Insights into Working Mechanism of Alkali Metal Fluorides as Dopants

Subscriber access provided by UNIV OF LOUISIANA

C: Energy Conversion and Storage; Energy and Charge Transport

Insights into Working Mechanism of Alkali Metal Fluorides as Dopants of ZnO Films in Inverted Polymer Solar Cells Jing Qiu, Biao Guo, Hongwei Zhang, Chengzhuo Yu, and Fenghong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07694 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Insights into Working Mechanism of Alkali Metal Fluorides as Dopants of ZnO Films in Inverted Polymer Solar Cells Jing Qiu, Biao Guo, Hongwei Zhang, Chengzhuo Yu and Fenghong Li*

State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun 130012, P. R. China. *

E-mail: [email protected] (F. H. Li).

ABSTRACT: Alkali metal fluorides (AMFs) have been utilized as dopants of ZnO films in PTB7:PC71BM based inverted polymer solar cells (i-PSCs). As a result, power conversion efficiency (PCE) and device stability were obviously improved in i-PSCs with ZnO:AMF. In particular for the i-PSCs with ZnO:NaF (0.2 mol%), PCE of 8.64% was obtained in the fresh devices and the PCE still kept at 7.56% after i-PSCs were retained in the glove box for 80 days, which were much better than the only ZnO devices. Results of photocurrent density-effective voltage characteristics, electron mobility and capacitance-voltage characteristics demonstrated that doping ZnO with AMFs can effectively raise the charge carrier extraction, electron mobility, charge carrier density and built-in potential in the i-PSCs. X-ray photoemission spectroscopy (XPS) measurements indicated that oxygen defects on ZnO surface were reduced through doping with AMFs. Atomic force microscopy images presented that adding 0.2 mol% AMFs into ZnO didn’t lead to the lattice distortion of ZnO films. Scanning electronic microscopy-energy dispersive spectrum mapping and XPS depth profiling suggested that there were more Na+ at the top of ZnO:NaF surface than K+ 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

or Cs+ at the top of ZnO:KF or ZnO:CsF surface. Therefore Na+ have more chances to meet the defects on ZnO surface than K+ and Cs+ so that the i-PSCs with ZnO:NaF shows the supreme PCE. These results reveal that NaF is an effective, competitive and prospective dopant of ZnO in i-PSCs.

1. INTRODUCTION Polymer solar cells (PSCs) have been widely investigated thanks to some particular features for example low cost, slight weight, flexibility, solution processing and the potential of fabricating large-area PSCs.1-5 Power conversion efficiency (PCE) has been quickly boosted via synthesis of original photo-active materials,6-10 modification of interface layers,11-13 morphology control of active layers14, 15 and design of new device structures.16, 17 So far champion PCE of 14.62% has been achieved in the ternary inverted PSCs (i-PSCs).18 Recently i-PSCs have been developed rapidly due to their higher PCE and better device stability than conventional PSCs.19 Generally there exists an electron transport layer (ETL) between ITO and organic active layer to reduce the energy barrier and polish up the contact in the i-PSCs. Because of high transparency, low cost, suitable energy level, good stability and environmental friendliness,20-22 zinc oxide (ZnO) has been known as a promising ETL in i-PSCs. However a poor interfacial contact between active layer and pure ZnO film obtained by sol-gel method is a big challenge for ZnO as an ETL. High series resistance at the interface limits the electron transport

2


Page 2 of 30

Page 3 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ability, leading to a low short-circuit current density (JSC) and fill factor (FF).23, 24

In addition, as centers of interfacial recombination of photo-generated charge

carriers, the surface defects of the ZnO film severely damage PCE and device stability.25, 26 At the same time conductivity of ZnO film prepared by sol-gel method is poor, which would deteriorate the electron transport process and inevitably compromises the performance of the devices.27 Consequently, it is imperative to devote some efforts to solve these problems. One of effective approaches is directly doping ZnO precursor solution with appropriate materials to not only improve conductivity of ZnO film and compatibility between photo-active layer and the ZnO film but also inhibit surface traps. Reported dopants of ZnO mainly include organic and inorganic materials. Compared to organic dopants, inorganic dopants of ZnO should be more desired for practical application due to lower cost, more easily processing and environmental friendliness. Currently inorganic dopants of ZnO films mainly consist of alkali metal (AM) salts,28, 29 alkaline earth metal salts30, 31 and salts with main group-III/IV elements.32-38 In particular Li2CO3 as a dopant of ZnO has improved the PCE of the i-PSCs up to 10.08% which is the highest PCE of the PTB7:PC71BM based devices with ZnO as an ETL.28 Li (or Na) prefers basically interstitial sites to substitutional sites of Zn in the ZnO matrix.39, 40 The majority of the AM ions must exist as interstitial donors in the structure rather than as substitutional acceptors.41, 42 Therefore AM salts have demonstrated a big potential as a good n-type cationic dopant of ZnO 3



Page 4 of 30

films in the i-PSCs. Moreover the fluorine (F) is considered as a candidate suitable for anion doping with lower lattice distortion because its radius is close to the radius of oxygen.43 Recent investigations demonstrated that incorporating a few F anions into ZnO film obviously enhanced the mobility of charge carries and the transmittance in visible region.44 Therefore the alkali metal fluorides (AMFs) should be promising candidates for doping ZnO. Unfortunately systematic investigations of working mechanism for the AMFs as dopants of ZnO in the i-PSCs have not been reported yet even though they have been widely applied to cathode interlayers in the organic light-emitting devices and photovoltaic devices.45-52

In this contribution, sodium fluoride (NaF), potassium fluoride (KF), cesium fluoride (CsF) are used as dopants of ZnO in the i-PSCs. PCEs of 8.64% for ZnO:NaF, 8.39% for ZnO:KF and 8.28% for ZnO:CsF have been gained in the devices based on PTB7:PC71BM because of simultaneously enhanced open circuit voltage (VOC), JSC and FF compared to the control device with pure ZnO. Combined measurements of photocurrent density-effective voltage (Jph-Veff), electron

mobility

(µe)

and

capacitance-voltage

(C-V)

characteristics

demonstrated that doping ZnO with AMFs can effectively raise exciton dissociation and charge extraction, electron mobility, charge carrier density and built-in potential in the i-PSCs. O1s X-ray photoemission spectroscopy (XPS) spectra presented that incorporating the AMFs led to a reduction of oxygen-deficient

defects

at

the

ZnO

surfaces.

4


Scanning

electron

Page 5 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


microscope-energy dispersive spectrum (SEM-EDS) mapping and XPS depth profiling suggested that the smaller the ionic radius, the more likely the alkali metal ions appeared on the surface of ZnO films. Therefore Na+ have more chances to meet the defects of ZnO surface than K+ and Cs+ so that the i-PSCs with ZnO:NaF shows the supreme PCE.

2. EXPERIMENTAL SECTION

2.1 Materials and preparation of ZnO and ZnO:AMF precursor solutions Zn(CH3COO)2·2H2O,

ethanolamine,

2-methoxyethanol,

NaF,

KF,

CsF,

chlorobenzene (CB) and 1,8-diiodooctane (DIO) were purchased from Sigma-Aldrich. PTB7 and PC71BM were obtained from 1-material Inc. (Canada) and American Dye Source, respectively. All materials were used as delivered. ZnO precursor solution was prepared according to the previous reports, namely adding 1.0 g Zn(CH3COO)2·2H2O into the mixture of 280 µL ethanolamine and 10 mL 2-methoxyethanol and then stirring 24h in air.53 The ZnO:AMF precursor solutions were prepared by adding AMF into ZnO precursor solution. Molar ratios of the AMFs and ZnO were 0.05%, 0.1%, 0.2%, 0.5%, 1.0% and 2.0%, respectively.

2.2 Fabrication of devices and Characterization Device structure of the i-PSCs is ITO/ZnO or ZnO:AMF/PTB7:PC71BM/MoO3/Al as shown in Fig. 1. The structure of the electron-only devices is ITO/Al/ZnO or ZnO:AMF/PTB7:PC71BM/LiF/Al. Detailed device fabrication procedures can be 5



Page 6 of 30

found in Supporting Information. The current density-voltage characteristics, external quantum efficiency (EQE) spectra and C-V characteristics of the i-PSCs have been measured.

The

transmittance

spectra,

XPS

and

ultraviolet

photoemission

spectroscopic (UPS) experiments of ZnO and ZnO:AMF films have been carried out. Atomic force microscopy (AFM) images and SEM-EDS mapping have been taken. More detailed information for these measurements can been found in Supporting Information.

3. RESULTS AND DISCUSSION

Fig. 1 Molecular structures of PTB7 and PC71BM, and inverted device configuration (AMF = NaF, KF or CsF).

In this work, we select three AMFs namely NaF, KF and CsF as the dopants of ZnO except LiF due to its insolubilization in the mixture solution of 2-methoxyethanol and ethanolamine. UV-visible transmittance spectra of ZnO or ZnO:AMFs have been recorded as displayed in Fig. S1. As expected, adding AMFs into ZnO increased the transmittances of ZnO films in visible region indeed. In order to optimize out the best doping ratios of the AMFs in the ZnO films, we first fabricated the i-PSCs (Fig. 1) with ZnO:NaF, ZnO:KF or ZnO:CsF where molar 6


Page 7 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


doping ratios of the AMFs are 0.05%, 0.1%, 0.2%, 0.5%, 1.0% and 2.0%. The J-V curves of all i-PSCs are displayed in Fig. S2 and device parameters are listed in Table S1-S3. As a result, optimized molar doping ratios for all the AMFs are 0.2%. Therefore only the devices with ZnO:AMF (0.2%) will be presented and discussed in the following. The i-PSCs with ZnO:NaF, ZnO:KF and ZnO:CsF are named as Device 2, Device 3 and Device 4, respectively. The i-PSC with pure ZnO (Device 1) is regarded as the control device. The J-V characteristics of Device 1-4 under illumination has been measured and presented in Fig. 2a. Table 1 shows the corresponding photovoltaic parameters of the devices. More than 30 devices for all device configurations were measured to ensure the validity and repeatability of data. Device 1 displays an average PCE of 7.31% comparable to the reported values by others under similar conditions.11 When the ZnO was doped by AMF, simultaneous improvements in VOC, JSC and FF have been observed in the i-PSCs. As a result, the Max. PCEs of Device 2, 3, and 4 reach 8.64%, 8.39% and 8.28%, respectively. The EQE spectra of the devices from 350 nm to 800 nm were measured (Fig. 2b) to check discrepancy of JSC.54 The JSC deviation from JSC-EQE derived from the EQE spectra of all the devices is below 5% (Table 1), indicating that the JSC values are not overvalued and acceptable.

7



Fig. 2 (a) J-V characteristics under illumination. (b) EQE spectra of Device1-4. (c) Jph-Veff characteristics of the devices. (d) Fitted dark injected current Jinj versus applied voltage characteristics of the devices. Device 1-4 are the PTB7:PC71BM based i-PSCs with ZnO, ZnO:NaF, ZnO:KF and ZnO:CsF, respectively.

Dependence of the Jph on the Veff can support the improved performance of the devices with ZnO:AMF. Fig. 2c shows Jph-Veff characteristics, where Jph is difference of the current density between under illumination (Fig. 2a) and in the dark (Fig. S3) and Veff is difference between V0 (the voltage at Jph = 0) and V (the applied voltage).55 When Veff is sufficiently high, Jph approaches saturation (Jph,sat) without recombination. Therefore the Jph/Jph,sat ratio can represent

8


Page 8 of 30

Page 9 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


probabilities of exciton dissociation and charge collection. Under short-circuit conditions, Jph,sc/Jph,sat ratios of Device 2, 3 and 4 are 98.4% , 95.3%, and 95.0%, respectively while Device 1 only has a Jph,sc/Jph,sat ratio of 93.0%. It implies that the i-PSCs with ZnO:AMF have better exciton dissociation. Under maximum power output conditions, the i-PSCs with ZnO:AMF (Jph,max/Jph,sat = 83.3% for Device 2, 81.8% for device 3 and 80.4% for device 4) demonstrates the higher Jph,max/Jph,sat ratios than Device 1 (Jph,max/Jph,sat = 75.8%). It suggests that Device 2-4 exhibit enhanced charge extraction and collection efficiency. Accordingly we infer that both the enhanced exciton dissociation and the reduced bimolecular recombination contributed to the better photovoltaic performance of Device 2-4.

Diode ideality factor n is a key parameter (departure from unity) to describe trap-assisted and tail state recombination, which can be obtained from the dark J-V curve according to Eq. S1.56 The n values of Device1-4 have been calculated from the linear slopes in Fig. 2d derived from the dark J-V curves (Fig. S3). Compared to Device 1 (n = 2.05), the n values of Device 2, 3 and 4 are 1.69, 1.70 and 1.71, respectively. It suggests that trap-assisted charge carrier recombination during charge transport is less in the i-PSCs with ZnO:AMF as an ETL.

Compared with Device 1, Device 2-4 with ZnO:AMF films demonstrates better FF in Table 1. In particular the FF of Device 2 reaches 72.0%. It is possible that

9



Page 10 of 30

adding AMFs into ZnO leads to an enhancement of electron transport ability. To verify this speculation, we carried out the measurements of electron mobility using the space charge limited current (SCLC) method57 as shown in Fig. S4a. As a result, all the devices with ZnO:AMF show higher electron mobility than the control device in Table 1. Moreover the conductivities of ZnO:AMF films are higher than that of the pure ZnO film (Fig. S4b and Table 1).

Table 1 Performance of i-PSCs based on PTB7:PC71BM with ZnO or ZnO:AMF. JSC

JSC-EQE

VOC

FF

[mA cm-2]

[mA cm-2]

[V]

[%]

14.89

14.78

0.735

15.89

15.60

15.87 15.72

PCE [%]

µe

Conductivity

[cm2 V-1 s-1]

[S m-1]

2.57×10-4

2.47×10-4

Device

1 2 3 4

Max.

Aver.

68.0

7.44

7.31

0.755

72.0

8.64

8.35

4.39×10-4

5.38×10-4

15.31

0.745

71.0

8.39

8.26

4.27×10-4

4.96×10-4

15.16

0.745

70.7

8.28

8.18

3.68×10-4

4.75×10-4

Device 1: ITO/ZnO/PTB7:PC71BM/MoO3/Al; Device 2: ITO/ZnO:NaF/PTB7:PC71BM/MoO3/Al; Device 3: ITO/ZnO:KF/PTB7:PC71BM/MoO3/Al; Device 4: ITO/ZnO:CsF/PTB7:PC71BM/MoO3/Al

Fig. 3 Mott–Schottky charts of capacitance versus voltage in i-PSCs with ZnO (Device 1), ZnO:NaF (Device 2), ZnO:KF (Device 3) and ZnO:CsF (Device 4) in the 10


Page 11 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dark.

The VOC values of all the devices with ZnO:AMF are higher than that of the device with pure ZnO in Table 1. The enhanced VOC can be attributed to the decreased work function (WF) of ZnO films. UPS spectra of ZnO and ZnO:AMF films have been measured in Fig. S5. WF values derived from the UPS spectra are 4.20 eV for ZnO film, 4.04 eV for ZnO:NaF film, 4.14 eV for ZnO:KF film and 4.12 eV for ZnO:CsF film. Unsurprisingly, the VOC of Device 2 is the highest in all the devices due to the lowest WF. It can be further explained by a change in the built-in voltage (Vbi).58, 59 In order to gain Vbi, we carried out C-V measurements of Device 1-4. (C/A)-2 - V curves of the devices in the dark are displayed in Fig. 3. Vbi can be estimated from the Mott-Schottky relationship (Eq. 1). 1 2(Vbi − V ) = 2 C qε 0ε r NA 2

(1)

where Vbi is obtained from the intercept of linear (C/A)-2 - V, q, ε0 and εr are the elementary charge, the dielectric constant of vacuum and the relative dielectric constant of the semiconductor, respectively, and A is the active area of the device. The Vbi values from Eq.1 of Device 2-4 are 0.780 V, 0.763 V and 0.763 V, respectively while the Vbi of Device 1 is 0.750 V. Apparently the Vbi of the devices with ZnO:AMF are higher than Device 1. Furthermore charge carrier density N can be calculated from the slope of linear (C/A)-2 - V. Compared to Device 1 (N = 2.23×1016 cm-3), Devices with ZnO:AMF present higher N due 11



to N = 4.58×1016 cm-3 for Device 2, 4.07×1016 cm-3 for Device 3 and 3.82×1016 cm-3 for Device 4. Obviously, there are higher N in Device 2-4 than Device 1.

In addition to high efficiency, device stability is an important consideration for practical applications of PSCs. To explore the influence of AMFs in ZnO films on stability of the devices, we performed a parallel comparison of stability for non-encapsulated Device 1-4 stored in a nitrogen-filled glovebox. Fig. S6 shows normalized JSC, VOC, FF and PCE of Device 1-4 versus storing time in the dark. Device 2-4 degraded slower than Device 1 within 80 days (Fig. S6, S7 and Table S4). It indicates that doping ZnO with AMF can effectively improve stability of the i-PSCs. Thus doping ZnO with AMF can improve not only PCE but also life time of the i-PSCs.

Fig. 4 (a) Zn2p, (b) O1s core-level XPS spectra of ZnO and ZnO doped by different alkali metal fluorides.

To figure out the reasons why doping ZnO with AMF gave rise to the improved 12


Page 12 of 30

Page 13 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


device performance, we carried out XPS measurements of the ZnO, ZnO:NaF, ZnO:KF and ZnO:CsF films. The peaks at 1072.4 eV for Na1s, 294.0 eV for K2p and 725.6 eV for Cs3d have been observed in Fig. S8, which means the presence of Na, K and Cs ions in the ZnO films. Fig. 4 presents the Zn2p (a) and O1s (b) XPS characterization of ZnO and ZnO:AMF films. Obviously adding AMF did not bring about any change of Zn2p XPS spectra of ZnO. However the change for O1s XPS spectra of ZnO is apparent after adding AMF. The O1s spectra can be de-convoluted into two peaks which are at 531.1 eV associated with the O atoms for Zn-O bonding and at 532.5 eV belonged to the oxygen-deficient defects, like hydroxyl groups, carboxylate groups and oxygen vacancies.53, 60, 61 The area ratios of the peak at 532.5 eV relative to the whole area of the O1s spectra were calculated to be 48.3%, 39.0%, 42.1% and 42.6% for pure ZnO, ZnO:NaF, ZnO:KF and ZnO:CsF, respectively. It indicates that the presence of additional AMF may promote a more efficient formation of Zn-O bonds in ZnO layers, thus resulting in a reduction of the concentration of oxygen deficient species, especially NaF. This behavior may benefit the properties of these ZnO films as ETL in photovoltaic devices.

13



Fig. 5 SEM-EDS mapping images of ZnO:NaF, ZnO:KF and ZnO:CsF films, where white dots for F element in ZnO:NaF, ZnO:KF and ZnO:CsF films shown in (a-c) respectively, green dots for Na element in ZnO:NaF film (d), yellow dots for K element in ZnO:KF film (e) and blue dots for Cs element in ZnO:CsF film (f).

AFM images of ZnO and ZnO:AMF layers in Fig. S9 reveal that the films are uniform, smooth and well-continuous, The root-means-square (RMS) roughnesses of ZnO, ZnO:NaF, ZnO:KF and ZnO:CsF are 1.58 nm, 1.09 nm, 1.38 nm and 1.67 nm, respectively. It indicates that adding NaF and KF improved the morphology of ZnO while adding CsF made the morphology of ZnO become slightly rougher probably due to a bigger ionic size of Cs+. Basically adding 0.2 mol% AMF into ZnO did not result in the lattice distortion of ZnO films.

To discover the distribution of AMF on ZnO:AMF surface, we gained some information using SEM-EDS mapping as shown in Fig. 5. The white dots in Fig. 5a, 5b and 5c represent F element in ZnO:NaF, ZnO:KF and ZnO:CsF 14


Page 14 of 30

Page 15 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


films, respectively. The green dots in Fig. 5d, the yellow dots in Fig. 5e and the blue dots in Fig. 5f are Na element in ZnO:NaF layers, K element in ZnO:KF layers and Cs element in ZnO:CsF layers, respectively. Obviously, the amount of fluorine on the ZnO:AMF surface is approximately equal. However the distribution of the three alkali metal elements on the surfaces of ZnO:AMF films is quite different. It implies that alkali metal cations and F anions independently exist in the ZnO:AMF films because they maybe were separated during the fabrication of the ZnO:AMF films. Moreover the green dots for Na are much more than the blue dots for Cs and the dot counts for K are between Na and Cs. This order matches the ionic radii of Na (102pm), K (138 pm) and Cs (174 pm). Namely the smaller the ionic radius, the more likely the alkali metal ion appears on the surface of ZnO. Negatively charged hydroxyl and carboxyl ions are main defects on the surface of ZnO films.62, 63 The orders of the electronegativity and the ionic radius of the alkali metal ions are Na+ > K+> Cs+ and Na+ < K+ < Cs+, respectively. The larger electronegativity benefits AM+ to be effectively moved towards ZnO surface enriched more negatively charged, and a smaller ionic radius helps AM+ to be more mobile due to little steric hindrance and form a more compact passivation layer on ZnO surface.64, 65

15



Fig. 6 XPS depth profiling of ZnO:AMF films. (a) Na1s XPS spectra of ZnO:NaF film, (b) K2p XPS spectra of ZnO:KF film, (c) Cs3d XPS spectra of ZnO:CsF film and (d) Vertical AM/Zn ratio in the ZnO:AMF films. AM: alkali metal.

To further investigate the vertical distribution of the AM ions in ZnO:AMF films, we employed XPS depth profiling analysis. Fig. 6a-c show the XPS spectra of Na1s, K2p and Cs3d, respectively, obtained from ZnO:AMF films etched by an Ar ion beam. Na1s, K2p and Cs3d peaks represent Na, K and Cs ions, respectively. Zn2p peak represents ZnO. Peak area ratios of Na1s/Zn2p, K2p/Zn2p and Cs3d/Zn2p are proportionally related to the percentage of Na, K and Cs ions in the ZnO films, respectively. Therefore, the molar ratios of AM ions to ZnO in the vertical direction can be evaluated by using AM/Zn peak area ratios in XPS depth profiling. Fig. 6d 16


Page 16 of 30

Page 17 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shows the dependence of AM/Zn ratio on etching time for ZnO:AMF films. Since the etching rate is 5 nm/min, 0, 1, 2, 3, 4 and 5 minute etchings approximately correspond to 30 nm (top surface), 25 nm, 20 nm, 15 nm, 10 nm and 5 nm (bottom). (NAM/NZn)mea. is NAM/NZn measured by quantitative analysis of XPS spectra. (NAM/NZn)cal. is NAM/NZn calculated according to molar doping ratio and molecular formula of AMF and ZnO under the assumption that vertical distribution of AMF and ZnO is average and uniform from top to bottom. For the ZnO:NaF film, (NNa/NZn)mea. at the top surface is nearly 20 times of one at the bottom, suggesting that more Na ions enrich at the top surface. For the ZnO:KF film, (NK/NZn)mea. at the top surface is nearly 6.5 times of one at the bottom while (NCs/NZn)mea. at the top surface is only 3.2 times of one at the bottom for the ZnO:CsF film. Namely the concentration of K or Cs ions decreased at the top surface and increased at the bottom. It indicates that Na ions mainly focus on the surface of ZnO while Cs ions have a more uniform vertical distribution than Na and K because all the (NCs/NZn)mea. points are closer to the theoretical average (NAM/NZn)cal. = 0.002 as shown in Fig. 6d. The results are in agreement with the SEM-EDS mapping images. Therefore Na ions have more chances to meet the surface defects of ZnO than K and Cs ions so that the device with ZnO:NaF shows the highest PCE. Moreover we also studied the vertical distribution of F in the ZnO:NaF, ZnO:KF and ZnO:CsF shown in Fig. S10. A similar vertical distribution of F anions can be observed in the three ZnO:AMF films. Namely the F anions have a uniform vertical distribution in the films. It proves once more that the cations and anions in the AMFs are separated in the sol-gel ZnO:AMF films. 17



4. CONCLUSION AMFs have been successfully applied in i-PSCs as dopants of ZnO. PCEs of 8.64%, 8.39% and 8.28% were obtained in the PTB7:PC71BM based devices with ZnO:NaF, ZnO:KF and ZnO:CsF, respectively. Moreover doping AMF into ZnO improved the device stability as well. Combined measurements of Jph-Veff, electron mobility and C-V characteristics demonstrated that doping ZnO with AMFs can effectively raise exciton dissociation and charge extraction, electron mobility, charge carrier density and built-in potential in the i-PSCs. O1s XPS spectra presented that adding AMFs led to a reduction of oxygen-deficient defects on the ZnO surfaces. AFM images showed that adding AMF didn’t cause the lattice distortion of ZnO films. SEM-EDS mapping and XPS depth profiling suggested that the smaller the ionic radius, the more likely the alkali metal ions appeared on the surface of ZnO. Therefore Na+ have more chances to meet the defects of ZnO surface than K+ and Cs+ so that the i-PSCs with ZnO:NaF shows the supreme PCE. These results demonstrated that NaF was an available, rival and hopeful dopant of ZnO in the i-PSCs.

ASSOCIATED CONTENT Supporting Information.

The transmittance spectra of ZnO or ZnO:AMFs, J-V curves of i-PSCs with in various ratios in the dark and under the illumination, I-V characteristics for conductivity measurements and J-V curves of electron-only devices, stability measurements of the i-PSCs, UPS/XPS spectra of different ZnO:AMF films on 18


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ITO, AFM phase images and height images of ZnO films with and without AMFs, XPS depth profiling study F1s peak of ZnO:AMF films, and supplementary tables for performances parameter and stability parameter of the i-PSCs before and after 80 days.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by grants from the National Basic Research Program of China (2014CB643505), the National Natural Science Foundation of China (51273077) and the Natural Science Foundation of Jilin Province, China (20170101169JC).

REFERENCES (1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Hegger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789-1791.

(2) Thompson, B. C.; Fŕechet, J. M. J. Polyer-Fullerene Composite Solar Cells. Angew. 19



Page 20 of 30

Chem. Int. Ed. 2008, 47, 58-77.

(3) Lin, X. F.; Yang, Y. Z.; Nian, L.; Su, H.; Ou, J. M.; Yuan, Z. K.; Xie, F. Y.; Hong, W.; Yu, D. S.; Zhang, M. Q.; et al. Interfacial Modification Layers Based on Carbon Dots for Efficient Inverted Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Nano Energy 2016, 26, 216-223.

(4) Zhang, S. Q.; Ye, L.; Hou, J. H. Breaking the 10% Efficiency Barrier in Organic Photovoltaics: Morphology and Device Optimization of Well-Known PBDTTT Polymers. Adv. Energy Mater. 2016, 6, 1502529-1502548.

(5) Liang, Z. Q.; Zhang, Q. F.; Jiang, L.; Cao, G. Z. ZnO Cathode Buffer Layers for Inverted Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 3442-3476.

(6) Chen, S. S.; Yao, H. T.; Li, Z. K.; Awartani, O. M.; Liu, Y. H.; Wang, Z.; Yang, G. F.; Zhang, J. Q.; Ade, H.; Yan, H.; Surprising Effects upon Inserting Benzene Units into a Quaterthiophene-Based D-A Polymer-Improving Non-Fullerene Organic Solar Cells via Donor Polymer Design. Adv. Energy Mater. 2017, 7, 1602304-1602309.

(7) Hou, J. H.; Inganas, O.; Friend R. H.; Gao, F. Organic Solar Cells Based on Non-fullerene Acceptors. Nat. Mater. 2018, 17, 119-128.

(8)

Ma, Y. L.; Kang, Z. J.; Zheng, Q. D.; Recent Advances in Wide Bandgap

Semiconducting Polymers for Polymer Solar Cells. J. Mater. Chem. A 2017, 5, 1860-1872.

(9)

Zheng,

Q.

D.;

Jung,

B.

J.;

Sun,

J.;

Katz,

H.

E.

Ladder-Type

Oligo-p-phenylene-Containing Copolymers with High Open-Circuit Voltage and Ambient 20


Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photovoltaic Activity. J. Am. Chem. Soc. 2010, 132, 5394-5404.

(10)

Wang, M.; Qin, H.; Wang, L. X.; Wei, J. J.; Cai, D. D.; Yin, Z. G.; Ma, Y. L.; Chen, S. C.;

Tang, C. Q.; Zheng, Q. D. Ladder-type Tetra-p-phenylene-based Copolymers for Efficient Polymer Solar Cells with Open-circuit Voltages Approaching 1.1 V. J. Mater. Chem. A, 2015, 3, 21672-21681.

(11)

Guo, B.; Han, J. X.; Qiu, J.; Yu, C. Z.; Sun, Y. Q.; Li F. H.; Hu, Z. H.; Wang, Y. Alcohol-

Soluble Isoindigo Derivative IIDTh-NSB as a Novel Modifier of ZnO in Inverted Polymer Solar Cell. ACS Appl. Mater. Interfaces 2017, 9, 42969-42977.

(12)

Chen, Y. C.; Wang, S.; Xue, L. W.; Zhang, Z. G.; Li, H. L.; Wu, L. X.; Wang, Y.; Li, F.

H.; Zhang, F. L.; Li, Y. F. Insights into the Working Mechanism of Cathode Interlayers in Polymer Solar Cells via [(C8H17)4N]4[SiW12O40]. J. Mater. Chem. A 2016, 4, 19189-19196.

(13)

Yin, Z. G.; Wei, J. J.; Zheng, Q. D. Interfacial Materials for Organic Solar Cells: Recent

Advances and Perspectives. Adv. Sci. 2016, 3, 1500362-1500398.

(14)

Richardson, B. J.; Wang, X. Z.; Almutairi, A.; Yu, Q. M. High Efficiency PTB7-based

Inverted Organic Photovoltaics on Nano-ridged and Planar Zinc Oxide Electron Transport Layers. J. Mater. Chem. A 2015, 3, 5563-5571.

(15)

Guo, B.; Zhou, W. L.; Wu, M. C.; Lv, J. J.; Yu, C. Z.; Li, F. H.; Hu, Z. H. Improving the

Efficiency of Polymer Solar Cells via a Treatment of Methanol: Water on the Active Layers. J. Mater. Chem. A 2016, 4, 9644-9652.

(16)

Huang, J.; Wang, H. Y.; Yan, K. R.; Zhang, X. H.; Chen, H. Z.; Li, C. Z.; Yu, J. S. 21



Highly Efficient Organic Solar Cells Consisting of Double Bulk Heterojunction Layers. Adv. Mater. 2017, 29, 1606729-1606737.

(17)

Shi, X. L.; Zuo, L. J.; Jo, S. B.; Gao, K.; Lin, F.; Liu, F.; Jen, A. K. Design of a Highly

Crystalline Low-Band Gap Fused-Ring Electron Acceptor for High-Efficiency Solar Cells with Low Energy Loss. Chem. Mater. 2017, 29, 8369-8376.

(18)

Li, H.; Xiao, Z.; Ding, L. M.; Wang, J. Z. Thermostable Single-junction Organic Solar

Cells with a Power Conversion Efficiency of 14.62%. Science Bulletin 2018, 63, 340-342.

(19)

Cao, H. Q.; He, W. D.; Mao, Y. W., Lin, X.; Ishikawa, K.; Dickerson, J. H.; Hess, W. P.

Recent Progress in Degradation and Stabilization of Organic Solar Cells. J. Power Sources 2014, 264, 168-183.

(20)

Cowan, S. R.; Schulz, P.; Giordano, A. J.; Gaicia, A.; Macleod, B. A.; Marder, S. R.;

Kahn, A.; Ginley, D. S.; Ratcliff E. L.; Olson, D. C. Chemically Controlled Reversible and Irreversible Extraction Barriers via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole-based Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 4671-4680.

(21)

Dkhil, S. B.; Duche, D.; Gaceur, M.; Thakur, A. K.; Aboura, F. B.; Escoubas, L.; Simon,

J.; Guerrero, A.; Bisquert, J.; Belmonte, G. G.; et al. Interplay of Optical, Morphological, and Electronic Effects of ZnO Optical Spacers in Highly Efficient Polymer Solar Cells. Adv. Energy Mater. 2014, 4, 1400805-1400816.

(22)

Huang, J.; Yin, Z. G.; Zheng, Q. D. Applications of ZnO in Organic and Hybrid Solar 22


Page 22 of 30

Page 23 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cells. Energy Environ. Sci. 2011, 4, 3861-3887.

(23)

Liao, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Fullerene Derivative-Doped Zinc

Oxide Nanofilm as the Cathode of Inverted Polymer Solar Cells with Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766-4771.

(24)

Eom, S. H.; Baek, M. J.; Park, H., Yan, L.; Liu, S. B., You, W.; Lee, S. H. Roles of

Interfacial Modifiers in Solar Cells: Inorganic/Polymer Bilayer vs Inorganic/Polymer:Fullerene Bulk Heterojunction. ACS Appl. Mater. Interfaces 2014, 6, 803-810.

(25)

Wang, K.; Zhang, Z.; Liu, C.; Fu, Q.; Xu, W. Z.; Huang, C. W.; Weiss R. A.; Gong, X.

Efficient Polymer Solar Cells by Lithium Sulfonated Polystyrene as a Charge Transport Interfacial Layer. ACS Appl. Mater. Interfaces 2017, 9, 5348-5357.

(26)

Jia, X. K.; Jiang, Z. Y.; Chen, X. H.; Zhu, J. P.; Pan, L. K.; Zhu, F. R.; Sun, Z.; Huang, S.

M. Highly Efficient and Air Stable Inverted Polymer Solar Cells Using LiF-Modified ITO Cathode and MoO3/AgAl Alloy Anode. ACS Appl. Mater. Interfaces 2016, 8, 3792-3799.

(27)

Stubhan, T.; Salinas, M.; Ebel, A.; Krebs, F. C.; Hirsch, A.; Halik M.; Brabec, C. J.

Increasing the Fill Factor of Inverted P3HT:PCBM Solar Cells Through Surface Modification of Al-Doped ZnO via Phosphonic Acid-Anchored C60 SAMs. Adv. Energy Mater. 2012, 2, 532-535.

(28)

Nho, S.; Baek, G.; Park, S.; Lee, B. R.; Cha, M. J.; Lim, D. C.; Seo, J. H.; Oh, S. H.;

Song, M. H.; Cho, S. Highly Efficient Inverted Bulk-heterojunction Solar Cells with a Gradiently-doped ZnO Layer. Energy Environ. Sci. 2016, 9, 240-246.

(29)

Peng, H.; Xu, W. P.; Zhou, F.; Zhang, J.; Li, C. N. Enhanced Efficiency of Inverted 23



Polymer Solar Cells using Surface Modified Cs-doped ZnO as Electron Transporting Layer. Synthetic Metals 2015, 205, 164-168.

(30)

Yin, Z. G.; Zheng, Q. D.; Chen, S. C.; Cai, D. D.; Zhou, L. Y.; Zhang, J. Bandgap

Tunable Zn1-xMgxO Thin Films as Highly Transparent Cathode Buffer Layers for High-Performance Inverted Polymer Solar Cells. Adv. Energy Mater. 2014, 4, 1301404-1301409.

(31)

Pachoumi, O.; Li, C.; Vaynzof, Y.; Banger, K. K.; Sirringhaus, H. Improved

Performance and Stability of Inverted Organic Solar Cells with Sol-Gel Processed, Amorphous Mixed Metal Oxide Electron Extraction Layers Comprising Alkaline Earth Metals. Adv. Energy Mater. 2013, 3, 1428-1436.

(32)

Jagadamma, L. K.; Senani, M. A.; Labban, A. E.; Gereige, I.; Ndjawa, G. O. N.; Faria, J.

C. D.; Kim, T.; Zhao, K.; Cruciani, F.; Anjum, D. H.; et al. Polymer Solar Cells with Efficiency ＞10% Enable via a Facile Solution-Processed Al-Doped ZnO Electron Transporting Layer. Adv. Energy Mater. 2015, 5, 1500204-1500215.

(33)

Liu, X. H.; Li, X. D.; Li, Y. R.; Song, C. J.; Zhu, L. P.; Zhang, W. J.; Wang, H. Q.; Fang,

J. F. High-Performance Polymer Solar Cells with PCE of 10.42% via Al-Doped ZnO Cathode Interlayer. Adv. Mater. 2016, 28, 7405-7412.

(34)

Stubhan, T.; Oh, H.; Pinna, L.; Krantz, J.; Litzov, I.; Brabec, C. J. Inverted Organic Solar

Cells using a Solution Processed Aluminum-doped Zinc Oxide Buffer Layer. Org. Electron. 2011, 12, 1539-1543.

(35)

Oh, H.; Krantz, J.; Litzov, I.; Stubhan, T.; Pinna, L.; Brabec, C. J. Comparison of 24


Page 24 of 30

Page 25 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Various Sol-gel Derived Metal Oxide Layer for Inverted Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2011, 95, 2194-2199.

(36)

Hu, Z. Y.; Zhang J. J.; Zhu, Y. J. Inverted Polymer Solar Cells with a Boron-doped Zinc

Oxide Layer Deposited by Metal Organic Chemical Vapor Deposition. Sol. Energy Mater. Sol. Cells 2013, 117, 610-616.

(37)

Shin, K. S.; Lee, K. H.; Lee, H. H.; Choi, D.; Kim, S. W. Enhanced Power Conversion

Efficiency of Inverted Organic Solar Cells with a Ga-Doped ZnO Nanostructured Thin Film Prepared Using Aqueous Solution. J. Phys. Chem. C 2010, 114, 15782-15785.

(38)

Kyaw, A. K. K.; Sun, X. W.; Zhao, D. W.; Tan, S. T.; Divayana, Y.; Demir, H. V.

Improved Inverted Organic Solar Cells With a Sol-Gel Derived Indium-Doped Zinc Oxide Buffer Layer. IEEE J. Sel. Top. Quantum Electron. 2010, 16, 1700-1706.

(39)

Chang, J. J.; Lin, Z. H.; Zhu, C. X.; Chi, C. Y.; Zhang, J.; Wu, J. S. Solution-Processed

LiF-Doped ZnO Films for High Performance Low Temperature Field Effect Transistors and Inverted Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 6687-6693.

(40)

Choi, J.; Kim, Y.; Jo, J. W.; Kim. J.; Sun, B.; Walter, G.; Arquer, F. P. G.; Bermudez, R.

Q.; Li, Y. Y.; Tan, C. S.; et al. Chloride Passivation of ZnO Electrodes Improves Charge Extraction in Colloidal Quantum Dot Photovoltaics. Adv. Mater. 2017, 29, 1702350-1702354.

(41)

Krithiga, R.; Selvi, N.; Subhashree, G.; Sankar, S. Oxygen Vacancies Induced Room

Temperature Ferromagnetism in Li, Na and K Co-doped ZnO Synthesized by Solution Combustion Technique. J. Mater. Sci-Mater EL 2018, 29, 5124-5133. 25



(42)

Huang, G. Y.; Wang, C. Y.; Wang, J. T. First-principles Study of Diffusion of Li, Na, K

and Ag in ZnO. J. Phys.: Condens. Matter 2009, 21, 345802-345810.

(43)

Liu, B.; Gu, M.; Liu, X. L.; Huang, S. M.; Ni, C. First-principles Study of

Fluorine-doped Zinc oxide. Appl. Phys. Lett. 2010, 97, 122101-122104.

(44)

Ilican, S.; Caglar, Y.; Caglar, M.; Yakuphanoglu, F. Structural, Optical and Electrical

Properties of F-doped ZnO Nanorod Semiconductor Thin Films Deposited by Sol-gel Process. Appl. Surf. Sci. 2008, 255, 2353-2359.

(45)

Brabec, C. J.; Shaheen, S. E.; Winder, C.; Sariciftci, N. S.; Denk, P. Effect of LiF/metal

Electrodes on the Performance of Plastic Solar Cells. Appl. Phys. Lett., 2002, 80, 1288-1291.

(46)

Lu, Z.; Chen, X. H.; Zhou, J. P.; Jiang, Z. Y.; Huang, S. M.; Zhu, F. R.; Piao, X. Q.; Sun,

Z. Performance Enhancement in Inverted Polymer Solar Cells Incorporating Ultrathin Au and LiF Modified ZnO Electron Transporting Interlayer. Org. Electron 2015, 17, 364-370.

(47)

Yu, H. Z.; Huang, X. X.; Li, Y. P.; Wu, Z. P. Effect of Methanol Treatment on the

Performance of P3HT:PC71BM Bulk Heterojunction Solar Cells with Various Cathodes. J. Mater. Sci: Mater. Electron 2017, 28, 12909-12915.

(48)

Hung, L. S.; Tang, C. W.; Mason, M. G. Enhanced Electron Injection in Organic

Electroluminescence Devices using an Al/LiF Electrode. Appl. Phys. Lett. 1997, 70, 152-155.

(49)

Lee, J.; Park, Y.; Kim, D. Y.; Chu, H. Y.; Lee, H.; Do, L. M. High Efficiency Organic

Light-emitting Devices with Al/NaF Cathode. Appl. Phys. Lett. 2003, 82, 173-176.

(50)

Keller, J.; Chalvet, F.; Joel, J.; Aijaz, A.; Kubart, T.; Riekehr, L.; Edoff, M.; Stolt, L.; 26


Page 26 of 30

Page 27 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Torndahl, T. Effect of KF Absorber Treatment on the Functionality of Different Transparent Conductive Oxide Layers in CIGSe Solar Cells. Prog. Photovolt. Res. Appl. 2018, 26, 13-23.

(51)

Jiang, X. X.; Xu, H.; Yang, L. G.; Shi, M. M.; Wang, M.; Chen, H. Z. Effect of CsF

Interlayer on the Performance of Polymer Bulk Heterojunction Solar Cells. Sol. Energy Mater. Sol. Cells 2009, 93, 650-653.

(52)

Cai, P.; Zhong, S.; Xu, X. F.; Chen, J. W.; Chen, W.; Huang, F.; Ma, Y. G.; Cao, Y. Using

Ultra-high Molecular Weight Hydrophilic Polymer as Cathode Interlayer for Inverted Polymer Solar Cells: Enhanced Efficiency and Excellent Air-stability. Sol. Energy Mater. Sol. Cells 2014, 123, 104-111.

(53)

Sun, Y. M.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar

Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679-1683.

(54)

Zimmermann, E.; Ehrenreich, P.; Pfadler, T.; Dorman, J. A.; Weickert, J.;

Schmidt-Mende, L. Erroneous Efficiency Reports Harm Organic Solar Cell Research. Nat. Photonics 2014, 8, 669-672.

(55)

Chi, D.; Lu, S. D.; Xu, R.; Liu, K.; Cao, D. W.; Wen, L. Y.; Mi, Y.; Wang, Z. J.; Lei, Y.;

Qu, S. C.; et al. Fully Understanding the Positive Roles of Plasmonic Nanoparticles in Ameliorating the Efficiency of Organic Solar Cells. Nanoscale 2015, 7, 15251-15257.

(56)

Zhang, Q.; Kan, B.; Liu, F.; Long, G. K.; Wan, X. J.; Chen, X. Q.; Zuo, Y.; Ni, W.;

Zhang, H. J.; Li, M. M.; et al. Small-molecule Solar Cells with Efficiency over 9%. Nat. 27



Photonics 2015, 9, 35-72.

(57)

Goh, C.; Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J.

Molecular-Weight-dependent Mobilities in Regioregular Poly(3-hexyl-thiophene) Diodes. Appl. Phys. Lett. 2005, 86, 122110-122113.

(58)

Zhou, H. Q.; Zhang, Y.; Seifter, J.; Collins, S. D.; Luo, C.; Bazan, G. C.; Nguyen, T. Q.;

Heeger, A. J. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Adv. Mater. 2013, 25, 1646-1652.

(59)

O’Malley, K. M.; Li, C. Z.; Yip, H. L.; Jen, A. K. Y. Enhanced Open-Circuit Votage in

High Performance Polymer/Fullerene Bulk-Heterojunction Solar Cells by Cathode Modification with a C60 Surfactant. Adv. Energy Mater. 2012, 2, 82-86.

(60)

Kumar, S.; Panigrahi, D.; Dhar, A. Efficiency Enhancement of ZnO based Inverted BHJ

Soalr Cells via Interface Engineering using C70 Modifier. Org. Electron. 2016, 38, 1-7.

(61)

Li, D.; Qin, W. J.; Zhang, S. C.; Liu, D. Y.; Yu, Z. Y.; Mao, J.; Wu, L. F.; Yang, L. Y.; Yin,

S. G. Effect of UV-ozone Process on the ZnO Interlayer in the Inverted Organic Solar Cells. RSC Adv. 2017, 7, 60400-6045.

(62)

Liao, S.; Jhuo, H.; Yeh, P.; Cheng, Y.; Li, Y.; Lee, Y.; Sharma, S.; Chen, S. Single

Junction Inverted Polymer Solar Cell Reaching Power Conversion Efficiency 10.31% by Employing Dual-doped Zinc Oxide Nano-Film as Cathode Interlayer. Sci. Rep. 2014, 4, 6813-6819.

(63)

Wang, Y. L.; Fu, H. Y.; Wang, Y.; Tan, L. C.; Chen, L.; Chen, Y. W. 3-Dimensional 28


Page 28 of 30

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ZnO/CdS Nanocomposite with High Mobility as an Efficient Electron Transport Layer for Inverted Polymer Solar Cells. Phys. Chem. Chem. Phys. 2016, 18, 12175-12182.

(64)

Li, S.; Ye, S.; Liu, T. H.; Guo, Z.; Wang, H. Y.; Wang, D. P. Dual Role of the Reactant

MOH (M=Li, Na or K) in the Growth of ZnO Quantum Dots under a Sol-Gel Process: Promoter and Inhibitor. RSC Adv. 2016, 6, 50173-50179.

(65)

Sharma, A.; Dhar, S.; Singh, B. P. Role of the Surface Polarity in Governing the

Luminescence Properties of ZnO Nanoparticles Synthesized by Sol-gel Route. Appl. Surf. Sci. 2013, 273, 144-149.

29



TOC graphic

Insights into Working Mechanism of Alkali Metal Fluorides as Dopants of ZnO Films in Inverted Polymer Solar Cells

30


Page 30 of 30

Insights into Working Mechanism of Alkali Metal Fluorides as Dopants

Recommend Documents