Realizing High Performance Inverted Organic Solar Cells via a

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Realizing High Performance Inverted Organic Solar Cells Via a Non-Conjugated Electrolyte Cathode Interlayer Yaru Li, Xiaodong Li, Xiaohui Liu, Liping Zhu, Wenjun Zhang, and Junfeng Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10558 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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The Journal of Physical Chemistry

Realizing High Performance Inverted Organic Solar Cells Via a Non-Conjugated Electrolyte Cathode Interlayer Yaru Li,a Xiaodong Li,a Xiaohui Liu,a Liping Zhu,a Wenjun Zhanga and Junfeng Fanga* Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201 (China). Corresponding Author E-mail: [email protected]; Telephone: +86-574-87617770 Abstract Organic electrolyte interlayer has been widely used in organic solar cells (OSCs). However, previous reports mainly focused on conventional OSCs despite with poor stability. In more stable inverted OSCs, the application of organic electrolyte interlayer is rarely reported and the device efficiency is low. Here, a simple non-conjugated small molecule electrolyte (SME), ethylene diamine tetraacetic acid tetrasodium (EDTA-4Na), is successfully introduced into inverted OSCs as a cathode interlayer, leading to a high efficiency of 9.69%. Besides efficiency, the device stability is also improved and almost 85% of the efficiency can be maintained even after storage of 46 days. More importantly, EDTA-4Na tremendously alleviates the hysteresis phenomenon

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happening in organic optoelectronic devices based on ionized interlayer. As a result, stabilized power output of ~9.5% is obtained in OSCs with EDTA-4Na. These inspiring results prove that electrolytes with simple structure can also achieve excellent performance and that nonconjugated SME is also a competitive candidate as cathode interlayer, especially in inverted OSCs. Introduction Bulk-heterojunction (BHJ) organic solar cells (OSCs) have been receiving a great attention due to their advantages of low cost, light weight, flexibility and ease of large area fabrication through roll-to-roll printing.1-2 Improving the power conversion efficiencies (PCEs) of OSCs is one of the most important issues and the PCE has been promoted over 10% in the past decades.35

Besides new materials design and morphology optimization of the photoactive layer,6-7

interfacial modification between active layer and electrode has also been proved to be an efficient approach to enhance device PCEs.8-15 Among various interlayer materials, organic electrolytes have been widely used in organic photoelectric devices due to their low temperature solution processability and work function (WF) adjustability through changing the chemical structure.16 However, previous reports mainly focus on conjugated system since π-delocalized structure is considered crucial for the conductivity of organic materials. Recently, we found that organic small molecule electrolytes (SMEs) without any conjugated unit also worked well as the interlayer in both OLEDs and OSCs, further broadening the design conception of interlayer materials from conjugated to non-conjugated system.17-18 The advantages of the non-conjugated SMEs lie in the simple synthesis process and well-defined chemical structure, which could lead to high purity materials and good device reproducibility.17, 19-20


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Stability is another important topic in OSCs study apart from efficiency. To solve this issue, inverted device architecture is invented; where high WF metals (Au, Al) combined with airstable metal oxides (NiO, MoO3) are used as the top anode to collect holes. As a result, the device performance, in great extent, is related to the interlayer engineering between ITO cathode and active layer.8, 21 Generally, neutral conjugated polymer interlayers, such as PFN and other amine-functional materials, are adopted in inverted OSCs with good device performance.

3 22-26

However, their similar solubility to active layer brings serious solvent erosion and interface mixing during device fabrication, which will be a great hindrance for large area device fabrication and reproducibility. To avoid this problem, conjugated electrolyte through quaternization of neutral amines is a well-known choice to realize orthogonal solubility.27-28 But this introduction of electrolyte interlayers will affect the morphology of the active layer on top, decreasing device PCEs, which has been proven in our previous study.29 As a result, most electrolytes interlayers can only work well in conventional OSCs28, 30-32 and limited electrolytes interlayers are reported in inverted OSCs so far,13,

33-35

especially for top-level efficient

devices.36-37 For example, through side-chain design of conjugated electrolyte interlayer, our group overcame the erosion problem and simultaneously realized efficient inverted OSCs with PCE ~8% due to the control of interlayer film wetting properties.38 Recently, inverted OSCs with PCE of ~9% are reported by introducing fullerene based electrolytes interlayer.36-37 However, these limited reports all focused on conjugated system with complicated synthesis and purification process, which will increase the manufacture cost of OSCs. Great efforts are still needed to explore better electrolytes interlayer and further improve device efficiency.28,

39-40

Under the guidance of above rules, we find that disodium edetate (EDTA-2Na), which has extremely simple chemical structure without any conjugated unit, also works well as cathode


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interlayer in inverted OSCs.41 But the device efficiency could vary from 6.0% to 9.0% under the different measurement conditions such as measuring times, direction and range of the scanning bias, exhibiting severe device hysteresis, which would limit its practical application.42-43 In this communication, we introduce a simple non-conjugated small molecule tetra sodium edetate (EDTA-4Na) as cathode interlayer in OSCs. The hysteresis like phenomenon is significantly alleviated and the PCE can further increase to 9.69%. The stabilized power output under voltage of the maximum power point presents an outstanding PCE of ~9.5%. Importantly, the inverted devices with EDTA-4Na show excellent stability and over 80% of its initial PCE can be maintained even after storage for 46 days. Our results confirm the great potential of nonconjugated small molecule interlayer materials once more and it simultaneously proves that “simple” can also mean “excellent”. Experimental Materials: The tetra sodium salt of EDTA (EDTA-4Na) was from Aladdin (China). PTB7-Th and PC71BM were purchased from 1-Material Chemscitech Inc. (Canada), American Dye Source, Inc. (USA), respectively. Devices fabrication: The ITO/glass electrode was cleaned by successive sonication in detergent, deionized water, acetone and isopropyl alcohol, and then dried by nitrogen flow. The cleaned ITO electrodes were then treated by UV-Ozone for 20 minutes. Next, the organic small molecules of EDTA-4Na solution (3mg/mL in water) was spin-coated on pre-ITO glass at 4000 O

rpm for 60 s and heated at 140 C for 15 min in air. The active layer fabrication is reported previously.5 Finally, 10 nm MoO3 (0.25 Å/s) and 100 nm Al (1 Å/s) were deposited to form the anode. The device area is 6 mm2.


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Device characterization: The J-V measurements were obtained with Keithley 2440 source meter. Illumination was provided by a solar simulator (Oriel So1 3A) with AM 1.5G spectra at 100 mW/cm2. Ultraviolet photoelectron spectroscopy (UPS) was operated by a Kratos AXIS ULTRA DALD system. Results & Discussion

Figure 1. a) Chemical structure of EDTA-4Na; b) energy level diagrams of OSCs; c) J–V characteristics of inverted OSCs with bare ITO, EDTA-4Na and PFN interlayer under AM 1.5 G illumination of 100 mW cm−2. d) PCE distribution of inverted OSCs with EDTA-4Na interlayer.


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Table 1. Devices parameters of inverted OSCs with bare ITO, EDTA-4Na and PFN interlayer. Interlayer

Voc

Jsc

PCE

Rs

Rsh 2

(Ω•cm ) (Ω•cm2)

(V)

(mA/cm )

(%)

(%)

Bare ITO

0.330

15.04

50.8

2.52

7

170

EDTA-4Na

0.794

17.00

71.8

9.69

3

1033

4

1018

a

(0.788±0.004) PFN

0.794 (00.788±0.006)

a

FF 2

(17.02±0.24) (69.8±1.18) (9.34±0.17) 16.58

71.5

9.41

(16.72±0.32) (69.4±1.33) (9.15±0.26)

the average devices parameters are among 48 separated devices. The inverted device configuration and chemical structure of EDTA-4Na are depicted in

Figure 1a. When introducing EDTA-4Na as cathode interlayer, the work function of ITO is greatly reduced from 4.78 eV to 4.07 eV (Figure S1), which is much close to the LUMO level of PC71BM and favorable for electrons transfer to ITO cathode (Figure 1b). As a result, the devices with EDTA-4Na interlayer exhibit excellent device performance and the PCE can reach 9.69% with Voc of 0.794 V, Jsc of 17.00 mA cm-2, and FF of 71.8% (Figure 1c, Table 1). The device performance is even slightly better than that with PFN interlayer (PCE of 9.41%). In addition, the OSCs with EDTA-4Na also show good reproducibility and the average PCE is 9.34±0.17% (9.15±0.26% for PFN based devices) among 48 separated devices (Figure 1d).


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Figure 2. a) Normalized J-V curves of OSCs with EDTA-4Na under different light intensity; b) FF and c) Voc variation with light intensity for OSCs with bare ITO, PFN and EDTA-4Na; d) Electrical impedance spectra of OSCs in dark conditions with bare ITO, PFN and EDTA-4Na; Inset: spectra of OSCs with bare ITO. To gain deeper insight into the working mechanism of EDTA-4Na interlayer, the J-V curves are measured under different light intensity ranging from 6.25 to 100 mW/cm2. To investigate the relationship between photo-generated current density and carriers extraction, J-V curves are normalized by the reverse saturation current (Jsat).44 In the range from reverse bias to maximum power point, almost no obvious difference in normalized J-V curves is observed for EDTA-4Na based device as shown in Figure 2a. And this indicates that carriers extraction is not determined by its generation rate and the carriers recombination is minimal in the range from short circuit to


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maximum power point.44-45 While in range from maximum power point to Voc (open circuit), the J-V curves under different light intensity starts to change, thus affecting the FF of OSCs. As shown in Figure 2b, FF gradually reduces with the increase of light intensity, especially at intensity around 100 mW/cm2, which can be attributed to the trap states between active layer and ITO cathode. In OSCs with EDTA-4Na, the decrease of FF is minimal, while in OSCs with PFN, FF decrease is moderate, and in OSCs with bare ITO, the FF decrease is maximum (Figure S3 and S4), implying interfacial traps are greatly reduced due to the introduction of EDTA-4Na interlayer.44 The decrease of interfacial traps will suppress the traps-assisted recombination in OSCs, thus improving devices performance. In addition, the relationship between Voc and light intensity is also studied to further investigate the recombination in EDTA-4Na based OSCs (Figure 2c). If bimolecular recombination is dominant in OSCs, the devices Voc yields a slope of kT/q versus natural logarithm of light intensity, where k, T and q represent the Boltzmann constant, Kelvin temperature, elementary charge.13, 46 If extra traps assisted recombination is involved in OSCs, the slope of Voc versus natural logarithm of light intensity will become larger than kT/q.13, 46 In OSCs with EDTA-4Na, the slope is 1.04 kT/q (very close to kT/q), and slope is 1.14 kT/q in OSCs with PFN, whereas it increased to 1.27 kT/q in OSCs with bare ITO, indicating that EDTA-4Na reduces the density of interfacial traps, thus suppressing traps-assisted recombination in OSCs. Electrical impedance spectra (EIS) under dark conditions are further conducted to indepth study the carriers transfer and recombination mechanism. From previous reports, the lowfrequency arc in EIS mainly depends on the recombination in OSCs.47 To quantify the recombination resistance (Rrec) of OSCs, the EIS spectra is fitted as shown in Fig. 2d and large EIS arc size means large Rrec, thus suppressing recombination in OSCs. For OSCs with bare


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ITO, the size of semicircle arc is quite small (Figure 2d inset), indicating the existence of small Rrec and severe interfacial recombination. When introducing EDTA-4Na, the arc size is greatly increased, even larger than that of OSCs with PFN, leading to a much increased Rrec, thus suppressing the interfacial recombination. All these results reveal that besides WF regulation, EDTA-4Na can also work well as an effective barrier to reduce interfacial recombination, agreeing with its best device performance.

Figure 3. a) J-V curve of OSCs with EDTA-4Na under forward scan (Voc to Jsc) and reversed scan (Jsc to Voc); b) J−V curves of OSCs with EDTA-4Na after repeated measurements under the scan range from 1.5 V to −1.5 V; c) The J−V curves of EDTA-4Na based OSCs with 2 V bias treatments; d) The stabilized power output under voltage of the maximum power point (0.615V).


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In our previous studies with EDTA derivatives interlayer (EDTA-2Na), the devices show severe hysteresis. That is, the device performance greatly depends on the measurement conditions, such as measuring times, direction and range of the scanning bias, and external bias treatment, which limits its practical application.41, 48 To further confirm whether this hysteresis phenomenon still exists in our OSCs with EDTA-4Na, the device hysteresis is investigated as shown in Figure 3. Interestingly, the hysteresis is completely eliminated when adopting EDTA4Na interlayer. The J-V curve is almost the same whether under forward (PCE of 9.6%) or reverse scan (PCE of 9.5%) conditions, indicating the hysteresis-free performance (Figure 3a). In addition, no obvious variation of device performance is observed even after 10 times repeated measurements (Figure 3b). Importantly, different to our previous studies in EDTA-2Na, external bias treatment (2 V bias) has no impact on the performance of devices with EDTA-4Na (Figure 3c), further confirming the eliminated device hysteresis. The power output under maximum power point (0.615 V) stabilizes at ~9.52% as shown in Figure 3d, indicating the reliable performance in our EDTA-4Na based devices. In previous studies, ions motion49 in EDTA-2Na interlayer under external bias is considered to be the reason for device hysteresis.48 However, in EDTA-4Na, the movable H+ ions are replaced with Na+ ions. The 8 times larger ions radius of Na+ (0.098 Å) than that of H+ (0.012 Å) makes the ions motion difficult in EDTA-4Na interlayer, especially in solid state, thus eliminating device hysteresis. The specific relationship between device hysteresis and interfacial modification will be another subject of future investigation.


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Figure 4. a) Stability test of the non-encapsulated inverted and conventional devices with EDTA-4Na interlayer in glove box (dark conditions); b) J–V characteristics of conventional OSCs with EDTA-4Na interlayer. Apart from efficiency, device stability is another important issue in OSCs. As shown in Figure 4a, inverted OSCs with EDTA-4Na exhibit good stability. 84.3% of its original PCE value can be maintained even after storage for 46 days without encapsulation. In comparison, conventional devices with structure of ITO/PEDOT:PSS/PTB7-Th:PC71BM/EDTA-4Na/Al are also fabricated as shown in Figure 4b. Though good device efficiency with PCE of 9.74% is obtained (Figure S5), the conventional devices show poor stability. Only 32.7% of its initial PCE can be maintained in the same conditions, which further confirms the stability advantages of inverted devices. The lifetime decay in conventional devices is mainly ascribed to the decrease of Jsc and FF (Figure S6). Conclusions In conclusion, we have fabricated high performance OSCs by using a simple nonconjugated small molecule interlayer (SMEs, EDTA-4Na) as cathode interlayer. The high performance is attributed to the effective regulation of ITO WF and suppressed interfacial recombination by introduction of EDTA-4Na. As a result, the device efficiency can be increased


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to 9.69%. In addition, inverted devices with EDTA-4Na show better stability than conventional devices and 84.3% of its original PCE can be maintained after storage for 46 days. Importantly, the application of EDTA-4Na can completely alleviate the device hysteresis in previous reports and a stabilized power output of 9.52% is obtained. Our results prove that organic ion materials, especially non-conjugated small molecule electrolytes with simple chemical structure, can also be competitive candidate as the cathode interlayer in inverted OSCs. Supporting Information. Additional characterization (UPS, EQE, SEM, AFM). The normalized J-V curves of i-OSCs under different light intensity. The distribution of the PCEs for conventional OSCs. The Jsc and FF decay of conventional and inverted devices with EDTA-4Na. Thermal stability of EDTA-4Na based i-OSCs under 40 oC and 60 oC. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author E-mail: [email protected]; Telephone: +86-574-87617770 Notes Yaru Li and Xiaodong Li contributed equally to this work. The authors declare no competing financial interests. Acknowledgments The Project is supported by National Natural Science Foundation of China (61474125, 51273208), National Youth Top-notch Talent Support Program, Hundred Talent Program of


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Realizing High Performance Inverted Organic Solar Cells via a

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