Interface Engineering in Quantum-Dot-Sensitized Solar Cells

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Interface Engineering in Quantum Dot Sensitized Solar Cells Ganga Halder, Dibyendu Ghosh, Md. Yusuf Ali, Atharva Sahasrabudhe, and Sayan Bhattacharyya Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00293 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Interface Engineering in Quantum Dot Sensitized Solar Cells Ganga Halder, Dibyendu Ghosh, Md. Yusuf Ali, Atharva Sahasrabudhe and Sayan Bhattacharyya* Department of Chemical Sciences, and Centre for Advanced Functional Materials, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur – 741246, India *Email for correspondence: [email protected] Abstract The unique properties of II-VI semiconductor nanocrystals such as superior light absorption, size dependent optoelectronic properties, solution processability and interesting photophysics prompted quantum dot sensitized solar cells (QDSSCs) as promising candidates for next generation photovoltaic (PV) technology. The QDSSCs have advantages such as low-cost device fabrication, multiple exciton generation and possibilities to push over the theoretical power conversion efficiency (PCE) limit of 32%. In spite of dedicated research efforts to enhance the PCE, optimize individual solar cell components and better understanding of the underlying science, QDSSCs have unfortunately lacked behind promise due to shortcomings in the fabrication process and with the QDs themselves. In this feature article, we briefly discuss the QDSSC concepts and mechanisms of the charge carrier recombination pathways that occur at multiple interfaces viz. (i) metal oxide (MO)/QDs (ii) MO/QDs/electrolyte and (iii) counter electrode (CE)/electrolyte. The so far developed rational strategies to minimize/block these charge recombination pathways are elaborated. The review concludes with a discussion on the present challenges to fabricate efficient devices and the future prospects of QDSSCs. Keywords: Quantum dot; solar cell; interface; charge recombination; photoanode; metal oxide; counter electrode; electrolyte

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Introduction Renewable energy being a global issue in the recent years,1 with increasing drive to seek alternative energy sources across the world, there is a growing interest in low-cost, easily manufactured and efficient energy sources. One of those sources is the harnessing of sun’s energy through the excitation of semiconductor materials. Over 130,000TW of solar radiation falls upon the earth and if only 0.1% is harvested for electrical power it would easily sustain the world’s energy needs.2 The challenge for researchers is to develop a system that can convert sunlight into electricity at the same cost as fossil fuels. Chronologically, the PV technologies have evolved into three generations.3 First generation PV cells are based on a single crystalline semiconductor wafer while the second generation devices are based on semiconductor thin-films. Both these classes of solar cells are based on solid-state semiconductor technologies fabricated by sophisticated techniques requiring high pressure and temperature conditions and state-of-the-art clean room facilities. These very requirements that make the first and second generation devices so efficient on one hand, severely limit their practical implementation. By combining the knowledge from the first two classes of solar cells, the blueprint for third generation PV devices was sketched to produce devices that may push the conversion efficiencies beyond the Shockley-Queisser (SQ) limit while maintaining a low production cost in order to remain competitive. The development of liquid junction sensitized solar cell configuration of the third generation PV technology seems highly promising in this regard. Among the third generation PV systems, QDSSCs have become very promising and attractive technology in recent years.4 This partly comes from the excellent intrinsic properties of II-VI QD light absorbers such as size tunable band gap, quantum confinement, solution processability and high absorption coefficient to name a few. Moreover, unique photophysical processes such as multiple exciton generation and hot-carrier extraction open


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up the possibility of pushing PV device performance beyond the theoretical SQ limit of 32.9% in these systems.5 For example, a PbSe based solar cell with an external quantum efficiency of 100% was recently demonstrated which provides a great promise for design of QDSSCs crossing the SQ limit.2 Indeed QDSSCs have seen a remarkable progress in PV performance over the past few years as the PCE of these systems was improved from an initial less than 1% in 2008 to 12% in 2017.6 A major contributing factor for such a rapid increase of PCE in a relatively short time is due to better understanding of the underlying photophysical processes and barriers for device performance. A detailed understanding of QDSSC photophysics has allowed researchers, including our group, to systematically improve device performance by exploring various routes such as developing new methods for QD loading, interface engineering through post-treatments, electrolyte and CE optimization etc. Before reviewing some of these interesting approaches to improve device performance, it’s prudent to introduce the anatomy and basic working principles of a QDSSC. To begin with, QDSSCs belong to the photoelectrochemical solar cell family and share some of their basic working principles and design paradigms with that of dye-sensitized solar cells (DSSCs). In particular, a QDSSC is composed of a wide band-gap, MO semiconductor such as TiO2 as the electron transport layer (ETL) which is sensitized with a low-band gap semiconductor QD. This QD sensitized mesoporous TiO2 comprises the photoanode of the solar cell. The cathode or CE of the cell is a metal sulfide (typically CuxS) film. The photoanode and the cathode are separated by a separator and a sulfide/polysulfide redox couple electrolyte which completes the cell. To successfully extract electric power from such a device configuration at considerably high conversion efficiencies, a series of charge transfer steps need to act in tandem. As shown in Figure 1a photoexcited electrons are injected from the conduction band (CB) of QD into the low lying CB of MO and finally extracted out into the external circuit through the transparent conducting current collector. 3 ACS Paragon Plus Environment

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The photogenerated holes in the valence band (VB) of QD oxidize the redox couple in the electrolyte and the oxidized species in turn gets reduced back to its native state at CE, thereby completing the circuit.7 While the photophysical processes just described are all favorable for the working of a QDSSC, there are several other scavenging pathways counteracting the favorable ones that deteriorate the device performance. These charge recombination processes mainly occur at the interfaces of MO/QD, MO/QD/electrolyte and CE/electrolyte. Some of the well-studied and important recombination pathways are depicted in Figure 1a: (1) charge recombination in photoexcited QD; (2) and (3) the photoexcited electrons in the CB of QD and MO, respectively can get trapped in the interfacial defect states between MO and QD; (4) back transfer of electrons from CB of MO to the holes in VB of QD; (5) electrons from CB of MO can be trapped in the interfacial defect states between MO nanoparticles (NPs); (6) recombination of photoexcited electrons in the CB of QD with the oxidized species of the redox couple in the electrolyte; and (7) recombination of photoinjected electrons in MO with oxidized species of the redox couple.8 As can be seen from Figure 1a, most of the recombination events are facilitated at the triple junction boundary between the MO/QD/electrolyte interfaces and as such these interfaces have received significant attention from researchers for improving PCE of QDSSCs in past few years. The overall performance of a QDSSC is highly dependent on the rates of different recombination processes vis-à-vis rates of charge injection and charge extraction. The formidable challenge is to suppress the rates of unwanted recombination events without affecting any of the favorable processes described above. Although the working principles of a QDSSC is analogous to other photo electrochemical cells such as DSSCs, there is one conceptual difference in their underlying photophysics.9 The intra-bandgap trap states in QDs originating from unsatisfied valence of the surface atoms acts as recombination centers in contrast to dyes which have a molecular nature and hence do not give rise to such centers. The presence


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of trap states in the light absorber gives rise to additional recombination pathways that are not relevant in a conventional DSSC. Photoexcited electrons in the CB of a QD can be trapped at such sites and if the trap state is sufficiently low lying in energy, it significantly hinders charge injection into the CB of MO and thereby lowers the photocurrent output of the device. Moreover, a trapped electron has a higher probability of recombining with a photogenerated hole in the VB of QD or with the oxidized species in the electrolyte. Similarly, a photoexcited hole can also be trapped at such centers which can lead to scavenging of photoexcited electrons from the CB of MO or QD.

Such hole trapping process further hinders the

inherently slow rate of hole transfer and has been implicated in deleterious QD photooxidation.7,8 This subtle but profound difference calls for novel surface engineering approaches to improve QDSSC performance. Another important, but often neglected, interface responsible for promoting charge recombination is the CE/electrolyte interface. QDSSCs have traditionally suffered from a poor fill factor that significantly reduces the overall PCE even for cells recording high short circuit current density (JSC) and open circuit voltage (VOC) values. The oxidized electrolyte is regenerated at the CE/electrolyte interface from photogenerated electrons through the external circuit. This necessitates facile charge transfer kinetics at CE and the use of better electrocatalysts so as to sustain large JSC in a high performing QDSSC. A high resistance to interfacial charge transfer at the CE, generally due to poor electrocatalytic activity, results in a high overpotential cost that in-turn creates a bottleneck for electron flow and promotes back electron transfer at the photoanode. Understanding of the recombination physics in a working QDSSC has allowed the researchers to formulate novel approaches to overcome these bottlenecks. The effect of such an effort is visible in the rapid increase in device efficiencies which have now touched > 12%.6 In this feature article we focus on each of the above mentioned interfaces and discuss some of the adopted strategies to control the undesirable charge recombination processes in 5 ACS Paragon Plus Environment

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QDSSCs. Based on the mechanistic understanding of charge transfer processes we also discuss some possible future directions to further enhance the PV performance. Fluorine doped Tin Oxide (FTO)/MO Interface In QDSSCs, MO layer plays an important role in influencing the PV performance of the cells in three distinct ways: (i) creates pathways for transferring photo-injected electrons to the conducting FTO (ii) supplies large geometric surface area to achieve high loading of QDs and (iii) harvest more energy through scattering MO layer.10-13 Among several MOs, SnO2, ZnO and TiO2 have been mostly used by the research community due to fulfillment of the above three criteria especially in QDSSCs. When QDs interact with the incident photons, electron−hole pairs are generated that rapidly separate into electrons and holes at the interface between MO/QD and QD/electrolyte, respectively. The separation of charge carriers of course depends on the energy band positions of both QDs and MO. A suitable choice of QDs includes the mid-bandgap (1−2.4 eV) semiconductors whereas MOs have a large bandgap (3−3.5 eV) whereby the condition for choosing the right combination of MOs and QDs depends on their relative VB and CB positions (Figure 1b). For effective and faster electron transfer from QDs to MO, it is necessary that the CB position of MOs should be lower than the CB of QDs whereas the VB of MOs needs to be much lower than the QDs to block holes from undergoing recombination. In this respect Figure 1b validates the preference for SnO2, ZnO and TiO2. Morphology control of MO nanostructures was the key research topic in the last two decades even much before QDSSC came into relevance. Various nanostructures have been adapted to design the photoanode (Figure 2a-l).10-22 Among them, one-dimensional (1D) nanostructures attracted significant attention because of having unidirectional pathway for electron transport. Electron mobility (µe) is the key parameter which controls transfer of


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photo-generated electrons through the MOs. SnO2 has higher µe (~100–200 cm2V-1S-1) than TiO2 (0.1–1 cm2V-1S-1) resulting faster electron diffusion.23 1D nanostructures offer better µe compared to the bulk or three dimensional (3D) counterparts.24 Electron diffusion length (Ln) is determined by Ln = (Dnτn)1/2

(1)

where Dn is the electron diffusion coefficient and τn is the electron lifetime. 1D nanostructures offer large Ln than spherical NPs, for example in TiO2 nanowires Ln is ~ 60 µm whereas for NPs it is 10−14 µm.25 A high Ln results in better collection efficiency for QDSSCs. In spite of high µe and large Ln, the devices based on 1D nanostructure suffer from poor device efficiency due to their insufficient surface area for QD loading. Hierarchical branched nanostructures are much more promising candidates than 1D nanostructures,26 as it can enhance the surface area for loading of the QDs. Moreover its light scattering effect helps to absorb a large number of solar photons improving the PCE. The surface area of MO films can be further enhanced by using hierarchical aggregate structures.26 TiO2 mesoporous beads with a combined advantage of light scattering and large surface area can achieve PCEs up to ~4% for CdS/CdSe co-sensitized solar cells. The PCE of this solar cell can be further enhanced to ~5% to construct a bilayer photoelectrode with ZnO NP-film and ZnO microsphere as the light scattering layer.13 The most well established MO ETL is a tri-layer of TiO2 comprising of (i) 10−20 nm thick compact TiO2 (c-TiO2), (ii) 8−10 µm thick mesoporous TiO2 (m-TiO2), and (iii) 4−5 µm thick scattering TiO2 (s-TiO2) (Figure 2m). Each of these layers has its own promises for interfacial charge transport mechanism.27 Nanocrystalline compact layer transfers the photogenerated electrons to conducting FTO as well as blocks the electrolyte (Figure 2m) to come in contact with FTO and thereby reduce the recombination pathway. The FTO/c-TiO2 interface works as a hole blocking layer in QDSSCs. The thick 8−10 µm mesoporous MO 7 ACS Paragon Plus Environment

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(m-TiO2) layer acts as an active layer to achieve high loading of QDs through the mesoscale pores. Mesoporous TiO2 usually consists of microspheres with suitable pore size (5−10 nm) achieving a high surface area (150−200 m2/g) for QD loading. The deposited QDs are capable of electron-hole pair generation driving subsequent transfer of electrons to conducting FTO and holes to the electrolyte redox couple. The s-TiO2 layer acts as a scattering center for the transmitted light and thereby aids harvesting more solar light in the device. MO/QD Interface The presence of grain boundaries and interface defects between adjacent MO NPs in the photoanode film can trap the photogenerated electrons and provide easy pathways for charge recombination as shown in Figure 1a.23 The undesired charge recombination pathways in the MO/QD interface can be minimized by multiple surface modifications of MO and the QDs. Surface Modification of MO Nanostructures The most convenient way to passivate the MO surface is post treatment of the photoanode by thin overlay of a wide bandgap oxide such as SiO2, Al2O3, MgO and amorphous TiO2.28 Besides, fluorine treatment on TiO2, nanostructured electrodes also show a positive impact on QDSSC performance.29 To consider one case study, the surface modification of ZnO photoelectrode was successfully performed by Cao et al. through a facile chemical approach with a very thin layer of TiO2 NPs.16 To overcoat, ZnO photoanode was dipped into an aqueous solution of 0.04 M H3BO3 and 0.1 M (NH4)2TiF6 at room temperature for 30 min followed by rinsing with deionized water for several times and finally annealed at 400°C. In this chemical approach, H3O+ ions enforce the dissolution of ZnO exposed facets while the passivation by TiO2 takes place simultaneously via chemical bond 8 ACS Paragon Plus Environment

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formation with the newly created surface facets of ZnO. The QD precursors can pass through the newly created apertures and enlarged pores to allow a homogeneous QD distribution within the film (Figure 3a). Figure 3b shows a higher absorbance of CdS/CdSe QD loaded TiO2-passivated ZnO film than the QD loaded unpassivated film, confirming better loading of QDs in the former. Consequently the QDs over the passivated electrode show higher PCE (4.68%) over the unpassivated film (2.38%) as shown in Figure 3c. Apart from better QD loading, the thin TiO2 passivating layer acts as a barrier layer and decreases back electron transfer from photoanode to the electrolyte (6 and 7 pathways in Figure 1a) in order to suppress the charge recombination (Figure 3d, e). In another case study, taking ZnO nanorods as MO, the nanorod surface modification can be performed with self-assembled monolayers (SAMs) of phosphonic acid molecules e.g. 3-phosphonopropionic

acid

(3-PPA),

butylphosphonic

acid

(BPA)

and

3-

aminopropylphosphonic acid (APPA).30 Incorporating SAMs not only passivates the ZnO surface but can alter the band alignment by tuning the surface work function. The surface modification of ZnO nanorods was carried out by dipping the ZnO photoanode into ethanolic solutions of 3-PPA, BPA and APPA at pH 5.4 for 1 min followed by rinsing with ethanol to remove excess molecules. The optical absorbance spectra of CdS/ZnO and CdS/SAMs/ZnO photoanodes show no change in absorbance (Figure 4a) which indicate that SAMs do not affect the loading of QDs onto modified nanorods. The photoluminescence (PL) spectra in Figure 4b show two emission bands which are associated with band edge emission (378 nm) and defect state emission (435-640 nm). The reduction of defect band emission after SAM modifications clearly indicates the passivation of nanorod surface. As shown in Figure 4c an improved PV performance was observed for CdS/3-PPA (1 min)/ZnO QDSSCs with a PCE of 1.41% when compared to CdS/ZnO QDSSCs (1.01%). However, PCE decreases for CdS/BPA/ZnO QDSSCs (0.95%) and CdS/APPA/ZnO QDSSCs (0.80%). These results were


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explained from the change in work function of ZnO nanorods modified with different SAMs. The electron withdrawing –COOH group of 3-PPA molecule creates an interface dipole and the direction of this dipole moment is towards ZnO surface which reduces the work function of 3-PPA modified ZnO photoanode and creates an energy barrier between CdS and ZnO (Figure 4d). This retards back transfer of electrons and improves the PCE. However, as BPA and APPA molecules consist of electron donating –CH3 and –NH2 groups, respectively, an unfavourable interface dipole is generated which results an increment of work function of ZnO and hinders the transfer of electrons from CdS to ZnO. Doping of MO Nanostructures Another strategy to alter the charge recombination dynamics in QDSSCs is to modify the intrinsic properties of MO nanostructures by doping a suitable metal ion. The electron transport in TiO2 nanocrystals can be enhanced dramatically by this approach. Several metals like Al, Cu, Nb, W, Cr, Ta and Zn have been reported to change the intrinsic conductivity of the MOs.31 Fujishima et al.32 reported that Nb-doped anatase TiO2 films can achieve metallic conductivity and this concept was borrowed by researchers for preparation of a desired photoanode.33,34 A similar photoanode was used in CdS/CdSe co-sensitized QDSSCs by Deng et al.35 where the highest PCE of 3.3% was obtained for 2.5mol% Nb-doped TiO2 photoanode based QDSSCs which is 94.1% higher than with the undoped TiO2 photoanode. Besides the observed correlation between the J-V curves and Incident Photon to Current conversion Efficiency (IPCE) spectra, QDSSCs based on an optimized Nb-doped TiO2 have lower chances of charge recombination and better charge collection efficiency. Apart from metallic doping, nitrogen doping,36 and boron/sulfur co-doping,37 can broaden the light absorption of MOs from UV to visible region. The photoresponse of TiO2 can be enhanced in the near-UV and visible region by combining N-doping and CdSe QD sensitization.37


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QD Sensitization In principle, the essential criteria for designing of a high performance QDSSC is to choose QD sensitizers such that the QDs have a broad light harvesting range and suitable bandgap alignment with MO and the redox electrolyte to achieve effective interfacial charge separation. In general, narrow bandgap cadmium and lead based chalcogenide binary semiconductor QDs like CdS, CdSe, PbS, and PbSe are popularly used as sensitizers.38 However, these binary QDs have lower PCE due to their limiting light harvesting range and/or poor charge injection efficiency. In comparison, the ternary and quaternary alloyed QDs e.g. CdSexS1-x,39 CdSexTe1-x,40 CuInSexS2-x,41 CuInSexTe2-x,42 ZnCuInSe,43 ZnAgInSe,44 and CuInGaSe,45 are more efficient as they have broader light harvesting range and their bandgap can be tuned only by altering their composition without affecting the particle size. Sequential deposition or co-deposition of QDs onto the MO films has also been performed to improve the light harvesting range.46 Kamat et al. have demonstrated the design of a tandem architecture (Figure 5a) with CdSexS1-x QD sensitizers having ‘x’ dependent bandgaps that can improve the light harvesting range as well as charge transport better than the mixed QD architecture (Figure 5b).39 This type of tandem architectures was created by sequential deposition of different bandgap semiconductors to allow transfer of electrons from the larger to smaller bandgap QDs and finally to MO in a cascade fashion helping in better charge separation (Figure 5c). Surface Modification of QDs The termination of dangling bonds of the QD lattice at its surface and imperfect coverage of capping ligands during their synthesis, invariably leads to a high concentration of surface defects.47 These defect states open several charge recombination channels both in the QDs as well as at the interfaces of MO/QD/electrolyte which hinder the desired charge 11 ACS Paragon Plus Environment

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transfer processes. Therefore, the quality of QDs plays a significant role in their PV performance. Although, the widely used in-situ deposition of metal chalcogenide nanocrystals by SILAR (Successive Ion Layer Adsorption and Reaction) and CBD (Chemical Bath Deposition) techniques provide good surface coverage and better connectivity between MO and QDs, due to the lack of control over particle size distribution, morphology and relatively low synthesis temperatures, the density of defect states cannot be controlled.24 The preparation of colloidal QDs at relatively higher temperatures can improve their crystallinity and reduce the density of defects. Since ternary QDs demonstrate low PCEs due to the presence of defect states at their internal atom vacancies, alloying such QDs is a promising approach, wherein the diffusion of foreign metal ions into the vacant sites of the semiconductor lattice can improve their crystallinity and carrier mobility. Recently, Zndiffused AgInSe2 colloidal QD was successfully synthesized by our group using a high temperature synthesis method. Through different optimization steps, the best possible PCE of 3.07% was achieved when Zn/(Ag+In) ratio is 48.2% (AZ5-8Z) which is 84% higher than the unalloyed AgInSe2 QDs (AZ0-8Z).44 The high quality colloidal QDs with relatively less defect density could be deposited onto the MO film to make the photoanode by several techniques e.g. drop casting, dip coating or spin coating. However, due to poor surface coverage and non-uniform distribution of particles over the films, these techniques lead to insufficient absorption of light and a large exposed area of TiO2 comes into direct contact with the electrolyte for charge recombination. A reduced electrical connection between the QDs and MO due to presence of long chain surfactant molecules also hinders the charge transfer efficiency. Because of such limitations QDSSCs fabricated with colloidal QDs were not able to show a remarkable improvement in PCE. To address these issues, a capping ligand induced self-assembly approach was developed by Zhong’s group.48 In this approach, the organic surfactant capped colloidal QDs were first made water dispersible by surface


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modification with the short chain bifunctional linker molecules (e.g. thioglycolic acid, mercaptopropionic acid (MPA) and cysteine). Thereafter the dispersions were drop casted onto the MO films. The role of these linker molecules is to bind with both the QD and MO chemically and electronically with the help of their thiol (–SH) and carboxylic acid (–COOH) groups, respectively. This procedure enhances surface coverage of QDs onto the MO film electrodes which helps to capture a larger number of solar photons to construct high efficiency solar cells. On the other hand, the undesirable contact between the exposed surface of TiO2 and electrolyte reduces to alter the recombination dynamics.49 Core/Shell QDs Another efficient way to suppress the charge recombination in QDSSCs is the growth of an epitaxial shell by an inorganic semiconductor material around the QD core in colloidal state. Depending upon the relative band alignment of the core and shell materials, core-shell QDs can be classified into the three types: type-I, reverse type-I and type-II as shown in Figure 6a. In case of type-I, band gap of the core lies within that of the shell and the photogenerated electron and hole pair remain confined at the core. In reverse type-I, band gap of the shell lies within the core and the photogenerated charge carriers remain partially or completely confined in the shell depending on the shell thickness. In type-II core-shell QDs, both VB and CB edges of the core material are either lower or higher than that of the shell. The separation of electrons and the holes occurs at different regions of the core and shell, broadening the spectra in case of core-shell heterostructures.50 Type I core/shell QDs Due to the complex crystal structures and nonstoichiometric compositions in ternary and quaternary QDs, along with surface traps states, several defect energy levels are created in between the bandgap which are responsible for their poor PV performance in spite of 13 ACS Paragon Plus Environment

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having a broad light harvesting range.51 These defect states can be passivated by growing a ‘thin’ shell around the core QDs with a wide band gap semiconductor e.g. ZnS or CdS to get type-I core/shell structure which is a common approach to eliminate the defect states. The thin shell minimizes the non-radiative decay channels and enhances the PL quantum yield,52 while it enhances the charge collection and injection efficiencies which helps to improve the PCE. CuInS2/ZnS,53 PbS/CdS,54 CdSeTe/Mn-CdSe,55 and CdSeTe/CdS,56 are few examples of type-I core/shell QDs. A systematic control of the shell thickness is necessary for efficient charge separation since thicker shell hinders the tunneling of charge carriers already confined within the core. Recently, a dramatic improvement of PCE is reported in ternary CuInS2 (CIS) “green” QDs by applying a very thin layer of ZnS.53 The oleylamine capped CIS QDs were synthesized at high temperature and the shell was grown by cation exchange procedure at a moderate temperature. The shell thickness can be optimized by varying the concentration of shell precursor and reaction time. Adding the ZnS shell generates higher photocurrent of 20.26 mA/cm2 than CIS alone (17.92 mA/cm2) (Figure 6b). By applying an optimal shell thickness, CIS QDSSCs has achieved 6.66% PCE which is also reflected by a higher charge recombination resistance (Rrec) of CIS-Z (Figure 6c) due to the passivation of surface trap states around the CIS core by ZnS shell and thereby minimizing the pathways of back electron transfer (Figure 6d). Reverse Type I core/shell QDs QDSSCs made by reverse Type-I core/shell QDs is another approach to reduce charge recombination processes at the photoanode/electrolyte interface. CdS/CdSe core/shell QD is a common example,57 in which bandgap of the shell lies within the core QDs and creates a cascade band alignment (Figure 6e). This helps to distribute the electrons and holes in the shell region improving the charge injection efficiency from QDs to TiO2 and suppressing the charge recombination to achieve better PCE and IPCE over the core QDs (Figure 6f,g). 14 ACS Paragon Plus Environment

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Type II Core/Shell QDs Type-II core/shell QDs simultaneously demonstrate a wide absorption range, fast electron injection and slower recombination rate. Herein the hole is localized at the core and the electron is localized at the shell i.e. spatial separation of charge carriers occurs which helps to enhance the rate of electron transfer from the QDs to MO as well as reduce the charge recombination dynamics. CdTe/CdSe and ZnTe/CdSe are two model systems of typeII core/shell QDs.58,59 Due to formation of “spatially indirect energy bandgap” or “exciplex state”, the effective bandgap of the QDs is reduced, which helps to expand the light harvesting range from visible to the infrared region (Figure 7a,b). Due to this spatial separation of the electron−hole pair, the rate of electron injection is enhanced and recombination probabilities are reduced. Using MPA capped water soluble QD sensitizers, PCEs of 7.17, 6.67 and 5.42% were achieved for ZnTe/CdSe, CdTe/CdSe and CdSe champion cells, respectively under 1 sun illumination (Figure 7c). Indeed a remarkable improvement of Jsc is observed in the core/shell QDs (19.35 mA/cm2) than only CdSe QDs (15.08 mA/cm2). Moreover ZnTe/CdSe showed a higher VOC of 0.646 V than CdTe/CdSe QDs (0.597 V). The enhancement of Voc can be explained by EIS measurement which was performed both in the dark and in presence of light to investigate the effect of TiO2 CB edge upshift by ZnTe/CdSe sensitization under illumination relative to that of CdTe/CdSe sensitization. It was observed that both ZnTe/CdSe and reference CdTe/CdSe QDSSCs show similar chemical capacitance (Cµ) values which indicate that the two sensitizers have similar effects on the TiO2 CB under dark conditions (Figure 7d). However under illumination ZnTe/CdSe cells exhibit lower Cµ than CdTe/CdSe at the same applied bias voltage (Figure 7e). Keeping Cµ fixed, the photovoltage in the ZnTe/CdSe cell is ~50 mV higher than that for the reference cell under illumination. Thus ZnTe/CdSe QD sensitized film shifts the CB of TiO2 in the upward direction by ~50 mV which helps to enhance the VOC. ZnTe/CdSe type-II 15 ACS Paragon Plus Environment

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core/shell QDs also have larger CB offset compared to that of the CdTe/CdSe (1.22 versus 0.27 eV) which is shown in Figure 7f. This extension of band offset helps to accumulate charge carriers at the QD/TiO2 interface inducing stronger dipole effects under illumination and upshifts the TiO2 CB edge to enhance the photovoltage over the reference cell. Thus appropriate band engineering can improve the PCE not only by enhancing the light harvesting zone but also by increasing the VOC. Core/Shell/Quasi-shell Configuration To further reduce surface charge recombination, our group developed an interface engineering approach involving a dual sensitization strategy with core/shell CdTe/CdS QDs as the primary light absorber layer over a mesoporous TiO2 scaffold.60 An additional CdS layer, which we termed as quasi-shell, was introduced to further improve the device performance. CdTe/CdS core/shell QDs is an interesting light absorber system owing to the type-II electronic interface of the heterostructure in which both the VB and CB of the CdTe core are located higher than the CdS shell. Such a cascaded band alignment is known to facilitate the electron-hole separation and electron injection into the TiO2 CB. Moreover, the exciplex state of such a type-II system causes a red-shift in the absorption spectrum which results in a larger spectral window of the sensitized device.61 Moreover, CdTe/CdS core/shell QDs is a particularly interesting system owing to the well-established low-temperature aqueous synthesis route and tunable thickness of CdS shell which provides a fine knob over the electronic and optical properties of QDs. We synthesized MPA capped CdTe/CdS QDs of varying shell thicknesses and increasingly red-shifted absorption spectra and utilized time resolved PL decay to identify the emergence of type-II interface for this system. MPA capped CdTe/CdS QDs were then self-assembled onto the TiO2 photoanode and the loading of QDs was optimized by adjusting the solution pH. Evidently higher loading of QDs resulted in higher PCEs. Besides the obvious optical contribution of QDs at higher loading, the higher 16 ACS Paragon Plus Environment

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PCE with increasing loading also had a contribution from reduced interfacial recombination as observed by the increasing trend of Rrec values obtained from EIS measurement. However, the maximum PCE at optimized loading of CdTe/CdS QDs increased only up to 3.6%. We hypothesized that further passivating the surface defect states originating from exposed TiO2 sites could significantly boost the PCE by blocking the recombination pathway as well as by increasing the optical absorbance of the photoanodes. To realize this, a CdS layer of increasing thickness was introduced in a controlled fashion by SILAR. Since the growth of this CdS layer is non-epitaxial in nature we termed this as a “quasi-shell”. Interestingly, a non-obvious trend of an initial increase in PCE up to 4 SILAR cycles was observed followed by a decrease in PCE upon introduction of further SILAR cycles (Figure 8a). EIS measurements could elucidate the underlying mechanism by which PCE increases upon introduction of a quasi-shell. Interestingly, a similar inverted V-shaped trend was observed for Rrec values obtained after fitting the spectra at varying bias voltages (Figure 8b). Thus up to 4 SILAR cycles the resistance to recombination increases and this is believed to be due to passivation of TiO2 surface trap states. Upon further addition of SILAR layers both PCE and Rrec drops and this was attributed to the over-loading and consequent pore-blocking by CdS particulates. Such a pore blocking reduces the interfacial contact between electrolyte and TiO2/QD porous electrode which can further slow-down the hole extraction process and promote recombination. The presence of both the epitaxial CdS shell around CdTe cores and the SILAR deposited quasi-shell significantly improved the PCE of devices as compared to control cells with either one shell present or total absence of any shell layers (Figure 8c). Using the dynamic technique of VOC decay it was understood that while the CdS epitaxial shell improves the charge carrier lifetime in the device, carrier lifetime is only maximized in the presence of both the CdS shell and the quasi-shell (Figure 8d). While the epitaxial CdS shell passivates the defects of the CdTe core QDs, the CdS quasi-shell passivates the TiO2


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trap states. Thus when present simultaneously these layers work in tandem to block two major recombination pathways in a QDSSC as shown in the schematic in Figure 8e and help in improving the PCE. This dual sensitization approach enabled PCE as high as 6.7% in QDSSCs fabricated reproducibly. Doping of Transition Metal Ions Doping is another common strategy to manipulate the optoelectronic properties of a semiconductor QD. A variety of transition metal ion dopants such as Ni2+, Mn2+, Cu+, and Co2+ are reported in the literature.62 Among the above mentioned dopants, doping of Mn2+ has been most widely studied in a large variety of nanocrystals.63-69 Doping of Mn2+ can create an electronic energy level in the mid bandgap and changes the electronic and photophysical properties. Since Mn d−d transition (4T1−6A1) is both Laporte and spin forbidden, it enhances the lifetime of the photogenerated charge carriers and reduces the electron-hole recombination to enhance the PCE of QDSSCs. Kamat’s group has successfully established that the efficiency of Mn-doped co-sensitized CdS-CdSe QDSSCs could reach up to 5.4%.70 However SILAR and CBD are two common approaches to grow Mn-doped QDs onto photoanode whereby the type of doping namely lattice doping, QD surface segregation or ion clustering responsible for achieving higher efficiency cannot be elucidated. To address this problem, colloidal Mn-doped QDs with different doping concentrations were successfully synthesized by our group and the doped QDs were deposited over MO film by electrophoretic deposition to fabricate the QDSSCs.71 The colloidal doped QDs show higher lifetime than undoped QDs as obtained from PL decay measurements (Figure 9a). To confirm the presence of Mn2+ into CdS nanocrystals and the nature of the doping, Electron Paramagnetic Resonance (EPR) is an useful technique which shows that the nature of doping can change with a corresponding change in doping concentration.72 EPR spectra in Figure 9b showed a sextet of hyperfine lines with hyperfine splitting constant (64.4−65.4) × 10−4 cm−1 18 ACS Paragon Plus Environment

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for 0.92-1.44 at% due to Mn2+ lattice doping. From Mn at% of 1.92 to 2.96, hyperfine splitting constant did not change significantly. A broad resonance signal was observed for 2.96 at% due to Mn2+-Mn2+ cluster formation. The doping concentration and the type of doping had a significant role in the device performance (Figure 9c). The highest PCE of 2.1% was achieved when lattice doping was maximum at Mn at% of 1.44. The QDSSCs made by doped QDs showed higher Rrec and better electron lifetime than undoped QDs as shown in Figure 9d,e. PCE decreased with higher doping concentration when exchange coupled Mn2+Mn2+ pairs appear as non-radiative recombination centers (Figure 9f). Thus, increasing the lifetime of excited charge carriers by intentional doping of transition metal ions is a notable boost for designing high efficiency QDSSCs. MO/QD/Electrolyte Interfaces In QDSSCs, the surface coverage of QDs onto MO is usually quite low 12% PCE in less than a decade,96 and with an overhaul of the aforementioned drawbacks, the technology can be boosted tremendously since QDSSCs are already benefited from low-cost solution phase preparation methods and high speed printing techniques. Indeed the progress and commercialization in QDSSC research has to go hand-inhand with the progress of other solar cell types, mainly the lead halide perovskite solar cells (PSCs).97 Both PSCs and QDSSCs engage lesser quantity of the sensitizer material in contrast to the silicon solar cells. In the case of low cost QDSSCs, QDs are much more stable under operational conditions and this PV technology got a major boost when its PCE reached >12% with Zn-Cu-In-Se “green” QDs which is visibly comparable to DSSCs and the all-inorganic CsPbI3 “toxic” solar cells.98,99 However, this value is still low when compared to commercialized silicon solar cells and state-of-the-art organic-inorganic bulk PSCs. Long term instability due to leakage of liquid electrolyte is another issue that needs active research to bring them nearer to commercialization. To make QDSSCs viable for long term applications, the development of solid electrolyte devices may play a decisive role. Barring the elimination of possibilities of electrolyte leakage in the solid state QDSSC devices, proper sealing of the device is essential to prevent leakage of Cd- and Pb-based toxic photoactive materials in the environment. Future improvements may also come from developing strategies to increase loading of QDs onto MO, engineering of the QDs to further increase light harvesting efficiency in solar spectrum, ordered arrangement of QDs such as a tandem structure, modification of MO, QD and photoanode by passivation and reduction of intrinsic trap states of QDs, employing better hole transporting material and superior CE with lower RCT. Considering the huge promise to substitute the existing solar cell technologies, Texas 27 ACS Paragon Plus Environment

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based Solterra Renewable Technologies have invested in QDSSCs, concentrating on low-cost synthesis methods to produce four armed QDs, which holds potential to better charge transport and superior PCE than conventional spherical QDs of the same material.100 Acknowledgements The Department of Science and Technology - Science and Engineering Board (DST-SERB), Government of India is duly acknowledged for the financial support under grant no. EMR/2016/001703. GH thanks University Grants Commission (UGC), New Delhi, DG thanks SERB National Postdoctoral Fellowship (NPDF) Scheme and MYA thanks IISER Kolkata for their fellowships. References (1) Nayak, P. K.; Garcia-Belmonte, G.; Kahn, A.; Bisquert, J.; Cahen, D. Photovoltaic Efficiency Limits and Material Disorder. Energy Environ. Sci. 2012, 5, 6022–6039. (2) Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H. –Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science 2011, 334, 1530–1533. (3) Kramer, I. J.; Sargent, E. H. The Architecture of Colloidal Quantum Dot Solar Cells: Materials to Devices. Chem. Rev. 2014, 114, 863–882. (4) Kamat, P. V.; Tvrdy, K.; Baker, D. Semiconductor Nanoarchitectures for Rev. 2010, 110, 6664–6688.

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and Interfaces of Metal Oxide Solar Cells. J. Phys. Chem.

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(35) Jiang, L.; You, T.; Deng, W. –Q. Enhanced photovoltaic performance of a quantum dotsensitized solar cell using a Nb-doped TiO2 electrode. Nanotechnology 2013, 24, 415401 (1– 6). (36) López-Luke, T.; Wolcott, A.; Xu, L. –P.; Chen, S.; Wen, Z.; Li, J.; Rosa, E. D. L.; Zhang, J. Z. Nitrogen-doped and CdSe quantum-dot-sensitized nanocrystalline TiO2 films for solar energy conversion applications. J. Phys. Chem. C 2008, 112, 1282–1292. (37) Li, L.; Yang, X.; Zhang, W.; Zhang, H.; Li, X. Boron and Sulfur Co-Doped TiO2 Nanofilm as Effective Photoanode for High Efficiency CdS Quantum-Dot-Sensitized Solar Cells. J. Power Sources 2014, 272, 508–512. (38) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion J. Phys. Chem. C 2007, 111, 2834–2860. (39) Santra, P. K.; Kamat, P. V. Tandem Layered Quantum Dot Solar Cells. Tuning the Photovoltaic Response with Luminescent Ternary Cadmium Chalcogenides. J. Am. Chem. Soc. 2013, 135, 877–885. (40) Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSexTe1-x Alloyed Quantum Dot Sensitized Solar Cells with More than 6% Efficiency and High Stability. ACS Nano 2013, 7, 5215–5222. (41) McDaniel, H.; Fuke, N.; Pietryga, J. M.; Klimov, V. I. Engineered CuInSexS2−x Quantum Dots for Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 355−361. (42) Kim, S.; Kang, M.; Kim, S.; Heo, J. –H.; Noh, J. H.; Im, S. –H.; Seok, S. I.; Kim, S. –W. Fabrication of CuInTe2 and CuInTe2-xSex Ternary Gradient Quantum Dots and Their Application to Solar Cells. ACS Nano 2013, 7, 4756–4763. (43) Du, J.; Du, Z.; Hu, J. –S.; Pan, Z.; Shen, Q.; Sun, J.; Long, D.; Dong, H.; Sun, L.; Zhong, X.; Wan, L. –J. Zn-Cu-In-Se Quantum Dot Solar Cells with aCertified Power Conversion Efficiency of 11.6%. J. Am. Chem. Soc. 2016, 138, 4201–4209. (44) Halder, G.; Bhattacharyya, S. Zinc-Diffused Silver Indium Selenide Quantum Dot Sensitized Solar Cells with Enhanced Photoconversion Efficiency. J. Mater. Chem. A 2017, 5, 11746–11755. (45) Peng, W.; Du, J.; Pan, Z.; Nakazawa, N.; Sun, J.; Du, Z.; Shen, G.; Yu, J.; Hu, J. –S.; Shen, Q.; Zhong, X. Alloying Strategy in Cu−In−Ga−Se Quantum Dots for High Efficiency Quantum Dot Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 5328−5336. (46) Li, L. –B.; Wang, Y. –F.; Rao, H. –S.; Wu, W. –Q.; Li, K. –N.; Su, C. –Y.; Kuang, D. – B. Hierarchical Macroporous Zn2SnO4-ZnO Nanorod Composite Photoelectrodes for Efficient CdS/CdSe Quantum Dot Co-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 11865–11871. (47) Hines, D. A.; Kamat, P. V. Recent Advances in Quantum Dot Surface Chemistry. ACS Appl. Mater. Interfaces 2014, 6, 3041−3057.


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(48) Zhang, H.; Cheng, K.; Hou, Y. M.; Fang, Z.; Pan, Z. X.; Wu, W. J.; Hua, J. L.; Zhong, X. H. Efficient CdSe Quantum Dot-sensitized Solar Cells Prepared by a Postsynthesis Assembly Approach. Chem. Commun. 2012, 48, 11235−11237. (49) Li, W.; Zhong, X. Capping Ligand-Induced Self-Assembly for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 796−806. (50) Lo, S. S.; Mirkovic, T.; Chuang, C. –H.; Burda, C.; Scholes, G. D. Emergent Properties Resulting from Type-II Band Alignment in Semiconductor Nanoheterostructures. Adv. Mater. 2011, 23, 180–197. (51) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756– 3776. (52) Chaudhuri, R.G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373–2433. (53) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203–9210. (54) Jiao, S.; Wang, J.; Shen, Q.; Li, Y.; Zhong, X. Surface Engineering of PbS Quantum Dot Sensitized Solar Cells with a Conversion Efficiency Exceeding 7%. J. Mater. Chem. A 2016, 4, 7214–7221. (55) Wang, G.; Wei, H.; Luo, Y.; Wu, H.; Li, D.; Zhong, X.; Meng, Q. A Strategy to Boost the Cell Performance of CdSexTe1-x Quantum Dot Sensitized Solar Cells over 8% by Introducing Mn Modified CdSe Coating Layer. J. Power Sources 2016, 302, 266–273. (56) Yang, J.; Wang, J.; Zhao, K.; Izuishi, T.; Li, Y.; Shen, Q.; Zhong, X. CdSeTe/CdS TypeI Core/Shell Quantum Dot Sensitized Solar Cells with Efficiency over 9%. J. Phys. Chem. C 2015, 119, 28800−28808. (57) Pan, Z.; Zhang, H.; Cheng, K.; Hou, Y.; Hua, J.; Zhong, X. Highly Efficient Inverte Type-I CdS/CdSe Core/Shell Structure QD-Sensitized Solar Cells. ACS Nano 2012, 6, 3982– 3991. (58) Wang, J.; Mora-Seró, I.; Pan, Z.; Zhao, K.; Zhang, H.; Feng, Y.; Yang, G.; Zhong, X.; Bisquert, J. Core/Shell Colloidal Quantum Dot Exciplex States for the Development of Highly Efficient Quantum-Dot-Sensitized Solar Cells. J. Am. Chem. Soc. 2013, 135, 15913– 15922. (59) Jiao, S.; Shen, Q.; Mora-Sero´, I.; Wang, J.; Pan, Z.; Zhao, K.; Kuga, Y.; Zhong, X.; Bisquert, J. Band Engineering in Core/Shell ZnTe/CdSe for Photovoltage and Efficiency Enhancement in Exciplex Quantum Dot Sensitized Solar Cells. ACS Nano 2015, 9, 908–915. (60) Sahasrabudhe, A.; Bhattacharyya, S. Dual Sensitization Strategy for High-Performance Core/Shell/Quasi-shell Quantum Dot Solar Cells. Chem. Mater. 2015, 27, 4848–4859. (61) Zhao, K.; Pan, Z.; Zhong, X. H. Charge Recombination Control for High Efficiency Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2016, 7, 406–417. 32 ACS Paragon Plus Environment

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Figure 1. (a) Schematic illustration showing the working principles of QDSSC. Green solid arrows indicate the enlarged view of MO/QD, MO/QD/electrolyte and CE/electrolyte interfaces. Solid arrow, solid curve arrow and dashed arrows show excitation, electron transfer, hole transfer and recombination processes, respectively. (b) Energy level diagram of some well studied materials applied in QDSSCs.


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Figure 2. Field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) images of different nanostructuring of MOs: (a) TiO2 donuts; reproduced with permission from ref 17, copyright (2014) Elsevier. (b) TiO2 nanosheets; reproduced with permission from ref 11, copyright (2012) WILEY-VCH Verlag GmbH & Co. (c) TiO2 nanorod arrays; reproduced with permission from ref 10, copyright (2013) WILEY-VCH Verlag GmbH & Co. (d) TiO2 microsphere precursors (surface area of 2.64 m2/g) whereas the inset shows mesoporous microspheres (surface area 86.6 m2/g ) by hydrothermal crystallization of the precursor microspheres; reproduced with permission from ref 18, copyright (2014) Elsevier. (e) ZnO hierarchical tetrapod; reproduced with permission from ref 14, copyright (2012) Royal Society of Chemistry. (f) ZnO microspheres; reproduced with permission from ref 13, copyright (2014) American Chemical Society. (g) 3-dimensional nanostructured ZnO nanorod arrays; reproduced with the permission from ref 12, copyright (2013) Royal Society of Chemistry. (h) ZnO nanorods; reproduced with permission from ref 19, copyright (2014) American Chemical Society. (i) SnO2 NPs; reproduced with permission from ref 20, copyright (2013) American Chemical Society. (j) SnO2/TiO2 core/shell NPs; reproduced with permission from ref 15, copyright (2015) American Chemical Society. (k) SnO2 nanofiber; reproduced with permission from ref 22, copyright (2016) Elsevier. (l) SnO2 NPs; reproduced with permission from ref 21, copyright (2015) Elsevier. (m) Schematic of tri-layer TiO2 photoanode showing electrolyte migration (orange lines) through m-TiO2 and light scattering (red small arrows) in s-TiO2 layer.


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Figure 3. (a) Surface passivation strategy of ZnO mesoporous electrode. Comparison of (b) absorbance spectra, (c) J–V characteristics, (d) Nyqiust plot and (e) Bode phase diagram of the QDSSCs under forward bias of -0.6 V in dark. Reproduced with permission from ref 16, copyright (2013) The Royal Society of Chemistry.


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Figure 4. (a) UV–vis absorption spectra of CdS/ZnO and CdS/SAMs(1 min)/ZnO. (b) Room temperature PL spectra of ZnO nanorods, 3-PPA(1 min)/ZnO, BPA(1 min)/ ZnO and APPA(1 min)/ZnO. (c) J–V characteristics of CdS/ZnO nanorods, CdS/3-PPA(1 min)/ZnO, CdS/BPA(1 min)/ ZnO and CdS/APPA(1 min)/ZnO QDSSCs. (d) Schematic illustration of energy diagrams of ZnO nanorods, 3-PPA(1 min)/ZnO, BPA(1 min)/ZnO and APPA(1 min)/ZnO solar cells. Reproduced with the permission from ref 30, copyright (2015) Elsevier Ltd.


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Figure 5. Schematic representation of QDSSCs constructed by (a) sequential deposition (b) mixed deposition of green, orange, and red CdSeS QDs constituting the rainbow structure. (c) Schematic illustration showing electron transfer processes between QDs assembled in a tandem architecture. Reproduced with permission from ref 39, copyright (2012) American Chemical Society.


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Figure 6. (a) Schematic diagram of band alignment of Type-I, Reverse Type-I and Type-II core/shell QDs, respectively. (b) J−V characteristics of QDSSCs under the illumination of 100 mW/cm2 sunlight. (c) Nyquist plots of CIS-Z and CIS-based champion cells at −0.55 V forward bias. (d) Schematic representation showing the importance of ZnS shell. Reproduced with permission from ref 53, copyright (2014) American Chemical Society. (e) Schematic representation of band edge alignment of the inverted type-I CdS/CdSe core/shell structure in QDSSCs. (f) J-V curves and (g) IPCE spectra of QDSSCs based on different CdS/CdSe QD sensitizers. Subscripts denote λmax of excitonic bands of the representative QDs. Reproduced with permission from ref 57, copyright (2012) American Chemical Society.


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Figure 7. (a) Normalized absorption (solid lines) and PL spectra (dashed lines) of CdTe and CdTe/CdSe QDs in toluene (λex = 360 nm). (b) Schematic illustration of CdTe/CdSe core/shell QD showing the relative band positions. Solid lines indicate the position of VB and CB. Arrows indicate the bandgap (Eg) for CdTe, CdSe QDs and the exciplex state. Reproduced with permission from ref 58, copyright (2013) American Chemical Society. (c) J-V curves of CdSe, CdTe/CdSe and ZnTe/CdSe QDs based QDSSCs. Dependence of Cµ on applied voltage (Vapp) of QDSSCs under (d) dark (e) illumination conditions. (f) Schematic representation of the band gap and band offsets (in eV) at the interfaces of bulk ZnTe/CdSe and CdTe/CdSe. Reproduced with permission from ref 59, copyright (2015) American Chemical Society.


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Figure 8. (a) J−V characteristics of CdTe/CdS QD loaded films with varying number of quasi-shells. The selected QDs have an emission maximum at 645 nm. (b) Variation of Rrec with shifted potential scale. (c) J−V characteristics (d) photogenerated charge carrier lifetime obtained from OCVD plots for core (C), core/shell (CS), core/quasi-shell (CQS4), and core/shell/quasi-shell (CSQS4). Here 4 indicate four quasi-shell layers deposited by SILAR. (e) Schematic illustration of the band diagram showing the utility of the dual sensitization strategy: (i) CdTe core, (ii) CdTe/CdS core/shell, and (iii) CdTe/CdS/CdS core/shell/quasishell QDSSCs, respectively. Reproduced with permission from ref 60, copyright (2015) American Chemical Society.


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Figure 9. Mn-doped CdS QDSSCs: (a) PL decay curves and (b) EPR spectra of the colloidal QDs measured at 100 K. (c) J−V characteristics of QDSSCs based on different doping concentration of QDs, (d) Nyquist plot of the representative QDSSCs under −0.6 V forward bias in dark, (e) electron lifetime. (f) Schematic representation of the band diagram of Mndoped QDSSC showing different charge transfer and recombination pathways. Solid, dashed and curved arrows indicate excitation, recombination and nonradiative decay processes, respectively. Reproduced with permission from ref 71, copyright (2015) American Chemical Society.


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Figure 10. (a-d) Device parameters of CdS and CdS/ZnS QDSSCs treated with different surface passivation agents. (e) Rrec and (f) Cµ for CdS/ZnS QDSSCs under dark conditions. Reproduced with permission from ref 75, copyright (2013) American Chemical Society.


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Figure 11. (a) Schematic illustration showing the effect of photoanode with different metal chloride treatments. (b) J−V characteristics of different metal chlorides treated CdSeTe-based QDSSCs. (c) Calculation of density of states for (i) bare TiO2 (101) electrode and the surface passivated with (ii) 1 ML ZrO2, (iii) 1 ML Nb2O5, and (iv) 0.5 ML Fe2O3. Reproduced with permission from ref 73, copyright (2016) American Chemical Society.


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Figure 12. Zn-alloyed AgInSe2 QDSSCs: (a) J–V characteristics of the representative QDSSCs under 1 sun illumination. Plots of the (b) Rrec and (c) Cµ of the QDSSCs with applied potential. (d) Bode phase diagram of the QDSSCs. (e) Variation of dark current with applied potential in the log scale. Schematic diagram of the AZ5-8ZTS based QDSSC showing the electron transfer (solid curved arrows) and recombination pathways (dotted straight arrows). Reproduced with permission from ref 44, copyright (2017) The Royal Society of Chemistry.


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Figure 13. (a) Schematic of the working mechanism of CEs in QDSSC. FESEM and TEM images of nanostructured CE: (b) CuxS hexagonal nanoplates; reproduced with permission from ref 87, copyright (2016) The Royal Society of Chemistry. (c) Skeletal Cu7S4 nanocages; reproduced with permission from ref 88, copyright 2014 Elsevier. (d) CuS nanosheets; reproduced with permission from ref 89, copyright (2015) The Royal Society of Chemistry. (e) Cu2-xSe nanotubes; reproduced with permission from ref 90, copyright (2015) WILEYVCH Verlag GmbH & Co. (f) Schematic of rGO supported Cu2S NPs; reproduced with the permission from ref 92, copyright (2011) American Chemical Society. (g) CuxS/GOR composite; reproduced with the permission from ref 87, copyright (2016) The Royal Society of Chemistry. (h) CuS electrospun carbon nanofibers; reproduced with the permission from ref 93, copyright (2014) American Chemical Society. (i) Graphene hydrogel; reproduced with the permission from ref 94, copyright (2017) Royal Society of Chemistry. (j) Graphitic porous carbon; reproduced with permission from ref 95, copyright (2016) Elsevier. FESEM images of (k) CNT, (l) GOR and its schematic structure, (m) GO and its schematic structure. (n) J-V plots, (o) Nyquist plot and (p) Tafel polarization curve of symmetric cells for different composite CEs. Inset of (o) shows the equivalent circuit used to fit the Nyquist plot. (k) Schematic showing band position modification via CuxS/GOR composite for faster electron transfer to polysulfide electrolyte. Reproduced with permission from ref 87, copyright (2016) The Royal Society of Chemistry.


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Figure 14. (a) FESEM image of human hair and inset showing its digital image. Photograph of (b) pre-carbonized hair obtained by acid treatment and (c) final carbonized GPC obtained at high temperature in N2 atmosphere. (d) J-V characteristics, (e) Voc decay curves and (f) Bode plots of a CdS QDSSC using GPC CEs prepared at different carbonization temperatures. (g) Nyquist plots of symmetric cells with the simplified Randle’s circuit shown in inset. Reproduced with permission from ref 95, copyright (2016) Elsevier.


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Table of Contents (TOC):

Author Biographies

Ganga Halder completed her B.Sc. from the Katwa College, University of Burdwan, India in 2010 with Chemistry in major. She received her M.Sc. degree in Chemistry from Indian Institute of Technology, Bombay, India in 2012. She joined as a Ph.D. scholar in the Department of Chemical Sciences at Indian Institute of Science Education and Research (IISER) Kolkata, India under the supervision of Dr. Sayan Bhattacharyya. Her current research focuses on development of advanced materials for photovoltaic devices and understanding the charge recombination and charge transfer processes in quantum dot sensitized solar cells.

Dibyendu Ghosh is a postdoctoral research fellow at Indian Institute of Science Education and Research (IISER) Kolkata, India, since 2015, when he joined Dr. Bhattacharyya’s 50 ACS Paragon Plus Environment

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research group. He obtained his Ph.D. in Physics from Jadavpur University, India in 2015. His Ph.D. work was on III-V semiconductor thin films for device application under the supervision of Professor A. K. Pal and Professor R. Bhar. His research is centered on the controlled synthesis of II-VI quantum dots, halide perovskite nanocrystals using methods of colloidal chemistry and highlighting these nanomaterials for energy conversion systems.

Md. Yusuf Ali is currently a Masters student in Department of Chemical Sciences at Indian Institute of Science Education and Research (IISER) Kolkata, India under the supervision of Dr. Sayan Bhattacharyya. He obtained his B.Sc from Ramakrishna Mission Vivekananda Centenary College, Rahara, India with Chemistry major in 2015. His research interests include the development of functional materials for photovoltaic devices.

Atharva Sahasrabudhe received Integrated BS-MS degree (Gold Medalist) in Chemical Sciences at the Indian Institute of Science Education and Research (IISER) Kolkata, India. He worked towards his Masters' Thesis under the guidance of Dr. Sayan Bhattacharyya at the Advanced Functional Materials Laboratory, where his research was focused on understanding structure-function correlation of nanostructured materials for developing improved renewable energy devices such as quantum-dot solar cells and electrolyzers. Currently he is pursuing his PhD in the Bioelectronics group at the Massachusetts Institute of Technology, USA.


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Sayan Bhattacharyya is currently an Associate Professor in the Department of Chemical Sciences and Chair of the Centre for Advanced Functional Materials at Indian Institute of Science Education and Research (IISER) Kolkata, India. He was born in Kolkata where he did his B.Sc. in Maulana Azad College, University of Calcutta, India in 1996. After obtaining his M.Sc. degree from University of Kalyani, West Bengal, India in 1998 he completed his Ph.D. with Prof. N. S. Gajbhiye at the Indian Institute of Technology, Kanpur, India in 2006. After his postdoctoral research with Prof. (Emeritus) Aharon Gedanken at Bar-Ilan University, Israel during 2006-2008 and Prof. Yury Gogotsi at Drexel University, USA during 2008-2010 he joined IISER Kolkata as Assistant Professor in April 2010. He is a Materials Chemist interested in photovoltaics, catalysis for energy, magnetism and biological applications. A combination of wet-chemical synthesis and self-assembly of smart solid state nanomaterials, structure-property correlation and device applications are used to attain these research goals.


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Interface Engineering in Quantum-Dot-Sensitized Solar Cells

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