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Aqueous-Processed Polymer/Nanocrystal Hybrid Solar Cells with Efficiency of 5.64%: The Impact of Device Structure, Polymer Content and Film Thickness Gan Jin, Haotong Wei, Zhongkai Cheng, Henan Sun, Hai-Zhu Sun, and Bai Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07171 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 11, 2017

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The Journal of Physical Chemistry

Aqueous-Processed Polymer/Nanocrystal Hybrid Solar Cells with Efficiency of 5.64%: The Impact of Device Structure, Polymer Content and Film Thickness Gan Jin, †§ Haotong Wei, †§ Zhongkai Cheng, † Henan Sun, ‡ Haizhu Sun ‡* and Bai Yang †* †

State Key Laboratory of Supramolecular Structure and Materials, College of

Chemistry, Jilin University, Changchun 130012, People’s Republic of China. ‡

College of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China.

To whom correspondence should be addressed. E-mail:[email protected].; [email protected] Tel: +86-431-85099667. Fax: +86-431-85099667. †

‡

Jilin University

Northeast Normal University

1 ACS Paragon Plus Environment


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ABSTRACT

Due to their environmentally friendly ideology, aqueous-processed hybrid solar cells (HSCs) are favored for industrial production. However, relatively low device performance urges the progress in power conversion efficiency (PCE) for their further applications. In this work, the function of polymer and nanocrystal (NC) is studied by investigating three different device structures. The polymer (low hole mobility), which plays an important role even the content is extremely low, is mainly responsible for enhancing the Voc and FF while the NC (high hole mobility) is the principle part in light absorbing, carrier generating and transporting. The intensive study of polymer and NC makes it possible achieving high performance through adjusting the thickness of different active layers by using device structures of Cathode/Electron transport layer (ETL)/NC/Polymer:NC/Hole transport layer (HTL)/Anode. Efficient aqueous-processed HSC with PCE of 5.64% is obtained which presents the highest performance among polymer/NC HSCs to date.

1. INTRODUCTION Polymer/nanocrystal (NC) hybrid solar cells (HSCs) have been investigated for 20 years since the pioneer work reported by Alivisatos and co-workers.1-5 The introduction of various polymer and NCs promotes the development of HSCs in performance and mechanism.6-13 As a special member, aqueous-processed HSCs have attracted much attention due to the distinctive characteristic that the solvent is nontoxic.14-29 This leads aqueous-processed HSCs to be a popular candidate in 2 ACS Paragon Plus Environment

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industrial production. However, the power conversion efficiency (PCE) of HSCs is still lower than that of organic polymer/fullerene or inorganic NC solar cells (NCSCs).30-35 Therefore, further improving the photovoltaic performance of HSCs is imperative. In 2013, our group developed the n-i structure of HSC devices and obtained the highest PCE of 4.76% among HSCs.36 However, limited knowledge results in incomprehensive understanding of this device structure. At that time, the main role of poly(p-phenylenevinylene) (PPV) was known as donor which absorb light and generate excitons dissociated at the interface of PPV and CdTe NCs.37-39 However, the content of PPV in absorb layer is extremely low compared with CdTe. This means that most incident light is absorbed by CdTe while only little light is absorbed by PPV. Recent reports aimed at polymer/CdTe or PbS, PbSe hybrid system have proved that conjugated polymer contributes little to the photocurrent.40-42 Because the exciton dissociation efficiency from polymer to NCs is low,19, 43 the broader the absorption of polymer, the more inferior the device performance is. Therefore, it is necessary to reconsider the superiority of polymer and NCs in efficient HSCs. Herein, the function of PPV and CdTe NCs is re-annotated through systematically investigating three devices with structures of ITO/TiO2/CdTe/MoO3/Au (Figure 1a, Device

A),

ITO/TiO2/PPV:CdTe/MoO3/Au

(Figure

1b,

Device

B)

and

ITO/TiO2/CdTe/PPV:CdTe/MoO3/Au (Figure 1c, Device C). We discover that the optimal thickness of CdTe device (A) is larger than that of PPV:CdTe device (B) and the PPV:CdTe HSCs (B) showed higher Voc than that of CdTe NCSCs (A). The 3 ACS Paragon Plus Environment


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device performance of PPV:CdTe (B) is improved under thin film (50 and 100 nm) and degraded under thick film (150 and 200 nm) compared with that of CdTe (A). It is found that the PCE isn’t obviously influenced by the content of PPV when it is extremely low, which is different from the previous cognitions.30, 36 The experimental results show that the hole mobility of PPV (hole acceptor) is much lower than that of CdTe, and the CdTe NCs contribute to light absorption while PPV is mainly responsible for the improvement of Voc and FF. Therefore, through reasonably optimizing the thickness of CdTe and PPV:CdTe films, PCE as high as 5.64% is obtained using Device C, which is the highest PCE among polymer/NCs HSCs to date.

Figure 1. The used three device structures. (a) Device A: ITO/TiO2/CdTe/MoO3/Au, (b)

Device

B:

ITO/TiO2/PPV:CdTe/MoO3/Au,

(c)

Device

C:

ITO/TiO2/CdTe/PPV:CdTe/MoO3/Au 2. EXPERIMENTAL SECTION 2.1. Materials. Tellurium powder (200 mesh, 99.8%), R, R0-dichloro-p-xylene (98%) and tetrahydrothiophene (99%) were all purchased from Aldrich Chemical Corp. 2-Mercaptoethylamine (MA, 98%) was obtained from Acros. Tetrabutyl 4 ACS Paragon Plus Environment

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titanate (98%), hydrochloric acid (36~38%), sodium hydroxide (NaOH, 96%), sodium borohydride (NaBH4, 99%) and CdCl2 (99+%) were commercially obtained. All of the solvents were of chemical pure grade and used as received. 2.2. Preparation of TiO2 Precursor. Typically, 4 mL tetrabutyl titanate was dissolved in 2 mL isopropanol in a conical flask for 5 min. 210 µL water and 17 µL concentrated HCl were mixed with 4 mL isopropanol for 5 min, and then this solution was dropped into the conical flask over about 10 min, and the mixture was stirred for 12 h at room temperature. Before use, the resultant TiO2 precursor was diluted with isopropanol. 2.3. Preparation of Aqueous MA-capped CdTe NCs. In a typical synthesis, a freshly prepared solution of NaHTe was injected into 12.5 mM N2-saturated CdCl2 solutions in the presence of MA at a pH range of 5.5–6.0. The molar ratio of Cd/MA/Te was set as 1:2.4:0.2. The resultant precursor solution was refluxed at 100 o

C for 60 min to maintain the growth of CdTe NCs. After preparation, the NCs

solution was centrifuged at a speed of 8000 r per min (rpm) with the addition of isopropanol to remove superfluous salts and MA. Subsequently, the NCs was dried in a vacuum oven and then dissolved in deionized water. 2.4. Preparation of PPV Precursor and PPV. Briefly, 10 ml of 0.4 M NaOH was added into 10 ml of 0.4 M p-xylylenebis (tetrahydrothiophenium chloride) methanol solution. This solution was cooled to 0–5 oC in an ice bath. The reaction proceeded for 1 h and then was terminated by the addition of 0.4 M HCl aqueous solution to neutralize the reaction solution. The PPV precursor aqueous solution was dialyzed 5 ACS Paragon Plus Environment


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against deionized water for one week. The obtained PPV precursor solution is ~5 mg mL-1. The PPV precursor can be converted to conjugated PPV after annealing under nitrogen, and the synthesis routes are presented in Scheme S1. 2.5 Preparation of Aqueous PPV:CdTe Solution. The aqueous PPV:CdTe solution was prepared by adding MA-capped CdTe NCs solution into PPV precursor solution and then stirred sharply for a uniform mixed solution. These solutions with weight ratios of polymer:NCs equal to 1:0, 1:0.35, 1:1.75, 1:3.5, 1:7, 1:10.5, 1:14, 1:17.5, 1:21, 1:28, 1:31.5, 1:35, 1:56 and 1:84 were obtained by adding 0, 1, 5, 10, 20, 30, 40, 50, 60, 80, 90, 100, 160, 240 µL of CdTe NCs aqueous solution with concentration of 70 mg mL-1 into 40 µL of PPV precursor solution, respectively. 2.6. Preparation of the Aqueous-Processed Devices. The photovoltaic devices were constructed by combining the spin-coating and vacuum evaporation methods. ITO-coated glasses first underwent ultrasonic treatment in chloroform, acetone, isopropyl, and then were rinsed by deionized water before drying in N2 flow, followed by the oxygen plasma treatment for 5 min. Then spin-coating of the TiO2 layer (~30 nm) at 2000 r per min (rpm) for 10 s, followed by annealing at 450 oC for 20 min in ambient condition to convert the TiO2 precursor into anatase-phase TiO2. For the CdTe devices, the active layer was fabricated by multiple spin coating of the aqueous solution of CdTe NCs at different speeds in ambient condition for required thickness, and subsequently annealed at 300 oC in the glove box for 2 min per layer except that the last layer is annealed for 60 min. For the PPV:CdTe devices, the active layer was fabricated by multiple spin coating of the PPV:CdTe solution at different speeds in 6 ACS Paragon Plus Environment

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ambient condition for required thickness, and subsequently annealed at 300 oC in the glove box for 2 min per layer except that the last layer is annealed for 60 min. For the CdTe/PPV:CdTe devices, the active layer was fabricated by multiple spin coating of the CdTe and PPV:CdTe solution at different speeds in ambient condition for respective required thickness, and subsequently annealed at 300 oC in the glove box for 2 min per layer except that the last layer is annealed for 60 min. Finally, a 5 nm MoO3 film and 60 nm Au electrodes were evaporated on top of the active layer through a mask at a pressure below 10−5 Torr, leading to an active area of 5 mm2. 2.7. Controlling the Thickness of PPV, CdTe and PPV:CdTe Film. The CdTe NCs films with thickness of 50 and 130 nm were obtained by spin coating CdTe aqueous solution with concentration of 70 and 210 mg mL-1 at roate speed of 700 and 900 rpm for 1 min, respectively. The CdTe NCs films with thickness of 100, 150, 200, 250, 300, 350 and 400 nm were constructed by multiple spin coating of 70 mg mL-1 CdTe solution at roate speed of 700 rpm for 1 min with 2, 3, 4, 5, 6, 7 and 8 layers, respectively. The CdTe NCs films with thickness of 520 nm was constructed by multiple spin coating of 210 mg mL-1 CdTe solution at roate speed of 900 rpm for 1 min with 4 layers. The PPV:CdTe (1:28) films with thickness of 50, 100, 150 and 200 nm were obtained by multiply spin coating PPV:CdTe (1:28) aqueous solution at roate speed of 900 rpm for 1 min with 1, 2, 3 and 4 layers, respectively. The PPV:CdTe films under thickness of 50 nm with weight ratios of polymer:NCs equal to 1:0, 1:0.35, 1:1.75, 1:3.5, 1:7, 1:10.5, 1:14, 1:17.5, 1:21, 1:28, 1:31.5, 1:35, 1:56 and 1:84 were obtained by spin coating corresponding PPV:CdTe aqueous solutions 7 ACS Paragon Plus Environment


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at roate speed of 1000, 1000, 1000, 1000, 1000, 900, 900, 900, 900, 900, 900, 900, 800 and 800 rpm for 3 min, 3 min, 3 min, 2min, 2 min, 1min, 1min, 1min, 1min, 1min, 1min, 1min, 1min, 1min, respectively. The PPV film with thickness of 1 nm was obtained by spin coating diluted PPV precursor solution (1 mg mL-1) at roate speed of 3000 rpm for 1 min. The PPV film with thickness of 500 nm was obtained by multiply spin coating PPV precursor solution at roate speed of 1000 rpm for 3 min with 10 layers. The 80 nm PPV:CdTe films in Device C was obtained by spin coating the hybrid solution (40 µL of CdTe solution with concentration of 210 mg mL-1 mixed with 20 µL of PPV precursor solution) at roate speed of 900 rpm for 1min. All of the PPV, CdTe and PPV:CdTe layers were annealed at 300 oC in the glove box for 2 min to enhance the anti-aqueous solubility before spin coating the subsequent layer. 2.8. Characterization. UV-vis spectra were acquired on a Shimadzu 3600 UV-vis-NIR spectrophotometer. Fluorescence spectra were acquired on a Shimadzu RF-5301 PC spectrofluorimeter and the excitation wavelength was 365 nm. Atomic force microscopy (AFM) images were recorded in tapping mode with a Digital Instruments NanoScopeIIIa under ambient conditions. The film thicknesses were measured on an Ambios Tech. XP-2 profilometer. The current density-voltage (J-V) characterization of PV devices under dark condition and white-light illumination from an SCIENCETECH 500-W solar simulator (AM 1.5G, 100 mW cm−2) was carried out on computer-controlled Keithley 2400 Source Meter measurement system in air. EQE was measured under illumination of monochromatic light from the xenon lamp using a monochromator (JobinYvon, TRIAX 320) and detected by a computer-controlled 8 ACS Paragon Plus Environment

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Stanford SR830 lock-in amplifier with a Stanford SR540 chopper. The electrochemical impedance spectra (EIS) measurements were conducted from a CHI 660E electrochemical workstation in dark conditions at zero bias voltage with a frequency ranging from 1 Hz to 100 kHz. The TA setup consisted of 400 nm pump pulses doubled from 800 nm laser pulses (~100 fs duration, 250 Hz repetition rate) generated from a mode-locked Ti:sapphire laser/amplifier system (Solstice, Spectra-Physics) and broadband white-light probe pulses generated from 2 mm-thick water. The relative polarization of the pump and the probe beams was set to the magic angle. The TA data were collected using a fiber coupled spectrometer connected to a computer. For TPV measurements, the sample chamber consisted of a platinum wire gauze electrode (with a transparency of ca. 50%) as a top electrode, a glass substrate covered with ITO as a bottom electrode, and a 10 µm thick mica spacer as an electron isolator. The samples were excited with a laser radiation pulse (a wavelength of 532 nm and a pulse width of 5 ns) from a third harmonic Nd:YAG laser (Polaris, New Wave Research, Inc.). The intensity of the pulse was regulated with a neutral grayfilter and determined using an EM500 single-channel joulemeter (Molectron, Inc.). The TPV signals were recorded using a 500 MHz digital phosphor oscilloscope (TDS 5054, Tektronix) with a preamplifier. All the other measurements were performed under ambient atmosphere at room temperature. 3. RESULTS AND DISCUSSION 3.1. The Impact of Film Thickness on Device A



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Figure 2a presents the J-V curves of Device A with different thickness of CdTe thin film under AM 1.5 G illumination (100 mW cm-2). The corresponding parameters (Voc, Jsc, FF and PCE) are listed in Table 1. Within a range of thickness (50 ~ 200 nm), the PCE increases with increasing film thickness, which contributes to the simultaneous improvement of Voc, Jsc and FF. The increase of Jsc is due to more light absorbed. The enhancement of Voc and FF (except 200 nm) is because thicker film is beneficial to

Figure 2. (a) J-V curves of CdTe NCSCs with different film thickness under AM 1.5 G illumination. (b) J-V curves of CdTe and PPV:CdTe devices with film thickness of 50, 100 and 150 nm under dark condition. (c) Electrochemical impedance spectra of the CdTe and PPV:CdTe devices with film thickness of 50, 100 and 150 nm under dark condition. (d) J-V curves of PPV:CdTe HSCs with different film thickness under AM 1.5 G illumination. 10 ACS Paragon Plus Environment

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Table 1. CdTe NCSCs of different thickness with champion PCE and corresponding Jsc, Voc and FF. Values in [] are averages, 6 devices are tested to obtain average values. Thickness (nm) 50

100

150

200

250

300

350

400

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0.419

8.64

45.0

1.63

[0.405 ± 0.017]

[8.29 ± 0.51]

[42.5 ± 3.8]

[1.28 ± 0.35]

0.464

11.7

46.8

2.54

[0.449 ± 0.015]

[11.3 ± 0.4]

[44.6 ± 3.2]

[2.24 ± 0.30]

0.491

15.1

49.1

3.63

[0.496 ± 0.013]

[14.5 ± 0.6]

[46.6 ± 3.1]

[3.33 ± 0.30]

0.532

16.1

46.7

4.01

[0.523 ± 0.014]

[15.6 ± 0.7]

[46.1 ± 2.8]

[3.89 ± 0.18]

0.520

16.3

47.4

4.01

[0.515 ± 0.015]

[15.7 ± 0.8]

[46.0 ± 3.1]

[3.86 ± 0.20]

0.521

15.6

43.0

3.50

[0.507 ± 0.021]

[14.9 ± 0.7]

[41.8 ± 2.8]

[3.23 ± 0.27]

0.513

15.1

42.8

3.32

[0.504 ± 0.022]

[14.3 ± 1.2]

[40.4 ± 2.7]

[2.96 ± 0.37]

0.500

14.3

42.5

3.04

[0.475 ± 0.028]

[13.6 ± 0.8]

[39.7 ± 2.8]

[2.75 ± 0.29]

avoid pinholes, which decreases the leakage current and simultaneously increases the shunt resistance (Rsh). These results are confirmed by the measurement of dark current and electrochemical impedance shown in Figure 2b and c. From Figure 2b, the dark current in the reverse region distinctly decreases as the film thickness increases. This 11 ACS Paragon Plus Environment


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is due to the reduction of pinholes during the multi-layer preparation process. Therefore, the current flowing through diode (junction) is enhanced, which is proved in the electrochemical impedance spectra (EIS). It is seen from Figure 2c that the Rsh largely enhances with increasing film thickness. This means the power loss caused by the current that bypasses the device junction and loads is relieved through an alternate current path. As a consequence, the Voc and FF (except 200 nm) both increase. The FF has a little decrease from 150 nm to 200 nm because thicker layer leads to more charge-carrier recombination and a part of charges cannot reach to the electrodes. The PCE with thickness of 200 nm is almost the same with that of 250 nm, indicating charge-carrier recombination becomes more and more obvious when thickness exceeding 200 nm. Therefore, the decline of the photovoltaic performance is detected with film thickness of 300, 350 and 400 nm. 3.2. The Impact of Film Thickness on Device B The Device B with different film thickness is studied, and the weight ratio of PPV to CdTe is set as 1:28. The J-V curves are shown in Figure 2d and the details are listed in Table 2. The device with thickness of 50 nm presents excellent FF (＞ 60%) because PPV accepts holes from CdTe and simultaneously decreases leakage current (Figure 2b). Transient absorption (TA) measurement (shown in Figure 3a) is used to characterize charge transfer. The exciting light with wavelength of 800 nm only excites the CdTe NCs, and the probed wavelength is 825 nm, the wavelength at which the bleaching peak of CdTe located.19 PPV:CdTe exhibits faster decay rate than CdTe itself, which demonstrates that a number of holes are accepted by PPV. The half 12 ACS Paragon Plus Environment

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lifetime for CdTe1s state in CdTe and PPV:CdTe is about 100 and 6 ps, respectively. Therefore, separated electrons and holes form which is beneficial to minimize charge-carrier recombination. Furthermore, the lifetime of charge carriers is lengthened after introducing PPV into CdTe. Transient photovoltaic (TPV, Figure 3b) technique is used to characterize the charge dynamics. PPV:CdTe shows longer recombination time (32 ms) than that of CdTe NCs (16 ms), implying that separated charge carriers suppress the opportunity of recombination. Tapping mode AFM is used to characterize the surface morphology of PPV (Figure 3c), CdTe (Figure 3d) and PPV:CdTe (Figure 3e) films. The PPV film is very smooth which exhibits root-mean-square (RMS) roughness of 0.715 nm. The CdTe film shows relative rough surface with RMS roughness of 4.780 nm, and this value decreases to 2.191 nm on PPV:CdTe film. Therefore, it is suggested that pinholes or voids among spherical CdTe NCs are filled in by PPV. The reduced pinholes is beneficial to lower the leakage current and simultaneously improve the Rsh, which are confirmed through the dark current (Figure 2b) and electrochemical impedance (Figure 2c) measurements. From Figure 2b, the dark current of PPV:CdTe devices in the reverse region obviously decreases compared with CdTe device on identical thickness (50, 100, 150 nm). The EIS shown in Figure 2c presents larger Rsh after introducing PPV under identical thickness (50, 100, 150 nm). As a result, effective charge transport especially at high bias voltage is possible, which contributes to the high FF. It is worth noting that the Jsc of PPV:CdTe hybrid device is lower than that of pure CdTe NCs device under the same thickness of 50 nm. The main absorb light material is CdTe NCs, 13 ACS Paragon Plus Environment


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Figure 3. (a) Transient absorption decay kinetics of CdTe NCs and PPV:CdTe hybrid films on a quartz substrate excited at 800 nm and probed at 825 nm. (b) Transient photovoltage responses of CdTe NCs and the PPV:CdTe hybrid films. AFM height images (2 × 2 um) of (c) PPV polymer film, (d) CdTe NCs film and (e) PPV:CdTe hybrid film. (f) J-V characteristics of CdTe NCs and PPV polymer devices measuring the hole mobility. 14 ACS Paragon Plus Environment

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Table 2. PPV:CdTe HSCs of different thickness with champion PCE and corresponding Jsc, Voc and FF. Values in [] are averages, 6 devices are tested to obtain average values. Thickness (nm) 50

100

150

200

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0.578

6.69

61.3

2.37

[0.581 ± 0.012]

[6.45 ± 0.32]

[59.1 ± 1.2]

[2.24 ± 0.13]

0.580

11.2

47.9

3.11

[0.580 ± 0.018]

[10.8 ± 0.6]

[47.6 ± 2.5]

[2.98 ± 0.13]

0.578

13.1

45.3

3.43

[0.586 ± 0.017]

[12.7 ± 0.7]

[44.7 ± 2.1]

[3.17 ± 0.26]

0.613

12.7

34.0

2.65

[0.598 ± 0.021]

[12.5 ± 0.6]

[33.7 ± 2.2]

[2.46 ± 0.19]

which will be systematically discussed in the following part about polymer content. The existence of PPV in PPV:CdTe composite film occupies a part of volume, and hence leads to the decrease of light absorption. The devices with thickness of 100 nm and 150 nm exhibit higher PCE with increased Jsc and decreased FF. The Jsc increases because more light is absorbed and converted into photocurrent. However, the FF declines due to inferior hole mobility of PPV, which means some of the accepted holes from CdTe are lost during transport and will not reach to the hole transport layer (HTL). Therefore, the PCE of PPV:CdTe device is lower than that of CdTe device under thick film (150 and 200 nm). This is confirmed by the measurement through space-charge-limited-current (SCLC) method which detects the hole mobility of PPV 15 ACS Paragon Plus Environment


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and CdTe (Figure 3f). The device structure is ITO/PPV or CdTe/ Au. The hole mobility is calculated according to the Mott-Gurney equation: J = 9ε0εrµ(V − Vbi − Vr)2 /8L3 where ε0 is the permittivity of free space (8.85 × 10-12 F m-1), εr is the dielectric constant of PPV (assume to 3) and CdTe (assume to 10), µ is the hole mobility, V is the applied voltage, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, Vbi is the built-in voltage and L is the film thickness. The thickness of PPV and CdTe film are about 500 nm and 520 nm, respectively, detected by the Ambios Tech. XP-2 profilometer. The calculated hole mobility of PPV and CdTe is about 4.00 × 10-6 cm2 V-1 s-1 and 9.66 × 10-5 cm2 V-1 s-1, respectively, the hole mobility of PPV is much lower than that of CdTe NCs. Therefore, the PCE of CdTe increases until the thickness reaches 200 nm and maintains to 250 nm and this phenomenon is unpractical on PPV:CdTe HSCs devices. Overmuch recombination results in decreased FF and thus reduces the Jsc when thickness of PPV:CdTe film reaches 200 nm. The decreased FF is accordance with the Rsh calculated in Figure 2a and d, the results are shown in Table S1. The trend of PPV:CdTe devices is inversed under illumination compared with that under darkness because of the light induced carriers recombination which competes with the decreased leakage current. Even so, the Voc of PPV:CdTe device (HSCs) is higher than that of CdTe device (NCSCs). The improvement on Voc is an incontrovertible fact and should be investigated and explained in the future, the possible reason maybe


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interfacial dipole,42,

44

which leads to decreased interfacial resistance, exciton

recombination and enhanced built-in electrical field. 3.3 The Impact of Polymer Content on Device B In previous reports, the content of polymer in the hybrid plays fatal role on device performance. Excessive or too few polymer does negative impact on device performance. Herein, we find that the most admissive weight ratio of polymer:NCs (1:18) is not accurate and the understanding of the ratio needs a recognition. The selected weight ratios are 1:0, 1:0.35, 1:1.75, 1:3.5, 1:7, 1:10.5, 1:14, 1:17.5, 1:21, 1:28, 1:31.5, 1:35, 1:56 and 1:84, respectively. The J-V curves about different ratios are shown in Figure S1 and the details including Voc, Jsc, FF and PCE are listed in Figure 4a and Table 3. The film thickness is kept the same and selected at ~50 nm to exclude unnecessary disturbance. The energy levels are presented in Figure 4b for explaining the Voc. For the pure PPV device, the Voc and Jsc are 0.775 V and 0.099 mA cm-2. The high Voc is attributed to large difference (1.3 eV) between CB (-4.2 eV) of TiO2 and HOMO level (-5.5 eV) of PPV. The low Jsc is resulted from inferior exciton dissociation due to the nature of the materials (PPV, TiO2) and limited surface contact area between TiO2 and PPV. The photoluminescence (PL) spectra (Figure 5a) are used to characterize the charge transfer from PPV to TiO2 and CdTe. From Figure 5a, PPV exhibits obvious fluorescence ranges from 500 nm to 650 nm. After introducing TiO2 or CdTe to form the PHJ (TiO2/PPV or CdTe/PPV), the fluorescence is partly quenched which indicates charge transfer from PPV to TiO2 and CdTe. The quenched degree demonstrates that the charge-transfer efficiency of PPV/CdTe is much 17 ACS Paragon Plus Environment


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Figure 4. (a) Parameters including Voc, Jsc, FF and PCE about PPV:CdTe devices with different PPV contents. (b) Energy levels of the used materials including ITO, TiO2, CdTe, PPV, MoO3 and Au.[23, 45, 46]


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Table 3. PPV:CdTe HSCs of different polymer contents with champion PCE and corresponding Jsc, Voc and FF. Values in [] are averages, 6 devices are tested to obtain average values. Weight Ratio (Polymer : NCs) 1:0

1 : 0.35

1 : 1.75

1 : 3.5

1:7

1 : 10.5

1 : 14

1 : 17.5

1 : 21

1 : 28

1 : 31.5

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0.775

0.099

28.8

0.0222

[0.755 ± 0.020]

[0.068 ± 0.031]

[26.8 ± 2.0]

[0.0168 ± 0.0054]

0.649

0.062

31.5

0.0127

[0.639 ± 0.017]

[0.046 ± 0.025]

[29.7 ± 1.8]

[0.0096 ± 0.0031]

0.393

0.326

22.0

0.0281

[0.385 ± 0.016]

[0.285 ± 0.045]

[21.3 ± 1.5]

[0.0226 ± 0.0056]

0.604

2.43

21.5

0.316

[0.596 ± 0.018]

[2.13 ± 0.31]

[20.7 ± 2.7]

[0.286 ± 0.030]

0.607

4.10

20.7

0.514

[0.593 ± 0.021]

[3.75 ± 0.39]

[20.1 ± 2.6]

[0.466 ± 0.048]

0.644

5.45

32.7

1.15

[0.629 ± 0.015]

[5.51 ± 0.25]

[31.6 ± 3.1]

[0.93 ± 0.22]

0.581

5.82

44.6

1.51

[0.585 ± 0.012]

[5.63 ± 0.42]

[44.1 ± 2.6]

[1.26 ± 0.25]

0.579

6.23

51.3

1.85

[0.586 ± 0.017]

[6.03 ± 0.34]

[50.1 ± 1.6]

[1.71 ± 0.14]

0.586

6.39

56.4

2.12

[0.583 ± 0.016]

[6.35 ± 0.38]

[55.2 ± 1.2]

[2.01 ± 0.11]

0.578

6.69

61.3

2.37

[0.581 ± 0.012]

[6.45 ± 0.32]

[60.1 ± 1.2]

[2.24 ± 0.13]

0.592

6.22

60.7

2.24

[0.589 ± 0.018]

[6.47 ± 0.36]

[58.9 ± 1.9]

[2.11 ± 0.13] 19

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1 : 35

1 : 56

1 : 84

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0.591

6.49

58.1

2.23

[0.584 ± 0.016]

[6.43 ± 0.33]

[57.3 ± 1.7]

[2.08 ± 0.15]

0.594

6.38

61.6

2.33

[0.589 ± 0.014]

[6.47 ± 0.35]

[60.1 ± 1.5]

[2.14 ± 0.19]

0.595

6.28

58.9

2.20

[0.583 ± 0.012]

[6.47 ± 0.28]

[58.6 ± 1.1]

[2.05 ± 0.15]

effective than that of PPV:TiO2. When forming the BHJ (PPV:CdTe), even importing extremely few CdTe (1:0.35), the fluorescence of PPV is quenched more obviously compared with the PHJ which implies that charge transfer in BHJ is more efficient than that in PHJ. When the ratio is higher than 1:1.75, no distinct fluorescence is detected. The PCE decreases with introducing tiny amount of CdTe (1:0.35) because too few CdTe leads to discontiguous CdTe islands which play as recombination center and do not contribute to the photocurrent. This is confirmed by absorption spectra (Figure 5b) and EQE curves (Figure 5c). The absorption regions of PPV and CdTe are from 300 nm to ~530 nm and 300 nm to 900 nm, respectively. The EQE values of devices with ratios of 1:0 and 1:0.35 both ends up at ~530 nm, which demonstrates that too few CdTe contributes negligible photocurrent. When the ratio reaches 1:1.75, the Jsc increases because CdTe plays an important role in absorbing light, separating excitons generated from PPV, accepting electrons and simultaneously forming the interpenetrating network. The EQE curve shown in Figure 5c demonstrates that a fraction of photocurrent is resulted from CdTe because of the evident EQE values ranges from 530 nm to 795 nm. However, the content of CdTe is too low to form 20 ACS Paragon Plus Environment

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sturdy interpenetrating network, the Jsc is still low. It is worth mentioning that the Voc decreases largely because the Voc is determined by complex heterojunctions including PPV/TiO2, TiO2/CdTe and PPV/CdTe heterojunctions. In the ratio range from 1:3.5 to 1:17.5, the Jsc increases with

increasing CdTe content because of the formation of

more and more sturdy interpenetrating network and the great photocurrent is resulted from CdTe. From Figure 5c, the EQE values increase in the range from 530 nm to 900

Figure 5. (a) The PL spectra of PPV, PPV/TiO2, PPV/CdTe and PPV:CdTe films with different PPV contents under excitation wavelength of 365 nm. (b) UV-vis-NIR spectra of PPV, CdTe and PPV:CdTe (1:14) films. (c) EQE curves of PPV:CdTe devices with different PPV contents. (d) J-V curve of planar ITO/TiO2/CdTe (50 nm)/PPV (1 nm)/MoO3/Au device under AM 1.5 G illumination and corresponding parameters. 21 ACS Paragon Plus Environment


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nm with increasing CdTe content demonstrates that more and more CdTe contributes to the photocurrent. The increased FF (except 1:7) and PCE also indicate that the formation of interpenetration network. It is interesting that the Voc reaches a relatively large value (0.644 V) when the ratio is 1:10.5, this may be attributed to the stronger effect of interfacial dipole because PPV is discovered tend to gathering on the upper surface of the PPV:CdTe hybrid. In the ratio range from 1:21 to 1:84, the Jsc, Voc, FF and PCE are all relatively stable in spite of different polymer content. The constant Jsc is resulted from the same film thickness. The stable Voc offers an evidence demonstrating the determinant of Voc is not the PPV:CdTe BHJ. The Voc is slightly higher than CdTe devices, which maybe resulted from the interfacial dipole effect on surface of CdTe/PPV. Even an extremely thin layer of PPV on surface of CdTe improves the Voc obviously, which is confirmed by designing planar heterojunction (PHJ) solar cell such as ITO/TiO2/CdTe(~50 nm)/PPV(~1 nm)/MoO3/Au, the J-V curves are shown in Figure 5d. The Voc is 0.581 V, which is consistent with that of the BHJ solar cells. It is worth noting that 1 nm of PPV and 50 nm of CdTe corresponds to the weight ratio of about 1:300 because the density ratio of PPV to CdTe is nearly 1:6. Compared to the thicker TiO2 (~30 nm), the TiO2/CdTe p-n heterojunction seems to be more dominant instead of the PPV/CdTe heterojunction. This phenomenon indicates that recognition and proper utilization about polymer and NCs is necessary in constructing the HSCs.


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3.4 The Impact of Film Thickness on Device C Considering that introduction of PPV is beneficial to separate charges, prolong the charge-carrier lifetime, increase the Voc, and the hole mobility of CdTe itself is excellent, it is reasonable to combine CdTe with PPV:CdTe, that is to say, device C maybe superior to device A or device B. In our previous report, the application of prior structure already obtained PCE of 4.76%, which is better than 4% (the latter structure). However, the understanding of this structure is not comprehensive. According to our new recognition about the respective function of conjugated polymer and NCs, thickness of CdTe and PPV:CdTe plays pivotal roles. Because the PCE of CdTe NSCSs reaches peak value around 200 ~ 250 nm, partial introduction of PPV into CdTe in this thickness range is rational. The J-V curves about CdTe/PPV:CdTe devices with different CdTe and PPV:CdTe thickness are shown in Figure 6a and the details are listed in Table 4. The best PCE reaches 5.64% including Voc of 0.607 V, Jsc of 17.6 mA cm-2 and FF of 52.8% with total thickness of 210 nm, the thickness of CdTe and PPV:CdTe is 130 nm and 80 nm, respectively. The external quantum efficiency (EQE) curve of this champion device is presented in Figure 6b which exhibits a wide photoresponse. The EQE values exceed 50% with broad range from 350 to 740 nm and reaches 74% in the range from 400 to 440 nm. The integral photocurrent density from EQE measurement is 16.719 mA cm-2, which is within the error range (5%). The Jsc increases while the Voc and FF decreases with increasing the CdTe or PPV:CdTe film. The Jsc decreases obviously with decreasing the CdTe or PPV:CdTe film thickness because thinner films lead to insufficient absorption. 23 ACS Paragon Plus Environment


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Figure 6. (a) J-V curves of ITO/TiO2/CdTe/PPV:CdTe/MoO3/Au devices with different thicknesses of CdTe and PPV:CdTe films. (b) EQE curve of the champion device with CdTe and PPV:CdTe thickness of 130 nm and 80 nm.


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Table 4. CdTe/PPV:CdTe HSCs of different thickness with champion PCE and corresponding Jsc, Voc and FF. Values in [] are averages, 6 devices are tested to obtain average values. Voc (V)

Thickness of CdTe/PPV:CdTe (nm) 100/80

Jsc (mA cm-2)

FF (%)

PCE (%)

0.588

15.9

46.5

4.35

[0.587 ± 0.015]

[14.9 ± 1.2]

[46.3 ± 1.6]

[4.15 ± 0.20]

0.607

17.6

52.8

5.64

[0.589 ± 0.018]

[17.6 ± 0.4]

[51.5 ± 1.9]

[5.51 ± 0.13]

0.570

18.9

44.7

4.82

[0.565 ± 0.015]

[17.7 ± 1.5]

[43.6 ± 1.1]

[4.68 ± 0.14]

0.566

16.9

48.2

4.61

[0.551 ± 0.018]

[15.8 ± 1.7]

[48.5 ± 1.4]

[4.42 ± 0.19]

0.572

19.1

29.8

3.26

[0.573 ± 0.015]

[17.6 ± 1.5]

[30.5 ± 1.6]

[3.04 ± 0.22]

130/80

150/80

130/50

130/100

4. CONCLUSIONS In conclusion, through investigating the function of conjugated polymer, the CdTe and PPV:CdTe solar cells are systematically studied by using different device structures, adjusting different film thickness and polymer contents. The respective merits of NCs and conjugated polymer are disscussed. The optimal device structure is Cathode/ETL/NC/Polymer:NC/HTL/Anode with resonable film thickness. The HSCs with structure of ITO/TiO2/CdTe(~130 nm)/PPV:CdTe(~80 nm)/MoO3/Au exhibits superior performance with PCE of 5.64% including Voc of 0.607 V, Jsc of

17.6 mA 25

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cm-2 and FF of 52.8%. This PCE presents the highest photovoltaic performance among polymer/NCs HSCs to date.

ASSOCIATED CONTENT

Supporting Information.

The scheme of the synthesis routes of PPV precursor and PPV, J-V curves of PPV:CdTe devices with different polymer contents, EQE curves of all the devices except the presented ones in main text, the change of extinction coefficients of PPV, CdTe and PPV:CdTe (1:14) films with the wavelength. This material is available free of charge via the Internet at http://pubs.adcs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]

Present Addresses Author Contributions §

G.J. and H.W. contributed equally to this work.

Funding Sources Notes ACKNOWLEDGMENT


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This work was financially supported by the NSFC (51433003, 21574018), the National Basic Research Program of China (2014CB643503), the Science Technology Program of Jilin Province (20130204025GX), Jilin Provincial Education Department (543) and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University).

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Nanocrystal Hybrid ... - ACS Publications

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