Top Electrode Interface Generated by Two

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SEMICONDUCTOR POLYMER/TOP ELECTRODE INTERFACE GENERATED BY TWO DEPOSITION METHODS AND ITS INFLUENCE ON ORGANIC SOLAR CELL PERFORMANCE Enrique Pérez-Gutiérrez, Denisse Barreiro-Argüelles, Jose-Luis Maldonado, Marco Antonio MenesesNava, Oracio Barbosa-Garcia, Gabriel Ramos-Ortiz, Mario Rodriguez, and Canek Fuentes-Hernandez ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08970 • Publication Date (Web): 28 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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ACS Applied Materials & Interfaces

SEMICONDUCTOR POLYMER/TOP ELECTRODE INTERFACE GENERATED BY TWO DEPOSITION METHODS AND ITS INFLUENCE ON ORGANIC SOLAR CELL PERFORMANCE Enrique Pérez–Gutiérrez1,*, Denisse Barreiro–Argüelles1, José–Luis Maldonado1,*, Marco– Antonio Meneses–Nava1, Oracio Barbosa–García1, Gabriel Ramos–Ortíz1, Mario Rodríguez1, Canek Fuentes–Hernández2. *

1

E. mail: [email protected], [email protected]

Research Group of Optical Properties of Materials (GPOM), Centro de Investigaciones en

Óptica A.P. 1-948, CP 37000 León, Guanajuato, México. 2

Center for Organic Photonics and Electronics (COPE), School of Electrical and Computer

Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.

Abstract In this manuscript the effect of two techniques for top–electrode deposition in organic photovoltaics

(OPVs)

cells

with

the

configuration

ITO/PEDOT:PSS/PTB7–

Th:PC71BM/PFN/Top–electrode is analyzed. One deposition was made by evaporation under high vacuum, meanwhile the other was carried out at normal room atmosphere; for the former, a double layer of Ca and the eutectic alloy Field´s metal (FM) was thermally evaporated, while for the latter FM was deposited just by melting and dropping it on top of the delimited active area at temperatures about 90 ºC. The average short–circuit photocurrent density, open circuit voltage and fill factor for devices with either Ca/FM (evaporated) or FM (by dripping) cathode, were very similar: around 13.20 mA/cm2, 840 mV and 0.6, respectively. Average efficiency for devices with the mentioned evaporated cathode was of 6.4 % (largest value 7.0 %), meanwhile for devices with the cathode deposited by dripping, it was of 6.1 % (largest value 6.5 %). Morphological analysis, by atomic force microscopy on the surface of a FM electrode, detached from an OPV device, shows inhomogeneities and pinholes in its surface with an average roughness of 16 nm. OPV photo–current was studied by means of laser beam induced current (LBIC), it showed that OPVs devices with FM top electrode exhibits an inhomogeneous response. An impedance analysis was also carried out and results were correlated with defects observed 1 ACS Paragon Plus Environment


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at the studied interface. In spite of the mentioned deficiencies at FM interface, overall PV performance of devices with this electrode highlights the convenience of using FM due to its easy, fast and low cost deposition (vacuum free) characteristics.

Keywords: Organic solar cells, Field´s metal electrode, Laser beam induced current, Impedance spectroscopy, PTB7–Th–based devices.

1. Introduction Organic photovoltaics (OPVs) are a promising technology for the generation of renewable energy because of their potential for low–cost and continued improvement of their power conversion efficiency.1–6 Development of low–cost novel fabrication methods for electrode deposition, that avoid vacuum systems, while preserving the performance of OPV devices is also an important component in advancing this emerging technology and its economic viability. Some efforts for having top–electrode of easy deposition include metal nanowires, conductive polymers and blends of them,1,7–12 as well as graphene.13,14 However, the device performance by using these electrodes could be diminished by undesired phenomena at the polymer–electrode interface and, for non-metallic back– electrodes the lack of reflectivity could also be another bad issue. With this fact in mind it is important to study other additional alternatives, which could produce cells with acceptable power conversion efficiencies (PCE).

Regarding these mentioned issues, we have recently reported the use of the eutectic alloy Field´s metal (FM) made of bismuth, indium and tin (32.5 %, 51 %, 16.5 %, M.P. = 65 °C) and other similar alloy called Wood´s metal, as top metal electrodes in OPVs cells.15–19 Eutectic alloys are attractive because they provide an easy and fast way for fabricating top– metal electrodes since they melt at low temperatures (below 100 °C), and allow deposition under a regular atmosphere. Moreover, the material and deposition cost is significantly low compared with that for traditional metals and their implementation by high vacuum evaporation procedure. Some examples of the used active films and efficiencies in our previous OPV devices are: i) based on the blend P3HT:PC71BM, PCE was 2.68 %;17 ii) based on P3HT:PC71BM:SPFGraphene, PCE = 2.15 %;20 and iii) based on PTB7:PC71BM 2 ACS Paragon Plus Environment

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and, doping with tiny amounts of PVK and ECZ in order to obtain more device stability, the best achieved efficiency was 4.8 %.18

PCE in an OPV device is proportional to the product of the Voc, Jsc and FF. All these parameters are highly sensitive to properties in the bulk of the organic photoactive layer, such as, the photogeneration efficiency, charge recombination,20–23 space charge effects,24,25 charge carriers mobility and balance between them,26,27 among others, as well as to the properties of the charge–collecting interfaces.1,28 In the context of OPVs with metallic top electrodes, it has been reported that their performance can be highly influenced by the used deposition technique, which is typically thermal evaporation under high vacuum conditions. For instance, D. Gupta et al. 29,30 carried out studies about the effects produced on the OPV performance by defects introduced by variations on the evaporation conditions of the top– electrode, such as evaporation rate and metal selection. The parameters reported for P3HT:PCBM based OPV devices with slow evaporation rate of Al top–electrode where Voc = 0.52 V, Jsc = 4.3 mA/cm2, FF = 0.45 and PCE = 1.25 %. While for high evaporation rates, identical OPV devices displayed Voc = 0.4 V, Jsc = 1.47 mA/cm2, FF = 0.12 and PCE = 0.47 %.30 Therefore, it is important to study those interfaces, their defects and influence on the performance of OPVs. In general, it is known that variations of the conditions for thermal–deposition under vacuum can lead to partial metal coverage (empty sites in between semiconductor polymer layer and top–electrode), diffusion of the metal into the photoactive layer and/or even to influence chemical modifications of the polymer layer.

At this polymer layer and top–electrode interface, not only defects determine the charge carrier extraction and OPV performance, also, for a maximum charge harvesting it is necessary that the Fermi level of the top electrode be aligned with the lowest unoccupied molecular orbital of the acceptor material.31,28 In the last years, several polymers with polar groups have been used for modifying the cathode work function (and Fermi Level) due to the formation of dipoles at the polymer-electrode interface.32–35 As reported by Zhou et al.,34 with the polymer PEIE a low work function electrode can be fabricated regardless the used metal; also PEIE was reported as a modifier for polymer–top electrode interface for high efficient OPVs.36 Another example is the report of He et al.,35 where it was 3 ACS Paragon Plus Environment


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demonstrated a simultaneous enhancement of Voc, Jsc, and FF in OPVs with PFN as the cathode interfacial layer; devices were based on the PTB7 polymer.

In this work, we report a comparative study on the properties of OPV cells using two different deposition techniques. In the first approach and as a reference, a Ca/FM top– electrode was fabricated through the standard deposition technique based on high–vacuum evaporation; the evaporated FM layer was used here only to cover and protect the Ca layer. On the second method a FM top–electrode was fabricated using a melted metal dropping technique. This work aims to study and elucidate the possible parasitic effects arising from the use of a melted metal dropping technique in comparison with the standard vacuum evaporation and to take advantage of this FM alloy in OPV devices. However, it is also true that some eutectic alloy deterioration could take place in real applications because devices must tolerate temperatures up to 65 °C. Other eutectic alloys with melting point about 140 °C, such as that one composed by Bi and Sn (58 % and 42 %, respectively), could be used to avoid any inconvenience with the sun light heating under real conditions; tests of this alloy for the top electrode implementation on OPVs devices are currently taking place in our group. OPV devices in this study, under a direct configuration, are based on a BHJ blend of the high–performance polymer poly[4,8–bis(5–(2–ethylhexyl)thiophen–2– yl)benzo[1,2–b;4,5–b']dithiophene–2,6–diyl–alt–(4–(2–ethylhexyl)–3–fluorothieno[3,4– b]thiophene)–2–carboxylate–2–6–diyl)] commonly known as PTB7–Th,37-39 and the fullerene derivative PC71BM.

2. Experimental Section

2.1. Materials. ITO/glass substrates with 10–15 Ω/square were acquired from Delta Technologies. Polymers PTB7–Th and PFN were purchased from 1–Materials, Inc., the electron acceptor PC71BM was purchased from American Dye Source, Inc. PEDOT:PSS (Clevios PVP AI4083) was obtained from Heraeus–Clevios. All solvents were acquired from Sigma Aldrich and used as received. Ca and Field’s metal were acquired from Aldrich and Rotometals, respectively. 4 ACS Paragon Plus Environment

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2.2. Device fabrication. ITO/glass substrates were cleaned with ethanol in an ultrasonic bath and were rubbed with alcohol wetted cotton, after that, substrates were dried with clean and dry air and kept at 85 °C over 12 h. Before deposition of polymer layers, substrates were treated with UV oxygen plasma for 5 min. PEDOT:PSS was spin–coated onto the ITO and was annealed at 120 °C for 15 min in air. Blend solution of the donor polymer PTB7–Th with PC71BM (1:1.5 wt % in chlorobenzene, 13.50 mg/mL of polymer) was stirred for 12 h and was deposited by spin-coating at 1500 rpm for 30 s in a glovebox, thickness of the active layer was about 110 nm. Samples were then annealed at 85 °C in N2 atmosphere during 15 minutes. The electron transport layer was spin–coated at 5000 rpm in air from a solution of 2 mg of the polymer PFN in 7 ml of methanol with 14 µl of acetic acid. For Ca/Field´s metal cathode, subsequent evaporations of Ca (50 nm) and Field´s metal (150 nm) were carried out at 2 × 10-6 torr (active area 0.09 cm2), in this case the FM serves as covering layer and Ca works as electrode due to its thickness.

When only Field´s metal was used as cathode, small pellets were placed on a metal container, set on a hot plate and heated about 90 ºC in air. The melted material was deposited by dripping onto the upper organic layer and the amount of the metallic alloy determines the final thickness of the FM layer (typically in the range of 250 µm–1 mm). The active area (0.07 cm2) was delimited by a tape mask. Along the deposition process device was heated at the same temperature to avoid freezing, all this process was done in air atmosphere. The work function of FM is 4.2 eV, it was calculated by theoretical methods15 and also measured by Kelvin probe; this value is quite similar to that of Al or Ag.

2.3. Characterization and Measurements. For device characterization a solar simulator Sciencetech SS150 was used, light intensity was calibrated to 100 mW/cm2 using an Oriel reference cell. The I–V curves were recorded with a source-meter Keithley 2450 under N2 atmosphere. Morphology and thickness were analyzed by means of atomic force microscopy (AFM) with a microscope easyscan2 from 5 ACS Paragon Plus Environment


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Nanosurf, with a maximum square scanning area of 110 µm, operating in contact mode under ambient conditions. Deposition by dropping enables us to remove a deposited drop and analyzes the interface of electrode after testing the cell; for that, a drop was removed from a characterized cell and the surface was washed with chloroform and dried with clean air. It was analyzed by AFM.

For EQE measurements a xenon lamp from Oriel model 66902, was used as excitation source, a monochromator from Princeton Instruments Acton series SP2500 was used to choose the wavelength and a power meter from Thorlabs PM100 was used to measure the incident power. LBIC analysis,40–42 was carried out with a laser diode of 470 nm as excitation source with a power of 0.5 mW, the spot size was 7 µm and the response was acquired with an acquisition data card handled with Matlab software. Impedance measurements were performed using a potentiostat PARSTAT 2273 with a frequency analyzer module.

3. Results and discussion

Fig. 1 shows a comparison of the J–V curves for the best OPV devices with Ca/FM and FM top–metal electrodes. Table 1 summarizes average values of photovoltaic parameters measured on 16 devices of each electrode. As shown in Table 1, OPV devices with Ca/FM or FM top–metal electrodes display nearly identical performance with only a slight variance in the fill factor values. Globally, the PCE values in these OPVs (under direct configuration) as well as the best efficiency value for the studied devices of near 7 %, are comparable with OPVs based on PTB7–derivatives reported in the literature, in the range 5–9 %.37

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Fig. 1. Representative J–V curve for the best devices based on PTB7–Th:PC71BM with Ca/Field´s metal and Field´s metal eutectic alloy (Bi 32.5 %, In 51 %, Sn 16.5 %) as cathode. The inset is a picture of four cells in a single substrate with the cathode deposited by dropping (scale is in cm).

Table 1. Parameters for OPVs cells based on PTB7–Th:PC71BM active layer with Ca/Field´s metal and Field´s metal as cathode. Average values come from 16 devices along with standard deviations. Cathode Field´s metal

Voc (mV) 804 ± 6

Jsc (mA/cm2) 13.1 ± 0.6

FF

η

η

0.57 ± 0.02

(%) 6.1 ± 0.4

(best) 6.5

803 ± 6

13.2 ± 0.3

0.60 ± 0.01

6.4 ± 0.2

7.0

(by dripping) Ca/Field´s metal (evaporation)

It is worth noting that in our approach the metallic top–electrode is deposited in air using a manual procedure at low–temperature that yields a mirror–like surface to the naked eye (see photograph in the inset of Fig. 1). As alternative top–electrode, FM achieved good performance compared with other reported materials. 7–14 For instance, Nickel, F. et al.,11 reported the use of AgNWs as top electrode for flexible OPVs devices based on the blend 7 ACS Paragon Plus Environment


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PTB7:PC71BM, the achieved efficiency was of 4.8 %. AgNWs have also been reported for OPVs with the active layer P3HT:PCBM with efficiencies ranging 2.0–2.6 %.7–9 When the highly conductive polymer PEDOT:PSS or graphene were used as top–electrodes, efficiencies about 3 % were reported for OPV devices based on P3HT:PC71BM.10,13 Other recent work, for semi–transparent and flexible OPVs cells based on PTB7:PC71BM and using graphene for both electrodes, reached an efficiency of 3.4 %.14

The high performance of OPVs devices using FM electrode suggests that potential defects in morphology and contact at the interface between PFN and the top–metal do not play, necessarily, a crucial role in the device performance. A closer examination of FM surface on OPVs cells, after testing, was enabled by the possibility of detaching the dropping FM top–electrode due to its thickness, >2 mm. Although the detaching procedure usually resulted in damage of the active layer, due to a good adhesion between metal and organic layer, a rinse with chloroform removes the polymer residues without touching the surface and allows analyze the morphology of the FM electrode. Figure 2 shows the FM surface morphology captured by AFM. In the analyzed area, of 50 × 50 µm, an average rms surface roughness of 16 nm was measured, however, it is clear that the electrode displays an irregular morphology with some flat and homogeneous areas but also some pinholes randomly distributed at the metal surface; in some cases these pinholes can be as deep as 100 nm. The possible origin of these inhomogeneities comes from the fact that, when the melted metal drop is cooled down and passes from liquid to solid state, a reordering of its structure generates dislocations on the metal surface. In fact these dislocations generate tiny fissures on the top surface of the electrode, these fissures could occur also in the surface in contact with the polymer layers, however, they are smaller due to the adhesion forces in between the cathode and the organic material. In contrast with previous reports,29,30 where empty or uncovered areas between the top–electrode and polymer were correlated with a concave shape for J–V curves, thinking on these areas as traps and recombination zones, here it is clearly shown that the presence of surface defects at the nanometer scale does not immediately lead to poor J–V characteristics or device performance. As can be observed in the AFM image (Fig. 2), aside from defects on the FM surface, there are some flat and homogeneous areas that are as large as tens of a few micrometers. We believe that all these 8 ACS Paragon Plus Environment

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areas are responsible for collecting the photo–generated charge in a similar way that sharp tips could easily attract electrical charges and thus, enable the good performance of devices with FM top–electrode.

Fig. 2. Morphology and line topography of a detached drop, from and OPV cell, of Field´s metal used as top electrode.

In addition to AFM studies, the photocurrent response over the active area of our devices was analyzed by means of laser beam induced current (LBIC). This technique is widely used to analyze defects and grain boundaries for polycrystalline inorganic solar cells.40–42 Fig. 3 shows representative LBIC photocurrent maps for OPVs devices with Ca/FM (Fig. 3a) and dropped FM (Fig. 3b) as top–electrode, the resolution of these maps is about 10 µm, which is determined by the laser spot. In these maps, a higher photocurrent is depicted by the red color while the blue color represents no photocurrent response. In all cases OPVs cells with Ca/FM top–electrode display more uniform LBIC photocurrent maps across the active area of the device. In contrast, OPVs devices with FM top–electrodes display inhomogeneous regions within the active area. For instance, in the representative example shown in Fig. 3b, the middle part of the active area, depicted in yellow, produces significantly lower photocurrent values than in the rest of the contact area and green areas generate little to no current. Similar behavior was observed for other organic cells with FM top–electrodes analyzed in this work. LBIC photocurrent maps suggest that the slightly 9 ACS Paragon Plus Environment


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smaller FF values observed in OPVs with the dropped FM top–electrodes, could arise from an increased overall resistance attributed to surface defects at the microscopic level leading to an increased charge recombination and trapping issues.

This effect can be compared with some photocurrent maps reported for polycrystalline inorganic solar cells,43,44 where the grain boundaries have been shown to lead to inhomogeneous contact with the cathode and reduced photocurrent values due to recombination or charge accumulation. However, it must be emphasized that nanoscale defects observed by AFM or the inhomogeneous contact exhibited by LBIC on OPVs with FM top–electrodes, according to data in Table 1, do not lead to a drastically different charge carrier extraction properties or FF values than those displayed by OPVs devices with Ca/FM top–electrodes.

Previously, a similar behavior was also observed by LBIC when the degradation of OPVs cells was studied.45,46 For instance in the report of Lloyd et al.,45 regarding degradation of inverted P3HT–PCBM based OPVs devices, the photocurrent map showed, before aging, small defects and these devices exhibited a good performance (Jsc = 7.8 mA/cm2, FF = 0.57, PCE = 2.6%) with homogeneous photo–current response. However, after the prolonged exposure of devices to illumination (700 h), the photocurrent map presented bigger defects at the microscale with a differentiated response and a decrease of Jsc and FF meanwhile Voc remained almost constant. In fact the J–V curve after degradation ends with an S–shape due to the provoked defects.


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Fig. 3. LBIC images for OPVs devices based on PTB7–Th:PC71BM fabricated with (a) evaporated Ca/Field´s metal (active area = 0.09 cm2) and (b) drop cast Field´s metal (active area = 0.07 cm2) cathode.

To have a quantitative resistances values in both types of the studied devices, the series (Rs) and shunt (Rsh) resistance where extracted in the dark from the semilogarithmic dark J–V curve (Fig. 4). These values were determined by using the experimental data to adjust an equivalent one–diode circuit model; this model can be used in order to extract a first estimation of the device parameters.47–50 The average calculated values for Rs and Rsh were 14 Ω cm2 and 2.3×106 Ω cm2, respectively for devices with Ca/FM and, of 16 Ω cm2 and 4.6×105 Ω cm2 for devices with FM. Small difference in the average value for Rs is in agreement with the observed performance of devices, more specifically with the shape of the representative J–V curve (Fig. 1), in which the slope for J–V characteristics above 0.6 V denotes higher Rs for devices with the drop cast Field´s metal, and thus, a slightly lower value for FF and ƞ. A similar value for Rs has been reported for the same donor polymer; K. Tada,51 report about 25 Ω cm2 for the active film of PTB7–Th:C70 and 140 Ω cm2 for Rsh, the reached efficiency was ~ 5 %. Also, similar Rsh values, to those mentioned in this work, are reported by Z. He et al.,52 for the blend PTB7:PC71BM. It should be noted that since OPVs with different top–electrodes display very similar device performance, it is reasonable to assume that the large difference in work function values between an evaporated Ca/FM (ca. 2.9 eV) and the dripping FM (ca. 4.2 eV by Kelvin probe) electrode does not originate large differences in Rs. This feature is in part because of the reported effect of PFN, which creates an interfacial electrical dipole due to the strong interaction and alignment of its polar amino group with the active layer and, leads to a vacuum level shift at the interface with the active film.35



0.1 2

Current density (A/cm )

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Ca/Field´s metal Field´s metal

0.01 1E-3 1E-4 1E-5 1E-6 1E-7 -1.0

-0.5

0.0

0.5

1.0

1.5

Bias (V)

Fig. 4. Semi logarithmic plot of the dark current density–voltage behavior for OPVs devices based on PTB7–Th:PC71BM with Ca/Field´s metal or drop cast Field´s metal as cathode.

On the other hand, electrochemical impedance spectroscopy (EIS) was used to analyze the behavior of our devices in the frequency domain. It is well known that the usual performance of an OPV cell can be modeled as an RC circuit that results in a semicircle, with a radius that depends on the illumination and bias conditions when the imaginary part of the impedance (Z´´) is plotted against the real part (Z´).53–55 At lower frequencies the impedance plot is a large semicircle and its size is related to the so called recombination resistance Rrec (derived of carriers recombination).56 The theory described by Bisquert about impedance spectroscopy,53 correlates this Rrec with two kinds of boundary conditions: a reflecting or absorbing one, the bigger the Rrec the bigger the reflecting condition and lower absorption. Bisquert defines the reflecting boundary like the charge accumulation at the cathode for dye sensitized solar cells or OPVs and the absorbing boundary condition like a good charge carrier extraction in such devices. On the contrary, at higher frequencies, and bias voltages approaching to Voc, Rrec decreases, it is because under high bias the carrier density inside the device increases significantly and the absorption condition at the boundary predominates.


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Figure 5(a) shows the impedance response at a bias voltage of 0 V under illumination for devices with Ca/FM and FM. Under this condition the recombination resistance determines the diameter (impedance) of the semicircle and differences between devices become more evident; the impedance for devices with FM is almost twice than that for Ca/FM. Higher recombination resistance can be related with defects at the PFN/top–electrode interface where recombination processes can occur and as we have shown, the density of these defects is bigger in OPVs having the drop cast FM top electrodes.

80 Field´s metal Ca/Field´s metal

4

Ca/Field´s metal Field´s metal

(a)

(b)

60

Z" (Ω)

Z" (kΩ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

40

20

0

0 0

3

6

50

100

150

Z´ (Ω)

Z´ (kΩ)

Fig. 5. Impedance response for devices based on PTB7–Th:PC71BM with Ca/Field´s metal and drop cast Field´s metal as cathodes (a) at V= 0 and under illumination, (b) biased near to Voc. Figure 5(b) shows the impedance response with an applied voltage near to Voc under illumination, as expected, the applied voltage decreases the impedance. Once again the impedance is higher for the OPV device with FM but in this case the difference is not so large with respect to that one in Fig. 5(a). This feature means a similar boundary condition under bias near to Voc where maximum charge extraction is achieved. The effect of the cathode on impedance can be compared with the work of Qi et al.,57 they analyzed the impedance by EIS for OPVs devices based on PTB7 polymer with and without electron transport layer (ETL) based on modified C60 fullerene. Devices without ETL reported by Qi and coworkers exhibited lower impedance but its photocurrent density 13 ACS Paragon Plus Environment


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and fill factor were lower (Voc = 519 mV, Jsc = 14.62 mA/cm2, FF = 0.57, PCE = 4.3%) compared with those with ETL (Voc = 740 mV, Jsc = 15.79 mA/cm2, FF = 0.71, PCE = 8.3%). EIS analysis showed an increment in the devices impedance by the insertion of the ETL; however, the PV performance was improved too. When they measured impedance with an applied voltage near to Voc, for both kinds of devices, impedances decreased and values were almost identical between them, which shows that higher impedance does not correlate directly with lower power conversion efficiency. In our case, the larger impedance for the drop cast FM electrode, measured at 0 V, could be related with the morphological defects observed by AFM analysis on the surface of this electrode and the inhomogeneous photo–current response showed by LBIC. Interfaces defects may induce parasitic effects that increase the impedance and could affect the overall PV performance. On the contrary, for the measured impedance with an applied voltage near to Voc, the bias helps to increase the charge extraction in spite of defects and thus the impedance decreases.

Fig. 6. External quantum efficiency measured on PTB7–Th:PC71BM–based devices with Ca/Field´s metal and drop–cast Field´s metal cathodes.

Figure 6 shows a comparison of the external quantum efficiency (EQE) for devices with both types of top–electrodes. The spectral shape and EQE values are comparable with other reports for similar materials in the active layer.58,59 It can be seen that devices with a FM top–electrode exhibit slightly higher EQE than those with a Ca/FM top–electrode. Since both types of devices share the same architecture except for the cathode, the smaller EQE values displayed by devices with a Ca/FM cathode is attributed to absorption losses in the Ca layer,60,61 which would cause the Ca/FM cathode to reflect less light than the FM cathode. This is consistent with observations in the literature wherein a direct comparison 14 ACS Paragon Plus Environment

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of the performance of devices with Ca, Ag or Al top–electrodes yield consistently higher Voc and FF values on device with Ca cathodes but lower Jsc values than in devices with Ag and Al cathodes.36,62 Indeed behavior observed in Fig. 1, is consistent with these observations. This result confirms that the FM is a suitable material to be used as top– electrode and that it leads to devices with a performance that is similar to other metal electrodes.

4. Conclusions Ca/Field´s metal (deposited by thermal evaporation) and Field´s metal (deposited by dripping method) were used as top–electrodes in efficient OPVs cells. The analysis of the semiconductor polymer layer–Field´s metal interface through AFM showed a high density of nanoscale defects, also, LBIC showed a non homogeneous contact at the microscale. Despite of these issues, devices with Field´s metal cathode showed just a slightly lower average performance, mainly ascribable to changes in FF. Analysis of the experimental data showed slightly changes in resistance and impedance for devices with Field´s metal, which can explain the slight change in FF. However, in spite of the lack of homogeneity for the semiconductor polymer layer–Field´s metal interface, the good values for electrical parameters reached for devices with this electrode, easy, cheap and of fast implementation, suggest that the quality of this interface is not so crucial for the overall PV cells performance.

Acknowledgments. This work was supported by CONACYT-SENER (Mexico) Project 153094 and CeMie–Sol 207450/27 (Mexico) call 2013–02, Fondo Sectorial CONACYT– SENER–SUSTENTABILIDAD ENERGETICA. Authors also thank Martín Olmos for his technical assistance. Also we thank to Olivia Amargos for her help with EQE measurements.

Conflicts of Interest: The authors declare no conflicts of interest.



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Fig. 1. Representative J–V curve for the best devices based on PTB7–Th:PC71BM with Ca/Field´s metal and Field´s metal eutectic alloy (Bi 32.5 %, In 51 %, Sn 16.5 %) as cathode. The inset is a picture of four cells in a single substrate with the cathode deposited by dropping (scale is in cm). 204x168mm (96 x 96 DPI)

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Fig. 2. Morphology and line topography of a detached drop, from and OPV cell, of Field´s metal used as top electrode. 86x139mm (96 x 96 DPI)


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Fig. 3. LBIC images for OPVs devices based on PTB7–Th:PC71BM fabricated with (a) evaporated Ca/Field´s metal (active area = 0.09 cm2) and (b) drop cast Field´s metal (active area = 0.07 cm2) cathode. 70x36mm (96 x 96 DPI)



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Fig. 4. Semi logarithmic plot of the dark current density–voltage behavior for OPVs devices based on PTB7– Th:PC71BM with Ca/Field´s metal or drop cast Field´s metal as cathode. 69x54mm (300 x 300 DPI)


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Fig. 5. Impedance response for devices based on PTB7–Th:PC71BM with Ca/Field´s metal and drop cast Field´s metal as cathodes (a) at V= 0 and under illumination, (b) biased near to Voc. 734x329mm (96 x 96 DPI)



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Fig. 6. External quantum efficiency measured on PTB7–Th:PC71BM–based devices with Ca/Field´s metal and drop–cast Field´s metal cathodes. 69x53mm (300 x 300 DPI)


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Abstract Graphic 130x150mm (150 x 150 DPI)


Top Electrode Interface Generated by Two

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