Naphthalene Diimide Based n-Type Conjugated Polymers as Efficient

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Naphthalene Diimide-Based N-type Conjugated Polymers as Efficient Cathode Interfacial Materials for Polymer and Perovskite Solar Cells Tao Jia, Chen Sun, Rongguo Xu, Zhiming Chen, Qingwu Yin, Yaocheng Jin, Hin-Lap Yip, Fei Huang, and Yong Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10365 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Applied Materials & Interfaces

Naphthalene Diimide-Based n-type Conjugated Polymers as Efficient Cathode Interfacial Materials for Polymer and Perovskite Solar Cells Tao Jia†, Chen Sun†, Rongguo Xu, Zhiming Chen, Qingwu Yin, Yaocheng Jin, Hin-Lap Yip*, Fei Huang* and Yong Cao Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China

ABSTRACT: A series of Naphthalene diimide (NDI)-based n-type conjugated polymers with aminofunctionalized side groups and backbones were synthesized and used as cathode interlayers (CIL) in polymer and perovskite solar cells. Owing to controllable amine side groups, all the resulting polymers exhibited distinct electronic properties such as oxidation potential of side chains, charge carrier mobilities, self-doping behaviors and interfacial dipoles. The influence of the chemical variation of amine groups on the cathode interfacial effects were further investigated in both polymer and perovskite solar cells. We found that the decreased electron-donating property and enhanced steric hindrance of amine side groups substantially weaken the capacities of work function altering of cathode and trap passivation of perovskite film, which induced the ineffective interfacial modifications and declining device performance. Moreover, with further improvement of the backbone design through the incorporation of rigid acetylene spacer, the resulting polymers substantially exhibited the enhanced electron-transporting property. When used as CILs, high PCEs of 10.1% and 15.2% were respectively achieved in polymer and perovskite solar cells. Importantly, these newly developed n-type polymers were allowed to be processed over a broad thickness range of CILs in photovoltaic devices and a prominent PCE of over 8%

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for polymer solar cells and 13.5% for perovskite solar cells can be achieved with the thick interlayers over 100 nm, which is beneficial to roll-to-roll coating processes. Our findings contribute towards better understanding of the structure-performance relationship between CIL material design and solar cell performance, and provide important insights and guidelines for the design of high-performance n-type CIL materials for organic and perovskite optoelectronic devices. KEYWORDS: Polymer solar cells, Perovskite solar cells, Cathode interlayers, Amine side groups, Naphthalene diimide

INTRODUCTION Polymer and perovskite solar cells have attracted significant interests due to their unique features including lightweight, flexibility, low-temperature process and the potential of large-scale roll-to-roll manufacturing.1-3 Over the past decade, remarkable boosts in device efficiency have been made in bulk heterojunction (BHJ)-based polymer solar cells (PSCs)4-5 and hybrid organometal trihalide perovskite solar cells (PVKSCs).6-7 The performance improvements were attributed to the combined efforts including the optimization of the light harvesting materials optimization of device architecture

12-13

8-10

and their thin film structures11, the

and processing methods.14-15 In addition to these factors, the

development of efficient interlayers was also very important, which could be used to control the energy level alignments at the interfaces, enhance charge selectivity and extraction at the electrodes and thereby maximizes the solar cell performance.16-17 Therefore, a lot of efforts have been focused on the development of novel interfacial materials for electrode modification, especially for the cathode interfacial materials.18-20 A variety of interfacial materials have been proven to be effective cathode interlayers for PSCs, including metal salts (such as LiF,21 Cs2CO3,22 CsF,23), n-type metal oxide semiconductors (such as ZnO,24 TiOx25) and organic CIL materials.26-27 Among different types of interface materials, aminofunctionalized small molecules and polymers have attracted extensive attention due to their unique properties of orthogonal solvent processing and effective interfacial modification capability.28-30 It has 2 Environment ACS Paragon Plus

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been shown that amino-functionalized CILs, such as poly[(9,9-bis(3'-(N, N-dimethylamino)propyl)-2,7fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)31-34 and ethoxylated polyethyleneimine (PEIE)35 could provide multiple interfacial modification effects for PSCs including (1) the reduction of work function (WF) of the metal electrode through the formation of interfacial dipole induced by the pendant polar groups, leading to better energy-level alignment at the electrode interface to enhance the device performance,36-37 (2) n-doping of PCBM acceptors at the BHJ/CIL interface in fullerene-based devices through electron transfer from the amine side chains,38 (3) enhanced hole-trapping effect introduced by the stable oxidation states of the aliphatic amines, which could improve the electron selectivity at the cathode interface,39-40 and (4) the smaller surface energy difference between amino-functionalized materials and PCBM favoured the formation of desired vertical phase separation in the BHJ film and facilitated better electron transport and extraction at the interface.41 Additionally, when these aminofunctionalized materials were used as CILs in PVKSCs, the amine side groups were proven to have dual functionalities of both modifying the cathode’s WF and passivating the surface traps on the perovskite crystal surfaces and grain boundaries, which help to form a better energy alignment and reduce the interfacial resistance to achieve high efficiency devices.42-43 Although polymers with amine side groups show generally good interfacial modification in both PSCs and PVKSCs, further tuning of the chemical environment of the amine side groups in these CIL materials and exploring their contributions to the interfacial modification were basically unexplored and had been rarely discussed. Actually, all the aforementioned interfacial functions in PSCs and PVKSCs might be closely related with the amine side groups in

amino-functionalized polymers. Therefore, it is essential to investigate the structure-function relationship between the chemical variations of amine side groups and the resulting interfacial effect in order to provide better design guidelines for CIL materials used in PSCs and PVKSCs. Herein, a series of novel n-type copolymers comprising the fluorene and naphthalene diimide (NDI) units, P1~P6, were designed and synthesized (Scheme 1). NDI, which has a large electron affinity, was chosen as it has been widely used to construct n-type polymers with excellent electron mobility for field-effect transistors44 and all-polymer solar cells.45 Moreover, the additional self-doping behavior be3 Environment ACS Paragon Plus


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tween amine side groups and NDI units could further improve the electron transporting property46 and make them more compatible for roll-to-roll printing processes. As shown in Scheme 1, P1~P4 are composed of identical backbone but with different pendants of dimethylamino, N-methylbenzylamino, dibenzylamino and pure alkyl chains. These modifications gradually tune the chemical environment of the amine side groups through inductive effect and steric hindrance introduced by the additional benzyl groups, providing a series of new polymers for systematic study of the influence of the chemical variation of amine side groups on the resulting interfacial effect. In addition, since the acetylene spacers can introduce better planarity of the conjugated backbone and improve charge transport property, P5 and P6 without the acetylene spacer were also synthesized as reference materials. It is found that the resultant polymers indeed show very distinct properties, such as the oxidation potential of the side chains, charge carrier mobilities, self-doping behavior and interfacial dipole formation property, due to the chemical variation of the amine side groups. Furthermore, we incorporated these polymers into inverted PSCs based on the generally used

poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-

fluoro-2[(2-ethylhexyl) carbonyl] thieno [3,4-b] thiophenediyl]] (PTB7-Th) and [6,6]-phenyl C71butyric acid methyl ester (PC71BM) BHJ, and studied the impact of the different amine side groups on the device performance. Additionally, we also explore the applicability of the optimized polymer interlayer

to

PSCs

based

on

poly[thieno[3,2-b]thiophene-alt-(5,10-bis(2-octyldodecyl)thiophen-5-

yl)naphtho[1,2-c:5,6c]bis[1,2,5]thiadiazole] (NT812) /PC71BM BHJ, which is another high performance BHJ system.47 Our results suggested that the decreased electron-donating property and enhanced steric hindrance of amine side groups will substantially weaken the capacities of WF’s altering, which introduced the declining device performance. Eventually, the P1 derived devices exhibit the most promising performance with PCE as high as 10.09 %, outperforming the devices based on P6 interlayer (9.45 %). Importantly, P1 can yield highly efficient PSCs with the PCE still over 8% when the thickness of the CIL is up to 100 nm, which is attributed to the substantially improved electron mobility by the introduction of acetylene spacers. Our results suggested that it is feasible to develop novel thickness-insensitive CIL materials via effective structure modulation. 4 Environment ACS Paragon Plus

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In addition to PSCs, P1~P4 were also employed as CILs in PVKSCs to further investigate the relationship between the chemical environment of the amine side groups and their corresponding trap passivation effect to perovskite. Similar to the trend observed in PSCs, the gradually chemical variations of amine side groups resulted in the declining performance of PVKSCs due to the reduced effectiveness of WF modification of cathode and trap passivation of perovskite, which was evidenced by the combinatorial studies including Kelvin probe measurement, photoluminescence (PL), time-resolved PL decay and transient photovoltage measurement. Likewise, a thick P1 interlayer can be also used without significantly sacrificing the device efficiency, and a prominent PCE of 13.5 % with CIL thickness of 150 nm are achieved, which is more compatible for roll-to-roll printing processes. RESULTS AND DISCUSSIONS Synthesis and Characterization The procedure for the syntheses of new polymers is shown in Scheme 1, while the complete experimental details can be found in the Supporting Information. 2,7-Diethynyl-9,9-bis(3-(N,Ndimethylamino)-propyl)fluorine

(M1),

bromopropyl)-2,7-diethynyl-9H-fluorene

9,9-dibutyl-2,7-diethynyl-9H-fluorene (7),

(M4),

2,6-dibromo-N,N ′ -bis(2-butyloctyl)-

9,9-bis(31,4,5,8-

naphthalenediimide (8), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluoren (M5), and

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-bis(3-(N,N-dimethylamino)-

propyl)fluorine (M6) were synthesized and purified according to the published procedures.48-52 Compound 7 was then reacted with N-methylbenzylamine and dibenzylamine in dimethyl formamide (DMF) with potassium carbonate as base to produce compound 3,3'-(2,7-diethynyl-9H-fluorene-9,9-diyl)bis(Nbenzyl-N-methylpropan-1-amine)

(M2)

and

3,3'-(2,7-diethynyl-9H-fluorene-9,9-diyl)bis(N,N-

dibenzylpropan-1-amine) (M3), respectively. P1~P4 were then synthesized by Sonogashira coupling polymerization of the corresponding diyne monomers M1~M4 and compound 8, whereas P5 and P6 were synthesized by Suzuki coupling polymerization of the corresponding diboronic ester monomers 5 Environment ACS Paragon Plus


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M5, M6 and compound 8, respectively. The resultant polymers exhibited excellent solubility in chloroform (CF) and chlorobenzene (CB). It should be noted that M2 and M3 can be purified in column chromatography without the addition of triethylamine (TEA), which is different from M1. This result demonstrates that the chemical structures of the amine side groups can severely affect the molecular polarity of the resulting compounds. Another interesting phenomenon is that P1 and P6 can be soluble in alcohols in the presence of trace amount of acetic acid, while P2 and P3 were totally insoluble in alcohols even with the addition of acetic acid. The difference in solubility can be explained by both the decrease in electron density of side nitrogen atoms and increase in steric hindrance around the amines introduced by the benzyl groups, which may screen the electrostatic interactions between the acid and the amino-terminal groups and thereby decreases the solubility of the polymers in polar solvents.53 The molecular weights of the copolymers were evaluated by gel permeation chromatography (GPC) measurement, which was carried out with CF as the eluent using linear polystyrene standards. The number average molecular weights (Mn) of the polymers are ranged from 11.3 to 20.4 kDa, with details summarized in Table S1. In order to qualitatively elucidate the chemical differences of the amine pendants, 15N HHMBC was performed to determine the resonances of side nitrogen atoms in M1, M2 and M3 (Figure S1). With urea as the internal reference, the

15

N signals at 26.1, 37.9 and 48.0 ppm were observed for

M1, M2 and M3, respectively, indicating that the incorporation of benzyl groups led to different chemical environment of side nitrogen atoms. The thermal properties of the polymers were evaluated by thermogravimetric analysis (TGA), with relevant characteristics and detailed data shown in Figure S2 and Table S1, respectively. We found that all the obtained polymers exhibited good thermal stability with 5% weight-loss temperature (Td) from 300 to 360 °C.


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Scheme 1. Synthetic routes of the copolymers Optical, Electronic, and Electrical Properties The normalized UV-Vis absorption spectra of the copolymers in CF solution and as thin films are shown in Figure 1a and 1b, with the relevant data summarized in Table 1. P1~P4 exhibited similar absorption spectra with two absorption bands, while P5 and P6 exhibited two main blue-shifted absorption bands. The absorption band appears below 450 nm was attributed to the π–π* transition absorption, and the other band covered from 450 to 650 nm was resulted from the intramolecular charge transfer (ICT) effect between the fluorene and NDI units. Since the improved planarity of the conjugated backbone introduced by the additional acetylene spacers, P1~P4 showed stronger ICT characteristic with absoprtion peaks shifted bathochromically when compared with P5 and P6. Moreover, the absorption spectra of the resultant polymers in solid states were very similar to their solutions except for a slight red shift due to the enhanced intermolecular interactions in the solid states. The optical bandgaps (Egopt) of 7 Environment ACS Paragon Plus


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P1~P6 were measured from the absorption onsets of the corresponding thin films and determined to be 1.93, 1.94, 1.95, 1.95, 2.12 and 2.13 eV, respectively. Cyclic voltammetry (CV) measurements were used to evaluate the frontier molecular orbital energy levels of the resultant polymers. The CV curves are shown in Figure 1c and the redox potentials as well as the derived HOMO and LUMO energy levels were summarized in Table 1. Ferrocene/ferrocenium (Fc/Fc+) reference was used as an internal standard, which delivered a potential onset of 0.39 V with respect to the saturated calomel electrode (SCE) electrode. The HOMO and LUMO energy levels were calculated by the following equations: EHOMO= -e(Eox+4.41) (eV), ELUMO= -e(Ere+4.41) (eV), where Eox and Ere denote the onset of oxidation and reduction potentials, respectively. As can be seen in Figure 1c, all the resultant polymers exhibited one-step irreversible oxidation and two-step reversible reduction processes, corresponding to the oxidation of fluorene units and the reduction of naphthalene diimide units, respectively. The Eox/Ere for P1~P4 were estimated to be 1.55/-0.49 V, 1.51/-0.47 V, 1.49/-0.48 V, and 1.57/-0.44 V, respectively. Thus, the EHOMO/ ELUMO for P1~P4 were calculated to be -5.96/ -3.92 eV, -5.93/ -3.94 eV, -5.90/ -3.93 eV, and -5.98/ -3.97 eV, respectively. The energy levels of these polymers were quite comparable due to their identical conjugated backbones, which dominate the electronic properties despite different side groups were presented. The Eox/ Ere for P5 and P6 were determined to be 1.55/ -0.57 V and 1.57/ -0.59 V, corresponding to the EHOMO / ELUMO of -5.96/ -3.84 eV and -5.90/ -3.82 eV, respectively. The results showed that the LUMO energy levels of P1~P4 are slightly lower than those of P5 and P6, which could be attributed to the improved planarity of the backbone as well as the slightly electron withdrawing nature of the acetylene spacers. It is worth to note that no signal for the oxidation of amine side groups can be observed, and this result can be potentially assigned to the formation of charge transfer complexes between the lone pair electrons of the nitrogen atoms and the electron-deficient NDI units.39 To better justify this claim, electrochemical oxidative behaviors for the monomers of M1~M4 were also studied by using CV measurement in dichloromethane (DCM) solutions. The CV curves of M1~M4 and three other different amines as reference materials are 8 Environment ACS Paragon Plus

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shown in Figure S3a and S3b, respectively. In Figure S3a, M1~M3 clearly show two oxidation processes. The one at lower potential can be attributed to the oxidation of side amine groups, which are further confirmed by the oxidation process of triethylamine (TEA), N,N-dimethylbenzylamine (DMeBA) and N-methyldibenzylamine (MeDBA) (Figure S3b), respectively. In the absence of amine pendants, M4 exhibits no oxidation process at low potential. It should be noted that the oxidation potentials of amine side groups for M1~M3 were gradually increased, indicating that the benzyl substitutions can decrease the electron density around the nitrogen atoms and making them more difficult to be oxidized. Nevertheless, due to the relatively small oxidation potentials of the amine side chains in comparison with the HOMO of the polymers, which mainly determined by the conjugated backbone, the amino groups keen to trap holes as hole transfer from the deeper HOMO of the polymer is energetically favorable. This hole trapping effect can improve the electron selectivity at the cathode interface by preventing holes from migrating to the cathode.39-40 The increasing oxidation potentials of the amine groups in M2 and M3 indeed will reduce the driving energy for aforementioned hole transfer phenomenon, and weaken their hole trapping effect as well as the cathode modification capability.

Figure 1. Optical and electrochemical properties of the resultant polymers. UV–Vis absorption spectra of the resultant polymers in CF solution a) and as thin films b); c) Cyclic voltammetry curves of the resultant polymers.



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Table 1. UV-Vis absorption and electrochemical properties of the polymers λmax (nm)

Eg opt (eV)

Eox (V)

Ere (V)

EHOMO (eV)

ELUMO (eV)

367, 586

1.93

1.55

-0.49

-5.96

-3.92

363, 573

366, 587

1.94

1.51

-0.47

-5.93

-3.94

P3

363, 572

367, 586

1.95

1.49

-0.48

-5.90

-3.93

P4

361, 572

367, 586

1.95

1.57

-0.44

-5.98

-3.97

P5

322, 516

323, 522

2.12

1.55

-0.57

-5.96

-3.86

P6

322, 516

323, 521

2.13

1.57

-0.59

-5.90

-3.84

Polymers

Solution

Film

P1

365, 572

P2

The hole transfer process mentioned above indeed is an equivalent process for an electron transfer from the amine to the HOMO level of the polymer that is occupied by a single electron, such situation can be achieved when the polymer is photoexcited, leaving a hole in the HOMO and an electron in the LUMO.46 When the electron from the amines fills up the hole state in the HOMO of the polymer, the remaining electron in the LUMO of the polymer generates a n-doped state and the degree of n-doping of the polymer can be studied by electron paramagnetic resonance (EPR).54 Therefore, we further study the n-doping effect of P1~P4 by EPR spectroscopy to evaluate the relationship between the different amine side groups and the degree of charge transfer in these polymers under illumination. Equal amounts of the polymers in solid state were prepared and the EPR was measured under the same conditions. The result was shown in Figure S4. P1~P3 with different amine side groups showed a gradual decrease in EPR signal intensities with the same g values of ~2.001, while no EPR signal was detected for P4 with pure alkyl side chains, indicating that the degree of charge transfer is depending on the electrondonating capacity of amine side groups since the inductive effect introduced by the benzyl groups will decrease electron density of the amines as well as their electron donating power. In addition, the EPR signal of P1 was higher than that of P6, which could be attributed to the slightly deeper LUMO energy level of P1 than P6. The deeper LUMO level of P1 corresponds to stronger electron affinity, which will be more favorable to accept electron from amine groups.55


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To study the charge transport properties of the resultant polymers, electron-only devices based on the architecture of ITO/Al/CIL/Al were prepared and the electron mobilities were extracted by fitting the data using the space-charge-limited current (SCLC) model. The results are shown in Figure S5 and the electron mobilities data are summarized in Table S3. The electron mobilities of P1 and P4 were 2.9810-4 and 9.810-5 cm2 V-1 s-1, respectively, whereas P2 and P3 delivered lower mobilities of 7.110-5 and 2.510-5 cm2 V-1 s-1, indicating that the presence of additional bulky benzyl groups impedes electron transport.56 The electron mobilities of P5 and P6 are 1.010-5 and 2.410-5 cm2 V-1 s-1, respectively, indicating that the rigid acetylene spacers can improve the charge transport property of the polymer probably through promoting better planarity of the conjugated backbone of the polymers.57 Noted that the higher electron mobility of P1 will substantially allow a thicker interlayer used in devices, which is beneficial to large-area manufacturing processes. The morphologies of these CIL materials were studied by tapping mode atom force microscopy (AFM). The thin films were prepared by spin-coating their fresh CB solutions onto indium tin oxide (ITO) substrates. The AFM topographies are shown in Figure S6. It can be clearly seen that P1~P4 formed homogeneous films with root mean square (RMS) surface roughness of ~1 nm, suggesting that different side pendants have negligible influences on the morphology of the resultant polymers. P5 and P6 can even deliver more smooth surfaces with RMS surface roughness of ~ 0.5 nm, suggesting all the polymers can form high quality film and fulfill the film formation requirement as a good interlayer. The interfacial compatibility of the CIL materials was critically important during the sequential deposition of the multi-layered organic devices. Thus, the wettability of the new polymer interlayers was investigated by the contact angles measurements. As shown in Figure S7, it was found that P1~P4 demonstrated similar contact angles values, indicating the chemical variation of amine side chains exhibited very limited effect on the resulting polymers’ wetting property. In addition, P1~P4 exhibited relatively greater contact angles than P6 due to the increased rigidity of backbones. Noted that the contact angles values de-



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rived from these polymers were overall approximate to that of PC71BM films, which can indeed minimize the wetting effect on the device performance in inverted PCBM based PSCs.41 To qualitatively illustrate the electron density of the nitrogen atom in the amine side groups, the model molecules C1~C3 which correspond to P1~P3 were chosen and optimized at the B3LYPD3BJ/6-311G(d) level58-60 using a suite of Gaussian 09 programs, where the empirical dispersion with Becke-Johnson damping was included61. The atomic dipole moment corrected Hirshfeld (ADCH) charges62, calculated by the Multiwfn 3.3.9 program63, were employed to reflect the electron density of the N1 atom in C1~C3. The negative ADCH charges indicate electron accumulation from other atoms. As shown in Figure S8, the negative ADCH charges of N1 atom of C1~C3 were -0.272, -0.238 and 0.217, respectively, which means the electron density of N1 atom in C1~C3 was gradually decreased. This result can be used to interpret the aforementioned distinct properties including chemical shift of nitrogen atom in

15

NH-HMBC characterization, oxidation potential of side chains and self-doping be-

haviors. Furthermore, we also performed density functional theory (DFT) calculations with B3LYP / 631G(d, p) basis sets to gain insight into the geometrical configuration of the resultant polymers. Dimer molecules DM1 and DM2 which corresponded to P1 and P6 were used as models to investigate the influence of the local acetylene spacers on the molecular configuration. As shown in the structural model (Table S4), the intrinsic sp-hybridized nature of acetylene spacer offers DM1 better coplanarity with much smaller dihedral angle than DM2. In general, this better coplanarity can introduce a better intrachain charge transport and afford a higher electron mobility. Therefore, the result of DFT calculation was consistent with the characterization of electron mobility. Photovoltaic Properties To demonstrate the potential application of the polymers as CILs in PSCs and explore the influence of the different amine side groups on the resulting interfacial effects, we fabricated devices based on P1~P4 CILs using an inverted architecture of ITO/CIL/PTB7-Th: PC71BM/MoO3/Al (Figure 2a). The current-voltage (J-V) characteristics of the devices are shown in Figure 2b, and the detailed photovolta12 Environment ACS Paragon Plus

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ic data are summarized in Table 2. Photovoltaic devices without CIL delivered a low PCE of 3.83 % (Voc = 0.42 V, Jsc = 16.57 mA cm–2, and FF = 0.55). After the incorporation of CILs (~10 nm), the devices based on P1 achieved a PCE of 8.93% (Voc = 0.77 V, Jsc = 15.97 mA cm –2, and FF= 0.73), while the devices based on P2 and P3 delivered the PCE of 7.61% and 3.08%, respectively. In the meanwhile, the control devices based on P4 exhibited a low PCE of 2.16%. Photovoltaic devices with P1~P3 CILs showed a drop in performance, which mainly caused by the decrease in both Voc and FF. As it is well known that amine can form interfacial dipole with metal and metal oxide surface and alter their WFs, we therefore conducted Kelvin probe measurements to investigate the WF modification effect on ITO, which is a cathode in our inverted PSCs. The measured WFs are listed in Table S5. After deposition of a thin layer (~10 nm) of P1~P4, the WFs of the modified ITO were changed from its original value of 4.74 eV to -4.16, -4.31, -4.49 and -4.73 eV, respectively. The reduction in WF of cathode can increase the built-in potential of the PSCs and therefore enhance the Voc. In additon, the reduced WF also offers better energy level alignment between the active layer and cathode, which facilitates better electron transport and collection to achieve high FF.64 The observed trend of WF reduction of the cathode after coating with different polymers can be explained by the interfacial dipole formation property between the amine side groups and the ITO. The relatively strong electron-donating capacity and small size of the dimethyl amine in P1 can form intimate contact with ITO surface and facilitate a strong dipole formation, leading to the largest magnitude of WF reductions. The incorporating of benzyl groups in P2 and P3 not only reduce the electron density around the amine but also increase the contact distance to the ITO surface, leading to a descending capability in forming interfacial dipoles and resulted in a smaller reduction of ITO’s WF. The alkyl chains in P4 barely form any interfacial dipole, so the WF of ITO essentially remains the same. The trend of WF reduction of ITO cathode indeed correlates well with the Voc obtained in the PSCs with different polymer CILs as shown in Table 2. P1-based PSCs show the highest Voc of 0.77 V while the Vocs continue to decrease from P2- to P4-based devices. The other major photovoltaic parameter that is subsequently decreased from P1- to P4-based PSCs is the FF, which is governed by both the shunt and series resistances of the devices. A PSC with good 13 Environment ACS Paragon Plus


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charge selectivity for the electrodes typically can result in large shunt resistance and is beneficial to improve device performance. The decreasing capability for hole trapping from P1 to P4 can give rise to the reducing effectiveness in electron selectivity when they are used as CILs, and therefore lead to the reduction of shunt resistance (Table 2). The series resistance is associated to the ohmic loss during charge transport through the different layers and their interfaces within the device. With the electron mobility decreased from P1~P3 and a poor interface formed between the ITO and P4, the series resistances were therefore increased from P1- to P4-based devices (Table 2). As a result of these factors, the overall FFs were decreased from P1- to P4-based PSCs. The PSCs based on P6 CIL, which bore side dimethylamino groups but without the acetylene spacer on the backbone, was also tested. Despite the lower electron mobility of P6, the corresponding PSC indeed showed a higher PCE of 9.45 % when compared to the P1 based PSC of 8.93 %. As the Voc and FF were very similar in both cases, the major parameter that contributed to the lower PCE of P1 based PSC was the reduced Jsc. We attributed the result to the stronger ICT characteristic of P1, which showed absorption peak nearly four times higher than that of P6 (Figure S9). The stronger absorption of the P1 CIL could reduce the overall light intensity in the BHJ and therefore affect the photocurrent of the PSCs. This result was verified by the external quantum efficiency (EQE) spectra (Figure 2c), showing a most significant reduction in EQE at the 300-400 nm and 500-600 nm regions where P1 exhibited strong absorption.


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Figure 2. a) Device structure of the inverted PSCs and active layer materials used for devices. b) J-V curves of the inverted PSC devices based on different CILs. c) EQE spectra of the corresponding PSCs. Table 2. Device parameters of the inverted PSCs based on PTB7-Th/PC71BM BHJ with different CILs under AM 1.5G illumination with light intensity of 100 mW cm-2 CILs

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

P1

0.77±0.00

15.71±0.26

72.53±0.18

8.75±0.16

8.93

1.4

1265.7

P2

0.72±0.00

15.94±0.12

64.72±0.42

7.51±0.12

7.61

3.1

1187.5

P3

0.37±0.00

16.09±0.05

51.05±0.78

3.05±0.06

3.08

4.0

745.0

P4

0.31±0.00

15.62±0.23

44.18±0.29

2.09±0.11

2.16

5.0

341.9

P6

0.77±0.00

16.47±0.27

72.85±0.26

9.33±0.15

9.45

1.7

1241.8

None

0.42±0.00

16.43±0.14

55.12±0.15

3.60±0.22

3.83

4.8

904.3

Best PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

To further compare the performance of P1 and P6 as the CILs, we investigated their effect on another BHJ system of NT812:PC71BM, which could enable high-performance PSCs with active layer thicknesses

of

over

300

nm.

PSCs

were

fabricated

with

a

structure

of

ITO/PEDOT:PSS/NT812:PC71BM/CIL/Al (Figure 3a). By using a thick BHJ film and locating the CIL far away from the transparent electrode, we could minimize the light absorption problem introduced by the CIL and therefore better compared the property of P1 and P6 in terms of their thickness effect on the performance of the PSCs. The J–V characteristics and the EQE spectra were shown in Figure 3b and 3c, with the photovoltaic data summarized in Table 3. When a 10 nm CIL was used, PSCs derived from P1 delivered a high PCE of 10.09 %, which outperformed those based on P6 with a PCE of 9.45 %. When 15 Environment ACS Paragon Plus


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the thickness of CIL was increased to 50 nm, PSCs based on P1 maintained a high PCE of 8.61 %, while the PSCs based on P6 showed a PCE of 7.25 %, which corresponded to a 15 % and 23 % loss of PCE with respected to the PSCs based on 10 nm CILs. These results suggest that the charge transport property of the CIL becomes more critical when a thicker interlayer is used and in such case P1 is much better than P6. The excellent charge transporting property of P1 triggered us to further increase its thickness to 100 nm and the corresponding PSC still showed PCE of over 8%, indicating that P1 is a promising interlayer material with good processing window for eventually R2R printing of PSCs.

Figure 3. (a) Device structure of the conventional PSC and the chemical structures of NT812 and PC71BM used for the devices. (b) J-V curves of the conventional PSC devices based on NT812:PC71BM with various thicknesses of P1 and P6 CILs. c) EQE spectra of the corresponding PSCs. Table 3. Device parameters of the PSCs based on NT812/PC71BM system with P1 and P6 as CILs in various thicknesses under AM 1.5G illumination with light intensity of 100 mW cm-2 CILs

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

Best PCE (%)

P1 (10nm)

0.73±0.00

19.96±0.23

66.31±1.62

9.67±0.32

10.09

P1 (50nm)

0.73±0.00

18.74±0.37

61.82±0.84

8.45±0.24

8.61


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ACS Applied Materials & Interfaces P1 (100nm)

0.73±0.00

17.70±0.38

59.94±1.82

7.74±0.23

8.03

P6 (10nm)

0.73±0.00

19.84±0.09

65.08±0.16

9.37±0.08

9.45

P6 (50nm)

0.72±0.00

16.75±0.34

58.93±2.32

6.93±0.45

7.25

Amino-functionalized polymers are also efficient CILs for PVKSCs and it has been proven that they can serve dual functionalities to not only tune the WF of cathode for a better electron extraction but also passivate the surface traps of perovskite for a reduction of charge recombinations.43 Therefore, we also fabricated PVKSCs based on the device configuration of ITO/PEDOT:PSS (~40 nm)/CH3NH3PbI3xClx(~400

nm)/CIL(~50 nm)/Ag and studied the effect of our polymer CILs on their photovoltaic per-

formance. The schematic of the device structure and the associated J-V curves are shown in Figure 4a and 4b, with the photovoltaic parameters summarized in Table 4. The PCEs of the PVKSCs with P1~P4 as the CILs are 15.2%, 11.5%, 7.7% and 1.5%, respectively, which follows well with the trend observed in the i-PSCs. Encouraged by the case that a thick CIL could be used in i-PSCs, we also fabricated the PVKSCs with a broad thickness range of 50 to 150 nm to examine the dependency of PCE on thickness of the selected P1 interlayer. The performance of the PVKSCs was shown in Figure S10. For an optimal thickness of 50 nm, the devices based on P1 interlayer produced a PCE over 15%. When the thickness was increased to 100 nm, the PCE values only dropped slightly. Even with a thick P1 CIL of 150 nm, the PCE of the devices still remained at the high values of 13.5%. These results further confirmed that P1 indeed possessed good electron transporting property, making it good candidate for potential large-area manufacturing. Additionally, the declining trend of the performance of PVKSCs similar to that of PSC also triggered us to further analyze the underlying factors. As shown in Table 4, the drop of PCEs was mainly attributed to the sharply decrease in FFs. The S-shaped J-V curves are clearly shown in the devices based on P3 and P4 CIL, while P2 based devices also suffers from poor diode characteristic. To study the reasons behind that possibly caused these poor J-V characteristics, we investigated interfacial properties of both the CIL/Ag and the perovskite/CIL.



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Figure 4. (a) Schematic diagram of the perovskite solar cells and (b) J-V curves of the perovskite devices with different CILs. Table 4. Photovoltaic parameters of PVKSCs with different CILs CIL

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

P1

0.93±0.01

20.7±0.8

0.73±0.02

14.0±1.2

15.2

P2

0.92±0.02

19.6±1.0

0.58±0.02

10.1±1.4

11.5

P3

0.91±0.01

18.7±0.8

0.42±0.03

6.2±1.5

7.7

P4

0.84±0.02

6.0±0.9

0.26±0.02

1.1±0.4

1.5

Best PCE (%)

As discussed in the cases of PSCs, the alignment of the WF of the cathode for the light harvesting layer and the electron transport layer is critically important to maximize the Voc and improve electron extraction in the solar cells. We therefore also studied this effect in our PVKSCs. Considering that Ag was used as the cathode in the PVKSCs, while our previous analysis of the polymer-induced WF modulation effect in inverted PSCs was based on ITO cathode, we therefore further tested the WF of the polymer coated Ag films using Kelvin probe measurement for a better comparison. Our results showed that after coating with a 50 nm of the P1~P4 polymer films, the WF of the pristine Ag film changed from 4.66 eV to -4.00, -4.13, -4.24 and -4.36 eV, respectively. The trend of the WF shifts was in good agreement with that observed in the ITO cases although the degree of WF changes was generally larger in the Ag cases, suggesting that the interfacial dipole formed on Ag might be even more effective. This can also explain the Vocs are less dependent on the different CILs in the cases of PVKSCs, showing relatively small shifts of Voc from 0.93 V to 0.84 V for the P1 to P4-based devices (Figure 4b, Table 4). Since the effect of WF modulation at the CIL/Ag interface probably could not be the sole reason that caused the severe S-shaped J-V curve in the PVKSCs observed in our study.65 Therefore, we also investigated the perovskite/CIL interface and studied their effects on the overall performance of the PVKSCs. The presence of defeat states in the perovskite films had been identified as a key factor limit18 Environment ACS Paragon Plus

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ing the performance of PVKSCs via enhanced trap-assisted charge recombinations.66-67 One type of defeats in perovskite film was the surface defeat, which could be formed when a halide atom was missing on the crystal surface, leaving a positively charged Pb atom which acted as an electron trapping state. Therefore, surface trap passivation became very important to ensure good performance for PVKSCs. In our previous study, we had proven that amino-functionalized CIL materials can effectively passivate the surface traps at the perovskite/CIL interface by forming a coordinate or dative-covalent bond between the electron-rich amine side group and the positively charged Pb atom, leading to the improved device performance. Therefore, in order to study the influence of chemical variation of the amine side groups on their surface trap passivation effect in PVKSCs, a complementary set of experiments including steady-state PL, time-resolved PL decay, transient photovoltage measurement were performed. PL measurement is a simple measurement to provide qualitative analysis on the surface passivation property of perovskite as the presence of surface states that located within the bandgap and close to the band edges can result in a smaller apparent bandgap, which lead to a red shift of the emission spectra.68 In contrary, removal of these trap states will blue shift the emission spectra. In the PL measurement, an excitation wavelength of 406 nm was used to excite the perovskite films either from air side or from ITO side of the ITO/PEDOT/perovskite/CIL/air sample. As shown in Figure 5a and 5b, the pristine perovskite film showed a PL peak at 776 nm. When excited from the ITO side, all the samples showed an emission peak at 776 nm, which was identical to observation in the pristine film and suggested that the PEDOT barely showed any passivation effect to the perovskite. When the samples were excited from the air side, perovksite films coated with P1 showed an obvious blue-shifted PL peak from 776 to 772 nm, while P2- and P3-coated perovskite films showed a similar blue-shift emission peak with a smaller shift from 776 to 774 nm. Meanwhile, there was no shift of PL peak for P4-coated perovskite film. These results revealed that P1, P2 and P3 all could passivate the pervoskite surface traps with the former one showing the strongest effect. The consequence for trap passivation was the reduction of the probability for charge recombination, leading to a prolonged PL lifetime that can be studied by transient PL decay measurement. In our study, the PL lifetimes were determined by fitting the exponential decay with double exponential functions. As shown in Figure 5c, our results revealed that the PL lifetime τ1 was similar in all of the cases with lifetimes < 10 ns. However, the more relevant lifetimes that associated with the trap induced recombination were τ2, which were 96.2 ns, 79.9 ns, 63.8ns and 35.3 ns for the P1~P4 coated perovskite films, respectively. The trend of measured PL lifetimes was in good agreement with the trap filling capability revealed by the steady-state PL study. In conjunction with the PL study, transient photovoltage (TPV) measurement based on the completed devices could also provide valuable information on the recombination property in the devices as the transient photovoltage decay time increased with the reduced recombinations. The TPV characteristics of the PVKSCs with P1~P4 CILs 19 Environment ACS Paragon Plus


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were shown in Figure 5d. Due to the reduced trap densities in the perovskite film, PVKSCs using P1 as CIL showed a slower photovoltage decay time of 0.47 µs, while the devices based on P2, P3 and P4 are 0.39, 0.30 and 0.10 µs, respectively. A longer electron lifetime generally indicated more efficient trap passivation property, which could reduce the charge recombination rate in the devices69. Therefore, the results obtained in the steady state PL, transient PL and also the TPV study were all in good agreement with each other and they all drew the same conclusion that the chemical environment of amine side groups played a critical role on interacting with the perovskite surface, leading to drastic difference in recombination dynamic at the perovskite/CIL as well as the PVKSC performance.

Figure 5. Photoelectronic properties of perovskite films with different CILs. PL spectra of perovskite/CIL films excited by a 406 nm light source a) from the air side and b) from the ITO side; c) Timeresolved PL decay transients measured at 770 nm for perovskite /CILs after excitation at 406 nm. The


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solid lines are the double-exponential fits of the PL decay transients. d) Photovoltage decay determined from different CIL-based perovskite solar cells. CONCLUSION In summary, a series of novel NDI-based conjugated polymers with different amine side groups and polymer backbones were synthesized and applied as CILs in both PSCs and PVKSCs. The chemical variations of the amine side groups from dimetylamino to N-methylbenzylamino and then to dibenzylamino introduced a decrease in electron-donating property and an increase in steric hindrance. These subtle modifications were proven to not only alter the intrinsic physical properties of the polymers including their electron mobility and self-doping capability, but also significantly affect their interfacial modification capability including the WF modulation of the cathode and also the surface trap passivation effect for perovskite films. As a result, the distinct interfacial effects and a declining performance were observed in these CIL-based devices. In addition, the manipulating of rigid acetylene spacer in polymeric backbone could indeed endow these polymers good electron-transporting property. Consequently, the introduction of optimized polymer CILs can enable the high-performance PSCs and PVKSCs with PCEs over 10% and 15%, respectively, and a thick interlayer for both PSCs and PVKSCs could be used without significantly sacrificing the device performance, which made them ideal candidate with good processing window for future large-area printing of PSCs and PVKSCs. Therefore, we believe our study will provide important insights and guidelines for developing versatile n-type conjugated polymers that can be applied as efficient cathode interfacial materials for high-performance organic and hybrid optoelectronic devices. SUPPORTING INFORMATION

Supporting Information is available free of charge on the ACS Publication website at http://pubs.acs.org Materials and instruments; synthetic details of the monomers and copolymers; device fabrication of PSCs and PVKSCs;

15

N H-HMBC characterizations of the monomers; TGA curves; CV curves of the

monomers and the reference materials; EPR spectra of the resulting polymers; J-V curves of electrononly devices; AFM topography; Contact angle measurements; The atomic dipole moment corrected 21 Environment ACS Paragon Plus


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Hirshfeld (ADCH) charges calculated by the Multiwfn 3.3.9 program; The contrastive absorption spectra of P1 and P6 in dilute solution; The effects of the thickness of P1 CIL on the PCE of PVKSC; Molecular weights and thermal properties; The Eox of the monomers and the reference materials; Electron mobilities; Optimized geometry and dihedral angles from DFT calculations; The work function (WF) of the modified ITO and Ag. AUTHOR INFORMATION

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

T.J. and C.S. contributed equally to this work

ACKNOWLEDGEMENT The work was financially supported by the Natural Science Foundation of China (No. 21634004, 21490573, 21761132001), the Ministry of Science and Technology (No. 2014CB643501), and the Science and Technology Program of Guangzhou, China (No. 201707020019 and 201607020010).

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Naphthalene Diimide Based n-Type Conjugated Polymers as Efficient

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