Organic “donor-free” dye with enhanced open circuit voltage in

solid-state sensitized solar cells

Antonio Abate,a‡, Miquel Planells,b‡ Derek J. Hollman,a Samuel D. Stranks,a Annamaria

Petrozza,c Ajay Ram Srimath Kandada,c Yana Vaynzof,d† Sandeep K. Pathak,d Neil

Robertsonb* and Henry J. Snaitha*

aDepartment of Physics, University of Oxford, Oxford, Parks Road,OX1 3PU, U.K.

bEaStCHEM – School of Chemistry, University of Edinburgh, Kings Buildings, Edinburgh

EH9 3JJ, U.K.

cCenter for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Via

Pascoli 70/3, 20133 Milano, Italy

dCavendish Laboratory, Department of Physics, University of Cambridge, 19 JJ Thomson

Avenue, CB3 0HE, United Kingdom

†Present address: Centre for Advanced Materials, Im Neuenheimer Feld 227, Heidelberg

University, Heidelberg, 69120, Germany

‡ These authors contributed equally.

* Corresponding authors: HJS h.snaith1@physics.ox.ac.uk, NR neil.robertson@ed.ac.uk.

KEYWORDS. Organic dyes / solid state dye sensitized solar cells / oligo(3-

hexylthiophene) / P3HT / DSSC

Abstract

The predominant recent trend in designing new organic dyes for sensitized solar cells has

been to combine an electron-withdrawing group (acceptor, A) on one side of a conjugated π-

spacer and one or more electron-donating groups (donor, D) on the other side, which generate

a donor-pi-acceptor (D-π-A) dye. Here, we reported solid-state cells sensitized with

cyanoacrylic end-functionalized oligo(3-hexylthiophene) as a “donor-free” dye (π-A) with

power conversion efficiency competitive with the state-of-the-art D-π-A dyes. We show that

without an electron donor group a significantly higher open circuit voltage (Voc) is achieved,

while maintaining short circuit current. By combining experimental and theoretical

investigation, we conclude that improved Voc is due to higher steady-state concentration of

oxidized dye, which increases the potential offset across the TiO2-hole transporter

heterojunction. This work questions the basic premise of the operating principles of solid-

state dye-sensitized solar cells and opens a new direction for organic dye-sensitizer design

strategy.

Solid-state dye-sensitized solar cells (ss-DSSCs) employing organic hole-conductors

to regenerate the photo-oxidized dye were first introduced at the end of the 1990’s.1,2 For the

pioneering molecular hole-conductor based ss-DSSC, the I-/I3- redox electrolyte was replaced

with 2,2’-7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-

OMeTAD) without significantly changing the device structure.1 In particular, a 4.2 μm thick

mesoporous TiO2 sensitized with a Ru(II) based dye (N719)3 was used, generating a light-to-

power conversion efficiency (PCE) below 1% under 10 mWcm-2 A.M. 1.5 simulated solar

irradiance; N719 had already exhibited a PCE over 10%, when utilized with the standard

liquid redox electrolyte.3 After the development of this ss-DSSC, new, more efficient devices

have been prepared using spiro-OMeTAD while reducing the thickness of the TiO2 to less

than 2 μm.4 Indeed, it has been recently demonstrated that the device series resistance

significantly reduces the PCE.5 There are many factors, pore-filling, charge collection

efficiency and parasitic absorbance, which collude to limit the solar cell thickness to around

2 μm.6-9 Reducing the thickness of the TiO2 may help mitigate these losses,10 and in addition

increase the open-circuit voltage by increasing the average charge density in the film, but this

requires dyes with much higher extinction coefficients to fully absorb the light in a shortened

optical path.11

In addition to Ru(II) based dyes, organic dyes have been successfully utilized with a TiO2

layer of less than 2 μm due to their high molar absorptivity.12-14 The predominant recent trend

in designing new dyes has been to combine an electron-withdrawing group (acceptor, A) on

one side of a conjugated π-spacer and one or more electron-donating groups (donor, D) on the

other side, which generate a push-pull system, or donor-pi-acceptor (D-π-A).15,16 In this

configuration, the photoexcitation should be associated with a charge transfer through the π-

spacer from donor to acceptor, on which the lowest unoccupied molecular orbital (LUMO) is

mainly localized.17,18 Cyanoacrylic acid is a commonly employed acceptor,12,19 as it can

directly link onto the TiO2 surface and provide effective electron injection from the dye

LUMO to the TiO2 conduction band.20 Several donor groups such as indoline,21

triphenylamine,22 carbazole,23 and cumarine24 have been successfully applied in dyes for ss-

DSSCs. However, it is important to note that most D-π-A dyes were first designed for

DSSCs based on redox electrolytes.25-29 For this liquid based redox electrolyte, the electron-

donating groups could be necessary to stabilize the oxidized dye cation and to ensure

reversible redox activity of the sensitizer in the presence of strong oxidants.30,31 Accordingly,

the donor group may be unnecessary for ss-DSSCs, where the redox electrolyte is replaced by

mild oxidant hole conductors.32-34

Here, we investigate cyanoacrylic end-functionalized oligo(3-hexylthiophene) (oligo-

3HT, Figure 1a) as a donor-free light-harvester for use in ss-DSSC applications. We report

oligo-3HT ss-DSSCs with a power conversion efficiency competitive with the state-of-the-art

D-π-A dyes (i.e. Y123, see Supplementary Information).35 We show that without an electron

donor group, given the same base molecular structure (see Figure 1a-a’), a significantly

higher open circuit voltage (Voc) is achieved while maintaining the same short circuit current

(Jsc). By combining experimental and theoretical techniques, such as small-perturbation

photovolatage-photocurrent decay, ultraviolet photoelectron spectroscopy, density functional

theory calculations, and ultra-fast transient absorption, we elucidate the basis for

improvement of the Voc.

Figure 1. (a-a’) oligo-3HT and MK236 molecular structure, (b-b’) cyclic voltammetry traces at different scan rates, measured in a 0.3 M [TBA][PF6] CH2Cl2 solution, and (c-c’) molecular orbital distribution of HOMO (bottom) and LUMO (top) at the B3LYP/6-31G(d) level of theory (isodensity = 0.04). EHOMO was extracted from electrochemical measurements and ELUMO by subtracting from the optical band gap, which was obtained from the crossing point between the excitation and emission spectra.37

In Figure 1, we report the electrochemical characterization of oligo-3HT and the

corresponding D-π-A dye with a carbazole donor unit (MK2).36 oligo-3HT was synthesised

in good yield according to the experimental procedures reported in the Supplementary

Information (SI). The two dyes have similar molecular backbone (see Figure 1a-a’), as they

both use cyanoacrylic acid as an acceptor group and four thiophene derivatives as a π-spacer.

MK2 has a terminal carbazole donor group, completing the D-π-A structure, while oligo-3HT

has an additional thiophene, elongating the π-system to achieve a molecular size similar to

MK2. In Figure 1b-b’, cyclic voltammetry shows that oligo-3HT is reversibly redox active,

with the oxidation potential more positive than that of MK2. In Figure 1c-c’, DFT

calculations show that for both dyes the highest occupied molecular orbital (HOMO) is

delocalized over the thiophene π-spacer and, for MK2, additionally over the carbazole group.

Conversely, the LUMO is localized predominantly over the cyanoacrylic acid group and its

first adjacent thiophene for both dyes. These HOMO and LUMO distributions and their

associated energy values are expected to ensure a strong directionality of the photoexcitation

towards the TiO2 and, consequently, an efficient electron injection.18

Figure 2. Current-voltage curves for devices employing oligo-3HT and Y12335 under AM 1.5 simulated sunlight of 100 mW cm-2 equivalent solar irradiance.38 The reported JV curves are the best device out of a series of 10 devices for each dye (see SI).

Table 1. Device performance parameters for oligo-3HT, Y12335 and MK2.36

Jsc (mA cm-2) PCE (%)

Voc (V) FF

Y123 8.21 4.6 0.84 0.67oligo-3HT 7.69 4.4 0.92 0.62

MK2 6.89 2.8 0.70 0.58

To compare oligo-3HT to the traditional D-π-A dyes in ss-DSSCs, we prepared a set

of devices (see SI for data distribution and device preparation) sensitizing 1.5 μm thick

mesoporous TiO2 films with oligo-3HT, Y123,35 or MK236 while using spiro-OMeTAD as the

hole transporting material.1,37 The current-voltage (JV) characteristics of devices measured at

AM 1.5 simulated sun light (100 mW cm-2)38 are shown in Figure 2, with the corresponding

performance parameters summarized in Table 1. The best device prepared using oligo-3HT

shows a PCE of 4.4 % with a very high Voc of 920 mV. In ss-DSSCs, a high Voc has been

observed before by adding strong oxidants such as Co(III) complexes in the spiro-

OMeTAD,39 which increases both the Voc and the fill factor (FF), achieving a PCE greater

than 7%.39 However, we utilized the common additives combination for ss-DSSCs, lithium

bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP).5, 31 In Figure 2,

devices prepared with MK2 show comparable Jsc, but 200 mV lower Voc, to those with

prepared with oligo-3HT. We also reported the JV for the best device prepared in our

laboratory using the state-of-the-art organic dye Y123, which has the typical D-π-A

structure.40 The power conversion efficiency of oligo-3HT is similar to that of Y123, and,

notably, the Voc is significantly higher for oligo-3HT (see Table 1).

Figure 3. Photovoltage and photocurrent decay measurements41 for devices prepared with oligo-3HT and MK2. (a) Charge density at short circuit (Φsc) against short circuit current (Jsc); (b) recombination lifetimes at open-circuit conditions (τrec) against charge density at open-circuit (Φoc); (c) voltage against the differential capacitance; (d) transport lifetimes at short circuit conditions (τtrans) against charge density at short circuit (Φsc).

To investigate the cause of the increased Voc, we first performed small-perturbation

photovoltage and photocurrent decay measurements, comparing oligo-3HT and MK2 devices

(Figure 3).41 In ss-DSSCs, the Voc is produced by the difference between the quasi-Fermi

level for electrons in the TiO2 and holes in the spiro-OMeTAD.41 At the same

photogenerated charge density (Φ), this energy difference may be affected by i. the total

number of electron traps below the conduction band in TiO2,42 ii. the charge recombination

rate,42 and iii. dipole generation between the TiO2 and the spiro-OMeTAD, which are mainly

due to the sensitizer and the additives (Li-TFSI and tBP) in direct contact with TiO2.5,43 As

reported by O’Regan and co-workers, the relative change in number of trap states can be

estimated by plotting the charge extracted under short circuit condition (Φsc) against the

short-circuit current (Jsc).42 Figure 3a clearly shows that devices containing with oligo-3HT

or MK2 have a nearly identical number of TiO2 electron traps. Since there is no difference in

the number of traps, we can directly compare the recombination lifetimes at the same charge

density, as reported in Figure 3b.42,44 The two curves overlap significantly; therefore, changes

to charge recombination does not appear to contribute to the increased Voc with oligo-3HT.

In Figure 3c, we plotted the voltage against the differential capacitance, which qualitatively

shows the charge density distribution for the electron in TiO2.41 Since we have established

that the number of trap states (Figure 3a) does not change, any change in capacitance at the

same voltage is directly correlated to the dipole build up, or a shift in surface potential at the

dye-sensitized heterojunction.45 This variation is often attributed to i. the surface density of

sensitizing molecules adsorbed onto the TiO2 (dye loading), ii. the ground-state molecular

dipole of the sensitizer, or iii. the amount of additives (Li-TSFI, tBP) in direct contact with

TiO2.45,46 However, we could not observe distinct differences between any of these. In

particular, dye loading and the ground-state molecular dipoles were found to be similar (see

SI). Any difference in the additives (Li-TSFI, tBP) accessing the TiO2 surface can be

rejected considering that the density of TiO2 trap states (Figure 3a) and the charge transport

lifetimes (Figure 3d),44 which are both sensitive to the concentration of Li-TSFI and tBP,45 do

not show significant differences.

Figure 4. a) Transient absorption (TA) spectrum of oligo-3HT on TiO2. b) TA kinetics at probe = 570 nm and 720 nm for oligo-3HT and MK2. The pump used in the experiments is tuned to 520 nm with an incident fluence of 20 μJ/cm2.

To shed light on this surprizing increase in Voc, we investigated the charge generation

processes in the fs-ps time scale through transient absorption (TA) spectroscopy. In Figure

4a, we show TA spectra for oligo-3HT adsorbed on a TiO2 mesoporous film. At 500 fs after

photoexcitation, the TA spectrum shows a positive band for wavelengths () shorter than

620 nm and a photoinduced absorption (PA) band for longer than 620 nm. The two bands

are formed immediately following photoexcitation. We assign the positive band (i.e.

enhancement in probe light transmission) to the ground state photobleaching (PB) of the

molecule and the PA to the oxidized dye (dye cation) absorption. When the oligo-3HT is cast

on glass or adsorbed on a high band gap oxide (i.e. mesoporous Al2O3 ), the PB band (and a

relative photoinduced absorption feature) decays during the probing time, indicating ground

state recombination of the photogenerated molecular excitation (see SI). However, when the

dye is adsorbed on the TiO2 the PB does not decay; rather, it shows a slight growth in the first

5 ps, which may be due to energy transfer from aggregate species.47 We conclude that oligo-

3HT injects electrons into the TiO2 conduction band on an ultrafast timescale (less than

200 fs) and hence the decay of the PB due to ultrafast recombination of the photo-excited

species on the dye is arrested.48 The energy transfer from the aggregate species on the other

hand, bleaches the molecular absorption and hence increases the PB in 5 ps. In Figure 4b, we

compare the PB and PA band dynamics for MK2 and oligo-3HT. While MK2 shows a decay

of the primary photoexcitation in the picosecond time scale, as commonly observed for many

D-π-A organic dyes, the oligo-3HT shows an ultrafast electron-injection and no electron back

transfer for up to 400 ps. This result was quite unexpected, as it is commonly believed that a

D-π-A structure is necessary for efficient charge generation and sustained long lived oxidized

dye (dye cation) at the interface with the TiO2.49

Figure 5. Photoinduced absorption (PIA) spectra of mesoporous TiO2 film sensitized with oligo-3HT and MK2, both infiltrated with the hole transporter (spiro-OMeTAD, Li-TFSI and tBP). Samples were excited using a 514.5 nm laser line, chopped at 23 Hz.

To study the impact of a long-lived dye cation on the device performances in steady state

conditions, we performed photoinduced absorption (PIA)50 on mesoporous TiO2 film

sensitized with oligo-3HT and MK2, both infiltrated with the hole transporter (spiro-

OMeTAD, Li-TFSI and tBP, Figure 5). In the MK2 spectrum, as reported for many other D-

π-A organic dyes at interface between TiO2 and spiro-OMeTAD,51 we observe the negative

feature at 570 nm due to dye bleaching11,52 and two positive features at 730 and 1410 nm (the

latter spanning from 1100 to 1650 nm), which have been attributed to oxidized spiro-

OMeTAD (spiro-OMeTAD+).53 In the oligo-3HT spectrum, on top of the absorption features

due to spiro-OMeTAD+, we observe more pronounced peaks at 800 and 1410 nm, which

correspond to the oxidized oligo-3HT (oligo-3HT+, see SI). Notably, as reported by Zhang

and co-workers,51 MK2+ has the same absorption feature with similar oscillator strength at

800 nm (see simulated UV-Visible absorption and oscillator strength of oligo-3HT and MK2

in SI), which barely shows up in Figure 5. This suggests that, at the same level of

photogenerated charge (same concentration of spiro-OMeTAD+ calculated at 1600 nm) in

steady state condition, the concentration of oligo-3HT+ is higher than the MK2+. This result

could explain the high Voc measured for the oligo-3HT. Indeed, an oxidized dye generates a

dipole moment (about 50 D pointing away from the TiO2 surface) significantly stronger than

the neutral dye (about 10 D, see SI),52 which originates from the separation of the negative

charge injected into the TiO2 and the positive charge left on the dye. A dipole moment

pointing away from the TiO2 surface will cause an increase in the energy splitting between

the quasi-Fermi level for electrons in the TiO2 and that for holes in the spiro-OMeTAD,

which results in a larger Voc.54 Following the procedure reported by Nüesch and co-

workers,55 we calculated that to get an enhancement of 200 mV in Voc, as we observed

moving from MK2 to oligo-3HT (Figure 2 and 3c), we require around 7% steady state

concentration of dye cation at TiO2 surface (about 2.5 x 1012 dye cm-2 see SI). This value is in

good agreement with that reported from Howie and co-workers, which demonstrated that a

similar displacement in the TiO2 conduction band required a difference in surface charge

concentration in the order of 3-4 x 1012 cm-2.52

In summary, we have demonstrated oligo-3HT as an effective sensitizer for ss-

DSSCs, with a power conversion efficiency comparable to the state-of-the-art organic dye

Y123.35 Our approach challenges the fundamental premise that has directed the molecular

engineering of efficient sensitizers for ss-DSSCs. Indeed, the majority of the organic dyes

have been designed following the D-π-A model. We have shown that oligo-3HT, with a

similar molecular backbone to that of the D-π-A dye, MK2, exhibits an increased open-circuit

voltage while maintaining the same short-circuit current. Once more, in contrast to the

assumed design criteria, the oligo-3HT dye sustains a significant fraction of oxidized dye

under operating conditions, which appears to beneficially enhance the open-circuit voltage.

Hence the premise that we require a high hole-transfer yield should come under scrutiny. We

propose that a new strategy to obtain much more efficient solid-state dye sensitized solar

cells, may be to design dyes with significantly longer oxidized lifetimes so that the hole

density can equilibrate between the dye and the hole-transporter, obviating the requirement

for an energy level offset, and hence loss, at this heterojunction.

Experimental section

Electrochemical characterization. All cyclic voltammetry measurements were carried out in

freshly distilled CH2Cl2 using 0.3 M [TBA][PF6] electrolyte in a three-electrode system, with

each solution being purged with N2 prior to measurement. The working electrode was a Pt

disk. The reference electrode was Ag/AgCl and the counter electrode was a Pt rod. All

measurements were made at room temperature using a μAUTOLAB Type III potentiostat,

driven by the electrochemical software GPES. Ferrocene was used as the internal standard in

each measurement.

Computational details. The molecular structures were optimized first in vacuum without any

symmetry constrains, followed by the addition of CH2Cl2 solvation via a conductor-like

polarizable continuum model (C-PCM).56 The presence of local minimum was confirmed by

the absence of imaginary frequencies. All calculations were carried out using the Gaussian 09

program57 with the Becke three parameter hybrid exchange, Lee Yang-Parr correlation

functional (B3LYP) level of theory. All atoms were described by the 6-31G(d) basis set. All

structures were input and processed through the Avogadro software package.58

Solar cell fabrication. FTO substrates (15 Ω/sq, Pilkington) were etched with zinc powder

and HCl (2 M aqueous solution) to give the desired electrode patterning. The substrates were

cleaned with Hellmanex (2% by volume in water), de-ionized water, acetone, and ethanol.

The last traces of organic residues were removed by a 10 min oxygen plasma cleaning step.

The FTO sheets were subsequently coated with a compact layer of TiO2 (70 nm) by aerosol

spray pyrolysis deposition at 270°C, using oxygen as the carrier gas. Films of 1.5 µm thick

mesoporous TiO2 were then deposited by screen-printing a commercial paste (Dyesol 18NR-

T). The TiO2 films were slowly heated to 500°C and allowed to sinter for 30 min in air.

Once cooled, the samples were immersed into a 15 mM TiCl4 aqueous solution for 45 min at

70°C and then heated to 500°C for another sintering step of 45 min. After cooling to 70°C,

the substrates were immersed in a 0.5 mM dye solution, in 1:1 mixture of acetonitrile and

tert-butyl alcohol, for one hour at room temperature. After the dyed films were rinsed in

acetonitrile, the hole conductor matrix was applied by spin-coating at 1000 rpm for 45 s in

air. The solutions for spin coating consisted of 80 mM of hole transporter, 15 mM of lithium

bis(trifluoromethylsulfonyl)imide salt and 70 mM of 4-tert-butylpyridine in anhydrous

chlorobenzene. After drying overnight, back contacts were applied by thermal evaporation of

150 nm of silver.

Solar cells characterization. Simulated sunlight was generated with a class AAB ABET solar

simulator calibrated to give simulated AM 1.5, of 100 mW cm-2 equivalent irradiance, using

an NREL-calibrated KG5 filtered silicon reference cell. The current-voltage curves were

recorded with a source meter (Keithley 2400, USA) scanning at 0.15 V/s from 1.4 to 0 V.

The solar cells were masked with a metal aperture defining the active area (0.065 cm2) of the

solar cells. All devices were stored in air and in dark for 12 hours before testing.

Photovoltage and photocurrent decay measurements. Photovoltage and photocurrent decay

measurements were performed by a similar method to O’Regan and co-workers, as described

elsewhere.41, 42

Ultrafast Transient Absorption measurements. The laser train pulses comes from a Ti-

Sapphire source Coherent Micra 18 + Rega 9040, with a maximum output energy of about

5 μJ, 250 kHz repetition rate and pulse duration of about 40 fs. The fundamental train is

divided into two branches: the pump and the probe beams. The pump pulse has been tuned at

520 nm via a collinear optical parametric amplification, leading a pulse duration around

100 fs. Pump pulses, delayed with respect to the probe with a motorized optical stage, are

focused in a 200 μm diameter spot. The probe beam is obtained in the visible and near IR

region by generating white light-supercontinuum in a 2 mm thick sapphire plate. Chirp-free

transient transmission spectra are collected by using a fast optical multichannel analyser with

a dechirping algorithm. The measured quantity is the normalized transmission change: ΔT/T.

Excitation energy has been held around 20 nJ in order to prevent saturation of the optical

transitions. The system has sensitivity of the order of 10-4 and a time resolution of 100 fs. All

measurements are performed in vacuum to prevent any oxygen effect and/or sample

degradation.

Photoinduced absorption spectroscopy. Films for cw-photoinduced absorption (PIA) were

excited with an Ar-ion laser tuned to 514.5 nm (maximal intensity of 50 mW cm-2) and

chopped at 23 Hz. A continuous white light probe produced by a halogen bulb (~1 sun

intensity) was passed through the sample and detected using a monochromator (Spectra Pro

2300i, Acton Research Corp.) coupled to diodes for detection in the visible (PDA10A,

Thorlabs) and in the NIR (ID-441-C, Acton Research Corp.). Data were acquired using a

lock-in amplifier (SR830, Stanford Research Systems) locked to the light modulation

frequency and a NI USB-6008 (National Instruments) acquisition card. A customized

Labview (National Instruments) program provided an automated interface to control hardware

and record spectra.

Supplementary Information

Y123 molecular structure; Synthetic scheme and chemical characterisation; Electrochemical

characterisation; Optical characterization; TD-DFT computational details; Device

performance parameters data distribution; Photovoltaic action spectra; Ultraviolet

photoelectron spectroscopy; Dye loading on TiO2; Dye ground-state molecular dipole; fs-ps

time scale TA spectrum of oligo-3HT adsorbed on Al2O3; oligo-3HT PIA spectrum.

Acknowledgment

We thank the Engineering and Physical Sciences Research Council (EPSRC) APEX project

for financial support. We thanks Nagatoshi Koumura and Michael Grӓtzel for providing

MK2 andY123 dyes respectively.

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