Abstract

The effect of nanoplasmonic (Ag) on the performance of DSSCs has been studied in doped and undoped ZnO (DZ and UZ) NPs, which were prepared by the urea-assisted combustion route. Different techniques were conducted to characterize DZ and UZ NPs. XRD patterns were indexed to the hexagonal wurtzite structure of ZnO NPs (ICSD-52362). The values of average crystalline size of UZ and DZ (1.0 mol% Ag) NPs were 20.45 and 22.30 nm, respectively. HR-TEM micrograph revealed good crystallization with an intermediate or poor agglomeration with distribution of semispherical morphologies of ZnO NPs. The energy bandgap of UZ and DZ NPs was changed from 3.21 to 3.31 eV. The deconvolution of the PL spectra recognized eight peaks into near ultraviolet (NUV) and visible regions. The PL emission of visible region overshadowed NUV transition. The photovoltaic cell with the doped photoanode DZ:1.0 mol% Ag exhibited the best performance parameters: Voc = 0.46 V, Jsc = 7.81 mA.cm<sup>2</sup> , Pm = 1.91, FF = 51%, and η = 1.91%. A double exponential function was used as a powerful fitting function for the TOCVD data. The results revealed that τ<sup>n</sup> in the UZ NPs photoanode was longer than that in the DZ:1.0 mol% Ag NPs photoanode.

Keywords: combustion synthesis, eosin Y, nanoplasmonic, solar energy conversion device, renewable energy, defects, lifetime, electron recombination

## 1. Introduction

The exacerbation of energy worldwide crisis in addition to two bottlenecks: (i) the inevitable exhaustion of fossil fuel in the coming fifty years and (ii) the global environmental damage caused by the combustion of large amounts of fossil fuels [1], spurs us to find a new pollution-free, eco-friendly, renewable, clean and sustainable energy source [2]. There are many types of renewable energy resources—such as wind, biomass, and tidal, geothermal, and solar energy [3]. Accordingly, solar energy has been increasingly exploited, which is the most promising one to meet the future demands as it is in limitless supply [2]. Consequently, it is imperative to develop solutions to broaden light absorption and suppress charge recombination toward efficient solar energy conversion (SEC) [2]. Hence, SEC into a most usable form, i.e., electrical energy, is highly important to fulfill the everincreasing demand for energy [1]. Photoelectrical conversion (PEC) is a promising

and environmentally friendly technology to solve the energy crisis faced because sunlight gives a power of 106 TW/year continuously [4]. Photovoltaic cells (PVs) are an essential component in smart grids and mobile commerce or in the development of integrated photovoltaics (BIPVs) and vehicles [5]. Dye-sensitized solar cells (DSSCs) are a class of mesoporous solar device which have evolved as credible alternative to conventional solid-state p-n junction photovoltaics [6, 7]. The DSSCs have several advantages, showing higher performance with relatively lower cost, lower handling expenses, lower strength of optical and incidence angles, higher mechanical durability, lighter weight, and more aesthetically pleasing and transparent design [1, 5]. With the advantages of being easily manufactured, colored, and flexible, this type of solar cells has become available and considerable for reducing the cost of electricity generation [8]. Nevertheless, their low efficiencies remain a major roadblock to commercialization in DSSCs [5]. In addition, light absorption, charge recombination, and dye pickup are still a challenge in the field of DSSC technology [9]. Numerous strategies have been investigated to enhance PCE of DSSC by improvement of light harvesting, carrier collection and noble metal NPs, using semiconductor quantum dots, developing new dyes and designing new morphology [10]. Moreover, different approaches were designed such as large surface area mesoporous photoanode; hierarchically Nano-structured and scattering top-layer photoanode; new panchromatic absorbing dyes; photonic crystal photoanode and plasmonic photoanode [5, 6]. In PEC, semiconductors play an important role in capturing light to generate electron–hole pairs for subsequent electricity and chemical fuel production. Also, they generate photocurrent and produce H2 under visible light illumination [4]. Recent researches on pure and alloyed metal NPs demonstrated high potential in this field to improve light absorption/scattering properties of semiconductor network. This is due to their unique response under particular wavelengths known as plasmonic effect [9]. Newly, ZnO has been used as an alternative n-type semiconducting photoanode material for DSSCs. This is because ZnO has high electron dynamics, and a versatile nanostructure morphology relative to others [11]. ZnO plays an important role in the optoelectronic devices. This is because of its excellent properties of piezoelectric, electron conductivity, and a large exciton binding energy of 60 meV [12]. One general advantage of incorporating NPs into DSSCs is that they provide far-field scattering and concomitant enhancement. This is due to longer effective optical path lengths [13]. Their outstanding light trapping and electromagnetic field concentrating properties proved very effective [9]. Further modifications have been performed by mixing with other nanostructured metal oxides or with metallic NPs. These metallic NPs include noble metals (NMs) Au, Ag, and Cu, i.e., nanoplasmonic (NPL) metals), for improving their charge carrier and absorption properties [11]. NMs in nanoscale have widely been applied in various research fields, including optical sensors, catalysts, and surface-enhanced Raman scattering. This is due to their characteristic optical and electrical properties, which are not available with bulk analogues [14]. NPL has unique optical characteristics that can be harnessed for several technological applications from spectroscopy to nanomedicine to photovoltaics [7]. NPL is a rapidly growing research field that exploits enhancement of concentrated optical energy on the nanoscale in nanostructured metal [11]. Also, NPL enhanced photocatalytic including water splitting, artificial photosynthesis, and photodegradation of organic pollutant and photovoltaic systems [2, 4, 13]. On the other hand, NPL introduction in photoactive layers of DSSC, to trap or confine light inside the active layer and enhance the absorption in the semiconductor film could provide superior performances presumably due to their unique electronic, optical and magnetic properties [15]. Moreover, NPL enhancement in solar cells is achieved by four possible mechanisms. They are far-field coupling of scattered

#### Nanoplasmonic for Solar Energy Conversion Devices DOI: http://dx.doi.org/10.5772/intechopen.84953

light, which can be trapped in the mesoporous semiconductor, near-field coupling of electromagnetic fields, plasmon resonance energy transfer from nanostructures to semiconductor, and hot-electron transfer [4, 7, 14]. As sun light possesses less energetic photons, the hot electrons cannot be produced just by the solar spectrum. In DSSC technology, this problem can be solved by the use of exothermic chemical processes [9]. In another study, quantum capacitance-like effect has been observed in the device, which accelerates the carrier transport via additional electric field [9]. The enhanced electromagnetic field is highly dependent upon the wavelength of incident light, also, the shape, size, and aggregation state of the NPs [10]. The increasing of the light-harvesting efficiency was done by introducing NPL structures into various types of PVs such as silicon, thin-film, organic solar, and polymer solar cell [10, 11, 13, 14]. Among the NMs silver, which have the critical advantages and is considered as one of the most suitable candidates for practical applications due to its facile easy preparation and lower cost than others [10]. As to the electrical aspect, Ag NPs could play a role as the electron transport separation center, which accelerated the electron to move through the photoanode network and reduced the charge recombination [8]. However, the exact mechanism for the plasmon-induced charge separation process in plasmonic photovoltaic devices is not clarified yet [14]. In conclusion, the possibility of photosensitization of MOs using plasmonic metal NPs is tantalizing [7]. While there is increasing evidence for hot-electron injection, there is no consensus to date. Further research is necessary to establish more firmly the possibility [4]. More work is also needed to better understand what factors determine the mechanism of operation. The strength of interaction between the metal NP and MOs is likely a critical factor, which may in turn depends on the detailed structures of both constituents down to the molecular or atomic level [4]. The merits designate the implementation of plasmonic nanostructures into photocatalysis and photovoltaics as a promising strategy for solar energy conversion [2]. Several methods have been reported to deposit Ag NPs onto DSSC photoanode. However, none is able to prevent the aggregation of Ag NPs and keep structures uniform during the deposition process [10]. Until now, there has been strong controversy regarding the role of plasmonic NPs in DSSCs. In addition, more indepth investigation for the enhancement mechanism is still required [14]. Guided by this principle, in this study, we designed a kind of unique Ag-doped ZnO NPs as a photoanode in DSSCs, to investigate the plasmonic effects and elucidate the working principle more systematically. Nevertheless, this method has never been used for doping metallic particles for plasmon effect in DSSCs. The present work is the first effort to employ solution combustion method for enhancing plasmon effect in DSSCs. The plasmonic effect of Ag on the performance of DSSCs has been studied. A dramatic increase in efficiency is achieved in the device of Ag NPs/ZnO NPs compared with that of ZnO NPs.
