Effectiveness of a polyvinylpyrrolidone interlayer on a zinc oxide film for interfacial modification in inverted polymer solar cells

Abstract

This paper investigate the effectiveness of non-conjugated polymer polyvinylpyrrolidone (PVP) at the interface of an n-type metal oxide buffer layer and the photoactive layer in inverted bulk heterojunction solar cells. A 15% enhancement in power conversion efficiency (PCE) is realized after the incorporation of a thin PVP layer between zinc oxide (ZnO) and polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₀BM) based photoactive layer in inverted polymer solar cells. The fabricated devices with the PVP layer show enhanced PCE as high as 7.30% under simulated AM 1.5 G (100 mW cm⁻²) illumination. The ZnO/PVP improves the electron extraction property of the ITO electrode, effectively blocks holes from the highest occupied molecular orbital of the donor, suppresses charge recombination at the interface of ZnO and the photoactive layer, and decreases the interfacial contact resistance.

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1. Introduction

Recently, significant improvements have been made to optimize and increase the performances of polymer bulk heterojunction (BHJ) solar cells but further enhancement especially in terms of stability and power conversion efficiency (PCE) are still needed for their commercialization. Several studies have been carried out towards the development of organic solar cells, such as exploring the addition of additives in the photoactive layer, insertion of a metal oxide buffer layer at the anode and cathode interfaces or even the use of a self-assembled monolayer.^1–5 Among them the role of the anode/cathode buffer layer in inverted polymer solar cells has been the most widely studied concept to improve the performances as well as stability of solar cells.^5–7 The tungsten oxide (WO₃) and Zinc oxide (ZnO) are the most promising and widely studied anode and cathode buffer layer materials in inverted polymer solar cells due to their high transparency in the visible range, their adequate conductivity and their moderate charge mobility. Likewise, it also allows much better air stability thus enhancing performances and especially lifetime of devices.^7,8

Even though metal oxide buffer layers in inverted BHJ solar cells show decent performances, appropriate interface engineering is still needed in order to form an ohmic contact between photoactive layer and respective charge collecting electrodes. Recently, this major decisive element has been studied and improvement of this interfacial contact was made by replacing the n-type metal oxide with water/alcohol soluble conjugated and non-conjugated polymer layer such as conjugated polyelectrolyte (CPE), poly [9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene-alt-2,7-(9,9-dioctylfluorene)] (PFN), polyethylene oxide (PEO), polyethylenimine (PEI).^9–13 Such conjugated/non-conjugated polymer as an electron extraction layer in polymer solar cells not only forms surface dipole moment to reduce the work function of ITO electrode but also forms an efficient contact with the active layer. It also improves the adhesion with the active layer and enhances charge transfer and charge carrier extraction of the respective electrodes.^9,10,13,14 Previously, Kippelen et al. reported polyvinylpyrrolidone (PVP) modified indium-tin-oxide (ITO) electrode for the efficient electron extraction in inverted BHJ solar cells. They observed reduction in the work function of ITO after spin coating ultrathin PVP layer and also revealed comparable solar cell performances with respect to the widely used ZnO buffer layer.¹⁵ These conjugated/non-conjugated polymers show favorable performances in inverted BHJ solar cells and it could be very promising candidates for low temperature solution processed devices. However, few studies have been reported on improving the interfacial contact between inorganic metal oxide buffer layer (electron extraction layer) and organic photoactive layers.^16–22 Steven Hau et al. reported a novel approach of self-assembled monolayer (SAM) to improve the charge selectivity and also to reduce the charge recombination losses at the organic photoactive and inorganic buffer layer interfaces.^23,24 Also, significant performances enhancement of BHJ solar cells were achieved by tuning the interface properties of organic photoactive and inorganic buffer layers (ZnO).^23,24 The barrier-less contact and the worthy adhesion of inorganic and organic layers in BHJ solar cells provide advantages of excellent charge extraction and transport, as well as reduction in charge recombination at the interface and decrease in contact resistance. T. Yang et al. and A. Heegar et al. also reported PFN-Br and PEIE as an interfacial layer to engineer the ZnO and photoactive layer contact for efficient electron extraction.^16,22 Additionally, such interfacial layer between n-type inorganic layer and organic active layer provides a good interface adhesion and enhances the charge extraction and transport.¹⁶

In this paper, we demonstrate a suitability of inexpensive, environmentally stable, non-toxic and easily processable highly transparent PVP layer at the interface of solution processed ZnO and PTB7:PC₇₀BM layer in inverted BHJ solar cells. Similarly, we also compare and discuss the effect of thin PVP layer on the interfacial properties of ZnO and photoactive layer. The PVP polymer is an easily soluble in water and other polar solvents due to their polar amide group and non-polar methylene groups, respectively located in the backbone and in the ring of the molecule.^15,25

2. Experimental

2.1. Materials and solution preparation

The polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7), Indene-C₆₀ bisadduct (ICBA) and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₀BM) were purchased from luminescence technology corporation. The PVP, poly(3-hexylthiophene) (P3HT), zinc acetate dehydrate were purchased from Sigma-Aldrich. The P3HT and ICBA were dissolved in 1,2-dichlorobenzene with a concentration of 1 : 0.6 and stirred for 24 h in a nitrogen atmosphere. The detail recipe for making a blend of photoactive layer has been reported elsewhere.²⁶ All solutions were filtered through 0.45 μm polytetrafluoroethylene (PTFE) membrane filter prior to using.

2.2. Solar cell fabrication

The schematic of solar cell architecture along with their energy level diagram and molecular structure of studied donor and acceptor materials are shown in Fig. 1. The inverted solar cells were fabricated on commercially available ITO glass substrates having an active area of 0.2 × 0.2 cm² with a sheet resistance of 15 Ω sq⁻¹. All the glass substrates were cleaned by ultra-sonication in an acetone, isopropyl alcohol, rinsed in deionized water and dried using nitrogen followed by UV-ozone treatment for 10 min. Primarily a thin layer of ZnO (∼30 nm) was spin coated onto pre-cleaned ITO substrates at 600 rpm followed by annealing at 150 °C for 30 min in ambient atmosphere. The ZnO solution was made according to the reported method.²⁷ Afterword samples were transferred into nitrogen filled glove box for further deposition. Subsequently, we incorporated a thin layer of water/alcohol soluble PVP (6 nm) polymer. The PVP was dissolved in 2-ethoxyethanol to a weight concentration of 0.2 wt% and deposited on top of the ZnO layer at 3000 rpm followed by annealing at 110 °C for 10 min in nitrogen filled glove box. The PTB7:PC₇₀BM (1 : 1.5) was dissolved in mixed solvents 1,2-diclorobenzene (99%, DCB) and 1,8-diiodooctane at 97 : 3% with a total concentration of 25 mg ml⁻¹. The 100 nm film of PTB7:PC₇₀BM was spin coated at 1000 rpm onto thin layer of PVP. Finally, solar cell devices were completed with deposition of 5 nm of WO₃ and 100 nm of aluminium (Al) by thermal evaporation system through a shadow mask under a high vacuum pressure (∼3.0 × 10⁻⁷ Torr) with a deposition rate of ∼0.3 Å s⁻¹ and 2.5 Å s⁻¹. All devices were encapsulated by a transparent glass cover by using an UV curable epoxy as sealing material, in an N₂-filled glove box.

Fig. 1 Schematic of fabricated inverted polymer BHJ solar cells, (b) schematic of the energy level diagram, (c & d) chemical structures of donor PTB7 and acceptor PC₇₀BM used in the study.

2.3. Device measurements

The current density–voltage (J–V) characteristics of the fabricated devices were measured using a computer controlled Keithley 2400 source-meter, in the dark and under illumination with calibrated AM 1.5 G (100 mW cm⁻²) sun simulator (Asahi Japan, Model HAL-302) in ambient conditions. A xenon light source was used to give simulated irradiance of 100 mW cm⁻² (equivalent to an AM 1.5 G irradiation) at the surface of solar device. The EQE/IPCE measurements were performed using EQE system (Model 74000) obtained from Newport Oriel Instruments USA and HAMAMATSU calibrated silicon cell photodiode was used as a reference diode. The wavelength was controlled with a mono-chromator in the range of 200–1600 nm. The surface morphology of the film was studies by using atomic force microscopy (AFM). All thickness measurements were analysed using alpha-step surface profiler. All electrical measurements were performed in ambient atmosphere.

3. Results and discussion

The current density versus voltage (J–V) characteristics of the fabricated solar cells were measured under the illumination of AM 1.5 G (100 mW cm⁻²) are shown in Fig. 2. For the valid comparison of solar cell performances, three separate inverted polymer BHJ solar cells were fabricated with and without incorporation of thin PVP interlayer between ZnO and photoactive layer as well as PVP alone on ITO surface. Fabricated device configurations are as follows:

Fig. 2 The current density versus voltage characteristics of Device A and Device B measured under AM 1.5 G illuminations with intensity of 100 mW cm⁻² in ambient air.

Device A: glass/ITO/ZnO/PTB7:PC₇₀BM/WO₃/Al,

Device B: glass/ITO/ZnO/PVP/PTB7:PC₇₀BM/WO₃/Al and

Device C: glass/ITO/PVP/PTB7:PC₇₀BM/WO₃/Al.

Device A exhibits a PCE of 6.18%, an short circuit current density (J_sc) of 14.0 mA cm⁻², open circuit voltage (V_oc) of 0.71 V, and a fill factor (FF) of 62.48%. When ITO/PVP is applied as an electron selective electrode, the Device C shows s-shaped J–V characteristics (see Fig. S1†) with very poor solar cell performances, which could be due to the non-conformal surface coverage of thin PVP layer on the ITO surface.¹⁵ It has been previously reported that the UV exposure on the ITO/PVP surface improves the electron selectivity of the ITO electrode by reducing their work function.¹⁵ Hence, to confirm the effect of UV treatment on the performance of low band gap polymer based solar cells, we performed 3 min UV treatment on the solar devices through glass side. As per the previous report, s-shaped curved completely removed after post UV treatment, the PCE of 2.89%, and V_oc, FF increases to 0.72 V and 37.76% but, there is a significant decrease in the J_sc from 11.67 to 10.59 mA cm⁻², respectively. Therefore further study is required to know the evidence of decrease in J_sc and overall performances. In contrast, the device with thin PVP layer on ZnO surface (Device B) demonstrates an enhanced PCE of 7.30% with V_oc, J_sc, and FF respectively equal to 0.72 V, 15.17 mA cm⁻² and 66.73%. Indeed the studied inverted solar cell architecture does not require any further post processing or UV treatment on the devices. This provides an effectiveness of PVP layer at the interface of inorganic buffer and organic photoactive layer. Over 6 solar cell devices were fabricated to check the reproducibility of device performances. The detailed performances of inverted BHJ solar cells are summarized in Table 1. These results particularly demonstrate the significant improvement in J_sc and FF following the incorporation of thin PVP layer. In order to confirm the improvement in J_sc values of the Device B over Device A, external quantum efficiency (EQE) measurements were examined and are shown in Fig. 3. The improved EQE spectrum is observed throughout the spectral range from 350 to 700 nm for Device B as compared to Device A. The maximum EQE obtained for Device B and Device A are 65% and 59% respectively, at the wavelength of 620 nm. The calculated J_sc value from the integration of EQE spectrum (300 nm to 800 nm) shows a good agreement with the experimentally measured J_sc value. In addition, the shape of the EQE spectrum of both devices show negligible change, which indicates the identical morphology and crystallinity content of the photoactive layer. Hence, the improvement in the J_sc of Device B is not due to the effect of photoactive layer but it is related to the incorporation of thin PVP layer.²⁸

Table 1 Photovoltaic performance parameters of PTB7:PC₇₀BM solar cells with and without PVP (measured under AM 1.5 G illumination with intensity of 100 mW cm⁻²)

Device	Voc (V)	Jsc (mA cm^−2)	FF (%)	Eff. (%)	Rs (Ω)
Device A	0.71 ± 0.01	14.00 ± 0.04	62.47 ± 0.1	6.18 ± 0.08	160.1
Device B	0.72 ± 0.01	15.17 ± 0.06	66.73 ± 0.2	7.30 ± 0.1	131.2

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Fig. 3 The external quantum efficiency (EQE) of device with (Device B) and without (Device A) PVP layer.

Moreover, to provide further evidence to confirm the effect of ZnO/PVP layer on the crystallinity content of PTB7, UV-visible spectroscopy measurements were performed at room temperature. Fig. 4 shows the UV-visible absorption spectra of photoactive layer (PTB7:PC₇₀BM) with and without PVP layer on ZnO. The maximum absorption point is observed approximately at 650–680 nm wavelength. This is the maximum absorption wavelength of PTB7. The absorption spectra is nearly identical for both samples, which indicate no effect on the crystallinity content of photoactive layer even after the incorporation thin PVP layer at the interface of ZnO and PTB7:PC₇₀BM.^28,29 Hence, this optical result reveals that the improvement in the performance of Device B is associated with the superior electron extraction ability of ZnO/PVP layer instead of the effect of photoactive layer.

Fig. 4 UV-visible spectra of PTB7:PC₇₀BM photoactive layer with and without PVP layer on ZnO.

We also believe that the performances enhancement of Device B is mainly due to the ideal interfacial contact between inorganic ZnO and organic PTB7:PC₇₀BM photoactive layer. The surface modification of ZnO with PVP layer might assist to improve the charge carrier extraction by reducing the charge recombination at the interface. Thus morphological and electrical studies have been carried out to corroborate our argument. In order to investigate the morphology of ZnO surface, devices with and without thin PVP layer on top of ZnO surface were studied using atomic force microscopy (AFM). Fig. 5 illustrates the AFM images of ZnO samples as well as photoactive layer with and without PVP layer. Cleaned bare glass substrates were used to prepare AFM samples. Sample with and without thin PVP layer on ZnO shows smooth surface morphology with no significant changes in their surface roughness values. The ZnO surface with and without PVP layer exhibits root mean square (RMS) roughness values of 2.91 nm and 3.20 nm, respectively. The small variations in the surface morphology may be caused due to thin PVP layer. Similarly, the surface morphology of PTB7:PC₇₀BM on ZnO and ZnO/PVP also follows the same tendency with slight increase in their RMS roughness values to 3.49 nm and 3.10 nm, respectively. Such smooth morphology of ZnO with thin PVP layer could help to form excellent contact with the photoactive layer and to reduce the leakage current. To gain further insight into the interfacial contact of ZnO/PVP and PTB7:PC₇₀BM, water contact angle test were considered (Fig. S2†). The water contact angle of ZnO and ZnO/PVP surface is 22^° and 15^°, which clearly indicate the more hydrophilic behaviour of modified ZnO surface. Such modified surface could provide a good wettability.³⁰

Fig. 5 The atomic force microscopy images in non-contact mode (a) ZnO (30 nm), (b) ZnO (30 nm)/PTB7:PC₇₀BM (100 nm), (c) ZnO (30 nm)/PVP (6 nm) and (d) ZnO (30 nm)/PVP (6 nm)/PTB7:PC₇₀BM (100 nm). All the images were obtained for 3 μm × 3 μm surface area.

The inclusion of thin PVP layer in Device B shows significant enhancement in FF as compared to Device A due to noteworthy decrease in contact resistance of the device. Thus, in order to investigate the quality of interfacial contact made between the photoactive and ZnO layer, series resistance (R_s) of both devices were calculated. R_s indicates the contact resistance at the interface, as well as the bulk resistance of each layers. The R_s of the fabricated solar cell was decreased from 160.1 Ω to 131.2 Ω with the modification ZnO surface with thin PVP layer. This is a strong evidence for decrease in interfacial contact resistance, resulting in notable improvement in the fill factor of Device B. Similarly, leakage current also reflects the loss of charge carriers at the interfaces; hence, dark J–V characteristics of both Device A and Device B were measured and are presented in Fig. 6. The Device B shows significant improvement in dark current density in the forward direction with excellent diode characteristics and low leakage current in the reverse direction as compare to Device A. This clearly indicates the excellent electron extraction and suppression of leakage current ability of ZnO/PVP layer.

Fig. 6 The current density versus voltage characteristics of Device A and Device B under dark condition.

Another expected reason explaining the increased J_sc of Device B over Device A is believed to be reduction in the work function of ZnO with the incorporation of thin PVP layer. Hence, improved electron extraction and transporting characteristics of ZnO. It may also form a surface dipole moment at the ZnO and active layer interface, which helps to reduce the work function of ZnO. To provide an evidence for this assumption ultraviolet photoelectron spectroscopy (UPS) analysis was carried out to find the electronic energy levels of ZnO and ZnO/PVP interlayers. Fig. S3 (see ESI†) shows the UPS characteristics of ZnO with and without PVP layer. There is a 0.3 eV reduction observed in the work function of ZnO layer after depositing very thin PVP layer. The pyrrolidone groups of PVP helps to form a strong surface dipole by making an ionic double layer at the interfacial layer results in the reduction of work function of ZnO layer.²⁴ The work function of ZnO and ZnO/PVP using UPS measurement is approximately 4.4 and 4.1 eV respectively. This also shows valid agreement with reported data, which reveals that the thin PVP layer on ITO induced a vacuum level shift, thus reduction in the work function of ITO electrode.¹⁵ The change in the work function of modified ZnO layer not only improves their electron extraction property but also reduce the recombination of charge carrier at the interface.

Additionally, in order to confirm the suitability of ZnO/PVP as an interfacial layer, inverted BHJ solar cells were fabricated with widely used photoactive layer (P3HT:ICBA). Similar, improvement in the performances of this solar cells were observed. We have also optimized the thickness of PVP layer for P3HT:ICBA (80 nm) based device. Three devices were fabricated for the different thickness of PVP (4, 6, 8 nm) layer by keeping the constant thickness of ZnO (30 nm) and WO₃ (5 nm). The J–V characteristics of P3HT:ICBA based devices with different thickness of PVP layer are shown in Fig. S4 (see ESI†), and their photo-performances are summarized in Table S1 (see ESI†). The PCE of the optimized device with and without PVP layer for P3HT:ICBA based photoactive layer were found to be 5.46% and 5.13% respectively. The J–V characteristics of the P3HT:ICBA based solar cell with and without thin PVP layer are shown in Fig. 7. The device with PVP layer shows an improved current density from 9.16 to 9.33 mA cm⁻² and FF from 64.94 to 66.32%, respectively as compare to the reference devices.

Fig. 7 Comparative J–V characteristics of device with and without PVP in P3HT:ICBA based photoactive layer under 1.5 G illumination.

4. Conclusions

In summary, we demonstrated ZnO/PVP interlayer to form efficient interfacial contact with hydrophobic photoactive layer in inverted BHJ solar cells. Insertion of thin PVP layer between ZnO and PTB7:PC₇₀BM solar cell shows an overall power conversion efficiency of 7.30%. Thin PVP layer not only forms a worthy interface between n-type metal oxide and organic photoactive layer but also help to improve the electron extraction and transport property from PC₇₀BM to ZnO. We believe that such water/alcohol soluble non-conjugated polymer will be an excellent alternative for improving the interfacial contact area between metal oxide and photoactive layer in future BHJ solar cells.

Acknowledgements

This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSET) (NRF-2009-0093323), and the Human Resources Development program (No. 20134010200490) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy.

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Footnote

† Electronic supplementary information (ESI) available: J–V characteristics of Device C, UPS characteristics, J–V characteristics under light condition for determination of optimal thickness of PVP layer; electrical parameters of fabricated devices with and without PVP layer for P3HT:ICBA based photoactive layer. See DOI: 10.1039/c4ra08613a

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