Passivation of PbS Quantum Dot Surface with l ... - ACS Publications

Subscriber access provided by FLORIDA ATLANTIC UNIV

Article

Passivation of PbS Quantum Dot Surface with L-glutathione in Solid-State Quantum-Dot-Sensitized Solar Cells Askhat N. Jumabekov, Niklas Cordes, Timothy D Siegler, Pablo Docampo, Alesja Ivanova, Ksenia Fominykh, Dana D Medina, Laurence M Peter, and Thomas Bein ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10953 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on February 14, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

ACS Applied Materials & Interfaces

Passivation of PbS Quantum Dot Surface with L-glutathione in Solid-State Quantum-Dot-Sensitized Solar Cells

Askhat N. Jumabekov,a Niklas Cordes,a Timothy D. Siegler,a,b Pablo Docampo,a Alesja Ivanova,a Ksenia Fominykh,a Dana D. Medina,a Laurence M. Peterc and Thomas Beina,*

a

Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-University

Munich (LMU), 81377 Munich, Germany b

Department of Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame,

Indiana 46556, United States c

Department of Chemistry, University of Bath, Bath, BA2 7AY, U.K.

ABSTRACT: Surface oxidation of quantum dots (QDs) is one of the biggest challenges in quantum dotsensitized solar cells (QDSCs), since it introduces surface states that enhance electron-hole recombination and degrade device performance. Protection of QDs from surface oxidation by passivating the surface with organic or inorganic layers can be one way to overcome this issue. In this study, solid-state QDSCs with a PbS QD absorber layer were prepared from thin mesoporous TiO2 layers by the successive ionic layer adsorption/reaction (SILAR) method. Spiro-OMeTAD was used as the organic p-type hole transporting material (HTM). The effects on the solar cell performance of passivating the surface of the PbS QDs with the tripeptide Lglutathione (GSH) were investigated. Current-voltage characteristics and external quantum efficiency measurements of the solar cell devices showed that GSH-treatment of the QDsensitized TiO2 electrodes more than doubled the short circuit current and conversion efficiency. Impedance spectroscopy, intensity-modulated photovoltage and photocurrent spectroscopy 1 ACS Paragon Plus Environment


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 2 of 28

analysis of the devices revealed that the enhancement in solar cell performance of the GSHtreated cells originates from improved charge injection from PbS QDs into the conduction band of TiO2. Time-resolved photoluminescence decay measurements show that passivation of the surface of QDs with GSH ligands increases the exciton lifetime in the QDs.

KEYWORDS: Lead sulfide, quantum dots, surface passivation, solar cells, impedance spectroscopy.

1. INTRODUCTION Interest in quantum dot-sensitized solar cells (QDSCs) - particularly those sensitized with PbS quantum dots (QDs) and utilizing both liquid and solid hole mediators - has increased substantially over the past few years due to the size-dependent tunable band gap, high absorption coefficient and the possibility to generate and split multiple excitons.1-7 The AM 1.5 power conversion efficiencies of QDSCs have reached values as high as 8.2% with CdSexTe1-x QDs and 9.2% for planar solid-state TiO2/PbS bilayer colloidal quantum dot (CQD) devices.8-10 However, these values are still significantly smaller than the efficiencies of dye-sensitized solar cells (DSCs).11 One of the major performance-limiting factors in QDSCs compared to DSCs is the presence of sub-bandgap states in QDs (Fig. 1) originating from defect sites and/or surface states formed by surface oxidation or corrosion.12, 13 These sub-bandgap states enhance charge trapping (processes 2 and 3 in Fig. 1) and lead to nonradiative recombination (process 4) of the electronhole pair in the QD. The rate constant of nonradiative recombination is around ~109 s-1, whereas for the radiative recombination (process 5 in Fig. 1) it is around ~107-109 s-1.14,

15

The

nonradiative and radiative recombination of the electron-hole pair in the QD is often referred to as “geminate recombination”, and this process competes with charge injection and separation events in QDSCs (processes 1 and 7). Usually, process 1 is rather fast with transfer rate constants in the range ~1010-1011 s-1.16-20 However, hole transfer from QDs into the electrolyte (process 7) 2 ACS Paragon Plus Environment

Page 3 of 28

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


is rather slow, with transfer rate constants of ~107-108 s-1. This process can be slowed down by hole trapping (process 2).15,

21, 22

Additionally, QD sub-bandgap states act as centers for

recombination (processes 6 and 8), which competes with hole transfer to the electrolyte (process 7).23, 24 Process 6, a nonradiative recombination path from the conduction band or surface states of QD to the redox mediator/hole transport layer, is faster than process 8 and is electrolytedependent.23 Back electron transfer of electrons from the conduction band of TiO2 to the electrolyte (process 9) is a much slower process, and it will not be influenced by the presence of sub-bandgap states in the QDs.20, 23 Overall, sub-bandgap states in QDs have a negative effect on charge injection and separation processes and cause a significant deterioration of device performance.

Figure 1. Schematic illustration of photoinduced charge-transfer processes following absorption of quanta of light by PbS QD on TiO2.

The surface treatment of QDs with different inorganic and organic agents has been shown to passivate the surface of QDs and effectively protect them from oxidation.13, 25, 26 For instance, Tachan et al.26 showed that the deposition of a thin MgO layer onto QD-sensitized porous TiO2 electrodes helps to increase the short circuit current and the open circuit voltage. In their report, the authors stress the importance of tuning the interface between the QDs and the metal oxide 3 ACS Paragon Plus Environment


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 4 of 28

(MO) in order to improve efficiencies.26 Recently, Zhao et al.8 showed that coating the surface of QDs with a double ZnS/SiO2 inorganic layer substantially suppresses recombination losses and boosts PCE of the cells beyond 8%. Another recent study reported by Abdellah et al.15 demonstrated that formation of a inorganic shell layer around QDs can inhibit the hole trapping process and can result in improved solar cell characteristics. Related studies were also reported by de la Fuente et al.27; the authors explored the effect of a range of surface-adsorbed species including

dimethylamine,

ethanedithiol,

hexadecyl

trimethylammonium

chloride

and

tetrabutylammonium iodide on the performance of QDSCs with polysulfide electrolyte. It is clear that a better understanding of the surface properties of QDs and of tuning the interfaces between QDs, electron acceptor and electron donor materials is crucial for the fabrication of efficient QDSCs. Nevertheless, most of the surface passivation studies on QDSCs were performed with liquid polysulfide electrolyte based solar cells, and not many studies have been conducted with cells employing solid hole conductors.26-29 One of the few studies on surface passivation

in

solid-state

QDSCs

with

spiro-OMeTAD

(tetrakis(N,N-di-4-

methoxyphenylamino)-9,9′-spirobifluorene) hole transporting material (HTM) was reported by Brennan et al.30, in which the authors employed an atomic layer deposition (ALD) technique to grow an ultrathin alumina barrier layer either between mesoporous TiO2 and PbS QDs (TiO2/Al2O3/PbS) or between TiO2/PbS and Spiro-OMeTAD HTM (TiO2/PbS/Al2O3). The authors reported a factor of two improvement in device performance for the cells with TiO2/Al2O3/PbS electrodes.

The L-glutathione (GSH) ligand is particularly interesting for passivating the surface of PbS QDsensitized TiO2 in solid-state QDSCs because it is able to coordinate both with the PbS QDs via the SH group and with the TiO2 surface via the COOH groups. Hence, it can provide passivation of the surface of the QDs and protection from oxidation, as well as a barrier layer between TiO2 and HTM.31, 32 In our previous work, we have shown that GSH-protected pre-synthesized PbS QDs can be used to fabricate liquid electrolyte-based as well as solid-state QDSCs.31, 32 Here we 4 ACS Paragon Plus Environment

Page 5 of 28

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


report an investigation of the successful application of L-glutathione (GSH) as a passivation agent for in situ synthesized PbS QDs in solid-state QDSCs incorporating mesoporous TiO2 electron transport and spiro-OMeTAD hole transport layers. We found that the treatment of PbS QD-sensitized mesoporous TiO2 electrodes with GSH significantly improves the short circuit current, thus, resulting in higher efficiencies for the GSH-treated cells compared to non-treated reference cells. The solid-state QDSCs were characterized with current-voltage (I-V), external quantum efficiency (EQE) and impedance spectroscopy (IS) measurements. Electron lifetimes were measured by open circuit photovoltage decay (OCVD), intensity-modulated photovoltage spectroscopy (IMVS) and IS measurements. The charge collection efficiencies of the devices were estimated from IMVS and intensity-modulated photocurrent spectroscopy (IMPS) measurements. The recombination of electron-hole pairs in bare and GSH-protected PbS QDs was studied with time-resolved photoluminescence (PL) decay measurements.

2. EXPERIMENTAL SECTION Fabrication of Solid-state QDSCs. Fluorine-doped tin oxide glass substrates (FTO, Pilkington, TEC7) were cut to 1.5×2 cm2. The FTO substrates were patterned by selectively etching the FTO layer and were coated with a compact TiO2 blocking layer (BL) via spray pyrolysis. The mesoporous TiO2 electrodes with 3±0.1 µm thickness were prepared by spin-coating TiO2-paste with around 30 nm sized TiO2 nanocrystallites onto FTO substrates with freshly made BLs and calcining at 450 °C. The mesoporous TiO2 electrodes were then sensitized with PbS QDs by the successive ion layer adsorption/reaction (SILAR) process and one half of the QD-sensitized TiO2 electrodes were treated with L-glutathione (GSH). Finally, the fabrication of solar cells was completed by spin-coating the spiro-OMeTAD solution onto the sensitized mesoporous TiO2 electrodes with and without GSH-treatment and depositing 150 nm thick silver contacts onto the

5 ACS Paragon Plus Environment


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 6 of 28

electrodes via evaporation through a mask. The detailed description of the experiments and solar cell fabrication is given in the Supporting Information.

Characterization Techniques. The absorbance spectra of the PbS QD-sensitized TiO2 electrodes were measured with a UV-Vis-NIR spectrophotometer in transmission mode (Lambda 1050, PerkinElmer). The structure and phase of the mesoporous TiO2 films were characterized by wide angle X-ray diffraction (XRD) using a Bruker D8 Discover diffractometer with Cu-Kα radiation (λ = 1.54 Å) and equipped with a LynxEye position sensitive detector. The thickness of the mesoporous TiO2 films was measured with a Dektak150 profilometer. The cross-section of the mesoporous TiO2 films was examined using a JEOL JSM-6500F scanning electron microscope (SEM) with a field emission gun operating at 0.5−30 kV. The I-V characteristics of the solid-state QDSCs were measured under simulated solar light using a 300 W xenon lamp with an AM 1.5 irradiance of 100 mW cm-2 (XPS 400, Solar Light Company Inc.). The I-V characteristics were recorded using a Keithley 2400 source meter with 0.11 cm2 active cell area. The EQE spectra of the cells were measured using chopped monochromatic light provided by a 150 W xenon lamp (Lot-Oriel) and monochromator (Micro HR) with order sorting filters (Thorlabs). Measurements were made using bias light with AM 1.5 irradiance of 11.5 mW cm-2, and the signal was detected by a DSP lock-in amplifier (7230, Signal Recovery). The EQE data were recorded using the Keithley 2400 source meter. The OCVD, IMVS/IMPS and IS measurements were carried out using a potentiostat equipped with a frequency response analyzer (PGSTAT302N, Autolab, Metrohm) and LED driver. The measurements were carried out under cool white LED (LDCCW, Metrohm) irradiation at open-circuit conditions for IMVS and IS and short circuit conditions for IMPS over the frequency range from 10 kHz to 1 mHz and 1MHz to 1 mHz, respectively. In IMVS/IMPS measurements the amplitude of the AC modulation current was 10% of the DC current applied to the LED and in IS measurements the amplitude of the AC modulation voltage was 10% of open circuit potential at which IS measurements were performed. Time-resolved PL measurements were performed with a Fluotime 300 6 ACS Paragon Plus Environment

Page 7 of 28

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


Spectrofluorometer (Picoquant). A green laser with a 508 nm wavelength and ~3 µJ cm-2 pump fluence at 5 MHz repetition rate was used for the excitation. The emission for the time resolved measurements was monitored at the maximum (at 760 nm) of the steady state photo emission.

3. RESULTS AND DISCUSSION Mesoporous TiO2 anatase electrodes (cf. Figure S1 in the Supporting Information) with 3±0.1 µm thickness (cf. Figure S2 in the Supporting Information) deposited onto fluorine-doped tin oxide glass substrates (FTO, Pilkington, TEC7), pre-coated with a compact TiO2 BL, were sensitized with PbS QDs grown by the SILAR method. 5.5 SILAR cycles were performed to sensitize the electrodes, resulting in PbS QDs with Pb2+ terminated surface deposited onto the walls of mesoporous TiO2. One half of the sensitized mesoporous TiO2/PbS electrodes were then treated with GSH dissolved in ethanol (see Supporting Information). TEM images of TiO2/PbS treated with GSH are shown in Figure S3 in the Supporting Information. It is expected that the GSH-treatment results in the PbS being covered with a monolayer of GSH molecules in which the GSH molecules are attached to PbS QD sites via Pb-S bond formation between Pb2+ surface ions of the QDs and the mercapto group of the GSH molecule.33 At the same time, it can be expected that bare TiO2 sites on the TiO2/PbS electrode will also be covered with a monolayer of GSH, in which GSH molecules are attached to TiO2 via –COO- groups as illustrated in Figure 2a.34 Figure 2b shows the UV-Vis absorbance spectra of mesoporous TiO2/PbS electrodes with (GSH-treated) and without (reference) GSH-treatment. Since the GSH molecules do not absorb in the visible region of the spectrum, the absorbance of the GSH-treated TiO2/PbS electrode does not differ much from the absorbance of the bare TiO2/PbS electrode.



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 8 of 28

Figure 2. (a) Schematic illustration of PbS QD-sensitized TiO2 electrodes with and without GSH-treatment; (b) UV-Vis spectra of PbS QD-sensitized TiO2 electrodes with and without GSH- treatment. 8 ACS Paragon Plus Environment

Page 9 of 28

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


We used the absorbance spectra A(λ) of the reference and GSH-treated TiO2/PbS electrodes to calculate the maximum (theoretical) short circuit currents, JSCtheor, by using the following relationships:19

= Φ = 1 − 10

(1)

(2)

where q is the electron charge, F(λ) is the incident photon flux density, LHE(λ) the light harvesting efficiency, Φinj is the electron injection efficiency and ηc is the electron collection efficiency. Here, Φinj and ηc are assumed to be unity (Φinj=1; ηc=1) for the maximum short circuit condition. Integration was performed over the wavelength range from 400 to 900 nm. Our calculation resulted in JSCtheor values of 13.8 and 15.2 mA cm-2 for the reference and GSH-treated electrodes, respectively. The JSCtheor value for the GSH-treated electrode is slightly higher than the one for the reference electrode due to a minor difference in their absorbance (see Fig. 2b). The I-V curves of solid-state QDSCs were recorded under standard AM 1.5 irradiance of 100 mW cm-2, and results are presented in Figure 3. The I-V measurements show that surface treatment of PbS QD-sensitized mesoporous TiO2 electrodes with GSH significantly increases the short circuit current. The short circuit current of the GSH-treated cells is more than a factor of two higher (3.84 mA cm-2) compared to the reference cell (1.7 mA cm-2). The open circuit voltage of the GSH-treated cell is slightly higher than for the reference cell, which may originate from the barrier effect of GSH layers between TiO2/HTM or/and FTO/HTM. The fill factor for the GSH-treated cell appears to be slightly lower compared to the reference cell for reasons that are unclear. Overall, the performance of the GSH-treated cell was superior to that of the reference cell, resulting in about 1% efficiency at 1 sun AM 1.5. The solar cell parameters of the reference and GSH-treated cells are contrasted in Table 1. Comparison of the experimental values for short circuit current with calculated values, JSCtheor, shows that the performance of 9 ACS Paragon Plus Environment


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 10 of 28

both solar cell devices (reference and GSH-treated cells) is rather moderate, which also suggests that Φinj and ηc of the cells are, in fact, not unity (Φinj≠1; ηc≠1).

Figure 3. I-V characteristics of solid-state QDSCs with and without GSH-treatment.

Table 1. Summary of the solar cell performance of the reference and GSH-treated cells under AM 1.5 at 100 mW cm-2 illumination. Cell

JSCtheor (mA cm-2)

JSC (mA cm-2)

VOC (mV)

FF (%)

(%)

Reference

13.8

1.70

547

50

0.47

GSH-treated

15.2

3.84

557

44.5

0.95


Page 11 of 28

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


The external quantum efficiency (EQE) spectra of the cells were recorded in order to obtain the spectral response of the devices (Fig. 4). The EQE spectra of both cells follow similar trends as the UV-Vis absorbance spectra (Fig. 2b), and the GSH-treated cell has a factor of two higher EQE throughout the entire visible range relative to the reference cell. This indicates either improved injection or collection efficiency of charge carriers in the GSH-treated device since the absorbance spectra of both electrodes are almost the same. The GSH-treated cell gives a maximum EQE of 35% at 435 nm, whereas the reference cell shows only 18% at the same wavelength. The onset of the EQE spectrum of the GSH-treated cell is slightly red shifted compared to the spectrum of the reference cell. This may indicate that the bare TiO2/PbS electrode has slightly smaller sized QDs, possibly as a result of shrinkage due to partial oxidation creating shells of PbSO3 and PbSO4 around the PbS QDs.24, 35 Additionally, GSH-treatment may also facilitate better energy band alignment between the TiO2 and the PbS QDs, enabling electron injection from deeper QD surface states into the TiO2, thus contributing to the increased as well as red-shifted spectral response of the GSH-treated cell compared to the reference cell. The calculation of the AM 1.5 short circuit currents from the EQE spectra resulted in values of 1.62 and 3.72 mA cm-2 for the reference and GSH-treated cells, respectively. These values are in excellent agreement with the data in Table 1.



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 12 of 28

Figure 4. EQE spectra of solid-state QDSCs with and without GSH-treatment. To gain a better understanding of the behavior of the devices, impedance spectroscopy measurements were performed on solid-state QDSCs fabricated with and without a GSHtreatment (Fig. 5). The impedance data were analyzed using ZView software with a model proposed by Dualeh et al..36 In the model, a transmission line (TL) represents the distributed recombination resistance and chemical capacitance. The product Rrec×Cµ corresponds to the so called ‘electron lifetime’, which is determined by the transfer of electrons from the TiO2 to the HTM. Rsub and Csub represent the photoanode, RHTM and constant phase shift element, CPEHTM, represent the spiro-OMeTAD, and Rcathode, CPEcathode and Rs represent the HTM/contact resistance and capacitance and series resistance, respectively (see inset in Fig. 5). The inclusion of Rsub and Csub, which represent the direct contact between the TiO2 and the HTM at the base of the mesoporous layer, had little influence on the overall fitting result, and therefore these elements were removed from the equivalent circuit to simplify the fitting process. Bode plots and 12 ACS Paragon Plus Environment

Page 13 of 28

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


phase angle plots of the experimental impedance responses of the cells together with fitting results are shown in Figure S4 in the Supporting Information.

Figure 5. Example of the impedance response of the GSH-treated cell measured at open circuit voltage of 351 mV, and the equivalent circuit used to fit the experimental impedance spectra of the cells (inset).

The values of chemical capacitance, Cµ, and recombination resistance, Rct, extracted from fitting the experimental impedance data measured at different open circuit voltages, are shown in Fig. 6a and 6b.



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 14 of 28

Figure 6. (a) Chemical capacitance and (b) recombination resistance of solid-state QDSCs with and without GSH-treatment measured at open circuit under different illumination intensities. 14 ACS Paragon Plus Environment

Page 15 of 28

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


Analysis of IS data shows that the chemical capacitance of the reference and GSH-treated cells stays almost the same throughout the measured voltage range, only showing a small difference (a factor of 2 or less) at higher voltages, which suggests that the GSH ligands have little influence on the distribution of electron trap states of the mesoporous TiO2 electrode except at higher voltages.32 The comparison of recombination resistances, on the other hand, shows that at higher voltages the values of Rct for the reference cell become larger than those for GSH-treated cell. The origin of this effect is unclear since one would expect the opposite situation if the GSH ligands form a barrier layer between TiO2 and HTM, thus suppressing recombination of electrons in the conduction band of TiO2 with holes in the HTM. The slightly higher values of Cµ for GSH-treated cells above 300 mV compared to the reference cell could be due to the enhanced accumulation of electrons/holes in passivated QDs, which contributes to the total density of states of the TiO2-QD system.23,

26, 37

This would influence the kinetics of charge carrier

recombination (process 9 in Fig. 1) and might therefore explain the notable difference between Rct for the reference and GSH-treated cells at higher voltages as well (see Fig. 6b).37,

27

The

electron lifetimes, τn, of the cells were calculated from the results of IS measurements by using the relationship τnIS = Rct × Cµ proposed by Bisquert et al.38-42 The voltage dependence of the values for τn for solid-state QDSCs with and without GSH-treatment are shown in Figure 7. We also obtained electron lifetimes of the cells (Fig. 7) using two additional methods; open circuit photovoltage decay (OCVD) and intensity-modulated photovoltage spectroscopy (IMVS) techniques (cf. Figures S5 and S6 in the Supporting Information).43 The electron lifetimes for the reference cell obtained by the IMVS measurements, τnIMVS, agree well with values obtained by IS, τnIS, for the entire measurement range. However, it can be seen that the electron lifetimes obtained by OCVD, τnOCVD, deviate slightly from the values τnIS and τnIMVS at lower voltages (below 200 mV). The electron lifetimes for the GSH-treated cell, on the other hand, show excellent agreement between τnIS, τnIMVS and τnOCVD throughout the entire measurement range. The reason for τn of the GSH-treated cell obtained by OCVD being similar to τnIS and τnIMVS 15 ACS Paragon Plus Environment


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 16 of 28

unlike the reference cell could be due to GSH ligands acting as an additional barrier layer between FTO and HTM, preventing shunting via the substrate.32 At lower voltages in the dark the role of the TiO2 network in the recombination of electrons is negligible and the main recombination pathway would be via the substrate.26 Thus, an additional ligand layer on top of the dense ~60 nm TiO2 layer between FTO and HTM can slow down the recombination rate. This can be seen from the photovoltage decay curve in figure S5 in Supporting Information, which shows slower decay for the GSH-passivated cell at lower voltages.

Figure 7. Voltage dependence of the electron lifetimes of solid-state QDSCs with and without GSH-treatment. It is clear from these results that the electron lifetimes of the reference and GSH-treated cells do not show significant differences, which indicates that the kinetics of the loss of electrons from the TiO2 by transfer to the hole transport layer are similar for both devices. It is important to note at this point that the OCVD, IMVS and IS techniques detect the recombination of electrons in the 16 ACS Paragon Plus Environment

Page 17 of 28

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


TiO2 with holes in the HTM, but not the recombination of photogenerated electrons and holes in the PbS QDs via surface states (geminate recombination). The latter process happens at much faster time scales and does not affect the relaxation of electrons in the TiO2.44,

45

Instead,

recombination of electron-hole pairs in the PbS QDs via surface states will be apparent as a reduction of the injection efficiency in the device. In order to estimate the charge collection efficiency, ηc, of the cells with and without GSHtreatment we measured IMVS/IMPS responses of the devices at three different illumination conditions (cf. Figure S7). The ηc of the ss-QDSCs with and without GSH-treatment can be estimated from the approximate relationship: ηc=1-(τd/τn).46,

47

Here, τd is the charge transport

time which is obtained from the IMPS measurements, whereas τn is the electron lifetime obtained from the IMVS measurements (see Fig. 7). This estimation of ηc gave values of ~0.6 for both devices (with and without GSH-treatment) and did not vary with the light illumination intensity (cf. Figure S8). This shows that the GSH-treatment does not have any apparent influence on the efficacy of charge collection in the devices. It is also possible to estimate the charge injection efficiency, Φinj, for the solar cell devices using the values for JSC (experimental short circuit current obtained from I-V measurements), JSCtheor (maximum short circuit current calculated from absorbance spectrum) and collection efficiency (ηc=0.6) by the following relationship: Φ =

!"# $ %

(3)

The estimation of Φinj from equation (3) resulted in values of ~0.2 and ~0.42 for the reference and GSH-treated cells, respectively. These findings suggest that the cells, particularly the reference cell, suffer from poor net charge injection efficiencies. Also, the values for collection efficiency show that close to half of all the electrons in TiO2 are lost to back-reaction (processes 8 and/or 9 in Fig. 1) in both cells. Overall, our findings adequately explain the difference in the performance between the reference and GSH-treated cells and are consistent with the results of the I-V and EQE measurements. 17 ACS Paragon Plus Environment


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 18 of 28

In order to gain a better understanding of the recombination kinetics of electrons and holes in the QDs via surface states, we studied the photoluminescence (PL) decay of bare and GSH-protected PbS QDs using the time-correlated single photon counting technique. For this purpose bare and GSH-treated PbS QD-sensitized mesoporous Al2O3 electrodes were prepared with a thickness of 3.2±0.1 µm on glass substrates (see Supporting Information for details), and the PL decays of the samples were recorded (Fig. 8). The PL decays obtained were deconvoluted from the instrument response function (IRF) and fitted with a biexponential function (cf. Table S1 in the Supporting Information). The fitting shows that the decay curves have both fast, τ1, and slow, τ2, components (inset Table in Fig. 8). The fast component, τ1, could be due to fast charge trapping and/or defect-related decays and subsequent non-radiative recombination.14,

15

However, the fast

component (particularly for the bare QDs) is below the time resolution limit of the instrument (70 ps) and therefore cannot be fully deconvoluted from the laser response. The slow component, τ2, is assigned to radiative recombination of electrons and holes in the PbS QDs.14 The small peak at 1.275 ns in the decay curves originates from the instrumental response. The PL decay results reveal that bare PbS QDs have a significantly faster decay compared to GSH-protected QDs. The decay lifetime, τ2, for the GSH-protected QDs is higher than for the bare QDs by a factor of six. This suggests that GSH-protection decreases the number of surface states (reduced geminate recombination) and increases the carrier lifetimes in the QDs.15 We believe that this could be the main reason for improved charge injection from GSH-protected PbS QDs into the conduction band of TiO2, thus resulting in higher short circuit currents as shown in I-V and EQE measurements.


Page 19 of 28

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


Figure 8. PL decays of the bare and GSH-treated PbS QD-sensitized Al2O3 electrodes (black and red dots), the corresponding biexponential fits (black and red lines), and the instrument response function curves (green and dark green dots). In summary, solid-state QDSCs with SILAR grown PbS QDs and spiro-OMeTAD HTM were fabricated. The effect of GSH-treatment as a method for surface passivation has been examined. The comparison of solar cell performance of the devices with and without treatment showed superior performance of the GSH-treated cells. GSH-treatment of the QD-sensitized electrodes results in improvements in the short circuit current and conversion efficiency by more than a factor of two. A maximum EQE value of 35% at 435 nm was recorded for the GSH-treated cell compared to only 18% for the reference cell. IS studies show that GSH-treatment has little influence on the distribution of electron trap states in the mesoporous TiO2, except at higher voltages. The recombination resistance for the reference device shows slightly higher values at higher voltages compared to the GSH-treated device. These differences may have originated 19 ACS Paragon Plus Environment


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 20 of 28

from the contribution of long-lived charge carriers in passivated QDs to the device chemical capacitance and recombination kinetics, although differences in HTM infiltration associated with different wettability of the mesoporous TiO2/PbS electrodes with and without GSH-treatment may also play a role. OCVD, IMVS and IS techniques showed that there is not much difference in the electron lifetimes of the GSH-treated and reference cells except for a slight deviation of OCVD results from IMVS and IS data at lower voltages (below 200 mV) for the latter cell. Estimation of the charge collection efficiency from IMVS/IMPS measurements revealed that charge collection is the same for both the reference and GSH-treated devices, whereas the net charge injection efficiency for the GSH-treated cell is a factor of two higher compared to the reference cell. Time resolved PL decay studies reveal a significant difference in charge carrier lifetimes between bare and GSH-protected PbS QDs, where the latter exhibits an increase by almost one order of magnitude. Overall, combining the findings of all the experimental measurements, we conclude that the enhanced cell performance of the GSH-treated cells originates from the suppression of surface state-mediated electron-hole recombination in the PbS QDs, leading to improved charge injection and separation.

ASSOCIATED CONTENT Supporting Information. Experimental procedure for cell preparation, wide-angle XRD patterns and cross-section image of mesoporous TiO2 electrode, impedance spectra, OCVD, IMVS and IMPS spectra, collection efficiency of the cells as well as the details of fitting of the time resolved PL decay experiments. This material is available free of charge via the Internet at http://pubs.acs.org.


Page 21 of 28

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


AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by LMU Excellent, the Nanosystems Initiative Munich (NIM cluster), and the SolTech research network funded by the State of Bavaria. Askhat N. Jumabekov is grateful to the German Academic Exchange Service for a DAAD scholarship. The authors thank Dr. Steffen Schmidt for his help with SEM measurements.

REFERENCES 1.

Rhee, J. H.; Chung, C.-C.; Diau, E. W.-G. A Perspective of Mesoscopic Solar Cells Based on Metal Chalcogenide Quantum Dots and Organometal-Halide Perovskites. NPG Asia Mater. 2013, 5, e68.

2.

Sambur, J. B.; Novet, T.; Parkinson, B. A. Multiple Exciton Collection in a Sensitized Photovoltaic System. Science 2010, 330, 63-66.

3.

Lee, J.-W.; Son, D.-Y.; Ahn, T. K.; Shin, H.-W.; Kim, I. Y.; Hwang, S.-J.; Ko, M. J.; Sul, S.; Han, H.; Park, N.-G. Quantum-Dot-Sensitized Solar Cell with Unprecedentedly High Photocurrent. Sci. Rep. 2013, 3, 1-8.

4.

Lee, H.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nüesch, F.; Geiger, T.; Zakeeruddin, S. M.; Grätzel, M.; Nazeeruddin, M. K. PbS and CdS 21 ACS Paragon Plus Environment


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 22 of 28

Quantum Dot-Sensitized Solid-State Solar Cells: “Old Concepts, New Results”. Adv. Funct. Mater. 2009, 19, 2735-2742. 5.

Im, S. H.; Kim, H.-J.; Kim, S. W.; Kim, S.-W.; Seok, S. I. All Solid State Multiply Layered PbS Colloidal Quantum-Dot-Sensitized Photovoltaic Cells. Energy Environ. Sci. 2011, 4, 4181-4186.

6.

Moreels, I.; Lambert, K.; Smeets, D.; De Muynck, D.; Nollet, T.; Martins, J. C.; Vanhaecke, F.; Vantomme, A.; Delerue, C.; Allan, G.; Hens, Z. Size-Dependent Optical Properties of Colloidal PbS Quantum Dots. ACS Nano 2009, 3, 3023-3030.

7.

Hens, Z.; Moreels, I. Light Absorption by Colloidal Semiconductor Quantum Dots. J. Mater. Chem. 2012, 22, 10406-10415.

8.

Zhao, K.; Pan, Z.; Mora-Seró, I.; Cánovas, E; Wang, H.; Song, Y.; Kuga, Gong, X.; Wang, J.; Bonn, M.; Bisquert, J.; Zhong, X. Boosting Power Conversion Efficiencies of Quantum-Dot-Sensitized Solar Cells Beyond 8% by Recombination Control. J. Am. Chem. Soc. 2015, 137, 5602-5609.

9.

Labelle, A. J.; Thon, S. M.; Masala, S.; Adachi, M. M.; Dong, H.; Farahani, M.; Ip, A. H.; Fratalocchi, A.; Sargent, E. H. Colloidal Quantum Dot Solar Cells Exploiting Hierarchical Structuring. Nano Lett. 2015, 15, 1101-1108.

10.

Carey, G. H.; Levina, L.; Comin, R.; Voznyy, O.; Sargent, E. H. Record Charge Carrier Diffusion Length in Colloidal Quantum Dot Solids via Mutual Dot-To-Dot Surface Passivation. Adv. Mater. 2015, 27, 3325-3330.

11.

Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; CurchodBasile, F. E.; AshariAstani, N.; Tavernelli, I.; Rothlisberger, U.; NazeeruddinMd, K.; Grätzel, M. DyeSensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247.

12.

Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778-17787. 22 ACS Paragon Plus Environment

Page 23 of 28

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.

Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; Kemp, K. W.; Kramer, I. J.; Ning, Z.; Labelle, A. J.; Chou, K. W.; Amassian, A.; Sargent, E. H. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577-582.

14.

Omogo, B.; Aldana, J. F.; Heyes, C. D. Radiative and Nonradiative Lifetime Engineering of Quantum Dots in Multiple Solvents by Surface Atom Stoichiometry and Ligands. J. Phys. Chem. C 2013, 117, 2317-2327.

15.

Abdellah, M.; Marschan, R.; Žídek, K.; Messing, M. E.; Abdelwahab, A.; Chábera, P.; Zheng, K.; Pullerits, T. Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance. J. Phys. Chem. C 2014, 118, 25802-25808.

16.

Tvrdy, K.; Frantsuzov, P. A.; Kamat, P. V. Photoinduced Electron Transfer from Semiconductor Quantum Dots to Metal Oxide Nanoparticles. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 29-34.

17.

Hines, D. A.; Kamat, P. V. Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions. J. Phys. Chem. C 2013, 117, 14418-14426.

18.

Guijarro, N.; Campina, J. M.; Shen, Q.; Toyoda, T.; Lana-Villarreal, T.; Gomez, R. Uncovering the Role of the ZnS Treatment in the Performance of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2011, 13, 12024-12032.

19.

Yang, Z.; Chen, C.-Y.; Roy, P.; Chang, H.-T. Quantum Dot-Sensitized Solar Cells Incorporating Nanomaterials. Chem. Commun. 2011, 47, 9561-9571.

20.

Kamat, P. V. Quantum Dot Solar Cells. The Next Big Thing in Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 908-918.

21.

Chakrapani, V.; Baker, D.; Kamat, P. V. Understanding the Role of the Sulfide Redox Couple (S2–/Sn2–) in Quantum Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2011, 133, 9607-9615.



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

22.

Page 24 of 28

Christians, J. A.; Kamat, P. V. Trap and Transfer. Two-Step Hole Injection Across the Sb2S3/CuSCN Interface in Solid-State Solar Cells. ACS Nano 2013, 7, 7967-7974.

23.

Hod, I.; González-Pedro, V.; Tachan, Z.; Fabregat-Santiago, F.; Mora-Seró, I.; Bisquert, J.; Zaban, A. Dye versus Quantum Dots in Sensitized Solar Cells: Participation of Quantum Dot Absorber in the Recombination Process. J. Phys. Chem. Lett. 2011, 2, 3032-3035.

24.

Tang, J.; Brzozowski, L.; Barkhouse, D. A. R.; Wang, X.; Debnath, R.; Wolowiec, R.; Palmiano, E.; Levina, L.; Pattantyus-Abraham, A. G.; Jamakosmanovic, D.; Sargent, E. H. Quantum Dot Photovoltaics in the Extreme Quantum Confinement Regime: The Surface-Chemical Origins of Exceptional Air- and Light-Stability. ACS Nano 2010, 4, 869-878.

25.

Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T. Effect of ZnS coating on the photovoltaic properties of CdSe quantum dot-sensitized solar cells. J. Appl. Phys. 2008, 103, 084304-5.

26.

Tachan, Z.; Hod, I.; Shalom, M.; Grinis, L.; Zaban, A. The Importance of the TiO2/Quantum Dots Interface in the Recombination Processes of Quantum Dot Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2013, 15, 3841-3845.

27.

de la Fuente, M. S.; Sánchez, R. S.; González-Pedro, V.; Boix, P. P.; Mhaisalkar, S. G.; Rincón, M. E.; Bisquert, J.; Mora-Seró, I. Effect of Organic and Inorganic Passivation in Quantum-Dot-Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 1519-1525.

28.

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.


Page 25 of 28

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


29.

Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203-9210.

30.

Brennan, T. P.; Trejo, O.; Roelofs, K. E.; Xu, J.; Prinz, F. B.; Bent, S. F. Efficiency Enhancement of Solid-State PbS Quantum Dot-Sensitized Solar Cells with Al2O3 Barrier Layer. J. Mater. Chem. A 2013, 1, 7566-7571.

31.

Jumabekov, A. N.; Deschler, F.; Böhm, D.; Peter, L. M.; Feldmann, J.; Bein, T. Quantum-Dot-Sensitized Solar Cells with Water-Soluble and Air-Stable PbS Quantum Dots. J. Phys. Chem. C 2014, 118, 5142-5149.

32.

Jumabekov, A. N.; Siegler, T. D.; Cordes, N.; Medina, D. D.; Böhm, D.; Garbus, P.; Meroni, S.; Peter, L. M.; Bein, T. Comparison of Solid-State Quantum-Dot-Sensitized Solar Cells with ex Situ and in Situ Grown PbS Quantum Dots. J. Phys. Chem. C 2014, 118, 25853-25862.

33.

Deng, D.; Xia, J.; Cao, J.; Qu, L.; Tian, J.; Qian, Z.; Gu, Y.; Gu, Z. Forming Highly Fluorescent Near-Infrared Emitting PbS Quantum Dots in Water Using Glutathione as Surface-Modifying Molecule. J. Colloid Interface Sci. 2012, 367, 234-240.

34.

Sakai, N.; Tatsuma, T. Photovoltaic Properties of Glutathione-Protected Gold Clusters Adsorbed on TiO2 Electrodes. Adv. Mater. 2010, 22, 3185-3188.

35.

Clifford, J. P.; Konstantatos, G.; Johnston, K. W.; Hoogland, S.; Levina, L.; Sargent, E. H. Fast, Sensitive and Spectrally Tuneable Colloidal-Quantum-Dot Photodetectors. Nat. Nanotechnol. 2009, 4, 40-44.

36.

Dualeh, A.; Moehl, T.; Nazeeruddin, M. K.; Grätzel, M. Temperature Dependence of Transport Properties of Spiro-MeOTAD as a Hole Transport Material in Solid-State DyeSensitized Solar Cells. ACS Nano 2013, 7, 2292-2301.



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

37.

Page 26 of 28

Boix, P. P.; Larramona, G.; Jacob, A.; Delatouche, B.; Mora-Seró, I.; Bisquert, J. Hole Transport and Recombination in All-Solid Sb2S3-Sensitized TiO2 Solar Cells Using CuSCN As Hole Transporter. J. Phys. Chem. C 2012, 116, 1579-1587.

38.

Fabregat-Santiago, F.; Bisquert, J.; Cevey, L.; Chen, P.; Wang, M.; Zakeeruddin, S. M.; Grätzel, M. Electron Transport and Recombination in Solid-State Dye Solar Cell with Spiro-OMeTAD as Hole Conductor. J. Am. Chem. Soc. 2009, 131, 558-562.

39.

Bisquert, J.; Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Giménez, S. Electron Lifetime in Dye-Sensitized Solar Cells: Theory and Interpretation of Measurements. J. Phys. Chem. C 2009, 113, 17278-17290.

40.

Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J. Characterization of Nanostructured Hybrid and Organic Solar Cells by Impedance Spectroscopy. Phys. Chem. Chem. Phys. 2011, 13, 9083-9118.

41.

Mora-Seró, I.; Giménez, S.; Fabregat-Santiago, F.; Gómez, R.; Shen, Q.; Toyoda, T.; Bisquert, J. Recombination in Quantum Dot Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1848-1857.

42.

González-Pedro, V.; Xu, X.; Mora-Seró, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 4, 5783-5790.

43.

Cameron, P. J.; Peter, L. M. How Does Back-Reaction at the Conducting Glass Substrate Influence the Dynamic Photovoltage Response of Nanocrystalline Dye-Sensitized Solar Cells? J. Phys. Chem. B 2005, 109, 7392-7398.

44.

Ehrler, B.; Musselman, K. P.; Böhm, M. L.; Morgenstern, F. S. F.; Vaynzof, Y.; Walker, B. J.; MacManus-Driscoll, J. L.; Greenham, N. C. Preventing Interfacial Recombination in Colloidal Quantum Dot Solar Cells by Doping the Metal Oxide. ACS Nano 2013, 7, 4210-4220.


Page 27 of 28

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.

Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. J. Am. Chem. Soc. 2006, 128, 2385-2393.

46.

Hwang, D.; Jo, S. M.; Kim, D. Y.; Armel, V.; MacFarlane, D. R.; Jang, S.-Y. HighEfficiency, Solid-State, Dye-Sensitized Solar Cells Using Hierarchically Structured TiO2 Nanofibers. ACS Appl. Mater. Interfaces 2011, 3, 1521-1527.

47.

Duan, Y.; Fu, N.; Zhang, Q.; Fang, Y.; Zhou, X.; Lin, Y. Influence of Sn Source on the Performance of Dye-Sensitized Solar Cells Based on Sn-Doped TiO2 Photoanodes: A Strategy for Choosing an Appropriate Doping Source. Electrochim. Acta 2013, 107, 473480.



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 28 of 28

TOC GRAPHICS


Passivation of PbS Quantum Dot Surface with l ... - ACS Publications

Recommend Documents