High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells

Organic Electronics 14 (2013) 2826–2832

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High open circuit voltage in efficient thiophene-based small molecule solution processed organic solar cells Núria F. Montcada a,1, Beatriz Pelado b,1, Aurelien Viterisi a, Josep Albero a, Julieta Coro b,c, Pilar de la Cruz b, Fernando Langa b,⇑, Emilio Palomares a,d,⇑ a

Foundation Institute of Chemical Research of Catalonia (ICIQ), Avda. Països Catalans 16, Tarragona E-43007, Spain Instituto de Nanociencia, Nanotecnologia y Materiales Moleculares (INAMOL), Universidad de Castilla La Mancha, Campus de la Fábrica de Armas, Toledo 45071, Spain c Laboratorio de Síntesis Orgánica, Facultad de Química, Universidad de La Habana, 10400 La Habana, Cuba d Institució Catalana de Recerca I Estudis Avançats (ICREA), Passeig. Lluís Companys 23, E-08010 Barcelona, Spain b

a r t i c l e

i n f o

Article history: Received 21 April 2013 Received in revised form 2 August 2013 Accepted 3 August 2013 Available online 20 August 2013 Keywords: Small molecule solar cells Solution processing solar cells High open circuit voltage

a b s t r a c t We have synthesized and fully characterized an oligothiophene small organic molecule for its use as electron donor moiety in solution processed bulk-heterojunction organic solar cells. Our results show that device solvent annealing process of the conjugated oligothiophene molecule leads to a light-to-energy conversion efficiency of 3.75% under standard illumination conditions. The solar cell presents open-circuit voltage and fill factors as high as 1.01 V and 63.05% respectively, which are among the highest values obtained for small molecule solution processed organic solar cells. Ó 2013 Elsevier B.V. All rights reserved.

Solution processed bulk-heterojunction organic solar cells based on small molecules (smOSC) have achieved great progress in recent years. Latest results from Chen [1] and Bazan [2] groups have demonstrated independently light-to-energy conversion efficiencies over 8%. Indeed, such results have fuelled the interest of other research groups on the synthesis and characterization of small molecules and their use in solution processed solar cells. Yet, most of the examples found in the literature still fall in the range of 2–4% [3–9] light-to-electricity conversion efficiency under sun simulated light irradiation (100 mW/cm2, 1.5AM G), similar values as the ones reported for semiconductor polymer P3HT:PC60BM solar cells (P3HT: poly-3-hexyl thiophene and PC60BM: [6,6]-

⇑ Corresponding authors at: Instituto de Nanociencia, Nanotecnologia y Materiales Moleculares (INAMOL) Universidad de Castilla La Mancha, Campus de la Fábrica de Armas, Toledo 45071, Spain. (Emilio Palomares). Tel.: +34 977920241. E-mail addresses: [email protected] (F. Langa), epalomares@ iciq.es (E. Palomares). 1 These authors contributed equally to this work. 1566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2013.08.004

phenyl-C61-butyric acid methyl ester). In one hand, these molecules do not require extensive synthetic steps in contrast with the molecules leading to record solar-to-energy conversion efficiencies (Fig. 1) and, moreover, can be prepared and purified in gram scale what it is understood as an advantage for photovoltaic market applications [10]. On the other hand, however, the electronic properties of these small organic molecules lead to poor or low charge mobility in the thin organic bulk-heterojunction film leading to reduced open-circuit voltage (VOC) due to fast interfacial charge recombination processes under illumination and low device fill factor (FF). We have previously demonstrated that solvent vapour annealing (SVA) of the thin organic bulk-heterojunction active layer can lead to optimized nano-morphology in smOSC and increase the organic solar cells fill factor from 40% to 64% that is a value among the highest reported in solution processed smOSC [11]. In the present work, we have applied the SVA process to a ‘‘economic-to-synthesize’’ [12] oligothiophene small organic molecule for their use in solution processed smOSC and obtain solar to energy conversion efficiencies up to 3.8% at 1 sun.

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X-ray diffraction (see Supporting information). The presence of the dicyanovynilene moiety at the extremity of the molecule’s backbone appears to be a key element in inducing long-range order in the solid state through intermolecular supramolecular interactions. Indeed, in a very similar fashion as in previously reported oligothiophenes incorporating a dicyanovynilene moiety [16], smL01 is seen to form extended linear networks of smL01 molecules through hydrogen bonding between the vinyl proton and the cyano group of an adjacent molecule. Such feature was revealed by the single crystal X-ray diffraction structure depicted in Fig. 2. The bond length between the vinyl protons and the cyano groups are of 2.40 Å and 2.50 Å being consistent with what would be expected for hydrogen bonds. This interaction is presumably favored by the fact that he vinyl proton is rendered electron deficient by the presence of the two neighboring electron withdrawing cyano groups. The side view in Fig. 2c, additionally shows that the linear networks of smL01 are stacked with respect to one another, with the stacks being separated by a distance of 3.46 A. The UV–Visible spectra and the LHE (Light Harvesting Efficiency) of the smL01/PC70BM film (film thickness 75 ± 5 nm) is shown in Fig. 3. The most relevant parameters related to the absorption, the fluorescence emission and the electrochemical properties of the smL01 molecule in solution are listed in Table 1. The fluorescence emission of thin bulk-heterojunction smL01/PC70BM films was also analyzed. As can be seen in Fig. 4, upon light excitation at the maximum of the smL01 absorption, the small molecule fluorescence emission is quenched in the presence of the electron acceptor molecule PC70BM. Thus, this result suggests efficient electron transfer from the small organic molecule to the fullerene derivate, as further confirmed later using L-TAS. We ruled out the presence of energy transfer processes between the small molecule smL01 and the PC70BM as no emission from the fullerene was observed and, moreover, no overlap between the dye fluorescence emission and the fullerene absorption occurs. We also would like to notice that the fluorescence quenching efficiency was much

Fig. 1. Chemical structures for the molecules used by Bazan (1) and Chen (2) on the efficiency record smOSC.

The molecule utilized in our study, smL01 was built around a common central 3,4-ethylenedioxythiophene (EDOT) moiety, substituted on both sides by thienylenevinylene moiety terminated by dicyanovinylene groups. smL01 was readily synthesized from the EDOT precursor 2 [13]. Consecutive Horner-Emmons and Vielsmeyer formylation reactions using the phosphonated thiophene precursor 1, [14,15] yielded carboxaldehyde-functionalized building block 3, as trans isomer according to the NMR spectra, which after Knoevenagel condensation with malonitrile under basic conditions led to smL01 (Scheme 1). The new compound was satisfactorily characterized by 1H and 13C NMR, FT-IR, MALDI-MS spectrometry and

C6H13

C6H13

O

O

+ S

1

PO(OEt)2

OHC

S

O

i), ii)

O S

S

OHC

CHO

S

CHO

C6H13

C6H13

C6H13

2

C6H13

3 iii)

O

O S

S NC CN

S C6H13 C6H13

C6H13 C6H13

CN CN

smL01 Scheme 1. Synthesis of smL01. Reactions and conditions: (i) NaH, DMF, r.t., 57 %; (ii) POCl3, DMF, dichloroethane, 57%; (iii) Malonitrile, NEt3, CHCl3, 87%.

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Fig. 2. X-ray diffraction single crystal structure of smL01 (Cambridge Crystallographic Data Centre, Cambridge, UK, CCDC: 949621). (a) View of the unit cell (space group: P-1, a (red) = 14.853(11), b (green) = 15.3990(11), c (blue) = 21.596(3), (b) upper view showing two molecules of smL01 part of the hydrogen bonded network of smL01, the hydrogen bonds are symbolized by the red and green doted lines and (c) side view of the stacked smL01 molecules. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Further confirmation of the efficient interfacial electron transfer reaction was obtained from L-TAS as illustrated in Fig. 5. Upon excitation at kex = 680 nm a decay transient was recorded extending from micro-second to hundreds of milliseconds. The recorded signal was fitted to a power-law exponential decay (Eq. (1)) with a half-lifetime of 2 ls and a a of 0.87.

s ¼ sDn0  na

Fig. 3. The UV–Visible absorption spectra (solid line) and the LHE (dash line) of a thin smL01/PC70BM film. The thickness (75 ± 5 nm) is identical to the used in the solar cells.

Table 1 UV–Visible, steady-state fluorescence and electrochemical data for the molecule used in this study.

a b

Small Absorption molecule kmax (nm)b

Log (e)

smL01

4.87 709

618

Emission kmax (nm)

Ered (V)a

Eox (V)

DE E0–0 (eV) (eV)

1.22 0.50 1.72 1.87

+

Measured values versus Fc/Fc . Measured in dichloromethane and in a concentration of 5  106 M.

lower for the SVA thin bulk-heterojunction films suggesting that, upon SVA, self-segregation occurs in the thin film leading to pure crystalline domains of the small organic molecules responsible for the higher emission yield. Moreover, no changes on the fluorescence emission of the small molecule excited state lifetime were observed and also, measuring the decays at the same acquisition time results in perfect agreement with the steady-state emission measurements.

ð1Þ

Moreover, we observed that the L-TAS signal amplitude was also sensitive to the SVA time as indicated in Fig. 6. As can be seen, the higher signal amplitude corresponds to a SVA time of 30 s that can be directly correlated with high device photocurrent observed in the solar cells devices as described later on this work. Further SVA time leads to a decrease in L-TAS signal amplitude, as well as, a decrease in device photocurrent. We have previously shown [11], for solution processed smOSC, that the SVA treatment leads to the formation of pure crystalline domains in the donor phase of the bulk-heterojunction thin film. The longer the SVA times the bigger the crystallites of the donor molecules and, thus, a concomitant decrease of the donor–acceptor interface necessary to separate the formed excitons after light excitation. A decrease in the L-TAS signal amplitude can be correlated with the decrease of polaron generation yield in the bulk-heterojunction thin film and, hence, to the formation of pure crystalline domains of the small molecules. We assign the L-TAS signal to the polarons at the smL01/PC70BM films as no signal was observed, under nitrogen or oxygen atmosphere, in pristine smL01 thin films that could lead us think about the presence of triplet excited states. The spectrum of the L-TAS measurements can be seen at the SI (Fig. S16). After the characterization of the thin bulk-heterojunction films using fluorescence emission measurements and time-resolved L-TAS we focused on the device preparation. The smL01 small molecule resulted in device efficiencies of 3.5% under standard measurement conditions with a VOC of 1.01 V and a FF of 63.70%. Fig. 7 shows the photocurrent versus voltage curves in this study.

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Fig. 4. Left, steady-state emission spectra of the different samples used in our study. SVA corresponds to solvent annealing. The SVA time is indicated in the Figure legend. Right, time correlated single photon counting decays for all samples measured maintaining identical acquisition time (356 s).

ΔOD

0.0001 8 10

-5

6 10

-5

4 10

-5

2 10

-5

0

10

-6

10

-5

0.0001

Time (s) Fig. 5. L-TAS decay of the 60 nm thin smL01/PC70BM film at upon excitation at kex = 680 nm and kprobe = 800 nm. The film was solventannealed for 30 s.

The first striking observation is the measured high open-circuit voltage (Voc) in these solar cells. The measured oxidation potential versus Fc/Fc+ for smL01 is 0.5 ± 0.05 V (Table 1) that corresponds to a HOMO (Highest Occupied Molecular Orbital) of 5.3 eV calculated following the equation and procedures reported by Forrest and coauthors [17]. The device maximum theoretical Voc value in organic solar cells can be calculated as the difference between the LUMO (Lowest Unoccupied Molecular Orbital) of the acceptor molecule (PC70BM), which is close to 4.0 eV and the HOMO of the donor molecule. Hence, the expected maximum would be close to 1.3 V if the splitting of the quasi-Fermi levels of both materials would be equal to the HOMODONOR–LUMOACEPTOR energy difference. However, the observed device Voc is limited by the charge losses due to the different recombination reactions occurring at the device interfaces and, hence, the difference in energy of the quasi-Fermi levels is always lower (Scheme 2). The evaluation of the charge losses due the recombination at the device under operation conditions are discussed later on this work.

0.0001

non-A 30 sec 7 min

-5

8 10

-5

ΔOD

6 10

-5

4 10

-5

2 10

0

-6

10

-5

10

0.0001

Time (s) Fig. 6. L-TAS decay of the 60 nm thin smL01/PC70BM films at upon excitation at kex = 680 nm and kprobe = 800 nm. The legend shows the SVA time for the bulk-heterojunction thin film.

Fig. 7. Measured current density versus voltage (J–V) curves for smL01 at 1 sun (100%) and in dark without the solvent-annealing process.

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Further optimization of the best device was carried out by changing different preparation conditions as, for example, the small molecule smL01:PC70BM ratio, spin-coating revolution speed and solvent annealing time as shown in Table TS1 at the SI. For example, the optimized devices were obtained with smL01:PC70BM ratio of 1:1 by weight (smL01 20 mg/mL) in chloroform at 8000 rpm and with a solvent annealing time of 60 s. Fig. 8 shows the J–V curves for the optimized device. Upon solvent annealing treatment the device photocurrent was increased close to 6 mA for a device with a total photoactive layer of 75 ± 5 nm. Interestingly, the solvent annealing process did not affect the device Voc. To understand better the observed high Voc we measured the device losses due to the charge recombination processes occurring under device operation. We employed charge extraction (CE) and transient photovoltage (TPV) techniques as reported before [18,19]. As illustrated in Fig. 9, either non-solvent annealed devices (non-SVA) or the solvent annealed (SVA) ones show a linear relationship between the measured device charge density and the light induced device Voc (so called ‘‘light bias’’). Only at voltages close to 0.9 V the charges show an exponential increase with voltage. As can be seen, in both type of devices the accumulated charge is very similar with small differences within the experimental error of the measurements. Our group [19] and others [20] have reported before that this linear relationship is indicative of most charges being stored at the device electrodes rather than at the organic bulk-heterojunction and only at high light intensities close to 100 mW/cm2 (1 sun) the materials quasi-Fermi level splits and the device Voc depends strongly on the non-geminate recombination kinetics (Fig. 10).

Fig. 8. J–V curves of the optimized smL01:PC70BM smOSC at 1 sun (100%) and dark.

Fig. 9. Measured charge density at different light induced device open circuit voltage for non-solvent annealed (non-SVA) and solvent annealed (SVA) solar cells. The solid lines are used as a guide to the eye.

Scheme 2. Pictorial description of the origin of the device Voc. The difference in energy between the quasi-Fermi levels (qF) is due to the different charge density of holes at the donor material (smL01) and electrons at the acceptor material (PC70BM). The Voc theoretical is larger than the observed due to the presence of charge recombination reactions (krec) during device operation that reduce the overall charge density that sustain the qF apart.

The measured charge density (Fig. 9) and charges lifetime (Fig. 10) for the non-SVA and SVA smL01 solar cells agree with the experimental observation of equal device Voc. We discuss now the moderate photocurrent observed. Unfortunately, the device thickness is thin enough to achieve high fill factors and high open circuit voltage, however, limiting the light harvesting of the solar cells as shown in Fig. 3. The measured EQE (External Quantum Efficiency) in Fig. 11 is in good agreement with the device photocurrent. The use of thicker films lead to poorer device performance mainly due to low charge mobility of the blend organic film. Hole only and electron only devices were prepared and the space charge limited current (SCLC) in these devices was measured to obtain the values for hole and electron mobility. The hole and electron only devices were fabricated following standard reported procedures

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Fig. 10. Measured charges lifetime under different light bias for nonsolvent annealed and solvent annealed smL01:PC70BM organic solar cells.

Fig. 11. EQE spectrum for the smL01:PC70BM.

and the SCLC region was measured applying sufficient large potentials (Fig. 12), however, the data could not be fitted to the known Mott-Gurney equation but it was possible to obtain the mobility value using the field dependent mobility equation (Eq. (2)) as reported before [21,22], where l (cm2 V1 s1) is the mobility coefficient, d (cm) is the film thickness, Veff (V) is the applied voltage, b (cm1/2 V1/2) is the electric field parameter and e (e0er  3) is the media permittivity.

J SCLC

pffiffiffiffiffiffiffiffi! 2 0:89b V eff 9 V eff pffiffiffi ¼ el 3 exp 8 d d

ð2Þ

The calculate hole mobility, lh, and the electron mobility, le, were 1.2 ± 0.7  106 cm2 V1 s1 and 2.6 ± 1  103 cm2 V1 s1 , respectively. The low value for the hole mobility will have definitively an impact on the low efficiency of our EQE and explains, as mentioned above, the inferior results obtained with thicker smL01:PC70BM films.

2831

Fig. 12. Current–voltage curves for a hole only device under irradiation and in the dark. The fitting to Eq. (1) is shown as a solid line.

In conclusion, we have designed and synthesized a novel oligothiophene small molecule through straightforward synthetic steps leading to optimized device efficiencies close to 3.8% under standard conditions. The devices show outstanding open circuit voltage and high fill factor for solution-processed smOSC. Moreover, we have studied by using steady-state fluorescence and L-TAS spectroscopies the influence of the solvent vapour annealing process on the polaron generation yield and its correlation with the smOSC short-circuit photocurrent. We have demonstrated that increasing SVA times lead to a decrease in polaron yield due likely to the formation of pure crystalline domains of the small molecules, as observed before for other smOSC. Moreover, from L-TAS measurements and steady-state emission studies we have proven that efficient charge transfer occurs when the solvent annealing time is precise. However, the solar cells show limited photocurrent, close to 6 mA/cm2, in good agreement with moderate EQE values. Further studies on device optimization lead us to conclude that thicker films were not an option as the overall device efficiency was much lower. Additional studies with hole only and electron only devices confirmed low hole mobility values which limit the charge collection efficiency and, thus, the EQE for smL01:PC70BM solar cells. On the other hand, the measurement of the charge density and the carriers lifetime under operation conditions agree with the experimental observation of high device Voc either in solvent annealed devices or non-solvent annealed solar cells. The observed high voltage is due mainly to the HOMO energy level for the smL01 molecule and the fact that the charge carriers are mostly accumulated at the device electrodes and the kinetics of recombination being more dependent on the device internal electric field than the non-geminate recombination kinetics.

Acknowledgments Eduardo Escudero and Marta Martínez are acknowledged for solving the X-Ray diffraction structure of

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smL01. The Spanish MINECO under grants CTQ201018859, CTQ2010-1749 and CONSOLIDER CDS-007 HOPE2007 supports this work. EP would like also thanks the EU for the ERCstg PolyDot, and the Catalan government for the 2009 SGR-207 projects. B.P. thanks to the MINECO for a FPI grant. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2013.08.004. References [1] J. Zhou, Y. Zuo, X. Wan, G. Long, Q. Zhang, W. Ni, Y. Liu, Z. Li, G. He, C. Li, B. Kan, M. Li, Y. Chen, Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit, J. Am. Chem. Soc. 135 (2013) 8484–8487. [2] Y. Sun, G.C. Welch, W.L. Leong, C.J. Takacs, G.C. Bazan, A.J. Heeger, Solution-processed small-molecule solar cells with 6.7% efficiency, Nat. Mater. 11 (2012) 44–48. [3] Y. Kim, S. Cook, J. Kirkpatric, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, M. Heeney, R. Hamilton, I. McCulloch, Effect of the end group of regioregular poly(3-hexylthiophene)polymers on the performance of polymer/fullerene solar cells, J. Phys. Chem. C 111 (2007) 8137– 8141. [4] J.A. Mikroyannidis, D.V. Tsagkournos, S.S. Sharma, Y.K. Vijay, G.D. Sharma, Conjugated small molecules with broad absorption containing pyridine and pyran units: synthesis and application for bulk heterojunction solar cells, Org. Electon. 11 (2010) 2045–2054. [5] Y. Lin, P. Cheng, Y. Liu, X. Zhao, D. Li, J. Tan, W. Hu, Y. Li, X. Zhan, Solution-processable small molecules based on thieno[3,4-c] pyrrole-4,6-dione for high-performance solar cells, Sol. Energy Mater. Sol. Cells 99 (2012) 301–307. [6] G.D. Sharma, M.S. Roy, J.A. Mikroyannidis, K.R. Justin Thomas, Synthesis and characterization of a new perylene bisimide (PBI) derivative and its application as electron acceptor for bulk heterojunction polymer solar cells, Org. Electon. 13 (2012) 3118– 3129. [7] X. Wan, Y. Liu, F. Wang, J. Zhou, G. Long, Y. Chen, Improved efficiency of solution processed small molecules organic solar cells using thermal annealing, Org. Electon. (2013). http://dx.doi.org/0.1016/ j.orgel.2013.03.006. [8] J.A. Mikroyannidis, A.N. Kabanakis, S.S. Sharma, G.D. Sharma, Low band-gap phenylenevinylene and fluorenevinylene small molecules containing triphenylamine segments: synthesis and application in bulk heterojunction solar cells, Org. Electon. 12 (2011) 774–784.

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