Toward Efficient Solid-State p-Type Dye-Sensitized Solar Cells: The

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Towards Efficient Solid-State p-Type DyeSensitized Solar Cells: The Dye Matters Trang T. T. Pham, Sudip Kumar Saha, David Provost, Yoann Farré, Mahfoudh Raissi, Yann Pellegrin, Errol Blart, Sylvain Vedraine, Bernard Ratier, Dmitry Aldakov, Fabrice Odobel, and Johann Bouclé J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10513 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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The Journal of Physical Chemistry

Towards Efficient Solid-State p-Type DyeSensitized Solar Cells: the Dye Matters Trang T. T. Pham,1 Sudip K. Saha,1 David Provost,2 Yoann Farré,2 Mahfoudh Raissi,2 Yann Pellegrin,2 Errol Blart,2 Sylvain Vedraine,1 Bernard Ratier,1 Dmitry Aldakov,3 Fabrice Odobel,2* and Johann Bouclé 1* 1

XLIM UMR 7252, Université de Limoges/CNRS, 123 Avenue Albert Thomas, 87060 Limoges

Cedex, France 2

Université LUNAM, Université de Nantes, CNRS, Chimie et Interdisciplinarité: Synthèse,

Analyse, Modélisation (CEISAM), UMR 6230, 2 rue de la Houssinière, 44322 Nantes cedex 3, France 3

Univ. Grenoble Alpes, INAC-SPRAM, F-38000 Grenoble, France; CNRS, INAC-SPRAM, F-

38000 Grenoble, France; CEA, INAC-SPRAM, F-38000 Grenoble, France * Corresponding authors: [email protected] ; [email protected]

ABSTRACT. Photo-electrochemical devices based on p-type nanostructured semiconducting materials show strong potentialities for various applications, such as photovoltaics and photocatalysis. While only one study was reported on the use of the reference dye P1 for solidstate p-type dye-sensitized solar cells (DSSC), in this work we have systematically investigated two diketopyrrolopyrrole (DPP) derivatives as sensitizers for solid-state p-type DSSC based on

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NiO and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as solid-state electron transporter material. We report on the performance in solid-state p-type DSSC of a simple DPP dye bearing a thienyl carboxylic acid as binding group and a parent compound substituted by a pyromellitimide (PYRO) playing the role of secondary inner electron acceptor. By focusing on the dye/PCBM interface, we specifically show using transient photoluminescence measurements that the presence of a secondary electron acceptor unit can efficiently favor the formation of the (dye+/PCBM-) state, owing to its significant reducing ability and lifetime of the charge separated state. As a consequence, using these DPP derivatives leads to unprecedented photocurrents up to 0.45

mA cm-2, which are 10 times larger than previously reported values for the system

based on P1. Our analysis also demonstrates the strong correlation between the ability of the dyes to efficiently generate charge carriers and the resulting photocurrents.

INTRODUCTION Dye-sensitized solar cells (DSSC) have been explored for over twenty years and allowed to get a deep understanding of the photo-generated physical processes associated with nanostructured metal oxide films for electrochemical applications.1-3 Nonetheless, among DSSC, p-type devices have attracted significantly lower attention compared to their n-type counterparts.4-7 In most ptype devices, nanostructured nickel oxide (NiO) is used as sensitized nanoporous photoelectrode, in contrast to n-type TiO2 photo-anode used for conventional DSSC. Although fast photo-induced hole transfer from the excited sensitizer (S*) to the valence band of NiO is generally evidenced, the relatively short-lived charge-separated state (e.g. hole in NiO and electron on the sensitizer, or / state) remains an intrinsic limitation to efficient photovoltaic energy conversion.5, 8-10 In addition, poor physical properties of the p-type photo-


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cathode, as well as a non-optimal energetic configuration at the NiO-dye-electrolyte interface, led to device performance up to 2.5% under standard solar emission,11 far below that of n-type DSSC.12 Despite poor reported efficiencies, p-type DSSC remain highly promising for the fabrication of tandem DSSC devices, and important efforts are made in the international community to develop alternative p-type inorganic semiconductors,13-17 organic dyes,18-21 or electrolyte systems.11, 22 Recently, the transposition from liquid to solid-state electrolytes was proposed using the commercial 4-(Bis-(4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl)-amino)-benzoic acid (P1) dye and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as electron transporter.23 A first photo-physical study evidences sub-picosecond hole injection from the excited P1* to NiO (< 200-300 fs), while electron transfer from 1 to PCBM competes with charge recombination. Consequently, poor photocurrent generation (~50 μ. ) is attributed to slow dye regeneration by PCBM, owing to a short lifetime of the reduced dye. Although this study was not focusing on the more suitable compounds for efficient charge generation, it however paves the way for researchers towards more rational design of new molecular species to demonstrate better performing solid-state p-type DSSC. In the quest for highly efficient organic dyes, we recently proposed dyads based on diketopyrrolopyrrole derivatives (DPP) to improve the electron transfer efficiency from the photo-reduced sensitizer to the redox shuttle, thanks to their long-lived charge-separated state induced from the presence of an inner electron acceptor unit.24-25 DPP are also highly relevant in the field of DSSC as they possess an excellent electron withdrawing ability leading to high oxidizing power in the excited state, are relatively photo-stable, and can be easily synthesized.2627

When combined with triarylamine electron-rich moieties and a secondary electron acceptor


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unit formed by naphthalene diimide (NDI), these DPP dyes exhibited long-lived charge separated states ( / ) up to 250 µs, resulting in efficient p-type DSSC based on notoriously slow Co(III) polypyridine electrolytes, and demonstrating the strong potentialities of these compounds.24 In this work, we have synthesized a new DPP derivative (DPP-PYRO) more specifically designed for solid-state p-type DSSC in the presence of PCBM as electron transporter medium (ETM). Shifting from NDI to pyromellitimide (PYRO) as secondary electron acceptor unit, we target both a long-lived charge separated state and a larger driving force to reduce PCBM from the secondary inner acceptor. Indeed, PYRO exhibits a larger reduction potential than NDI (see below). The properties and associated device performance of DPP-PYRO are compared to its analogous compound bearing no secondary electron acceptor unit (DPP-Br), as well as to the commercial P1 reference (the chemical structures of the dyes and PCBM are given in (Figure 1). Our study especially focuses on the PCBM electron transporting medium (ETM), by optimizing its infiltration in the dye-sensitized NiO electrode and by monitoring the electron transfer process occurring at the dye-PCBM interface by time-resolved photoluminescence (TRPL) measurements. A significant improvement in photocurrent is demonstrated for the three dyes when a two-step infiltration procedure is used. Our approach also evidences the critical influence of the dye on the charge generation mechanisms. The optimal compounds towards efficient solid-state p-type DSSC should bring together a suitable energetic configuration at the interface with NiO and PCBM, a long-lived charge-separated state, in combination with an appropriate interface with PCBM so that quantitative electron transfer can occur.


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Figure 1. Chemical structures of the diketopyrrolopyrrole dyes DPP-PYRO and DPP-Br, along with those of P1 and PCBM.

EXPERIMENTAL Synthesis of intermediates and DPP-PYRO dye. 1

H and

13

C spectra were recorded on a AVANCE 300 UltraShield BRUKER and AVANCE

400 BRUKER. Chemical shifts for 1H and

13

C NMR spectra are referenced relative to residual

protium in the deuterated solvent (CDCl3 δ = 7.26 ppm for 1H and δ = 77.16 ppm for 13C; THFd8 δ = 3.57, 1.72 ppm for 1H and δ = 67.21, 25.31 ppm for 13C; C6D6 δ = 7.16 ppm for 1H and δ = 128.06 ppm for 13C) or to an internal reference (TMS, δ = 0 ppm for both 1H and 13C). NMR spectra were recorded at room temperature, chemical shifts are written in ppm and coupling constants in Hz. High-resolution mass (HR-MS) spectra were obtained either by electrospray ionization coupled with high resolution ion trap orbitrap (LTQ-Orbitrap, ThermoFisher Scientific,) or by MALDI-TOF-TOF (Autoflex III de Bruker), working in ion-positive or ionnegative mode. Electrochemical measurements were made under an argon atmosphere in the


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mixture of dichloromethane:dimethylformamide (CH2Cl2:DMF, 95:5 volume ratio) with 0.1 M Bu4NPF6. Cyclic voltammetry experiments were performed by using a SP300 Bio-Logic potentiostat/galvanostat. A standard three-electrode electrochemical cell was used. Potentials were referred to a saturated calomel electrode as internal reference. All potentials are quoted relative to SCE. The working electrode was a glassy carbon disk and the auxiliary electrode was a Pt wire. In all the experiments the scan rate was 100 mV.s-1. UV-visible absorption spectra were recorded on a Varian Cary 300, using 1 cm path length cells. Emission spectra were recorded on a Fluoromax-4 Horiba Jobin Yvon spectrofluorimeter (1 cm quartz cells). Chemicals were purchased from Sigma-Aldrich or Alfa Aesar and used as received. Thin-layer chromatography (TLC) was performed on aluminium sheets pre-coated with Merck 5735 Kieselgel 60F254. Column chromatography was carried out with Merck 5735 Kieselgel 60F (0.040-0.063 mm mesh). The synthesis of compound 1 was described in a literature procedure.28 Compound 2. Pyromellitimide 1 (107 mg, 0.202 mmol) was dissolved in 23 mL of toluene and 5 mL of N,N-Diisopropylethylamine (DIPEA) under argon atmosphere and the resulting mixture was freed from oxygen by bubbling argon while sonicating. Then, copper iodide (2 mg, 0.010

mmol),

ethynyltrimethylsilane

(142

µL,

0.910

mmol)

and

tetrakis(triphenylphosphine)palladium (23 mg, 0.020 mmol) were added and the mixture was stirred at 50°C for 2 hours. Solvents were evaporated and the crude product was purified by flash column chromatography (silica gel, dichloromethane) to obtain the title product as a pale yellow solid (84 mg, 83%). 1H NMR (300 MHz, CDCl3, 25°C), δ (ppm):.8.37 (s, 2H), 7.60 (d, 2H, 3J = 8.8 Hz), 7.44 (d, 2H, 3J = 8.8 Hz), 3.76 (t, 2H, 3J = 7.4 Hz), 1.71 (m, 2H), 1.40-1.20 (m, 10H), 0.87 (t, 3H, 3J = 6.8 Hz), 0.27 (s, 9H) .13C NMR (75 MHz, CDCl3, 25°C), δ (ppm): 166.2, 165.1,


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137.8, 136.8, 132.9, 131.0, 126.0, 123.7, 118.9, 104.0, 96.1, 39.0, 31.9, 31.1, 29.3, 29.2, 28.6, 27.0, 22.7, 14.2, 0.0. MS (EI) m/z: 500.3. Compound 3. To a solution of pyromellitimide 2 (6.7 mg, 13.4 µmol) in 4 mL of tetrahydrofuran (THF), tetrabutylammonium fluoride (14 µL (3M in THF), 14 µmol) was added at 0°C. The mixture was stirred at 0°C for two minutes, then water was poured and the product was extracted with dichloromethane. Organic layer was dried over anhydrous magnesium sulfate, and the resulting crude product was purified by flash column chromatography (silica gel, dichloromethane) to give the title product as a white solid (2.0 mg, 35%). 1H NMR (300 MHz, CDCl3, 25°C), δ (ppm):.8.38 (s, 2H), 7.65 (d, 2H, J = 8.6 Hz), 7.47 (d, 2H, J = 8.6 Hz), 3.75 (t, 2H, J = 7.4 Hz), 3.16 (s, 1H) 1.71 (m, 2H), 1.40-1.20 (m, 10H), 0.87 (t, 3H, J = 6.8 Hz).

13

C

NMR (75 MHz, CDCl3, 25°C), δ (ppm): 166.2, 165.1, 137.9, 136.8, 133.1, 131.4, 126.1, 122.6, 118.9, 82.7, 78.7, 39.0, 31.9, 29.3, 29.2, 28.6, 27.0, 22.8, 14.2. MS (EI) m/z: 428.3. Compound 5. In a sealed tube, compound 4 (DPP) (42.0 mg, 0.0406 mmol, 1.00 eq) and compound 3 (23.0 mg, 0.0537 mmol, 1.30 eq) were solubilized in a mixture of toluene:triethylamine (6 mL, 5:1 volume ratio) under argon atmosphere. The mixture was degassed carefully. Copper iodide (2.0 mg, 0.0062 mmol, 0.20 eq) and Pd(PPh3)4 (5.0 mg, 0.0041 mmol, 0.10 eq) were added quickly. The solution was stirred at 80°C for 18h. After cooling to room temperature the solvents were evaporated under reduced pressure. The crude product was purified by column chromatography (eluent:petroleum spirit:dichloromethane then pure dichloromethane). The recovered material was then taken in a dichloromethane (1 mL) and methanol was added. The precipitate was recovered by filtration as a red product (26 mg, 46%). 1

H NMR (400 MHz, C6D6, 25°C), δ (ppm):.8.08 (d, J=8.83 Hz, 4H), 8.03 (d, J=8.41 Hz, 2H),

7.89 (d, J=8.35 Hz, 2H), 7.82 (s, 2H), 7.53 (m, 4H), 7.48 (d, J=8.50 Hz, 2H), 7.41 (d, J=8.50 Hz,


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2H), 7.32 (d, J=8.62 Hz, 2H), 6.99 (d, J=8.65 Hz, 6H), 3.82-4.02 (m, 4H), 3.57-3.63 (m, 2H), 1.56-1.72 (m, 4H), 1.51 (s, 18H), 1.01-1.31 (m, 26H), 0.90 (t, J=6.96 Hz, 3H), 0.76-0.85 (m, 6H), 0.67-0.75 (m, 6H).

13

C NMR (100 MHz, C6D6, 25°C), δ (ppm): 165.81, 165.03, 164.52,

162.75, 150.62, 148.71, 147.43, 146.52, 142.54, 137.34, 136.38, 136.00, 132.44, 132.01, 131.80, 131.34, 129.92, 129.26, 129.00, 127.22, 126.87, 126.04, 125.75, 125.58, 123.25, 123.07, 118.03, 110.82, 110.42, 91.45, 90.74, 80.26, 44.99, 44.87, 38.88, 38.59, 32.01, 30.69, 29.47, 29.33, 28.65, 28.48, 28.09, 27.09, 24.08, 23.07, 22.90, 14.18, 14.06, 10.48.HRMS (ESI+) m/z: [M]+ calculated for C88H95N5O10, 1381.7073; found, [M]+ 1381.7076. ∆= 0.2 ppm. Sensitizer DPP-PYRO. In a round bottom flask containing compound 5 (36 mg, 0.025 mmol, 1 eq) under argon was added dichloromethane (4.0 mL) and then trifluoroacetic acid (1.0 mL) slowly. The resulting solution was then stirred for 3 h at room temperature. After completion of the reaction monitored by TLC, the crude was concentrated under reduced pressure and purified by precipitation in a mixture of dichloromethane:methanol (1:1 volume ratio) by addition of hexane (20mL). The product was obtained as a red powder in 91% yield (30 mg, Figure 2). 1H NMR (400 MHz, THF-d8, 25°C), δ (ppm):. 8.30 (s, 2H), 7.99 (d, J=8.51 Hz, 2H), 7.93 (m, 6H), 7.80 (d, J=8.35 Hz, 2H), 7.74 (d, J=8.61 Hz, 2H), 7.66 (m, 4H), 7.59 (d, J=8.74 Hz, 2H), 7.24 (d, J=8.69 Hz, 2H), 7.15 (d, J=8.86 Hz, 4H), 3.85 (m, 4H), 3.70 (m, 2H), 1.50 (m, 2H), 1.07-1.41 (m, 28H), 0.89 (m, 3H), 0.81 (m, 6H), 0.74 (m, 6H).

13

C NMR (100 MHz, THF-d8, 25°C),

δ (ppm): 166.88, 166.72, 165.63, 162.76, 162.66, 151.45, 148.76, 147.32, 147.23, 143.07, 138.46, 137.69, 136.85, 133.05, 132.90, 132.60, 131.80, 130.20, 129.63, 129.49, 128.92, 128.25, 127.21, 126.77, 126.26, 125.98, 123.54, 123.06, 118.27, 111.02, 110.54, 91.62, 90.45, 45.22, 39.41, 38.92, 32.52, 31.08, 29.92, 29.86, 29.08, 28.96, 27.58, 24.42, 23.50, 23.28, 14.17, 14.07,


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10.54. HRMS (ESI-) m/z: [M-2H]2- calculated for C80H77N5O10, 1267.5670; found, [M-2H]21267.5640. ∆= 2.4 ppm.

Figure 2. Synthesis of dye DPP-PYRO. Reagents and conditions: a) Pd(PPh3)4, CuI, Et3N, toluene, 80°C, 18 h, 46%; b) TFA, DCM, TA, 3 h, 91%.

Solar cell fabrication and characterization FTO substrates (Solaronix, 15Ω/square) were etched by HCl and subsequently cleaned in an ultrasonic bath with distilled water, ethanol, acetone and isopropanol for 10 min each, followed by 10 min of UV-ozone treatment. A NiO blocking layer was spin-coated at 2000 rpm for 30 s on the FTO substrate from a 0.5M solution of nickel acetate tetrahydrate and ethanolamine in 2methoxyethanol, followed by a sintering step at 500oC in air for 30 min. NiO porous layer were prepared from a home-made NiO paste, whose fabrication is described in published reports.25 The paste, based on particles presenting a mean diameter of around 20 nm (Inframat), was diluted with absolute ethanol in a 1:1 weight ratio and continuously stirred before use. NiO mesoporous layers are then obtained by spin-coating at 4000 rpm the previous diluted paste, and the films were sintered in air at 400oC for 30 minutes. The reference P1 dye (Dyenamo AB, Sweden) was used, in addition to DPP-PYRO and DPP-Br, the latter being synthesized


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according to a published procedure.25 In all cases, the dye solutions were prepared at 0.2 mM from a CH2Cl2:THF solvent mixture (2:1 volume ratio). Chenodeoxycholic acid (CDCA, 0.6 mM) was used as co-adsorbent. The sensitization was performed overnight. To improve the dye coverage on the NiO surface, the samples were rinsed with acetonitrile after the sensitization, followed by a spin-coating cycle using 20 μl of CDCA solution in acetonitrile (2 mM for P1 and DPP-Br and 10 mM for DPP-PYRO; 2000 rpm, 30 s). No rinsing after spin-coating was needed. Further discussion about this treatment is presented in the supporting information (ESI, Table S1). PCBM (American Dye Source, Inc., Canada) was dissolved at various concentrations from 5 to 40 mg/mL in chlorobenzene, using 3 vol% of 1,8-diiodooctane as additive. PCBM infiltration was performed in a nitrogen-filled glovebox by first dropping 20 µL of solution on the dyesensitized NiO electrodes, followed by a resting time of 20 s and a gradual spin-coating cycle from 500 to 3000 rpm over 40 s. The samples were then annealed under inert atmosphere for 10 min before a second PCBM deposition cycle was performed. A final annealing at 100°C was then applied. A TiOx thin layer was then deposited on top of PCBM by spin-coating from a 0.23 M titanium (IV) isopropoxide solution in isopropanol containing 0.013 M of HCl, followed by 15 min annealing at 100oC. Finally, a 120 nm thick Al electrode was deposited by thermal evaporation under vacuum (10-6 mbar) through a shadow mask, defining two independent devices with an active area of 0.18 cm² each. Solar cells were characterized in the dark and under calibrated simulated solar emission using an AM1.5G-filtered NEWPORT Class A solar simulator and a certified reference monocrystalline silicon cell. The equivalent irradiance was adjusted at 100 mW.cm-2. The electrical signal was recorded using a Keithley 2400 source-


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measure unit. We point out that several (>5) independent sets of cells were characterized for all dyes. Average devices illustrating the general trends are presented.

Photoluminescence studies Photo-physical studies on non-injecting substrates were performed by first depositing thin and transparent porous Al2O3 films on glass from a diluted home-made alumina paste. Briefly, 5 g of an Al2O3 nanoparticle (Sigma-Aldrich, used as purchased) suspension in isopropanol was added to a 5 g solution of ethyl cellulose initially dissolved at 10 wt% in ethanol. 3.33 g of α-terpineol was then added and the resulting paste was stirred with an ultrasonic homogenizer. The paste was further diluted by adding absolute ethanol in a 1:1 ratio, and thin Al2O3 films were deposited by spin-coating on pre-cleaned glass substrates at 2000 rpm for 60 s. The films were annealed at 380°C before sensitization by the three dyes using the above-mentioned recipes. UV-visible transmission spectra were recorded on the bare and PCBM-infiltrated dye-sensitized substrates (Al2O3 or NiO) by an AGILENT Cary 300 reflectometer using an integrating sphere. Steadystate photoluminescence (PL) spectra were recorded on an FLS 980 spectrometer (Edinburgh Instruments, UK) using a monochromated Xenon lamp (450 W) and a cooled Hamamatsu R928P photodetector. Samples were placed at 45° in a specific sample holder, and the spectral bandwidth was defined at 1.5 nm. Transient photoluminescence spectra were recorded on the same apparatus using a 510 nm picosecond laser diode (temporal width of 150 ps) and a fast response photodetector. The detection was made through time-correlated single photon counting (TCSPC) and the traces were adjusted by taking into account the instrument response function


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(IRF), estimated using a diffusive reference sample. The IRF of the spectrometer (full width at half maximum of ~ 520 ps) of is given in ESI (Figure S7).

Scanning electron microscopy Top view and cross-section morphology of the substrates before and after deposition of PCBM was investigated using a Zeiss Ultrascan 55 scanning electron microscope (SEM) at different accelerating voltages. EDX profiles for Ni, Sn, C, and Al have been recorded on the device cross-section, with a spatial resolution of approximately 100 nm. These profiles are qualitatively discussed in order to monitor the penetration of HTM on the dye-sensitized NiO electrode. Several EDX images taken at the cross-sections of various samples show similar concentration profiles.

RESULTS AND DISCUSSION The synthesis of the DPP-PYRO dye relies on the assembly of two key building blocks, namely the pyromellitimide 3 and the TPA-DPP 4.24 The latter was synthesized from compounds 128 and 4,24 whose preparations were already published in the literature. Compound 3 was synthesized in two steps starting with a Sonogashira cross-coupling reaction between ethynyltrimethylsilane and iodo-pyromellitimide 1 using Pd(PPh3)4 and CuI as the catalytic system followed by cleavage of the silyl protecting group by tetrabutyl ammonium fluoride (Figure 3). DPP-PYRO was prepared from the Sonogashira cross-coupling between reaction between TPA-DPP 4 and ethynyl substituted pyromellitimide derivative 3 using the classic catalytic system Pd(PPh3)4 and CuI in presence of trimethylamine as base and afforded the


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compound 5 in 46% yield. Trifluoroacetic acid was employed to hydrolyze the tert-butyl ester groups in 5 to synthesize the DPP-PYRO dye in 91% yield. All the new compounds gave satisfactory NMR and mass spectrometry analyses (see experimental part).

Figure 3. Synthesis of intermediate 3. Reagents and conditions: a) Pd(PPh3)4, CuI, TMSA, DIPEA, toluene, 50°C, 2 h, 83%; b) TBAF, THF, 0°C, 2 min. 35%.

Electronic UV-visible absorption and emission spectroscopy The absorption and emission spectra of the DPP-PYRO, DPP-Br and P1 dyes were recorded in dichloromethane solution and are shown in Figure 4. The spectroscopic data are collected in Table 1. DPP-PYRO and DPP-Br show an absorption band at 492 nm (ε≈22000 M-1cm-1) attributed to a π-π* transition mostly localized on the DPP core. For DPP-PYRO there is an additional intense band at 325 nm (ε≈56300 M-1cm-1) attributed to the transitions on the pyromellitimide moiety. On the other hand, the spectrum of P1 is dominated by a strong and blue-shifted charge transfer transition located at 486 nm.29 All the three dyes are fluorescent but DPP-PYRO exhibits an emission band at 576 nm, which is partially quenched compared to the


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DPP-Br dye, most certainly due to electron transfer to the pyromellitimide unit as this reaction is exergonic (see below).

Figure 4. Electronic absorption (straight line) and normalized emission (dashed line) spectra of the dyes DPP-PYRO (red), DPP-Br (black) and P1 (blue) recorded in dichloromethane solution.

Table 1. Wavelengths of maximal absorption (λabs) with extinction coefficient (ε), wavelength of maximal emission (λem) recorded at room temperature in dichloromethane and zero-zero energy level of the lowest singlet excited state (E00) of the DPP moiety. Dyes

λabs / nm (ε / M-1 cm-1)

λem / nm

a

E00 / eV


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DPP-PYRO

492 (21900), 325 (53600)

576

2.29

DPP-Br

487 (17800), 355 (27100)

563

2.33

P1

486 (42800), 352 (24000)

618

2.25

Electrochemical study and electron transfer driving forces An electrochemical study by cyclic voltammetry was undertaken to determine the redox potentials of the new dye DPP-PYRO in order to calculate the driving forces for hole injection in NiO (∆G°inj, see Table 2). The redox potentials of the two other dyes (DPP-Br and P1) were taken from the literature.24, 29

Table 2. Redox potentials recorded by cyclic voltammetry at room temperature in dichloromethane/DMF [95:5] solution with Bu4NPF6 (0.1 M) as supported electrolyte and referenced versus saturated calomel electrode (SCE). The values in parentheses correspond to the potential difference (in meV) between the cathodic and anodic waves associated with the electrochemical process. E1/2(TPA+/TPA)

E1/2(dye+/dye)

E1/2(dye/dye-)

E1/2(Pyro/Pyro-)

a

(V) ∆E (mV)

(V) ∆E (mV)

(V) ∆E (mV)

(V) ∆E (mV)

(V)

(eV)

DPP-PYRO

1.14c

1.06c

-1.18 (140)

-0.80 (90)

1.11

-0.81

DPP-Br d

1.19 (90)

1.13 (120)

-1.26

-

1.03

-0.73

P1e

1.13

-

-1.01

-

1.24

-0.96

Dyes

E(dye*/dye-)

b

∆Ginj


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a

Calculated according to the equation: E1/2(DPP*/DPP-) = E1/2(DPP/DPP-) + E00. b Calculated according to the equation: ∆G°inj = EBV(NiO)- ERed(DPP*/DPP-) with EBV(NiO) = 0.30 V vs SCE. c Non reversible peak, determined as the peak potential. d Taken from reference 24. e Taken from reference 29.

For DPP-PYRO, two weakly reversible oxidation processes occur successively. The first one is located on the DPP core and the second on the triphenylamine moiety at 1.06 and 1.14 V vs SCE respectively. The DPP core is reduced with a reversible wave at -1.18 V vs SCE while pyromellitimide moiety is reduced at -0.80 V vs SCE. The 0.38 V difference between the reduction potentials of pyromellitimide and DPP indicates that there is a significant electron transfer driving force for electron shift after the hole injection in NiO. The calculated Gibbs free energy for hole injection process indicates an exergonic reaction, which therefore is thermodynamically favourable for all the dyes (Table 2). Finally, the optical and electrochemical properties of the dyes can enable a first approximation of the driving force for dye regeneration through electron transfer to PCBM which are found to be in the order of -0.1, -0.5, and -0.2 eV for DPP-PYRO, DPP-Br, and P1, respectively. As a consequence, the energetic configuration at the NiO/dye/PCBM interface appears favorable for current generation in the presence of the solid-state ETM (Figure 5).


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Figure 5. Energy level diagram the pertinent states of the components of NiO-based p-type solid-state DSSC sensitized with DPP-PYRO, DPP-Br, and P1 dyes and PCBM as ETM. The energy values are taken from reference 23 and Table 2.

Optimization and performance of solid-state p-type DSSC Solid-state p-type DSSC fabricated in this study are based on the following structure: FTO / Dense NiO / Porous NiO / Dye / PCBM/ TiOx / Al. The detailed fabrication procedure is described in the experimental section. A dense NiO layer, deposited from solution by spincoating, was used to prevent current leakage induced by direct contact between PCBM and NiO or FTO.30-31 Similarly, a sol-gel TiOx film formed from a titanium (IV) isopropoxide solution was used as hole blocking layer on top of PCBM, to enhance the selectivity of the electroncollecting electrode. This layer is also crucial to prevent electrical shunts induced by the relatively high roughness of the NiO porous film and the difficulty to achieve its perfect coverage by PCBM.32 To address this issue, we specifically optimized the deposition procedure for PCBM infiltration, resulting in a two-step sequential deposition method. The thickness of the NiO porous layer was also varied from 650 nm up to 1.7 µm, the best device performance being obtained for thinner NiO layers, as illustrated in the ESI (Table S2). Figure 6a presents the optical absorption spectra of solid-state p-type DSSC based on PCBM and the three dyes, while Figure 6b summarizes their typical electrical characteristics measured under standard solar illumination (100 mW cm-2, AM1.5G). The two DPP derivatives exhibit a comparable absorption profile, with features similar to that observed in solution. In particular,


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the main absorption band of DPP-PYRO is once again found slightly red-shifted compared to that of DPP-Br. We also clearly evidence the presence of PCBM for the three dyes through the weak contribution at 740 nm. Finally, the optical densities of the three samples at the maximum absorption of the dye are found in good correlation with the molar extinction coefficient measured in solution (Table 1), which suggests that a comparable dye loading is achieved in the three cases.


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Figure 6. a) Optical absorption spectra of solid-state p-type DSSC based on PCBM and on DPP-PYRO (red triangles), DPP-Br (blue circles), and P1 (black squares) dyes. No Aluminum top contact was deposited in this case. The spectrum of bare NiO electrode, including dense and porous NiO layers deposited on FTO, is also given (grey dashed line). b) Corresponding current density – voltage characteristics under standard solar emission (100 mW cm-2, AM1.5G).

Concerning their photovoltaic performance, both DPP derivatives show larger open-circuit voltages (VOC) and short-circuit current densities (JSC) than P1, illustrating their better suitability for solid-state devices based on NiO and PCBM. The corresponding incident-photon-to-charge carrier efficiency (IPCE) spectra (Figure 7) matches the observed trend in photocurrent, with a peak value of 4.3 % for DPP-Br.


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Figure 7. IPCE spectra of solid-state p-type DSSC based on PCBM and on DPP-PYRO (red triangles), DPP-Br (black circles), and P1 (blue squares) dyes.

The shape of IPCE spectra also matches the optical absorption profile of the dye-sensitized NiO electrodes. Remarkably, a noticeable contribution of PCBM to current generation through direct hole transfer to NiO is observed, which is in agreement with previous reports.23 Its contribution on the overall photocurrent is, however, likely to be only minor compared to that of the dye, considering the very low photocurrent observed without dye (ESI, Table S2). Moreover, the dye-free NiO/PCBM device exhibits string leakages, which confirm the poor quality of our dense NiO layer. The largest photocurrent is achieved by DPP-Br (0.45 mA cm-2), while DPPPYRO exhibits the largest photo-voltage (225 mV). These large photocurrents are an order of magnitude larger than those reported by Zhang et al. for the system based on P1.23 Considering the fact that P1 possesses a much larger (by a factor of two) molar extinction coefficient than DPP-Br and DPP-PYRO, these observations are the signatures of more efficient charge transfer reactions, driven by the electronic properties of the two DPP derivatives. However, a drawback of these devices, limiting their performance, is their low photo-voltage, which is far below the 620 mV reported by Zhang et al.23 Indeed, all devices reported here show significant current leakages, leading to low VOC values. These leakages, which are also observable in the dark J(V) characteristics of the cells (ESI, Figure S1), are likely to be related to the lower quality of the dense NiO layer, which was deposited from solution by spin-coating in our case, while chemical spray pyrolysis was previously used.23 The other important factor influencing device shunts is associated with the morphology of the layers, and especially the homogeneity of the porous NiO film and the filling fraction of PCBM


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in the dye-sensitized NiO electrode. Scanning electron microscopy images of the samples are presented in Figure 8. Porous NiO electrodes are fairly rough, as can be seen from the top image in Figure 8b. More in-depth investigations on the morphology of NiO electrodes used for p-type DSSC were reported in our previous work,33 showing that a suitable porosity is usually achieved in such films. After sensitization and infiltration by the ETM, a significant fraction of PCBM is observed in the depth of the pores, indicating a reasonable filling by PCBM for the three dyes (see the ESI Figure S2 for the cross-section image corresponding to sample based on DPP-Br). Additional energy dispersive X-ray (EDX) spectrometry profiles confirm the effective penetration of PCBM into the network of the NiO electrodes (ESI, Figure S3 and Figure S4) following the two-steps procedure adopted in this work. Despite these relevant features, top-view SEM images also reveal a strong PCBM aggregation (ESI, Figure S5). In addition to the roughness of our NiO porous electrodes, we believe that the presence of such inhomogeneous PCBM capping layer can be responsible for important current leakages, resulting in modest photovoltaic behaviour of our devices, through low fill factors and VOC. Further efforts towards more efficient solid-state p-type devices should therefore focus on the deposition process of PCBM, by shifting for example towards different solvents or solvent mixtures in order to more efficiently disperse the fullerene molecules or better control their segregation during the evaporation of the solvent. Also, varying the porosity of the NiO electrode could enable a more complete infiltration of PCBM, leading to more homogeneous capping layer. The use of better film-forming compounds as electron transporters is probably also to be explored.


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Figure 8. SEM images of samples: (a) cross section and (b) top view of bare porous NiO film deposited on FTO substrate; Cross-section of Dense NiO/Porous NiO /dye/PCBM deposited on FTO in the case of (c) dye DPP-PYRO and (d) P1. The thin TiOx interlayer is thus critical to prevent the devices from direct short-circuits, however, it is probably not sufficient to achieve a smooth interface with the metallic top electrode. Further efforts on interface engineering are therefore still needed to improve photovoltaic response of solid-state p-type DSSC. A last feature that we point out is associated with the light-soaking properties of the cells when characterized under constant solar illumination. We systematically observe a significant increase of device performance over time when the cells are placed under light (in ambient atmosphere in our case), as reported in Figure S6 (ESI) for device based on P1 dye. Jsc linearly increases over time up to approximately 20 min, while the open-circuit voltage decreases, before both parameters finally stabilized. We also evidence a counter-diode-like behaviour in the J(V) curve at the initial stage of the


22

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characteristics. Such features relate to observations made by Click et al on NiO-based p-type DSSC,34 which also indicate a light-, or polarization-activated current generation in such devices. We did not specifically focus on this aspect in this work and complementary experiments are still required in order to shed light on this particular issue.

Photo-physics at the dye/PCBM interface Fast hole injection kinetics in the sub-picosecond range (0.2 to 20 ps) have been resolved both for P110,

23

and DPP derivatives24 on NiO electrodes using transient absorption spectroscopy.

Charge generation starts with the initial formation of the / charge separated state, which should live for a sufficient time so that the electron transfer from the reduced to PCBM can occur before charge recombination. In the presence of P1, it was shown that, following the fast hole injection, the reduced dye ( 1 ) transfers an electron to the PCBM layer within approximately ~ 50 ps, leading to the long-lived (≫ 1ns) / state.23 However, the rate of the electron shift step towards PCBM hardly outcompetes charge recombination between the reduced P1 and holes in NiO.23 In the case of DPP dyads, the presence of a secondary electron acceptor such as naphthalene diimide (NDI) significantly increases the lifetime of the charge separated state from the picosecond regime up to 250 µs, while the hole injection into NiO still remains very fast (0.2 to 10 ps timescale).24 DPP-PYRO, which follows exactly the same molecular design as DPP-NDI, most probably also leads to a long-lived charge separated state ( /

− ), presumably situated in the microsecond timescale since the electron shift from the reduced DPP to PYRO is exergonic (∆ = -0.38 eV) and this acceptor is located at the same distance from the DPP core and has a similar structure compared to the DPP-NDI dyads. Assuming a comparable hole injection rate constant


23


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Page 24 of 47

for the three sensitizers (which is reasonable since the Gibbs free energies for the injection process are very exergonic for all dyes), and keeping in mind the aforementioned trends in lifetime of charge separated states, the magnitude of current generation is finally strongly dependent on the electron injection efficiency between the reduced dyes and PCBM. Consequently, we decided to more specifically focus on the dye/PCBM interface by performing time-resolved photoluminescence measurements on Al2O3 mesoporous substrates. It can be considered as a reference material since the valence and conduction bands of aluminum oxide are beyond the reach of the dye and PCBM excited states, allowing thus to study the dye/PCBM interface alone. Using this strategy, which enables us to directly monitor the electron transfer process from the excited dye ( ∗ ) to PCBM, a relative comparison between P1, DPPBr and DPP-PYRO is possible, giving additional information on the interfacial charge kinetics at the interface. Indeed, because the electron is transferred onto the PYRO unit before being scavenged by PCBM, then the situation after photo-excitation, whether on alumina or NiO, is quite identical, further justifying that we can safely compare both situations for DPP-PYRO. Furthermore, calculating ∆ for both ∗ / → / and

/ →

/ reactions show comparable values, suggesting similar relative kinetics with this set of dyes.

Figure 9a, b, and c present the steady-state photoluminescence spectra corresponding to the three dyes grafted on Al2O3 alone and in the presence of PCBM, the latter being infiltrated using the same process as for devices. First, in the absence of PCBM, the lifetime of the DPP-PYRO dyad is significantly shortened compared to that of DPP-Br, supporting the occurrence of a photo-induced electron transfer to the PYRO unit, because energy transfer is an uphill process


24

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(the excited state of PYRO can be estimated from the optical absorption of the most red-shifted absorption band, giving a value of around 0.8 eV above that of DPP). For all the dyes, a significant quenching of the dye emission is monitored in the presence of PCBM. As a noninjecting electrode is used here, this observation is consistent with efficient non-radiative deexcitation pathways for the photo-excited dye, which can be safely assigned to electron transfer from the dye excited state to PCBM, leading to the / charge separated state. Measurements depicted in Figure 9 indicate that the electron transfer efficiency ("#$ ) is larger in the case of DPP-Br and DPP-PYRO, than in the case of P1. Indeed, DPP-Br and DPP-PYRO exhibit PL quenching of 85% and 70%, respectively, while that of P1 is limited to around 30%.


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Figure 9. Steady-state photoluminescence spectra for dyes grafted on Al2O3 non-injecting substrates, without and in the presence of PCBM: (a) P1, (b) DPP-Br, and (c) DPP-PYRO. The excitation is performed at 492 nm in all cases. (d) Time-resolved photoluminescence traces for system based on dye DPP-PYRO grafted on Al2O3, without (black data) and in the presence of PCBM (red data). The excitation is performed at 510 nm, and the emission is detected at 626


26

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nm. The deconvolutions of the spectra using bi-exponential decay functions and taking into account the IRF, are plot as solid blue lines. To get a more quantitative analysis of this electron transfer process to PCBM, transient PL measurements (TRPL) were performed on the same systems using a picosecond laser diode as pulsed excitation source (510 nm). Figures S8, S9, and Figure 9d present the decay traces for the three dyes on Al2O3, without and in the presence of PCBM. Bi-exponential decay functions were used to fit the traces using the conventional expression: %&' = )* . exp %−

& & ' + ) . exp %− ' .* .

where .* and . correspond to the decay time constants, and )* and ) to the amplitude of both components, respectively. These fitting parameters are summarized in Table S3 (ESI), while Table 3 presents the average decay time (.0 ), electron transfer rate constants (1#$ ), and electron transfer efficiencies ("#$ ). These values are determined from the fitting parameters using the following expressions:35-36 .0 =

"#$ = 1 −

1#$ =

)* .* + ) . )* .* + ) . .0 %dye + PCBM' .0 %dye alone'

1 1 − .0 %dye + PCBM' .0 %dye alone'

The PL lifetime of DPP-PYRO (0.68 ns) is found much shorter than that of DPP-Br (2.67 ns). This observation is a direct consequence of the presence of the PYRO unit, acting as a secondary electron acceptor unit. The photo-excited electron is indeed shifted away from the DPP core, leading to a quenched fluorescence and a shorter PL lifetime. Such behavior is accompanied by a


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larger electron transfer efficiency to PCBM, illustrating the benefit of this synthetic strategy towards more efficient solid-state p-type DSSC. Table 3. Summary of average decay times, electron transfer rate constant, and electron transfer efficiency extracted from TRPL data for the three dyes on Al2O3 without and in the presence of PCBM. .0 % )'

.0 % + '

1#$

"#$

(ns)

(ns)

(108 s-1)

(%)

P1

0.85

0.73

2.0

15

DPP-Br

2.67

0.91

7.2

66

DPP-PYRO

0.68

0.35

13.5

48

Dye

Finally, both DPP derivatives exhibit much larger electron transfer efficiencies from their excited state to PCBM than dye P1. In general, "#$ is dependent on several factors. First, a reasonable difference between the LUMO levels of the dye and PCBM (in other words, the driving force for dye regeneration when considering a working cell in the presence of NiO) is required, so that the electron transfer to the fullerene acceptor is thermodynamically favorable. In this context, and as mentioned previously, the DPP-Br/PCBM interface presents the largest driving force (0.5 eV) for electron transfer, leading to the fastest formation of the / charge separated state among the investigated compounds taken on non-injecting substrates. DPP-PYRO, which does not show such beneficial energetic configuration with a driving force of only 0.1 eV, is however associated with the largest electron transfer rate among the three dyes. This observation points out the second major factor that governs the interfacial charge kinetics with PCBM, that is its ability to efficiently separate the photo-generated charges due to the


28

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presence of a second electron acceptor unit (PYRO). Despite a relatively favorable energetic configuration, P1 is associated with a poor "#$ of only 15%. Such modest ability to transfer its electron to PCBM in its excited state with regard to the DPP derivatives is likely to be transposed to fully working cells based on NiO where electron transfer occurs from the reduced dye. In fact, we believe that the relatively high molar extinction coefficient of P1 does not counterbalance the slow dye regeneration kinetics suggested by our analysis, and evidenced by Zhang et al.23 When comparing P1 with DPP-Br, this observation emphasizes the net advantage of the DPP unit as efficient electron acceptor over the dicyano-vinyl groups.

Finally, device performance demonstrated by solid-state p-type DSSC based on the three dyes can be rationalized by summarizing their main features (Table 4). Applying a simple three-level scoring scheme for the three compounds allows us to point out their relative ability for photocurrent generation. We assume in this case that hole injection to NiO from the excited dye is not a limiting step for the three compounds, as the hole injection quantum yield is expected to be virtually quantitative for the three dyes, based on the fast injection rate constant compared to the dye excited state lifetime.10,

23-24

Analysing the following parameters: i) light harvesting

efficiency of the sensitizers, ii) lifetime of charge separated state; iii) the driving force for electron transfer to PCBM; and finally iv) the electron transfer efficiency measured on noninjecting substrates, we found that there is a good correlation between the experimental photocurrents (DPP-Br > DPP-PYRO > P1) and the ranking of the above features. In the light of these findings, we clearly point out the relevance of our strategy based on the introduction of a secondary electron acceptor with significant reducing power in order to slow down charge recombination processes while maintaining a sufficient driving force to reduce the


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Page 30 of 47

electron transporter medium (PCBM in this case). We also suggest that further improvement in device operation will require the development of sensitizers presenting a sufficiently cathodic reduction potential in order to efficiently transfer an electron to PCBM. In addition, new solidstate p-type DSSCs could also take benefit from new molecular or polymeric electron acceptor transporting materials such as those recently reported in the field of organic bulk heterojunctions for example.37-38 Alternatively, significant advances in the field are also expected through a proper interfacial engineering strategy, particularly regarding the NiO blocking layer and electron-selective contact.

Table 4. Comparison of the ability of the three dyes for efficient current generation in solid-state p-type DSSC based on NiO and PCBM. A simple 3-level scoring scheme is adopted. The photocurrents (@AB ) were measured under standard solar illumination. P1

DPP-Br

DPP-PYRO

Light harvesting efficiencya

+++

++

++

Lifetime of / charge separated stateb

+

++

+++

++

+++

+

Electron transfer efficiency ("#$ 'd

+

+++

++

Summary (ability for current generation)

+

+++

++

0.22

0.45

0.33

Driving force for electron shift to PCBM

c

Experimental @AB (mA cm-2)


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a

Estimated from Figure 6. b Estimations based on NiO from reference 24 for DPP derivatives, and from reference 23 for dye P1. Similar trends can be deduced on Al2O3 with PCBM from Table 3. c Estimated from Figure 5. d Estimated from Table 3.

CONCLUSIONS New diketopyrrolopyrrole derivatives (DPP-Br and DPP-PYRO) have been synthesized and successfully implemented in solid-state p-type dye-sensitized solar cells based on NiO and PCBM as hole and electron transporting materials, respectively. Taking benefit from suitable electronic and optical properties, promising performance has been achieved with the DPP derivatives with regard to the reference P1 dye. By specifically focusing on the dye/PCBM interface, we have evidenced the benefit of a secondary electron acceptor unit (pyromellitimide, PYRO) on the electron injection efficiency, elucidated by transient photoluminescence measurements on non-injecting substrates. The possibility to displace the photo-excited electron of the dye towards the PYRO unit leads to the rapid formation of a / charge separated state. The relatively low photocurrent evidenced for dye P1 can be attributed to its poor electron injection efficiency to PCBM. Transposing these findings to working solar cells, our analysis emphasizes a good correlation between the photocurrents produced by the dyes and their ability to enable efficient electronic processes during charge carrier generation. Finally, we believe that important improvements remain to be demonstrated in the field of solid-state p-type photo-electrochemical devices by designing organic compounds presenting tailored electronic properties, so that a proper energetic configuration and a strong ability to separate the photoexcited charges can be combined. Interface engineering remains also to be more systematically explored in this emerging field, towards alternative electron transporting materials and selective contacts.


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Supporting information. Influence of CDCA treatment and NiO electrode thickness on photovoltaic performance, PL fitting parameters used for PL analysis, IPCE spectra, dark J(V) curves of the cells, complementary SEM images, EDX profiles, light-soaking properties of the cell based on P1, IRF for TRPL deconvolution, and TRPL traces.

Acknowledgements. This work was supported by the Agence Nationale de la Recherche, project QuePhelec (ANR-13-BS10-0011-01). This work has been performed with the use of the PLATINOM facility at the University of Limoges and XLIM Research Institute (CNRS UMR 7252). The work was partially conducted in the framework of the LabEx Sigma-Lim, which is acknowledged. The authors are thankful for financial support from the ‘‘Région Limousin’’ through the thematic project EVASION. SKS is thankful to a regional postdoctoral fellowship from Région Limousin.

Author Contributions. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. T. T. T. Pham and S. K. Saha mainly contributed to the main experimental developments of the manuscript, including solar cell fabrication and characterization, under the supervision of J. Bouclé, S. Vedraine and B. Ratier. They also contributed to the preparation of the manuscript. Y. Farré and D. Provost were involved in the synthesis of the sensitizers DPP-Br and DPP-PYRO, under the supervision of E. Blart, Y. Pellegrin, and F. Odobel. M. Raissi and Y. Pellegrin were involved in the synthesis of


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the NiO paste used for electrode preparation, as well as in the review of the manuscript. S. Vedraine and B. Ratier were involved in the analysis of data and review of the manuscript. T. T. T. Pham and J. Bouclé performed the PL studies and analyses. D. Aldakov performed the SEM measurements and EDX profiles, and participated to the review of the manuscript. Finally, F. Odobel and J. Bouclé designed and supervised the experiments and the final version of the article.

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25. Favereau, L.; Warnan, J.; Pellegrin, Y.; Blart, E.; Boujtita, M.; Jacquemin, D.; Odobel, F., Diketopyrrolopyrrole Derivatives for Efficient Nio-Based Dye-Sensitized Solar Cells. Chemical Communications 2013, 49, 8018-8020. 26. Kaur, M.; Choi, D. H., Diketopyrrolopyrrole: Brilliant Red Pigment Dye-Based Fluorescent Probes and Their Applications. Chemical Society Reviews 2015, 44, 58-77. 27. Robb, M. J.; Ku, S. Y.; Brunetti, F. G.; Hawker, C. J., A Renaissance of Color: New Structures and Building Blocks for Organic Electronics. Journal of Polymer Science Part A: Polymer Chemistry 2013, 51, 1263-1271. 28. Redmore, N. P.; Rubtsov, I. V.; Therien, M. J., Synthesis, Electronic Structure, and Electron Transfer Dynamics of (Aryl) Ethynyl-Bridged Donor-Acceptor Systems. Journal of the American Chemical Society 2003, 125, 8769-8778. 29. Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L., Design of an Organic Chromophore for P-Type Dye-Sensitized Solar Cells. Journal of the American Chemical Society 2008, 130, 8570-8571. 30. Gibson, E. A.; Smeigh, A. L.; Le Pieux, L.; Fortage, J.; Boschloo, G.; Blart, E.; Pellegrin, Y.; Odobel, F.; Hagfeldt, A.; Hammarström, L., A P-Type Nio-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V. Angewandte Chemie - International Edition 2009, 48, 4402-4405. 31. Ho, P.; Bao, L. Q.; Cheruku, R.; Kim, J. H., Improved Performance of P-Type Dscs with a Compact Blocking Layer Coated by Different Thicknesses. Electronic Materials Letters 2016, 12, 638-644. 32. Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J., Efficient Organometal Trihalide Perovskite Planar-Heterojunction Solar Cells on Flexible Polymer Substrates. Nat Commun 2013, 4, 2761. 33. Wood, C. J., et al., A Comprehensive Comparison of Dye-Sensitized Nio Photocathodes for Solar Energy Conversion. Physical Chemistry Chemical Physics 2016, 18, 10727-10738. 34. Click, K. A.; Beauchamp, D. R.; Garrett, B. R.; Huang, Z.; Hadad, C. M.; Wu, Y., A Double-Acceptor as a Superior Organic Dye Design for P-Type Dsscs: High Photocurrents and the Observed Light Soaking Effect. Physical Chemistry Chemical Physics 2014, 16, 2610326111. 35. Aldakov, D.; Sajjad, M. T.; Ivanova, V.; Bansal, A. K.; Park, J.; Reiss, P.; Samuel, I. D. W., Mercaptophosphonic Acids as Efficient Linkers in Quantum Dot Sensitized Solar Cells. Journal of Materials Chemistry A 2015, 3, 19050-19060. 36. Sajjad, M. T., et al., Novel Fast Color-Converter for Visible Light Communication Using a Blend of Conjugated Polymers. ACS Photonics 2015, 2, 194-199. 37. Qin, Y.; Uddin, M. A.; Chen, Y.; Jang, B.; Zhao, K.; Zheng, Z.; Yu, R.; Shin, T. J.; Woo, H. Y.; Hou, J., Highly Efficient Fullerene-Free Polymer Solar Cells Fabricated with Polythiophene Derivative. Advanced Materials 2016, doi : 10.1002/adma.201601803. 38. Li, S.; Ye, L.; Zhao, W.; Zhang, S.; Mukherjee, S.; Ade, H.; Hou, J., Energy Level Modulation of Small Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Advanced Materials 2016, doi : 10.1002/adma.201602776.


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TOC Graphic


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Chemical structures of the diketopyrrolopyrrole dyes DPP-PYRO and DPP-Br, along with those of P1 and PCBM Figure 1 261x170mm (150 x 150 DPI)



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Synthesis of dye DPP-PYRO. Reagents and conditions: a) Pd(PPh3)4, CuI, Et3N, toluene, 80°C, 18 h, 46%; b) TFA, DCM, TA, 3 h, 91%. Figure 2 158x36mm (300 x 300 DPI)


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Synthesis of intermediate 3. Reagents and conditions: a) Pd(PPh3)4, CuI, TMSA, DIPEA, toluene, 50°C, 2 h, 83%; b) TBAF, THF, 0°C, 2 min. 35%. Figure 3 86x51mm (300 x 300 DPI)



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Electronic absorption (straight line) and normalized emission (dashed line) spectra of the dyes DPP-PYRO (red), DPP-Br (black) and P1 (blue) recorded in dichloromethane solution Figure 4 90x59mm (300 x 300 DPI)


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Energy level diagram the pertinent states of the components of NiO-based p-type solid-state DSSC sensitized with DPP-PYRO, DPP-Br, and P1 dyes and PCBM as ETM. The energy values are taken from reference 23 and Table 2 Figure 5 133x91mm (300 x 300 DPI)



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Optical absorption spectra of solid-state p-type DSSC based on PCBM and on DPP-PYRO (red triangles), DPPBr (blue circles), and P1 (black squares) dyes. No Aluminum top contact was deposited in this case. The spectrum of bare NiO electrode, including dense and porous NiO layers deposited on FTO, is also given (grey dashed line) Figure 6a 90x67mm (300 x 300 DPI)


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Corresponding current density – voltage characteristics under standard solar emission (100 mW cm-2, AM1.5G) Figure 6b 90x66mm (300 x 300 DPI)



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IPCE spectra of solid-state p-type DSSC based on PCBM and on DPP-PYRO (red triangles), DPP-Br (black circles), and P1 (blue squares) dyes. Figure 7 90x70mm (300 x 300 DPI)


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SEM images of samples: (a) cross section and (b) top view of bare porous NiO film deposited on FTO substrate; Cross-section of Dense NiO/Porous NiO /dye/PCBM deposited on FTO in the case of (c) dye DPPPYRO and (d) P1. Figure 8 301x258mm (72 x 72 DPI)



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Steady-state photoluminescence spectra for dyes grafted on Al2O3 non-injecting substrates, without and in the presence of PCBM: (a) P1, (b) DPP-Br, and (c) DPP-PYRO. The excitation is performed at 492 nm in all cases. Figure 9 99x53mm (300 x 300 DPI)


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(d) Time-resolved photoluminescence traces for system based on dye DPP-PYRO grafted on Al2O3, without (black data) and in the presence of PCBM (red data). The excitation is performed at 510 nm, and the emission is detected at 626 nm. The deconvolutions of the spectra using bi-exponential decay functions and taking into account the IRF, are plot as solid blue lines. Figure 9d 99x67mm (300 x 300 DPI)


Toward Efficient Solid-State p-Type Dye-Sensitized Solar Cells: The

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