3 Thin-Film Colloidal Quantum Dot Solar Cells Lenore Kubie, Matthew C. Beard NATIONAL RENEWABLE E NERGY LABORATORY, GOLDEN, CO, UN ITED STATES
1. Introduction to Colloidal Quantum Dots 1.1 The Tunable Optoelectronic Properties of Quantum Dots The quantum dots (QDs) discussed here are spherical colloidal semiconductor nanocrystals with a radius smaller than the Bohr exciton radius of their bulk equivalents. Because the amount of confinement felt by excitons formed in QDs varies with the QD size, the optical and electronic properties of QDs can be tuned by simply changing the diameter of the QD [1]. These properties include size-dependent bandgaps, changes in Auger-type processes [2, 3], and increased surface-to-volume ratio effects (e.g., size-dependent phase transitions). The size-tunable and material-dependent properties of QDs provide unique ways to approach designing materials for solar energy conversion. The design and synthesis of the resulting QDs lie at the interface of materials science, chemistry, and physics— QDs preside where molecules meet materials and bonds meet bands. A significant portion of research on QD solar cells has focused on PbSe or PbS due to their well-suited bandgap (bulk PbSe has a bandgap of 0.28 eV [4]) and the ability to strongly confine excitons (the Bohr exciton radius of PbSe is 46 nm [4]). Fig. 1A demonstrates the ability to access a large range of bandgaps in PbSe QDs by simply tuning the QD diameter. As the radius of a PbSe QD increases from 3.3 to 8.1 nm, the position of the absorption onset shifts from 1200 to 2100 nm. This lowest-energy absorption feature corresponds to the formation of a 1S exciton (Fig. 1B), or bound electron-hole pair (e-h). We label these excitons based on their principle quantum number, n, and their angular momentum, L. Note that the differences in confinement felt by the electronic wavefunction affect both the fundamental 1S exciton energy and the spacing of higher-energy electronic states. Because of the anisotropy of the effective mass in PbSe, the P-states show further splitting into their parallel and perpendicular components [5, 6].
1.2 MEG Pathways, Their Competition, and Their Optimization in QDs Like bulk materials, QDs are able to absorb photons of light with energies above their excitonic resonances. These absorption processes generate “hot” excitons within the Advanced Micro- and Nanomaterials for Photovoltaics. https://doi.org/10.1016/B978-0-12-814501-2.00003-7 © 2019 Elsevier Inc. All rights reserved.
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FIG. 1 (A) Absorbance spectra of PbSe QDs of various diameters. As the diameter of the PbSe QDs increases, the position of the 1S exciton peak shifts to longer wavelengths (lower energy). (B) Schematic showing how the change in QD size changes the confinement felt by the electronic wavefunction. Transitions are denoted by their principle quantum number, n, and angular momentum, L. Anisotropy of electron/hole effective masses cause splitting of P-transitions. QD size decreases from left to right. (C) Sizing curve demonstrating the relationship between QD radius and the position of the lowest-energy (1S) transition of PbSe QDs. As the QD radius increases, the lowest energy transition approaches the bulk bandgap of PbSe. Adapted from O.E. Semonin, J.M. Luther, M.C. Beard, Quantum dots for next-generation photovoltaics, Mater. Today 15(11) (2012) 508–515. Copyright 2018, with permission from Elsevier.
QD with excess energy 4 Eex ¼ hv Eg. Similarly, the excess energy of the electron and hole can be calculated as 4 Ee ¼ Ee ECB and 4Ee ¼ EVB Eh+ (Fig. 2). In bulk semiconductors, hot charge carriers relax to the band edge through interactions with the bulk crystal, resulting in the emission of longitudinal optical (LO) phonons. These LO-phonon pathways are very fast (lifetime of 100 fs) and result in the emission of heat [6–8]. However, if one or both hot carriers has excess energy greater than the bandgap (4 Eh+ Eg or 4Ee Eg), then a second relaxation pathway is available in which the hot carrier produces one or more additional electron-hole pairs. This process is termed impact ionization (II) in bulk materials [7, 8]. Optimization of II was investigated as a method of increasing overall photovoltaic device efficiencies; however, the inefficiency of II in bulk semiconductors is now generally considered to be too great
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 37
FIG. 2 MEG in a quantum-confined QD. Due to the small dimensions of the crystal, Bloch states with defined momentum are present, not eigenstates. Therefore, only energy, and not momentum, needs to be conserved. Hot carries with energy ΔEe > Eg (or ΔEh+ > Eg; MEG process via hole not shown) formed in QDs undergo one of two competing processes: [1] generation of a second exciton (kMEG) or [2] cooling of the single exciton state (kcool) while producing heat. The same two processes apply for hot holes (not shown).
to allow significant improvements to photovoltaic devices [9]. The inefficiency stems from not only the rapid relaxation of the hot carriers via the phonon cooling described previously, but also the conservation of energy and momentum. These constraints raise the II onset in bulk materials from the theoretical 4Eex ¼ 2Eg to between 3 and 4Eg, and thus absorption of photons with energies of greater than 4 to 5Eg, which are generally of low abundance in the solar spectrum. Furthermore, such high energy onsets imply a low energy efficiency, and thus are of less interest in achieving high energy conversion efficiencies. In QDs, a process similar to II can occur; this process is termed multiple exciton generation (MEG). In contrast to bulk semiconductors, MEG can have a lower onset threshold than II due to several advantages offered by the small size of QDs. Firstly, their small physical size removes the conservation of crystal momentum constraint, as this is a product of the long-range repeating atomic potentials found in bulk crystals that are not found in QDs. Secondly, quantum confinement causes carrier-carrier interactions
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to be larger than carrier-phonon interactions, and thus one can take advantage of and modify the cooling pathways of hot carriers [3, 10–12]. These effects allow the rate of MEG (kMEG) to be competitive with and greater than the rate of hot-exciton cooling (kcool) (Fig. 2). In addition, the ever-growing range of compositions, shapes, and heterostructures obtainable in nanocrystals offer near-infinite freedom to conceptualize, engineer, and fabricate materials with even higher MEG efficiencies and thus higher limiting power-conversion efficiencies. The competition between MEG or II and carrier cooling via phonon emission can be expressed as [13] QY ¼ 1 +
kMEG ð1Þ
+
kMEG + kcool
ð1Þ
ð2Þ
kMEG kMEG + …, ð1Þ ð2Þ kMEG + kcool kMEG + kcool
(1)
where QY is the number of e-h pairs generated per absorbed photon, k(n) MEG is the rate of producing (n + 1) excitons from (n) hot excitons, and kcool is the cooling rate that is in competition with MEG. Successive terms in Eq. (1) represent higher-energy processes, and thus are only valid with sufficient excitation energies (i.e., k(n) MEG ¼ 0 when hv < n Eg). From work on II in bulk semiconductors, we can approximate the competitive cooling rate (kcool) to be a constant with increasing excess energy [14], while kMEG increases with increasing excess energy above a threshold photon energy, hvth(vide infra) [15, 16]. We can therefore evaluate Eq. (1) as a function of excess photon energy using a phenomenological expression relating kMEG and kcool [17] kMEG ¼ Pkcool
hvex 2 hvth
(2)
using a parameter, P, which encompasses the competition between kMEG and kcool, where hvex ¼ hv hvth. In Fig. 3, we plot QY for various values of P ranging from 0.1 to 10,000. When MEG is much greater than cooling (P ¼ 10,000), the QY approaches the energy conservation limit where each addition bandgap worth of energy produces an additional exciton (i.e., 2 e-h pairs are produced at 2Eg, 3 e-h pairs at 3Eg). Experimental results (markers) as well as general trends (lines) for bulk PbSe and PbSe QDs are also shown in Fig. 3. We find that P increases from 0.42 to 1.5 when comparing bulk PbSe to PbSe QDs, demonkMEG strating increases in quantum-confined systems compared with the bulk [13, 18]. kcool Further refinements of shape [19], internal structure (core/shell particles) [20], morphology, and composition [21] are pushing and exploring the limits of MEG in nanoscale systems. An interesting structure that may provide some advantages involves the use of Janus-like nanostructures [22]. These nanocrystals are asymmetric compositionally, yet structurally spherical (Fig. 3B). The spherical structure allows for close packing and good charge conduction pathways, while the asymmetric heterostructure component should allow for additional degrees of freedom for tailoring charge relaxation processes and enhancement of MEG.
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 39
FIG. 3 (A) Lines are the calculated QY at various values of P. The data points compare the measured QYs for bulk PbSe and PbSe QDs. Reprinted with permission from Accounts of Chemical Research, 2013, 46, 1252. Copyright 2018 American Chemical Society. (B) Janus-QDs of CdS and PbS.
1.3 Advantages of Multiple Exciton Generation in Solar Photoconversion In this section, we will discuss the maximum theoretical efficiency improvements to solar cells by switching from bulk semiconductor to QD systems. First, we must understand the three assumptions inherent in the Shockley-Queisser (S-Q) analysis of limiting efficiencies when converting sunlight to free energy in a single-junction photovoltaic cell [23–25]. They are as follows: 1. Photons with energy less than Eg are not absorbed and thus do not contribute. 2. Conduction band electrons and valence band holes created by absorption of highenergy photons immediately relax to their band minima, leaving only carriers at the band edge where excess energy is lost as heat. 3. The solar cell radiates excess light energy (energy above Eg) as a black-body. The largest energy loss is that of hot-carrier relaxation, which generates heat rather than usable free energy. The most successful way of overcoming these losses in bulk semiconductor systems is the use of a second p-n junction with a larger-bandgap semiconductor. In such multijunction or tandem solar cells, high-energy photons are absorbed by the larger bandgap semiconductor, and the lower-energy photons are absorbed by the smaller bandgap semiconductor. For an infinite number of junctions in the stack, the theoretical conversion efficiency reaches 68% at one-sun intensity [26]. Ross and Nozik have also demonstrated that this high conversion efficiency could also be obtained by capturing the excess kinetic energy of hot photogenerated carriers in a single bandgap device by transporting and collecting hot carriers at energy-selective contacts [25].
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FIG. 4 (A) The detailed balance-derived maximum allowable PCE. Reprinted with permission from Accounts of Chemical Research, 2013, 46, 1252. Copyright 2018 American Chemical Society. (B) Effect of solar concentration on power conversion efficiency (PCE) for various MEG characteristics denoted by the threshold photon energy. For each concentration value, the maximum possible PCE is selected from the plot of PCE versus bandgap; bandgap is a free variable. Reprinted with permission from JPCL, 2012, 3 (19), 2857. Copyright 2018 American Chemical Society.
For the case of MEG, a detailed balance analysis of the photoconversion efficiency (Fig. 4A) for various MEG characteristics denoted by the value of P (see Section 1.2 and Fig. 3) [27] demonstrates considerable benefits if the MEG process can be optimized. Another method of increasing power conversion efficiency in photovoltaic devices is by concentrating the incident sunlight [28]. Concentrated sunlight reduces some entropy losses and increases the Voc relative to the bandgap. The lower bandgap for a given Voc allows for higher multiplication and thus higher power conversion efficiency. Fig. 4B demonstrates the relationship between calculated ideal power conversion efficiencies and concentration of light for various MEG threshold (kvth) values. In all cases, concentrated sunlight increases PCE. However, when MEG is present and accounted for, the increase in efficiencies are even more pronounced. Currently, PbS and PbSe QDs show an MEG onset near 3Eg with P ¼ 1.5. From Fig. 4, we can see that MEG should not increase the PCE in a PbS QD solar cell at one sun (thus the need to further increase the MEG efficiency). However, under concentrated light, there is a great benefit to the PCE, even for the case with the threshold photon energy of 3Eg (red curve, Fig. 4B), achieving a limiting PCE of 45% at 300–500 suns. If the threshold could be further reduced to 2.5Eg, then the benefits could be quite significant (limiting PCE of 50% at 300–400 suns).
2. Quantum Dot Arrays for Solar Light Conversion 2.1 Construction of QD Solar Cells In early work inspired by organic photovoltaics, solution casting of QDs layers in conjunction with organic polymers was implemented to produce bulk heterojunction composite films [29]. Another method of cell fabrication that was developed to overcome low charge mobility was attachment to mesoporous TiO2, using a redox couple [30, 31]. By mixing
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 41
charge accepting materials with the QD photosensitizers, one mitigates the need to rely on a long-range depletion region to separate charges, and instead forms many short-range heterojunctions. Along these lines, several methods of interlacing PbS QDs with electron accepting materials have been successfully implemented. These methods include utilization of large porous TiO2 [32], n-type Bi2S3 nanocrystals [33], and metal oxide nanowires [34–36]. All of these systems allow for thicker active layers and thus higher cell optical densities, which can collect more photons to convert to free energy. The current champion QD solar cells use p-n junctions to create a large depletion layer across the QD film that aids in charge separation and mobility (Fig. 5). In the QD photovoltaic systems using polymer or mesoporous TiO2, efficient charge collection relies on efficient charge separation between the QDs and acceptor layer (polymer or TiO2) at one of the many interfaces present, whereas the later approach of a single thick film between an electron and hole transport layer relies on efficient charge migration across the QD film followed by efficient charge injection into the transport layer interfaces.
FIG. 5 Model of a p-n junction PbS solar cell. The layers of the cell construction are overlaid with the band energies of the materials (not to scale). Note how the PbS bands are bent across the entire PbS layer, driving the photoexcited electron and holes to migrate to opposite sides of the PbS layer. This is because the p-type PbS QDs form a p-n junction with the n-type ZnO. MoO3 is deposited along with a low work function metal as the back contact forming a Schottky barrier at the back side of the PbS layer.
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QD solar cell design has evolved over the years. Originally the fabrication of PbS, PbSe, or alloy PbSxSe(1 x) film devices used only a low work function metal contact evaporated onto the back of the QD layer, which created a simply Schottky barrier. Now, both hole transport layers (e.g., MoO3 [37, 38], V2O5 [37], or NiO [39]) and electron transport layers (e.g., ZnO [40, 41]or TiO2 [42]) are used to more efficiently separate generated charges. It is also common to form p-n heterojunctions with the p-type QD layer [43], and an n-type electron transport layer such as ZnO [41]. For example, the Sargent group developed a TiO2 contact that capitalizes on drift transport in the induced depletion region of the QD layer [44]. Complex QD solar cell architectures have also been developed, wherein QDs of different sizes (and thus different bandgaps) are used to funnel charges across the QD layer [45]. Many issues in QD solar cells arise not from the QD core or interfaces between QDs and charge transport layers but instead from the QD ligands. In order to maintain synthetic control and obtain QD samples with narrow size distributions, as-synthesized QDs are coated with long-chain insulating ligands (e.g., tri-n-octylphosphine, oleylamine, or oleic acid). In order to maximize charge transport across the QD film, ligand exchange methods have been developed. In 2003, Guyot-Sionnest et al. developed a soaking method for QD films to enhance QD coupling using 1,6-hexanedithiol [46]. These methods have evolved over time to include other dithiols [47], diamines [48, 49], and mercaptocarboxylic acids [50]. Removal of long ligands and replacement with short, electronically coupling ligands creates a large change in film volume, which can result in undesirable film cracking. In order to avoid this, layer-by-layer approaches have been developed, in which ligands are replaced following each of several depositions of QDs [51, 52]. A key development of thin-film QD solar cells was the introduction of dual ligand treatments. In 2011 Semonin et al. found that an EDT treatment followed by a hydrazine treatment increased the Voc [53]. The precise mechanism for the increased Voc was not detailed. In 2014 Bawendi and coworkers developed a double treatment from EDT and TBAI [54]. They found that the increased Voc and increased efficiency resulted from differences in the work function of the QD films when treated individually with EDT and TBAI. Therefore, the dual treatment produces an energy landscape within the QD absorber layer that aids in charge separation and reduces charge recombination near the contacts. Recent activity in the fabrication of QD solar cells is evolving away from the layer-bylayer approaches and toward a single-step solution casting approach. In these approaches, the ligand exchange, from the insulating oleate ligand to a short inorganic ligand, is done in solution prior to QD deposition. New polar solvents are used to support the shorter polar ligands prior to film formation. In 2014, Sungwoo et al. developed a onestep layer deposition procedure employing an ammonium iodide (NH4I) treatment to wash off the organic ligand and transfer the QDs to a polar phase. The QDs are then spin cast to achieve photovoltaic quality PbS quantum dot (QD) layers [55]. Ammonium iodide is used to replace the long alkyl organic native ligands binding to the QD surface, resulting in iodide terminated QDs. However, in this case, the films were still fairly thin, and the
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 43
resulting solar cells could not be optimized. Subsequently, PbI2 in DMF was found to remove nearly all of the as-produced organic ligand and form close-packed, dense PbS films with PbI2 decorating the surface [56]. The thickness of the absorber layer could be increased to 800 nm, nearly thick enough to absorb all of the incident solar irradiance. In all of the recent work, halide surface treatments render the QD films more stable toward oxidation [57–59]. Recent work from Sargent and coworkers has further extended the PbI2 treatments and produced record PbS QD solar cells [60].
2.2 Multiple Exciton Collection in QD Solar Cells Multiple exciton collection (MEC) from a QD photovoltaic system was first reported by Sambur et al. in 2010. By sensitizing atomically flat single crystals with a monolayer of PbS QDs, precise internal quantum efficiency (IQE) measurements could be performed, and it was shown that IQE exceeded 100% at 2.9Eg of the PbS QDs [61]. It was also found that under a large external bias, PbS QD photodetectors showed very large photoresponses under UV illumination, and this was explained as MEC [62]. In 2011, the first full p-n heterojunction solar cell was produced that has >100% external quantum efficiency (EQE) at high-energy illumination (Fig. 6A) [53]. In good agreement with Sambur et al., this threshold was at 2.8Eg. The addition of an antireflectance coating produced a solar cell with a peak EQE of 114% and IQE of 130%. Since the development of PbS QD MEG devices, MEG has also been detected in devices made from PbSe nanorods [63] and PbTe QDs [64]. Impact ionization thresholds for silicon and germanium are 3.5Eg (3.9 eV) and 4.1Eg (2.8 eV), respectively, whereas MEG onsets for PbS QDs have been found to be at 2.8Eg (2 eV). Compounded with the solar irradiance at 2 eV compared with germanium’s 2.8 eV or silicon’s 3.9 eV, MEG in quantum dot systems is far more efficient for solar light harvesting [65].
2.3 Tandem Cells A multijunction approach is another method that can be used to exceed the single junction Shockley-Queisser limit. A cascading interconnected stack of solar cells with progressively smaller bandgaps is used to reduce losses due to the thermalization of carriers. The size tunability of quantum dots allows for precise optimization of constituent cell bandgaps and provides a path for a multijunction cell made from a single absorber material. Notably, PbS quantum dots can be tuned to the ideal bandgaps for a triple junction cell: 0.71, 1.16, and 1.83 eV [66]. Two initial reports of tandem (or double junction) quantum dot solar cells were published in 2011. Choi et al. reported a tandem cell using an interconnecting layer of ZnO/ Au/PEDOT:PSS between the 1 and 1.6 eV PbS QD cells [67]. The thin gold layer was found to be crucial for minimization of series resistance from the interlayer. Wang, Koleilat, and colleagues reported a depleted heterojunction tandem using a graded recombination layer consisting of a stack of metal oxides with a progression of work functions [68].
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FIG. 6 (A) Spectral characterization of a PbSe QD solar cell exhibiting MEG. The EQE (normalized by reflectance cell reflectance) and the extracted IQE are shown. Importantly, the IQE and EQE begin to rise at 3Eg (arrow), indicating a MEG onset. Adapted with permission from O.E. Semonin, J.M. Luther, S. Choi, H.Y. Chen, J. Gao, A.J. Nozik, M.C. Beard, Peak external photocurrent quantum efficiency exceeding 100% via MEG in a quantum dot solar cell, Science 334 (2011) 1530–1533. Copyright 2018 American Chemical Society. (B) Cross section TEM of the solar cell in part (A). (C) JV characteristics of the PbS/CdTe tandem devices. (D) Cross section TEM of the PbS/CdTe tandem device. Reproduced with permission from Nano Lett., 2017, 17, 1020. Copyright 2018 American Chemical Society.
This layer allowed efficient recombination of electrons from the back cell with holes from the front cell and led to cells with PCE of 4.2% and open circuit voltages of 1.06 V. Besides the design of the interconnecting recombination layer, optimization of the individual subcells was also important for realization of the tandem cell [69, 70]. Another approach incorporating thin-film QD layers in tandem configuration involves pairing the QD layer with a thin-film technology that is already developed. For example, CdTe thin-film solar cells hold the greatest market share in photovoltaics next to Si, and have recently exceeded multicrystalline Si in performance and costs [71]. While generally underexplored, combining CdTe in a tandem configuration could retain the low-cost structure of CdTe devices, yet greatly improve the amount of power extracted. Luther and coworkers [72] explored the possibility of combining PbS QD layers with a solution
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 45
deposition route to thin-film CdTe [73]. They explored the effects of tuning the thickness of the top cell in tandem configuration. They found that thinner top films affect the range at which near optimal performance can be achieved. They developed a ZnTe/ZnO recombination layer (Fig. 6C and D) that sums the voltage of ink-based CdTe and PbS QD subcells. They find that with present-day quality of CdTe, a PbS QD bottom cell (Eg ¼ 0.95) and overall efficiency >9% can add substantial efficiency gains.
2.4 Nanomaterials in Photocatalysis The small volume of quantum dots not only causes them to have desirable optical and electrical properties for photocatalysis, but their large surface-to-volume ratio is also beneficial, as less material is needed for the same active surface leading to lower materials costs. Quantum dots have broad optical absorptions, making them more favorable than dyes for light-driven reactions. Also, MEG in quantum dots means that catalysis has the potential to occur very efficiently. Finally, because the band edges of QDs change with QD diameter, the oxidation and reduction potentials of QDs also change with changes in QD size, meaning reaction driving forces can be tuned with particle size. Using platinum as the catalytic site, and getting photogenerated electrons from QDs, researchers have produced robust hydrogen photocatalytic systems producing 6% photon-to-hydrogen efficiency [74, 75]. Capitalizing on the high surface area of QDs while also moving away from an expensive platinum catalyst, Han et al. generated an extremely stable solution-based system that generated hydrogen at >36% quantum efficiency using a nickel-centered catalyst [76]. This field has quickly been gaining more attention. In addition, other catalysis reactions include carbon-carbon bond formation [77] and oxidation reactions [78, 79]. Recently, MEG has been utilized for the photocatalytic production of hydrogen with quantum yields above unity [80]. Such a system is a proof of principle that MEG and QDs may also impact the generation of fuels from sunlight. Currently, low-cost materials exhibit very low efficiencies, and systems that exhibit high efficiency have a very high cost. If it is possible to capture multiple exciton generated electrons in a fuel-forming reaction, this could be a way to increase the efficiency of the process. Two or more photoelectrodes connected in series or in parallel with each able to take advantage of MEG is a potential area of future research (Fig. 7).
2.5 Record Cells In order to be considered a record cell, a cell must be certified by an independent laboratory. The National Renewable Energy Laboratory (NREL) maintains a table of certified “champion” solar cells for all different photovoltaic disciplines (e.g., monocrystalline silicon, CIGS, and quantum dot cells). The most up-to-date chart can be found at https:// www.nrel.gov/pv/. The first QD solar cell to be included in the champion solar cell chart was developed at NREL and certified in 2010. This cell only had a 2.94% efficiency and used ZnO as an
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FIG. 7 (A) Schematic of the experimental setup. Photogenerated holes oxidize sulfide, while electrons reduce hydrogen at the dark Pt cathode. (B) The energy levels of the QD photoelectrode with respect to NHE (V). (C) SEM images of typical PbS QD photoelectrodes. (D) Schematic of the MEG process that occurs within the individual QDs. Reprinted with permission from Nature Energy, 2017, 2, 17052, Copyright 2018.
electron transport layer and silver as a back contact to the PbS QD film. In a year’s time, NREL increased the efficiency of the QD solar cell to 4.4% by implementing some of the ligand modifications discussed in Section 2.1, incorporating ethanedithiol and mercaptopropionic acid. The Sargent Group out of the University of Toronto currently holds the record for a metal chalcogenide quantum dot solar cell. This cell uses an ex situ ligand exchange process to achieve efficient ligand exchange and tight QD packing in the QD film, and has an efficiency of 11.28% [81]. However, the current overall champion quantum dot solar cell is not a metal chalcogenide device, but in fact a CsPbI3 perovskite quantum dot device developed at NREL [82].
3. Conclusions and Outlook Quantum dots are an entire class of material with countless ways to modify their optical and electronic properties for solar energy conversion. New developments in the field require complex inorganic synthesis, ligand chemistry, theoretical descriptions, linear and time-resolved spectroscopies, and device/architecture engineering, all working in symbiosis to push advances. Greater understanding of ligand exchange chemistries, electronic coupling in QD arrays, and the chemistry and physics of defects (either surface
Chapter 3 • Thin-Film Colloidal Quantum Dot Solar Cells 47
related, matrix related, or core) will contribute to improvements in solar cell performance, stability, and ease of fabrication. Greater understanding of the reaction mechanisms that form QDs is opening new doors for QD design [83]. In the field of photocatalysis, complex dot-in-rod structures are being utilized to drive the separation of the electron away from the hole and into a platinum catalyst [84, 85]. By improving the rate at which excitons form dissociated charges, we will further improve solar cell efficiencies. As conventional solar energy technologies begin to reach their limiting values, approaches that go beyond those limits will become increasingly important. MEG allows a device made from a singular material to exceed the Shockley-Queisser limit for a singlejunction cell and has already been realized in solar cells. No longer are we limited to core/ shell QD structures; we now have access to more advanced Janus-like QD structures [22]. These CdS/PbS structures have already shown improved MEG efficiencies compared with PbS QDs. While the Janus structures are one method of accomplishing this, choosing materials with large differences in effective masses of electrons and holes (see Section 1.2) or with smaller bandgaps will also decrease MEG thresholds. We expect more advances in both the MEG efficiency as well as the solar cell efficiencies for Quantum Dot solar cells.
Acknowledgments We gratefully acknowledge funding from the Office of Basic Energy Sciences, Office of Science within the US Department of Energy under contract No. DE-AC36-08GO28308. Our work on QD solar cells was part of the Center for Advanced Solar Photophysics, an Energy Frontier Research Center, while the work on MEG in QDs was funded by the Solar Photochemistry program. The views expressed in the article do not necessarily represent the views of the US Department of Energy or the US government. The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for US government purposes.
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