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A Step Towards Efficient Panchromatic MultiChromophoric Sensitizers for Dye Sensitized Solar Cells Fadi M. Jradi, Daniel O'Neil, Xiongwu Kang, Jinsze Wong, Paul Szymanski, Timothy C. Parker, Harry L. Anderson, Mostafa A. El-Sayed, and Seth R. Marder Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02006 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on August 27, 2015

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Chemistry of Materials

A Step Towards Efficient Panchromatic Multi-Chromophoric Sensitizers for Dye Sensitized Solar Cells Fadi M. Jradi,†,§ Daniel O’Neil,‡,§ Xiongwu Kang,‡ Jinsze Wong,ǁ Paul Szymanski,‡ Timothy C. Parker,† Harry L. Anderson,*,ǁ Mostafa A. El-Sayed,*,‡ Seth R. Marder*,† §

The authors wish to declare that F.M.J. and D.O. contributed equally to this body of work. School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States. ‡ Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States. ǁ Department of Chemistry, Oxford University, Oxford OX1 3TA, United Kingdom. †

Abstract: Panchromatic dyes with absorption profiles extending into the near infra-red are of interest to researchers in the field of dye sensitized solar cells (DSSCs), as they offer potential access to a wide energy range of photons necessary to enhance solar to electric power conversion efficiencies (PCEs). In this report, a porphyrin with a Soret band absorbing at high energy is combined with a squaraine absorbing at low energy via an acetylene linker to form a bichromophoric sensitizer with molar extinctions on the order of 105 M-1 cm-1 and an incident photon-to-current efficiency (IPCE) onset of ~850 nm. Various bulky substituents were installed on both the porphyrin and squaraine moieties, and conjugation was increased with π-bridge spacers to achieve a PCE of 7.6%, which is up to 15% higher than a comparable squaraine-only dye. For the most part, charge injection dynamics indicate slower charge injection rates and lower injection quantum yields for these bichromophoric sensitizers compared to non-porphyrin squaraine-based DSSC sensitizers. Nevertheless, higher PCE was observed for most porphyrincontaining dyes due largely to increased panchromaticity.

Introduction Dye sensitized solar cells (DSSCs) have been of major interest in the field of photovoltaics since the first efficient DSSC was reported in 1991.1 Central to the function of a DSSC is the chromophore sensitizer and over the years a wide range of organic sensitizers have been tested,2 with several relatively high performing classes of dyes identified such as indolines,3 triarylamines,4 squaraines (Sq),5 and porphyrins (Por),6 with the later holding the record solar-toelectric power conversion efficiency (PCE) of 13.0%. The PCE in a DSSC is in part dependent on the photocurrent density determined at the short circuit current (JSC), which is dependent on the optical properties of the sensitizer such as absorptance: the ratio of the radiant energy absorbed by the sensitizer to that incident on it.7 High absorptance requires good overlap of the absorption spectrum of the dye, with the incident solar spectrum especially between 400 and 920 nm where the spectral irradiance peaks,8 with 920 nm being the upper limit, as it was determined that the PCE peaks at ~1.4 eV in a single junction photovoltaic device.9 This being said, a sensitizer with an absorption onset at 920 nm, theoretically, has the potential of achieving JSC equal to 33 mA/cm2 under AM1.5 G, the standard solar spectrum used to test solar cells. A value that is almost double what is achieved by the best performing porphyrin dye.6

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Early panchromatic sensitizers were ruthenium-based,10 and achieved JSC on the order of 20.5 mA/cm2, with an absorption onset at around 900 nm and an incident photon-to-current conversion efficiency (IPCE) with a maximum of about 80%. However due to the low molar absorptivities of these classes of dyes, the toxicity of ruthenium and ruthenium being a nonabundant element, organic chromophores have been pursued as an alternative. Organic chromophores may have molar absorptivities ~10 × higher than typical ruthenium complexes because of the large transition dipole moments between π-orbitals; however, the intense transitions are also often narrow, which leads to lack of panchromaticity. Several approaches to increase the panchromaticity of organic dyes ranged from co-sensitization,5, 11 the use of energy relay dyes that utilize Förster resonant energy transfer from unattached chromophores in the electrolytes to tethered sensitizers,12 the use of light harvesting antennas linked to chromophores, mimicking biomolecules13 and the use of covalently linked bichromophoric,14 and even trichromophoric sensitizers,15 with complimentary absorption profiles to achieve increased JSC. In pursuing panchromaticity, red to near infra-red light absorbing sensitizers, such as squaraines16 and phthalocyanines17 are of special interest since they absorb photons that are inaccessible to most other sensitizers. Squaraine dyes,18 a subclass of polymethine dyes, have high molar absorptivities on the order of 105 M-1 cm-1 in the red to near infrared region (600 to 650 nm) of the solar spectrum, and have the potential to act as promising building blocks toward panchromatic absorption if absorption at higher energy can be achieved through chromophore design. Several structural modifications to squaraine based dyes have been reported,19 in which groups that absorb high energy photons have been introduced either as bridges between the squaraine and anchoring group,5, 16 donors on the opposite end of the squaraine relative to the anchoring group,20 or in multichromophoric systems.14a, 15 Porphyrins may provide increased panchromaticity in squaraine-based multichromophoric dyes since porphyrins have an absorption profile that compliments that of squaraines, typically a sharp Soret band (ε > 105 M-1 cm-1) between 400 and 450 nm and up to four Q bands (ε > 104 M-1 cm-1) between 500 to 700 nm.21 In addition, extended conjugation in Por-Sq-Por two-photon absorbing chromophores has been shown to red-shift the squaraine absorption by ~100 nm, which may potentially enable improved harvesting of infrared photons in a Por-Sq bichromophoric system.15 Herein, the design of an asymmetric bichromophoric sensitizer that links a porphyrin to a squaraine via an acetylene bridge is presented. Earlier, acetylene linkages have been shown to promote unidirectional charge flow from the porphyrin to the squaraine,21 which in the case of DSSCs, may presumably result in charge injection into TiO2 via proper positioning of a cyanoacrylic (CA) anchoring group on the squaraine moiety of a bichromophoric Por-Sq-CA system. The structures of four porphyrin-squaraine dyes are shown in Figure 1 and are conjugated through an acetylene having either thiophene (T) (PButyl-SC2-T, PButyl-SC12-T and PSil-SC12-T) or 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene (DTS) (PSilSC12-DTS) π-bridging units, and cyanoacetic acid as an anchoring group. Alkyl chains of varying length (ethyl and dodecyl) have been introduced on the indoline portion of the squaraine units along with tert-butyl and trihexylsilyl groups on the meta positions of the meso phenyl rings of the porphyrin to decrease aggregation in squaraines5, 22 and porphyrins,23 which can form H-aggregates when deposited on TiO2 surfaces. Additionally, 2-ethylhexyl (EtHex) groups are present in the DTS π-bridge, which has been previously shown in DTS-CA (Figure 1) both to


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decrease aggregation and to decrease charge recombination between the I−/I3− electrolyte and TiO2 in liquid electrolyte-based DSSCs,16 thus increasing both short circuit current (JSC) and open circuit voltage (VOC). As π-bridges, T and DTS are mainly utilized because of their relatively low degree of aromaticity, which is expected to facilitate intramolecular charge transfer to the anchoring group/acceptor and then on to TiO2.5

Figure 1: Molecular structures of the porphyrin-squaraine-cyanoacetic acid (Por-Sq-CA) dyes, YR6, and DTS-CA are shown for comparison. Results and discussion Materials The synthesis of asymmetrical porphyrins 1 and 2, Scheme 1, followed the stepwise approach that was developed by Senge and others.24 The synthesis of 1 and 2 are outlined in the Supporting Information (SI) and are briefly described here, referring to Scheme S1: dipyrromethane (S1’), which was prepared from pyrrole and formaldehyde, was condensed with benzaldehyde derivatives S11’ and S12’ to give the free-base porphyrins S2’ and S3’, respectively. The reaction of these porphyrins with phenyl lithium proceeds via an additionoxidation mechanism, where an initial attack of the phenyl lithium at the meso-position to form a porphodimethene is followed by protonation and oxidation to give the meso-functionalized asymmetrical free-base porphyrins S4’ and S5’ in high yields. The asymmetrical free-base porphyrins then undergo facile metallation with zinc acetate to give S6’ and S7’ (Scheme S1), which are then brominated using N-bromosuccinimide to afford porphyrins S8’ and S9’ in


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greater than 90% yields. Finally, Sonagashira coupling with trimethylsilyl (TMS) acetylenes gives the desired porphyrins 1 and 2 in 83% and 78% yields, respectively. The symmetrical squaraine building blocks, 3 and 4, in Scheme 1, were synthesized according to a literature procedure,25 in which a squaric acid was condensed with two equivalents of 1-alkyl-5-iodo-3,3-dimethyl-2-methyleneindoline (S4’’: alkyl = ethyl and S5’’: alkyl = dodecyl) to yield the symmetrical squaraines, Scheme S2, which were in turn coupled to 1 and 2 via a Sonogoshira coupling reaction that included an in-situ deprotection step with tetrabutylammonium fluoride (TBAF), to yield compounds 5 to 7. A Suzuki-Miyaura cross coupling of compounds 5-7 with thiophene boronic acid gave 8-10, which after Knoevenagel condensation with cyanoacetic acids yielded the (Por)-(Sq) dyes PBut-SC2-T, PBut-SC12-T, and PSil-SC12-T in Scheme 1. The synthesis of PSil-SC12-DTS, required the synthesis of the 4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene bridge DTS, Figure 1, separately and including it towards the end of the synthesis to insure minimal losses to the precious squaraine porphyrin precursor. DTS, whose complete synthesis is described in Scheme S3 in the Supporting Information, was prepared from the advanced intermediate S13’ which was protected with 2,2-dimethyl-1,3-propanediol to yield S14’,26 which then underwent a halogen-lithium exchange reaction and trapped with 2-isopropoxy-dioxaborolane to get DTS,26 Scheme S3. Suzuki-Miyaura cross coupling reaction between 7 and DTS, Scheme 1, resulted in the protected dye 11 which was then deprotected using trifluoroacetic acid to get 12 which after Knovenagel condensation reaction with cyanoacetic acid to get PSil-SC12-DTS.


4

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Scheme 1. Synthesis of Porphyrin-squaraine dyes.a

a

(a) TDBAPd, PPh3, TBAF, CuI, (iPr)2NH, tetrahydrofuran, 45 oC, 1 hour. (b) (5-formylthiophen-2-yl)boronic acid, Pd(dppf)Cl2.CH2Cl2, K2CO3, methanol, toluene. (c) Cyanoacetic acid, piperidine, chloroform, acetonitrile. (d) 7, DTS, Pd(dppf)Cl2.CH2Cl2, K2CO3, methanol, toluene. (e) Trifluoroacetic acid, room temperature, overnight.

Optical and Redox properties The absorption spectra of porphyrins are usually comprised of two bands; a relatively low intensity Q-band extending from 500-650 nm and a relatively intense Soret band from 400-460 nm, while those of squaraines are characterized by a relatively intense transition at 650 nm. As shown in Figure 2, and compared to the parent squaraine YR6, PBut-SC12-T has a 60 nm redshifted squaraine-based absorption band at 720 nm with an onset at 760 nm and an order of magnitude increase of high energy absorption from 400 to 500 nm due to the porphyrin’s Soret band. Upon increasing the length of the alkyl chain on the squaraine indole groups from ethyl in


5


PBut-SC2-T to dodecyl in PBut-SC12-T, a slight bathochromic shift was observed. The side chains on the porphyrin have little effect on the solution absorption, with tert-butyl PBut-SC12T and trihexylsilyl PSil-SC12-T having similar spectra. Variation of the π-bridge from T in PSilSC12-T to DTS in PSil-SC12-DTS red-shifted the squaraine and porphyrin bands by 6 nm. In addition, increased absorption in the region around 450-500 nm in PSil-SC12-DTS is likely the result of absorption of the DTS π-bridge. The optical density of the dyes adsorbed on TiO2 were determined and are presented in Figure 2 to gain an understanding of the aggregation effects in the surface-bound dyes. General broadening of the absorption profiles for all four dyes, along with red shifting of the squaraine-based absorption band was observed, consistent with what have been previously reported in literature.5, 22 PBut-SC2-T, having the shorter alkyl chain on the squaraine moiety, showed a shoulder around 670 nm, which is usually assigned to the Haggregate.5, 22 The shoulder at ~670 nm was reduced with increased alkyl chain length on the squaraine from ethyl (PBut-SC2-T) to dodecyl (PBut-SC12-T), presumably due in some part to decreased aggregation. The absorption between ~480-550 nm in DTS dyes (DTS-CA and PSilSC12-DTS) compared to the other dyes adsorbed on TiO2 is likely due to the DTS bridge. 3x10

2x10

1x10

YR6 PBut-SC2-T PBut-SC12-T PSil-SC12-T DTS-CA PSil-SC12-DTS

5

5

5

0 400

500

600


1.2

Optical Density (O.D.)

Molar Extinction (cm-1M-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Wavelength (nm)

700

800

1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

Wavelength (nm)

800

Figure 2. (Left) UV-vis absorption spectra of PBut-SC2-T, PBut-SC12-T, PSil-SC12-T and PSil-SC12-DTS in THF, also shown are YR6 and DTS-CA for comparison; (Right) UV-vis absorption spectra of the same series adsorbed on TiO2. The films were soaked in a dye solution of 0.05 mM dye with 10 mM 3α,7α-dihydroxy-5β-cholic acid (CDCA) for 1 hour. Legends are of the same size. The ground-state oxidation potential of the dye E( S + / S ) compared to the electrochemical potential of the electrolyte and the excited-state oxidation potential of the dyes E( S + / S * ) compared to the conduction band edge (CBE) of TiO2 are of major importance in solar cell operation. The CBE of TiO2 is usually placed at -0.5 V vs. NHE.27 However, it has been shown that this potential can be widely tuned by factors such as the surface charge, dipoles of the adsorbed molecules,27a pH of the electrolyte solution,28 and the presence of cations in the electrolyte.29 Hence, a CBE closer to -0.7 V vs. NHE (200 mV more negative) has been suggested more recently.30 The density of the semiconductor acceptor states have been shown to increase exponentially,30a which means a higher E( S + / S * ) with respect to the CBE should result in higher electron injection rates (kei). As seen in Table 1, compared to squaraine dyes, the (Por)-(Sq) dyes


6

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have E( S + / S * ) that are on average 140 mV less negative, which could translate to lower injection rates even though they are still at least 120 mV more negative than TiO2 CBE (-0.7 V vs. NHE). Similarly, to provide ample room for dye regeneration by the I−/I3− redox couple (0.30 V vs. NHE) an E( S + / S ) higher than 0.65 V vs. NHE31 is recommended. Table 1 lists the E( S + / S ) and E( S + / S * ) for the current sensitizers as well as for YR6 and DTS-CA. The E( S + / S ) of the Por-Sq dyes were around ~0.90 V vs. NHE while those of the squaraine dyes were ~0.82 V vs. NHE, providing driving forces for dye regeneration of ~600 mV and ~520 mV, respectively. Thus, to a first approximation, both Por-containing and non-Por dyes should have sufficient driving force for dye regeneration. Table 1. Optical and electrochemical properties of YR6, DTS-CA, PBut-SC2-T, PBut-SC12-T, PSil-SC12-T and PSil-SC12-DTS.

ε [M-1cm-]1a)

E0−0opt [eV]b)

E( S + / S ) [V]c)

E( S + / S * ) [V]d)

Dyes

λ max

YR6

659

279,000

1.76

+0.80

-0.96

PBut-SC2-T

718

182,000

1.72

+0.90

-0.82

PBut-SC12-T

713

246,000

1.72

+0.88

-0.84

PSil-SC12-T

717

272,000

1.72

+0.89

-0.83

DTS-CA

657

257,000

1.82

+0.84

-0.98

PSil-SC12-DTS

722

317,000

1.72

+0.89

-0.83

a)

Derived from absorption spectra in THF, with the exception of YR6, and DTS-CA from ethanol; b) Determined from the intersection of the normalized absorption and emission spectra, refer to SI. c) Halfwave ground state oxidation potentials determined via cyclic voltammetry, refer to SI. d) Calculated according to the following equation

E( S + / S * ) = E( S + / S ) - E0−0opt . Photovoltaic characterization A mesoporous photoanode purchased from SOLARONIX (ref. no. 74101) consisting of a 10 µm thick 20 nm TiO2 particles and a 4 µm 100 nm TiO2 scattering layer on fluorine-doped tin oxide glass was treated with TiCl4 solution (40 mM) at 70 °C for 30 minutes. After washing with deionized water and absolute ethanol, the photoanode was thermally annealed at 500 oC for 30 minutes. After cooling, the photoanode was dipped in a dye solution in ethanol/chloroform (4:1) containing 3α,7α-dihydroxy-5β-cholic acid (chenodeoxycholic acid, CDCA) for 4 hours at room temperature. After washing with ethanol and drying with high purity nitrogen gas, a drop of electrolyte and a 60 µm thick Surlyn spacer were placed on the stained TiO2 electrodes. A counter electrode was then placed on top and held in place with clamps. Counter electrodes were prepared by adding two drops of platinum salt solution (Dyesol, MS006220) on FTO and heated at 400 °C for 20 min. The liquid electrolyte consisted of 0.6 M 1,3-dimethylimidazolium iodide, 0.03 M iodine, 0.05 M LiI, 0.05 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine in 15/85 (v/v) mixture of valeronitrile and acetonitrile. The IPCE spectra of dye cells based on the sensitizers in Figure 1 with the respective dye/CDCA ratios given in Table 2 are shown in Figure 3. The IPCE curve of each dye generally follows the spectral features seen in the optical spectra of the surface bound dyes shown in


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Figure 2. The Por-containing dyes all have nearly equivalent or somewhat greater peak currents in their high energy absorption areas (~500 nm) compared to their low energy absorption areas (~750 nm) whereas both non-Por dyes (YR6 and DTS-CA) have greater peak currents in their low energy absorption areas (~700 nm); however, these slightly more balanced peak currents are not necessarily translated into overall higher PCEs as summarized in Table 2. The photovoltaic performance was characterized under simulated AM1.5 one sun illumination on cells with 0.36 cm2 active areas. It should be noted that the cells were not masked during illumination, which may lead to some overestimation of absolute PCE values;32 however, subsequent studies on the same characterization equipment using masked cells has shown an overestimation of at most 10%. Within this series of dyes, thiophene π-bridged YR6 may be compared to thiophene πbridged PBut-SC2-T, PSil-SC12-T, and PSil-C12-T to assess the effect of synthetic addition of Por on overall DSSC performance. Amongst the Por-containing dyes, and as illustrated by space filling models (Figures S36 to S40), the steric profile increases from PBut-SC2-T to PButSC12-T to most sterically encumbered PSil-SC12-T. Compared to non-Por YR6, the reduced JSC in PBut-SC2-T is most likely due at least in part to increased dye aggregation that is apparent in the optical spectra of TiO2-adsorbed PBut-SC2-T; however, since YR6 also displays aggregation, the reduced JSC of PBut-SC2-T may be from lower charge injection efficiency φ(ei) (as discussed below). Both PBut-SC12-T and PSil-SC12-T, two Por-dyes that are more sterically demanding than YR6, each show greater JSC. Additionally, higher VOC of +12 mV and +7 mV were observed for PBut-SC12-T and PSil-SC12-T, respectively, which is somewhat consistent with increased charge collection from decreased electrolyte/injected charge recombination due to the greater steric shielding from the combination of the Por and Sq moieties in PBut-SC12-T and PSil-SC12-T compared to those in PBut-SC2-T and YR6. Similarly, the most sterically demanding Por-containing dye in this series, PSil-SC12-DTS, leads to ~30 mV higher Voc than that seen in PBut-SC12-T and PSil-SC12-T due to the additional 2-ethylhexyl groups on the DTS bridge. This is consistent with what has been seen in DTS-CA, which has been attributed in part to reduced electron/injected charge recombination due to steric shielding of the 2-ethylhexyl groups.16 In both cases with the bulkier [3,5-bis(trihexylsilyl)phenyl]-substituted Por dyes, lower JSC was observed compared to the 3,5-bis(tert-butyl)phenyl analogue in the case of PSil-SC12-T and PBut-SC12-T or the non-Por analogue in the case of PSil-SC12-DTS and DTS-CA. Although not studied in-depth herein, this lower JSC in PSil-SC12-T vs. PBut-SC12-T may be due to the bulkier 3,5-bis(trihexylsilyl)phenyl groups shielding the hole of the oxidized dye from the electrolyte in PSil-SC12-T compared to the less sterically shielding 3,5-bis(tert-butyl)phenyl in PBut-SC12-T. Such shielding could reasonably be expected to reduce dye regeneration and hence reduce JSC. This effect may be greater in comparing PSil-SC12-DTS to DTS-CA, since DTS-CA contains no shielding Por moiety. One interesting feature of the IPCE curve of PSilSC12-DTS is that the high-energy region (~400-600 nm) has a noticeably higher IPCE peak than the low-energy region (~600-850), which is in contrast to the optical absorption of PSil-SC12DTS on TiO2 in Figure 2. For the other Por-containing dyes, the IPCE curves more closely follow the adsorbed TiO2 optical spectra. The high-energy IPCE observation of PSil-SC12-DTS may be from a combination of: 1) Por-based absorption of a photon that internally converts to the low energy excited state that injects with the efficiency of the lower excited state; and 2) a DTSbased absorption to a higher excited state that hot injects with somewhat greater efficiency due to proximity of the DTS-based orbital to the TiO2 (note that the shape of the PCE curve of PSil-


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SC12-DTS in the range ~500-550 somewhat follows that of DTS-CA). The lower peak IPCE of PSil-SC12-DTS in the low energy region compared to PBut-SC12-T and PSil-SC12-T is a matter that may require additional investigation since the aggregation seen in PBut-SC2-T, which is presumably responsible for the low peak IPCE of PBut-SC2-T, should not be expected to be as significant a factor in the relatively low peak IPCE seen in the more sterically shielded PSil-SC12-DTS.


100

IPCE (%)

80 60 40 20 0 400

500 600 700 800 Wavelength (nm)

900

20

2

Current Density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 10 5 0 0.0


0.2

0.4 Voltage (V)

0.6

Figure 3. Left: IPCE scans for optimized dye cells. Right: Photovoltaic characteristics of optimized cells with TiO2 films soaked in dye solutions whose compositions are stated in Table 2. The TiO2 films, in both measurements, were soaked in dye solutions for 4 hours. Legends are of the same size Table 2. Photovoltaic performancea of the optimized dye cells

YR6

Dye Conc. [mM]

CDCA Conc. [mM]

Voc [V]

JSC [mA/cm2]

FF [%]

PCE [%]

0.1

10

0.65 ± 0.01

14.2 ± 0.3

70.4 ± 0.2

6.5 ± 0.1


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PBut-SC2-T

0.1

10

0.65 ± 0.01

13.4 ± 0.1

70.4 ± 0.8

6.1 ± 0.1

PBut-SC12-T

0.05

10

0.66 ± 0.01

16.3 ± 0.4

70.1 ± 0.6

7.5 ± 0.1

PSil-SC12-T

0.05

10

0.65 ± 0.01

15.2 ± 0.1

71.2 ± 0.3

7.1 ± 0.1

DTS-CA

0.05

10

0.68 ± 0.01

19.1 ± 0.2

68.3 ± 0.7

8.9 ± 0.2

PSil-SC12-DTS

0.1

10

0.69 ± 0.01

16.0 ± 0.2

69.6 ± 0.7

7.6 ± 0.1

a

Based on measurements of 3 devices for all dyes with upper/lower measured values shown. The photovoltaic measurements were conducted in the absence of a mask and with a cell active area of 0.36 cm2.

Electron injection kinetics and rates Device dynamics were measured using standard femtosecond pump-probe experiments. A

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