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Cs2SnI6 Encapsulated Multi-Dye Sensitized All Solid-State Solar Cells Byunghong Lee, Yamuna Ezhumalai, Woongkyu Lee, MingChou Chen, Chen-Yu Yeh, Tobin J. Marks, and Robert P. H. Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19778 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Applied Materials & Interfaces

Cs2SnI6 Encapsulated Multi-Dye Sensitized All Solid-State Solar Cells Byunghong Lee,1* Yamuna Ezhumalai,2 Woongkyu Lee, 1 Ming-Chou Chen,2 Chen-Yu Yeh,3 Tobin J. Marks,1,4 and Robert P. H. Chang 1* 1Department

of Materials Science and Engineering and the Materials Research Center, the

Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States 2Research

Center of New Generation Light Driven Photovoltaic Modules, National Central

University, Taoyuan, 32001 Taiwan 3Department

of Chemistry, Research Center for Sustainable Energy and Nanotechnology

(RCSEN), and Innovation and Development Center of Sustainable Agriculture (IDCSA), National Chung Hsing University, Taichung, Taiwan 4Department

of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois

60208, United States

KEYWORDS. Photovoltaic Cell, Perovskite, Cs2SnI6, Sn-TiO2, porphyrin, donor (D)-πbridge-acceptor (A) organic sensitizers, multi sensitization, solid state hole conductor

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ABSTRACT The design of a dye-sensitized solar cell (DSSC) based on the simultaneous incorporation of multiple dyes is examined. By investigating the use of the porphyrin based YD2-o-C8 and YDD6, and the organic chromophore TTAR, which can act as complementary absorbers, we are able to enhance the capture of incoming light across the solar spectrum. This is demonstrated first by using a conventional DSSC architecture with a liquid electrolyte, and performed a power conversion efficiency (PCE) of 11.2%, representing an improvement over cells based on each of the independent dyes. Next, we used Cs2SnI6 as encapsulating layer over the sensitizing molecules to reduce charge leakage across the dye layers and also added to the absorption of longer wavelengths up to one micron. Finally, we fabricated a cell utilizing a Cs2SnI6/ Succinonitrile solid hole-transport electrolyte and achieved a PCE of ~8.5%. It is expected that the all solid-state design will go a long way towards improving long-term device stability.

1. Introduction A simple model that gives the quality of a solar cell relies on measuring the cell J-V curve and extracting the values for the short circuit current (Jsc), the open circuit voltage (Voc) and the fill factor (FF). From these values, the cell efficiency can be determined. However, these bulk measurements do not take into account the dynamic properties of the solar device, or their long-term stability. In most devices there are inherent losses due to trap states, recombination sites, and shorts across the interfaces through nano-channels between negative and positive charges. In addition, there is the desire to have the device absorb light across the whole solar spectrum thus optimizing the total efficiency. This can be met by the use of tandem cells, but these increase the cost of the system.1 This paper seeks to address these issues through the design of an all solid-state DSSC incorporating multiple dyes. In a traditional DSSC, a 2


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monolayer of dye molecules coats the surfaces of a TiO2 nano-particle film. This serves as the sensitizing layer where light is converted into charge pairs, with the electrons migrating to the anode, such as fluorine- or indium-doped tin oxide glass (FTO or ITO). The electrons are then transported through an external load to the cathode side of the DSSC. The internal side of the cathode usually has a layer of platinum which acts as a catalyst to help activate the redox couple reaction of the iodine liquid electrolyte to recharge the ground state of the dye molecules with electrons that were depleted.2 Since the advent of DSSCs, ruthenium-based dyes have been considered as the most effective photosensitizer with a good PCE (~11%) due to their strong metal to ligand charge transfer (MLCT) process, a broad absorption spectrum with a relatively long excited-state lifetime and good electrochemical stability. 3 Although the certified incident-photon-to-current efficiency (IPCE) values for the very efficient dyes, N3 and N719, are close to 80%, these dyes enable energy conversion up to only about 600-700nm. Thus, in recent years many new dyes have been developed with the goals of being environmentally benign, inexpensive, and having efficient absorption, particularly in the solar spectrum from 350~1,000nm. However, such an ideal single sensitizer has not yet been developed. Thus, many research groups have attempted the challenge by combining two or more dyes with different absorption range, known as a “dye cocktail”.4,5 This strategy extends and increases the light harvesting ability and hence the photocurrent. For an efficient “dye cocktail”, the sensitizers should fulfill the following essential conditions: i) strong molar extinction coefficients to make a thin layer of the mesoscopic TiO2 film, ii) a suitable structure to inhibit unfavorable dye aggregation, and iii) reduction of the recombination of electrons in the TiO2 film with I3– and other acceptor materials through the formation of a compact molecular monolayer covering the bare 3



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TiO2 nanoparticle surface. Multi-sensitized DSSCs have achieved 11.5% efficiency with liquid state electrolytes, but the corresponding results with a solid-state electrolyte have not yet been reported.6 To this end, we apply multi-sensitization based on the use of the porphyrin (YD2-o-C8 ) and donor-π-bridge-acceptor (D-π-A) typed dyes (TTAR).7-8 We will show that these dyes have complementary geometries to allow optimal surface packing on TiO2. We have also engineered the band alignments of the TiO2 layers doped by adding Sn to develop the electron transfer properties between the TiO2 and sensitizers. For further improvement, a thin layer of inorganic perovskite-derivative Cs2SnI6, which we have reported on previously, is inserted between the co-sensitized layer and the electrolyte to reduce recombination losses. This also helps to further extend the light absorption to 1000nm due to the smaller bandgap of the Cs2SnI6 material. Additionally, we replace the liquid electrolyte with our solid-state electrolyte. We adopted the previous method of using an intermixed Cs2SnI6 and succinonitrile (SN) film. This composite layer is placed adjacent to our large area polyaromatic hydrocarbon (LPAH) catalytic layer, which is deposited on top of the FTO cathode of our DSSC device. We anticipate that this all solid-state device will be just as efficient and more stable than some of the best DSSCs reported to date.9-10 The main contribution of this work is to report on the optimization of these multiple porphyrin dyes as absorbers, as well as the best approach to minimize the losses at the interface between the dye layers, the solid electrolyte material, and the TiO2 nanoparticles on the anode side. We first report on the assembly and performance of a multi-dye DSSC using a traditional liquid electrolyte. Next, we discuss the effect of inserting the Cs2SnI6 encapsulating layer. It is very difficult to map the surface structure of the multiple dye layer and detect the nano4


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channels. This is especially true in the case of the liquid electrolyte. Therefore, to investigate this interface, we adopted a simulated model, and used Conducting AFM (C-AFM) tips for a simulated study. Finally, we report on the fabrication and testing of our all solid state DSSC and show that it achieves an efficiency of ~8.5%, comparable to traditional liquid-electrolyte cells.

2. Multi-Sensitization Based DSSCs Using a Liquid Electrolyte Designing sensitizers with higher molar extinction coefficients has led to significant improvement of cell performance.11 However, further efficiency boosts can be obtained by extending the absorption range while not negatively impacting other parameters, such as the intrinsic stability, optical (eg, absorption ability) and chemical properties (eg, ground- and excited-state redox potential). Therefore, a good understanding of the intermolecular interactions as well as complementary light-harvesting between the co-adsorbing dyes must take precedence for efficient multi-sensitizaion. In addition, significant dye aggregation on the surface of TiO2 must be avoided to prevent the decrease of electron injection efficiency.12-15 Here, the photosensitizers investigated were the Zn-based porphyrin dyes, YD2-o-C8 and YDD6, and the more conjugated TTA-based small molecule, TTAR, which holds the record for solar performance as a single dye.7-8, 16 These three dyes absorb in complementary positions within the solar spectrum. In this system, weak absorption above 520nm for YD2-o-C8 can be compensated for by TTAR dye molecules. YDD6 can extend the light-harvesting ability out to near 800nm.17 Several papers reported that different molecular sizes allowed a better surface coverage, yielding a high Jsc and Voc and resulting in a high PCE.16, 18-19 Therefore, inserting

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small sized TTAR dye molecules (estimated molecular length = 24.9Å) into the gaps within the YD2-o-C8 (about 167Å) and saturating the TiO2 surface can help to boost PCE. In our experiements, the conduction band of TiO2 photoanode was firstly modified by the use of Sn as the doped materials. For state-of-the-art TiO2 photoelectrodes, 0D hierarchically structured TiO2 nanosphere (NS) film is employed as a photoelectrode due to its followed benerfical properties: (1) large surface area and dye adsorption; (2) fast electron transport at reduced grain boundaries; (3) light scattering effects of the submicrometer sized structures; and (4) better penetration of electrolytes through the relatively large pores between spheres. Many different chemcial bath or spray pyrolysis approaches have been developed to prepare sphere typed metal oxides. However, these techniques are not practical or cost-effective for the largescale production or flexiblity of DSSC. In our research, a polymer binder free TiO2 NS have been developed by E-spraying technique.20 This film with the high intrinsic surface area of each TiO2 NSs surface as well as the crack free surface morphology demonstrated the effective photoelectrode for the solid state system. Further, for better interconnection between TiO2 NSs and the enhanced effective electron injection from photogenerated sensitizer to the conduction band of TiO2,21 we modified TiO2 NSs surface through the addition of Sn as a doping element by a simple hydrothermal method and E-spraying technique.9 The low and high mangificatio SEM images of the different shaped TiO2 film are shown in Figure S1(a). The TiO2 prepared from E-spraying system converts the nanoparticle to a highly micro- and nanoporous crystalline sphere with a range of diameter between 250 and 500nm. With Sn doping, urchinlike TiO2 NSs are formed and they have uniform size with diameteres of 300~600nm. This omnidirectionally morphology evolution provide the improved interparticle connectivity among TiO2 NSs as well as more efficient electron tranfer pathway compared with smooth 6


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surfaced NSs film. The surface area and pore sizes are calculated with the Brunauer–Emmett– Teller (BET) and Barrett–Joyner–Halenda (BJH) models. Figure S1(b) indicate the Sn-TiO2 show a high surface area of 123.0 m2g-1, 2.2 times and 0.6 times higher than that of NPs and SPs. For more accurate experiment, the surface coverage of sensitizers adsorbed to the surface of each samples at 7μm thick film, the surface-bound sensitizers are desorbed in 0.1M NaOH (aq) to hydrolyze the carbonxlyate surface linkages.22 The number of molecules is calculated from UV-vis absorption spectra of desorbed sensitizers using the extinction coefficient of the N719 sensitizer (ε = 3748 cm-1M-1 at 535nm). It is determined that about 106% and 1.7% more dye molecules are attached to the surface of Sn doped TiO2 SPs (≈ 4.08 × 10-8 mol mg-1) compared to NPs (≈ 1.98 × 10-8 mol mg-1) and SPs (≈ 4.02 × 10-8 mol mg-1), respectively. Furthermore, the cumulative BJH mesopore volume of NSs (0.723 cm3g−1 ) and Sn-NSs (0.755 cm3g−1 ) show almost similar, but Sn-TiO2 exhibit about 51.2% decreased larger pores (26.5 nm) that are formed by the interstitial voids among the spheres and about 10% enlargered the tiny internal pores (10.1nm) that are formed inside the TiO2 NS, compared to the undoped TiO2 NS. (see Figure S1(c)) Both of these types of pores can shorten the diffusion paths of ions and further facilitate efficient diffusion of the electrolyte. From the suggested our impedance models, we confirm the electron diffusion coefficient rate of triiodide D1 for Sn-TiO2 NS film is 4.8 × 10-7 cm2S-1, 157% and 6.7% higher than that obtained from nanoparticles (NP) film ( 2.1 × 10-7 cm2S-1 ) and NSs film ( 4.5×10-7 cm2S-1 ), respectively. The porosities (P) and roughness factor (R) of each films can be also calculated by referring to this formula; 23 P = Vp/ ρ-1 + Vp and R = ρ (1-P)S , where Vp is the specific cumulative pore volume (cm3 g-1) and ρ-1 is the inverse of the density of anatase TiO2 (ρ-1=0.257 cm3g-1). The estimated porosity is about 57.5%, 73.8 % and 74.5% for NPs, NSs and Sn-NSs, respectively, 7



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indicating that NSs film have a great advantages for efficient solid state electrolyte. In addition, about 31.3% and 4.5% increased roughness factor in the Sn-NS films compared to NPs and NSs film leads to an increase in the charge harvesting efficiency from the complete coverage of the inner surface of the TiO2 porous film with a monolayer of dye. Hence, Sn-TiO2 NSs film consider the promising photoelectrode for high performance solid-state DSSC. The fabrication of a multi-sensitized mesoporous TiO2 film is performed by the modified stepwise deposition method. The detailed process can be seen in the experimental section. Figure 1(a) displays the absorption coefficient (ε) of the individual dyes and multi-sensitized systems. The absorption spectrum of TTAR is found in the range of 350~560nm with the amount of light absorbed by this sample for a 449.5nm (log ε/M−1 cm−1=5.01). As zinc porphyrin series, YD2-o-C8 and YDD6 exhibit a broad split feature for the Soret band with an intense maximum at 448nm (log ε/M−1 cm−1= 5.33) for YD2-o-C8 and at 491nm (log ε/M−1 cm−1=5.51) for YDD6 and the Q-band absorption with a maximum at 644nm (log ε/M−1cm−1 = 4.49) for YD2-o-C8 and at 741nm (log ε/M−1 cm−1= 4.94) for YDD6. Therefore, the use of YDD6 can help to cover the lack of light-harvesting ability beyond 700nm (near-IR). As a result, the combination of complementary porphyrin (YD2-o-C8 and YDD6) and organic dye (TTAR) produces a panchromatic absorption promoting the PCE of the DSSC. Figure 1(b) shows the JV curves and the corresponding IPCE action for each single dye and multi-(TTAR/ YD2-o-C8/ YDD6) sensitized DSSC, respectively. An impressive PCE of ca. 11.2% (Jsc=18.6 mAcm-2, Voc=0.818 V, FF =72.8%, 998 mWcm-3) is attainable when the multisensitization is used. Note the Jsc increases from 14.3 mAcm-2 and 6.32 mAcm-2 to 18.6 mAcm2,

seen for both the individual TTAR and YD2-o-C8. For the multi-sensitized DSSCs, the IPCE

spectra shows two major differences compared to the YD2-o-C8 only DSSCs: no dip is 8


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observed in the visible region as the efficiency remained between 75% to 85% from 410nm to 670nm and there is some current above 710nm. This effect is consistent with the absorption shown in Figure 1(a). To understand the charge transport kinetics of cell, electro-chemical impedance spectroscopy (EIS) is performed using a fitting model.24,25 The direct current resistance at ω=0 is given by a simple function of both the electron transport resistance in TiO2 , Rw= kBT/q2Ans × L/Deff = Con × L/Deff, and the charge-transfer resistance in recombination of electron at TiO2/electrolyte interface, Rk=Con × 1/Lkeff , where kB, T, q, A, L and ns represent Boltzmann constant, absolute temperature, charge of an electron, the electrode area, thickness of TiO2 film, and the steady-state electron density in the conduction band, respectively. The first-order reaction rate constant for the loss of electrons, keff , which is estimated to be equal to the peak frequency of the central arc, ωmax , electron lifetime (τeff = 1/keff), and the effective electron diffusion coefficient, Deff = (Rk/Rw) × L2keff. Our calculation and data fitting provide us with some physical insight into the differences in the transport properties of the multi-sensitized solar cell. From this equation, low keff, high Rk/Rw, high Deff and high ns are necessary condition to attain highly efficient DSSCs. The estimated total resistance (RIR) of the multi-sensitized DSSC in the 1~0.3 MHz frequency region is about 18.51 Ω, which is 1.58, 2.23 and 3.41 times lower values than that of the individual TTAR, YD2-o-C8 and YDD6, respectively. From the fitted data, the multisensitized DSSC shows 40.2%, 74.9% and 84.9% lower interfacial recombination rates (keff) in TiO2 and 0.8%, 129% and 294% higher Rk/Rw values compared to those of TTAR, YD2-oC8 and YDD6 based DSSCs. The higher keff of porphyrin dyes when compared with TTAR can be explained by the recombination of iodide ions and oxidized electrons at the TiO2 surface 9



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between dyes. This charge recombination process is illustrated in Figure 1(c). For similar reasons, a multi-sensitized DSSC gives rise to a denser packing and coverage and leads to the highest keff. Therefore, the charge density in the TiO2 conduction band (ns) with multisensitized system is increased by about 124% and 562% over TTAR and YD2-o-C8 single dyes. The relation of relative tradeoff between charge recombination and electron diffusion can be expressed by the diffusion length, Ln, given by the equation Ln = (Deff × τeff)1/2, where Deff and τeff are the carrier diffusion length and lifetime, respectively. The effective Ln of the conduction band electrons for single-(TTAR, YD2-o-C8, YDD6) and multi-sensitized DSSC can be calculated from this equation. The Ln for all samples is estimated to be ~24.6μm for TTAR, 16.3μm for YD2-o-C8, 9.52μm for YDD6, and ~32.1μm for multi-sensitization, respectively. This calculated data suggests that multi-sensitized DSSC leads to enhance the collection of being photo-generated electrons in comparison to those in single sensitizer. The best fit parameters are tabulated in Table I, which includes the parameters for electron kinetics in DSSCs. The impedance graph of each of samples is given in S-2. Based on these achievements, our efforts is headed for achieving an energy efficiency of over 11.2% by combining 3D architecture DSSC using ITO nanorod array (NRA) and photonic crystal concept.25-27 From an effective approach to improve the light-harvesting ability and to delay the electron-hole recombination, the maximum PCE is about 13.26%, with photo current = 3.4 mA, Voc = 0.811 V, FF=73.3% and maximum power (Pmax) = 1.99mW. (see Fig. 1(d))

3. Cs2SnI6 Encapsulated Multi-Sensitized All Solid State DSSC In spite of its impressive PCE using the liquid electrolyte in a conventional DSSC, there are issues regarding the stability, particularly due to concern regarding leakage of the electrolyte 10


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and dye aggregation. In order to overcome these issues, a solid state hole conductor is employed to replace the liquid electrolyte. In the case of the liquid electrolyte, YDD6 sensitizer was used to supplement absorption in the near IR region, but the complexity of synthesis and fabrication of this compound causes issues for scale up and reproducibility.16, 28-29 In addtion, the large πsystems in these molecules induce a strong tendency to stack, which dramatically diminishes their efficiencies when used in DSSCs due to the rapid deactivation of the dye excited state. Therefore, for the further improvement of the multi-sensitized dye, the inorganic perovskite Cs2SnI6 is used, which exhibits absorption in the near IR range, out to 1000 nm.9 As described in the experimental section, TTAR and YD2-o-C8 sensitizers are first deposited into a Sn doped TiO2 film. Then, a homogenuous Cs2SnI6 film is used to cover the organic photosensitizers/ TiO2 substrate without deterioration of the photosenstizers, as shown in Figure 2(a). In eariler research, we focused on the two-step solution process for photosensitizer application.9 Although the two-step process produced high PCE for a single sensitizer system, the high temperature and the complexity of the process has a negative effect on the multi-sensitized DSSC. Therefore, we rely on a one-step deposition process because it is a faster and simpler technique. Finally, a solid state flexible Cs2SnI6/ SN matrix film is applied as the electrolyte to complete the solid-state DSSC. In processing this new electrolyte, we sought to optimize for two conditions: i) minimizing the damage to the organic photosensitizers and ii) obtaining complete coverage of the dye coated surface with the Cs2SnI6 film. To find the best conditions for meeting both cases, different solvents (N,N-Dimethylformamide (DMF), Ethanol (EtOH), Isopropyl Alcohol (IPA), and Methanol (MeOH)) were used to dissolve the Cs2SnI6 powder. The solution was then deposited onto FTO glass by spin-coating or electrospraying (E-spraying). In the case of 11



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spin-coating, it was very difficult to make uniform films under ambient conditions. Therefore, we focused on the E-spraying method. We attempted two strategies: i) co-spraying, where CsI and SnI4 are electrosprayed simultaneously from separate syringe pumps, which can help to control the ratio of CsI and SnI4, and ii) E-spraying one mixed solution, where different molar ratios of CsI and SnI4 are prepared using each solvent and then mixted together. The mixture of CsI and SnI4 solution is then E-sprayed onto the substate. Both cases offer the ability to control the film composition, but there are still issues for obtaining a highly crystalline Cs2SnI6 film. The detailed results and studies are not listed in this paper. Through various experiments, we found that the best way of fabricating the multi-sensitized system was to instead use presynthesized Cs2SnI6 powder. A solution prepared from the Cs2SnI6 powder is directly sprayed onto the substrate by the force of gravity. Figure S3 shows the XRD analysis and the corresponding SEM images for E-sprayed films using different solvents to dissolve Cs2SnI6. In the case of DMF, the powder is well dissolved, and the ochahedral structure with an average size of ~860 nm is observed. The XRD peaks of film were characteriaized by the first (111) peak, and the next (222) and (444) peaks to the 2θ values of 13.1, 26.5 and 54.6o, respectively. The XRD peaks and SEM images of film match that of the bulk Cs2SnI6 powder prior to dissolution. From Hall measurements, the carrier mobility and concentration are about 3.9cm2/V·s and 1.57×1014cm-3, respectively. The carrier concentration of Cs2SnI6 films is similar to the value of the powder, while the mobility is about 100 times lower. This reason can be attributed to the final film coating morphology. Among the many parameters which affect E-spraying, dielectric constant, viscosity, evaporation rate, and solvent quality are the most prominent solvent properties that affect the final coating morphology.30,31 If the solution evaporation time is insufficient, the ground plates form droplet 12


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aggregation instead of a nanocluster film. Therefore, the selection of a proper solvent is an important step in controlling the film morphology and quality. DMF is much less volatile (boiling point is 146°C) and has a high dielectric constant (36.71 at 25oC).30 Althouth Espraying is performed at a high working distance of 15cm and high voltage of 15~18kV, droplet aggregation is observed on the substrate. This wet surface state turn into the Cs2SnI6 film through the heating process (at 130oC). As a result, the dried Cs2SnI6 film is susceptitible to roughness and pin-holes or empty space on the surface. In addtion, the dye-TiO2 interface is vulnerable to degradation in the presence of DMF or even trace amount of DMF. Alcholic solvent with a relatively fast evaporation rate (≈1.4) and low boiling point (64.7~82.6 oC) is a good candidate for the E-spraying process. Therefore, a series of alcholic solvents is tested. From XRD analysis, the peak position of samples using IPA and MeOH match the bulk Cs2SnI6 powder. However, samples produced by EtOH exhibit the presence of a strong CsI peak near 27.7o and Cs2SnI6 at 26.5 o with the preferential orientation along the (110) and (222) diffraction plane, respectively. In addition, Cs2SnI6 powder is not well dissolved in IPA solvent. This is an issue for continuous deposition, leading to a sparse Cs2SnI6 film. The main problem of solubility between powder and solvent and formation of completely covered film can be solved by using MeOH solvent. As seen in Figure S3(c), high dielectric constant (ε =32.70 at 25oC) and fast evaporation rate results in smaller octahydral structure (~270nm) and densely covered Cs2SnI6 film. From the Hall measurement, MeOH based film show about 33.8 cm2/V·s for the carrier mobility and 5.35×1013cm–3 for the carrier concentration (c.a. EtOH: μ= 4.20 cm2/V·s, ni = 5.35×1014cm–3 and IPA: μ= 21.1 cm2/V·s, ni= 7.10 × 1013cm–3) This value is close to the bulk Cs2SnI6 powder. Thus, for multi-sensitization system, MeOH is used for Cs2SnI6 deposition. 13



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Cross-sectional SEM images of the TTAR/ YD2-o-C8/ Cs2SnI6 coated Sn-TiO2 film reveal that the sensitizers completely cover the surface of mesoporous Sn-TiO2 spheres from top to bottom, which is shown in Figure 2(b). The elemental mapping images on the rectangular area demonstrates excellent distribution of Ti, Zn, S, Sn (as the unique elements for each material) throughout the 5-mm-thick nanoporous TiO2. Figure 2(c) displays the molecular structures of the YD2-oC8 and TTAR senstiziers and the corresponding absorption spectra for the indiviual dyes, the co-sensitized and the multi-sensitized Sn-TiO2 film. From the more detailed absorbance study, increasing the YD2-o-C8/ TTAR ratio makes a stronger Soret band (400~500nm), but a decreased Q-band (600~700nm) absorption is observed in all the cosensitized samples. The highest absorption can be observed with a YD2-o-C8/ TTAR ratio of 1/2, which shows about 16% and 29.3% improved absorption in the range 380nm~1000nm, compared with pure TTAR and YD2-o-C8, respectively. This result reveals that the total dye coverage has been improved by insering small sized TTAR dye molecules into the gaps within the YD2-o-C8 saturated TiO2 surface. Coating with a small amount of

Cs2SnI6 significantly enhanced the absorption in the 700~1000nm region.

4. Effects of the 216 Encapsulating Layer Due to the possible aggregation of molecules and/or poor geometrical fitting between molecule types, there will exist nano-channels which will result in the unwanted recombination of charges leading to decreased device efficiencies. To monitor the dye coverage ratio on the film, AFM is firstly used for investigating the topography of the dye molecule covered 14


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TiO2 surface and determine the root mean square roughness (Rq), mean roughness (Ra) and the maximum vertical distance between the highest and lowest data points in the image (Rmax) as result of using different sensitizers. This is shown in Figure 3(a). As expected, the Ra value increases with increasing molecular size. Specifically, the multi-sensitized surface shows about 4.37 times higher roughness than that for the TTAR-coated surface. The Rmax of multi-sensitized TiO2 surface is comparable to the YD2-o-C8 coated surface, which is the larger of the two dye molecules. To directly investigate the electrical properties of films, we applied the Conductive Atomic Force Microscopy (C-AFM). 32 This technique allows topography image by monitoring of the pathways of electrical conduction between a AFM tip and a conductive substrate.32-33 In Figure 3(b), we compare the current images for a single organic dye (YD2-o-C8, left) and Cs2SnI6 covered same dye films (right) on top of ITO substrate. The current map is strongly related to the sample and its electrical properties because the conductive tip is scanned over such a surface by measuring point by point the current flowing vertically. Higher contact area means higher current (assume the empty space between dyes), so nanoscale slopes and variations in topography contribute to the appearance of spot-like features.33 While the current maps describe neither features correlated with the ITO topography, nor any other aggregation feature, and thus show a good coating, where now the whole surface is semiconducting. In addition, we show that with Cs2SnI6 coating, the photoluminescence (PL) decay is 25% slower, indicating slower recombination (see Figure 3(c)) To calculate the energy level alignment of each sensitizer and the TiO2 film, ultraviolet photoelectron spectroscopy (UPS) measurements were performed to determine the highest occupied molecular orbital (HOMO) energy level. The estimated HOMO position can be seen 15



Page 16 of 33

in Figure 3(d), which also shows the energy diagram with the measured values of the samples and the operating process of a multi-sensitized DSSC. This is in agreement with the cyclic voltammetry measurements and DFT calculation from solution.8-9,

34

Since the lowest

unoccupied molecular orbital (LUMO) levels of TTAR (-3.14eV), YD2-o-C8 (-2.70eV) and Cs2SnI6 (-4.23eV) lie above the conduction band edge of Sn-TiO2 ( -4.32 eV) the electron injection from the photo-excited dye states to TiO2 is thermodynamically favorable. Therefore, multi-sensitization with the whole visible region of the solar spectrum improves the photon absorption and carrier generation thereby making the dye combination more effective than its individual constituents.

5. Solar Cell Performance and Analysis Figure 4(a) shows the JV characteristics of the solar cells fabricated in this study. Devices sensitized with only TTAR as the absorber with the solid state ionic conductor Cs2SnI6 yielded a Jsc of 10.36 mAcm-2, a Voc of 717 mV, a FF of 72.6%, and a resultant η of 5.39%. Devices sensitized with only YD2-o-C8 dye and the same cell design exhibit Jsc, Voc, FF and efficiencies of 11.38 mAcm-2, 619 mV, 70.0% and 4.943%, respectively. The increased Jsc and decreased Voc of YD2-o-C8 can be explained by the higher positioning of the energy band. The TTAR/ YD2-oC8 based multi-sensitized DSSC shows a high Jsc of 14.95 mA cm-2 in agreement with the enhanced absorption. 6, 35,36 The enhanced light-harvesting efficiency can be also confirmed by IPCE measurement. (see Figure 4(b)) The overall phenomenon is similar to the liquid based multi-sensitized DSSC. The integrated IPCE value of TTAR/ YD2-o-C8 sensitized solar cell is increased by about 11.9% over pure TTAR and 32.4% over pure YD2-o-C8 samples. Further improvement can be observed with the Cs2SnI6 coating. With a unique Cs2SnI6 coating process, 16


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we observed a phenomenal response in the red part of the spectrum, with a peak IPCE close to 60% and 30 % at 700nm and 750nm, respectively. The broad spectrum of the IPCE leads to about 17.3 % enhancement in the integrated IPCE value. Like the trend with the IPCE, a significant improvement in the JV characteristics can be observed by coating with the Cs2SnI6 solution. (Jsc =17.79 mAcm-2, a Voc=674 mV, a FF=69.7% and a resultant η =8.536%) This outstanding performance is attributed to better coverage, better passivation of the TiO2 surface, and reduction in recombination due to electron back-transfer between TiO2 and the electrolyte. In our model calculation and data fitting, the TTAR/ YD2-o-C8/ Cs2SnI6 sensitized cell shows the highest τeff and ns value, leading to the significantly increased Jsc. However, the slight decrease in Voc can be explained by the intermolecular recombination from the decreased Rk/Rw value.37 The estimated total resistance (RIR) of multisensitized DSSC at 1~0.3MHz frequency region is about 35 Ω, which is 1.37 , 1.56 and 1.18 times lower values than that of TTAR, YD2o-C8 and the co-sensitized (TTAR/ YD2-o-C8) cell, respectively, as shown in Figure 4(c). From simulated data, the Cs2SnI6 assisted multi-sensitized DSSC shows 50.5%, 36.9% and 20.6% lower interfacial recombination rates in TiO2, and 102%, 58.5% and 25.4% higher τeff value than those of the TTAR, YD2-oC8, and co-sensitized cells. (see Table 2) From this we infer that a multi-sensitized DSSC gives rise to a denser packing and coverage, leading to a higher keff by reducing recombintion of injected electrons with oxidized electrons at the TiO2 open space surface between dyes. Therefore, the charge density in the TiO2 conduction band (ns) in the multi-sensitized system increased by about 230% and 107% over the TTAR and YD2-o-C8 single dyes. The effective diffusion length, Ln, of the conduction band electrons for single TTAR, YD2-o-C8, co-sensitized, and multisenstitized DSSCs is estimated to be ~12.7 μm, 10.7 μm, 17.4 and ~21.1μm, respectively. The multi-sensitized cell exhibits the maximum Ln, 17



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indicating identified low recombination effects. This calculated data suggests that multisenstitization leads to enhanced collection of photogenerated electrons in comparison to those in the single sensitizers. Open-oircuit voltage decay (OCVD) dynamics also demonstrate the lifetimes of photogenerated electrons.38 The OCVD technique is a method that consists of turning off steady-state illumination and monitoring the subsequent decay of Voc. The response time is obtained by the reciprocal of the derivative of the decay curve normalized by the thermal voltage: 𝜏𝑛 = ―

𝑘𝐵𝑇 𝑑𝑉𝑜𝑐 ―1 𝑒

(

𝑑𝑡

)

where kB is the Boltzmann constant and T is the temperature.

Analysis of the OCVD dynamics reveals that of the four solar cells at present, the slowest decay dynamics are observed for the TPAT/ YD2-o-C8/ Cs2SnI6 cell. (see Figure 4(d))

6. Conclusion In this study, we examined two strategies for improving the performance and stability of dyesensitized solar cells, the use of multiple dyes and the fabrication of an all solid-state DSSC. First, we investigated the feasibility of using multiple dyes to increase the range of the solar spectrum absorbed. We used three dyes, YD2-o-C8, YDD6, and TTAR, each of which absorbs primarily in different ranges of the solar spectrum. We demonstrated that when all three were used together to sensitize TiO2 films, the absorption, IPCE, and cell efficiency were improved compared to any of the dyes individually. Subsequently, we substituted Cs2SnI6 for YDD6 as the primary absorber at longer wavelengths. This was implemented as an encapsulating layer over the top of the YD2-o-C8/ TTAR layers, and helped reducing unwanted charge recombination between the dyes and electrolyte. This is demonstrated by the decrease in the presence of conductive spots, as examined by conducting AFM, when the encapsulating layer was used. Finally, we fabricated an all solid-state DSSC using a Cs2SnI6/ SN mixture as the 18


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solid-state electrolyte. These devices achieved a PCE of ~8.5%, indicating that they are comparable to traditional devices using a liquid electrolyte. It is anticipated that these cells will possess vastly improved long-term stability, due to elimination of electrolyte leakage and reduction of electrical shorts across the device. It is our hope that this investigation will aid in the development of the next generation of scalable, efficient, and stable solar cells.

7. Experimental Section Anode Preparation

All experiment were carried out under ambient laboratory conditions.

As mentioned in our paper, Sn-TiO2 Sphere films were producing by a simple hydrothermal method as a first step. 9 100mL of Titanium(IV) isopropoxide (Aldrich, 97%, Ti(Oi-Pr)4,) was separately mixed with a solution containing appropriate amounts of SnCl2·2H2O (Aldrich, 99.99%) in water. As controlled the Sn/Ti ratio in the precursor solution, 2wt%~10wt % of SnTiO2 were successfully synthesized by the hydrothermal method. Next, the dispersed Sn-TiO2 solution was directly electro-sprayed onto a dense TiO2 blocking layer coated FTO glasses after replacing the solvent of water based hydrothermal Sn-TiO2 solution with ethanol. 9 As the electrospray condition, 10~16kV of the electric field was applied at 30~45 μl/min of the feed rate. The electrosprayed TiO2 films were annealed at 500°C for 45 min. To improve the cell performance, a 0.02M aqueous solution of TiCl4 was grown onto an extra layer of E-sprayed TiO2 film. After flushing with deionized water and drying, the electrodes were gradually sintered again at 150oC for 15 min, at 320oC for 10 min, at 500oC for 30min. The liquid electrolyte composed of 0.6 M of 1- butyl-3-methylimidazolium iodide (BMII), 0.03 M of iodine, 0.1M of guanidinium thiocyanate (GSCN) and 0.5 M of 4-tert-butylpyridine (tBP) in acetonitrile and valeronitrile (85:15 v/v) was injected between two electrode. 19



Multi-Sensitization Coating

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Considering the size of sensitizer, the TiO2 electrode is

immersed first into a 1M solution of YDD6 16in a mixture of 1,2-Dichlorobenzene (DCB) and Ethanol (EtOH) (volume ratio= 1:10) and kept at room temperature for 1 hour. After spincoating at 3000 rpm for 45sec, a small quantity of a YD2-o-C8 solution was prepared from the same ratio of mixed solvents (DCB: EtOH) and then dropped onto the YDD6/TiO2 and left for 5 min before spinning at 3000 rpm for 60sec. The well covered sensitizer layer is obtained by repeating the coating procedure about three times. After the YD2-o-C8 7sensitizer coating, assynthesized TTAR8 dissolved in the same solvent mixture is deposited onto the YD2-oC8/ YDD6/ TiO2 using the same procesure. For the further improvement of multi-sensitized dye, inorganic perovskite Cs2SnI6, was coated by electrosprayed technique.

Cathode Preparation

For the purpose solid state work, the electro-sprayed large-effective-

surface-area polyaromatic hydrocarbon (LPAH) materials was used for our designed cell10, while a thin layer of a 5 mM solution of H2PtCl6 coated FTO glass was empolyed at a liquid electrolyte based cell.25 The LPAH material can be produced by the arc technique (Power: 100A, 27V in the range 50 to 400 Torr under pure hydrogen pressure. A 2g of LPAH materials were dispersed in 1000 mL IPA by sonication. The LPAH suspension was electrosprayed and sintered at 400oC for 20 min.

Cell Fabrication Process

The Cs2SnI6 powder was prepared first from oxidized fresh B-

γ CsSnI3 powder.39-40 Then 5wt% of mixed succinonitirile (Sigma-Aldrich, 99%) and inorganic materials i.e. CuSCN, oxidized CsSnI3, Cs2SnI6 powders (so called "matrix") were prepared 20


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for 10h at 75oC. Following our earlier process, solid state cell can be assembled in a stable sandwich configuration. 9

Characterization

Powder and film X-ray diffraction patterns were collected in-house

using a Rigaku MiniFlex 600 X-ray diffractometer (Cu Kα, 1.5406 Å) operating at 40 kV and 15 mA. Synchrotron radiation (APS, 11BM-B) was used for selected annealing temperatures of Cs2SnI6 to identify the decomposition temperature of the compounds. The obtained data were analyzed using the JANA2006 software package. Optical diffuse-reflectance spectra were collected at room temperature using a Shimadzu UV-3600 PC double-beam, doublemonochromator spectrophotometer on powdered samples using BaSO4 as a 100% reflectance reference. XPS measurements were obtained using a monochromatic Al Kα source at hν = 1486 cm–1. A typical operating pressure for XPS and UPS measurements was 1 × 10–8 Torr. XPS spectra were collected at a pass energy of 20.0 eV and a dwell time of 10 ms. An electron flood gun was used to eliminate charging effects. All binding energies were referenced to an adventitious C(1s) energy of 285.0 eV. UPS spectra were collected with a 21.2 eV He(1) source with a pass energy of 2.0 eV and a step size of 0.01 eV. A sample bias of −10 V was applied to determine the secondary electron cutoff value via curve fitting. A Newport-Oriel ® IQE-200 ACDC was used to measure incident photon to charge carrier efficiency (IPCE). The devices were evaluated under 100 mW/cm2 AM1.5G simulated sunlight with a class A solar cell analyzer (Spectra Nova Tech.). A silicon solar cell fitted with a KG3 filter tested and certified by the National Renewable Energy Laboratory (NREL) was used for calibration. The KG3 filter accounts for the different light absorption between the perovskite based solar cell and the silicon solar cell, and it ensures that the spectral mismatch correction 21



Page 22 of 33

factor approaches unity. The electrochemical impedance results were measured under the same light illumination with an impedance analyzer (Solartron 1260), and a potentiostat (Solartron 1287) when the device was applied at its Voc. An additional low amplitude modulation sinusoidal voltage of 10 mVrms was also applied between an anode and cathode of a device over the frequency range of 0.05150k Hz.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS publications website.

AUTHOR INFORMATION Corresponding Author *

Byunghong Lee, E-mail address: [email protected]

* Robert

P.H Chang, E-mail address: [email protected]

ORCID Chen-Yu Yeh: 0000-0002-7815-5681 Woongkyu Lee: 0000-0001-7106-9564

Author Contributions 22


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B. Lee designed the overall experiments and fabricated the solar cells. B. Lee and W. Lee performed the all related characterization and analyses. Y. Ezhumalai, M.-C. Chen, and C.-Y. Yeh synthesized the molecules used in the article and characterized the basic properties of them. T. J. Marks advised on the concept of the experiment. R. P. H. Chang supervised all the experiments and took charge in writing the manuscript.

ACKNOWLEDGMENT The Northwestern University authors acknowledge the support of the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under Award Number DE-SC0001059. This work made use of the EPIC, Keck-II, and SPID facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS), the MRSEC program (NSF DMR-1121262 and 1542205) at the Materials Research Center, the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. C.-Y. Yeh thanks the financial support for this work from the Ministry of Science and Technology (MOST) and the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

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FIGURES

27



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Figure 1. (a) Molar absorption coefficients measured in 10-5 M o-C6H4Cl2 (insert: photographs of dye films), (b) IPCE analysis (top) and JV characteristics (bottom) for cells made from single and multiple dyes, (c) Schematic view of the charge recombination in the individual dye and multi-sensitized DSSCs, and (d) I-V and P-V curve for the best performance DSSC with the optimized condition.

28


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Figure 2. (a) Schematic showing the experimental procedure for forming the multi-sensitized cell. (b) A cross-sectional SEM image of the TTAR/YD2-o-C8/Cs2SnI6 /TiO2 layers on FTO glass. On the right is the main element mapping by EDS. (c) Absorption spectrum of each single sensitizer and multi-sensitized system.

29



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Figure 3. (a) AFM surface images of the dense TiO2 films coated with the different photosensitizers, and (b) C-AFM topography images, and (c) the photoluminescence (PL) lifetime and for single and Cs2SnI6 assisted single dye coated sensitized film. (d) Operating schematic drawing with UPS analysis with the different photosensitizers 30


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Figure 4. (a) JV and (b) IPCE characteristics, (c) Nyquist plot (inserted in Bode phase plots) with the best-fit model and (d) Normalized open circuit voltage decay (OCVD) for each single sensitizer (a. TTAR b.YD2-oC8), co-sensitized (c. TTAR/YD2-oC8) and multi-sensitized (d. TTAR/YD2-oC8/Cs2SnI6) solar cell at the solid state ionic conductor Cs2SnI6/SN (inserted in schematic representation and surface SEM image for Cs2SnI6 encapsulated multi-sensitized film)

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Table 1. Parameters for the Best Fit of the Impedance and Photovoltaic Data for a single dye (TTAR, YD2-oC8 and YDD6) and multi sensitized DSSC. Deff k (10-5 cm2s- eff (Hz) 1)

τeff (ms)

Rk

Rw

Rk/Rw

Con Rd (Ωcms-1) (Ω)

ns R (1018cm- IR (Ω) 3)

VOC (V)

Jsc (mA/cm2)

FF (%)

EFF (%)

TTAR

1.918

19.9

314.7

8.3

3.3

2.5

0.102

3.5

5.378

29.40

0.842

14.3

72.7

8.73

YD2-oC8

2.014

47.5

132.1

10.3

9.3

1.10

0.302

4.3

1.822

41.21

0.769

14.3

67.5

7.45

YDD6

1.142

79.1

79.42

7.9

12.3

0.64

0.387

5.7

1.121

63.21

0.687

6.42

68.7

3.03

Triple

1.952

11.9

527.1

6.1

2.4

2.52

0.044

2.9

12.07

18.52

0.818

18.6

72.8

11.2

Table 2 Parameters for the Best Fit of the Impedance and Photovoltaic Data for a single dye (TTAR, YD2-oC8 and YDD6) and multi sensitized DSSC at the solid state ionic conductor. Deff keff (10-5 (Hz) cm2s-1)

τeff (ms)

Rk/Rw

D1 Con ns R (10-6 d (Ωcms(1019cm(Ω) 3 cm2s1) ) 1)

Rtotal (ohm)

VOC (V)

Jsc (mA/cm2)

FF (%)

EFF (%)

(a)

TTAR

2.103

15.172 65.91 2.375 0.066 3.5 0.719

0.42

49.06

0.717 10.36

72.6 5.392

(b)

YD2-oC8

1.724

11.915 83.93 2.478 0.052 3.5 1.148

0.57

55.91

0.619 11.38

70.0 4.943

(c)

Multiple

1.392

9.464

106.6 2.520 0.043 2.9 1.635

0.59

42.60

0.697 14.95

68.5 7.139

(d)

Multiple/Cs2SnI6

1.014

7.518

133.0 2.312 0.031 2.4 2.372

0.86

35.77

0.674 17.79

69.7 8.356


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