Efficient Organic Photovoltaics with Improved Charge Extraction and

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Efficient Organic Photovoltaics with Improved Charge Extraction and High Short Circuit Current Minu Mohan, Vikas Nandal, Sanish Paramadam, Kasala Prabhakar Reddy, Sekar Ramkumar, Sumanshu Agarwal, Chinnakonda S. Gopinath, Pradeep R Nair, and Manoj AG Namboothiry J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01314 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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The Journal of Physical Chemistry

Efficient Organic Photovoltaics with Improved Charge Extraction and High Short Circuit Current Minu Mohan,a Vikas Nandal,b Sanish Paramadam,a Kasala Prabhakar Reddy,d Sekar Ramkumar,a Sumanshu Agarwal,c Chinnakonda S Gopinath,d Pradeep R Nair,b and Manoj A G Namboothiry*a a School of Physics, Indian Institute of Science Education and Research (IISER TVM) CET campus, Engineering College P. O., Thiruvananthapuram, Kerala 695016, India. b Department of Electrical Engineering, Indian Institute of Technology, Bombay Powai, Mumbai 400076, India c.Department of Energy Science and Engineering, Indian Institute of Technology, Bombay Powai, Mumbai 400076, India. d.Catalysis Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India

ACS Paragon Plus Environment

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ABSTRACT

Exciton generation, dissociation, free carrier transport and charge extraction play an important role in the short circuit current (Jsc) and power conversion efficiency of an organic bulk heterojunction (BHJ) solar cell (SC). Here we study the impact of band offset at the interfacial layer and morphology of active layer on the extraction of free carriers. The effects are evaluated on inverted BHJ SC using Zinc oxide (ZnO) as a buffer layer, prepared via two different methods-ZnO nanoparticle dispersed in mixed solvents (ZnO A) and sol gel method (ZnO B). Device with ZnO A buffer layer improves the charge extraction and Jsc. The improvement is due to the better band offset and morphology of the blend near to the ZnO A/Active layer interface. Further, the numerical analysis of current-voltage characteristics illustrates the morphology at the ZnO A/Active layer interface has a dominant role in improving OPV performance than the band offset.


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1. INTRODUCTION The efficiency of organic BHJ SC depends on factors such as light absorption, exciton creation, diffusion and dissociation, free carrier transport and transfer to the external circuit through electrodes.1 Even though there are reports of 100% internal quantum efficiency in terms of absorbed photons and exciton generation, the external quantum efficiency resulting in the short circuit current is limited by free carrier transport and its transfer to their respective electrodes.2 Organic photovoltaics (OPVs) with power conversion.3-4 efficiency (PCE) above 10% were reported during the last decade but the Jsc were not very high. It has been mostly attributed to the limited extraction of the free carrier from the active layer to the external circuit due to the band offset at the interfacial layers and morphology of the active layer.5-6 Doped buffer layers and additional interfacial layers were used to address the band offset whereas, high boiling point additives were added to the active layer to control the morphology.7-8 But the above-mentioned efforts were not effective in making a large improvement in the extraction efficiency and Jsc. Here, we address the effect of these parameters by fabricating OPVs in the inverted structure using ZnO buffer layer. The contribution of band offset at the interfacial layers and the active layer morphology towards high-efficiency solar cell are discussed in detail. ZnO is one of the preferred electron transporting layer (ETL) due to its attractive properties like high electron mobility (~0.066 cm2V-1s-1),9 better environmental stability,10 non-toxicity, and low work function.11 For large-area coating, solution-processed ZnO is preferred over other deposition techniques like atomic layer deposition,12 chemical vapor deposition,13 sputtering,14 etc. due to its ease of fabrication. Here we have studied, in detail, the OPV performance of inverted devices using two types of ZnO buffer layer- one processed using nanoparticle


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dispersed in mixed solvents (ZnO A) and another via high-temperature sol-gel method (ZnO B). A mixed solvent with different boiling points was used to prepare ZnO thin films from NP dispersion, as it has been reported that the solvents play an important role in the quality of the ZnO film.15 Poly (3-hexylthiophene-2,5-diyl) (P3HT) blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) and polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) blended with PC71BM were used as active layers. The OPVs fabricated using ZnO A shows an enhanced free carrier extraction as observed in external quantum efficiency and saturated photocurrent measurements based on Onsager model compared to that made using ZnO B as ETL. Optimized PTB7:PC71BM and P3HT:PC71BM BHJ solar cells processed in ambient conditions showed the highest PCE of 9.1 % (Jsc~19.50 mAcm-2) and 5.6 % (Jsc~14.70 mAcm-2) respectively. To the best of our knowledge, these are the highest PCE and Jsc value reported for such solar cells using undoped ZnO as ETL.16-17

2. EXPERIMENTAL METHODS Synthesis of Zinc oxide (ZnO A) Nanoparticles. ZnO nanoparticles were prepared according to the literature procedure.18 Zinc acetate dihydrate (Zn(CH3COO)2•2H2O, Aldrich, 99.999%, 2.95 g) and methanol (125 ml) were mixed in a two-necked flask connected with a water condenser and stirred at 65oC. A solution of KOH (1.48 g) in methanol (65 ml) was then added drop wise at 60–65oC over a period of 10-15 minutes. After heating the solution at 65oC for 2.5 hours, it was kept overnight for precipitation. The obtained precipitate was washed twice with methanol (20 ml). To apply as an electron transport layer, the precipitate was dispersed in a mixture of n-butanol (70 ml), methanol (5 ml) and chloroform (5 ml).


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Synthesis of Zinc oxide (ZnO B) Sol gel. Zinc acetate dihydrate solution of 0.1M concentration was prepared in ethanol and vigorously stirred for 2-3 hours at 80oC. An ethanolamine stabilizer (28% by weight )was added, and the solution was continued stirring for further 12-15 hours at 60oC.19 Solar cell fabrication. Devices were fabricated on cleaned indium tin oxide (ITO) coated glass plates (10 Ω/cm2, Delta Technologies Inc, USA). ITO coated glass plates were cleaned by ultrasonicating in a bath with deionized water, acetone, and isopropanol sequentially for 30 minutes each and was kept in the vacuum oven for drying. Before spin coating, ITO plates were exposed to UV ozone for 30 min. A 30 nm thick ZnO A was spin-coated on top of the ITO substrate by spin coating. For ZnO B layer formation, spin coating was followed by an annealing at 200oC for 1 hour. For P3HT:PC71BM device, solution containing a mixture of poly(3hexylthiophene-2,5-diyl):[6,6]-phenyl

C71

butyric

acid

methyl

ester

(P3HT:PC71BM)

(concentration:32mg/ml, blend ratio:1:0.6) in chlorobenzene was spin-cast on top of ZnO layer and annealed at 155oC for 5 minutes to get a thickness of ~120 nm. For PTB7:PC71BM device, polythieno[3,4-b]-thiophene/benzodithiophene (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) are mixed in the ratio 1:1.5 (33 mg/ml) in chlorobenzene and diiodo octane (3%) and spin cast on top of ZnO layer. The thicknesses were optimized for best performance and were measured using a Dektak 6M stylus profilometer with an accuracy of ~ ±5 nm. Then 5nm molybdenum trioxide (MoO3, Sigma Aldrich) and 100 nm Ag (99.99% purity, Alfa Aesar) were deposited on top by thermal evaporation at a pressure of ~10-6 torr. The cross-sectional area of each cell is 9 mm2, and all the measurements were done in ambient conditions. Solar cell characterization. The current–voltage characteristics of the devices were performed using a Keithley 6430 source meter in the dark and under the illumination of a 1000


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W m2 AM1.5G spectrum using an Oriel 3A solar simulator tested with an NREL calibrated silicon solar cell. The external quantum efficiency (EQE) measurements were carried out by a lock-in technique using a 250 W xenon lamp coupled to a Newport monochromatic and chopped at 40 Hz using a light chopper blade as a light source. A lock-in amplifier (SRS 830, Stanford Research Systems Inc USA) was used to measure currents, and an NIST calibrated silicon photodiode was used to find the power spectral response of the incident light. The measurements were performed using shadow masks to avoid edge effects, and an appropriate mismatch factor was taken to square off the spectral mismatch for calculating the PCE and EQE.20-23 Material and Thin Film characterization. Powder X-Ray diffraction (XRD) pattern was measured on X-Pert Pro PANalytical diffractometer.

Morphology and microstructure

characterizations were performed by Field emission scanning electron microscopy (Nova NanoSEM NPE 206 high resolution SEM). UV-Vis absorption spectra were acquired using SHIMADZU UV-3600 Spectrophotometer. Atomic force microscopy (AFM) measurements were done using Veeco NanoScope V in tapping mode. Ultra-violet photoelectron spectroscopy (UVPES) measurements have been made with R3000HP (VG Scienta) analyzer at a pass energy of 50 eV for XPS and 5 eV for UVPES.24-25

3. RESULTS AND DISCUSSION The valence band edge of the ZnO layers is evaluated by probing the electronic structure using photoelectron spectroscopy (PES). ZnO coated on ITO substrates are used to evaluate the valence band (VB) (Figure 1a) and core level (Figure 1b and 1c) details. PES results are shown in Figure 1 for ZnO B, and ZnO A thin films. Both results shows similar VB features (Figure 1a), derived from O 2p orbitals and fully-filled shallow Zn 3d10 level.26 Although the features


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look similar, the results indicate several finite differences and the most important feature is the difference in VB edge energy. ZnO B exhibits a VB shift of about 0.4 eV towards lower binding energy (BE) or fermi energy (EF) than ZnO A. VB maximum is observed at 7.75 eV and 7.3eV for ZnO A and ZnO B, respectively. Fairly well-developed valley found between VB and Zn 3d levels for ZnO B denotes more energy separation between them; which is not entirely correct for ZnO A. A distinct feature observed at BE = 3 eV for ZnO B indicates the creation of a new electronic state, which is likely due to electron rich oxygen vacancy defects.27 Features observed above 11 eV is dominated by secondary electrons caused by inelastic scattering and does not give reliable information. However, work function measured for both ZnO materials turns out to be the same, at 4.8 eV.


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Figure 1. (a) Ultra violet photoelectron valence band spectrum, (b) Zn 2p3/2, and (c) O 1s core level x-ray photoelectron spectra of ZnO thin films coated on ITO substrate.

Electronic properties obtained from ultraviolet (UV)PES and UV-Vis spectral (analyzed using Tauc plot) data is compared in Table S1. A distinct difference of about 0.4 eV is observed in UVPES as well as in the absorption data, which confirms the shift of VB edge by 0.4 eV in the case of ZnO B than ZnO A. Core level spectra recorded for Zn 2p and O 1s do not show significant changes in the BE for ZnO prepared by two different methods. Zn 2p3/2 and O 1s core level appear at ~1021.8 and ~530 eV (Figure 1b and 1c), respectively, which is typical for ZnO.28 However, corollary information obtained from UVPES and XPS is that the surfaces are Zn-terminated in both cases. O 2p and Zn 3d shows comparable intensity, in spite of three times higher photoionization cross section (σ) of O 2p (10.67 Mb) than Zn 3d (3.57 Mb) with 21.2 eV photons.29 Even though σ is low for Zn 3d, comparable intensity suggests three times higher Zncontent on the surface than O. The argument is further supported by Zn 2p and O 1s core level results, which show very high intensity for the former. The lower energy gap between O 2p VB and Zn 3d level observed in Figure 1 and high surface content of Zn from the above calculation indicate the significant role of Zn 3d also in the interfacial properties. Hence the observed VB offset of 0.4 eV of ZnO A below that of ZnO B facilitate two processes- (1)The hole blocking property of ZnO A/Active layer interface and (2) better extraction of electrons to ITO through ZnO A/Active layer interface. Another factor which plays an important role is the morphology of active layer on top of the ETL and is analyzed using AFM studies. Figure 2 shows the AFM images of the blend (PTB7:PC71BM) on top of the ETL. The phase distribution of thin (~10 nm) active layer


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(PTB7:PC71BM) on top of ZnO A (Figure 2a, c) is more homogeneous compared to the thin layer above ZnO B (Figure 2b,d). AFM image shows that a better blend ordering has taken place at the active layer/ZnO A interface. But the AFM of thick active layer (~120 nm) shows comparable phase distribution of the blend on top of ZnO A and ZnO B (Figure S1). It has been reported that an ordered active layer improve its absorption.30 Hence the enhanced UV-Vis absorption of the active layer grown on ZnO A (as shown in Figure S2) can be attributed to the ordering of active layer confirming AFM analysis. The thickness of ZnO and the active layer are ~30 nm and ~120 nm respectively.


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Figure 2. AFM phase image of 10 nm thick PTB7:PC71BM film on (a, c) ITO/ZnO A and (b, d) ITO/ZnO B respectively. The scanning area is 2µm x 2µm. Here we use ZnO A and ZnO B to study the effect of band offset at the ZnO/Active layer interface and morphology of active layer on the performance of BHJ OPVs. Device structures such as ITO/ZnO A/PTB7:PC71BM or P3HT:PC71BM/MoO3/Ag and ITO/ZnO B/PTB7:PC71BM or P3HT:PC71BM/MoO3/Ag are fabricated to study the above-mentioned effects. Inverted BHJ structure has been used as a standard device architecture, as it is known to have higher stability in air processing.31 The chemical structure of donor polymers and PC71BM acceptor, the schematic energy level diagram of P3HT:PC71BM and PTB7:PC71BM devices are depicted in Figure S3. ZnO layer and active layer are optimized by measuring photovoltaic properties of the devices for various thickness of a particular layer, keeping the thickness of other layers constant. ZnO A and ZnO B layer thickness optimization data are shown in Figure 3a. Optimized thickness of ZnO layer is ~30 nm as the PCE and Jsc shows a peak value at this thickness. The active layer thickness is also optimized (~120 nm) keeping the ZnO layer thickness at 30 nm. The photovoltaic performance of the devices made using ZnO A and ZnO B are evaluated by current-voltage (I-V) characteristics of the devices under simulated 1 Sun AM 1.5G irradiation (Figure 3b). Devices made with ZnO A as a buffer layer (DEVICE A) shows the best performance and is as shown in Table 1. To check the consistency of the device, we have fabricated about 50 of each device under same conditions. Histograms of the cell performance characteristics are shown in Figure S4. Table 1 gives the comparison of photovoltaic parameters of all the best devices. Even though P3HT:PC71BM and PTB7:PC71BM devices show improved Jsc for both ZnO A and ZnO B buffer layers, DEVICE A shows an increased Jsc than devices


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made with ZnO B buffer layer (DEVICE B). One of the reasons for such a difference can be due to the band offset or the defects/traps of the ZnO buffer layer affecting the extraction of free carriers. The influence of defects/traps of the ZnO buffer layer is studied using photoluminescence (PL) spectroscopy of ZnO layer. The PL spectra of both ZnO A and ZnO B are shown in Figure 3c. The PL spectra of ZnO shows the effect of size, shape, preparation method and temperature.32 The intense peak at UV region refers to near band edge emission and the broad peak at the visible region is due to deep level or trap state emission.32-33 ZnO A has a less intense visible peak than ZnO B, where the green emission is due to the radiative recombination of the electron in the oxygen vacancy and the photogenerated hole.34 Thus ZnO A has less radiative recombination as compared to ZnO B which can give rise to enhanced Jsc for DEVICE A.

Figure 3. (a) Variation of PCE and Jsc with respect to ZnO A and ZnO B thickness (b) I-V characteristics with current density plotted in y-axis (c) Photoluminescence spectra of ZnO A


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and ZnO B on glass plate (d) The external quantum efficiency of ZnO A and ZnO B buffer layered PTB7:PC71BM and P3HT:PC71BM inverted device.

Table 1. Photovoltaic parameters of PTB7:PC71BM and P3HT:PC71BM inverted device with ZnO A and ZnO B as electron transporting.

Voc (V)

Device

ZnO A ZnO B

Jsc (mAcm2 )

FF (%)

Rs Rsh PCE (%) (Ωcm2 (kΩcm2) )

Theoretical Jsc (mAcm2 )

PTB7:PC71BM

0.731 19.50

63.50 9.1 ±0.4

19.42

9.05

19.25

P3HT:PC71BM

0.635 14.70

60.59 5.6±0.2

6.75

40.92

13.26

PTB7:PC71BM

0.721 18.30

49.40 6.5±0.3

21.15

1.80

17.80

P3HT:PC71BM

0.570 13.20

54.70 4.1±0.1

19.57

3.64

12.73

Further, the origin of improved Jsc has been evaluated using external quantum efficiency (EQE) measurements. EQE spectra of the devices nearly follow the UV-Vis absorption spectra of the thin films of PTB7:PC71BM and P3HT:PC71BM blend (as shown in Figure S5a). DEVICE A shows an increased EQE compared to DEVICE B (Figure 3d). ZnO A buffer layered P3HT:PC71BM device has an EQE of 90.70% at wavelength ~500nm and that for PTB7:PC71BM device shows a maximum of 87.07% EQE at wavelength ~470nm. The ZnO B devices attain a peak EQE of 86.81% and 77.87% for P3HT:PC71BM and PTB7:PC71BM devices respectively. EQE shows less photocurrent below 350 nm which rules out the role of ZnO in carrier photogeneration as the absorption of ZnO is below 350 nm, which is evident from Figure S5b.35 P3HT:PC71BM device shows much EQE enhancement in the wavelength region 600 nm to 720 nm compared to the low light absorption of the P3HT:PC71BM blend at the absorption edge. It is


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to be noted that P3HT and PTB7 have an absorption coefficient of 105cm-1 and PC71BM has a value of 104 cm-1.36-38 In the wavelength region 600–720 nm, as the absorption coefficient is low, light get absorbed into the blend and the penetration depth extends to the blend/MoO3 interface. The higher EQE values in the absorption edge region can be due to better exciton dissociation followed by an efficient electron transport to the ITO through the blend and ZnO buffer layer. Similarly, for the PTB7:PC71BM device, EQE has a broad spectrum ranging from wavelength ~335 nm to ~800 nm with a nearly constant response from wavelength ~350 to ~680 nm. Here also, the devices show improved EQE numbers for light with higher penetration depth reaching the MoO3/Active layer interface and light with lower penetration depth reaching ZnO/active layer interface. This can be due to improved transport of free carriers through the bulk without much recombination. The extended EQE edge to wavelength ~720 nm can also be attributed to the charge transport complexes (CTCs) formed at the interface of polymer and PC71BM.39 The observed enhancement in EQE of PTB7:PC71BM and P3HT:PC71BM can be due to an ordered active layer grown on ZnO A which helps in better free carrier transport and extraction. Such a possibility is further evidenced by our electron mobility studies on active layers grown on both ZnO surfaces. One of the limiting factors of the improved efficiency of an OPV BHJ is the electron transport through the bulk as it depends on the ordered arrangement of PC71BM in the polymer matrix. An ordered structure thus should be able to improve the electron mobility. Here we use the space charge limited current (SCLC) method to determine the improvement in electron mobility. For this purpose, electron-only devices with structure ITO/ETL/active layer/ETL/Al are fabricated. PTB7:PC71BM and P3HT:PC71BM are used as active layers. ZnO A and ZnO B are used as ETL. SCLC studies are limited to electron-only devices as we are interested in understanding the


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possible improvement in free carrier transport of the active layer grown on two different ZnO surfaces. SCLC uses the Mott-Gurney square law,40

=

(1)

where is the dielectric constant of the material; is the permittivity of free space; L is the distance between the cathode and anode, and V is the applied voltage. Mobility was obtained from the - curve (Figure S6). The electron mobility of P3HT:PC71BM device with ZnO A buffer layer is obtained as 2.52 x 10-3 cm2V-1s-1 and 1.61 x 10-3 cm2V-1s-1 with ZnO B. Similarly, for PTB7:PC71BM device the electron mobilities are 7.08 x 10-4 cm2V-1s-1 and 4.4 x 10-4 cm2V-1s-1 for ZnO A and ZnO B buffer layers respectively. The enhancement in electron mobility for DEVICE A confirms a better charge extraction through the active layer/ZnO A interface and can be correlated to the improved active layer ordering as observed in AFM (Figure 2) and UV-Vis studies (Figure S2). This inference supports the observed enhancement in EQE as mentioned previously. The theoretical Jsc calculated from the EQE spectra of P3HT:PC71BM and PTB7:PC71BM device are in good agreement with the experimental values as is evident from Table 1.20 The improved EQE is further evaluated using generation rate and dissociation probability. The underlying process responsible for the enhanced performance of the DEVICE A is evaluated using the maximum exciton generation rate (Gmax) and exciton dissociation probability (P(T, E)) studies based on Onsager model.41 The photocurrent under AM 1.5G 1 sun illumination from DEVICE A and DEVICE B are measured by applying a reverse voltage sweep from 1 to 10 V.42 Figure S7 shows the dependence of the photocurrent density (Jph) on the effective voltage (V0-V) under illumination for P3HT:PC71BM device with ZnO A buffer layer. Here,


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= −

(2)

where JL and JD are the current densities under illumination and dark respectively, = −

(3)

where V0 is the compensation voltage at which Jph=0 and Va is the applied voltage. Saturated photocurrent density (Jsat) is determined from the Figure S7 and S8. On the assumption that there is no recombination, the values of Gmax is obtained using the equation42 =

(4)

where q is the electronic charge and L is the active layer thickness. The values of Gmax for the ZnO A/ P3HT:PC71BM and ZnO B/P3HT:PC71BM devices are obtained as 9.11 x 1027 m-3 s-1 and 7.81 x 1027m-3s-1 respectively. Similarly, for PTB7:PC71BM devices, Gmax are obtained as 1.32 x 1028 m-3 s-1 and 7.02 x 1027m-3s-1 for DEVICE A and DEVICE B respectively (Figure S7). Even though Gmax is a measure of the maximum number of photons absorbed, all photogenerated exciton will not dissociate into free charge carriers, but only a fraction of Gmax dissociates. The generation rate G(T, E) of free charge carriers is given by42 , ! = ", !

(5)

where P(T, E) is the exciton dissociation probability. The values of P(T, E) under short circuit condition (Va = 0) are obtained from the plot of P(T, E) vs Veff, and is found to be 90.3% for P3HT:PC71BM and 97.8% for PTB7:PC71M devices made on ZnO A (Figure S7 and S8). P(T, E) values for DEVICE B is found to be 89.3% and 96.7 % for P3HT:PC71BM and PTB7:PC71BM devices respectively (Figure S7 and S8). A comparison is made in Table S2 and S3 of the Jsc, Voc, PCE, FF, Gmax and P(T, E) values with the currently available devices of similar structure. Fill factor (FF) is another important parameter that affects the PCE of photovoltaic devices. FF reflects the charge transport and charge extraction efficiency of the device.43-44 Further, FF is affected by series resistance (Rs) and shunt resistance (Rsh) of the photovoltaic device.

45

Very


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high Rsh and very low Rs are ideal for improved FF values. It is observed from the I-V characteristics that DEVICE A has higher FF than DEVICE B. Better free carrier extraction observed in DEVICE A is one of the reasons for such an improvement.44 The FF enhancement can be further attributed to the low Rs value and high Rsh of DEVICE A compared to DEVICE B (Table 1). The low Rs value indicates better charge collection and large Rsh value shows a controlled recombination loss and reduced leakage current. 46 The improved Rs and Rsh values of DEVICE A can be attributed to the morphology of ZnO A layer which results in enhanced FF and Jsc values.47 Morphology of ZnO layer decides the contact quality between the buffer layer and active layer. This is studied using scanning electron microscopy (SEM) and AFM.48


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Figure 4. SEM image of (a) ZnO A thin film (b) ZnO B thin film. (c) AFM (height) image of ZnO A and (d) ZnO B.

Figure 4a illustrates the SEM image of ZnO A film coated on glass substrate. SEM image shows a uniform distribution of spherical nanoparticles throughout the substrate area with a grain size of ~30 nm. The SEM image of ZnO B (Figure 4b) exhibits a non-uniform layer with grains forming clusters. The AFM image of ZnO A film (Figure 4c ) reveals that the surface has a uniform distribution with a root mean square (rms) roughness of 2.44 nm. Similarly, the AFM image of ZnO B (Figure 4d) shows a rms roughness of 3.05 nm. Hence the improvement in Rs and Rsh can be attributed to the relative smooth surface of ZnO A which also helps in better exciton generation and separation as discussed earlier.49 The dense and homogeneous layer of ZnO A, as seen in the phase image (Figure S9a), is assumed to improve the contact between the buffer layer and the active layer.50 But the phase image of ZnO B does not show such homogeneity and hence results in poor contact between the layers (Figure S9b). The crystallinity of buffer layers also plays a significant role in tuning the Rs and Rsh values and hence in the improvement of FF.51 This is analyzed using X Ray diffraction (XRD) studies. The XRD patterns of both the ZnO thin films (shown in Figure S10a and b) are in good agreement with the JCPDS Data Base.52 There are three distinct peaks at 31.7o, 34.4o, and 36.3o corresponding to the planes (100), (002) and (101) respectively, which confirm the formation of polycrystalline structure. ZnO A is more crystalline than ZnO B which is evident from the sharp, narrow and intense diffraction peaks of ZnO (Figure S10a-b).53 This is further evidenced by the improved conductivity of ZnO A using four-probe measurements. It is observed that ZnO A shows a better


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conductivity (1.78 x 10-6 S cm-1) compared to ZnO B (4.76 x 10-7 S cm-1) and can be due to the improved crystallinity of ZnO A as discussed above. Even though experimental measurements and analysis illustrate that band offset at the ZnO/Active layer interface and morphology of the active layer play important roles in the improved photovoltaic performance, it will be interesting to understand which factor dominates in influencing the OPV performance. Dark and light I-V characteristics of DEVICE A and DEVICE B are further analyzed using numerical simulation to understand the above-mentioned factors. It is evident from the light I-V characteristics (Figure 3b) that the FF significantly improves for DEVICE A. Classical literature on p-i-n solar cells and our previous results indicate that FF of such devices could depend on parameters like the mobility and lifetime of carriers in the active layer.54-55 Further, our results indicate that the electronic structure of ZnO A and ZnO B are different (change in the conduction band and valence band levels, see Figure 1). This could create a barrier in terms of carrier injection to the active layer and hence could influence the device characteristics in a non-intuitive way. To obtain additional insights on device physics, the dark I-V characteristics of these devices are provided in Figure S11. The dark I-V characteristics imply that the parasitic shunt effects are greatly reduced in DEVICE A as compared to DEVICE B. In addition, the reverse saturation current significantly reduces for DEVICE A. Hence, in general, the device performance can be summarized as follows: DEVICE A shows better FF, with improved dark I-V characteristics as compared to DEVICE B and the improved performance could be due to combination of the critical parameters like carrier mobility, lifetime and band offset.


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To explore device physics in detail, we performed numerical simulations of a three-layer heterostructure device (see Figure S3b for schematic), where active layer (P3HT:PC71BM) is sandwiched between contact buffer layers (ETL and hole transporting layer (HTL)). It is assumed that the metal contacts are ohmic in nature. HTL is highly doped p-type material whereas n-type doping concentration of ETL is adjusted in such a way that it can account for series resistance effect in the high bias regime. A self-consistent solution is achieved at each voltage bias step by solving Poisson’s, and continuity equations for electrons and holes.


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Figure 5. Simulation results to determine the relative contribution of various factors like carrier mobility (#), carrier lifetime ($), and conduction band offset (%&' , at the interface of ETL and Active layer P3HT:PC71BM) on the Current-Voltage characteristics. The left panel shows the dark characteristics while the right panel shows the results under illumination. For (a) and (b) # is varied with $ = 10*+ , and %&' = 0.2 /0. For (c) and (d) $ is varied with # = 10*1 231 /0, and %&' = 0.2 /0, while for (e) and (f) %&' is varied with # = 10*1 231 /0, and $ = 10*+ ,. The simulations results indicate an improvement in the quality of the active blend (i.e., # and $) is the dominant factor that results in the experimental observation of simultaneous improvement in dark and light I-V characteristics. Material parameters like mobility µ, life time τ of charge carriers and band offset δEC (at the interface of active and buffer layers) could play a significant role in determining charge transport. Effect of each material parameter stated above is observed on dark and illuminated I-V characteristics by keeping the remaining two at a fixed value. Figure 5 shows the simulated I-V characteristics of the modelled system. It is observed that an increase in carrier mobility or lifetime can result in better dark characteristics and better FF (see Figure 5 a-d, also note that lifetime improves the VOC as well) – which are in accordance with our experimental observations. Interestingly, however, an increase in band offsets typically results in better dark characteristics but reduced FF (see Figure 5 e-f). But the experimental results show improvement in both dark I-V and FF. Therefore, we can infer that the performance improvement with ZnO A as ETL layer is due to improved quality of active blend as mentioned in EQE analysis and observed in AFM images (improvement in µτ product). This improvement is a significant effect to counter any ill effects introduced by band offsets. The results shown in Figure 5 can be understood in terms of the


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charge collection efficiency of solar cells. Efficient collection of photo-generated carriers depends on the material properties of the active layer and electric field, E. Indeed, the charge collection length is given as µτE, which explains why an increase in mobility or lifetime results in improved performance. However, an increase band offset δEC reduces the electric field inside the active layer thus resulting in degraded device performance. Similar results are obtained with PTB7:PC71BM layer sandwiched between contact buffer layers (Figure S12).

4. CONCLUSION In conclusion, ZnO A and ZnO B provide necessary band offset and active layer morphology modification to study their effects on BHJ OPV performance using DEVICE A and DEVICE B. DEVICE A shows better performance such as high Jsc and improved FF, compared to that of DEVICE B. The enhanced extraction of free carriers in DEVICE A gives rise to high Jsc, as evaluated using saturated photocurrent measurements, which also results in better EQE values and enhanced FF. The FF improvement is further attributed to the low Rs and high Rsh values resulting from improved surface properties of ZnO A as inferred from AFM and SEM studies. All these improvements have resulted in highly efficient inverted BHJ solar cells using PTB7:PC71BM (PCE~9.1%) and P3HT:PC71BM (PCE~5.6%) blend. The improved performance is due to (1) a better hole blocking and an electron transporting properties of ZnO A due to the valence band offset measured using PES and (2) enhanced morphology of the active layer of DEVICE A as observed in AFM and UV-Vis studies. Further evaluation of these factors using numerical simulation studies of dark and light IV shows that morphology of the active layer plays a significant role than the band offset at the interface.


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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Additional AFM images, device structure, generation rate and dissociation probability data for both PTB7:PC71BM and P3HT:PC71BM devices, simulation data for PTB7:PC71BM device.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Telephone number: +91 471 2599426 Fax numbers: +91 471 2597427 ACKNOWLEDGMENT Manoj A G Namboothiry acknowledge the financial support from Solar Energy Research Initiative (Department of Science and Technology, Government of India) Ministry of Human Resorce Development (Government of India) and Indian Institute of Science Education and Research Thiruvananthapuram (IISER TVM), Kerala, India. The modeling section of this paper is based upon work supported in part by the Solar Energy Research Institute for India and the United States (SERIIUS), funded jointly by the U.S. Department of Energy (under Subcontract DE-AC36-08GO28308) and the Govt. of India's Department of Science and Technology (under Subcontract IUSSTF/JCERDC-SERIIUS/2012). CSG thank CSIR, New Delhi for financial support through CSC-0404 project.


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We dedicate this article to (late) Ms. Ishwarya G Chinnakonda (daughter of Dr. Chinnakonda S Gopinath).

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BRIEFS A study of the competition between the effect of band offset at the interfacial layer and morphology of active layer on the extraction of free carriers of a bulk heterojunction organic solar cell.


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Figure 1. (a) Ultra violet photoelectron valence band spectrum, (b) Zn 2p3/2, and (c) O 1s core level x-ray photoelectron spectra of ZnO thin films coated on ITO substrate. 208x291mm (300 x 300 DPI)



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Figure 2. AFM phase image of 10 nm thick PTB7:PC71BM film on (a, c) ITO/ZnO A and (b, d) ITO/ZnO B respectively. The scanning area is 2µm x 2µm. 213x185mm (300 x 300 DPI)


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Figure 3. (a) Variation of PCE and Jsc with respect to ZnO A and ZnO B thickness (b)I-V characteristics with current density plotted in y-axis (c) Photoluminescence spectra of ZnO A and ZnO B on glass plate (d) The external quantum efficiency of ZnO A and ZnO B buffer layered PTB7:PC71BM and P3HT:PC71BM inverted device. 246x184mm (300 x 300 DPI)



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Figure 4. SEM image of (a) ZnO A thin film (b) ZnO B thin film. (c) AFM (height) image of ZnO A and (d) ZnO B. 227x190mm (300 x 300 DPI)


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A study of the competition between the effect of band offset at the interfacial layer and morphology of active layer on the extraction of free carriers of a bulk heterojunction organic solar cell. 99x94mm (150 x 150 DPI)


Efficient Organic Photovoltaics with Improved Charge Extraction and

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