Lithium-Induced Defect Levels in ZnO Nanoparticles To Facilitate

Jul 6, 2016 - In this work, lithium-doped zinc oxide nanoparticles (LZO NPs) with different Li/Zn molar ratios (Li/Zn = 0, 0.05, 0.2) are successfully...
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Lithium-Induced Defect Levels in ZnO Nanoparticles To Facilitate Electron Transport in Inverted Organic Photovoltaics Wen-Hui Cheng,† Jau-Wern Chiou,‡ Meng-Yen Tsai,† Jiann-Shing Jeng,*,§ Jen-Sue Chen,*,† Steve Lien-Chung Hsu,† and Wei-Yang Chou∥ †

Department Department § Department ∥ Department ‡

of of of of

Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan Materials Science, National University of Tainan, Tainan 700, Taiwan Photonics, National Cheng Kung University, Tainan 701, Taiwan

S Supporting Information *

ABSTRACT: In this work, lithium-doped zinc oxide nanoparticles (LZO NPs) with different Li/Zn molar ratios (Li/Zn = 0, 0.05, 0.2) are successfully prepared to form an electron transporting layer (cathode buffer layer) in the inverted-type P3HT:ICBA organic photovoltaic (OPV) devices. As compared with the undoped ZnO NPs buffer layer, a considerable improvement OPVs from 2.344% to 2.946% is obtained by using 5%-LZO NPs as a buffer layer, which owns Jsc of 7.22 mA/cm2, Voc of 0.86 V, and FF of 47.4%. X-ray absorption near-edge structure (XANES) spectra show the increase of unoccupied O 2p-derived states in 5%-LZO NPs, which leads to better carrier conductance. The energy levels of defects in 5%-LZO NPs analyzed by photoluminescence are found to facilitate electron extraction to the cathode. Impedance measurement results indicate that the carrier lifetime is effectively increased to 2176 μs by applying the 5%-LZO NPs buffer layer, showing the improvement of carrier extraction efficiency and resulting in its progressive performance.

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INTRODUCTION Pursuing higher performance with larger working area as well as longer stability of organic photovoltaic (OPV) devices is always the most important issue for applying OPV in our daily life.1 In addition, based on its flexible nature and capability of large-area fabrication, organic solar cells have attracted much attention.2 The blend of P3HT (poly(3-hexylthiophene-2,5-diyl)) and PCBM ([6,6]-phenyl-C61 butyric acid methyl ester) is commonly used as active layer owing to the complete knowledge about the mechanism,3,4 though the higher electron affinity of PCBM limit the open circuit voltage (Voc) to 0.6 V. A fullerene derivative ICBA (indene-C60 bisadduct) with a higher LUMO energy level is applied to replace PCBM as the donor,5−7 which results in higher Voc but sacrifices the short circuit current (Jsc),8 and the final OPV performance will be enhanced. As compared with conventional OPV structure, the inverted OPV structure possesses much higher stability than conventional OPV cells by preventing the corrosion of anode indium tin oxide (ITO) from the hole-transporting material poly(3,4ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PEDOT:PSS),9 which affects the long-term stability. In addition, the insertion of a buffer layer between the active layer and cathode electrode can resolve the degradation phenomena regarding the oxygen, moisture, and metal diffusion into the © XXXX American Chemical Society

active layer. ZnO cathode buffer layer is selected because, with its suitable energy level, ZnO can enhance electron transport and retard hole transport to the cathode because of a good band matching among the ZnO, the cathode and the LUMO of the electron acceptor.10 However, the defects and traps in ZnO and the poor contact between the inorganic ZnO layer and the organic active layer lower the performance of inverted cells.11 Therefore, a variety of n-type electron transporting layer (ETL) materials such as TiO2,12 ZnO/CdS nanocomposite,13 ultrathin zinc oxide (ZnO),14 and nanostructured ZnO15 have been introduced. In this study, an inverted OPV structure with ZnO nanoparticles (NPs) or Li-doped ZnO NPs as the cathode buffer layer is investigated. Lloyd et al.16 synthesized a lithium incorporated ZnO layer by sol−gel process with tunable work function to improve the efficiency of P3HT/ZnO planar organic−inorganic hybrid solar cells. Ruankham et al.17 also incorporated lithium into ZnO during hydrothermal growth of nanorods to enhance the performance of P3HT/ZnO hybrid solar cells. Kim et al.18 demonstrated that a solution-processed lithium-doped zinc oxide (LZO) as the electron transport layer Received: April 11, 2016 Revised: June 25, 2016

A

DOI: 10.1021/acs.jpcc.6b03656 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

fluorescence spectrophotometer (Hitachi F-4500). The impedance analysis of solar cells in the dark condition was measured with Agilent 4294A Precision Impedance Analyzer in the frequency range from 50 Hz to 1 MHz with an oscillating voltage of 25 mV.

in inverted organic photovoltaics and a 2 wt % Li doping in ZnO was found to optimized the OPV performance. However, the LZO film needed to be heat treated (at 245 °C for 10 min) and the role of Li doping was not analytically resolved. We take a low-temperature solution route (i.e., without further annealing) to synthesize Li-doped ZnO NPs for its advantage of low-temperature process and convenient composition control. The mechanism on the PCE improvement of the organic solar cells using Li-doped ZnO NPs as cathode buffer layers is systematically explored.

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RESULTS AND DISCUSSION To characterize the effect of lithium incorporation in ZnO nanoparticles as cathode buffer layer, LZO NPs with the precursor molar ratio of Li/Zn = 0, 0.05, 0.2 are applied in the P3HT:ICBA solar cells, and the devices are denoted as ZnO NPs device, 5%-LZO NPs device, and 20%-LZO NPs device, respectively. The microstructure of nanoparticles is examined by TEM and the images are displayed in Figure 1, parts a, c, and e. Both

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EXPERIMENTAL METHODS The solar cell devices were fabricated on patterned ITO glass with a sheet resistance of 7 Ω/square. The substrates were cleaned in an ultrasonic bath with detergent, acetone, and isopropyl alcohol for 15 min each and then dried under a nitrogen stream. Lithium incorporated ZnO NPs were prepared following the method similar to the reported method of preparing ZnO nanoparticles.19 The chemical sources zinc acetate dihydrate (Zn(CH3COO)2·2H2O) (SHOWA, 99%) and lithium acetate dihydrate (Li(CH3COO)·2H2O) (SigmaAldrich, 97%) were dissolved in methanol with the different Li/ Zn molar ratios (Li/Zn= 0, 0.05, 0.2). The amount of zinc precursor is invariable. During reaction with KOH, the precipitation of LZO NPs could be observed. After centrifugation and washing process, the nanoparticles were dried and dispersed in 1-butanol. To form the cathode buffer layer, LZO NPs were spin coated on ITO glass without further annealing. The substrate was later transferred to the Ar-filled glovebox for following deposition. P3HT (poly(3-hexylthiophene-2,5diyl)) (UniRegion Bio-Tech, UR-P3H001) and ICBA (indeneC60 bisadduct) (UniRegion Bio-Tech, UR-ICBA) were dissolved in 1,2-dichlorobenzene (Sigma-Aldrich, 99%) in 1:1 w/w ratio to form a 40 mg/mL solution. P3HT:ICBA solution was spin-coated onto the LZO NPs at 500 rpm for 10 s and 1500 rpm for 1 min, and then dried in a Petri dish for 20 min, followed by annealing at 110 °C for 10 min on a hot plate in the glovebox. PEDOT:PSS was then spin-coated on the top at the spin rate of 5000 rpm for 60 s, followed by direct drying on a hot plate at 160 °C for 10 min. Finally, 45 nm Au/35 nm Ag stack electrode was deposited through thermal evaporation. The working area of the ITO/LZO NPs/P3HT:ICBA/ PEDOT:PSS/Au/Ag devices was defined to be 0.16 cm2 by a shadow mask. The chemical composition of LZO NPs was examined by secondary ion mass spectrometer (SIMS, ION-TOF TOF.SIMS IV), with Ga+ ion source and O2+ sputtering ion source. Cross-sectional transmission electron microscopy (TEM) images of devices and nanoparticles were captured by JEOL JEM-2100F, with an acceleration voltage of 200 keV. The device cross-sectional TEM samples were prepared with focused ion beam (FIB) and nanoparticles TEM samples were prepared by dropping diluted solution on Formvar/ carbon covered copper grids. The photovoltaic characteristics of solar cell devices were measured with a Keithley 2400 source measure unit. An AM 1.5 G illumination with an intensity of 100 mW/cm2 was simulated by Newport’s Oriel class A solar simulator. The O K-, Zn L3-edge XANES spectra of LZO NPs were obtained in total fluorescence yield (TFY) mode at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. The photoluminescence spectra were conducted with a Xe lamp and an excitation wavelength of 325 nm using a

Figure 1. (a) TEM image and (b) diffraction pattern of ZnO NPs. (c) TEM image and (d) diffraction pattern of 5%-LZO NPs. (e) TEM image and (f) diffraction pattern of 20%-LZO NPs.

ZnO NPs and LZO NPs exhibit crystalline structure with clear lattice fringes. The average diameter of ZnO and LZO NPs is carefully assessed from transmission electron microscopy (TEM) images, which are obtained from 20 nanoparticles. The average diameters of ZnO, 5%-LZO NPs, and 20%-LZO NPs are 5.29 ± 0.98, 6.28 ± 1.6, and 6.58 ± 0.27 nm, respectively. The diffraction patterns of each samples, as shown in Figure 1, parts b, d, and f, are pertaining to all diffraction rings of crystalline ZnO phase. TEM analysis confirms that only B

DOI: 10.1021/acs.jpcc.6b03656 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C zinc oxide phase is present in LZO NPs. Because the synthesis of nanoparticles involves precipitation process, the effectiveness of lithium incorporation is confirmed by using secondary ion mass spectroscopy (SIMS) (Figure S1, Supporting Information). Figure 2 shows the cross-sectional images of inverted

Figure 2. Cross-sectional image of P3HT:ICBA solar cells with 5%LZO NPs cathode buffer layer. Figure 3. (a) Illuminated and (b) dark J−V characteristics of P3HT:ICBA solar cells with ZnO and LZO NPs as cathode buffer layers.

solar cell devices with layered structure from bottom to up: ITO/LZO NPs/P3HT:ICBA/PEDOT:PSS/Au/Ag. The cathode buffer layer formed by LZO nanoparticles is continuous with thickness of 15−25 nm. To examine the effect of lithium incorporation in cathode buffer layer to the performance of inverted P3HT:ICBA solar cells, the J−V characteristics of devices under illumination and dark conditions are measured, as shown in Figure 3, parts a and b, respectively. The photovoltaic parameters are listed in Table 1. The data shown in Table 1 are obtained from at least five cells using the same batch processed at the same time. The average values and errors (i.e., the standard deviation) are added for all extracted photovoltaic parameters. Under illumination, the power conversion efficiency of ZnO NPs device is 2.344%. With addition of 5% lithium into precursor solution during preparing ZnO NPs for cathode buffer layer, the open circuit voltage (Voc) of the device increases slightly from 0.82 to 0.86 V, short circuit current (Jsc) increases from 6.40 to 7.22 mA/cm2, and fill factor FF improves from 44.7% to 47.4%, leading to an enhanced efficiency of 2.946%. However, with further addition of lithium precursor to 20%, the Voc decreases back to 0.82 V, Jsc drops to 6.18 mA/cm2, and FF decreases to 40.1%, so as to reduce the PCE to 2.024%. The FF of our solar cells is not perfect because of high Rs. Generally, the FF decreases when the Rs increases. Table 1 shows this trend exactly. Rs can be attributed to the ohmic loss in the whole device, which includes the bulk resistance and the contact resistance.20,21 Therefore, the Rs is affected not only by the resistivity of LZO NPs cathode buffer layer but also other factors such as the contact resistance between different layers. As a result, the FF is not significantly improved although the Li doping reduces the ZnO NP resistivity (see Table S1 in Supporting Information). To avoid the environmental factors (such as temperature and moistness) that influence the OPV performance, all devices in this study were fabricated in the same batch. Our motivation of investigating the effect of Li doping in ZnO NP cathode buffer layer on the PCE

improvement is well carried out although the FF values may not be excellent. The electronic characteristics of OPVs are generally modeled to an equivalent circuit, which consists of a series resistance (Rs), a shunt resistance (Rsh), and a photocurrent source (Jph) in parallel with a diode.22 Rs and Rsh can be obtained from the inverse of the J−V curve slopes at J = 0 and V = 0, respectively.23 The Rs and Rsh of individual device are also summarized in Table 1. As seen in Table 1, 5%-LZO NPs device exhibits the smallest Rs (26.0 Ω cm2) and the largest Rsh, which means the resistance for carrier transport in device is minimized and carrier loss by undesired path (for example, recombination) is effectively blocked, leading to the improvement of Jsc and FF. On the contrary, 20%-LZO NP device shows the highest Rs (44.1 Ω cm2) and the smallest Rsh, which indicates a lot of photogenerated carriers are lost and device performance is deteriorated. The X-ray absorption near-edge structure (XANES) spectra were obtained in total fluorescence yield (TFY) mode to analyze the electronic structure of LZO NPs, as shown in Figure 4, parts a and b. Features A1−D1 of O K-edge spectra in Figure 4a can be ascribed to the transitions of electrons from O 1s to unoccupied O 2pπ (along the c-axis) and O 2pσ (in the bilayer) states.24,25 The higher intensity of the O K-edge spectrum of 5%-LZO NPs shows the increase of unoccupied O 2p-derived states, which results in better carrier conductance.26 Therefore, the smallest Rs of 5%-LZO NPs device can be explained. On the contrary, the O K-edge spectra of 20%-LZO NPs represents reduction of unoccupied O 2p states, leading to the worse conductance and highest Rs. This result suggests the occupation of O 2p-derived states in the cathode buffer layer is an important factor affecting Rs of the solar cell device. Features A2−C2 of the normalized Zn L3-edge spectra in Figure 4b can be ascribed to electron transitions from Zn 2p states to C

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The Journal of Physical Chemistry C Table 1. Photovoltaic Performance of P3HT:ICBA Solar Cells with Various Li-Doped ZnO Nanoparticles ETLa

a

device

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rs (ohm cm2)

Rsh (ohm cm2)

ZnO NPs 5%LZO NPs 20%LZO NPs

0.82 ± 0.00 0.86 ± 0.00 0.82 ± 0.02

6.40 ± 0.24 7.22 ± 0.38 6.18 ± 0.12

44.7 ± 0.7 47.4 ± 1.1 40.1 ± 0.9

2.344 ± 0.06 2.946 ± 0.19 2.024 ± 0.07

35.0 ± 3.6 26.0 ± 1.6 44.1 ± 4.0

481.4 ± 80.0 914.8 ± 277.6 252.9 ± 67.4

Errors present the standard deviation of multiple independent organic solar cells.

substitutional site(LiZn ′ ), and accompanied by an oxygen vacancy (V··O). This behavior is depicted by the following defect equations.27,28 Li 2O ⎯⎯⎯→ Li·i + Li′Zn + OOx

(1)

Li·i

(2)

ZnO

→

Li′ZnV ··O

Incorporation of lithium in zinc oxide will lead to formation of oxygen vacancies. The defect levels of positively charged oxygen vacancy locate near conduction band edge of ZnO.29 To determine the concentration of oxygen vacancies in ZnO and LZO NPs, X-ray photoelectron spectroscopy (XPS) was performed to measure the oxygen bonding configuration of the NPs. It is found that the oxygen deficiency ratio [i.e., OII/(OI + O II + O III )] increases with increasing the Li-doping concentration in ZnO (see Supporting Information, Figure S4 and Table S1). For the Li-doped ZnO NPs, Li atoms may passivate oxygen dangling bonds. Once oxygen is bonded with Li, its core level electron binding energy shall be attributed to OI subpeak of O 1s XPS spectrum. However, the Li−O bonding (i.e., passivation of O by Li) may also lead to formation of oxygen vacancies according to the defect eqs eq 1 and eq 2. Therefore, we observed an increase of OII subpeak intensity for both 5%-LZO and 20%-LZO NPs, as compared to undoped ZnO NPs. Therefore, the existence of an optimal Lidoping concentration in ZnO can be observed, which may passivate the dangling bonds of oxygen as well as provide a suitable amount of oxygen vacancies. To further prove the existence of defect levels, the PL spectra of ZnO NPs, 5%-LZO NPs, and 20%-LZO NPs are shown in Figure 5. The emission at 375 nm is originated from band-to-

Figure 4. (a) O K-edge and (b) Zn L3-edge X-ray absorption nearedge structures (XANES) spectra of ZnO and LZO NPs.

unoccupied Zn 4s/3d states.25 The Zn L3-edge XANES spectra of all three LZO NPs samples nearly overlap. We infer that the incorporation of lithium in ZnO NPs will not apparently influence occupancy of Zn 4s/3d states and the main structure is similar to that of ZnO.26 Figure S2 in the Supporting Information shows the atomic force microscopy (AFM) topography images of ZnO NPs with and without Li additives. Notably, the additive of 5% Li in ZnO NPs decreases the rootmean-square (RMS) roughness from 3.16 nm for undoped ZnO NPs to 2.72 nm for 20%-LZO NPs. As increasing the Li to 20%, the roughness increases to 4.10 nm (Table S1, Supporting Information). In general, a smooth surface will enhance carrier transport and decrease the carrier recombination probability from trapping. Hence, a 5%-LZO NPs with an optimal surface roughness would lead to the OPV possessing a higher PCE. In order to know the electrical conductivity after doping Li, the resistivity of LZO NPs was measured by four-point probe. To achieve a thick NPs films, the spin coating and heattreatment processes were repeated several times to obtain the film thickness of 150 nm. The resistivity decreases with increasing Li % in ZnO NPs (Table S1, Supporting Information). The result can also be proven by the XANES spectra (Figure 4), where the amount of the unoccupied O 2p state is increased after Li doping in ZnO and gives us more insight into the conductivity benefit from Li doping. There are two possible positions for lithium in ZnO structure, substitutional site (LiZn ′ ) and interstitial site (Li·i). The lithium at interstitial site (Li·i) may further proceed to the

Figure 5. Photoluminescence spectra of ZnO and LZO NPs.

band transition. The feature around 400 nm can be attributed to the transition from oxygen vacancy related defect levels to valence band. We can observe the increase of intensity after lithium incorporation, which tallies with our assumption that there are more defect levels near conduction band edge (i.e., the positively charged oxygen vacancies) with higher doping concentration. However, the broad green band 480−560 nm in the PL spectrum which can be attributed to the existing native D

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The Journal of Physical Chemistry C defects in ZnO29 is obviously enhanced in the condition of 20% LZO NPs. In addition, the optical band gaps of ZnO NPs incorporating 0%, 5%, and 20% Li from their transmittance spectra (see Supporting Information, Figure S3 and Table S1), resulting in values of 3.40, 3.43, and 3.33 eV, respectively. The optical band gaps are not significantly different, which is supported by the similar main peak positions in PL spectra (see Figure 5). The Li doping can create oxygen vacancies (see Supporting Information, Figure S4 and Table S1) and populate electron carriers in the conduction band of 5%-LZO NPs. Therefore, the bandgap increases in the 5%-LZO NPs as explained in terms of Burstein−Moss shift.30 On the other hand, based on the results of PL, more deep defect states are found in the bandgap of 20%-LZO NPs, which will affect the optical transition. As a result, the minor optical bandgap narrowing of 20%-LZO NPs can be attributed to the creation of more defect states in 20% Li doping as compared to the 5%LZO NPs.31,32 The schematic band diagrams of P3HT:ICBA solar cells are displayed in Figure 6a−6c to illustrate the role of defect levels in cathode buffer layer. In 5%-LZO NPs device, there are more lithium induced oxygen vacancies in the cathode buffer layer than in ZnO NPs device. The lithium induced defect levels located right under the conduction band edge could act as passageway to facilitate electron transport. Therefore, the electron−hole recombination probability decreases as the electrons move to the cathode expediently and results in a higher Rsh. The lower Rs (owing to more unoccupied O 2pderived states) and higher Rsh leads to enhanced Jsc and FF, and the suppressed recombination restores Voc, leading to better performance. In 20%-LZO NPs device, even though more defect levels under conduction band edge will help electron transport to the cathode, there are many more defect levels located deep in LZO bandgap, which is near the HOMO of P3HT and will expedite holes to transport to the wrong direction. This undesired transport path will severely increase carrier recombination, resulting in a small Rsh. Combining with its higher Rs (owing to the reduction of unoccupied O 2p states), the Jsc and FF of 20%-LZO NPs device diminish. Therefore, we cannot further promote the device performance by applying higher lithium incorporation. We further demonstrate our idea of lithium doping altered recombination by impedance analysis. The Nyquest plot is shown in Figure 7 with the equivalent circuit show in inset.33,34 The fitting parameters are list in Table 2. The equivalent circuit can divided into three parts: R0 is the resistance series with solar cell device; R1∥C1 is associated with the bulk resistance and capacitance of the solar cell and will respond under high frequency; Rrec∥CPE is associated with the semicircle in the low frequency part of the Nyquest plot and relates to carrier recombination resistance and chemical capacitance of the internal charge transfer events at interfaces. The constant phase element (CPE) is a chemical capacitance with specific phase represented by n, and the magnitude of the CPE is expressed by QCPE. A value of n = 1 corresponds to an ideal capacitor. Recombination lifetime can be calculated by formula shown below.

τavg = (R recQ CPE)1/ n

Figure 6. Schematic band diagrams of P3HT:ICBA solar cells with (a) ZnO NPs; (b) 5% LZO NPs; (c) 20% LZO NPs cathode buffer layers.

Figure 7. Impedance spectra of P3HT:ICBA solar cells with ZnO and LZO NPs as cathode buffer layers.

(3)

According to the impedance analysis, the 5%-LZO NPs solar cell presents the longest recombination lifetime of 2176 μs, E

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The Journal of Physical Chemistry C Table 2. Impedance Fitting Parameters of P3HT:ICBA Solar Cells with LZO NPs as Cathode Buffer Layers ZnO NPs 5%-LZO NPs 20%-LZO NPs

R0 (Ω)

R1 (Ω)

Rrec (kΩ)

C1 (nF)

QCPE (nF)

CPE-n

τavg (μs)

28.81 ± 1.86 24.73 ± 0.29 99.09 ± 2.85

732.4 ± 48.20 226.4 ± 0.60 1412.0 ± 41.5

117.37 ± 0.78 182.73 ± 1.76 78.94 ± 0.31

5.41 ± 0.13 7.95 ± 0.40 3.78 ± 0.06

21.79 ± 0.77 17.20 ± 0.22 20.46 ± 0.53

0.90 ± 0.01 0.94 ± 0.00 0.91 ± 0.01

1318 ± 11.00 2176 ± 30.28 855 ± 5.50

indicating a higher probability for carrier extraction. On the contrary, the recombination lifetime of 20%-LZO NPs device decreases dramatically to 855 μs, which infers more recombination occur in the device. We also find that variation of R0 and R1 with lithium incorporation concentration shows the same tendency with Rs calculated from the J−V curve. Rrec also shows the same tendency with Rsh, which means that recombination is an important factor to alter shunt resistance. Impedance measurement demonstrates that the recombination lifetime of 5%-LZO NPs device is substantially extended, which again suggests the defect levels near conduction band edge in the cathode buffer layer restrain recombination and lead to its advanced performance. In this study, the LZO nanoparticles (NPs) was directly spincoated from the suspension precursor onto the ITO substrate without any further post thermal treatment. The convenient composition control and low-temperature process (i.e., without further annealing) signify the practical advantages. As for the role of Li doping in ZnO cathode buffer layer, we ponder the idea of defect level assisted charge transfer and recombination mechanism, which is experimentally supported by photoluminescence (PL) analyses and impedance measurement. In addition, defect states in the Li-doped ZnO NPs layer are revealed by XANES analyses, which well explains the electron conduction mechanism and provides a guideline for enhancing the ZnO cathode buffer layer performance via incorporating impurities.

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CONCLUSIONS

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-S.J.). Telephone: +886-62133111 ext. 592. *E-mail:[email protected] (J.-S.C.). Telephone: +886-6-2757575 ext. 62948. Fax: +886-6-2762541. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to express appreciation for the financial support from the Ministry of Science and Technology of Taiwan (Grant Nos. MOST 102-2221-E-006-071-MY3, MOST 102-2221-E024-021-MY3, and MOST 103-2221-E-006-086-MY3).

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REFERENCES

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LZO nanoparticles with different Li:Zn molar ratios as cathode buffer layers are effectively applied in inverted-type P3HT:ICBA bulk heterojunction OPVs. The doping concentration of Li impacts the performance of OPV significantly. As compared with the ZnO NPs device (PCE = 2.344%), the power conversion efficiency of inverted P3HT:ICBA solar cell gains a 26% enhancement when using an optimized 5%-LZO NPs cathode buffer layer (PCE = 2.946%). The enhanced performance results from the defect level assisted carrier extraction, and demonstrates that improvement in the value of recombination lifetime. However, higher Li doping concentration in ZnO produces more deep defect levels, which leads to more carrier recombination. Lithium incorporation will influence defect levels generated in the ZnO band structure and contribute to the difference of performance.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03656. Additional results of the secondary ion mass spectroscopy, AFM topography, transmittance, Tauc plot, and XPS O 1s spectra (PDF) F

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DOI: 10.1021/acs.jpcc.6b03656 J. Phys. Chem. C XXXX, XXX, XXX−XXX