Undoped and Manganese2+-doped polycrystalline Cd1−xInxTe sensitizer for liquid-junction solar cell devices
Graphical abstract
Introduction
Conventional dye-sensitized solar cells (DSSCs) have been extensive studied which still an active and effective research field and also it can be a promising low-cost production, high transparency, ease of fabrication and yielded a good power conversion efficiency, replacing silicon-based photovoltaic devices [1], [2], [3], [4], [5]. Typically, the DSSCs comprise of a mesoporous TiO2 as a photoelectrode and coated a thin layer of an organic dye as a photosensitizer. Nevertheless, organic dyes are relatively highly toxic, unstable in the long term and the main absorption range only covers the visible region (i.e. it cannot reach the near-infrared region), leading to energy loss in DSSCs. Thus, to solve this problem, the narrow-band-gap semiconductor nanoparticles (NPs), referred to as quantum dots (QDs), as an alternative material are recommended to be explored.
The binary II–VI metal chalcogenides have been offered as new opportunity sensitizers such as PbS, Sb2S3, Bi2S3, Ag2S, Cu2S, Cu2−xTe and MnTe [6], [7], [8], [9], [10], [11], [12]. The several advantages of inorganic sensitizers such as tunable absorption bands due to the quantum-size effect [13], [14], a high extinction coefficient and multi electron–hole pair generation (MEG) by a single incident photon [15], [16] and the extraction of hot electrons which can be delivered in the cells to boost the operational efficiency [17]. Narrow-band-gap semiconductors also absorb photons in the full sunlight spectrum. Recently, the highest efficiency of 11% was yielded by single InAs QDs with a sol–gel-grown single layer Ta2O5 anti-reflection coating [18]. However, the theoretical efficiency of SSCs has been studied and it was expected to achieve up to 44% [19], which is much higher than the rate of 31% for semiconductor solar cells according to the Shockley–Queisser limit [20].
Apart from binary semiconductors, ternary semiconductor NPs have also been attracted much interest as a strong potential candidate for photovoltaic materials due to their direct band gaps providing a well-matched solar spectrum which will be of benefit for energy absorption and have high absorption coefficients and high stability. However, there have been only a few studied due to the greater difficulty of synthesis because the corrected three-element stoichiometry of the atom is involved. For example, H.P. Ho and Prasad et al. synthesized a I–III–VI chalcopyrite semiconductor-AgInS2; it was found that this semiconductor absorbed light in the visible region and produced an efficiency of 0.05% accordingly [21]. Recently, Lee et al. synthesized lead antimony sulfide (Pb5Sb8S17) semiconductor-sensitized solar cells with the highest efficiency of 2.51% under 0.118 sun [22], while silver antimony sulfide (AgSbS2) as a semiconductor nanoparticle sensitizer produced the best efficiency of 1.28% under 0.148 sun [23]. Furthermore, a new ternary-AgBiS2 was synthesized for liquid-junction solar cells, producing an efficiency of 0.53% under 1 sun and reached 0.76% at a light intensity of 0.148 sun [24]. These result can be explained that the I–V curves measured under 1 sun generate more photoelectrons, which produces more carrier recombinations. By reducing the light intensity, the number of photoelectrons is greatly reduced and hence, carrier recombination is also decreased, yielding a higher η. Chang et al. prepared CdHgTe NCs, which covered an absorption range of 400–1200 nm and produced an efficiency of 1.0%. When the material acted with CdTe QDs, the efficiency increased to 2.2% under 1 sun [25]. To date, Zhong et al. have produced the highest efficiency of 6.36% under full 1 sun illumination from the ternary semiconductor-CdSe0.45Te0.55 system [26].
One material, i.e. cadmium indium telluride (CdIn2Te4) is a II–III–VI ternary compound which as a candidate semiconductor has attracted attention for study in optical absorption, spectroscopic ellipsometry (SE) and thermo reflectance (TR) measurements as it can be applied as an optoelectronic and a nonlinear optical device [27]. CdIn2Te4 has also a direct-energy gap of ∼1.1–1.2 eV at 300 K which is close to that of the optimal energy gap (∼1.49 eV) of a solar absorber and its structure is basically a defect chalcopyrite structure with half of one of the cations removed [28], [29], [30], [31]. The absorption coefficient α as a function of photon energy (∼1.2 eV) at room temperature is higher than 200 cm−1 [30]. Thus, these reasonable features suggest that the Cd1−xInxTe compound incorporated with the CdIn2Te4 has potential for utilization as a good solar absorber. Previously, Cd0.6In0.4Te with an energy gap of 1.37 eV was synthesized using a wet chemical method for solar cell applications and produced the best power conversion efficiency of 1.89% [32]. The optical properties and energy gaps of Cd1−xInxTe were studied by doping with various Mn2+ concentrations; the results showed that the energy gaps decreased with increasing Mn2+ doping concentration [33]. Thus, this technique can be applied to a solar absorber for solar cell devices.
In this work, we synthesized Cd1−xInxTe NPs using the chemical bath deposition (CBD) process. The photovoltaic properties of CBD cycles were investigated and a Mn2+-doped Cd1−xInxTe sensitizer was also compared with the optimal CBD cycle. UV–Vis–NIR absorption was employed to determine the absorption edge and used to infer an energy gap. X-ray photoelectron spectroscopy (XPS) was used to investigate the composition information with the coupling-related energy states and also to determine the band alignments of a semiconductor interface (i.e. CBM and VBM), which provided the main evidence of such photovoltaic performance via electron injection in solar cells.
Section snippets
Preparation of TiO2 photoelectrodes
The TiO2 powder (particle-size dispersion ∼115 nm) was mixed with polyethylene glycol (PEG, 20 wt%) and TritonX-100. The ratio of mixed materials of TiO2 to PEG to TritonX-100 = 1:2:0.1 by weight and then the solution was stirred for 30 min at room temperature. The TiO2 photoelectrode was prepared by spreading TiO2 paste on fluorine-doped tin oxide glass (FTO, 13 Ω/cm2, Aldrich) using the doctor blading technique. The as-prepared sample was dried at 120 °C for 6 min, then fired at 450 °C for 30 min [12],
Characteristics of undoped and Mn2+-doped Cd1−xInxTe-sensitized TiO2 films
Fig. 1a shows the SEM image of bare TiO2 with an average diameter of ∼85 nm. After coating with Cd1−xInxTe NPs as in Fig. 1b, the surface of the TiO2 becomes rougher. For coating with Mn2+-doped Cd1−xInxTe NPs (Fig. 1c), it can be seen that the TiO2 surface was covered by the larger size of the interconnected Mn2+-doped Cd1−xInxTe NPs. The interconnected particles are advantageous for efficient charge transfer between particles upon photoexcitation. Due to the low resolution, the size
Conclusions
Semiconductor sensitized-Cd1−xInxTe NPs were synthesized using the CBD process and deposited on TiO2 photoelectrodes. The calculated individual Mn2+-doped Cd1−xInxTe size was approximately 7 nm, which was consistent with that determined via using TEM (∼10 nm). An increase in the CBD cycle led to a decrease in the energy gap and after Mn2+ doping, the energy gap narrowed from 1.47 to 0.90 eV, which provides an advantage for broader absorption to the NIR region, leading to a reduced energy loss of
Acknowledgments
The authors are grateful to the Kasetsart University Research and Development Institute (KURDI) for financial support (Grant no. 142.57) and proof reading. Also, we would like to thanks the Center for Alternative Energy (CAE), Faculty of Science, Mahidol University for instrument facilities.
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