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Hot Charge Carrier Extraction from Semiconductor Quantum Dots Pallavi Singhal, and Hirendra N. Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03980 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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The Journal of Physical Chemistry

Hot Charge Carrier Extraction from Semiconductor Quantum Dots Pallavi Singhal±$ and Hirendra N. Ghosh±#!* ±

Homi Bhabha National Institute, Mumbai 400085, India

$

#

Health Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

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

Institute of Nano Science & Technology, Habitat Centre, Mohali, Punjab- 160062, India *Corresponding Author: [email protected], [email protected]

Abstract This article focus on the recent work carried out on hot charge carrier extraction from colloidal quantum dots (QDs). It has been contemplated that the quantum dot based solar cell (QDSC) can reach to exceptional high efficiencies if hot charge carriers can be extracted. However, the process requires excellent charge carrier separation from energetically hot states as compared to relaxation within QDs energy levels. Also, most of the studies were being carried out on thermal charge carrier extraction from QDs and very limited information was available on hot charge carrier extraction. In this feature article, we have discussed the work carried out on hot charge carrier extraction from QDs and the challenges involved in the process.

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1. Introduction 1.1. Motivation The motivation to write this article arises from the future energy requirement; being one of the burning topics discussed in current times. Today, around 85% of the world's energy requirement depends on the fossil fuels which are depleting at a faster rate, and have detrimental consequences on human health and the environment1. Moreover, it is also predicted that the global energy demand will be double by 20502. Considering these facts, mankind must explore other non-conventional sources of energy such as, wind, hydrothermal, solar etc., since they are clean and do not adversely impact the environment. Out of these sources; solar energy is one of the environmental friendly sources with wider availability3-6. The work started after the creation of the first photovoltaic cell in the early 1950s and since then, the evolution of solar technologies continues at an unprecedented speed7. It has been predicted that by 2050, solar photovoltaics would contribute about 16% of the worldwide electricity consumption, and solar would be the world's largest source of electricity8. Despite these predictions and great efforts by numerous researchers, currently, solar power provides just 1% of the total worldwide electricity production. The reason for such low contribution is the unavailability of suitable materials which can absorb a large portion of solar radiation and convert the photo-generated charge carriers to electricity at a faster rate. Also, except for silicon solar cells, most solar cell devices have relatively low power conversion efficiency, stability issues, involvement of toxic substances and therefore, are still far from real-world applications9. 1.2. Evolution of Solar Cell Technology Solar cell technology can be broadly classified into three generations. Single crystalline silicon solar cells, also known as the first-generation solar cells, are already commercialized, 2


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and have an efficiency of 15−20%10,

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(with a certified high efficiency reaching 25%12).

These solar cells have great charge transport properties, excellent stability and are environmentally benign as well. However, the complicated manufacturing processes, high production cost, and long energy payback time have slowed down their implementation worldwide9. The second generation solar cells, known as thin-film solar cells uses amorphous or polycrystalline material13, 14. Here the main advantage is the large reduction in amount of semiconductor materials required and therefore, the cost is highly reduced. CdTe and CIGS thin-film cells are typical examples of this kind. However, the efficiencies of these solar cells are less and needs to be further improved. Third generation solar cells target the use of inexpensive, abundant, and environment-friendly materials as well as fabrication using cheap manufacturing processes9. Dye sensitized solar cells (DSSC)15-19, quantum dot solar cell (QDSC)20-31, organic solar cell32 and perovskite solar cells33 are typical examples of third generation solar cells. DSSC, which use dye molecules as light absorbers and produced with inexpensive equipment, have been studied since 1988 and have achieved over 14% efficiency to date34. In organic solar cells, different organic molecules are used as light harvesting materials, and the best value of 13% has been reported recently32. However, both of these solar cells are not satisfactorily stable for long-term use. In addition to this, DSSC have limitations such as limited light absorption in solar region and absorption tunability. To overcome these limitations, QDSC have attracted much attention. The unique properties of quantum dots (QDs) such as band-gap tunability35-39, multiexciton generation (MEG)40-45, high extinction coefficient, hot carrier generation46-51 and longer exciton lifetime46 make these materials ideal candidates for solar cell application21-23, 52. Presently QDSC have achieved an efficiency of 13.43%53 as compared to < 1% as reported in 199854. This shows the great efforts from numerous researchers and potentiality of these materials for solar cell application. Very recently, perovskite solar cells which uses metal halide perovskite materials 3



as solar radiation absorber has shown much attention55-60. Their efficiency has already been reached to 22%61 from 3.8% since 200933. In addition, they also share some benefits with QDSC, such as low-cost solution processability. Despite astoundingly high efficiencies, this technology is still in its nascent stage and face challenges such as long-term durability33. In this feature article, different processes involved in QDSC and by which its efficiency can be increased are explained. Importance of charge carrier extraction and role of molecular adsorbate to achieve high efficiency QDSC is demonstrated with a specific focus on the hot charge carrier extraction from QDs; its importance in QDSC and the challenges involved in the process.

2. Results and Discussions 2.1. Design of a QDSC Figure 1 demonstrates the basic design of a QDSC where QDs act as solar radiation absorber. Unlike most traditional solar cells, QDSC operates based on the principle of photo-induced electron transfer since the length scale of the QDs is too small to support field-driven charge separation62. On solar radiation absorption, electron and hole are generated within QDs materials. The photo-generated electron from QDs is transferred to the electron acceptor which is generally a mesoporous metal oxide substrate such as TiO 2 since it facilitates charge

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Figure 1: Schematic Diagram of QDSC and different processes involved: (1) Photoexcitation of QDs by solar radiation; (2) Electron transfer from excited QDs into TiO 2 ; (3) Transport of electrons to the collecting electrode surface; (4) Hole transfer to the redox couple; (5) Recombination of electron and hole in photo-excited QDs; (6) Recombination between electrons from QDs and the oxidized form of the redox couple; and (7) interfacial recombination between electrons from TiO 2 and the oxidized form of the redox couple. transport to the transparent contact24. The hole remains at the surface of the QDs where the reduced species of the electrolyte donates an electron to regenerate the sensitizer. Finally, transported electrons and oxidized electrolyte species rejoin and complete the circuit at the counter electrode as shown in Figure 1. The first and foremost process in any QDSC is absorption of solar radiation by QDs to generate charge carriers. Both, high solar radiation absorption and fast charge carrier extraction to the respective electrode/electrolyte system are the major requirements to achieve a high efficiency QDSC. However, there are many competing deleterious processes with this such as recombination between the electrons from QDs and the oxidized form of the redox couple, interfacial recombination of electrons from TiO 2 and the oxidized form of the redox couple, and electron-hole recombination as shown in QDSC (Figure 1). To achieve high efficiency QDSC, these deleterious processes should be minimized. It has been reported that electron transfer62 from QDs to the TiO 2 matrix occur with rate constants of ~ 1010 - 1011 s−1. However, the hole transfer62 from QDs to polysulfide or CuSCN is much slower process and found to take place with rate constant ~ 107 - 109 s−1. This bottleneck in hole transfer reaction increases the probability of processes 5 and 6 in Figure 1 which hinder the ability to realize higher efficiency QDSC since majority of the carriers are lost before their collection into the TiO 2 transport matrix63, 64. 5



A more detailed view of QDSC depicts that there are many processes involved within a photo-excited QDs which can either increase or decrease its efficiency. These processes are summarized in Figure 2. Also, to overcome the limitation of slow charge transfer rate, many researchers have monitored the interaction of QDs with different molecular adsorbates which can extract charge carriers (both electrons and holes) at a much faster rate. A number of processes can occur during this interaction and are summarized in Figure 2. Here first we will consider the condition, 2E g >E hν >E g where E hν is the energy of solar photon. Under this regime, hot charge carrier (both electron and hole which are present in energetically higher states) can be generated65 and fate of these charge carriers can takes several paths such as: (1) Extraction of hot charge carriers (both electron and hole) to the respective molecular

Figure 2: Different processes involved after generation of hot charge carriers in QDs, (1) Extraction of hot charge carriers by suitable molecular adsorbates; (2) Hot charge carrier cooling to the lowest energy level; (3) Extraction of thermal charge carriers by suitable molecule adsorbates; (4) Trapping of charge carriers to the trap states; (5) Charge carrier recombination. 6


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adsorbate prior to cooling; (2) Relaxation to the lowest state by dissipation of excess energy as heat through electron-phonon interactions66; (3) Thermal charge carrier extraction by the molecular adsorbate; (4) Trapping of charge carriers to the long lived states and; (5) Recombination of charge carriers. It has been reported that the efficiency of QDSC can reach to 66% if hot charge carriers can be extracted67 breaking the Shockley-Queisser limit68 of 33%. Realization of such a hot charge carrier QDSC requires extraction of hot charge carriers at a much faster rate before cooling to the band edges. Alternate to hot charge carrier generation, if the excitation energy; E hν >2E g , multiple exciton generation (MEG) can be realized. MEG is one of the fascinating property of QDs where absorption of one photon generate more than one e-h pair, as a result a huge increment in QDSC efficiency can be realized43,

44, 69-71

. Since, more than 2Eg of excitation energy is

necessary, MEG is more favoured in low band gap materials such as PbS, PbSe. To demonstrate this process in PbSe QDs, special experiments were designed by Nozik group44 where the QDs were excited at different excitation energy and decay rate of exciton was monitored. It was observed that when the samples were excited at >2Eg a fast decay component along with a slow decay component appeared. Interestingly, the time constant of the slow decay component is always same when the samples were excited at low energy where no MEG was realized. However, one of the main competing processes with MEG is the Auger assisted charge recombination process. In Auger relaxation pathway the electron uni-directionally transfers its energy to the hole72, 73. This process can be fast in QDs due to a large electron-hole wave function overlap and reduction in momentum conservation requirements due to spatial localization. If the hole spatially decoupled from the electron, Auger recombination process can be reduced and MEG efficiency can be enhanced. This can be done by extracting the hole from photo-excited QDs material. Here, it is to be noted that the advantages of both, MEG and hot charge carrier extraction can be combined in a single 7



system if a suitable hole transporting molecule which can extract the hot hole from the QDs is used. These processes are shown in Figure 3. For this process to occur (1) The coupling between hole transporting molecule and QDs should be strong; (2) Process should be thermodynamically favourable; (3) The relaxation within the hole energy levels should be slow; and (4) Interaction between photo-excited electron and hole should be less. A number of studies have been carried out where researchers have shown hot charge carrier extraction from QDs materials. We will be discussing about these studies subsequently.

Figure 3: Schematic diagram of different processes involved after photo-excitation of QD materials with E hν > 2Eg. Process 1: Generation of hot charge carriers, Process 2: Carrier multiplication (CM) followed by MEG, Process 3: Non radiative auger recombination, Process 4: Charge carrier dissociation in presence of hole accepting adsorbate. Adapted from ref49. 2.2. Thermal Charge Carrier Extraction from QDs: Role of Molecular Adsorbate As mentioned above, to realise high efficiency QDSC, it is necessary to separate the photo-generated charge carriers from the QDs and a number of studies have been carried out 8


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in this direction where different molecular adsorbates are used to extract photo-generated charge carriers from the QDs. For this process to occur, two major criteria; thermodynamic viability of charge transfer process and effective overlap between donor and acceptor orbitals should take place74. A number of studies have been carried out where researchers have reported the electron transfer from QDs to the molecular adsorbates75-79. Few to be mentioned: Wang et. al. have chosen a composite system of CdSe QDs and methyl viologen where they have shown that photo-excited CdSe QDs can transfer electron to methyl viologen in 150 fs time scale80. Yang et. al. have shown an electron transfer time of 6 fs from photo-excited PbS QDs to TiO 2 film81. Zhu et. al. have taken a composite system of CdSe QDs and anthraquinone and observed an electron transfer time of 3.4 ps82. Boulesbaa et. al. have taken a composite system of CdSe QDs and Rhodamine B and observed an electron transfer time of 54 ps from photo-excited CdSe QD to Rhodamine B83. Yang et. al.84 have investigated on a composite system of CdSe nanorod and methylene blue and observed an electron transfer time of 3.5 ± 0.1 ps. Interestingly, along with electron transfer they have also shown that the hole quickly localizes in the coulomb potential well generated by the reduced electron acceptor near the nanorod surface. Here in a large activation energy is required to detrap the hole from the potential well. While they have reported electron transfer from QDs in picoseconds-sub-picosecond time scale, hole extraction from QDs remains a challenging task and limits the efficiency of QDSC. This is due to the less delocalization of hole wave-function as compared to electron in QDs which imposes stringent conditions on coupling of hole orbitals with molecular adsorbate. Recently, a number of studies have been carried out where hole transfer from QDs has been reported in picoseconds-sub-picosecond time scale. Since the hole extraction from QDs is a tedious process and is much slower as compared to electron transfer, recently, few reports have come where researchers used a 9



coupled system of QDs with different molecules and hole extraction was observed at a comparable rate to that of electron85-91. We have investigated a series of molecules for this purpose. During this investigation, we designed our system in such a way that the phenomenon of super-sensitization85-88 can be realized. In a super-sensitized system, both QDs and molecular adsorbate absorb solar radiation and energy levels are such that on photoexcitation of QDs, holes are transferred to molecular adsorbate and on photo-excitation of molecular adsorbate, electrons are transferred to QDs resulting in grand charge separation. This type of systems has dual advantages of being high solar radiation absorption and greater charge separation; both of which are important to achieve high QDSC efficiency. This phenomenon was demonstrated in a composite system of CdSe QDs and Pyrogallol red (PGR) where both absorb solar radiation and energy levels are such that grand charge separation was realized87 (Figure 4A). Ultrafast transient absorption measurements were performed and formation of PGR+. was observed in transient spectra of CdSe/PGR composite

Figure 4A: Schematic diagram of a super-senitization process in CdSe/PGR composite system. Adapted from ref.87 4B: Upper panel: Transient absorption spectra of CdSe/PGR composite materials at different time delays after excitation at 400 nm laser light. Lower panel: Kinetic decay traces at 690 and 900 nm for the CdSe/PGR system after exciting the samples at 400 nm laser light. Adapted from ref.87 10


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which confirmed hole transfer process from CdSe QDs to PGR (Figure 4B). Hole transfer and charge recombination time was observed to be 500 fs and >200 ps respectively confirming fast hole extraction and grand charge separation in CdSe/PGR composite system (Figure 4B). Similar studies were also performed in CdS QDs/Dibromofluorescein (DBF) composite86 and CdSe QDs/Aurin tricarboxylic acid (ATC) composite systems85 where hole transfer rate from QDs to dye molecule was observed to be 800 fs and 900 fs respectively. Interestingly, along with fast hole extraction and grand charge separation; charge transfer (CT) complex formation between QDs and molecular adsorbate was also observed in these systems (Figure 5A, 5C and 5D). Such complex formation enhances the solar radiation absorption probability, which is one of the major criteria to achieve a high efficiency QDSC. Formation of DBF cation radical was also observed in CdS/DBF complex in the transient absorption spectrum of CdS/DBF complex (Figure 5B). Also, in CdSe QDs/ATC composite system, effect of type-1 and type-2 shell on hole transfer dynamics was studied and it was observed that hole transfer occurs in both, CdSe/ZnS type-I core–shell QDs/ATC and

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Figure 5A: Optical absorption spectra of (a) DBF dye, (b) CdS QD, and (c−i) DBF−CdS complex at constant DBF concentration (0.3 µM) with increasing CdS QDs concentration [CdS] are (c) 0.049, (d) 0.098, (e) 0.147, (f) 0.196, (g) 0.294, (h) 0.392, and (i) 0.49 μM. Inset: Benesi−Hilderband (B−H) plot of the DBF−CdS CT complex. Adapted from ref.86 5B: Transient absorption spectra of DBF−CdS complex at different time delays after excitation at 400 nm laser light. Adapted from ref.86 5C: Optical absorption spectra of (a) ATC dye (b) CdSe QDs, and (c–h) ATC-CdSe complex at constant ATC concentration (20 μM) with increasing CdSe QD concentration, [CdSe] are (c) 0.002, (d) 0.004, (e) 0.02, (f) 0.04, (g) 0.08, and (h) 0.1 μM. Inset: B–H plot of the ATC–CdSe CT complex. Adapted from ref.85 5D: Upper Panel: Steady state optical absorption of (a) ATC b) CdSe/ZnS core–shell, (c) ATC-CdSe/ZnS type-I core–shell composite, and (d) (a)+(b). Lower panel: Steady state optical absorption of e) ATC, (f) CdSe/CdTe core–shell, (g) ATC-CdSe/CdTe type-II core– shell composite, and (h) (e)+(f). Adapted from ref.85 CdSe/CdTe type-II core–shell QDs/ATC complexes. However, in presence of type-1 shell, hole transfer is slow (6 ps) while in type-II shell, hole transfer is fast (3 ps). Super-sensitization was also shown by Kamat and co-workers89 in a composite system of CdS QDs, Al 2 O 3 and squaraine dye (JK-216). They have shown that the overall conversion efficiency (η) is much higher (3.14%) in CdS/Al 2 O 3 /JK-216 system as compared to pure CdS QDs (1.01%); realizing the importance of super-sensitized system in QDSC (Figure 6A and 6B).

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Figure 6A: IPCE spectra and 6B: J-V curves for nanostructured TiO 2 film electrodes modified with (a) CdS, (b) JK-216, (c) CdS/JK-216, and (d) CdS/Al 2 O 3 /JK-216. Adapted with permission from ref89. Copyright 2011 American Chemical Society. In another studies, researchers have chosen a series of molecules as hole transporting mediators for QDs. In these studies, effect of thermodynamic parameter on hole transfer rate was monitored. For this purpose, Alivisatos group92 has chosen a series of ferrocene

Figure 7A: Energy diagram of CdSe and CdS valence (VB) and conduction bands (CB) relative to the six ferrocene ligands. Adapted with permission from ref92. 7B: Schematic diagram of general behaviour predicted by the Auger-assisted model for charge transfer. Plot between driving force vs rate constant. Adapted with permission from ref92. Copyright 2015 American Chemical Society. 7C: Different catechol derivatives used in the study. 7D: Fluoresence upconversion decay traces of CdSe QD (λ ex - 400 nm and λ em – 620 nm) with and without catechol derivatives. (a) pure QD (b) CdSe/3-CH 3 (c) CdSe/4-CH 3 (d) CdSe/Cat (e) CdSe/4-CHO (f) CdSe/4-NO 2 (g) CdSe/3-OCH 3 . Inset: Plot of change of hole transfer rate with Gibbs free energy of charge transfer reaction. Adapted from ref50. 13



molecules and monitored the relationship between driving force and interfacial hole transfer rate (Figure 7A). They have observed that the Auger-assisted mechanism explain charge transfer reactions and the relationship can be used to design QDs − molecular systems that maximize interfacial charge transfer rates while minimizing energetic losses associated with the driving force (Figure 7B). We have also studied the effect of driving force on charge transfer rate49,

50

. For this purpose, series of catechol derivatives with different electron

donating and withdrawing groups were selected (Figure 7C) and their redox potentials were determined using cyclic voltammetry studies50. Hole transfer dynamics from CdSe QDs to catechols were monitored using ultrafast transient absorption and Femtosecond Upconversion studies and it was observed that hole transfer rate follows Marcus’s electron transfer theory except for those derivatives which forms complex with QDs50 (Figure 7C). Interestingly, it was also observed that electron donating group increases hole transfer rate in a remarkable way, suggesting them as better hole transporting molecules as compared to other derivatives. Effect of temperature on hole transfer rate was also studied by Alivisatos group93 and it was observed that the hole transfer rate is independent of temperature at low temperature while increases at higher temperatures (Figure 8A). They have shown that the dominant pathway for hole transfer to the ferrocene moiety is via a shallow and reversible trap and not via direct transfer from the QDs excitonic state (Figure 8B). Effect of QDs size on hole transfer rate was also demonstrated in one of the investigation where two different size of CdTe QDs were synthesized and hole transfer rate to poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylenevinylene] (MEHPPV) polymer94 was monitored (Figure 8C). It was observed that the hole transfer rate depends on size of QDs and for smaller size QD, hole transfer rate determined to be faster. In another study, Weiss and co-workers95 have demonstrated hole transfer from PbS QDs to aminoferrocene and observed that hole transfer only occurs if the 14


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ligand shell of the QDs allows aminoferrocene to gain direct access to the inorganic core of the QDs. This suggests that the permeability of the ligand shell is important in determining the probability of interfacial charge transfer95. They have also shown the hole transfer from a photo-excited CdS QDs to phenothiazine, an exciton delocalizing molecule96. It was observed that upon interaction of QDs with phenothiazine, the band gap of the QD decreases due to delocalization of the exciton and this delocalization enables hole transfer from the QD to phenothiazine in the time scale of

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