Improvement in performance of inverted organic solar cell by rare earth element lanthanum doped ZnO electron buffer layer

Materials Chemistry and Physics 240 (2020) 122076

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Improvement in performance of inverted organic solar cell by rare earth element lanthanum doped ZnO electron buffer layer Muhammad Zafar a, BongSoo Kim b, **, Do-Heyoung Kim a, * a b

School of Chemical Engineering, Chonnam National University, 300 Youngbong-dong, Gwangju, 500-757, Republic of Korea Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), 50, UNIST-gil, Ulsan, 44919, Republic of Korea

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

� La-doped ZnO was successfully applied as an ETL for organic solar cells. � Improved efficiency was observed compared to that of cells with undoped ZnO. � The ETLs were formed by a simple, costeffective, eco-friendly solution approach.

A R T I C L E I N F O

A B S T R A C T

Keywords: Electron buffer layer Zinc oxide Inverted organic solar cell Lanthanum Doping

In the past decade of robust innovation, the inverted organic solar cells (IOSCs) have been considered as a substitute photovoltaic technology with the potential to provide comparable power conversion efficiencies (PCEs) combined with low processing cost and ease in fabrication. The doping of metal oxides is an expedient technique for controlling the electronic band gap configurations of the electron buffer layer (EBL) in inverted organic solar cells for better performance. In addition, the sol-gel method is utilized for doping various functional materials as EBLs in IOSCs due to its cost effectiveness and uniform nanoscale film deposition. In this report, we analyzed the sol-gel-based ZnO films as EBLs for P3HT: PCBM based IOSCs. The ZnO film thickness was opti­ mized and we studied the effect of lanthanum doping into the ZnO films by measuring the power conversion efficiency of the devices. In our study, lanthanum nitrate hexahydrate was selected as a potential lanthanum dopant. The IOSC device made with 1.57 atomic.%-lanthanum-doped ZnO (La-ZnO B) EBL showed a PCE of 4.34%, which is an increment of 12% as compared to the reference cell device containing a pure ZnO EBL. Therefore, we demonstrated that the lanthanum doping enhanced the interfacial electrical properties in terms of conductivity and carrier density.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B. Kim), [email protected] (D.-H. Kim). https://doi.org/10.1016/j.matchemphys.2019.122076 Received 5 January 2019; Received in revised form 8 May 2019; Accepted 27 August 2019 Available online 28 August 2019 0254-0584/© 2019 Published by Elsevier B.V.

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1. Introduction

earth-doped TMOs are also as luminescent down-shifting layers in solar cells [25]. We chose lanthanum nitrate hexahydrate as a principal La dopant for ZnO films. Recent reports have used lanthanum nitrate hexahydrate as a dopant for elemental La and oxidized La2O3 [19–24]. Lanthanum doping using a solution-processing route has been achieved with various lanthanum precursors, like lanthanum sulfate, lanthanum chloride, lanthanum bromide, lanthanum iodide and lantinum fluoride [19–28]. However, there are limitations to these methods including the use of expensive chemicals, solvent miscibility, elevated-temperature condition, and the additional processing steps required to remove pre­ cipitates, metal species and unavoidable by-products. In view of the abovementioned factors, we present a simple, low-temperature based sol-gel route for utilizing La-doped ZnO films for IOSCs. We have constructed IOSCs using solution-process route to synthe­ size all of the operational layers in ambient air, with the exception of a silver electrode deposited using vacuum thermal evaporation. A trans­ parent La-doped ZnO film (La-ZnO) acted as an EBL that enabled the functioning of a complete organic-solar-cell device We observed that a ZnO film thickness and La doping of 25 nm and 1.57 atomic.%, respectively, gave the highest IOSC PCE of 4.34%. Furthermore, as per best of our knowledge this is the first report for utilizing La doped ZnO EBL in IOSC application.

In today’s world of renewable energy, solar cells are considered as the top most alternatives for clean and affordable energy generation [1]. Generally, among the family of solar cells, the silicon solar cells are well known and are preferred due to their high power conversion efficiency (40%) [2]. However, they also possess some drawbacks like high fabrication costs, complicated processing and scarcity of important in­ termediate production chemicals [3]. To overcome these issues the organic solar cells (OSCs) have gained much attention by researchers because of their outstanding features such as low fabrication cost, simple processing, thin lightweight construction, and potential for the use in flexible plastic solar cell fabrication, which can be further extended to industrial roll-to-roll processing and spray pyrolysis [1–3]. Inverted structure organic solar cells (IOSCs) have been frequently adopted as they show superior stability, reliability under ambient air conditions and much better performance than their predecessor con­ ventional structure organic solar cells [4,5]. IOSCs have a structure in the sequence of cathode/electron-buffer-layer/photoactive-material /hole-transport-layer/anode, where the electron buffer layer (EBL) is an important electrical connection between the electrode and the pho­ toactive material (PAM); it plays a vital role of extracting and propa­ gating the transport of photo generated electrons. It should allow the movement of the electrons only, and block the passage of the holes through an energy band-gap mismatch, which leads to the improvement of the power conversion (PCE). The n-type conductive metal oxides (CMO), such as titanium dioxide (TiO2) or Zinc oxide (ZnO) have been used as good EBLs. Moreover, insertion of these metal oxides can considerably increase the stability of IOSCs, and also minimize the recombination rate near the interface between the EBL and PAM [4–8]. Different strategies have been implemented to enhance the perfor­ mance and quality of the EBL interface. Most conveniently for EBLs, using a nano-engineered interface (nanotubes, nanobelts, nanosheets, nanoflakes, nanostars) or doping of thin films resulted in the improve­ ment of photovoltaic performance [7–32]. ZnO based EBLs are commonly used due to their good band gap alignment within the OSC structure and relatively high electron mobility [2,3]. The pure ZnO films contains native intrinsic defects such as oxygen vacancies and zinc in­ terstitials, so these defects open an opportunity for elemental doping. The intrinsic transport properties of electrons and holes in CMO films, such as ZnO, can be altered by the insertion of dopants. The main pur­ pose of doping the CMOs, either with anions or cations can not only optimize their band gap but also improve the electrical properties. In one of the most widely known approaches, Al doping the ZnO-based buffer layer is used to increase the performance of solar cells [8–11]. Hypothetically we can enforce the privilege to reallocate the conduction band (CB) or valence band (VB) to a desired position with respect to the fermi level by selecting an appropriate dopant [7–32]. In this study we chose rare earth element lanthanum (La) as a dopant, as there are many reports on introducing La atoms into the ZnO lattice through substitutional doping in place of Zn atoms; these studies were primarily conducted to enhance the electron charge transport properties and catalytic activation of ZnO under visible light [19]. Goel et al. have recently doped La into ZnO nanorods for dye-sensitized solar cells, which increased the light harvesting capability of the device but unfortunately, the device power conversion efficiency (PCE) was less than 0.5%. Likewise, Sun et al. have doped La into the TiO2 lattice for IOSC application and reported a promising result (PCE ¼ 2.45%) [20, 21]. Therefore, similar results would be expected in case of La doped ZnO films for IOSC application. Moreover, this strategy has been applied in various nano-scaled optoelectronic devices such as photodetectors, sensors, catalysts, thin film transistors (TFTs) and hybrid solar cells [19–24]. In addition, sol-gel method is a reliable expedient technique to dope specific elemental salts into the TMOs. An optimized doping ratio can improve the electron mobility, increase charge-carrier concentra­ tion, and elongate electron lifetimes [14–24]. Moreover, the use of rare

2. Experimental 2.1. Materials The FTO glass (with a SR � 15 Ω/square and a T > 90%) was pur­ chased from MTI Korea. The principal photoactive materials, i.e., P3HT and PCBM, were purchased from Solaris Chem Canada. Lanthanum ni­ trate hexahydrate (LNH), isopropyl alcohol (IPA), chlorobenzene, zinc acetate dihydrate (ZAD), ethanol amine (EA), ethyl alcohol, and vana­ dium oxytriisopropoxide were procured from Aldrich and were of 99% purity. 2.2. Fabrication of lanthanum-doped ZnO films The ZnO EBL was fabricated from equimolar solutions (0.23 M) of ZAD and EA in ethyl alcohol. Lanthanum nitrate hexahydrate (LNH) was then added to the solution as a dopant at 2, 5, and 10 wt%. The solution was mixed at 85 � C until it becomes clear. Afterwards, the solution was spin-coated at 4000 rpm for 35 s on the clean FTO substrate and consequently heated at 150 � C for 1 h. The fabricated film thickness was 25 nm. Furthermore, the XPS analysis showed that the films formed with 2, 5, and 10 wt% lanthanum nitrate hexahydrate solutions contained 0.70, 1.57 and 2.48 atomic.% La respectively. 2.3. Fabrication and characterization of IOSC devices FTO glass substrates were sonicated sequentially under DI water, IPA, and acetone for 20 min each. Four types of EBLs were fabricated separately: ZnO, La-ZnO A (2 wt% LNH), La-ZnO B (5 wt% LNH), and LaZnO C (5 wt% LNH), The respective solutions were spin-coated onto cleaned FTO substrates and annealed at 150 � C for 1 h. The PAM (P3HT: PCBM) was then mixed at a ratio of 1:0.8 by weight in chlorobenzene and deposited onto the FTO/EBL structure by spin coating. Afterwards, the hole transport layer (HTL) of V2O5 was prepared at a ratio of 1:150 by volume of vanadium oxytriisopropoxide in IPA, spin coated at 6000 rpm for 45 s onto the PAM, and then heated at 160 � C for 10 min. Lastly, the silver electrode was thermally evaporated under vacuum on top of the HTL. The effective area of the device was 3 mm2. The average PCE % was taken from 20 fabricated devices. Surface morphology of the films was examined using scanning electron microscope (SEM, JSM-7500 F). Surface roughness in terms of RMS values was determined by using atomic force microscopy (AFM, XE-100 system) and the thickness of the films was measured by laser2

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ellipsometry (HORIBA). The hall measurement effect of the films was evaluated on HMS-3000 system. In addition, optical measurements were performed with a UV/Vis spectrophotometer (OPTIZEN POP). Current density–voltage (J–V) characteristics of the fabricated devices were measured with a mask under an AM 1.5G solar simulator at 100 mW/ cm2 using a Keithley 2400 source measurement unit. The use of the mask was intended to avoid the parasitic current arising from illuminated areas external to the device. Lastly, external quantum efficiency (EQE) measurements were carried out using a McScience potentiostat, with a 200 W Xe lamp in combination with a monochromator for dispersing light within an area of 3 mm2.

100

Transmittance %

80

3. Results and discussion A schematic IOSC structure is shown in Fig. 1. The electrons created by the splitting of excitons in the PAM (P3HT:PCBM) were collected by the ZnO or La-ZnO films spin-coated onto the FTO substrate to give 25 nm thickness and annealed subsequently. The ZnO films were doped with La to investigate the impact of doping on the device performance indicators. Thin films are generally selected for solar device manufacturing as they minimize the interfacial distance between the PAM and the ZnO layers, hereby improving the electron capture capa­ bility. However, in previous studies, the IOSC performance decreased when the EBL interface thickness was increased to more than 25 nm; this was due to rise in series resistance at the interface between PAM and sluggish electron collection times [33–37]. An IOSC with a pure-ZnO-film EBL was used as reference and the performance of IOSCs with La-ZnO A, La-ZnO B and La-ZnO C EBLs was determined. The La content in the La-doped ZnO films was estimated using the XPS, which will be discussed later in the text. We optimized ZnO based film annealing condition to 150 � C for 1 h because higher temperature re­ gimes did not showed any significant improvement in the device per­ formance parameters [33,34]. In the UV-spectroscopy analysis, the transmittance of the films is dependent on the doping content and thickness of the films. In our case the thickness of the film was fixed (25 nm) and the doping content was changed. As shown in Fig. 2, when the transmittance spectra of pure ZnO and La-ZnO films were obtained in the visible range of 350–500 nm wavelength; the pure ZnO film shows the lowest transmittance and the La-ZnO B film showed the highest transmittance; whereas the La-ZnO A film transmittance was slightly

60 40 20

ZnO La-ZnO A La-ZnO B La-ZnO C

0 200

300

400

500

600

700

800

Wavelength (nm) Fig. 2. Transmission spectra of ZnO and La-ZnO films.

higher than that of La-ZnO C film. So this means at 2 and 5 wt% La doping content, the transmittance started to increase but later on at 10 wt% it started decreasing with no further improvement in trans­ mittance. This minor decrease in transmittance can hurdle the effective absorption of photons in the PAM and eventually affect the IOSC photovoltaic parameters [13]. Chen et al. also observed similar trends in the transmittance spectra of sol-gel derived La-ZnO films and concluded that the impregnation of La3þ ions into the ZnO lattice improves visible light transmittance at optimized doping ratio [38]. We also performed the SEM analysis of the spin coated ZnO or La-ZnO films, which revealed that there was no significant change in the surface morphology as shown in Fig. 3. The photovoltaic performance indicators determined for the IOSCs containing the pure ZnO and Lanthanum doped La-ZnO EBLs are mentioned in Table 1. The respective J-V curves are presented in Fig. 4. The basic reference cell containing the pure ZnO EBL exhibited an opencircuit voltage (Voc) of 0.61 V, a short-circuit current density (Jsc) of 10.76 mA/cm2, a fill-factor (FF) of 58.39%, and a PCE of 3.85%. Comparatively to the reference device, the La-ZnO A IOSC showed slightly better performances, though they were inferior to those exhibited by the La-ZnO B IOSC, the performance of which was considerably improved by the 5 wt% La doping of the EBL. The IOSC PCE decreased on increasing the lanthanum doping content from 5 to 10 wt%. This trend is attributed to the corresponding slight decrease in transmittance and hurdle in the absorption of the photons by the PAM. This decrement of PCE is also related to the dominant bonding charac­ teristics of La-O-Zn in the lattice structure then O-Zn-O bonding. This excess incorporation of La3þ ions will result in the expansion of the ZnO lattice and decrement of Zn2þ ions, which will deteriorate the film crystallization structure and lead to the phenomenon of interfacial charge traps and recombination [38,39]. Meanwhile, the La-ZnO B IOSC Table 1 Photovoltaic properties of inverted organic solar cells with different EBLs.

Fig. 1. Schematic structure of ZnO-based inverted OSC. 3

Device EBL

VOC (V)

JSC (mA/ cm2)

FF (%)

Rshunt (Ω)

Rseries (Ω)

Best PCE (%)

Average PCE (%)

ZnO La-ZnO A La-ZnO B La-ZnO C

0.61 0.61

10.76 11.18

58.39 57.14

14452 14436

258 268

3.85 3.93

3.7 � 0.2 3.9 � 0.1

0.63

11.65

59.00

16465

294

4.34

4.1 � 0.3

0.61

10.19

57.41

14163

283

3.57

3.5 � 0.2

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Fig. 3. SEM images of ZnO/La-ZnO films: (a) ZnO film with 15–30 nm nanoparticle diameter, (b) La-ZnO A film with 12–20 nm nanoparticle diameter, (c) La-ZnO B film with 10–18 nm nanoparticle diameter, and (d) La-ZnO C film with 10–15 nm nanoparticle diameter.

Current Density (mA/cm2)

12 10 8 6 4 2 0 0.0

ZnO La-ZnO A La-ZnO B La-ZnO C 0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage (V) Fig. 4. J-V characteristics of inverted OSC with different EBLs.

exhibited the highest PCE (4.34%) and Voc, Jsc, and FF values of 0.63 V, 11.65 mA/cm2, and 59.00%, respectively. The shunt resistance (Rsh) increased on doping the La-ZnO B EBL with 5 wt% La, while minor in­ crease was observed in the series resistance (Rs). The Rsh value is linked to the overall current leakages and recombination within the device while the Rs value is related to the contact resistance at the various interfaces of IOSC [13,36]. However, in our study, only the EBLs

Fig. 5. AFM images of the surface of (a) ZnO film, (b) La-ZnO A film, (c) LaZnO B film, and (d) La-ZnO C film.

interface was altered, and the remaining device configuration was the same, so the change in Rsh value is related to the interfacial properties of the ZnO based EBLs. In addition, the Voc values were almost in the 4

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incorporating La to increase Voc and forming the better charge carrier transport channels through the cascade energy level structure and optimizing phase separation of donor/acceptor materials at interface between EBL and PAM [26,27]. Spin coating is a cost effective technique to deposit uniform films [33]. Fig. 5 displays the AFM profile of the pure ZnO and La-doped ZnO films. Once the La doping level was raised to 5 wt% for La-ZnO B film, the root mean square (RMS) value marginally decreased from 0.56 to 0.48 nm. However when the doping level was exceeded to 10 wt%. (La-ZnO C) the RMS slightly increased again to 0.50 nm. This indicates that the chance of surface defects forming between the PAM and the EBL decreases as the doping level reached the optimum value of 5 wt% (La-ZnO B). A decrease in the number of surface defects will ultimately reduce the current leakages and charge recombination at the interface between the PAM as mentioned in the data in Table 1 [35,36]. The optical band gap of the ZnO, La-ZnO A, La-ZnO B and La-ZnO C films was calculated from their transmission spectra. The pure ZnO is a direct-band gap material, however doping can cause fluctuations in the band gap. The optical band gap of the films was deduced by the wellknown Tauc relationship [40]. Eg (band gap) of the as deposited films was determined by plotting (α � hν) 2 against the photon energy (hν) and extrapolating the straight-line portion of this plot to the photon-energy axis, as specified in Fig. 6 The Eg dropped from 3.34 eV for the ZnO film to 3.21 and 3.08 eV for the La ZnO A (La 2 wt%) and

20000

( hv)

2

15000

ZnO La-ZnO A La-ZnO B La-ZnO C

10000

5000

0 1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Energy (eV)

Fig. 6. (αhν)2 vs photon energy for ZnO and La-ZnO films.

similar range for the four IOSCs as they consist of the same PAM and HTL. Meanwhile, doping with 5 wt% La (La-ZnO B) attributed better Jsc and FF values. These improvement in Voc, Jsc and FF are mainly due to the harvesting of more photons, optimizing the energy level of EBL by Table 2 Electrical Properties of ZnO and La-ZnO EBL films. EBL

Resistivity � 10

ZnO La-ZnO A La-ZnO B La-ZnO C

1.16 0.30 0.29 1.22

4

(Ω cm)

Conductivity � 103 (1/Ω cm)

Carrier density � 1021 (cm 3)

Hall electron mobility (cm2 V

8.60 33.3 34.4 8.19

2.30 7.25 9.49 1.16

22.80 22.50 22.50 23.70

Fig. 7. XPS spectra of (a) ZnO film, (b) La-ZnO A film, (c) La-ZnO B film, and (d) La-ZnO C film. 5

1

s 1)

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Materials Chemistry and Physics 240 (2020) 122076

to larger ionic radii. Meanwhile the broadening of band gap resulted from losing ZnO intrinsic properties and shifting to La2O3 formation by quantum refinement effect [38–41]. The band gap is related to the quantum refinement effect on the basis of doping content, particles size and lattice parameters [39]. Shakir and Korake both observed similar band gap narrowing effect at their particular La-ZnO doping concen­ trations [19,42]. The improved photovoltaic performance of La-ZnO B, compared with pure ZnO, is related to this band gap narrowing impact. This minor fluctuation of band gap also ensured the improvement of the La-ZnO film electrical properties as explained in the following paragraph. The as deposited films electrical properties were determined by using the measurements of the Hall (Table 2), which further confirmed the ntype conductivity of the ZnO and La-ZnO films. The hall electron mobility of the pure and doped samples was almost in the same range. However, the conductivity and carrier density of La-ZnO B were greater than those of the pure ZnO. Subsequently, when the La-doping content was increased to 10 wt% in the La-ZnO C film the conductivity and carrier density dropped. The increase in electrical conductivity could originate from the increase in the large carrier concentration introduced by La3þ cation [39]. Meanwhile, the excess influence of charged impu­ rities (La3þ) in pure material (ZnO) can affect electron drift velocity, low-field mobility and diffusion coefficient; which altogether reduces conductivity [43]. In similar context, Goel et al. also reported the in­ crease in conductivity of DSSCs at optimum La-ZnO doping ratio [20]. Thus, 5 wt% La-doping content was the optimized ratio which improved the electron transport properties and in turn, contributed towards the enhancement of Jsc value in the La-ZnO B based IOSC compared with the pure ZnO based IOSC [14,20]. The XPS quantitative analysis was performed to confirm the forma­ tion of the ZnO and La-ZnO films as shown in Fig. 7. These XPS results explain the doping mechanism of La3þ into the ZnO lattice. Sharp Zn and O peaks can be observed in the XPS spectra for all four samples, while the La 3d5/2 peaks are only present in La-ZnO A, La-ZnO B and La-ZnO C survey spectra. The major peaks in the ZnO sample are Zn 2p3/2 and O 1s at 1021.3 and 531.1 eV, respectively; the binding energies of these peaks corresponds to ZnO [41–46]. The Zn 2p3/2, O 1s, and La 3d5/2 peaks in La-ZnO A were at 1021.7, 531.1, and 835.4 eV respectively. The Zn 2p3/2, O 1s, and La 3d5/2 peaks in La-ZnO B were at 1021.7, 531.0, and 835.3 eV, respectively. The Zn 2p3/2, O 1s, and La 3d5/2 peaks in La-ZnO C were at 1020.9, 530.2, and 834.4 eV respectively. Collectively these samples binding energies matches the binding energy of ZnO with oxidized La [42,45]. The La 3d5/2 peaks indicates the La3þ ions into the ZnO lattice [44]. The under discussion La-doped ZnO samples show approx. the same high-resolution (HR) XPS spectra for O 1s and La 3d5/2. Therefore, we only short-list the La-ZnO B sample for doping mechanism study as it showed the best PCE performance. Fig. 8 shows the HR-XPS spectrum for O 1s and La 3d5/2 for sample La-ZnO B. In Fig. 8 (a), the peak centered at 529.5 eV could be attributed to the coordination of oxygen in Zn–O–Zn, whereas that at 531.2 eV could be ascribed to the coordination of oxygen in La–O–Zn [41,46]. The HR scans in Fig. 8 (b), also show that the binding energy of La 3d5/2 is located at 835.4, 833.7 and 838.3 eV, which corresponds to the bonding states of La2O3, La (OH)3 and LaxOx lanthanum oxide clusters respectively [39]. Hence­ forth, it was confirmed that the La element mostly existed in the form of þ3 valence in the sample. It can be expected that the La–O bond length should be decreased when La ions have been incorporated into the ZnO lattice and substituted the Zn ion sites because the ionic radius of La3þ (103.2 p.m.) is much bigger than that of Zn2þ (74 p.m.). Such a shrinkage of the La–O bond length increases the interaction between the ions [39,41]. Thus, the XPS results reveal that the obtained La-ZnO B film was composed of a main ZnO phase combined with a small amount of impurities, i.e. La2O3, which contributed towards the enhancement of the device performance. At this optimized doping ratio the La-ZnO B film showed superior electrical properties, as proven by the aforesaid hall measurement results. However, when the doping level was

Fig. 8. High-resolution XPS spectra of (a) O 1s La-ZnO B film, and (b) La 3d5/2 La-ZnO B film.

ZnO La-ZnO A La-ZnO B La-ZnO C

100

EQE (%)

80 60 40 20 0 300

400

500

600

700

800

Wavelength (nm) Fig. 9. EQE spectra of IOSCs with ZnO, La-ZnO A, La-ZnO B and La-ZnO C EBLs.

La-ZnO B (La 5 wt%) films, respectively. However when the La doping content was increased to 10 wt% in La-ZnO C film the band gap reverted again to 3.27 eV. The band gap narrowing of La doping is attributed to La atoms deliberately replacing Zn sites in the ZnO lattice structure due 6

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Materials Chemistry and Physics 240 (2020) 122076

increased (La content ¼ 10 wt%), the excess of La3þ ions into the ZnO lattice led to the deterioration of the optical transmittance and electrical conductivity of the films as ascribed to the electron-impurity interaction and the coulomb interaction between the carriers [39]. The lanthanum content in the La-ZnO A, La-ZnO B and La-ZnO C films was 0.70, 1.57 and 2.48 at.%, respectively. Interestingly, the aforementioned results suggest that the chemical binding state of the spin coated ZnO/La-ZnO EBL is close to that of lanthanium-doped ZnO hierarchical nano­ structures by electrochemical synthesis [44]. To further investigate the origin of the IOSC performance presented here, we measured the external quantum efficiency (EQE) of the four fabricated devices, as indicated in Fig. 9 [47,48]. The La-ZnO B IOSC EQE is much greater than that of the ZnO IOSC between 300 and 400 nm. The La-ZnO A IOSC EQE is intermediate between those of the La-ZnO B and ZnO IOSCs. However the La-ZnO C IOSC showed the most poor EQE. An improved EQE indicates a longer minority-carrier lifetime, removal of charge traps and reduced recombination; which altogether results in an overall increase in the Jsc and PCE values [5,34].

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4. Conclusions IOSCs based on a FTO/La-ZnO/P3HT:PCBM/V2O5/Ag structure were fabricated using sol-gel-deposited ZnO electron buffer layers (25 nm thick). We optimized the lanthanum-doping ratio of La-ZnO EBL films relative to the PCE and consequently investigated the impact of La doping the ZnO EBL on the IOSC parameters. An IOSC with a La-ZnO B EBL (La content 5 wt%) showed the champion PCE (4.34%) along with other improved performance parameters; this contributed to enhance the electron transport properties accomplished by narrowing the band gap at the interface of the EBL and the PAM. The sol-gel technique presented here provides a promising approach for fabricating solar-cell devices due to its ease in operation, simple controllable parameters, and significant low cost. Acknowledgments This research was supported by the National Research Foundation of Korea (NRF-2017R1E1A1A03070930), funded by the Ministry of Edu­ cation, Science and Technology, Korea (NRF-2015M3A7B4050424). We also thank the Korea Basic Science Institute (KBSI) at the Gwangju Center for assistance with SEM and XRD analyses. References [1] H. Kim, S. Nam, J. Jeong, S. Lee, J. Seo, H. Han, Y. Kim, Organic solar cells based on conjugated polymers : history and recent advances, Korean J. Chem. Eng. 31 (2014) 1095–1104. [2] M.A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Solar cell efficiency tables (Version 45): solar cell efficiency tables, Prog. Photovolt. Res. Appl. 23 (2015) 1–9. [3] H. Naz, R.N. Ali, Q. Liu, S. Yang, B. Xiang, Niobium doped zinc oxide nanorods as an electron transport layer for high-performance inverted polymer solar cells, J. Colloid Interface Sci. 512 (2018) 548–554. [4] C.-D. Kim, N.T.N. Truong, V.T.H. Pham, Y. Jo, H.-R. Lee, C. Park, Conductive electrodes based on Ni–graphite core–shell nanoparticles for heterojunction solar cells, Mater. Chem. Phys. 223 (2019) 557–563. [5] M.-S. Song, C.H. Jeong, D.-H. Kim, Application of sol–gel processed titanium oxide for inverted polymer solar cells as an electron transport layer, Sci. Adv. Mater. 8 (2016) 75–79. [6] C.-C. Lin, S.-K. Tsai, M.-Y. Chang, Spontaneous growth by sol-gel process of low temperature ZnO as cathode buffer layer in flexible inverted organic solar cells, Org. Electron. 46 (2017) 218–225. [7] J.M. Bjuggren, A. Sharma, D. Gedefaw, S. Elmas, C. Pan, B. Kirk, X. Zhao, G. Andersson, M.R. Andersson, Facile synthesis of an efficient and robust cathode interface material for polymer solar cells, ACS Appl. Energy Mater. 1 (2018) 7130–7139. [8] M. Prosa, M. Tessarolo, M. Bolognesi, O. Margeat, D. Gedefaw, M. Gaceur, C. Videlot-Ackermann, M.R. Andersson, M. Muccini, M. Seri, J. Ackermann, Enhanced ultraviolet stability of air-processed polymer solar cells by Al doping of the ZnO interlayer, ACS Appl. Mater. Interfaces 8 (2016) 1635–1643.

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