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Inverted Type Polymer Solar Cells with Self Assembled Monolayer Treated ZnO Ye Eun Ha, Mi Young Jo, Juyun Park, Yong-Cheol Kang, Seong Il Yoo, and Joo Hyun Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp311148d • Publication Date (Web): 21 Jan 2013 Downloaded from http://pubs.acs.org on January 21, 2013

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The Journal of Physical Chemistry

Inverted Type Polymer Solar Cells with Self Assembled Monolayer Treated ZnO Ye Eun Ha,† Mi Young Jo,† Juyun Park,‡ Yong-Cheol Kang,‡ Seong Il Yoo, † and Joo Hyun Kim†,*

†

Department of Polymer Engineering and ‡Department of Chemistry, Pukyong National

University, Busan 608-739, Korea. KEYWORDS Bulk heterojunction Solar cell, Self-assembled monolayer, Interface dipole, Surface property

ACS Paragon Plus Environment

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ABSTRACT

The work function and surface property of ZnO can be simply tuned by the self-assembled monolayer (SAM) molecules derived from benzoic acid such as 4-methoxybenzoic acid (MBA), 4-tert-butylbenzoic acid (BBA), and 4-fluorobenzoic acid (FBA), which have different dipole orientation and magnitude. MBA, BBA and FBA treated ZnO layer used as a electron injection/transporting layer for inverted type polymer solar cells (PSCs) with a structure of ITO/SAM treated ZnO/active layer (P3HT:PC61BM)/MoO3/Ag. The power conversion efficiency (PCE) of PSC based on MBA and BBA treated ZnO reaches at 3.34 and 2.94%, respectively, while the PCE of the device based on untreated ZnO is 2.47%. In contrary, the PCE of the device with FBA treated ZnO is 1.81%. The open circuit voltage (Voc) of the device with MBA, BBA, and FBA treated ZnO is 0.63 and 0.62 V, respectively, while the Voc of PSC with untreated ZnO is 0.60 V. Contrarily, the Voc of the device with FBA treated ZnO is 0.53 V. The PCE and Voc of PSCs based on MBA and BBA treated ZnO are better than those of the other devices. This seems to be related with the direction of dipole moment of benzoic acid derivatives. Also, morphology of active layer seems to be affected by the substituent on 4-position of benzoic acid. The active layer on MBA treated ZnO shows optimized morphology and its device shows the best performances. We demonstrate that the work function and morphology of active layer can be controlled by SAMs treatment of the ZnO surface with different dipole orientation and substituent on 4-position of benzoic acid. These are very simple and effective method for improving the performances of PSCs. The results provide an alternative strategy to improve the interface property between inorganic and organic materials in organic electronic devices.


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1. INTRODUCTION

Polymer solar cells (PSCs) based on π-conjugated polymers are considered as a energy source because they can be fabricated by cost effective, large area printing, and coating process on flexible substrate.1-4 The bulk heterojunction (BHJ) solar cells based on π-conjugated polymer and fullerene derivatives blend layer sandwiched between transparent conducting electrode and low work function metal electrode are effective structure of polymer solar cells.5-7 In the past few years, tremendous results have been reported to improve the performances of PSCs by the development of new materials8-11 and optimization of morphologies by processing methods.12-15 The photo-induced charge separation, transporting, collection properties are very important factors for influencing the performances of PSCs. Thus, the interfacial properties between the active layer and the cathode or anode are crucial factor for governing performances as well,16 because series resistance (Rs) of PSC is important parameter for performances of PSCs and determined by the electrical resistivity of each layer and the contact resistance between layers. The charge collections from active layer to each electrodes are one of the fundamental steps, which are strongly related with the contact resistance. A thin layer of poly(3,4ethylenedioxylenethiophene):poly(styrenesulfonic acid) (PEDOT:PSS)17 on ITO, crosslinkable arylamine derivatives18-22 on ITO, and self-assembled monolayers (SAMs) modified ITO23, 24 are mainly used for improving the interface properties between ITO and active layer. As for the cathode, introducing a thin layer of LiF25-27, poly(ethyleneoxide)28, water soluble π-conjugated polymers,29-31 alcohol soluble neutral conjugated polymers,32 water soluble non-conjugated polyelectrolyte based on viologen33 between active layer and metal cathode have been used for decreasing the work function of the cathode. These improve the device performances through the formation of favorable interface dipole across the junction. In addition, TiOx

34-36

and ZnO37-43


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prepared by the sol-gel process between active layer and metal cathode have been used for improving the device performances. The surface property of thin film of ZnO can be easily tuned by SAMs.44 By using this advantage, the contact properties between ZnO/cathode and ZnO/active layer can be modified by SAMs. The direction of interface dipole between ZnO/metal can be modified by the conjugated carboxylic acid derivatives.12 To control of contact properties between ZnO or TiO2/active layer in inverted type PSC (ITO/ZnO/active/MoO3/Ag), mixed SAM with a different molecular species were applied to ZnO surface,45 carboxylic acid derived from fullerene SAM was applied to ZnO surface,24 and various materials were applied to ZnO or TiO2 surfaces.46, 47 The optimum surface energy for the optimum BHJ morphology can be controlled by the mixed SAM without sacrificing work function of ZnO. The performances of inverted type PSCs are improved by insertion of ultrathin layers of TiO2 (< 3 nm) between ZnO and active layer. TiO2 layers act as barrier for electron collection and reduce recombination of electrons and holes at ZnO surfaces by the thin layer of TiO2.47 M. Bruening et. al. report that the surface potential of inorganic semiconductor such as CdTe and CdSe can be controlled by the adsorption of a series of 4-substituted benzoic acid derivatives on semiconductor surface. The surface potential is directly proportional to the effective work function. Also, the surface potential and work function of semiconductor are decreased by the strong electron donating substituent on benzoic acid.48-50 In this research, we introduce SAMs derived from benzoic acid (BA) such as 4methoxybenzoic acid (MBA), 4-tert-butybenzoic acid (BBA), and 4-fluorobenzoic acid (FBA), and between ZnO and active layer. We refer to MBA treated ZnO, BBA treated ZnO, and FBA treated ZnO as ZnO/MBA, ZnO/BBA, and ZnO/FBA, respectively. As illustrated in Figure 1, the work function of ZnO will be changed by the SAM treatment with different magnitude and


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direction of permanent dipole moment of BA derivatives. FBA, BBA, and MBA have different dipole moment, which are 1.55, -0.55, and -1.45 D, respectively.48-50 The performances of PSCs will depend on SAMs with different substituent on BA derivatives modified ZnO. Moreover, the morphology of active layer will be changed by the surface property of ZnO, which will be depend on the chemical structure of SAM molecules. Here, we report the photovoltaic and physical properties of the inverted type PSC with SAMs modified ZnO as electron injection/transporting layer.

Figure 1. Schematic illustration of polymer solar cells with SAM-modified ZnO and schematic energy level diagram of the devices with SAM-modified ZnO. For ZnO/FBA, interfacial dipole


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directed toward ZnO. As for ZnO/BBA and ZnO/MBA, interfacial dipole directed way from ZnO. There is no net interfacial dipole for ZnO without SAM. 2. EXPERIMENTAL Materials. 4-fluorobenzoic acid, 4-tert-butylbenzoic acid, 4-methoxybenzoic acid, zinc acetate dihydrate, triethanol amine, methoxyethanol were purchased from Alfa Aesar and used as received unless otherwise described. Regioregular poly(3-hexylthiophene) (P3HT) and (6,6)phenyl-C61-butyric acid methyl ester (PCBM) were purchased from Rieke Metals Inc. and nanoC Inc., respectively. Measurements. The thickness of film was measured by Alpha-Step IQ surface profiler (KLA-Tencor Co.). Elemental analysis of before and after SAM treatment by the benzoic acid derivatives was performed using (THERMO VG SCIENTIFIC (UK), MultiLab2000) an X-ray photoelectron spectroscopy (XPS) and recorded using Al Kα X-ray line (15 kV, 300 W). The surface energy (γ) of the ZnO layer before and after SAM treatment was evaluated by the measurements of the static advancing contact angle with deionized water and diiodomethane. The contact angle (KRUSS, Model DSA 100) were entered in the Wu model (harmonic mean) for the calculation of the dispersive and polar components of the surface energy. The effective work function was obtained by Kelvin probe (KP) measurements (McAllister Technical Services. KP 6500) of the contact potential difference between the sample and the KP tip. The KP tip work function was 5.203±0.011 eV. TEM images of the P3HT:PCBM active layer were obtained with a JEM-2010 using an accelerating voltage of 80 kV. Active layer was delaminated from the ITO substrate by dissolving ZnO layer in HCl solution. The typical thickness of delaminated films for TEM was ca. 200 nm. The AFM topography images were taken using a Digital Instruments (MultiModeTM SPM) operated in the tapping mode. The current density–voltage


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measurements under 1.0 sun (100 mW/cm2) condition from a 150 W Xe lamp with an 1.5G filter were performed using a KEITHLEY Model 2400 source-measure unit. A calibrated Si reference cell with a KG5 filter certified by National Institute of Advanced Industrial Science and Technology was used to confirm 1.0 sun condition. The incident photon to collected electron efficiency (IPCE), external quantum efficiency, was calculated by: λ IPCE % 1240 J / I where Jsc (µA/cm2) is the short circuit current density measured at the wavelength λ (nm) and IP (W/m2). Fabrication of PSCs. For fabrication of PSCs with a structure of ITO/before and after SAM treated ZnO/active layer/MO3/Ag, a layer of 40 nm-thick of ZnO film on pre-cleaned and UV/O3 treated ITO (sheet resistance = 13 ohm/square) was deposited by using the sol-gel process. The sol-gel solution was prepared with 0.164 g of zinc acetate dihydrate and 0.05 mL of ethanolamine dissolved in 1 mL of methoxyethanol. The solution was stirred for 30 min at 60 °C prior to deposition. Thin film of ZnO precursor was cured at 300 oC for 10 min to partly crystallize the ZnO film, which is prepared by the literature procedures.44,47 To deposit of selfassembled molecules, a 1.0 mg/mL solution of benzoic acid derivative in methanol was spincoated on the ZnO film at 4000 rpm for 60 s. To remove physically absorbed molecules, the SAM treated ZnO surface washed using pure methanol then dried by the stream of nitrogen. Active layer was spin-cast from the blend solution of P3HT/PCBM (20 mg of P3HT and 20 mg of PCBM were dissolved in 1 mL of o-dichlorobenzene (ODCB)) at 600 rpm for 40 s and dried in covered petri dish for 1 h. Prior to spin coating, the active solution was filtered through 0.45 µm membrane filter. The typical thickness of active layer was 200 nm. Before deposition of


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MoO3/Ag, active layer was thermally annealed at 150 oC for 20 min in the glove box (N2 atmosphere). Finally, 20 nm-thick of MoO3 and 100 nm-thick Ag were deposited successively onto the top of active layer through a shadow mask with a device area of 0.13 cm2 at 2 x 10-6 Torr. 3. RESULTS AND DISCUSSION Characterization of SAMs Modified ZnO. XPS (X-ray photoelectron spectroscopy) spectra were measured to confirm covering by SAM of ZnO surface. As shown in Figure 2, the oxygen peaks in XPS is asymmetric, indicating that there are two oxygen species in ZnO surface. The peaks at 542 and 543 eV are attributed oxygen in ZnO (Figure 2 (a)). The peak at 542 eV is due to oxygen in ZnO crystal lattice and the peak at 543 eV corresponds to chemisorbed oxygen caused by surface hydroxyl.51 As shown in Figure 2 (b) - (d), the position of peaks correspond to oxygen in SAMs modified ZnO are shifted to lower binding energy. This indicates that the ZnO surface is fully covered by BA derivatives.


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Figure 2. XPS spectra of (a) ZnO, (b) FBA treated ZnO, (c) BBA treated ZnO, and (d) MBA treated ZnO. For polymer solar cells (PSCs), charge injection barrier is very important factor for improving the device performances. As illustrated in Figure 1, the electron injection barrier can be modified by the formation of interface dipole between ZnO and active layer. The interface dipole is induced by the SAM molecule with permanent dipole moment. To confirm the formation of interface dipole by SAM treatment, we measure the effective work function of ZnO and SAMs treated ZnO surface by using a Kelvin Probe Microscopy (KPM). As shown in Figure 3, the effective work function of ZnO/FBA is 4.31 ± 0.04 eV, which is larger than that of ZnO


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(4.17 ± 0.01 eV). The effective work function of ZnO/BBA and ZnO/BBA are 3.97 ± 0.01, 3.94 ± 0.04 eV, which are smaller than that of untreated ZnO. The direction of interface dipole across the junction depends on the permanent dipole orientation of BA derivatives. The dipole orientation of FBA and BBA (or MBA) is the exact opposite direction. For FBA, interface dipole between ZnO and active layer is directed toward ZnO. As for BBA and MBA, interface dipole is directed away from ZnO. Therefore, the effective work function of ZnO treated with FBA is larger than that of ZnO. In contrary, the effective work function of BBA and MBA treated ZnO is smaller than that of untreated ZnO. It is known that the surface potential of inorganic semiconductor such as CdTe, CdInSe2, CdSe, and GaAs48-50 can be controlled by the adsorption of a series of 4-substituted benzoic acid derivatives on semiconductor surface. The surface potential difference is directly proportional to the effective work function difference. The change in the semiconductor surface potential varies linearly with the electron affinity of the substituent of the benzoic acid derivatives. The dipole moment of benzoic acid derivatives reflects the electron withdrawing and donating power of substituent. The strong electron donating power substituent on benzoic acid reduces the surface potential and work function of semiconductor. Very similar correlations are observed in SAMs modified ZnO with benzoic acid derivatives. From the KPM results, we confirm that direction of the formation of interface dipole and variation of the work function of ZnO illustrated in Figure 1 are reasonable. As shown in Figure 3, the surface energy reflects the substituent on benzoic acid derivative. The hydrophobic fluorine and tert-butyl substituent decreases the surface energy of ZnO surface.


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Figure 3. Effective work function (square) and the surface energy (triangle) of ZnO/FBA, ZnO, ZnO/BBA, and ZnO/MBA. Photovoltaic Properties. Figure 4 shows current density–voltage curves of inverted type PSCs under AM 1.5G simulated illumination with an intensity of 100mW/cm2 and under the dark condition. The photovoltaic parameters and efficiency of the best PSCs with various SAM treated ZnO are summarized in Table 1 and Figure 5. As shown in Table 1 and Figure 5 (a), the Voc data of the device based on ZnO/MBA is 0.63 V, which is higher than those of PSC with ZnO/BBA (0.62 V), untreated ZnO (0.61 V) and ZnO/FBA (0.53 V). This is because the effective work function of ZnO treated with MBA shows the smallest value than the others. As for ZnO/FBA, Voc is smaller than that of the device based on untreated ZnO. This is due to the


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formation of unfavorable interface dipole between ZnO and active layer. The power conversion efficiency (PCE) of PSC with ZnO/MBA reaches at 3.34%, which is higher than that of the device based on ZnO/FBA (1.81%), ZnO (2.49%), and ZnO/BBA (2.94%). As seen in Table 1 and Figure 5 (b), the Jsc and FF value of PSC with MBA/ZnO show higher than those of the other devices as well.


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Figure 4. Current density–voltage curves of inverted type PSCs (a) under AM 1.5G simulated illumination with an intensity of 100mW/cm2 and (b) under the dark condition. (square: FBA SAM modified, circle: without SAM, triangle: BBA SAM modified, inverted triangle: MBA SAM modified).


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Figure 5. (a) Voc (square) and PCE (triangle), (b) Jsc (square) and FF (triangle), and (c) Rs (square) and Rp (triangle) vs. with and without SAM treated ZnO, and (d) Voc vs. effective work function of SAM treated ZnO. As seen in Figure 5 (b) and Table 1, short circuit current (Jsc) data of the devices with ZnO/FBA, ZnO, ZnO/BBA, and ZnO/MBA are -7.55, -7.58, -7.88, and -8.77 mA/cm2, respectively. The device with ZnO/MBA and ZnO/BBA show better Jsc data than that of the device with untreated ZnO. In contrary, the Jsc data of PSC with ZnO/FBA is very comparable to that of the device based on untreated ZnO. The fill factor (FF) data of the device with ZnO/FBA is 45.1%, which is lower than that of the device based on untreated ZnO (51.5%). The FF of PSCs with ZnO/BBA and ZnO/MBA are 60.4 and 60.5%, respectively, which are higher than that of the device based on untreated ZnO. Brabec et. al. reported that the diode's ideality factor (n) and saturation current density (Jo) reflect the performances of PSCs as well. The diode’s ideality factor (n) reflects the density of donor/acceptor interfaces in which recombination processes take place. Therefore, n is representative of the morphology between the polymers and the fullerenes. The saturation current density (Jo) reflects the number of charges that can overcome the barriers under reverse bias. Therefore, Jo represents the minority charge density in the donor/acceptor interface of bulk heterojunction solar cells.52 As for PSC with FBA treated ZnO, the n and J0 shows highest values. This presumably due to that the best performances of PSC with MBA treated ZnO arises from the lower ideality factor and saturation current. However, there are still lots of debates about relationship between n, Jo and the performance of PSC. Figure 6 shows the incident photon to collected electron efficiency (IPCE) of the PSCs in this research, which are showed a maximum of IPCE at 540 nm. Among the devices, PSC with MBA treated ZnO shows highest valve of 65.1%, which is higher than that of PSC without SAM


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(62.9%), with FBA treated ZnO (60.9%), and BBA treated ZnO (64.7%). The IPCE results also strongly demonstrate how the photovoltaic parameters are related to the performances of PSCs with various SAM treated ZnO. The series resistance (Rs) and parallel resistance (Rp) of PSC are important parameters of PSCs. The Rs and Rp were calculated from the inverse slope near high current regime and slope near lower current region in the dark J–V curves (Figure 4 (b)).53 As shown in Figure 5 (c) and Table 1, the Rs of the device based on ZnO, ZnO/FBA, ZnO/BBA and ZnO/MBA are 13.7, 5.17, 3.15 and 2.82 Ω cm2, respectively. The Rs reduces in the devices with ZnO/BBA and ZnO/MBA, while Rs is increase in the device based on ZnO/FBA. The Rs of the device with ZnO/FBA is much higher than the other devices. Moreover, parallel resistance (Rp) of the PSC with FBA/ZnO is 1.03 kΩ cm2, which is much smaller than the PSC without SAM (2.45 kΩ cm2), with BBA treated ZnO (6.22 kΩ cm2), and with MBA treated ZnO (10.1 kΩ cm2). The Voc data in ZnO/FBA is much different from the device with untreated ZnO as change of the effective work function of FBA treated ZnO. Whereas, the Voc data of PSC with BBA and MBA treated ZnO are not much different from the device with untreated ZnO regardless of sharp change in the effective work function of ZnO compared to those of ZnO/BBA and ZnO/MBA. The highly drop of Voc data in ZnO/FBA compared to that of the device with untreated ZnO seems to be attributed to the highest Rs and lowest Rp value of the device with ZnO/FBA.54 This is due to the formation of unfavorable interface dipole between ZnO and active layer in FBA treated ZnO. FBA SAM with ZnO forms unfavorable dipole across the ZnO and active layer results in Schottky contact and shows poor device performances. In contrary, BBA and MBA treated ZnO have favorable dipole across the ZnO and active layer and generate better contact so that the devices show better performances.


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Table 1. The best photovoltaic parameters and efficiencies of PSC with various SAM treated ZnO. The averages for photovoltaic parameters of each device are given in parentheses with mean variation.

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.53

-7.55

45.1

1.81

(0.55 ± 0.01)

(-7.23 ± 0.15)

(45.0 ± 0.43)

(1.78 ± 0.03)

0.60

-7.83

53.0

2.49

(0.61 ± 0.01)

(-7.80 ± 0.07)

(51.6 ± 1.14)

(2.44 ± 0.07)

0.62

-7.88

60.4

2.94

(0.62 ± 0.004)

(-8.11 ± 0.17)

(57.6 ± 1.40)

(2.88 ± 0.05)

0.63

-8.77

60.5

3.34

(0.62 ± 0.004)

(-8.51 ± 0.13)

(59.8 ± 0.82)

(3.16 ± 0.06)

FBA

No SAM

BBA

MBA

Rs (Ω•cm2)a

Rp (kΩ•cm2)b

nc

J0 (µA/cm2)d

13.7

1.03

2.37

0.93

5.17

2.45

2.22

0.091

3.15

6.22

1.77

0.033

2.82

10.1

1.66

0.059

a: series resistance (estimated from the device with best PCE value). b: parallel resistance (estimated from the device with best PCE value). c: ideality factor (estimated from the device with best PCE value). d: saturation current density (estimated from the device with best PCE value).


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Figure 6. IPCE spectra of PSCs with SAMs modified ZnO. Morphology of the Active Layer. We confirm that the Voc data are vary by the direction of interface dipole, which is easily tuned by the SAM molecules with different dipole moment. However, the variation of Jsc and FF data are not explained by the change of interface dipole. Among the photovoltaic parameters, the PCE of PSC with ZnO/MBA is significantly higher than that of the device with untreated ZnO regardless of small change in Voc data of PSC with ZnO/MBA compared to that of PSC with untreated ZnO. This is due to the big change of Jsc and FF value, which are strongly related with morphological property of active layer.55 For efficient charge separation and transporting in PSC, it should have phases of P3HT and PCBM in the order of 10-20 nm.1,56 To obtain morphology of active layer, transmission microscopy (TEM)


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and atomic force microscopy (AFM) were taken to investigate the morphology of active layer on SAMs treated ZnO. Figure 7 show TEM images of active layers. Active layer was delaminated from the ITO substrate by dissolving ZnO layer in HCl solution. The bright region in TEM images (Figure 7) indicate P3HT-rich local phase. Figure 7 (a) shows very good interpenetrating network and P3HT:PCBM phase separated morphology. However, the size of PCBM aggregates is 80 - 130 nm and TEM image shows very big size of P3HT domains. The maximum size of PCBM aggregate is very close to the thickness of active layer (~ 200 nm). The r.m.s. roughness of active layer on ZnO/FBA (13.39 nm) (Figure 8 (a)) also supports that a FF of the device with ZnO/FBA is much lower than that of the device with untreated ZnO. Figure 7 (c) and (d) show TEM images of active layer on ZnO/BBA and ZnO/MBA, respectively. The size of PCBM aggregates of ZnO/BBA and ZnO/MBA are 30 - 40 nm and 10 - 20 nm, respectively, which are smaller than those of the active layer on untreated ZnO (40 - 60 nm) and FBA treated ZnO. Moreover, the boundaries between PCBM aggregates and P3HT domains of active layers on ZnO/BBA and ZnO/MBA are more sharper than those of the active layer on untreated ZnO. This indicates that the FF data of ZnO/BBA and ZnO/MBA are much higher than those of the other devices. The distribution of PCBM aggregates on ZnO/MBA are more uniform than the case of ZnO/BBA. Moreover, the size of PCBM aggregates on ZnO/MBA is very close to the optimum condition, indicating that Jsc and FF of the device with ZnO/MBA is significantly improved than the other devices. Even though, TEM images do not provide exact information about vertically phase separated structures across both electrodes, we confirm that ZnO/MBA exhibit optimized phase separated morphology among the devices by TEM images. As shown in Figure 8, the r.m.s. roughness of the active layer on ZnO/MBA is 4.24 nm, which is not much different from the r.m.s. roughness data of active layer on ZnO (2.89 nm) and ZnO/BBA (1.62 nm). The


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performances of the device seems to be unaffected by the surface energy of SAM modified ZnO. However, the morphology of the active layer seems to be affected by the substituent on 4position of benzoic acid. The effective work function data and morphological changes of active layer strongly support that the device based on ZnO/MBA exhibit best performances.

Figure 7. TEM images of active layer deposited on (a) ZnO/FBA, (b) ZnO, (c) ZnO/BBA, and (d) ZnO/MBA.


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Figure 8. AFM topography images of thermally annealed P3HT/PCBM film on (a) ZnO/FBA, (b) ZnO, (c) ZnO/BBA, and (d) ZnO/MBA. (x, y = 1 µm/div., z = 100 nm/div.)

4. CONCLUSION We have fabricated inverted polymer solar cells with a series of benzoic acid derivatives SAMs treated ZnO as electron injection/transporting layer. The work function and surface property of ZnO can be successfully tuned by the interfacial modification with a series of benzoic acid derivatives SAMs treatment. The work function of ZnO depends on the orientation of dipole moment of SAM molecules. Also, we have observed the substituent on 4-position of benzoic acid affect the morphology of active layer. The performances of inverted type polymer


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solar cells can be improved by the appropriate choice of SAM molecule. Our results in this paper provide an alternative strategy to improve the performances of PSCs by the control of interface property between inorganic and organic materials in polymer solar cells.

AUTHOR INFORMATION Corresponding Author * [email protected] ACKNOWLEDGMENT This research was supported by Converging Research Center Program through the Ministry of Education, Science and Technology (2012K001279) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012-0001356).

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Inverted Type Polymer Solar Cells with Self ... - ACS Publications

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