Optoelectronic Properties of Semiconductor Quantum Dot Solids for

Perspective pubs.acs.org/JPCL

Optoelectronic Properties of Semiconductor Quantum Dot Solids for Photovoltaic Applications A. A. Chistyakov,† M. A. Zvaigzne,† V. R. Nikitenko,*,† A. R. Tameev,†,‡ I. L. Martynov,† and O. V. Prezhdo*,†,§ †

National Research Nuclear University “MEPhI” (Moscow Engineering Physics Institute), Moscow 115409, Russia A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences, 31-building 4 Leninsky Prospect, Moscow 119071, Russia § Department of Chemistry, Department of Physics, and Department of Astronomy, University of Southern California, Los Angeles, California 90089, United States ‡

ABSTRACT: Quantum dot (QD) solids represent a new type of condensed matter drawing high fundamental and applied interest. Quantum confinement in individual QDs, combined with macroscopic scale whole materials, leads to novel exciton and charge transfer features that are particularly relevant to optoelectronic applications. This Perspective discusses the structure of semiconductor QD solids, optical and spectral properties, charge carrier transport, and photovoltaic applications. The distance between adjacent nanoparticles and surface ligands influences greatly electrostatic interactions between QDs and, hence, charge and energy transfer. It is almost inevitable that QD solids exhibit energetic disorder that bears many similarities to disordered organic semiconductors, with charge and exciton transport described by the multiple trapping model. QD solids are synthesized at low cost from colloidal solutions by casting, spraying, and printing. A judicious selection of a layer sequence involving QDs with different size, composition, and ligands can be used to harvest sunlight over a wide spectral range, leading to inexpensive and efficient photovoltaic devices.

only short-range order, while superlatticies are anisotropic and characterized by three-dimensional order. Several factors may influence the degree of ordering of a particular structure, including nanocrystal size, type and length of the QD surface ligands and details of the approach chosen for structure manufacturing, for example, colloidal stability of QDs in a given solvent and the rate of solvent evaporation. The most effective methods of producing QD superlatticies include drop casting and slow QD planting from solution.22,24 The structures obtained by these methods possess long-range order enduring for micrometers in the lateral plane. However, both methods have a significant drawback; they are useful only for application to small areas.

Fabrication Methods and Optical Properties of Quantum Dot Solids. Colloidal quantum dots (QDs) can be used to create QD films on various substrates. A new type of material generated by this inexpensive procedure and known as a QD solid has attracted strong attention due to both novel fundamental properties of artificial atom structures observed in such condensates1−14 and versatile and technologically simple applications.15−21 QD solids produced today can be divided into two types according to the spatial distribution of the nanocrystals, namely, disordered (Figure 1a) and ordered (Figure 1b) structures. The latter are known as superlattices. The disordered structures possess

The distance between the adjacent particles plays a crucial role for practical applications of QD solids in photovoltaic devices and photosensors as it directly affects the charge and energy transfer. Received: March 20, 2017 Accepted: August 11, 2017 Published: August 11, 2017

Figure 1. TEM image of (a) a three-dimensional disordered QD solid22 and (b) a three-dimensional superlattice.23 © 2017 American Chemical Society

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DOI: 10.1021/acs.jpclett.7b00671 J. Phys. Chem. Lett. 2017, 8, 4129−4139

The Journal of Physical Chemistry Letters

Perspective

Recent works reported replacement of the original ligands in solution. In this process, the long organic ligands that stabilize QDs in relatively nonpolar solvents are exchanged with shorter ligands that allow QD dispersion but now, typically, by electrostatic stabilization in more polar solvents.37,38 A significant problem in “solution” ligand exchange is QD aggregation due to the decrease of colloidal stability. In this case, QD solids are made by applying QDs onto a substrate from the solution with new ligands. Unfortunately, the length of the ordered areas within such structures is not large. It is particularly important for characterization of QD solids to determine the degree of ligand replacement and the sample structure. Infrared (IR) spectroscopy is widely used to examine the composition of the prepared sample. Typical IR spectra of QD films with different surface ligands are shown in Figure 2.

Other common methods of creating QD solids are spraying,25 spin-coating,26 and dip-coating.25 In addition to ease of application, these methods allow one to control simply the thickness of produced samples by varying the number of layers. Besides, spraying and dip-coating allow application of QD layers on areas of various sizes. Given the above advantages, these methods also have a drawback, which is rapid evaporation of the solvent that leads to amorphous structure. In addition, the creation of QD structures by spin-coating is associated with high material consumption. Unlike the spraying and spin- and dip-coating methods, the Langmuir−Blodgett technique27 allows one to create a more ordered structure by gradually applying QD monolayers on the substrate surface. However, this method requires a long time to create a single sample. Doctor blade (blade casting) has become a promising method to create highly ordered structures of semiconductor QDs. This method is based on “smearing” a QD solution over the substrate surface by a mechanical blade, which enables precise control of the thickness of the applied layer by changing the gap between the substrate and the tip of the blade, the blade speed, and the substrate temperature. Recently, Maul et al.28 have shown the ability to create well-ordered (strongly coupled) QD solids of PbS QDs for photodetection purposes. It is worth noting that this technique requires a highly concentrated solution of QDs (nanoink) with concentrations on the order of 150 mg/mL. Production of ordered QD films depends significantly on the QD crystal structure, size variation,22 as well as properties of the nanocrystal surface. The size dispersity of QDs depends directly on the synthesis method. The most common methods of colloidal synthesis today are the “heat-up method” and “hot-injection method”, which enable one to obtain QD samples with size distributions of 5−10%.22,29 The size dispersity of QDs can be further narrowed to xc ≅ 1, where z is the number of nearest neighbors. For the simple cubic lattice, z = 6, and for random close-packing of QDs, z = 7. Indeed, the calculations71 show that among all materials mentioned above only PbSe QD solids can exhibit extended states. Although the s/t0 ratio

Similarly to organic semiconductors, it is almost inevitable that QD solids exhibit energetic disorder, which prevents band-like transport of charge carriers and excitons. First, consider the inhomogeneity of QD size. The presence of quantum confinement energy levels is a fundamental and valuable property of QD solids. These discrete levels depend of nanocrystal size. Considering a QD as a very deep 4132



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ln[μ(F)/μ(0)] = const√F. One has to note that the GDM gives this dependence only approximately and over a limited range of field strengths. QDs also have large dipole moments, even much larger than those of organic molecules. Compare 25 and 47 D for CdSe QDs with diameters of 0.34 and 0.46 nm, respectively,52,93 versus 3−5 D for typical polar organics. (D is Debye, a unit of dipole moment, 1 D ≈ 3.34 × 10−30 C · m ≈ 0.21 e · Å.) However, large dielectric constants of semiconductors (for example, ε = 9 for CdSe and ε = 210 for PbSe) leads, according to eq 3, to a sufficiently small value of “dipolar” disorder, σP = 0.014 eV for R = 1.7 nm, p = 25D, and ε = 9, and even less than that for lead salts. Thus, we should expect that the energy correlations do not have the same importance for QD solids as those for organic materials. Perhaps this fact is more important for the description of transport in QD solids than a particular kind of the energy level distribution g(E), eqs 2 and 3. Depending on the carrier concentration and temperature, there are two basic modes of hopping conductivity in disordered organic materials: variable-range hopping (VRH) between LSs near the quasi-Fermi level and nearest-neighbor hopping (NNH). In the first case, one can describe the temperature dependence of conductivity by the Mott’s law, GVRH ∝ exp[−(T0/T)1/4].94 Here, the activation energy of hopping is much less than the scale of disorder, ΔE ≪ s, and one can derive the shape of the energy level distribution function g(E) near the quasi-Fermi level EF from measurements of conductivity temperature dependence because T0 = const/ g(EF). In the NNH regime, hopping to the nearest neighbors (in space) is the prevailing mechanism, although the hopping length is not a fixed value due to spatial disorder. Even small variations in the hopping length lead to large fluctuations of transition rates as a result of the exponentially strong dependence on the length. As a rule, ΔE > s in this regime, considering rather deep LSs, that is, LSs having low energy E relative to the center of the distribution g(E). Moreover, hopping from deep LSs occurs mainly to the states proximal to a certain energy, called the transport level, providing a sufficiently sharp decrease of g(E) for deep states (e.g., Gaussian or exponential distribution).87 One can introduce an effective transport level EC, even if the energy width of the range of LSs that govern charge transport is not small relative to the width of the energy level distribution g(E). It is required that the rate of escape from deep states controlling the mobility, that is, states with energies E proximal to − σ2/kT for the Gaussian distribution or E ≈ EF if g(E) is exponential or a slower function of energy,87 can be described as ω(E) = ω0 exp[−(EC − E)/kT].95−97 This expression results from an interplay of the activation energy and hopping distance.97 The conductivity follows the expression GNNH ∝ exp[−(EC − EF)/ kT].87 The effective transport level and deeper states are analogous to the mobility edge and traps, respectively. If the energy distribution is Gaussian, the transition from the NNH to the VRH regime occurs under the condition98 (T0(EF0)/T)1/4kT < σ, where EF0 is the low-temperature limit of the quasi-Fermi level, which depends on charge carrier concentration. The transition from the NNH to the VRH regime with decreasing temperature has been traced in the QD solid formed by PbS nanocrystals.72 The energy dependence of the deep tail of g(E), which determines the temperature dependence of mobility at low carrier concentrations86,87 is not known at present in QD solids. An exponential tail has been reported from calculations71

There is significant similarity between the transport of electrons in QD solids and that in conventional disordered semiconductors, especially organic materials. decreases with decreasing R, it is sufficiently large even for small QDs with R = 1.5 nm. The characteristic size of small organic molecules is 1 nm, which is slightly smaller than QDs but still of the same order of magnitude.74 Organic semiconductors are characterized by weak intermolecular interactions and strong disorder, t0 ≅ 0.01 eV, s ≅ 0.1 eV. Hence, similarly to QD solids, the Anderson criterion is fulfilled for organic semiconductors, and all states are localized. The Gaussian disorder model (GDM)86,87 forms the basis for description of charge transport in organic semiconductors. In accord with GDM, electron (hole) transport is of hopping nature, that is, it is carried by incoherent tunneling hopping between molecular LSs.86,87 Energy conservation is satisfied for transitions between LSs of various energies due to interaction with phonons. A consistent description of the transition rates is a complicated quantummechanical problem.88 Usually, simplified phenomenological variants of the Miller−Abrahams model86,87,89 or the Marcus model90 are employed. The latter model is more justified microscopically, while the former one is mathematically easier while consistent with experimental data. It has been shown theoretically that the interaction of electrons with optical phonons is rather strong to provide hopping between QDs in accord with the Marcus model,91 although Auger recombination, perhaps, is important for energy balance of hopping in QD solids. According to the GDM, the energy distribution of LSs is Gaussian g (E ) =

⎡ E2 ⎤ exp⎢ − 2 ⎥ ⎣ 2σ ⎦ 2πσ 2 1

(2)

and energies of neighbor states are noncorrelated. In the past decade, the correlated disorder model (CDM) has been widely accepted. According to the CDM, the energy of a carrier is calculated as the sum of the interaction energies of the charge with permanent molecular dipole moments that are randomly oriented in space.92 As a result, the Gaussian distribution (eq 2) is also obtained in the CDM (with a possible exception of the very deep tail that has little influence on the mobility). In this case, the variance is equal to92 σ = σp = 2.35ep /(4πεε0a 2)

(3)

where p is molecular dipole moment and a is a typical intermolecular distance. An even more important result of the CDM is a strong and long-range correlation of LS energies, indicating, in particular, that neighbor states are most likely to have the same energy. The reasons are the long-range nature of the charge−dipole interaction and small values of dielectric constants of organic materials, requiring one to consider the interaction with a large number of surrounding dipoles not only with the nearest neighbors. It is the correlation of the energies that has provided the explanation of the so-called Poole−Frenkel dependence of mobility on the strength of the electric field, F, which is widely observed in organic materials, 4133



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In eq 4, f i is the occupation probability of state i and νij is the transition rate from state i to state j. In eq 5, pc(x,t) is the concentration of carriers in the conductive states, ρ(E,x,t) is the energy distribution of occupied traps, t(E) = ν0−1 exp(E/kT), ν0 is a frequency factor, and r(E) is the model-dependent dimensionless capture rate from “conductive states” to “traps”. The coefficient K0, which is also model-dependent, quantifies the fraction of conductive states. It is known from percolation theory that conductive states form a network with a characteristic size rc that is the correlation radius of the percolation cluster.87 Equation 5 is valid provided that the functions ρ and pc are averaged on the spatial scale of rc, which is temperature- and disorder-dependent but typically is much smaller than 100 nm, and the respective time scale tc. Simple analytic expressions of the temperature-, field-, and concentration-dependent mobility and diffusion coefficient can be obtained on the basis of the transport level concept.96,97 It is worth noting that the MT description is valid if a QD solid contains a band of extended states and a mobility edge. It is applicable not only for Gaussian and exponential energy distributions g(E) but also for any distribution that decreases faster than E−4 with increasing energetic depth E.95

characteristic for disordered inorganic materials. However, the reported results correspond to weak energy disorder, for which a real mobility edge exists, and exclude the deep tail of the energy distribution. The temperature dependence of conductivity or mobility, measured in the NNH regime, is interpreted usually in accord with the Arrhenius law,72,74 ln μ ∝ −Ea/kT, where Ea is the activation energy. This dependence is valid not only for exponential but also for the Gaussian distribution (eq 2) under the condition EF > −σ2/kT,99 while ln μ ∝ −const · (σ/kT)2 in the limit of low concentration.86,87 Considering the field-effect mobility, one has to remember that the disorder within the surface layer at the interface with the gate dielectric may be different from the disorder within the bulk material. The QD size distribution is usually regarded as Gaussian.99,100 Transition from the Gaussian size distribution D(L) to the energy distribution g(E), according to the equation g(E) dE = D(L) dL and eq 1, yields the deep tail that decreases with energy faster than the Gaussian function. However, a more slowly decaying “tail” can arise from another source of disorder. The Gaussian distribution (eq 2) appears most natural for QD solids with moderate disorder, in accordance with the central limit theorem and by analogy with organic materials.86,87 The transport level concept, mentioned above, is known as an effective analytic tool, which permits a description of hopping transport in the NNH regime by the relatively simple formalism of the multiple trapping (MT) model, which was developed originally for the case of inorganic disordered materials.87,95,97 The transport level87 or the effective transport energy,95,97 EC, is an analogue of the mobility edge, which separates extended states and LSs (traps) in the MT model. Up to now, the transport level concept has been used in combination with the Miller−Abrahams model for the rates of hopping between LSs. However, the MT description of hopping transport does not require application of the Miller−Abrahams model. Moreover, the MT description does not require introduction of the transport energy EC explicitly. The key assumption is that a small fraction of charge carriers contributes to charge transport at any time, while other carriers are waiting for release from deep states, irrespective of details of the hopping mechanism. This is the essence of the MT model, and it is typical for disordered organics and QD solids. Indeed, the hopping rates between various pairs of LSs are widely distributed because of the exponentially strong dependence on distance and energy in any reasonable model and because a fraction of LSs contributes to transport in general. The assumptions include a quasi-equilibrium distribution of populations of the “conductive states” and small occupation numbers for these states resulting in Boltzmann statistics. At the same time, the deep states (traps) can be filled or their occupations can be far from equilibrium. On the basis of these assumptions and the detailed balance principle, one can reduce the well-known balance equation of hopping transport ∂fi /∂t =

∑ [νjif j (1 − fi ) − νijfi (1 − f j )] j≠i

The multiple trapping model is rather universal and appears adequate for description of charge transport in disordered QD solids. A combination of steady-state (field-effect mobility, I−V characteristics) and transient (time-of-flight, charge carrier extraction by linearly increasing voltage) measurements can provide information about the distribution of deep LS energies g(E). Thus, investigation of optical and transport properties of QD solids can give important information on the type of disorder in these materials, allowing one to perform an adequate theoretical analysis. This information is very much necessary for improvement of transport properties of electronic devices based on QD solids and for improving control over QD solid synthesis. Applications in Photovoltaic Cells. Large light absorption cross sections, quite efficient energy and charge carrier transport, and low cost make QD solids excellent candidates for PV applications.15,101−103 A typical PV cell contains a photoactive layer, in which the key photophysical processes take place, and a charge transport layer used for collecting electrons and holes at the electrodes. QDs can be used for development of both photoactive and transport layers. The photoactive layer can be prepared using QD−polymer mixtures104,105 and ternary blends.106 For example, in the latter case, doping of graphitic carbon nitride QDs into polymer−fullerene blends increases the solar cell efficiency to 9.18% due to the combined improvements in nanomorphology, electron transfer, and charge carrier transport.107 QDs were first used as a solid layer in a Schottkytype QD PV cell having the ITO/QD solid/metal structure.108 Yet, the efficiency of such a device is known to be limited, with the record-performing solar cell based on a PbS nanocrystal Schottky junction showing a power conversion efficiency (PCE) of 5%.109 A low built-in voltage at junctions is one of the reason for the limitation. By using the surface photovoltage spectroscopy, a relatively low built-in potential of 0.45 V was

(4)

to the balance equation of the MT model ∂ρ(E , x , t ) −1 r (E) = ν0K 0 ∂t p (x , t ) ρ (E , x , t ) × [g (E) − ρ(E , x , t )] c − Nt t (E )

(5) 4134



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fast, on a picosecond time scale, if the nanoparticle size is less than 4 nm. The electron transfer is slow for QD sizes ranging from 4 to 10 nm. The transfer is inhibited for nanoparticles that are larger than 10 nm (Figure 4). On the contrary, in the case

measured for the Schottky junction formed between the CdSe QD layer and the ITO electrode. This was attributed to the formation of In−Se interfacial states resulting in Fermi level pinning.110

Light absorption in a wide spectral range is ensured by the sequential arrangement of QDs layers having (1) varying size (conductivity level), (2) different ligands, and (3) different QD composition. Due to quantum confinement effects, the QD size directly determines the light absorption properties. Incorporation of QDs of different sizes in the photoactive layer allows one to cover a wide range of solar radiation. For instance, the tunable bandgaps of PbS chalcogenides can be widened to 1.5 eV from bulk bandgaps of less than 0.5 eV, and such QD solids provide PCEs ranging within 5−9%.109,111−113 Perhaps the most rational use of QDs of different sizes in the photoactive layer involves formation of a QD size gradient.102 In this case, a uniform sequence of transport energy levels is created in order to ensure efficient transfer of charge carriers along the size gradient. Another method of controlling the transport of energy and charge carriers in QD solid layers is based on variation of ligands. The effect of ligands on the properties of QD PV cells is reviewed in refs 114 and 115. The use of halide ligands during solid-state QD ligand exchange improves carrier mobility and decreases the number of charge recombination centers.115 Short ligands, such as metal chalcogenide complexes, lead to much improved carrier mobility and film conductivity.38 Controlling the QD size and length of ligand molecules allows one to control disorder. Indeed, the energy disorder, δE, produced by the QD size variation, δR, is proportional to the mean energy of the miniband E, δE/E = 2δR/R.69 Therefore, the gradient of the QD size R produces a gradient of the energy disorder δE and a gradient of the disorder-controlled mobility. The energy disorder gradient can be used additionally to facilitate charge transfer in QD PV cells. One can use the methods and simulation tools,116,117 developed for organic semiconductors and described in the previous section, to model the transport energy and the disorder-controlled mobility for optimization of QD PV cells. It is important to note that not only the disordered structure of QD solids but also the intrinsic disorder of QDs can have an important and nontrivial impact on the operation of QD solar cells. Thus, the presence of defects and amorphous structures in QD systems, compared to crystalline materials, has been shown to increase the MEG efficiency.65,118 PV devices require efficient charge extraction from the photoactive layer, that is, collection of charge carriers on electrodes. Formation of charge transfer excitons, for example, in closely packed silicon QDs is supported by analysis of the transition density matrix.119 The relative positions of the work functions of the electrode material and the charge transport level in the QD layer determine the collection efficiency of charge carriers on the electrodes. In particular, the interface between a TiO2 electrode and a QD solid layer has been studied in ref 60 for QDs with sizes ranging from 1 to 14 nm. Electron transfer from a PbS nanoparticle to the electrode is

Figure 4. Scheme of electronic band alignment for bulk TiO2 and bulk CdS and PbS QDs of different size.60

of a CdS layer, fast electron injection occurs even for CdS bulk and, thus, for CdS QDs of any size. It has also been shown that a photoactive layer prepared from a mixture of PbS and CdS QDs increases the efficiency of the PV cell. Low-temperature-processed PV cells show inferior performance. One of the main reasons is the loss of photogenerated electrons in the electron collection layer (ECL) and related interfaces, that is, indium tin oxide/ECL and ECL/perovskite (Figure 5). Tailoring the energy level and electron transport in

Figure 5. Schematic illustration of the strategy for improving the collection of photogenerated electrons in perovskite PV cells.123

oxide ECLs using Zn2SnO4 nanoparticles and QDs notably minimizes the loss of photogenerated electrons in the lowtemperature-fabricated flexible PV cells. The results provide an effective method for fabricating high-performance, low-temperature solution-processed flexible PV cells based on perovskites120−122 with a PCE of 16%.123 Structures employing a QD solid as a photoactive layer and a perovskite as a thin-shell passivator of QD surfaces have led to reduced defect densities and increased carrier diffusion lengths. As a result, the PCE was enhanced to almost 9%.124 The passivation of QDs starting from the solution phase led to PbS QD solar cells with an efficiency of 10.6%.21 4135



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In addition to the core−shell QD structures, QDs composed of multiple elements provide additional advantages in the development of photoactive layers based on QD solids. Solar cell based on high-quality ∼4 nm Cd- and Pb-free Zn−Cu−In− Se-alloyed QDs with an absorption onset extending to ∼1000 nm yielded the efficiency of 11.6%.125 At present, it is a record value for QD solids certified by NREL.

include photonic crystals and photophysics of semiconductor nanostructures. Oleg Prezhdo is a Professor of Chemistry, Physics and Astronomy at USC. He is Editor for The Journal of Physical Chemistry Letters, The Journal of Physical Chemistry, and Surface Science Reports. A recipient of multiple awards and visiting positions around the world, he works on quantum and semiclassical dynamics with application to nonequilibrium processes in nanoscale and biological systems.

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QD layers are formed from liquid colloidal QD solutions, using simple and cost-effective methods of layer deposition (casting, printing, and spray).

ACKNOWLEDGMENTS The work was supported by the “improving the competitiveness” program of the National Research Nuclear University “MEPhI”. Preparation and characterization of QD solids was supported by the Russian Foundation for Basic Research, Grant No. 16-29-09623 (OFI_M). Photovoltaics was supported by the Russian Science Foundation, Grant No. 15-13-00170. O.V.P. acknowledges support of the U.S. Department of Energy, Grant No. DE-SC0014429, and the U.S. National Science Foundation, Grant No. CHE-1565704.

Last but not least, it is important to emphasize the manufacturing advantages of QD PVs cell over the classic Si-based PV cells. The technology underlying formation of QD layers is simple and affordable, largely because of the straightforward solution processing techniques that require only inexpensive equipment. Further advantages arise because QD PV cells are lightweight and can be developed and manufactured using flexible substrates.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (V.R.N.). *E-mail: [email protected] (O.V.P.). ORCID

O. V. Prezhdo: 0000-0002-5140-7500 Notes

The authors declare no competing financial interest. Biographies Alexander Chistyakov is a Professor at MEPhI. He received his Ph.D. in Physics and Mathematics from MEPhI under the supervision of Prof. Yury Bycovskiy in 1983 and his Dr. Sci. in 1998. With over 30 years of experience in laser photophysics of molecules and semiconductor nanostructures, he is currently interested in development of hybrid nanomaterials for photovoltaics and sensing. Mariya Zvaigzne is a Ph.D. student in the Department of Physics of Micro- and Nanosystems in MEPhI. Her research interests are in energy and charge transfer in nanohybrid materials based on semiconductor quantum dots and organic polymers. Vladimir Nikitenko is a Professor ar MEPhI, where he obtained his Ph.D. (1990) and Dr. Sci (2006) in Condensed Matter Physics. He was a visiting scientist with Prof. H. Bässler, Marburg, Germany (1996−2002) and an AvH Fellow at Darmstadt University of Technology, Germany (2003−2004). His scientific interests are in theoretical modelling of charge transport and recombination in disordered organics and quantum dot solids. Alexey Tameev received his Ph.D. in Physical Chemistry (1992) and Dr. Sci in Physics and Physical Chemistry (2012) at the A. Frumkin Institute of Physical Chemistry and Electrochemistry of the Russian Academy of Sciences. His work is dedicated to experimental research on charge carrier transport in organic and nanocomposite materials and to development of solar cells based on such materials. Igor Martynov is an Associate Professor in the Department of Physics of Micro- and Nanosystems in MEPhI. He received his Ph.D. in Physics and Mathematics from MEPhI in 2011. His research interests 4136



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Optoelectronic Properties of Semiconductor Quantum Dot Solids for

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