DOI: 10.1002/cssc.201500309
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Molecular Engineering of Organic Dyes with a HoleExtending Donor Tail for Efficient All-Solid-State DyeSensitized Solar Cells Jianfeng Lu,[a] Yu-Cheng Chang,[b] Hsu-Yang Cheng,[b] Hui-Ping Wu,[b] Yibing Cheng,[a, c] Mingkui Wang,*[a] and Eric Wei-Guang Diau*[b] We report a new concept for the design of metal-free organic dyes (OD5–OD9) with an extended donor–p–acceptor (D–p–A) molecular framework, in which the donor terminal unit is attached by a hole-extending side chain to retard back electron transfer and charge recombination; the p-bridge component contains varied thiophene-based substituents to enhance the light-harvesting ability of the device. The best dye (OD9) has a D–A–p–A configuration with the hexyloxyphenylthiophene (HPT) side chain as a hole-extension component and a benzo-
thiadiazole (BTD) internal acceptor as a p-extension component. The co-sensitization of OD9 with the new porphyrin dye LW24 enhanced the light-harvesting ability to 800 nm; thus, a power conversion efficiency 5.5 % was achieved. Photoinduced absorption (PIA) and transient absorption spectral (TAS) techniques were applied to account for the observed trend of the open-circuit voltage (VOC) of the devices. This work provides insights into the molecular design, photovoltaic performance, and kinetics of charge recombination.
Introduction Dye-sensitized solar cells (DSSCs) have attracted great attention because they are regarded as promising next-generation photovoltaic devices for indoor applications.[1–3] Conventional DSSCs suffer from leakage of liquid electrolytes such that the enduring stability of the devices under light-soaking conditions becomes a critical issue to be resolved;[4] thus, all-solid-state DSSCs (ss-DSSCs) have been developed as a viable alternative.[5–7] In ss-DSSCs, the liquid electrolyte is replaced with a hole-transport material (HTM) in a solid phase to circumvent the problems of leakage of liquid electrolyte and the corrosion of metal contacts associated with volatile solvents.[8, 9] Thus, an advantage of ss-DSSCs is their superior stability over those of their liquid-based counterparts, and they are attractive for the realization of the roll-to-roll production of flexible photovoltaic devices.[10, 11] All-solid-state perovskite solar cells (PSC) are based on a similar device structure and have shown remark[a] J. Lu, Prof. Y. Cheng, Prof. M. Wang Michael Grtzel Center for Mesoscopic Solar Cells Huazhong University of Science and Technology 1037 Luoyu Road, Wuhan 430074 (PR China) Fax: (+ 86) 27-87792225 E-mail: [email protected] [b] Dr. Y.-C. Chang, H.-Y. Cheng, Dr. H.-P. Wu, Prof. E. W.-G. Diau Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung University Hsinchu 30010 (Taiwan) Fax: (+ 886) 3-5723764 E-mail: [email protected] [c] Prof. Y. Cheng Department of Materials Engineering Monash University Melbourne, Victoria, 3800 (Australia) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500309.
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able progress in device performance, but the present PSC technology requires improved sensitivity to air and moisture and the removal of the lead component of perovskite for its future commercialization.[12–14] Owing to the incomplete filling of pores in the mesoporous TiO2 electrode with the solid-state HTM (e.g., spiro-OMeTAD) and the small diffusion length associated with it, the thickness of the TiO2 layer in an ss-DSSC is limited to approximately 2 mm.[15, 16] In this case, dyes with large molar absorption coefficients are necessary to achieve sufficient light harvesting.[17–19] In addition to ruthenium-based photosensitizers, metal-free organic dyes have been applied to ss-DSSCs with a TiO2 layer with a thickness less than 2 mm owing to their large molar absorptivity.[20, 21] A simple principle to design a new organic dye is based upon a molecular framework with a donor–p–acceptor (D–p–A) structure, for which the dye is composed of an electron-donating group, a conjugated p spacer, and an electron-withdrawing group.[22–25] In this molecular framework, two strategies have been commonly deployed to improve the performance of ss-DSSC devices. The first strategy is to modify the donor site,[26–28] because the electronic and steric traits of the electron-donating moiety play a pivotal role in determining the energy levels of the dye molecules adsorbed on the surface of titania nanocrystals.[29] Several donor groups, such as triphenylamine (TPA), indoline, carbazole, and cumarine, have been utilized for this purpose.[30, 31] For example, an ss-DSSC device with the TPA-based organic dye Y123 featuring a 2,4bis(hexyloxy)benzene moiety as an extended donor attained a power conversion efficiency (PCE) of 7.2 %.[7] The second strategy is to incorporate specific aromatic units into the p bridge to enhance the light-harvesting ability of the dyes.[32–34] Wang and co-workers reported a certified efficiency of 6.09 %
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Scheme 1. Molecular structures of OD5–OD9 designed according to a hole-extending D–p–A configuration.
with dye C220 bearing a 4H-cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) unit as a p bridge.[35] Zhu and co-workers reported new organic dyes with a D–A–p–A configuration incorporating an additional electron-withdrawing unit into the p bridge as an internal acceptor. This D–A–p–A framework is beneficial to tune the molecular energy levels, and redshifted spectral features enhance the corresponding photovoltaic performance and device stability.[36, 37] We report here a new concept based on those two strategies for the design of a new series of metal-free organic dyes based on a hole-extending D–p–A configuration to improve the PCE of ss-DSSCs. First, from an organic dye with a typical D–p–A configuration featuring TPA as a donor group and 3,4ethylenedioxythiophene (EDOT) as a p unit, either 5-hexyl-2,2’bithiophene (HBT) or 2-[4-(hexyloxy)phenyl]thiophene (HPT) was attached to the donor group as a hole-extending side chain to retard charge recombination in the device; the molecular structures based on the HBT-TPA and HPT-TPA units are shown in Scheme 1 (labeled OD5 and OD6, respectively). We demonstrated that the use of HPT as a hole-extending side chain in OD6 improved the output photovoltage of the corresponding devices through retardation of the kinetics of charge recombination. Second, bithiophene (BT), CPDT, or a benzothiadiazole-thiophene (BTD-thiophene) moiety can replace the EDOT-thiophene unit in OD6 to extend the p bridge; the corresponding molecular structures are shown in Scheme 1 (OD7, OD8, and OD9, respectively). The ss-DSSC fabricated with organic dye OD9 exhibited an incident photon-to-current efficiency (IPCE) spectrum broadened to l = 750 nm. OD9 was cosensitized using the new push–pull porphyrin dye LW24 (the molecular structure is shown in Scheme 2) in an ss-DSSC, and the device performance attained a PCE of 5.5 % with a shortcircuit current density (JSC) of 10.6 mA cm¢2, an open-circuit voltage (VOC) of 810 mV, and a fill factor (FF) of 0.64. ChemSusChem 2015, 8, 2529 – 2536
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Scheme 2. Molecular structures of the zinc porphyrin co-sensitizer LW24 and the hole-transport material spiro-OMeTAD.
Results and Discussion The dyes OD5–OD9 were synthesized according to the experimental procedures reported in the Supporting Information; the synthetic approaches are summarized below. First, the p bridges with varied conjugation moieties were synthesized. The bridges were then substituted with extended donating groups through Pd-catalyzed Suzuki coupling reactions.[23] Through Knoevenagel condensations of the aldehyde precur-
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Full Papers sors with cyanoacetic acid,[36] target dyes OD5–OD9 were eventually obtained. As depicted in Scheme 1, dyes OD5 and OD6 possess a similar p bridge and acceptor, whereas the donors are extended by either HBT or HPT. Dyes OD7–OD9 feature the same donor and acceptor moieties as OD6, but the p bridges between the TPA-based donor and the cyanoacetic acid acceptor are replaced with BT, CPDT, and BTD-thiophene, respectively. The UV/Vis absorption spectra and electrochemical characterization of OD5–OD9 are presented in Figure 1 a and 1 b, re-
bandgap and improves the light-harvesting efficiency.[37] The optical properties of these organic dyes sensitized on thin films of TiO2 are presented in Figures S1–S3. To evaluate the reduction potentials of the OD dyes, we employed cyclic voltammetry (CV); the results are shown in Figure 1 b, and the corresponding data are listed in Table 1. All of the dyes exhibited reversible oxidation waves, which are attributed to the removal of an electron from the amine segment. The oxidation potential observed for OD5 is notably shifted more positively than that for OD6, which indicates that the electron density on the nitrogen atom is depleted by the thiophene unit owing to the electron negativity of the sulfur atom.[40] In contrast, OD9 has a more positive EOX than OD7 and OD8, consistent with the reported trend.[37] The CV data are depicted in an energy-level diagram in Figure 2: the highest occupied molecular orbital (HOMO) levels of dyes OD5–OD9 are ¢5.71, ¢5.45, ¢5.47, ¢5.41, and ¢5.48 eV respectively; the corresponding lowest unoccupied Figure 1. (a) UV/Vis absorption spectra of OD5–OD9 in CHCl3 at 25 8C. (b) Cyclic voltammetry measurements permolecular orbital (LUMO) energy formed in CHCl3 solution at 25 8C. levels are ¢3.44, ¢3.37, ¢3.41, ¢3.42, and ¢3.55 eV. For all of spectively. The UV/Vis spectra of all of these dyes in solution the dyes, the LUMO levels are more negative than the conducexhibited two broad absorptions with maxima between l = tion band of TiO2, whereas the HOMO levels are more positive 350 and 550 nm. In accordance with the spectral features for than that of spiro-OMeTAD; this provides sufficient driving p-conjugated donor–acceptor molecules, the absorption band force for electron injection and dye regeneration in ss-DSSC in the long-wavelength region results from charge-transfer (CT) devices. transitions from the donor to the cyanoacrylic acid acceptor.[38] To evaluate the device performance of these OD dyes, we The absorption band in the wavelength region 350–450 nm prepared ss-DSSC devices (the fabrication details are provided arises from the p–p* transition, for which the wavelength of in the Supporting Information, and the side-view SEM image the absorption maximum is affected by the HBT or the HPT of a typical device is shown in Figure S4) sensitized with OD5– long chain at the terminus of the donor group.[39] For example, OD9 on mesoporous TiO2 films of thickness1.7 mm using spiroOD5 and OD6 exhibit similar spectral features in the CT transiOMeTAD (Scheme 2) as the HTM layer. The current–voltage (J– tion (e 3 Õ 104 L mol¢1 cm¢1 at l = 515 nm), but the maximum V) characteristics and the corresponding IPCE action spectra of of the p–p* transition of OD6 is blue-shifted by approximately the devices are shown in Figure 3 a and b, respectively; the 10 nm with respect to that of OD5 (Table 1), which shows the effect of p conjugation between the HBT Table 1. Summary of optical and electrochemical properties of dyes OD5–OD9. and the HPT terminal groups at the donor site. In contrast, OD7 shows spectral features similar to Dye lmax [nm] lmax, emission[b] Eox[c] E0¢0[d] Eox¢E0¢0 those of OD6, which indicates that the two thio(Absorption [103 L mol¢1 cm¢1])[a] [nm] [V vs. NHE] [V vs. NHE] [V] phene units in OD7 are not coplanar to affect the pOD5 516 (36.4) 626 1.06 2.27 ¢1.21 conjugation. Dye OD8 shows a significantly redshiftOD6 515 (30.9) 623 0.95 2.18 ¢1.23 OD7 511 (52.0) 659 0.97 2.06 ¢1.09 ed CT band. This phenomenon indicates that the OD8 542 (58.4) 670 0.91 1.99 ¢1.08 fixing of the two thiophene units enhances the p OD9 535 (42.8) 665 0.98 1.93 ¢0.95 conjugation responsible for the redshift of the CT [a] The absorption and emission data were measured for samples in CHCl3 at 25 8C. band in OD8. The substitution of the thiophene unit [b] Excitation wavelength [nm]: OD5, 516; OD6, 515; OD7, 511; OD8, 542; OD9, 535. in OD7 with the electron-accepting group BTD to [c] The electrochemical measurements were performed at 25 8C with each dye give dye OD9 also red-shifted the low-lying CT transi(0.5 mm) in CHCl3/0.1 m NBu4PF6/N2, Pt disk working and counter electrodes, Ag/AgCl reference electrode, scan rate = 50 mV s¢1; EOX is the first oxidation value NHE = normal tion, and the band edge extended to approximately [38] hydrogen electrode. [d] Estimated from the intersection wavelengths of the normal700 nm. Hence, the incorporation of the HPT ized UV/Vis absorption and the fluorescence spectra. donor with the BTD unit in OD9 decreases the ChemSusChem 2015, 8, 2529 – 2536
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Full Papers The ss-DSSC device made of OD9, which is based on a molecular structure with a hole-extending D–A–p–A configuration, achieved a significantly improved photovoltaic performance. The magnitude of VOC of the OD9 device is explained by retarded charge recombination, as is evident from the measurements of the photoinduced absorption (PIA) spectra and nanosecond transient absorption kinetics, which are described in the following section. The impressive JSC of the OD9 device is attributed to the superior light-harvesting properties of the dye shown in Figure 3 b. The IPCE action spectrum of the OD9 device is much broader than those of the OD5–OD8 devices, in agreement with their corresponding absorption spectra for solid films (Figure S1). The IPCE spectrum of OD9 exhibits a broad plateau that covers the visible spectral region from Figure 2. Energy [eV]/potential [V vs. SHE] levels and associated potentials of OD5–OD9 compared with the CB of TiO2 and HOMO of the hole-transport 400 to 600 nm; the spectral onset extends to approximately material spiro-OMeTAD. 750 nm. Thus, the introduction of an electron-withdrawing group, BTD, into the p-conjugated skeleton broadens the IPCE corresponding photovoltaic parameters are summarized in spectrum of OD9 significantly by nearly 50 nm relative to Table 2. The OD5 device exhibited a PCE of 4.2 % with VOC = those of the other dyes. 850 mV, but the OD6 device further improved the PCE to 4.5 % To investigate the spectral properties of the dye cations with VOC = 870 mV. The superior device performance and VOC of upon photoexcitation, we measured the photoinduced absorpthe OD6 device indicates the advantage of HPT as a hole-extion spectra of TiO2 films sensitized with OD5–OD9.[41] The PIA tending side chain in the organic dyes. The effects of the thiospectra in the range 550-1800 nm for the films of TiO2/dye and phene-based substituents on the photovoltaic performance TiO2/dye/spiro-OMeTAD are shown in Figure 4 a and b, respecare negative from OD6 to OD8, but the IPCE spectrum of OD9 tively. The PIA spectra of films without spiro-OMeTAD feature shows that the BTD unit in the OD9 device shows a positive two absorption bands at l = 600 and 1400 nm, whereas those effect in terms of light-harvesting capability, which extends of films with spiro-OMeTAD layer show an additional spectral over 700 nm (Figure 3 b). As a result, device OD9 exhibited feature at approximately 1300 nm, which broadens into the a performance with JSC = 8.84 mA cm¢2, VOC = 900 mV, FF = 0.58, near-IR band, as shown in Figure 4 b. As reported previously, and an overall PCE of 4.7 %. the absorption in the 700–1200 nm region is assigned to the oxidized spiro-OMeTAD species (spiro-OMeTAD + ),[42] but the PIA signals of spiro-OMeTAD + greatly overlap those of the dye Table 2. Photovoltaic parameters of ss-DSSCs made of OD5–OD9 meacations. A greater concentration of the dye cations on the TiO2 sured under simulated AM 1.5G one-sun illumination. surface at the steady state might induce a larger dipole Device JSC [mA cm¢2] VOC [V] FF PCE [%] moment pointing away from the TiO2 surface.[42, 43] This situation causes an increased splitting of energy between the OD5 8.28 0.85 0.60 4.2 OD6 8.53 0.87 0.60 4.5 quasi-Fermi level of TiO2 and the HOMO level of spiro-OMeTAD, OD7 8.88 0.82 0.58 4.2 which leads to an increased VOC of the device. As Figure 4 b OD8 6.61 0.86 0.64 3.7 shows, the PIA spectrum of the OD9 film exhibits enhanced OD9 8.84 0.90 0.58 4.7 absorption characteristic of spiro-OMeTAD + accompanied with an apparent blueshift of the near-IR band maximum at l = 1400 nm, consistent with the dipole-moment hypothesis aforementioned such that the concentration of the oxidized OD9 dye might be greater than those of the others. The observed blueshift of the OD9 spectrum in the presence of the HTM layer indicates a possible potential downward shift of spiro-OMeTAD to account for the larger VOC of the OD9 device than those of the other devices. To understand the kinetics of Figure 3. (a) J–V curves and (b) IPCE spectra of ss-DSSC devices with OD5–OD9 measured under simulated AM 1.5G full sunlight. charge recombination in relation ChemSusChem 2015, 8, 2529 – 2536
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Full Papers spiro-OMeTAD can approach to the TiO2 surface.[7, 39, 46] Thus, our TAS results justify the molecular design with the HPT as a holeextended tail in the D–A–p–A configuration to form a compact surface-blocking layer for the retardation of CR kinetics to enhance the VOC of the device.[47, 48] The internal acceptor BTD in OD9 also plays an important role to decrease the rate of charge recombination between TiO2 and spiro-OMeTAD to obtain the largFigure 4. Photoinduced absorption (PIA) spectra of mesoporous TiO2 films sensitized with OD5–OD9 (a) without est VOC in this series of metalhole transporter (spiro-OMeTAD, Li-TFSI, and TBP) and (b) infiltrated with the HTM layer. Samples were excited at free organic sensitizers. l = 450 nm from a laser chopped at 19 Hz. The photovoltaic performance of the device made of OD9 was to the device performance, we measured nanosecond transient furthered improved by adjusting the dye immersion periods absorption spectra (TAS) for the same titania films sensitized (Figure S5 and Table S1), the amounts of the additives inside with organic dyes OD5–OD9 in the absence or presence of spiro-OMeTAD (e.g., the molar ratios of TBP and lithium salt in spiro-OMeTAD.[44] For all TAS measurements, the pumping and the solid-state electrolyte, see Figure S6 and Table S2), and the probing wavelengths were 450 and 1342 nm, respectively. As molar ratios of the co-adsorbent chenodeoxycholic acid the PIA spectra of the TiO2/dye films shown in Figure 4 a reflect (CDCA; Figure S7 and Table S3). Ionic additives influence many the absorption signals of e(TiO2)/dye + (mainly the cationic dye aspects of the bulk properties of spiro-OMeTAD as well as its species), the TAS decays reflect the back transfer of the injected electrons (BET) from the conduction band of TiO2 to the HOMO level of the dye.[44, 45] In contrast, in the presence of HTM, the PIA spectra in Figure 4 b reflect the absorption signals of e(TiO2)/dye/HTM + because dye regeneration occurred rapidly so that the contribution of e(TiO2)/dye + /HTM was negligible (no increasing characteristics were observed). Thus, the observed TAS decays correspond to the charge recombination (CR) between the e(TiO2) and the spiro-OMeTAD + species. As shown in Figure 5, all normalized TAS profiles (DA vs. t) display a multiexponential decay feature; they were well fitted with a simple kinetic model, a stretched exponential function (DA = b e¢(t/t) ). For the TiO2/dye films, the fitted effective BET lifetimes (tBET) are 6.7, 13.5, 8.7, 9.4, and 11.8 ms for dyes OD5–OD9, respectively. The results indicate that the molecular design with the HPT as a side chain of the donor (OD6–OD9) retards the BET kinetics relative to those of OD5, but the insertion of the bithiophene substituents in OD7 and OD8 accelerates the BET kinetics relative to those of OD6. As the BTD unit is introduced in OD9, the BET kinetics becomes slower than those for OD7 and OD8 but still slightly faster than those for OD6. For the TiO2/ dye/HTM films in the presence of spiro-OMeTAD, the fits of the TAS signals yields CR lifetimes (trec) of 5.7, 8.2, 5.2, 8.2, and 9.1 ms for OD5–OD9, respectively. In general, the values of trec exhibit a systematic variation with the trend OD9 > OD6 ~ OD8 > OD5 > OD7, which is consistent with the trend in their corresponding VOC values (Table 2). Our results indicate that the newly designed HPT unit can extend hole propagation of the dye relative to those of the alkyl or 2,4-bis(hexyloxy)benFigure 5. Transient absorption spectra (TAS) of OD5–OD9-sensitized mesopozene donor terminuses to maintain the spiro-OMeTAD + remote rous TiO2 with or without HTM. The traces with scatter are raw data; the from the e(TiO2) beyond the minimum distance ( 2 nm) that solid curves are the results fitted to the equation DA = e¢(t/t) . b
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Full Papers interfacial characteristics.[49] For example, lithium bis(trifluoromethane)sulfonimide (Li-TFSI) plays a primary role in determining the conductivity of the spiro-OMeTAD. The influence of the concentration of Li-TFSI on charge transport, density of states, and charge generation results in an overall compromise for the maximum efficiency of power conversion.[50–53] We performed current–voltage measurements for six OD9 devices fabricated under the same experimental conditions (Figure S8 and Table S4), which confirmed that the device performances were within a small uncertainty. Upon performance optimization, the OD9 device achieved the best performance with JSC = 9.36 mA cm¢2, VOC = 834 mV, FF = 0.68, and PCE = 5.3 %. Consistent with previous reports,[52] the JSC and FF values are clearly improved, but VOC decreases to some extent, which will be discussed in the following. We attribute this effect to an effective improved conductivity of spiro-OMeTAD as well as a decreased series resistance of the devices upon increasing the Li-TFSI concentration. The variation of the amounts of tert-butylpyridine (TBP) and CDCA additives (Figure S7 and Table S3) results in negative effects produced by Li-TFSI, such that the formation of a dipole moment shifts the vacuum level of TiO2 downward; the intercalation of lithium ions induces an increased number of electron-trap states and a smaller diffusivity of negative charges.[54, 55] To improve the device performance further, we applied the concept of co-sensitization in our ss-DSSC system. Co-sensitization implies the use of multiple dyes with complementary absorption spectra sensitized on a semiconductor film to enhance the light-harvesting ability of a DSSC device.[56–59] In our case, OD9 was co-sensitized with the new porphyrin dye LW24 (Scheme 2). As an analogue of the porphyrin dye LD14,[60, 61] the LW24 dye is functionalized with an electron-withdrawing BTD unit to extend the conjugation length in the p bridge for the same purpose of OD9. The BTD unit is inserted between the porphyrin core and the benzoic acid. This molecular design extends the light-harvesting properties effectively and improves the performance of porphyrin-sensitized solar cells.[62, 63] As Figure S3 shows, the normalized absorption spectra of OD9 and LW24 complement each other well in the visible spectral region. The current–voltage characteristics of the ss-DSSC devices made of only OD9 dye, only LW24 dye, and the co-sensitized OD9/LW24 mixture adsorbed on TiO2 films are shown in Figure 6 a (Table 3); the corresponding IPCE action spectra appear in Figure 6 b. The LW24 device features a photovoltaic performance with JSC = 6.75 mA cm¢2, VOC = 790 mV, FF = 0.56, and PCE = 3.0 %. The tail of the IPCE spectrum of LW24 extends beyond 800 nm, but the overall performance of the device was unsatisfactory because of its small photocurrent. As reported previously,[22, 64, 65] porphyrin works well in a liquid-type DSSC with a cobalt-based electrolyte, but in the solid-state DSSCs the device performance is poor because of the poor ability to transfer holes from the porphyrin to spiro-OMeTAD. The reported PCE values are 1.5 % for YD2 and 3.0 % for Zn-2 porphyrins.[66–68] In our case, the device performance of the ss-DSSC made of only the LW24 dye is comparable with that of the best porphyrin-based ss-DSSC. Moreover, for OD9 co-sensitized ChemSusChem 2015, 8, 2529 – 2536
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Figure 6. (a) J–V curves and (b) IPCE spectra of ss-DSSC devices made of OD9, LW24, and co-sensitized OD9/LW24 measured under simulated AM 1.5G one-sun illumination.
Table 3. Photovoltaic parameters of ss-DSSCs made of OD9, LW24, and co-sensitized OD9/LW24 measured under simulated AM 1.5G one-sun irradiation. Device
JSC [mA cm¢2]
VOC [V]
FF
PCE [%]
OD9 LW24 OD9 + LW24
9.36 6.75 10.59
0.83 0.78 0.81
0.68 0.56 0.64
5.3 3.0 5.5
with LW24, the PCE of the device increased to 5.5 %. The major improvement of the device performance was cause by the enhanced JSC value, which reached as much as 10.6 mA cm¢2. The extension of the IPCE spectrum of the OD9/LW24 device in the range 670–770 nm accounts for the improved JSC relative to that of the OD9-only device.
Conclusions Through systematic molecular engineering, we designed triphenylamine-based organic dyes in a molecular framework featuring a hole-extending donor–p–acceptor (D–p–A) configuration for all-solid-state dye-sensitized solar cells. We demonstrate that a device made of organic dye OD9, which incorporates our newly designed 2-[4-(hexyloxy)phenyl]thiophene (HPT) as a hole-extending side chain of the donor terminus and benzo[c][1,2,5]thiadiazole (BTD) as a p-extending internal acceptor to form a D–A–p–A configuration, attained an impressive efficiency of 5.3 % for power conversion. We have shown that a compact surface-blocking layer was achieved through steric hindrance by modification of the side chain with HPT as a hole extender, which retards back electron transfer and charge recombination to improve the open-circuit voltage (VOC) of the device. The BTD unit inserted in the p component not only extends the p conjugation to enhance the lightharvesting ability for improved short-circuit current (JSC) of the device but also effects the dipole moment to shift downward the potential of spiro-OMeTAD for improved VOC. With organic dye OD9 co-sensitized with porphyrin dye LW24, the OD9/ LW24 device shows a remarkable photovoltaic performance with JSC = 10.6 mA cm¢2, VOC = 810 mV, FF = 0.64, and PCE = 5.5 %. This work provides new insight into the molecular design of metal-free organic dyes for highly efficient ss-DSSCs.
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Full Papers Acknowledgements J.L., Y.C., and M.W. thank the Director Fund of WNLO, the 973 Program of China (2014CB643506, 2013CB922104, and 2011CBA00703) and NSFC (21161160445 and 21173091) for financial support; they thank also the Analytical and Testing Centre at HUST for performing characterization of various samples. Y.C.C., H.C.C., H.P.W., and E.W.G.D. received support from National Chiao Tung University (NCTU), the Ministry of Science and Technology (MOST) of Taiwan, and the Ministry of Education (MOE) of Taiwan. J.L. thanks NCTU (Hsinchu, Taiwan) and WNLO (Wuhan, China) for supporting of his visit to NCTU. Keywords: donor–acceptor systems · dyes · porphyrinoids · sensitizers · solar cells [1] B. E. Hardin, H. J. Snaith, M. D. McGehee, Nat. Photonics 2012, 6, 162 – 169. [2] H. Pettersson, K. Nonomura, L. Kloo, A. Hagfeldt, Energy Environ. Sci. 2012, 5, 7376 – 7380. [3] A. Fakharuddin, R. Jose, T. M. Brown, F. Fabregat-Santiago, J. Bisquert, Energy Environ. Sci. 2014, 7, 3952 – 3981. [4] A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595 – 6663. [5] U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissçrtel, J. Salbeck, H. Spreitzer, M. Grtzel, Nature 1998, 395, 583 – 585. [6] J.-H. Yum, P. Chen, M. Grtzel, M. K. Nazeeruddin, ChemSusChem 2008, 1, 699 – 707. [7] J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N. Cevey-Ha, C. Yi, M. K. Nazeeruddin, M. Grtzel, J. Am. Chem. Soc. 2011, 133, 18042 – 18045. [8] B. Li, L. Wang, B. Kang, P. Wang, Y. Qiu, Sol. Energy Mater. Sol. Cells 2006, 90, 549 – 573. [9] P. Docampo, S. Guldin, T. Leijtens, N. K. Noel, U. Steiner, H. J. Snaith, Adv. Mater. 2014, 26, 4013 – 4030. [10] L. GonÅalves, V. Bermudez, H. Ribeiro, A. Mendes, Energy Environ. Sci. 2008, 1, 655 – 667. [11] M. Ye, X. Wen, M. Wang, J. Iocozzia, N. Zhang, C. Lin, Z. Lin, Mater. Today 2015, 18, 155 – 162. [12] M. A. Green, A. Ho-Baillie, H. J. Snaith, Nat. Photonics 2014, 8, 506 – 514. [13] J. Rhee, C. Chung, E. W. G. Diau, NPG Asia Mater. 2013, 5, e68. [14] M. Grtzel, Nat. Mater. 2014, 13, 838 – 842. [15] T. Bessho, S. Zakeeruddin, C. Yeh, E. Diau, M. Grtzel, Angew. Chem. Int. Ed. 2010, 49, 6646 – 6649; Angew. Chem. 2010, 122, 6796 – 6799. [16] P. Docampo, A. Hey, S. Guldin, R. Gunning, U. Steiner, H. J. Snaith, Adv. Funct. Mater. 2012, 22, 5010 – 5019. [17] M. Wang, J. Liu, N. Cevey-Ha, S. Moon, P. Liska, R. Humphry-Baker, J. Moser, C. Grtzel, P. Wang, S. Zakeeruddin, M. Grtzel, Nano Today 2010, 5, 169 – 174. [18] C. Chen, M. Wang, J. Li, N. Pootrakulchote, L. Alibabaei, C. Ngoc-le, J. Decoppet, J. Tsai, C. Grtzel, C. Wu, S. Zakeeruddin, M. Grtzel, ACS Nano 2009, 3, 3103 – 3109. [19] D. Chen, Y. Hsu, H. Hsu, B. Chen, Y. Lee, H. Fu, M. Chung, S. Liu, H. Chen, Y. Chi, P. Chou, Chem. Commun. 2010, 46, 5256 – 5258. [20] A. Mishra, M. Fischer, P. Buerle, Angew. Chem. Int. Ed. 2009, 48, 2474 – 2499; Angew. Chem. 2009, 121, 2510 – 2536. [21] A. Dualeh, R. Humphry-Baker, J. H. Delcamp, M. K. Nazeeruddin, M. Grtzel, Adv. Energy Mater. 2013, 3, 496 – 504. [22] L. Li, E. W. G. Diau, Chem. Soc. Rev. 2013, 42, 291 – 304. [23] E. Gabrielsson, H. Ellis, S. Feld, H. Tian, G, Boschloo, A. Hagfeldt, L. Sun, Adv. Energy Mater. Adv. Energy. Mater. 2013, 3, 1647 – 1656. [24] P. Gao, H. N. Tsao, C. Yi, M. Grtzel, M. K. Nazeeruddin, Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201301485. [25] M. Liang, J. Chen, Chem. Soc. Rev. 2013, 42, 3453 – 3488. [26] J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, P. Comte, K. Pei, T. W. Holcombe, M. K. Nazeeruddin, J. Hua, S. M. Zakeeruddin, H. Tian, M. Grtzel, J. Am. Chem. Soc. 2014, 136, 5722 – 5730.
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Received: March 1, 2015 Revised: April 4, 2015 Published online on June 26, 2015
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Molecular Engineering of Organic Dyes with a HoleExtending Donor Tail for Efficient All-Solid-State DyeSensitized Solar Cells Jianfeng Lu,[a] Yu-Cheng Chang,[b] Hsu-Yang Cheng,[b] Hui-Ping Wu,[b] Yibing Cheng,[a, c] Mingkui Wang,*[a] and Eric Wei-Guang Diau*[b] We report a new concept for the design of metal-free organic dyes (OD5–OD9) with an extended donor–p–acceptor (D–p–A) molecular framework, in which the donor terminal unit is attached by a hole-extending side chain to retard back electron transfer and charge recombination; the p-bridge component contains varied thiophene-based substituents to enhance the light-harvesting ability of the device. The best dye (OD9) has a D–A–p–A configuration with the hexyloxyphenylthiophene (HPT) side chain as a hole-extension component and a benzo-
thiadiazole (BTD) internal acceptor as a p-extension component. The co-sensitization of OD9 with the new porphyrin dye LW24 enhanced the light-harvesting ability to 800 nm; thus, a power conversion efficiency 5.5 % was achieved. Photoinduced absorption (PIA) and transient absorption spectral (TAS) techniques were applied to account for the observed trend of the open-circuit voltage (VOC) of the devices. This work provides insights into the molecular design, photovoltaic performance, and kinetics of charge recombination.
Introduction Dye-sensitized solar cells (DSSCs) have attracted great attention because they are regarded as promising next-generation photovoltaic devices for indoor applications.[1–3] Conventional DSSCs suffer from leakage of liquid electrolytes such that the enduring stability of the devices under light-soaking conditions becomes a critical issue to be resolved;[4] thus, all-solid-state DSSCs (ss-DSSCs) have been developed as a viable alternative.[5–7] In ss-DSSCs, the liquid electrolyte is replaced with a hole-transport material (HTM) in a solid phase to circumvent the problems of leakage of liquid electrolyte and the corrosion of metal contacts associated with volatile solvents.[8, 9] Thus, an advantage of ss-DSSCs is their superior stability over those of their liquid-based counterparts, and they are attractive for the realization of the roll-to-roll production of flexible photovoltaic devices.[10, 11] All-solid-state perovskite solar cells (PSC) are based on a similar device structure and have shown remark[a] J. Lu, Prof. Y. Cheng, Prof. M. Wang Michael Grtzel Center for Mesoscopic Solar Cells Huazhong University of Science and Technology 1037 Luoyu Road, Wuhan 430074 (PR China) Fax: (+ 86) 27-87792225 E-mail: [email protected] [b] Dr. Y.-C. Chang, H.-Y. Cheng, Dr. H.-P. Wu, Prof. E. W.-G. Diau Department of Applied Chemistry and Institute of Molecular Science National Chiao Tung University Hsinchu 30010 (Taiwan) Fax: (+ 886) 3-5723764 E-mail: [email protected] [c] Prof. Y. Cheng Department of Materials Engineering Monash University Melbourne, Victoria, 3800 (Australia) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500309.
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able progress in device performance, but the present PSC technology requires improved sensitivity to air and moisture and the removal of the lead component of perovskite for its future commercialization.[12–14] Owing to the incomplete filling of pores in the mesoporous TiO2 electrode with the solid-state HTM (e.g., spiro-OMeTAD) and the small diffusion length associated with it, the thickness of the TiO2 layer in an ss-DSSC is limited to approximately 2 mm.[15, 16] In this case, dyes with large molar absorption coefficients are necessary to achieve sufficient light harvesting.[17–19] In addition to ruthenium-based photosensitizers, metal-free organic dyes have been applied to ss-DSSCs with a TiO2 layer with a thickness less than 2 mm owing to their large molar absorptivity.[20, 21] A simple principle to design a new organic dye is based upon a molecular framework with a donor–p–acceptor (D–p–A) structure, for which the dye is composed of an electron-donating group, a conjugated p spacer, and an electron-withdrawing group.[22–25] In this molecular framework, two strategies have been commonly deployed to improve the performance of ss-DSSC devices. The first strategy is to modify the donor site,[26–28] because the electronic and steric traits of the electron-donating moiety play a pivotal role in determining the energy levels of the dye molecules adsorbed on the surface of titania nanocrystals.[29] Several donor groups, such as triphenylamine (TPA), indoline, carbazole, and cumarine, have been utilized for this purpose.[30, 31] For example, an ss-DSSC device with the TPA-based organic dye Y123 featuring a 2,4bis(hexyloxy)benzene moiety as an extended donor attained a power conversion efficiency (PCE) of 7.2 %.[7] The second strategy is to incorporate specific aromatic units into the p bridge to enhance the light-harvesting ability of the dyes.[32–34] Wang and co-workers reported a certified efficiency of 6.09 %
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Scheme 1. Molecular structures of OD5–OD9 designed according to a hole-extending D–p–A configuration.
with dye C220 bearing a 4H-cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) unit as a p bridge.[35] Zhu and co-workers reported new organic dyes with a D–A–p–A configuration incorporating an additional electron-withdrawing unit into the p bridge as an internal acceptor. This D–A–p–A framework is beneficial to tune the molecular energy levels, and redshifted spectral features enhance the corresponding photovoltaic performance and device stability.[36, 37] We report here a new concept based on those two strategies for the design of a new series of metal-free organic dyes based on a hole-extending D–p–A configuration to improve the PCE of ss-DSSCs. First, from an organic dye with a typical D–p–A configuration featuring TPA as a donor group and 3,4ethylenedioxythiophene (EDOT) as a p unit, either 5-hexyl-2,2’bithiophene (HBT) or 2-[4-(hexyloxy)phenyl]thiophene (HPT) was attached to the donor group as a hole-extending side chain to retard charge recombination in the device; the molecular structures based on the HBT-TPA and HPT-TPA units are shown in Scheme 1 (labeled OD5 and OD6, respectively). We demonstrated that the use of HPT as a hole-extending side chain in OD6 improved the output photovoltage of the corresponding devices through retardation of the kinetics of charge recombination. Second, bithiophene (BT), CPDT, or a benzothiadiazole-thiophene (BTD-thiophene) moiety can replace the EDOT-thiophene unit in OD6 to extend the p bridge; the corresponding molecular structures are shown in Scheme 1 (OD7, OD8, and OD9, respectively). The ss-DSSC fabricated with organic dye OD9 exhibited an incident photon-to-current efficiency (IPCE) spectrum broadened to l = 750 nm. OD9 was cosensitized using the new push–pull porphyrin dye LW24 (the molecular structure is shown in Scheme 2) in an ss-DSSC, and the device performance attained a PCE of 5.5 % with a shortcircuit current density (JSC) of 10.6 mA cm¢2, an open-circuit voltage (VOC) of 810 mV, and a fill factor (FF) of 0.64. ChemSusChem 2015, 8, 2529 – 2536
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Scheme 2. Molecular structures of the zinc porphyrin co-sensitizer LW24 and the hole-transport material spiro-OMeTAD.
Results and Discussion The dyes OD5–OD9 were synthesized according to the experimental procedures reported in the Supporting Information; the synthetic approaches are summarized below. First, the p bridges with varied conjugation moieties were synthesized. The bridges were then substituted with extended donating groups through Pd-catalyzed Suzuki coupling reactions.[23] Through Knoevenagel condensations of the aldehyde precur-
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Full Papers sors with cyanoacetic acid,[36] target dyes OD5–OD9 were eventually obtained. As depicted in Scheme 1, dyes OD5 and OD6 possess a similar p bridge and acceptor, whereas the donors are extended by either HBT or HPT. Dyes OD7–OD9 feature the same donor and acceptor moieties as OD6, but the p bridges between the TPA-based donor and the cyanoacetic acid acceptor are replaced with BT, CPDT, and BTD-thiophene, respectively. The UV/Vis absorption spectra and electrochemical characterization of OD5–OD9 are presented in Figure 1 a and 1 b, re-
bandgap and improves the light-harvesting efficiency.[37] The optical properties of these organic dyes sensitized on thin films of TiO2 are presented in Figures S1–S3. To evaluate the reduction potentials of the OD dyes, we employed cyclic voltammetry (CV); the results are shown in Figure 1 b, and the corresponding data are listed in Table 1. All of the dyes exhibited reversible oxidation waves, which are attributed to the removal of an electron from the amine segment. The oxidation potential observed for OD5 is notably shifted more positively than that for OD6, which indicates that the electron density on the nitrogen atom is depleted by the thiophene unit owing to the electron negativity of the sulfur atom.[40] In contrast, OD9 has a more positive EOX than OD7 and OD8, consistent with the reported trend.[37] The CV data are depicted in an energy-level diagram in Figure 2: the highest occupied molecular orbital (HOMO) levels of dyes OD5–OD9 are ¢5.71, ¢5.45, ¢5.47, ¢5.41, and ¢5.48 eV respectively; the corresponding lowest unoccupied Figure 1. (a) UV/Vis absorption spectra of OD5–OD9 in CHCl3 at 25 8C. (b) Cyclic voltammetry measurements permolecular orbital (LUMO) energy formed in CHCl3 solution at 25 8C. levels are ¢3.44, ¢3.37, ¢3.41, ¢3.42, and ¢3.55 eV. For all of spectively. The UV/Vis spectra of all of these dyes in solution the dyes, the LUMO levels are more negative than the conducexhibited two broad absorptions with maxima between l = tion band of TiO2, whereas the HOMO levels are more positive 350 and 550 nm. In accordance with the spectral features for than that of spiro-OMeTAD; this provides sufficient driving p-conjugated donor–acceptor molecules, the absorption band force for electron injection and dye regeneration in ss-DSSC in the long-wavelength region results from charge-transfer (CT) devices. transitions from the donor to the cyanoacrylic acid acceptor.[38] To evaluate the device performance of these OD dyes, we The absorption band in the wavelength region 350–450 nm prepared ss-DSSC devices (the fabrication details are provided arises from the p–p* transition, for which the wavelength of in the Supporting Information, and the side-view SEM image the absorption maximum is affected by the HBT or the HPT of a typical device is shown in Figure S4) sensitized with OD5– long chain at the terminus of the donor group.[39] For example, OD9 on mesoporous TiO2 films of thickness1.7 mm using spiroOD5 and OD6 exhibit similar spectral features in the CT transiOMeTAD (Scheme 2) as the HTM layer. The current–voltage (J– tion (e 3 Õ 104 L mol¢1 cm¢1 at l = 515 nm), but the maximum V) characteristics and the corresponding IPCE action spectra of of the p–p* transition of OD6 is blue-shifted by approximately the devices are shown in Figure 3 a and b, respectively; the 10 nm with respect to that of OD5 (Table 1), which shows the effect of p conjugation between the HBT Table 1. Summary of optical and electrochemical properties of dyes OD5–OD9. and the HPT terminal groups at the donor site. In contrast, OD7 shows spectral features similar to Dye lmax [nm] lmax, emission[b] Eox[c] E0¢0[d] Eox¢E0¢0 those of OD6, which indicates that the two thio(Absorption [103 L mol¢1 cm¢1])[a] [nm] [V vs. NHE] [V vs. NHE] [V] phene units in OD7 are not coplanar to affect the pOD5 516 (36.4) 626 1.06 2.27 ¢1.21 conjugation. Dye OD8 shows a significantly redshiftOD6 515 (30.9) 623 0.95 2.18 ¢1.23 OD7 511 (52.0) 659 0.97 2.06 ¢1.09 ed CT band. This phenomenon indicates that the OD8 542 (58.4) 670 0.91 1.99 ¢1.08 fixing of the two thiophene units enhances the p OD9 535 (42.8) 665 0.98 1.93 ¢0.95 conjugation responsible for the redshift of the CT [a] The absorption and emission data were measured for samples in CHCl3 at 25 8C. band in OD8. The substitution of the thiophene unit [b] Excitation wavelength [nm]: OD5, 516; OD6, 515; OD7, 511; OD8, 542; OD9, 535. in OD7 with the electron-accepting group BTD to [c] The electrochemical measurements were performed at 25 8C with each dye give dye OD9 also red-shifted the low-lying CT transi(0.5 mm) in CHCl3/0.1 m NBu4PF6/N2, Pt disk working and counter electrodes, Ag/AgCl reference electrode, scan rate = 50 mV s¢1; EOX is the first oxidation value NHE = normal tion, and the band edge extended to approximately [38] hydrogen electrode. [d] Estimated from the intersection wavelengths of the normal700 nm. Hence, the incorporation of the HPT ized UV/Vis absorption and the fluorescence spectra. donor with the BTD unit in OD9 decreases the ChemSusChem 2015, 8, 2529 – 2536
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Full Papers The ss-DSSC device made of OD9, which is based on a molecular structure with a hole-extending D–A–p–A configuration, achieved a significantly improved photovoltaic performance. The magnitude of VOC of the OD9 device is explained by retarded charge recombination, as is evident from the measurements of the photoinduced absorption (PIA) spectra and nanosecond transient absorption kinetics, which are described in the following section. The impressive JSC of the OD9 device is attributed to the superior light-harvesting properties of the dye shown in Figure 3 b. The IPCE action spectrum of the OD9 device is much broader than those of the OD5–OD8 devices, in agreement with their corresponding absorption spectra for solid films (Figure S1). The IPCE spectrum of OD9 exhibits a broad plateau that covers the visible spectral region from Figure 2. Energy [eV]/potential [V vs. SHE] levels and associated potentials of OD5–OD9 compared with the CB of TiO2 and HOMO of the hole-transport 400 to 600 nm; the spectral onset extends to approximately material spiro-OMeTAD. 750 nm. Thus, the introduction of an electron-withdrawing group, BTD, into the p-conjugated skeleton broadens the IPCE corresponding photovoltaic parameters are summarized in spectrum of OD9 significantly by nearly 50 nm relative to Table 2. The OD5 device exhibited a PCE of 4.2 % with VOC = those of the other dyes. 850 mV, but the OD6 device further improved the PCE to 4.5 % To investigate the spectral properties of the dye cations with VOC = 870 mV. The superior device performance and VOC of upon photoexcitation, we measured the photoinduced absorpthe OD6 device indicates the advantage of HPT as a hole-extion spectra of TiO2 films sensitized with OD5–OD9.[41] The PIA tending side chain in the organic dyes. The effects of the thiospectra in the range 550-1800 nm for the films of TiO2/dye and phene-based substituents on the photovoltaic performance TiO2/dye/spiro-OMeTAD are shown in Figure 4 a and b, respecare negative from OD6 to OD8, but the IPCE spectrum of OD9 tively. The PIA spectra of films without spiro-OMeTAD feature shows that the BTD unit in the OD9 device shows a positive two absorption bands at l = 600 and 1400 nm, whereas those effect in terms of light-harvesting capability, which extends of films with spiro-OMeTAD layer show an additional spectral over 700 nm (Figure 3 b). As a result, device OD9 exhibited feature at approximately 1300 nm, which broadens into the a performance with JSC = 8.84 mA cm¢2, VOC = 900 mV, FF = 0.58, near-IR band, as shown in Figure 4 b. As reported previously, and an overall PCE of 4.7 %. the absorption in the 700–1200 nm region is assigned to the oxidized spiro-OMeTAD species (spiro-OMeTAD + ),[42] but the PIA signals of spiro-OMeTAD + greatly overlap those of the dye Table 2. Photovoltaic parameters of ss-DSSCs made of OD5–OD9 meacations. A greater concentration of the dye cations on the TiO2 sured under simulated AM 1.5G one-sun illumination. surface at the steady state might induce a larger dipole Device JSC [mA cm¢2] VOC [V] FF PCE [%] moment pointing away from the TiO2 surface.[42, 43] This situation causes an increased splitting of energy between the OD5 8.28 0.85 0.60 4.2 OD6 8.53 0.87 0.60 4.5 quasi-Fermi level of TiO2 and the HOMO level of spiro-OMeTAD, OD7 8.88 0.82 0.58 4.2 which leads to an increased VOC of the device. As Figure 4 b OD8 6.61 0.86 0.64 3.7 shows, the PIA spectrum of the OD9 film exhibits enhanced OD9 8.84 0.90 0.58 4.7 absorption characteristic of spiro-OMeTAD + accompanied with an apparent blueshift of the near-IR band maximum at l = 1400 nm, consistent with the dipole-moment hypothesis aforementioned such that the concentration of the oxidized OD9 dye might be greater than those of the others. The observed blueshift of the OD9 spectrum in the presence of the HTM layer indicates a possible potential downward shift of spiro-OMeTAD to account for the larger VOC of the OD9 device than those of the other devices. To understand the kinetics of Figure 3. (a) J–V curves and (b) IPCE spectra of ss-DSSC devices with OD5–OD9 measured under simulated AM 1.5G full sunlight. charge recombination in relation ChemSusChem 2015, 8, 2529 – 2536
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Full Papers spiro-OMeTAD can approach to the TiO2 surface.[7, 39, 46] Thus, our TAS results justify the molecular design with the HPT as a holeextended tail in the D–A–p–A configuration to form a compact surface-blocking layer for the retardation of CR kinetics to enhance the VOC of the device.[47, 48] The internal acceptor BTD in OD9 also plays an important role to decrease the rate of charge recombination between TiO2 and spiro-OMeTAD to obtain the largFigure 4. Photoinduced absorption (PIA) spectra of mesoporous TiO2 films sensitized with OD5–OD9 (a) without est VOC in this series of metalhole transporter (spiro-OMeTAD, Li-TFSI, and TBP) and (b) infiltrated with the HTM layer. Samples were excited at free organic sensitizers. l = 450 nm from a laser chopped at 19 Hz. The photovoltaic performance of the device made of OD9 was to the device performance, we measured nanosecond transient furthered improved by adjusting the dye immersion periods absorption spectra (TAS) for the same titania films sensitized (Figure S5 and Table S1), the amounts of the additives inside with organic dyes OD5–OD9 in the absence or presence of spiro-OMeTAD (e.g., the molar ratios of TBP and lithium salt in spiro-OMeTAD.[44] For all TAS measurements, the pumping and the solid-state electrolyte, see Figure S6 and Table S2), and the probing wavelengths were 450 and 1342 nm, respectively. As molar ratios of the co-adsorbent chenodeoxycholic acid the PIA spectra of the TiO2/dye films shown in Figure 4 a reflect (CDCA; Figure S7 and Table S3). Ionic additives influence many the absorption signals of e(TiO2)/dye + (mainly the cationic dye aspects of the bulk properties of spiro-OMeTAD as well as its species), the TAS decays reflect the back transfer of the injected electrons (BET) from the conduction band of TiO2 to the HOMO level of the dye.[44, 45] In contrast, in the presence of HTM, the PIA spectra in Figure 4 b reflect the absorption signals of e(TiO2)/dye/HTM + because dye regeneration occurred rapidly so that the contribution of e(TiO2)/dye + /HTM was negligible (no increasing characteristics were observed). Thus, the observed TAS decays correspond to the charge recombination (CR) between the e(TiO2) and the spiro-OMeTAD + species. As shown in Figure 5, all normalized TAS profiles (DA vs. t) display a multiexponential decay feature; they were well fitted with a simple kinetic model, a stretched exponential function (DA = b e¢(t/t) ). For the TiO2/dye films, the fitted effective BET lifetimes (tBET) are 6.7, 13.5, 8.7, 9.4, and 11.8 ms for dyes OD5–OD9, respectively. The results indicate that the molecular design with the HPT as a side chain of the donor (OD6–OD9) retards the BET kinetics relative to those of OD5, but the insertion of the bithiophene substituents in OD7 and OD8 accelerates the BET kinetics relative to those of OD6. As the BTD unit is introduced in OD9, the BET kinetics becomes slower than those for OD7 and OD8 but still slightly faster than those for OD6. For the TiO2/ dye/HTM films in the presence of spiro-OMeTAD, the fits of the TAS signals yields CR lifetimes (trec) of 5.7, 8.2, 5.2, 8.2, and 9.1 ms for OD5–OD9, respectively. In general, the values of trec exhibit a systematic variation with the trend OD9 > OD6 ~ OD8 > OD5 > OD7, which is consistent with the trend in their corresponding VOC values (Table 2). Our results indicate that the newly designed HPT unit can extend hole propagation of the dye relative to those of the alkyl or 2,4-bis(hexyloxy)benFigure 5. Transient absorption spectra (TAS) of OD5–OD9-sensitized mesopozene donor terminuses to maintain the spiro-OMeTAD + remote rous TiO2 with or without HTM. The traces with scatter are raw data; the from the e(TiO2) beyond the minimum distance ( 2 nm) that solid curves are the results fitted to the equation DA = e¢(t/t) . b
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Full Papers interfacial characteristics.[49] For example, lithium bis(trifluoromethane)sulfonimide (Li-TFSI) plays a primary role in determining the conductivity of the spiro-OMeTAD. The influence of the concentration of Li-TFSI on charge transport, density of states, and charge generation results in an overall compromise for the maximum efficiency of power conversion.[50–53] We performed current–voltage measurements for six OD9 devices fabricated under the same experimental conditions (Figure S8 and Table S4), which confirmed that the device performances were within a small uncertainty. Upon performance optimization, the OD9 device achieved the best performance with JSC = 9.36 mA cm¢2, VOC = 834 mV, FF = 0.68, and PCE = 5.3 %. Consistent with previous reports,[52] the JSC and FF values are clearly improved, but VOC decreases to some extent, which will be discussed in the following. We attribute this effect to an effective improved conductivity of spiro-OMeTAD as well as a decreased series resistance of the devices upon increasing the Li-TFSI concentration. The variation of the amounts of tert-butylpyridine (TBP) and CDCA additives (Figure S7 and Table S3) results in negative effects produced by Li-TFSI, such that the formation of a dipole moment shifts the vacuum level of TiO2 downward; the intercalation of lithium ions induces an increased number of electron-trap states and a smaller diffusivity of negative charges.[54, 55] To improve the device performance further, we applied the concept of co-sensitization in our ss-DSSC system. Co-sensitization implies the use of multiple dyes with complementary absorption spectra sensitized on a semiconductor film to enhance the light-harvesting ability of a DSSC device.[56–59] In our case, OD9 was co-sensitized with the new porphyrin dye LW24 (Scheme 2). As an analogue of the porphyrin dye LD14,[60, 61] the LW24 dye is functionalized with an electron-withdrawing BTD unit to extend the conjugation length in the p bridge for the same purpose of OD9. The BTD unit is inserted between the porphyrin core and the benzoic acid. This molecular design extends the light-harvesting properties effectively and improves the performance of porphyrin-sensitized solar cells.[62, 63] As Figure S3 shows, the normalized absorption spectra of OD9 and LW24 complement each other well in the visible spectral region. The current–voltage characteristics of the ss-DSSC devices made of only OD9 dye, only LW24 dye, and the co-sensitized OD9/LW24 mixture adsorbed on TiO2 films are shown in Figure 6 a (Table 3); the corresponding IPCE action spectra appear in Figure 6 b. The LW24 device features a photovoltaic performance with JSC = 6.75 mA cm¢2, VOC = 790 mV, FF = 0.56, and PCE = 3.0 %. The tail of the IPCE spectrum of LW24 extends beyond 800 nm, but the overall performance of the device was unsatisfactory because of its small photocurrent. As reported previously,[22, 64, 65] porphyrin works well in a liquid-type DSSC with a cobalt-based electrolyte, but in the solid-state DSSCs the device performance is poor because of the poor ability to transfer holes from the porphyrin to spiro-OMeTAD. The reported PCE values are 1.5 % for YD2 and 3.0 % for Zn-2 porphyrins.[66–68] In our case, the device performance of the ss-DSSC made of only the LW24 dye is comparable with that of the best porphyrin-based ss-DSSC. Moreover, for OD9 co-sensitized ChemSusChem 2015, 8, 2529 – 2536
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Figure 6. (a) J–V curves and (b) IPCE spectra of ss-DSSC devices made of OD9, LW24, and co-sensitized OD9/LW24 measured under simulated AM 1.5G one-sun illumination.
Table 3. Photovoltaic parameters of ss-DSSCs made of OD9, LW24, and co-sensitized OD9/LW24 measured under simulated AM 1.5G one-sun irradiation. Device
JSC [mA cm¢2]
VOC [V]
FF
PCE [%]
OD9 LW24 OD9 + LW24
9.36 6.75 10.59
0.83 0.78 0.81
0.68 0.56 0.64
5.3 3.0 5.5
with LW24, the PCE of the device increased to 5.5 %. The major improvement of the device performance was cause by the enhanced JSC value, which reached as much as 10.6 mA cm¢2. The extension of the IPCE spectrum of the OD9/LW24 device in the range 670–770 nm accounts for the improved JSC relative to that of the OD9-only device.
Conclusions Through systematic molecular engineering, we designed triphenylamine-based organic dyes in a molecular framework featuring a hole-extending donor–p–acceptor (D–p–A) configuration for all-solid-state dye-sensitized solar cells. We demonstrate that a device made of organic dye OD9, which incorporates our newly designed 2-[4-(hexyloxy)phenyl]thiophene (HPT) as a hole-extending side chain of the donor terminus and benzo[c][1,2,5]thiadiazole (BTD) as a p-extending internal acceptor to form a D–A–p–A configuration, attained an impressive efficiency of 5.3 % for power conversion. We have shown that a compact surface-blocking layer was achieved through steric hindrance by modification of the side chain with HPT as a hole extender, which retards back electron transfer and charge recombination to improve the open-circuit voltage (VOC) of the device. The BTD unit inserted in the p component not only extends the p conjugation to enhance the lightharvesting ability for improved short-circuit current (JSC) of the device but also effects the dipole moment to shift downward the potential of spiro-OMeTAD for improved VOC. With organic dye OD9 co-sensitized with porphyrin dye LW24, the OD9/ LW24 device shows a remarkable photovoltaic performance with JSC = 10.6 mA cm¢2, VOC = 810 mV, FF = 0.64, and PCE = 5.5 %. This work provides new insight into the molecular design of metal-free organic dyes for highly efficient ss-DSSCs.
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Received: March 1, 2015 Revised: April 4, 2015 Published online on June 26, 2015
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