Efficient Dye-Sensitized Solar Cells with Voltages Exceeding 1 V

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Letter

Efficient Dye-sensitized Solar Cells with Voltages Exceeding 1 V Through Exploring Tris(4-alkoxyphenyl)amines Mediators in Combination with the Tris(bipyridine) Cobalt Redox System Yan Hao, Wenxing Yang, Martin Karlsson, Jiayan Cong, Shihuai Wang, Xing Li, Bo Xu, Jianli Hua, Lars Kloo, and Gerrit Boschloo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00872 • Publication Date (Web): 15 Jul 2018 Downloaded from http://pubs.acs.org on July 15, 2018

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ACS Energy Letters

Efficient Dye-sensitized Solar Cells with Voltages Exceeding 1 V Through Exploring Tris(4alkoxyphenyl)amines Mediators in Combination with the Tris(bipyridine) Cobalt Redox System Yan Hao,1ǁ* Wenxing Yang,2ǁ*# Martin Karlsson,1 Jiayan Cong,1 Shihuai Wang,2 Xing Li,3 Bo Xu,2 Jianli Hua, 3Lars Kloo,1 and Gerrit Boschloo2 1

Department of Chemistry, Applied Physical Chemistry, KTH Royal Institute of Technology,

SE-10044 Stockholm, Sweden. 2

Department of Chemistry – Ångström Laboratory, Physical Chemistry, Uppsala University,

SE-75120 Uppsala, Sweden. 3

Key Laboratory for Advanced Materials, Institute of Fine Chemicals, School of Chemistry and

Molecular Engineering, East China University of Science and Technology, 200237 Shanghai, P. R. China. #

Present Address: Department of Chemistry, Emory University, 1515 Dickey Drive, NE, Atlanta,

GA 30322, USA.

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ABSTRACT. Tandem redox electrolytes, prepared by the addition of a tris(p-anisyl)amine mediator into classic tris(bipyridine)cobalt based electrolytes, demonstrate a favorable electron transfer and reduced energy loss in dye-sensitized solar cells. Here, we have successfully explored three tris(4-alkoxyphenyl)amines mediators with bulky molecular structures and generated more effective tandem redox systems. This series of tandem redox electrolytes rendered solar cells with very high photovoltages exceeding 1 V, which approaches the theoretical voltage limit of tris(bipyridine)cobalt based electrolytes. Solar cells with power conversion efficiencies of 9.7% to 11.0% under one sun illumination were manufactured. This corresponds to an efficiency improvement of up to 50% as compared to solar cells based on pure tris(bipyridine)cobalt based electrolytes. The photovoltage increases with increasing steric effects of the tris(4-alkoxyphenyl)amine mediators, which is attributed to a retarded recombination kinetics. These results highlight the importance of structural design for optimized charge transfer at the sensitized semiconductor/electrolyte interface and provide insights for the future development of efficient dye-sensitized solar cells.


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ACS Energy Letters

TOC GRAPHIC


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Since O’Regan’s and Grätzel’s seminal paper on dye-sensitized solar cells (DSSCs) in 1991,1 a large research effort has been devoted to the insight and improvement of this type of molecular photovoltaic (PV) devices. DSSCs represent a feasible alternative to silicon-based solar devices due to reasonable efficiencies, possibilities for transparent and multiple color devices and competitive low-cost materials and processing.2-3 Furthermore, there is an increasing interest in indoor applications for this technology. More recently, however, the DSSC development has been shadowed by the emergence of perovskites solar cells (PSCs), originally derived from the solid state DSSC platform.4-5 The impressive progress of PSCs in just a few years rendering power conversion efficiencies (PCEs) exceeding 20%6-9 makes the development of DSSCs, with a reported record PCE of 14.0% after 25 years of research, look outdated.10-14 However, DSSCs, as well as organic photovoltaics (OPV), can definitely compete in the field of outdoor PV applications providing that stable efficiencies exceeding 15.0% can be achieved. In terms of the output voltage, the performance of DSSCs and OPV is relatively poor. For DSSCs the absorption threshold of the sensitizing dyes is typically in the range 1.7 to 2.0 eV, while the open-circuit potential typically recorded is well below 1.0 V. In essence, this means that about 50% of the absorbed photon energy is lost in the device charge separation and transfer processes. As we will demonstrate in this work, there is a clear potential to significantly minimize such energy losses. The redox mediator is a key component in the electrolyte of DSSCs and essential for generating high conversion efficiencies. Its function is to regenerate the oxidized dye after photoinduced electron injection, and to transport charge between the two electrodes in the solar cell.1517

Dye regeneration must occur rapidly to avoid charge recombination losses between the photo-

oxidized dye and injected electrons in the TiO2 substrate.18-20 Slow kinetics of dye regeneration, for instance due to an insufficient driving force for this process, may result in both low open-


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circuit voltage (VOC) and low short-circuit current density (JSC) caused by the fast recombination of the electrons in TiO2 with the oxidized dye molecules. On the other hand, an excessive driving force for the regeneration process leads to a lower VOC, limiting the power conversion efficiency. Hence, much effort has been devoted to identify an optimal driving force for dye regeneration,12, 21-25

as well as aiming for an efficient non-corrosive electron transfer mediator, both of which are

very important for the practical application of DSSCs. The iodide/triiodide system initially used in DSSCs remained the most versatile redox mediator for many years because of the slow recombination kinetics and relatively fast dye regeneration rates. Iodine-based electrolytes have shown near-unity internal quantum efficiencies. In particular, the small size of the I-/I3- redox components allows for relatively fast diffusion within the mesoporous substrate, and the two-electron system allows for a high current to be passed through the device. The favorable electron transfer kinetics of the I-/I3- redox system contributed to its unrivalled position for 20 years, and energy conversion efficiencies of over 11.0% have been reached.26-27 Unfortunately, the corrosive nature and the substantial thermodynamic loss in the dye regeneration process of the I-/I3- system limit its overall performance and thus applicability. Iodine-free redox mediators have emerged as interesting alternatives. Excellent progress in efficient devices has been made by exploring new redox mediators, such as Co2+/Co3+ complexes, going from 6.7% to 14.0% in just a few years,10-11, 22organic redox mediators, giving devices with a performance over 6.0%,21, 23, Ferrocene (Fc/Fc+) at 7.5%24 and recently Cu+/Cu2+ complexes with device efficiencies reported up to 10%.28 Undoubtedly, one-electron transfer mediators, such as cobalt and copper complex systems, have shown particular promises offering efficiencies over 10.0%. However, the Co2+/Co3+ complex based electrolytes have suffered from


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fast recombination loss and slow dye regeneration kinetics, which appear to be unavoidable energy losses limiting the maximum PCE of the corresponding solar cells.29 Even though significant progress has been made for devices based on copper complexes in recent years, such systems still show disadvantages related to the chemical instability of the copper complexes caused by ligand exchange and unwanted redox reactions,30 none of which are beneficial for the industrialization of such solar cells. The very interesting strategy of using redox mediator mixtures was reported by Bignozzi and co-workers in 2006,31 in which a co-mediator, characterized by a fast electron transfer reaction, was mixed with a cobalt complex redox system characterized by rather slow kinetics. But at that time, the resulting solar cell performance was low. Later, Cong and co-workers32 added 2,2,6,6tetra-methyl-1-piperidinyloxy (TEMPO) into a cobalt-based electrolyte, which gave a significant improvement of efficiency in devices based on the classic organic dye LEG4, from 7.1% to 8.4%. Unfortunately, the stability of this system was rather poor because of the intrinsic chemical instability of TEMPO.21, 33 In a recent work from our lab, we showed that the addition of tris(p-anisyl)amine (TPAA) to a classic tris(bipyridine)cobalt (Co(bpy)32+/3+) based electrolyte led to a large improvement of the performance of the DSSCs, from 7.2% to 9.1% in devices based on the dye LEG4.34 This is due to the fast dye regeneration reaction primarily mediated by TPAA, on a nanosecond timescale, which effectively reduces electron recombination losses to the oxidized dye. In this tandem electrolyte, the processes of the dye regeneration and the charge transport were demonstrated to be separated; dye regeneration is primarily mediated by TPAA, while Co(bpy)32+/3+ mainly mediates the charge transport in the electrolyte. In this work, we have explored three tris(4-alkoxyphenyl)amines mediators with a consecutive increase in the length of the substituted alkoxy chains: tris(4-ethoxyphenyl)amine


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(TPEA), tris(4-propoxyphenyl)amine (TPPA) and tris(4-butoxyphenyl)amine (TPBA), to form the tris(4-alkoxyphenyl)amines/tris(bipyridine)cobalt (TPA/Co) (Figure 1) based tandem redox electrolytes. The steric effects by lengthening the TPA alkoxy chain are expected to cause a retardation of electron transfer reactions.22, 35 The naive reasoning behind these arguments is based on the assumption that longer inter-system distances applying a Marcus model cause a higher barrier for electron transfer and thus slower reaction rates. By combining these tandem redox electrolytes with a D–A–π–A organic dye (AQ310) (Figure 1), which shows a broad light absorption in the visible spectrum,36 efficient DSSCs have been fabricated with total power conversion efficiencies of 9.7% to 11.0% under standard AM 1.5 G illumination. A significant improvement of the DSSC performance was observed for all tandem redox systems, 35% - 50% improvement, with respect to that of the pure tris(bipyridine)cobalt based electrolyte (Co). The improvement is caused by both an increase of the JSC and in the VOC, with the latter demonstrating over 1.0 V for devices based on the TPPA/Co and TPBA/Co electrolytes. Detailed transient absorption spectroscopy measurements confirm an improvement in dye regeneration efficiency, over 91% in contrast to 71% for the pure Co electrolyte. This is attributed to an accelerated dye regeneration rate induced by the TPA mediators in the tandem redox system, which counteracts the fast recombination process between the oxidized A310 and the injected electrons in TiO2. In particular, it is the recombination kinetics in the tandem redox systems that are significantly slowed down and that display a dependency on the length of the TPA alkoxy chains. The main effect is observed in the VOC. We believe that the strategy of designing tandem redox electrolytes with different components optimized for different functions in the electrolyte may pave the way to a renaissance of dye-sensitized solar cells.


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Figure 1. The molecular structures of a series of tris(4-alkoxyphenyl)amines (TPA), [Co(bpy)3]2+/3+ and the and the organic dye used in this work (AQ310). Initially, TPEA, TPPA and TPBA were synthesized according to the general synthetic route shown in Scheme S1 and detailed synthesis method in the supporting information. The TPA molecules differ only in the length of the substituted alkoxy chains. The structural changes show only minor impact on the optical and electrochemical properties of the molecular materials. The increase in the alkoxy chain length slightly weakens the electron donating properties of the oxygen atom of the benzyl group. All TPAs show ground state absorption peaks at around 290 nm and extinction coefficients of ~22600 M-1 cm-1 (Figure S1), while the oxidized TPAs all show distinct new peaks at 375, 610 (shoulder) and 720 nm (Figure S2). Further electrochemical studies confirm that the redox potentials of all the TPA molecules are ~ (0.78 ±0.01) V (vs. NHE) and the diffusion coefficients are estimated to ~ 1*10-5 cm2/s in acetonitrile (Figure S3).


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Figure 2. DSSCs based on the AQ310 dye and TPA/Co tandem redox electrolytes. (a) Current density versus applied potential under 100 mW cm-2 AM 1.5G illumination (solid lines) and in the dark (dashed lines). (b) Incident photon-to-current conversion efficiency (IPCE) spectra. The solar cells were fabricated with four TPA/Co tandem redox electrolytes, as well as the pure cobalt-based electrolytes (Co) in combination with the single D-A-π-A organic dye (AQ310) as sensitizer36. The standard Co electrolyte contained 0.22 M Co(bpy)3(PF6)2, 0.05 M Co(bpy)3(PF6)3, 0.10 M LiClO4 and 0.20 M TBP in acetonitrile, and the addition of 0.10 M TPAA, TPEA, TPPA and TPBA into this standard Co electrolyte generated the TPAA/Co, TPEA/Co, TPPA/Co and TPBA/Co tandem redox electrolytes, respectively. The current density-voltage (J-V) characteristics of the DSSCs devices are shown in Figure 2a and the corresponding photovoltaic parameters are summarized in Table 1. The solar cells fabricated with Co electrolyte demonstrate a moderate efficiency of 7.3%. The tandem redox electrolyte TPAA/Co demonstrated a significantly improved solar cell performance of 10.0%, representing a 37% of increase relative to that of Co electrolyte. This


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Table 1. Photovoltaic performance of DSSCs based on the AQ310 dye and different electrolytes and at different light intensities (AM 1.5G spectral distribution and an active area of 0.25 cm2).

Electrolytes

Light intensity (mW cm-2)

VOC (mV)

JSC (mA cm-2)

IPCE Integral current JSC (mA cm-2)

Fill Factor (%)

Co

100

775

12.7

12.4

74.3

7.3

TPAA/Co

100

915

14.6

14.5

74.9

10.0

TPEA/Co

100

950

15.5

15.4

74.5

11.0

50 33 11

925 910 870

8.2 6.0 2.0

75.3 76.1 73.9

11.4 12.6 11.7

TPPA/Co

100

1010

14.4

14.3

72.2

10.5

TPBA/Co

100

1030

13.7

13.7

69.0

9.7

PCE (%)

enhancement in efficiency is much higher than compared to devices based on another single dye, LEG4, with the same electrolyte.34 This result does not only confirm the significant enhancement of the solar cells performance by the addition of the TPA mediator, but also points to the importance of the combination of dye and the tandem redox system (see discussion below). Most important in this work, by lengthening the alkoxy chain, the tandem redox electrolytes TPEA/Co and TPPA/Co rendered solar cells showing even higher efficiencies of 10.5% and 11.0%, respectively, while the electrolyte TPBA/Co offered a comparable efficiency, of 9.7%, to that of the TPAA/Co electrolyte. The clear improvement of both photocurrent and photovoltage was shown by the tandem redox systems. The different TPAs show a difference in the VOC of the corresponding devices. Increasing of the length of the alkoxy chains from 1 to 4 carbon atoms, the photovoltage of the devices improves almost linearly from 915 mV (TPAA) to 1030 mV (TPBA) (Figure S5). This VOC dependence on alkoxy chain length could be related to the recombination processes, since the VOC depends on the electron concentration in the TiO2 substrate.16


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(a)

(b)

Figure 3. (a) Electron lifetimes as a function of voltage under open-circuit conditions for DSSCs sensitized with AQ310 employing the Co and TPA/Co electrolytes. (b) Transient absorption spectra of AQ310-sensitized TiO2 electrodes in contact with an inert electrolyte (0.2 M TBP, 0.1 M LiClO4 in acetonitrile; blue line) or a TPAA electrolyte (addition of 0.1 M TPAA into the inert electrolyte; red line). The dashed line represents the absorption spectrum of TPAA•+ in acetonitrile obtained by spectroelectrochemistry. Laser excitation wavelength: 620 nm. Laser intensity: 1.8 mJ/pulse. Assuming a flat band potential of the conduction band of TiO2 at -0.5 V (vs. NHE) and a redox potential of the cobalt bipyridyl redox mediator at 0.56 V (vs. NHE), the maximum achievable photovoltage is estimated to be 1.06 V, Vmax=VCB-Vredox. The obtained high VOC of the devices based on the TPBA/Co electrolyte, being very close to the maximum achievable VOC, indicates an efficient retardation of charge-recombination loss reactions in the system. The reduction of recombination losses by lengthening the alkoxy chain of TPAs was further confirmed by combining three experimental techniques: electron lifetimes (Figure 3a), determined by the voltage decay of the DSSCs in response to a small light intensity modulation, dark currents


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(dashed lines in Figure 2a), as well as impedance spectra (Figure S6). Figure 3a shows that an increase of alkoxy chain length on the TPA molecules shifts the voltage-electron lifetime curves, giving a longer lifetime at a specific voltage. Similar lifetimes were obtained from the impedance spectra and lead to the same conclusion. The dark currents of the DSSCs reflect a significant retardation of electron recombination rate between the FTO back contact and the oxidized redox species in the tandem redox electrolytes.35 A noticeable increase of the onset potential and a reduced dark current at the same potential (>0.7 V) were noted to correlate with a longer alkoxy chains of the TPAs. Overall, devices with the tandem redox electrolyte of TPEA/Co demonstrate a highest solar cell performance of 11.0% under 100 mW cm-2 of simulated sunlight and a further improved efficiency of 12.6% at 33 mW cm-2. These efficiencies are among the best reported for DSSCs and highlight the potential of DSSCs under low light intensity. Especially for a single organic dye based DSSCs, only very few studies have so far reported efficiencies up to 11.0%. To our knowledge, only Peng Wang’s group has reported similar efficiencies through the design of quite complex dye molecules.37-38 The statistics and the average values of a large number of devices are shown in Figure S4 and Table S6, which demonstrate good reproducibility of TPA/Co tandem redox electrolyte based DSSCs. It should be noted that the investigated devices did not have any anti-reflecting coating, which is likely to further increase the PCEs of these systems. The incident photon-to-current efficiency (IPCE) spectra of the DSSCs were recorded and are shown in Figure 2b. The trend of the IPCE is consistent with that of the JSC’s of all DSSCs. Compared to DSSCs containing the Co electrolyte, the maximum IPCE increases from approximately 65% to over 80% for the best system containing the TPEA/Co electrolyte. The integrated currents obtained from the IPCE spectra (Table 1) show good agreement with the


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JSC’s obtained from the J-V measurements. The low IPCE of AQ310 in combination with the Co electrolyte is noticeable and will be further discussed below. In the following section, we describe results from transient absorption spectroscopy measurements to elucidate the effect of the TPA alkoxy chain length on the charge transfer processes involved. Figure 3b shows the transient absorption spectra of AQ310-sensitized TiO2 electrodes, abbreviated as AQ310/TiO2, in contact with two types of electrolytes: an inert electrolyte (0.2 M TBP, 0.1 M LiClO4 in acetonitrile) and an electrolyte coded as TPAA (0.1 M TPAA added to the inert electrolyte). In the case of the inert electrolyte, photo-excitation of AQ310/TiO2 results in the formation of AQ310+/TiO2(e-) after an ultrafast charge injection from the excited state of AQ310 into TiO2, a process typically occurring in the fs to ps regime.39 The appearance of the broad absorption signal between 700 nm to 900 nm (Figure 3b) therefore represents the transient absorption from AQ310+, as further confirmed by the previous photo-induced absorption study by Xing Li et al.36 Noticeably, the addition of TPAA into the inert electrolyte, i.e. TPAA electrolyte, leads to both the formation of a new peak at ~730 nm, as well as a simultaneous decrease of the peak intensity at ~820 nm. Previous studies have shown that TPAA•+ has a characteristic absorption peak at 720 nm. It is, therefore, reasonable to attribute the peak at 730 nm to the spectral overlap between oxidized AQ310+ and TPAA•+. This co-occurrence of TPAA•+ and AQ310+ in the electrolyte after only 10 ns excitation implies that the dye regeneration of AQ310+ by TPAA is not complete within a 10 ns timescale. With the above spectral assignments, additional kinetic studies (Figure 4) were performed to investigate the mechanism of the device improvement when combining AQ310 with the tandem


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redox electrolytes, and particularly the key role of the alkoxy chains of the TPAs. The electrolytes used for the kinetic measurements include inert, TPA electrolytes (0.1 M TPAA, TPEA, TPPA or TPBA in the inert electrolyte) and the TPA/Co tandem redox electrolytes mentioned above. In all electrolytes, the decay at 825 nm (Figure 4a, 4c) originates from AQ310+ alone, and can therefore be used to characterize the decay of AQ310+. For the inert electrolyte, the only possible decay path for AQ310+ is from the charge recombination between AQ310+ and TiO2(e-), characterized by a decay half time constant (t1/2) of ~1 µs (Figure 4a). The TPA electrolytes show a significant and 10-fold increase of the decay rate at 825 nm, with a reduced decay t1/2 of 0.1 µs. This result shows that the TPA mediators can significantly accelerate the regeneration of AQ310+. The kinetic decay traces show no apparent dependence on the type of TPA, indicating that the difference in the length of the alkoxy chains in the TPAs studied has little impact on the AQ310+ regeneration processes. Even in the tandem redox TPA/Co electrolytes (Figure 4c), the decay t1/2 of AQ310+ is of the same order as that of pure TPA electrolytes. This leads to the conclusion that the regeneration of AQ310+ is mainly mediated by the TPAs in the tandem redox electrolytes.


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Figure 4. The normalized kinetic decays of AQ310/TiO2 in contact with different TPA and TPA/Co electrolytes monitored at 825 nm (a, c) and 720 nm (b, d). Herein, TPA represents electrolytes with 0.1 M TPAs added to the inert electrolytes, while TPA/Co tandem redox electrolytes stands for electrolytes with 0.1 M TPAs added into the cobalt-based electrolyte. The yellow lines mark the bi-exponential models fitted to the experimental decays (parameters are shown in Tables S1 and S2), with details described in the main text. Laser excitation wavelength: 620 nm. Laser intensity: 1.8 mJ/pulse.


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The kinetic decays at 720 nm (Figure 4b, 4d) attributed to the absorption signal from both TPAA•+ and AQ310+ were also studied. For all the TPA type electrolytes (Figure 4b), a clear biexponential decay process was observed regardless of the type of TPA, as well as a clear dependence of the decay traces on the TPA alkoxy chain length. The fast decay appears immediately after the laser excitation and lasts until ~1 µs, with a t1/2 ~0.1 µs for all the TPAs. These decays are analogous to those recorded at 825 nm and discussed above. Therefore, it is reasonable to assume that they originate from the regeneration of AQ310+ by the TPA mediator. In contrast, the slow decay path persists up to 1000 µs and must therefore originate from the reaction of TPA•+ with the electrons in TiO2(e-). Modeling of the second decay process (Note S3) indicates a significant increase of the decay t1/2 constant in the order of TPAA (20 µs) < TPEA (64 µs) < TPPA (98 µs) < TPBA (111 µs). The results show that the longer alkoxy chain on TPA efficiently retards the charge recombination loss between TPA•+ and TiO2 (e-). However, for the tandem redox TPA/Co electrolytes, as a result of the redox potential difference between the TPA and the tris(bipyridine)cobalt redox system (ΔE ~ 0.22 eV), the generated TPA•+ will react with the Co(bpy)32+, and most likely through an outer-sphere electron transfer mechanism. We further studied the effects of the alkoxy chains on the reaction between the TPA•+ and the Co(bpy)32+ (Figure 4d with the original data shown in Figure S7). The decay trace at 720 nm from the TPA/Co tandem redox electrolytes demonstrates a clear dependence on the type of TPA in the slow decay process, but also a bleach feature. This bleach feature is attributed to the Stark effect of the AQ310, as the prolonged electron lifetime of the system enhances the intrinsic electric field between TiO2(e-) and cations in the electrolyte affecting the dye (for details, see Figure S8, and discussions in Note S2). Therefore, the slow component of the decay at 720 nm represents the spectral envelope of charge compensation of the local electric


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field

40

and the regeneration of the TPA by the Co(bpy)32+. The charge compensation of the

system can be presumed to be the same for all the TPA/Co electrolytes.41 The time at the minimum of the bleach can therefore serve as an indication of the relative reaction rates of the TPA•+ with the Co(bpy)32+, following the order TPAA (15 µs) < TPEA (29 µs) < TPPA(31 µs) < TPBA (56 µs). The approach described above most likely leads to an overestimation of the time constant for the reaction between TPA•+ and the Co(bpy)32+. Nevertheless, it is clear that the reaction between the TPA•+ and the Co(bpy)32+ is retarded as a function of the alkoxy chain length. Overall, the above results conclude that an increase of the TPA alkoxy chain length can both retard the charge recombination between TPA•+ and TiO2(e-), as well as the charge transfer between TPA•+ and the Co(bpy)32+. Given the competitive nature of these two processes, it is important to know the effective regeneration of TPA•+ by the Co(bpy)32+, and its dependence on the TPA alkoxy chain length. In order to further quantify the overall regeneration efficiency of TPA•+ by Co(bpy)32+, we applied another dye, LEG4, sensitizing TiO2 (LEG4/TiO2), whose Stark effect bleach is clearly separated from the absorption of the TPA•+, to facilitate the study of the impact of the length of alkoxy chain on the TPA•+ regeneration efficiency. As the charge transfer between TPA•+ and TiO2(e-) or Co(bpy)32+ are presumed to be independent of the dye molecule used under the condition that the fast dye regeneration process takes place at approximately ns timescale for both two dyes but depends on the electron concentration in TiO2(e-). We applied a similar laser intensity to ensure a comparative amount of charge carriers in the TiO2, as well as amount of TPA•+ formed. Indeed, just as for the AQ310/TiO2 system, both the charge transfer rates, defined as k = 1/t1/2, for the reactions between TPA•+ and TiO2(e-) as well as between TPA•+ and


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Co(bpy)32+, show a decrease with increasing length of the TPA alkoxy chains (Figure 5a, Figure S9 and Table S3 and S4, with details discussed in Note S3, ) following an empirical exponential equation 1: k ~ A exp (-β * n)

(1)

where A represents the limiting charge transfer rate between the reactants and n is the length of the alkoxy chain unit, while the value β represents the sensitivity of the charge transfer reaction to the length of the alkoxy chain. The results obtained from least-squares fits of eq. (1) show that the β values are 0.62 and 0.30 for the recombination reaction with TiO2(e-) and for the regeneration by Co(bpy)32+, respectively. These results indicate that the recombination process between TiO2(e-) and TPA•+ shows a stronger dependence on the length of the alkoxy chain of the TPAs than the regeneration of TPA•+ by Co(bpy)32+ (Figure 5b). In other words, this indicates that the recombination with TiO2(e-) is stericallymore affected by the TPA chain length.

Figure 5. (a) The change of reaction rates between the TPA•+ with TiO2(e-) and Co(bpy)32+, respectively, as a function of the alkoxy chain length on the donor part of the TPAs. (b) the


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ACS Energy Letters

schematic summary of the effects of the alkoxy chains on the corresponding charge transfer reactions discussed in the manuscript. Furthermore, the regeneration efficiency of the TPA•+ by Co(bpy)32+ was estimated and shows the order TPAA•+ (88%) < TPEA•+ (94%) ≈ TPPA•+ (92%) ≈ TPBA•+ (94%). Hence, it is reasonable to conclude that even that both the charge transfer process between TPA•+ and TiO2(e-), as well as with Co(bpy)32+, are retarded, the process of TPA•+ reduction by Co(bpy)32+ in all four TPA/Co electrolytes is ‘fast enough‘. In contrast, the significantly retarded recombination between the TPA•+ and TiO2(e-) contributes significantly to the high VOC and high performance of the resulting DSSCs. The improvement triggered by all TPA/Co tandem redox electrolytes (as high as 50%) of the DSSC performance in the AQ310 sensitized systems in the present study should be noted. This is much higher than the 25% improvement reported for the previously studied LEG4 sensitized system. Therefore, further kinetic studies of the LEG4/TiO2 and AQ310/TiO2 electrodes in contact with both the inert and the Co electrolyte were undertaken in order to elucidate the reasons behind the difference. Figure S11 shows the kinetic decay of ∆A at 720 nm, where both LEG4+ and AQ310+ (see Figure 3) show strong absorption, for the LEG4/TiO2 and AQ310/TiO2 electrodes after photoexcitation in both types of electrolytes. The dye regeneration rate of both dyes by the Co electrolyte is comparable, with a t1/2 of 1.2 µs and 1.5 µs for AQ310 and LEG4, respectively. However, AQ310+ demonstrates a much faster dye recombination rate with electrons in TiO2 (t1/2 ~ 4.1 µs) when in contact with the inert electrolyte. This is substantially faster than for LEG4+ (t1/2 ~ 16.1 µs). This implies more severe charge recombination loss problems in the


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AQ310+/TiO2(e-) system as compared to that in the LEG4+/TiO2(e-) system. As a result, the dye regeneration efficiency of AQ310 was calculated to be only ~71% in contact with the Co electrolyte (91% for LEG4, as seen in Table S5), which therefore explains the low IPCE of the AQ310 in combination with the Co electrolyte in the DSSC devices discussed above. However, as shown previously, upon applying the tandem redox electrolytes, the dye regeneration efficiency of AQ310 sensitized DSSCs improved to more than 90%. It is, therefore, reasonable to attribute the large improvement of the performance of devices based on AQ310 to the effective suppression of recombination losses by the tandem redox electrolytes allowing exploitation of the AQ310 superior light absorbing properties in spite of its regeneration loss sensitivity. Furthermore, the newly designed TPA mediators with more pronounced steric effects may tune the recombination kinetics, therefore allowing higher VOC and JSC. These results highlight that the tandem redox electrolytes should be more applicable to sensitizers suffering from severe recombination loss problems. In Summary, we have designed and synthesized three novel tris(4-alkoxyphenyl)amine mediators for the application in tandem redox systems for DSSCs. When combined with a broad absorption dye of D-A-π-A type, AQ310, these mediators lead to solar cells with efficiency up to 12.6% and 11.0% under 0.3 sun and 1 sun illumination, respectively, which is promising for DSSCs based on a single organic sensitizer. The photovoltaic performance was improved by as much as 50% as compared to DSSCs based on a pure tris(bipyridine)cobalt redox electrolyte. The improvement of the photovoltaic performance is attributed to an increase in both VOC and JSC, particular the VOC higher than 1 V approaching to the theoretical voltage limit. Detailed kinetic studies show unambiguously that, although the oxidized AQ310+ suffers from severe recombination loss reactions with TiO2(e-), application of the TPA mediators in tandem redox


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ACS Energy Letters

electrolytes can effectively overcome the recombination problem by an even faster dye regeneration process. The strategy of tuning the length of the TPA alkoxy chains developed here can give further flexibility to retard charge recombination and optimize the charge transfer scheme. These results pave the way to a renaissance of dye-sensitized solar cells by the combination of high-performing sensitizers and controlled electron transfer processes. ASSOCIATED CONTENT Supporting Information Supporting information includes the experimental section, supporting figures and notes. The supporting Information is available free of charge on the ACS Publications website at DOI: XXX. AUTHOR INFORMATION Corresponding Author *Correspondence to: [email protected], [email protected]. ORCID Yan Hao: 0000-0002-0996-1794 Wenxing Yang: 0000-0002-3440-9416 Gerrit Boschloo: 0000-0002-8249-1469 Lars Kloo: 0000-0002-0168-2942


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Notes The authors declare no competing financial interests. ǁ

These authors contributed equally to this work

ACKNOWLEDGMENT We gratefully acknowledge the Swedish Energy Agency, the Swedish Research Council (VR), the STandUP for Energy program, the Knut and Alice Wallenberg and Stiftelsen Olle Engkvist Byggmästare Foundations for the financial support. Dr. Biaobiao Zhang is acknowledged for HRMS and cyclic voltammetry measurement. Ms. Linqin Wang is acknowledged for transporting chemicals. Mr. Roger Jiang is thanked for transient current measurements. Dr. Leif Häggman is thanked for his great technical support. REFENRENCES: 1. O'Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar Cell Based on DyeSensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. 2. Goncalves, L. M.; de Zea Bermudez, V.; Ribeiro, H. A.; Mendes, A. M. Dye-Sensitized Solar Cells: A Safe Bet for The future. Energy Environ. Sci. 2008, 1, 655-667. 3. Nazeeruddin, M. K.; Baranoff, E.; Grätzel, M. Dye-Sensitized Solar Cells: A Brief Overview. Sol. Energy 2011, 85, 1172-1178. 4. 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-SolidState Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep 2012, 2, 591. 5. Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. 6. Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; et al. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206209. 7. Anaraki, E. H.; Kermanpur, A.; Steier, L.; Domanski, K.; Matsui, T.; Tress, W.; Saliba, M.; Abate, A.; Gratzel, M.; Hagfeldt, A.; et al. Highly Efficient and Stable Planar Perovskite Solar Cells by Solution-Processed Tin Oxide. Energy Environ. Sci. 2016, 9, 3128-3134.


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Efficient Dye-Sensitized Solar Cells with Voltages Exceeding 1 V

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