Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a

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Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer Kai Wang, Zhan Zhang, Chang Liu, Qiang Fu, Wenzhan Xu, Chongwen Huang, Robert A. Weiss, and Xiong Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13642 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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ACS Applied Materials & Interfaces

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer Kai Wang,1 Zhan Zhang,1 Chang Liu,1 Qiang Fu,1 Wenzhan Xu,1,2 Chongwen Huang,1 R. A. Weiss1 and Xiong Gong1,2* 1) Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, USA 2) State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China. Abstract: In this paper, we report the highly efficient bulk heterojunction (BHJ) polymer solar cells (PSCs) with an inverted device structure via utilizing an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer to reengineer the surface of the solution-processed zinc oxide (ZnO) electron extraction layer (EEL). The unique lithium-ionic conductive LiSPS contributes to enhanced electrical conductivity of the ZnO/LiSPS EEL, which not only facilitate charge extraction from BHJ active layer, but also minimize the energy loss within the charge transport processes. In addition, the organic-inorganic LiSPS ionomer well circumvents the coherence issue of the organic BHJ photoactive layer on the ZnO EEL. Consequently, the enhanced charge transport and the lowered internal resistance between the BHJ photoactive layer and the ZnO/LiSPS EEL give rise to a dramatically reduced dark saturate current density and significantly minimized charge carrier recombination. As a result, the inverted BHJ PSCs with the ZnO/LiSPS EEL exhibit an approximatively 25% increase in power conversion efficiency. These results indicate our strategy provides an easy but effective approach to reach high performance inverted PSCs.

Corresponding author, Email: [email protected]; Fax: (330) 972-3406 KEYWORDS: ionomer, inverted polymer solar cells, charge transport, interfacial modification, charge carrier recombination

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Introduction Bulk heterojunction (BHJ) polymer solar cells (PSCs) have been considered as promising alternatives to the traditional silicon solar cells owning their unique potentials in low-cost mass production, high throughput manufacturing for green, flexible and large-scale applications.1-5 In the past decades, various strategies including development of novel efficient conjugated polymers, optimization of thin film morphologies of BHJ photoactive layer and amelioration of device structures especially the interfacial engineering, have made power conversion efficiencies (PCEs) beyond 10% from single-junction PSCs and ~12% from tandem PSCs with an conventional device structure.6,7 However, the conventional-structured PSCs always exhibit inferior device stabilities due to the acidic poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) anode buffer layer and the low work function (WF) active metal cathode.8-11 Furthermore, the dilemma where the energy-intensive deposition of top electrodes and lack of low-WF metal inks for low-temperature printing technology makes the conventional-structured PSCs far away from industrializations.12,13 For circumventing above issues, an inverted device structure with robust electrode buffer layers and stable high WF metal electrodes (e.g., Ag and Au) has been invented.14-17 To facilitate charge extraction, transport and collection as well as to maintain good device stability, metal oxides and metal oxide composites including zinc oxide (ZnO), titanium oxide (TiOx) and cesium carbonate (Cs2CO3), as the electron extraction layers (EEL)18-21 have been employed in the inverted PSCs. 2


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Nevertheless, the intrinsic surface defects and poor surface electrical conductivities of solution-processed metal oxides22-26 still limit the charge extraction and transport efficiencies, lowering the charge carrier collection efficiency at the electrode and thus resulting in a low short-circuit current density (JSC) and a small fill factor (FF). In addition, the poor coherence at the interface between the metal oxides EEL and BHJ photoactive layer would further deteriorate the electron extraction process and cause serious trap-assisted charge carrier recombination, which inevitably compromises the resulting device performance.27 To address these issues, an ultra-thin layer from conjugated polyelectrolytes (CPEs),28,29 and various self-assemble monolayers (SAM)30-33 has been used to reengineer the interface between the EEL and BHJ photoactive layer. However, previous researches mainly focused on the utilization of CPEs and SAMs to tune the WFs of the EELs to enlarge the open circuit voltage (VOC) via dipole momentum induced by these materials.28,30,32 The utilization of the highly electrical conductive amphiphilic materials as the interfacial layers to facilitate the charge extraction and to reduce the energy loss during the charge transport processes are rarely reported. Recently, our group reported the utilization of CPEs and water-soluble neutral fullerene derivatives as the interfacial modification layer to reengineer the EEL in the inverted PSCs,28,33 where the dramatically enlarged JSC but the invariant VOC revealed the importance of the surface electrical conductivity of the EEL on the device performance.

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Similar to CPEs and SAMs, organic-inorganic ionomers, which consists of neutral repeating units and highly ionized electrical conductive units, can also be used as the interfacial modification layer to bridge the metal oxide EEL and BHJ photoactive layer. Furthermore, high thermal stability at solid state and high electrical conductivity of ionomers make itself more preferable materials for efficient and stable inverted PSCs. In this study, we employ an ultrathin layer of lithium sulfonated polystyrene (LiSPS) ionomer, as an interfacial modification layer to reengineer the surface

of

the

solution-processed

ZnO

EEL.

By

using

the

poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-ﬂuoro-2-[ (2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7)34 as the electron donor and the [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) as the electron acceptor to form BHJ photoactive layer, 25% enhancement in PCE has been obtained from the inverted device using the ZnO/LiSPS EEL, in comparison with that of the inverted device using the ZnO EEL.

Experimental Details Materials. Both PTB7 and PC71BM were purchased from 1-Material Inc. and used as received. The LiSPS was synthesized as follows: low molecular weight polystyrene (PS) with polydispersity index of 1.06 was sulfonated randomly in dicholoroethane (o-DCB) by acetyl sulfate. After the sulfonation, a 50% excess of lithium hydroxide or acetate was added to SPS acid derivative in mixed solvents of methanol and toluene with a volume ratio of 9:1 to conduct the complete conversion. The 4


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neutralizing base in methanol was then added by drop-wise for making an agitated polymer solution. Then the steam distillation was used to recover the neutralized SPS, which was further washed and dried.35 The degree of sulfonation (DS) was determined by elemental analysis (Galbraith Laboratories, Inc.). The weight percentage of lithium (wt% Li) in 50 mg samples of lyophilized LiSPS was 0.3%. Thus the DS is determined to be 4.8 mol % in LiSPS. ZnO and ZnO/LiSPS thin films: ZnO thin film was solution-processed on the ITO (indium tin oxides) substrates. Zinc acetate (Sigma-Aldrich, 99.99%) and ethanolamine

(Sigma-Aldrich,

≥99.5)

were

dissolved

in

2-methoxyethanol

(Sigma-Aldrich, 99.8%) then the solution was kept stirring for 12 hours. The precursor ZnO solution was spin-casted on the pre-cleaned ITO glasses and then annealed at 200 ºC in air for 60 minutes. Afterwards, the ZnO coated ITO substrates were ultra-sonicated in isopropyl alcohol and then dried in oven for 12 hours. The thicknesses were measured to be ~30 nm. The ZnO/LiSPS thin film was prepared by deposition of a thin layer of LiSPS from LiSPS solution. The thickness of LiSPS was controlled to be from 0 to 3, to 5, to 10 nm via tuning the precursor solution concentration and spin-speed during the deposition. Accordingly, the thickness of the ZnO/LiSPS thin film was measured to be from 30 to 40 nm for each thickness of LiSPS layer, where the thickness of ZnO for each condition is kept invariant of ~ 30 nm while the LiSPS ultrathin layer is in the range of 0 to 10 nm. Thin film characterization. The absorption spectra of ZnO and ZnO/LiSPS thin films were measured by a HP Agilent 8453 UV-visible spectrophotometer. The 5



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surface morphologies of ZnO, ZnO/LiSPS were obtained using atomic force microscopy (AFM) measurement in tapping mode, from Nano-Scope NS3A system. The thickness of photoactive layer of PTB7:PC71BM films, interfacial modification layers of ZnO and ZnO/LiSPS and LiSPS were characterized by Dektak 150 surface profilometer. Space charge limited current (SCLC) method was applied to estimate the electron mobility of the electron-only device with a device structure of ITO/Al/EELs/PTB7:PC71BM/Al,36,37 where the EEL is either ZnO or ZnO/LiSPS thin layers. The mobilities of PTB7:PC71BM BHJ composite were calculated according to the Child’s Law:

=

(1)

where is the dielectric constant of free space, is the dielectric constant PTB7:PC71BM BHJ composite, which can be assumed to be 3 as the typical value of conjugated polymers.36,37 The d is the thickness of PTB7:PC71BM photoactive layer in PSCs. Inverted PSCs fabrication. The device structure of the inverted PSCs is ITO/ZnO/PTB7:PC71BM/MoO3/Ag and ITO/ZnO/LiSPS/PTB7:PC71BM/MoO3/Ag. The ZnO and ZnO/LiSPS thin films were fabricated as described above. The photoactive layer was spin-coated from a solution of PTB7:PC71BM (1:1.5 wt/wt %, 1 wt % in 1, 2-dichlorobenzene) with 1,8-diiodoctane (3 vol %) as the processing additive. The spin-rate was controlled to be 1000 rpm to give a ~80 nm thick active layer. After that, an 8 nm thick molybdenum oxide (MoOx) layer and 100 nm Ag electrode were deposited on the top of BHJ photoactive layer sequentially by thermal 6


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evaporation under high vacuum atmosphere. The device area was measured to be 0.16 cm2. Inverted PSCs characterization. The inverted PSCs devices were measured under one-sun solar simulator (with Newport Air Mass 1.5 Global (AM 1.5G)) with an irradiation intensity of 100 mW/cm2. The light intensity was calibrated by a standard Si-cell. The current density versus voltage (J-V) characteristics were measured by Keithley 2400 Source Measure Unit. The external quantum efficiency (EQE) spectra of the devices were characterized by a commercial photomodulation spectroscopic setup (DSR100UV-B). The impedance spectroscopy (IS) was obtained by the Impedance/gain-phase analyzer (HP 4194A). All the PSCs were measured in dark with the frequency from 5 Hz to 100 kHz and an oscillating voltage of 10 mV. To construct the RC circuit, PSCs were tested at their VOC conditions during the IS measurement. The transient photocurrent measurements were performed by using an optical chopper controlled 532 nm laser pulse at different frequencies to ensure the response reaching a steady state.

Results and discussion Scheme 1a shows the molecular structures of PTB7, PC71BM and LiSPS. The contribution of the additional Li+ in LiSPS can enlarge the carrier concentration and render high electrical conductivity of the ZnO/LiSPS EEL. Thus ameliorated charge transfer process within the electron pathways is anticipated in the inverted PSCs.35 The ionic nanodomains in LiSPS can induce physical crosslinking in the ionomer, 7



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resulting in a robust thin film after solvent drying, which would allow the solution-deposition of upper layer free from destroying the bottom ionomer layer. In addition, the organic main-chain in LiSPS ionomer ensures good wettability for the upper BHJ photoactive layer on the ZnO/LiSPS EEL.38 Such optimized active layer-EEL coherence are expected to reduce trap-assisted recombination at the interfaces. Thus ameliorated charge transfer process within the electron pathways and reduced trap-assisted recombination at the interfaces would result in enhanced PCEs for the inverted PSCs. Scheme 1b shows the device structure of the inverted PSCs with LiSPS modification layer, where the ITO acts as the cathode, the ZnO/LiSPS acts as the EEL, the PTB7:PC71BM BHJ composite acts as the light absorber, MoOx acts as the hole extraction layer (HEL) and the Ag acts as the anode. Figure 1a presents the transmittance of ZnO and ZnO/LiSPS thin films. Both the ITO/ZnO and ITO/ZnO/LiSPS thin films possess high transmittance from 420 nm to 800 nm. Although the ITO/ZnO/LiSPS multilayer shows a slight lower transmittance with a negligible loss of 3%, the overall transmittance is still high over 80%. Such feature indicates that the additional LiSPS layer does not significantly compromise the number of photons being absorbed by BHJ photoactive layer; hence the light absorption by BHJ photoactive layer will not be significantly affected. Besides the light transmittance, charge transport property is another key factor determining the effectiveness of the EELs for solar cells. In order to estimate the electrical conductivities of ZnO and ZnO/LiSPS thin films, the J-V characteristics of the device with a structure of ITO/ZnO (or ZnO/LiSPS)/Ag are investigated (the thin 8


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film thickness and device area are consistent with those used in solar cell devices), where the high-WF Ag electrode is used to ensure the ohmic contact is formed for minimizing the charge loss at the interface. As shown in Figure 1b, the linear dependence J-V curves implies a well formed ohmic contact between the electrode and the film of interest, and the slopes from J-V curves are corresponding to the electrical conductivities of the devices. Since both ITO and Ag electrodes have dramatically larger electrical conductivity over 104 S/m, the significantly smaller electrical conductivity estimated from the slopes are mainly contributed by the thin films of interest. The electrical conductivity can be estimated by,39 ≈ =

(2)

where σ is the electrical conductivity, G0 is the conductance which can be derived from the slope of J-V curve, d0 is the thickness of ZnO or ZnO/LiSPS thin film and S is the device area, the electrical conductivities of the ZnO thin film and ZnO/LiSPS thin film are estimated to be 3.02×10-4 S/m and 4.69×10-4 S/m, respectively. Higher electrical conductivity from the ZnO/LiSPS thin film is probably originated from the ionic contribution of LiSPS layer. Such high electrical conductivity would facilitate charge transport and reduce the series resistance for PSCs. Figures 2a & 2c exhibit the phase and height images of the ZnO EEL observed from the AFM measurement. A root-mean-square (RMS) roughness (Rq) of 1.1 nm is obtained from the ZnO EEL. Figure 2b shows the phase image of the ZnO/LiSPS EEL, where the unique phase separation pattern of ionomer is observed, indicating a complete coverage of the LiSPS ionomer layer on the ZnO EEL surface. Furthermore, 9



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a grain-like microstructural film morphology was observed in the ZnO/LiSPS EEL. Such unique feature is in good consistence with the ionic nanodomain structure in typical alkali metal ionomer thin films,40 indicating the presence of the LiSPS ionomer layer at the surface. In addition, as shown in Figure 2d, the ZnO/LiSPS EEL exhibits an increased Rq of 2.7 nm. The enlarged Rq would provide improved interfacial adhesion between the ZnO EEL and BHJ photoactive layer, rendering additional electron extraction pathways from BHJ photoactive layer to the EEL.36 The improved interfacial coherence of the ZnO/LiSPS EEL with BHJ photoactive layer would facilitate the charge extraction from BHJ photoactive layer to the ZnO/LiSPS EEL; meanwhile the improved electrical conductivity of the ZnO/LiSPS EEL are expected to decrease the charge loss in the charge transport within the EEL itself. Thus more charge carriers are expected to be collected by the electrodes in the inverted PSCs with a device structure of ITO/ZnO/LiSPS/BHJ composite/MoO3/Ag. Figure 3a shows the J-V characteristics of the inverted PSCs under one-sun illumination. Table 1 lists the device performance parameters of the inverted PSCs with different EELs. The PSCs using ZnO EELs show a JSC of 16.72 mA/cm2, a VOC of 0.70 V, a FF of 60.6% and a PCE of 7.09%. The PSCs using the ZnO/LiSPS EEL show a JSC of 18.07 mA/cm2, a VOC of 0.71 V, a FF of 68.3% and a PCE of 8.76%. Approximatively 25% enhancement in PCE has been achieved from the inverted PSCs with the ZnO/LiSPS EEL in contrast to that from the inverted PSCs using ZnO EEL. The improved PCE from the inverted PSCs using the ZnO/LiSPS EEL mainly comes from the significantly enlarged JSC and FF, indicating the charge carrier 10


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collection efficiencies at the electrodes are boosted by the novel ZnO/LiSPS EEL,36,37 which is consistent with our hypothesis described above. In addition, the effect of ionomer ultrathin layer with different thicknesses on device performance is further investigated. Table 2 lists device performance parameters of the inverted PSCs with the ZnO/LiSPS EEL, where the thickness of LiSPS is different. As the film thickness increases from 0 to 5 nm, the VOC, JSC, and FF show a simultaneous increase, rendering an optimal PCE of 8.76 (±0.16)%; while further increase the thickness to 10 nm, the efficiency draw back to 7.00 (±0.43)%. This can be understood by that at smaller thicknesses, an ultra-thin LiSPS can help modifying the surface of ZnO by passivating the surface trap-states,41 offering external ionic conductivity,42 allowing the tunneling effect for charge transport43; while at a larger thickness, the drastically increased internal series resistance is expected to compromise the charge transport throughout the layer. To investigate the enhancement in the photocurrent, the external quantum efficiencies (EQEs) of the inverted PSCs with different EELs are measured. As shown in Figure 3b, in contrast to that from the device using the ZnO EEL, the device using the ZnO/LiSPS EEL exhibit greatly increased EQE responses in the visible region from 420 nm to 720 nm, and show an EQE exceeding 80% from 550 nm to 790 nm. By integrating the EQE spectra, JSC of 16.54 mA cm-2 and 17.95 mA cm-2 are estimated from the device using the ZnO EEL and ZnO/LiSPS EEL, respectively. These integrated JSC are in good agreement with those observed from the J-V characteristics. Noted the slight difference in JSC obtained in different methods is 11



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probably due to the imperfect solar spectrum of different solar simulators. The enhanced EQE from the inverted PSCs using the ZnO/LiSPS EEL is probably originated from the enhanced charge transport, extraction and collection efficiencies. To confirm it, series resistance (RS) and shunt resistance (RShunt) of the inverted PSCs with the ZnO EEL and the ZnO/LiSPS EEL are investigated. The estimated RS and RShunt are summarized in Table 1. After depositing the LiSPS modification layer on the ZnO EEL, the RS is reduced from 7.25 Ω cm2 to 4.66 Ω cm2, while the RShunt is increased from 413.80 Ω cm2 to 955.18 Ω cm2. The smaller RS comes from the reduced contact resistance and/or lowered bulk resistance after introducing the LiSPS interfacial modification layer; while the larger RShunt suggests less leakage current.44,45 All these results indicate that the inverted PSCs with the ZnO/LiSPS EEL possesses high JSC, as expected. To study the enlarged FF, the photocurrent (Jph = Jillum.-JD) versus the effective voltage (Veff = V0-V),46 where Jillum. is the current density under illumination, JD is the current density in dark, V0 is the voltage where Jph = 0, and V is the applied bias, is quantitatively investigated. As shown in Figure 4a, at the large reverse bias (Veff > 0.9 V), the photocurrents Jph of PSCs with either the ZnO EEL or the ZnO/LiSPS EEL are saturated with an identical value of Jsat =19.21 mA cm-2, which suggests the total amount of photon-induced electron hole pairs are the same in both PSCs (Jsat = eGmaxL, where e is the elementary charge, L is the thickness of BHJ photoactive layer and Gmax is the generation rate of the electron-hole pairs),47 giving the almost identical thicknesses of BHJ photoactive layers. While at the lower bias (Veff < 0.4 V), at 12


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maximum power output condition (Veff = 0.2 V), the Jph/Jsat are 70.8% and 86.5% for the device with ZnO EEL and ZnO/LiSPS EEL, respectively. The large ratio suggests enhanced charge carrier extraction at BHJ photoactive layer/EEL interface and improved charge carrier collection efficiency at the electrodes in the inverted PSCs with the ZnO/LiSPS EEL. The improved charge carrier collection efficiency in the inverted PSCs using the ZnO/LiSPS EEL also indicates less non-geminate recombination within the photodiode.47 Light intensity-dependent JSC and VOC are further investigated (Figure S3) for studying the influence of the highly electrical conductive EEL and enhanced interfacial coherence on the charge carrier recombination in the inverted PSCs. Figures 4b & 4c exhibit the light-intensity dependence of JSC and VOC for PSCs at steady-states. The JSC can be correlated with the light intensity by,48 ∝ ( ≤ 1)

(3)

where I is the light intensity and is an index. At short-circuit condition, owning to the maximum charge sweep out, the bimolecular recombination goes to a minimal value ( ≈ 1). While for < 1, the deviation of from integer originates from the bimolecular recombination loss due to insufficient sweep out, and a larger deviation suggests more severe bimolecular recombination.49 Figure 4b shows the JSC dependence on I in log-log scale with a power law fitting according to Equation (3). The PSCs with the ZnO EEL show a large deviation of = 0.90 from 1, indicating a server bimolecular recombination; while the PSCs with the ZnO/LiSPS EEL show a = 0.98 , indicating a minimized bimolecular recombination. These are well 13



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correlated to the observed increase in the shunt resistance (Table 1) and high collection probability at the short-circuit condition. The reduced bimolecular recombination is probably originated from the well electrical coherence between each interlayer, where the free carriers are easily to be sweep out and the reduced trap-state will facilitate charge carriers to escape the shallow traps. At the open-circuit condition, VOC can be correlated with the light intensity by,49 $% ∝ & ∙ ()()

(4)

where S is the slope. Around one-sun illumination, the recombination mechanism is mainly dominated by the bimolecular recombination (S=kT/q, where k is the Boltzmann constant, T is the absolute temperature and q is the elementary charge). Figure 4c shows the VOC dependence on I for the PSCs with different EELs. The slope of 0.032 (1.23kT/q), which is quite deviate from 0.026 (kT/q), is observed from the device using the ZnO EEL; while a slope of 0.027 (1.04kT/q), which is close to 0.026 (kT/q), is observed from the device using the ZnO/LiSPS EEL, suggesting an alleviated geminate recombination in the devices using the ZnO/LiSPS EEL around the one-sun illumination.49 The impedance spectra are further applied to study the underlying charge transport physics in the inverted PSCs with different EELs.50,51 The RS incorporates the sheet resistance (RSheet) and the charge-transfer resistance (RCT). The RSheet is correlated to the electrodes and the RCT is correlated with the resistance at the interface between the electrodes and the charge extraction layers, the charge extraction layers and BHJ photoactive layer, and inside of BHJ photoactive layer.51, 14


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In this work, the difference in RCT is only at the interface between the ZnO EEL and BHJ photoactive layer. Figure 4d shows the Nyquist plots of the inverted PSCs with either the ZnO EEL or the ZnO/LiSPS EEL. The RCT of 1080 Ω and 790 Ω are observed from the PSCs with the ZnO EEL and ZnO/LiSPS EEL, respectively. Smaller RCT from the inverted PSCs with using the ZnO/LiSPS EEL indicate the charge transfer properties within PSCs have been improved by employing the ZnO/LiSPS EEL. The reduced resistance in the PSCs using the ZnO/LiSPS EEL can be ascribed to the optimized interface between EEL and active layer. The LiSPS ionomers can sufficiently modify the inhomogeneous surface of the inorganic ZnO, passivating the trap-states and make intimate contact with the active layer. Further, the ionic nature of the LiSPS ultrathin layer also benefits the charge behavior extraction owning to the reduced energy barrier induced by the ionic interfacial modification effect.52 To further understand the underlying physics of the enlarged FF, J-V characteristics of PSCs devices are measured in dark condition. The results are shown in Figure 5a. Over three times smaller dark current densities under the large reverse bias is observed from the device using the ZnO/LiSPS EEL as compared to those using the ZnO EEL. To visualize the role of the LiSPS modification layer in the diode, we employ the general single exponential diode equation53 (in dark the light-induced current JSC=0) of

= exp -./0 ($ − 2)3 + 5$ −

(5)

where J is the current flowing through the external circuit, J0 is the dark saturate 15



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current density, A is the ideality factor, k is the Boltzmann constant, T is the temperature, R is the series resistance, G is the shunt conductance, V is the applied bias. Figure 5b shows the derivative 6($) ≡ 8/8$ (g(V) becomes flat with the value in reverse bias equal to G) verses V in the near region of the short-circuit and at the reverse bias where the derivative term is negligible.53 The G of the inverted PSCs using the ZnO/LiSPS EEL is 0.1 mS cm-2, which is smaller than that of 0.5 mS cm-2 of the device using the ZnO EEL. The smaller G corresponds to a reduced internal leakage current throughout the diode.53 Figure 5c presents the :() ≡ 8$/8 verses (J+JSC)-1, rendering the R by the linear fitting. A smaller R of 0.9 Ohm cm2 in the PSCs with the ZnO/LiSPS EEL indicates enhanced charge transport process within the diode. By employing the plot of J+JSC-GV verses (V-RJ) with the R obtained from Figure 5c, the J0 for different diodes is estimated. The linear region, as shown in Figure 5d, suggests a good fitting with the equation. The inverted PSCs with the ZnO/LiSPS EEL shows a dramatically reduced J0 of 7.5×10-8 mA cm-2 compared with that of 1.6×10-5 mA cm-2 from the device using the ZnO EEL. The dramatically reduced J0 indicates reduced leakage current and thus minimized charge carrier recombination along within the electron pathways from BHJ photoactive layer to the cathode. As a result, the slightly enlarge VOC is generated. Above results demonstrate that the improved device performance in the device using ZnO/LiSPS EEL mainly comes from the enlarged photocurrent and FF, which are originated from the improved electrical coherence between interlayers and optimized electron percolation pathways from BHJ active layer throughout the EEL to 16


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the electrodes. To understand the ionic contribution from the LiSPS ionomer to the ameliorated charge transport properties within the solar cells, we further study the electron mobility and transient photocurrent response in the inverted PSCs. The electron-only device with an architecture of ITO/Al/EELs/PTB7:PC71BM/Al are processed and tested. Figure 6a shows the log-log plots of the current density verses effective voltage in dark, where the effective voltage equals to the difference between the built-in potential (Vbi) and the applied voltage (Vappl).53 The fitting (straight lines with slope of 2) in Figure 6a indicates that the J-V characteristics of both electron-only devices follow the Child’s law in the space charge limited region from 0.5 V to 1.5 V.54 Figure 6b shows the plot of J1/2 verses (Vappl - Vbi) characteristic in the space charge limited region and Child’s law is further employed to obtain the electron mobilities for both devices. BHJ composite from the inverted PSCs with the ZnO EEL possess an electron mobility of 2.3×10-4 cm2 V-1 s-1, while BHJ composite from the inverted device using the ZnO/LiSPS EEL exhibit an electron mobility of 7.2×10-4 cm2 V-1 s-1. The enhanced electron mobility is attributed to the enhanced electrical properties of the ZnO/LiSPS EEL. To further understand the differences of charge transport behavior in each device, the time-dependent photocurrent response measurement is conducted. The results are shown in Figures 6c & 6d. A dramatically slower response time for the device using ZnO/LiSPS EEL is observed in comparison with that using ZnO EEL. For the case of single-type charge carriers, the retarded transport can be ascribed to the trap-filling/detrapping mechanism;55 while in this case, owing to the ion participation, such reduced response time are most likely owning to 17



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the slower migration rate for the ions from the ionomers.56,57 Consequently, the ion involved charge transport further facilitates the charge transport from BHJ photoactive layer throughout the EEL and to the electrode.

Conclusion In conclusion, we reported an ultrathin interfacial modification layer of lithium sulfonated polystyrene (LiSPS) ionomer being used to reengineer the interface between the ZnO EEL and BHJ photoactive layer, rendering an improved electrical coherence between each interlayer. The ion involved charge transport process further facilitates the charge transport and minimizes the charge carriers’ loss during the transport process. Thus, the elevated charge transport properties, reduced internal resistance and optimized contact between the ZnO EEL and BHJ photoactive layer give a dramatically reduced dark saturate current density and minimized charge carrier recombination. As a result, a 25 % enhanced PCE is obtained from the inverted PSCs using the ZnO/LiSPS EEL in contrast to those using the ZnO EEL. Our method provides a simple but effective way to approach high PCEs from the inverted PSCs.

Author information Corresponding Author *E-mail: [email protected] Author Contributions 18


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X.G. developed the idea for this work; C.W.H synthesized the polymers; K. W and Z. Z prepared the samples, and performed all the measurements; K.W. analyzed the results and wrote the final version of the manuscript; X.G. supervised the work. All authors were involved in the discussion. Notes The authors declare no competing financial interest.

Acknowledgements The authors would like to thank NSF (EECS 1351785) and Air Force Scientific Research (FA9550-15-1-0292) for financial support. The authors at South China University of Technology would like to acknowledge the Natural Science Foundation of China (No. 51329301) for financial support.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The estimated electrical conductivity of electron extraction layers of different thicknesses; dark current density-voltage characteristics of polymer solar cells using ionomer of different thicknesses; charge carrier recombination study by measuring current density-voltage characteristics at different light intensities; solar cell performance based on PTB7:PCBM system spin-casted from the chlorobenzene solution. 19



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Table 1. Device performance of the inverted PSCs with different EELs EELs

VOC

JSC*

JSC# -2

FF -2

RS

RShunt 2

PCE 2

(V)

(mAcm )

(mAcm )

(%)

(Ω·cm )

(Ω·cm )

(%)

0.70

16.72

16.54

60.6

7.25

413.80

7.09

ZnO/LiSPS 0.71

18.06

17.95

68.3

4.66

955.18

8.76

ZnO

JSC*: JSC observed rom the J-V curve; JSC#: JSC estimated from EQE measurement.

Table 2. Device performance of the inverted PSCs with LiSPS of different thicknesses. LiSPS thickness (nm)

VOC (V)

JSC (mAcm-2)

FF (%)

PCE (%)

0

0.70 (±0.01) 16.72 (±0.21) 60.6 (±3.7) 7.08 (±0.19)

3

0.70 (±0.01) 17.75 (±0.15) 65.7 (±4.1) 8.16 (±0.21)

5

0.71 (±0.01) 18.06 (±0.32) 68.3 (±2.9) 8.76 (±0.16)

10 0.72 (±0.02) 16.33 (±0.57) 59.6 (±5.2) 7.00 (±0.33) The data are quoted as the average ± one standard deviation (obtained from 20 devices for each group).

28


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Scheme 1. (a) The molecular structures of PTB7, PC71BM and LiSPS; (b) device architecture of inverted polymer solar cells with the ZnO/LiSPS electron extraction layer (EEL).

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Figure 1. (a) Transmittance spectra of ZnO and ZnO/LiSPS thin films; (b) The current density-voltage characteristic of the metal/EEL/metal devices, with device structure of ITO/ZnO EEL/Ag and ITO/(ZnO/LiSPS) EEL/Ag.

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Figure 2. AFM phase images (10 µm×10µm) of (a) ZnO, (b) ZnO/LiSPS thin films; and height images (10 µm×10µm) of (c) ZnO, (d) ZnO/LiSPS thin films.

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Figure 3. (a) The J-V characteristics, (b) The external quantum efficiency (EQE) versus wavelength of, the inverted PSCs with the ZnO EEL or the ZnO/LiSPS EEL under AM1.5 illumination.

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Figure 4. (a) The photocurrent density (Jph ) versus the effective voltage (Veff) of the inverted PSCs with different EELs under AM1.5 illumination; (b) JSC verses the light intensity of the inverted PSCs with different EELs; (c) VOC verses the light intensity of the inverted PSCs with different EELs; (d) Nyquist plots of the of the inverted PSCs with different EELs at V≈VOC.

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Figure 5. Dark J-V characteristics of the inverted PSCs with the ZnO EEL or the ZnO/LiSPS EEL: (a) lnJ verses V; (b) the shunt characterization G(V) verses V; (c) r(J) verses (J+JSC)-1 with fit used to determine R; (d) ln(J+JSC-GV) verses (V-RJ) with fit used to determine J0.

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Figure 6. The effect of the ionomer interlayer on charge transport properties. (a) Log-log plots of current density verses voltage (data points) and fitting (solid line) for Child’s regime; (b) J1/2 verses (Vappl-Vbi) characteristic of diodes ITO/Al/EELs/PTB7:PC71BM/Ca/Al. The transient photocurrent response of inverted PSCs with (c) the ZnO EEL and (d) the ZnO/LiSPS EEL.

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Table of Contents (TOC) Graphic

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Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a

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