Versatile Device Architectures for High-Performing Light

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Versatile Device Architectures for High Performing Light-Soaking-Free Inverted Polymer Solar Cells Yu Yan, Feilong Cai, Liyan Yang, Wei Li, Yanyan Gong, Jinlong Cai, Shuang Liu, Robert R Gurney, Dan Liu, and Tao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08130 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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ACS Applied Materials & Interfaces

Versatile Device Architectures for High Performing Light-Soaking-Free Inverted Polymer Solar Cells Yu Yan1,2, Feilong Cai1,2, Liyan Yang1,2,Wei Li1,2,Yanyan Gong1,2, Jinlong Cai1,2, Shuang Liu1,2, Robert R. Gurney1,2, Dan Liu1,2, Tao Wang1,2* 1

School of Materials Science and Engineering, Wuhan University of Technology, Wuhan, 430070,

China 2

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology,

Wuhan, 430070, China * E-mail: [email protected]

ABSTRACT：：Metal oxide charge transport layers have been widely employed to prepare inverted polymer solar cells with high efficiency and long lifetime. However, the intrinsic defects in the metal oxide layers, especially those prepared from low-temperature routes, overshadow the high efficiency that can be achieved and also introduce “light-soaking” issues to these devices. In this work,

we

have

employed

polyethyleneimine

(PEI)

and

poly(9,9-bis(6 ′ -(N,N-

diethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) to modify our low-temperature-processed TiO2 electron-transport-layer(ETL), and demonstrated that the light-soaking issue can be effectively eliminated by PEI modifications due to the formation of abundant dipole moments, whilst PFN-OX was ineffective due to deficient dipole moments at the interface. Excitingly, PEI modifications enable versatile device architectures to obtain lightsoaking-free, inverted PTB7-Th:PC71BM solar cells with efficiencies over 10%, by adding PEI either in the bulk of or as an adjacent layer beneath or above the TiO2 ETL.

KEYWORDS: Polymer solar cells, Light-soaking, TiO2, PEI, PFN-OX,

1

ACS Paragon Plus Environment


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1. INTRODUCTION Solar cells capture and convert solar energy into electricity. Polymer solar cells (PSCs), as one type of photovoltaic device, can be made from low-cost solution processing. Dedicated efforts from molecular tailoring1-5, morphology control 6-10 and interfacial engineering 11-14 have driven a steady increase of the power conversion efficiency (PCE) of PSCs to over 10% in the past decade. Notably, ternary 15,16, tandem 17,18 and non-fullerene 19-21 PSCs have demonstrated great potential to further improve device PCE with better performance, indicative of a bright future for organic photovoltaics. Despite the encouraging achievements, numerous challenges still exist; issues include rigorous processing conditions,22 low operational lifetime,23 and unstable operational efficiency.24,25 Further efforts are required to be able to fabricate high efficiency PSCs with superior stability at low cost on large scales. It has been widely observed that most of those state-of-the-art PSCs adopt an inverted device architecture (referred as inverted polymer solar cells (i-PSCs)).26 In these i-PSCs, indium tin oxide (ITO) often acts as the cathode to collect photo-generated electrons. However, its intrinsic high work function (WF) cannot effectively block holes, and will degrade the device performance dramatically due to substantial recombination at the electrode interface. Interfacial materials were commonly applied on top of ITO to reduce its WF for effective charge injection. Among them, polymer electrolytes with either conjugated or non-conjugated backbones represent one of the most effective interfacial materials.27 For example, water-/alcoholsoluble poly (9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorine)(PFN) emerged and achieved an efficiency record in 2012.28 Then, a series of PFN analogues, e.g. poly[9,9-bis(6’-(N,Ndiethylamino)propyl)-fluorene-alt-9,9-bis-(3-ethyl(oxetane-3-ethyloxy)-hexyl)-fluorene] (PFN-OX) and

poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-

dioctylfluoren)] dibromide (PFN-Br))29-32, which have conjugated backbones and polar or ionic pendant groups, were applied to various optoelectronic devices. The delicate chemical structures of these polymer electrolytes endow them with orthogonal solubility and interfacial dipoles, which realized the possibility of multi-layer processing and reduced interfacial barriers for efficient charge extraction. Similarly, the most representative non-conjugated electrolytes, such as polyethylenimine (PEI)33,34 and polyethylenimineethoxylated (PEIE)35, which consisted of simple aliphatic amine groups, showed universal and valid modification effects to enhance the device performance by lowering the WF of cathode and subsequently forming favorable electron transporting pathways. 2


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Alternatively, transparent metal oxides such as ZnOx, TiOx and SnOx, benefiting from proper optical and electronic properties, also act as a classical type of electron transport layer (ETL) in iPSCs.36,37 However, devices using TiOx or ZnOx as ETLs often showed S-shaped current densityvoltage (J-V) curves during the initial J-V scans, where the current density reduced dramatically close to the open-circuit voltage. This S-kinked J-V curve will gradually recover with improved device metrics upon continuous AM 1.5G (100 mW cm-2) light illumination in a time scale ranging from several seconds to hours, a phenomenon that is called the light-soaking effect. Under realworld operations, devices may take a longer time to recover due to the low intensity of sunlight in the morning and evening or in a cloudy day, and the instable power output during the recovering process might damage electrical devices that were used to collect charges. Substantial efforts have been devoted to exploring the origins of light-soaking as well as to eliminate this phenomenon in i-PSCs. Early work stated that the imbalanced mobility caused the Sshaped J-V curve in planar heterojunction organic solar cells.38 However, this explanation may be logically impossible for the nowadays commonly explored bulk heterojunction solar cells, since the hole and electron mobility should differ by several orders of magnitude to cause a distinct S-shape. Model simulations 39,40 suggested that these atypical J-V curves originated from the reduced charge extraction efficiency close to one electrode which then caused charge accumulation and recombination. The latter can be verified by advanced characterizations such as in-situ scanning Kelvin probe 41, electron spin resonance (ESR) spectroscopy 42 and charge extraction by a linearly increasing voltage (CELIV)43. Some research attributed the appearance of the S-kink to the dangling groups or surface absorbed oxygen from the environment that acts as surface trap site in the metal oxide ETL44,45, which can be filled by the UV light excited electron along with the lowered WF and enhanced photoconductivity.46 This UV activation phenomenon was also believed to be effective through photo-induced rearrangement of the Fermi levels between ITO and TiOx47, however, this was challenged by further research concluding that the barrier existed at the interface between the active and electron-selective layers.48-51 Despite these debates, the consensus is that inefficient electron transport from the active layer to ITO induces the light-soaking effect, and the approach to overcome is to optimize electron-transporting and hole-blocking, e.g. by using n-type doping of metal oxides and achieving a favorable contact with the photoactive layer52-54, or lowering the WF to reduce the energy barrier at interfaces.55,56 In this work, we have employed low-temperature-processed, crystalline TiO2 nanoparticles as 3



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the electron transport material in i-PSCs using poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b′]dithiophene-co-3-fluorothieno [3,4-b]thiophene-2-carboxylate] (referred as PBDTTTEFT or PTB7-Th) as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electorn acceptor. PEI and PFN-OX were employed to modify the interface between TiO2 and the active layer. We found that the light-soaking issue in inverted glass/ITO/TiO2/PTB7Th:PC71BM/MoO3/Ag devices can be effectively eliminated by putting PEI on top of TiO2 due to the formation of abundant dipole moments, whilst a PFN-OX layer on top of TiO2 has been ineffective due to relatively deficient interfacial dipole moments. Excitingly, PEI modifications enable versatile device architectures to obtain light-soaking-free, inverted PTB7-Th:PC71BM PSCs with efficiencies over 10%, by adding PEI either in the bulk of or as an adjacent layer beneath or above the TiO2 ETL. 2. EXPERIMENTAL SECTION 2.1 Methods. TiO2 nanoparticles were synthesized following our previous report. The assynthesizedTiO2liquor was precipitated and purified three times with diethylether. The precipitate was collected and dispersed in ethanol by ultra-sonication to prepare a TiO2 dispersion with a solid content of ~5 mg/ml. To modify the TiO2 dispersion, different amounts of PEI (50% H2O solution, branched, Mn=60k by GPC, Aldrich) were added in and sited on a shaking table for 2h to allow efficient blending. All dispersions were stored in a refrigerator before use. 2.2 Device fabrication and testing. To fabricate OPV devices, pre-patterned ITO-glass substrates (resistance ca.15Ωper square) were cleaned by ultrasonication sequentially in water, acetone, ethanol and isopropyl alcohol for 10 min each, and then dried at 140 oC on a hotplate. Cleaned ITO substrates were further treated with UV-Ozone for 10 min. The TiO2 electron transport layer was cast from the TiO2 dispersion at 3000 rpm, followed by thermal annealing at 125 oC for 20 minutes to create a ca. 20 nm thick thin film. For the preparation of PEI/TiO2 orTiO2/PEI ETLs, the PEI solution was diluted with 2-methoxyethanol (2-ME) to give a 0.01 wt % solution and then cast onto ITO or TiO2 at 5000 rpm (40 ul) for 40 s with acceleration of 1000 rpm to form a 2 nm-thick thin film. The films were subsequently heated on a hotplate at 110 oC for 10 min. PFN-OX was dissolved in a mixed solution of methanol and acetic acid (v/v=100:1) to form a 0.5 mg/ml solution, and then spin-cast onto ITO or TiO2 at 3000 rpm (40 ul) for 40 s to form a ~3 nm film. The films were then heated at 155 oC under a fluorescent lamp to allow crosslinking. All these processes were 4


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performed in an ambient environment. The films were then transferred into an N2-filled glovebox to deposit the photoactive layer. The PTB7-Th (14.4 mg/ml) (purchased from Solarmer Materials Inc.) and PC71BM (21.6 mg/ml) solutions were prepared using chlorobenzene (CB) as solvent, and were mixed together with a 1:1.5 w/w ratio to create blend solution with a solid content of 18 mg/ml. 3 vol.% of 1,8diiodooctane (DIO) was then added to the blend solution and stirred for 3 hours before use. The photoactive layer was cast at 600 rpm onto the TiO2 layer create a film thickness of ~100 nm. The device substrates were transferred into an evaporation chamber and kept overnight under a high vacuum (~10-7 torr) to completely remove the residual solvent. Finally, 10 nm MoO3 and 100 nm Ag were deposited onto the photoactive layer through shadow masks by thermal evaporation. Each device substrate contains 8 pixelated sub-devices, with the size of each active area being 4 mm2 as defined by the shadow mask. All the devices were encapsulated with UV-curable epoxy glue and glass slides before removing from the glove box for device testing. XPS measurements were performed by using a Thermo Fisher Scientific PHI Quantera II system with a monochromatic Al Kα source. Surface morphologies of the TiO2 films were characterized by SPM (NT-MDT, Russia). Contact potential difference of ETLs was conducted with SPM (NT-MDT, Russian) equipped with Scanning Kelvin Microscopy-AM PASS II, and used the freshly exfoliated HOPG (4.62eV) as reference to calibrate the probes (NSG01/Pt/15). Film thickness was measured using a Dektak XT surface profiler (Bruker, USA) and cross-checked using spectroscopic ellipsometry. Water contact angle were measured with OneAttension measurement instrument (Biolin Scientific, Finland). Device characterization was performed under AM 1.5G(100 mW cm-2) conditions using a Newport 3A solar simulator in air at room temperature. The light intensity was calibrated using a standard silicon reference cell certified by the National Renewable Energy Laboratory (NREL)(Colorado, USA). J-V characteristics were recorded using J-V sweep software developed by Ossila Ltd. (Sheffield, UK) and a Keithley 2612B Source Meter unit (SMU). The active area of the i-PSCs was 4 mm2 defined by the shadow mask of the cathode， and the J-V characterization was performed with an aperture mask with accurately defined area of 2.12 mm2. External quantum efficiency (EQE) was measured with a Zolix EQE system equipped with a standard Si diode. The life-time measurements were conducted under ISOS-L-1 conditions. The encapsulated devices were subjected to the simulator (AM 1.5, 100 mW/cm2) in ambient humidity and temperature with 5



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continuous irradiation for 48h, and tested automatically every 15 min. Impedance measurements and dark charge extraction were performed on the Solartron ModuLab-XM (Ametek, Inc.) under various biases with the amplitude of 10 mV. The PSCs before and after light-soaking under a simulated one sun were measured under dark conditions from high to low frequency (1M to 100 Hz). Equivalent circuit simulations were conducted using the software package ZView 3.1 (Scribner Associate, Inc.). 3. RESULTS AND DISCUSSION

Figure 1. (a) The schematic diagram of the inverted PSCs with PTB7-Th:PC71BM as the active layer, (b) The molecular structures of interfacial materials PEI and PFN-OX.

We first compare the devices incorporating a thin PEI or PFN-OX interlayer on top of the TiO2 ETL, and the device with an unmodified TiO2 ETL serves as the reference. Figure 1 illustrates the configuration of our i-PSCs and the molecular structures of the donor, acceptor and interfacial materials in this work. The molecular structure of the non-conjugated PEI contains numerous amine groups in both the backbone and side chains. Whilst the conjugated PFN-OX has a rigid polyfluorene backbone and cross-linkable side chains, it contains less amine groups in the side chains. Previous studies have explored the effect of backbone to device performance

57

and

therefore this is outside of the focus of this work. The TiO2 nanoparticles (NPs) were prepared in our previous work and have been demonstrated to be an effective ETL to prepare high performance 6


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i-PSCs after compositional and surface modifications.53 In this work, TiO2 NPs was dispersed in ethanol, and then spin cast on pre-cleaned ITO substrate for further modifications using PEI or PFN-OX. Through device efficiency studies, we found that the optimum thickness of the PEI and PFN-OX interfacial layer in our inverted devices were 2~3 nm, which is consistent with previous reports.35,57,58

Figure 2. The evolutions of J-V curves of i-PSCs using (a) TiO2, (b) TiO2/PFN-Oxand (c) TiO2/PEI as the ETL. Comparisons of the (d) stabilized J-V curves and (e) external quantum efficiency (EQE) of the i-PSCs with TiO2/PEI、TiO2/PFN-Ox and TiO2 as the ETL. (f) Photoluminescence spectra of the active layer cast on TiO2/PEI,TiO2/PFN-OX and TiO2 ETLs. Table 1. Stabilized device metrics of i-PSCs with different ETLs. The average PCEs were obtained based on 12 individual devices.

PCEmax ETLs

FF

Jsc

Voc 2

Rs

Rsh 2

WF 2

(PCEave)(%)

(%)

(mA/cm )

(V)

(Ω cm )

(Ω cm )

(eV)

ITO/TiO2

9.49 (9.37 ± 0.10)

69.89

-17.25

0.796

6.58

462.02

4.82

ITO/TiO2/PEI

10.06 (9.76 ± 0.08)

71.85

-17.20

0.814

5.48

520.71

4.13

ITO/TiO2:PEI

10.05 (9.89 ± 0.12)

70.05

-17.68

0.812

5.42

449.84

4.42

ITO/PEI/TiO2

10.30 (10.13 ± 0.03)

71.72

-17.64

0.814

5.56

514.19

4.50

ITO/PEI/TiO2/PEI

9.97 (9.83 ± 0.08)

73.39

-17.46

0.778

6.12

532.24

4.24

ITO/TiO2/PFN-OX

10.05 (9.82 ± 0.04) 9.78 (9.68 ± 0.10)

70.46 70.06

-17.65 -17.75

0.796 0.794

6.87

560.13

4.64

ITO/PFN-OX/TiO2

6.57

430.90

4.80

ITO/PFN-OX

9.44 (9.40± 0.03)

66.54

-18.11

0.783

8.24

736.09

4.33

ITO/PEI

9.62 (9.43± 0.18)

67.02

-18.57

0.773

6.47

471.53

3.97

7



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For i-PSC employing unmodified TiO2 as the ETL, the device exhibited a serious lightsoaking effect, as shown in Figure 2a. The J-V curves in the initial scans have a typical S-shape, resulting in the low fill factor (FF) and open circuit voltage (Voc) and consequently low PCE. By adding a PFN-OX interfacial layer on top of the TiO2 ETL, the light-soaking effect still existed (see Figure 2b), the J-V sweep presented an S-shape and the FF and Voc gently recovered upon continuous light illumination. A remarkable contrast was obtained after adding a PEI layer on top of the TiO2 ETL, the S-shape disappeared and good reproducibility was obtained between different JV scans, as illustrated in Figure 2c. The stabilized J-V characteristics of the i-PSCs with various ETLs under simulated AM 1.5G illumination (100 mW cm-2) are plotted in Figure 2d and the detailed device parameters are summarized in Table 1. After stabilisation, the i-PSCs employing unmodified TiO2 ETL have a maximum PCE of 9.49%, with a FF of 69.89%, Jsc of 17.2mA/cm2 and

Voc of 0.796 V. The i-PSC devices with PEI modified TiO2 (TiO2/PEI) as the ETL showed enhanced FF and Voc compared to the pure TiO2-basedi-PSC, achieved a maximum PCE of 10.06%. The stabilized i-PSC device based on PFN-OX modified TiO2 (TiO2/PFN-OX) obtained slightly higher

Jsc and FF compared with the unmodified TiO2-based device, demonstrating a similar performance with the TiO2/PEI-based i-PSC. Figure 2e showed the EQE data of devices presented in Figure 2d, and the integrated Jsc values from EQE spectra support that a slightly higher Jsc can be achieved in the TiO2/PFN-OX-based i-PSCs. Steady-state photoluminescence (PL) spectra illustrated in Figure 2fwere obtained to examine the extraction of charge-carriers by different ETLs. Assuming the absence of any morphological changes within the bulk of the photoactive layer after casting on different ETLs, a lower PL spectrum intensity can be associated to better charge extraction from the photoactive layer by the ETLs. The results here suggested that superior charge carrier extraction can be achieved after surface modifications of the TiO2 ETL using ultrathin PEI or PFN-OX interfacial layers, which we think is the reason for the enhanced FF or Jsc. To elucidate the underlying mechanism, we firstly conducted the topography and microstructure characterizations. As illustrated in Figure 3a, pristine TiO2 ETL showed a relatively high RMS roughness (~5.8 nm) from the topographic atomic force microscopy (AFM). After putting an ultra-thin PEI layer on top, the roughness reduced slightly to 5.2 nm, an effect that is consistent with literature report on PEI coated ZnO.58 The TiO2 ETL coated with a thin PFN-OX layer was smoothest, with the lowest RMS roughness of only 4.1 nm. It is obvious that the hydrophilic and hydrophobic segments of the PEI and PFN-OX backbones respectively bring 8


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distinct differences to the ETL surfaces. The PEI coated TiO2 showed a reduced water contact angle (WCA) of 32o compared to 50o of the as-cast TiO2 film, due to the presence of a large amount of amine groups from PEI molecules, and demonstrated enhanced hydrophilicity. The PFN-OX coated TiO2 film, on the other hand, showed enhanced hydrophobicity with a WCA ca. 70o. The changes of the WCA indicate effective coating of the thin modification layers on top of TiO2. Although the PEI and PFN-OX modified TiO2 ETLs showed different hydrophilic properties, the photovoltaic solution (CB/DIO solution) wetted well on both surfaces during spin coating.

Figure 3. (a) Surface morphology of TiO2、TiO2/PEI and TiO2/PFN-OX ETLs with RMS roughness values and (b) water contact angles on these respective surfaces. (c) N 1s and (d) Ti 2p spectra of XPS obtained from PEI and PFN-OX modified TiO2, (e) contact potential distribution obtained from PEI and PFN-OX modified TiO2. HOPG was used as a reference to determine the work function of SKPM tips.

To further explore the modifications of PEI and PFN-OX to TiO2 surfaces, we have performed X-ray photoelectron spectroscopy (XPS) measurements. The N1s XPS spectrum is not detectable from the TiO2 surface, but displays peaks between 398 to 400 eV (see Figure 3c) on the PEI and PFN-OX coated TiO2 surfaces, confirming the presence of PEI and PFN-OX. The intensity of the N1s spectrum of the PEI coated TiO2 is much stronger compared to that of the PFN-OX coated 9



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TiO2, due to a much higher fraction of the N element in the molecular structure of PEI. The asymmetric N1s peaks in the PEI coated TiO2 can be deconvoluted to two individual peaks locating at 398 and 399.5 eV through a Gaussian-Lorentzian fitting, and are assigned to the neutral amine groups and the newly formed Ti-N bond after doping the underlying TiO2. This newly formed Ti-N bond presents in the PFN-OX coated TiO2 as well, and a weak shoulder at the higher binding energy ca.402.3 eV is associated with the protonated amines which might form due to the presence of acetic acid that has been introduced in order to crosslink PFN-OX. These results are consistent with the report by Zhou et al, who also reported the protonation of amines in an acidic medium, and concluded that the reduction of WF was mainly attributed to the neutral amines.34 The Ti 2p1/2 and Ti 2p3/2 peaks of the pristine TiO2 locate at the binding energy of 458.4 and 464.1eV with a fixed interval of 5.7 eV,53 and these peaks didn’t change after coating TiO2 with PFN-OX. However, the Ti 2p peaks both shift to a lower binding energy after coating TiO2 with PEI (see Figure 3d). Intuitively, these results would closely relate to the WF reduction of TiO2. We have therefore conducted the contact potential difference (∆V) characterization using scanning Kelvin probe microscopy (SKPM). As shown in the inset in Figure 3e, ∆V and the WF of SKPM tip (φtip) and sample (φtip) following the equation q ∆V=φtip-φsample. Stable and highly oriented pyrolytic graphite (HOPG) with a known WF of 4.62 eV was used as the reference to calibrate φtip. As shown in Figure 3e, the calculated WFs of TiO2/PEI and TiO2/PFN-OX from the CPD measurements are 4.13 and 4.64 eV respectively, both being smaller than that of the TiO2 film (4.82 eV). Surface modification by PEI is therefore more effective at reducing the WF of TiO2, and we attribute this to the increased amount of amino groups in the molecular structure of PEI compared to PFN-OX, as the amino groups will introduce interface dipoles to lower the WF for efficient charge extraction. The smallest surface WF of the TiO2/PEI film indicates a higher build-in voltage and therefore the enhanced Voc of TiO2/PEI-based i-PSCs, our results support this conclusion (see Table 1) and is consistent with results reported by Li et, al. 59 The light-soaking effect is present in the TiO2-based i-PSCs due to inefficient charge extraction and charge accumulation at the cathode interface. Although the PFN-OX modification to TiO2 can reduce its WF slightly, charge extraction is not as efficient and the light-soaking issue remains until a more effective surface modification using PEI is executed. When an S-shape appears in a J-V curve (see Figure 2 for instance), a distinct current density retardation occurs when the bias was closer to the Voc, suggesting that this behavior relates to the 10


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bias that was applied. Here, we have conducted the electrochemical impedance spectra (EIS) characterization under varying DC biases before (Figure 4a) and after (Figure 4b) light-soaking for TiO2-, TiO2/PEI- and TiO2/PFN-OX- based i-PSCs. The EIS spectra were fitted using the equivalent circuit model containing numerous resistance and capacitance (R and C) elements (see inset of Figure 4a). 53 The fitted elements R1 and C1 in the low-frequency range (from 100 to 2x105 Hz) (Figure 4c, e and g) represent contributions from the photoactive layer, and the fitted Rµ and Cµ in the high-frequency (from 2x105 Hz to 1 MHz) range (Figure 4d, f and h) represent contributions from device interfaces. Rs is the series resistance, R2 and Q are modification factors of the bulk photoactive layer. The resistance R1 associated with the BHJ layer is much higher than the interfacial resistance Rµ, and is consistent with the results reported by Kuwabara et al.49,50 As shown in Figure 4c, e and g, the changes of R and C elements for all three devices showed similar trends as a function of applied bias before and after the light-soaking treatments upon a period of 80 s illumination to completely eliminate any light-soaking issues in three i-PSCs). The R1 and C1 elements from all three devices reduced slightly after light-soaking, suggesting that there were marginal changes to the photoactive layer which shouldn’t account for the light-soaking phenomenon. However, for the high frequency Rµ and Cµ elements, Rµ showed a four-fold decrease after light-soaking treatments of the TiO2- and TiO2/PFN-OX- based devices (see Figure 4d and f), therefore it is the interfacial property changes that eventually eliminate the S-shaped J-V curve. The Rµ of the TiO2/PEI-based device demonstrated marginal changes (see Figure 4h), as light-soaking is absent from this device because PEI modification has enhanced the charge transport at the interface.

11



(a)

-1 V -0.8 V -0.75 V -0.70 V -0.65 V -0.60 V F ittin g

-15000

Z '' ( )

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-30000 -20000

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50000

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Z '( )

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C ap ac itan c e, in itial C ap ac itan c e after lig ht-s o akin g R es is tanc e, in itial R es is tanc e after lig h t-s o ak in g

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C a p ac itan c e ,in itial C ap a c itan c e , after lig h t-s o ak in g R es is tanc e, initial R es is tanc e, after lig ht-s oak in g

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Figure 4. Typical Nyquist plot of the impedance data before (a) and after (b) light soaking of pristine TiO2-based i-PSCs. Plots of fitted capacitance and resistance at various biases in the low-frequency range for (c) TiO2-, (e) TiO2/PFN-OX- and (g) TiO2/PEIbased i-PSCs, which represent contributions from the active layer. Plots of fitted capacitance and resistance at various biases in the high-frequency range for (d) TiO2-, (f) TiO2/PFN-OX- and (h) TiO2/PEI- based i-PSCs, which represent contributions from the interface.

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When the light-soaking issue presents in the i-PSC, we found that the interfacial capacitance Cµ increased after light-soaking (see Figure 4d of the TiO2-based device as an example), and this is also present in the capacitance-frequency plot in Figure S1. Figure S1 also shows the frequency dependence of the electrical loss of the TiO2-based device before and after light-soaking (obtained at a constant DC bias of -0.75 V at which a distinct S-kink would show, see Figure 2a). Before light-soaking, a plateau appeared in the intermediate frequency region of the capacitance-frequency plot. This is a characteristic of devices with inefficient interfacial charge extraction and the trapped charges would increase the capacitance.60 After light-soaking, the plateau disappeared as a result of better electron extraction but the capacitance and dielectric loss increased, which we tentatively attribute to the increased ability of the device ETL interface to block holes. For the TiO2-based device, hole carriers can flow to the cathode under such a forward bias of 0.75 V in dark, which means that the high WF TiO2 cathode has poor carrier selectivity and causes substantial recombination with a dramatically decreased fill factor. After the light-soaking process, holes can be effectively blocked and will accumulate at the cathode interface to increase the capacitance. This electronselectivity can be enabled by the light-soaking treatment (e.g. for TiO2-based i-PSCs) or interfacial modifications (e.g. for TiO2/PFN-OX-based i-PSCs). This interfacial carrier selectivity can be clearly illustrated through the capacitance-voltage (C-V) measurement.61 As shown in Figure S1, a distinct difference can be observed in the phase-bias plot of the Mott-Schottky curve before and after light-soaking. A distinct hysteresis of the phase angle under a number of forward biases was observed and indicates poor rectification properties of the device before light-soaking, and the device returned to normal after light-soaking when a notable built-in voltage started to exist. The results therefore suggest that the elimination of the light-soaking effects is also associated with enhanced interfacial carrier selectivity by executing a notable built-in voltage either after lightillumination or interlayer modification to the metal oxide ETL. The results from our experiments conclude that PEI modification on top of the TiO2 can decrease the energy barrier and enhance charge extraction from the photoactive layer to the ETL, eliminating the light soaking effect as well as maximizing the stabilized device efficiency. Alternatively, a series of polymers, such as PVP,62 PEO,63 PEG,64 PEI65 and PFN,66 have been introduced to the bulk of metal oxide ETL to enhance device efficiency. Here, we have also blended the TiO2 NPs with PEI solution directly and evaluated the modifications of PEI to the dispersion of TiO2 and the subsequent impact on device performance and the light-soaking phenomenon. The dispersion of the TiO2 after adding different amounts of PEI up to 35% (w/w) was firstly examined. The optical images of these dispersions are shown in Figure S2. Our original TiO2 dispersion was 13



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opaque, and gradually turned transparent until 10% (w/w) PEI was introduced, then turned opaque again with the presence of more PEI and eventually appeared chocolate-like when 35% of PEI was added. The Tyndall effect is a simple method to distinguish a dispersion from a solution.67 A green laser was used to penetrate these dispersions/solutions, and a similar trend was observed. The addition of PEI helps to disperse and stabilize TiO2 NPs with an optimal fraction around 10% (w/w) when a clear Tyndall effect was observed and the green laser can go through the vial. When the fraction of PEI is less than or higher than 10% (w/w), the green laser cannot penetrate through the vials due to a strong scattering effect from the TiO2 NPs or aggregates. With excessive PEI in the TiO2 dispersion, large TiO2 clusters will form due to the viscous nature of PEI. The surface morphology of films cast on ITO using dispersions with different amounts of PEI supported our conclusion. As can be seen from Figure S3, large clusters can be observed on the film surface cast from the dispersion with 7.5% (w/w) of PEI. Meanwhile, the films cast from dispersion with 10% PEI are cluster-free, and aggregates can be observed at high magnification (bottom right of Figure S3) on the film cast from dispersion with 12.5% PEI. Comparison of the device efficiency using TiO2:PEI as the ETL supports our morphological observations that 10% (w/w) PEI produced the best ETL to achieve the most efficient i-PSCs (see Figure S4). In fact, devices with clustered TiO2 film as the ETL showed poor reproducibility and notable standard deviations. The WCAs of as-cast TiO2 films with different amounts of PEI are showed in Figure S5. The WCAs started to decrease with the presence of a small amount of PEI (i.e. ~2.5%), and then exhibited abnormal variations with further addition of PEI. We attribute this abnormal behavior to the changes of surface roughness caused by the presence of TiO2 clusters. When the PEI content is higher than 12.5%, the film surface turns more hydrophilic due to the hydrophilic nature of PEI. With the presence of the optimum amount of PEI in the TiO2 dispersion, TiO2:PEI-based i-PSC was also light-soaking free (see Figure S7b) and a maximum PCE of 10.05% was achieved (see Table 1 and Figure 5). To further demonstrate the versatile device architectures that PEI modifications can enable, we prepared further i-PSCs by putting a thin PEI layer on top of ITO and underneath the pristine TiO2 film (see Figure 5c). Again, light-soaking-free i-PSC devices can be prepared (see Figure S6c) and a maximum PCE of 10.3% was achieved (see Table 1). We found that the PEI interfacial layer can reduce the WF of ITO from ~4.62 to ~3.97 eV, therefore reducing the energy barrier at the ITO interface to facilitate charge extraction. 68 Meanwhile, as PEI is soft in a viscous state, the surface 14


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part of the layer might easily penetrate into and modify the adjacent TiO2 layer to reduce its WF (~4.50 compared to ~4.82 eV without an underlying PEI layer, see Table 1), and is therefore most effective to achieve the highest efficiency as our results have demonstrated. We have also fabricated an i-PSC based on the ETL of PEI/TiO2/PEI, with a 2 nm PEI layer sitting on the top and underneath of the TiO2 layer simultaneously. The device exhibited a high FF but reduced Jsc and Voc, and consequently failed to achieve the a high PCE. We attribute this to the insulating nature of the PEI layers, whose multi-presence in the device architecture would inhibit charge transport although the WF of this ETL is also low (4.24 eV). As a comparison, we have also modified the ITO surface using a thin PFN-OX layer, we found that the light-soaking effect persisted in the device (see Figure S7). However, after the light-soaking process, the stabilized device showed improved PCE compared with the pristine TiO2-based i-PSC (see Table 1 for comparison). (b)

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The large amount of amino groups in the molecular structure of PEI certainly counts for this contrasting effect between PEI and PFN-OX modifications. For completeness, i-PSCs incorporating PEI or PFN-OX interlayers (ca. 2 nm) between ITO and the active layer were also conducted, and both type of devices delivered a moderate PCE of ca. 9.5%, a result that is consistent with previous work showing that these two interfacial layers can modify the ITO electrode to prepare efficient light emitting diodes and photovoltiacs. 69 Our observation here challenges the previous conclusion that the ITO/metal oxide interface is not the origin of the light-soaking issue in i-PSCs.64 As our 15



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results suggest, if this interface is properly modified using PEI, the light-soaking effect can be effectively eliminated enabling preparation of i-PSCs with enhanced efficiency. We note however that although PFN-OX modification is less effective at eliminating the light soaking issue, inverted TiO2-based polymer solar cells incorporating PFN-OX as the interfacial layer can also enhance the stabilized efficiency as well as the life-time compared to the TiO2-based i-PSC without any interfacial modifications, due to the conjugated backbone and crosslinking nature of PFN-OX. This is supported by our life-time testing measured under ISOS-L-1 conditions (continuous irradiation under one sun, with encapsulated devices been tested in ambient temperature and humidity) shown in Figure S8, where both TiO2/PFN-OX- and TiO2/PEI- based i-PSCs demonstrated enhanced stability with the latter being most stable.

4. CONCLUSION We have therefore demonstrated versatile device structures to prepare light soaking-free i-PSCs with efficiencies over 10%, by putting the PEI interlayer either underneath, in the bulk or on top of the TiO2 ETL. For PEI on top of TiO2, strong dipoles formed at the active layer/ETL interface can facilitate the extraction of photogenerated electrons. Doping of TiO2 via the electron-donating N atoms from PEI will also reduce traps and facilitate electron transporting. The higher WF and the relatively weak dipoles offered by PFN-OX modification lead to a light-soaking process for i-PSCs using either TiO2/PFN-OX or PFN-OX/TiO2 as ETLs, and inhibit the maximum achievable device PCE. However, after UV activation during the light-soaking process, an i-PSC employing TiO2/PFN-OX as the ETL can deliver enhanced performance. And for i-PSCs receiving PEI modifications, strong dipole moments can be present either on top of, in the bulk of or underneath the TiO2 surface to favor charge extraction, and enable versatile device architectures to fabricate light-soaking-free, high performance PSCs. Our investigation provides a feasible solution to modify other types of metal oxide interfaces to eliminate light-soaking effects in inverted polymer solar cells. ASSOCIATED CONTENT Supporting Information Capacitance and dielectric loss spectra of TiO2-based device before and after light-soaking; Tyndall effects for TiO2 dispersions with different amounts of PEI; Optical surface images of ETLs cast 16


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from TiO2 dispersions with different amounts of PEI; PCE of TiO2:PEI-based i-PSCs as a function of PEI content in the TiO2 dispersion; Water contact angles of TiO2 films cast from dispersions having different content of PEI; J-V curves of i-PSCs employing (a) TiO2/PEI, (b) TiO2:PEI and (c) PEI/TiO2 as the ETL; J-V characteristics of PFN-OX/TiO2-based i-PSC showing light-soaking phenomenon; Life-time results tested under ISOS-L-1 condition. ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (Grant No. 21504065, 21774097), and the Fundamental Research Funds for the Central Universities (WUT: 2017IVA002) of China. TW also acknowledges support from the Recruitment Program of Global Experts (1000 Talents Plan) of China. We thank Prof. Fei Huang at South China University of Technology for providing the PFN-OX material. REFERENCES (1) Albrecht, S.; Janietz, S.; Schindler, W.; Frisch, J.; Kurpiers, J.; Kniepert, J.; Inal, S.; Pingel, P.; Fostiropoulos, K.; Koch, N.; Neher, D. Fluorinated Copolymer PCPDTBT with Enhanced OpenCircuit Voltage and Reduced Recombination for Highly Efficient Polymer Solar Cells. J. Am. Chem. Soc., 2012, 134(36), 14932-14944. (2) Zhong, H.; Li, C. Z.; Carpenter, J.; Ade H.; Jen, A. K.-Y. Influence of Regio-and Chemoselectivity on the Properties of Fluoro-Substituted Thienothiophene and Benzodithiophene Copolymers. J. Am. Chem. Soc. 2015, 137(24), 7616. (3) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Accounts Chem. Res.2012, 45(5), 723-733. (4) Ye, L.; Zhang, S.; Huo, L.; Zhang, M.; Hou, J. Molecular Design Toward Highly Efficient Photovoltaic Polymers Based on Two-Dimensional Conjugated Benzodithiophene. Accounts Chem. Res. 2014, 47(5), 1595-1603. (5) Yao, H.; Ye, L.; Zhang, H.; Li, S.; Zhang, S.; Hou, J. Molecular Design of BenzodithiopheneBased Organic Photovoltaic Materials. Chem. Rev. 2016, 116(12), 7397-7457. (6) Collins, B. A.; Li, Z.; Tumbleston, J. R.; Gann, E.; McNeill, C. R.; Ade, H. Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7: PC71BM Solar Cells. Adv. Energy Mater. 2013, 3(1), 65-74. (7) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nature Commun. 2014, 5, 5293. 17



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(43) Sundqvist, A.; Sandberg, O. J.; Nyman, M.; Smatt, J. H.; Österbacka, R. Origin of the SShaped JV Curve and the Light-Soaking Issue in Inverted Organic Solar Cells. Adv. Energy Mater. 2016, 6, 1502265. (44) Nam, C. Y. Ambient Air Processing Causes Light Soaking Effects in Inverted Organic Solar Cells Employing Conjugated Polyelectrolyte Electron Transfer Layer. J. Phys. Chem. C 2014, 118(47), 27219-27225. (45) Zhou, Y.; Cheun, H.; Potscavage Jr, W. J.; Fuentes-Hernandez, C.; Kim, S. J.; Kippelen, B. Inverted Organic Solar Cells with ITO Electrodes Modified with an Ultrathin Al2O3 Buffer Layer Deposited by Atomic Layer Deposition. J. Mater. Chem. 2010, 20(29), 6189-6194. (46) Lin, Z.; Jiang, C.; Zhu, C.; Zhang, J. Development of Inverted Organic Solar Cells with TiO2 Interface Layer by Using Low-Temperature Atomic Layer Deposition. ACS Appl. Mater. Inter. 2013, 5(3), 713-718. (47) Kim, J.; Kim ,G.; Choi, Y.; Lee, J.; Park, S. H.; Lee, K. Light-Soaking Issue in Polymer Solar Cells: Photoinduced Energy Level Alignment at the Sol-Gel Processed Metal Oxide and Indium Tin Oxide Interface. J. Appl. Phys. 2012, 111(11), 114511. (48) Cowan, S. R.; Schulz, P.; Giordano, A. J.; Garcia, A.; MacLeod, B. A.; Marder, S. R.; Kahn, A.; Ginley, D. S.; Ratcliff, E. L.; Olson, D. C. Chemically Controlled Reversible and Irreversible Extraction Barriers via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole-Based Organic Solar Cells. Adv. Funct. Mater. 2014, 24(29), 4671-4680. (49) Kuwabara, T.; Kawahara, Y.; Yamaguchi, T.; Takahashi, K. Characterization of Inverted-Type Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by ac Impedance Spectroscopy. ACS Appl. Mater. Inter. 2009, 1(10), 2107-2110. (50) Kuwabara, T.; Iwata, C.; Yamaguchi, T.; Takahashi, K. Mechanistic Insights into UV-Induced Electron Transfer from PCBM to Titanium Oxide in Inverted-Type Organic Thin Film Solar Cells using AC Impedance Spectroscopy. ACS Appl. Mater. Inter. 2010, 2(8), 2254-2260. (51) Trost, S.; Zilberberg, K.; Behrendt, A.; Polywka, A.; Görrn, P.; Reckers, P.; Maibach, J.; Mayer, T.; Riedl, T. Overcoming the “Light-Soaking” Issue in Inverted Organic Solar Cells by the Use of Al: ZnO Electron Extraction Layers. Adv. Energy Mater. 2013, 3(11), 1437-1444. (52) Lim, F. J.; Set, Y. T.; Krishnamoorthy, A.; Ouyang, J.; Luther, J.; Wei H. G. Addressing the Light-Soaking Issue in Inverted Organic Solar Cells using Chemical Bath Deposited Fluorinated TiOx Electron Transport Layer. J. Mater. Chem. A 2015, 3(1), 314-322. (53) Yan, Y.; Cai, F.; Yang, L.; Li, J.; Zhang, Y.; Qin, F.; Xiong, C.; Zhou, Y.; Lidzey, D. G.; Wang, T. Polymer Solar Cells: Light-Soaking-Free Inverted Polymer Solar Cells with an Efficiency of 10.5% by Compositional and Surface Modifications to a Low-Temperature-Processed TiO2 ElectronTransport Layer. Adv. Mater. 2017, 29(1), 1604044. 21



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For Table of Contents use only

Versatile Device Architectures for High Performing Light-Soaking-Free Inverted Polymer Solar Cells

Yu Yan, Feilong Cai, Liyan Yang, Wei Li, Yanyan Gong, Jinlong Cai, Shuang Liu, Robert R. Gurney, Dan Liu, Tao Wang*

PEI PEI

TiO2

TiO2

ITO

ITO 10

T iO 2 :P E I

8 P C E (% )

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TiO2 TiO2/PEI

P E I/T iO 2 T iO 2 /P E I

6 4 2

PEI

ITO

TiO2:PEI

0 0

2

4 6 8 10 12 Irrad iatio n tim es (s )

24


14

Versatile Device Architectures for High-Performing Light

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