TBP precursor agent passivated ZnO electron transport layer for highly efficient polymer solar cells

Organic Electronics 76 (2020) 105458

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TBP precursor agent passivated ZnO electron transport layer for highly efficient polymer solar cells Zhongqiang Wang a, *, Zongtao Wang a, Ruqin Zhang a, Kunpeng Guo a, **, Yuezhen Wu a, Hua Wang a, Yuying Hao a, Shihe Yang a, b, *** a

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

b

A R T I C L E I N F O

A B S T R A C T

Keywords: Electron transport layer ZnO Polymer solar cells Defects passivation High efficiency

Defects passivation in electron transport layer (ETL) is a key issue to optimize the performance of polymer solar cells (PSCs). In this work, a novel strategy is developed to form defects passivated ZnO ETL by introducing 4-tertbutylpyridine (TBP) agent into precursor. While the power conversion efficiency (PCE) of the inverted PSCs based poly{4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl) carbonyl]thieno [3,4-b]thiophene-4,6-diyl}:[6,6]-phenyl C71-butyric acid methyl ester (PTB7:PC71BM) with the pure ZnO ETL is 8.02%, that of the device with modified ZnO ETL is dramatically improved to 10.26%, with TBP accounting for ~28% efficiency improvement. Our study demonstrates that the precursor agent significantly affect the surface morphology and size of ZnO in ETL. Furthermore, it proves that the ZnO ETL with TBP (T-ZnO) is beneficial to polish interfacial contact between ETL and active layer and depress exciton quenching loss, resulting in enhanced exciton dissociation, efficient carrier collection and reduced charge recombination, thus improving the device performance. To verify the universality of T-ZnO ETL, the champion photovoltaic per­ formance with a PCE of 11.74% (10% improvement) is obtained in the PBDB-T-2F:IT-4F based nonfullerene PSCs using T-ZnO as ETL. Our work developed a new, universal and facile strategy for designing highly efficient PSCs based on fullerene and nonfullerene blend systems.

1. Introduction Bulk heterojunction (BHJ) solar cells based on conjugated polymers blended with soluble acceptors have the merit of low-cost fabrication and easy processing, which have attracted great attention from re­ searchers [1,2]. To date, the power conversion efficiency (PCE) of BHJ solar cells has exceeded 14% for single junction devices thanks to the great advances in functional materials innovation, interfacial engi­ neering and device design [3–6]. In the past, the most important source of PCE improvement has been on the light absorber material in BHJ solar cells. It should be recognized that the charge transport layer also plays a key role because it can mediate the interface contact, enhance the charge carrier extraction efficiency and reduce the charge carrier recombination in polymer solar cells (PSCs) [7–10]. In the conventional

device structure, acidic PEDOT:PSS, air-sensitive electron transport layer materials and low work function metal are usually selected as the hole-collecting layer, the electron-collecting layer and the cathode, respectively, leading to instability in PSCs. To improve the PCE and stability of PSCs, an inverted device structure has been designed, which adopting a metal having a high work function as the anode on top of the device to collect holes, and ITO as the cathode in the bottom to collect electrons [11]. In the inverted PSCs, the n-type metal oxide such as zinc oxide (ZnO), titanium oxide (TiOx), tin oxide (SnO2) and cesium oxide (CsOx) are usually deposited on the ITO as an electron transport layer (ETL) to reduce the energy barrier between the active layer and ITO [12]. Among these electron transporting materials, ZnO possesses several merits making it a more prominent candidate, including its environmentally

* Corresponding author. ** Corresponding author. *** Corresponding author. Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Research Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, China. E-mail addresses: [email protected] (Z. Wang), [email protected] (K. Guo), [email protected] (S. Yang). https://doi.org/10.1016/j.orgel.2019.105458 Received 7 August 2019; Received in revised form 12 September 2019; Accepted 23 September 2019 Available online 26 September 2019 1566-1199/© 2019 Elsevier B.V. All rights reserved.

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Organic Electronics 76 (2020) 105458

friendly nature, facile fabrication via Sol-Gel method, as well as its remarkable electron mobility and good transparency [12–14]. Never­ theless, ZnO as a promising electron transport material is still faced with great challenges to overcome in order to make more efficient PSCs. In Sol-Gel preparing ZnO, for example, there are defects forming both on the surface and in the bulk, such as oxygen vacancies, which may be potential electron trap sites leading to recombination loss [15]. Addi­ tionally, the hydrophilic nature of pristine ZnO usually makes poor interfacial contact with the hydrophobic organic active layer resulting in a high contact barrier [16]. Many strategies have been developed to passivate the surface defects of solution-processed ZnO ETLs. For example, introducing an interfacial layer with strong dipole or an ionic liquid surfactant, such as poly­ electrolyte, self-assembled molecular aggregates, alcohol/water-soluble conjugated polymer and alkali metal carbonate, on top of the ZnO sur­ face to prepare the ETLs with a double-layer configuration [17–20]. Then the ZnO transport layer can effectively avoid direct contact with the active layer, tuning the work function of the ETLs, and further reducing the energy barrier from the active layer to ETLs. Another method is to directly blend ZnO and polymers with polar functional group, such as poly (ethyleneoxide) (PEO), poly (ethylene glycol) (PEG), poly [(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9, 9-dioctylfluorene)] (PFN) and polyethylenimine (PEI), through the surface electrostatic interaction to form ETLs composite layer [19–25]. This method can effectively passivate the defects of ZnO films and thus decrease charge recombination, eventually improve the PCE of inverted PSCs with such composite ETLs. However, these methods have led to new interfacial contacts in BHJ solar cells, which increase complexity in PSCs [11]. This together with the insulating property of the organic interfacial materials have limited the application of ZnO based bilayer and blend ETL in organic PSCs. In this context, we propose a novel strategy to perfect the forming of ZnO ETL, diminish the defects in the interior and on the surface of the ZnO ETL film by introducing appropriate agent to the ZnO precursor solution. To realize this goal, we turn to the additive with lone pair of electrons on the nitrogen atom that can form coordinate bonds with the Lewis acid sites of metal oxide, aiming to suppress the defects of ZnO ETL. Meanwhile, the additive should have peripheral alkyl group that provide a hydrophobic surface to polish the contact between ETL and active layer. The candidate of PEI with non-conjugated backbone and functional amine has been reported as interfacial layer to tune the work function of cathode and polish the contact between cathode and active layers in PSCs [26], which shows stronger electron-donating property compared with 4-tert-butylpyridine (TBP). The strong electron-donating property is promising to passivate the defects of Sol-Gel processed ZnO ETL. However, low conductivity was observed in PEI film, leading to high series resistance in thick film, which limited the electron extraction and transport process in cells. Hence, commercial TBP with electron withdraw nature of pyridine ring was selected as the precursor agent in this study. As a result, the ZnO ETLs with TBP agent showed less exciton quenching, better interface contact with the active layer, more efficient carrier collection and weaker charge recombination when compared with pristine ZnO ETLs in inverted PSCs. Finally, the PCE of the device based on poly{4,8-bis [(2-ethylhexyl)oxy]benzo [1,2-b:4,5-b’]dithio­ phene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b] thiophene-4,6-diyl} (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) with modified ZnO ETL was improved to 10.26% due to the impressive enhancement of the short circuit current density (Jsc). Furthermore, a champion PCE of 11.73% was achieved in PBDB-T-2F: IT-4F based solar cells with a modified ZnO ETL to demonstrate the universal advantage of our strategy in inverted PSCs.

2. Experimental section 2.1. Preparation of the ZnO precursor The ZnO precursor was prepared by dissolving anhydrous zinc ace­ tate (Zn(CH3COO)2, Energy Chemical, 99.5%, 0.836 g) and ethanol­ amine (NH2CH2CH2OH, Aldrich, 99.9%, 0.28 g) in 2-methoxyethanol (CH3OCH2CH2OH, Energy Chemical, 99%, 10 mL) under vigorous stir­ ring for 12 h for the hydrolysis reaction in air. Then, 4-tert-butylpyridine (TBP, Energy Chemical, 98%) was added to the precursor solution in different volume proportion and stirred at room temperature for another 4 h. 2.2. Fabrication of inverted PSCs MoO3 was purchased from Rieke Company. PTB7 and PC71BM were achieved from 1-Material of U.S. and Luminescence Technology Cor­ poration of Taiwan, respectively. PBDB-T-2F and IT-4F were purchased from Solarmer Materials Inc. of China. Inverted solar cells were fabri­ cated on ITO-coated glass substrates. The ITO-coated glass substrates were cleaned with detergent, acetone and isopropanol under ultrasonication, and subsequently dried overnight in an oven. ZnO precur­ sor was spin-coated on the UV ozone-treated ITO and then annealed at 200 � C for 1 h in air. The samples were quickly transferred to a N2protected glove-box. Then, a solution containing a mixture of PTB7: PC71BM ¼ 1:1.5 in mixed solvent of chlorobenzene (CB) and 1,8-diio­ doctane (DIO) (97:3 by volume) with a total concentration of 25 mg mL 1, or a solution of PBDB-T-2F:IT-4F ¼ 1:1 in CB and DIO (95:5 by volume) with a total concentration of 20 mg mL 1 was spin-coating on top of ZnO films with thickness about 95 nm, and 100 nm, respec­ tively. After spin-coating, the PBDB-T-2F:IT-4F films were annealed at 100 � C for 20 min. In a chamber with base pressure of 3 � 10 4 Pa, 5 nm MoO3 was thermally deposited at a rate of 0.2 Å s 1. Finally, 80 nm Al was evaporated with a shadow mask. A shadow mask was used during the thermal evaporation of Al to define active area of 0.12 cm2. All materials and solvents were used as received without further purification. 2.3. Device characterization Both efficiencies (PCE and EQE) and J-V characteristics were recor­ ded using the Newport system. The illuminated state J-V characteristics were measured with the simulated AM 1.5 G light source at 100 mW cm 2. A calibrated mono-silicon diode was used as a reference, which has a response in the spectral response of 300–800 nm. 3. Results and discussion To facilitate the presentation and discussion below, the thin films prepared from the precursor solution without TBP were named P–ZnO, and the thin films prepared from the precursor solution with TBP agent were named T-ZnO:X (X represents the proportion of TBP). Firstly, to manifest that the interact between TBP and ZnO, Fourier-transform infra-red (FT-IR) spectra were employed. As shown in Fig. 1, the pres­ ence of bands at 821 cm 1, 1598 cm 1 and 1409 cm 1 have attributed to – N) for TBP, respec­ the vibration band of ν (C–H), ν (C–N) and ν (C– tively. In the T-ZnO:10% complex spectrum, these bands are present and have shifted to 840 cm 1, 1615 cm 1 and 1424 cm 1, respectively. These shifts are attributable to the formation of coordinate bond be­ tween the lone pair of electrons on the nitrogen atom of the pyridine ring and the Lewis acid sites of the ZnO surface. With the above result in hand, the P–ZnO and T-ZnO:X films were applied as ETL in PSCs with the same device structure of ITO/ETL/PTB7: PC71BM/MoO3/Al (Fig. 1 (b)). The optimized thickness of ZnO was set to 40 nm in our study. The energy level diagrams of PTB7:PC71BM and PBDB-T-2F:IT-4F based inverted cells are also displayed in Fig. 1 (c). The 2

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Organic Electronics 76 (2020) 105458

Fig. 1. a) FT-IR spectra of ZnO, T-ZnO: 10% complex and TBP. b) The identical device architecture. c) Energy level diagram of PTB7:PC71BM and PBDB-T-2F:IT-4F based inverted cells.

chemical structures of the PTB7, PBDB-T-2F, PC71BM, TBP and IT-4F are shown in Fig. S1 (ESIy). First, the volume ratio of TBP to CH3OCH2CH2OH in the ZnO pre­ cursor was optimized for optimal device performance. The range of variation was from 0% to 15%. The current density versus voltages (J-V) and external quantum efficiency (EQE) characteristics are shown in Fig. 2. The corresponding performance parameters were summarized in Table 1. The reference device featuring P–ZnO as the cathode buffer layer displayed an open circuit voltage (Voc) of 0.74 V, a Jsc of 15.83 mA cm 2, a FF of 68.5% and a PCE of 8.02%. The Jsc values increased with increasing TBP content from 0% to 10%, then decreased with further increase of the TBP content. The further increase TBP content leading to PCE decreases could be attributed to the morpho­ logical effect, this will be discussed later. An optimal device perfor­ mance was obtained when the ratio of TBP was 10% with a Voc of 0.74 V, a Jsc of 19.81 mA cm 2, a FF of 70.0% and a PCE of 10.26%. In com­ parison with the reference device, the Jsc and PCE in T-ZnO:10% based PSCs were improved by 25% and 28%, respectively. Fig. 2b shows the EQE spectra of the cells prepared with different ETLs. The trends reflecting by EQE spectra were consistent with those the values of Jsc. The cells using T-ZnO:X as ETLs exhibited higher effi­ ciencies. Furthermore, it was noted that the shape of spectra showed no change regardless of whether TBP was applied. The values of EQE increased within the range from 350 nm to 700 nm. Moreover, the EQE with a value of 82% at 650 nm was obtained in PSCs using T-ZnO:10% as ETLs, which was impressive in PTB7:PC71BM based cells. From the EQE spectra, it is the addition of TBP that has contributed to the photovoltaic performance enhancement. It has been proved that the surface morphology of ETL in PSCs plays a critical role in determining the performance of device. Hence, the morphology changes of the ETLs without and with the TBP agent were studied by atomic force microscopy (AFM), and the 3-D top images are

Table 1 Photovoltaic performance of PTB7:PC71BM based inverted PSCs. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM (95 nm)/ MoO3(5 nm)/Al. 2

X

Voc(V)

Jsc (mA cm

0 2% 5% 8% 10% 12% 15%

0.74 � 0.01 0.74 � 0.01 0.74 � 0.01 0.74 � 0.01 0.74 � 0.01 0.74 � 0.01 0.74 � 0.01

15.83 � 0.11 16.61 � 0.19 17.77 � 0.16 18.52 � 0.21 19.81 � 0.14 19.19 � 0.12 18.51 � 0.19

)

FF(%)

PCE(%)

68.5 � 0.3 69.3 � 0.6 70.2 � 0.5 69.9 � 0.7 70.0 � 0.4 70.4 � 0.3 70.3 � 0.5

8.02 � 0.05 8.49 � 0.21 9.22 � 0.16 9.54 � 0.16 10.26 � 0.07 9.95 � 0.12 9.67 � 0.14

displayed in Fig. 3. The Root-Mean-Square (RMS) roughness of P–ZnO film was 2.371 nm. The RMS roughness was decreased to 1.361 nm, 1.220 nm and 1.251 nm in T-ZnO:5%, T-ZnO:10% and T-ZnO:15% films, respectively. The smooth surface might polish the contact between ETLs and active layers, and influence the charge transport and collection in PSCs, resulting in an improved photocurrent. Therefore, the morphology change should be partly responsible for the performance improvement. The interface contact between ETLs and active layers can be deter­ mined by the surface wettability of ETLs in PSCs. To check this aspect, the contact angle of water droplets on P–ZnO and T-ZnO:X films were measured in this study. As shown in Fig. 4, the contact angles of water on P–ZnO, T-ZnO:5%, T-ZnO:10% and T-ZnO:15% films were 26� , 28.5� , 33� and 31� , respectively. In comparison with P–ZnO film, the T-ZnO films were more hydrophobicity, indicating more efficient contact be­ tween ETLs and photoactive layers. Again, the decreased contact angle of T-ZnO:15% based film can be attributed to its increased roughness. Thus, the efficient contact is beneficial to the performance improvement of PSCs.

Fig. 2. a) Illuminated state J–V characteristics and b) external quantum efficiency (EQE) characteristics of the PSCs for different volume ratios of TBP to CH3OCH2CH2OH. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM(95 nm)/MoO3(5 nm)/Al. 3

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Fig. 3. Three-dimensional (3-D) top images (5.0 � 5.0 μm2) of the a) P–ZnO, b) T-ZnO:5%, c) T-ZnO:10%, d) T-ZnO:15% films. The structure of samples is ITO/ P–ZnO or T-ZnO:X (40 nm).

Fig. 4. Contact angles of water droplets on the a) P–ZnO, b) T-ZnO:5%, c) T-ZnO:10% and d) T-ZnO:15% films. The structure of samples is ITO/P–ZnO or T-ZnO: X (40 nm).

could be investigated by space-charge-limited current (SCLC) method. Hence, the electron mobility in P–ZnO and T-ZnO:X based solar cells were extracted by SCLC model. The electron-only device structures were ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM(95 nm)/BCP (8 nm)/Al. As shown in Fig. S2 (ESIy), the electron-only device J-V characteristics were fitted by SCLC model. The electron mobility of inverted PSCs using P–ZnO, T-ZnO:5%, T-ZnO:10% and T-ZnO:15% as ETL were 3.18 � 10 4, 5.98 � 10 4, 1.60 � 10 3 and 1.16 � 10 3 cm2 V 1 S 1, respectively, which is consistent with the increase of ZnO size in SEM study. The enhanced electron mobility is beneficial to polish the

The grain size in ZnO films upon thermal annealing was revealed by the scanning electron microscope (SEM) study. The SEM images in Fig. 5 display the film morphology of the ZnO films after 200 � C thermal annealing. The ZnO films without and with TBP are continuous and compact after 1 h of thermal annealing. Compared with P–ZnO film, the increase of ZnO grain size in TBP augmented T-ZnO:10% was confirmed by SEM study. It is noted that the grain size increase could minimize the grain boundary in the ZnO ETL, which contributes to a higher efficiency in the T-ZnO based solar cells. The effect of TBP on charge transport behavior inside the solar cells 4

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Fig. 5. The SEM images of the a) P–ZnO and b) T-ZnO:10% films. The structure of samples is ITO/P–ZnO or T-ZnO:X (40 nm).

electrical properties of T-ZnO:X ETL, which is attributed to the better contact between ETL and active layer, resulting in reduced Rs and increased Rsh. To further reveal the charge recombination mechanisms in devices using P–ZnO and T-ZnO ETLs, we measured the dependence of Jsc on incident power intensity to reveal the charge carrier recombination behavior in PSCs [27,28]. A power law dependence of Jsc with incident power intensity can be written as: Jsc ∝I α

Table 2 The values of the fitted α and slopes for the devices fabricated using different ETLs. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM (95 nm)/MoO3(5 nm)/Al. X

0

2%

5%

8%

10%

12%

15%

α

0.945 1.235

0.947 1.195

0.952 1.152

0.959 1.123

0.969 1.080

0.967 1.087

0.963 1.102

S

(1)

assisted Shockley-Read-Hall (SRH) recombination behavior was sup­ pressed in T-ZnO:X based PSCs [33]. We can conclude that the intro­ duction of TBP effectively reduces the trap assisted SRH recombination loss in cells. The diminished charge recombination losses also improved the Jsc of T-ZnO:X based PSCs. To further understand the improvement of Jsc, the maximum exciton generation rate (Gmax) was calculated in PSCs. Fig. 7a and Fig. S5 (ESIy) display the dependence of Jph on the effective applied voltage (Veff). The Jph is expressed as:

where I is the incident power intensity and α is the exponential factor. In an efficient photovoltaic cell, α is close to unity [29,30]. As shown in Fig. 6a and Fig. S3 (ESIy), the characteristics of Jsc-power intensity are plotted in log-log scale and fitted with Equation (1). The fitted α for the devices with different ETLs are listed in Table 2. All values of α are slightly lower than 1, and the value of α after adding TBP has been improved, where α showed 0.969 using T-ZnO:10% as ETL. The improved α in T-ZnO:X based devices indicates weaker bimolecular recombination at short circuit condition in these devices [31]. Under open circuit condition, all photo-generated charge carriers recombine again without extraction in the cells, which directly reveal the recombination process. Therefore, the dependence of Voc upon power intensity can be used to reveal the inside recombination behavior. The Voc as a function of power intensity was shown in Fig. 6b and Fig. S4 (ESIy). The slope of Voc versus the logarithm of the power intensity displays a linear dependence with kT=q in PSCs, where k the Boltzmann constant, T the temperature in Kelvin, q the elementary charge [32]. The extracted data were fitted with a function with slope S. As listed in Table 2, the slope of cells with P–ZnO, T-ZnO:5%, T-ZnO:10% and T-ZnO:15% were 1.235 kT=q, 1.152 kT=q, 1.080 kT=q and 1.102 kT= q, respectively. A stronger dependence of Voc on light intensity with a slope of 1.235 kT=q was observed in P–ZnO based device. The weaker dependence of Voc-power intensity characteristic implied that the application of TBP reduced the trap density of ZnO ETLs, and thus trap

Jph ¼ JL

JD

(2)

where JL is the current density under illumination condition (100 mW cm 2, AM 1.5G), JD is the current density under dark condi­ tion. Veff is determined by the equation: Veff ¼ V0

Va

(3)

where V0 is the voltage at which the Jph ¼ 0, Va is the voltage applied on the device [33,34]. Two different regimes are clearly reflected in Fig. 7a. In the first regime, the Jph linearly increases with the increasing Veff. Then, the Jph gradually becomes saturated at high Veff. The saturated current density is equal to the Jph due to the application of high Veff, which should be independent of applied electrical field and temperature [35]. Therefore, the Jsat is expressed as:

Fig. 6. a) The Jsc dependence on incident power intensity (symbols) on a logarithmic scale. b) The Voc dependence on incident power intensity (symbols) on a logarithmic scale. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM (95 nm)/MoO3(5 nm)/Al. 5

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Organic Electronics 76 (2020) 105458

Fig. 7. a) Photocurrent density (Jph) as a function of effective bias (Veff) in the devices using P–ZnO and T-ZnO:X as the ETLs. b) Exciton dissociation probability as a function of the effective bias (Veff) in the devices with P–ZnO and T-ZnO:X as the ETLs. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM (95 nm)/MoO3(5 nm)/Al.

Jsat ¼ qGmax L

ITO/ZnO (40 nm)/PTB7(50 nm)/MoO3(5 nm)/Al (10 nm). In compari­ son with P–ZnO, the PL intensity increased when T-ZnO:X films were used as ETLs, indicating diminished exciton quenching at the interface of T-ZnO:X/PTB7, which should be attributed to the passivated surface defects of T-ZnO:X films. The defects passivation in T-ZnO:X films should be responsible for the Jsc improvement. Electric impedance spectroscopy (EIS) measurement is usually used to analyze the interface properties in solar cells, such as transmission resistance and charge recombination. Hence, EIS was employed to provide further insight properties at ZnO film and PTB7:PC71BM inter­ face. Fig. 8b and Fig. S7 (ESIy) displayed the Nyquist plots of the ISat zero bias under dark condition. Compared with P–ZnO based device, the TZnO:5%, T-ZnO:10%, and T-ZnO:15% presented shorter diameters, indicating lower transport resistance. The decreased transport resistance means lower contact resistance in T-ZnO based cells, leading to more efficient charge carrier transport. This result further confirmed the modified interfacial contact between ETL and active layers after intro­ ducing TBP into the ZnO precursor. To further confirm the advantage of TBP, the nonfullerene system of PBDB-T-2F:IT-4F was also used as light absorber. The J-V characteristic and EQE spectra were shown in Fig. 9a and Fig. 9b. A PCE of 10.73% with a Voc of 0.86 V, a Jsc of 16.68 mA cm 2, a FF of 74% was obtained in the reference cell based on P–ZnO. As anticipated, the champion and impressive PCE of 11.74% with a Voc of 0.86 V, a Jsc of 18.28 mA cm 2, a FF of 74.7% was observed in cell using TBP doped ZnO as ETL having a TBP ratio of 10%. Compared to the performance in reference cell, the Jsc and PCE were improved about 10% after the introduction of TBP.

(4)

where q is the elemental charge, L is the thickness of the active layer [34, 36]. Hence, the Gmax can be calculated from the measured Jsat. The values of Gmax were 1.25 � 1028, 1.39 � 1028, 1.45 � 1028 and 1.44 � 1028 m 3 s 1 for the device fabricated with P–ZnO, T-ZnO:5%, T-ZnO:10% and T-ZnO:15%, respectively. In general, the Gmax is gov­ erned by charge carrier transport and collection in organic solar cells. The enhanced Gmax reflected efficient charge transport and collection in T-ZnO:X based cells, which also contributed to the improved Jsc. In organic solar cells, only a portion of excitons can dissociate into free charge carrier under natural conditions due to the unique photo­ electric conversion mechanism. The values of Jph under the short circuit condition were divided by the Jsat (Jph/Jsat) [34,37]. The values of Jph/Jsat are highly revealing of the exciton dissociation probability under bias. The exciton dissociation probabilities under short circuit condition were 77.3%, 86.3%, 95.1% and 89.9% in P–ZnO, T-ZnO:5%, T-ZnO:10%and T-ZnO:15% based devices, respectively. We infer that T-ZnO:X films can facilitate exciton dissociation, increasing the photo­ current in T-ZnO:X based devices. The normalized photocurrent density (Jph/Jsat) was shown in Fig. 7b and Fig. S6(ESIy). Thus, the introduction of TBP increased the exciton dissociation probability, thereby enhancing the photocurrent of PSCs [38]. Exciton quenching is highly related to the photovoltaic performance in PSCs, the photoluminescence (PL) from the donors must be effectively quenched by acceptors in order to achieve high efficiency [38,39]. Therefore, to study the harmful exciton quenching at the ZnO/active layer interface, pure PTB7 films rather than the PTB7:PC71BM blend films were employed in the following PL quenching experiment. Fig. 8a shows the PL quenching spectra of the PTB7 films with a structure of

Fig. 8. a) Photoluminescence spectra of the ITO/ZnO(40 nm)/PTB7(50 nm)/MoO3(5 nm)/Al (10 nm) samples under photo-excitation at 500 nm. b) Impedance spectra (Nyquist plot) of the devices using P–ZnO and T-ZnO:X as ETLs. The device structure is ITO/P–ZnO or T-ZnO:X (40 nm)/PTB7:PC71BM (95 nm)/ MoO3(5 nm)/Al. 6

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Fig. 9. a) Illuminated state J–V characteristics and b) EQE characteristics of the PSCs using PBDB-T-2F:IT-4F as light absorber. The device structure is ITO/P–ZnO or T-ZnO:10% (40 nm)/PBDB-T-2F:IT-4F (100 nm)/MoO3(5 nm)/Al.

4. Conclusion

[8] H. Ma, H.-L. Yip, F. Huang, A.K.-Y. Jen, Adv. Funct. Mater. 20 (2010) 1371–1388. [9] J.J. Intemann, K. Yao, Y.-X. Li, H.-L. Yip, Y.-X. Xu, P.-W. Liang, C.-C. Chueh, F.Z. Ding, X. Yang, X. Li, Y. Chen, A.K.Y. Jen, Adv. Funct. Mater. 24 (2014) 1465–1473. [10] F.-X. Xie, W.C.H. Choy, W.E.I. Sha, D. Zhang, S. Zhang, X. Li, C.-W. Leung, J. Hou, Energy Environ. Sci. 6 (2013) 3372–3379. [11] Y. Sun, J.H. Seo, C.J. Takacs, J. Seifter, A.J. Heeger, Adv. Mater. 23 (2011) 1679–1683. [12] Y.S. Kim, B.-K. Yu, D.-Y. Kim, W.B. Kim, Sol. Energy Mater. Sol. Cells 95 (2011) 2874–2879. [13] Z. Liang, Q. Zhang, L. Jiang, G. Cao, Energy Environ. Sci. 8 (2015) 3442–3476. [14] N. Sekine, C.-H. Chou, W.L. Kwan, Y. Yang, Org. Electron. 10 (2009) 1473–1477. [15] C.E. Small, S. Chen, J. Subbiah, C.M. Amb, S.-W. Tsang, T.-H. Lai, J.R. Reynolds, F. So, Nat. Photonics 6 (2011) 115–120. [16] Z. Wu, T. Song, Z. Xia, H. Wei, B. Sun, Nanotechnology 24 (2013) 484012. [17] R. Zhang, M. Zhao, Z. Wang, Z. Wang, B. Zhao, Y. Miao, Y. Zhou, H. Wang, Y. Hao, G. Chen, F. Zhu, ACS Appl. Mater. Interfaces 10 (2018) 4895–4903. [18] S. Woo, W.H. Kim, H. Kim, Y. Yi, H.-K. Lyu, Y. Kim, Adv. Energy Mater. 4 (2014) 1301692. [19] S. Shao, K. Zheng, T. Pullerits, F. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 380–385. [20] W. Yu, L. Huang, D. Yang, P. Fu, L. Zhou, J. Zhang, C. Li, J. Mater. Chem. A 3 (2015) 10660–10665. [21] S.H. Liao, H.J. Jhuo, Y.S. Cheng, S.A. Chen, Adv. Mater. 25 (2013) 4766–4771. [22] S.B. Jo, J.H. Lee, M. Sim, M. Kim, J.H. Park, Y.S. Choi, Y. Kim, S.-G. Ihn, K. Cho, Adv. Energy Mater. 1 (2011) 690–698. [23] H.-L. Yip, A.K.Y. Jen, Energy Environ. Sci. 5 (2012) 5994–6011. [24] T. Hu, L. Chen, K. Yuan, Y. Chen, Nanoscale 7 (2015) 9194–9203. [25] N. Wu, Q. Luo, Z. Bao, J. Lin, Y.-Q. Li, C.-Q. Ma, Sol. Energy Mater. Sol. Cells 141 (2015) 248–259. [26] H. Kang, S. Hong, J. Lee, K. Lee, Adv. Mater. 24 (2012) 3005–3009. [27] R.A. Street, M. Schoendorf, A. Roy, J.H. Lee, Phys. Rev. B 81 (2010) 205307. [28] Z. Wang, Z. Wang, Z. Wang, M. Zhao, Y. Zhou, B. Zhao, Y. Miao, P. Liu, Y. Hao, H. Wang, B. Xu, Y. Cheng, S. Yin, 2D Mater. 6 (2019) 4. [29] P. Schilinsky, C. Waldauf, C.J. Brabec, Appl. Phys. Lett. 81 (2002) 3885–3887. [30] Z. Wang, T. Sano, T. Zhuang, X.-F. Wang, H. Sasabe, J. Kido Org. Electron. 50 (2017) 191–197. [31] Z. Wang, Z. Hong, T. Zhuang, G. Chen, H. Sasabe, D. Yokoyama, J. Kido, Appl. Phys. Lett. 106 (2015), 053305. [32] S.R. Cowan, A. Roy, A.J. Heeger, Phys. Rev. B 82 (2010) 245207. [33] M.M. Mandoc, W. Veurman, L.J.A. Koster, B.D. Boer, P.W.M. Blom, Adv. Funct. Mater. 17 (2007) 2167–2173. [34] V.D. Mihailetchi, H.X. Xie, B.D. Boer, L.J.A. Koster, P.W.M. Blom, Adv. Funct. Mater. 16 (2006) 699–708. [35] K.S. Tan, M.K. Chuang, F.C. Chen, C.S. Hsu, ACS Appl. Mater. Interfaces 5 (2013) 12419–12424. [36] V.D. Mihailetchi, L.J. Koster, J.C. Hummelen, P.W.M. Blom, Phys. Rev. Lett. 93 (2004) 216601. [37] M.F. Xu, X.Z. Zhu, X.B. Shi, J. Liang, Y. Jin, Z.K. Wang, L.S. Liao, ACS Appl. Mater. Interfaces 5 (2013) 2935–2942. [38] J.-L. Wu, F.-C. Chen, Y.-S. Hsiao, F.-C. Chien, P. Chen, C.-H. Kuo, M.H. Huang, C.S. Hsu, ACS Nano 5 (2011) 959–967. [39] P. Anger, P. Bharadwaj, L. Novotny, Phys. Rev. Lett. 96 (2006) 113002.

In conclusion, we have successfully demonstrated a facile strategy of adding TBP agent into the precursor solution of ZnO to improve the efficiency of inverted PSCs. The introduction of the precursor agent changed the morphology, contact angle and grain size of ZnO ETLs, resulting in their efficient contact with the active layer. In addition, the agent application suppressed the exciton quenching and diminished the SRH recombination in PSCs by passivating the defects of ZnO ETLs. Compared with the P–ZnO based device, the T-ZnO:X based PTB7: PC71BM BHJ device showed enhanced Jsc (from 15.83 to 19.81 mA cm 2) and PCE (from 8.02% to 10.26%). Moreover, a cham­ pion PCE of 11.74% was obtained in PBDB-T-2F:IT-4F based devices due to the contribution of TBP. Our work offers how to design efficient ETL with a facile strategy for developing highly efficient PSCs based on fullerene and nonfullerene light absorbers. Acknowledgements This study was supported by National Natural Science Foundation of China (Grant No. 61704118). This study was also supported by the Qualified Personal Foundation of Taiyuan University of Technology (800101-02030017). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.orgel.2019.105458. References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789–1791. [2] J. Hou, O. Ingan€ as, R.H. Friend, F. Gao, Nat. Mater. 17 (2018) 119–128. [3] H. Zhang, H. Yao, J. Hou, J. Zhu, J. Zhang, W. Li, R. Yu, B. Gao, S. Zhang, J. Hou, Adv. Mater. (2018) 1800613. [4] Z. Xiao, X. Jia, L. Ding, Sci. Bull. 62 (2017) 1562–1564. [5] F. Zhao, S. Dai, Y. Wu, Q. Zhang, J. Wang, L. Jiang, Q. Ling, Z. Wei, W. Ma, W. You, C. Wang, X. Zhan, Adv. Mater. 29 (2017) 1700144. [6] M. Li, K. Gao, X. Wan, Q. Zhang, B. Kan, R. Xia, F. Liu, X. Yang, H. Feng, W. Ni, Y. Wang, J. Peng, H. Zhang, Z. Liang, H.-L. Yip, X. Peng, Y. Cao, Y. Chen, Nat. Photonics 11 (2016) 85–90. [7] R. Po, C. Carbonera, A. Bernardi, N. Camaioni, Energy Environ. Sci. 4 (2011) 285–310.

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