P3HT hybrid inverted solar cells by interfacial modification

Solar Energy Materials & Solar Cells 96 (2012) 160–165

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Improving the performance of CdS/P3HT hybrid inverted solar cells by interfacial modification Min Zhong a,b,1, Dong Yang a,b,1, Jian Zhang a, Jingying Shi a, Xiuli Wang a, Can Li a,n a

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b Graduate University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 May 2011 Received in revised form 16 September 2011 Accepted 17 September 2011 Available online 26 October 2011

This paper reports the effects of interfacial modification on the performance of the inverted hybrid solar cells consisting of P3HT as an electron donor and CdS nanoporous film as an electron acceptor. The device performance can be enhanced by grafting N719 dye molecules onto the surface of CdS. By optimizing the P3HT thickness, the power conversion efficiency (PCE) of the CdS/N719/P3HT hybrid solar cell is increased to 1.06% from 0.06% of the CdS/P3HT hybrid solar cell, with a champion device efficiency of 1.31% under AM1.5G of 100 mW/cm2 intensity. It is revealed that the interface modification by N719 dye can promote exciton dissociation between the two components, reduce interfacial charge recombination, and form a dipole layer at the interface that modulated the interface energy level, which enhances the open circuit voltage and short circuit current coinstantaneously. This work may provide a general method to achieve high performance organic–inorganic hybrid solar cells by interfacial modification. & 2011 Elsevier B.V. All rights reserved.

Keywords: Hybrid solar cells Interfacial modification CdS nanoporous film Interfacial charge recombination

1. Introduction Hybrid polymer/inorganic solar cells have attracted a growing interest in developing stable, low cost, and mechanically flexible photovoltaic devices due to the advantage of the high carrier mobility and stability from inorganic semiconductors and flexibility from the polymer [1,2]. Hole-conducting polymers have been combined with a wide range of inorganic nanomaterials, such as TiO2 [3,4], ZnO [5,6], CdSe [7,8], and so on. The active layers of these devices are formed by phase separation during the spin-coating of mixtures of the polymers and the inorganic materials. Because the disordered, interpenetrating networks of the two phases result in tortuous conduction pathways, low carrier mobilities, and trapped charge in isolated phases, charge transport in these bulk heterojunctions may be inefficient. To further improve the efficiency, over the past several years, many groups have built hybrid polymer/inorganic cells with inverted structure [9–12], where a conjugated polymer is inserted into inorganic network. Both organic and inorganic phases are continuous, and the transport of generated electrons and holes is facilitated in two separated phases [11]. The power conversion efficiency (PCE) of hybrid polymer/inorganic solar cell with

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Corresponding author. Tel: þ86 411 84379070. E-mail address: [email protected] (C. Li). 1 Both authors contributed equally to this work.

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.09.041

inverted structure has reached as high as 5.1% [12], and promising prospect will be anticipated. CdS nanostructures have attracted considerable interest because of their good electron acceptable and transportable characteristics [13]. Theoretically, well aligned one dimensional (1D) arrays that enable a fast carrier pathway could be the ideal structure for the hybrid solar cells [14]. Crystalline CdS nanowire/ nanorod arrays have been grown and used [15,16], and a PCE of 1.73% had been achieved in hybrid solar cells with inverted structure [17]. However, the growth of CdS nanowires with precise control remains a technological challenge and the fabrication of CdS single crystalline nanowires needs to be at high processing temperature (800 1C). It is necessary to find facile routes for preparation of hybrid solar cells with CdS nanostructures. Apart from the creation of organic–inorganic heterojunction with desired morphology, modifying the surface of the inorganic component with a thin layer of molecules may induce a significant improvement of the performance of the hybrid solar cells. Promising results have been achieved in photovoltaic devices based on metal oxide nanocrystals and P3HT with effective interface modifications [18–20]. However, only a few papers have reported on the interface modification in CdS/polymer hybrid composite [21,22]. The CdS/polymer hybrid solar cells after interfacial modification show relatively low PCE. It is believed that there is much room for further improvement in the performance of CdS/polymer hybrid cells by interfacial modification.

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Fig. 1. (a) TEM images of CdS powder. (b) XRD pattern of the CdS film sintered at a temperature of 380 1C. (c) SEM image of a CdS porous film. (d) SEM cross section of a CdS/P3HT hybrid solar cell. The device structure is FTO/dense TiO2/CdS/P3HT/Au.

In this work, we present a facile route to fabricate CdS/P3HT hybrid inverted solar cells. Nanoporous CdS films prepared by doctor-blade method serve as an electron acceptor and P3HT as an electron donor. The CdS/P3HT interface was modified by N719 dye, leading to significant enhancement of performance in this hybrid solar cell. The effect of the interface modification on the performance of the hybrid solar cell is investigated by combining the analysis by photoluminescence (PL), electrochemical impedance spectroscopy (EIS), and Mott–Schottky analysis.

2. Experimental 2.1. Preparation of CdS nanoparticles and CdS films CdS nanoparticles were prepared by a hydrothermal method previously described [23]. Briefly, an aqueous solution of Na2S (800 ml, 0.14 M) was added slowly to Cd(OAc)2 solution (1000 ml, 0.14 M) under vigorous stirring. The yellow mixture was stirred for 24 h before it was suspended in pure water (120 ml) and transferred to a Teflon-lined stainless steel autoclave (150 ml) and heated at 473 K for 72 h. Filtration and washing with water and ethanol subsequently afforded white CdS powder, which was further dried under vacuum at 368 K for 24 h. Cleaned fluorine-doped tin oxide (FTO, 14 O/sq, Ashima) glass is first coated with a  50 nm dense TiO2 layer (as the holeblocking layer) by dip-coating in the titanium(IV) n-butoxide (Ti(OBu)4) and petroleum ether solution with a volume ratio of 1:50. The samples were subsequently calcined at 450 1C for 30 min. Then the as-prepared CdS nanoparticles were used to

prepare the porous films on top of the compact layer by doctorblade method [24,25]. CdS powder (0.4 g), acetylacetone (0.06 ml), TX-100 (0.4 ml), and deionized H2O (8 ml) were grid in an agate mortal into a diluted paste, which was applied for the film preparation. Subsequent annealing was then performed in air at 380 1C for 30 min to remove organic components and improve the CdS particle connectivity. 2.2. Device fabrication Some of the sintered CdS porous films are modified with N719 dye (cis-bis(isothiocyanato)bis(2,20 -bipyridyl-4,40 -dicarboxylato)ruthenium(II) bis-tetrabutyl-ammonium). The samples are immersed in 0.5 mM N719 dye solution in ethanol for 12 h. A layer of P3HT is infiltrated in each CdS nanoporous film by spincoating P3HT solution (Reike Metal, Mw  48000, regioregularity 493%, 24 mg ml  1 in dichlorobenzene) for 60 s and subsequent thermal annealing is done at 100 1C for 20 min in a glove box. Then 50 nm thick Au front electrodes were deposited on top of the film through a shadow mask containing 3.14 mm2 circular openings by thermal evaporation in a high vacuum chamber (6  10  4 Pa). The parameters of the devices are obtained from an average value of a set of five regions on each sample, which reflect the generally observed trend. 2.3. Characterization XRD patterns were measured on a Rigaku diffractometer equipped with a Cu Ka radiation source. UV–Vis absorption spectra were obtained on Shimadzu UV-2550. SEM images were

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obtained on a Quanta 200F microscope (FEI Company) with an accelerating voltage of 0.5–30 kV. The photoluminescence spectra were measured on combined fluorescence spectrometers (FLS920 Edinburgh) using a 450 W Xe lamp (the excitation wavelength: 470 nm). Power conversion efficiency measurements under AM1.5G conditions (100 mW/cm2) were performed using a solar simulator (Newport 92250 A, 300 W). J–V characteristics were measured using a Keithley 2400 SourceMeter. The light intensity was calibrated using a silicon solar cell (SRC-1000-TC-Quartz, NREL calibrated). Electrochemical impedance spectroscopy (EIS) and Mott–Schottky measurements of the hybrid solar cells were recorded on an electrochemical workstation of IM6ex (Germany, Zahner Company), EIS was carried out at a varied forward bias with an AC amplitude of 10 mV and a frequency range of 0.5 Hz to 2 MHz. Mott–Schottky measurements were carried out from 1.5 V to 0.1 V at a frequency of 1 kHz. All measurements were carried out in air at room temperature.

3. Results and discussion TEM in Fig. 1a shows that the average grain size of CdS nanoparticles prepared by hydrothermal method is about 50 nm. The XRD pattern shows that CdS on the FTO are cubic

Fig. 2. (a) UV–Vis absorption spectra of CdS/P3HT hybrid and CdS/N719/P3HT hybrid. Inset: absorption spectra of CdS film and P3HT. (b) PL emission spectra of P3HT conjugated polymer, CdS/P3HT hybrid and CdS/N719/P3HT hybrid.

(see Fig. 1b). Fig. 1c shows the SEM image of a CdS film fabricated by the doctor-blade method, and the nanoporous structure of CdS film can be clearly seen. From the cross-sectional image in Fig. 1d, P3HT interpenetrates inside the CdS porous film. A remaining  50 nm thick P3HT layer is observed on top of the structure, which can potentially reduce the dark (leakage) current. The inset in Fig. 2a shows absorption spectra of P3HT and CdS film. The pristine P3HT exhibits a broad absorption spectrum that ranged from 450 to 650 nm and CdS film have an absorption spectrum that ranged from 400 to 550 nm. The absorption bands of CdS and P3HT complement each other making the two materials appropriate to enlarge the absorption spectrum of the hybrid. Fig. 2a shows the UV–Visible absorption of the hybrid materials with CdS/ P3HT and CdS/N719/P3HT. For the samples with N719 dye, no significant change in the absorption spectra has been found, indicating that the photocurrent enhancement may not be due to the sensitizing effect of the dye. Fig. 2b shows the photoluminescence emission spectra of P3HT, CdS/P3HT, and CdS/N719/P3HT hybrid. The PL intensity of the CdS/P3HT hybrid is significantly reduced compared to that of the pristine P3HT. Strong quenching of the PL emission occurring in CdS/P3HT hybrid implies that charge separation effectively occurred before the exciton recombination within P3HT. Furthermore, the PL intensity is significantly reduced by more than an order of magnitude after being modified by N719 dye, indicating that more efficient charge separation can be achieved at the CdS/P3HT interface. Fig. 3a shows the schematic structure of the CdS/P3HT hybrid solar cell. The compact TiO2 layer prevents direct contact between the polymer and the substrate and serves as a hole-blocking layer. P3HT acts as the electron donor (D) and CdS as the electron acceptor (A). Fig. 3b shows the J–V characteristics of CdS/P3HT and CdS/N719/ P3HT hybrid solar cells. The detailed parameters were summarized in Table 1. CdS/P3HT solar cell gave an open-circuit voltage (Voc) of 493 mV, a short-circuit current density (Jsc) of 0.57 mA/cm2, a fill

Fig. 3. (a) Schematic structure of a CdS/P3HT hybrid solar cell. (b) J–V characteristics of CdS/P3HT and CdS/N719/P3HT hybrid solar cells under AM1.5G simulated illumination (100 mW/cm2). Inside the bracket is the rotation speed of P3HT layer.

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factor (FF) of 0.23, and a power conversion efficiency (PCE) of 0.06%, similar to those reported devices with similar structure [26,27], which is possibly attributed to the poor interfacial contact owing to the hydrophilic CdS surface and the hydrophobic P3HT (see Fig. S1). When the CdS films were modified by N719 dye, significant improvement of Voc, Jsc, FF, and PCE were achieved. The rotation speed of spin coating has great influence on the efficiency of solar cells [28]. P3HT layer becomes thinner with the increase of spincoating speed. The amount of exciton generation decreases with thickness due to less of total absorption of light (see Fig. S2). However, the recombination may also decrease for thinner P3HT films due to the decreased distance required for the charges to reach the electrodes, and more electrons transport to the circuit, resulting in an increase of the overall power conversion efficiency. The optimal thickness is the tradeoff between absorption of the films and recombination in the device. When the rotation speed of P3HT layer was 600 rpm, the PCE of CdS/N719/P3HT hybrid solar cells was dramatically improved to 1.06%. The best performing device was measured at 1.31% under 100 mW/cm2 of AM1.5G illumination. The device performance enhancement may result from the improvement of the CdS/P3HT heterojunction interface. In the following section, the optimized CdS/N719/P3HT solar cell with

Table 1 Photovoltaic parameters of CdS/N719/P3HT hybrid solar cells compared to CdS/ P3HT solar cells and showing the effect of the rotation speed of spin coating. Solar cell

Jsc (mA/cm2)

Voc (mV)

FF

PCE (%)

CdS/P3HT CdS/N719/P3HT(400 rpm) CdS/N719/P3HT(500 rpm) CdS/N719/P3HT(600 rpm) CdS/N719/P3HT(700 rpm) CdS/N719/P3HT(800 rpm)

0.57 3.69 4.45 5.34 4.17 4.11

493 567 521 518 389 344

0.23 0.21 0.28 0.38 0.36 0.35

0.06 0.44 0.66 1.06 0.58 0.49

The parameters of the devices are obtained from an average value of a set of five regions on each sample under 100 mW/cm2 (simulated AM1.5G illumination). Inside the bracket is the rotation speed of spin coating of P3HT layer.

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the efficiency of 1.06% was investigated for comparison with the CdS/P3HT solar cell. EIS is used for understanding of the role of the interface modification on the device performance. Fig. 4a shows the Nyquist plots of CdS/P3HT and CdS/N719/P3HT solar cells characterized at a forward bias of  0.5 V in dark. Each curve is composed of two semicircles: the left semicircle in the high-frequency region is related to the charge exchange process at the P3HT/Au interface, and the right one at lower frequency corresponds to the charge-transfer process at the CdS/P3HT interface, similar to Ref. [29]. The recombination resistance (Rct) and the electron lifetime (tn) were obtained by fitting with the impedance data using the transmission line model [30,31]. Fig. 4b shows the fitted recombination resistance of the two devices under different bias. Rct exhibits the expected exponential behavior. CdS/N719/P3HT cell gives much higher recombination resistance in comparison with that of CdS/P3HT cell at the same bias voltage. The same trend in the semi-logarithmic plots of electron lifetime (tn) is shown in Fig. 4c, and the enhanced electron lifetime can be attributed to the reduced charge recombination at the interface between CdS and P3HT. The interface modification can also adjust the band offset by creating a dipole layer, which can change the effective Ec–HOMO gap and consequently affects the maximum attainable Voc [32]. The dipoles direct away from CdS (as described in Fig. 5a) is due to the protonation effect of the carboxylic acid group, similar to the observation in Ref. [32]. Accordingly, the increase of Voc after interfacial modification may be due to the increase in the effective Ec–HOMO gap in this CdS/P3HT hybrid. Information on band-edge displacement can be sought through differential capacitance measurements with the help of the Mott–Schottky analysis [30,33,34]. Fig. 5b shows the Mott–Schottky plot measured at an AC frequency of 1 kHz for various solar cells. The flatband potential of CdS can be obtained from the intercept (extrapolating the linear plot to C  2 ¼0). From the Mott–Schottky plots, the flatband potential of CdS has been estimated as 0.19 and  0.32 V, respectively, for CdS/P3HT and CdS/N719/P3HT cell. In the case of the CdS/N719/P3HT cell, an upward shift of the flatband potential of approximately 130 mV was observed

Fig. 4. Impedance spectra of (a) CdS/P3HT and CdS/N719/P3HT (with the efficiency of 1.06%) at the bias of  0.5 V in the dark, (b) recombination resistance (Rct) and (c) electron lifetime (tn) in the CdS/P3HT and CdS/N719/P3HT solar cell measured at different biases obtained from impedance measurements in dark.

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Scheme 2. Schematic energy level diagram for FTO/CdS/N719/P3HT/Au device.

Scheme 2 shows the schematic energy levels of CdS/N719/P3HT. The lowest unoccupied molecular orbital (LUMO) levels (3.85 eV) [35] of N719 dye also sit between the LUMO of P3HT (  3.0 eV) and the conduction band edge of CdS ( 4.3 eV). N719 dye can possibly mediate charge transfer in one direction from P3HT to CdS for electron accepting and result in a more efficient charge separation due to the cascaded energy levels. This is supported by the PL measurement. Therefore, N719 dye serves multiple functions to improve the performance of hybrid solar cells.

4. Conclusions

Fig. 5. (a) Schematics of band diagram of CdS/P3HT and CdS/N719/P3HT hybrid solar cells. (b) Mott–Schottky plots of the capacitance of CdS/P3H and CdS/N719/ P3HT (with the efficiency of 1.06%) solar cells.

In summary, hybrid inverted solar cells based on doctor-bladed CdS nanoporous films and P3HT were fabricated. We have found that modification of the interface between the inorganic CdS and the organic P3HT layer with N719 dye can improve the performance of solar cells. Depending on the functionality of the N719 dye, it can help to improve exciton dissociation, reduce interfacial charge recombination, and enhance the band edge offset, thereby the efficiency of the CdS/P3HT hybrid solar cells was increased by 16 times than that of the unmodified one by interface modification with optimized thickness of P3HT films. This work may provide an useful method for increasing the efficiency of organic–inorganic hybrid solar cells by interface modification.

Acknowledgment This work was financially supported by the National Natural Science Foundation of China under Grant no. 20904057 and Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences under Grant no. KGCX2–YW–395.

Appendix A. Supporting information

Scheme 1. Schematic view of CdS/N719/P3HT hybrid.

compared to CdS/P3HT cell, which possibly resulted in the increase of Voc of CdS/N719 hybrid solar cell with N719 dye modification. Additionally, N719 dye is chemically bonded on the CdS surface with carboxylate group and the p-conjugated structure of N719 dye may also interact with the thiophene rings of P3HT (as described in Scheme 1) [20,21], which may further improve the compatibility between the hydrophilic CdS surface and the hydrophobic P3HT [22]. N719 dye may also act as a ‘‘bridge’’ enhancing the charge transfer between the two components.

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2011.09.041.

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