Symposium EN19 : Novel Inorganic Semiconductors for Optoelectronics and Solar Energy

The United States’ Materials Genome Initiative (MGI), announced in 2011, helped launch the era of Materials-by-Design which combines theory, computation, experiment and data sciences to accelerate materials development and deployment. So far, Materials-by-Design research has primarily focused on the intrinsic properties of equilibrium materials. However, many technologically relevant materials are metastable and, further, roughly 1/3 or more of new materials discovered each year are metastable rather than equilibrium materials. Therefore, for new materials to help in addressing energy needs, we must directly address the research challenges of both including metastable materials and moving from Materials-by-Design to Solutions-(or Devices)-by-design. Our approach to and progress towards this challenge will be demonstrated by examples taken from current research on opto-electronic semiconductors and transparent conductors.

Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Both Pb²⁺ and Bi³⁺ possess a similar soft polarisability and form a wide range of compounds with rich structural diversity, such as BiX₆ clusters, 1D ribbons, and layered perovskite type structures. Whilst both are composed of earth-abundant materials and experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced.[1]

Here, we use hybrid density functional theory, with the addition of spin orbit coupling (SOC), to examine a range of bismuth containing V-VI-VII candidate photovoltaic (PV) absorbers.[2-5] We show that BiSI and BiSeI possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this, we have suggested alternative device architectures expected to result in improved power conversion efficiencies. Lastly, we explore the defect properties of BiSI and suggest ideal growth conditions for optimised film properties.

References

[1] A. M. Ganose, C. N. Savory and D. O. Scanlon, Chem. Commun. 53, 20–44 (2017)
[2] K. T. Butler, J. M. Frost, and A. Walsh, Energy Environ. Sci. 8, 838 (2015)
[3] A. M. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A 4, 2060 (2016).
[4] A. M. Ganose, M. Cuff, K. T. Butler, A. Walsh and D. O. Scanlon, Chem. Mater. 28, 1980 (2016)
[5] D. S. Bhachu et. al., Chem. Sci. DOI: 10.1039/C6SC00389C (2016)

Given rapid advances in photovoltaics, transparent electronics, and other emerging energy technologies, the development of p-type transparent conductors (TCs) has been a relatively slow-moving front. Both n-type and p-type TCs are conventionally oxides, but there is increasing evidence that chalcogenide (S, Se, Te) semiconductors should have higher hole transport and probability of p-type doping than oxides (in exchange for lower transparency). We use a high throughput computational framework to screen a large database of chalcogenide compounds likely to have a high hole conductivity and high optical transparency. Our ultimate goal is to synthesize promising compounds in the laboratory for use in stable devices. To this effect, we investigate potential metrics for TC performance including various weighting methods for Boltzmann transport calculations, defect formation energies, dopant selection criteria and global thermodynamic stability. From these criteria, we discover over one hundred computationally stable multi-anionic compounds with indirect computed gaps E_G > 1.5 eV (since PBE underestimates the gap) and average effective masses 0 < m_h* < 1 by screening the Materials Project database. Several compounds studied previously as TCs emerge from our screening, including ZnS and sulvanites TaCu₃X₄ (X = S, Se, Te). We further pare down this list for synthesis by selecting only single anionic compounds, removing compounds with toxic and highly reactive elements, and estimating p-type dopability. A refined list of ten top experimentally-favorable candidates emerges and includes spinel ZnAl₂S₄, distorted rocksalt BaSnS₂, and several other rocksalt structures. We will also present our initial attempts to combinatorially synthesize and characterize a few of the candidates, and lay out a roadmap for a future high-throughput screening, synthesis, characterization closed loop to enable new device paradigms using transparent conducting chalcogenides.

The use of advanced machine learning algorithms for design and discovery of novel inorganic semiconductors for optoelectronics energy conversion applications requires large datasets amenable to data mining. Whereas a number of computational materials property databases exists (e.g. materialsproject.org, materials.nrel.gov), the machine learning based on experimental data is limited by the lack of large and diverse data resources. Here, we report on our progress towards a publicly open High Throughput Experimental Materials (HTEM) Database (htem.nrel.gov). Presently, this database contains 130,000 sample entries, characterized by structural (100,000), chemical (70,000), optical (50,000) and electrical (40,000) properties of novel inorganic thin film semiconductor materials, grouped in >4,000 sample entries across >100 materials systems. More than a half of these data are publicly available. In addition to showing how HTEM database may enable scientists to explore materials by browsing web-based user interface, this presentation will discuss the underlying laboratory information management system (LIMS). Also, this presentation will illustrate how advanced machine learning algorithms can be adopted to materials science problems of predicting materials conductivity using random forest methods, and clustering unrelated samples into groups based on composition similarity.

The discovery of earth abundant, functional materials is critical for sustainable technological advancement. There is a concerted global effort to reduce the time it takes to realize such materials via databases, high-throughput screening, informatics, and mapping out the ‘‘materials genome.’’ But what fraction of theoretical chemical space is represented by the number of known compounds that have been thoroughly characterized to date? Forming a four-component compound from the first 103 elements results in excess of 10¹² potential combinations. Such a search space is intractable to high-throughput experiment or first principles calculations.

We present a hierarchical screening approach that is capable of dealing with such a search space, consisting of three key stages: First, we employ an arsenal of simple chemical rules that are the product of centuries of research, in order to filter out chemically implausible element compositions. This is implemented using the open-source SMACT package.¹ Second, we use supervised machine learning and data mining to rapidly filter for target properties and suggest likely structures of leading candidates. Finally, we apply density functional theory (DFT) calculations in order to verify stability, structure and target properties. At each stage, the size of the search space is drastically reduced, ensuring that as computational cost increases, the number of candidate materials remains feasible.

We demonstrate the power of this approach by discovering new quaternary oxide materials for solar energy applications. SMACT is used to reduce the search space of billions to ~1 million chemically sensible compositions. A gradient boosting regression machine learning model is trained on a database² of high-quality bandgap calculations, then used to identify ~20,000 oxide compositions that are most likely to have useful bandgaps. A recent statistics-based approach to structure prediction using a probabilistic model proposed by Hautier et al.³ is employed to suggest ~500,000 likely crystal structures, which are then fed to the AFLOW-ML⁴ model to predict thermodynamic stability via machine-learnt DFT total energies. All of these steps can be carried out in a matter of hours to days, using minimal computing resources. The result is a series of potential new energy materials; our methodology can be applied to materials design in a range of contexts and is an important new tool in the quest for accelerated materials discovery.

1. D. W. Davies, K. T. Butler, A. J. Jackson, A. Morris, J. M. Frost, J. M. Skelton, A. Walsh, Chem, 2016, 1, 617.
2. I. E. Castelli, F. Hüser, M. Pandey, H. Li, K. S. Thygesen, B. Seger, A. Jain, K. A. Persson, G. Ceder, K. W. Jacobsen, Adv. Energy Mater., 2015, 5, 1400915
3. G. Hautier, C. Fischer, V. Ehrlacher, A. Jain, G. Ceder, Inorg. Chem., 2011, 50, 656.
4. O. Isayev, C. Oses, C. Toher, E. Gossett, S. Curtarolo, A. Tropsha, Nat. Commun., 2017, 8, 15679.

Potentially low-cost high-throughput approaches have been demonstrated that form inorganic semiconductor films directly from nanoparticle or molecular inks. The highest efficiency devices have been prepared with CuInGaSe2 utilizing hydrazine as a solvent and complexing agent. Here, we present our progress to develop of a class of solution-phase routes to Cu₂ZnSn(S,Se)₄, Cu(In,Ga)(S,Se)₂ and most recently bismuth halides that do not use suspensions of nanoparticles or hydrazine. We have discovered new effects of alloying and doping using a combinatorial ultrasonic spray coater and high-throughput screening method to map the optoelectronic properties of absorber layers. The presentation will focus on: (i) the formation of films and elimination of deleterious elements, (ii) incorporation of dopants and their effects of absorber properties and grain boundaries, (iii) alloying to form record high open circuit voltage relative to the maximum theoretical open circuit voltage for the bandgap, (iv) a new understanding of grain growth and impurity removal in kesterites, and recent results to yield tandem solar cells with hybrid perovskites.

[1] Xin, H., Vorpahl, S.M., Collord, A.D., Braly, I.L., Uhl, A.R., Krueger, B.W., Ginger, D.S., Hillhouse, H.W., Phys. Chem. Chem. Phys. 17, 23859-23866 (2015).
[2] Uhl, A.R., Katahara, J.K., Hillhouse, H.W., Energy & Environmental Science 9, 130-134 (2016).
[3] Collord, A.D., Hillhouse, H.W., Chem. Mater. 28, 7, 2067–2073 (2016).
[4] Clark, J.A., Uhl, A.R., Martin, T., Hillhouse, H.W., Chem. Mater. ASAP, DOI: 10.1021/acs.chemmater.7b03313, (2017).
[5] Uhl, A.R., Yang, Z., Jen, A.K.-Y, Hillhouse, H.W., J. Mater. Chem. A 5, 3214-3220 (2017).
[6] Williamson, B.W., Eickemeyer, F.T., Hillhouse, H.W., Submitted.

In this work, we present the effects of annealing on the structural and optical properties of polycrystalline ZnGeP₂ films on Si substrates. As structural analogs to III-V materials, II-IV-V₂ materials have the potential for optoelectronic applications beyond the present III-V materials. Currently, III-V optoelectronic devices enable fiber communications, solid-state lasers, light emitting diodes and high efficiency photovoltaics, but they rely on epitaxial heterostructures limited by lattice matching considerations. In contrast, II-IV-V₂ materials can be lattice matched to silicon and have the potential for tunable electronic properties for fixed composition through control of cation ordering. Therefore, implementation of a material with similar properties to the III-Vs and lattice matched with silicon could be transformative for tandem photovoltaics. ZnGeP₂ is one such material with a lattice matching within 1% of silicon and a band gap of 2.1 eV.
In order to control crystallinity and ordering independently of composition, stoichiometric amorphous ZnGeP₂ films were grown and then annealed ex-situ. The crystallinity and ordering of the annealed films was studied with x-ray diffraction and transmission electron microscopy. To gain an understanding of how optical properties change with structure, spectroscopic ellipsometery was used to determine the optical constants and absorption coefficient of the films. Using these characterization techniques, we have confirmed the ability to control crystallinity and ordering of the films as a function of anneal temperature and time. Subsequently, with changes in crystallinity and ordering we observed variations in the optical properties of the films. From our results we can conclude that ZnGeP₂ shows large potential for integration in Si-based devices.

Tunable electrical conductivity – the ability to change carrier concentration over a wide range of useful values while maintaining superior mobility – is arguably the central technological advantage of an Amorphous Oxide Semiconductor (AOS) such as ternary or quaternary oxides of post-transition metals, for example, In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O. Compared to the crystalline counterparts, where the electron mobility is governed primarily by scattering on ionized impurities, phonons, and grain boundaries, the nature of and the relationship between the carrier generation and transport in AOSs are more complex. Although amorphous materials lack grain boundaries, the strong local distortions in the Metal-Oxygen (M-O) polyhedra associated with a weak ionic M-O bonding as well as long-range structural correlations in the disordered system give rise to entangled transport phenomena at different length scales [1-3]. Given the many degrees of freedom in the amorphous structure, the long-range structural characteristics and the electronic properties of the donor defects in AOSs differ fundamentally from those in the crystalline transparent conducting oxides. Therefore, defects in AOSs must be considered along with the structural evolution of the disordered system.

In this work, we report the results of computationally intensive ab-initio molecular dynamics (MD) simulations combined with accurate density functional electronic structure calculations for amorphous In-based and Sn-based oxide semiconductors. Novel approaches for non-stoichiometric-melt cooling and time-dependent statistical analysis not only show a significant improvement over the standard oxygen vacancy models but also allow us to simultaneously address the structural morphology, evolution, and the dynamics of defect formation, thereby providing the necessary integral framework to comprehensively understand the fundamental materials properties of AOSs. We demonstrate that the approach provides a statistically complete defect picture of conducting amorphous oxides by capturing the formation of both shallow defects that produce carriers and localized deep defects that limit carrier mobility via electron trapping or scattering. The scheme also allows us to study the long-range structural reconstruction in AOSs and the defect dynamics during quenching or annealing processes providing the necessary information for optimizing the electronic and optical properties of AOSs toward their application in optoelectronic technologies.

[1] D. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J.E. Medvedeva, and R.P.H. Chang,
Chemistry of Materials, 26, 5401-5411 (2014).
[2] R. Khanal, D.B. Buchholz, R.P.H. Chang, and J.E. Medvedeva, Physical Review B, 91, 205203 (2015).
[3] J.E. Medvedeva, D.B. Buchholz, and R.P.H. Chang, Advanced Electronic Materials, 3, 1700082 (2017).

Ternary oxides represent a large but relatively little-explored class of semiconductors that are of interest for photoelectrochemical energy conversion applications. One of the best performing candidates thus far is BiVO₄, an n-type photoanode with a bandgap of 2.4 eV. One well known ‘trick’ to improve this material is doping. Donor doping with tungsten (W) supposedly improves the photocurrent by increasing the conductivity, but is also found to reduce the magnitude as well as the lifetime of the photoconductivity. Another dopant that was recently found to greatly improve the photocurrent of BiVO₄ is hydrogen [1]. We found that in contrast to tungsten, hydrogen actually increases the carrier lifetime and diffusion length in BiVO₄ [2]. This is due to the passivation of trap states, the possible origins of which will be discussed. A second ‘trick’ to improve the performance of BiVO₄ is by deposition of a cobalt phosphate (CoPi) co-catalyst on the surface. We recently showed that the main role of this ‘co-catalyst’ is not to enhance the charge transfer kinetics, but to passivate surface defects on BiVO₄ [3]. To get a better understanding of the chemical nature of these states, we employed ambient pressure resonant photoemission (AP-resPES) and operando XPS methods. These experiments revealed the presence of two separate states in the bandgap of BiVO₄ and a redistribution of the phosphate species in the electrolyte under illumination [4]. These initial results are the first steps towards a molecular-level understanding of the BiVO₄/electrolyte interface that may eventually help to design efficient solar fuel generators. In the last part of the talk I will discuss recent results on CuBi₂O₄, a photocathode material with a bandgap of ~1.7 eV. We developed a modified solution chemistry that enables us to spray highly homogeneous films that show photocurrent densities up to 2 mA/cm² [5]. To enhance the charge separation in these films, we introduced a gradient of copper vacancies across the film thickness. This results in the formation of a homojunction in CuBi₂O₄ that increases the carrier diffusion length and reduces charge recombination [6]. The resulting films showed AM1.5 photocurrent densities of up to 2.5 mA/cm² at +0.6 V vs. RHE in the presence of a hole scavenger, which is a new benchmark for this material.

References
[1] J. Cooper et al., Chem. Mater. 28, 5761 (2016)
[2] J.W. Jang et al., Adv. Energy Mater. 1701536 (2017)
[3] C. Zachäus et al., Chem. Sci. 8, 3712 (2017)
[4] M. Favaro et al., J. Phys. Chem. B (in press, DOI:10.1021/acs.jpcb.7b06942)
[5] F. Wang et al., J. Mater. Chem. A 5, 12838 (2017)
[6] F. Wang et al., J. Am. Chem. Soc. 139, 15094 (2017)

Improving the efficiency of solar-power oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier stymied by the difficulty in computational modelling of substitutional alloys and the high dimensionality of co-alloying composition spaces. Since substituional alloying simultaneously changes multiple materials properties, understanding the underlying cause for performance improvements is also challenging, motivating our application of combinatorial materials science techniques to map photoelectrochemical performance of 948 unique bismuth vanadate alloy compositions comprising 0 to 8% alloys of various p-block, alkalai earth, rare earth, and 4d & 5d elements (e.g Bi-V-A) along with a variety of compositions from each pairwise combination (e.g. Bi-V-A-B) of these elements. Upon identification of substantial improvements in the co-alloying Bi-V-A-B spaces, structural mapping was performed to reveal a remarkable correlation between performance and a lowered monoclinic distortion. First-principles density functional theory calculations indicate that the improvements are due to a lowered hole effective mass and hole polaron formation energy, and collectively, our results identify the monoclinic distortion as a critical parameter in the optimization and understanding of bismuth vanadate-based photoanodes.

Inorganic and hybrid organic–inorganic main-group halides that adopt the perovskite structure combine excellent performance in photovoltaic applications, ease of preparation, and abundant constituent elements, but the origins of their remarkable properties are a matter of debate [1]. Here, we address two unusual aspects of the crystal structure and dynamics of these materials which suggest the primacy of the group IV–halogen sublattice in dictating performance, and apply these insights to discover new optoelectronic materials via high-throughput computational screening.

First, X-ray scattering studies reveal local off-centering of the group 14 cations within their coordination octahedra across the materials class reflecting a preference for lower symmetry coordination than that implied by crystallographic approaches [2,3]. Ab initio calculations, optical measurements, and analogies to existing theory implicate the ns² lone pair electrons as the origin of this phenomenon, which we propose leads to enhanced defect screening, reduced thermal conductivity, and unusual temperature-dependence of the electronic bandgap [2]. We further demonstrate control of the strength of this phenomenon by chemical substitution on all sites of the perovskite [3].

Second, solid state nuclear magnetic resonance and dielectric spectroscopies reveal the full temperature-dependent dynamics of molecular reorientation in the high-performance formamidinium lead iodide [4]. Despite markedly different barriers for molecular rotation compared to those in the homologous methylammonium lead iodide, both systems exhibit similar dynamics at room temperature [4]. Together with the vastly different dipole moments for the two molecules, this result sheds light on emerging hypotheses of polaronic transport and transient Rashba–Dresselhaus effects.

Using design criteria based in part on these findings about the unusual electronic structure and lattice polarizability of the halide perovskites, we screen 54,000 compounds in the Materials Project database to identify candidate optoelectronic materials. Subsequent ab initio calculations and experimental preparation and screening are employed to test the validity of these criteria as predictors of high performance.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award number DE-SC-0012541.

1. D. H. Fabini, J. G. Labram, A. J. Lehner, J. S. Bechtel, H. A. Evans, A. Van der Ven, F. Wudl, M. L. Chabinyc, R. Seshadri, Inorg. Chem. 56 (2017).
2. D. H. Fabini, G. Laurita, J. S. Bechtel, C. C. Stoumpos, H. A. Evans, A. G. Kontos, Y. S. Raptis, P. Falaras, A. Van der Ven, M. G. Kanatzidis, R. Seshadri, J. Am. Chem. Soc. 138 (2016).
3. G. Laurita, D. H. Fabini, C. C. Stoumpos, M. G. Kanatzidis, R. Seshadri, Chem. Sci. 8 (2017).
4. D. H. Fabini, T. A. Siaw, C. C. Stoumpos, G. Laurita, D. Olds, K. Page, J. G. Hu, M. G. Kanatzidis, S. Han, R. Seshadri, J. Am. Chem. Soc. 2017, DOI:10.1021/jacs.7b09536.

P-type transparent conducting oxides (TCOs) present many exciting problems for materials scientists due to low conductivity arising from large hole effective masses. The applicability of TCOs to technologies like flat panel displays and photovoltaic cells establishes the need for complementarity between the well documented and commercially available n-type TCOs and p-type TCOs and motivates the search for a means to delocalize the O-2p states that limit shallow acceptors and lead to large hole effective masses. CuAlO₂ and AgAlO₂ (XAO) show promise as p-type TCOs due to the presence of X-3d/4d states, which hybridize with O-2p states and delocalize the valence states. Additionally, p-doping with Mg may further enhance conductivity of pristine XAO.
XAO exists as three polymorphs: the delafossites 2H and 3R, and an orthorhombic polymorph which is the least stable polymorph energetically. This study is restricted to the 2H polymorph since it is the least studied of the two delafossites. In this work, a theoretical study based on first-principles calculations is presented on pure and Mg-doped (replacing Al) 2H-XAO using density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP) code. Projector augmented wavefunction pseudopotentials with a cutoff energy of 400 eV are employed and the exchange correlation energy is treated using the generalized gradient approximation with the addition of a Coulomb interaction energy (Hubbard correction U) for the Cu-3d and Ag-4d states. Results are also obtained using the hybrid functional (HSE06) approach. Pure 2H-XAO is initially modelled using an 8-atom hexagonal primitive cell, which is used to relax the crystal structure. Band structure, density of states, and the complex dielectric function are obtained. By calculating the total energy per atom as a function of V/V₀, where V₀ is the relaxed volume, it was observed that the 2H(3R) polymorphs modelled using hexagonal primitive(unit) cell are equal in total energy down to 1 meV/atom at V=V₀, with the rhombohedral 3R model 90(37) meV higher for CuAlO₂(AgAlO₂). This suggests that the 2H polymorph may have a slightly lower total energy. Hole effective masses are determined using parabolic curve fitting of the lower band gap edge around the high symmetry k points of the first Brillouin zone.
Once the electronic structure of the bulk using the primitive cell is well described, 64-atom hexagonal supercells are used to model pristine and Mg-doped 2H-XAO. The electronic structures, densities of states, optical properties and hole effective masses for these systems are presented and discussed in the context of experimental results from literature. A discussion of the effects of Mg-doping on the optical properties and its effectiveness in reducing hole effective masses and increasing conductivity is also presented.

Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) due to its high carrier mobility (up to ~100 cm² V⁻¹ s⁻¹) and low absorption of visible-light. However, In-free TCOs have been sought for some time due to the cost and relative scarcity of In. Among these, alloys of ZnO and SnO₂ (commonly referred to as ZTO) offer acceptable electron mobility (>10 cm² V⁻¹ s⁻¹) and high thermodynamic stability [1]. Importantly, the large diameter spherical orbitals of the Zn²⁺ and Sn⁴⁺ cations in ZTO cause the carrier mobility to be insensitive to structural disorder [1]. Amorphous ZTO (a-ZTO), produced with low growth temperatures, therefore exhibits similar carrier mobility to its polycrystalline counterpart.

Here, we investigate the characteristics of a-ZTO deposited energetically using high power impulse magnetron sputtering (HiPIMS) [2]. This technique has proved capable of producing high quality metal oxide layers suitable for electronic devices [3, 4]. Reactive co-deposition from Zn (HiPIMS mode) and Sn (DC magnetron sputtering mode) targets yielded a-ZTO with varying Zn:Sn composition across a 4-inch diameter sapphire substrate. The electrical and optical properties of this film were then studied as a function of composition. As-deposited, the films were amorphous, transparent and semi-insulating. Hydrogen (a known n-type dopant in ZnO and SnO₂) was introduced into the a-ZTO by post-deposition annealing (1 h, 500 °C, 100 mTorr H₂) and resulted in significantly increased conductivity with no measurable structural alterations. After annealing, Hall effect measurements revealed n-type carrier concentrations of ~1 × 10¹⁷ cm⁻³ and mobilities up to 15 cm² V⁻¹ s^–1. These characteristics proved both stable and suitable for device applications. Results from transistors and memristors based on energetically deposited a-ZTO will be presented. These suggest that HiPIMS can produce dense, high quality a-ZTO suitable for electronic applications.

REFERENCES

[1] H. Chiang, J. Wager , R. Hoffman, J. Jeong and D. A. Keszler, High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layers, Appl. Phys. Lett. 86 (2005) 013503
[2] V. Kouznetsov, K. Macák, J. M. Schneider, U. Helmersson and I. Petrov, A novel pulsed magnetron sputter technique utilizing very high target power densities, Surface and Coatings Technology 122 (1999) 290–293
[3] J. G. Partridge, E. L. H. Mayes, N. L. McDougall, M. M. M. Bilek, D. G. McCulloch Characterization and device applications of ZnO films deposited by high power impulse magnetron sputtering (HiPIMS) , Journal of Physics D: Applied Physics 46 (16), 165105 (2013)
[4] B. J. Murdoch, D. G. McCulloch, R. Ganesan, D. R. McKenzie, M. M. M. Bilek, and J. G. Partridge, Memristor and selector devices fabricated from HfO_2-xN_x, Appl. Phys. Lett. 108, 143504 (2016)

Semiconductor compounds are widely used for photocatalytic hydrogen production applications, where photogenerated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr_1-xNbO₃, 0.03 < x < 0.20) were reported to show competitive photocatalytic efficiencies under visible light which was attributed to interband absorption. This discovery expanded the range of materials available for optimized performance as photocatalysts. Here we have studied epitaxial thin films of SrNbO_3+δ and found that their bandgaps are ~4.1 eV. Surprisingly the carrier density of the conducting phase exceeds 10²² cm^-3 and the carrier mobility is only 2.47 cm² V^-1 s^-1. Contrary to earlier reports, the visible light absorption at 1.8 eV (~688nm) is due to the plasmon resonance, arising from the large carrier density. We propose that the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation.

Reference:
1. D.Y. Wan, Y.L. Zhao, Y. Cai, et al., Nature Communications 8, 15070 (2017).
2. T.C. Asmara, D.Y. Wan, Y.L. Zhao, et al., Nature Communications 8, 15271 (2017).

The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Solar cells with the CZTS-based (Cu₂ZnSnS₄, Cu₂ZnSnSe₄, Cu₂ZnSn(S,Se)₄) absorbers are advantageous for lowing the cost, but the development of solar cells based on flexible substrates has been relatively unexplored. In this work, a flexible CZTSSe solar cell applying the Mo foil substrate was developed. The characteristics of solar cells were examined by applying the sputtering process for four precursor stacking orders (Cu/Sn/Zn/Mo foil, Cu/Zn/Sn/Mo foil, Zn/Sn/Cu/Mo foil, and Sn/Cu/Zn/Mo foil). In addition, NaF was deposited on the Mo foil, and the effects of Na were investigated. The samples were characterized by means of scanning transmission electron microscopy-energy dispersive spectrometry (STEM-EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), a solar simulator, external quantum efficiency (EQE) measurements, depth resolved Raman spectroscopy, and admittance spectroscopy (AS).
Acknowledge: This work was supported by by the DGIST R&D Program of the Ministry of Science and ICT (18-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).

Photon upconversion is a promising strategy to boost the conversion efficiencies of commercially available photovoltaics beyond the Shockley-Queisser limit by harvesting photons with energy below the band gap of a host cell and converting pairs of such low-energy photons into single high-energy photons that can be returned to and absorbed by the host cell. Existing upconversion materials based on lanthanide-doped nanocrystals and sensitized triplet-triplet annihilation have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency.¹ II-VI based semiconductor nanostructures offer a new paradigm for photon upconversion in which bandgap, relative band alignment, and carrier dynamics can be tailored through material composition, size, and morphology to achieve high quantum yield at desired wavelengths with broad spectral absorption.² Recent advances in the synthetic capabilities of semiconductor nanoparticles has enabled the fabrication of complex heterostructures composed of various materials. One class of new semiconductor heterostructures are colloidal double quantum dots (QDs), where two spatially separated semiconductor nanoparticles are electronically coupled.³ Colloidal synthesis of such semiconductor heterostructures offers a route to solution-compatible, low-cost processing. We report a semiconductor nanostructure upconverting platform consisting of coupled quantum dots (QDs)³ that efficiently upconverts low-energy photons under continuous-wave illumination. The nanostructure consists of a narrower bandgap tellurium-doped cadmium selenide QD absorber and a wider bandgap cadmium selenide emitter spatially separated by cadmium sulfide nanorod. The double quantum dot nanostructure is designed such that electrons promoted by a first photon absorption in the Te-doped CdSe absorber are delocalized over the entire structure but the holes remain confined. A second low-energy photon excites the hole via intraband absorption, allowing it to escape into the cadmium sulfide and ultimately relax into the emitter QD. We will present synthesis conditions, structural characterization, and detailed optical spectroscopy of the upconversion photophysics that collectively demonstrate control over the photon energies absorbed and the corresponding photon energy gain through control over the composition and size of the absorbing QDs.

1. Zhou, J., Liu, Q., Feng, W., Sun, Y. & Li, F. Upconversion Luminescent Materials: Advances and Applications Jing. Chem. Rev. 115, 395–465 (2015).
2. Sellers, D. G. et al. Novel nanostructures for efficient photon upconversion and high-efficiency photovoltaics. Sol. Energy Mater. Sol. Cells 155, 446–453 (2016).
3. Deutsch, Z., Neeman, L. & Oron, D. Luminescence upconversion in colloidal double quantum dots. Nat. Nanotechnol. 8, 649–653 (2013).

Compositionally graded In_xGa_1-xN layers of high structural and optical quality grown on Si have the potential for cheap and efficient solar harvesting systems¹. The theoretical maximum efficiency for tandem photoelectrochemical water-splitting systems could reach over 22.5% with Si as bottom cell and In_xGa_1-xN as top cell with a bandgap between 1.6-1.8eV, an indium content of 0.37-0.44, respectively^1,2. However, the use of In_xGa_1-xN as a water-splitting photoelectrode (PE) for solar hydrogen production has yet to show promising performance as initial demonstrations of In_xGa_1-xN on a GaN substrate have exhibited photocurrents below 0.1mA/cm² at AM1.5G irradiation, i.e. a current even below what has been reported for pure n-GaN³. We developed a numerical model of In_xGa_1-xN water-splitting PEs which aims at identifying and quantifying the losses in the system. The model included electromagnetic wave propagation calculations for the determination of the locally resolved light absorption and charge generation terms, which built the generation terms in the charge transport and conservation equations solved in the semiconductor. The charge transfer at the semiconductor-electrolyte interface, a boundary condition to our model, accounted for Fermi level pinning and a potential drop in the Helmholtz layer due to surface states. The complete numerical model was validated using linear sweep voltammograms of In_xGa_1-xN PEs grown by plasma-assisted molecular beam epitaxy with varying indium content (0.095, 16.5, 23.5, 33.3 and 41.4) and under varying light irradiance (1%, 10%, 50% and 100% of AM1.5G). Parametric analyses were performed on optical and electronic properties to identify key performance parameters and evaluate their impact on the performance of compositionally graded In_xGa_1-xN PEs.

Low cost and large-scale production of semiconducting materials requires development of solution processing routes. While developing such solution processing routes for photovoltaic systems, it is also important to achieve sufficient efficiencies of photovoltaic devices which will ultimately reduce the cost of power generation.
Numerous solution processing routes have been tried for fabricating various semiconducting films for solar energy conversion. Amongst which, Hydrazine has shown highest conversion efficiencies for materials like CIGS and CZTS. However, because of its explosive and carcinogenic nature, it is not feasible to use this chemical in any scale up production. On the other hand, amine-thiol system which is safer than Hydrazine has shown promising results in dissolving array of metals, metal salts, oxides, chalcogenides. Even though devices made with amine-thiol system have demonstrated promising efficiencies, the hydrocarbon chains in amine and thiol species results in formation of carbonaceous fine grain layer in the final film which affects the performance of photovoltaic devices.
Herein, we present a novel approach in utilizing amine-thiol chemistry for film fabrication. Understanding amine-thiol chemistry using various analytical techniques has enabled us in fabricating better quality films. This presentation will exclusively focus on fabrication of such two semiconducting thin film materials viz. CISe and Se-Te alloy. Amongst which, CISe is a low bandgap material and is of great interest for its application in fabrication of tandem solar architectures with high band gap materials like perovskite. We will discuss the effect of various precursor selection and various selenization conditions on CISe film quality and also will demonstrate the fabrication of working device from CISe system with more than 11% efficiency.
Along with novel approach for fabricating conventional material like CISe, we will also discuss our work in synthesizing a novel thin film alloy of Se and Te for photovoltaic application. While people have fabricated working devices with selenium as an absorber material for indoor applications, we will demonstrate an alloying process of Se with Te to tune the bandgap of the absorber material. Our understanding of chalcogen dissolution mechanism in amine-thiol system has made it possible to alloy two chalcogens to form crystalline phase material. This presentation will include synthesis as well as thin film fabrication from Se-Te alloy followed by primary attempt of making a working photovoltaic device with various architectures.
In conclusion, we have come up with better understanding of amine-thiol system which enabled us to develop novel approach for its use in better quality semiconducting films. The results of which are applicable to the synthesis of nanoparticles and films of a vast array of chalcogenides including Cu₂S, WSe₂, SnS, SnSe, FeS₂, CZTS and other Kesterites.

Solar energy is an abundant, clean and free access resource, but it require harvesting and storage for sustainable future. Photovoltaics or photocatalysis technologies dedicated to sun light conversions frequently involve photo-visible-responsive semiconductors [1], such us materials with formula BiMOS (M; Cu [2] or Ag). In this study, we applied a strategy of substitution of Cu by Ag to produce a new family of oxysulfide BiAgOS [3]. We were interested to address how the total substitution of Cu by Ag in BiCuOS system affect its crystal structure, optical and electronic properties by using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared by a hydrothermal method. Its structural, morphological, and optoelectronic properties were investigated and compared with those of BiCuOS. Rietveld refinement of the powder X-ray diffraction confirmed that BiAgOS has the same crystal structure as BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, are shifted toward lower angles, indicating an increase in the cell parameters. Combined with experimental measurements, density functional theory calculations employing the range-separated hybrid HSE06 exchange-correlation functional with spin–orbit coupling quantitatively elucidated photophysical properties such as absorption coefficients, effective masses, and dielectric constants. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals is located at a more positive potential than that of S and Cu leads to widening band gap. Both materials possess high dielectric constants and low electron and hole effective masses. BiAgOS has dielectric constant larger than BiCuOS making it very interesting for photoconversion applications because the material could efficiently screen photogenerated charges. By combining the UV-Vis, Mott-Schottky, and photoelectron spectroscopy in air measurements, the relatively low bandgap of and their p-type character, BiCuOS and BiAgOS can be considered as interesting starting compositions for the development of new semiconductors for photovoltaics or Z-scheme photocatalytic applications. Moreover, this study opened the window toward new oxychalcogenides materials BiAgOCh (Ch; Se, Te) which are not yet synthesis or investigate.

[1] P. Hoffmann, Tomorrow’s Energy; MIT Press: Cambridge, MA, 701, 2004; Vol. 1.
[2] W.C. Sheets, et al., Inorg. Chem., 2007, 46, 10741.
[3] A. BaQais, et al., Chem. Mater., 2017, 29 (20), 8679.

The high diffusivity of Cu⁺ ions in the hexagonal-close-packed structure of Cu_2-xS often leads to interesting and unexpected observations. Herein, we present the etching of oxide and sulfide thin film underlayers during the atomic layer deposition of Cu_2-xS thin films. The infiltration of the underlayers by Cu⁺ ions is an essential step that precedes the etching process. However, it is suspected that the eventual etching of the underlayer, and the etch rate strongly depend on the lattice (bond dissociation energy) of the underlayer material. Thin films of ZnS, ZnO, SnS, and SnO were etched to different degrees during the deposition of Cu_2-xS while SnO₂ exhibited a high resistance to etching. Interestingly, a selective removal of Zn²⁺ was observed when a ternary Zn_1-xSn_xO film was used as underlayer. Based on XPS results and findings from other supplementary experiments, a possible reaction mechanism was proposed for the etching process. Finally, the observation was extended to the synthesis of Cu_2-xS nanowires that can be used as effective absorbers for photovoltaic cells. Details of the findings of this work will be presented at the conference.

Solar cell efficiency could potentially be increased by exceeding the Shockley-Queisser limit through singlet fission. The Shockley-Queisser limit on photovoltaic efficiency is the theoretical maximum efficiency of a p-n junction. It states that nearly two-thirds of the light energy incident on a conventional photovoltaic material is not converted to electrical energy. Some of this lost energy could be harvested through singlet fission. Via the Dexter process, inorganic colloidal PbS nanocrystals are used to harvest the energy from triplet excitons generated by organic 1,6-diphenyl-1,3,5-hexatriene (DPH). Singlet fission (SF) is a spin-allowed process in which a high energy photon creates a singlet state in one chromophore, moiety of organic molecule responsible for absorption and emission of light. The singlet state splits into two triplet states—one in the original chromophore and one in a neighboring chromophore that is correctly orientated for electronic coupling. The Dexter process is a simultaneous, correlated transfer of an excited electron on one molecule (donor) to another molecule (acceptor) via a non-radiative pathway which depends on the wavefunction overlap between donor and acceptor. SF material has previously been made from organics, specifically polyacenes such as tetracene and pentacene. Polyacenes have been observed to have shorter triplet lifetimes and energy triplets compared to DPH. This makes DPH a prominent candidate to produce a more efficient SF material with a higher SF yield. The Wittig Reaction is utilized to synthesize DPH from derivatives with specific functional groups that allow DPH to bind to PbS. Energy transfer from DPH (donor) to PbS (acceptor) is characterized by absorbance and emission measurements.

In past few years, transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), have attracted great research interests due to their superior electronic, optical, and mechanical properties. In particular, excellent absorbance (5-10 %)^[1] and high quantum efficiency (~10⁴ %)^[2] have allowed TMDs to be exploited as channel materials in high-performance photodetectors with high responsivity and high detectivity. However, most TMD photodetectors operated in the limited detection range from ultraviolet to visible region (10 ~ 780 nm) because TMDs generally have an energy bandgap between 1 and 2 eV. Although it was possible to detect 1064 nm and 980 nm lights by utilizing rhenium diselenide (ReSe₂) and multi-layer MoS₂ with relatively small bandgap (1 and 1.3 eV, respectively)^[3], no further researches to detect lights with longer wavelength by using TMD materials were reported yet.

Here, we demonstrate a highly responsible IR photodetector operating based on the interlayer optical transition phenomenon. This IR photodetector formed on ReS₂/ReSe₂ heterostructure not only extended the detection range up to IR region (1310 nm) that individual ReS₂- and ReSe₂- photodetectors cannot cover, but also maintained high responsivity even under IR region (3.64 × 10⁵ A/W at λ = 980 nm and 1.58 × 10⁵ A/W at λ = 1310 nm). The small interlayer bandgap of 0.62 eV formed at the ReS₂/ReSe₂ heterojunction interface lead to the interlayer optical transition phenomenon, consequently enabling photocarriers to be easily generated with less energy than the band-to-band transition.


References

[1] J. Shim, H.-Y. Park, D.-H. Kang, J.-O. Kim, S.-H. Jo, Y. Park, J.-H. Park, Adv. Mater. 29 39 (2017)
[2] S. Yang, S. Tongay, Q. Yue, Y. Li, B. Li, F. Lu, Sci. Rep. 4 5442 (2014)
[3] S.-H. Jo, H.-Y. Park, D.-H. Kang, J. Shim, J. Jeon, S. Choi, M. Kim, Y. Park, J. Lee, Y. J. Song, S. Lee, J.-H. Park, Adv. Mater. 28 6711 (2016)

Rare earth (RE) elements are frequently employed in many applications, such as catalysis, magnetism, phosphor lighting etc., due to their unique properties stemming from the steady 4f energy levels. Recently, there has been a push to replace these RE elements due to increased economic and national security issues. One proposed family of alternatives; are transition metal (TM) dopants, but are typically avoided because of their field dependent properties. For example, the strong overlap of the TM d orbitals with surrounding ligand is often viewed as an unfavourable phenomenon for luminescent or magnetic applications. In the current project, the field dependent hybridization of TM is utilized to engineer the d energy levels, allowing for controlled optoelectronic properties of TM ion.
In this work, thin films of TiO₂:Ni (~ 50 nm) were deposited onto Si substrates by employing sol-gel chemistry and spin-coating techniques. Weak localized external fields were created by surface functionalization of these films with benzoic acid ligands. Initial structural and optical characterization on TiO₂:Ni was performed to determine the crystal structure and crystal field splitting. X-ray photoelectron and soft X-ray absorption spectroscopy studies were used to investigate the ability to manipulate Ni core and valence levels without affecting the oxidation states or crystal structures. Additionally, the band structure of surface modified TiO₂:Ni, investigated by a combination of ultraviolet photoelectron and X-ray emission spectroscopy techniques, revealed that the interband Ni 3d states suffered a shift towards/away from the valence band depending upon the electron withdrawing/donating nature of the ligand. Furthermore, this phenomenon was theoretically supported by the ligand field multiplet calculations and time-dependent density functional theory simulations on bulk-mimicking TiO₂:Ni²⁺ clusters. These preliminary results on TiO₂:Ni demonstrate that the reversible tuning of TM d energy levels can be exploited to as the first step to the substitution of RE elements in phosphors that can have controlled luminescent properties.

Inorganic semiconductor based pn junctions constitute an integral part of most optoelectronic devices, such as photovoltaic (PV) solar cells and light emitting diodes (LEDs). Although the inorganic materials offer higher stability and superior electro-optic properties relative to organics, they present exorbitant cost and scale-up challenges. This paper presents a radically different approach to circumvent such challenges. It takes advantage of naturally formed nanocrystalline pn junctions, resulting from judicious coupling of semiconductor materials and electrochemical processes. Certain combinations can lead to highly practical inorganic material systems, comprising process-induced bulk pn junction nanostructures. The paper will illustrate an exemplary material system, based on single-step electrodeposited copper-indium-selenide (CISe) ordered defect chalcopyrite compounds. Thermodynamically driven reactions enable the electrodeposition of highly-ordered, interlinked, space-filling CISe films, in a single step. This approach creates a low-cost processing platform to produce nanocrystalline films, with all the attributes necessary for efficient bulk homojunction (BHJ) operation. Surface analytical microscopies and spectroscopies reveal unusual phenomena and extraordinary electro-optical properties that could potentially maximize spectral absorption and reduce recombination loss. They support the fortuitous formation of surprisingly ordered, sharp, abrupt 3-dimensional, nanoscale CISe pn BHJs. The specific BHJ structure enables efficient separation and transport of free carriers and essentially performs the same functions as planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals used in state-of-the-art BHJs; they exhibit mixed conductivity and high doping densities. These distinctive innate attributes of CISe films naturally create ordered nanoscale morphology and facilitate interconnections between the nanocrystals to form the BHJ structure. This totality manifests a highly significant advance in semiconductor processing as it creates an accessible, low cost solution-based method to fabricate high quality pn BHJ nanocrystalline material systems that can be directly used in PV or LED devices. Furthermore, with the incorporation of finely band-aligned contact electrode materials, the CISe BHJ film can transition into high performance flexible devices and roll-to-roll processing in simple thin-film form factor for easy scale-up.

Mid-infrared detection devices, which are employed in medical imaging, gas sensing, security surveillance and navigation sensing system applications, are essential components in an internet of things platform. InSb is one of the promising candidates for mid-infrared detection devices. It has a room temperature bandgap energy of 0.17 eV and is capable of detecting photon with a wavelength up to 7.3 µm. Growth of InSb on Si overcomes the restrictions of InSb substrate, which are expansive, small size (< 4 inches in diameter) and low ruggedness [1]. Hetero-epitaxy growth of InSb devices on Si is also one of approaches to realize the integration of mid-infrared InSb with Si-based electronic devices on a single wafer, without requiring any wafer bonding process. However, the growth of InSb on (100) Si substrate is challenging due to their large lattice mismatch (19.3 %) and different lattice structures (zinc blende vs. diamond), resulting in high density of defects. Furthermore, direct growth of InSb on (100) Si surface is prohibited because of the formation of In metallic islands on Sb-terminated Si surface instead of InSb film [2]. Therefore, an intermediate buffer layer between InSb layer and Si substrate is needed. In this report, we demonstrated two different intermediate buffer: Ge/GaAs buffer and AlSb/GaSb buffer.
Growth of InSb on a 6° offcut Si substrate using an AlSb/GaSb intermediate buffer was carried out using a molecular beam epitaxy (MBE) system. A 5nm AlSb island was firstly grown, followed by a 100 nm GaSb layer. Subsequently, a 50 nm AlSb layer was grown. Finally, a 0.8 µm InSb was grown on the AlSb surface using interfacial dislocation array to accommodate lattice mismatch. At the initial growth stage of InSb, a spotty RHEED pattern was observed and changed to a clear (1×3) after ~1 minute of growth. Using this InSb layer, an InSb photoconductor was fabricated and its photo-response was measured.

In Ge/GaAs buffer, growth of Ge on Si substrate was carried using a MOCVD system. Growth of GaAs and InSb was carried using a MBE system. Lattice-mismatch (14.6%) strain between GaAs and InSb was accommodated by an interfacial misfit (IMF) array formed at InSb/GaAs interface, which consisted of uniformly distributed 90° misfit dislocations [3]. TEM observation exhibited a low defect density in InSb layer. An InSb p-i-n photo-detector structure was grown. Spectral response of the InSb photodetector with the detector area of 0.0285 mm² was measured using a Fourier Transform Infrared (FTIR) spectrometer with a KBr beam splitter from 80 K to 200 K.


Reference
[1] J.I. Chyi, D. Biswas, S. Iyer, N. Kumar, H. Morkoc, R. Bean, K. Zanio, H.Y. Lee, H. Chen, Appl. Phys. Lett. 54(11) (1989) 1016-1018.
[2] G. Franklin, D. Rich, H. Hong, T. Miller, T.-C. Chiang, Phys. Rev. B 45(7) (1992) 3426.
[3] Jia, B. W.; Tan, K. H.; Loke, W. K.; Wicaksono, S.; Yoon, S. F., Mater. Lett. 2015, 158, 258-261.

Zinc oxide (ZnO) nanorods (NRs) have attracted lots of attention as a potential material for light-emitting diode (LED) applications, due to its outstanding characteristics such as large direct band gap and high exciton binding energy. However, despite these advantages, ZnO NRs are not used in LEDs due to several problems. Since ZnO NRs have naturally n-type conductivity owing to donor type defects in the lattice, the achievement of p-type conductivity is difficult. As a result, most studies have employed heterojunctions which use alternative p-type materials to fabricate LEDs using ZnO NRs. But, heterojunction LEDs generally exhibit poor luminescent efficiency due to lattice mismatch at the p-n junction interface. Another difficulty in fabricating ZnO NRs-based LEDs is the initial growth control of vertical ZnO NR on conventional electrodes, which is due to different crystal structure between the ZnO NR and the electrodes.
In this report, to overcome the problems of ZnO NRs, an Ag bottom electrode was employed as a multiple role implementing layer, which acted as a dopant source for p-type ZnO NRs and a growth template for ZnO NRs. During growth of ZnO NRs on the Ag bottom electrode without seed layer, Ag⁺ ions are dissolved from the Ag bottom electrode and then incorporated into the ZnO lattice. Then, the n-type ZnO NRs were homoepitaxially grown on these Ag-doped p-type ZnO NRs to fabricate a highly efficient p-n homojunction LED with minimum interface defects. These ZnO NRs p-n homojunction LEDs showed a typical rectifying behavior with a turn-on voltage of 3.5 V and a high rectifying ratio of 1.5 × 10⁵ at 5 V. Furthermore, under a forward bias of 9 V, the LED exhibited a wide yellow EL emission centered at 645 nm, which was attributed to the various emission sites of ZnO deep-level defects.

Keywords: Ag doping, p-type ZnO NRs, Homoepitaxial p-n junction, Solution process, Light-emitting diode.

A typical perovskite solar cell consists of a hole transporting layer (HTL), a perovskite light absorber and an electron transporting layer (ETL). The most commonly studied HTL’s are organic/polymeric materials such as Spiro-OMeTAD(2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene ) or PTAA(poly(triaryl amine)), which are expensive and unstable in the presence of water vapor.

This presentation describes the application to perovskite solar cells of a new kind of inorganic HTL synthesized using atomic layer deposition (ALD). By alloying TiO₂ with IrO_x in a super-cycle ALD process, we found that the electron transporting material TiO₂ becomes an effective HTL. Atomic layer deposition offers the prospect of depositing such hole contact materials over a wide variety of substrate materials and geometries, and with excellent control of layer thickness and composition. Previous research [1] on ALD-grown alloys of TiO₂ and RuO₂ showed that a large work function (~ 5.2 eV) can be achieved as result of a ~ 10 mol% RuO₂ addition. In the current work, a Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ PSC including a TiO₂-IrO_x HTL of composition 20mol% IrO_x and ~ 10 nm thickness achieved a high power conversion efficiency of 13.4% with 1.004 V open circuit voltage, which is 100 mV higher than the V_oc of the reference device using a PEDOT:PSS. These results indicate that ALD-grown TiO₂-transition metal oxide alloys are promising HTL’s for perovskite photovoltaics.

References
[1] O.L. Hendricks, A.G. Scheuermann, M. Schmidt, P.K. Hurley, P.C. McIntyre, and C.E.D. Chidsey, “Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO₂–RuO₂ Alloy Schottky Contacts for Silicon Photoanodes,” ACS Appl. Mater. Interfaces 8, 23763-73 (2016).

In the search for low-cost, high efficiency solar cells, researchers have been investigating new materials to use as carrier-selective contact layers. Sub-stoichiometric molybdenum trioxide (MoO_3-x ) is a promising material for interface contacts due to its hole selectivity and low parasitic absorption. It has been investigated as a contact for a number of different photovoltaic devices using different absorber materials, including ZnO/PbS quantum dot, CdS-CdTe nanotubes, CuInSe, and kesterite, as well as organic materials, perovskites, and crystalline silicon.

The performance of these contacts depends on the electrical and chemical properties of MoO_3-x. Of particular interest are the bandgap, electron affinity, and the presence of a defect band, the latter imparting n-type semiconducting properties to the metal oxide. These material properties have been found to be sensitive to the presence of intrinsic defects, especially oxygen vacancies. An understanding of how the defect chemistry of MoO_3-x varies with preparation conditions could allow the material properties of MoO_3-x to be optimised for interfaces with different absorber materials.

This paper reports on the use of density functional theory (DFT) simulations to predict defect concentrations as a function of temperature and oxygen partial pressure for crystalline MoO_3-x, by constructing Brouwer diagrams. Additionally it is shown that samples prepared in contact with silicon may be prone to contamination under common preparation conditions, and this contamination can affect the electronic structure of the material. It is therefore reasonable to assume that contamination of MoO_3-x may also occur for other absorber materials and the implications of this contamination in terms of device performance and durability needs to be considered. We then extend this analysis into the properties of amorphous MoO_3-x by means of reverse Monte Carlo modelling based on experimental thin film diffraction data. This extension is of particular value because most commonly-used preparation methods typically result in an amorphous metal oxide. This study provides a critical theoretical contribution to our understanding of the role of defects in transition metal oxide functionality as a carrier-selective contact for photovoltaic devices.

Thin-film solar cells provide an alternative to conventional silicon-based photovoltaics, and modules based on thin film cadmium telluride (CdTe) are cost-competitive with silicon. Device efficiency has increased from 16% to 21% in the last 5 years. Key factors enabling the improvements have been alterations in the device structure, including replacement of the traditional cadmium sulphide (CdS) window layer with higher band-gap alternatives like magnesium-doped zinc-oxide (MZO), and grading/alloying of the near-interface region in the CdTe absorber layer with selenium. However, research on devices incorporating these changes is limited. Little high resolution microstructural/chemical/electronic characterisation has been published. Improved characterisation, and hence understanding, of these devices, can lead to improved cell design/processing for enhanced performance, taking record cell efficiencies closer to the thermodynamic limit of ~30%.

In this work, devices with three architectures are characterised by high resolution dynamic SIMS (‘NanoSIMS’) giving 3-Dimensional chemical maps of the cells at nanometre resolution. This is supplemented by correlative high resolution EBSD and cathodoluminescence measurements, to establish the effects of observed microstructural and chemical features on performance. The three absorber/buffer layer architectures are: 1) traditional CdS/CdTe; 2) MZO/CdTe; and 3) high efficiency, MZO/CdSeTe/CdTe, selenium-graded devices. This set spans the evolution from more conventional device structures to the newer, high-efficiency structures.

The advantage of NanoSIMS is that it’s high sensitivity, combined with high resolution, can directly detect the behaviour of chlorine in grain interiors. This means that, where present, variations in chlorine concentrations can be observed, both between grains and within grains. For instance, data shows concentration gradients within grains that indicate ingress of chlorine from grain interiors to the grain bulk. These local variations in grain interior chlorine concentration can then be correlated to the local luminescence intensity, to ascertain whether chlorine effects carrier recombination activity in the grain bulk. Another interesting behaviour observed is the segregation of chlorine to linear or ribbon-shaped features within many of the grain interiors in all three device types. The 3D nature of the data enables behaviour of chlorine at the three different interfaces to be tracked.

In addition to chlorine, the chemical maps show the three-dimensional behaviour of sulphur and selenium alloying in each of the device types.

A fundamental understanding of carrier transport is imperative for efficient semiconductor electronics and optoelectronics. Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its armchair and zigzag directions. Here we report a direct observation of carrier transport transition dynamics from phonon scattering transport to multiple trapping and release transport mechanism along the armchair direction, resulting from the relaxation of free carriers above the band edge to the band-tail states. We identified that the suppression of phonon scattering effects, a characteristic by Hall and field effect transistor measurements, is due to carrier transport in band tail states. Along the zigzag direction, only multiple trapping and release transport in band-tail states is observed, which might be due to low carrier mobility.

2D van der Waals heterostructures with different types of band alignment have recently attracted great attention due to their unique optical and electrical properties. Most 2D heterostructures are formed by transfer-stacking two monolayers together, but the interfacial quality and controllable orientation of such artifcial structures are inferior to those epitaxial grown heterostructures. Herein, for the frst time, a direct vapor phase growth of high-quality vertically stacked heterostructure of SnS2/MoS2 monolayers is reported. An extremely Type II band alignment exists in this 2D heterostructure, with band offset larger than any other reported. Consistent with the large band offset, distinctive optical properties including strong photoluminescence quenching in the heterostructure area are observed in the heterostructure. The SnS2/MoS2 heterostructures also exhibit well-aligned lattice orientation between the two layers, forming a periodic Moiré superlattice pattern with high lattice mismatch. Electrical transport and photoresponsive studies demonstrate that the SnS2/MoS2 heterostructures exhibit an obvious photovoltaic effect and possess high on/off ratio (>106), high mobility (27.6 cm2 V-1 s-1) and high photoresponsivity (1.36 A W-1). Effcient synthesis of such vertical heterostructure may open up new realms in 2D functional electronics and optoelectronics.

High quality WS₂ film with the single domain size up to 400 microns was grown on Si/SiO₂ wafer by atmospheric pressure chemical vapor depos