Ligand field states and defect levels synergism: A close look at the band alignment of 4T1‑Mn-CdS/Bi2S3-co-sensitized photoanodes
Introduction
The photoconversion efficiency (PCE) has been the most remarkable feature for obtaining promising solar cells during the last years. However, the actual research is focused on the improvement of the optical properties of several photoanode materials, and the decrease of charge carrier recombination into the devices [1]. By controlling these characteristics, a high energy conversion can be achieved. The latter is the case of quantum dot sensitized solar cells (QDSSCs), which have reached the PCE of their equivalent dye-sensitized solar cells around 11% [2]. As soon as the photophysical and electrical properties of sensitizers were progressively understood, the PCE was increased over 16.6% [3]. Advantages of narrow band gap-semiconductor based quantum dots (QDs) such as generation of multiple electron-hole pairs, high molar extinction coefficient, and tunable band structures have upgraded the sunlight harvesting capability and the carrier transport into solar cells [3, 4]. Despite of these improvements, some semiconductors do not show one of the latter two abilities, producing the eventual lowering of PCE from the devices, and the loss of interest from the industry. Thus, it has been explored low-cost and nontoxic (or low-toxicity) chalcogenides such as Sb2×3 (X = S, Se, S/Se), CuInSe2, Zn−Cu−In−Se alloys, among others, because these materials exhibit attractive optical properties (specially, a low band gap between 1 and 1.7 eV), providing solar devices with PCE values comparable with their Pb-counterparts [5, 6]. In this context, within the most attractive chalcogenide materials, we highlight bismuth sulfide (Bi2S3). This low-toxic chalcogenide displays a low band gap (Eg) around 1.2–1.7 eV, meaning a high visible-light absorption [7, 8]. Unfortunately, Bi2S3-based solar cells using TiO2 based photoanodes have produced PCEs lower than 1.0%, caused mainly for a poor band alignment in the TiO2/Bi2S3 interface [9], [10], [11], [12]. Therefore, the co-sensitization has been reported to be efficient for inducing a suitable band structure between binary sensitizers such as CdS and CdSe [13,14]. For instance, the introduction of CdS QDs has been widely studied as a suitable strategy to improve the light harvesting of QDSSCs in the visible region, with external quantum efficiencies near to 90% and increase the electron injection to reach the back contact, besides of its facile preparation [15], [16], [17], [18]. However, some reports have also demonstrated that the use of CdS mixed with other sensitizers can also provide an energy barrier for charge carrier separation and mobility, achieving low PCEs [19], [20], [21].
An alternative to overcome the latter issue is the incorporation of new electronic states into the binary CdS coming from Mn-doping. Interestingly, Mn2+ electronic states are considered as efficient electron storage sites, where their carrier transfer mechanism can be explained according to the ligand field theory [22, 23]. Mn2+ cation shows a coordination number of 4 into chalcogenide rich-ambients (e.g. S2−, Se2−), generating a weak tetrahedral field. In this context, 4T1→6A1 ligand field emission transitions into Mn2+ can be achieved. This type of d-d transitions is known to be spin and orbitally forbidden, resulting in a photoexcited electron lifetime around milliseconds [22], [23], [24]. Additionally, these d-d transitions are located into the band gap of CdS, exhibiting a low energy gap around 2.1 eV [25]. This feature has been revealed to red-shift the Eg of CdS [22], which can explain the enhancement of its charge carrier separation and electron transfer dynamics [4,[26], [27], [28], [29]]. However, an excess of this cation also provides non-radiative recombination sites and a poor band alignment into the composites [22,26,30]. Therefore, the amount of Mn2+ into CdS plays an important role in the photovoltaic performance of the QDSSCs. The density of Mn2+ can be easily adjusted by controlling the Mn precursor through low-cost successive ionic layer adsorption and reaction (SILAR), using alcoholic solutions during the formation of Mn–doped CdS QDs. Furthermore, an adequate amount of Mn2+ into inner CdS can increase electron lifetime into the composite material, preventing charge carrier recombination and loss of PCE in QDSSCs [31].
The generation of photocurrent into Mn–CdS sensitized solar cells has been studied in function of the density of Mn2+, achieving higher PCE values than that of CdS-devices [27,29,31]. Nonetheless, the literature is limited about how the Mn2+ligand field electronic states such as 4T1 or/and 6A1 impact on the carrier transfer ability of Mn–CdS. It seems that the presence of Mn2+ is the only key factor to dictate the carrier mobility. But the CdS QDs synthesis itself encloses the formation of hidden energy levels that mitigate the carrier dynamics into photoanodes [32,33]. Maybe, this is one of the main aspects that have not allowed to reach high PCEs in CdS-QDSSCs. By considering some parameters as the concentration of cationic/anionic solutions, washing process and the number of deposition cycles during SILAR process, CdS can form structural defects known as Cd-Cd energy levels. These states are produced by direct interaction between two adjacent Cd atoms caused by sulfur vacancies into CdS lattice. The sulfur release transfers two electrons to Cd atoms for establishing a Cd-Cd bond through the Cd sp2 hybridization [32]. In terms of energy, Cd-Cd states are located above to the valence band (VB) of CdS, lowering its Eg [32,33]. Cd-Cd energy levels have been early reported to impact the photovoltaic properties of CdS/CdS1-xSex co-sensitized photoanodes [33]. Clearly, if the influence of the Cd-Cd energy levels has not been deeply investigated, their interaction with Mn2+ electronic states for improving the carrier transport/separation into heterostructures has not been revealed yet. By using photoelectrochemistry to analyze both the light harvesting and charge carrier transport abilities of composites, we can observe how the synergism between Cd-Cd energy levels and Mn2+ ligand field electronic states (1) facilitates the band engineering to produce a more suitable band structure using unmatched sensitizers, and (2) promotes carrier flow in order to achieve a high photocurrent. Thus, we consider that the photovoltaic performance of low-band gap based QDSSCs could be improved.
In this work, we studied the impact of the synergism between Mn2+ 4T1 electronic states and Cd-Cd defects on the photophysical and photo(electro)chemical properties of Mn–CdS/Bi2S3 co-sensitized boron, nitrogen and fluorine-co-doped TiO2 nanotubes (X–Mn–Y–CdS–Bi2S3). For understating the band structure and carrier transfer processes. Materials were modified by the presence of ligand field electronic states and structural defects, an efficient carrier transport pathway, high electron lifetime and photocurrent were achieved into X–Mn–Y–CdS–Bi2S3 composite photoanodes. This contribution opens the door to visualize more clearly the carrier transfer mechanisms occurring into composite photoanodes that limit their potential application in QDSSCs.
Section snippets
Fabrication of BNF–TNT arrays
B-, N- F-co-doped TiO2 nanotubes (BNF-TNT) were grown on Ti foils according to an established procedure [33,34]. Briefly, Ti anodization was performed in an electrolyte solution composed by 0.06 wt% H3BO3, 0.45 wt% NH4F and 2.0 wt% deionized (DI) water in ethylene glycol, applying a bias potential of 60 V for 2.5 h. A two-electrode cell, using a Cu foil as the cathode, were used for the material preparation. The as-prepared BNF-TNT were rinsed with DI water, dried at 100 °C and annealed at
Morphological and physicochemical characterization of composite photoanodes
FESEM images of the BNF–TNT, 0.7–Mn–4–CdS and 0.7–Mn-4–CdS–2–Bi2S3 photoanodes were obtained, where it was observed the top-view and cross section of the materials surface. BNF–TNT were vertically oriented, showing cavities with an average diameter around 116.6 ± 2.6 nm (Fig. 1a) and an average tube length around 16.7 ± 1.0 nm (Fig. 1a’). The width of the cavities was decreased after the deposition of sensitizer nanoparticles, achieving an increase of the wall thickness in the tubular
Conclusions
The incorporation of Mn2+ into CdS for synthesizing Mn-CdS provided the improvement of the light harvesting and electron transport ability into X–Mn–Y–CdS–2–Bi2S3 composites. This fact was caused by the presence of both structural defects such as Cd-Cd energy levels and ligand field electronic states such as Mn2+ 4T1 into the Mn-CdS QDs. In this context, the photophysical and photoelectrochemical properties of composites were modified, altering their band structure. The formation of an adequate
CRediT authorship contribution statement
Andrés F. Gualdrón-Reyes: Conceptualization, Methodology, Data curation, Formal analysis, Investigation, Visualization, Writing - original draft. Johan S. Ríos-Niño: Methodology, Data curation, Investigation. Angel M. Meléndez: Conceptualization, Supervision, Investigation, Validation, Writing - review & editing. Jhonatan Rodríguez-Pereira: Methodology, Data curation, Investigation, Validation. Mario Alejandro Mejía-Escobar: Methodology, Investigation, Writing - review & editing. Franklin
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Authors are graceful with Universidad Industrial de Santander-Colciencias project 110265843664 (VIE 8836). Andrés F. Gualdrón-Reyes acknowledges to COLCIENCIAS for the Ph.D. 617-scholarship (Doctorado Nacional COLCIENCIAS).
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