried out by the p-SILAR, and the procedure is described in detail in the Experimental Section and Scheme 1. After deposition of PbS, the dominant peak of PbS appears at 30.1° (Figure 2B), confirming the formation of the PbS phase. A wide broadening of the PbS peak (Figure S1) indicates a low degree of crystallinity and the very small nature of the PbS particles deposited onto the TiO2 surface. To obtain additional evidence of the presence of PbS QDs on P25 TiO2, TEM analysis coupled with EDS analysis was carried out. Figure 3A shows the presence of PbS on the TiO2 surface, which is marked by black circles and has a dark shade compared to TiO2. The dark-field TEM image reveals even

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RESULTS AND DISCUSSION As a starting point, a commercial TiO2 powder is an obvious choice to synthesize solar paint for the one-coat fabrication of 17660


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The Journal of Physical Chemistry C

Figure 3. TEM images of different TiO2 powders after depositing PbS by p-SILAR: (A−E) P25 TiO2 and (F−J) p-TiO2. (A and F) Bright-field TEM images (black circles highlight the presence of PbS, and white rectangles highlight the areas used for EDS mapping), (B and G) dark-field images (white circles indicate the presence of PbS), (C and H) HRTEM images, and (D and E and I and J) EDS elemental mappings (the markers in E and J are equal to 10 and 50 nm, respectively).

more interesting features. In addition to larger QDs, the surface of P25 TiO2 is covered with small flakes of PbS (Figure 3B). The chemical nature of the flakes was confirmed by the EDS analysis. Due to the very small size of these flakes and without any protection by a passivation layer, they may oxidize very easily. We will discuss the implications of the presence of these flakes in more detail later. The high-resolution TEM (HRTEM) image (Figure 3C) shows the presence of ZnS phase in addition to PbS on the TiO2 surface. Instead of covering the surface of the TiO2 decorated with PbS, ZnS is present as particles which are randomly distributed on the TiO2 surface. Some of these particles are deposited onto the PbS QDs and provide partial protection for the QDs from oxidation and photocorrosion. The EDS elemental mapping (Figure 3D and 3E) of the dark areas in the TEM image in Figure 3A further confirms the presence of Pb and S in these areas. In addition to the localized high concentrations of Pb and S, which result from large QDs, the presence of Pb and S is also uniformly distributed on the TiO2 surface. This uniform distribution of PbS on the P25 TiO2 surface is attributed to the presence of PbS flakes, as seen in the dark-field TEM image (Figure 3B). Additional TEM images and EDS analysis is provided in Figure S2. P25 TiO2 decorated with PbS QDs was used to make a paste with ethyl cellulose as a binder and terpineol as a solvent. This paste will hereafter be referred as P25-SP (solar paint). P25-SP was applied on FTO glass with a TiO2 blocking layer, and to remove binder and solvent, heat treatment was carried out in air at 250 °C. This heat treatment process also causes the rapid oxidation of PbS to lead oxide (PbO) and oxysulfides (PbSOx). Fortunately, these oxidized species can be converted, to some extent, back to PbS by treating it with a polysulfide electrolyte.36 To study the changes on the TiO2 surface during this whole process, XPS analysis of the P25-SP powder was performed after every step (Figure 4). As given in Figure 4, the Pb 4f and S 2p XPS spectra were analyzed after three distinct stages of the solar paint fabrication from P25-SP as mentioned in the caption of Figure 4. The Pb 4f XPS spectra in Figure 4A can be fitted to two peaks. The peak at 136.85 eV (peak 1, blue) was assigned to the S-

Figure 4. XPS spectra of (A−C) Pb 4f and (D−F) S 2p of P25-SP at various stages of solar paint fabrication. PbS-sensitized P25 TiO2 (A and D) after drying at 60 °C, (B and E) after heat treatment at 250 °C, and (C and F) after regeneration of PbS by polysulfide treatment. In A, B, and C, doublets 1, 2, and 3 correspond to S-coordinated Pb (PbS), O-coordinated Pb (PbO), and oxysulfides-coordinated Pb (PbSOx), respectively. In D, E, and F, doublets 1, 4, and 3 represent Pb-coordinated S, Zn-coordinated S, and the oxidation state of S in PbSOx, respectively.

coordinated Pb (PbS),44 and the peak at 137.90 eV (peak 2, olive) is attributed to the O-coordinated Pb (PbO).45 The PbO may have been formed during the drying process due to the oxidation of PbS. The Pb peak for PbS appeared at relatively lower energy than the usual PbS peak. This negative shift could be a result of unpassivated PbS on the surface of P25 TiO2.44,46 It should be noted that in Figure 4A the signal of the PbO peak (peak 2, olive) is stronger than that of the PbS peak (peak 1, blue), suggesting that PbS, which was deposited onto the P25 via p-SILAR, is prone to oxidation. This vulnerability of PbS might result from inadequate ZnS passivation, which will be discussed in detail later. After the heat treatment at 250 °C, the 17661


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The Journal of Physical Chemistry C peak at 136.85 eV completely disappeared due to the oxidation of PbS to oxide and oxysulfides (Figure 4B). However, another peak at 138.50 eV (peak 3, orange) appeared which was attributed to the Pb coordinated with oxysulfides (PbSO3 and PbSO4).47 When P25-SP was treated with a polysulfide solution, the peak at 136.85 eV reappeared with a very strong signal and the peak at 137.90 eV was diminished, which suggests that the polysulfide treatment can efficiently convert most of the oxides and oxysulfides of Pb that were formed during the heat treatment back to PbS (Figure 4C). The regeneration of PbS can also be supported by the S 2p XPS spectra (Figures 4D−F), which can be resolved into three peaks: the peak at 160.4 eV (peak 1, blue) belongs to Scoordinated Pb, the peak at 161.0 eV (peak 4, cyan) corresponds to S-coordinated Zn, and the peak at 167.3 eV (peak 3, orange) shows the presence of oxysulfides of Pb, which has PbSO348 as a dominant PbSOx species (Figure 4D). After the heat treatment, peak 1 disappeared completely, indicating oxidation of PbS, but the peak 4 was mostly intact, which points toward the resistance of ZnS to oxidation (Figure 4E). On the other hand, the PbSOx peak becomes stronger and shifts toward higher energy, which could result from the formation of PbSO449 during heat treatment. When the polysulfide treatment was carried out, the PbS was regenerated that caused the reappearance of the peak at 160.4 eV (peak 1, blue, Figure 4F). These results support the conclusions made from the Pb 4f XPS spectra that the polysulfide electrolyte can regenerate the oxidized Pb species during the heat treatment into PbS. It is noteworthy that increasing the number of ZnS SILAR cycles or increasing the concentration of ZnS precursors did not cause any regeneration issues. Because the ZnS is deposited as particles randomly placed on the TiO2 surface, some of the PbS surface would always be exposed for the regeneration. However, increasing the ZnS passivation by p-SILAR can cause the decrease in electrical contacts between TiO2 particles that results in the degradation of the solar cell performance. When P25-SP was used to make the photoanode, the resulting solar cell yields a PCE of 1.0% with short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF) values of 3.14 mA/cm2, 527 mV, and 0.59, respectively (Figure 5A, Table 1). This PCE value is comparable to the best previously reported PCE of QDSSCs assembled with a singlestep fabrication of a photoanode.22 However, it is noteworthy that this result was achieved even though we employed an oxygen environment unlike in the previous report that required an oxygen-free environment for certain fabrication steps.22 These QDSSCs show a maximum IPCE value of 9.35% at 500 nm, and it decreases with increasing wavelength, almost fading away at 950 nm (Figure 5B). As PbS-based liquid-junction QDSSCs are known to be unstable, especially without a proper passivation layer, this inherent instability is also evident in the current case. The photocurrent of the P25 solar paint-based QDSSCs is relatively unstable. The photocurrent degrades to nearly 60% after 300 s of continuous illumination (inset of Figure 5A). The main reason for the low stability of these devices is the inefficient passivation of PbS with ZnS. As discussed earlier, the TEM images showed that ZnS provides only partial coverage of PbS QDs, and hence, the PbS flakes on the surface of P25 TiO2 particles are largely unprotected and can oxidize very quickly. As seen in the TEM images of PbS-coated P25 TiO2, its surface is covered with the unprotected flakes of PbS, which may have formed due to the faster adsorption of Pb2+ ions on

Figure 5. (A) J−V curves and (B) IPCE spectra of QDSSCs based on different solar paints. (Inset of A) Photocurrent stability of the respective solar cells.

the P25 TiO2 surface. To control the growth rate of PbS, we decided to use a TiO2 powder that has a lower surface charge than P25 TiO2. Another factor that may lead to the lower stability is the charge extraction ability of P25 TiO2. As P25 TiO2 particles are very small and have a solid spherical shape, the low-temperature heat treatment that we used cannot fully guarantee the formation of good electrical contact, which is a prerequisite for efficient charge extraction from the semiconducting mesoporous network. To circumvent these hurdles, we chose p-type TiO2 particles with a thorn-ball-like structure and a very high charge conductivity.38 The zeta potential of the p-TiO2 particles was measured to be −11.2 mV in a neutral aqueous environment, and that of the P25 TiO2 surface was −26.4 mV. Hence, the lower surface charge of the p-TiO2 particles ensured slower growth of PbS with smaller sizes and a larger band gap. Also, the needle-like structures on the surface can lead to physical entanglement of neighboring particles that can enhance the charge transport properties throughout the TiO2 network. Another advantage of using these TiO2 particles is that they possess a very high charge conductivity, which is very important in the current scenario because a low heat treatment temperature has to be used for sintering of the TiO2 film. Figure 1B shows the thorn-ball-like structure of p-type TiO2 with a very rough surface (BET surface area = 56.2 m2/g), which is desirable for making multiple contacts with the neighboring particles. Unlike P25 TiO2, which has n-type conductivity, the Mott−Schottky measurements show that this TiO2 has p-type conductivity (Figure S3); this is consistent with the results of a previous report.38 The XRD analysis revealed that it mainly consists of an anatase phase with a very small portion of a rutile phase (Figure 2A). After deposition of PbS, the main peak of PbS appeared at 30.1° (Figure 2B), but 17662


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The Journal of Physical Chemistry C Table 1. Solar Cell Performance Parameters of QDSSCs Based on Different Solar Paints.a sample

JSC (mA/cm2)

VOC (mV)

FF

PCE (%)

P25-SP p-TiO2-SP

3.14 ± 0.22 5.23 ± 0.20

527.1 ± 4.0 494.2 ± 8.9

0.587 ± 0.017 0.524 ± 0.034

0.97 ± 0.04 1.35 ± 0.05

Based on the average of a minimum of 10 devices. The numbers on the right side of the ± sign are the standard deviations of the respective parameters.

a

this peak of PbS is broader than that observed in P25-SP (Figure S1). This result suggests that the average QD size in the paste made from the p-type TiO2 powder decorated with PbS QDs (p-TiO2-SP) is smaller than that in P25-SP. This fact was also supported by the apparent difference in the color of the as-synthesized solar paint powders (Figure 2 and Figure S4). The P25-SP had a blackish color, whereas p-TiO2-SP a brownish color. To further confirm the above deduction, a Tauc plot was obtained to calculate the band gap, which is inversely related to the QD size.50−52 The band gap of p-TiO2SP was 1.36 eV, and that of P25-SP was 1.24 eV (Figure S5). These band gaps correspond to average PbS QD sizes of 4.5 and 4.1 nm for P25-SP and p-TiO2-SP, respectively.52 Hence, it is reasonable to conclude that the average QD size in p-TiO2SP is smaller than that in P25-SP. The presence of PbS in p-TiO2-SP was visually confirmed by TEM analysis. The dark areas in the TEM image (Figure 3F) and bright areas in the dark-field TEM image (Figure 3G) are PbS, which was verified by EDS elemental mapping (Figure 3I and 3J). The HRTEM image also showed the partial coverage of PbS QDs by ZnS (Figure 3H), as was the case in P25-SP. It is worth mentioning that the TEM images are highly localized; hence, they cannot be used to estimate the average QD size. Contrary to P25 TiO2, the surface of p-TiO2 cannot be clearly resolved in the TEM images. Thereby, in the TEM images we observed aggregates of PbS instead of individual QDs. In addition, HRTEM is a very localized method that cannot give an accurate picture of the size distribution (additional TEM images of p-TiO2-SP are provided in Figure S6). The peaks of PbS in the XRD results are also not intense enough to make a quantitative assessment of the size of the PbS QDs. However, the peak broadening of PbS peaks provides a qualitative comparative assessment (Figure S1). As discussed earlier, the best quantitative estimate of the average QD size could only be made by measuring the band gap of the PbS QDs (Figure S5). In terms of spectral features, p-TiO2-SP XPS spectra are very similar to those of P25-SP. The Pb 4f spectra (Figures 6A−C) show that the PbS peak at 136.4 eV (peak 1, blue) disappeared after the heat treatment, but the PbO peak (peak 2, olive) becomes stronger with additional emergence of PbSOx peak (peak 3, orange, Figure 6B). Just like in P25-SP, the polysulfide treatment resulted in the regeneration of PbS that is evident from the reappearance of the PbS peak (peak 1, blue) in Figure 6C. The same conclusion can be deduced from the S 2p spectra (Figures 6D−F) that show the presence of PbS (peak 1, blue), ZnS (peak 4, cyan), and PbSOx (peak 3, orange) in the assynthesized sample (Figure 6D), but after heat treatment the PbS peak (peak 1, blue) disappeared and the PbSOx (peak 3, orange) signal became stronger (Figure 6E). The polysulfide treatment was also effective on p-TiO2-SP, as it resulted in the reappearance of the PbS peak (peak 1, blue) in Figure 6F. However, it is noteworthy that there are two main differences between the XPS features of P25-SP and p-TiO2-SP. The first disparity is the ratio of sulfide and oxide peaks (PbS/(PbO + PbSOx)), which is given in Table S1 so that the results can be

Figure 6. XPS spectra of (A−C) Pb 4f and (D−F) S 2p of p-TiO2-SP at various stages of solar paint fabrication. PbS-sensitized p-TiO2 (A and D) after drying at 60 °C, (B and E) after heat treatment at 250 °C, and (C and F) after regeneration of PbS by the polysulfide treatment. (A, B, and C) Doublets 1, 2, and 3 correspond to S-coordinated Pb (PbS), O-coordinated Pb (PbO), and oxysulfides-coordinated Pb (PbSOx), respectively. (D, E, and F) Doublets 1, 4, and 3 represent Pb-coordinated S, Zn-coordinated S, and the oxidation state of S in PbSOx, respectively.

better visualized quantitatively. As can be seen in Table S1, the relative amount of oxide and oxysulfides on P25-SP is more than 1.5 times higher than those on p-TiO2. This suggests that PbS on P25-SP is more prone to oxidation. This might also play a crucial role in charge transfer through the TiO2 layer. The presence of a large amount of oxides on the TiO2 surface can act as recombination centers and could result in a lower performance of the solar cell.53,54 Another difference is the ratio of ZnS and PbS peaks in the S 2p XPS spectra (Table S2). pTiO2-SP have a considerably higher ZnS/PbS ratio as compared to P25-SP, suggesting the presence of a better passivation layer in the p-TiO2-SP. To gain further insight into the state of the ZnS passivation layer in both solar paints, we analyzed the Zn 2p peaks from the XPS spectra of regenerated samples (Figure S7). The Zn 2p peaks were fitted to the Scoordinated Zn (ZnS). The relative signal intensity of Zn in pTiO2-SP was almost three times higher than that in P25-SP. This result indicates that ZnS passivation in p-TiO2-SP must be relatively more effective. We speculate that the better passivation on the p-TiO2 surface was a result of more uniform deposition of ZnS which was caused by the less negative surface charge. As previously reported by our group,5 more uniform PbS can be deposited on the TiO2 surface by reducing the ion adsorption rate. We believe that same argument can be extended here; however, the surface charge of the TiO2 surface plays a more important role as a controlling factor. A less negative surface charge (−11.2 mV) on the p-TiO2 surface as 17663


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Figure 7. Time-resolved transient absorption spectra recorded following 387 nm laser pulse excitation of (A) PbS on ZrO2 (PbS/ZrO2), (B) PbS on P25 TiO2 (PbS/P25), and (C) PbS on p-TiO2 (PbS/p-TiO2). (D) Normalized absorption difference (ΔA)−time profiles of PbS/ZrO2, PbS/P25, and PbS/p-TiO2 recorded at 420 nm.

(Figure S7) and better charge extraction due to the better conductivity of TiO2 particles. To confirm the hypothesis that the inadequate passivation was the cause of the lower PCE and poor stability in P25-SP solar cells, a control experiment was performed to check the performance of ZnS passivation by the traditional SILAR. The results are given in Figure S8 and Table S3. The PbS solar cells without any passivation performed poorly and showed a PCE of only 0.38%. This PCE can be enhanced to 0.98% and 1.15% if the PbS QDs were passivated with ZnS by p-SILAR and the traditional SILAR, respectively. Application of ZnS passivation by SILAR on P25-SP can further increase the PCE to 1.69%. The photocurrent stability of P25SP was also improved significantly by the deposition of the ZnS layer using the conventional SILAR (Figure S9). These results confirm that the incomplete passivation in these solar paints is the main culprit for the relatively lower PCE and poor stability. In addition, PbS-sensitized QDSSCs were fabricated by the conventional method (deposition of QDs by SILAR on mesoporous P25-TiO2 film), so that a fair comparison of performance can be made with QDSSCs based on solar paint. A PCE of 2.73% was obtained with photocurrent stability similar to the solar cells made of solar paint with ZnS passivation deposited by SILAR (Figures S8 and S9, Table S3). The higher performance likely results from a better TiO2 film obtained by the high-temperature annealing and improved protection from the slow oxidation of PbS. Given these observations, to further advance the performance of the solar paint technology, the issue of inadequate passivation needs to

compared to the P25 TiO2 surface (−26.4 mV) may have resulted in slower but more uniform deposition of ZnS on the p-TiO2 surface. Due to the better passivation of PbS QDs on pTiO2, they may have lower surface defects and are better protected from oxidation.16,55 Both these factors can reduce the charge recombination and may result in a higher PCE in pTiO2-SP-based solar cells. The p-TiO2-SP-based solar cells showed an average JSC value of 5.23 mA/cm2, which is indeed more than 60% higher than that of the P25-SP solar cells (Figure 5A, Table 1). The average VOC and FF values for p-TiO2-SP were 494 mV and 0.524, respectively, which are slightly lower than those of P25-SP. However, a strong boost of the JSC value resulted in a 35% increase of the PCE of p-TiO2-SP, and an average PCE of 1.35% was obtained for p-TiO2-SP, where the champion cell showed a PCE value of 1.41%. The IPCE values of the PbS QDs in p-TiO2-SP are higher than those of P25-SP over the whole spectral range in which PbS QDs absorb light (Figure 5B). The highest IPCE value of 22% was observed at 510 nm, and it decreased with increasing wavelength before fading away at 1050 nm. The photocurrent stability of the p-TiO2-SP QDSSCs was much better than that of the P25-SP QDSSCs (inset of Figure 5A). The photocurrent in the p-TiO2-SP QDSSCs decreased to only 85% after more than 500 s of continuous light exposure. On the other hand, the photocurrent of the P25-SP QDSSCs dropped to 60% after just 300 s of light exposure. This better stability of the p-TiO2-SP QDSSCs could be a result of the relatively better passivation of PbS QDs 17664


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absorption feature recorded in the visible region. In other words, the decay in transient absorption signals shown in Figure 7D cannot be attributed solely to the electron injection kinetics from PbS to TiO2. However, the noticeable difference in decay trends between PbS/ZrO2 and PbS/TiO2 confirms the presence of charge transfer at the interface of PbS and TiO2. The ultrafast decay shown in Figure 7D along with this fact does not allow us to make a clear quantitative distinction of the relative charge kinetics in the two solar paints. However, it is evident from this observation that the electron transfer takes place effectively in both solar paints and occurs in a very fast rate. According to Lian and co-workers, the electron injection from PbS to TiO2 is ultrafast on the order of femtoseconds (6.4 ± 0.4 fs).57 Therefore, the performance of solar cells made from the solar paints is more likely to be governed by charge recombination dynamics at the photoanode/electrolyte interface. A detailed EIS study was carried out to understand the various optoelectronic processes that may be responsible for limiting the performance of different solar paints. The Nyquist plots of the QDSSCs made from two solar paints are given in Figure S11 at various bias conditions. To decouple the charge transfer at the counter electrode and recombination at the TiO2/QDs/electrolyte interface, two parallel RC circuits were used in a series for an equivalent circuit (Figure S12). Another series resistance was also added to compensate for the resistance of FTO and electrical connections. The EIS spectra were fitted to this equivalent circuit to extract the various physical parameters. Before any parameter can be compared, the effects of the series resistance (Rs) and counter electrode resistance (RCE) on the voltage drop across the solar cell were removed (VF = Vappl − VS − VCE, where VF is the Fermi voltage, Vappl is the applied voltage, and VS and VCE are the voltage drops across the solar cell due to RS and RCE, respectively). VF is a measure of the change of the Fermi level (EFn) from its equilibrium state (EF0) and is given as (VF = EFn − EF0)/q, where q is the positive elementary charge. The chemical capacitance (Cμ) is a measure of the carrier concentration in the ECB of TiO263 and is directly related to the difference of the EFn and ECB of TiO2, as given by eq 1.5,64,65

be addressed in future research. Also, engineering of the TiO2 particles and the annealing process should be carefully exercised to further boost the PCE of the solar paint-based QDSSCs. An equilibrium energy band diagram (Figure S10) was constructed to identify various energetic factors that may have contributed to the increase of the JSC of p-TiO2-SP. The band gaps of the TiO2 powders and solar paints were calculated by performing diffuse reflectance UV−vis spectroscopy and constructing Tauc plots (Figure S5). The band gaps of P25 and p-TiO2 are very similar, and their band positions were assigned according to a previous report.52 The band gap of PbS changes with the QD size, and most of this change is accompanied by a displacement in the conduction band (ECB) of PbS. However, if the size is very small then the valence band position also starts to shift dramatically. The method described by Hyun and co-workers52 was used to calculate the expected band positions of PbS QDs in different solar paints. The difference between the conduction bands of PbS and P25 (Figure S10A, ΔE = 0.24 eV) is small. Hence, it may result in less favorable energetic conditions for the electron transfer from the ECB of PbS to the ECB of P25. On the other hand, the gap between the ECB of PbS in p-TiO2-SP and the ECB of pTiO2 (Figure S10B, ΔE = 0.32 eV) is relatively larger with a value of ∼0.08 eV compared to that in P25-SP. This increased ΔE could provide relatively better energetics for the electron transfer from the excited PbS QDs to p-TiO2. It should be noted that while p-TiO2 shows p-type conductivity with holes as majority carriers, the Fermi level in p-TiO2 should not be too low compared to the intrinsic Fermi level, as the doping level and carrier concentration in p-TiO2 are very low.38 Hence, electrons still have a high chance of conduction through the conduction band of p-TiO2 as minority carriers. Although it is expected that the recombination of electrons and holes may be higher in p-TiO2-SP QDSSCs, we speculated that more favorable characteristics (better optoelectronic properties of PbS QDs, favorable electron transfer energetics, and relatively better passivation in p-TiO2-SP) were sustainable due to the higher conductivity of p-TiO2.38 Further discussion of this issue will be provided later with the aid of EIS analysis. To probe the electron transfer process at the interface between PbS and TiO2, femtosecond transient absorption spectroscopy was carried out. This ultrafast spectroscopic technique has proven very powerful in examining various charge separation and recombination events occurred in lightharvesting systems.56 The time-resolved transient absorption spectra were recorded following a laser pulse excitation at 387 nm and are displayed in Figure 7. Photoinduced absorption, i.e., positive signals throughout the whole visible region, appeared instantaneously after excitation regardless of the substrate on which PbS was deposited. This can be attributed to the red shift of higher energy bands driven by the presence of excited electrons.57,58 The appearance of the photoinduced absorption in our results is consistent with previous reports.57,59 However, the relaxation dynamics of these transient features are different depending on the substrate (Figure 7). The transient signals from PbS on TiO2 were substantially shorter lived (less than 1 ps) than those from PbS on insulting ZrO2 (∼1 ns). Note that the same effective masses of electrons and holes in PbS (0.09m0 each where m0 is the electron rest mass) lead them to contribute equally to the transient signals unlike CdSe.57,58,60−62 Therefore, it is very difficult to discern the contributions of electron and of hole transfer to the decay dynamics of the induced absorption by examining the transient

⎛ E − E Fn ⎞ Cμ ∝ exp⎜ − CB ⎟ kBT ⎝ ⎠

(1)

Hence, a narrow difference of ECB and EFn would lead to a higher concentration of electrons in the ECB of TiO2. This also implies that a higher Cμ at the same EFn would be a result of lowering the ECB of TiO2. When the Cμ values of both solar paints are compared, p-TiO2-SP showed a much higher Cμ compared to P25-SP (Figure 8A), which suggests that the ECB of TiO2 in the p-TiO2-SP-based solar cells is lower than that in the P25-SP-based solar cells. This difference of the ECB position (ΔECB) can be estimated by shifting the VF scale in the Cμ vs VF plot and making the two Cμ values overlap (Figure 8B).65−68 The dip in the ECB of p-TiO2 (∼0.15 eV) may have led to the better charge separation in p-TiO2 SP, which contributed to the higher JSC in these QDSSCs. However, a lower ECB would also lead to a lower VOC in the cell. To fairly compare the recombination resistance (Rr) and electron lifetime (τn), the ΔECB was removed to obtain the equivalent conduction band condition (Vecb).5,64−67 The pTiO2-SP showed a better Rr compared to P25-SP (Figure 8C). 17665


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Figure 9. Electron lifetime calculated from the OCVD data along with fitted lines.

Here, Ccμ,Cbμ, and Csμ are the chemical capacitances due to conduction band states, bulk trap states, and surface trap states, respectively,τc is the electron lifetime due to the electron transfer from conduction band states, and esox(EF) is the transition probability of electrons from surface traps that are below the quasi-Fermi level. The mathematical and physical details of the various parameters in eq 2 are provided in the Supporting Information (eqs S1−S9) and elsewhere.69 The parameters extracted by fitting eq 2 to the electron lifetime extracted from the OCVD data are provided in Table 2. It was

Figure 8. Comparison of various physical parameters extracted from the EIS spectra. (A) Chemical capacitance (Cμ) vs Fermi voltage (VF), (B) C μ vs equivalent conduction band voltage (V ecb ), (C) recombination resistance (Rr) vs Vecb, and (D) electron lifetime (τn) vs Vecb.

Table 2. Parameters Extracted from Fitting the OCVD Modela parameter −3

Nb (cm ) Ns (cm−3) T0 (K) T1 (K) λ (eV) Ec − Eredox (eV) Acb (s−1)b Ast (s−1)b kcb (cm3 s−1)c kst (cm3 s−1)c

The higher Rr in p-TiO2-SP may be attributed to the relatively better passivation of PbS QDs in p-TiO2-SP. It should be noted that a higher Rr usually leads to a higher VOC in a solar cell, but p-TiO2-SP showed a slightly lower VOC compared to P25-SP. As the increases of Cμ and Rr have opposite effects on the VOC value, we speculate that Cμ is the dominant factor that resulted in the slightly lower VOC in p-TiO2-SP. The electron lifetime (τn), which is the product of Rr and Cμ (τn = CμRr), was also longer for p-TiO2-SP (Figure 8D). All of these physical parameters extracted from the EIS analysis (Cμ, Rr, and τn) contributed to the substantially increased JSC value in p-TiO2SP. The OCVD measurements also showed that the overall recombination in p-TiO2-SP is slower compared to P25-SP because p-TiO2-SP showed a longer lifetime (Figure S13). This is consistent with the conclusions made from the EIS analysis. It should be noted that the electron lifetime measured from the OCVD analysis is lower than the lifetime measured by EIS. Typically, in good performing DSSCs, both techniques give similar lifetimes, but in QDSSCs, due to the light-sensitive nature of the QDs, the lifetime measured by OCVD is generally lower.65 Recombination occurs within the QDs due the presence of trap states in the QDs. To obtain more insight into the trap states, the mathematical model developed by Bisquert and co-workers42,69 was fitted to the OCVD data to estimate the bulk and surface trap densities and recombination rates from different states (Figure 9). The equation used for the fitting is given below. τn =

1.69 × 1.89 × 645 553 0.470 0.643 7.10 × 8.30 × 4.72 × 5.52 ×

19

10 1018

105 104 10−15 10−16

p-TiO2-SP 5.96 × 5.29 × 774 484 0.485 0.578 7.45 × 5.05 × 4.95 × 3.35 ×

1019 1017

105 104 10−15 10−16

a Nb and Ns are the concentrations of bulk traps and surface traps, respectively. T0 and T1 are the temperature parameters for the distribution of bulk and surface traps, respectively. λ is the reorganization energy of oxidized species in electrolyte, Ec − Eredox is the difference between the conduction band states of TiO2 and electrolyte redox potential, and Acb and Ast are the rates of recombination from conduction band states and surface trap states, respectively. Kcb and kst are the rate constants for conduction band states and surface trap states, respectively. bAcb = kcbcox and Ast = kstcox, where cox is the concentration of oxidized species in electrolyte. c Calculated with cox = 1.51 × 1020 (0.25 M).

found that the bulk trap density (Ns) in p-TiO2 is higher than that in P25 TiO2, which is somewhat expected because p-TiO2 contains Ti vacancies.38 However, the surface trap density (Ns) was suppressed in p-TiO2 due to the relatively better passivation, and this resulted in the lower recombination in p-TiO2-SP through the surface traps (Ast, Table 2). This OCVD analysis along with the EIS analysis clearly suggests that the recombination in p-TiO2-SP was suppressed, which led to the improved JSC and eventually the higher PCE in the p-TiO2SP-based QDSSCs.

Cμc + Cμb + Cμs s Cμcτc−1 + Cμs eox (E F )

P25-SP

(2) 17666


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CONCLUSION AND OUTLOOK Given complex multistep processes performed with the aid of various expensive equipment for the fabrication of solar cells, developing a way to manufacture solar cells that can be fabricated in a facile, rapid, and inexpensive fashion could be a revolutionary idea to address this issue. While a solar paint based on a mixture of CdS- and CdSe-coated TiO2 nanopowder was proposed as a potential approach to achieve this goal a few years ago,22 the requirement of an inert atmosphere for the process of this first-generation material has limited further advance of this concept. In this work, we demonstrate a new solar paint based on a PbS-coated TiO2 nanopowder that can be processable in air and also seek a comprehensive understanding about this second-generation material to figure out factors limiting solar cell performance thereof for a next leap. The unique property of PbS that its oxidized species can quickly be recovered to PbS upon a brief exposure to a polysulfide solution renders the solar paint made of the PbScoated TiO2 nanopowder feasible for heating in air. Using two solar paints based on a commercial Degussa P25 TiO2 and a ptype TiO2, we carried out in-depth characterization to elucidate the characteristics of PbS-based solar paint governing the performance of the paint-on solar cells. Our investigation reveals that the controlled growth of PbS on the TiO2 surface, which seems to be influenced primarily by the characteristics of TiO2, is the most important factor dictating the PCE and stability of solar cells. It determines the uniformity of PbS QDs deposited on TiO2 and the degree of surface passivation by ZnS. While our paint-on solar cells have not yet been optimized, they are capable of achieving a PCE of 1.41%, which is greater than that of the first-generation cells fabricated under an inert atmosphere (1%). This improvement sounds trivial at a quick glance; however, it is noteworthy that when the Cd-based solar paint was processed in air, the PCE of solar cells thereof was merely 0.16% (Figures S14−S16). Despite such advance, the paint’s efficiency still lags behind that of the stateof-the-art PbS-based QDSSCs (5.73%).36 Note that the comparison with solid-state PbS QD solar cells based on Schottky junction should be excluded, because this type of solar cell cannot be implementable for the fabrication of a low-cost, large-area solar cell without the use of expensive equipment and electrode materials. However, there is more room for further improvement of the PbS-based solar paint, and this work provides several important future directions for those improvements. First, it is critical for enhancing the solar cell performance to develop a new methodology that can delicately control the size, monodispersity, and coverage of PbS QDs on TiO2. Since the low positioned conduction band of PbS limits the effective electron injection from PbS to TiO2,52 keeping the particle size below 4−5 nm and ensuring the monodispersity is very important. Once these requirements are fulfilled, increasing the coverage of PbS would play a pivotal role in boosting the solar cell performance. The electron injection from PbS to TiO2 is ultrafast as evidenced in our transient absorption spectroscopic analysis; hence, the performance of PbS-based solar cells is primarily governed by charge recombination at the photoanode/electrolyte interface, which is proven by the EIS and OCVD analyses. Increasing the loading of PbS QDs on TiO2 not only improves light absorption capability but also substantially suppresses the undesirable charge recombination at the interface. The pSILAR method we demonstrated in this report worked to some

extent for these purposes, but it was still unsatisfactory to meet the demand. As a possible remedy, a large-scale synthesis of PbS-decorated TiO2 nanopowder reported by Stark and coworkers70 attracts our attention. This flame spray synthesis allows for a mass production (∼15 g/h) and also offers the homogeneous deposition of fairly monodisperse PbS QDs (less than 2 nm) up to 33 vol %. We speculate that if this material were combined with our protocols for solar cell fabrication, a significant improvement in performance would be anticipated. Second, improving surface passivation by ZnS is another important consideration. This requirement seems to be easily fulfilled as demonstrated in Figures S8 and S9. The simple ZnS postcoating, which can be done by a simple paint brushing of a Zn2+-containing solution and of a S2−-containing solution over the PbS film, works well for this purpose. Third, it is necessary to demonstrate the fabrication of solar cells composed solely of solar paints. While making a highly efficient photoanode is the most challenging task in the fabrication of solar cells, two other main components (counter electrode and electrolyte) are also essential parts of the solar cell. A material for the counter electrode can be readily made into a paint, and there are several options available for a solid-state electrolyte that can be applied by a paint brush;71−74 hence, utilizing these materials for a fully paint-on solar cell would not be a task of great difficulty. Efforts toward these directions are currently underway, and we hope this work could revive the interest of the scientific community for a more facile and convenient method of fabricating solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b05207. Zoom-in of PbS XRD peaks, additional TEM data, Mott−Schottky plots, digital photographs of solar paint powder, Tauc plots, Zn 2p XPS spectra, additional J−V and photocurrent stability curves, energy band diagram, Nyquist plots, equivalent circuit, and equations for the OCVD model; characterization of P25-CdS solar paints (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Muhammad A. Abbas: 0000-0002-8338-9802 Prashant V. Kamat: 0000-0002-2465-6819 Jin Ho Bang: 0000-0002-6717-3454 Present Address

§ Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, 44000, Pakistan

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 17667


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ACKNOWLEDGMENTS This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016R1A1A1A05005038, NRF2015R1A5A1037548, and 2008-0061891). P.V.K. acknowledges the grant support of Toyota Motor Europe. This is contribution number NDRL No. 5143 from the Notre Dame Radiation Laboratory which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533.

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