Balanced Dipole Effects on Interfacial Engineering for Polymer/TiO2 Array Hybrid Solar Cells
© The Author(s). 2017
Received: 12 October 2016
Accepted: 28 January 2017
Published: 3 February 2017
The polymer/TiO2 array heterojunction interfacial characteristics can be tailored by balanced dipole effects through integration of TiO2-quantum dots (QDs) and N719 at heterojunction interface, resulting in the tunable photovoltaic performance. The changes of V oc with interfacial engineering originate from the shift of the conduction band (E c) edge in the TiO2 nanorod by the interfacial dipole with different directions (directed away or toward the TiO2 nanorod). The J sc improvement originates from the enhanced charge separation efficiency with an improved electronic coupling property and better charge transfer property. The balanced dipole effects caused by TiO2-QDs and N719 modification on the device V oc are confirmed by the changed built-in voltage V bi and reverse saturation current density J s.
TiO2 is mainly used in photocatalytic and photoelectrode for photocurrent because of its nontoxicity, high electron mobility, and high chemical and thermal stability [1, 2]. Hybrid solar cells (HSCs) based on conjugated conducting polymers (donor) and TiO2 nanocrystals (acceptor) have received extensive attention, as they have the potential to offer low-cost, mechanically flexible, and up-scalable alternatives to conventional photovoltaics [3, 4]. A promising photovoltaic device structure for HSC consisting of a direct and ordered path, instead of disordered three-dimensional networks of interconnected nanoparticles for electron transport to the collecting electrode, has been proposed [5, 6]. Single-crystalline rutile TiO2 nanorod arrays (NRAs) are hydrothermally grown directly on fluorine-doped tin oxide (FTO) substrates as acceptors to dissociate excitons and collect electrons in a HSC, which demonstrates an enhanced power conversion efficiency compared with that of the dense TiO2 film-based device [7, 8]. However, in general, the polymer/pristine TiO2-NRA solar cells perform poorly, wherein most of the open-circuit voltage (V oc) is 0.30–0.44 V and the short-circuit current (J sc) is between 0.28–2.20 mA/cm2 [7–10]. It was demonstrated that the interfaces between the polymer and the nanocrystals play a crucial role in determining the photovoltaic performance. The relatively poor performance of the polymer/pristine TiO2-NRA solar cells can be partly attributed to the undesirable interfacial properties between the polymer and TiO2-NRAs [11, 12].
Optimization of the polymer/nanocrystal interface can enhance the charge separation efficiency and reduce the charge recombination and is an important issue for efficient HSC devices . Therefore, to improve device performance, various studies have been performed on modified TiO2-NRA surfaces. For example, TiO2-NRA modified with an organic molecule (i.e., D149) has improved the J sc to 3.93 mA/cm2 and V oc to 0.60 V due to the improved compatibility of the interface morphology ; inorganic modification of TiO2-NRA, such as with crystalline CdS-quantum dots (QDs), normally results in an increase in J sc to 1.51 mA/cm2 and V oc of 0.45 V ; and modification with crystalline CdSe-QDs normally results in an increase in J sc to 1.15 mA/cm2 and V oc of 0.62 V . It is obvious that both the organic and inorganic modifications differentially affect the polymer/TiO2-NRA devices’ performance. At present, few studies on interfacial engineering of combinations of the organic and inorganic material in the polymer/TiO2-NRA HSCs have been reported. Zhang et al. studied the composite interfacial modification in the P3HT/TiO2-NRA interface using inorganic (CdSe) and organic (N719 dye, pyridine) materials as modifiers . At present, there are some limitations to improve the device performance by the method of monomodification, which leads to the moderate improvement in device efficiency. In their results, the performance of composite interfacial modification was superior to that of modifications based on a monolayer. Obviously, engineering the heterojunction interface using organic and inorganic materials simultaneously in polymer/TiO2-NRA HSCs is a method for further improving the photovoltaic performance.
Synthesis of TiO2-NRA
TiO2-NRA was hydrothermally grown on FTO-coated glass (14 Ω/sq, 400 nm FTO thickness, Nippon Sheet Glass Co.) according to the reported procedure . Deionized water (30 mL) was mixed with 30 mL of concentrated hydrochloric acid (35%) to reach a total volume of 60 mL in a Teflon-lined stainless steel autoclave (100 mL volume). The mixture was stirred in ambient conditions for 5 min, the cleaned FTO substrate was put upside down in the Teflon liner, and 1 mL of titanium (IV) isopropoxide was added. After 10 min of ultrasonic solving, the autoclave was sealed and autoclaving was conducted at 180 °C for 2 h in an electric oven to produce TiO2-NRA.
Synthesis of TiO2-NRA@TiO2-QDs
The TiO2-NRA substrate was removed, rinsed extensively with deionized water, and dried under airflow. Subsequently, the TiO2-NRA substrate was put upside down in the Teflon liner and added 0.1 M titanium isopropoxide ethanol solution. The sealed autoclave was heated to 200 °C in an electric oven for another 4 h to produce TiO2-CSA. Once it cooled, the substrate was removed and dried under airflow after carefully rinsing it with anhydrous alcohol several times.
Synthesis of TiO2-NRA@TiO2-QDs@N719
The dried TiO2-NRA@TiO2-QD substrate was immersed in ethanol solution of N719 (5 × 10−6 M) in an autoclave and heated to 80 °C for 8 h in an electric oven. After the autoclave was cooled to room temperature, the substrate was removed and rinsed with alcohol several times to remove the excess dye, providing the sample TiO2-NRA@TiO2-QDs@N719.
The procedure used for fabrication of solar cells was similar to that described in previous works [17, 18]. Poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) (average Mn = 40000 − 70000, Aldrich) and poly(3,4-ethylene dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) (Clevios P HC V4, H. C. Starck) were commercially obtained. The MEH-PPV layer was deposited on the top of the array by spin-coating (1500 rpm, 40 s) the MEH-PPV solution in chlorobenzene (10 mg/mL) under ambient conditions. Active layer deposition was followed by annealing at 150 °C under N2 atmosphere for 10 min. Subsequently, a PEDOT:PSS film was spin coated (2000 rpm, 60 s) over the polymer layer. After the deposition of PEDOT:PSS, the sample was sequentially heated for 10 min at 100 °C in a N2 glove box. Finally, a gold electrode (100 nm) was evaporated through a shadow mask to form an overlapped area of 3 mm × 3 mm between the indium tin oxide (ITO) and Au, which was defined as the effective device area.
Characterizations and Measurements
Scanning electron microscopy (SEM) measurements of nanostructures were performed with field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) studies were performed on a JEOL-2010 microscope under an acceleration voltage of 200 kV. The room temperature photoluminescence (PL) properties were measured in ambient conditions. PL measurements were made with a Hitachi F-7000 spectrofluorophotometer. The steady-state J−V curves were measured with AM 1.5 illumination under ambient conditions using a 94023A Oriel Sol3A solar simulator (Newport Stratford, Inc.) with a 450 W xenon lamp as the light source. Incident photon-to-current efficiency (IPCE) spectra of the solar cells were measured by using a QE/IPCE measurement kit (Zolix Instruments Co., Ltd.) in the spectral range of 300 − 900 nm.
Results and Discussion
Photovoltaic parameters of solar cells under the AM 1.5 illumination of 100 mWcm−2
V oc (V)
J sc (mA/cm2)
NRA@QDs (4 h)
NRA@QDs (8 h)
NRA@QDs (4 h)@N719 (4 h)
NRA@QDs (4 h)@N719 (8 h)
where N is the dipole concentration, μ the dipole moment, θ the angle the dipole makes to the TiO2 nanorod surface normal, ε r the dielectric constant of TiO2, and ε 0 the permittivity of free space. If dipoles are directed away from the TiO2 nanorod, cosθ > 0 and leading to δE > 0; if dipoles are directed toward the TiO2 nanorod, cosθ < 0, leading to δE < 0. Therefore, the magnitude of E c shifting correlates with the dipole concentration and direction in the shell/polymer interface. With the presence of dipoles directed away from the TiO2 in the shell/polymer interface (i.e., cosθ > 0) due to the TiO2-QD shell (Fig. 8b), the E c of the TiO2 nanorod core will be shifted toward the local vacuum level of the polymer due to the δE > 0 (Fig. 5b).
The obtained V oc of 0.54 and 0.63 V for the MEH-PPV/TiO2-NRA@TiO2-QDs&N719-based device, however, are somewhat lower than the value of 0.69 V for the MEH-PPV/TiO2-NRA@TiO2-QD-based counterpart device. This results from the modification of the ZnO surface by N719, stemming from the dissociative adsorption of the carboxylic acid group to form a carboxylate bond, in which the positive proton charge on the surface and the negative charge on the carboxylic group together form an interfacial dipole [21, 22]. A theoretical calculation has demonstrated the direction of the dipoles generated by the adsorbed N719 molecules on the oxide surface with the monodentate anchoring mode directed to the oxide surface (i.e., cosθ < 0) (Fig. 5b) . In this case, the dipole concentration generated by the modification with N719 will change the E c of the TiO2 nanorod with δE < 0 based on eq. (1). That means the V oc will be reduced by shifting the band edge potential of TiO2 closer to the polymer E vac [20, 24]. The V oc in the MEH-PPV/TiO2-NRA@TiO2-QDs&N719 device with 8 h of N719 bonding time was further confirmed in this conclusion (Fig. 2 and Table 1). The dipole concentration in the sample MEH-PPV/TiO2-NRA@TiO2-QDs&N719 with 4 h of N719 bonding time should be lower than the counterpart with 8 h. The magnitude of suppressed band edge shifting of TiO2 (i.e., δE < 0) should be smaller than the 8-h sample based on eq. (1), which causes the V oc in the 8-h device MEH-PPV/TiO2-NRA@TiO2-QDs&N719 to be lower than MEH-PPV/TiO2-NRA@TiO2-QD&N719 with 4 h. Therefore, there is a balance of dipole effects (i.e., positive or negative of δE and its magnitude) by QD layer and N719 modification on device V oc.
Equation (4) indicates that a plot of lnJ d versus V should yield a straight line. Therefore, J s and n can be extracted from the lnJ d−V curves in the linear region, which q/nkT and lnJ s corresponds to the slope and y-intercept, respectively (Fig. 8). Therefore, we extracted the approximate value of dark reverse saturation current Js from the dark J−V curve based on eq. 4 .
Based on eq. 2, we calculated the interface energy barrier Φ B values of all devices (Fig.7b). The energy barrier φ for this process is correlated, but not necessarily just equal, to the difference between the E c and E HOMO (E c−E Homo in Fig. 7a) due to the complicated interfacial dynamic processes [28, 30]. If there was a shift of the E c edge in the TiO2 nanorod, it would affect the E c−E Homo (i.e., φ), and thereby influence the J s, based on eq. 2. The changes of device J s and V oc with interfacial engineering are depicted in Fig. 7b. It is observed that J s decreased from 2.76 × 10−3 mA/cm2 in the MEH-PPV/TiO2-NRA device to 2.03 × 10−4 mA/cm2 in the MEH-PPV/TiO2-NRA@TiO2-QD device; therefore, the energy barrier φ (or E c−E Homo) increased based on eq. 2, which agrees with the expectation on the up-shift of the E c edge in the TiO2 nanorod after the growth of the QD layer in Fig. 5b. Additionally, the little increase of J s (3.35 × 10−4 mA/cm2) after the engineering of N719 agrees with the small downshift of the E c edge in the TiO2 nanorod (i.e., φ) due to the adsorbed N719 molecules on the TiO2-QD surface with monodentate anchoring mode directed to the TiO2 surface in Fig. 5b.
The heterojunction interfacial engineering in polymer/TiO2 nanorod array (NRA) hybrid solar cells was performed in two steps: first, we grew TiO2-quantum dots (QDs) on a TiO2-NRA surface to form the TiO2-NRA@TiO2-QD structure. Next, the TiO2-NRA@TiO2-QD structure was further bonded with organic molecules (N719) on its surfaces to form the TiO2-NRA@TiO2-QDs@N719 composite array through the solvothermal method. By controlling the interfacial engineering for polymer/TiO2-NRA solar cells through the integration of TiO2-QDs and N719 molecules, the V oc and J sc in polymer/TiO2-NRA@TiO2-QDs@N719 solar cells can be tuned, improving the device efficiency nearly four times compared with that of pristine TiO2-NRA-based solar cells. The tunable device performance is resulted from the balanced interfacial dipoles, which is confirmed by the changed built-in voltage V bi and reverse current J s. These results therefore provide information crucial to the optimization of interface in HSCs.
We acknowledge the “1112 Talents Project” of Huzhou City and the valuable suggestions from the peer reviewers.
The role of the National Natural Science Foundation of China (21607041; 11547312; 11647306) is designing the work; the role of the Zhejiang Provincial Natural Science Foundation of China (LQ14F040003) is purchasing the materials; and the role of the Science and Technology Planning Project of Zhejiang Province (2017C33240), Seed Fund of Young Scientific Research Talents of Huzhou University (RK21056), and Foundation of Science and Technology Innovation Activities & Emerging Talents Plan of Zhejiang Province (2015R427005; 2016R42707) is the collection, analysis, and interpretation of the data.
FW carried out the experiments and drafted the manuscript. YZ, XY, and JX participated in the device preparation. XL participated in the design of the study. YT conceived of the study and helped to draft the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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