Influences of CdSe NCs on the photovoltaic parameters of BHJ organic solar cells

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 194 (2018) 50–56

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Influences of CdSe NCs on the photovoltaic parameters of BHJ organic solar cells Fatih Ongul a,⁎, Sureyya Aydin Yuksel a, Cagdas Allahverdi b, Sinem Bozar a, Mehmet Kazici a,c, Serap Gunes a a b c

Yildiz Technical University, Faculty of Arts and Science, Department of Physics, Davutpasa Campus, 34210 Esenler, Istanbul, Turkey Toros University, Faculty of Engineering, Department of Electrical and Electronics Engineering, Nanomaterial Production Laboratory, 33140 Yenişehir, Mersin, Turkey Siirt University, Faculty of Arts and Science, Department of Physics, 56100, Siirt, Turkey

a r t i c l e

i n f o

Article history: Received 16 August 2017 Received in revised form 12 December 2017 Accepted 3 January 2018 Available online 5 January 2018 Keywords: CdSe Nanocrystals BHJ Organic solar cells

a b s t r a c t In this study, the high quality CdSe nanocrystals (NCs) capped with stearic acid were synthesized in a solvent and then purified four times by using the precipitation and redissolution process. The average size of the synthesized CdSe NCs was determined ~3.0 nm via transmission electron microscopy (TEM) measurement and their corresponding optical band edge energy was also calculated as ~2.1 eV using ultraviolet-visible (UV–Vis) absorption spectroscopy. The bulk heterojunction (BHJ) hybrid solar cells based on a ternary system including P3HT, PCBM and CdSe NCs at different weight concentrations (0 wt%, 0.1 wt%, 0.5 wt%, 1 wt% and 2 wt%) were fabricated by spin-casting process. The effect of the concentration of CdSe NCs on the photovoltaic parameters of these BHJ organic solar cells was investigated. The surface morphology of the photoactive layer modified by the incorporation of CdSe NCs into P3HT:PCBM matrix was observed with scanning electron microscopy (SEM). It was shown that when the concentration of CdSe NCs increases above 0.1 wt% in this ternary system, the photovoltaic performance of the devices significantly decreases. The power conversion efficiency of the organic photovoltaic (OPV) device was enhanced ~20% by incorporating CdSe NCs with 0.1 wt% with respect to those without CdSe NCs. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Excessive consumption of fossil fuels (a non-renewable energy resource) such as coal, crude oil and natural gas causes to harmful impacts on the environment. Some of these harmful effects are climate change as a result of global warming, air pollution and toxic wastes. Organic photovoltaic (OPV) solar cells have attracted enormous attention because of growing demand for renewable energy sources. OPV solar cells offer a variety of advantages like low manufacturing cost and easy handling and processing [1–4]. On the other hand, there are some disadvantages like lower efficiency and shorter lifetime as compared to inorganic ones. Therefore, new electron acceptor and donor materials are especially needed to enhance the efficiency of OPV solar cells. The concept of the bulk heterojunction (BHJ), based on a nanoscale blend of a conjugated polymer as an electron donor and a fullerene derivative as an electron acceptor material, comes into prominence to increase the efficiency of organic solar cells by way of providing a chargeseparating network throughout the absorption layer, because the exciton diffusion lengths are generally very short in organic semiconductors [5–8]. The Poly(3-hexylthiophene) (P3HT) which is widely used in many organic electronic applications because of its unique properties ⁎ Corresponding author. E-mail address: [email protected] (F. Ongul).

https://doi.org/10.1016/j.saa.2018.01.012 1386-1425/© 2017 Elsevier B.V. All rights reserved.

have been mostly investigated. The power conversion efficiency of the BHJ with the blends of poly(3-hexylthiophene-2, 5-diyl) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) has approached to 4% [9–11]. Hybrid solar cells comprising of conjugated polymers and inorganic semiconductor nanocrystals are of great interest in the literature of third generation solar cells due to the high electric permittivity and low binding energy within the inorganic component to reduce the energetic driving force for charge separation. However, the poor, hopping type electron transport in the inorganic component of binary hybrid solar cells employing inorganic nanocrystals and conjugated polymers and also the requirement for high nanocrystals loading of the polymer to reach the percolation threshold are the main drawbacks towards higher efficiencies [12]. On the other hand, the surfactant, which prevents the further growth of nanocrystals, is an insulating layer, which blocks the electrical transport between the nanocrystals. Therefore, tailoring of such surfactants is an important issue [13]. For their efficient use in hybrid solar cells organic ligand shell surrounding the nanocrystals needs to be removed. Ligand exchange route is followed in the literature [14]. However, it has some disadvantages such that it can cause a tendency of the nanocrystals to form large aggregates, which may affect the charge transfer and morphology in hybrid solar cells. In hybrid polymer/CdSe solar cells chlorobenzene/pyridine or chloroform/pyridine binary solvents are used, if the ligand shell of

F. Ongul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 194 (2018) 50–56

51

Fig. 1. Schematic description of the device structure with Glass/ITO/PEDOT:PSS/P3HT:PCBM:CdSe-NCs/Al (a) and its energy level diagram (b).

nanocrystals is replaced by pyridine, as a result the film morphology becomes dependent on binary solvent ratio. Inorganic semiconductors such as CdSe, CdS and CdTe have a high dielectric constant, high charge mobility and high light absorption. So, hybrid solar cells comprising of these inorganic semiconductors or their nanocrystals and organic materials are of great interest thanks to providing advantages of both materials [15–16]. CdSe nanocrystals (NCs) having different shapes like rods, tetrapods have used as electron acceptors with electron donor polymers in hybrid solar cells [17–19]. Long chain fatty acids such as oleic acid (OA) and stearic acid (SA) are used often to passivate surfaces of the nanocrystals successfully. However, they must be removed from the surfaces of the nanocrystals as much as possible or replaced with short chain ligands using ligand exchange reactions to improve their charge transport [20–21]. The most commonly used method for purification of nanocrystals is precipitation and redissolution process. The purification process of long chain fatty acid stabilized CdSe NCs also helps to improve the efficiency of BHJ solar cells [22–23]. In previous studies, the hybrid solar cells involving CdSe NCs which act an electron acceptor and P3HT which acts an electron donor have been mostly investigated for binary systems but not that much for ternary systems including PCBM [24–29]. In this research, CdSe NCs capped with stearic acid were synthesized by using hot injection method [30]. Then, CdSe NCs were purified via successive precipitation from methanol and finally stored in chlorobenzene (CB). We fabricated BHJ organic photovoltaic based on a ternary system consists of a blending of P3HT, PCBM and CdSe NCs. The structure of photovoltaic device consisting of ITO/PEDOT:PSS/P3HT:PCBM:CdSe-NCs/Al was created and its photovoltaic parameters were measured depending on the various concentrations of CdSe NCs. The energy levels of CdSe NCs having LUMO ~3.5 eV and HOMO ~5.7 eV are suitable for LUMO energy level of PCBM ~3.8 eV and HOMO energy level of P3HT ~ 5.2 eV. The values

Indium Tin Oxide (ITO) coated substrates bought from Kintec Company which have a thickness of around 120 nm, a sheet resistance of 12 Ω/sq. and a size of 1.5 × 1.5 cm2 were used as anode electrode. The onethird of ITOs was patterned by etching with an acid mixture (HCl:HNO3: H2O) and then each substrate was cleaned with Hellmanex™, distilled water, acetone and isopropanol in an ultrasonic bath for 20 min, respectively. The aqueous solution of poly(3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS, Clevious PH1000) filtered with 0.45 μm Whatman filter was deposited on top of the ITO substrates by spin coating at 1500 rpm for 60 s. The PEDOT:PSS coated samples were annealed in an oven at 150 °C for 4 min under atmospheric condition in order to remove residual moisture. The solutions used in purification process of CdSe NCs had been completely removed by centrifugation and drying and then pure CdSe NCs free of solvents was extracted. The purified CdSe NCs were dried under vacuum for evaporating their solvents and then obtained solid precipitate was weighed as 3.7 mg. This solid precipitate was dissolved in 4 ml CB to prepare a stock solution with a concentration of 0.925 mg/ml. The homogeneous stock solution

40

0.8

(a)

1.2 Absorbance (a.u.)

2. Experimental

0.6

30

1 0.4

0.8

20

0.2

0.6 0.4 0.2 0

(b)

(α2((hν)2)(cm-2(eV)2)x108

1.4

of the energy levels of materials used in this structure were taken from references [31–32]. Schematic description of the device structure and its energy level diagram are shown in Fig. 1. Here, we used a simple centrifuging method to prepare dry CdSe nanocrystals and the novelty of this work lies in the fact that we were able to dissolve them in chlorobenzene without needing any binary solvent mixtures and we mixed these CdSe NCs in P3HT:PCBM blend to fabricate ternary hybrid solar cells. Our aim was to study the effect of CdSe NCs on optical, morphological and electrical properties of the active layer and also the power conversion efficiency of the devices.

P0 P2 P4

10

0 500

550

600

650

300 400 500 600 700 800 900 1000 1100 Wavelenght (nm)

0 1.5

2 hν(eV)

2.5

Fig. 2. (a) The absorption spectra of the synthesized CdSe NCs. The black curve labelled as P0 is the absorption spectrum of the unpurified CdSe NCs. The blue (P2) and red (P4) curves are the absorption spectra of the CdSe NCs purified for 2 times and 4 times, respectively. (b) Tauc plot of the purified CdSe NCs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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F. Ongul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 194 (2018) 50–56

Fig. 3. TEM image of ~3.0 nm CdSe NCs synthesized with the hot injection method. The scale bar is 20 nm.

of CdSe NCs was diluted with CB at different ratios (1:50, 1:10, 1:5 and 1:2.5) in order to obtain various weights of CdSe NCs (0.1 wt%, 0.5 wt%, 1 wt% and 2 wt%) in P3HT:PCBM polymers. 12 mg of P3HT from Rieke Metals and 6.5 mg of PCBM from Solenne BV were mixed with various weight ratios of the purified CdSe NCs (0.1 wt%, 0.5 wt%, 1 wt% and 2 wt%) dispersed in 1 ml CB. The each mixture solution of P3HT: PCBM:CdSe-NCs was spin coated on the PEDOT:PSS layers at 800 rpm for 60 s and heated on a hotplate at 120 °C for 3 min in a glove box. The prepared samples were transferred to a vacuum thermal evaporation system (10−5 mbar) for coating of aluminium (Al) as a cathode electrode with thickness of 100 nm on top of them. The top Al electrode was deposited through a shadow mask with periodic rectangular pattern to form a 2 × 10 mm2 area cathode with three devices per substrate as shown Fig.1a. The active area for all devices defined by the overlap of ITO and Al electrodes was about 0.1 cm2. The optical absorption spectra of CdSe NCs were measured using UV/ VIS Spectrometer Lambda 2 (PerkinElmer) with wavelength range from 320 nm to 1100 nm. The average size of CdSe NCs was calculated theoretically with the empirical formula given in Eq. 1 where D is the average diameter in nanometers and λ is the wavelength of the first excitonic peak in nanometers [33].

 D ¼ 1:6122 10−9 λ4 − 2:6575 10−6 λ3
 þ 1:6242 10−3 λ2 −ð0:4277Þλ þ ð41:57Þ

ð1Þ

where α is the absorption coefficient, hν is the photon energy, Eg is the optical band edge energy and B is a constant. The value of optical band edge energy was obtained from the extrapolation of the straight line to the hν axis. The Fourier transform infrared (FTIR) spectra of P3HT:PCBM:CdSeNCs were recorded using Perkin Elmer spectrometer with ATR accessory in the wavenumber range of 650–4000 cm−1. The surface morphologies of the photovoltaic layers were examined by using FEI Quanta 450 FEG scanning electron microscope. Transmission electron microscopy (TEM) measurement of the synthesized CdSe NCs was also realized with JEOL JEM 2100F HRTEM working at 200 kV to determine their average diameter directly. Current-voltage (I–V) characteristics of the devices were measured using a computer controlled Keithley 2400 source meter under dark and illumination. The lighting was provided by Luzchem solar simulator set at 100 mW/cm2 (AM1.5G) calibrated using a standard crystalline silicon diode. Short-circuit current density Jsc (mA/cm2), open-circuit voltage Voc (V), fill-factor FF and parasitic resistances which include the series resistance (Rs) and the shunt resistance (Rsh) were determined from the current density-voltage (J-V) curve under illumination in order to characterize the photovoltaic performance of devices. FF is given by the ratio of the maximum power (VmppxJmpp) to the product of Voc and Jsc (Eq. 3). In the Eq. 4, the power conversion efficiency (PCE or ƞAM1.5) is defined as output electric power (Pout) to input light power (Pin). FF ¼

V mpp x J mpp V oc x J sc

ηAM1:5 ð%Þ ¼

ð3Þ

  P out FFxV oc x J sc x100 x100 ¼ P in P in

ð4Þ

The potential barrier height was calculated with the Eq. 5 [35].   qϕ J 0 ¼ A T 2 exp − b kb T

ð5Þ

Here, ϕb is the potential barrier height, kb is the Boltzmann constant, T is the temperature, q is the electron charge, A* is the effective Richardson coefficient and Jo is the saturation current derived from the straight line intercept of Ln(J) at zero potential (V = 0) in the dark at room temperature. The incident photon to current conversion efficiency (IPCE) was measured using a quantum efficiency system (Newport). The photocurrent density (Jsc) was calculated by integrating the IPCE(λ) spectrum using Eq. 6 [36]. Z

The optical band edge energy of the purified CdSe NCs was obtained using the Eq. 2 which is known as Tauc's eq. [34].

Jsc ¼

  2 α2 ¼ B hν−Eg =ðhνÞ

Where F(λ) is the incident photon flux density at AM1.5 condition, IPCE

ð2Þ

100

qFðλÞ IPCEðλÞdλ

ð6Þ

(a)

Transmittance (%)

100

80

99

0wt%

60

0.1wt% 0.5wt%

40

1.0wt% 2.0wt%

20

98 97

(b)

96 95 2800

2900

3000

3100

2900 2925 2950 3200

CdSe-NCs

0 500

1000

1500

2000 2500 3000 Wavenumbers (cm-1)

3500

4000

Fig. 4. The FTIR transmittance spectra of P3HT:PCBM solution with CdSe NCs for different concentrations and (b) Normalized FTIR peaks at 2925 cm−1.

F. Ongul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 194 (2018) 50–56

(a)

10 000x

(b)

10 000x

(c)

53

10 000x

Fig. 5. The SEM images of photoactive layers (a) without CdSe NCs and with CdSe NCs for different concentrations (b) 0.1% and (c) 2% by weight.

Fig. 6. The current density-voltage characteristics in dark and under illumination for the OPV devices based on ternary blends of P3HT, PCBM and CdSe NCs with concentrations of 0, 0.1, 0.5, 1, 2 wt% and comparison of current density-voltage curves in linear scale of the investigated devices under illumination.

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Table 1 The photovoltaic parameters of the OPV devices with different weight of CdSe NCs (0%, 0.1%, 0.5%, 1% and 2%). CdSe NCs

ϕb (eV)

Jo dark (mA/cm2)

Voc (mV)

Jsc (mA/cm2)

Jıpce (mA/cm2)

FF

PCE (%)

Rs (Ωcm2)

Rsh (Ωcm2)

0 wt% 0.1 wt% 0.5 wt% 1.0 wt% 2.0 wt%

0.71 0.71 0.73 0.75 0.76

1.12 10−2 1.37 10−2 4.86 10−3 2.76 10−3 1.67 10−3

555 553 538 541 532

4.56 5.21 4.10 3.28 2.34

4.21 4.82 3.84 3.03 2.17

0.51 0.53 0.49 0.40 0.37

1.28 1.53 1.08 0.71 0.46

21 18 34 78 116

376 424 475 439 469

(λ) is the spectral responsivity, q is the electron charge and λ is the wavelength of the light. 3. Results and Discussion

Voc (mV)

Jsc (mA/cm2)

The absorption spectra of the CdSe NCs are shown in Fig. 2a. In this figure, the black curve (P0) is the absorption spectrum of the unpurified CdSe NCs and the blue and red curves (P2 and P4) are the absorption spectra of the CdSe NCs purified for 2 times and 4 times, respectively. The lowest energy peak seen in the absorption spectra of the CdSe NCs is called as the first exciton peak. The first excitonic absorption peak of the unpurified sample was determined at ~ 554 nm and did not shift with the purification process as shown in the inset graph. This indicates that average NC size did not change with the purification. The average CdSe NC size can be found from an empirical formula given in Eq. 1 using the first exciton peak position. According to this formula, it is calculated to be ~3.1 nm. The TEM photo of the unpurified CdSe NCs is given in Fig. 3. 1766 nanocrystals were counted from other TEM image

6 4 2 0 560 540 520

0.5 0.4

40

0.3 1.5

30 IPCE (%)

PCE (%)

FF

500

of the sample to determine the average size by using ImageJ processing program. The average size of the CdSe NCs is ~3.0 nm with its standard deviation ~±1.3 nm. The absorption onset energy of the CdSe NCs was determined via Tauc plot analysis. The square of the product of absorption coefficient and photon energy is plotted against wavelength in Fig. 2b. The absorption onset energy of the sample, which is sometimes called as the optical band edge energy, is about ~2.1 eV which is greater than that of bulk CdSe (1.74 eV) because of quantum confinement effect. The FTIR transmittance spectra of P3HT:PCBM:CdSe-NCs blends with different weight of CdSe NCs (0%, 0.1%, 0.5%, 1% and 2% by weight) together with that of the solution of the CdSe NCs are shown in Fig. 4. The absorption peaks at 2857 cm−1 and 2925 cm−1 correspond to stretching of CH2 of stearic acid [37]. As seen from the inset of Fig. 4, increasing concentration of CdSe NCs in P3HT:PCBM accompanies to the decrease of the transmittance at 2925 cm−1 by the virtue of CdSe NCs are coated with the stearic acid. The SEM measurements of the photoactive layer consisting of P3HT: PCBM:CdSe-NCs are illustrated in Fig. 5. As understood from the figure, the photoactive layer without CdSe NCs exhibited rather compact and smooth surface morphology (Fig. 5a). The photoactive layer with 0.1% CdSe NCs showed smooth surface which indicated that CdSe NCs were uniformly distributed in the ternary system (Fig. 5b) and but for 2% CdSe NCs a rough surface appeared because of agglomeration of the CdSe NCs (Fig. 5c). The current density-voltage (J–V) characteristics of the OPV devices with ITO/PEDOT:PSS/P3HT:PCBM:CdSe-NCs/Al configuration are shown in Fig. 6. The photovoltaic parameters of these devices are summarized in Table 1. The prepared device without using CdSe NCs as a reference sample exhibited Jsc of 4.56 mA/cm2, Voc of 555 mV, FF of 0.51 and PCE of 1.28%. The device performance was improved by incorporating CdSe NCs with 0.1 wt% concentration into ternary system and exhibited Jsc of 5.21 mA/cm2, Voc of 553 mV and FF of 0.53 which led to PCE of 1.53%. When compared with other studies in the literature based on ternary P3HT:PCBM hybrid bulk heterojunction solar cells, the increase of 20% in PCE seems to be meaningful since 15% [38] to 23% [39] increase in PCE after addition of the third component to P3HT:PCBM solar cells

1 0.5 0 -0.5

0

0.5

1

1.5

2

Weight fraction of CdSe NCs (%) Fig. 7. The average values of photovoltaic parameters (Jsc, Voc, FF and PCE) of devices based on ternary blends of P3HT, PCBM and CdSe NCs with concentrations of 0, 0.1, 0.5, 1 and 2 wt% with their standard deviations.

20

2 1 3 4

10

5

0wt% 0.1wt% 0.5wt% 1.0wt% 2.0wt%

0 300

400

500 600 Wavelength (nm)

700

800

Fig. 8. The IPCE spectra of the OPV devices based on ternary blends of P3HT, PCBM and CdSe NCs with concentrations of 0, 0.1, 0.5, 1 and 2 wt%.

F. Ongul et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 194 (2018) 50–56

have also been reported. The increment of Jsc is directly proportional to the number of charges photogenerated and collected at the electrodes. The incorporation of CdSe NCs with low concentration into P3HT: PCBM matrix could be attributed to better charge extraction at the P3HT/PCBM interface. The energy barrier height of the device (ϕb) calculated with using its dark current increased from 0.71 eV to 0.76 eV and its saturation current (Jo) decreased from 1.12 × 10−2 mA/cm2 to 1.67 × 10−3 mA/cm2 with increasing concentration of CdSe NCs. The photovoltaic performance of the devices is limited by the fill factor (FF) related with the two parasitic resistances which are series resistance (Rs) and shunt resistance (Rsh). Rs is directly associated with the charge carrier transport between the photoactive layer and contacts. Rsh is related with manufacturing defects and these defects cause leakage of current in the device. In general, a low value of Rs and a high value of Rsh are desired for solar cells. As it can be seen in Table 1, the highest FF was achieved for the device comprising of CdSe NCs with concentration of 0.1 wt% due to the lowest value of Rs (18 Ωcm2) and a moderate value of Rsh (424 Ωcm2) as compared to those of other devices. However, when the concentration of CdSe NCs in the ternary system increased more than 0.1 wt%, the efficiency of the device gradually decreased from 1.53% to 0.46% associated with decreasing of Jsc from 5.21 mA/cm2 to 2.34 mA/cm2 and also decreasing of FF from 0.53 to 0.37. The Rs value of the device significantly rose with increasing concentration of CdSe NCs in the photoactive layer after 0.1 wt% and the photovoltaic performance was remarkably reduced. It can be explained by that the residual capping ligands on the surface of CdSe NCs after the purification prevent the charge transport and then increase in Rs value. The experiments for the same weight ratio of CdSe NCs were repeated three times at different times. The average value calculated for each parameter and its standard deviation were plotted in Fig. 7 against weight fraction of CdSe NCs in device. As understood from the figure, the device performance was improved by incorporating 0.1 wt% CdSe NCs as compared to the other devices, and also these results indicated the good reproducibility. The incident photon-to-current conversion efficiency (IPCE) is defined as the ratio of the number of charges in the external circuit and incident photons. The shape of the IPCE spectrum gives information about spectral sensitivity of the photovoltaic device. The incident photons to current conversion efficiency (IPCE) spectra of these devices are shown in Fig. 8. The Jsc values given in Table 1 were calculated by integrating the IPCE spectra of the devices. These IPCE results were confirmed by the photovoltaic parameters. The IPCE spectra of all devices exhibited a broad response in the range of 300–650 nm. The IPCE value of the device having 0.1% CdSe NCs was found to be 37% around 500 nm. It was enhanced about 16% with respect to that of reference sample. However, it gradually decreased for greater concentration of CdSe NCs because of decreasing charge transport. 4. Conclusion In summary, the hybrid solar cells comprising of low dimensional inorganic semiconductors and organic materials are designed to take advantages of both materials. For this purpose, CdSe NCs whose average size is ~3.0 nm were synthesized and then purified for removing their insulation layer. When the concentration of CdSe NCs in P3HT:PCBM matrix was increased above 0.1 wt%, the series resistance of the prepared OPV devices significantly increased and thus their photovoltaic parameters gradually got worse, which may be caused by residual insulating ligands leading to insufficient charge transport. The photovoltaic parameters of these devices could be affected by the surface morphology of the photoactive layer; the roughness of the photoactive layer was remarkably increased with higher concentration of CdSe NCs which lead to reduction in the photovoltaic performance of devices. The power conversion efficiency of the OPV devices was enhanced ~20% by incorporating CdSe NCs with a concentration of 0.1 wt% compared to those without CdSe NCs. Optimizing material quality and

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