Fabrication of nanostructured ZnO film as a hole-conducting layer of organic photovoltaic cell
© Kim et al.; licensee Springer. 2013
Received: 18 July 2012
Accepted: 18 April 2013
Published: 16 May 2013
We have investigated the effect of fibrous nanostructured ZnO film as a hole-conducting layer on the performance of polymer photovoltaic cells. By increasing the concentration of zinc acetate dihydrate, the changes of performance characteristics were evaluated. Fibrous nanostructured ZnO film was prepared by sol-gel process and annealed on a hot plate. As the concentration of zinc acetate dihydrate increased, ZnO fibrous nanostructure grew from 300 to 600 nm. The obtained ZnO nanostructured fibrous films have taken the shape of a maze-like structure and were characterized by UV-visible absorption, scanning electron microscopy, and X-ray diffraction techniques. The intensity of absorption bands in the ultraviolet region was increased with increasing precursor concentration. The X-ray diffraction studies show that the ZnO fibrous nanostructures became strongly (002)-oriented with increasing concentration of precursor. The bulk heterojunction photovoltaic cells were fabricated using poly(3-hexylthiophene-2,5-diyl) and indene-C60 bisadduct as active layer, and their electrical properties were investigated. The external quantum efficiency of the fabricated device increased with increasing precursor concentration.
KeywordsElectronic materials Polymers Vapor deposition Sol-gel process ZnO Nanostructured fibrous film
Clean and renewable energy has been a considerable issue in the last decade. For this reason, organic photovoltaic cells (OPCs) have been attractive devices as next-generation substitute energy sources [1–4]. At present, the performance of OPCs has been reported up to power conversion efficiency (PCE) of 10% and above [5, 6]. There have been reports that polymer solar cells have many advantages of cost effectiveness in the fabrication process, and the mechanical flexibility and polymeric materials provide a wide field of applications. Furthermore, the advantage of organic photovoltaic cells has a high potential to be manufactured using continuous coating technology capable of producing large areas at a low cost [7, 8]. Poly(3,4-ethylenedioxythiophene:poly(4-styrenesulfonate)) (PEDOT:PSS) is the most widely utilized as hole-conducting layer material in organic light-emitting diodes and photovoltaic cells . The advantages of PEDOT:PSS include low temperature, excellent stability, large area processing, low cost, and flexibility. However, the efficiency of this material is limited by their low carrier mobility . Therefore, hole mobility is a key parameter for photovoltaic devices with respect to their adaption in device applications.
ZnO has received much attention over the past few years because of its wide range of properties that depend on doping, including a range of conductivity from metallic to insulating (including n-type and p-type conductivity), high transparency, piezoelectricity, wide-bandgap semiconductivity, room-temperature ferromagnetism, and huge magneto-optic and chemical-sensing effects. Without much effort, it can be grown into many different nanoscale forms, thus allowing various novel devices to be achieved . ZnO, a II-VI semiconductor, is now recognized as a promising candidate for blue and ultraviolet light-emitting diodes or laser diodes due to its wide bandgap of 3.37 eV and large exciton binding energy of 60 meV [12–17]. Its large exciton binding energy allows excitonic absorption and recombination even at room temperature, which makes this material appealing . A lot of methods have been extensively used for oriented ZnO film synthesis, including laser molecular beam epitaxy, pulsed laser deposition, metal-organic chemical vapor deposition, sputtering , cathodic magnetron sputtering and reactive electron beam evaporation, spray pyrolysis, and electrodeposition. However, sol-gel processes are particularly adapted to produce ZnO colloids and films in a simple, low-cost, and highly controlled way. The sol-gel process, also called soft chemistry (‘chimie douce’), allows elaboration of a solid material from a solution by using a sol or a gel as an intermediate step and at much lower temperatures than is possible by traditional methods of preparation . It enables the powderless processing of glasses, ceramics, and thin films or fibers directly from a solution. The synthesis of solid materials via chimie douce often involves wet chemistry reactions and sol-gel chemistry based on the transformation of molecular precursors into an oxide network by hydrolysis and condensation reactions [19, 20].
Recently, poly(3-hexylthiophene) (P3HT) has been used as a hole transporter in combination with ZnO nanostructures. These devices have an efficiency of approximately 0.5% under standard solar conditions (AM 1.5, 100 mW/cm2) and show a current density of Jsc = 2.2 mA/cm2, an open-circuit voltage of Voc = 440 mV, and a fill factor of 0.56. This cell performance can be significantly improved to Jsc = 10.0 mA/cm2, Voc = 475 mV, and a fill factor of 0.43, leading to an efficiency of 2% by using a blend of P3HT and (6,6)-phenyl-C61-butyric acid methyl ester. The low open-circuit voltage in hybrid solar cells using ZnO as the electrode material is not yet fully understood. Certainly, more investigation is necessary to find the leakage, and then higher cell efficiencies can be expected .
In this work, we have investigated the structural, morphological, and optical properties of ZnO nanostructured fibrous film spin coated on indium-tin oxide (ITO) glass. We fabricated polymer solar cells that have the structure of ITO/ZnO/PEDOT:PSS/active layer (P3HT:ICBA)/Al. Poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA) were blended and used as an active layer in polymer bulk heterojunction (BHJ) photovoltaic cells. The performance characteristics of polymer photovoltaic cells using ZnO nanostructured fibrous film as a hole-conducting layer have been investigated.
ITO thin films are a highly degenerate n-type semiconductor which have a low electrical resistivity of 2 to 4 × 10−4 Ω cm. The low resistivity value of ITO film is due to high carrier concentration. It has a wide-bandgap semiconductor (3.5 to 4.3 eV), which shows high transmission in the visible wavelength (80% to 90%) and relatively high work function (4.7 eV). The ITO glass substrates were supplied from Samsung Corning Precision Materials Co. Ltd (Seoul, Korea). PEDOT:PSS aqueous solution (1.3 wt.%) as a buffer layer material was purchased from Baytron® (Hanau, Germany). Zinc acetate dihydrate as a precursor material was purchased from Junsei Chemical (Tokyo, Japan). P3HT as an electron donor and ICBA as an electron acceptor were purchased from 1-material Co. (Quebec, Canada). 1,2-Dichlorobenzene and isopropanol as a solvent were purchased from Sigma-Aldrich (Seoul, South Korea). Monoethanolamine as additive was purchased from Junsei Chemical (Tokyo, Japan).
Preparation of ZnO nanostructured fibrous film
The pre-patterned ITO glass substrates were cleaned with acetone, ethanol, and isopropyl alcohol (1:1:1) for 1 h by sonication and then rinsed with ethanol. After cleaning, the ITO glass substrates were annealed at 230°C for 10 min in vacuum and served as high-work function electrode. ZnO nanostructured fibrous films were prepared by sol-gel process in which zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O) was added to a solution of isopropanol and monoethanolamine. The molar ratio of zinc acetate dihydrate and monoethanolamine was 1:1, and the zinc concentration in isopropanol was set from 0.2 to 1.0 M. The mixture was stirred at 60°C for 2 h to yield a clear homogeneous solution. After stirring, the solution was spin coated at 3,000 rpm for 20 s on the pre-patterned ITO glass. The films were then dried at various temperatures for 3 h and then cooled to room temperature on a hot plate. The ZnO nanostructured fibrous films were observed under scanning electron microscopy (SEM; S-4800, Hitachi, Tokyo, Japan). The crystal structures of the samples were characterized using an X-ray diffractometer (XRD; D8 Advance, Bruker AXS GmbH, Ettlingen, Germany) with CuKa (k = 1.5418 Å) radiation.
PEDOT:PSS was used as a buffer layer material and filtered using a 0.45-μm Millipore polytetrafluoroethylene syringe filter (Millipore Co., Billerica, MA, USA). PEDOT:PSS was stirred for 1 h and then spin coated on the ZnO nanostructured fibrous film at 3,000 rpm for 60 s using a digitalized spin coater (MS-A10, Mikasa Co. Ltd., Tokyo, Japan). The PEDOT:PSS thin films were annealed for 20 min at 120°C in vacuum to remove the water. After the annealing process, the devices were cooled down to room temperature. The bulk heterojunction active layer was prepared via solution process. P3HT and ICBA were dissolved in 1,2-dichlorobenzene in a weight ratio of 1:1 and concentration of 20 mg/ml solution. The blend of P3HT and ICBA was stirred for 24 h at 40°C. The blend of P3HT:ICBA solution was spin coated on the PEDOT:PSS buffer layer at 2,000 rpm for 60 s. After spin coating the active layer, Al cathode was thermally evaporated onto the active layer in vacuum with a thickness of 100 nm. The thickness was measured using a well-calibrated quartz crystal thickness monitor (CRTM-600, ULVAC Kiko Co. Ltd., Saito Japan). The vacuum pressure was under 3 × 10−5 Torr, and the deposition rate of aluminum was controlled from 1 to 5 Å/s. The fabricated devices were subsequently post-annealed for 10 min at 150°C in vacuum condition.
Results and discussion
X-ray diffraction spectra
Scanning electron microscopy
UV-visible absorption spectra
Performance characteristics of the photovoltaic devices
Short-circuit current (mA/cm2)
Open-circuit voltage (V)
Power conversion efficiency (%)
0.2 M precursor
0.4 M precursor
0.6 M precursor
0.8 M Precursor
External quantum efficiency
In this work, we synthesized ZnO fibrous nanostructure by sol-gel process with various precursor concentrations. We have investigated the performance characteristics of organic photovoltaic cells using nanostructured ZnO film as a hole-transporting layer. ZnO film-based photovoltaic cells were focused on the dependency of Zn2+ precursor concentration with morphology. By adding ZnO fiber film, the conductivity and carrier mobility of the device were improved. As the precursor concentration increased, ZnO (002) orientation was observed. In a morphological aspect, with increasing concentration of precursors (0.2 to 1.0 M), the fibrous structure grew with a thickness of 300 to 600 nm and a maze-like structure. Fibrous structures have more effective surface area than smooth surface; ZnO fibrous structure is expected to be used in photovoltaic devices. For the photoluminescence aspect, the UV and green-yellow PL intensities increase with increasing concentration of precursor from 0.2 to 1.0 M. The UV-visible spectra studies show that a rapid increase of intensity at the whole wavelength area was observed. Especially, intensity at the ultraviolet area increased rapidly. The external quantum efficiency of the device was improved at the whole wavelength. The performance characteristics of polymer BHJ photovoltaic cells using ZnO fiber film as a hole-conducting layer and a P3HT:ICBA blended active layer have been investigated. As the concentration of Zn2+ precursors increased from 0.2 to 0.6 M, Voc, Jsc, and PCE increased. This improvement can be explained by an increased charge carrier mobility of holes and electrons. However, as the concentration of Zn2+ precursor reached 0.8 M, all values of the characteristic parameters decreased. The polymer photovoltaic cells with the structure ITO/PEDOT:PSS (180°C for 1 h annealing)/P3HT:ICBA (20 mg/ml) (1:1 wt.%)/Al (100 nm) were investigated with the maximum power conversion efficiency of 6.02%.
HK and YK are MSc students at the Chemical Engineering Department, Pusan National University, South Korea. YC is a professor in the Chemical Engineering Department, Pusan National University, South Korea.
polymer bulk heterojunction
Scanning electron microscopy
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010–0003825) and the Brain Korea 21 project.
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