Recent Progress of Electron Storage Mn Center in Doped Nanocrystals

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Recent Progress of Electron Storage Mn Centre in Doped Nanocrystals Tushar Debnath, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11055 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Recent Progress of Electron Storage Mn Centre in Doped Nanocrystals Tushar Debnath† and Hirendra N. Ghosh† ‡* †

Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085,

India ‡

Institute of Nano Science and Technology, Mohali, Punjab 160062, India * E-mail: [email protected], [email protected];

Abstract: Mn doping in semiconductor nanocrystals (NCs) has been reviewed for their unequivocal relevance to solar energy harvesting in terms of both mechanistic approach of electron storage in Mn centre and improved power conversion efficiency (PCE). Mn doped NCs measured to have significantly higher PCE than the undoped counterpart which has been reviewed for a number of different host NCs including different alloy and core-shell structures. Such increment of PCE has been attributed to electron relaxation from the hot states of the host NCs via Mn state (that occurs at hundreds of picosecond timescale) which acts as electron storage centre. This can avoid the detrimental charge recombination significantly and at the same time, fast electron injection from Mn state to TiO 2 leads to enhance PCE performance. These phenomenons are clearly demonstareted by analysing the feedback from femtosecond transient absorption, time resolved photoluminescence and impedance spectroscopy.

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1. Introduction: Semiconductor nanocrystals (NCs), often termed as quantum dots (QDs), offers tunability of optoelectronic properties owing to their easy solution processability that makes them desirable candidate for nanoelectronics, bio-imaging and quantum dot solar cell (QDSC).1-9 Research on QDSC increases manifold with the aid of research on multiple exciton generation (MEG) as it is believed that a single photon can generate multi electron-hole pair (exciton) which can enhance the solar cell efficiency to a great extent.10-14 By separating the multiexcitons before ultrafast exciton-exciton annihilation, one can enhance the efficiency of the device significantly. Exciton dissociation in ultrafast timescale has been reported in many QD/TiO 2 systems.15-17 Reports are also available where dissociation of multi-exciton take place by introducing molecular adsorbates that act as either electron or hole acceptors.18-25 Formation of heteronanocrystals (HNCs) through three or more elements together either in core-shell26-29 or in alloy structure30-35 can also dissociate multi-excitons and thereby modify the optoelectronic properties. Due to smooth variation of potential from inner part of the alloy towards the surface, the charge separation yield is higher in alloy NCs as compared to the coreshell NCs, where charges can be trapped in the core/shell interface. One of the major advantages of the alloy NCs that by changing only the composition of the constituents, the optical band gap as well as different optoelectronic properties can be varied significantly keeping the same size of the NCs. Ultrafast charge carrier dynamics of alloy NCs was also performed to understand the underlying mechanism for improved optoelectronic applications.35 Intentional impurity doping in the NCs can provide another extra degree of freedom for tuning the optoelectronic properties of the host such as magnetic property, electrical conductivity etc.36-39 Doping of 3d transition metal such as Mn, Cu etc. in the semiconductor host are widely investigated in recent years which in turn can tune the optical and electrical properties.40,41 Particularly Mn doping in different semiconductors has been explored by


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several researchers as a photoluminescence activator for their high emission quantum yield (QY). The longer lifetime of the excited Mn2+ prepared by sensitization in the doped NCs has been revealed for energy storage application.42-45 The atomic like 4T 1 -6A 1 transition, originates from Mn d-d transition, leads to such high QY of the Mn-doped NCs. Due to involvement of the Mn 4T 1 -6A 1 state46-48, the PL spectra of Mn-doped NCs usually appear at ~2.1 eV irrespective of NC size.49,50 In contrast, for the Cu-doped NCs the PL involves both Cu and the semiconductor (CB) state and can appear in different region.51-54 Due to both spin and orbitally forbidden transition the lifetime of the excited Mn2+ prepared by sensitization found to be in the range of milliseconds. Recently in several literature it appears that Mn centre can also act as electron storage centre.55-62 Probably, this is the leading factor of Mn dopant over the other dopants (such as Cu) which makes Mn dopant directly applicable in solar energy conversion. For example, it has been observed that incorporation of Mn dopant in the host NCs has enormous consequence for boosting the power conversion efficiency (PCE) of the QDSC made with such NCs.55,56,58,59,61,62 Here the electron is stored on the Mn d-state, thereby reducing the electron trapping as well as recombination to a great extent. Several researchers are involved for the investigation of Mn-doped NCs to make them suitable for energy storage application for more than a decade. However, no report was found to use such Mn-doped NCs for boosting the PCE till the seminal work reported by Santra and Kamat in 2012.55 In this report, for the first time, they speculated Mn centre can act as electron storage centre for boosting the PCE.55 Till now people have engaged themselves for exploration of the role of Mn centre as electron storage centre for both mechanistically as well as PCE enhancement in different host NCs. Mndoping has been successfully investigated in different binary as well as multinary alloy NCs, where in some cases formation of charge transfer (CT) state has been observed. Such CT state can alter the electron relaxation dynamics through Mn state to large extent that leads to improvement of the PCE.60,62 Using CdSeTe alloy NCs as a host, PCE of as high as 9.4% was



recently reported for Mn-doped CdSeTe alloy by Zhong and co-workers.59 Some of the recent reviews of the doped NCs41 have indicated the possibility of Mn centre acting as electron storage centre, however, there is no detailed and exclusive review aricle is available on the electron storage Mn centre in the literature. Although in the recent scenario people are working on the field of electron storage Mn centre very actively over the last 5-6 years. In the present article, therefore, the recent progress of synthesis and mechanistic features through optical, ultrafast transient absorption and impedance spectroscopy of Mn-doped NCs for improved PCE have been reviewed where Mn centre has been found to play a major role. The the host environment can influence the crystal field and electronic states of Mn2+ ions in the doped NCs.36 Addressing of such energy and electronic structure of the host forms the strong foundation for the fundamental understanding of Mn doped NCs. 2. Energy Structure of the II-VI Nanocrystals and the Electronic States of Mn2+ Ions within those Nanocrystals The physics behind the energy structure of II-VI semiconductor NCs, which are commonly explain related to strong mixing of electronic states, have been a fundamental research interest over the last few decades. In the absence of any band mixing effect, bulk band leads to several independent quantized states within the spherical NCs.11,63-68 Two quantum number can be classified for each quantum states, e.g. angular momentum of an envelope wave function 𝐿𝐿𝐿, that describes the carrier motions in the NC and the number of states in a series of states of given symmetry (n). Electron and hole states in semiconductor NCs therefore can be expressed as nL e and nLh . Typical notation for L values are S (L=0), P (L=1), D (L=2) etc. and n values are 1, 2, 3 etc. Therefore, the electron states are defined as 1S e , 1P e , 1D e etc and the hole states are as 1S h , 1P h , 1D h etc. However, in real system, one cannot avoid the band mixing effect; particularly in the valence band of many semiconductors, the multi band sub structure can be explained by confinement induced mixing between different sub bands. The Hamiltonian of


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the valence band contains both crystal lattice and NC confinement potential; therefore, the total angular momentum can be represented as 𝐹𝐹𝐹 = 𝐿𝐿𝐿 + 𝐽𝐽𝐽 where 𝐽𝐽𝐽is the sum of Bloch function

angular momentum. The valence band states, in general, are denoted by nL F . For example, the size dependent hole energy calculation for CdSe shows the lowest energy hole states are 1S 3/2 , 1P 3/2 , 2S 3/2 . Thus the lowest energy excitons can be expressed as 1Se-1S 3/2 (1S), 1Se-2S 3/2 (2S) and 1Pe-1P 3/2 (1P). For good quality of colloidal CdSe NC, in optical absorption spectrum all these states are well resolved (Figure 1A and B). Now, when Mn atom is doped in such NCs, the local electronic structures of Mn2+ can be described by the ligand field theory.36,69 With five 3d electrons, Mn2+ has a six-fold degenerate free-ion singlet ground state orbital (6S). However, the spin-flip transition produces lowest free-ion excited state 4G term. In weak tetrahedral field, the five-fold degenerate 3d orbital of Mn2+ is split to e and t 2 . As a result, although the ground state remains totally symmetric i.e. 6A 1 ; but, it splits the orbital degeneracy of the higher terms (including 4G). The lowest free-ion excited 4G term thus split in to 4T 1 , 4T 2 and 4A 1 terms in weak tetrahedral field (Tanabe-Sugano diagram, Figure 1C). The higher free-ion terms (4P, 4D, 2I etc.) also split into several terms in presence of weak tetrahedral field. The lowest energy 4T 1 -6A 1 transition is responsible for luminescence of Mn2+ in the doped NCs (weak tetrahedral filed). Although there is spin forbiddenness for the 4T 1 -6A 1 transition, however, such can be partially overcome due to spin-orbit coupling at the Mn2+ and the anion (e.g., S2-, Se2-, Te2-) and Mn2+-Mn2+ exchange interaction at higher Mn concentration.70,71 However, such processes are very inefficient, thereby the 6A 1 - 4T 1 (and higher excited terms) transition largely remain forbidden.72 As a result, the optical absorption spectrum does not contain any signature due to ligand filed transition for Mn-doped NCs. The relaxation dynamics of Mn-doped NCs can be explained by an energy transfer mechanism that act between the NC and the Mn2+ state (Figure 1D).36,38,45,49 On photo-



excitation, electrons from the valence band (VB) in the NC excited to the conduction band (CB) and thereby, during the relaxation the exciton transfers its excess energy to Mn ion which leads to population in the 4T 1 state. The energy transfer mechanism found to involve exchange interaction which occurs on sub-nanosecond timescale.73,74 Following energy transfer, the 4T 1 -6A 1 transition occurs on micro- to millisecond timescale to provide the Mn characteristic luminescence. Therefore, one of the obvious criteria to observe Mn luminescence in doped NCs is that the the exciton transition energy of the host NC has to be higher in energy as compared to the 4T 1 -6A 1 ligand field transition energy. This indeed enables the host to dopant energy transfer process viable. Figure 1E shows bulk band gap of different II-VI semiconductor host and these are correlated with Mn 4T 1 -6A 1 energy gap (~2.1 eV). Among the listed semiconductors, the bulk band gaps of CdSe and CdTe have lower energy as compared to that of Mn2+ 4T 1 -6A 1 transition energy gap, while for the other common II-VI semiconductors have bulk band gap energy higher than that of Mn2+ 4T 1 -6A 1 transition energy gap (Figure 1E). However, while their size reduced to nanodimensions, the quantum confinement widens the band gap for the NCs. In any circumstances, the II-VI semiconductors other than CdSe and CdTe, always have band gap energy higher than the Mn2+ 4T 1 -6A 1 ligand field transition energy gap (provided there is no other effect on 4T 1 -6A 1 energy gap). As a result the energy transfer mechanism remains active and gives characteristic Mn d-d photoluminescence (PL). However, for both CdSe and CdTe, on quantization, there is a certain probability that the NC band gaps become larger than the Mn2+ 4T 1 -6A 1 energy gap which leads to activate the energy transfer mechanism and can give Mn PL. On the other hand, if the size of the NC becomes larger than certain threshold, then the band gap of the NCs becomes smaller than the 4T 1 -6A 1 energy gap and restricts the energy transfer process (see below for more details). Under such condition, exclusively exciton PL can be observed in Mn-doped CdSe or CdTe NCs. Therefore, the appearance and disappearance mechanism of the Mn2+ PL in Mn-doped NCs largely depend


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on the synthetic condition employed by which one can tune the band gap of the host NCs. In the next section we have reviewed some established synthetic strategy of different doped NCs.

Figure 1. (A) Optical absorption spectrum of colloidal CdSe NC and assignment of different excitonic states. (B) In the schematic diagram, the lowest energy allowed interband (valence band to conduction band) transitions are shown for CdSe NC. (C) Tanabe-Sugano diagram of d5 system (Mn2+) in weak tetrahedral field. Lowest and several spin-forbidden ligand field excited terms are shown. Also, the emissive 4T 1 -6A 1 transition that is observed for Mn2+ doped II-VI NCs, is indicated by orange arrow. (D) Illustration of energy transfer mechanism from host NC to Mn2+ 4T 1 state that leads to observation of Mn PL. (E) Comparison of bulk band gap energies of several II-VI NCs26 to the Mn2+ ligand field transition energy gap i.e. 4T 1 6

A 1 , which approximately appears at ~2.1 eV for most of the Mn2+ doped NCs.



3. General Synthetic Approach: Growth mechanism and Optical Spectroscopy The synthetic chemistry plays a major role for governing diverse attractive properties in doped NCs.36,41,49,75-79 Synthesis of Mn doped NCs can be achieved either by simultaneous introduction of host and dopant precursor or by growth of host NCs followed by dopant introduction. In the former, co-nucleation takes place where the growth of dopant and host are coupled to each other leading to uniform doping (Figure 2A); however the growth of dopant is completely decoupled from the growth of host in the later. Here precise adsorption of dopant in the appropriate location of the host can be achieved. Although after sufficient growth time end product of both the approach leads to internal doped NCs (Figure 2B).80-82 Such synthetic procedure often results dual emitting NCs where the emission have been detected both from dopant as well as excitonic states (see Figure 2). Dopant precursor also have important role for governing the reaction via internal or surface doped intermediate as shown in Figure 2A where generally the less reactive dopant precursor proceeds preferably via surface doped intermediate. In another approach the Mn dopants are allowed first for the nucleation growth followed by host nucleation. This approach generally leads to core doped NCs having emission exclusively from the dopant (Figure 2C).39,80 Doping in the interface or at different location of the shell in a core-shell NCs also has been successfully achieved by many researchers (Figure 2D).42,43,47,81,83 One such example of Mn-doped ZnSe/CdSe/ZnSe NCs is shown in Figure 1D, in which interestingly, shows tuning of Mn d-emission from 480-600 nm which supposed to be fixed at ~585 nm due to 4T 1 -6A 1 transition.47


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Figure 2. Schemes of different doping approach show successful incorporation of the dopant in the host NCs. (A) Co-nucleation doping leads to internal as well surface doped NCs by controlled reaction. (Right) Mn-doped ZnSe and Mn-doped ZnSe/ZnS shows efficient Mn demission (orange arrow) after photoexcitation of wide band gap host ZnSe and ZnSe/ZnS semiconductor NCs. Over coating the host with smaller band gap CdS NCs leads to overall narrowing down the band gap of the NCs. Narrowing band gap allows thermal population of the excitonic states from 4T 1 state of Mn by back energy transfer which results appearance of



excitonic PL. Finally, dual emitting NCs with ~40% QY has been observed. (B) Growth doping: here dopant is introduced after growing the host NCs, leading to surface as well as internal doping. (Right) Growth doping of Cu-doped ZnSe NCs, showing enhancement of dopant emission (with concomitant reduction of exciton emission) as the doping time increases indicating incorporation of dopant into the host. (C) Dopant nucleation doping leads to core doped NC with PL exclusively appears from the dopant. (Right) PL from Mn core-doped NCs, the QY of which enhances with growth time. (D) Representation of interface as well as location dependent doping in core-shell structure. (Right) a) Schematic route for the synthesis of Mndoped ZnSe/CdSe/ZnSe QDs. b) Luminescence from Mn-doped NCs. c) Mn-PL bands for Mndoped at exactly interface (1-4), at different distance from core (5-8) and at ZnSe (9) NCs. d) TEM image of Mn-doped core-shell NCs. A, B, C and D are reprinted (adapted) with permission from ref 82, 80, 39 and 47 respectively. Copyright American Chemical Society. Surface Mn doped NCs can be synthesized successfully by controlling the reaction growth as shown in Figure 2A and 2B in a co-nucleation growth process or by growing first the host nucleus followed by introduction of dopant molecules. Sometimes such surface rich doped NCs are found to emit band edge emission instead of dopant emission. For example, Figure 3A shows the optical spectrum of a surface Mn doped CdSe NCs synthesized via conucleation growth of CdO and Mn-acetate followed by Se injection.57 The emission spectrum of the NCs has maxima ~560 nm with quantum yield (QY) 3.3 nm (Figure 3B and C).36 It has been observed that at the size region of CdSe 3.3 nm the Mn d-emission disappears; only exciton PL survives in that size regime. In that size regime the exciton band gap reduces as a result the ligand field transition energy of Mn becomes larger than exciton band gap. Under such circumstances, the energy released during the non-radiative electron-hole recombination of the host is not sufficient to excite the Mn atom further to 4T 1 state. As a result, Mn PL was not observed in such case. However, in the case of surface doped QD, absence of Mn PL cannot be explained on the similar argument as above. The surface rich doping can easily be distinguished from the core rich doping by recording the EPR spectrum. Figure 3D shows the typical EPR spectrum of a surface and core rich Mn doped ZnSe NC where six hyperfine lines are observed for both the cases due to five Mn unpaired electrons. The hyperfine coupling constant (A) is much higher (~90 G) in the surface rich QD as compared to the core rich one (~65 G).75 Having such high ‘A’ value leads to existence of Mn2+ ion as isolated ion on the surface. To understand the absence of Mn PL in such surface doped NCs, the work by Sarma and co-workers46 has been analysed as discussed below. They demonstrated that in Mn doped Zn 0.25 Cd 0.75 S NCs, the ligand field splitting enhances gradually i.e. 4T 1 -6A 1 splitting amplifies as the dopant (Mn) moves from core of the NCs to the surface of the NCs. Thereby they could tune the Mn PL to a large extent (i.e. from 530 nm to 630 nm) as compared to its original luminescence band ~590 nm (Figure 3E). This was explained based on having large number of inequivalent dopant sites in the host NCs, where dopant atom can be placed from the core to the surface of the NCs. Furthermore, they extended the work of Mn d-emission tunability to a strain induced core-shell system viz from the ensemble of Mn-doped ZnSe/CdSe/ZnSe NCs as discussed earlier (Figure 2D).47 Therefore, keeping these factors in mind, a model was proposed, as shown in Figure 3F.



Here in the surface doped NCs due to enhance ligand filed transition energy (as compared to the exciton band edge energy) the 4T 1 -6A 1 transition energy gap of Mn becomes larger than the band gap of CdSe NCs in spite of having the size of CdSe 1.8% doping) has been explained due to Mn2+-Mn2+ interaction at excess number of Mn2+ ions in the single lattice, which introduces defects in the mid band gap region to trap the charge carrier.58 Earlier it was reported that at higher Mn2+ concentration the PL intensity drastically quenches due to Mn2+-Mn2+ interaction.87 Although in the present case, such Mn2+ clustering does not have any drastic effect in PL intensity, they significantly reduce the QDSC efficiency and NC lifetime by non-radiative electron trapping. Direct sensitization also has been employed on SnO 2 surface for Mn doped CdS NCs to ascertain such improvement in PCE due to Mn doping (Figure 4D).88 There again it was revealed, although on Mn doping the PCE of the host NCs enhances, at higher Mn concentration reduction in the PCE was observed.


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Direct sensitization also has been carried out for Mn doped core-shell NCs where the deposition has been carried out through SILAR technique by Santra et.al.55 They have fabricated QDSC by depositing CdS NCs along with Mn2+ through SILAR technique on FTO coated TiO 2 photoanode followed by SILAR deposition of CdSe layer on it. Control sample solar cells without Mn, without CdSe and without both Mn and CdSe also have been fabricated for comparison after following same technique. The champion photovoltaic performance was found for Mn doped CdS/CdSe NCs (5.42 %) which was highest QDSC efficiency in the literature at the time of the report (Figure 4E). Tthey have observed more than 20% enhancement of PCE for Mn doped NCs as compared to the undoped counterpart (Table 1). The underlying mechanism for such improvement in presence of Mn has been explained through electron transfer to TiO 2 from photo-excited CdS/CdSe NCs through Mn state. Here Mn centre can store the electron for longer time, thus reduces detrimental charge recombination in the solar cell device (Figure 4F). Therefore, one of the obvious criteria for improved PCE is to minimize the electron-hole recombination reaction by dissociating the exciton in ultrafast timescale.



Figure 4. (A) Steady state PL spectra of 1.76 Mn atomic % doped CdS NCs (C4) as a function of temperature (77K to 250K) showing excitonic PL of CdS (blue region) as well as Mn-d emission (red region). (B) Variation of efficiency and open circuit voltage as a function of Mn:Cd atomic %, showing maximum efficiency obtained for C3. The wine red dotted line shows the relative intensity of the Mn d−d transitions for different percentage of Mn. (C) Computed electron lifetime of different QDSCs vs. applied voltage through impedance measurement. (D) Current-voltage relationship of different concentration Mn-doped CdS NCs sensitized SnO 2 electrode. (E) J−V characteristics under AM 1.5 global filter of 100 mW/cm2 sunlight for different working electrodes. (F) Schematic illustration of the electron transfer (k et ) from doped CdS into TiO 2 nanoparticles via Mn state. A to C are reprinted (adapted) with permission from ref 58, while E and F are from ref 55. Copyright (2014 and 2012) American Chemical Society. D is reprinted (adapted) with permission from ref 88. Copyright (2013) Springer. 4.2. Super sensitization Now, it is widely reported in literature that using suitable molecular adsorbate for cosensitization with desirable NCs, the exciton dissociation can be reality either through electron or hole transfer from the NCs to the molecular adsorbate.18,20,22-25,89-92 Exciton dissociation in ultrafast timescale mainly through electron transfer from different NC to molecular adsorbate has been investigated widely. However intrinsic problem in exciton dissociation in QD based devices has been observed due to poor hole transfer rate. Recently a number of research investigation was reported where it has been observed the sensitized molecule can act as electron donor as well as hole acceptor to the NC of interest, producing a ‘grand charge separated’ state, also called ‘super sensitization’.20,23-25,93,94 In such super sensitization the photoexcited molecule can donate electron to the NC and at the same time it can also accept the photo generated hole from the NC due to their favourable energy levels. A typical super


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sensitization behaviour has shown in Figure 5A between CdS NC and di-bromo fluorescein (DBF) molecule.94 Here the LUMO of the DBF molecule resides above the CB of the CdS NCs, thereby allowing photo-excited electron from DBF to be transferred to CdS NCs. At the same time, the HOMO of the DBF molecule lies above the VB of CdS NCs too; this makes the molecule available for accepting the photoexcited hole from CdS NCs. As a consequence a ‘grand charge separated’ state is created where all the photoexcited electrons are localized on CdS NCs and the photoexcited holes are localized on the DBF molecule. Kamat and co-workers have demonstrated CdS/JK-216 dye composite can at as supersensitizer, where the CdS/JK216 show much higher PCE as compared to both CdS and JK-216 alone due to dual charge transfer processes (Figure 5B).93 Keeping in mind that super-sensitization play a major role for exciton dissociation for higher PCE, super-sensitization studies have also been carried out in surface Mn-doped CdSe NC in presence of bromo pyrogallol red (BrPGR) molecule.57 Figure 5C compares the optical absorption spectra of super sensitized CdSe/BrPGR and MnCdSe/BrPGR composite which shows emergence of new broad red-shifted band peaking ~600 nm due to formation of strong charge transfer (CT) complex. Ultrafast transient absorption spectra of both the systems also show presence of transient bleach at the CT complex position41. Interestingly, the closer observation implies that the CT absorption band (both optical and transient bleaching) for Mn doped CdSe NC is red shifted as compared to the undoped one by 5-10 nm in spite of having similar 1S excitonic absorption in both doped and undoped one. This is believed to be due to involvement of the Mn atom in the charge transfer process. Furthermore, the analysis of transient kinetic traces at both 1S excitonic as well as CT position for both the composite systems unravels the involvement of the Mn atom more quantitatively (Figure 5D). Timeresolved kinetic trace analysis implied that at 1S excitonic position both CdSe/BrPGR and MnCdSe/BrPGR systems behave quite similarly, however the electron-hole recombination



dynamics as monitored at the CT bleach position show much slower (25%) dynamics when Mn atom is involved. Here Mn doped QDs provide further assistance for electron storage by directly taking electron from the BrPGR molecule, thus decoupled the electron-hole wavefunction overlap. Such superior behaviour of slow charge recombination in super sensitized doped system is expected to improve the solar efficiency performance when embedded to TiO 2 . Although enhancement of PCE for super sensitizer has been reported for undoped NCs system20,93, till date no literature has been reported for such doped super sensitizer.

Figure 5. (A) Typical super-sensitization scheme between CdS QD and di-bromo fluorescein (DBF) molecule where exchange of photoexcited charge carrier has been shown. Path 3 shows direct charge transfer from DBF to CdS in the relevance of CT complex formation. (B) J-V characteristics for working electrodes (a) CdS, (b) JK-216, (c) CdS/JK-216, and (d) CdS/Al 2 O 3 /JK-216. (C) Absorption spectra of (a) Br-PGR molecule, (b) CdSe QD, (c) CdSe/Br-PGR composite, (d) Mn-doped CdSe d-Dot, and (e) Mn-doped CdSe/Br-PGR composite. (D) Recovery time traces at 1S excitonic (top) and CT (bottom) state respectively; (a,c) CdSe/Br-PGR and (b,d)Mn-CdSe/Br-PGR. A, B, C and D are reprinted (adapted) with


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permission from ref 94, 93 and 57 respectively. Copyright (2013, 2011 and 2014) American Chemical Society. 4.3. Formation of Doped-Ternary Alloy The exciton confinement of different NCs can be achieved in a more controlled way when three or more elements are implanted together to form a heteronanocrystal (HNC). The HNCs can be realized by either having a sharp or a diffuse interface. Core-shell NCs offers a straininduced interface where the carrier confinement potential varies abruptly from core to shell.9598

Alloying with three or more elements can smoothen the interface in the HNCs; thereby

carrier confinement potential varies smoothly from the inner part to the surface of the HNCs.3135,99-102

This leads to decrement of interfacial defects due to reduced strain in the alloy structure

which in other word improves the PL quantum yield of the NCs and makes them more viable for different technological applications. Such structure also improves facile charge separation (for type-II) leading to long lived charge separated state. Different alloy NCs specially made with combination of II-VI semiconductors have been gained considerable attention in terms of facile charge transfer as well as improved PCE.99-102 Doping in such alloy structure can do wonder in terms of optoelectronic properties as it has one more degree of freedom to control the charge carrier and charge transfer processes. At the same time, the dopant can act as electron storage centre which leads to reduced charge recombination and higher solar conversion efficiency. Very recently researcher’s started working on Mn doping in alloy NCs to control the charge carrier dynamics which eventually altered the exciton dynamics as well as higher PCE as discussed in the subsequent sections.59-62 To control the exciton dynamics in CdTeSe alloy NCs, which itself shows promising photovoltaic performance,103,104 CdTeSe alloy have been doped with Mn atom. Figure 6A, B shows the absorption and emission spectrum (de-convoluted) of undoped and Mn doped



CdTeSe alloy NCs respectively.60 The general observation is that both alloy show 1S exciton as well as CT state (weak) absorption. Interestingly the luminescence spectra are dominated by CT states for both undoped as well as Mn-doped alloy. Formation of CT state in the alloy has been explained on the basis of type-II CdTe/CdSe structure in CdTeSe NCs. Due to higher reactivity of Te-TOP (tri-octyl phosphine) as compared to Se-TOP with Cd-oleate, the core of the alloy becomes Te-rich while the surface becomes Se-rich, leading to a type-II CdTe/CdSe core-shell like structure. While the interface becomes gradient in nature since the reaction temperature was kept low (~250 ºC). A pictorial representations of CT state has been shown in Figure 6C which indicates direct electron (in Se-rich shell) - hole (in Te-rich core) recombination across the gradient interface leads to formation CT luminescence state. However, in case of Mn-doped alloy, the emission from the band edge state is more prominent as compared to undoped alloy which also can be visualized as reduced CT intensity. The factor that responsible for such reduction of CT intensity instead of having such gradient structure in Mn-doped alloy is primarily assumed due to involvement of the Mn atom in the electronic transition. EPR analysis of Mn-CdTeSe alloy confirmed that Mn atom is surface doped in the alloy, which results more ligand field splitting of 4T 1 - 6A 1 state. As a result, the ligand field transition energy of Mn atom becomes larger than the band gap of the CdTeSe NCs and therefore, no Mn d-emission was observed (due to absence of the energy transfer from host to the Mn). Furthermore, the involvement of Mn state in the electronic transition to CT state has been demonstrated in ultrafast timescale using femtosecond time resolved absorption technique.60 Figure 6D, E compares the ultrafast transient absorption spectrum of undoped and Mn doped CdTeSe alloy NCs respectively. The notable observation is the appearance of pulsewidth limited (i.e. 700 ps) electron transfer to CT state. Due to involvement of Mn atom, decoupling of electron-hole take place and charge recombination process become very sluggish. In addition to that it’s also accompanied by slow hole transfer from CdSe rich surface to the CdTe rich core of the NCs by a factor of 2 as compared to the undoped NC and thereby the charge recombination rate further reduced.

Figure 6. De-convoluted UV-vis absorption and PL (inset) spectra of (A) undoped and (B) Mndoped CdTeSe alloy NCs. (C) Charge transfer across Se-rich shell and Te-rich core in CdTeSe gradient alloy NCs. Ultrafast TA spectra at different time delay (0.5ps to 1ns) and kinetic decay trace of CT state (inset) of (D) undoped and (E) Mn-doped CdTeSe alloy NCs. (F) Schematic illustration of several carrier relaxation in Mn doped CdTeSe alloy. All the figures are reprinted (adapted) with permission from ref 60. Copyright (2014) American Chemical Society. Electron storage in the Mn state for such longer time and extremely slow electron relaxation to CT state has been predicted to contribute towards high efficient QDSC. Interestingly Zhong



et. al. reported a champion performance of 9.4% PCE on Mn doped CdSeTe based QDSC.59 To demonstrate the role of Mn in alloy NCs, they designed different electrodes with or without Mn (e.g. CdSeTe/ZnS, CdSeTe/Mn-ZnS, Mn-CdSeTe/ZnS and Mn-CdSeTe/Mn-ZnS) and carried out open circuit voltage decay (OCVD), impedance spectroscopy in addition to solar cell efficiency measurement. Table 1 demonstrates the PCE performances of different solar cells which clearly reveal the role of Mn for improved performances. Further evidences come from OCVD measurements which provide information related to charge recombination i.e. the electron recombination with the oxidized species in the redox electrolyte. After switching off the illumination the voltage decay is measured in OCVD measurement (Figure 7A). It is evidenced that Mn doped alloy NCs exhibited slower voltage decay than the undoped NCs. The electron lifetime has been determined via following equation using open circuit voltage decay rate and shown in Figure 7B: τ n = - (k B T/e) (dV oc /dt)-1 A linear dependence of lifetime reduction with increase in voltage was observed. The higher electron lifetime of Mn-doped alloy as compared to the undoped one (about four times) can well demonstrate the suppression of charge recombination processes in presence of Mn through electron storage. In addition, complementary impedance spectroscopy measurements were also carried out from which chemical capacitance (C µ ) and recombination resistance (R rec ) were computed. The C µ values were independent on Mn doping which was ascribed due to unaltered conduction band states of TiO 2 with or without Mn. However greater R rec values were found for Mn doped NCs as compared to undoped one, which has been attributed due to reduced charge recombination as R rec has inverse relationship to charge recombination. In addition to that the calculated electron lifetime (R rec x C µ ) also follows similar trend (1.46 s vs. 0.45 s for Mn-doped vs. undoped) as determined through OCVD measurement. Finally Nyquist plots of


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various solar cells also have been demonstrated in Figure 7C, which concludes better charge collection efficiency at the photoanode for Mn-doped alloy NCs.

Figure 7. (A) Open circuit voltage decay (OCVD), (B) computed electron lifetime as a function of V OC and (C) Nyquist plots under bias of -0.6 V for different solar cell modules. Figures are reprinted (adapted) with permission from ref 59. Copyright (2016) Royal Society of Chemistry. 4.4. Interface Chemistry in Doped-Quaternary Alloy In the previous section we have shown Mn doped gradient CdTeSe alloy NCs as a better material for improved solar conversion efficiency. However, the presence of interfacial states in such alloy NCs can have significant role in the charge transfer process, especially charge transfer to Mn state followed by to TiO 2 .62, 105-108 To play with the interface chemistry, we have demonstrated high quantum yield (>50%) undoped and surface Mn-doped quaternary gradient CdZnSSe alloy NCs as a function of reaction temperatures.62, 105 By controlling the reaction temperature, one can easily synthesize significantly gradient (G-250, at relatively low injection temperature 250 °C) and minimal gradient (G-300, high temperature 300 °C) alloy NCs. Surprisingly the absorption spectra of the G-250 alloy is much red shifted as compared to the G-300 alloy although the later was synthesize at high temperature (Figure 8A-D). The deconvoluted absorption spectra of the G-250 (both undoped and Mn-doped) exhibit CT state at the red region of the spectra in addition to the excitonic states, which were re-confirmed by the ultrafast transient absorption studies.62 On the other hand, in the G-300 alloy NCs, only exhibit



excitonic absorptions and no such CT absorption has been observed even in transient absorption studies. At relatively low reaction temperature, as suggested in many literature reports,32,35,60,105 due to difference in reactivity between metal and chalcogenide precursors gradient alloy having core-shell like structure is formed. In the aforementioned situation, being Cd and Se have highest reactivity can move towards the core whereas less reactive Zn and S prefers to stay in the periphery of the NC. Therefore a gradient structure composed with CdSe/CdS/ZnSe/ZnS like structure is formed (Figure 8E) in which absorption and PL is dominated by the inner core CdSe/CdS quasi-type-II structure.105 Presence of this quasi-typeII structure, the constituent NCs (i.e. CdSe and CdS) of which are having very low conduction band (CB) offset, facilitates electron delocalization in the CB of both the constituents, however hole remains in the valence band of CdSe. As a result, partial charge separation occurs even in the ground state that results the formation of CT state. However, at higher reaction temperature the interionic diffusion plays the major role than the reactivity of ions, causing formation of significantly less gradient structure at the hetero-interface removing of the core-shell like structure. Thus Zn and S get incorporated towards the core of the NCs along with other ions. Since Zn-based NCs have blue shifted absorption due to large band gap,109-112 and the absorption of the G-300 alloy moves to the blue region as compared to the G-250 analogue which indeed endorses the incorporation of Zn to the core of the NCs. Therefore, the absence of any core-shell like arrangement particularly the absence of inner core quasi-type-II behaviour in G-300 alloy results no CT state formation. It has been observed CT state play a major role is slowing down the electron cooling process, especially when the NC is doped with Mn. The electron cooling in undoped G-250 alloy is found to be very slow (~8 ps) which further slows down on doping with Mn (~10 ps) due to Mn mediated electron cooling to CT state. At the same time recombination dynamics also becomes slower for doped NCs where Mn screens the charge recombination process. However due to absence of any CT state in G-300 alloy the


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slow electron cooling time constant found to be absent. A typical electron cooling time constant ~800 fs was observed which is similar to II-VI NCs and does not change much on Mn doping. However, photoluminescence quantum yield (QY) for G-300 alloy is higher as compared to the G-250 alloy. To correlate the slow electron cooling and recombination dynamics in the presence of Mn, photovoltaic measurements was also carried out. Interestingly ~30% relative enhancement of PCE was observed on Mn doping for the G-250 alloy (Table 1). On the other hand, although G-300 alloy has blue shifted absorption (absorbs less visible radiation) and faster electron cooling as compared to G-250 alloy, still the PCE of G-300 alloy was found to be much higher as compared to the G-250 NC. In fact the PCE of G-300 alloy is higher than the Mn doped G-250 alloy also (see Table 1). These results clearly suggest that interface in the alloy NCs play a major role for electron trapping which can be improved noticeably by homogeneous mixing of all the components. Figure 8F demonstrates the typical schematic of electron transfer to TiO 2 for Mn-doped G-250 and G-300 alloy where carrier trapping and restricted electron transfer has been shown for the former due to presence of interface. However minimal trapping and smooth electron transfer for the G-300 alloy is the underlying reason for such remarkable improvement. In case of Mn doped G-300 alloy increment in PCE was found to be ~3% as compared to the undoped G-300 alloy. These observations illustrate that Mn doped less gradient (e.g. G-300) alloy can open new insight for QDSC application for which further optimization is needed.



Figure 8. De-convoluted spectra of CdZnSSe G-250 alloy (A) undoped, (B) Mn-doped and CdZnSSe G-300 alloy (C) undoped and (D) Mn-doped. Top: De-convoluted UV-vis absorption and bottom: de-convoluted TA spectrum at 1ps (G-250) and 0.5ps (G-300) time delay respectively. The wine red (only for G-250, A and B), red, olive and blue Gaussians represents CT state, 1S, 2S and 1P (and 1S state of ZnSe) states respectively. (E) Schematic design of G250 CdZnSeS alloy NC which has a core-shell structure with gradient composition of CdSe/CdS/ZnSe/ZnS. (F) Schematic illustration of Mn assisted electron transfer to TiO 2 film and neutralization of hole through polysulphide electrolyte for both G-250 and G-300 alloy NCs. A, B, E and F (partially) (G-250 alloy) are reprinted (adapted) with permission from ref 62. Copyright (2017) Wiley Materials. C and F (partially) is reprinted (adapted) with permission from ref 105. Copyright (2018) American Chemical Society. 4.5. Doping in the Shell in a Core-Shell NCs Mn doping in ZnS shell (acts a passivation layer only) can enhance the PCE performance to some extend as discussed in earlier (Table 1) where Mn is believed to retard the recombination process. To explore such effect of doping in the shell, CdSe active layer is grown on CdSeTe alloy NCs by SILAR technique where Mn has been doped in the CdSe shell.61 Here


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the PCE performances of CdSeTe, CdSeTe/CdSe and CdSeTe/Mn-CdSe were recorded where a significant enhancement was observed due to Mn doping as compared to the undoped NCs (Table 1). In addition to the ultrafast transient absorption measurements a series of other studies e.g., OCVD, impedance measurement (Nyquist plots), PL decay trace have been carried out to understand the charge transfer mechanism in the above systems. A slight increase of ohmic resistance (R s ) was observed on (Mn)-CdSe coating on the alloy NCs which is expected to be the reason of reduction of FF (0.587 vs. 0.552) of the devices. Other parameters like recombination resistance and electron lifetime also was found to be increased on Mn doping in the QD materials which eventually improved the PCE. Faster PL decay was observed for Mn doped NCs as compared to undoped one, when embedded on TiO 2 and Al 2 O 3 substrate (Figure 9A, B). Moreover relatively faster decay was observed on TiO 2 surface (3.96, 3.74 and 2.63 ns) as compared to Al 2 O 3 surface (7.64, 5.96 and 5.86 ns) for CdSeTe, CdSeTe/CdSe and CdSeTe/Mn-CdSe NCs respectively. This observation is believed to be due to facile electron transfer to TiO 2 interface as compared to wide bandgap Al 2 O 3 surface. It has also been observed that the presence of Mn can facilitate the CdSe deposition rate, which eventually increases the light absorption cross-section of the film as shown in Figure 9C. To record the absorption spectra, all the NCs are directly grown on 2 mm-thickness Al 2 O 3 films. As revealed from the absorption spectra, the introduction of Mn accelerates the CdSe deposition, thereby enhance the PCE.61 The second and most important factor is that the ligand field transition energy of Mn is larger than the band edge transition energy of CdSe since MnCdSe shell was grown by SILAR technique where the size of CdSe cannot be controlled and thus becomes >3.3 nm.36 In addition to that the energy level of the conduction band edge of CdSeTe is expected to be same or lower as compared to the conduction band edge of CdSe of similar size.61 Therefore the Mn state can effectively store the photoexcited electrons, as



revealed in earlier sections, thus avoiding much more electron trapping as well as recombination.

Figure 9. Time resolved emission decay traces on (A) TiO 2 and (B) Al 2 O 3 substrates and (C) UV-vis absorption spectra on 2 mm-thickness Al 2 O 3 films of CdSeTe, CdSeTe/CdSe and CdSeTe/Mn-CdSe NCs respectively. A to C are reprinted (adapted) with permission from ref 61. Copyright Elsevier (2016). Another important class of nanomaterials, ternary I-III-VI e.g. CuInS 2 (CIS), CuInSe 2 (CISe) etc., are promising scientific and technological interest due to their low band gap, easy solution processability, less toxicity for an emerging alternative of the thin-film photovoltaics.113-119 Using CuInSe 2 as core material and by introducing Zn and Ga into the crystal structure, incredible PCE ~12% have been achieved very recently.120,121 This shows the supremacy of these materials as the promising candidate for QDSC. Here in this review it has already been demonstrated that introduction of Mn dopant has significant role for enhancing PCE for several II-VI semiconducting materials, primarily by electron storing in the Mn state and retarding the detrimental charge recombination. From the above discussions it can be visualized that introduction of Mn dopant in such I-III-VI materials can enhances the PCE further. In fact there have been a few reports of Mn-doped CIS/Se based materials for preliminary understanding of optical properties,122 still we need to investigate further for developing the champion photovoltaic performances with such materials. Few years back, Meng and co-workers reported photovoltaic performances of shell doping in CIS-based core,


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where the PCE found to be as high as 5.38%, one of the highest reported QDSC efficiency at the time of the publication.56 Specifically, they have synthesized CIS NCs by aqueous route and then deposited on TiO 2 thin film. Finally, (Mn)-CdS was grown on the CIS thin film by SILAR technique (i.e. by alternative deposition of the previously prepared CIS film into an aqueous solution of (Cd2+ + Mn2+ and S2-). The J-V and IPCE characteristic curves of all the controlled samples (undoped and doped) are shown in Figure 10A,B. Huge jump in PCE was found after putting CdS shell as compared to core-only sample. Interestingly additional 15% enhancement of PCE was found on Mn doping in the CdS shell. Schematic diagram as shown in Figure 10C can explain the reason for such huge improvement of PCE which was justified in accordance with IPCE (Figure 10B) and absorption (not shown) measurement. Actually, in thin film significant coupling of CIS and CdS results stronger wave-function interaction due to indirect transition (i.e. from CIS to CdS, see Figure 10C) as CIS/CdS forms a type-II core-shell alignment. Thereby a charge separated state is formed that leads to significant red-shift in optical absorption. Consequently, the IPCE spectrum gets significant red-shift, in accordance with the PCE enhancement. In addition, the deposition of CdS shell gets accelerated when Mn is introduced, as discussed earlier, thereby further broaden the IPCE spectra to red region. However the promising enhancement in PCE after Mn doping was mainly attributed to spatial separation of electrons by Mn state to reduce the charge recombination considerably (Figure 10C) which eventually collected to the external circuit. Although excited state dynamics for some of these kinds of materials have been performed,114, 117, 118, 123, 124 however such charge separation dynamics involving Mn dopant has not been investigated till date in the litrature, without which further optimization of such solar module would be difficult.



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Figure 10. (A) J-V and (B) IPCE characteristics of CIS-based solar cells. (C) Plausible schematic diagram of charge transfer processes in CIS/Mn-CdS solar cell. Figures are reprinted (adapted) with permission from ref 56. Copyright Royal Society of Chemistry (2013). Table 1. Photovoltaic performance of different NC samples in terms of J-V measurement. a The experiments performed in SnO 2 electrode, whereas in all other cases TiO 2 electrode is used as photoanode. NC sample

J sc [mAcm2

V oc [V]

FF [%]

PCE [%]

]

% PCE

Ref

increment

CdS

5.25

0.484

49.4

1.25

Mn-CdS

8.39

0.501

49.4

2.08

CdS/CdSe

17.2

0.516

47

4.19

Mn-CdS/CdSe

20.7

0.558

47

5.42

a

CdS

7.06

0.678

38

1.83

a

Mn-CdS

10.17

0.683

40.4

2.80


58 66 55 30 88 53

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CdSeTe/ZnS

20.43

0.652

63.3

8.44 (8.55)

59

CdSeTe/Mn:ZnS

20.57

0.666

63.7

8.72 (8.82)

3

Mn:CdSeTe/ZnS

20.72

0.670

64.2

8.91 (9.04)

6

Mn: CdSeTe/Mn :ZnS G-250

20.83

0.685

64.7

9.23 (9.40)

10

12.57

0.59

44

3.3

Mn: G-250

17.28

0.59

42

4.29

G-300

17.1

0.61

43

4.4

Mn: G-300

17.43

0.59

44

4.52

3

CdSe x Te 1-x / CdSe/ZnS

21.17

0.632

57.6

7.71

6

CdSe x Te 1-x / Mn-CdSe/ZnS CIS–CdS

22.55

0.653

55.2

8.14

17.38

0.574

47

4.69

CIS–Mn-CdS

19.29

0.581

48

5.38

62 30

61

15 56

5. Summary and Future Outlook The present review summarizes the recent development of Mn doped NCs which have explicit application to solar energy harvesting where Mn centre act as an electron storage centre. We summarizes the different strategic synthetic approaches of Mn doped NCs that includes core, surface and interface doped NCs. Due to Mn 4T 1 -6A 1 transition, core and different interface doped NCs can exhibit strong PL at ~2.1 eV. However, in surface Mn doped CdSe NCs no PL due to Mn 4T 1 -6A 1 transition was observed, only CdSe exciton emission was observed. This observation is explained due to increased ligand field transition energy (increased 4T 1 -6A 1 ) as compared to the exciton band gap energy of CdSe, thereby unable to do the Mn excitation by exciton energy transfer. Direct involvement of Mn atom for the enhancement of PCE in Mn doped QDSC has been clearly demonstrated. For example, Mn



doped CdS found to exhibit more than 50% PCE enhancement as compared to the undoped NC when directly embedded either to TiO 2 or SnO 2 . Even Mn doped CdS/CdSe also shows about 30% PCE increment with promising 5.42% PCE. All of the above features have been speculated due to electron storing in Mn centre which eventually retards the charge recombination process. Effect of Mn atom has also been demonstrated in charge transfer dynamics of Mn-CdSe/Br-PGR super-sensitized system, where both photo-excited electron and hole get localized in Mn-CdSe and Br-PGR respectively. In fact, such surface Mn doped CdSe NCs can easily accept electron at the Mn state from the photo-excited external molecule (Br-PGR) due to better orbital overlap, in addition to the thermodynamical viable energetics. Transient CT bleach recovery kinetics found to be slower for Mn-doped CdSe which reflect slow charge recombination process. This is due to screening of electron by Mn, that eventually can enhance the PCE performance. Mn doping in different alloy NCs also have been summarized in terms of improved PCE. The formation of CT state in Mn doped CdTeSe gradient alloy NCs within the material has been explained due to gradient type-II CdTe/CdSe alignment. It has been observed that for undoped CdTeSe alloy, evolution of CT state as bleaching signal in TA occurs within 700 ps) is found to take place from the upper conduction band of the Mn-doped alloy. This observation is explained due to Mn mediated electron cooling to CT state, where Mn centre act as an electron storage centre. Mn mediated such slow electron cooling can be well complemented for unprecedented 9.4 % PCE in Mn doped CdTeSe NCs. Nyquist plots from impedance measurements for such undoped and Mn doped solar cells also reveal better charge collection efficiency for Mn doped NCs. Mn doping in quaternary CdZnSSe G-250 and G-300 alloy NCs also have been reviewed and the consequences of interface on PCE has been demonstrated. Due to presence of inner core CdSe/CdS quasi type-


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II structure, the G-250 alloy can form CT state which results slow electron cooling to CT state while doping with Mn. As a result 30% PCE enhancement for Mn doped G-250 alloy was observed as compared to the undoped one. Interestingly in G-300 alloy such CT state is absent as a result fast electron cooling (6600) and is recipient of several awards and honours including INSA young scientist medal (1998), A. K. Bose Memorial Award (2000), Homi Bhabha Science & Technology Award (2010), CRSI Bronze Medal (2011) and DAE-SRC Outstanding Investigator Award (2012). He is the fellow of three major academia of India like Fellow of National Academy of Science (F.N.A.Sc.), Fellow of Academy of Science (F.A.Sc.), and Fellow of Indian National Science Academy (F.N.A.). Currently he is a Member of Editorial Advisory Board, Chemical Physics, Journal of Physical Chemistry (A/B/C) and Letters.


Recent Progress of Electron Storage Mn Center in Doped Nanocrystals

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