Carrier Dynamics of Dye Sensitized-TiO2 in Contact with Different

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C: Energy Conversion and Storage; Energy and Charge Transport

Carrier Dynamics of Dye Sensitized-TiO2 in Contact with Different Cobalt Complexes in the Presence of Tri(p-anisyl)amine Intermediates Wenxing Yang, Yan Hao, Lars Kloo, and Gerrit Boschloo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03395 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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The Journal of Physical Chemistry

Carrier Dynamics of Dye Sensitized-TiO2 in Contact with Different Cobalt Complexes in the Presence of Tri(panisyl)amine Intermediates Wenxing Yang†#, Yan Hao†*, Lars Kloo†, Gerrit Boschloo†* †Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE 75120 Uppsala, Sweden. † Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden. #Present address: Department of Chemistry, Imperial College London, London SW7 2AZ, UK (L.S.) Abstract: Heterogeneous charge transfer processes at sensitized wide bandgap semiconductor surfaces are imperative for both fundamental knowledge and technical applications. Herein, we focus on the investigation of carrier dynamics of a triphenylamine-based dye, LEG4, sensitized TiO2 (LEG4/TiO2) in contact with two types of electrolyte systems: pure cobalt-based electrolytes and in combination with an organic donor, tri(panisyl)amine (TPAA). Four different cobalt redox systems with potentials spanning a 0.3 V range were studied, and the carrier recombination and regeneration kinetics were monitored both at low and at high TiO2 (e-) densities (1.3×1018 cm-3 and 1.3×1019 cm-3 , respectively). The results reveal that the introduction of the TPAA intermediate more effectively suppress the recombination loss of TiO2 (e-) under high charge conditions, close to open-circuit, as compared to low charge conditions. As a result, the charge transfer from the cobalt complexes to the oxidized dyes is significantly improved by the addition of TPAA. Dye-sensitized solar cells fabricated with the TPAA-containing electrolytes demonstrate remarkable improvement in both VOC and JSC, and lead to more than 25 % increase of the light-to-electricity conversion efficiency. Furthermore, an unprecedented detrimental impact of TPAA on the device performance was identified when the redox potential of the TPAA donor and the cobalt complexes are close. This is ascribed to the formation of TPAA+ which can act as an active recombination centers and thus lower the solar cell performance. 1 ACS Paragon Plus Environment

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These insights point at a strategy to enhance the lifetimes of electrons generated in sensitized semiconductor electrodes by overcoming the charge recombination between TiO2 and the oxidized dye under high carrier densities in the semiconductor substrate and offer practical guidance to the design of future efficient electrolyte systems for dye-sensitized solar cells.

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1. INTRODUCTION During the past decades, dye-sensitized wide band-gap semiconductors have achieved enormous application in various photo-electrochemical devices, noticeably in dye-sensitized solar cells (DSSCs)1 and dyesensitized photo-electrochemical cells2. For example in DSSCs, the application of organic dyes-sensitized titanium dioxide (TiO2) in combination with cobalt tris(bipyridyl) (Co(bpy)32+/3+) redox electrolytes have led to the development of the solar to electricity conversion efficiency up to 14%.3 In addition, such sensitized electrodes have also demonstrated the capability to split water4, to reduce CO25 and to generate solar fuels6. The principles of operation in such electrodes involve dye excitation by the incoming light, which is then followed by a rapid charge injection from the dye molecules (electrons or holes) into the semiconductors support materials.1 Since the light absorption is dominated by the molecular sensitizer and the semiconductors essentially act as a conducting medium, the recombination of the bulk electron-hole pair generated is suppressed.7 Instead, recombination losses mainly take place at the surface of the sensitized semiconductors.8 Surface recombination comprise both of recombination from the injected electrons, TiO2 (e-), to the oxidized dye (µs to ns, the process a2 in Scheme 1a) and from TiO2 (e-) to the redox mediators (ms to µs, the process a3 in Scheme 1a). Due to the closer spatial location of TiO2 (e-) to the oxidized dye molecules, the former recombination is commonly much faster and therefore more critical. Furthermore, the recombination between TiO2 (e-) and the oxidized dye commonly shows strong dependence of its decay kinetics on the carrier densities9,10. A high carrier density in the TiO2 substrate has been shown to follow to an empirical power law increase in the recombination kinetics: 𝑡0.5 ∝ [𝑒 − ]−𝛽 , where t0.5 is the decay half time, and β = 2 to 4 depending on the electrolyte employed11. As a result, the dye regeneration efficiency often shows a dramatic decrease under maximum power point and open-circuit conditions which limits the device performance. For example, in previous studies of DSSCs a dramatic drop in dye regeneration caused by a Co(bpy)32+/3+)-based electrolyte, from 100% to 28%, was observed by merely moving from shortcircuit to open-circuit conditions.10 Such a strong carrier dependent recombination rate fundamentally must limit the maximum achievable charge-carrier density in TiO2 and, as a consequence, also the photo-voltage. In order to mediate this bottleneck of device performance, new approaches to reduce the recombination loss



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between TiO2 (e-) and the oxidized dye/redox electrolytes, especially under high carrier density, are therefore highly needed. Scheme 1. The electron transfer pathways involved at a dye-sensitized TiO2 interface for systems with (a) Co-based redox systems and (b) Co-based redox systems in combination with TPAA. Herein, four cobalt electrolytes have been studied with the respective redox potentials shown in Figure 1. (The light blue and red dashed lines represent the recombination of TiO2 (e-) with the oxidized dye/redox mediator. The light blue and red lines solid lines represent the desired forward processes in both electrolyte systems, with the details given in the main text.)

Recently, we demonstrated a simple strategy to improve the efficiency of this type of photoelectrodes in DSSCs by simply adding an organic intermediate donor, tri(p-anisyl)amine (TPAA)12, into the standard Co(bpy)32+/3+-based electrolyte13. As demonstrated in Scheme 1b, the key of success of such tandem electrolyte relies on a two-step electron transfer cascade process introduced by TPAA during the charge transfer processes from the cobalt complex to the oxidized dye: Firstly, TPAA is thought to regenerate the dye within a few ns (extremely fast, close to the diffusion-limit charge transfer), forming the oxidized TPAA (TPAA+). Secondly, the formed TPAA+ is then regenerated by the cobalt mediator. The aim of the current work is to elucidate the kinetic mechanisms in the tandem electrolytes by expanding the combination of TPAA with four different cobalt complexes systems (Figure 1) whose formal redox potentials span a range of 0.3 V.14 This difference in formal reduction potentials of the cobalt complexes employed is expected to allow screening of the impact of driving force in the second regeneration step described above on the overall regeneration efficiency, as well as on the device performance. Furthermore, we compare throughout the charge transfer kinetics of both the tandem electrolytes and the pure cobalt complex based electrolytes under two different TiO2(e) densities of 1.3×1018 cm-3 and 1.3 ×1019 cm-3 (± 10 %), characterized by transient absorption spectroscopy. As shown by the charge extraction measurements of fully operational DSSC devices in Figure S1, the investigated photo-electrodes show typical charge densities 4 ACS Paragon Plus Environment

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in the order of ~ 1.4 to 2.2 ×1018 cm-3 under open-circuit conditions and one-sun illumination intensity. The two charge densities, therefore, represent two cases of charge accumulation in TiO2 (a) lower/comparable to an open-circuit carrier density (b) higher than an open-circuit carrier density; both referring to electron densities in a DSSC device under 1 sun illumination. A comparison at these two charge densities with respect to the resulting kinetics will allow identification of the limiting rate of dye regeneration relevant to the device operation.10 Finally, the kinetics insights obtained have been correlated with the more complex DSSC photovoltaic performance. The achieved insights are expected to provide both an important fundamental understanding of the interaction between the sensitized photo-electrodes and the redox electrolytes with additional intermediate donor and also serve as a general guideline for further device improvement of sensitized photo-electrochemical cells. 2. EXPERIMENTAL SECTION AND METHODS 2.1. Chemicals. All the chemicals are purchased from Sigma Aldrich unless otherwise mentioned. The chemicals are used without further purification. LEG4, TPAA, [Co(bpyPY4)]n+, [Co(bpy)3]n+, [Co(phen)3]n+ and [Co(phenCl)3]n+ were purchased from Dyenamo AB, Sweden. 2.2. Solar cell fabrication. The solar cells were fabricated by sandwiching the TiO2 photo-electrodes and counter electrodes, between of which was filled with redox electrolytes. The TiO2 photo-electrodes were made with carefully cleaned fluorine-doped tin oxide (FTO) glass (TEC 15). The procedure of cleaning for FTO glass is followed with detergent, water, acetone and ethanol for 30 min, respectively. After that, the FTO glass was pre-treated with TiCl4 solution (40 mM in water) for 90 min at 70 ˚C to form a blocking layer. Then a screen printing technique was applied to prepare mesoporous TiO2 films with an area of 5 × 5 mm2 on the FTO glass. The film consists of one transparent layer (4 µm) which was printed with colloidal TiO2 paste (Dyesol DSL 30 NRD-T) and one opaque layer (4 µm) prepared by another TiO2 paste (Dyesol DSL WER2-O). Between layers, the newly printed layer was always dried at 120 ˚C for 2 min before next layer. After the screen printing, the photo-electrodes were sintered in an oven (Nabertherm Controller P320) in the air atmosphere followed by a temperature gradient program, 180 ˚C (10 min), 320 ˚C (10 min), 390 ˚C (10 min) and 500 ˚C (30 min). After the cooling process, the electrodes need to be treated again with TiCl 4 water solution (20 mM) for 30 min at 70 ˚C, followed by heating at 500 ˚C for 30 min. During the cooling process, the electrodes were immersed in a dye bath for overnight when its temperature is around 90 ˚C. In the present study, the dye bath contains LEG4 (0.2 mM) in tert-butanol: acetonitrile (1:1 v/v). Counter electrodes were prepared by depositing 10 μL of an H2PtCl6 solution (5 mM in ethanol) on the pre-drilled FTO glass (TEC 8) followed by heating in air at 400 ˚C for 30 min. Finally, the solar cells were assembled by sealing the photo-electrode and the counter electrode using a 25 μm thick thermoplastic Surlyn frame. An 5 ACS Paragon Plus Environment


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electrolyte solution was then injected through the hole on the counter electrode by vacuum filling method, and the hole was sealed with thermoplastic Surlyn cover and a microscope glass coverslip. The four cobalt electrolytes consist of Co(L)3(PF6)2 (0.22 M), Co(L)3(PF6)3 (0.05 M), LiClO4 (0.1 M) and 4-tert butylpyridine (TBP, 0.2 M) in acetonitrile and TPAA/Co electrolyte is made by adding 0.1 M TPAA into four varied cobalt electrolyte. 2.3. Solar cells characterisation. Current-voltage (IV) characteristics of the solar cells were recorded by a source measurement unit (Keithley 2400) under the illumination of a solar simulator (Newport, model 91160). The solar simulator gives light with AM 1.5 G spectral distributions and was calibrated to an intensity of 100 mW cm-2 using a certified reference solar cell (Fraunhofer ISE). During the measurement, a black metal mask with an aperture of 5 × 5 mm2 was applied. Incident photon-to-current conversion efficiency (IPCE) spectra of the cells were measured with a computer-controlled setup comprising a xenon light source (Spectral Products ASB-XE-175), a monochromator (Spectral Products CM110) and a Keithley multimeter (model 2700). The IPCE spectra were calibrated using a certified reference solar cell (Fraunhofer ISE). 2.4. Electron lifetime and charge extraction measurement. Electron lifetimes of solar cells are measured by a home-made “toolbox” as described previously15. Briefly, a white LED (Luxeon Star 1W) was used as a light source, and its intensity is controlled by the applied bias voltages. In the electron lifetime measurement, a small change of bias voltage was applied onto the LED, which therefore causes the modulation of the light intensity. The transient photovoltage signal is recorded using a 16-bit resolution digital acquisition board (National Instruments), and the lifetime can be read as the time constant of the single exponential fitting of the transient photovoltage signal. For the charge extraction measurements, the solar cells were firstly kept at open-circuit under illumination for 5 s and then the circuit was switched to short circuit, with the integrated current giving the stored charge within the solar cells. Both measurements are conducted at various bias voltages of LED to illuminate the dependence of lifetime and accumulated charge on light intensity. 2.5. UV-vis measurement. The UV-Vis measurement is performed on with an HR-2000 Ocean Optics spectrophotometer with the baseline correction. 2.6. Laser spectroscopic measurement. The samples preparation for the laser measurements follows similar procedures as in the solar cells manufactures, except for the absence of a scattering layer. This guarantees the light transmission of the sample. Before the measurement, we checked the photovoltaic performance of the laser samples. They display smaller efficiencies but similar trends as those fabricated with the scattering layers. The laser measurements were performed with a laser flash photolysis spectrometer (Edinburgh Instrument LP920). A continuous wave xenon-light was used as the probe light source. The laser pulses were generated by frequency tripled Nd: YAG laser (Continuum Sunlight II, 10Hz repetition rate, ten ns pulse width) and tuned to 620 nm by using an OPO (Continuum Sunlight). The excitation wavelength was chosen where the light absorbance of films are around ~ 0.5, which guarantees a uniform light illumination across the films. Two excitation energy, 0.18 mJ cm-2 and 1.8 mJ cm-2 per pulse, 6 ACS Paragon Plus Environment

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was adjusted by using neutral density. These two laser pulse intensities were estimated to generate an excitation densities of ~1.3 ×1018 and 1.3 ×1019 cm-3, with the film thickness, area and porosity to be 6 µm, 0.25 cm2 and 0.5, respectively. Kinetic traces were monitored at 720 and 825 nm measured by a photomultipliers tube detector. The signal was averaged from 100 to 500 pulses to reduce the noise. 3. RESULTS AND DISCUSSION 3.1 Charge transfer reactions of the LEG4/TiO2 electrode in contact with pure cobalt based electrolytes. Figure 1 shows the chemical structures of the dye molecules and the cobalt redox mediators

Figure 1. The chemical structures of molecules used in the present study together with their corresponding HOMO energy levels (dyes) or redox potential (redox mediators) in parentheses (vs NHE); values taken from previous publications12,14,16. used in the present study. The triphenylamine-based dye used, LEG4, and the cobalt redox mediators have been demonstrated to give a wide solar spectral response17, fast regeneration kinetics16 and lead to highly efficient DSSCs3,17,18. Therefore, these systems involve the state-of-the-art photosensitized electrodes. In this study, we firstly investigated the electron transfer kinetics of LEG4 sensitised-TiO2 (LEG4/TiO2) contacting with pure cobalt-based electrolytes (Co) after photoexcitation at two different excitation densities. Upon laser excitation, dye molecules chemisorbed onto TiO2 will get excited and rapidly inject electrons into the 7 ACS Paragon Plus Environment


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TiO2 substrate (normally within few ps)19. These excitation and injection reactions result in a transient absorption spectra difference (TAS) of the sensitized electrodes (Δ𝐴 = 𝐴𝑏𝑠𝐸𝑥 − 𝐴𝑏𝑠𝐺𝑟𝑜𝑢𝑛𝑑 ), which can be monitored to trace the time evolution of the species involved by their characteristic absorption features. Figure 2a shows the TAS of the LEG4/TiO2 electrode in contact with the Inert electrolyte (0.1 M LiClO4, 0.2 M TBP in ACN), where two photo-induced absorption peaks at 670 nm and 770 nm can be clearly resolved. These two peaks have been previously demonstrated to emerge from the formation of LEG4+,20,21 after an ultrafast charge injection from the excited dye to TiO2, together with a bleach attributed to both the dye ground state bleach as well as Stark effects.20,22 The comparison of kinetic traces in Figure 2b shows identical decays at 720 and 825 nm when the photoelectrode is in contact with the Inert as well as four Co electrolytes, confirming their same spectral origin, i.e. LEG4+. (The wavelengths of 720 nm and 825 nm were chosen since the oxidized TPAA absorbs at 720 nm, but not at 825 nm, see the next section below.) The recombination reaction between TiO2 (e-) and the redox mediators (Process a3, scheme 1a) has been previously shown to be slow, ~ ms timescale.13 This process will therefore be neglected in the discussion of the ~ µs TAS carrier dynamic. Thus, when the LEG4/TiO2 electrode is in contact with the Inert electrolyte, the only path to dissipate LEG4+ originates from the back recombination between TiO2 (e-) and LEG4+ (Process a2, Scheme 1a). The decays at 720 nm/825 nm thus provide a way to quantify this charge recombination rate. On the other hand, when the LEG4/TiO2 electrode is in contact with the Co electrolytes, the decays of LEG4+ are expected to represent not only the above-mentioned process but also the dye regeneration of LEG4+ by the redox-active electrolytes (Process a1, Scheme 1a). As a result, these decay kinetics are commonly fitted by a bi-exponential mathematical model. 10,14,23 3.1.1 Low electron density.


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(a)

(b)

(c)

(d)

Low Excitation

High Excitation

Density: 1.3×1018 cm-3

Density: 1.3×1019 cm-3

Figure 2. (a) Transient absorption spectra of the LEG4/TiO2 electrode in contact with the Inert electrolyte (0.1 M LiClO4, 0.2 M TBP in ACN) (b) Decay at 720 nm and 825 nm for the LEG4/TiO2 electrode in contact with both the Inert electrolyte and different Co electrolytes (0.22 M Co2+, 0.05 M Co3+ added into the Inert electrolyte). PY4: Co(bpyPY4)32+/3+, BPY: Co(bpy)32+/3+, Phen: Co(phen)32+/3+, PhenCl: Co(phenCl)32+/3+ (c) Comparison of the normalized decay traces of the LEG4/TiO2 electrode at 720 nm for all the electrolytes at low carrier densities, 1.3×1018 cm-3. The yellow lines represent the bi-exponential models. (d) Comparison of the normalised kinetic decays of LEG4/TiO2 at 720 nm for all the electrolytes under high excitation densities, 1.3×1019 cm-3. Excitation wavelength: 620 nm. Repetition rates: 10Hz. In Figure 2c, the normalised decays at 720 nm of the LEG4/TiO2 electrodes in contact with both the Inert and different Co electrolytes are compared at 1.3×1018 cm-3 electron density. The decay traces are shown to be properly modeled by a biexponential function (parameters of a least-square fit are summarised in Table S1). The amplitude-weighted average decay time constants in the presence of the Co electrolytes show an order of tBPY (2.2 µs) < tPY4 (3.4 µs) < tPhen (4.0 µs) < tPhenCl (11. 0 µs), all of which demonstrate much faster regeneration than that of the Inert electrolyte, tInert (38.0 µs). The significant reduction in time constants in the presence of the redox mediators concludes that the dye regeneration of LEG4+ by the cobalt redox 9 ACS Paragon Plus Environment


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mediators is fast and effectively out-compete the direct recombination process. In order to quantify the kinetic competition between the dye regeneration (process a2, Scheme 1a) and dye recombination (process a3, Scheme 1a), the dye regeneration efficiencies of LEG4/TiO2 by four Co electrolytes can be estimated using Eq. 1,10,23 and the results are summarised in Table 1. 𝜂𝑟𝑒𝑔 = 𝑘

𝑘𝑟𝑒𝑔 𝑟𝑒𝑔 +𝑘𝑟𝑒𝑐

𝜏𝑟𝑒𝑔

=1−𝜏

𝑟𝑒𝑐

(Eq. 1)

The data show that, despite the difference in driving force of the cobalt complexes with respect to LEG4+ regeneration, PY4 (0.61 eV), BPY (0.51 eV) and Phen (0.46 eV), the dye regeneration efficiencies are all close to 1 (94 %, 91 % and 89 %, respectively). These results are coherent with the previous study by Feldt et.al23, which concludes that a driving force of ~ 0.5 eV should be sufficient for efficient regeneration of a triphenylamine-based dye, D35 in that case, by the Co electrolytes. The dramatic decrease in dye regeneration efficiency noted for PhenCl (71%), however, may indicate that the low driving force, i.e. the difference between the PhenCl redox potential and the LEG4 HOMO energy level, ~ 0.35 eV, is insufficient for a fast dye regeneration process. Therefore, the competing charge recombination reaction between TiO2 (e-) and LEG4+ in the case of the PhenCl electrolyte leads to a bad solar cell performance (vide infra). 3.1.2 High electron density. As introduced previously, the low carrier density represents a case where the electron density in TiO2 is lower or comparable to electrodes in a fully operational DSSC at 1 sun illumination under open-circuit condition. In order to investigate the dye regeneration efficiency at higher electron densities, kinetic studies were further performed at a charge density of ~ 1.3 ×1019 cm-3. All the decay traces of the LEG4/TiO2 electrode at 720 nm in contact with different electrolytes show dramatically faster rates as compared to those determined at low light excitation (Figure 2d). The recombination rate between the oxidized dye and TiO2 (e) is well-known to increase dramatically at higher TiO2 (e-) concentrations, and it is, as a result, the shape of the decays resembles that of the recombination of TiO2 (e-) and LEG4+. The dispersive nature caused by kinetic inhomogeneity makes it difficult to model the decays with a bi-exponential function.24 For simplicity, we use the decay half-time to quantify the decay rates at high excitation density, just as done in previous studies.9 One should note that the different way of quantifying the time constants under low and high light excitation intensity (electron density) limits the rigidity of a direct comparison between the times 10 ACS Paragon Plus Environment

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constants across light intensities. However, the analyses still allow the estimation of the dye regeneration efficiency acquired at the same excitation intensity, for which the time constants were all processed in the same way. The results show the half-time of the decay at high electron density to follow the order of tPY4 (0.5 µs) < tBPY (0.7 µs) < tPhen(0.6 µs) < tPhenCl (1.3 µs) < tInert (1.4 µs). In contrast to the rates determined at low electron

density, the proximity between tInert and tredox under the high excitation densities indicates a larger contribution of the recombination processes to the decay of LEG4+. Indeed, the dye regeneration efficiency, as estimated using eq.1, shows an order of PY4 (64 %) > Phen (50 %) > BPY ≥ (57 %) > PhenCl (7 %) (Table 1), demonstrating dramatically lower efficiencies as compared to the results obtained at low electron density. These results indicate that, although the dye regeneration efficiency of PY4, BPY and Phen is high at low TiO2(e-), an increase of TiO2(e-) at a high density can cause the accelerate the charge recombination rate between TiO2 (e-) and LEG4+ and result in a dramatic decrease of the dye regeneration efficiency. This aspect is, however, often overlooked in the literature since dye regeneration is commonly investigated at low carrier densities obtained under low light excitation levels. As a result, this critical charge recombination loss can be expected to suppress the further accumulation of charges in TiO2 and should account for the limitation of device performance. Table 1. Comparison of the overall dye regeneration efficiency from the Co electrolytes with respect to the LEG4+ at low and high light intensity. Low electron intensity High electron intensity PY4 BPY Phen PhenCl

w/o TPAA

w TPAA

w/o TPAA

w TPAA

91% 94% 89% 71%

99% 98% 94% 73%

64% 50% 57% 7%

97% 96% 91% 69%

3.2 Charge transfer of the LEG4/TiO2 electrode in contact with tandem electrolytes.



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(b)

(a)

(c)

(d)

Low excitation

High excitation

density: 1.3×1018 cm-3

density: 1.3×1019 cm-3

Figure 3. (a) Transient absorption spectra of the LEG4/TiO2 electrode in contact with the Inert/TPAA electrolytes (0.1 M TPAA added to the Inert electrolyte) (b) Decays at 720 nm and 825 nm for the LEG4/TiO2 electrode in contact with both the Inert/TPAA electrolyte and different Co/TPAA electrolytes (0.22 M Co2+, 0.05 M Co3+ added into the Inert/TPAA electrolyte). (c) Comparison of the normalized decay traces of the LEG4/TiO2 electrode at 720 nm for all the tandem electrolytes at low excitation densities, 1.3×1018 cm-3. The yellow lines represent the biexponential models. (d) Comparison of the normalized kinetic traces of the LEG4/TiO2 electrode at 720 nm for all the tandem electrolytes at high electron density, 1.3×1019cm-3. Excitation wavelength: 620 nm. Repetition rate: 10 Hz. After evaluation of the kinetic processes at the LEG4/TiO2 electrode in contact with pure cobalt complex electrolytes, we further investigated electron transfer kinetics of the LEG4/TiO2 electrode contacting electrolytes with the addition of TPAA at two excitation densities. Figure 3a shows that, in stark contrast to Figure 2a, the addition of the TPAA results in only one peak at 720 nm for all the TAS spectra of the LEG4/TiO2 electrode immediately after ~ 10 ns photo-excitation, regardless if the Inert/TPAA or Co/TPAA electrolytes are used (Figure S4b). The peak at 720 nm can be assigned to the absorption spectrum of the oxidized TPAA, as shown in the previous report12, with minor contribution from intra-band 12 ACS Paragon Plus Environment

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absorption of electrons in TiO2 as a broad featureless absorption25. The immediate appearance of TPAA+ in all TPAA-containing electrolytes, directly after photo-excitation, concludes the rapid occurrence of 𝐿𝐸𝐺4+ + 𝑇𝑃𝐴𝐴 → 𝐿𝐸𝐺4 + 𝑇𝑃𝐴𝐴+ . The above conclusion is in good agreement with the observation that the spectral transition from LEG4+ to TPAA+ occurs on a sub-nanosecond time scale as revealed by ultrafast spectroscopic measurements.12 Furthermore, as shown in Figure 3b, the kinetic studies at 720 nm, for all the TPAA-containing electrolytes, span a range of 1 to 100 µs, while the decays at 825 nm only exhibit a negligible initial amplitude and a very fast response within few 10 to 100 ns. Overall, the electrolyte-independent TAS and the kinetic features of the LEG4/TiO2 electrode at 720/825 nm lead to the conclusion that TPAA completely dominates the regeneration kinetics of LEG4+, with only negligible contribution from the cobalt mediators. Further experiments show that even at high electron densities, the kinetic difference at 720/825 nm (Figure S6) remains the same as that in Figure 3b. These results conclude that at high electron densities, the regeneration of LEG4+ by TPAA is still fast enough to outperform the recombination loss reaction between LEG4+ and TiO2 (e-), although the estimated 10-fold rate increases of the reaction between LEG4+ and TiO2 (e-) at high electron densities (See SI Note I). As a result, in the Co/TPAA electrolyte systems, the overall charge transfer efficiency, from the cobalt electrolyte to the oxidized dye, can be defined as: 𝜂𝑟𝑒𝑔 = 𝜂𝑟𝑒𝑔 × 𝜂𝑟𝑒𝑔 (Eq. 2) 1

2

Where 𝜂𝑟𝑒𝑔 represents the regeneration efficiency of step 1 (scheme 1b) and 𝜂𝑟𝑒𝑔 the regeneration 1

2

efficiency of TPAA+ in step 2 (scheme 1b), while 𝜂𝑟𝑒𝑔 can be treated as 1 in all cases. 1

In other words, the overall charge transfer processes from the cobalt complexes to LEG4+ is only depicted by the efficiency of step 2 in the Co/TPAA electrolytes. Recent studies suggest that the kinetics of the charge recombination between TiO2 (e-) and TPAA+ (Process b3, Scheme 1b) also shows a clear dependence on the TiO2 (e-) concentration.26 Therefore, a similar electron density study of the regeneration efficiency of TPAA+ in the tandem electrolyte systems was performed (Scheme 1b, step 2). For the Inert/TPAA electrolyte, the decay behavior at 720 nm now represents the recombination processes of TiO2 (e-) with TPAA+ (process b2, Scheme 1b), while the decay behavior at 720 nm for the Co/TPAA 13 ACS Paragon Plus Environment


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electrolytes is attributed to the regeneration kinetics of TPAA+ by the cobalt systems and in combiantion with the recombination of TiO2 (e-) with TPAA+ (process b3, Scheme 1b). A similar approach as the dye regeneration efficiency calculated above in the single redox mediator was used to evaluate the efficiency of process 2, or equivalently, the entire charge transfer efficiency. 3.2.1 Low electron density. Figure 3c and 3d shows that the normalized decays of LEG4/TiO2 at 720 nm under low light and high light excitation, together with the biexponential decay fittings to the decay (Table S2 and Table S3, respectively). Under the low light intensity, the decay time constants at 720 nm show an order of Inert/TPAA (133.5 µs) > PhenCl/TPAA (36.5 µs) > Phen/TPAA (7.9 µs) > BY4/TPAA (2.5 µs) >PY4/TPAA (1.7 µs), which result in a TPAA+ regeneration efficiency or overall charge transfer efficiency (𝜂𝑟𝑒𝑔 /𝜂𝑟𝑒𝑔 ) of BY4/TPAA (98%) ~ 2

PY4/TPAA (99%) > Phen/TPAA (94%) > PhenCl/TPAA (73 %). Noticeably, these regeneration efficiencies show a universal increase of charge transfer efficiency from the cobalt electrolyte to LEG4+ than that of the single redox mediators under low light intensities, indicating that the addition of TPAA can optimize the charge transfer and could result in high performance devices, as shown in previous studies. 3.2.2 High electron intensity In comparison, at high electron density, the decays at 720 nm of the LEG4/TiO2 electrode in the TPAAcontaining electrolytes show an order of Inert/TPAA (68.3 µs) > PhenCl/TPAA (20.8 µs) > Phen/TPAA (6.1 µs) > BY4/TPAA (2.8 µs) >PY4/TPAA (2.1 µs). In strong contrast to the 10-fold increase of the recombination rate between TiO2 (e-) and LEG4+, the recombination rate between TiO2 (e-) and TPAA+ only demonstrates a moderate 2-fold increase of kinetics, with the time constant of 68.3 µs and 133.5 µs, respectively. As a result, the TPAA+ regeneration efficiency/overall charge transfer efficiency (𝜂𝑟𝑒𝑔 /𝜂𝑟𝑒𝑔 ) 2

remains similar to that determined at low electron density, following the order of BY4/TPAA (97%) ~ PY4/TPAA (96%) > Phen/TPAA (91%) > PhenCl/TPAA (69 %). To conclude, the above results collectively show that the addition of TPAA essentially replace the strong carrier dependent recombination pathway between TiO2 and oxidized dye (a2, Scheme 1a) with a weakened carrier dependent recombination path between TPAA+ and TiO2 (e-) (b3, Scheme 2b). This can therefore


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provide an essential way to overcome the former kinetic processes and potentially break the bottleneck of tremendous photo-sensitized systems. Table 2. Photovoltaic performances of solar cells fabricated with four cobalt electrolytes with and without TPAA. Electrolytes

VOC(mV)

JSC (mA/cm )

FF (%)

PCE (%)

PY4

w/o TPAA w TPAA w/o TPAA w TPAA w/o TPAA w TPAA w/o TPAA w TPAA

755 820 785 820 800 845 920 850

12.2 13.1 12.6 13.3 10.6 12.4 11.0 10.0

69.6 73.4 74.4 74.7 71.6 71.7 73.9 71.1

6.4 7.9 7.4 8.2 6.1 7.6 7.5 6.1

BPY Phen PhenCl

(a)

2

Cobalt mediators

(b)

Figure 4. The current-voltage curves (a) and electron lifetimes (b) of solar cells containing the four different cobalt-based electrolytes with and without the addition of 0.1 M TPAA into the electrolytes. 3.3 Device performance. In order to propagate the implication of the above findings to fully operational solar cells, DSSCs based on LEG4/TiO2 electrodes were fabricated, and the performance of the devices investigated. As shown in Figure 4a and Table 2, the addition of TPAA is found to dramatically improve the solar cell efficiency, ~ 11 % to 25% on a relative scale, for all the solar cells investigated except for those based on the PhenCl/TPAA electrolyte system (~ 20% decrease; IPCE shown in Figure S7). Both a substantial enhancement of VOC and JSC are generally observed. For the PY4/TPAA electrolyte, as an example, the addition of TPAA results in a VOC improvement from 755 to 820 mV and a JSC increase from 15 ACS Paragon Plus Environment


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12.2 to 13.1 mA/cm2, which eventually leads to an overall 25% relative increase of the conversion efficiency. These results illustrate the strong relevance of the enhanced charge transfer efficiency observed above and scrutinized in the preceding sections to the device performance. A faster dye regeneration at LEG4/TiO2 allows a prolonged carrier lifetime in the TiO2 substrate and therefore a higher accumulated charge densities in the TiO2. Indeed, additional electron lifetime measurements under quasi steady-state conditions (Figure 4b) show a large increase of lifetime upon addition of TPAA to the electrolytes, around 7-, 5-, and 4- fold increase for the device based on the PY4/TPAA, BPY/TPAA and PhenCl/TPAA electrolytes, respectively. Intriguingly, the addition of TPAA into the PhenCl electrolyte, however, instead leads to a decrease in solar cell performance, as well as a shorter lifetime (~ 2-fold).


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(a)

(c)


(b)

(d)

W/O TPAA

W

Figure 5. UV-Vis spectra of the four cobalt electrolytes with (red line)and without addition (black line) of TPAA: (a) Co(bpyPY4)32+/3+, (b) Co(bpy)32+/3+, (c) Co(phen)32+/3+, (d) Co(phenCl)32+/3+. The electrolyte composition used was 0.22 M Co(L) 32+, 0.05 M Co(L) 33+, 0.2 M TBP, 0.1 M LiClO4 in acetonitrile. The inset graph in panel (c) shows the magnified spectral region between 650 nm and 900 nm. The blue dashdot curve in panel (d) shows the absorption profile of TPAA+ taken from previous studies, while the inset images highlight the color difference of the PhenCl electrolyte with and without added TPAA. In order to further understand this deviation observed for the devices based on the PhenCl/TPAA electrolyte, the optical absorption properties of the electrolytes were investigated when adding TPAA. The appearance of a distinct peak at 720 nm is noted when adding TPAA into the PhenCl electrolyte, while this peak is absent or very weak in the other tandem electrolytes (Figure 5a, b, c). A spectral comparison between the new peak and that of the TPAA+ confirms the likely formation TPAA+ in the PhenCl/TPAA electrolytes. The concentration of the newly formed species is calculated to be ~ 1.7 mM by taking the extinction coefficient of TPAA+ (~ 8.2 ×103 cm-1 M-1) into account, as estimated from a previous report12. These results indicate the occurrence of the following reaction in the system: 17 ACS Paragon Plus Environment


𝐶𝑜[𝐿]3

3+

+ 𝑇𝑃𝐴𝐴 → 𝐶𝑜[𝐿]3

2+

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+ 𝑇𝑃𝐴𝐴+

In the electrolyte mixture causes the redox potentials to equilibrate, i.e., Eredox,Co = Eredox,TPAA. Accordingly, the concentration of the species can be estimated using Eq.5, derived from the Nernst equation (SI): 𝐸𝑟𝑒𝑑𝑜𝑥,𝐶𝑜 − 𝐸𝑟𝑒𝑑𝑜𝑥,𝑇𝑃𝐴𝐴 = 0.059𝑙𝑜𝑔

[𝐶𝑜[𝐿]3

2+

][𝑇𝑃𝐴𝐴+ ]

[𝑇𝑃𝐴𝐴][𝐶𝑜[𝐿]3

3+

]

(Eq.5)

The calculated TPAA+ concentration in all the Co/TPAA electrolytes (Table S4) predict that the largest concentration of TPAA+, ~ 2 mM, to be formed in the PhenCl/TPAA electrolytes. This is in close agreement with the above UV-Vis spectroscopic estimate. Previous studies have shown that, although mediating an ultrafast dye regeneration, the TPAA/TPAA+ (0.22 M/0.05M) redox electrolytes only result in a very poor solar cell performance ( ~ 2 % conversion efficiency) due to dramatic recombination losses from the FTO electrode to the TPAA+ component. Therefore, a steady-state formation of TPAA+ in the electrolyte system can act as a strong recombination loss agent for charges in the FTO electrode and/or TiO2 substrate, leading to bad overall device performance. Therefore, this most likely accounts for the above noted bad performance devices based on the PhenCl/TPAA electrolyte. These results imply that a low driving force between the TPAA and the cobalt mediator will not only result in a low regeneration efficiency of TPAA+, but it will also lead to the oxidation of the electron donor (Co(II)), potentially enhancing the recombination loss. This aspect is of crucial important to the design of the tandem electrolyte systems. From kinetic point of view, a low driving force may still be overcome by the application of redox mediators with very fast dye regeneration efficiency, e.g. TEMPO/TEMPO+16 or copper-based redox complexes. However, the formation of TPAA+ due to the thermodynamic equilibrium constraints may enforce the necessity of a driving force at least higher than 0.2 eV between the redox mediators and TPAA, regardless of the kinetics properties of the redox mediators. 4. CONCLUSION The strong TiO2 (e-) dependent recombination loss between TiO2 and the oxidized dyes is well-known to account for a significant loss of energy in many photoelectrochemical systems. In this work, we have demonstrated that the addition of a small organic donor into ordinary cobalt-based electrolytes substantially reduce the problematic recombination loss between TiO2 (e-) and the oxidized dye especially at illumination levels corresponding to high charge carrier densities, which can boost the solar cell performance by more 18 ACS Paragon Plus Environment

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than 25%. In order to achieve that, a fast regeneration between the oxidized dye and the electron donor need to be fast enough to compete the recombination between TiO2 (e-) and the oxidized dye under high carrier accumulation. On the other hand, the redox potentials of the cobalt electrolytes need to be sufficiently negative (on an NHE scale) to provide a large driving force to ensure the efficient generation of TPAA+ and avoid a significant steady-state concentration of oxidized TPAA in the electrolyte. These insights are crucial for the further exploration of tandem redox electrolytes to overcome the current efficiency bottleneck in many photoelectrochemical systems. Supporting Information Experimental details; Calculation of the carrier densities at the open-circuit condition; UV-Vis spectra of the LEG4/TiO2 film and TPAA in acetonitrile; Transient absorption spectra of the LEG4/TiO2 electrode in the inert and redox electrolytes with and without addition of TPAA. Kinetic comparison at 720 nm and 825 nm of LEG4 in contact with different cobalt-based electrolytes with added TPAA under high electron densities; IPCE of the solar cells; Model fit parameters. Calculation notes. Corresponding authors Yan Hao: [email protected] Gerrit Boschloo: [email protected] Notes The authors declare no competing financial interest ORCID: Yan Hao: 0000-0002-0996-1794 Wenxing Yang: 0000-0002-3440-9416 Gerrit Boschloo: 0000-0002-8249-1469 Acknowledgement: The authors acknowledge the the Stiftelsen Olle Engkvist Byggmästare, Swedish Research Council and the Swedish Energy Agency for the funding supports. 19 ACS Paragon Plus Environment


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Reference: (1)

Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595–6663.

(2)

Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Visible Light Water Splitting Using Dye-Sensitized Oxide Semiconductors. Acc. Chem. Res. 2009, 42, 1966–1973.

(3)

Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient DyeSensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894–15897.

(4)

Shan, B.; Sherman, B. D.; Klug, C. M.; Nayak, A.; Marquard, S. L.; Liu, Q.; Bullock, R. M.; Meyer, T. J. Modulating Hole Transport in Multilayered Photocathodes with Derivatized P-Type Nickel Oxide and Molecular Assemblies for Solar-Driven Water Splitting. J. Phys. Chem. Lett. 2017, 8, 4374–4379.

(5)

Bachmeier, A.; Hall, S.; Ragsdale, S. W.; Armstrong, F. A. Selective Visible-Light-Driven CO2 Reduction on a P-Type Dye-Sensitized NiO Photocathode. J. Am. Chem. Soc. 2014, 136, 13518– 13521.

(6)

Click, K. A.; Beauchamp, D. R.; Huang, Z.; Chen, W.; Wu, Y. Membrane-Inspired Acidically Stable Dye-Sensitized Photocathode for Solar Fuel Production. J. Am. Chem. Soc. 2016, 138, 1174–1179.

(7)

Peter, L. “Sticky Electrons” transport and Interfacial Transfer of Electrons in the Dye-Sensitized Solar Cell. Acc. Chem. Res. 2009, 42, 1839–1847.

(8)

O’Regan, B. C.; Durrant, J. R. Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real. Acc. Chem. Res. 2009, 42, 1799–1808.

(9)

Haque, S. A.; Tachibana, Y.; Klug, D. R.; Durrant, J. R. Charge Recombination Kinetics in DyeSensitized Nanocrystalline Titanium Dioxide Films under Externally Applied Bias. J. Phys. Chem. B 1998, 102, 1745–1749.


Page 21 of 23 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

(10)


Robson, K. C. D.; Hu, K.; Meyer, G. J.; Berlinguette, C. P. Atomic Level Resolution of Dye Regeneration in the Dye-Sensitized Solar Cell. J. Am. Chem. Soc. 2013, 135, 1961–1971.

(11)

Durrant, J. R.; Haque, S. A.; Palomares, E. Towards Optimisation of Electron Transfer Processes in Dye-Sensitised Solar Cells. Coord. Chem. Rev. 2004, 248, 1247–1257.

(12)

Hao, Y.; Yang, W.; Zhang, L.; Jiang, R.; Mijangos, E.; Saygili, Y.; Hammarström, L.; Hagfeldt, A.; Boschloo, G. A Small Electron Donor in Cobalt Complex Electrolyte Significantly Improves Efficiency in Dye-Sensitized Solar Cells. Nat. Commun. 2016, 7, 13934.

(13)

Feldt, S. M.; Gibson, E. a; Gabrielsson, E.; Sun, L.; Boschloo, G.; Hagfeldt, A. Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 16714–16724.

(14)

Feldt, S. M.; Lohse, P. W.; Kessler, F.; Nazeeruddin, M. K.; Grätzel, M.; Boschloo, G.; Hagfeldt, A.; Gratzel, M.; Boschloo, G.; Hagfeldt, A. Regeneration and Recombination Kinetics in Cobalt Polypyridine Based Dye-Sensitized Solar Cells, Explained Using Marcus Theory. Phys. Chem. Chem. Phys. 2013, 15, 7087–7097.

(15)

Boschloo, G.; Hagfeldt, A. Characteristics of the Iodide/triiodide Redox Mediator in Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1819–1826.

(16)

Yang, W.; Vlachopoulos, N.; Hao, Y.; Hagfeldt, A.; Boschloo, G. Efficient Dye Regeneration at Low Driving Force Achieved in Triphenylamine Dye LEG4 and TEMPO Redox Mediator Based DyeSensitized Solar Cells. Phys. Chem. Chem. Phys. 2015, 17, 15868–15875.

(17)

Gabrielsson, E.; Ellis, H.; Feldt, S.; Tian, H.; Boschloo, G.; Hagfeldt, A.; Sun, L. Convergent/Divergent Synthesis of a Linker-Varied Series of Dyes for Dye-Sensitized Solar Cells Based on the D35 Donor. Adv. Energy Mater. 2013, 3, 1647–1656.

(18) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. 21 ACS Paragon Plus Environment


(19)

Page 22 of 23

Koops, S. E.; Barnes, P. R. F.; O’Regan, B. C.; Durrant, J. R. Kinetic Competition in a Coumarin Dye-Sensitized Solar Cell: Injection and Recombination Limitations upon Device Performance. J. Phys. Chem. C 2010, 114, 8054–8061.

(20)

Yang, W.; Pazoki, M.; Eriksson, A. I. K.; Hao, Y.; Boschloo, G. A Key Discovery at the TiO2 /Dye/Electrolyte Interface: Slow Local Charge Compensation and a Reversible Electric Field. Phys. Chem. Chem. Phys. 2015, 17, 16744–16751.

(21)

Yang, W.; Vlachopoulos, N.; Boschloo, G. Impact of Local Electric Fields on Charge-Transfer Processes at the TiO2/Dye/Electrolyte Interface. ACS Energy Lett. 2017, 2, 161–167.

(22)

Cappel, U. B.; Feldt, S. M.; Schöneboom, J.; Hagfeldt, A.; Boschloo, G. The Influence of Local Electric Fields on Photoinduced Absorption in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2010, 132, 9096–9101.

(23)

Feldt, S. M.; Wang, G.; Boschloo, G.; Hagfeldt, A. Effects of Driving Forces for Recombination and Regeneration on the Photovoltaic Performance of Dye-Sensitized Solar Cells Using Cobalt Polypyridine Redox Couples. J. Phys. Chem. C 2011, 115, 21500–21507.

(24)

Nelson, J.; Chandler, R. E. Random Walk Models of Charge Transfer and Transport in Dye Sensitized Systems. Coord. Chem. Rev. 2004, 248, 1181–1194.

(25) Boschloo, G.; Fitzmaurice, D. Electron Accumulation in Nanostructured TiO2 (Anatase) Electrodes. J. Electrochem. Soc. 2000, 147, 1117. (26)

DiMarco, B. N.; Troian-Gautier, L.; Sampaio, R. N.; Meyer, G. J. Dye-Sensitized Electron Transfer from TiO2 to Oxidized Triphenylamines That Follows First-Order Kinetics. Chem. Sci. 2018, 9, 940– 949.


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TOC Graphic:


Carrier Dynamics of Dye Sensitized-TiO2 in Contact with Different

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