The Performance Enhancement of Polymer Solar Cells by Introducing

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The Performance Enhancement of Polymer Solar Cells by Introducing Cadmium-Free Quantum Dots Zhiqi Li, Xinyuan Zhang, Chunyu Liu, Zhihui Zhang, Yeyuan He, Jinfeng Li, Liang Shen, Wenbin Guo, and Shengping Ruan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08692 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 5, 2015

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The Journal of Physical Chemistry

The Performance Enhancement of Polymer Solar Cells by Introducing Cadmium-Free Quantum Dots Zhiqi Li, a Xinyuan Zhang,a Chunyu Liu, a Zhihui Zhang, a Yeyuan He, a Jinfeng Li, a Liang Shen,a,b Wenbin Guo, a,*, and Shengping Ruan a a

State Key Laboratory on Integrated Optoelectronics, Jilin University, 2699 Qianjin Street, Changchun 130012, China

ABSTRACT The utilization of inorganic nanocrystals is one of the key strategies to improve the performance of polymer solar cells (PSCs). In this paper, CuInS2/ZnS (CIS-Z) quantum dots (QDs) were employed to improve efficiency of PSCs composed of poly [N-9”-hepta-decanyl-2,7-carbazolealt-5,5-(4’,7’-di-2thienyl-2’,1’,3’-ben-zothiadiazole)](PCDTBT):fullerene

derivative

[6,6]-phenyl-C70-butyric

acid

methyl ester (PC71BM). The maximum power conversion efficiency (PCE) of 7.185% was achieved, accounting for 21.6% enhancement compared to the control device. The incorporation of CIS-Z QDs allowed not only enhancing exciton generation and dissociation but also improving charge transport property, leading to a higher short-circuit current density (Jsc) and fill factor (FF).

1. INTRODUCTION The growing potential in polymer solar cells (PSCs) is related to their unique advantages such as easy manufacturing, flexibility, the possibility of large surface printing, and low-cost alternative to the traditional inorganic-based solar cells.1-7 In terms of the increasingly serious energy crisis and the environmental contamination caused by the combustion of fossil fuels, further improvements of the effective conversion of solar energy into electricity are especially needed for mass production and practical applications of PSCs, and simultaneous enhancement of short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) are also in urgent need. Some approaches based on device physics have been studied and proved to be effective, such as incorporation of an additive,8 controlling the growth of different materials and optimizing film,9 proper annealing to improve the active layer morphology,10 and designing new device structures.11 Apart from these, the most common

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and simple strategy is incorporation of the quantum dots (QDs) materials to enhance effective harvesting of solar photons and maintain a high charge carrier mobility, leading to a higher power conversion efficiency (PCE).12-16 Although the solar cells doped with toxic cadmium (Cd) QDs have been developed to widen the absorption spectra and reduce the recombination of charge carriers,17-20 Cd QDs is seriously harmful to the environment and not conducive to sustainable development. From the perspective of making highly efficient PSCs through safe and environment-friendly procedures, processes involving harmful additive for the preparation of active layers must be changed. Recently, environmental-friendly cadmium-free QDs have been employed to fabricate photovoltaic devices and achieved high device performance. In this communication, we introduced CuInS2/ZnS (CIS-Z) QDs, possessing the nano-core-shell structure, into the active layer of PSCs based on poly [N-9”-hepta-decanyl-2,7-carbazolealt-5,5-(4’,7’di-2-thienyl-2’,1’,3’-ben-zothiadiazole)] (PCDTBT) as an electron donor and fullerene derivative [6,6]phenyl-C70-butyric acid methyl ester (PC71BM) as an electron acceptor.21-26 CIS-Z QDs has been extensively applied in PSCs and other photovoltaic devices.27-30 After doping different concentration of CIS-Z QDs into active layer, the PCE of PSCs were improved from 5.907% to 7.185%, accounting for a 21.6% enhancement. The incorporation of CIS-Z QDs increased not only charge generation and dissociation but also charge carrier transport capacity.

2. EXPERIMENTAL The ITO-coated glass substrates were cleaned with acetone, alcohol and deionized (DI) water respectively. Next, TiO2 was spin-cast at 3000 rotations per minute (rpm) on the top of the glass substrates and annealed at 450 oC for 2h, and then free cooling to the ambient temperature. Subsequently, the active layer solutions were made with different concentrations of CIS-Z QDs (purchased from NNCrystal Co. Ltd), and the doped ratios were 0, 0.01, 0.02, 0.03, and 0.04 mg/mL, respectively. The device configuration of ITO/TiO2/PCDTBT:PC71BM/MoO3/Ag and energy levels are depicted in Figure 1a and 1b, and the molecular structures of PCDTBT and fullerene derivative PC71BM are shown in Figure 1c. The mixture of PCDTBT, PC71BM, and CIS-Z QDs were dissolved 2


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into the 1, 2-dichlorobenzene solvent and spin-cast at 2000 rpm onto the TiO2 layer, and the active films were thermal annealed at 70 oC for 20 min. At last, MoO3 buffer layer with a thickness of 4 nm and Ag electrode with a thickness of 100 nm were developed on the top of active layer by thermal evaporation. As we know, MoO3 has been proved to be an effective hole buffer layer and has been extensively employed in PSCs.31,32 Simultaneously, the lowest unoccupied molecular orbital (LUMO) level of TiO2 is -4.2 eV, which is used as cathode buffer layer.33,34 The fabricated devices doped with 0, 0.01, 0.02, 0.03, and 0.04 mg/mL CIS-Z QDs are named as Device A, Device B, Device C, Device D, and Device E, respectively.

3. RESULTS AND DISCUSSION The transmission electron microscope (TEM) image of CIS-Z QDs is shown in Figure 2a, and the absorption and emission spectra are located in ultraviolet and visible region (Figure 2b). The emission spectrum of CIS-Z QDs is partial overlapping with the absorption area of active layer, which maybe increase the utilization of ultraviolet photons. The effect of CIS-Z QDs dopants on the morphological properties of the photoactive layer of PSCs is examined by atomic force microscopy (AFM). Figure 3 exhibits AFM topographic images of PCDTBT:PCBM films doped without and with CIS-Z QDs. The root-mean-squared (RMS) roughness of pristine PCDTBT: PC71BM film (Figure 3a) is about 1.55 nm, while for doped film, it shows a low RMS value of 1.10 nm (Figure 3b). It means that all the CIS-Z QDs are located within active layer thus the RMS roughness changed a little. Additionally, the minor variation of the film morphology would have negligible impact on the polymer film properties. The doped active layer surface presents an obvious and homogeneous phase separation, showing uniformly percolated structures and flat surfaces. It is worthy noting that continuous interpenetrating networks with proper domain size make great influence on exciton separation and charge transport. Herein, we studied the role of CIS-Z QDs doping on the performance of PSCs. Typical current density versus voltage (J-V) characteristics of PCDTBT:PC71BM based solar cells under AM 1.5 G illumination are presented in Figure 4a. The detailed performance parameters of fabricated devices are listed in Table 1, and all the values are typically average of 30 devices. After addition of CIS-Z QDs, a 3



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noticeable enhancement of Jsc and FF is observed. Unlike to the variation trend of Jsc, Voc is insensitive to the doping concentration of CIS-Z QDs. Compared to the control device, Jsc increased from 12.53 mA/cm2 to 14.60 mA/cm2, and FF moderately increased from 54.10% to 57.52% for the optimal Device C, which resulted in a PCE of 7.19%. In order to deeply understand the mechanism of the Jsc improvement, IPCE of solar cells doped with different concentration of CIS-Z QDs were measured. It can be found from Figure 4b that IPCE of doped devices demonstrates a consistent increase in the region of 350 to 650 nm, which increased the number of photogenerated charge carriers and resulted in the improvement of Jsc.35 The peak value of IPCE spectra reached up to 67% at the wavelength band of 440 nm for Device C, while IPCE peak of 58% at 420 nm, leading to an enhancement for light utilization. The J-V characteristics of devices with various doping amounts of CIS-Z QDs in dark are shown in Figure 5a. After incorporating of CIS-Z QDs, the dark current in the forward bias region (0.5-1 V) is dominated by fast charge carriers. Furthermore, the dark current at reverse bias and leakage current at zero bias are obviously reduced, which significantly increases the diode rectifying ratio of PSCs. The decreased dark current indicates a larger shunt resistance (Rsh) that could prevent the current from leakage and hence increase the Jsc and FF.36,37 CIS-Z QDs doping possesses an advantage to improve charge carrier transport and reduces charge recombination. In order to further explore the effect of CISZ QDs doping on the characteristics for doped devices, impedance spectroscopy was measured with an alternating current signal under open circuit voltage in the frequency range of 20 Hz to 20 MHz.38 Figure 5b presents the Cole-Cole curves of the impedance spectra for the devices without and with different concentration CIS-Z QDs doping in dark. At open circuit voltage condition, the built-in electric field was canceled out by the applied bias voltage, preventing the photogenerated charge carriers in the active layer from flowing toward electrodes. Charge carrier density can be calculated by

N =

1

e

Voc

Ad ∫

dark

C (V )dV , here e is elementary charge, A is the device area, d is the thickness of the

active layer, and C is the chemical capacitance. The chemical capacitance is simulated using Schottky

4


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equivalent circuit model.39-41 We found that the incorporation of CIS-Z QDs resulted in a noticeable enhancement of charge carrier density within the active layer. An accurately model was set up according to experimental data that resembles the typical semicircle shape.42-44 The shapes of impedance spectra are all semicircles, which are beneficial to investigate the interface resistance for the cells. The series resistance of photovoltaic cells is closely related to the diameter of the semicircles.45 It can be seen that impedance curve of Device C exhibits the smallest diameter, which reveals the smallest series resistance (Rs) shown in Table 1. It demonstrates that Rs of the optimal doped devices is dramatically decreased after introducing CIS-Z QDs, which contributes to the increase of Jsc and FF.4648

This phenomenon is consistent with the measured photocurrent and total solar-to-power conversion

efficiency of these devices in the order of Device C > Device D > Device A > Device E. For the purpose of directly investigating the effect of CIS-Z QDs on the electron and hole transport properties, we fabricated electron-only and hole-only devices and represented the results in Figure 6. Corresponding to the values in Figure 4a, Jsc increased after CIS-Z QDs were introduced, and the maximum Jsc and PCE were achieved for Device C, so we deduce that the incorporating of CIS-Z QDs made a great contribution to the enhancement of electron and hole mobilities. We extracted electron and hole mobilities at a typical applied voltage of 2.0 V using the space-charge limited circuit (SCLC) model.49,50 For Device A with pristine PCDTBT:PC71BM photoactive layer, we obtained electron mobility of 2.1454×10-5 cm2V-1s-1 (Figure 6a) and hole mobility of 1.8323×10-5 cm2V-1s-1 (Figure 6b). For the optimal Device C, electron and hole mobilities increased to 1.4736×10-3 cm2V-1s1

and 2.9352×10-4 cm2V-1s-1, respectively. Upon the incorporation of the CIS-Z QDs, hole and electron

mobilities are much higher than the undoped device. In order to investigate the mechanism of charge recombination for the optimal devices, the dependence of the photocurrent density (Jph) on the effective voltage (Veff) on a double-logarithmic scale was investigated and shown in Figure 7a, which was tested for the devices without and with the various concentrations of CIS-Z QDs. Jph=JL-JD

(1) 5



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Where JL are the current densities under illumination and JD in dark, and Veff=V0-V

(2)

Where V is the applied voltage and V0 is the compensation voltage at which Jph=0.51 Figure 7a shows that Jph linearly increases with increasing Veff, and it tends to saturate when value of the voltage is larger than 0.1 V. Compared with the device without doping, saturation current density (Jsat) have such a big boost for the doped device. Jsat is independent on the temperature and the bias, but depends on the absorbed incident photon flux. Therefore, the bigger Jsat corresponds to the more photoinduced charge carriers. The maximum exciton generation rate (Gmax) could be calculated from Jph = qGmaxL

(3)

where q is the electronic charge and L is the thickness of active layer, which means an impressive enhancement of Gmax occurred after incorporation of CIS-Z QDs. Since Gmax is related to maximum absorption of incident photons, the enhanced Gmax suggests the enhancement of the light utilization of the devices with CIS-Z QDs doping.52 The relationship between Jph and the exciton dissociation probability [P (E,T)] can be expressed by a power law equation: Jph ∝ Pα

(4)

where α is the recombination parameter, and the P (E,T) could be obtained from the ratio of Jph/Jsat. P (E,T) plotted with respect to effective bias (Veff) for these PSCs devices is presented in Figure 7b. P(E,T) values under Jsc condition increased for the devices with CIS-Z QDs, indicating that the excitation of CIS-Z QDs also benefits to the dissociation of excitons into free charge carriers. Therefore, the faster exciton split, faster response, and the larger FF are achieved in the PSCs with the CIS-Z QDs.

4. CONCLUSIONS In summary, we improved the performance of PSCs by incorporating CIS-Z QDs into PCDTBT:PC71BM active layer. The incorporation of CIS-Z QDs allows not only enhancing exciton generation and dissociation but also improving charge transport properties, leading to higher electron and hole mobilities. A highest PCE of 7.185% was achieved, accounting for a 21.6% efficiency enhancement. Our study demonstrates that cadmium-free QDs could play a general effect on the 6


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improving polymer photovoltaic device performance. We believe that the results of our study offer an effective approach to enhance the efficiency of PSCs.

AUTHOR INFROMATION Corresponding Author *Tel: +86 431 85168241-8221. Fax: +86 431 85168270. E-mail: [email protected]. a

State Key Laboratory on Integrated Optoelectronics, Jilin University, 2699 Qianjin Street, Changchun

130012, China b

Department of Mechanical and Materials Engineering and Nebraska Center for Materials and

Nanoscience, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0656, USA

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of China (61275035, 61274068, 11574110), Project of Science and Technology Development Plan of Jilin Province (20130206075SF), the Open Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2013KF10), Project of Graduate Innovation Fund of Jilin University (2015098) for providing support to the study.

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Using AC Impedance Spectroscopy. Appl. Mater. Interfaces. 2010, 2, 2254–2260. (43) Kim, G. H.; Song, H. K.; Kim, J. Y. The Effect of Introducing Buffer Layer to Polymer Solar Cells on Cell Efficiency. Sol. Energy Mater. Sol. Cells. 2011, 95, 1119–1122. (44) Perrier, G.; de Bettignies, R.; Berson, S.; Lemaitre, N.; Guillerez, S. Impedance Spectrometry of Optimized Standard and Inverted P3HT-PCBM Organic Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 101, 210–216. (45) Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J.; Durrant, J. R. Experimental Determination of the Rate Law for Charge Carrier Decay in A Polythiophene: Fullerene Solar Cell. Appl. Phys. Lett. 2008, 92, 093311. (46) Kim, J. Y.; Kim, S. H.; Lee, H. H.; Lee, K.; Ma, W. L.; Gong, X.; Heeger, A. J. New Architecture for High-Efficiency Polymer Photovoltaic Cells Using Solution-Based Titanium Oxide as An Optical Spacer. Adv. Mater. 2006, 18, 572–576. (47) Lee, K.; Kim, J. Y.; Park, S. H.; Kim, S. H.; Cho, S.; Heeger A. J. Air-Stable Polymer Electronic Devices. Adv. Mater. 2007, 19, 2445–2449 . (48) Cho, S.; Yuen, J.; Kim, J. Y.; Lee, K.; Heeger, A. J.; Lee, S. Multilayer Bipolar Field-Effect Transistors. Appl. Phys. Lett. 2008, 92, 063505. (49) Tsang, S. W.; Tse, S. C.; Tong, K. L.; So, S. K. PEDOT: PSS Polymeric Conducting Anode for Admittance Spectroscopy. Org. Electron. 2006, 7, 474-479. (50) Tsang, S. W.; So, S. K.; Xu, J. B. Application of Admittance Spectroscopy to Evaluate Carrier Mobility in Organic Charge Transport Materials. J. Appl. Phys. 2006, 99, 013706. (51) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94, 126602. (52) Chen, J. D.; Cui, C. H.; Li, Y. Q.; Zhou, L.; Ou, Q. D.; Li, C.; Li, Y. F.; Tang, J. X. Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency. Adv. Mater. 2015, 27, 1035–1041.

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Table 1. Photovoltaic parameters of polymer solar cells doped with various concentrations of CIS-Z QDs. Device

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

Rs(ohm)

Device A

0.87

12.53

54.10

5.91

223.98

Device B

0.87

13.58

56.97

6.70

124.33

Device C

0.87

14.45

57.52

7.19

119.75

Device D

0.87

14.04

57.77

7.01

133.38

Device E

0.87

13.27

52.84

6.10

259.07

Figure 1. (a) The structure diagram and (b) energy levels of all materials polymer solar cells, (c) molecular structure of PCDTBT and PC71BM.

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Figure 2. (a) The TEM image of CIS-Z QDs, (b) the absorption and emission spectra of CIS-Z QDs.

Figure 3. AFM images of active layer (a) without and (b) with optimal doping concentration of CIS-Z QDs.

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Figure 4. (a) J-V characteristic and (b) IPCE spectra of PSCs doped with different concentrations of CIS-Z QDs.

Figure 5. (a) J-V characteristics of PSCs doped with various amount CIS-Z QDs under no illumination, (b) impedance spectra of all completed devices in dark.

Figure 6. J-V characteristics of (a) hole-only device with configuration of ITO/MoO3/active layer/MoO3/Ag, and (b) electron-only device with structure of ITO/TiO2/active layer/BCP/Ag.

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Figure 7. (a) Photocurrent density (Jph), and (b) exciton dissociation probability [P(E,T)] plotted with respect to effective bias (Veff) for the reference and doped devices.

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Table of Contents

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The Performance Enhancement of Polymer Solar Cells by Introducing

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