Enhanced Electron Extraction Capability of Polymer Solar Cells via

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Enhanced Electron Extraction Capability of Polymer Solar Cells via Employing Electrostatically Self-Assembled Molecule on Cathode Interfacial Layer Zhiqi Li,† Xinyuan Zhang,† Chunyu Liu,† Zhihui Zhang,† Jinfeng Li,† Liang Shen,†,‡ Wenbin Guo,*,† and Shengping Ruan† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China ‡ Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska−Lincoln, Lincoln, Nebraska 68588-0656, United States ABSTRACT: In this paper, high-performance inverted polymer solar cells (PSCs) with a modified cathode buffer layer, titanium dioxide:polyethylenimine (TiO2:PEI), are demonstrated. The TiO2-O-PEI transport layer was fabricated by electrostatically self-assembled monolayers (ESAM) of PEI molecules. Protonated amine functional groups of PEI can combine protons (H+) hydrolyzing from its aqueous solution. Also, PEI could produce ESAM on the surface of hydroxylated TiO2 because of its cationic characteristics. The incorporation of the TiO2-O-PEI layer enhances the photocurrent and power conversion efficiency (PCE) due to the improved interfacial electron transport and extraction of the TiO2-O-PEI surface and the increased light absorption of the active layer. The enhanced PCE, low-cost materials, and solution process of TiO2-O-PEI buffer layers provide a promising method for highly efficient PSCs. KEYWORDS: polymer solar cells, electron transport and extraction, PEI, electrostatically self-assembled monolayers, power conversion efficiency

1. INTRODUCTION Polymer solar cells (PSCs) composed of electron-donor polymer and electron-acceptor fullerene offer a foreground for the lightweight, lower cost of large-area solar panels, mechanical flexible, and printable photovoltaic (PV) technology.1−11 Although the potential advantages of PSCs are fascinating, commercial adoption of PSCs is still limited by deficient light absorption, low electron mobility of polymer, and interfacial carrier recombination.12,13 Among them, poor electron transport capability has always been a serious issue for achieving high-performance PSCs for many years. Ideally, the solar cells should have balanced charge transfer and collection to ensure high efficient conversion of the solar photon to electron and a high electron and hole mobility for current carrier transport. In order to overcome this problem, several approaches including the choice of new conjugated polymers, device structures, interfacial engineering, etc., have been developed.14−16 In the most recent years, polymeric surfactants have been successfully employed as the interfacial layers, yielding highly efficient inverted PSCs. Although it improved the device performance slightly, the interfacial contact between organic materials and inorganic substance is not optimized. The intrinsic inorganic interfacial defects deteriorate the efficient electron extraction and transport, © XXXX American Chemical Society

resulting in a high interface recombination, and the device performance is very sensitive to these interlayers. Many techniques, including annealing treatment, controlling surface energy of substrates, cosolvent systems, and buffer layer modifications, have also been implemented in the quest for optimized morphology of the interface to further improve cells efficiency.17−26 Particularly, interfacial modifications with selfassembled water-soluble polymers have been carried out to improve interfacial contact, balance charge transport, and reduce interfacial contact resistance.27 These self-assembled interfaces optimized the wettability of the interfacial layers instead of the polar ITO surface and facilitated current carrier extraction from the active layer to the electrode. Additionally, water-soluble polymers have been proved to be promising candidates as efficient interfacial layer materials for high efficiency inverted PSCs.28−30 Solution processing capabilities are very beneficial to the commercialization of all-solution processed and massive production.31,32 To develop the fully printed PSCs, the influence of self-assembled water-soluble Received: December 18, 2015 Accepted: March 9, 2016

A

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Structure diagram of polymer solar cell and molecular structures of PCDTBT, PC71BM, and PEI.

Figure 2. Energy level structures of polymer solar cells (a) without and (b) with PEI layer. of 40 nm) was spin-coated on the cleaned ITO glass using sol−gel, and anatase TiO2 films were formed. Then, the fabricated TiO2 films were separated into two groups, the pristine TiO2 was used to fabricate the control devices, which was named as Device A. To deposit selfassembled molecules, the TiO2 films were immersed into the ethylene glycol (EG) solution of potassium hydroxide (KOH) for 20 h. Subsequently, PEI was dissolved in deionized water and deposited on the TiO2 films using a spin-coating process with the concentrations of 0.5, 0.75, 1, and 1.5 mg/mL, respectively. Afterward, TiO2-O-PEI films were thermally annealed at 100 °C, and the corresponding cells made by various concentrations of PEI were labeled as Device B, Device C, Device D, and Device E. The active layer (PCDTBT:PC71BM of 1:4 by weight, thickness = 100 nm) was spin-coated on top of TiO2 and TiO2-O-PEI films and homogenized inside a nitrogen-purged drybox for 20 min. Finally, the MoO3 (4 nm) and the Ag (100 nm) were evaporated on the active layer by a shadow mask of 2 mm × 3.2 mm at a vacuum of 1 × 10−5 Torr to finish PSCs. The Devices B−E were fabricated with the following structure, ITO/TiO2-O-PEI/PCDTBT: PC71BM/MoO3/Ag, and the control devices were manufactured with the configuration of ITO/TiO2/PCDTBT:PC71BM/MoO3/Ag. The schematic diagram and chemical formula of active layer materials are shown in Figure 1, and the energy level alignment of all materials is indicated in Figure 2. 2.2. Device Characterization. Current density−voltage (J−V) characteristics measurement of all devices was done by a Keithley 236 Source Measure Unit. Photovoltaic conversion characteristics were tested using an Air Mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 100 mW. AFM images in tapping mode were carried out using a Veeco multimode AFM with a nanoscope III

polymers on interfacial modification to enhance charge transfer and collection is still expecting for further research in depth. In this communication, PSCs based on a titanium dioxide (TiO2) interface coated by electrostatically self-assembled monolayers (ESAM) of polyethylenimine (PEI) molecules (a cationic nonconjugated polyelectrolyte) were studied. The amine functional groups of PEI can be partially protonated by combining with protons (H+) hydrolyzing from aqueous solution. This kind of cationic characteristics facilitates chemical combination between PEI and the hydroxylated surface of TiO2.33 The thickness of PEI nanolayers coated on the TiO2 films was optimized by controlling the concentration of PEI solutions. Consequently, fill factor (FF) and short-circuit density (Jsc) of PV devices were significantly enhanced, leading to an increased power conversion efficiency (PCE). To investigate the mechanism of the improved characteristics, surface wettability and the atom force microscope (AFM) measurement were employed to observe their feasibility as suitable candidates and the morphological properties, which indicate that incorporating ESAM of PEI molecules could result in a high electron mobility and high interfacial electron transfer rate at TiO2-O-PEI/active-layer interfaces.

2. EXPERIMENTAL METHODS 2.1. Device Fabrication. Commercial ITO-coated glass substrates were washed using acetone, anhydrous alcohol, and deionized water in turn and subsequently dried with nitrogen. The TiO2 layer (thickness B

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. (a) The hydroxylated surface of TiO2; (b) protonated functional PEI amines; (c) the electrostatic self-assembly of PEI; (d) the TiO2-OPEI interface. controller. All the photovoltaic parameters of cells in this study are typical average values of 32 cells from four repeated experiments. The parallel and series resistances were calculated from the J−V curve, respectively.

to the absorption of TiO2. In order to compare the hydroxylated surface of TiO2, surface wettability tests (SWT) of TiO2 films modified under different conditions were measured. SWT has been proved as an effective and welldefined diagnostic tool to investigate their feasibility as suitable candidates for efficient solar energy cells. The wetting of a solid with water, where air is the surrounding medium, is dependent on the relationship between the existing interfacial tensions, including water/air, water/solid, and solid/air. The ratio between these materials’ tensions determines the contact angle (õ) on a given surface. If the wettability is high, õ will be small, which is classed as hydrophilic.45 To form a superhydrophilic TiO2 surface, samples were immersed into the ethylene glycol (EG) solution of potassium hydroxide (KOH) for 20 h. As shown in Figure 5, compared with the

3. RESULTS AND DISCUSSION As shown in Figure 3, by incorporating protons (H+) hydrolyzing from aqueous solution, functional PEI amines can be partially protonated due to the amines’ strong basicity, leading to cationic characteristics. PEI could produce chemical combination with the hydroxylated TiO2 film (Figure 3b), which was named an electrostatic self-assembly (ESAM) of PEI (Figure 3c,d).44 To verify the formation of the TiO2-O-PEI layer, Fourier transform infrared spectroscopy (FTIR) of TiO2 and TiO2-O-PEI layers was performed (Figure 4). The intensity of the vibronic peak is found to increase with addition of PEI, which was employed to support the proposed bonding formation in the blend film. The peaks observed at 1000, 1100, 2800, and 2900 nm are due to PEI-O-TiO2 absorption, and those occurring at the short wavelength region correspond

Figure 5. Contact angles of the surface for (a) TiO2, (b) UV-treated TiO2, (c) hydroxylated TiO2, (d) TiO2-O-PEI.

pristine TiO2 surface with a contact angle of 33.95° (Figure 5a), it proposed a significant transform to form a superhydrophilic TiO2 surface with a water contact angle of 5.43°(Figure 5c) by hydroxylated treatment. Meanwhile, we also irradiated the TiO2 surface with a UV tube and got the water contact angle of 32.76° (Figure 5b) and confirmed that UV radiation changed the water contact angle a little. A high contact angle makes it difficult to deposit a solution on the surface of another material,

Figure 4. Fourier transform infrared spectra of TiO2 and TiO2-O-PEI layers. C

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. (a) J−V characteristics of devices without and with PEI layer, (b) J−V characteristics of devices in the dark.

Table 1. Parameters of Polymer Solar Cells with TiO2 and TiO2-O-PEI Electron Transport Layers Device A B C D E

Voc (V) 0.86 0.86 0.87 0.87 0.87

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

Jsc (mA/cm2) 13.52 14.81 14.96 15.13 14.15

± ± ± ± ±

0.14 0.20 0.10 0.08 0.10

FF (%) 49.24 50.63 53.81 56.81 55.45

± ± ± ± ±

0.30 0.17 0.24 0.37 0.30

PCE (%)

Rs (ohm)

Rsh(ohm)

± ± ± ± ±

302.53 237.99 193.83 152.86 170.12

6566.67 7038.58 6424.08 8008.62 6291.34

5.74 6.42 7.00 7.56 6.99

0.11 0.12 0.14 0.10 0.12

Figure 7. (a) The absorption and (b) transmission spectra of the PSCs without and with TiO2-O-PEI layers, (c) the transmittance of TiO2 and TiO2/PEI layers, (d) absorption spectra of the active layer on TiO2 and TiO2/PEI layers.

are −4.41 and −4.19 eV, respectively. Consequently, the formation of interfacial dipoles resulted in the decreased energy barrier, which is conducive to electron transfer from the active layer to TiO2. To clearly investigate the effects of ESAM of PEI molecules as an interfacial layer on device performances of PSCs, J−V characteristics of all devices were measured. As shown in Figure 6a, the TiO2-O-PEI modification layer presents a positive effect on device performance compared to Device A, including an increase of FF, Rsh, and Jsc. Among all the devices, Device D exhibits a Jsc of 15.128 mA/cm2 and an FF of 56.81%, resulting in the highest efficiency of 7.555%. All the photovoltaic parameters of PSCs in this study are summared in Table 1, which are an average of 32 devices. As

but the reduced contact angle enhances the spreading of the solution, which is beneficial to the occurrence of the ESAM of PEI. Compared with the pristine TiO2 surface, a decreased water contact angle of 22.74° (Figure 5d) is ascribed to the topographical surface morphology due to PEI ESAM. The reduced contact angle can also enhance the film-forming property of the PCDTBT:PC70BM solution on the TiO2-O-PEI layer. After the PEI layer is capped on the TiO2 film, the ESAM of functional PEI amine groups results in a spontaneous orientation, which leads to interfacial dipoles aligning at the surface of TiO2 (Figure 2b). The work functions of TiO2 and TiO2-O-PEI were measured using a Kelvin probe system, which D

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces mentioned previously, the modified TiO2-O-PEI layer has a different surface wettability due to the changed interface morphology, which leads to an optimized charge transfer channel; thus, electron collection capacity of modified devices could be improved. To further explore the effect of the TiO2-OPEI layer in depth and confirm the improvement of cell characteristics, the dark J−V characteristics for pristine TiO2and TiO2-O-PEI-based cells were measured, respectively. Figure 6b represents that devices with the TiO2-O-PEI interfacial layers possess smaller leakage current at negative voltages and rapidly increased current in positive voltages, which significantly increases the diode rectifying ratio. The decreased dark current indicates a larger shunt resistance (Rsh) and a decrease of series or contact resistance (Rs), which could prevent the current from leakage, resulting in the increase of Jsc and FF. In addition, the dark current in the forward bias region (1 V > Vapp > 0.7 V) rises rapidly, indicating an increase in charge transport. These enhanced dark J−V characteristics are reflected in the increase in the extraction of charge carriers from photoactive layers, and thereby enhanced device performance. Generated photocurrent in PV devices is one of the important factors for the enhanced device performance, which is usually attributed to the improved absorption of devices. To verify the TiO2-O-PEI layer effect on light harvesting, absorption spectra of the completed devices with TiO2 and TiO2-O-PEI layers are shown in Figure 7a. After TiO2 buffer layers were modified by PEI, a substantial enhancement of absorption in the range of 350−550 nm is observed for TiO2-O-PEI-based devices (the same active layer thickness of 100 nm). Especially, we observed a pronounced increase in the region of 300−480 nm, which may be ascribed to optical interference and/or decreased interface optical losses. As further proof of the light absorption enhancement, transmission spectra of different devices (Figure 7b) were also obtained, which are in good agreement with the absorption spectrum. To demonstrate the effect of the PEI layer on the optical absorption, the transmission spectra of ITO glass with TiO2 and TiO2-O-PEI layers were tested. As shown in Figure 7c, light transmittance of TiO2-O-PEI film is a little lower than that of pristine TiO2 film in the region of 400−700 nm. In order to verify that the optical absorption increase originates from the active layer, we measured the absorption spectra of TiO2 film, TiO2/PEI film, TiO2/PCDTBT:PC71BM film, and TiO2-O-PEI/PCDTBT:PC71BM film, respectively. The calculated absorption of only active layers was obtained (Figure 7d), which indicates a substantial absorption enhancement for the active layer on the TiO2-O-PEI film. Except for light absorption of the active layer, optical loss in thin film solar cells is mainly ascribed to the scattering and reflection effect of the interfaces. To deeply probe the role of the TiO2-O-PEI layer on light trapping of the active layer, we examined the morphological properties of TiO2 and TiO2-OPEI films using atomic force microscopy (AFM). As is displayed in Figure 8, TiO2 shows a very rough surface with a high root-mean-square (RMS) value of 2.54 nm (Figure 8a); by contrast, the TiO2-O-PEI film indicates a very smooth and featureless structure with a low RMS value of 0.965 nm (Figure 8b), which means that ESAM of PEI can effectively decrease the roughness of the TiO2 layer. Smooth interface morphology reduces interfacial defects of the TiO2 film, leading to a reduced light scattering and reflection. Furthermore, smooth and featureless interface morphology offers smaller charge recombination probabilities, and the minor variation of the film

Figure 8. AFM topography images of (a) TiO2 film for Device A, (b) TiO2-O-PEI film for Device D.

morphology has also a dramatic impact on charge transfer. Therefore, it is reasonable to speculate that electron transport is energetically favored at the TiO2-O-PEI interface, which is also confirmed by Jsc enhancement in the modified devices. In addition, the modified film morphology reduces contact resistance, accounting for the increased FF. The photocurrent generation depends on the gross of absorbed photons for the polymer (connected with the overall flux of the solar spectrum photons) and incident photon-toelectron conversion efficiency (IPCE).36−39 To verify the photocurrent enhancement of TiO2-O-PEI-based devices, we also measured the incident photon-to-current conversion efficiency (IPCE) of these devices. As shown in Figure 9a, devices with the TiO2-O-PEI layer demonstrate a higher IPCE throughout the visible range of 350−600 nm, especially at a wavelength of 530 nm. As we know, IPCE hinges on three procedures: (1) diffusion of photogenerated excitons into the interface of the PCDTBT/PC70BM, (2) interfacial excitons separation, and (3) electron and hole collection by cathode and anode.40−43 The nanoscale morphology of the TiO2-O-PEI interface can be described in step (3); interfacial defects and fluctuation film cause interfacial charge-trapping sites or “dead ends”, causing a concomitant loss of excitons. A smooth and featureless interface can form percolated contact with the PCDTBT:PC70BM layer with few charge trapping or defects. Photogenerated excitons will have less chance to recombine before reaching the electrodes. An enhanced photocurrent is decided by exciton separation and charge carriers injection into electrodes as much as possible. On the basis of this simple model, the integration of IPCE is equal to the Jsc; therefore, TiO2-O-PEI-based devices possess a higher short-circuit current. To further investigate the effects of the TiO2-O-PEI interfacial layer in PV devices, a linear dependence of photocurrent density (Jph) and the effective voltage (Veff) was calculated for the control and modified devices. Figure 9b reveals Jph−Veff curves in double-logarithmic coordinates, which was tested for the devices with TiO2 and TiO2-O-PEI buffer layers. Here Jph = JL − JD (1) where JL is the current densities under illumination and JD in the dark, respectively. Veff = V0 − V

(2)

where V and V0 are applied voltage and voltage at which Jph = 0.34,35 Apparently, Jph linearly increases with increasing Veff (Veff E

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 9. (a) IPCE of PSCs without and with PEI layers. (b) Photocurrent density (Jph) plotted with respect to effective bias (Veff) of different devices.

Figure 10. (a) Impedance spectra of Device A and Device D. (b) J−V characteristics of electron-only devices with TiO2 and TiO2-O-PEI layers.

PEI molecules for PSCs. Optimal interfacial morphological properties and good compatibility of the PEI ESAM-modified TiO2 with the active layers improved electron transport and decreased contact resistance, which increased the PCE of PCDTBTP:PC71BM-based PSCs from 5.742% up to 7.555%. High transparency, electron extraction improvement, and optimal interfacial morphological properties provide a promising way for realizing highly efficient PSCs.

< 0.1 V) and then saturates at a value more than 1 V. Compared with the TiO2-based device, Jph has such a big boost at the TiO2-O-PEI-based devices (Veff < 0.2 V), and it tends to saturate when the value of the voltage is larger than 0.4 V. Notably, the devices with the TiO2-O-PEI layers demonstrate higher saturation current density (Jsat), and Jsat is independent of the temperature and the bias but depends on the absorbed incident photon flux. Therefore, the bigger Jsat corresponds to the more photoinduced charge carriers.46,47 To deeply elucidate the TiO2-O-PEI effect on IPCE, we investigated charge transport and recombination dynamics of all devices using electrical impedance spectroscopy (EIS) and J−V characteristics of the electron-only devices in the dark. As is presented in Figure 10a, the EIS curves in the low-frequency region depend on charge transport at the interface between the active layer and electrode. Furthermore, the devices with TiO2O-PEI buffer layers exhibit slightly low charge transfer resistance than the device with the TiO2 layer, suggesting that the TiO2-O-PEI buffer layer effectively facilitates electron transport from the active layer to ITO.48 The J−V property for the electron-only device was obtained in the dark and is shown in Figure 10b. Compared with the TiO2-based device, Jsc of the TiO2-O-PEI-based devices are improved, which unravels that the TiO2-O-PEI interface is beneficial to electron transport. Across a thickness of 100 nm, electron mobility was calculated from the SCLC model at a typical applied voltage (Vapp = 10 V, an electric field of 105 V cm−1).49 Electron mobility of the control device is about 7.87 × 10−5 cm2 V−1 s−1, and that of the optimal device is 1.63 × 10−4 cm2 V−1 s−1, which attests to that the TiO2-O-PEI layer facilitates electron transort.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 431 85168241-8221. Fax: +86 431 85168270. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (61275035, 61370046, 11574110), the Project of Science and Technology Development Plan of Jilin Province (20130206075SF), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2013KF10).

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REFERENCES

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4. CONCLUSIONS In summary, we demonstrate the effective TiO2 interface modification via electrostatically self-assembled monolayers of F

DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b12394 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX