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

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Revival of Solar Paint Concept: Air-Processable Solar Paints for the Fabrication of Quantum Dot-Sensitized Solar Cells Muhammad A Abbas, Muhammad Abdul Basit, Seog Joon Yoon, Geun Jun Lee, Moo Dong Lee, Tae Joo Park, Prashant V. Kamat, and Jin Ho Bang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05207 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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The Journal of Physical Chemistry

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

Department of Advanced Materials Engineering, Hanyang University, 55 Hanyangdaehak-

ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea ‡

Department of Materials Science and Chemical Engineering, Hanyang University, 55

Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea §

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame,

Indiana 46556, United States #

ǁ

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States

Department of Bionano Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-

gu, Ansan, Gyeonggi-do 15588, Republic of Korea ξ

Department

of

Chemical

and

Molecular

Engineering,

Hanyang

University,

Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea

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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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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 year-round. 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 co-sensitization, 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 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 wellconnected 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 lowtemperature fabrication of a TiO2 film in air that is well-connected and mechanically stable while keeping the sensitizer intact. 3 ACS Paragon Plus Environment


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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. And 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. 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 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, timeconsuming 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 semi-transparent 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.


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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 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 PbS-based 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. 5 ACS Paragon Plus Environment


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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 pseudo-successive 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 mins and the supernatant was decanted. For further cleaning, 30 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 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 pSILAR 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.


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Scheme 1. Schematic Illustration of One p-SILAR Cycle for Solar Paint Synthesis.

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), SigmaAldrich #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 mins 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 mins 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 7 ACS Paragon Plus Environment


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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 X-ray 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). Semi-transparent 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-to8 ACS Paragon Plus Environment

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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). RESULTS AND DISCUSSIONS As a starting point, a commercial TiO2 powder is an obvious choice to synthesize solar paint for the one-coat fabrication of 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 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 dyesensitized 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 degrees (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.



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

(A)

p-TiO2

♦

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

♦

♦

o♦♦

o

♦♦

♦

♦♦

P25

♦

♦

o

20

♦

o♦♦ o

30

40

♦

♦♦

o

50

♦

60

♦♦

♦♦

70

80

2-Theta (degree)

Figure 2. XRD patterns of the (A) TiO2 powders used to make solar paints. The solid diamonds (♦) represent anatase peaks and the open circles (o) represent rutile peaks. (B) TiO2 powders after depositing PbS by p-SILAR (insets: the photographs of PbS-sensitized TiO2 powders). The plus (+) signs represent peaks from the PbS phase.


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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 darkfield TEM image reveals even 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 (Figures 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.



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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) are bright-field TEM images (black circles highlight presence of PbS and white rectangles highlight the areas used for EDS mapping), (B) and (G) are dark-field images (the white circles indicate the presence of PbS), (C) and (H) are HRTEM images, and (D-E) and (I-J) are EDS elemental mappings (the markers in (E) and (J) are equal to 10 and 50 nm, respectively).


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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).

4 f5/2

4 f7/2

32 1

32 1

3

41

(F)

Intensity (a. u.)

(C)

Intensity (a. u.)

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(B)

(A)

144

141

138

135

Binding Energy (eV)

(E)

(D)

175

170

165

160

155

Binding Energy (eV)

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 Figures 4A-C, doublets 1, 2, and 3 correspond to S-coordinated Pb (PbS), O-coordinated Pb (PbO), and oxysulfides-coordinated Pb (PbSOx), respectively. In Figures 4D-F, doublets 1, 4, and 3

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represent Pb-coordinated S, Zn-coordinated S, and the oxidation state of S in PbSOx, respectively. 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 the 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-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 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 details later on. After the heat treatment at 250 °C, the 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 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 S-coordinated Pb, the peak at 161.0 eV (peak 4, cyan) corresponds to Scoordinated Zn, and the peak at 167.3 eV (peak 3, orange) shows the presence of oxysulfides of


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Pb, which has PbSO348 as a dominant PbSOx species (Figure 4D). After the heat treatment, the peak 1 disappeared completely, indicating the oxidation of PbS but the peak 4 was mostly intact, which point towards the resistance of ZnS to oxidation (Figure 4E). On the other hand, PbSOx peak becomes stronger and shift towards 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.


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Figure 5. (A) J-V curves and (B) IPCE spectra of QDSSCs based on different solar paints. The inset of (A) shows the photocurrent stability of the respective solar cells. 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 single step 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


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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 (the 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. Table 1. Solar Cell Performance Parameters of QDSSCs Based on Different Solar Paints.a

a

Sample

JSC (mA/cm2)

VOC (mV)

FF

PCE (%)

P25-SP

3.14±0.22

527.1±4.0

0.587±0.017

0.97±0.04

p-TiO2-SP

5.23±0.20

494.2±8.9

0.524±0.034

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.

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 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


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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 degrees (Figure 2B), but this peak of PbS is broader than the one observed in P25-SP (Figure S1). This result suggests that the average QD size in the paste made from the ptype 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-TiO2-SP was 1.36


18

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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-TiO2-SP is smaller than that in P25-SP. The presence of PbS in p-TiO2-SP was visually confirmed by the 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 (Figures 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).


19


4 f5/2

4 f7/2

32 1

32 1

3

41

(F)

Intensity (a. u.)

(C)

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(B)

(A)

144

141

138

135

Binding Energy (eV)

(E)

(D)

175

170

165

160

155

Binding Energy (eV)

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. In Figures 6A-C, doublets 1, 2 and 3 correspond to S-coordinated Pb (PbS), O-coordinated Pb (PbO), and oxysulfides-coordinated Pb (PbSOx), respectively. In Figures 6D-F, doublets 1, 4, and 3 represent Pb-coordinated S, Zn-coordinated S, and the oxidation state of S in PbSOx, respectively.

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) shows 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 PbS peak (peak 1, blue) in Figure 6C. The same conclusion can be deduced from the S 2p spectra (Figures


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6D-F) that shows the presence of PbS (peak 1, blue), ZnS (peak 4, cyan), and PbSOx (peak 3, orange) in the as-synthesized 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 of p-TiO2-SP. The first disparity is the ratio of sulfides and oxides peaks (PbS/(PbO+PbSOx)), which is given in Table S1 so that the results can be 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. As 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). p-TiO2-SP have considerably higher ZnS/PbS ratio as compared to P25-SP, suggesting the presence of 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 S-coordinated Zn (ZnS). The relative signal intensity of Zn in p-TiO2-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 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


21


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surface plays a more important role as a controlling factor. A less negative surface charge (-11.2 mV) on p-TiO2 surface as compared to P25 TiO2 surface (-26.4 mV) may have resulted in slower but more uniform deposition of ZnS on P-TiO2 surface. Due to the better passivation of PbS QDs on p-TiO2, 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 p-TiO2-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 (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


22

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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 P25-SP was also improved significantly by the deposition of 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 be addressed in the future research. Also, engineering of the TiO2 particles and 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 UVvis 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


23


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Page 24 of 45

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 coworkers52 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, ∆E=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 p-TiO2 (Figure S10B, ∆E=0.32 eV) is relatively larger with a value of ~0.08 eV compared to that in P25-SP. This increased ∆E 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 pTiO2.38 Further discussion of this issue will be provided later with the aid of EIS analysis.


24

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(A)

a b c d e f

0.5 ps 1 ps 5 ps 10 ps 100 ps 1000 ps

PbS/ZrO2

6

a

4

b

4 c d

2 d c

0 a

e

0

-2

f

400

500

600

700

400

500

Wavelength (nm)

600

700

Wavelength (nm)

(D)

2.0

b

1.0

0.1 ps 0.25 ps 0.5 ps 1 ps

-3

1.0

a b c d

0.5

c

0.0

d

1.0

a

Normalized ∆ A

PbS/p-TiO2

a

Normalized ∆ A

1.5

0.1 ps 0.25 ps 0.5 ps 1 ps

-3

6

2

(C)

a b c d

PbS/P25

b

∆A (x 10 )

∆A (x 10 )

-3

(B)

10 8

∆ A (x 10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6

0.8

a

PbS/ZrO2

b c

PbS/P25 PbS/p-TiO2

0.6

b, c

0.4 0.2 0.0

a

0.4 0.2

-1

0

1

Time (ps)

b, c

2

3

a

PbS/ZrO2

b c

PbS/P25 PbS/p-TiO2

-0.5 0.0 -1.0 400

500

600

700

0

Wavelength (nm)

20

40

60

Time (ps)

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 pTiO2 (PbS/p-TiO2). (D) Normalized absorption difference (∆A)−time profiles of PbS/ZrO2, PbS/P25, and PbS/p-TiO2 recorded at 420 nm.

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 light-harvesting 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


25


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Page 26 of 45

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 absorption feature recorded in the visible region. In order 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 coworkers, the electron injection from PbS to TiO2 is ultrafast with 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.


26

Page 27 of 45

P25-SP p-TiO2-SP

10-2

10-3

10-4

(B)10-1 Cµ (F . cm-2)

Cµ ( F . cm-2)

(A)10-1

0.2

0.3

0.4

0.5

10-3

10-4 0.15 0.30 0.45 0.60 0.75

0.6

Vecb (V)

(D)

(C) 104

P25-SP p-TiO2-SP

3

1

10

τn ( s)

10

P25-SP p-TiO2-SP

10-2

VF (V)

Rr (Ω . cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


P25-SP p-TiO2-SP

100

102

101 0.15 0.30 0.45 0.60 0.75

Vecb (V)

10-1 0.15 0.30 0.45 0.60 0.75

Vecb (V)

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

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 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 (ܸி = ܸ௔௣௣௟ −


27


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Page 28 of 45

ܸௌ − ܸ஼ா , 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 ܸி = (‫ܧ‬ி௡ − ‫ܧ‬ி଴ )/‫ݍ‬, 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 equation 1.5,64,65 ‫ܥ‬ఓ ∝ exp ቀ−

ா಴ಳ ିாಷ೙ ௞ಳ ்

ቁ

(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 pTiO2-SP-based solar cells is lower than that in the P25-SP-based solar cells. This difference of the ECB position (∆ECB) 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 ∆ECB was removed to obtain the equivalent conduction band condition (Vecb).5,64-67 The p-TiO2-SP showed a better Rr compared to P25-SP (Figure 8C). 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


28

Page 29 of 45

lifetime (τn), which is the product of Rr and Cµ (߬௡ = ‫ܥ‬ఓ ܴ௥ ), 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-TiO2-SP. 101

Electron Lifetime τ (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

P25-SP P25-SP Fit p-TiO2 p-TiO2-Fit

10-1

10-2 -0.5

-0.4

-0.3

-0.2

-0.1

Voltage (V)

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

Table 2. Parameters Extracted from Fitting the OCVD Model.a Parameter

P25-SP

p-TiO2-SP

Nb (cm-3)

1.69 × 1019

5.96 × 1019

Ns (cm-3)

1.89 × 1018

5.29 × 1017

T0 (K)

645

774

T1 (K)

553

484

λ (eV)

0.470

0.485

Ec-Eredox (eV)

0.643

0.578

Acb (s-1)b

7.10 × 105

7.45 × 105

Ast (s-1)b

8.30 × 104

5.05 × 104

kcb (cm3 s-1)c

4.72 × 10-15

4.95 × 10-15

kst (cm3 s-1)c

5.52 × 10-16

3.35 × 10-16


29


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a

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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. b Acb = kcb cox and Ast = kst cox, where cox is the concentration of oxidized species in electrolyte.

c

Calculated

with cox = 1.51 × 1020 (0.25 M).

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 coworkers42,69 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. ஼ ೎ ା஼ ್ ା஼ ೞ

߬௡ = ஼ ೎ ఛషభഋ ା஼ഋೞ ௘ ೞ ഋ(ா ഋ ೎

ഋ ೚ೣ

ಷ)

(2)

Here, ‫ܥ‬ఓ௖ , ‫ܥ‬ఓ௕ , and ‫ܥ‬ఓ௦ are the chemical capacitances due to conduction band states, bulk trap states, and surface trap states, respectively, ߬௖ is the electron lifetime due to the electron transfer ௦ from conduction band states, and ݁௢௫ (‫ܧ‬ி ) is the transition probability of electrons from surface


30

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traps that are below the quasi-Fermi level. The mathematical and physical details of the various parameters in Equation 2 are provided in the Supporting Information (Equations S1-S9) and elsewhere.69 The parameters extracted by fitting Equation 2 to the electron lifetime extracted from the OCVD data are provided in Table 2. It was found that the bulk trap density (Ns) in pTiO2 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-TiO2-SP-based QDSSCs. CONCLUSION AND OUTLOOK Given complex multi-step 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 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 has been proposed as one of potential approaches 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 PbScoated 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 PbS-coated TiO2 nanopowder feasible for heating in air. Using two solar paints


31


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based on a commercial Degussa P25 TiO2 and a p-type 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 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% (Figure S14-S16). Despite such advance, the paint’s efficiency still lags behind that of the state-of-the-art PbS-based QDSSCs (5.73%).36 Note that the comparison with solidstate 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 lowly 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,


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hence the performance of PbS-based solar cells are primarily governed by charge recombination at 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 p-SILAR 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 the 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 we demonstrated in Figures S8-S9. The simple ZnS post-coating, 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, other two 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 towards 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 Supporting Information. The 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. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(JHB) E-mail: [email protected] *(TJP) E-mail: [email protected] Notes The authors declare no competing financial interest. Present Addresses &

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.


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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, NRF-2015R1A5A1037548, and 20080061891). PVK 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.

REFERENCES

(1) Lee, Y. L.; Lo, Y. S. Highly Efficient Quantum-Dot-Sensitized Solar Cell Based on CoSensitization of CdS/CdSe. Adv. Funct. Mater. 2009, 19, 604-609. (2) Luo, J.; Wei, H.; Li, F.; Huang, Q.; Li, D.; Luo, Y.; Meng, Q. Microwave Assisted Aqueous Synthesis of Core–Shell CdSexTe1−X–CdS Quantum Dots for High Performance Sensitized Solar Cells. Chem. Commun. 2014, 50, 3464-3466. (3) Jiao, S.; Shen, Q.; Mora-Seró, I.; Wang, J.; Pan, Z.; Zhao, K.; Kuga, Y.; Zhong, X.; Bisquert, J. Band Engineering in Core/Shell ZnTe/CdSe for Photovoltage and Efficiency Enhancement in Exciplex Quantum Dot Sensitized Solar Cells. ACS Nano 2015, 9, 908-915. (4) Wang, J.; Mora-Seró, I.; Pan, Z.; Zhao, K.; Zhang, H.; Feng, Y.; Yang, G.; Zhong, X.; Bisquert, J. Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient Quantum-Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2013, 135, 15913-15922.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 45

(5) Abbas, M. A.; Basit, M. A.; Park, T. J.; Bang, J. H. Enhanced Performance of PbS-Sensitized Solar Cells via Controlled Successive Ionic-Layer Adsorption and Reaction. Phys. Chem. Chem. Phys. 2015, 17, 9752-9760. (6) McDaniel, H.; Fuke, N.; Makarov, N. S.; Pietryga, J. M.; Klimov, V. I. An Integrated Approach to Realizing High-Performance Liquid-Junction Quantum Dot Sensitized Solar Cells. Nat. Commun. 2013, 4, 2887. (7) Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSexTe1–X Alloyed Quantum Dot Sensitized Solar Cells with More Than 6% Efficiency and High Stability. ACS Nano 2013, 7, 5215-5222. (8) Tian, J.; Lv, L.; Fei, C.; Wang, Y.; Liu, X.; Cao, G. A Highly Efficient (>6%) Cd1−XMnxSe Quantum Dot Sensitized Solar Cell. J. Mater. Chem. A 2014, 2, 19653-19659. (9) Panthani, M. G.; Stolle, C. J.; Reid, D. K.; Rhee, D. J.; Harvey, T. B.; Akhavan, V. A.; Yu, Y.; Korgel, B. A. Cuinse2 Quantum Dot Solar Cells with High Open-Circuit Voltage. J. Phys. Chem. Lett. 2013, 4, 2030-2034. (10) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High Efficiency of CdSe Quantum-DotSensitized TiO2 Inverse Opal Solar Cells. Appl. Phys. Lett. 2007, 91. (11) Li, L.; Yang, X.; Gao, J.; Tian, H.; Zhao, J.; Hagfeldt, A.; Sun, L. Highly Efficient CdS Quantum Dot-Sensitized Solar Cells Based on a Modified Polysulfide Electrolyte. J. Am. Chem. Soc. 2011, 133, 8458-8460. (12) Basit, M. A.; Abbas, M. A.; Jung, E. S.; Park, Y. M.; Bang, J. H.; Park, T. J. Strategic PbS Quantum Dot-Based Multilayered Photoanodes for High Efficiency Quantum Dot-Sensitized Solar Cells. Electrochim. Acta 2016, 211, 644-651.


36

Page 37 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(13) Bang, J. H.; Kamat, P. V. Quantum Dot Sensitized Solar Cells. A Tale of Two Semiconductor Nanocrystals: CdSe and Cdte. ACS Nano 2009, 3, 1467-1476. (14) Kim, J. Y.; Yang, J.; Yu, J. H.; Baek, W.; Lee, C. H.; Son, H. J.; Hyeon, T.; Ko, M. J. Highly Efficient Copper–Indium–Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers. ACS Nano 2015, 9, 11286-11295. (15) Sahasrabudhe, A.; Bhattacharyya, S. Dual Sensitization Strategy for High-Performance Core/Shell/Quasi-Shell Quantum Dot Solar Cells. Chem. Mater. 2015, 27, 4848-4859. (16) Ren, Z. W.; Wang, J.; Pan, Z. X.; Zhao, K.; Zhang, H.; Li, Y.; Zhao, Y. X.; Mora-Seró, I.; Bisquert, J.; Zhong, X. H. Amorphous TiO2 Buffer Layer Boosts Efficiency of Quantum Dot Sensitized Solar Cells to over 9%. Chem. Mater. 2015, 27, 8398-8405. (17) Yang, J. W.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. H. CdSeTe/CdS Type-I Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9%. J. Phys. Chem. C 2015, 119, 28800-28808. (18) Zhao, K.; Pan, Z.; Mora-Seró, I.; Canovas, E.; Wang, H.; Song, Y.; Gong, X.; Wang, J.; Bonn, M.; Bisquert, J. et al. Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602-5609. (19) Du, J.; Du, Z.; Hu, J. S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X. et al. Zn–Cu–In–Se Quantum Dot Solar Cells with a Certified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201-4209. (20) Ren, Z. W.; Wang, Z. Q.; Wang, R.; Pan, Z. X.; Gong, X. Q.; Zhong, X. H. Effects of Metal Oxyhydroxide Coatings on Photoanode in Quantum Dot Sensitized Solar Cells. Chem. Mater. 2016, 28, 2323-2330.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 45

(21) Zhao, K.; Pan, Z.; Zhong, X. Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2016, 7, 406-417. (22) Genovese, M. P.; Lightcap, I. V.; Kamat, P. V. Sun-Believable Solar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells. ACS Nano 2012, 6, 865-872. (23) Zhang, X.; Sun, H. X.; Tao, X. Y.; Zhou, X. F. TiO2@CdSe/CdS Core-Shell Hollow Nanospheres Solar Paint. RSC Adv. 2014, 4, 31313-31317. (24) Lan, X.; Voznyy, O.; García de Arquer, F. P.; Liu, M.; Xu, J.; Proppe, A. H.; Walters, G.; Fan, F.; Tan, H.; Liu, M. et al. 10.6% Certified Colloidal Quantum Dot Solar Cells Via SolventPolarity-Engineered Halide Passivation. Nano Lett. 2016, 16, 4630-4634. (25) Lan, X.; Masala, S.; Sargent, E. H. Charge-Extraction Strategies for Colloidal Quantum Dot Photovoltaics. Nat. Mater. 2014, 13, 233-240. (26) Koleilat, G. I.; Levina, L.; Shukla, H.; Myrskog, S. H.; Hinds, S.; Pattantyus-Abraham, A. G.; Sargent, E. H. Efficient, Stable Infrared Photovoltaics Based on Solution-Cast Colloidal Quantum Dots. ACS Nano 2008, 2, 833-840. (27) Lee, J.-S.; Kovalenko, M. V.; Huang, J.; Chung, D. S.; Talapin, D. V. Band-Like Transport, High Electron Mobility and High Photoconductivity in All-Inorganic Nanocrystal Arrays. Nat. Nanotechnol. 2011, 6, 348-352. (28) Kovalenko, M. V.; Scheele, M.; Talapin, D. V. Colloidal Nanocrystals with Molecular Metal Chalcogenide Surface Ligands. Science 2009, 324, 1417-1420. (29) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D. et al. Colloidal-Quantum-Dot Photovoltaics Using AtomicLigand Passivation. Nat. Mater. 2011, 10, 765-771.


38

Page 39 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30) Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Grätzel, M. et al. DepletedHeterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374-3380. (31) Xu, Z.; Li, T.; Zhang, F.; Hong, X.; Xie, S.; Ye, M.; Guo, W.; Liu, X. Highly Flexible, Transparent and Conducting CuS-Nanosheet Networks for Flexible Quantum-Dot Solar Cells. Nanoscale 2017, 9, 3826-3833. (32) Ke, W.; Fang, G.; Lei, H.; Qin, P.; Tao, H.; Zeng, W.; Wang, J.; Zhao, X. An Efficient and Transparent Copper Sulfide Nanosheet Film Counter Electrode for Bifacial Quantum DotSensitized Solar Cells. J. Power Sources 2014, 248, 809-815. (33) Kalanur, S. S.; Chae, S. Y.; Joo, O. S. Transparent Cu1.8S and CuS Thin Films on FTO as Efficient Counter Electrode for Quantum Dot Solar Cells. Electrochim. Acta 2013, 103, 91-95. (34) Gates, B. A Big Win for Cheap, Clean Energy. https://www.gatesnotes.com/Energy/Investing-in-Energy-Innovation (accessed 21-Sep-2.16). (35) Gebel, E. Solar Cells From a Paintbrush, Chemical & Engineering News. http://cen.acs.org/articles/89/web/2011/12/Solar-Cells-Paintbrush.html (accessed 21-Sep-2016). (36) Sung, S. D.; Lim, I.; Kang, P.; Lee, C.; Lee, W. I. Design and Development of Highly Efficient PbS Quantum Dot-Sensitized Solar Sells Sorking in an Aqueous Polysulfide Electrolyte. Chem. Commun. 2013, 49, 6054-6056. (37) Klem, E. J. D.; MacNeil, D. D.; Levina, L.; Sargent, E. H. Solution Processed Photovoltaic Devices with 2% Infrared Monochromatic Power Conversion Efficiency: Performance Optimization and Oxide Formation. Adv. Mater. 2008, 20, 3433-3439.


39


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 45

(38) Wang, S.; Pan, L.; Song, J. J.; Mi, W.; Zou, J. J.; Wang, L.; Zhang, X. Titanium-Defected Undoped Anatase TiO2 with p-Type Conductivity, Room-Temperature Ferromagnetism, and Remarkable Photocatalytic Performance. J. Am. Chem. Soc. 2015, 137, 2975-2983. (39) Ito, S.; Chen, P.; Comte, P.; Nazeeruddin, M.; Liska, P.; Péchy, P.; Grätzel, M. Fabrication of Screen‐Printing Pastes from TiO2 Powders for Dye‐Sensitised Solar Cells. Prog. Photovoltaics 2007, 15, 603-612. (40) Choi, H. M.; Ji, I. A.; Bang, J. H. Metal Selenides as a New Class of Electrocatalysts for Quantum Dot-Sensitized Solar Cells: A Tale of Cu1.8Se and PbSe. ACS Appl. Mater. Interfaces 2014, 6, 2335-2343. (41) Kim, C. S.; Choi, S. H.; Bang, J. H. New Insight into Copper Sulfide Electrocatalysts for Quantum Dot-Sensitized Solar Cells: Composition-Dependent Electrocatalytic Activity and Stability. ACS Appl. Mater. Interfaces 2014, 6, 22078-22087. (42) Kim, S. A.; Abbas, M. A.; Lee, L.; Kang, B.; Kim, H.; Bang, J. H. Control of Morphology and Defect Density in Zinc Oxide for Improved Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2016, 18, 30475-30483. (43) Yun, T. K.; Park, S. S.; Kim, D.; Shim, J. H.; Bae, J. Y.; Huh, S.; Won, Y. S. Effect of the Rutile Content on the Photovoltaic Performance of the Dye-Sensitized Solar Cells Composed of Mixed-Phase TiO2 Photoelectrodes. Dalton Trans. 2012, 41, 1284-1288. (44) Zhao, H.; Wang, D.; Zhang, T.; Chaker, M.; Ma, D. Two-Step Synthesis of High-Quality Water-Soluble near-Infrared Emitting Quantum Dots via Amphiphilic Polymers. Chem. Commun. 2010, 46, 5301-5303.


40

Page 41 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(45) Kanai, H.; Yoshiki, M.; Hayashi, M.; Kuwae, R.; Yamashita, Y. Grain‐Boundary‐Phase Identification of a Lead‐Based Relaxor by X‐Ray Photoelectron Spectroscopy. J. Am. Ceram. Soc. 1994, 77, 2229-2231. (46) Leiro, J. A.; Laajalehto, K.; Kartio, I.; Heinonen, M. H. Surface Core-Level Shift and Phonon Broadening in PbS(100). Surf. Sci. 1998, 412–413, L918-L923. (47) Zingg, D. S.; Hercules, D. M. Electron Spectroscopy for Chemical Analysis Studies of Lead Sulfide Oxidation. J. Phys. Chem. 1978, 82, 1992-1995. (48) Yashina, L. V.; Zyubin, A. S.; Püttner, R.; Zyubina, T. S.; Neudachina, V. S.; Stojanov, P.; Riley, J.; Dedyulin, S. N.; Brzhezinskaya, M. M.; Shtanov, V. I. The Oxidation of the PbS(001) Surface with O2 and Air Studied with Photoelectron Spectroscopy and Ab Initio Modeling. Surf. Sci. 2011, 605, 473-482. (49) Cant, D. J. H.; Syres, K. L.; Lunt, P. J. B.; Radtke, H.; Treacy, J.; Thomas, P. J.; Lewis, E. A.; Haigh, S. J.; O’Brien, P.; Schulte, K. et al. Surface Properties of Nanocrystalline PbS Films Deposited at the Water–Oil Interface: A Study of Atmospheric Aging. Langmuir 2015, 31, 14451453. (50) Moreels, I.; Lambert, K.; Smeets, D.; De Muynck, D.; Nollet, T.; Martins, J. C.; Vanhaecke, F.; Vantomme, A.; Delerue, C.; Allan, G. et al. Size-Dependent Optical Properties of Colloidal PbS Quantum Dots. ACS Nano 2009, 3, 3023-3030. (51) Xu, C.; Wu, J.; Desai, U. V.; Gao, D. Multilayer Assembly of Nanowire Arrays for DyeSensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 8122-8125. (52) Hyun, B. R.; Zhong, Y. W.; Bartnik, A. C.; Sun, L.; Abruña, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M.; Borrelli, N. F. Electron Injection from Colloidal PbS Quantum Dots into Titanium Dioxide Nanoparticles. ACS Nano 2008, 2, 2206-2212.


41


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 45

(53) Konstantatos, G.; Sargent, E. H. Colloidal Quantum Dot Optoelectronics and Photovoltaics; Cambridge University Press: New York, 2013. (54) van Sark, W. G. J. H. M.; Frederix, P. L. T. M.; Van den Heuvel, D. J.; Gerritsen, H. C.; Bol, A. A.; van Lingen, J. N. J.; de Mello Donegá, C.; Meijerink, A. Photooxidation and Photobleaching of Single CdSe/ZnS Quantum Dots Probed by Room-Temperature TimeResolved Spectroscopy. J. Phys. Chem. B 2001, 105, 8281-8284. (55) Tavakoli, M. M.; Mirfasih, M. H.; Hasanzadeh, S.; Aashuri, H.; Simchi, A. Surface Passivation of Lead Sulfide Nanocrystals with Low Electron Affinity Metals: Photoluminescence and Photovoltaic Performance. Phys. Chem. Chem. Phys. 2016, 18, 12086-12092. (56) Berera, R.; van Grondelle, R.; Kennis, J. T. M. Ultrafast Transient Absorption Spectroscopy: Principles and Application to Photosynthetic Systems. Photosynth. Res. 2009, 101, 105-118. (57) Yang, Y.; Rodriguez-Cordoba, W.; Xiang, X.; Lian, T. Strong Electronic Coupling and Ultrafast Electron Transfer between PbS Quantum Dots and TiO2 Nanocrystalline Films. Nano Lett. 2012, 12, 303-309. (58) Yang, Y.; Rodríguez-Córdoba, W.; Lian, T. Ultrafast Charge Separation and Recombination Dynamics in Lead Sulfide Quantum Dot–Methylene Blue Complexes Probed by Electron and Hole Intraband Transitions. J. Am. Chem. Soc. 2011, 133, 9246-9249. (59) Concina, I.; Manzoni, C.; Grancini, G.; Celikin, M.; Soudi, A.; Rosei, F.; Zavelani-Rossi, M.; Cerullo, G.; Vomiero, A. Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells. J. Phys. Chem. Lett. 2015, 6, 2489-2495. (60) González-Pedro, V.; Sima, C.; Marzari, G.; Boix, P. P.; Giménez, S.; Shen, Q.; Dittrich, T.; Mora-Seró, I. High Performance PbS Quantum Dot Sensitized Solar Cells Exceeding 4%


42

Page 43 of 45

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Efficiency: The Role of Metal Precursors in the Electron Injection and Charge Separation. Phys. Chem. Chem. Phys. 2013, 15, 13835-13843. (61) Bang, J. H.; Kamat, P. V. CdSe Quantum Dot–Fullerene Hybrid Nanocomposite for Solar Energy Conversion: Electron Transfer and Photoelectrochemistry. ACS Nano 2011, 5, 94219427. (62) Bang, J. H. Influence of Nanoporous Oxide Substrate on the Performance of Photoelectrode in Semiconductor-Sensitized Solar Cells. Bull. Korean Chem. Soc. 2012, 33, 4063-4068. (63) Bisquert, J. Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells. Phys. Chem. Chem. Phys. 2003, 5, 5360-5364. (64) Abbas, M. A.; Kim, T. Y.; Lee, S. U.; Kang, Y. S.; Bang, J. H. Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance. J. Am. Chem. Soc. 2016, 138, 390-401. (65) González-Pedro, V.; Xu, X.; Mora-Seró, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783-5790. (66) Barea, E. M.; Shalom, M.; Gimenez, S.; Hod, I.; Mora-Seró, I.; Zaban, A.; Bisquert, J. Design of Injection and Recombination in Quantum Dot Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 6834-6839. (67) Braga, A.; Gimenez, S.; Concina, I.; Vomiero, A.; Mora-Seró, I. Panchromatic Sensitized Solar Cells Based on Metal Sulfide Quantum Dots Grown Directly on Nanostructured TiO2 Electrodes. J. Phys. Chem. Lett. 2011, 2, 454-460. (68) Basit, M. A.; Abbas, M. A.; Bang, J. H.; Park, T. J. Efficacy of In2S3 Interfacial Recombination Barrier Layer in PbS Quantum-Dot-Sensitized Solar Cells. J. Alloy. Compd. 2015, 653, 228-233.


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(69) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Seró, I. Determination of Rate Constants for Charge Transfer and the Distribution of Semiconductor and Electrolyte Electronic Energy Levels in Dye-Sensitized Solar Cells by Open-Circuit Photovoltage Decay Method. J. Am. Chem. Soc. 2004, 126, 13550-13559. (70) Bubenhofer, S. B.; Schumacher, C. M.; Koehler, F. M.; Luechinger, N. A.; Grass, R. N.; Stark, W. J. Large-Scale Synthesis of PbS–TiO2 Heterojunction Nanoparticles in a Single Step for Solar Cell Application. J. Phys. Chem. C 2012, 116, 16264-16270. (71) Yang, Y.; Wang, W. A New Polymer Electrolyte for Solid-State Quantum Dot Sensitized Solar Cells. J. Power Sources 2015, 285, 70-75. (72) Duan, J.; Tang, Q.; Sun, Y.; He, B.; Chen, H. Solid-State Electrolytes from Polysulfide Integrated Polyvinylpyrrolidone for Quantum Dot-Sensitized Solar Cells. RSC Adv. 2014, 4, 60478-60483. (73) Feng, W.; Zhao, L.; Du, J.; Li, Y.; Zhong, X. Quasi-Solid-State Quantum Dot Sensitized Solar Cells with Power Conversion Efficiency over 9% and High Stability. J. Mater. Chem. A 2016, 4, 14849-14856. (74) Duan, J.; Tang, Q.; He, B.; Chen, H. All-Solid-State Quantum Dot-Sensitized Solar Cell from Plastic Crystal Electrolyte. RSC Adv. 2015, 5, 33463-33467.


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