Ag2S Quantum Dot-Sensitized Solar Cells by First Principles: The

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AgS Quantum Dot Sensitized Solar Cells by FirstPrinciples: The Effect of Capping Ligands and Linkers Javier Amaya Suárez, José J Plata, Antonio M. Márquez, and Javier Fernández Sanz J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07731 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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The Journal of Physical Chemistry

Ag2S Quantum Dot Sensitized Solar Cells by First-Principles: The Effect of Capping Ligands and Linkers Javier Amaya Suárez,† Jose J. Plata,‡,† Antonio M. Márquez,† and Javier Fernández Sanz∗,† †Departmento de Qu´ımica F´ısica, Universidad de Sevilla, 41012 Sevilla, Spain ‡Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA E-mail: [email protected]

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Abstract Quantum dots solar cells, QDSCs, are one of the candidates for being a reliable alternative to fossil fuels. However, the well studied CdSe and CdTe-based QDSCs present variety of issues for their use in consumer-goods applications. Silver sulfide, Ag2 S, is a promising material, but poor efficiency has been reported for QDSCs based on this compound. The potential influence of each component of QDSCs is critical and key for the development of more efficient devices based on Ag2 S. In this work, density functional theory (DFT) calculations have been carried out in order to study the nature of the optoelectronic properties for an anatase-TiO2 (101) surface sensitized with different silver sulfide nanoclusters. We have demonstrated how is possible a deep tune of its electronic properties modifying the capping ligands and linkers to the surface. Finally, an analysis of the electron injection mechanism for this system is presented.

Introduction Solar energy is one of the cleanest renewable energies, however, first (silicon wafers) and second (thin film technology) generation photovoltaics present an upper limit on the conversion efficiency of only 33%. Nanostructured particles or quantum dots (QDs) are one of the candidates for being the third-generation solar cells because they combine high performance and low cost. 1,2 The efficiency of QDs solar cells (QDSCs) has drastically growth during the last years 3–9 putting them on par with dye sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) photovoltaic cells. 10,11 Moreover, QDSCs present significant advantages compared with DSSCs such as the photostability of inorganic materials, high molar extinction coefficients or tunable energy gaps by controlling the QD size. Although CdSe and CdTe have been extensively used as QDs in QDSCs, 12 they present a variety of issues such as low abundance, high cost and toxicity 13 which are crucial for their use in consumer-goods applications. These concerns have led to the scientific community to look for alternative materials such as Cu2 S, 14–19 Ag2 S, 20–22 SnS, 23,24 Sb2 S3 , 25 and CuInS2 . 26 Ag2 S is a nontoxic semiconductor material which seems to be a good candidate with a band gap around 1 eV. 27 Recently, silver sulfide based materials have been proposed as one of the most promising alternatives for hot-carrier solar cells (HCSCs). 28,29 Lin et al. have measured long carrier cooling times in Ag2 S NPs that can potentially lead 29 to a efficiency around 33% in HCSCs. Silver sulfide has been also used as a light absorber in organic ACS Paragon Plus Environment

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BHJ solar cells with a maximum power conversion efficiency of 3.21%. 30 However, poor efficiency has been reported for QDSCs based on Ag2 S and TiO2 . 20–22 TiO2 nanotubes with a ZnO recombination barrier layer were synthesized by Chen et al. obtaining very low short-circuit current density, JSC (lower than 1 mA · cm−2 ). 20 Tubtimtae et al. achieved a higher performance for a Ag2 S-sensitized TiO2 solar cell with a JSC of 7.3 mA · cm−2 ) but with a low efficiency of 0.76%. 21 A Ag2 S-sensitized ZnO solar cell has been also reported, improving the JSC to 13.7 mA · cm−2 , but also with a low efficiency (0.49%) due to the lower open-circuit voltage, VOC , (100 mV) than the one in the cell based on TiO2 (about 350 mV). 22 Some authors have explained the poor performance of these systems despite its broad absorption spectrum by the alignment between the bands of the Ag2 S sensitizer and the oxide electrode. 31 To improve that, Ji et al. have synthesized Ag2 Sx Se1−x -sensitized TiO2 systems where the electron–hole recombination is minimized. 32 Despite the fact that silver sulfides QDs are potential candidates for substituting conventional QDSCs based on CdSe and PbS QDs, a deeper insight of the electronic structure of this system is needed to explain the previous poor results and improve its efficiency. The optimization of the performance of these devices remains as the main challenge. The different components (QD, the oxide, linkers and capping ligands) play an important role in the mechanisms that govern its performance and are the key to improve the efficiency of these devices. 33–39 However, to the best of our knowledge, there are not systematic studies about the effect of all these components to the electronic properties of the Ag2 S-TiO2 QDSC. In this article, we combine a methodology that has been proven to describe correctly the electronic structure of bulk Ag2 S, 40 with a bottom-up strategy for the modelling of the QDSC. 19 This study represents a step forward for the understanding of the electron transfer mechanism in the Ag2 S-TiO2 QDSCs and a progress to optimize and rationally design more efficient devices.

Computational details and model All the calculations were performed using the Vienna ab-initio simulation package code 41–43 with the projector-augmented wave method (PAW). 44,45 The energies were computed with the exchangecorrelation functional proposed by Perdew, Burke and Ernzerhof, PBE, 46 based on the generalized ACS Paragon Plus Environment

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layers were relaxed while last layer was fixed to its bulk positions.

Results and discussion Bare Ag2 S quantum dots Quantum dots are semiconductor nanoparticles made of a few hundreds or thousands of atoms that are small enough to show electronic confinement effects. Due to the difficulty of carrying out a first-principles study of this size, it is mandatory to look for models with a smaller size but able to qualitatively describe their properties. Two different strategies have been used to model the Ag2n Sn QDs. The structures of the smallest clusters (n = 1 − 12) were optimized taking as starting point the topology of Cu2n Sn clusters previously proposed by Dehnen et al. 52,53 The biggest clusters (n = 20 − 50), on the contrary, were modeled taking as reference the geometry of bulk α − Ag2 S. 54 From the bulk structure a sphere-like cluster is defined, modifying the cutoff radius to select the appropriate number of Ag2 S units. The optimized structures are depicted in Figure S1.

Figure 2: Ag2 S QD energy (EAg2 S ) per Ag2 S unit. The horizontal line represents the energy per Ag2 S unit of bulk α − Ag2 S The relative stability of QDs is represented in Figure 2. It can be observed as the bigger the cluster, the higher the stability, which is connected to the number of not fully coordinated atoms. The values in Figure 2 indicate that energyPlus perEnvironment Ag2 S unit starts to converge around 6 units and ACSthe Paragon 5



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Due to the high computational cost that such a long aliphatic chain would include in our model, we have selected a group of alternative thiols: Methanethiol (MT), 3-mercaptopropionic acid (MPA), ben-

zenethiol (BT), 4-(dimethylamino)benzenethiol (DAB), 4-methoxybenzenethiol (MBT), 4-mercaptobenzonitril (MBN) and 4-mercaptobenzoic acid (MBA) (see Figure 3). MT is the aliphatic thiol with the shortest chain and MPA has been widely synthesized and used as a linker molecule. 38,60 The rest of the ligands are aromatic molecules in which we have included different functional groups. These groups can present electron-donating and withdrawing effects and modify the optoelectronic properties of the QD stabilizing charges or holes that are produced when the electronic excitation occurs. 61,62 Each Ag atom of the clusters was coordinated to the thiol group of a ligand and the geometries were fully relaxed (see Figure 4). The HOMO-LUMO gap values obtained from the DOS calculations are shown in Table 1 as well as the energies for the first excitations in the absorption spectra (see Figure 5). The QD-ligands systems yields a band gap reduction of 0.08 - 0.38 eV due to the stabilization of the anti-bonding states. This stabilization also produces a red shift (0.52 - 1.05 eV) of the first absorption peaks, Eabs , in the absorption spectra compared to the bare quantum dots (see Figure S3). Different behaviors can be observed in absorption spectra depending on the nature of the chain. While Eabs keeps approximately constant for QDs saturated with aliphatic thiols regardless the carbon chain size (see Figure 5 (a)), these excitation energies highly depend on the substituents when aromatic ligands are involved (see Figure 5 (b)). Electron-donating groups, like −OCH3 or −N(CH3 )2 , tend to stabilize the photogenerated hole, which results in a red shift of the spectra. 62 In DAB and MBT, the electron-donating groups reduce the first excitation energy by 0.20 eV and 0.09 eV respectively. However, there is not a clear trend for ligands with electron-withdrawing groups (-CN, -COOH). For instance, 4-aminobenzonitrile (ABN) ligands shift the maxima to higher energies when coordinated to Cu2 S QDs, but a shift to lower energies was found when MBN was used. 19 The same tend was Table 1: Band gap energy , Eg , and first absorption peak energies, Eabs , for the saturated cluster with the different ligands. Energy values in eV. Ligand MT MPA BT DAB

Eg 2.37 2.35 2.28 2.19

Ligand MBT MBN MBA

Eabs 2.66 2.70 2.64 2.44

Eg 2.23 2.22 2.07


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Eabs 2.55 2.26 2.17


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found by Nadler et al. for the (CdSe)13 (ABN)6 system. 36 With Ag2 S QDs, MBN and MBA ligands also produce a red shift (0.38 and 0.54 eV respectively). The absorption spectra is clearly modified by the nature of the ligands but the origin of this modification is unclear. Bader charges of the QDs were calculated to analyze the impact of the charge transfer between the QD and the capping ligands on the optical properties (see Figure 6). There is a general trend in which the lower the negative charge of the cluster, the larger the red shift of the spectra. While in the aliphatic thiols, the lone pair of electrons of the sulfur atoms is strongly coordinated to the Ag atom of the QD, in the aromatic thiols, the benzene ring competes for these electrons. This competition can be tuned with electron-donating or withdrawing groups. MBA and MBN ligands present electron-withdrawing groups that reduce the electron donation of the thiol to the QD. However, ligands with electron donating groups such as DAB and MBT increase the charge transfer with respect the BT molecule. To demonstrate the complex and high impact of the ligands in the electronic structure of the QD, the absorption spectra of QD-MBA was calculated using three MBA-isomers (ortho, meta, and para). As it can be seen in Figure 7, there are strong modifications of the spectra depending on the isomer. The largest red shift is obtained for the o-MBA ligand and the smaller for the p-MBA molecules. These findings are in agreement with the experimental results obtained for MBA isomers adsorbed on Cu QDs, where the o-MBA ligand presents the largest shift to lower energies in the spectra. 63

Linkers and adsorption on TiO2 anatase (101) There are a different methods to link the nanoparticles on the oxide surface such as drop cast/spin cast, chemical bath and surface ionic layer deposition adsorption and reaction (SILAR). 1 Pre-synthesized QDs can also be adsorbed on the surface through an organic molecule that acts like an anchor which is known as the linker. This method shows some advantages over other methods because the number of attached QDs can be controlled as well as the linker can tailor the charge separation, recombination and transport mechanism. 56 For this study, three different linkers were selected as models. Cysteine (Cys) is a natural, abundant and inexpensive amino acid, which has been proven as excellent candidate not only theoretically but also experimentally. 16,19,56 The 3-mercaptopropionic acid (MPA) is frequently used for this purpose, for instance, attaching CdSe/CdTe core/shell nanoparticles on TiO2 . 60 As an ACS Paragon Plus Environment

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Table 2: First, second and third absorption energy, Eabs , values for the isolated QD-MT and QD-MT-linker-TiO2 systems with different linkers. Energy values in eV. Linker Cys (c1) Cys (c2) MPA MBA QD

1st 2.59 2.66 2.65 2.54 2.65

2nd 2.65 2.76 2.73 2.59 2.74

3rd 2.71 2.84 2.83 2.65 2.83

aromatic linker, we have chosen 4-mercaptobenzoic acid (MBA), which has been reported to accelerate the electronic injection process with respect to non-aromatics linkers. 38 The molecular structures for these three molecules are shown in Figure 8. The adsorption of the linker molecules on the anatase surface is the next step in the construction of the model. Different sites and configurations were tested. For cysteine, the lower energy conformation corresponds to deprotonation and interaction through the carboxylic oxygen atoms with two surface Ti cations. 19 MPA or MBA molecules do not present an amino group, so the most stable configuration is the adsorption through the deprotonated carboxylic group (see Figure 9). In all cases, the linkers are adsorbed strongly to the surface with adsorption energies greater than 1 eV. To study the QD-surface interaction, all the silver atoms of the “61 ” cluster were coordinated with MT molecules except one that was coordinated to the linker anchored to the anatase surface. MT was selected as capping ligand because of the computational cost associated to the size of the system (370 atoms). The adsorption of one QD over the anatase surface creates a QD monolayer with a distance between images of around 5 ˚ A (see Figure 10 (a)). The electronic structure of the semiconductor is strongly modified by the QD adsorption. While bare TiO2 shows a band gap value around 3 eV, the gap is almost closed when the QD is adsorbed. As it can be observed in Figure 10 (b), the QD states fill the anatase band gap. However, it seems that changing the linker does not affect significantly the electronic structure of the system. This severe modification of the electronic structure affects the absorption spectrum of the TiO2 . Due to its band gap value, anatase shows a wide band beyond 3 eV in the optical spectrum, however, new absorption peaks appear at lower energies when QD is adsorbed (Figure 10 (c)). Table 2 shows the position of the first three peaks in the absorption spectra using different linkers. The different values obtained for each linker demonstrate ACS Paragon Plus Environment

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the importance of this molecule not only as a anchor between the QD and the surface but also as a possible target to tune the electronic properties of the system. It is worth to mention that also the adsorption configuration of the linker can modify the electronic structure. While Cys(c1) induces a red shift in the spectra, Cys(c2) shows almost the same energy for the first absorption peak compared to the isolated QD/ligands system. MBA is the linker that produce the most significant shift to lower energies being in agreement with the effect that was observed when it was used as a capping ligand. It is possible to get information about the electron-injection mechanism comparing the position of the optical absorption maxima of the QD before and after being adsorbed on the surface. In a direct mechanism, electrons are excited from the QD HOMO state to the edge of the semiconductor conduction band. This transition requires a lower energy than the HOMO-LUMO transition energy so the absorption peak appears at a lower energy than the first peak of the spectra of the isolated QD/ligand system. On the other hand, the indirect mechanism means that electrons are excited from the QD HOMO state to the QD LUMO state. Then, the electron is injected into the semiconductor conduction band, since the QD LUMO state is overlapping with the conduction band. For this reason, the position of the optical maxima remains almost unaltered for an indirect mechanism. A schematic representation of these mechanisms is shown in Figure 10 (d). As it is shown in Figure 10 (c) and Table 2, the position of most the QD peaks remain almost unaltered especially for Cys (c2) and MPA. This fact points to a mainly indirect electron-injection mechanism. However, there is a shift of the first peaks to lower energies for Cys (c1) and MBA, which indicate some contribution of a direct mechanism too, especially for the MBA linker.

Conclusions The effect of ligands, linkers and support oxide on the behavior of Ag2 S-based QDSCs was examined using first principles DFT calculations. The model was build using a bottom-up approach to differentiate the effect of each element on the optoelectronic properties of the system. While there are not significant changes in the spectra using aliphatic ligands, there is a common shift to lower energies for most of the aromatic ligands. The effect of the ligand can be highly tuned even selecting different isomers. It was found that o-MBA presents a higher red shift in the spectra than m-MBA and p-MBA. ACS Paragon Plus Environment

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It seems that the linker selection does not severely modify the system’s electronic structure even selecting linkers that strongly interact with the anatase (101) TiO2 surface. The QD/ligands states are localized filling the band gap of the anatase, sensitizing the visible zone of the spectrum where the bare TiO2 does not show any absorption feature. Considering the position of the QDs peaks in the spectra before and after its adsorption on the anatase surface, a mainly indirect injection mechanism is proposed. However, a small contribution of a direct mechanism is observed notably when the MBA is used as linker.

Acknowledgement This work was funded by the Ministerio de Econom´ıa y Competitividad (Spain), the EU FEDER program, and the Junta de Andaluc´ıa, Grants CTQ2015-64669-P and P12-FQM-1595.

Supporting Information Available • QDs geometries and DOS and absorption spectra of “61 ” QD.

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Figure 8: Structure of the linkers: mercaptobenzoic acid (MBA).

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cysteine (Cys), 3-mercaptopropionic acid (MPA) and 4-

Figure 9: MPA adsorption geometry onto anatase (101). Colors: Ti, cyan; O, red; C, black; H, white; S, yellow.


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Ag2S Quantum Dot-Sensitized Solar Cells by First Principles: The

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