Enhanced photocurrent and stability of organic solar cells using solution-based TS-CuPc interfacial layer

Organic Electronics 37 (2016) 183e189

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Enhanced photocurrent and stability of organic solar cells using solution-based TS-CuPc interfacial layer M. Raïssi a, c, *, S. Leroy-Lhez b, B. Ratier a a

XLIM, UMR-CNRS 7252, University of Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France Laboratoire de Chimie des Substances Naturelles, Universit e de Limoges, 123 Av. Albert Thomas, 87060 Limoges, France c CEISAM UMR CNRS 6230, Universit e de Nantes, UFR sciences et techniques 2 rue de la Houssini ere BP 92208, 44322 Nantes Cedex 03, France b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 December 2015 Accepted 25 June 2016

Copper phthalocyanine-3,40 ,400 ,4000 tetra-sulfonated acid tetrasodium salt (TS-CuPc) are promising interfacial materials for organic photovoltaics (OPVs) because of their electrical properties, solution process ability and stability. In this article, we report the fabrication of poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) OPV devices incorporating solution-based TS-CuPc as electron transport layers that show promising enhancements of the device photocurrent and stability. We also discuss the impact of the mechanism of Photochemical and photophysical behavior and all parameters on device performance as well as the ambient degradation of these devices. In these results we showed charge extraction improvement by the use of the TS-CuPc. © 2016 Elsevier B.V. All rights reserved.

Keywords: Stability TSCuPc ETL OSCs Seebeck coefficient

1. Introduction Photovoltaic cells will hopefully lead to a clean and renewable energy source. Organic thin-film solar cells have potential advantages of low manufacturing cost, light weight, and mechanical flexibility. Nevertheless, the fundamental knowledge of organic semiconductors and photovoltaic devices was obtained through a long period of research. The first successful OPV device was reported as early as 1986 by Tang et al. using a bi-layer structure [1]. Thermally evaporated of tow thin layers of p-type small molecules (copper phthalocyanine) and n-type molecules (perylene diimide derivative), were deposited between two electrodes (indium tin oxide and silver). The charge separation at the donor-acceptor interface was found to be very efficient and an impressive efficiency (PCE) of ~ 1% and high fill factor (FF) of 65% were demonstrated. But, since ten years, organic blend solar cells of conjugated polymer and fullerene derivative have shown improved efficiencies [2e8]. In particular, a poly (3 hexylthiophene) (P3HT) and [6,6]phenyl-C71-butyric acid methyl ester (PCBM) blend system has attracted the most attention [4e8]. Recently, P3HT: PCBM blend solar cells achieving a power conversion efficiency (PCE)

* Corresponding author. XLIM, UMR-CNRS 7252, University of Limoges, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France. E-mail address: [email protected] (M. Raïssi). http://dx.doi.org/10.1016/j.orgel.2016.06.030 1566-1199/© 2016 Elsevier B.V. All rights reserved.

approaching 5% have been reported [9e11]. However, the major obstacles for the large-scale use of organic photovoltaic are their low efficiency and short lifetime. Some studies report the stability of devices under ambient conditions without encapsulation or with simple mechanical protection of the active layer but with no barrier properties. All of these studies report a relatively short lifetime [12e15]. Thus, many studies in the literature focus on the degradation of organic solar cells [15e17]. The morphology of the P3HTPCBM active layer seems to be a key factor as it allows important gains in terms of stability. Therefore, one can now realize device with stability measured in years rather than minutes [15e20]. In addition, the interface of organic active layer/metal electrode is another important key which is sensitive towards molecular oxygen and water. Furthermore, the device performance and stability depends also on devices’ architecture. The normal geometry of devices used poly (3, 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) [21,22] as hole transport buffer layer. Indeed, it has some advantages such as high work function, high transparency and good conductivity. But, the manly disadvantages of PEDOT: PSS layer is the intrinsic acidity, with a pH ranging from 1.5 to 2.5. That could dissolve indium ions from the ITO layer [23e27], allowing them to destroy the photoactive layer. The Aluminum or/and calcium (Al/ca) metal electrodes used in this geometry, have a low work function and are highly reactive toward the oxygen and water which leads to degradation of the devices.

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Today, the inverted BHJ PVCs geometry is dominating on organic devices with ITO as the cathode and silver (Ag, high work function metal) as the anode, because it has been demonstrated that it allows higher device stability [21,22]. To increase the performances and stability of organic devices, the interfaces layers between active layer and electrodes (Hole Transport Layer HTL and Electron Transport Layer ETL) are a key factor as it allows important gains in terms of electron collection and hole transport. The MoO3 thin film present the best candidate of HTL and it’s more used in inverted geometry. However, several thin films have been studied for the electron transport layer (ETL), such as Cs2CO3 [17,21], metal oxides (ZnO, TiOx) [25,28] Cs2CO3-doped C60 [29], metals (Ca, Mg) [30,31], In2S3 [32], etc., some of those films can show good electron collection ability. Nevertheless, TS-CuPc in aqueous solution enables the fabrication of printable electronics devices, as solar cells. This ink has a several potential advantages, such as light weight, flexibility, and low-cost manufacturing, as well as large-area feasibility which was not possible for other metal oxide (TiOx). Today the OPV technologies with a high efficiency and good stability are particularly attractive due to their potential in high throughput manufacture processes, like ink-jet printing and rollto-roll processing, that are more energy efficient, in comparison to common silicon photovoltaic production processes [9]. Herein, we report the results on stability of the organic solar cells with TS-CuPc as electron transport layer [9] and the effect of the exposition to air on the improvement of the performances. We have investigated the mechanism of the photochemical and photophysical behavior of the TSCuPc exposed to air which allows a strong stability toward the oxygen and water.

2. Experimental section All devices under investigation were prepared on Indium tin oxide (ITO)-coated glass with a sheet resistance of 10e15 U/▫ after cleaned by the process inside an ultrasonic bath, beginning with deionized water, followed by acetone and isopropanol and dried in oven at 120  C during overnight. The active layer (PCBM: P3HT 1:1 in o-dichlorobenzene) was sandwiched between TS-CuPc and MoO3 buffer layers, with a thickness of 140 nm and was deposited on top of the TS-CuPc layer by spin-coating as using the solvent annealing process [33,34]. This produces highly ordered thin films of polymer by slow evaporation of solvent in a partially closed container such as a Petri dish. This process allows the controlled separation of phases and crystallization of the components. The electron transport layer (ETL) chosen here is an aqueous solution of TS-CuPc (5 mg/ml). It was spin-coated with speeds at 1000 tr/min during 1 min on top of ITO (treated by UV-O3 during 15 min) and dried at 140  C for 10 min to form a thin interlayer with 15 nm thickness. Finally, the samples were transferred to a vacuum chamber, where MoO3 interface layer and Ag cathodes were deposited via thermal evaporation at a speed of 0.1 Å/s (MoO3) and 1e5 Å/s (Ag) through shadow masks at the base pressure of ~2  106 Torr. The active area of the devices was 18 mm2. Transfer of samples from the glove box to the evaporation chamber resulted into their exposure to ambient air for ~5 min. The performance and the stability studies of the devices were determined using AM 1.5G solar simulator at 100 mW/cm2 in ambient air. The current-voltage (J-V) curves were measured using a Keithley source meter. UVVis Absorption Spectra. The thin films of TSCuPc and TSCuPc/P3HTPCBM with different studies were casted from water and 1,2dichlorobenzene (o-DCB) solutions respectively onto quartz glass. UVvis absorption spectra of these thin films of solutions were measured using the HP 8453 spectrophotometer.

3. Results and discussion Indium tin oxide ITO is not well suited as an electron collecting electrode in established P3HT: PCBM systems due to the high work function (WF). To reduce the barrier, we introduced the TS-CuPc as interfacial layer whereas many work used metals oxides [19,22]. Fig. 1 show the architecture of devices (Fig 1a) and the energy level diagram with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the materials and the functional layers, as well as the electrodes. In the light of the energy diagram (Fig 1b) it is clear that both interfacial layers used in this particular device architecture (CuPc and MoO3 for electrons and holes respectively) are supposed to allow carrier extraction via the LUMO band. One advantage of such behavior is that materials used show very deep ionization potential and therefore an increased stability compared to most of organic and oxides interfacial layers. Fig. 2 show the J-V characteristics of two inverted geometry devices deposited by spin coating, one with TSCuPc thin film exposed in air during 60 min and the second device with TS-CuPc deposited in glovebox under nitrogen. A remarkably effect of the exposition to air of the TS-CuPc thin film on the improvement of the performances device was observed. Indeed, all parameters were improved especially the short circuit current density which increased from 10 mA/cm2 to 12 mA/cm2, the Voc (increasing from 0.23 V to 0.51 V) and the FF (increasing from 0.27 to 0.60). Table 1 summarized all PV results. Complementary of JeV characterization and external quantum efficiency (EQE) measurements were also conducted. In that purpose, the experimental photocurrent Jph (Jph ¼ JL  JD) as a function of effective applied voltage (Veff ¼ V0  V) for inverted solar cells containing TSCuPc as electron transport layer are shown in Fig. 3, JL, JD the current measured under illumination and dark respectively, V0 (Jph ¼ 0) corresponding exactly to the cell internal bias. When the voltage close of (V0  V < 0.1 V), the photocurrent is observed to increase linearly with voltage. In this regime, the transport rate is mainly governed by charge recombination. The curves in (V0  V < 0.1 V), show clearly that the slope of photocurrent increases with exposition time of TSCuPc. That means a better charge collection at the interfaces preventing the charge recombination. For V0  V > 0.1 V, the photocurrent entered a regime for which a square root dependence on the effective voltage is observed. However, where the photocurrent decrease in (V0  V > 0.1 V) has been attributed to the recombination effects or the blocking of the charges at the interfaces. Fig. 3 show in the square root regime (V0  V > 0.1 V), photocurrents were dependent of solar cells exposure time (illumination at 100 mW/cm2, white light (1sun)) since an exposure time of more than 9 min’ lead to higher photocurrents than those recorded after only 3 min of light exposure. This result suggests that TSCuPc, as electron transport layer, has been activated upon illumination and air and thus helped decrease the charges recombination at the interface as well as increase the charges transfer by reducing the series resistances. The EQE spectra showed a maximum at 550 nm with a high intensity value (EQE ¼ 80%) for the devices with TS-CuPc exposed to air presented in Fig. 4. This result suggests an enhanced charge transfer and exciton dissociation yield and/or reduced charge recombination. It is noted that the increase in the integrated EQE is consistent with the increase in Jsc and decrease in charge losses. However, the low value of the series resistances values (Table 1) proved that the high value of the conductivity of the TS-CuPc thin film exposed to air. For this reason, the conductivity of films over time in air were studied. The study of the conductivity vs seebeck coefficient as a function of the exposure time to ambient atmosphere (sun light and air) for TS-CuPc thin film deposed on glass are shown in Fig. 5. The

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Fig. 1. a) Device configuration and chemical structure P3HT and PCBM, (b) proposed energy level diagram of the polymer solar cell.

the majority carriers of charges transport. The values of Seebeck coefficient obtained on fresh film under vacuum or in open air are positives (ca. 440 mVK1), this is indicating that the holes are the majority carriers involved by transport [36]. When measurements were carried out on film maintained several hours under vacuum and dark or light conditions, a decrease of the Seebeck coefficient was observed, the values being still positive. However, the Seebeck coefficient values became negative when the TSCuPc film was exposed to ambient air under UV irradiation (lamp UV) with a value at ca. 300 mVK1. Thus, presumably, the conductivity of the TSCuPc film was based on a photo-induced oxidation phenomenon. This doping process is clearly visible comparing conductivity and Seebeck coefficient evolution under air and light exposure Fig. 5. High conductivity stability plateau is obtained when transport is assumed by the electrons. Moreover, these results gave obviously the proof that electrical transport is governed by electrons in the LUMO band of the TS-CuPc. These conductivity study results were in agreement with photovoltaic measurements that has underlined the need of a few minutes’ activation of the cell before recording satisfactory photovoltaic parameters. This oxygen and light Fig. 2. I-V characteristics of ITO/Ts-CuPc/P3HT-PCBM/MoO3/Ag.

Table 1 Photovoltaic values of tow devices. Solar cells

Voc (V)

FF

Jsc (mA/cm2)

PCE (%)

Rs U/cm2

Rsh U/cm2

TS-CuPc exposed in air (60 min) Ts-CuPc No exposed in air

0.51 0.235

0.60 0.27

12.02 10.34

3.7 0.65

42 105

2688 138

conductivity was measured by the four points probe setup. The conductivity value measured for the fresh film was very low (ca. 4  107 S cm1), but over time exposure, an increase of the conductivity was noticed (2  10- 4 S cm1 after 60 min). Moreover, the conductivity values became stables after 120 min of exposure time as the curve reached a plateau (ca. 5  103 S cm1). This feature evidenced that the layer became more and more conductive with exposure time (up to three order of magnitude compared to initial value) until its conductivity reached a limit value but then, the layer was stable. To get further insight in the effect of oxygen and light on the conductivity of TSCuPc film, the thermoelectric power of the film by using the two point’s setup was studied Fig. 6 [35] as the sign of Seebeck coefficient measured by this method indicates the sign of

dependent behavior have been partly described by Abramczyk et al. [37] The authors have indeed studied by resonance raman spectroscopy among other spectroscopic methods the photochemical and photophysical behavior of copper (II) phthalocyanine3,40 ,400 ,4000 -tetrasulfonate anion (Cu(tsPc)4-) under visible light irradiation and thus have identified transient species in the photoredox dissociation of this compound (eq. (1))

h i CuðTsPcÞ4 þ hy/CuIIðTsPc$ Þ5 þ CuIIðTsPc$ Þ3 2

(1)

This dissociation, occurring via photo-induced electron transfer between two molecules of phthalocyanine is determined mainly by intermolecular interaction. Indeed, this process, leading to the generation of ligand-centered radicals may occur only if there is a

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Fig. 3. Experimental photocurrent as a function of effective applied voltage, Veff ¼ V0eV, for ITO/TSCuPc/P3HT-PCBM/MoO3/Ag solar cells containing the active layer shown in. V0 represents the compensation voltage for which the photocurrent Jph ¼ JL  JD ¼ 0. The inset is zoomed-in on the region where Veff > 0.1 and the photocurrent display a square root dependence on voltage.

Fig. 6. Study of the Seebeck coefficient over time during the measure in four conditions: under vacuum and dark (black line) or light (red line) and in air and dark (blue line) or light (green line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

strong overlapping between p-electronic clouds of adjacent macrocycle rings. It also has been demonstrated that this photoinduced electron transfer can occur in the crystal phase in the case of dimers of Cu(TsPc)4- [38]. The existence of dimer of TS-CuPc can be evidenced by the splitting of the Q band in absorption spectrum. In our case, this behavior was observed in water solution but not in DMSO (Fig. 7 and Table 2), as described in the literature [37]. As expected, the B band lied between 300 and 350 nm in both solvents. In water, the Q band was split into two (one main band at 630 nm, characteristic of dimer emission and a shoulder at 665 nm, probably due to the monomer) whereas in DMSO, the absorption maximum was observed at 676 nm. Only a low intensity shoulder at 610 nm was observed. This difference induced by solvent disappear in the case of films Fig. 8, as absorption spectra recorded for both films obtained from H2O or DMSO solutions has the same profile. However, it is noteworthy to underline that absorption maxima in films are blue-shifted compared to the ones recorded for solution. This should be due to the formation of H aggregate rather than dimers. These dimers are described as not stable species under light illumination, for wavelength lower than 520 nm. In this case, the Fig. 4. EQE spectra of ITO/TS-CuPc/P3HT-PCBM/MoO3/Ag.

Fig. 5. Study of the conductivity during the exposition time in air and under light.

Fig. 7. Absorption spectra of TSCuPc solutions (room temperature, concentration. ca. 2,5  105 mol L1).

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Table 2 Absorption wavelength (nm) of both solution and thin film of TsCuPc. Water B band Solution Filmsb a b c

a

336 nm

DMSO Q band c

630 nm (665 nm) 612 nm

B band

Q band

350 nm

(610 nm)c 676 nm 614 nm

Room temperature, concentration ca. 25 mmol. Drop casting, 5 mg/mL, room temp. Shoulder in absorption spectrum.

Fig. 8. Absorption spectra of TSCuPc films from different solvent (drop casting, 5 mg/ mL, room temp.).

dimer dissociates onto Cu(tsPc)5- and Cu(tsPc)3- species [38]. Therefore, absorption spectra of TSCuPc films obtained from aqueous solution have been monitored as a function of UV light irradiation time. As it can be seen on Fig. 9, a slight decrease of the band at 615 nm was observed concomitantly with the increase of the shoulder at ca. 500 nm, this supporting the hypothesis of new species formation. No change in absorption feature has been observed under dark conditions. Thus, we assume that in our case, photochemical reaction occurs in presence of oxygen and that electron transport can be explained by this photo-induced process

Fig. 10. Studies of all parameters of ITO/TS-CuPc/P3HT-PCBM/MoO3/Ag devices over the time in air and under illumination AM1.5G, 100 mW/cm2.

Fig. 9. Evolution of absorption spectrum of TSCuPc films (drop casting, 5 mg/mL, room temperature) as a function of UV irradiation Time.

that should be, according to literature, be an electron transfer (see eq. (1)). The stability of organic solar cells is particularly difficult to study due to the large number of device parameters and experimental factors that may affect degradation, as well as the possibility for interplay among these factors. Recently, researchers have identified

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various potential avenues toward degradation, including photodegradation [39e43], ambient (H2O/O2) degradation [44e48] and thermal degradation [49e51] of the photo-active organic layer. Nevertheless, photo- and thermal-instability of the organicelectrode interfaces have also recently been highlighted as one of the most critical avenues toward OSC degradation. In this context, several different types of buffer layers have been introduced for increasing the lifetime of devices with low work function metal electrodes. Sol-gel processed titanium sub oxide (TiOx) [33,34] can be used, but is a relatively poor conductor in its amorphous form [52]. A recent study by Li et al. concluded that TiOx acts as a photochemically activated oxygen scavenger significantly enhancing the stability of P3HT: PCBM towards both UV and oxygen exposure [53]. M. Wang et al. have applied thermally evaporated chromium oxide (CrOx) between the active layer and the aluminum cathode to enhance stability [54]. In this work, we study the stability of the inverted devices by using our new interface layer of TSCuPc, Fig. 10 show the high stability of all parameters devices during 140 days in air, that prove the good stability of the TS-CuPc as electron transport layer and MoO3 as hole transport layer in air. It is worth to emphasis from this measurement that no noticeable exponential decay of the performance can be observed during all the observation period. This could mean that oxygen or moisture diffusion through electrode pinholes would be hindered compared to most of previous aging studies [55e57]. Further studies of the TS-CuPc/Ag electrode interface should be undertaken in order to explain such stability. However, both interfaces layers considering as the best alternative for encapsulation of the active layer against the diffusion of the oxygen and water which provoke the photodegradation of devices. 4. Conclusion In summary we have reported on the improvement of the TSCuPc conductivity under prolonged UV and air exposure. Increasing of the conductivity values of TSCuPc can greatly impact the performance of pristine solar cells, and its stability under illumination. In this study, it has been shown that the improvement of the FF and Voc in the JeV characteristics of solar cells under continuous exposure to UV light, the illumination of a solar simulator, or exposed to air, is correlated to changes of the seebeck coefficient value of the electron transport layer (TSCuPc). By using the two point’s setup under vacuum (sunlight or dark) and under irradiation by UV lamp in this studies for fully clarify the photo-induced oxidation mechanism behind the changing of the sign of Seebeck coefficient. For fresh film under vacuum or in open air positives values of the Seebeck coefficient (ca. 440 mVK1) were obtained, indicating that the majority of carriers involved by transport were holes. However, the Seebeck coefficient values became negative when the TSCuPc film was exposed to ambient air under UV irradiation (lamp UV) with a value at ca. 300 mVK1. Finally, the high conductivity of TSCuPc film as electron transport layer and a strong stability in air of the devices with a good performance allows a good new approach for the new devices technology. References [1] C.W. Tang, Two-layer organic photovoltaic cell, Appl. Phys. Lett. 48 (1986) 183. , S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, [2] S.H. Park, A. Roy, S. Beaupre K.H. Lee, A.J. Heeger, Bulk heterojunction solar cells with internal quantum efficiency approaching 100%, Nat. Phot. 3 (2009) 297e303. [3] Y. Liang, Z. Xu, J. Xia, S.T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, For the bright future bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%, Adv. Mater. 22 (2010) E135eE138. [4] G. Zhao, Y. He, Y. Li, 6.5% efficiency of polymer solar cells based on poly (3hexylthiophene) and indene -C60 bisadduct by device optimization, Adv.

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