Electrolytes as Cathode Interlayers in Inverted Organic Solar Cells

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Electrolytes as Cathode Interlayers in Inverted Organic Solar Cells: Influence of the Cations on Bias-Dependent Performance Yaru Li, Xiaohui Liu, Xiaodong Li, Wenjun Zhang, Feifei Xing, and Junfeng Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01240 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 23, 2017

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ACS Applied Materials & Interfaces

Electrolytes as Cathode Interlayers in Inverted Organic Solar Cells: Influence of the Cations on Bias-Dependent Performance Yaru Li,†‡ Xiaohui Liu,‡ Xiaodong Li,‡ Wenjun Zhang,*‡ Feifei Xing,† and Junfeng Fang*‡ †

Department of Chemistry, College of Sciences, Shanghai University, Shanghai

200444, China ‡

Key Laboratory of Graphene Technologies and Applications of Zhejiang Province,

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

KEYWORDS: organic solar cells, cathode interlayer, edetate electrolytes, external bias, ions motion

ACS Paragon Plus Environment


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ABSTRACT The performance of organic solar cells (OSCs) with edetate electrolytes depends on external bias and ions are speculated to be responsible for this phenomenon. To clarify the detailed relationship between the ions of electrolytes and the bias-dependent behaviors of devices, this work introduces four edetate cathode interlayers (EDTA-X, X = nH(4-n)Na, n = 0, 1, 2 and 4) containing different kind and number of cations into inverted OSCs. The results show that the devices initial and saturated (after external bias treatment) power conversion efficiencies (PCEs) both decrease with the increase of the number of H+. Moreover, the bias-dependent degrees increase with the increase of H+ number, with that the PCE increment of EDTA-4H device is 53.4% while EDTA-4Na device almost unchanged. The electrical impedance spectroscopy (EIS) and capacitance-voltage (CV) tests reveal that the interfacial recombination is greatly suppressed by external bias treatment which is not result from the decreased density of defect states. The results indicate that the ions motion, exactly the H+ motion, under external electrical field is responsible for the bias-dependent behavior, which is conducive to the design of new efficient electrolytic interlayers without bias-dependent performance.


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INTRODUCTION Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been successively studied owing to their potential application in flexible, light-weight and low-cost devices via low-temperature solution manufacturing.1-5 With materials design and morphology optimization of the photoactive layer, power conversion efficiency (PCE) over 11% for single junction OSCs has been achieved.6-13 To realize a more brilliant performance of OSCs, interfacial modification between active materials and electrodes to improve charge injection and extraction is a key issue.14-18 Organic interfacial materials with chemical diversity and mechanic flexibility are proved to be potential in adjusting the work function (WF) of cathode to reduce the interfacial energy barrier and facilitate charge extraction.19-21 Thus a series of efficient organic interlayers have been reported.22-24 Among various organic cathode interfacial materials, water/alcohol soluble organic electrolytes have attracted much interests.25-32 Their ionic or polar pendant groups will render the organic electrolytes soluble in polar solvents and enable the fabrication of multilayer polymer-based devices without interface mixing by solution-processing approach.33-34 In addition, the ions distribution in the electrolytes is helpful to form an aligned dipole moment at cathode interlayer/electrode interface, which can modify the WF of the electrode by inducing a vacuum-level shift.35-36 What’s more, their orthogonal solubility and good film-forming ability display their latent capacity of large-area processing by roll-to-roll or inkjet printing fabricating. When inverted OSCs (i-OSCs) using organic electrolytes as cathode interlayers, the



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surface dipoles will facilitate the extraction and transport of electrons from photoactive layer to indium tin oxide (ITO).35,37 Despite the remarkable role of the ions in organic electrolytes, certain issues are also caused by them. The ion migration could influence on the response time of organic semiconductor devices and the long-term stability of OSCs.38-42 However, limited researches have been focused on this topic. Recently, a simple and commercial edetate-based electrolyte, EDTA-Na, was successfully introduced into i-OSCs as a cathode interlayer by our group.42 As with other organic electrolytic interfacial materials, EDTA-Na was capable of reducing the WF of ITO cathode and improving the performance of i-OSCs. However, we found the device performance strongly depended on the external bias. By applying an external bias prior to the measurement for a short time, the PCE could impressively increase from 6.75% to 8.05%. In addition, the higher of the pre-bias, the shorter of the saturated time and the higher of the saturated PCE. The ions were suggested to be responsible for the improvement of device performance under the external bias, which should be further confirmed. Unquestionably, there are two kinds of cations in EDTA-Na, Na+ and H+, which one is mainly responsible for the unique performance of the EDTA-Na device should be also identified. In order to clarify the specific relevance between the bias-depended performance of devices and the cationic kind and number of electrolytes, we introduce four edetate-based electrolytes, named EDTA-X (X = nH(4-n)Na, n = 0, 1, 2 and 4, Figure 1a), as cathode interlayers for i-OSCs in this work. More importantly, through the


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deep research of the molecular structure and bias-dependent performance, we anticipate to dig some guidance of designing new organic electrolytes with excellent interfacial modification and none external bias dependence. The results show that the four interfacial materials with similar structure but different number of H+ and Na+, display significantly regular devices performance. Their initial PCEs decrease with the increase of H+ number (n) of EDTA-X, and their bias-dependent degrees increase with the increase of number of H+. Finally, the device with EDTA-4Na interlayer exhibits the highest PCE and eliminates the dependence of external bias.[43] EXPERIMENTAL SECTION Device fabrication. The ITO/glass substrates were cleaned by a sonication of 20 min in detergent, deionized water, acetone and isopropyl alcohol before being dried by nitrogen flow. The cleaned substrates were then treated by plasma for 10 minutes. Subsequently, aqueous solution (3 mg/mL) of the EDTA-X (X = nH(4-n)Na, n = 0, 1, 2, 4) (Aladdin, analytically pure, > 99%, China) was spin-coated on the prepared ITO/glass at 4000 rpm for 60 s and heated at 140 oC for 15 min in air. Then, the ITO/interlayer substrates were transferred into a N2-filled glove box and the photoactive layer was spin-coated at 2000 rpm for 120 s. The thickness of the EDTA-X interlayers is about 3 nm. The photoactive materials were blends of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-co-3-fluor othieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7-Th:PC71BM) (7:13 by weight) with a total concentration of 25 mg/mL in mixed solvents of chlorobenzene/1,8-diiodoctane (97/3 by volume). Additionally,



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p-chlorobenzoic (PCBA) was blended into the PTB7-Th:PC71BM with the proportion of 1:100 by weight as photoactive materials as a contrast. Finally, 10 nm MoO3 (0.20 Å/s) and 80 nm Al (1 Å/s) were deposited to form the anode. The device area was defined as 0.06 cm2. The electron-only devices were made with the structure of ITO/EDTA-X/Al and the concentration of EDTA-X interlayers were 15 mg/mL. The thickness of the interlayers in the electron-only devices is about 12 nm. Device characterization. Current density-voltage (J–V) characteristics of the devices are measured with a computer-controlled Keithley 2400 source meter and Newport 6279 NS solar simulator (450 W) with 100 mW/cm2 illumination. The electrical impedance spectroscopy (EIS) measurement is performed using a Solartron SI 1260 Impedance/gain-phase analyzer, and the impedance spectra is recorded by applying varied alternating current (AC) signal from 0.1 Hz to 10 MHz. The capacitance-voltage (CV) is performed using Keithley 4200-SCS semiconductor characteriztion system, and the measurement is recorded at a frequency of 10 kHz for extracting N. All the AC oscillating amplitudes are set as low as 20 mV to maintain the linearity of the response. RESULTS AND DISCUSSION To make it clear that the specific impact of ions on bias-dependent behavior of device, we thoroughly fabricate EDTA-nH(4-n)Na (n = 0, 1, 2 and 4) inverted devices. The molecular structures of EDTA-X and i-OSC device configuration are shown in Figure 1a and 1b. Their current density-voltage (J–V) curves under different bias are displayed in Figure S1-S4. First of all, we compare their initial device performance.


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As shown in Figure 1c and Table 1, when n = 0 (EDTA-4Na), the device exhibit the highest PCE value of 9.55%,43 simultaneously the highest Voc and FF values. On the contrary, when n = 4 (EDTA-4H), the device exhibit the poorest performance. In a word, the device performance decreases with the increase of H+ number (n) of EDTA-nH(4-n)Na. Then the research of the interfacial modification ability of EDTA-X interlayers is particularly necessary. Atomic force microscopy (AFM) was then carried out to compare the morphologies of the interlayers coated on ITO (Figure S5). We found that the surfaces of EDTA-X (X = nH(4-n)Na, n = 0, 1, 2 and 4) were rougher than bare ITO, and little difference was found between the four interlayers. Ultraviolet photoelectron spectroscopy (UPS) was also carried out to test the WF of ITO/interlayers. As shown in Figure S6, after modified by EDTA-nH(4-n)Na, the WF of ITO could be regulated from 4.80 eV to 4.40 eV (n = 4), 4.19 eV (n = 2), 4.12 eV (n = 1), and 4.07 eV (n = 0), respectively, making the ITO electrode more suitable for electron collection. It is obvious that the WF adjustability decreases with the increase of H+ number, which consists with the initial PCE and Voc values of corresponding devices.



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Figure 1. (a) Molecular structures of EDTA-X (X = nH(4-n)Na, n = 0, 1, 2 and 4). (b) I-OSCs device configuration. (c) The initial and saturated (after 2 V bias treatment) PCEs of the devices with EDTA-X interlayers. (d) The PCEs increment of EDTA-X devices after 2 V bias treatment represented in standard box plot. After the comparison of the initial performance of devices with EDTA-X interlayers, we further compare their bias-dependent behaviors. In detail, after 2 V bias treatment, the PCEs of EDTA-nH(4-n)Na devices increase to 7.21% (n = 4), 8.76% (n = 2), 8.96% (n = 1) and 9.56% (n = 0), respectively. Similar to the regularity of initial PCEs, the saturated PCEs of devices also decrease with the increase of H+ number (n) of EDTA-X. What’s more, through calculation we can see the PCE increment of each device with EDTA-nH(4-n)Na is 53.4% (n = 4), 25.1% (n = 2), 17.0% (n = 1) and 0.1% (n = 0), respectively. Apparently, the PCE increment


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gradually increases with the increase of n. Besides, the bias-treatment times required to reach saturation of the devices are also different. It will take about 240 s to reach saturation when n = 4, while it only needs 10 s when n = 0. With the increase of H+ number, the saturated time will be longer. All the above results indicate that the bias-dependent degree rely on the H+ number (also could consider as Na+ number) of EDTA-nH(4-n)Na. The more the number of H+, the stronger intensity of the device influenced by external bias. Table 1. The photovoltaic parameters of initial and saturated i-OSCs with EDTA-nH(4-n)Na interlayers.

Voc

Jsc

FF

PCE

Rsh

Time

[V]

[mA/cm2]

[%]

[%]

[Ω·cm2]

[s]

0.67

14.75

47.48

4.70

617

n

4a)

240 b)

0.77

14.82

62.86

7.21

707

2a)

0.69

15.98

63.59

7.00

1149

2b)

0.78

16.03

70.27

8.76

1281

1a)

0.74

15.95

65.03

7.66

1226

4

100

50 b

0.79

15.98

71.44

8.96

1458

0a)

0.79

16.77

71.99

9.55

1231

1

10 0 a)

b)

0.79

16.74

71.96

The initial parameters of fresh devices.

b)

9.56

1274

The saturated parameters of devices after

2 V bias treatment.



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Furthermore, the statistical data of ten devices fabricated with EDTA-X interlayers are presented in the form of a standard box plot, respectively. As shown in Figure 1d, the average of PCE increment of EDTA-nH(4-n)Na devices is 72.9% (n = 4), 22.4% (n = 2), 8.4% (n = 1) and 0.6% (n = 0), respectively. With the increase of the number of H+ in EDTA-X, the average PCE increment will enlarge, which further confirmed that the H+ would be a main reason for the bias-dependent performance of device. Moreover, the EDTA-X based devices performance under 1 V and 4 V (Figure S7) bias are also compared and same regularities exist, which exhibit the universal properties of the device under external bias. To gain deeper insight into the different bias-dependent behaviors of devices with EDTA-nH(4-n)Na interlayers, the electrical impedance spectra (EIS) of i-OSCs before and after external bias were measured. EIS is a usual tool to detect the dynamics of charge transfer and recombination at the interface. 44-46 In order to test the EIS variation of the fresh and bias-treated devices, we set the parameter of the direct current (DC) voltage to 0 V. The impedance spectras are only recorded by applying an alternating current (AC) signal with the oscillating amplitudes of 20 mV (rms) under dark condition. As shown in Figure 2, the impedance spectrums of the four devices are semicircles. At low frequencies, these plots intersect with lateral axis at different points which is related to recombination resistance (Rrec). Larger Rrec means lower recombination and higher shunt resistance (Rsh). For fresh devices with EDTA-nH(4-n)Na, the Rrec is about 8.1 KΩ (n = 4), 12.9 KΩ (n = 2), 19.0 KΩ (n = 1) and 29.1 KΩ (n = 0), respectively. With the decrease of the number of H+ (n), the Rrec


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(Rsh) increases, which agrees well with the initial photovoltaic performance. Then, we test the EIS of saturated devices that has been treated by external bias. The Rrec increase to 24.6 KΩ (n = 4), 30.7 KΩ (n = 2), 28.9 KΩ (n = 1) and 32.0 KΩ (n = 0), respectively. The improved Rrec means the suppressed interfacial recombination by external bias treatment. With the growth of n, the Rrec (Rsh) increament increases, resulting in the comparable Voc and FF values of devices with EDTA-X, which is also in accord with the bias-dependent degrees of the devices. These results show that the more the number of H+, the more recombination at the interface, and the more increament of recombination by external bias treatment.

Figure 2. The electrical impedance spectra of initial and saturated i-OSCs in dark condition with EDTA-nH(4-n)Na, (a) n = 4, (b) n = 2, (c) n = 1 and (d) n = 0.



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To study the mechanism of the suppressed interfacial recombination under external bias, we performed capacitance-voltage (C-V) measurement on the initial and saturated devices in a dark condition. C-V measurement is a helpful methode to explore the interface energetics between the cathode and active layer.47-49 From the Mott-Schottky (MS) equation we can make quantitative evaluation: ଵ ஼మ

=

ଶ(௏್೔ ି௏)

(1)

஺మ ௤ఌఌ೚ ே

where V is applied voltage, A is the device area, q is the elementary charge, ε is the dielectric constant, ε0 is vacuum dielectric constant, and N is the density of defect states. We assume the relative dielectric constant of 3 for the active layer of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-co-3-fluor othieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl-C71-butyric acid methyl ester (PTB7-Th:PC71BM). As provided in Figure 3, the values of N can be obtained using the linear slope of the C-2-V plot along with the MS equation. It is obvious that the density of defect states almost remain unchanged before and after external bias treatment. For example, the initial and saturated values of N for EDTA-4H device are 2.15×1016 cm-3 and 2.14×1016 cm-3 respectively. This means the suppressed interfacial recombination is not becuase of the reduced density of defect states.


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Figure 3. Mott-Schottky plots of i-OSCs in dark condition with EDTA-nH(4-n)Na, (a) n = 4, (b) n = 2, (c) n = 1 and (d) n = 0. Thus we inferred that ions migration under external electric field is responsible for the suppressed interfacial recombination, which is similar to the electric double layer in light-emitting electrochemical cells (LECs).50-52 The ions motion will lead to an extra internal electric field, which has the same direction with the built-in electrical field, resulting in the improved charge collection and the reduced recombination.51 Therefore, the stronger the bias-dependent intensity of the device, the more ions motion exists in EDTA-X interlayers. Since the bias-dependent intensity increases with the number of H+, we deduce that the motion of H+ rather than Na+ in EDTA-X interlayers is mainly occurred under external electric field condition.



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To detect the ions motion under external voltage directly, we further study the current density (J) response of the devices with the structure of ITO/EDTA-X/Al over time under constant external bias under dark conditions. As shown in Figure S8, the J of the EDTA-nH(4-n)Na (n = 1, 2, 4) based devices gradually decrease with time under 2 V bias, which match up with our previous hypothesis. The extra internal electric field induced by ions motion has the opposite direction with the external electric field, leading to the decreased of J.51 Directly, we compare the normalized J of the four interlayers based devices, as shown in Figure 4a. Distinctly, the decrement of J will enlarge with the growing number of H+ in EDTA-X. The result is quite consistent with the devices behaviors under external bias. The EDTA-4H affected by external bias most intensively, while the EDTA-4Na device influenced by external bias most slightly. Moreover, the more the number of H+, the much longer time for the device to reach the saturated value. As mobile H+ may also react with components of the active blends, we add p-chlorobenzoic (PCBA), which has one more carboxyl than the solvent of chlorobenzene, into the PTB7-Th:PC71BM as a contrast (Figure S9). According to the J-V curves, there is almost no change after adding protons into active layers, which could rule out the influence of mobile H+ to active layer. These contrasts sufficiently support the speculation that H+ motion induces the influence under eternal bias. In addition, the normalized stabilized power output of EDTA-X devices under the corresponding voltage of the maximum power point are compared (Figure 4b). It is clear that the response time of power output delay when n = 1/2/4. Their delay time also enlarge with the increase of H+ number (n). Finally, the device


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with EDTA-4Na, which possesses the most of the Na+ and the least amount of H+, shows most stable and bias-independent performance.

Figure 4. (a) The normalized current density curves of ITO/EDTA-X/Al devices under 2 V bias for 90 s. (b) The normalized stabilized power output of EDTA-X devices under the corresponding voltage of the maximum power point. Since the ion migration could influence the long-term stability of OSCs, we also studied the lifetime of devices with EDTA-X interlayers (all the devices were stored in glovebox filled with Ar without encapsulation). We recorded and normalized their initial and saturated PCEs. As shown in Figure S10, for EDTA-4H device, the normalized PCE improved from 51% to 100% after optimized by a 2 V bias treatment. After the device stored for 31 days, the PCE decreased to 66%. However, when a 2 V bias was applied, the PCE recovered to 75%. Similar tendencies are also found in other devices with EDTA-nH(4-n)Na (n = 1, 2) interlayers. It should be noted that the PCEs after being stored for a long time are still much higher than that of the fresh devices, which implied that the effects of the external electrical field still exist. At last, all the PCEs of devices exhibited slowly decay rates.



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CONCLUSION In conclusion, small molecule electrolytes EDTA-nH(4-n)Na (n = 0, 1, 2 and 4) interlayers are applied into i-OSCs to investigate the relationship between ions of electrolytes and bias-dependent behaviors of devices. The results show that the more number of H+ in EDTA-X, the lower the initial and saturated PCEs of devices. In addition, the bias-dependent degrees improve with the H+ number (n). Then we find that the improved performance of devices attribute to the suppressed interfacial recombination, but the density of defect states was unchanged at same time. The H+ motion under the external electric field is determined to be responsible for the bias-dependent performance of devices. The characterization of the J response of interlayer-only devices over time under external bias further demonstrated our conclusion. At last, our study provide a new sight into designing organic electrolytes for i-OSCs with excellent PCE and none bias-dependent performance. ASSOCIATE CONTENT Supporting Information J–V curves of devices with EDTA-X interlayers under different bias. AFM images of ITO and EDTA-X interlayers surface. UPS of ITO/EDTA-X interlayers. The variation of PCEs with bias time for the devices with EDTA-X interlayers under 1 V and 4 V bias. The current density curves of ITO/EDTA-X/Al devices over time under 2 V bias under dark conditions. The J−V curves for devices with and without adding PCBA into active layers. Stability tests of the non-encapsulated i-OSCs with


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EDTA-X interlayers for 31 days. The Supporting Information is available free of charge on the http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]; * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The Project is supported by National Natural Science Foundation of China (61474125, 51403222), National Youth Top-notch Talent Support Program, and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences.



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Electrolytes as Cathode Interlayers in Inverted Organic Solar Cells

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