Degradation study under air environment of inverted polymer solar cells using polyfluorene and halide salt as electron transport layers

Solar Energy 198 (2020) 419–426

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Degradation study under air environment of inverted polymer solar cells using polyfluorene and halide salt as electron transport layers

T

⁎

A. Sacramentoa, V.S. Balderramab, , M. Ramírez-Comoa, L.F. Marsalc, M. Estradaa a

Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV-IPN), Av. Instituto Politécnico Nacional 2508, 07360 Ciudad de México, Mexico b CONACYT- Center for Engineering and Industrial Development (CIDESI), Microtechnology Division (DMT), Av. Playa Pie de la Cuesta 702, Desarrollo San Pablo, 76125 Santiago de Querétaro, Querétaro, Mexico c Departament d’Enginyeria Electrònica Elèctrica i Automàtica Universitat Rovira i Virgili, Av. Països Catalans 26, 43007 Tarragona, Spain

A R T I C LE I N FO

A B S T R A C T

Keywords: Air environment Degradation ISOS-D1 protocols PTB7-Th:PC70BM blend Three-diode model

Analysis of stability and degradation of inverted organic solar cells (iOSCs) formed by the blend PTB7Th:PC70BM, with different stacks of electron transport layers (ETLs) PFN, PFN/LiF, LiF/PFN, LiF and without one are manufactured under N2 environment. The degradation study of the iOSCs is done following ISOS-D1 protocols under air environment, up to 7110 h. Performance parameter obtained from J-V curves such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and power conversion efficiency (PCE) are analyzed for each group of iOSCs with the above mentioned different stacks of ETLs and compared between of them. The cell manufactured with the stack PFN/LiF shows the highest stability over time and its TS80 is reached to 201 h. Cells with the stack LiF/PFN and PFN show their TS80 to 153 h and 144 h, respectively. The S-shape is presented on all J-V curves under study and are modeled using an equivalent circuit of three-diodes, in order to understand the physical mechanism related to it.

1. Introduction During the last few years, organic solar cells (OSCs) with polymer:fullerene blend as the active layer, have been intensively studied due to their improved performance parameters with efficiencies over 12% (Green et al., 2018). These devices can be fabricated on large areas, as well as on rigid and flexible substrates, low-temperature fabrication processes and low-cost. These are some of the advantages presented by OSCs compared to inorganic solar cells (Li et al., 2012; Dennler et al., 2009). However, this technology presents some disadvantages that limit the lifetime operation. If the cells are exposed to light, water and/or oxygen, the degradation process of their electrical characteristics starts (Balderrama et al., 2015). Therefore, this problem needs to be resolved to improve the electrical stability performance of the cells. In recent years, many of the investigations focused on OSCs have been dedicated to improve the power conversion efficiency (PCE) as the main electrical parameter (Arca et al., 2014; Krebs et al., 2014). This improved efficiency is attributed to the lower contact resistance between the cathode electrode and the active layer because of the decrease charge carrier extraction barrier to increase photocurrent

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extraction from the active layer by inserting an electron transport layer (ETL). This interfacial layer allows photons to pass through it with minimum absorption, providing efficient charge transport from the active layer to the corresponding electrode with minimum loss. It also helps to prevent the diffusion of oxygen or humidity into the active layer, thereby improving the lifetime of the unencapsulated devices (Yanagidate et al., 2014). One way to improve the stability and PCE in OSCs is selecting an efficient anode and/or cathode buffer layer, which is crucial (El Jouad et al., 2016; Xie et al., 2015). Materials such as vanadium oxide (V2O5) (Sánchez et al., 2017; Shen et al., 2012; Shen et al., 2011), nickel oxide (NiO) (Jiang et al., 2015; Kiriishi et al., 2015; Yang et al., 2017; Chen et al., 2011), tungsten oxide (WO3) (Brütsch et al., 2017; Guillain et al., 2014; Ji et al 2017) and molybdenum oxide (MoO3) (Shen et al., 2012; He et al., 2012; Xu et al., 2017; Zhang et al., 2014) are used as hole transport layer (HTL). While, poly [9,9-bis (3-(N, N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) (Sánchez et al., 2017; He et al., 2012; Zhang et al., 2014), titanium oxide (TiOx) (Sánchez et al., 2017; Yin et al., 2015) and zinc oxide (ZnO) (Chalal et al., 2016; Sánchez et al., 2018; Wu et al., 2017; Yu et al., 2017; Zhang et al., 2017, Li et al., 2018) are used as ETL.

Corresponding author. E-mail addresses: [email protected], [email protected] (V.S. Balderrama).

https://doi.org/10.1016/j.solener.2020.01.071 Received 9 December 2019; Received in revised form 18 December 2019; Accepted 26 January 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Meanwhile, thin halide salts layers (lithium bis(trifluoromethylsulfonyl)-amide (Li[CF3SO2]2N), lithium fluoride (LiF), and cesium carbonate (Cs2CO3)) have been proposed to be used as a buffer interlayer for OSCs to improve device performance and extend its long-term stability (Ahlswede et al., 2007; Balderrama et al., 2018). In this work, we continue with the study and analysis of OSCs previously reported by (Balderrama et al., 2018; Lastra et al. 2018, Lastra et al., 2019) in which is investigated the effect of ETLs on OSCs using a halide salt in combination with organic materials as the buffer layer. It is also demonstrated the good agreement with the experimental measurements by simulating the effect of different buffer cathode materials on the ITO surface on the performance parameters of the inverted PTB7-Th:PC70BM OSCs employing the metal-insulator-metal (MIM) model using the device simulator Silvaco/ATLAS technology computer-aided design (TCAD) to fit and extract the electrical parameters from the J–V curves and to simulate the degradation of OSCs. Here, we analyze the electrical parameters of different inverted organic solar cells (iOSCs) groups, which were stored for 1700 h (after fabrication) in darkness and N2 environment at 23 °C to stabilize their performance parameters. Afterwards, those one’s were again measured the J-V characteristics for the first time after stabilization, corresponding to the initial measurement (new reference time, 0 h). Then, all groups of devices are exposed under air environment for 7110 h, following the ISOS-D1 protocols. The active layer of iOSCs was based on a mix of low-bandgap polymer combined with fullerene material Poly [4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b́ ] dithiophene2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th) and [6,6]-phenyl-C71-butyric acid methyl (PC70BM), respectively. The influence of five stacks as ETLs and cathode contact was analyzed during the process degradation on the iOSCs, which were PFN, PFN/LiF, LiF/PFN, without ETL and LiF. The stacks were deposited between ITO and the active layer or known as bulk heterojunction (BHJ). Finally, the anode contact consisted of 5 nm of V2O5, and 100 nm of silver (Ag). It is also demonstrated that the presented S-shape effect on the J-V curves of studied devices can be modeled using an equivalent circuit of three-diodes to understand the physical mechanism related to it.

Fig. 1. Illuminated J-V characteristics of the five groups of inverted photovoltaic PTB7-Th:PC70BM solar cells corresponding to the initial measurement of the degradation process study under 1 sun (100 mW∙cm−2) equivalent illumination intensity corresponding to AM 1.5G spectrum. The inset shows the equivalent circuit used to model the J-V curves.

silicon photodiode, was applied to all the iOSCs under illumination. The devices were stored in dark under air environment after each measurement. The measurement took about 30 s. The degradation testing conditions were in accordance with ISOSD1 protocols (Reese et al., 2011).

3. Results and discussion Fig. 1 shows the J-V curves for all groups of iOSCs under illumination corresponding to the initial measurement (0 h of the degradation process study). Afterwards, the devices were left under ambient conditions, without encapsulation to continue the degradation study under ambient conditions. The total time for the degradation study for group I, group II and group III were of 7110 h, while that for group IV and group V was 1016 h. As we can see in Fig. 1, the S-shape is present in J-V curves for groups I and II. This is commonly associated to different aspects such as the presence of injection and extraction barriers at the contacts, creation of traps, formation of strong interfacial dipoles, and/or formation of space charge limited current (SCLC), as was reported elsewhere (Sims et al., 2014; Finck and Schwartz, 2013; Lechêne et al., 2014; Sundqvist et al., 2016; Romero et al., 2015). On the other hand, the S-shape curve cannot be reproduced with the standard one-diode model. For this reason, the equivalent circuit model of three diodes was used to fit the experimental curves under darkness and illuminated conditions (García-Sánchez et al., 2013; Gusain et al., 2016, Yu, et al., 2019; García-Sánchez et al., 2017). The circuital model is shown on the inset from Fig. 1. Diode D1 represents the PTB7-Th:PC70BM active layer, RS and RSH are series and shunt resistances associated with diode D1. Diodes D2 and D3 represent the interfaces between buffer layers and metallic contacts. The current flowing through the series-connected subcircuits (Fig. 1 inset) is given by either one of the following two equations (GarcíaSánchez et al., 2013; Gusain et al., 2016; García-Sánchez et al., 2017; Romero et al., 2014):

2. Experimental details 2.1. Device fabrication The iOSCs were fabricated and measured their J-V characteristics as were reported elsewhere (Balderrama et al., 2018; Lastra et al. 2018). Five groups of iOSCs were formed to study the behavior of their parameters over time, which were: Group Group Group Group Group

I – ITO/PFN/PTB7-Th:PC70BM/V2O5/Ag II – ITO/PFN/LiF/PTB7-Th:PC70BM/V2O5/Ag III – ITO/LiF/PFN/PTB7:Th:PC70BM/V2O5/Ag IV – ITO/PTB7-Th:PC70BM/V2O5/Ag V – ITO/LiF/PTB7-Th:PC70BM/V2O5/Ag

Two devices were taken from each group to follow the degradation study. All data shown in this work correspond to the device that showed the greatest stability in the variation of its electrical parameters during the degradation study, for each group. 2.2. Measurements and degradation testing conditions All current density-voltage (J-V) characteristic of the iOSCs under light and dark were measured at room temperature (23 °C with an average relative humidity (RH) of 54%) using a Keithley 2450 sourcemeasurement-unit. The samples were measured using a solar simulator under the AM 1.5G spectrum. A light intensity of 100 mW cm−2 (1 sun), calibrated with a Solarex Corporation certified monocrystalline

V1 ⎞ V I = I01 ⎡exp ⎛ − 1⎤ + 1 − IL ⎥ ⎢ n V R ∗ th SH 1 ⎠ ⎝ ⎦ ⎣ ⎜

420

⎟

(1)

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Table 1 Time constants and weighing data. Group

A1

T1 (h)

A2

T2 (h)

I II III IV V

0.45 0.38 0.42 0.40 0.39

220 278 1700 140 135

0.55 0.62 0.58 0.60 0.61

6000 7000 8500 300 150

follows an exponential law decay. The fast initial decay followed by a slow decay is modeled by the superposition of two exponential functions with different time constants as was previously done by (Balderrama et al. 2014; Yang et al., 2010).

PCE (t ) −t −t ⎞ = A1 ∗ exp ⎛ ⎞ + A2 ∗ exp ⎛ PCE (0) ⎝ T1 ⎠ ⎝ T2 ⎠

where PCE (0) is the relative initial power conversion efficiency (t = 0 h). (T1, T2) and (A1, A2) are the time constant of degradation and the degradation power factors, respectively. Groups I and II have a similar behavior regarding PCE curve. Groups IV and V have a more rapid decay than groups I, II and III. Table 1 shows the summary of values extracted from Eq. (4) (i.e. T1, A1 and T2, A2) for groups I, II, III, IV and V of iOSCs. The degradation mechanisms related to the first and the second time constants T1 and T2 are due to water and oxygen, respectively (Balderrama et al., 2014; Yang et al., 2010). Table 1 shows the highest T1 and T2 obtained for group II, 278 h and 7000 h, respectively. Following the same order of ideas, group I was to be of T1 = 220 h and T2 = 6000 h, group IV, T1 = 140 h and T2 = 300 h, and group V, T1 = 135 h and T2 = 150 h. In this sense, the addition of a LiF layer on top of the PFN layer (group II) makes more stable the parameters of the photovoltaic cells. The little increase observed in PCE for group I, II, III and IV in the first few hours of operation can be explained by the increment of Jsc, Voc and FF. This enhancement in Jsc can be due to the increment of the polymer conductivity generated by the doping concentration of the polymeric material reached after air exposition of the cells. Therefore, absorption of H2O and O2 in the active layer combined with photoinduced chemical reactions during the measurements are some of the mechanisms for the chemical transformation of the molecules in the active layer. Some Scientific reports have shown this effect on the polymeric materials (Mejia et al., 2007; Balderrama et al., 2016). The slightly increasing Voc is occasioned by a lower Langevin recombination rate due to decreased mobilities. The higher charge carrier densities and their more balanced distribution reduce the internal electric field and therefore increase the open circuit voltage (Balderrama et al., 2016; Schafferhans et al., 2010). It is well known that FF can be strongly affected by the series resistance (Rs). The Rs is related to the contact resistance between the cathode and the active layer, a good electrical contact leads to low Rs (Ramírez-Como et al., 2019). In this sense, large Rs tend to reduce the FF. The increment in FF can be explained by the slightly reduction in Rs in the first few hours of operation as can be seen in Table 3. Table 2 shows the initial PCE at 0 h for groups I, II, III, IV and V, which were 7.2%, 8.1%, 3.9%, 2.1% and 2.7%, respectively. Table 2 showed also the time decay to TS80, TS50, TS30 and TS10 (i.e. time when the PCE fall 20%, 50%, 70% and 90% from its initial value, respectively) for all iOSCs groups, defined by ISOS protocol (Reese et al., 2011). The highest TS80 was presented by group II with the stack PFN/ LiF corresponding to 201 h, followed by cell groups I and III with 153 h and 144 h, respectively. The groups IV and V their TS80 were less than that 100 h. The highest TS50, of 5357 h, was presented by the stack LiF/PFN. While that for the stack PFN/LiF and PFN their TS50 were to be of 1157 h and 864 h, respectively. For the rest of groups TS50 was less than 200 h. The TS30 for group III was the highest to be of 7110 h. Groups I

Fig. 2. Power conversion efficiency (PCE) over time for groups I, II, III, IV and V. All the devices were measured under AM 1.5G spectrum condition (100 mW cm−2).

−V2 ⎞ ⎡ ⎤ ⎤ ⎡ ⎛ V2 ⎞ I = −I02 ⎢exp ⎛⎜ ⎟ − 1 ⎥ + I03 ⎢exp ⎜ n3 ∗V ⎟ − 1⎥ n 2 ∗ V th ⎠ th ⎠ ⎝ ⎝ ⎣ ⎦ ⎦ ⎣

(2)

where I01, I02 and I03are the reverse saturation current of the diodes; Vth = kB T / q is the thermal voltage; n1, n2 and n3 are the ideality factor of the diodes; IL is the photogenerated current; kB is the Boltzmann constant; T is the temperature and q is electron charge. IL was determined by the expression (Phang et al., 1984):

−Voc ⎞ ⎡ R V I ∙R ∗ exp ⎛ sc s ⎞ − 1⎤ IL = Isc ⎛1 + S ⎞ + ⎛Isc − oc ⎞ ∗ exp ⎛ ⎢ ⎥ R R n 1 V SH SH th ⎠ ⎣ ⎝ n1Vth ⎠ ⎝ ⎝ ⎠ ⎝ ⎠ ⎦ ⎜

⎟

⎜

⎟

⎜

⎟

⎜

(4)

⎟

(3) Fig. 2 shows the PCE of all iOSCs groups as function of time. As we can see, the PCE of the cells, presents a rapid decay during the first few hundred hours of operation, which is known as “burn-in” (Upama et al., 2017; Peters et al., 2012), and is due to the photochemical reaction between the different layers of the cell (Balderrama et al., 2014). As shown in Fig. 3, the degradation of the groups I, II, III, IV and V

Fig. 3. Fit of the normalized PCE over time using Eq. (4) for the groups I, II, IV and V. The filled symbols are the experimental data and the lines are the fitting curves. 421

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Table 2 Summary Lifetime Data. Group

ηs (%)

TS80 (h)

TS50 (h)

TS30 (h)

TS10 (h)

I II III IV V

7.2 8.1 3.9 2.1 2.7

144 201 153 79 32

864 1157 5357 153 75

3783 6641 > 7110 223 153

> 7110 > 7110 > 7110 823 656

ηs is the power conversion efficiency after stabilization where all lifetime data were referenced.

and II presented values of 3783 h and 6641 h, respectively. Finally, TS10 values for groups I, II and III were higher than 7110 h, while for groups IV and V were less than that 823 h. In general, the degradation of PCE was less for the groups II and III than that group I. The combination of PFN with LiF material makes more stable the photovoltaic cells, but depends of its position. The performance parameter (i.e. short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF)) of iOSC were obtained from J-V measurements. Figs. 4–6 show the normalized values of Jsc, Voc and FF parameters over time for all groups of iOSCs, respectively. Fig. 4 shows that degradation of Jsc was very slow during the first 30 h for all iOSC groups. Afterwards, they degrade more rapidly for

Fig. 4. Normalized performance of short circuit current density (Jsc) with respect to its value at the initial time of the degradation study, obtained for all iOSCs groups, as a function of time. The devices were measured under AM1.5G spectrum condition (100 mW∙cm−2) under air environment.

Table 3 Modeling Parameters Obtained from Circuital Model. Group

Time (h)

PCEa (%)

Jscb (mA∙cm−2)

JLc (mA∙cm−2)

n 1d

J01e (mA∙cm−2)

n 2d

J02e (mA∙cm−2)

n 3d

J03e (mA cm−2)

RSf (Ω cm2)

RSHg (Ω cm2)

I

0 31 316 1064 2184 7110

7.2 6.2 4.6 2.9 2.3 1.3

18.1 17.5 16.2 13.4 12.8 8.9

18.19 17.60 16.38 14.14 14.97 11.16

1.66 1.66 1.67 1.83 1.93 2.08

1.47E-6 3.39E-6 6.76E-6 4.34E-5 8.28E-5 1.88E-4

2.13 2.27 2.49 2.55 2.60 2.65

4.84 3.35 2.25 2.07 1.96 1.80

2.39 2.79 3.09 3.18 3.19 3.21

0.97 0.85 0.79 0.66 0.64 0.58

3.2 2.3 2.7 6.1 15.9 21.7

697 512 244 127 98 90

II

0 31 243 864 2500 7110

8.1 7.4 5.8 4.6 3.2 1.9

18.1 17.8 16.8 15.9 15.5 13.6

18.17 17.87 16.92 16.10 16.50 14.14

1.65 1.65 1.66 1.66 1.67 1.73

1.52E-7 1.54E-7 1.91E-7 2.14E-7 2.37E-7 2.60E-7

1.84 2.29 2.54 2.83 2.96 3.21

1.47 0.86 0.52 0.37 0.34 0.30

2.53 2.58 2.65 2.81 2.98 2.99

0.12 0.12 0.09 0.07 0.06 0.05

2.1 1.2 1.6 2.2 15.3 15.9

369 352 336 287 254 82

III

0 31 243 864 2500 7110

3.9 3.7 3.3 3.2 3.3 1.3

15.7 16.3 16.3 14.1 14.9 8.1

18.17 17.18 16.75 14.96 15.65 8.68

1.39 1.46 1.63 1.72 1.79 1.99

5.65E-5 5.96E-5 6.02E-5 6.12E-5 6.17E-5 6.27E-5

1.56 1.66 1.75 1.85 1.98 2.22

4.36 4.36 4.24 9.56 3.86 1.17

0.50 0.10 0.60 1.95 1.99 2.05

10.03 6.89 5.84 0.02 0.02 0.01

5.7 7.1 3.8 5.6 7.8 9.0

48 143 214 98 105 95

IV

0 79 174 386 724 1016

2.1 1.7 0.9 0.4 0.2 0.1

16.3 13.8 9.5 5.6 4.3 3.6

17.50 14.85 10.42 6.46 5.11 5.82

1.04 1.06 1.09 1.15 1.17 1.22

2.15E-4 2.19E-4 2.53E-3 2.64 E-3 4.71 E-3 5.16 E-3

1.10 1.25 1.28 1.38 1.48 1.78

9.71 9.06 8.88 8.58 8.35 8.11

0.58 0.58 0.59 0.75 0.85 0.95

7.88 7.76 6.95 6.84 6.68 0.05

5.2 4.9 6.3 6.9 9.6 25.3

71 67 62 60 75 40

V

0 54 149 201 724 1016

2.6 1.8 0.9 0.4 0.3 0.2

16.5 13.9 9.9 5.9 3.9 3.5

16.82 14.27 10.46 6.34 4.50 5.46

1.41 1.42 1.43 1.44 1.83 1.86

6.83E-4 1.03 E-3 3.39 E-3 3.79 E-3 6.91 E-3 1.02 E-2

1.10 1.25 1.45 1.47 1.51 1.58

4.90 4.38 3.92 3.69 3.63 3.56

1.46 1.51 1.55 1.61 2.43 2.83

2.96 2.88 2.08 2.02 1.86 1.74

4.0 4.0 4.7 5.9 12.6 29.4

261 156 87 78 87 52

a

Power conversion efficiency. Short circuit current density. c Photogenerated current density. d Ideality factor of diode 1 (main diode), diode 2 and diode 3. e Saturation current density for diodes 1, 2 and 3. f Series resistance. g Shunt resistance per unit of area resultant for the overall cell structure. All parameters were extracted from the circuit model shown in Fig. 1 (inset) (GarcíaSánchez et al., 2013). b

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dominated by the overall series resistance (RS). The series resistance is related to resistance and thickness of active layer, to the contact resistance between the metal and active layer, and to the selected ETL (Balderrama et al., 2014, Ramírez-Como et al., 2019). Its value per unit area, RS0, can be calculated by the inverse slope of the J–V curve at the highest operating voltage range, where the curve becomes linear: RS0 = (J / V )−1. On the other hand, at low voltage/low current, near Jsc, current is dominated by the shunt resistance (RSH), which is related to the recombination of charge carriers near donor/acceptor interface of both organic materials PTB7-Th:PC70BM, will depend on the transport properties of the semiconductor. The value of the shunt resistance per unit area (RSH0) can be determined by calculating the inverse slope around 0 V of the J–V curve, RSH0 = (J / V )−1 (Balderrama et al., 2014). RS0 and RSH0 under illumination were extracted from J-V characteristics for each group of devices. The values are shown in Table 3. In general, all groups show an increase of RS0 with time. For groups I, II and III, the increase of Rs after 7110 h was of 3.28 Ω cm2 to 21.73 Ω cm2, 2.12 Ω cm2 to 15.9 Ω cm2, 5.73 Ω cm2 to 9.07 Ω cm2, respectively. For groups IV and V, after 1016 h, it was 5.25 Ω cm2 to 25.36 Ω cm2 and 4 Ω cm2 to 29.4 Ω cm2, respectively. On the other hand, as expected, RSH0 for the groups I, II, III, IV, and V of iOSCs under air, decreased from 697 Ω cm2 to 90 Ω cm2, 369 Ω cm2 to 82 Ω cm2, 214 Ω cm2 to 95 Ω cm2, 71 Ω cm2 to 40 Ω cm2, and 261 Ω cm2 to 52 Ω cm2, respectively. Finally, Table 3 shows also the extracted parameters from the modeled J-V curves for the different groups of devices. Fig. 7 shows the J-V curves measured (continued line represents experimental) and the modeled (opened symbols). The ideality factor of diode 1 (n1) represents the dominant transport mechanism in the active layer of the cell. The ideality factor for diodes 2 and 3 correspond the dominant transport mechanism between the buffer layers and the metallic contacts of the anode or cathode. If n = 1 indicates that the predominant transport mechanism is diffusion, n = 2 indicates that it is recombination and n between 2 and 3 indicates that the mechanism is a combination of diffusion, recombination and hopping (Balderrama et al., 2016). As we see on Table 3, for all groups of devices, n1 is between 1 and 2, and tends to increase as the cell degrades. n2 and n3 have the same behavior over time. It is well known that, when the OSCs are left under ambient conditions, oxygen and water molecules tend to diffuse into the device active layer and these devices start to degrade. In general, the increase of RS0 for the devices under air environment can be attributed to many factors, such as: degradation of the active layer of the iOSCs due to the reaction with oxygen or water after the devices were exposed to air; electro-chemical reaction of ETL with ITO electrode or the reaction between HTL with Ag contact; UV light when were exposed the devices to light, among others (Balderrama et al., 2014). In summary, the devices efficiencies decreasing due to degradation of the polymer and the electrode–polymer interface generating an increase on the series resistances. The degradation time of the solar cells was observed that depend of environment exposed, and the materials used as HTL or ETL, mainly. One way to increase the lifetime of the device is encapsulated. Finally, the different stacks as ETLs were studied in order to compare the stability over time of iOSCs. The cell manufactured with the stack PFN/LiF showed the best stability. The stability of the cells using LiF/PFN stack as ETL was also good, but the operation voltage of the cells was reduced.

Fig. 5. Normalized performance of open circuit voltage (Voc) with respect to its value at the initial time of the degradation study, obtained for all iOSCs groups, as a function of time.

Fig. 6. Normalized performance of fill factor (FF) with respect to its initial value at the initial time of the degradation study, obtained for all iOSCs groups, as a function of time.

groups I, II and III up to 1000 h. For groups IV and V, an abrupt decay of Jsc starts at 350 h, tending to reduce the degradation rate with tie. At 7110 h, Jsc reduced down to 36% for group II, while for groups I and III it falls to 54% and 51%, respectively. Group IV showed the greatest fall down to 60%. Fig. 5 shows the Voc behavior over time of all device groups. Groups II and I show a reduction in Voc during 7110 h, of 2% and 33%, respectively. The Voc for group III even increased to 12% which could be due to small chemical reaction generated between the BHJ and the ETL. Groups IV and V fall 46% and 39%, respectively within 1016 h. Variations of FF with time for all device groups are shown in Fig. 6. All photovoltaic devices groups presented a similar decay exponential law. After 7110 h, group III, I and II showed a decay of 37%, 49% and 61%, respectively. Groups IV and V reduced in 35% and 40%, respectively, during 1016 h. Group II presented the highest reduction on FF due to the formation of S-shape on J-V curve over time. Fig. 7 shows the J-V curves under illumination (experimental and modeled) for all groups of iOSCs exposed under air environment. It is well known that at high voltage and high current, the current is

4. Conclusion We analyzed the degradation behavior of iOSCs under air environment based on PTB7-Th:PC70BM as active layer and using different ETL stacks (PFN, PFN/LiF, LiF/PFN, without ETL and LiF), applying the 423

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Fig. 7. Illuminated J-V characteristics of PTB7-Th:PC70BM solar cells for different degradation times under air (a) group I, (b) group II, (c) group III, (d) group IV, and (e) group V. All the devices were measured under AM 1.5 G spectrum condition (100 mW∙cm−2).

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ISOS-D1 protocols. The degradation analyses of iOSCs showed that the cells manufactured with PFN and an additional thin layer of LiF improve the stability of the cell. It is worth noting that the PFN/LiF stack (group II) was more stable than that the LiF/PFN (group III). The values of TS80, for the PCE obtained of cells manufactured with the stack PFN/LiF, only with PFN and with the stack LiF/PFN were of 201 h, 144 h and 153 h, respectively. For TS30 the values were 6641 h, 7110 h and 3783 h, respectively. The analysis of Jsc showed that cells with stack PFN/LiF presented the lowest decay of 36% after 7110 h, while the decay for devices with the stack PFN and LiF/PFN was 54% and 51%, respectively. The Jsc for iOSC without ETL and only with a LiF layer fell very rapidly, approximately more than60% after 1016 h. Voc was the most stable parameter for group II, showing only a reduction of 2% during the overall time of analysis. Voc for group I fall 33%, for group III even increased to 12% which could be due to small chemical reaction generated between the BHJ and the ETL. Groups IV and V fall 46% and 39%, respectively in 1016 h. Regarding FF, all groups presented a similar decay exponential law. Groups I, II, III, IV and V presented a FF reduction of 48% (7110 h), 60% (7110 h), 37% (7110 h), 35% (1016 h) and 41% (1016 h), respectively regarding their initial values. We observed that group II presented a moderate decay of FF during the study degradation, as it was shown above. It was shown that the behavior of the J-V experimental curves of iOSCs can be modeled by the three-diode circuit model. The values extracted from the model served to establish the predominant degradation mechanism, depending on the used stack. Using the same model, it was possible to reproduce the S-shape effect presented on the J-V characteristics.

L.F., Estrada, M., 2016. Organic solar cells toward the fabrication under air environment. IEEE J. Photovolt. 6, 491–497. Balderrama, V.S., Sánchez, J.G., Lastra, G., Cambarau, W., Arias, S., Pallarès, J., Palomares, E., Estrada, M., Marsal, L.F., 2018. High-efficiency organic solar cells based on a halide salt and polyfluorene polymer with a high alignment-level of the cathode selective contact. J. Mater. Chem. A 6, 22534–22544. Brütsch, L., Czolk, J., Popescu, R., Gerthsen, D., Colsmann, A., Feldmann, C., 2017. Surfactant-free synthesis of sub-stoichiometry tungsten oxide nanoparticles and their use as anode buffer layers in organic solar cells. Solid State Sci. 69, 50–55. Chalal, D., Garuz, R., Benachour, D., Bouclé, J., Ratier, B., 2016. Influence of an electrode self-protective architecture on the stability of inverted polymer solar cells based on P3HT: PCBM with an active area of 2 cm2. Synth. Met. 212, 161–166. Chen, C.P., Chen, Y.D., Chuang, S.C., 2011. High-performance and highly durable inverted organic photovoltaics embedding solution-processable vanadium oxides as an interfacial hole-transporting layer. Adv. Mater. 23, 3859–3863. Dennler, G., Scharber, M.C., Brabec, C.J., 2009. Polymer-fullerene bulk-heterojunction solar cells. Adv. mater. 21, 1323–1338. El Jouad, Z., Barkat, L., Stephant, N., Cattin, L., Hamzaoui, N., Khelil, A., Ghamnia, M., Addou, M., Morsli, m., Bechu, S., Cabanetos, C., Richard-Plouet, M., Blanchard, P., Bernede, J.C., 2016. Ca/Alq3 hybrid cathode buffer layer for the optimization of organic solar cells based on a planar heterojunction. J. Phys. Chem. Solids, 98, 128–135. Finck, B.Y., Schwartz, B.J., 2013. Understanding the origin of the S-curve in conjugated polymer/fullerene photovoltaics from drift-diffusion simulations. App. Phys. Lett. 103, 143_1. García-Sánchez, F.J., Lugo-Munoz, D., Muci, J., Ortiz-Conde, A., 2013. Lumped parameter modeling of organic solar cells’ S-shaped I-V characteristics. IEEE J. Photovolt. 3, 330–335. García-Sánchez, F.J., Romero, B., Lugo-Muñoz, D.C., Del Pozo, G., Arredondo, B., Liou, J.J., Ortiz-Conde, A., 2017. Modelling solar cell S-shaped Iv characteristics with Dc lumped-parameter equivalent circuits a Review. Facta Univ. Ser. Electron. Energetics 30, 327–350. Green, M.A., Hishikawa, Y., Dunlop, E.D., Levi, D.H., Hohl-Ebinger, J., Ho-Baillie, A.W., 2018. Solar cell efficiency tables (version 52). Prog. Photovolt. 26, 427–436. Guillain, F., Tsikritzis, D., Skoulatakis, G., Kennou, S., Wantz, G., Vignau, L., 2014. Annealing-free solution-processed tungsten oxide for inverted organic solar cells. Sol. Energy Mater. Sol. Cells 122, 251–256. Gusain, A., Singh, S., Chauhan, A.K., Saxena, V., Jha, P., Veerender, P., Singh, A., Varde, P.V., Basu, S., Aswal, D.K., Gupta, S.K., 2016. Electron density profile at the interfaces of bulk heterojunction solar cells and its implication on the S-kink characteristics. Chem. Phys. Lett. 646, 6–11. He, Z., Zhong, C., Su, S., Xu, M., Wu, H., Cao, Y., 2012. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photon. 6, 591. Ji, R., Zheng, D., Zhou, C., Cheng, J., Yu, J., Li, L., 2017. Low-temperature preparation of tungsten oxide anode buffer layer via ultrasonic spray pyrolysis method for large-area organic solar cells. Mater. 10, 820. Jiang, F., Choy, W.C., Li, X., Zhang, D., Cheng, J., 2015. Post-treatment-free solutionprocessed non-stoichiometric NiOx nanoparticles for efficient hole-transport layers of organic optoelectronic devices. Adv. Mater. 27, 2930–2937. Kiriishi, K., Hashiba, K., Qiu, J., Fujii, S., Kataura, H., Nishioka, Y., 2015. Solution-derived NiO hole transport layers on the PTB7: PC71BM organic solar cells. ECS Trans. 69, 59–62. Krebs, F.C., Espinosa, N., Hösel, M., Søndergaard, R.R., Jørgensen, M., 2014. 25th Anniversary article: rise to power–OPV-based solar parks. Adv. Mater. 26, 29–39. Lastra, G., Balderrama, V.S., Reséndiz, L., Pallares, J., Garduno, S.I., Cabrera, V., Marsal, L.F., Estrada, M., 2018. High-performance inverted polymer solar cells: study and analysis of different cathode buffer layers. IEEE J. Photovolt. 8, 505–511. Lastra, G., Balderrama, V.S., Reséndiz, L., Pallares, J., Marsal, L.F., Cabrera, V., Estrada, M., 2019. Air environment degradation of a high-performance inverted PTB7-Th: PC70BM solar cell. IEEE J. Photovolt. 9, 464–468. Lechêne, B., Leroy, J., Tosoni, O., De Bettignies, R., Perrier, G., 2014. Origin of the Sshape upon aging in standard organic solar cells with zinc oxide as transport layer. J. Phys. Chem. C 118, 20132–20136. Li, J., Jian, H., Yao, L., Zhao, M., Shu, J., Xiao, X., Jiu, T., 2018. Highly efficient regular polymer solar cells based on Li-TFSI doping ZnO as electron-transporting interlayers. Sol. Energy 169, 49–54. Li, G., Zhu, R., Yang, Y., 2012. Polymer solar cells. Nature Photon. 6, 153–161. Mejia, I., Estrada, M., Cerdeira, A., Iñiguez, B., 2007. Reversible electrical characteristics in PMMA on P3HT OTFTs. ECS Trans. 9, 383–388. Peters, C.H., Sachs-Quintana, I.T., Mateker, W.R., Heumueller, T., Rivnay, J., Noriega, R., Beley, Z.M., Hoke, E.T., Saleo, A., McGehee, M.D., 2012. The mechanism of burn-in loss in a high efficiency polymer solar cell. Adv. Mater. 24, 663–668. Phang, J.C.H., Chan, D.S.H., Phillips, J.R., 1984. Accurate analytical method for the extraction of solar cell model parameters. Electron. Lett. 20, 406–408. Ramírez-Como, M., Balderrama, V.S., Sacramento, A., Marsal, L.F., Lastra, G., Estrada, M., 2019. Fabrication and characterization of inverted organic PTB7: PC70BM solar cells using Hf-In-ZnO as electron transport layer. Sol. Energy 181, 386–395. Reese, M.O., Gevorgyan, S.A., Jørgensen, M., Bundgaard, E., Kurtz, S. R., Ginley, D.S., Elschner, A., Olson, D.C., Lloyd, M.T., Morvillo, P., Katz, E.A., Elschner, A., Haillantf, O., Currier, T.R., Shrotriya, V., Hermenau, M., Riede, M., Kirov, K.R., Trimmel, G., Rath, T., Inganas, O., Zhang, F., Andersson, M., Tvingstedt, K., Lira-Cantu, M., Laird, D., McGuiness, D., Gowrisanker, S., Pannone, M., Xiao, M., Hauch, J., Steim, R., DeLongchamp, D.M., Rosch, R., Hoppe, H., Espinosa, N., Urbina, A., YamanUzunoglu, G., Bonekamp, J-B., van-Breemen, A.J.J.M., Girotto, C., Voroshazi, E., Krebs, F.C., 2011. Consensus stability testing protocols for organic photovoltaic

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported in part by CONACYT Project 237213 in Mexico. The work of Victor S. Balderrama (ID Researcher 7340 and CVU- 227699) was supported by “Cátedra-CONACYT Project Number 2 “Strengthening Scientific Capacity and Human Capital for the Development of MEMS for the Industry” in Querétaro, Mexico, Project “Laboratorio Nacional SEDEAM-CONACYT” under the Call 2019 No. 299061 and Project FORDECYT Number 297497. This work was supported by the Ministerio de Ciencia, Innovación y Universidades (RTI2018-094040-B-I00), by the Agency for Management of University and Research Grants (AGAUR) ref. 2017-SGR-1527, and by the Catalan Institution for Research and Advanced Studies (ICREA) under the ICREA Academia Award. References Ahlswede, E., Hanisch, J., Powalla, M., et al., 2007. Comparative study of the influence of LiF, NaF, and KF on the performance of polymer bulk heterojunction solar cells. Appl. Phys. Lett. 90, 163504. https://doi.org/10.1063/1.2723077. (in press). Arca, F., Loch, M., Lugli, P., 2014. Enhancing efficiency of organic bulk heterojunction solar cells by using 1, 8-diiodooctane as processing additive. IEEE J. Photovolt. 4, 1560–1565. Balderrama, V.S., Sánchez, J.G., Estrada, M., Ferre-Borrull, J., Pallares, J., Marsal, L.F., 2015. Relation of polymer degradation in air with the charge carrier concentration in PTB1, PTB7, and PCBM layers used in high-efficiency solar cells. IEEE J. Photovolt. 5, 1093–1099. Balderrama, V.S., Estrada, M., Han, P.L., Granero, P., Pallarés, J., Ferré-Borrull, J., Marsal, L.F., 2014. Degradation of electrical properties of PTB1:PCBM solar cells under different environments. Sol. Energy Mater. Sol. Cells 125, 155–163. Balderrama, V.S., Avila-Herrera, F., Sánchez, J.G., Pallares, J., Vigil-Galán, O., Marsal,

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Solar Energy 198 (2020) 419–426

A. Sacramento, et al.

Wu, F., Li, Z., Yang, D., Huan, Y., Wang, J., Yang, X., 2017. Characterization of spraycoated ZnO buffer layer for inverted polymer solar cells. Mater. Res. Bull 96, 47–52. Xie, H.J., Li, Y.Q., Ma, G.F., Wang, Q.K., Zhang, D.D., Tang, J.X., 2015. Enhanced performance of inverted organic solar cells by introducing a phosphorescence-doped electron extraction layer. IEEE J. Photovolt. 5, 885–888. Xu, C., Cai, P., Zhang, X., Zhang, Z., Xue, X., Xiong, J., Zhang, J., 2017. A wide temperature tolerance, solution-processed MoOx interface layer for efficient and stable organic solar cells. Sol. Energy Mater. Sol. Cells 159, 136–142. Yanagidate, T., Fujii, S., Ohzeki, M., Yanagi, Y., Arai, Y., Okukawa, T., Yoshida, A., Kataura, H., Nishioka, Y., 2014. Flexible PTB7: PC71BM bulk heterojunction solar cells with a LiF buffer layer. Jpn. J. Appl. Phys. 53, 02BE05. Yang, H.B., Song, Q.L., Gong, C., Li, C.M., 2010. The degradation of indium tin oxide/ pentacene/fullerene/tris-8-hydroxy-quinolinato aluminum/aluminum heterojunction organic solar cells: By oxygen or moisture. Sol. Energy Mat. and Sol. Cells 94, 846–849. Yang, W., Yu, Z., Liu, W., Li, C.Z., Chen, H., 2017. Aqueous solution-processed NiOx anode buffer layers applicable for polymer solar cells. J. Polym. Sci. A. 55, 747–753. Yin, Z., Zheng, Q., Chen, S.C., Li, J., Cai, D., Ma, Y., Wei, J., 2015. Solution-derived poly (ethylene glycol)-TiOx nanocomposite film as a universal cathode buffer layer for enhancing efficiency and stability of polymer solar cells. Nano Res. 8, 456–468. Yu, X., Yu, X., Zhang, J., Zhang, D., Chen, L., Long, Y., 2017. Light-trapping Al-doped ZnO thin films for organic solar cells. Sol. Energy 153, 96–103. Yu, F., Huang, G., Lin, W., Xu, C., 2019. An analysis for S-shaped IV characteristics of organic solar cells using lumped-parameter equivalent circuit model. Sol. Energy 177, 229–240. Zhang, K., Zhong, C., Liu, S., Mu, C., Li, Z., Yan, H., Huang, F., Cao, Y., 2014. Highly efficient inverted polymer solar cells based on a cross-linkable water-/alcohol-soluble conjugated polymer interlayer. ACS Appl. Mater. Interfaces 6, 10429–10435. Zhang, Y., Scarratt, N.W., Wang, T., Lidzey, D.G., 2017. Fabricating high performance conventional and inverted polymer solar cells by spray coating in air. Vacuum 139, 154–158.

materials and devices. Sol. Energy Mater. Sol. Cells 95, 1253–1267. Romero, B., Del Pozo, G., Destouesse, E., Chambon, S., Arredondo, B., 2014. Circuital modelling of S-shape removal in the current–voltage characteristic of TiOx inverted organic solar cells through white-light soaking. Org. Electron. 15 (12), 3546–3551. Romero, B., del Pozo, G., Arredondo, B., Reinhardt, J.P., Sessler, M., Würfel, U., 2015. Circuital model validation for S-shaped organic solar cells by means of impedance spectroscopy. IEEE J. Photovolt. 5, 234–237. Sánchez, J.G., Balderrama, V.S., Estrada, M., Osorio, E., Ferré-Borrull, J., Marsal, L.F., Pallarès, J., 2017. Stability study of high efficiency polymer solar cells using TiOx as electron transport layer. Sol. Energy 150, 147–155. Sánchez, J.G., Balderrama, V.S., Garduño, S.I., Osorio, E., Viterisi, A., Estrada, M., FerréBorrulla, J., Pallarès, J., Marsal, L.F., 2018. Impact of inkjet printed ZnO electron transport layer on the characteristics of polymer solar cells. RSC Adv. 8, 13094–13102. Schafferhans, J., Baumann, A., Wagenpfahl, A., Deibel, C., Dyakonov, V., 2010. Oxygen doping of P3HT: PCBM blends: influence on trap states, charge carrier mobility and solar cell performance. Org. Electron. 11, 1693–1700. Shen, L., Xu, Y., Meng, F., Li, F., Ruan, S., Chen, W., 2011. Semitransparent polymer solar cells using V2O5/Ag/V2O5 as transparent anodes. Org. Electron. 12, 1223–1226. Shen, L., Ruan, S., Guo, W., Meng, F., Chen, W., 2012. Semitransparent inverted polymer solar cells using MoO3/Ag/V2O5 as transparent anodes. Sol. Energy Mater. Sol. Cells 97, 59–63. Sims, L., Hörmann, U., Hanfland, R., MacKenzie, R.C., Kogler, F.R., Steim, R., Brütting, U., Schilinsky, P., 2014. Investigation of the s-shape caused by the hole selective layer in bulk heterojunction solar cells. Org. Electron. 15, 2862–2867. Sundqvist, A., Sandberg, O.J., Nyman, M., Smått, J.H., Österbacka, R., 2016. Origin of the S-shaped JV curve and the light-soaking issue in inverted organic solar cells. Adv. Energy Mater. 6, 1502265. Upama, M.B., Elumalai, N.K., Mahmud, M.A., Sun, H., Wang, D., Chan, K.H., Wright, M., Xu, C., Uddin, A., 2017. Organic solar cells with near 100% efficiency retention after initial burn-in loss and photo-degradation. Thin Solid Films 636, 127–136.

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