Lanthanum Hexaboride As Novel Interlayer for Improving the

Sylvain Chambon†‡§, Yolande Murat†‡§, Guillaume Wantz†‡§, Lionel Hirsch†‡§, and Pascal Tardy†‡§. † Univ. Bordeaux, IMS, UMR...
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Lanthanum Hexaboride As Novel Interlayer for Improving the Thermal Stability of P3HT:PCBM Organic Solar Cells Sylvain Chambon,*,†,‡,§ Yolande Murat,†,‡,§ Guillaume Wantz,†,‡,§ Lionel Hirsch,†,‡,§ and Pascal Tardy*,†,‡,§ †

Univ. Bordeaux, IMS, UMR 5218, F-33405 Talence, France CNRS, IMS, UMR 5218, F-33405 Talence, France § Bordeaux, INP, IMS, UMR 5218, F-33405 Talence, France ‡

S Supporting Information *

ABSTRACT: For efficient organic photovoltaic (OPV) solar cells, a low work function electrode is necessary to enhance the built-in voltage of the active layer, thereby improving the overall efficiency. Calcium is often used for this purpose in the laboratory; however, its development on a larger scale is impaired by its high reactivity with oxygen and water and the resulting low stability of solar cells under operation. The influence of a novel interlayer, lanthanum hexaboride (LaB6), on the electronic properties of OPV is studied in this work. Similarly to calcium, when LaB6 is used as an interlayer, it enhances the built-in voltage in the device, leading to a higher fill factor (FF) and optimal open circuit voltage (Voc). As a result, optimized LaB6based devices present significantly improved power conversion efficiencies. More importantly, while calcium/aluminum (Ca/Al) and aluminum (Al) cathodes lose their capacity to enhance the internal electrical field during thermal aging, the LaB6/aluminum (LaB6/Al) electrode remains stable. This remarkable effect results in a highly stable Voc and flat-band potential during aging. KEYWORDS: organic photovoltaic, stability, interlayer, lanthanum hexaboride, thermal degradation, lifetime

1. INTRODUCTION Organic photovoltaics is a continuously growing field of research, bringing together scientists from organic chemistry and fundamental and applied physics to improve the power conversion efficiency (PCE) and lifetime of solar cells. This research has led to the constant improvement of PCEs to up to 8−10% on lab-scale cells.1−4 The stability of solar cells is also a key factor in the commercialization of this technology.5,6 A big step forward was achieved in 2006 with the development of the inverted architecture.7−9 In this configuration, electrons are collected at the bottom electrode by the means of a metal oxide interlayer such as titanium oxide or zinc oxide. This kind of architecture has improved stability in shelf-lifetime conditions under air compared to the direct architecture in which electrons are collected by a low work function top electrode.10,11 However, the direct architecture is still necessary in some cases. Depending on the processing conditions (solvent, additives), some bulkheterojunction active layers exhibit vertical phase separation in which a donor-rich phase is located at the bottom of the active layer.12 In this situation, it is necessary to use a direct architecture to maximize charge collection. In these structures, a thin film of calcium is generally used between the active layer and the top aluminum electrode. Calcium is used as a low work function metal (ϕCa = 2.8 eV) to enhance the built-in potential necessary for efficient charge collection. However, calcium is a very © XXXX American Chemical Society

chemically sensitive material and reacts rapidly with oxygen or moisture13,14 to produce an electrically insulating calcium oxide or calcium hydroxide layer, which is detrimental to solar cell performance. Even though Kumar et al. show that when calcium is used in combination with silver, a partial recovery of the performances can be observed,14 it is still necessary to develop a more stable direct architecture for organic solar cells (OSC). To achieve this goal, the first step is to find an alternative to calcium as a low work-function electrode. Lanthanum hexaboride is a refractory ceramic which has metal-like behavior15 and a low work function (ϕLaB6 = 2.7 eV).16 This material is commonly used as an electron emitting material in cathode-ray tubes, microwave devices, electron microscopes, and plasma sources. Despite having never been used in organic solar cells before to our knowledge, these properties suggest that it may be a good challenger to calcium as interlayer material in OSC. However, its high melting point (2210 °C) makes it impossible to evaporate using classical thermal evaporation under vacuum, so electronbeam evaporation had to be used to grow LaB6 thin layers in this work. Received: August 17, 2015 Accepted: October 22, 2015

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DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Summary of PV Characteristics of OSC with Different Cathodes (LaB6/Al, Al, Ca/Al) top electrode

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

Rs (Ω)

Rsh (kΩ)

0.5 nm LaB6/Al 1 nm LaB6/Al 5 nm LaB6/Al 10 nm LaB6/Al Al 20 nm Ca/Al

10.97 ± 0.24 10.38 ± 0.15 10.11 ± 0.39 10.39 ± 0.35 10.77 ± 0.20 10.20 ± 0.49

0.555 ± 0.005 0.553 ± 0.006 0.503 ± 0.032 0.529 ± 0.012 0.535 ± 0.008 0.538 ± 0.011

0.61 ± 0.01 0.61 ± 0.01 0.35 ± 0.04 0.52 ± 0.01 0.56 ± 0.02 0.67 ± 0.01

3.74 ± 0.10 3.52 ± 0.09 1.78 ± 0.38 2.84 ± 0.16 3.24 ± 0.12 3.65 ± 0.17

23 26 344 148 38 14

1500 1430 1470 1690 1350 840

Figure 1. I−V characteristics of the different architectures fabricated: (brown) 0.5 nm LaB6/Al, (green) 1 nm LaB6/Al, (orange) 5 nm LaB6/Al, (purple) 10 nm LaB6/Al, (blue) Al, and (pink) Ca/Al (a) in the dark and (b) under 1 sun illumination. and 10 nm) and aluminum (80 nm) were deposited onto the samples (shadow masked) using an e-beam thermal evaporator under high vacuum (HV, 10−6 mbar). Calcium (20 nm)/aluminum (80 nm) electrodes were deposited by classical thermal evaporation under high vacuum (HV, 10−6 mbar). Thermal annealing was performed on a hot plate in a glovebox. The resulting active area of the devices was 0.12 cm2. 2.2. Current−Voltage Characterization. Current−voltage (I−V) characteristics were performed in a glovebox under inert atmosphere (O2 < 1 ppm, H2O < 1 ppm) in both the dark and under AM1.5 illumination (HMI lamp, 100 mW/cm2). The initial photovoltaic performances were averaged over 16 devices on four different substrates (four devices per substrate). Throughout the aging experiment, I−V characteristics were averaged over eight devices coming on two different substrates. The series (Rs) and shunt (Rsh) resistances were both extracted from the dark curves. Rsh was calculated at reverse bias (−0.5 V) and Rs at high forward bias (+0.9 V). 2.3. Thermal Aging Experiments. Thermal aging was performed on a temperature controlled hot plate at 85 °C in a glovebox under inert atmosphere (O2 < 1 ppm, H2O < 1 ppm) 2.4. Capacitance−Voltage Experiments. Capacitance−voltage (C−V) measurements were performed on a BioLogic SP-300 potentiostat controlled by EC-Lab software. A small AC perturbation (25 mV, 1 kHz) was superimposed on a DC voltage sweeping from −1 to +1 V. The parallel capacitance (Cp) was used in the Mott−Schottky plot to extract the Vfb. 2.5. Atomic Force Microscopy (AFM). A Bruker Innova Atomic Force Microscope was used in tapping mode to record the height and phase images. Olympus Micro Cantilevers (160 μm long; OMCL AC160-TS) were used at their resonant frequency (∼300 kHz). Scan speed was set between 0.7 and 1 Hz, and a resolution of 512 lines/image was used.

In this study, we report the use of lanthanum hexaboride as an electron injection layer material in poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) direct organic solar cells.17 Direct OSCs were fabricated that incorporated a LaB6 interlayer of variable thicknesses between the organic active layer and the aluminum top electrode. Reference devices incorporating a bilayer of Ca/Al and an Al top electrode were also fabricated for comparison purposes. The electronic properties of the devices integrating such novel interlayer for polymer solar cells were studied in terms of built-in voltage enhancement and photovoltaic properties. The LaB6 film formation onto the organic active layer was also monitored using atomic force microscopy (AFM). Finally, the stability of the different architectures against thermal aging was studied.

2. MATERIALS AND METHODS 2.1. Organic Solar Cell Fabrication. A batch of direct organic solar cells (OSC), including cells with bare Al, Ca/Al, and LaB6/Al top electrodes, was prepared via the following procedure. Commercial ITO/ glass substrates were cleaned in successive ultrasonic baths (acetone, ethanol and isopropanol) and subjected to UV-ozone treatment for 15 min in order to increase the hydrophilic nature of the surface and remove any residual organic contamination. A thin film (40 nm) of PEDOT:PSS (Clevios) was then spin-coated onto the substrates and annealed at 110 °C for 60 min under primary vacuum. The substrates were subsequently transferred to a nitrogen-filled glovebox (O2 < 1 ppm, H2O < 1 ppm) where they were spin coated with a PCBM (Solaris):P3HT (Plextronics Plexcore OS2100) active layer solution (1:1 ratio in o-dichlorobenzene, CP3HT = CPCBM = 20 mg mL−1) at 1000 rpm for 50 s. Before completely drying, the samples were placed in a covered Petri dish to undergo solvent annealing for 2 h.18,19 The active layer obtained was a 230 nm thick film. Lanthanum hexaboride (0.5, 1, 5, B

DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (A−E) AFM height and (F−J) phase images on 500 × 500 nm scan. (A and F) P3HT:PCBM bulk-heterojunction, (B and G) covered with 0.5 nm, (C and H) 1 nm, (D and I) 5 nm, and (E and J) 10 nmLaB6 layer. A common scale was used in order to be able to compare the different cases.

3. RESULTS AND DISCUSSION 3.1. Photovoltaic Characteristics of Lanthanum Hexaboride Based OSCs. 3.1.1. Current−Voltage Characterization. Different thicknesses of LaB6 layers were investigated in order to understand the influence of this interlayer on the photovoltaic (PV) performance. Table 1 summarizes the PV characteristics of the different solar cell architectures with LaB6/ Al, Ca/Al, or Al as electron collection electrodes. Photovoltaic devices with calcium−aluminum top electrodes were fabricated using thermal evaporation. They exhibit very good performances immediately after fabrication, with high fill factors (FF, 0.67) and low series resistance (Rs = 11 Ω). The resulting power conversion efficiency (PCE) was measured to be about 3.65%. These good performances were achieved because the active layer morphology was already optimized after the active layer deposition through the use of the solvent annealing technique.18,19 In all the other devices for which the top electrodes were deposited by e-beam evaporation (LaB6/Al and Al), an additional thermal treatment (140 °C - 10 min) was shown to be necessary to have fully functional cells, even though the active layer morphology had already been optimized (Figure S1). We attribute the initially poor performances to inefficient electrical contact between the organic semiconductor and the electrode (See Supporting Information for details). It is of interest to note that the short-circuit current (Jsc) is similar for all the devices. This result was confirmed by external quantum efficiency (EQE) measurements (Figure S2). This eliminates the possibility of an optical spacer effect from the LaB6 interlayer. This was expected as the thickness of the LaB6 interlayer (≤10 nm) is too thin to act as an optical spacer.20 Nevertheless, a thin layer of LaB6 (0.5−1 nm) improves the overall efficiency compared to the reference cell with only an aluminum electrode. The open-circuit voltage (Voc) is improved by 20 mV compared to aluminum samples, and the fill factor (FF) is 0.61, slightly above that of the aluminum samples (0.56). This increase impacts the power conversion efficiency which reaches values of 3.74 and 3.52% for 0.5 and 1 nm of LaB6, respectively (compared to 3.24% for aluminum-based devices). It was observed that further increasing the LaB6 thickness is detrimental for the solar cell device. With 5 nm thick LaB6, the FF

drops to 0.35, and Voc slightly decreases to 0.503 V, leading to a PCE of 1.5%. This very low FF is due to the presence of an Sshaped feature on the I−V curve (Figure 1). The S-shape phenomenon in organic solar cells has been extensively studied, and it is generally due to the presence of a potential barrier or poorly conductive layer at an interface in the device.21−24 The resistivity of LaB6 layer can vary greatly depending on the deposition method and the film thickness,25,26 and Winsztal et al. showed that the resistivity increases dramatically when the LaB6 film’s thickness is less than 50 nm (106 Ω·cm for 25 nm thick polycrystalline LaB6).25 A very low value for the conductivity of the deposited LaB6 layer could be the reason for the appearance of an S-shape feature in the devices with 5 nm thick LaB6 layer. However, interestingly, when the thickness of LaB6 is increased again, up to 10 nm, the S-shape feature disappears, and acceptable efficiencies of 2.84% are obtained. This improvement is also reflected in the Rs, which is lower in devices with 10 nm of LaB6 (148 Ω) than in devices with 5 nm of LaB6 (344 Ω). These results suggest that the conductivity through the LaB6 layer improves when the thickness is increased beyond 5 nm, which is not what one could intuitively expect (vide infra). 3.1.2. Atomic Force Microscopy Images. AFM images (height and phase) of the different LaB6 layers (0.5, 1, 2, 5, and 10 nm) on P3HT:PCBM bulk-heterojunction were taken to understand the formation of the LaB6 layer onto the active layer. These results are presented in Figure 2 with a common scale (see Figure S3 in the Supporting Information for the individual scales). On the phase image, the bare P3HT:PCBM layer shows P3HT-rich and PCBM-rich domains with typical dimensions on the orders of few tens of nanometers (Figure 2F).27 With only few Ångströms of LaB6, the phase image is very different from that of bare P3HT:PCBM. The image (Figure 2G) does not show a lot of contrast indicating a chemically homogeneous layer. This result suggests that, even with only 0.5 nm, the LaB6 has covered most of the surface. No pattern can be distinguished on the AFM images, indicating that the LaB6 layer is mostly amorphous. Increasing the LaB6 thickness further (1 nm) does not significantly change the phase image (Figure 2H). When the surface is fully covered by the LaB6 interlayer, ovoid shaped domains start to appear, as observed on the height and phase C

DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Proposed Growth Mechanism of LaB6 Layers on a P3HT:PCBM Bulk-Heterojunction

Figure 3. (a) Mott−Schottky plots (1/Cp2 = f (V), 1 kHz) and (b) photocurrent (Jph = JL - JD) as a function of the effective voltage (Veff = V0 − V) of the different architectures fabricated: (brown) 0.5 nm LaB6/Al, (green) 1 nm LaB6/Al, (orange) 5 nm LaB6/Al, (purple) 10 nm LaB6/Al, (blue) Al and (pink) Ca/Al.

analyzed to extract the flat-band potential of the different devices (Figure 3a and Table 2).

images (Figure 2C,H). At 5 nm, the LaB6 layer is composed of ovoid grains which fully cover the surface (Figure 2D). The absence of contrast between the grains on the phase images indicates that LaB6 is still covering the entire surface. The AFM images of the 10 nm thick layer of LaB6 are very different from the thinner ones, especially in the phase image. In particular, the image is not homogeneous anymore but exhibits visible cracks. This contrast suggests that the chemical composition is not the same on the surface. These cracks result from strain relaxation,28 and expose the active layer below the layer of LaB6 through the cracks. This phenomenon could explain the sudden recovery of the solar cells’ performances when going from 5 to 10 nm of LaB6. Direct contact of the aluminum electrode on the active layer would explain the lower series resistance (148 Ω) of the 10 nm thick LaB6 device compared to the 5 nm thick one (344 Ω). Scheme 1 shows a proposed formation mechanism for this kind of layer. 3.2. Effect of the LaB6 Interlayer on the Built-in Potential. To understand the impact of the insertion of a LaB6 layer between the organic active layer and aluminum electrode on the internal electric field, we performed capacitance−voltage (C−V) experiments. This technique allows the extraction of the flat-band potential (Vfb) from the Mott− Schottky plot (1/Cp2 = f (V)). Even though the extracted value is generally an underestimation of the actual built-in potential (Vbi), the two values are related, and for an identical active layer, a positive variation of Vfb (determined by C−V measurements) is the result of an increased Vbi.29 Mott−Schottky plots were

Table 2. Flat-Band Potential (Vfb) Extracted from Mott− Schottky Plots for the Different Solar Cell Architectures Studied, Initially and After 600 h of Aging at 85°C without Illumination top electrode

Vfb (V), initial

Vfb (V), after 600 h of aging

0.5 nm LaB6/Al 1 nm LaB6/Al 5 nm LaB6/Al 10 nm LaB6/Al Al 20 nm Ca/Al

0.340 ± 0.010 0.335 ± 0.010 0.350 ± 0.005 0.340 ± 0.001 0.270 ± 0.005 0.380 ± 0.005

0.310 ± 0.020 0.340 ± 0.014 0.310 ± 0.020 0.290 ± 0.005 0.170 ± 0.010 0.220 ± 0.015

The effect of the insertion of the LaB6 layer is clear on the Mott−Schottky plot. If only aluminum is used for electron extraction, Vfb is low, about 270 mV. Inserting even a thin layer of LaB6 raises Vfb up to 340 mV. Increasing the thickness of the interlayer further does not change the Vfb value dramatically, which remains around 340 mV. This improvement of the Vfb indicates the LaB6 deposited onto P3HT:PCBM active layer has a lower work function than aluminum, thereby improving the dissymmetry of the device contacts and the internal electric field. As the Vfb is similar for the different LaB6 thicknesses, one can suggest that the work function of LaB6 does not change significantly with the thickness and that 0.5 nm of LaB6 is sufficient to enhance the internal electric field in the device. This D

DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 4. (a) Evolution of the short-circuit current density, (b) open-circuit voltage, (c) fill-factor, and (d) power conversion efficiency of different kinds of solar cell architectures: (blue) aluminum, (pink) calcium/aluminum, (brown) 0.5 nm LaB6/aluminum, and (green) 1 nm LaB6/aluminum with thermal aging at 85 °C.

Jsat ratio in this study and that the improvement of the built-in potential due to the insertion of LaB6 reduces it, therefore explaining the higher FF.32,33 Thicker LaB6 layers (5−10 nm) do not show this improvement in FF, although they do present higher Vfb values as compared to aluminum samples. This lack of improvement was attributed to the low conductivity of the thick LaB6 layer, which adds an additional, highly resistive component to the device (increasing Rs, see Table 1), impacting dramatically the FF and giving rise to the S-shape. The calcium interlayer improves the built-in voltage in the device to an even greater extent than LaB6. This is reflected in the Vfb increasing by up to 380 mV, the highest value we measured, and the very high value determined for the FF (0.67). However, no clear increase of the Voc was observed in this case. We can explain this lack of improvement with the absence of thermal annealing. As calcium−aluminum samples do not tolerate thermal annealing (Figure S4), they were not submitted to the same thermal post-treatment as the other samples (Al and LaB6/ Al). As a result, the morphology of the active layer might not be exactly identical, explaining why the Voc is not fully comparable with the other thermally annealed devices.

increase impacts the Voc, which was shown to increase by 20 mV, as compared to aluminum reference devices. A similar trend in Voc and Vfb was observed by Guerrero et al. when comparing cathode materials with different work functions (Ca, Al, Ag, Au).30 But more importantly, the improved internal electrical field also plays a role in the improvement of the FF for a thin LaB6 layer (0.5−1 nm). In order to illustrate that, the net photocurrent (Jph = JL − JD) is plotted against the effective voltage (Veff = V0 − V) (Figure 3b), where JL and JD are the current density under illumination and in the dark, respectively, and V0 is the compensation voltage at which Jph = 0 mA cm−2.31 Photocurrents in the saturation regime (Jsat), that is, in strong reverse bias (Veff > 0.5 V), have the same value for all samples, as the devices have similar active layer morphologies and thicknesses. However, for low effective voltage (Veff < 0.2 V), Jph is higher in the case of Ca/ Al and LaB6/Al compared to only the Al top electrode. A higher Jph/Jsat ratio at low effective voltage can either be explained by an increase of the exciton dissociation ratio or a decrease of the bimolecular recombination.31 Because the active layers have the same morphology in every case, it is more likely that the bimolecular recombination at low effective voltage drives the Jph/ E

DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces 3.3. Thermal Aging Studies. The stability of the different solar cell architectures against thermal aging was studied in accordance with the ISOS-D-2 protocol,34 that is, 85 °C without illumination and in the absence of oxygen and moisture. This mild temperature protocol was chosen so that mainly interfacial effects would be observed and not the bulk heterojunction morphology changes, which require much higher temperatures.5 Figure 4 shows the evolution of the different photovoltaic parameters of the high performance architectures studied (Al, Ca/Al, 0.5 nm LaB6/Al and 1 nm LaB6/Al). Calcium/aluminum solar cells show very low resilience to thermal aging, even in absence of oxygen and moisture. After the first 6 h of aging, the fill factor, initially at 67% had already dropped down to 51%. Voc rapidly decreases, going from 0.54 to 0.47 V. As a result, the power conversion efficiency of the devices, initially around 3.65%, ended up at 2.34% after only 6 h of aging at 85 °C. After 600 h of aging, these devices had values of only 1.57% for power conversion efficiency (43% of the initial PCE). The flat-band potential decreased to 220 mV which is a 160 mV drop compared to its initial value (Table 2). This loss in the flatband potential clearly shows that the built-in voltage is not effective anymore. The calcium no longer acts to improve the dissymmetry between the two electrodes, leading to a significant decrease of Voc and FF. This very low durability of the solar cell might be due to the diffusion of calcium into the active layer. Janssen et al. showed that organic light emitting diodes (OLED) with calcium based electrodes rapidly lose their performances upon thermal treatment.35 They attributed this kind of degradation to the diffusion of calcium atoms into the active layer35,36 which would act as carrier traps and recombination centers. This phenomenon could explain the decrease of Voc and Vfb we observed in our degradation study. In contrast, the other kinds of devices present much better stability against thermal aging, especially the LaB6 based devices. Taking a careful look at Figure 4b, it can be seen that Voc remains very close to the initial value of around 0.55 V after 600h of aging (1% drop), while in the case of aluminum based devices it shifts from 0.535 to 0.495 V (a 7.5% drop). The same trend is observed in the flat-band potential Vfb (Table 2). In the case of LaB6 (0.5− 10 nm) based devices, Vfb did not change dramatically upon aging (between a 0 and 15% drop). This result indicates that the builtin voltage created by the high dissymmetry between the two electrodes is still effective after aging. The stability of the opencircuit voltage and flat-band potential is a strong indication that the LaB6 interlayer has not undergone any modification which would affect its work-function. On the other hand, the flat-band potential of aluminum-based devices suffered a drop of 35% of the initial value which is directly reflected in the decrease of the Voc. In this case, the thermal aging seems to have modified the active layer/aluminum interface. Aziz et al. extensively studied the organic active layer/metal interfaces by XPS and reported that photoinduced degradation leads to the reduction of bond density species at the interface between active layer and aluminum which would explain the loss of performance upon aging.37 Even though their aging protocol was different from the one employed in this study (photochemical vs thermal aging), this kind of reaction may also occur in our case and be the cause of the loss of built-in potential and, as a result, of open-circuit voltage. Further investigations of the chemical modification at the interface are being carried out to confirm these hypotheses (XPS analysis). As a result, the higher stability of the LaB6/Al top electrode can be directly observed in the performance of the device. After 600

h, the power conversion efficiency only drops down to 85 and 88% of its initial value for devices with LaB6 interlayer thicknesses of 0.5 and 1 nm respectively, compared with 74% for aluminum and 43% for calcium/aluminum-based electrodes.

4. CONCLUSION Lanthanum hexaboride is a novel material in the polymer solar cell field and has proven to be a good alternative to calcium as a low work function material for the electron injection layer. The optimum thickness of LaB6 was found around 0.5 nm, which showed a good compromise between built-in voltage enhancements and low series resistance. As a result, fill factor and opencircuit voltage were improved, leading to power conversion efficiencies higher than the reference devices with aluminum or calcium/aluminum. Higher LaB6 thicknesses result in increased series resistance due to the lower electrical conductivity of the deposited LaB6 layer. A mechanism for the formation of the LaB6 layer on a P3HT:PCBM active layer has been proposed in which 3D growth occurs when the layer reaches a critical thickness. In terms of thermal stability, the insertion of this interlayer improves the lifetime of the solar cell by stabilizing the Voc. The built-in voltage is fairly stable in the case of LaB6-based devices, while aluminum- and, more importantly, calcium/ aluminum-based devices lose their efficacy in terms of enhancing the internal electric field. In conclusion, we showed in this study that LaB6 is a good alternative to calcium as an electron injection layer in OPV devices, not only for improving the internal field in the device but also because it shows greater resilience to thermal aging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06475. Detailed description of the thermal post-treatment necessary for solar cells fabricated with e-beam evaporated electrodes; external quantum efficiency (EQE); additional AFM images; and the impact of thermal annealing on solar cells with calcium/aluminum top electrode. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the French National Research Agency (Agence Nationale de la Recherche) in the frame of the projects ANR-13-JS09-0014-01 “IN-STEP” and ANR-13-PRGE-0006-01 “HELIOS”. This work has also been supported by a public grant overseen by the French National Research Agency (ANR) as part of the Investissements d’Avenir program (reference: ANR-10-EQPX-28-01/Equipex ELORPrintTec). Finally, authors gratefully thank William Greenbank for his help to improve the language quality of this manuscript.

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REFERENCES

(1) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. High-Efficiency Inverted Dithienogermole-

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DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b06475 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX