Synthesis of hybrid solar cells using CdS nanowire array grown on conductive glass substrates
Introduction
Owing to distinct electrical and optical properties, organic–inorganic hybrid composites have shown strong potential to be used for high-performance devices including sensors, displays, light emitting diodes, solar cells, refractive and anti-refractive materials, and so on [1], [2]. In particular, organic–inorganic hybrid solar cells show technological importance due to advantages, such as low cost, easy fabrication, and the possibility to fabricate flexible devices, compared to inorganic solar cells [3], [4]. In accordance with the need in hybrid solar cells, various inorganic semiconductors, such as CdSe, CdTe, CdS, and PbS, have been actively investigated because of their application to serve as an acceptor for hybrid solar cells [5], [6], [7]. However, compared to the synthesis of CdSe nanomaterials for hybrid solar cells, the fabrication of CdS nanomaterials is still in a preliminary stage [8]. CdS is a II–VI semiconductor having a direct band gap of 2.4 eV at room temperature and it has been known as one of the most promising photo-sensitive materials owing to its unique photochemical activities and strong visible-light absorption and emission [8]. Hence, it has many commercial or potential applications in light-dependent resistors, solar cells, or other photoelectronic devices [9], [10], [11]. CdS nanostructured materials have attracted a great deal of interest in hybrid photovoltaic applications due to possible enhancement in power conversion efficiency arising from their large surface-to-volume ratio. Among various nanostructures, nanocrystals are the most frequently used in hybrid solar cells due to ease of fabrication [12]. However, electron transport along with the nanocrystals requires long pathways to reach an electrode and considerable portion of free electrons are apt to be trapped during the slow hopping transport [13]. On the other hand, one-dimensional NWs having high-aspect-ratio morphology can provide direct and efficient electron pathways from the light-absorbing materials to conductive substrates by avoiding a series of particle-to-particle-hopping transport that occurs in nanoparticle-based hybrid solar cells [14]. To obtain high photoconversion efficiency in conjugated polymer–CdS hybrid systems, photogenerated electrons should transfer from conjugated polymer to CdS prior to the electron-hole recombination, while holes should transfer from CdS to conjugated polymer. CdS NW array grown on a substrate is a desirable structure for the synthesis of organic–inorganic hybrids to be used for photovoltaic and other optoelectronic applications compared to agglomerated particle forms [15]. However, in spite of reports on the vapor-phase growth of CdS NWs, there still exist drawbacks to be overcome for their applications, such as deterioration of transparent electrodes by high processing temperature (∼800 °C) [11]. Low-temperature synthesis of CdS NWs that is compatible with conductive glass substrates has emerged as one of the key factors that enable the fabrication of high-efficiency NW solar cells [14]. So far, there is no report for the vapor-phase growth of CdS NWs on glass substrates and special strategies should be employed to achieve it.
In this study, we demonstrate the success in the vapor-phase synthesis of CdS NWs at a considerably reduced temperature that is compatible with transparent conducting oxide-coated soda-lime glass substrates. Also, heterojunctions of CdS NWs-conducting polymer which has type II band alignment formed as photovoltaic cells. The mechanism was discussed in detail for NW growth at a reduced temperature and the photovoltaic properties were investigated for polymer–CdS hybrid solar cells.
Section snippets
Experimental details
To grow the CdS NWs, 2–5 nm thick Bi catalyst layers were first sputtered in the area of 1 cm × 1 cm on pre-cleaned fluorine-doped tin oxide (FTO)-coated soda-lime glass substrates that were 1.5 cm × 1.5 cm in size. Polyvinyl alcohol (PVA) was formed over the Bi layer surface by dip coating. Next, cadmium sulfide (99.99%, Aldrich) and graphitized carbon black (99.9%, Aldrich) were mixed and ground in a mortar. The powder was dry-milled using an YSZ ball media (Tosoh Chemical, Tokyo, Japan) at 300 rpm for
Growth of CdS NWs on conductive glass substrates
CdS NWs were directly synthesized on FTO glass substrates by chemical vapor transport (CVT). FE-SEM cross-sectional view image of CdS NWs is shown in Fig. 1a, illustrating that the reaction produces high-density straight CdS NWs. Bi catalysts attached to the NW tips evidently reveal that the NW growth mechanism is vapor–liquid–solid (VLS) [16], [17], [18]. Single crystalline CdS NWs with 50–100 nm and 2–5 μm in diameter and length were formed at 450 °C for 10 min on FTO glass substrates. This is a
Conclusions
High-density and single-crystalline CdS NWs were synthesized at 450 °C by chemical vapor transport technique. Owing to the low synthesis temperature, CdS NWs were successfully grown on transparent conducting oxide (TCO)-coated glass substrates. Moreover, we demonstrate formation of MEH-PPV conjugated polymer–CdS NW hybrids via spin coating. This MEH-PPV–CdS heterojunction reveals noticeably enhanced visible-light absorption and suppressed visible-light emission. Photovoltaic cells fabricated
Acknowledgment
This work was supported by the Components and Materials Technology Development Program funded by the Ministry of Knowledge Economy, Republic of Korea (MKE, Korea) in 2008.
References (19)
- et al.
Adv. Colloid Interface Sci.
(2008) - et al.
Electrochem. Commun.
(2008) - et al.
Adv. Mater.
(2008) - et al.
Nano Lett.
(2007) - et al.
Science
(2002) - et al.
J. Phys. Chem. B
(2002) - et al.
Nano Lett.
(2003) - et al.
Appl. Phys. Lett.
(2005) - et al.
J. Phys. Chem. C
(2007)
Cited by (79)
Adjusting the structural, electronic and optical properties of CdS by the introduction of Be: A DFT study
2022, Materials Today CommunicationsCitation Excerpt :The Co doped CdS has been proven to have potential possibility for magnetic semiconductors and spintronics [54,58]. The doping of Cu in CdS helps to change the type of CdS from n to p semiconductor, which can be used in photovoltaic cells because of changes of band gap of CdS [49,57,59,60]. The fluorescence, magnetic, optical and optoelectronic properties of CdS can be improved by the introduction of Fe in CdS [48–52,61].
Preparation of CdS nanorods on silicon nanopillars surface by hydrothermal method
2019, Materials Research BulletinInsight into carrier transportation and hydrogen production activity of two novel morphological CdS films
2017, International Journal of Hydrogen Energy