Symposium EE1 : Emerging Materials and Phenomena for Solar Energy Conversion

With the record power conversion efficiency surpassing 20%, inorganic-organic hybrid perovskite has been the center of attention for many researchers in the field of photovoltaics and in the related fields. Different approaches are being used to prepare perovskite films such as vacuum evaporation, one-step solution process, or two-step solution process. In each approach, a great number of variations in the processing conditions are noted in the literature. Narrowing down to two-step process of perovskite films, where precursor solutions (PbI₂ and CH₃NH₃I solutions for CH₃NH₃PbI₃ perovskite) are sequentially spin-coated, differing ratios of molar concentration and volume of precursor solutions have been reported but discussion of their impacts is often absent or limited to the performance of the final solar devices. Here, we report a comprehensive study of how processing conditions in two-step solution process affects structural, chemical, and electrical properties of the perovskite films as well as the device performance of the resultant solar cells. In contrast to common anticipation, we find that the presence of a small amount of remnant PbI₂ that has not reacted with CH₃NH₃I to form perovskite helps improve the efficiencies as well as reduce hysteresis of current-voltage characteristics of the final solar cells. This is explained by the changes in fundamental materials properties of the perovskite films. Details of materials characterization of the perovskite by transmission electron microscopy, temperature-dependent capacitance and Hall measurements and the suggested links between materials properties and device performance will be discussed.

The recent success of methylammonium lead iodide perovskite solar cells has caused the photovoltaic research community to reconsider its criteria for what makes a promising absorber material. In particular, prior screening efforts have focused extensively on optical properties and often neglected transport. Our group has recently made an effort to further formalize selection criteria based not only on bandgap and absorption but also on computationally accessible properties that can enable improved carrier transport – even in the presence of defects. [1]

These properties include a high dielectric constant, disperse band edges (which can result from spin-orbit coupling prevalent in materials composed of heavier atoms), and antibonding character of the valence band maximum. The latter is observed in materials containing partially oxidized p-block cations, in which the cation lends antibonding s-orbital character to the VBM.

We have identified several classes of iodide materials predicted to exhibit these characteristics, in addition to the lead halide perovskites. In this work, we experimentally evaluate several of these compounds, including BiI₃ [2], SbSI, and BiOI. We grow these materials by a variety of methods, including physical vapor transport, solution synthesis, and Bridgman growth. We perform both spectral and time-resolved photoluminescence measurements to assess optical properties as well as minority carrier lifetime. In the present work, we identify several candidates with carrier lifetimes ranging from >200 ps to the nanosecond scale. By performing measurements on bare films without the need to fabricate devices, we can both speed up the cycle of learning and minimize the convoluting effects of other device layers. This efficient experimental validation in tandem with a computational search focused on minority carrier transport properties, may help accelerate the discovery of new promising PV absorbers.

[1] Brandt, R. E., Stevanović, V., Ginley, D. S., & Buonassisi, T. (2015). Identifying defect-tolerant semiconductors with high minority-carrier lifetimes: beyond hybrid lead halide perovskites. MRS Communications, 5(2), 1–11. doi:10.1557/mrc.2015.26

[2] Brandt, R. E., Kurchin, R. C., Hoye, R. L. Z., Poindexter, J. R., Wilson, M. W. B., Sulekar, S., … Buonassisi, T. (2015). Investigation of Bismuth Triiodide (BiI 3 ) for Photovoltaic Applications. The Journal of Physical Chemistry Letters, 151012112305004. doi:10.1021/acs.jpclett.5b02022

Perovskite based solar technologies are generating a great deal of interest in the materials community. In this work, we plan to discuss our ongoing work to characterize new synthesis routines and processes to generate sustainable and reliable perovskite-based materials whose properties are significantly better than the current state of the art. In particular, we are utilizing the latest advances in high-resolution analytical and in situ microscopy to characterize these emerging photovoltaic materials and their interfaces, defects, and discrete paths to crystallization.

There are longstanding interests in the relation between growth, microstructure, defects, electronic structure, and electro-optical activity for two reasons. The first reason is studies suggest there is a great deal of variability in the growth of these materials. The fundamental origins however remain nascent due to the current novelty of these materials and the complexities associated with studying beam-sensitive materials. Furthermore, beyond static conditions, observing transient behavior associated with the growth of these materials is of increasing importance to set future research directions for generating next generation perovskite-based solar cells. The second reason is a more complete understanding of the microstructure, growth defects, and doping behavior and how they affect the efficiency of devices will be crucial in developing both sustainable and efficient perovskite-based solar technology in the future.

This talk will present the correlations between our current ongoing observations and measurement of the optical properties, microstructure, defects, and crystallization associated with perovskite-based solar cells. The guided use of the latest high-resolution state-of-the-art high resolution analytical and in situ (scanning) transmission electron microscopy (S/TEM) techniques to examine growth, crystallization, and material stability of this exciting class of materials will be discussed at length.


This work was supported by the National Renewable Energy Laboratory as a part of the Non-Proprietary Partnering Program under Contract No. DE-AC36-08-GO28308 within the U.S. Department of Energy. Other parts of the TEM work were performed at the LeRoy Eyring Center for Solid State science at Arizona State University.

Hybrid organic-inorganic materials have recently generated considerable interest for photovoltaics, owing to the rapid rise in efficiency of methylammonium lead halide perovskite solar cells. But the widescale deployment of these perovskites is currently limited by their decomposition in the presence of humid air and environmental concerns with the lead content. It is essential to develop lead-free alternatives that are stable and efficient. Traditional methods for designing new solar absorbers have been empirical and slow, with many materials not reaching their theoretical performance, often limited by short minority carrier lifetimes. We have recently developed new strategies for computationally identifying materials likely to have favorable optical and transport properties based on their electronic structure, effective mass and dielectric constant. One material we predicted is methylammonium bismuth iodide (MBI). In our work, we show that Rietveld refinement of the powder X-ray diffraction pattern of MBI reveals a monoclinic unit cell consisting of Bi₂I₉^3- groups alternating with (CH₃NH₃)⁺ cations. We synthesize MBI thin films by low-temperature solution processing. These thin films are stable under ambient air (61±4% relative humidity and 21.8±0.7 °C temperature) for a month, whereas methylammonium lead iodide degrades to PbI₂ after only five days. Thermogravimetric analysis, X-ray photoelectron spectroscopy and X-ray diffraction are used to examine the causes for the differences in stability between the two materials. We also find that MBI has a high absorption coefficient approaching 10⁵ cm^-1 and indirect bandgap of 2.04 eV, which is suited to forming an efficient tandem with Si. MBI luminesces at room temperature, and we find that the photoluminescence decay times can be increased by treating the films with pyridine and using higher temperature vapor conversion to form larger grains. We use vapor growth to produce long-lifetime, continuous MBI thin films that we explore in devices. The features of MBI amenable to stability and long lifetimes are shared by other hybrid ternary bismuth halides, and this work identifies this family of materials as promising alternatives to the perovskites to produce efficient, lead-free, stable solar absorbers.

One of the most promising approaches to boost efficiency in photovoltaics is to make tandem solar cells. The challenge is to find a way to make this approach economically viable. Over the past few years, organic–inorganic lead halide perovskites have emerged in the PV technology scenario. The rapid progress in the field has led to certified solar energy conversion efficiencies above 20%, which brings this technology to the cohort of highly performing solar cells, while keeping the promise of low cost energy production. All-perovskite-based tandem solar cells may represent the ultimate goal considering the relatively easy tunability of the semiconductor’s bandgap. However, on a shorter term, the realization of hybrid systems which couple perovskite devices with market-leading technologies is highly attractive. For achieving these targets we need to obtain perovskite solar cells with high steady state efficiency (and thus reliable) and to deliver a device architecture and fabrication process that enables multi-layered structures.

We present¹ here a planar perovskite solar cell with a stabilized power conversion efficiency (PCE) of 17.6% at the maximum power point and a PCE of 17% extracted from quasi-static J–V with an open-circuit voltage of 1.11 V. Such excellent figures of merit can be achieved by engineering a solution-processed electron buffer layer that does not require high temperature steps. A compact thin film of perovskite absorber is grown onto a PCBM-based electron extraction layer by implementing a novel two-step procedure which preserves the soluble organic interlayer during the deposition of successive layers. . This has a broader impact on the design and optimization of future charge extracting layers as their choice will not be limited anymore by the processing of the top layers.
By using transient absorption spectroscopy we also evidenced the role of charge extraction in reducing the detrimental effects related to slow transient phenomena. Efficient charge extraction achieved with the use of 60-PCBM as electron extracting layer makes the device much less sensitive to the device polarization, thus producing an inherently more stable device. On the other hand, in the presence of a flat TiO₂ layer, the electron extraction is strongly dependent on the pre-polarization conditions, thus undermining device stability.

1. Chen Tao et al, Energy Environ. Sci., 2015, 8, 2365--2370

Numerous requirements are imposed on photovoltaic materials subject to their specific applications. Above all, high efficiency is of paramount importance. But beyond that, myriad issues such as production costs, temperature budgets that impact compatibility with flexible substrates, toxicity, availability and cost of constituent elements, and industrially expedient methods of fabrication can populate the list. While it is clear that no one specific photovoltaic technology can provide the answers to all these issues, earth abundant multinaries such as CZTS,Se and its variants are a sub-group of materials that hold great promise along with significant challenges. Comprised of relatively inexpensive and widely available elements, these materials were thought to provide the solution to terawatt level renewable energy generation. But, as we have come to understand, a high density of defects have conspired to limit open circuit voltage (Voc) and hence power conversion efficiency. In this talk I will review the status of our understanding of these voltage limiting defects. This knowledge has motivated our efforts along two lines; engineering of the back contact to increase Voc, and elemental substitution to reduce intrinsic defects. Back contacts consisting of high work function materials exploit electrostatics to drive higher Voc. These effects are supported by WXAMPS device simulations and confirmed with experimental results that show an increase in Voc when high work function back contacts are employed in conjunction with careful control of absorber thickness. In the realm of elemental substitution l will describe our recent results from alloying Ag with CZTS,Se over a range of concentrations. Both experiment and ab-initio calculations indicate that Ag provides as much as a 10X reduction in the density of Cu/Zn antisite defects that are primarily responsible for the low Voc that plagues CZTS,Se absorbers. Fundamental properties, electrical measurements and potential applications of multinary photovoltaic devices will be presented.

*Work done in collaboration with P. Antunez, T. Gershon, O. Gunawan, Y. Lee and T. Gokmen at IBM, A. Kummel, E. Chagarov and K. Sardashti at UC San Diego and D. Bishop at the University of Delaware. This work was carried out under support from the DoE Sunshot program under contract DE-EE0006334

Cu₂ZnGeSe₄ and Cu₂ZnSnSe₄ are quaternary semiconductors belonging to the adamantine compound family, contain only abundant elements, which makes these materials promising candidates for engineering on their base of different high-efficient and low-cost devices: solar cells, optical filters[1] and are considered as very interesting, also due to their possible applications in optoelectrics and non-linear optics [2,3]. Cu₂ZnSn(S_1-xSe_x)₄ solar cells with Ge alloying recently reached efficiency of 8.4% [2]. Cu₂ZnSnSe₄ crystallizes in the kesterite type structure which can be derived from the cubic sphalerite type structure by doubling the unit cell in the direction of the crystallographic c-axis and an ordering of the cations [4]. X-ray diffraction used for structural characterization of Cu₂ZnGeSe₄ was reported in the literature, and it suggests that Cu₂ZnGeSe₄ shows the tetragonal stannite type structure [5] In contrast to these findings recent first principal calculation predicts the kesterite type phase to be the ground state structure for this material [6]. A differentiation between the isoelectronic cations Cu⁺, Zn²⁺and Ge⁴⁺ and consequently kesterite and stannite is not possible using X-ray diffraction due to their similar scattering power. But neutrons diffraction can solve this problem; the coherent scattering lengths are sufficiently different [7]. By this method our group could show that both Cu₂ZnSnSe₄ and Cu₂ZnGeSe₄ occur in the kesterite structure. [4, 8].
A detailed structural analysis of Zn rich off-stoichiometric (B-F and F-D type mixtures) Cu₂Zn(Sn_1-xGe_x)Se₄ powder samples with x=0, 0.02, 0.16, 0.5, 0.55, 0.70, 0.76, 0.90, 1, grown by solid state reaction, was performed by neutron diffraction at the fine resolution neutron powder diffractometer E9 at BER II (λ = 1.7986 Å, RT). Rietveld refinement of diffraction data using the FullProf suite software [9] lead to accurate values of a and c lattice constants and site occupancy factors. The latter have given insights into the cation distribution within the crystal structure of Cu₂Zn(Sn_1-xGe_x)Se₄ solid solutions with different x values. The correlated information about changes in lattice parameters and cation site occupancies, details on the existing intrinsic point defects and their concentrations will be discussed.
This research was supported by KESTCELLS 316488, FP7-PEOPLE-2012 ITN, Multi-ITN project.
[1] K. Tanaka et. al., Sol. Energy Mater. Solar Cells 91 (2007) 1199
[2] Q. Guo et al., Sol. Energy Mater. Sol. Cells, 105 (2012)132.
[3] M. Ibañez et al., J. Am. Chem. Soc. 134 (2012) 4060.
[4] S.Schorr, Solar Energy Materials and Solar Cells, 95 (2011)1482.
[5] O.V. Parasyuk et. al., J. Alloys Comp. 329 (2001) 202–207.
[6] S. Chen et.al., Phys. Rev. B, 82 (2010), p. 195203 (8pp.)
[7] V.F. Sears, Neutron News 3 (3), 26–37, (1992).
[8] G. Gurieva, D.M.Többens, M. Valakh, S.Schorr, Phys and Chem of Solids. Submitted.
[9] Juan Rodriguez-Carvajal and Thierry Roisnel, www.ill.eu/sites/fullprof

We demonstrate by means of direct determination of the site occupancies from anomalous X-ray powder diffraction data at the Cu and Zn absorption edges the ordering of Cu⁺ and Zn²⁺ in B-type Cu₂ZnSnSe₄ (CZTSe) kesterite upon annealing at temperatures below 180°C.
Cu₂ZnSn(S,Se)₄ (CZTSSe) semiconductor material is a promising alternative for absorber layers in thin film solar cells. One structural property of particular interest is the distribution of the cations Cu¹⁺ and Zn²⁺ in the crystal structure. In a recent paper [1] ordering of Cu and Zn in the CZTS structure was reported at temperatures below 533 K; similar results were found for CZTSe below 473 K [2]. The implications of such a low critical temperature would be significant. CZTSSe is normally grown at 720–830 K, far above the critical temperature. The exact extent of ordering by the end of the synthesis will depend only on the part of the cooling that occurs below the critical temperature, in a temperature region in which cooling history often is neither controlled nor reported. Thus the concentration of Cu_Zn and Zn_Cu antisite defects in CZTSSe and from these the electrical properties of CZTS could be modified by relatively low temperature processes. Indeed, low temperature postdeposition annealing has been shown to increase photovoltaic power conversion efficiency significantly [3]. However, the low temperature ordering effects were only deducted indirectly, from changes in the Raman spectrum [1], from anomalous lattice parameter expansion [4], and from kinetics simulations [2].
Cu¹⁺ and Zn²⁺ cannot be distinguished by conventional X-ray diffraction, since they have essentially the same scattering factor for X-rays (same number of electrons). Anomalous diffraction has been used to overcome this, but currently published papers either restrict themselves to qualitative changes [5,6] or used single crystal data [7] with limited explanatory power for realistic samples. We used anomalous X-ray diffraction on the Cu- and Zn- K absorption edges to determine the distribution of Cu and Zn over the crystallographic sites in a B-type CZTSe kesterite (Cu_1.949Zn_1.059Sn_0.983Se₄) powder. Rietveld refinement allowed the quantitative determination of both Cu- and Zn-occupancy for all relevant sites. From this, the temperature dependency of a structure-based, quantitative order parameter could be determined. The critical temperature of the phase transition was confirmed at 460±10 K. The ordering mechanism is in agreement with a transition from disordered to ordered kesterite.

[1] J. J. S. Scragg et al., Appl. Phys. Let. 2014, 104, 041911
[2] G. Rey et al., Appl. Phys. Let. 2014, 105, 112106
[3] M. Neuschitzer et al., Chem. Mater. 2015, 27, 5279−5287
[4] S. Schorr, G. Gonzalez-Aviles, Physica Status Solidi A 2009, 206(5), 1054
[5] H. Nozakia et al., J. Alloys Comp. 2012, 524, 22
[6] T. Washio et al., J. Appl. Phys. 2011, 110, 074511
[7] A. O. Lafond et al., Acta Crystallographica B 2014, 70, 390

Thin film solar cell absorbers composed of earth-abundant elements such as Cu₂ZnSn(S,Se)₄ (CZTSSe) are particularly relevant due to their low toxicity and their record maximum power conversion efficiency of 12.6%. Despite promising results, further work is needed to understand how to improve this technology and enable its commercial-scale implementation. Recent efforts have identified the need to remove defect states by deliberate passivation or the introduction of dopants in order to improve upon the voltage deficits (compared to theoretical limits) exhibited by CZTSSe. The inclusion of Ag could reduce tail states that are introduced by the disorder caused in the random alternation of copper and zinc in the kesterite lattice (the copper cation is only 5% larger than the zinc cation). Herein we use a hydrazine solution growth procedure to deposit the CZTSSe absorbing layer, and use different alloying levels of Ag to substitute Cu. Compositional profiling across the absorber layer shows an almost homogeneous distribution of Ag, and the electrical characterization of the Ag-alloyed devices gives a record power conversion efficiency of 8.1% without the use of an antireflective coating. Changes in band gap, carrier density concentrations, and minority carrier lifetimes (measured by time-resolved photoluminescence, TRPL) are also used to assess the effects of Ag alloying. In addition, this study compares the use of Na and different Li salts and their effects on the absorber’s crystal grain structure, size, and overall device performance. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), time-resolved photoluminescence, and photoluminescence imaging are used to characterize the Ag-alloyed CZTSSe devices. This work is supported by the U.S. Department of Energy under DE-EE0006334.

Cu₂ZnSnS₄ (CZTS) has attracted significant attention for thin film solar cells because it is composed on earth abundant and non-toxic elements and, has an optimal band gap of 1.5 eV. CZTS absorbers are currently synthesized by annealing metallic precursors in sulfur atmospheres. Foreseeing large scale applications, one step processes, for example based on reactive magnetron sputtering, are currently developed [1-3]. The control of the film stoichiometry is critical during the CZTS growth since it influences the electrical properties of the material and its phase constitution. Indeed, it has been shown, based on phase diagram, that the experimental windows allowing to grow CZTS is very narrow [4,5]. Nevertheless, these diagrams are not relevant for low pressure processes operating outside the thermodynamical equilibrium. Therefore, the aim of the present work is to go further in the understanding of the influence of the Cu, Zn, Sn and S concentrations on the film phase constitution by drawing a phase diagram scalable to one step magnetron sputter processes.
We previously reported the growth of close to stoichiometric and phase pure CZTS thin films by sputtering a Zn and a CuSn alloy targets in Ar-H₂S atmospheres by magnetron sputtering [1]. The amount of metallic species (Cu, Zn, Sn and S) provided to the growing film is modified by varying: (i) the CuSn target composition and (ii) the power applied to this target, while the other experimental parameters are kept constant. The film phase constitution is determined by combining multi-wavelengths Raman spectroscopy and X-Ray diffraction analysis whereas the atomic concentration is measured by Energy-dispersive X-ray spectroscopy.
All the synthesized films are regrouped in a phase diagram which exhibits three groups of phase compositions: CZTS-ZnS, CZTS-SnS-ZnS and SnS-ZnS-Cu₄Zn-S₆. The main factor affecting the phase transformation is the sulfur concentration in the films. An atomic sulfur concentration < 50 % was found to be necessary to synthesize CZTS films. Its decrease associated to the increase of the Sn amount leads to the disappearance of CZTS to the profit of SnS phase. The electrical properties of the films have been evaluated by Hall effect measurements. All the films exhibited a p-type conductivity with a high hole concentration between 10¹⁷ and 10²⁰ cm^-3. The CZTS films have the highest holes concentration which could be explained by a high amount Cu or by the presence of the ZnS phase. A very low Hall mobility is found in comparison with the value usually reported in the literature. This is explained by a small grain size and defects in the film microstructure.

[1] Cormier et al., Acta Materiallia, 96 (2015) 80-88
[2] Liu et al., Sol. Energy Mater. Sol. Cells 94 (2010) 2431–2434.
[3] T. Ericson et al., Thin Solid Films 520 (2012) 7093–7099.
[4] C. Platzer-Bjorkman et al., Sol. Energy Mater. Sol. Cells 98 (2012) 110–117.
[5] Mitzi et al., Solar Energy Materials & Solar Cells 95 (2011

Hybrid perovskites became of high interest in the recent years due to their huge variety of element substitutions on both cation and anion sites and thus the tailoring of new materials for solar energy conversion. Our work is mainly focused on the solid solution CH₃NH₃PbI₃ – CH₃NH₃PbCl₃ which shows on the one iodine end member fast rising efficiencies of more than 20% solar conversion efficiencies.
The solid solution shows in case of 2.2% chlorine ratio a bandgap of about 1.1eV and additionally a higher air stability [1] which is still a problem in case of the iodine perovskite.
The CH₃NH₃PbI₃ (MAPbI₃) shows a tetragonal crystal structure (s.g. I4/mcm) while the chlorine perovskite (MAPbCl₃) belongs to the cubic space group Pm-3m. Both structures consist of corner linked PbX₆ – octahedra (X=I, Cl). However due to different electronic effects the iodine perovskite shows an octahedral tilting and disordering of the anions which influences the orientation of the organic methylammonium molecule and therefore photovoltaic properties.
Therefore we investigated as well the solid solution end members as systematically the solid solution possibilities with regard to the degree of substitution and miscibility.
The iodine end member was prepared by precipitation from hydroiodic solution [2] while MAPbCl₃ was prepared by precipitation of lead(II) acetate from a methyl amine solution [3]. Polycrystalline MAPbI_3-xCl_x was synthesized from an equimolar mixture of methylammonium halide and lead(II) halide in a mixture of γ-butyrolactone and N,N-dimethylformamide [4]. For structural investigations synchrotron X-ray and neutron diffraction experiments were performed at the Helmholtz-Zentrum Berlin für Materialien und Energie. In the presentation the main results of this complementary study concerning detailed structural parameters, especially the hydrogen positions and the orientation of the methylammine molecule, will be discussed.

1. Boix, P.P., et al., Current progress and future perspectives for organic/inorganic perovskite solar cells. Materials Today, 2014. 17(1): p.16-23.
2. Poglitsch, A. and D. Weber, Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter wave spectroscopy. The Journal of Chemical Physics, 1987. 87(11): p. 6373-6378.
3. Leguy, A.M.A., et al., Reversible hydration of CH₃NH₃PbI₃ in films, single crystals, and solar cells. CHEMISTRY OF MATERIALS, 2015. 27(9): p. 3397-3407.
4. Baikie, T., et al., Synthesis and crystal chemistry of the hybrid perovskite (CH₃NH₃)PbI₃ for solid-state sensitised solar cell applications. Journal of Materials Chemistry A, 2013. 1(18): p. 5628-5641.

Methylammonium lead trihalide perovskite (MAPbX₃ X is Cl, Br, or I) has emerged as a new generation of solution-processable optoelectronic materials for photovoltaics, light emitting diodes, lasers, and photodetectors. Most of these applications, however, are based on perovskite thin films which usually contain large density of charge traps at the grain boundaries. Since single crystals possess no grain boundaries, it is anticipated that perovskite single crystal based devices can possess much better optoelectronic performance. Recently, we have demonstrated that the carrier diffusion length in solution-grown MAPbI₃ single crystals is larger than 175 μm, more than two orders of magnitude longer than the thin film counterpart, which resulted in a nearly 100% internal quantum efficiency (IQE) near band edge in the thick perovskite single crystal solar cells.¹ However, the largely suppressed IQE at short wavelength range indicates the existence of severe surface charge recombination on the perovskite single crystals that would hinder the efficient charge collection.

Here, we first studied the surface charge recombination velocity of perovskite single crystals via photoconductivity measurement, and illustrated its origin and possible ways to reconcile it for photovoltaic application. Moreover, the surface charge recombination was intentionally utilized to fabricate highly narrow band perovskite single crystal photodetectors with full-width-at-half-maximum (FWHM) < 20 nm, and continuously tunable response spectra from blue to red by changing the halide composition and consequently, the bandgap of the single crystals.² The narrow-band photo-detection can be explained by the strong surface charge recombination of the excess carriers close to the crystal surfaces generated by short wavelength light. The excess carriers generated by below bandgap excitation locate away from the surfaces and can be much more efficiently collected by the electrodes assisted by the applied electric-field. To the best of our knowledge, this is the first time that perovskite single crystals were utilized for photodetector application and exhibited the unique narrow band light response in contrast to the thin film analogues. These are also the narrowest band photodetectors reported with a continuously tunable response spectrum. The new design paradigm presented in this work provides an alternative approach to realize the optical filter free UV, visible, or IR narrow band photodetection, and it is not limited to any specific material system.

Reference
Q. Dong, et al. "Electron-hole diffusion lengths> 175 μm in solution-grown CH3NH3PbI3 single crystals." Science 347, 967-970 (2015).
Y. Fang, et al. "Highly narrowband perovskite single-crystal photodetectors enabled by surface-charge recombination." Nature Photonics 9, 679–686 (2015).

Organic-inorganic hybrid CH₃NH₃PbI₃ perovskite solar cells have been investigated intensely in the past few years with efficiency exceeding 20%.[1] The substitution from I to Br in CH₃NH₃PbI₃ was proven to tune the band gap from 1.50 eV to 2.20 eV conveniently. This feature makes CH₃NH₃PbI_3-xBr_x very attractive in terms of fabricating colorful and semi-transparent devices. It was also found that CH₃NH₃PbI_3-xBr_x displays excellent photovoltaic property with improved long-term stability.[2]

However, it is not easy to fabricate uniform and well crystalized CH₃NH₃PbI_3-xBr_x planar films. Reaction between PbX₂ (X=Br or I) and CH₃NH₃X (X=Br or I) usually leads to films with rough surface and low coverage, resulting in poor performance of the solar cells.[3] Here we report a one-step solution method to fabricate more uniform and crystalized CH₃NH₃PbI_3-xBr_x (x=0, 0.45, 0.51, 0.60, 1, 2, 3) film on FTO/TiO₂ by using NH₄Cl additive. The films with improved coverage and crystallization were confirmed by SEM and XRD. UV-vis spectrum showed that the absorbance of the films using NH₄Cl was greatly enhanced comparing to those without NH₄Cl. Efficiencies of the resulting planar solar cells can be largely improved. Especially when x = 0.60, the efficiency increased from 6.40% to 12.10% by 89%. As far as we know, this is the highest efficiency of planar heterojunction solar cell based on CH₃NH₃PbI_3-xBr_x. Furthermore, we also conducted in-situ XRD to explore the role played by NH₄Cl in the film formation.

To sum up, this work provides a convenient approach for the fabrication of high efficiency CH₃NH₃PbI_3-xBr_x based planar heterojunction solar cells.

References:
1. Zhou, H., et al., Interface engineering of highly efficient perovskite solar cells. Science, 2014. 345(6196): p. 542-546.
2. Noh, J.H., et al., Chemical management for colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells. Nano Lett, 2013. 13(4): p. 1764-9.
3. Zhao, Y. and K. Zhu, Efficient planar perovskite solar cells based on 1.8 eV band gap CH3NH3PbI2Br nanosheets via thermal decomposition. J Am Chem Soc, 2014. 136(35): p. 12241-4.

In this study we fabricate the first epitaxially grown Cu₂O thin film photovoltaic devices, incorporating a Zn(O,S) buffer layer. We start by depositing 50 nm thin films of Au and Pt with sub-nanometer roughness on single crystalline MgO substrates. This ohmic back contact, which serves as a heteroepitaxial template, is then used to deposit phase pure, oriented Cu₂O thin films of 5 μm thickness by plasma-assisted molecular beam epitaxy (PA-MBE). Then, a 30 nm Zn(O,S) buffer layer is sputtered to form a stoichiometric Cu₂O-Zn(O,S) interface crucial for achieving high open circuit voltages, followed by an 80 nm layer of ITO as a top contact. Zn(O,S) has been developed as a Cd-free, earth-abundant, and scalably manufacturable window layer material for CIGS solar cells, and has a conduction band tunable by the relative O and S concentrations. Its thermodynamic properties make it a great heterojunction partner for Cu₂O.
We report current-voltage, spectral response, and light beam-induced current characteristics of thin film Cu₂O solar cells and correlate device performance with material structure and quality. The film and interface structure and defects are analyzed by high-resolution cross-sectional transmission electron microscopy and x-ray diffraction. Our material system has achieved open circuit voltages above 1 V, as well as short circuit currents of a few milliamps. We also demonstrate a scheme for replacing the precious metal contacts currently employed in Cu₂O photovoltaic devices.
We conclude by discussing the outlook for epitaxial Cu₂O thin film photovoltaic devices considering current trends in the PV market. Cu₂O has been regarded as a promising photovoltaic material for many decades, and most of the device results have been achieved on thermally oxidized, polycrystalline Cu₂O foils. Cu₂O’s bandgap of 2.1 eV makes it a strong wide bandgap candidate for a tandem solar cell with a crystalline Si bottom cell. The Cu₂O cell efficiency required to improve upon the efficiency of a crystalline Si cell in such a tandem configuration is around 8%. While the theoretical efficiency for a Cu₂O solar cell is around 21%, the experimental record efficiency for a Cu₂O device has tripled in the past few years and recently surpassed 6%. Our work presents a path to create an entirely earth-abundant, high efficiency tandem solar cell.

As the world’s demand for energy grows, the search for cost competitive and earth abundant photovoltaic materials becomes increasingly important. Cu₂SnS₃ (CTS) is a promising earth abundant absorber with demonstrated 4% device efficiency [1], and interest in this material has increased rapidly in the past few years. Our work on CTS has focused on understanding the defect physics of this material, and how these defects affect the carrier transport. Here we present a summary of our experimental and theoretical findings, including insights on the roles of point defects, cation disorder, and alloying in CTS.

We initially investigated defect behavior in CTS from the perspective of point defects. Theoretical calculations determined that the dominant acceptor defect in CTS is the Cu vacancy (V_Cu), which would be expected to control the hole concentration [2]. Experimentally, we control the V_Cu concentration by varying the S chemical potential during film growth [3]. Additionally, we observed that the as-grown films were cation disordered, as opposed to the theoretical predictions of a cation ordered structure.

Because of the importance that cation disorder has in similar materials such as CZTS, we investigated this issue in CTS. Theoretically, we found that cation disorder in CTS can cause regions of compositional inhomogeneity, or “clusters” of specific coordination motifs. These clusters may lead to electronic potential fluctuations and band tailing, ultimately affecting photovoltaic device performance [4]. Experimentally, we used terahertz spectroscopy to evaluate the minority carrier transport in ordered and disordered CTS. We found that even the ordered films showed poor minority carrier transport, including picosecond decay times and carrier localization. We couple this data with high-resolution TEM to conclude that even very low levels of structural defects (such as stacking faults or twins) can dominate the carrier transport in CTS [5].

Further theoretical investigations of disorder in Cu₂SnS₃ have revealed that cation disorder can significantly lower the formation energy of Cu_Sn antisite defects [6]. This can ultimately result in alloying with a semi-metallic Cu₃SnS₄ phase. Experimental spectroscopy techniques (Raman and NEXAFS) were used to detect changes in S and Cu bonding environments that result from formation of the predicted Cu₂SnS₃-Cu₃SnS₄ alloy.

[1] Kanai, et al. J. Journal of Appl. Phys 54, 08KC06 (2015).
[2] Zawadzki, et al. Appl. Phys. Lett. 103, 253902 (2013).
[3] Baranowski, et al. Chem. Mater. 2014, 26, 4951-4959.
[4] Zawadzki, et al. Phys. Rev. Appl. 3, 034007 (2015).
[5] Baranowski, et al. Phys. Rev. Appl. In press (2015).
[6] Zawadzki, et al. Submitted, 2015.

The project “Rapid Development of Earth-Abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

Among prospective thin film solar cell materials, antimony chalcogenides have a place owing to their availability, low toxicity and low cost. Even though the current production of antimony is concentrated in a few countries, antimony may be recovered more profitably once its solar cell application is developed. We used in this work antimony sulfide and antimony sulfide-selenide powder obtained by chemical precipitation of antimony salt, sodium thiosulfate, thioacetamide and sodium selenosulfate. Typical solar cell structure is simple: (i) TCO/Sb₂S₃/C-Ag or (ii) TCO/CdS/Sb₂S_1.2Se_1.8/C-Ag. Here TCO is a commercial SnO₂:F coating on 3 mm glass, CdS is chemically deposited thin film of 80 nm in thickness, the antimony chalcogenide film is 400 nm in thickness produced by thermal evaporation of the precipitate. Carbon electrodes are of 0.5 – 1 cm² in area, painted using colloidal graphite and heated at temperatures of 280-300 ^oC. Colloidal silver paint is applied on C-electrode after the heating. In cell (i), the open circuit voltage is 0.7 V, the short circuit current density is 9 mA/cm², fill factor, 0.37 and conversion efficiency, 2.3%. In cell (ii), these values are 0.53 V, 11 mA/cm², 0.36 and 2.1%. Together, the absorber films might improve upon these cell characteristics in a cell structure (iii) TCO/CdS/Sb₂S₃-Sb₂S_1.2Se_1.8/C-Ag.

Titanium dioxide (TiO₂) is considered one of the most promising semiconductors in sensors, solar cells, photodetectors, photocatalysis, etc. Due to its n-type conductivity and wide band gap (3.2 eV), it has been an important component in the dye sensitized solar cells (DSSC) during the last 25 years. However, the use of organic dyes to sensitize the mesoporous TiO₂ layers of the DSSCs make them unstable under continuous solar radiation. In order to avoid this, new inorganic sensitizers such as semiconductors have been developed to fabricate high efficient semiconductor-sensitized solar cells (SSSCs). In this work is presented the fabrication and optimization of SSSCs using a solid solution of antimony sulfide-selenide (Sb₂S_xSe_3-x). For the fabrication of the SSSCs an electron blocking layer of TiO₂ (bl-TiO₂) with 60 nm in thickness is deposited by spin-coating onto a conductive glass substrate (F-doped SnO₂, FTO). Then, a mesoporous TiO₂layer (mp-TiO₂) of about 300 nm is obtained by spin coating on the top of bl-TiO₂. Later, the mp-TiO₂ is sensitized with cadmium sulfide (CdS) and antimony sulfide-selenide by successive ionic layer absorption. The SSSCs are completed by placing graphite/silver contacts on the top of the Sb₂S_xSe_3-x absorber with a post-heating of the entire cell at 300 °C in nitrogen. The optical and electrical measurements of these cells indicate that the sensitizing process with CdS and Sb₂S_xSe_3-x absorbers improve significantly the light absorption and photogenerated current of the mp-TiO₂ layers, with respect to those SSSCs without CdS or Sb₂S_xSe_3-x. Furthermore, the presence of CdS between the mp-TiO₂/Sb₂S_xSe_3-x interface plays an important role to obtain solar cells with better photovoltaic parameters. The best solar cell showed an power conversion efficiency above of 1% under AM 1.5G solar radiation. Improvement of the photovoltaic characteristics of the SSSCs by optimizing the deposition of CdS and Sb₂S_xSe_3-x absorbers as well as increasing the porosity of the mp-TiO₂ layer are discussed.

Poly-crystalline CdTe-based photovoltaic (PV) devices are the forerunners in commercialized thin film solar cell technology. Despite the commercial success, present laboratory champion cells achieve ~21% power conversion efficiency and hence are still ~10% shy of its theoretical limit. Current research efforts are mainly focused on improving the open circuit voltage, V_oc, which is just 60% of CdTe band-gap /e in the most efficient solar cells. In this collaborative study we investigate effects of numerous randomly oriented grain boundaries and their role in limiting further improvements. It is very challenging to examine individual behavior of these sub-micrometer crystallites within as-grown poly-CdTe and to decouple surface effects. To overcome these difficulties we fabricate a number of artificial grain boundaries by CdTe wafer bonding which allow us to study CdTe interfaces in isolation. To reveal atomic structures of the interfaces we use aberration-corrected scanning –transmission electron microscope (STEM) and atomic-column resolved X-ray spectroscopy. Electronic and thermodynamic properties of these selected interfaces are then calculated using first-principles density-functional theory (DFT). To investigate role of the interfaces on charge carrier lifetimes 2-photon time-resolved photo-luminescence (TRPL) experiments are carried out which allow probing the crystals deep below influence of surface effects. Finite elements analysis simulations are used to extract interface recombination velocities based on the TRPL data and correlate with STEM imaging and DFT calculations to identify detrimental structures and ways to passivate them. A number of previous studies suggested that Cl segregation at grain boundaries after CdCl₂ treatment might be transforming the interfaces into effective charge carrier channels which assist their collection. We investigate the effects CdCl₂ treatment on these artificial grain boundaries using TRPL and DFT calculations.
Supported by Department of Energy SunShot BRIDGE Program (DOE DEEE0005956)

CdTe is a widely-used photovoltaic material, due to its high efficiency and low manufacturing cost. However, the practical efficiencies of polycrystalline CdTe photovoltaic cells are still well below the theoretical limit, indicating possible room for improvement. A fundamental understanding of the role of vacancies, interstitials, dislocations and grain boundaries on the electronic structure of CdTe may lead to efficiency improvements. Atomistic-level characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding. In the present work, we manufactured selected bicrystals of CdTe and revealed the grain boundary and dislocation core structures. We constructed atomistic models of grain boundaries and dislocation cores from image analysis and crystallographic information from STEM and modeled using first principles density functional theory (DFT) calculations. In addition, we carried out high-throughput search for twist and tilt grain boundary structures that have low interfacial energy using empirical potentials. We report the electronic density of states (DOS) and planar-averaged electrostatic potential profiles of different CdTe grain boundaries to understand charge interactions. We also present the results of point defects and pairs of point defects that are present on or near several representative grain boundary models. We discuss the thermodynamics of point defect and complex formation, as well as resultant changes in electronic structures. The implications of these electronic structure changes at grain boundaries on photovoltaic performance, and corresponding strategies to improve performance, are discussed.

ACKNOWLEDGEMENT: We acknowledge funding from the DoE Sunshot program under contract # DOE DEEE005956. Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The submitted abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.

Solar cells fabricated by simply coating n-type mono-crystalline silicon with carbon nanotubes (CNTs) have been reported [1]. Photovoltaic conversion efficiencies (PCE) exceed 11% and 17% with pristine and MnO_x-coated SWCNT films, respectively [2]. Such high PCEs, however, are achieved using highly crystalline SWCNT films captured on membrane filters by floating catalyst chemical vapor deposition (CVD) or SWCNT films processed in superaicd. Mild solution process of mass-produced CNTs is more promising for large-scale production of CNT films.
In this work, we studied the surfactant-based mild solution process for CNT films with a focus on removal of contaminated surfactants. Two kinds of CNT powders, MEIJO eDIPS (Meijo Nano Carbon Co., Ltd.) produced by enhanced Direct Injection Pyrolytic Synthesis method [3], and vertically aligned SWCNTs produced by on-substrate CVD [4], were used. Their dispersions were prepared by ultrasonication in 0.5 wt% sodium dodecylbenzene sulphonate aqueous solution and centrifugation. The dispersion was filtered onto a hydrophilic membrane filter, which was slowly immersed into purified water to let CNT film detach and float on the water surface. Water was heated to and held at ~70 °C for 3 h to remove surfactant residue from the film, and then the film was transferred onto SiO₂/n-Si substrate with a bare Si window of 1-2 mm in diameter. Gold anode and indium cathode were connected to the CNT film and n-Si, respectively. The as–fabricated cell with eDIPS CNT film of 80% optical transmittance showed a short-circuit current density of 30.0 mA/cm², an open-circuit voltage of 0.565 V, and a fill factor of 0.600, leading to a PCE of 10.2% (drastically improved from <5% without careful surfactant removal). Further doping of and impurity removal from the CNTs with 1 M HNO₃ considerably improved the fill factor, resulting in a PCE of 12.6%. The cells with on-substrate-produced SWCNTs showed a PCE of 9.1% despite of their inferior crystallinity than the eDIPS CNTs. Effective removal of surfactant is the key to realize solar cells using solution-processed CNT films.

Polycrystalline <100 nm thin CdS films used as n-type window layer play an important role in developing high efficiency heterojunction CuInSe₂, Cu₂ZnSnS₄, and CdTe solar cells. Though CdS film is formed by vacuum evaporation, electrodeposition and atomic layer deposition, by far the best efficiency results for these solar cells are obtained with the CdS buffer layer deposited by the chemical bath deposition (CBD) method. A key requirement for the window layer is high transmittance in the visible spectrum. This enables more photons reaching the absorber and thereby increase the photocurrent and quantum efficiency of solar cells. Though the high band gap materials ZnS or ZnO_1-x S_x meet this criterion, as a buffer layer these showed no improvement in solar cell performance.

In the present work, we propose an alternative window layer for the heterojunction solar cells based on cadmium oxysulfide (CdO_xS_1-x) thin films formed by the CBD method. The 100 nm thin CdO_xS_1-x films deposited by alloying CdS with CdO, which is a direct band gap n-type degenerate transparent conductor, we obtained high >90% optical transmittance over 550-1000 nm region. Corresponding pure CdS films show low ~ 70% transmission. It requires a ~20 nm thin CdS film to increase optical transmission for a gain of ~1.5 mA.cm^-2 in photocurrent density. The difficulty lies in depositing continuous CdS film without shunting paths at 20 nm thickness. We show that a CdO_xS_1-x film of ~100 nm thickness formed with highly conformal coverage over p-type absorber surface can easily meet the optical criterion. Direct optical band gap of CdO_xS_1-x films determined from Tauc plots is 2.25-2.3 eV (x=0.58-0.45) within the acceptable range for a buffer layer. Obeying the Vegard’s Law, it lies between the optical gaps of CdS (2.42 eV) and CdO (2.16 eV).

The CBD growth of CdS is via controlled release of S^2- and Cd²⁺ ions in an alkaline medium through complexing to prevent CdS or Cd(OH)₂ as colloids in the solution phase. In the present method, by altering the substrate conditions in which Cd²⁺ ions sheathed within micelle or surrounded by hydrophobic groups of sodium dodecyl sulfate surfactant, we carried out controlled Cd(OH)₂ deprotonation reaction to form CdO over the substrate concurrently with CdS to deposit CdO_xS_1-x films. These films are polycrystalline having cubic CdS structure identified by (111), (220) and (311) x-ray diffractions. Formation of CdO_xS_1-x phase is confirmed by Raman spectra which besides characteristic CdS LO A1 and A2 modes at 300 and 600 cm^-1, also show Raman lines from CdO at 432 and 925 cm^-1 assigned to 2LA and 2LO phonon modes. CdO_xS_1-x films show higher electrical conductivity and better structural stability compared to the CdS due to lesser S vacancy defects. This paper will present CBD growth, mechanistic aspects, structure, optical and electrical properties and evaluation of CdO_xS_1-x as a buffer layer in heterojunction solar cells.

Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies.¹⁻³ Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts,4 (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot electron injection for different particle diameters to elucidate potential quantum effects while keeping the plasmon resonance frequency unchanged. Interestingly, our studies did not reveal differences in the hot-electron generation and injection efficiencies for the investigated particle dimensions and plasmon resonances.

KEYWORDS: Plasmonic solar energy conversion, hot-electron, quantum efficiency, solar water splitting, Au/NiO_x, selective transport layer.

■ References
(1) Moskovits, M. Nat. Nanotechnol. 2015, 10 (1), 6.
(2) Brongersma, M. L.; Halas, N. J.; Nordlander, P. Nat. Nanotechnol. 2015, 10 (1), 25−34.
(3) Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10 (12), 911−921.

TiO₂ and Ta₂O₅ are both earth-abundant materials and efficient UV absorption semiconductors due to their large bandgaps and have shown interesting photocatalytic water-splitting behaviors when functionalized with suitable co-catalysts [1,2]. Characterization of these particulate catalyst systems is challenging since heterogeneity exists in the structure and performance of these nanoparticles. For water splitting applications, understanding surface hydroxylation processes on the surfaces of nanoparticles is also difficult. Electron energy-loss spectroscopy (EELS) in the electron microscopy has a unique advantage of studying materials’ response at high resolution allowing correlation of local optical and electronic properties with structural information including composition, morphology and surface facets. Recent advances in monochromation now allows EELS to achieve ultra-high energy resolution of 15 meV or better with spatial resolution below 1 nm [3]. This opens up the opportunity of detecting vibrational signals, inter-band and surface states and subtle bandgap changes at the nano-level, which is valuable to understand charge transfer and reaction mechanism at the interfaces/surfaces. Here we present results on the surface states that form on oxide nanoparticle surfaces. The effects of water and defects of the electronic structure of oxides is probed with EELS. Localized bandgap measurements at the co-catalyst-oxide interface is also discussed. To interpret the spectra, simulation of inter-band states and bandgap onset were performed using both dielectric models and density of states calculation.

[1] L. Zhang et al, J. Phys. Chem. C 119.13 (2015), 7207.
[2] Q. Liu et al, Appl. Catal. B: Environ.172 (2015), 58.
[3] O.L. Krivanek et al, Nature 514 (2014), 209.
[4] The support from US Department of Energy (DE-SC0004954) and the use of the microscope at John M. Cowley Center for High Resolution Microscopy at Arizona State University is gratefully acknowledged.

We have studied the optical, electrical, and structural properties of InGaN/GaN multiple quantum wells (MQWs) in a p-i-n solar cell grown by metalorganic chemical vapor deposition (MOCVD). Four equivalent structures were grown with 9, 20, 40, and 60x QWs in order to understand the limiting factors of the device performance. First, the onset for plastic relaxation as the total QW thickness increases from 40 to 60x (i.e. 120 to 180 nm) is observed by the opening of threading dislocation cores at a given thickness, when the film can no longer store the accumulated misfit strain energy between InGaN and GaN. The estimated critical thickness of ~129 nm is similar to that of a thick In_xGa_1-xN film with x=0.12 grown on GaN, according to the People and Bean model. Second, we observe an increase in the integrated absorption (from 280 nm to InGaN band gap) with an increase in the number of QWs. The integrated absorption does not improve considerably despite the increase in number of QWs from 40 to 60x, reaching a maximum of ~84%. This may be due to the degradation of the material quality beyond the critical thickness of InGaN which plays an important role in the absorption. Despite the higher absorption for the device with 60x QWs, the best cell performance under 300x suns is for 9x QWs with measured short-circuit current J_sc = 516 mA/cm², open-circuit voltage V_oc = 1.7 V, calculated fill factor of 0.71, and power of 623 mW/cm². Piezoelectric polarization fields and the non-radiative recombination centers limit the J_sc and V_oc, with the latter having a stronger effect for higher number of QWs due to material degradation. The correlation of these properties with the microstructure will be discussed.

Discovery and optimization of metal oxide based semiconductors for photoelectrochemical water splitting that are abundant, stable and non-toxic is challenging. The predominantly investigated binary oxides Fe₂O₃, TiO₂ and WO₃ do not show sufficient stability, a suitable bandgap, efficient light absorption, high catalytic activity or long charge carrier lifetimes so that new materials need to be identified. As the number of possible multinary mixtures of yet to be found materials is enormous, new methods of rapid synthesis and high-throughput/high-quality evaluation and optimization need to be implemented. Using this approach, synthesized Ti-W-O, Fe-W-O and Ti-Fe-W-O materials libraries were thoroughly characterized for potential photoelectrode materials by automated EDX, XRD, thickness and PEC measurements. For the Ti-W-O system exhibiting a compositional range between (Ti₈₀W₂₀)O_x and (Ti₉₇W₃)O_x, the PEC measurements revealed a maximum quantum efficiency of 37.5 % (at λ = 350 nm) and a maximum peak photocurrent density of 70.3 µA/cm² at 94.4 at.% Ti. Extensive phase analysis in the case of Fe-W-O indicated the presence of a ternary phase Fe₂O₆W at the local photocurrent maxima, while the sub-stoichiometric W₅O₁₄ phase (with 15 at.% Fe) demonstrated a maximum quantum efficiency of 45 % at λ = 300 nm and photocurrent densities of 65 µA/cm². Utilizing new ways for visualization of the obtained multidimensional datasets correlations between composition, morphology, thickness, crystallinity and PEC properties were made. An overview of optimization routes implemented in our combinatorial materials science framework by variation of deposition pressure, substrate temperature using an actively controlled step heater, and glancing angle deposition for nanostructuring will be shown.

Many research works reported that the rare earth ions is an advanced doping elements for optoelectronic ceramic materials which have potential applications on data storage, color display, multiplexed bio-imaging devices due to its excellent properties of having rich energy levels and their upconversion luminescence spectra are in a wide range from infrared to ultraviolet and tunable. However upconversion materials can be used for visible light producer via absorbtion of unabsorbed-infrared solar radiation for solar cells. Dye-sensitized solar cells (DSSC) have attracted great interest due to their large potential applications as a cheap alternative candidate to replace conventional silicon-based p–n junction solar cells. For many years, TiO₂ and ZnO absorber nanomaterials such as nanoparticles, nanowires and nanotubes and the organometallic ruthenium dye families such as N719, N3 and C101 are well known suitable most efficient materials to obtain highly efficient photo anodes. However, the maximum light conversion efficiency of DSSC has a barrier around 10% recorded. To alter this barrier, using light v layer, which is new subject matter to extend the light path, is performed to enhance the light harvesting efficiency. The suitable converter layer should be composed of Ba₂YbF₇ or doping whit Tm³⁺ or Er³⁺. In this study, to better understand of the up conversion emission effect on solar cell harvesting of Yb³⁺ concentration of Ba₂YF₇:xYb³⁺,0.02Er³⁺ (x = 0.2, 0.4, 0.6, 0.8, 0.98) used as upconversion layer to improve photo conversion efficiency of DSSC. Average size of converter layer particules is found as 10 nm by TEM. 45 % total improvements has been recorded on photovoltaic efficiency. In addition, highest IPCE value obtain DSSC whit upconversion layer than traditional DSSC. Surprisingly 25% improvements has been seen on decreasing serial resistance on photo anode. Positive improvements effect was seen on lifetime measurements by EIS. Applied upconversion layer obtains nearly two times higher recombination time. In addition, upconversion layer cells have higher diffusion length than conventional DSSC. in the light of this study, Ba₂YF₇:xYb³⁺,0.02Er³⁺ can be candidate an alternative material for upconversion layer applications for highly efficient dye sentized solar cell.

We demonstrate a highly efficient inverted bulk heterojunction (BHJ) polymer solar cell using a wet-chemically prepared doped ZnO with a self-organized ripple nanostructure as an electron extraction layer and a blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7) and [6,6]-phenyl C71 butyric acid methyl ester (PC₇₁BM) as an active light absorbing layer. In order to enhance the electron extraction efficiency, the ZnO ripple surface was modified with various alkali metal carbonate materials including Li₂CO₃, K₂CO₃, Na₂CO₃, Cs₂CO₃, and (NH₄)₂CO₃. The inclusion of an additional metal carbonate layer led to gradient doping of the ZnO ripple layer and improved the electron extraction properties by modifying the energy level without destroying the ZnO ripple structures. The highest performing solar cells were fabricated with Li₂CO₃ and yielded a maximum PCE of 10.08%; this value represents a ~14% increase in the efficiency compared to solar cells without a metal carbonate treatment.

Finding new strategies for transition away from dependence of fossil fuels is a global challenge of the 21^st century. Two-step thermochemical production of syngas, is an energy technology utilizing solar power to convert water and carbon dioxide into syngas, an attractive renewable fuel that can replace fossil fuels. Recently, perovskites have attracted much attention due to impressive solar fuel yields surpassing state-of-the-art material ceria.¹ Although higher yields can be obtained, it does not imply that it is the best choice for solar to fuel reactor. It is critical to compare the thermochemical fuel conversion efficiency, because it defines the actual useful chemical energy per input of solar energy.^{2, 3} Typically, this is calculated from oxygen nonstoichiometry equilibrium data derived from thermogravimetric measurements in the relevant temperature range, or extrapolations of data at lower temperatures. Performing experiments to test a few selected compositions is useful but time consuming and it does not give the best estimate of the thermochemical efficiency. We calculate thermochemical fuel conversion efficiency based on CALPHAD descriptions of defect chemistry of multi-component perovskites, based on critical review of all available experimental phase stability data. It is found that this defect chemistry model based on the weighted sum of optimized energies of various ionic compounds assembling the perovskite phase extrapolates well over the whole technologically relevant temperature and oxygen partial pressure range. Further, the thermodynamic properties are determined for a continuous variation of the doping concentration x, so a large compositional space can be tested. We compare and highlight advantages of this approach to previous predictions of thermochemical efficiency. The efficiency analysis is exemplified by using data on the perovskite composition La_0.6Sr_0.4Mn_1-xCr_xO_3-δ. Finally, we discuss how the presented approach can be generalized to effectively screen other dopants and select the most efficient material for various operating conditions.


1. A. H. McDaniel, E. C. Miller, D. Arifin, A. Ambrosini, E. N. Coker, R. O'Hayre, W. C. Chueh and J. Tong, Energy & Environmental Science, 2013, 6, 2424-2428.
2. A. H. Bork, M. Kubicek, M. Struzik and J. L. Rupp, Journal of Materials Chemistry A, 2015, 3, 15546-15557.
3. J. R. Scheffe, D. Weibel and A. Steinfeld, Energy & Fuels, 2013, 27, 4250-4257.

Semiconductor germanium (Ge) is an extensively studied element and widely used in the semiconductor industry. However, its use as an active medium in photovoltaics is significantly hampered by its inefficient light absorption due to the indirect band gap. It has been proposed that the indirect band gap of germanium can be switched to direct band gap by alloying with tin and thereby make them a better light absorber. However, the large lattice strain and low solubility of tin in germanium makes it challenging to obtain their alloys in bulk. Nonetheless, utilizing the nanocrystals’ ability to relax large lattice strain combined with a particular selection of precursor molecules, we have already achieved¹ the synthesis of Sn_xGe_1-x alloy nanocrystals with x up to 0.4. Here we report further improvements in the synthesis of Sn_xGe_1-x nanoalloys that yielded the product with tin content up to 95% pushing the composition limit, which has not been possible earlier. X-ray diffraction, TEM and EDX mapping analysis have confirmed the presence of homogenous alloys over the full composition range. Mechanistic and computational studies of the material formation point to the reaction pathway through the formation of intermediate cubane-like Sn-Ge imide clusters. Absorption spectroscopy of NCs of varying composition demonstrates a pronounced red-shift and increase in molar absorptivity as a more direct indicator of changes in the electronic structure toward direct band gap behavior with increasing Sn-content. The details of the synthetic approach together with results of the structural, optical characterizations and mechanistic investigation will be presented. Mechanistic implications of Sn-Ge nanoalloy formation allowed us to introduce silicon into the alloy composition and the preliminary results on the synthesis of Si_xGe_ySn_1-x-y nanoalloys will also be discussed.
References:
1. Ramasamy, K.; Kotula, P. G.; Fidler, A. F.; Brumbach, M. T.; Pietryga, J. M.; Ivanov, S. A. Chem Mater. 2015, 27, 4640-4649.

High quality ultrathin transition-metal dichalcogenide films and their Vander Waals heterostructures are very promising for fabrication of optoelectronic devices for their superior photovoltaic performance, extreme flexibility and long lifetime due to suitable band gap and excellent light absorption capability. Designing the appropriate and suitable band structures of these materials is crucial for its promising applications. Especially, thin molybdenum disulfide (MoS₂) film is further attractive as compare to its monolayer counterpart for broad-spectral photo-detectors and photovoltaic application. However, very little understanding regarding the underlying physics behind the photo-response behavior of thin MoS₂ films is known. Here, we demonstrate the effect of thickness for the generation of photocurrent in MoS₂ based solar cell which produces an enormous photo-generated carrier and enhance the photocurrent by forming a heterojunction with p-type Si substrate. MoS₂ based photovoltaic devices efficiently produce high short-circuit photocurrent values of 16 mA/cm² and reasonably high power-conversion efficiency of 4.2%. The present work will provide valuable scientific impetus of layered MoS₂ films for other emerging optoelectronic applications.

The current economic and ecological contexts push countries to find new solutions to the key issue of renewable energies. Among the different approaches, the improvement of solar cells efficiency is a promising way to increase the environmentally friendly produced energy. One of the solutions consists in developing down-converter layers compatible with the silicon based industry in order to absorb and convert one UV photon in two Infrared ones. Such frequency conversion films could be directly and easily deposited on commercial Si-based solar cells to improve the photovoltaic conversion efficiency at moderate cost.

To achieve such a goal, rare earth (RE) doped silicon nitride-based thin films have been deposited by reactive magnetron co-sputtering. This nitride host matrix allows a high RE incorporation while avoiding the clustering effect observed in silicon oxide matrices. Moreover it has been used as an efficient antireflective layer to increase the Si solar cell efficiency. Two different co-doping with RE ions have been chosen i.e.: Tb³⁺-Yb³⁺ and Ce³⁺-Yb³⁺.

The first step of this work consists in achieving an intense emission of Tb³⁺ or Ce³⁺ ions by optimizing the deposition parameters. For the former ions, the goal is to enhance the coupling between sensitizers and Tb³⁺, while for the later, the authorized 5d-4f transitions allow to achieve a direct efficient excitation of the RE. A comparison between the excitation efficiency of each co-doped system will be presented.

The second step concerns the incorporation of an increasing Yb³⁺content to optimize the Tb/Ce-Yb coupling with the aim at obtaining an intense emission at 980 nm just above the Si gap. The excitation mechanisms of Tb³⁺ and Yb³⁺ ions will be discussed with respect to the optical properties of co-doped systems. By a deep analysis of the optical properties of these systems, the excitation mechanisms of Tb³⁺ and Yb³⁺ ions will be discussed. The infrared quantum cutting efficiency of Tb³⁺–Yb³⁺ and Ce³⁺-Yb³⁺ co-doped silicon nitride-based systems found by photoluminescence decay time measurements above 180% will be presented. The internal quantum efficiency of these co-doped thin films will be estimate for different excitation wavelengths.

Finally, Si solar prototypes using these two optimized structures have been fabricated and tested to demonstrate the efficiency improvement expected under illumination conditions corresponding to AM1.5 Reference Solar Spectral Irradiance. The passivation effect through carrier lifetime analysis will be discussed.

Organometal halide perovskite absorbers such as methylammonium lead iodide chloride (CH₃NH₃PbI_3-xCl_x) have emerged as a new material family for photovoltaics. In a perovskite solar cell, absorption of photons results in the generation of electron-hole pairs that evolve towards the formation of highly delocalized Wannier type excitons, a fraction of which would dissociate spontaneously back into free carriers. The excitons and free carriers co-exist within the perovskite hence charge (electron-hole) recombination can take place within the perovskite layer. Ferroelectrics can be exploited as a driving force for efficient charge separation because spontaneous electric polarization and the resulted internal electric field promotes the desirable separation of photo-excited charge carriers and allows a higher photocurrent and a photovoltage, which may enable efficiencies beyond the current maximum in perovskite solar cells. Barium Titanate (BaTiO₃ –BTO) is an excellent ferroelectric material which has a high dielectric constant. In this work we explore a hybrid structure of CH₃NH₃PbI_3-xCl_x perovskite and ferroelectric BTO nanocomposite ITO/CH₃NH₃PbI_3-xCl_x:BTO/Al as a new type of photovoltaic device. Optimum growth conditions for the perovskite have been investigated and published in our previous work. A balance between perovskite coverage and ferroelectric nanoparticle concentration is important for efficient device function. Optimum BTO nanoparticle concentration for maximum remnant field within the hybrid structure was first investigated by fabricating BTO and polyvinylidene fluoride (PVDF) nanocomposites. Polarization measurements of composites deposited by a low-pressure spray process showed excellent ferroelectric properties. The process was extended to perovskite composite layers of CH₃NH₃PbI_3-xCl_x and BTO nanoparticles dissolved in dimethylformamide (DMF). A comparison of device performance with and without BTO will be presented.

Single-atom catalysts (SAC) are the limit of supported metals in downsizing heterogeneous catalysts. SACs maximize the efficiency of metal atom use, which is particularly important for supported noble metal catalysts. Moreover, with well-defined and uniform single-atom dispersion, SACs have recently demonstrated remarkable activity and selectivity for a variety of catalytic reactions. Although SACs are endowed with unique and excellent performance, so far, the study around SACs mainly concentrated in the field of heterogeneous catalysis, while little research had undertaken in the field of electrocatalysis. Electrocatalytic processes will undoubtedly be at the heart of energising future transportation and technology with the added importance of being able to create the necessary fuels in an environmentally friendly and cost effective manner. Consequently, the association between single-atom catalysts and electrocatalysis needs more attention and urgent research work.
The single-atom Pt₁/FeO_x catalysts with different Pt loadings as well as bare FeO_x were prepared and fabricated as counter electrodes (CEs) for dye-sensitized solar cells (DSCs). Electrocatalytic behaviors and photovoltaic behaviors of the SACs samples were investigated in-detail. With no Pt loading, the bare FeO_x show extremely low catalytic activity. Impressively, with very extremely low loading of single atoms, for example one Pt atom per 100 nm², a noticeably enhancement in catalytic activity and photovoltaic performance can be observed. DFT calculation was performed and demonstrated that ionization potential was responsible for the rate-determining step. Implicit acetonitrile solvent significantly reduced the ionization potential and enhanced electron donating ability. The charge density difference of adsorbed I and O₃-termination Pt₁/Fe₂O₃(001) surface showed that charge transfer occurs mainly between I atom and Pt atom anchored on the surface. The root of lowered ionization potential was the single atom Pt 5d orbital interacted with the support Fe₂O₃, accompanied with the much concentrated electronic states and higher density of occupied states of Pt₁/Fe₂O₃(001) around the Fermi energy. Thus, on the basis of experimental research, the role of single atom Pt in Pt₁/FeO_x during the I₃^- electro-catalytic reduction process was uncovered at the micro level.

Interface engineering of CdS/CZTS(Se) is an important issue for improving the performance of buffer/absorber heterojunction combination. By controlling thickness[1], soft baking temperature, or surface modification[2], the crossover phenomenon, which is attributed to interface recombination of this heterojunction, can be drastically eliminated; however, an in-depth study of the buffer/CZTS(Se) junction properties and its optimization has not been reported yet. In this work, we will present a detailed analysis of the effect of temperature in soft thermal process on the CdS/CZTSSe interface, and its consequence on the optoelectronic, micro-structural, and compositional properties. The CZTSSe precursor was prepared by stacked metallic layers and selenized using a fast ramping annealing (FRA) process; whereas the CdS buffer layers of about 60 nm thick were synthesized using a conventional chemical bath deposition (CBD) process, followed by soft baking process at various temperatures. By changing soft baking temperature on CZTSSe/CdS heterojunction the crossover phenomenon can be greatly improved. Based on photoemission studies, including XPS and UPS in depth, as well as scanning transmission electron microscopy (STEM) combined with energy dispersive spectroscopy (EDS) studies, we observed formation of Cd-Se bonding on the Cu-poor surface of CZTSSe due to interdiffusion during baking; concurrently, an n-type inversion layer[3] was also formed, as confirmed by depth profile XPS and STEM. However, if soft baking of CdS/ CZTSe junction sample was performed at low temperature, Cd would still diffuse into CZTSSe system, without having any bonding with CZTSSe system, but forming interface dipole, which deteriorated the CdS/CZTSSe junction properties. Therefore, the n-type inversion layer functioning as a Cd-CZTSSe/CZTSSe homojunction without detrimental recombination centers can lead to increases in both shunting resistance and fill factor, ultimately improve the power conversion efficiency from 3.2% to 7%.

Keywords: Kesterite structure, chalcogenide compound, thin film solar cell, heterojunction.

REFERENCES
[1] M. Neuschitzer, Y. Sanchez, S. López-Marino, H. Xie, A. Fairbrother, M. Placidi, S. Haass, V. Izquierdo-Roca, A. Perez-Rodriguez, E. Saucedo, Authors, Prog. Photovolt: Res. Appl. (2015), DOI: 10.1002/pip.2589
[2] K. B. Messaoud, M. Buffière, G. Brammertz, H. ElAnzeery, S. Oueslati, J. Hamon, B. J. Kniknie, M. Meuris, M. Amlouk, J. Poortmans, Prog. Photovolt: Res. Appl. (2015), DOI: 10.1002/pip.2599
[3] T. T. Wu, F. Hu, J. H. Huang, C. H. Chang, C. C. Lai, Y. T. Yen, H. Y. Huang, H. F. Hong, Z. M. Wang, C. H. Shen, J. M. Shieh, Y. L. Chueh, ACS Appl. Mater. Interfaces 2014, 6, 4842−4849

Solution processed Cu₂ZnSn(S,Se)₄ (CZT(S,Se)) quaternary thin film solar cells have attracted enormous interests as an economic alternative to other industrially-proven thin film technologies such as CIGS and CdTe. Recently, many laboratories have fabricated CZT(S,Se)-based solar cells with close to or above 10% power conversion efficiency using non-vacuum processes. Towards incorporation of thin film PV onto cost-effective, flexible and building material substrates such as metal or plastic foils, a simple solution-based approach for the fabrication of earth-abundant and non-toxic kesterite CZTS thin film solar cells is reported. Generally, toxic materials such as S, Se, H₂S and H₂Se are used to provide in excess during high-temperature annealing process to form proper CZT(S,Se) films. This drawback of using these toxic materials is an increase in process complexity, cost and safety hazard. To reduce the amount of chalcogen in the post-annealing process, we demonstrate significant increased sulphur participation into the pure sulphide CZTS film by further sulphur additions from a thiourea source, into the DMSO based precursor solution confirmed by Thermogravimetric analyses (TGA). The precursor solution was spin-coated on Mo-coated glass. Various heating profiles were attempted using a rapid thermal processing system (RTP) to investigate the sulphur losses from different S-rich precursors during post-annealing process. To understand whether the elemental distribution across the CZTS layer is varied by extra thiourea, a systematic compositional study using X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) is presented. XPS depth profiling and EDX reveal that an increase of sulphur incorporation in the final CZTS films is observed by adding more thiourea. In this study, the grain size was reported to reduce slightly with increased sulphur content while surface morphology was changed significantly. The effect on the surface of the CZTS film has been investigated using Raman spectroscopy, XPS and scanning electron microscopy (SEM). The annealed CZTS films have been attempted to fabricate solar cells devices using a standard Mo/CZTS/CdS/iZnO/ITO configuration.

CZTS/Se is a prospective compound semiconductor for applications in photovoltaic (PV) solar cells since it includes only abundant and non-toxic elements. Due to recent achievements in CZTS/Se processing, the current efficiency of conversion is above 12% in laboratory cells. However, higher efficiency is required for CZTS/Se PV panel commercialization. Comprehensive DFT-MD simulations of CZTS_xSe_1-x materials have been performed simulating various bulk defects and phase diagrams to develop an atomic level understanding of the defects and stability regions in the space of chemical potentials to steer engineering efforts in CZTS/Se processing.
Density-functional theory simulations of CZTS, CZTSe and CZTS_0.25Se_0.75 photovoltaic compounds have been performed to investigate the stability of the CZTS_0.25Se_0.75 alloy vs. decomposition into various secondary compounds. The vibrational component of the Gibbs energy was estimated by calculating phonon spectra and thermodynamic properties at finite temperatures. It was shown that the CZTS_0.25Se_0.75 alloy is stabilized not by enthalpy of formation but by vibration and mixing contributions to the Gibbs energy. A set of phase diagrams was built in the multidimensional space of chemical potentials at 300K and 900K temperatures to demonstrate alloy stability, boundary compounds at various chemical conditions and to steer experimental film processing. The Gibbs energy change for several decomposition reactions was calculated as a function of temperature with/without intermixing and vibration contributions to the Gibbs energy, demonstrating that even defect-free (no intermixing) CZTS_0.25Se_0.75 can be stable at typical processing temperatures.
Despite significant improvements in performance over the past decade, CZTS/Se solar cells demonstrate a low open-circuit voltage (Voc). This deficit in Voc is believed to be caused, at least in part, by a large density of bulk defects and compositional inhomogeneities, particularly atomic scale intermixing between Cu and Zn atoms in the kesterite crystal lattice. The Cu/Zn intermixing combined with the low dielectric constant of CZTS_xSe_1-x makes the material more prone to electrostatic potential fluctuations of the valence and conduction bands severely affecting the Voc.
One method to reduce the intermixing is to replace Cu with elements that have larger covalent radii such as silver (Ag) in order to increase the energy barrier for site exchange with Zn. Ag has a covalent radius of ~0.153nm which is approximately 16% larger than that of Zn (as opposed to Cu which is only approximately 5% larger than Zn). Ag incorporation into CZTSe increases the band gap and reduces the atomic disorder. In this study, DFT calculations were employed to evaluate the [Ag_Zn + Zn_Ag] defect pairs formation energy and the effect of Ag/Zn intermixing on the electronic properties of AZTS_xSe_1-x alloys. These intermixing energies are compared with the values calculated for CZTS_xSe_1-x compounds.

The quaternary compound semiconductor Cu₂ZnSnS₄ (CZTS) is a low cost absorber material for solar cells. Presently, solar cells using CZTSSe have reached efficiencies of about 12.6% thus pointing out promising future perspectives [1]. The polycrystalline layer exhibits an off-stoichiometric composition with intrinsic point defects. These defects have a significant influence on the electronic properties of the absorber material. [2]
This work focusses on the synthesis of phase-pure CZTS powders and layers via CZTS nanoparticle inks and the characterization of the composition and the structure.
Therefore, nanoparticle inks have been synthesized using the hot-injection method under inert conditions. To improve the synthesis different amounts of copper(II)acetylacetonate (Cu(acac)₂), zinc(II)acetylacetonate (Zn(acac)₂) and tin(IV)bis-acetylacetonate dichloride (Sn(acac)₂Cl₂) were solved in oleylamine. After heating the metal ion solution to an injection temperature of about 160 to 280 °C, a 1 M sulfur solution in oleylamine was added. For the following reaction, the mixture was heated up to 200 to 320 °C. After the syntheses, the dried nanoparticle powders were annealed at 600 to 750 °C for 5 days to 2 weeks, partially in sulfur atmosphere. The optimum reaction mixture to obtain pure CZTS powder after annealing was 2.0 mmol Cu(acac)₂, 1.2 mmol Zn(acac)₂ and 1.0 mmol Sn(acac)₂Cl₂. The best reaction conditions were an injection and a reaction temperature of 240 °C and an annealing temperature of 600 °C for 5 days in sulfur atmosphere. The nanoparticle inks were also processed into thin films on Mo coated glass slides with the aid of chemical mediators for solar cell applications.
To determine the phase purity of the CZTS nanoparticles and the annealed powders, an electron microprobe system equipped with wavelength dispersive X-ray analysis was used. For a detailed analysis of the nanoparticles X-ray diffraction, energy-dispersive X-ray spectroscopy, Raman spectroscopy, high-resolution transmission electron microscopy, and selected area electron diffraction were performed. With these methods, we were able to identify side products and to improve the CZTS ink synthesis. After annealing, the lattice parameters a and c of the CZTS powders were determined by Rietveld refinement of the X-ray diffraction data. Moreover, neutron powder diffraction was performed with our best CTZS sample at the BERII fine resolution neutron powder diffractometer E9 (HZB, Berlin, Germany). The neutron scattering length of Cu and Zn is different, thus it is possible to distinguish between Cu⁺ and Zn⁺ site occupation in the crystal structure. Rietveld refinements of the neutron diffraction data followed by an average neutron scattering length analysis allows to calculate the Cu/Zn order-disorder at the 2c and 2d sites and the formation of intrinsic point defects.

[1] Wang et al., Adv.Energy materials (2013) 1301465-1301470.
[2] Lafond et al., ZAAC 638 (2012) 2571-2577.

The solar absorptance property of W/WAlN/WAlON/Al₂O₃-based coating, deposited by DC/RF magnetron sputtering on W coated stainless steel substrate was studied by measuring the reflectance spectra in the wavelength range of 250-2500 nm. The effect of thermal annealing on the optical properties, microstructure and morphology of the solar selective absorber coating was also investigated. Annealing the coating at 350°C for 450 hrs in air did not show any significant change in the spectral properties of the absorber coating indicating the excellent thermal stability of the coating. However, annealing for longer duration (> 450 hrs) leads to a decrease in solar absorptance and a considerable increase in thermal emittance due to degradation of the coating. The experimental studies on the degraded coating are discussed in reference to the interdiffusion between the layers as well as oxidation of the tungsten layer. Taken together, the present study indicates the potential application of W/WAlN/WAlON/Al₂O₃-based selective coating in mid temperature photo thermal conversion systems.

References:
[1] H.L. Zhang, J. Baeyens, J. Degrève, G. Cacères, Concentrated solar power plants: Review and design methodology, Renewable and Sustainable Energy Reviews, 22 (2013) 466-481.
[2] D. Barlev, R. Vidu, P. Stroeve, Innovation in concentrated solar power, Solar energy materials and solar cells, 95 (2011) 2703-2725.
[3] A. Ambrosini, T.N. Lambert, M. Bencomo, A. Hall, N.P. Siegel, C.K. Ho, Improved high temperature solar absorbers for use in concentrating solar power central receiver applications, in, American Society of Mechanical Engineers, pp. 587-594.
[4] C.E. Kennedy, Review of mid-to high-temperature solar selective absorber materials, National Renewable Energy Laboratory Golden Colorado, 2002.
[5] N. Selvakumar, H.C. Barshilia, Review of physical vapor deposited (PVD) spectrally selective coatings for mid-and high-temperature solar thermal applications, Solar Energy Materials and Solar Cells, 98 (2012) 1-23.

We present examples of application of self-assembled nanostructures to organic electronic devices. We report the self-assembly of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) organogel films incorporating graphene quantum dots (GQDs). Owing to the electrostatic interaction between the GQDs and the PEDOT chains, GQD@PEDOT core-shell nanostructures are readily formed. We demonstrate that the GQDs affect the reorientation of PEDOT chains and the formation of interconnected structure of PEDOT-rich domains, improving the charge transport pathway. The power conversion efficiency of the organic photovoltaic device containing the self-assembled organogel as the hole transport layer (HTL) was 26% higher than the device with pristine PEDOT-PSS as the HTL. We also report that carbon allotropes can be dispersed in aqueous solution through non-covalent interactions with structurally similar self-assembled nanostructures from amphiphilic molecules. Furthermore, a mixture of self-assembled 2-D nanosheet structure + graphene and PEDOT:PSS was successfully introduced into the HTL to improve the performance of organic photovoltaic devices by increasing the photocurrent. These results demonstrate that the selective non-covalent functionalization of carbon allotropes by self-assembled nanostructures would be an attractive method for the fabrication of optoelectronic devices.

First Solar is a leading utility-scale PV company, with over 10 GW capacity installed worldwide. Using its proprietary CdTe thin-film technology, the company is now delivering solar energy that is cost competitive with fossil fuels in many areas of the world. From this vantage point, I will review alternative photovoltaic materials and devices, such as tandems or solution-based absorber layers, and put them into context of the cost/performance roadmaps of the PV industry. In addition, I will discuss how high the bar is that new technologies must clear to potentially replace today’s leading silicon and CdTe technologies.

The U.S. Department of Energy (DOE) SunShot Initiative is a collaborative national effort to reduce the price of solar energy to 6¢/kWh for utility scale installations by 2020. Thin-film photovoltaics offer a promising path to reach this goal due to low module manufacturing cost approaching $0.40/W with a potential for further decrease. This presentation will focus on the barriers that commercial thin-film photovoltaics still need to overcome to meet the SunShot goal. Module cost analysis will highlight the dependence of the path to SunShot goal on the module efficiency and lifetime. The challenges and opportunities identified for thin-film PV research at the 4th DOE SunShot Thin-Film Photovoltaic Workshop held at NREL in August 2015 will be summarized, and the areas where the gaps of knowledge and expertise still exist and a substantial effort is needed to move the state-of-the-art will be discussed. These areas include device design and material improvement to reach higher efficiency, reliability-enhancing module design, robust manufacturing processes with high yield and low cost, and integrated simulation, characterization, and device investigations. The presentation will also include highlights of recent progress in thin-film PV research funded by the SunShot Initiative.

Earth-abundant kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells have shown significant improvement in material quality and photovoltaic performance over the past few years. However, the growth in efficiency has slowed, due at least in part to the intrinsic limitations imposed by the bulk defects in CZTSSe and in particular the band tailing believed to be caused primarily by Cu-Zn disorder. Multiple spectroscopy techniques have experimentally shown that disorder exists on the Cu-Zn sublattice. One method to reduce the intermixing is to replace Cu with elements that have larger covalent radii such as silver (Ag) in order to increase the energy barrier for site exchange with Zn. Silver has a covalent radius of ~0.153nm which is approximately 16% larger than that of Zn (as opposed to Cu which is only approximately 5% larger than Zn). Ag incorporation into CIGSe has successfully improved the Voc by increasing the band gap and reducing the atomic disorder (and resulting band tailing). In this study, Density Functional Theory (DFT) calculations are employed to evaluate the energetics of Ag_Zn + Zn_Ag defect pair formation and the effect of Ag-Zn intermixing on the electronic properties of Ag₂ZnSn(S,Se)₄ (AZTSSe) as well as (Ag,Cu)₂ZnSnSe₄ alloys. Photoluminescence spectroscopy was employed to verify the low bulk defect density in Ag₂ZnSnSe₄. A second source of efficiency limitation is carrier recombination at grain boundaries. In both CZTS-Se and AZTS-Se these are not atomic scale structures but instead of nanoscale (>10nm) phases. To characterize the grain boundaries as a function of depth within the films, grazing angle cryo-Focused Ion Beam (at -180 °C to -190 °C) was employed to section the grain boundaries without perturbing their electronic structure. Afterwards, Auger nanoprobe microscopy (NanoAuger) was combined with high resolution ambient Kelvin Probe Force Microscopy (KPFM) to probe the chemical and electronic structure of the grain boundaries. Solution-deposited CZTS,Se grain boundaries showed negative charge (upward band bending) that is associated with the presence of SnOx phase formed during the film’s air anneal. In contrast thermally-evaporated AZTSe did not have a chemically distinct phase at its grain boundaries but small amounts of positive charge was detected at the grain boundaries. This work was supported by DoE contract DE-EE0006334.

The all-oxide solar cell is a promising concept for highly scalable next-generation photovoltaics (PV). An interesting candidate material is SnO, which has an excellent electron and hole mobility and can be doped both p-type and n-type, thereby opening the possibility of a homojunction device design. The main drawback of SnO as a PV material lies in the indirect nature of the band structure, with a band gap of only 0.7 eV and the minimum direct band gap being much larger at about 2.7 eV. In this work, we investigate isoelectronic cation-site alloying as a means to manipulate the band structure and optical properties.

Computationally, we performed first-principles calculations for alloying SnO with the oxides of the divalent cations Mg, Ca, Sr, and Zn. Related to the heterostructural nature of the alloying (litharge-rocksalt and litharge-wurtzite), the mixing enthalpies show a linear trend, which should suppress the tendency towards spinodal decomposition, and aid the formation of homogeneous films. The absolute mixing energies are about 0.9 eV per substituted Zn or Mg atom, and lower energies of about 0.5 and 0.2 eV are found for Ca and Sr, respectively, which have a smaller size mismatch with Sn(II). These energies suggest that non-equilibrium synthesis routes could be feasible for deposition of alloy compositions above the thermodynamic solubility limit. In order to assess the alloying effect on the band structure, we unfolded the band structure of the alloy supercells. This analysis showed that the alloying creates perturbations at a few hundred meV above the CBM or below the VBM, but keeping the dispersive nature of the band edges largely intact. At the same time, the band gap increases and the absorption onset decreases as a function of the alloy composition, such that alloys with as little as 10% cation substitution could be interesting as PV absorbers.

Experimentally, we deposited Sn_1-xZn_xO alloy thin films by combinatorial radio-frequency (RF) reactive magnetron sputtering from metallic targets, using a combinatorial approach with both composition and temperature gradients. The structure and composition was analyzed by spatially resolved techniques for all samples, and more in-depth characterization of the electrical and optical properties was performed for selected samples. The addition of Zn during the SnO synthesis induces a change of the Sn valence state from Sn⁰ via Sn²⁺ to Sn⁴⁺, which can be counteracted by reducing the oxygen partial pressure in the sputtering atmosphere. By simultaneously adjusting the Zn and the O content, we were able to prepare (Sn,Zn)O films with up to Zn/(Sn + Zn) = 10% in the tetragonal SnO-like structure without a significant formation of secondary phases according to XRD. Initial optical measurements showed a decrease in absorption onset, consistent with the theoretical predictions, but more detailed studies are needed to confirm that the optical properties are controlled by the alloy composition.

Solar energy conversion using emerging semiconductor materials is an important research direction in the area of photovoltaic (PV) and photoelectrochemical (PEC) solar cells. However, the progress to turn promising absorber materials into working photovoltaic devices is usually quite slow. A possible approach to accelerate this process is by high-throughput combinatorial experiments supported by first-principles theoretical calculations, referred to here as “Rapid Development”.

In this presentation, I will discus the general features of the “Rapid Development” approach, and illustrate it by recent example of chalcostibite photovoltaics. Chalcostibites CuSbQ₂ (Q=S,Se) is an interesting class of layered thin film solar cell absorbers that are chemically similar but structurally different than Cu(In,Ga)Se₂ (CIGS). Rapid development of chalcostibite photovoltaics, including the CuSbQ₂ self-regulated absorber growth [1,2] and combinatorial PV device development [2,3], recently led to ~5% solar cell efficiency with a clear path towards >10% PV devices.

Besides the rapid development of chalcostibite photovoltaics, recent progress in understanding effects of cation disorder in Cu₂SnS₃, Cu₂ZnSnS₄ (CZTS) and related tetrahedrally bonded absorber materials will be highlighted. I will also briefly discuss our ongoing efforts in absorber materials for oxide- and nitride-based photovoltaics.

This work was supported by U.S. Department energy, office of Energy Efficiency and Renewable Energy, as a part of the “Rapid Development of Thin Film Solar Cells” project within the SunShot initiative. Experimental contributions from Adam Welch, Lauryn Baranowski, Willian Lucas, as well as the theoretical collaborations with Pawel Zawadzki, Haowei Peng and Stephan Lany are gratefully acknowledged.


[1] Sol. En. Mat. Sol. Cel. 132, 499 (2015)
[2] Appl. Phys. Exp. 8, 082301 (2015)
[3] arXiv:1504.01345 (2015)

SnS (tin mono-sulfide) is currently generating great interest in materials science as it bears a huge potential for sustainable, low-cost and large-scale solar energy conversion. SnS is a non-toxic material consisting of abundant and cheap elements and possesses a high absorption coefficient and a band gap of 1.3 eV, which is beneficial for its application in solar cells. To date, the material is not yet explored in detail and there is only a limited number of studies reporting on SnS-based solar cells. The highest power conversion efficiency (4.4%) reported for solar cells using SnS as absorber material is very promising and was obtained in a thin film solar cell architecture in which the SnS layer was deposited in a vacuum-based process.^[1] In terms of low-cost processability using coating and printing techniques, the use of solution-based fabrication methods for SnS and the deposition of SnS in combination with organic semiconductors to form hybrid solar cells is particularly interesting. In this contribution, we introduce a facile solution-based route for the preparation of nanostructured SnS layers and demonstrate their suitability for efficient charge generation in hybrid photovoltaic devices. The hybrid films are fabricated using a precursor solution containing tin(II) chloride and thioacetamide which is deposited on a substrate to form a precursor layer. The precursor film is then converted into a SnS layer by thermal annealing in inert atmosphere. SEM images of the prepared films reveal that the layers consist of a porous nanoplate network, which can be readily infiltrated with a conjugated polymer to obtain a nanostructured hybrid solar absorber material.^[2] A transient absorption spectroscopic study on as prepared hybrid SnS/P3HT films revealed that long-lived charges are generated in the layers upon illumination, highlighting their potential for solar cell applications. Furthermore, hybrid solar cells were prepared in inverted device architecture and showed very promising short circuit currents up to 11-12 mA/cm². These high I_scs are based on current generation in a broad spectral range, which is due to a significant contribution of the SnS phase to charge generation in the hybrid solar cell.
[1] Sinsermsuksakul, P.; Sun, L.; Lee, S. W.; Park, H. H.; Kim, S. B.; Yang, C.; Gordon, R. G. Adv. Energy Mater. 2014, 1400496.
[2] Rath, T.; Gury, L; Sánchez-Molina, I.; Martínez, L.; Haque, S. A. Chem. Commun. 2015, 51, 10198-10201.

This work aims to explore a new class of chalcogenide semiconductors, tetrahedrite superabsorbers, for use in next-generation ultra-thin solar cells (TFSCs) where photoexcited charge carrier collection is enhanced by drift due to built-in electric field. In conventional TFSCs, a weak absorption onset near the band-gap (E_G) requires the absorber thickness to be 1-2 mm. In then, the absorber must exhibit high carrier mobility and a long minority carrier lifetime in order to approach the radiative recombination limit of efficiency. Reduction of the absorber thickness is an alternative and to be a large extent uninvestigated approach for improving TFSC performance. Reducing the absorber thickness allows for creation of a drift field across the absorber, aiding in the enhancement of photo-generated carriers. Improved minority carrier collection in a drift-aided TFSC relaxes the electrical performance requirements of the absorber. A drift-based TFSC is also more defect-tolerant, which should lead to simpler manufacturing. However, an efficient drift-aided TFSC requires that the absorber layer must exhibit a strong absorption coefficient > 10⁵ cm^-1 with an abrupt onset near E_G. Few inorganic materials exhibit these qualities. Very recently, a family of tetrahedrite compounds have identified as promising candidates through computational inverse design and subsequent experimental studies on the optical absorption and photovoltaic potential of materials. However, hole mobility of the materials could not be measured. Here we describe the properties of a synthetic mineral tetrahedrite-derivative that exhibits an unprecedented combination of strong absorption coefficient (α > 10⁵ cm^-1 at E_G + 0.1 eV) and good hole transport (m_p > 5 cm² V^-1 s^-1), as identified by conducting detailed electrical measurements and then modeling the results on the basis of impurity ion scattering, a major breakthrough in understanding the tetrahedrite transport properties. In addition, thermodynamic calculations and device simulations confirms that this unique combination of properties should enable high-efficiency (>20%) TFSCs in a drift-based operation mode with tetrahedrtie superabsorbers as thin as 200 nm.

Current Cu(In,Ga)Se₂-based solar cells are able to reach power-conversion efficiencies of more than 15% while exhibiting experimental dislocation densities up to 10¹⁰ to 10¹¹ cm^-2. This finding suggests that dislocations in this material are electrically not active or passivated by point defects.
In order to understand the role of dislocations in these absorbers, we focus on the structures found in experiments, namely the perfect screw and 60° dislocations and the interstitial Frank loop. We do so in both CuInSe₂ and CuGaSe₂ by means of first-principles calculations within density functional theory. We present and compare results on structural and electronic properties of different dislocation types. Our study reveal that all dislocations being investigated induce localized-shallow defect states, while deep defect states are only observed for the glide 60° dislocation in CuInSe₂. We also found evidence of charge accumulation in the surroundings of dislocation with and edge component. Interaction with point defects, known to be common and charged in CuInSe₂ and CuGaSe₂, is then caused by both elastic and electrostatic effects. In addition, based on calculated binding energies we conclude that, close to the cores and depending on the dislocation type, there is accumulation or depletion of copper. This non-stoichiometry induces the formation of a barrier for holes due to an offset between the valence band maximum at the cores and the corresponding value in the surrounding bulk. These results give an initial step towards understanding why high densities of dislocations appear not to be detrimental for the efficiency of such devices.

Despite the suitability of CdTe’s band gap and absorption coefficient for photovoltaic applications, its efficiency remains more than ten absolute percent below the predicted Shockley–Queisser limit. Current research efforts are directed towards improving open-circuit voltage (V_OC) and fill factor, since the short-circuit current density is already near the theoretical limit. The use of extrinsic dopants is expected to improve the V_OC substantially.
Indium (In) is currently the leading n-type dopant in CdTe and has been reported to achieve doping density of about 2x10¹⁸ cm^-3, but self-compensation is still a challenge with the heavy use of indium as a dopant in CdTe. Alternative candidates to indium as dopants in CdTe are group VII elements such as Bromine (Br), Chlorine (Cl) and Iodine (I). These have been used to achieve reasonable N_D, but Cl and Br seem to suffer from self-compensation and N_D limited to the low 10¹⁸ cm^-3. Iodine appears to be a capable alternative for n-type doping with potential to achieve N_D of 10¹⁹ cm^-3 and encouraging optical properties.

Iodine doped CdTe double heterostructures with variable doping concentration were grown with molecular beam epitaxy. SIMS characterization was used to measure doping concentration, while Hall measurement was used for estimating carrier concentration. Photoluminescence intensity and confocal photoluminescence techniques were used for optical characterization. We achieved heavy doping of Iodine in CdTe at levels up to 7.4x10¹⁸ cm^-3 without self-compensation and achieved a surface recombination velocity well below 200 cm/s. Studies suggest that compensation occurs with doping concentration much above this value. Dopant activation of about 80% was observed in most of the doped samples. These Iodine doped samples exhibit long lifetimes with evidence of photon recycling effects and no evidence of PL degradation with doping as high as 2x10¹⁸cm^-3 while Indium show substantial non-radiative recombination at levels above 5x10¹⁶ cm^-3. This suggests that iodine is a better alternative dopant to Indium in achieving doped n-type CdTe.

Hematite (α-Fe₂O₃) remains one of the most promising photoanode materials for photoelectrochemical (PEC) water splitting due to its unique combination of suitable optical and electrochemical properties combined with excellent stability and elemental abundance. This presentation will discuss results of hematite electrodes made via electrodeposition (ED) which are compared to planar analogs prepared by atomic layer deposition (ALD). Surprisingly, the ED films significantly outperformed the ALD made films despite comparable morphology and dimensions. The superior performance is attributed to variations in the crystallographic properties which results in enhanced hole transport and collection. These results indicate a non-zero hole diffusion length for the electrodeposited hematite thin films in contrast to the ALD counterparts, which resulting in the highest photocurrent among reported planar electrodes. Even for these electrodes, PEC and EIS measurements show the accumulation of holes in surface states which can recombine with conduction band electrons. The nature of such surface accumulated holes was studied with in-situ spectroelectrochemical measurements during PEC water oxidation. A bias dependent spectral feature was observed at 572 nm, which exhibited a quantitative correlation with the concentration of surface trapped holes. This absorbance is thus tentatively assigned as an iron oxo species at the surface. Operando infrared (IR) spectroscopy measurements were also performed which provide the first direct evidence that the surface trapped holes are indeed oxidized surface states which mediate PEC water oxidation on hematite electrodes. A potential and light-dependent IR absorption peak is reproducibly resolved at 898 cm^-1 which is correlated to surface trapped holes which build up during current-potential measurements. This peaks is assigned to a Fe^IV=O group, which is an intermediate in the PEC water oxidation reaction. Control experiments in contact with a hole scavenger and isotopically labeled water further corroborated the assignment of these spectral features to oxygen containing groups involved in the water oxidation reaction. In addition, a peak at 743 cm^-1 which appears in response to illumination and potential, but is insensitive to isotopically labeled water, is assigned as a surface trap state. These results establish the mechanism of PEC water oxidation on hematite by providing the first direct evidence of high-valent iron-oxo intermediates as the product of the first hole transfer reaction on the hematite surface.

For the design of tandem solar absorbers in photoelectrochemical absorber systems, stable, intermediate bandgap, p-type absorbers are needed. In this contribution new materials identified by utilizing combinatorial synthesis and high-throughput characterization of materials libraries from the systems Al-Cr-Fe-O and Cu-Si-Nb-Ti-O and subsequent optimization of identified champion materials will be discussed. In the Al-Cr-Fe-O system a new absorber material with a bandgap of 1.5 eV based on Al:Cr₂FeO_x was identified, which exhibits a promising photovoltage of 1 V and photocurrents of about 100 µA/cm². The origin of this exceptionally high photovoltage and routes to increase the photocurrent by nanostructuring will be addressed. In the second materials system the electrochemical instability of Cu₂O is addressed by alloying third elements in a combinatorial study that focused on the full quasi-ternary system Cu-Si-Ti-O. Two new p-type materials based on Si:Cu₃TiO_x and Ti:CuSiO₃ were found to show promising photoelectrochemical properties. Without a co-catalyst photocurrents of up to 430 µA/cm² at 416 mV vs. RHE were observed under 100 mW/cm² visible-light in Si:Cu₃TiO_x. Density functional theory calculations of Ti:CuSiO₃ show a promising electronic structure with the possibility of further doping by additional elements like Nb. In a second step microstructural optimization of the identified materials was performed by utilizing a novel step heater as well as by sputtering in a glancing incidence geometry to obtain nancolumnar morphologies. A roadmap for possible device integration of the newly identified materials will be shown.

A hybrid stack deposition-annealing (SDA) approach via atomic layer deposition (ALD) was successfully demonstrated for the synthesis of CuWO₄, where a stoichiometric amount of CuO was deposited on a stack of WO₃, followed by annealing to produce CuWO₄. The resulted CuWO₄ was characterized and examined as a photoanode for photoelectrochemical (PEC) water splitting. Promising water oxidation activity was observed for CuWO₄ thin films, affirming the promise of this approach. This methodology can in principal, be utilized for the ALD of many ternary and quaternary oxides for a variety of energy conversion/storage and electronics applications.
This synthetic method allowed us to carry out fundamental photoelectrochemical studies to investigate the rate-limiting steps of water oxidation on CuWO₄ thin films and develop strategies to improve its performance. Photocurrent density - voltage (JV) measurements of films with different thicknesses, comparison of incident-photon-to-current efficiencies (IPCE) illuminated from different sides and fitting monochromatic JV curve to Gartner’s model are conducted to quantify the minority carrier diffusion length of CuWO₄. Electrochemical impedance spectroscopy (EIS) and current transient are measured to study the role of surface states of our thin films in water oxidation. The effect of catalysts on the CuWO₄ surface is also studied.

Photoelectrochemical (PEC) water splitting is a promising strategy for converting solar energy to chemical fuels. Thermodynamically, the minimum energy required for splitting H₂O into H₂ and ½O₂ is 1.23 eV. In addition to the thermodynamic limit, additional activation energy is needed to overcome the kinetic barrier and increase the rate of electrochemical reaction. To minimize the requirement of activation energy, transition-metal oxides and hydroxides are frequently applied as low-cost water splitting catalysts, especially for oxygen evolution reaction (OER). Unlike the noble metal catalysts, these oxides and hydroxides are predominantly insulators. Introducing an insulating catalyst between the semiconductor photoelectrode and electrolyte will induce a significant drop of the total potential (defined by the initial Fermi level difference between semiconductor and electrolyte, denoted as φ₀) in the catalyst layer, and thus reduce the built-in potential (φ_bi) and depletion width (L_d) in the semiconductor. The reduced φ_bi and L_d impairs the efficiency of photoexcited charge separation and holes injection. To alleviate these drawbacks of applying an insulating catalyst layer, additional consideration should be given to enhancing the semiconductor/electrocatalyst interfacial electronic structure and properties.

The piezotronic effect is a recently-proposed principle for locally engineering the band structures of heterojunctions using piezoelectric polarization P_pz. P_pz can be generated by mechanically straining a piezoelectric material and considerably influences the free charge distribution and the band structure at the piezoelectric/semiconductor interface. Here, we presented a novel strategy for manipulating the interfacial band structure of a semiconductor/catalyst heterojunction using strain induced P_pz.¹ In a Ni(OH)₂ decorated ZnO photoanode system, appreciably improved photocurrent density of sulfite (SO₃^2-) and hydroxyl (OH^-) oxidation reactions were obtained by physically deflecting the photoanode. Both theoretical and experimental results suggested that the performance enhancement was a result of the piezoelectric P_pz-endowed enlargement of the built-in electric field at the ZnO/Ni(OH)₂ interface, which could drive additional amount of photoexcited charges from ZnO toward the interface for OER. This strategy demonstrates a new route for improving the performance of inexpensive catalysts-based solar-to-fuel production.

1. Li, H.; Yu, Y.; Starr, M.; Li, Z.; Wang, X. Piezotronic-Enhanced Photoelectrochemical Reactions in Ni(OH)₂ Decorated ZnO Photoanodes. J. Phys. Chem. Lett. 2015, 6, 3410-3416.

In this contribution, we will discuss the results of our effort at developing new polymeric materials for water splitting photocatalysis [1,2]. Traditionally, photocatalytic water splitting is the preserve of inorganic semiconductor based light absorbers combined with transition or noble metal co-catalysts. This restriction to inorganic materials is, however, neither inherent nor fundamental. Already in the 1980s, it was demonstrated that oligomers and polymers of p-phenylene under illumination with UV light could drive the reduction of protons to hydrogen [3], and the discovery six years ago that carbon nitride could do the same, as well as oxidise water [4], kick started the field of polymer photocatalysts in earnest. While as yet less explored than their inorganic counterparts, these materials offer the potential of photocatalysts based on earth abundant elements and whose properties can be easily tuned through copolymerisation [1,5].

In Liverpool and London we use a combination of experimental and computational high throughput screening to find (new) classes of polymers that drive photocatalytic proton reduction and/or water oxidation, an activity that has already resulted in the discovery of a range of polymers that evolve hydrogen [1,2]. We will review in our contribution our approach, as well as the new hydrogen-evolving materials. Specifically, we will discuss links between structure and activity, the stability of the materials under operating conditions and the lack of need for added metal co-catalysts.

[1] R.S. Sprick, J.X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, J. Am. Chem. Soc. 137, 3265, 2015.
[2] R.S. Sprick, B. Bonillo, R. Clowes, P. Guiglion, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, submitted for publication 2015.
[3] S. Yanagida, A. Kabumoto, K. Mizomoto, C. Pac, K. Yoshino, J. Chem. Soc. Chem. Commun. 474. 1985.
[4] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nature Mater. 8, 76, 2009.
[5] J.X. Jiang, A. Trewin, D.J. Adams, A.I. Cooper, Chem. Sci. 2, 1777, 2011.

Materials design for photovoltaic (PV) applications has focused largely on the identification of absorber materials with optimal properties. However, the performance of a photovoltaic device also depends critically on the properties of the interfaces between the absorber and other device components, such as buffer layers, charge-blocking layers, and electrodes. Taking interfacial behavior into account during the design process is thus an important challenge that the community has just begun to address. In this talk, I discuss several of our recent efforts in using first-principles-based approaches to understand and incorporate interfacial properties into the design of novel PV materials. In particular, I discuss the role of interfaces on the properties of a promising interface-stabilized ferroelectric photovoltaic system, ZnSnS₃/GaN; the role of defects on the behavior of Sn-doped Ga₂O₃ transparent conducting electrodes; and the design of a new class of self-assembled inorganic-organic nanostructured photovoltaics based on transition metal phosphates.

A promising family of photovoltaic solar cells, based solely on metal oxide thin films, has recently been gaining interest. To improve the inadequate electrical properties of the pure metal oxide materials, various metal oxides are mixed, to create new composite materials that may exhibit enhanced properties. The new materials are then examined as light absorbing layers in photovoltaic cells, stacked between other metal oxide layers in multi-layered structure. The structure is consisting of different metal oxide layers: transparent conductive layer, electron transport layer, absorber layer and hole transport layer. Because of the multi-layered structure, it is difficult to resolve the operating mechanism of these solar cells.
To meet this need, a home-built high-throughput incident photon to current efficiency (IPCE) measuring system was constructed. Using the new system and other high-throughput optical, electrical and structural scanning systems, we thoroughly studied the operating mechanisms of several all-oxide samples, including composite materials of Cu₂O, Fe₂O₃, Co₃O₄, Bi₂O₃, TiO₂ and others. In many of the cells we found evidence for two distinct processes with different operating mechanisms. The two mechanisms may work in parallel, compete, or even counter each other. As for their origin, the two mechanisms may result from photovoltaic activity of two different layers, or alternatively from activity of two separate bandgaps within the same material.
Once the different mechanisms are understood, it is possible to enhance or to suppress one of them. This will allow for further improvement of the all-oxide cells’ photovoltaic performance.

We report on the influence of bulk and surface defects on the carrier collection in cuprous oxide (Cu₂O) photovoltaic devices using photocurrent measurements on large grain polycrystalline Cu₂O solar cells and the first float zone, single crystalline Cu₂O photovoltaics. The devices consist of a Cu₂O substrate with a Zn(O,S) buffer layer, chosen for its earth abundance and ideal band alignment to Cu₂O, a 100 nm ITO front contact and an 80 nm Au back contact. With this device architecture we demonstrate devices with open-circuit voltages exceeding 1V.

Polycrystalline Cu₂O wafers are grown by thermal oxidation of copper foils and cleaned in situ during thin film deposition. Initial analysis of the polycrystalline Cu₂O devices using light-beam induced current (LBIC) measurements indicates a significant variation in photocurrent collection between different Cu₂O grains. We ascribe this difference to variation in crystallographic orientation between the grains, which influences the structure and chemistry of the Cu₂O/Zn(O,S) interface. Our LBIC measurements further demonstrate that grain boundaries in Cu₂O are photocurrent “dead zones” and act as electron traps or recombination centers.

To synthesize Cu₂O single crystals, we use the floating zone method to grow high quality, oriented Cu₂O single crystals as a system to study the fundamental mechanisms limiting Cu₂O photovoltaic efficiencies. Floating zone growth produces a single crystalline rod, which we then orient, cut, and polish to a thickness of 100 μm or less, to expose one of the major crystallographic orientations: (100), (110), or (111). We then define the intrinsic point defect landscape of these Cu₂O crystals by annealing in an inert atmosphere at temperatures ranging between 700 and 1000°C. This allows us to precisely control the concentration of copper and oxygen vacancies, and therefore the carrier concentration in our Cu₂O devices. Because the most efficient Cu₂O devices have stoichiometric Cu₂O interfaces we analyze the chemical character of the interface using x-ray photoelectron spectroscopy (XPS).

To understand the effect of the interfacial stoichiometry on device performance we use steady-state and time-resolved photoluminescence (PL) to elucidate the presence of interfacial recombination centers and to measure the carrier lifetime, respectively. Furthermore, we correlate the relationship between the copper vacancy concentration and the carrier concentration using steady-state PL and Hall effect measurements. The efficiency of the differently oriented devices is investigated by measuring the cell’s J-V characteristics and spectral response.

Thin film photovoltaics (PV), based on materials that absorb light 10-1000 times more efficiently than crystalline silicon, have the potential to lower PV module cost and facilitate broader deployment. To that end, new thin film solar absorber materials composed of Earth-abundant and non-toxic elements that would not pose obstacles to terawatt-scale production are currently the focus of significant interest. Among these materials are II-IV-nitride analogs of the famous group-III nitrides. ZnSnN₂ is one such material that has potential as a solar absorber due to its direct bandgap, steep absorption onset, and disorder-driven bandgap tunability. However, until recently, degenerate n-type carrier density (among other issues such as bandgap discrepancies) hindered further development of this material for PV applications. In this work, we demonstrate doping control in sputter-deposited Zn_1+xSn_1-xN₂ over three orders of magnitude as a function of cation composition and establish the direct bandgap of cation disordered (wurtzite) ZnSnN₂ at 1.1 eV. For 10% zinc-rich Zn_1+xSn_1-xN₂ films with no post-growth treatment, we report n-type carrier density of ~3x10¹⁸ cm^-3. [1] Films grown with the lowest carrier density were then subjected to in situ post-growth annealing under activated nitrogen atmosphere. In this way, we achieved a further decrease in free electron concentration to the PV-relevant value of 1.5x10¹⁷ cm^-3 while maintaining mobility above 2 cm²/Vs. The results of additional annealing treatments are in progress and will also be discussed, including ex situ annealing in forming gas and growth in the intentional presence of hydrogen followed by annealing. A brief discussion of future work will follow, including efforts toward fabricating devices based on zinc-rich Zn_1+xSn_1-xN₂ with both liquid and solid junctions.

[1] A.N. Fioretti, A. Zakutayev, H. Moutinho, C. Melamed, J.D. Perkins, A.G. Norman, M. Al-Jassim, E.S. Toberer, and A.C. Tamboli, J. Mater. Chem. C, DOI:10.1039/c5tc02663f, (2015)

The Group II-IV nitrides are an emerging semiconductor alloy series that are analogous to the well-characterized Group III-nitrides, with the Group III elements (In or Ga) replaced by a combination of a Group II element (Zn) and a Group IV element (Sn or Ge). ZnSn_xGe_1-xN₂ sputtered thin-films have demonstrated that the series has tunable band gaps from ~1.8eV-3.2eV across the entire x composition range, and that films have high absorption coefficients. Thus, there is potential to incorporate ZnSn_xGe_1-xN₂ alloys as absorbers in multijunction photovoltaic cells. Experimentally, ZnSn_xGe_1-xN₂ alloys have also demonstrated stability against phase segregation throughout the alloy series—a potentially significant advantage relative to In_xGa_1-xN alloys, where phase segregation occurs in In_xGa_1-xN with higher In content. ZnSn_xGe_1-xN₂ alloys also have the benefit of being composed of more abundant elements than In_xGa_1-xN alloys, which may become either too difficult or too expensive to produce as In consumption increases.
A current challenge is that fabrication methods such as reactive RF sputtering do not produce material with suitable electronic quality for devices. Improvement in crystallinity is needed to study the basic material properties of these alloys and further assess their applicability in multijunction photovoltaic cells, light emitting diodes, or optical sensors. We report on the fabrication of crystalline ZnSn_xGe_1-xN₂ thin-films by molecular beam epitaxy [MBE] and the structural and optoelectronic characterization of these films.
ZnSn_xGe_1-xN₂ films were grown on c-plane sapphire and GaN templates on c-plane sapphire substrates that were cleaned in-situ with nitrogen plasma from an atomic nitrogen source. 4N+ purity elemental Zn, Sn, and Ge sources at discrete temperatures were employed with a flux of atomic nitrogen pointed toward the substrate. Samples grown were analyzed in-situ with reflection high energy electron diffraction [RHEED]. RHEED images indicated crystalline films at the surface. RHEED images of samples grown for various time periods showed that initially growth occurred in islands and that over time the islands coalesced into larger grained films. Scanning electron micrographs of the surface and x-ray diffraction support this mechanism. X-ray diffraction also indicated the crystal structure of the films with a wurtzite-derived structure, while transmission electron micrographs display the epitaxy of the films to the substrates. Optoelectronic measurements describe the tunability of the band gaps and the carrier transport properties of these crystalline MBE ZnSn_xGe_1-xN₂ films.

Cu₂ZnSnS₄ (CZTS) has been considered as an absorber material to replace CuInGaSe2 (CIGS) in thin film solar cells owing to its similar crystal and electronic properties, and the relative abundance of its constituent elements. However, the highest efficiencies for CZTS cells are around 10%, much less than for CIGS equivalents. It has been suggested that cation antisite defects, such as Cu_Zn, Zn_Cu, Sn_Cu etc may be responsible, but evidence for these defects comes mainly from bulk techniques. In this paper we show that such defects can be directly identified by imaging and elemental microanalysis at sub-atomic resolution in an aberration-corrected transmission electron microscope (TEM). We have examined nominally-stoichiometric, micron-sized CZTS crystals produced by a hot-injection method followed by annealing at 550°C [1]. Individual crystals were oriented in the TEM along the kesterite [010] direction in the TEM, such that single element atom columns were viewed end-on. Atomic number contrast in high angle annular dark field (HAADF) images then distinguished the Sn and S columns from the Cu and Zn columns, and identified high densities of planar antisite domain boundaries. In all cases, these boundary had displacements which were a cation-cation vector, representing disorder on the cation sub-lattice, i.e. switching Cu columns to Zn sites and vice versa, or Sn columns to Cu or Zn sites and vice versa.
The paper will illustrate the different types of boundary present, and the role of some boundaries in effecting changes in the local stoichiometry. It will be shown that the chemical species in the individual atom columns can be distinguished by elemental mapping using energy dispersive X-ray spectroscopy, confirming the kesterite structure and the boundary species, as well as changes in stoichiometry towards the crystal edges. These observations will be discussed, with regard to the mechanisms of formation of antisite domain boundaries and their likely effect on cell efficiencies.
[1] N. Kattan et al: Applied Materials Today, article APMT-D-15-00081

The Shockley-Queisser (SQ) detailed balance treatment of a solar cell presents the best-case scenario for absorption and recombination in a PV device.¹ All photons above bandgap are absorbed, and the photogenerated carriers recombine radiatively. Thus a device in this limit would have a 100% photoluminescence quantum yield (PLQY). Any alternative non-radiative pathways for recombination will result in a drop in PLQY and concomitant losses in voltage. This relationship was quantified by Ross, who showed that the maximum voltage in photochemical systems decreases with the logarithm of the PLQY.² This relation matches experimental data for voltage and PLQY for materials like GaAs. However, the band-band luminescence of GaAs is close to ideal with very little sub-bandgap emission (Urbach Energy of 5-10meV). Many other materials, like the earth-abundant and inexpensive Cu₂ZnSn(S,Se)₄ (CZTSSe), exhibit significant sub-bandgap tails in absorption and emission (with energy broadening parameters closer to 50-60meV). This is readily apparent from the redshift of the PL peak from bandgap and the breadth of the emission spectrum.
Recently, we showed that PL spectra from materials with significant band tailing could be described using a generalized model for sub-bandgap absorption.³ We are able to extract quasi-Fermi level splitting, bandgap, local lattice temperature, and band tail character through this model. With these parameters in hand, it is possible to simulate PL curves for different models of absorption and varying magnitudes of band tails.
In this presentation I will demonstrate that use of PLQY alone as a gauge of material quality can be misleading in the presence of band tails. Band tails enable additional radiative pathways that enhance PLQY and can lead to the majority of luminescence being emitted below Eg. This complicates the translation of PLQY into V_OC, as one must account for the sub-bandgap absorption/emission in calculating the radiative-limited V_OC. Assumption of an ideal step (SQ) absorptivity simplifies calculations, but leads to overestimation of V_OC when using experimental PLQY, as all materials exhibit some band tailing. This overestimation is on the order of a few mV in GaAs, but CZTSSe samples, which have significant band tailing (and high V_OC deficits with respect to Eg), can suffer from a V_OC overestimation of 80 mV using the experimentally collected PLQY. We propose an additional loss term to account for sub-bandgap absorption and emission that can be paired with the SQ ideal V_OC to allow for the accurate estimation of potential V_OC from PLQY. This method is contact-free and can be done on absorbers rather than full devices.
¹ W. Shockley and H. J. Queisser, Journal of Applied Physics 32, 510 (1961).
² R. T. Ross, Journal of Chemical Physics 46, 4590 (1967).
³ J. K. Katahara and H. W. Hillhouse, Journal of Applied Physics 116, 173504 (2014)

The determination of minority-carrier lifetimes and surface recombination velocities is essential for the development of semiconductor technologies such as solar cells. The recent development of two-photon time-resolved microscopy allows for better measurements of bulk and subsurface interfaces properties. Here we analyze the diffusion problem related to this optical technique. Our three-dimensional treatment enables us to separate lifetime (recombination) from transport effects (diffusion) in the photoluminescence intensity. It also allows us to consider surface recombination occurring at a variety of geometries: a single plane (representing an isolated exposed or buried interface), two parallel planes (representing two inequivalent interfaces), and a spherical surface (representing the enclosing surface of a grain boundary). We provide fully analytical results and scalings directly amenable to data fitting, and apply those to experimental data collected on heteroepitaxial CdTe/ZnTe/Si.

CdTe is a promising material for solar cells because of its large absorption coefficient and favorable bandgap of 1.50 eV. First Solar demonstrated polycrystalline CdTe solar cells with a record efficiency of 21.5% in 2015, however it is still much lower than the detailed-balance-limit for CdTe (~32%). It is expected that with longer minority carrier lifetime, higher doping concentration, and better surface passivation, higher cell efficiency can be achieved. Both one-photon and two-photon excitation time-resolved photoluminescence (TRPL) measurements have shown that monocrystalline CdTe has much longer minority carrier lifetimes than polycrystalline CdTe. Moreover we recently demonstrated CdTe/MgCdTe double heterostructures grown on InSb substrates using MBE, with excellent crystalline quality, a record low interface recombination velocity (< 1 cm/s) and a record long carrier lifetime of 2.7 μs. The interface recombination velocity is found to be dependent on both the MgCdTe barrier height and thickness, and it is believed that both thermionic emission and tunneling processes can contribute to the effective interface recombination, aside from Shockley-Read-Hall recombination through interface trap states. Effective n-type doping is realized from 1×10¹⁶ cm^-3 to 1×10¹⁸ cm^-3 using indium. PL measurements show that the PL intensity is strongest with a 1×10¹⁷ cm^-3 doping level, and the carrier lifetimes are around 100 ns when doping density is in the range of 1×10¹⁶ cm^-3 ~ 1×10¹⁷ cm^-3. With long carrier lifetimes, moderate doping concentration, and good surface passivation, the CdTe/MgCdTe double heterostructures on InSb can be applied to high efficiency solar cells if a proper p-type window layer is developed. More studies are ongoing to further understand the interface recombination processes both experimentally and theoretically. The band offset between CdTe and MgCdTe will also be investigated and reported at the conference.

Hematite is a prospective photoanode material for photoelectrochemical cells (PEC) for solar water splitting. A deeper understanding of the polarization processes involved in the oxygen evolution reaction (OER) and the role of light is essential for guiding the efforts to improve the efficiency of hematite photoanodes.

This work examines model thin film hematite photoanodes using complementary characterization techniques that are well-suited to probe the charge carrier dynamics of complex photoelectrochemical reactions: photoelectrochemical impedance spectroscopy (PEIS), intensity modulated photocurrent spectroscopy (IMPS), and intensity modulated photovoltage spectroscopy (IMVS).

First, we verify the interrelation between PEIS, IMPS and IMVS measurements, where the electrical response to periodic oscillations in electrical potential and light intensity are measured in operando.

The IMPS measurements on hematite show two well-defined semicircles with different signs that are assigned to the static hole current (positive) and the recombination current (negative) with the help of a simple equivalent circuit model. Based on the fitting results of IMPS series varying the bias light intensity we are able to reconstruct the elusive recombination current for any operating point. We will further demonstrate how IMPS data can be used to simulate transient photocurrent experiments, namely light pulses and chopped light measurements, with superior quality and time resolution. A critical discussion of frequency domain measurements with respect to accuracy and experimental efforts is added.

The relations between PEIS and IMVS results are analyzed in detail. A deconvolution of the measured spectra is achieved by calculating the corresponding distribution of relaxation times (DRT). With the DRT we are able to separate the overall polarization into different contributions, and we examine how these contributions depend on operating conditions such as electrical potential and bias light intensity. The analysis reveals distinct common patterns in PEIS and IMVS. Beyond the onset potential these patterns only differ by a constant factor that can be linked to the quantum efficiency of the PEC with respect to the spectrum of the applied lamp. This way, combining electrical and optical excitation plus DRT analysis enables us to isolate the losses caused by the OER without any a priori assumptions.

In sum, this contribution introduces PEIS, IMPS and IMVS as valuable photoelectrochemical measurement techniques providing new insights into the rate limiting processes in PECs.

In this presentation, we will illustrate how modern first-principles computational approaches can be used to design, engineer, and optimize materials for solar energy conversion using several examples from our recent work. Regarding surface properties, we use first-principles simulations of chemical reaction pathways and mechanisms to elucidate how the presence of dopants on an otherwise pristine surface can change the energy landscape of the oxygen evolution reaction for hydrogen fuel production via photocatalytic watersplitting. We demonstrate this for the oxygen evolution reaction on doped and undoped lepidocrocite TiO₂ surfaces. Insights into the electronic structure and density of states illustrates why some dopant species work better than others to improve the kinetics of the reaction. We will also illustrate how combining disparate materials across a heterointerface can be used to obtain enhanced functionality for photovoltaic and photocatalytic material systems. Examples include the integration of TiO₂ with correlated metal oxides for enhanced light absorption, and the integration of TiO₂ with ferroelectric support layers. For the latter, switching the polarization of the substrate can change the reaction pathways, energy landscape, and modify rate limiting steps in the production of hydrogen fuel from water. Finally, since an important aspect of computational materials design is to understand the accuracy attainable with the most commonly used approaches, we will present some benchmark calculations of phase stability predictions for challenging thin-film photovoltaic materials such as the chalcopyrites and chalcopyrite alternatives. For this we compare the results obtained using conventional and beyond-conventional density functional theory to those of emerging high-accuracy techniques such as quantum Monte Carlo methods.

One of the main challenges associated with the search for novel, non-toxic and earth-abundant solar cell materials, is the development of alternative buffer layers.
For several novel absorber materials a limiting factor is a deficit of open circuit voltage, possibly in part due to an inferior band alignment at the front contact.
A promising alternative to the commonly used CdS can be found in In₂S₃. Besides eliminating the toxicity concerns associated with Cd, it features a slightly lower electron affinity and could therefore lead to an optimized conduction band offset for a variety of absorber materials such as SnS, CuSbQ₂, Sb₂Q₃ and CZTS, in all of which cases a higher conduction band (CB) of the buffer layer is desired.
For the application of In₂S₃ in thin film solar cells, in addition to a suitable band alignment, the main desired criteria are low deposition temperature, suitable optoelectronic properties, as well as the scalability of the process.
In this work the reactive sputter deposition of In₂S₃ is investigated utilizing a high throughput combinatorial screening of the temperature-flux parameter space with a variety of spatially-resolved characterization techniques including XRD, XRF and UV-VIS transmission measurements. It is demonstrated how changes in sulfur partial pressure controlled by the addition of H₂S to the sputtering gas, as well as deposition temperature influence the composition, structure and orientation of the In₂S₃ films. While for sputtering in pure Ar atmosphere deposition temperatures above 450°C were required to deposit films of phase pure In₂S₃, reactive sputtering with H₂S allowed for depositions without intentional heating. Depending on the substrate temperature, the resistivity of the films was found to vary over a broad range from 5 Ωcm to 1 MΩcm. The doping with Zr and Sn was investigated as an option to increase conductivity for lower deposition temperatures, but did not lead to the desired properties.
To investigate the feasibility of sputtered In₂S₃ layers for thin film solar cells, combinatorial PV device libraries were built on CZTS/Mo/SiO₂ substrates. Intentional In₂S₃ thickness gradients were applied to determine the ideal buffer layer thickness. In₂S₃/CZTS devices using In₂S₃ buffer layers deposited at 290°C with a thickness of 65 nm showed to be on par with CdS/CZTS reference devices with similar values for short circuit currents as well as open circuit voltages. Additionally an increase in external quantum efficiency could be observed for photon energies greater than 2.4 eV. This indicates lower parasitic absorption in the buffer layer, which makes In₂S₃ even more promising as an alternative buffer layer material.

The transition to a renewable energy economy requires development of low cost, highly efficient, scalable, and reliable solar energy generation materials. P-type transparent conducting materials (TCMs) have a wide array of unexplored applications in optoelectronics, particularly in photovoltaics as top contacts to thin film solar cells and as interconnect layers in tandem structures. To date, however, p-type TCMs have not yet been employed in commercial solar cells due to their low conductivities compared to n-type TCMs (ex: ITO or AZO). Additionally, nearly all p-type TCMs reported in the literature require processing temperatures >400 deg. C, which limit possible device configurations.

Recently, we have developed a room temperature synthesis process using earth-abundant, low cost materials that yields p-type Cu_xZn_1-xS films by pulsed laser deposition (PLD), among other methods. For PLD-synthesized films, we control Cu content from 0 < x < 0.75 and find optimized films within a “TCM regime” (0.10 < x < 0.35) have an optical band gap >3.0 eV, transparency in the visible of >60%, and hole conductivities up to 42 S cm^-1. Conductivity measurements performed from 15-450 K are consistent with the band conduction of holes expected from degenerate doping.

To better understand the conduction mechanism, we report a detailed study of the Cu_xZn_1-xS structure and morphology. In the “TCM regime,” synchrotron x-ray and electron diffraction reveal a nanocrystalline ZnS structure, but no crystalline Cu_yS phases; at Cu contents x > 0.45, films are amorphous and poorly conducting. Ab initio calculations and observed lattice constant shrinkage with Cu incorporation suggest the TCM regime conduction is due to some amount of substitutional copper into the wurtzite lattice, at most around 6-13%. The conductive Cu-doped phase is embedded in a less conducting amorphous Cu_yS network that dominates at higher Cu contents. This contrasts with the nanocomposite structure and conduction mechanism observed in Cu_xZn_1-xS deposited by equilibrium methods (eg: chemical bath deposition).

To demonstrate potential in solar cell devices, we have synthesized simple heterojunctions of PLD-Cu_xZn_1-xS and n-type silicon and show rectification in the dark and photovoltaic activity under illumination. Room temperature deposition of a p-type TCM is particularly attractive for applications in thin film and perovskite devices that require a low thermal budget (T_dep < 100 deg. C for perovskites), and device integration as well as manufacturability and scalability concerns will be discussed.

Graphene has attracted great deal of attention towards a wide range of applications, including transparent conducting electrodes (TCE) for optoelectronic and photovoltaic applications. The excitement towards graphene has been ignited as a result of its unique electrical properties, high optical transmittance, chemical stability and flexibility, making it a strong candidate as a replacement TCE material for the next generation of transparent optoelectronics and semitransparent photovoltaics.
Doping of graphene with molecular entities has been a desirable route to modifying its work function and increasing its charge carrier density, mainly through non-covalent charge transfer interactions. Metal-organic molecules were recently utilized to n- and p- dope the surface of inorganic electrodes as well as 2D materials, including single layer graphene and have proven to outperform other classes of molecules in terms of their stability, versatility and the ability to tune the energitics for a wide range of materials.
Herein, we investigate the doping of optically transparent few layers graphene films (FLG) with metal-organic molecules of a wide range of doping strength and compare their effect on the transport properties and work function shifts in graphene. Our study includes a set of solution-processed dimeric sandwich compounds acting as n-dopants and molybdenum tris(dithiolene) derivatives acting as p-dopants.
A direct correlation of work function shifts with the doping strength of n- and p- dopants has been established through ultraviolet photoelectron spectroscopy (UPS) with up to 1.5 eV and 0.6 eV shifts, respectively. A similar trend has been observed in conductivity, but, with small changes induced by the weaker dopants. To elucidate the changes in conductivity in terms of charge carriers density and mobility, we have performed Hall effect measurements; which showed the increase of the carrier density while decreasing the mobility in all cases due to coulombic scattering. X-ray photoelectron spectroscopy (XPS) and Raman scattering were used to elucidate the charge transfer doping mechanism of such molecules, which attach non-covalently to the surface of graphene without disrupting the basal plane. The optical transmittance of doped samples was maintained close to that of pristine FLG due to the submonolayer coverage of dopants, leading to an increase of the figure of merit of transparent conducting electrodes by up to 100%.

Transparent electrodes enable the minimization of shadowing and resistive losses in thin film photovoltaics. Recently an unusual interdigitated electrode design was proposed that could offer complete optical transparency at fifty percent metal areal coverage. This structure, called the catoptric electrode, functions by redirecting light reflected from the top surface of an embedded electrode line to an open surface area through total internal reflection. Numerical simulation of a silver based catoptric electrode with metal covering fifty percent of the surface predicts 84% optical transmission of unpolarized light averaged over the entire visible optical spectrum at fifty percent silver areal coverage. These remarkable figures were achieved under normal incidence illumination. This presentation will discuss the factors that affect the angle dependence of the performance of catoptric electrodes. Through numerical simulations of a realistic silver catoptric electrode structure with 1 um wide electrode fingers and a 1 um electrode gap it is demonstrated that larger than 75% optical transmission of unpolarized light averaged over wavelengths from 400nm-800nm can be achieved at angles between -60 degrees and +65 degrees off-normal at 50% metal coverage for a structure with a single tilt direction. Possible routes toward the fabrication of the proposed electrode design will be discussed.

The scale of renewable energy challenges not only calls for highly efficient photovoltaic (PV) or photoelectrochemical (PEC) technologies but also abundant, inexpensive, and robust materials. However, significant challenges in material performance, bulk and interface defects, and controlled doping and physical properties must be overcome before the potentials of earth-abundant materials can be fulfilled. New semiconductors for efficient light harvesting and charge separation, and highly active electrocatalysts must be discovered to enable the most efficient and sustainable production of energy using solar cells and PEC water splitting cells. Towards these goals, we have comprehensively elucidated the intrinsic defects in earth-abundant semiconductor iron pyrite (FeS₂) and conclusively explained the origin of its poor solar performance, which will enable us to rationally improve its solar performance. We also report several new earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and significantly enhanced their catalytic performance. By controlling the nanostructures and polymorphs of layered metal chalcogenide MS₂ (M = Mo, W) materials and metallic cobalt pyrite (CoS₂), we significantly enhanced their catalytic activity for HER. We further established ternary pyrite-type cobalt phosphosulfide (CoPS) as the best earth-abundant HER catalyst to date that does not contain expensive noble metals. Nanostructured CoPS can achieve catalytic activity for HER very close to that of platinum with outstanding long-term operation stability. These earth-abundant HER catalysts have been integrated with semiconductors to enable the most efficient solar-driven hydrogen generation devices using earth-abundant materials.

Quantum-dot-sensitized solar cell (QDSSC) has been considered as an alternative to new generation photovoltaics, but it still presents very low conversion efficiency. Rationally designing nanostructures aiming at solving the critical issues on counter electrodes would shed light on how to break through the conversion efficiency record of QDSSCs and push this field to move forward.
In this presentation, the designing three-dimensional nanowire Arrays as efficient counter electrodes for QDSSCs will be discussed. For example, tunnel junction arrays configured with degenerate n-type tin-doped indium oxide nanowire (ITO) core and degenerate p-type Cu₂S nanocrystal shell (ITO@Cu₂S) was designed and fabricated as new efficient counter electrode for QDSSCs. ITO nanowire array core provided a three dimensional conductive network. ITO core and Cu2S nanocrystal shell formed effective tunnel junctions with carrier transport path shorter than 100 nm. It was found that sheet resistance (R_h) was not only dependent on the electron conductivity of the substrate but also related to the semiconductor-electrode interface between CTO and catalysts. The high-quality tunnel junctions resulted in the considerable decrease in R_h of the device, and facilitated electron transfer from CTO to Cu₂S. Moreover, the chemical inert nature of ITO made this type of counter electrode stable in liquid electrolyte with no intrinsic issue like copper dissolution in state-of-the-art Cu/Cu₂S counter electrode. Compared with planar structures, the three-dimensional nanowire arrays presented higher surface area and easy accessibility of electrolyte, leading to higher catalytic activity of counter electrode as evidenced by apparently decreased charge transfer resistance. As a result, the power conversion efficiency of QDSSCs with the designed ITO@Cu₂S nanowire counter electrodes increased by 84.5% and 33.5% compared to that with Au and Cu2S counter electrodes, respectively.^[1] Furthermore, the influence of the interface of ITO/Cu₂S on the conversion efficiency of QDSSCs was further investigated by using different synthetic route and the following post-treatment to fabricate the ITO@Cu₂S nanowire counter electrodes.^[2] More interestingly, the hierarchical ITO@Cu₂S nanowire arrays were further fabricated and found to be able to further reduce the sheet resistance and charge transfer resistance between the electrolyte and the counter electrode. As a result, the QDSSCs with smaller series resistance, larger shunt resisitance and thus the power conversion efficiency of over 6% has been achieved by using this kind of hierarchical ITO@Cu₂S nanowire arrays as counter electrodes. ^[3]
[1] Y. Jiang, et. al, Nano Lett., 2014, 14, 365-372.
[2] Y. Jiang, et. al, ACS Appl. Mater. Interface, 2014, 6, 15448-15455.
[3] Y. Jiang, et. al, Nano Lett., 2015, 15, 3088–3095

Multiple-exciton generation—a process in which multiple charge-carrier pairs are generated from a single optical excitation—is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

The layered crystal structure of transition metal dichalcogenides (TMDs) enables their exfoliation into single or few-layer sheets which present superior optical and electronic properties compared to their bulk counterparts, making two-dimensional (2D) TMDs emerging and promising candidates for solar energy conversion. However, fabricating high-quality large-area 2D films remains challenging. Here we present advances in the solution-based fabrication of semiconducting 2D TMD thin films and their application in large-area photoelectrochemical (PEC) devices. Starting from solvent assisted exfoliation, [1] homogeneous 2D films consisting of all horizontally oriented TMD flakes are fabricated by creating strong spatial confinement from two non-solvents. [2] After transfer to conductive substrates, p-type WSe₂ electrodes are shown to be active for solar hydrogen generation while n-type MoS₂ and MoSe₂ electrodes can be used as photoanodes. Insights into the effects of thin film morphology, flake size, edges and defects on performances of the PEC cells are reported. Moreover, catalysts and heterojunction formation are shown to dramatically improve photocatalytic properties of 2D TMD films. Specifically, photocurrent of WSe₂ and MoS₂ electrodes achieve one order of magnitude enhancement afforded by catalyst and conjugated organic molecule coating, respectively, compared to the bare films. Internal (absorbed photon) quantum efficiencies up to 40% are observed with TMD layers only 10nm thick. The mechanisms of promoted charge transfer in these modified devices are discussed.
[1] Yu, X.; Prévot, M. S.; Sivula, K. Chem. Mater. 2014, 26, 5892.
[2] Yu, X.; Prevot, M. S.; Guijarro, N.; Sivula, K. Nat. Commun. 2015, 6, 7596.

Plasmonic nanoparticles and quasi-2D (Q2D) transition metal dichalcogenides (TMDs) have been identified as promising materials for solar-to-fuel energy conversion. Q2D MoS₂ is known to be catalytically active for driving the hydrogen evolution reaction (HER). Adding plasmonically active materials to Q2D MoS₂ is a promising strategy to enhance photocatalytic activity further, because their large absorption cross-sections and non-radiative decay of plasmons into hot electrons can enhance both, light absorption and charge carrier generation, in adjacent Q2D MoS₂. Recent work on MoS₂ has investigated the electron-hole recombination mechanisms with optically excited carriers. Here, we synthesize hybrid Au/Q2D MoS₂ nanostructures in liquid suspension and characterize their charge carrier dynamics by femtosecond transient absorption (TA) measurements. Understanding the dynamics of hot electron injection into MoS₂ and the ensuing dynamics is important to understand the potential for driving solar-to-fuel energy conversion processes using Au/Q2D MoS₂ nanoparticles.

A new model will be presented that shows how much the voltage of organic solar cells depends on factors such as the degree of intermixing between the donor and acceptor semiconductors, the binding energy of the charge transfer state, the lifetime of the charge transfer state and the degree of energetic disorder. Results from a device simulator will be used to show that it is now possible to accurately predict the performance of many organic solar cells and that it will be necessary to use materials with the charge carrier mobility of at least 10^-2 cm^2/Vs in order to achieve high fill factors in devices that are thick enough to absorb almost all of the light. Opportunities for increasing the voltage in organic solar cells will be presented.

Closing a photosynthetic cycle on a nanometer scale relaxes several materials’ requirements, broadens the spectrum of usable materials and makes new artificial photosynthesis device designs feasible. Conversely, at this length scale new hurdles emerge: while on any length scale spatial and chemical separation between the reaction sites is preferred, at nanometer scale, this is imperative. We suggest a design that compartmentalizes each reaction yet keep them electronically and protonically connected through an ultra thin hybrid organic-inorganic membrane. In this study we synthesize and study this membrane in planar configuration. The membrane is composed of ~2 nm long molecular wires attached to a water oxidation catalyst and embedded in ~2 nm thick silica by atomic layer deposition. Light absorber and reductive catalysis are positioned on the opposite side of the membrane. Electron microscopy, atomic force microscopy and spectroscopic methods (FTIR and XPS) confirm the wires are embedded in the silica layer, while (photo)electrochemical methods confirm charge transport through the wires.

Non-Fullerene Acceptors (NFAs), which are organic materials either small molecules or polymers, are considered as an alternative to the common fullerene derivatives (e.g. PCBM) for organic photovoltaics (OPVs). Unlike NFAs, PCBM has long been used as acceptor for many efficient OPV cells. However, its limited absorption region available for charge generation and high cost could stray it as a viable acceptor from the realisation of affordable commercial OPV devices in future. In addition, studies have shown that the thermal stability of OPV devices is primarily caused by the crystallisation of PCBM. While low cost NFAs have a broader absorption region and are deemed to have a higher thermal stability, the device efficiency achieved with some of NFAs previously is a couple of magnitudes lower than their PCBM analogues. Because of the big difference in PCE between NFAs and PCBM, the studies between these acceptors are incomparable (e.g. thermal stability study). Along with the successful effort in material designs, efficient OPV cells (PCE >5%) have been recently achieved using NFAs (highest reported PCE 8.3%^[1]) allowing a side-by-side study between NFAs and PCBM. In this connection, we compare the device thermal stability of these acceptors with the same donor and their charge generation kinetics by using photoluminescence (PL) and femto/micro-second transient absorption spectroscopies (fs/µs-TAS) in order to provide an insight of the role acceptors such as rhodanine small molecules and NDI polymers play in OPV devices.

[1] Y. Zhong, M. T. Trinh, R. Chen et al., Nat Commun 2015, 6.

Bulk heterojunction organic solar cells (OSCs) based on small molecules have recently attracted much attention as a promising alternative to conventional polymer-based OSCs, with the latest sunlight-to-power efficiency of ~10% [1]. The family of star-shaped donor-acceptor small molecules (SSMs) with triphenylamine (TPA) donor core and dicianovinyl (DCV) end groups have been proven as reliable and easy to synthetize donor molecules for OSCs, with the efficiency of more than 5% already achieved [2]. In a continuous hunt for the new material design it has been recently shown that the analog of TPA with methoxy substitutes (m-TPA) increases solubility and crystallinity of the star-shaped molecules [3], making m-TPA an attractive intramolecular donor unit. Among the intramolecular acceptor units, rhodanine (Rh) is competitive to DCV due to its strong electron-withdrawing character [4]. In this contribution, we compare four different SSMs with the TPA/m-TPA donor core and DCV/Rh acceptor groups, connected by a bithiophene pi-bridge, for their efficiency of the initial photon-to-charge conversion in the bulk heterojunction active layer with [70]PCBM fullerene acceptor. Charge separation dynamics are studied by ultrafast photoinduced absorption (PIA) spectroscopy [5], using a visible pump to mimic the sun photons and broadly tunable IR probe to monitor concentration of hole polarons at the SSM's conjugated system.
Introduction of the intramolecular donor and acceptor units might jeopardy efficient exciton dissociation due to energy level mismatch. We demonstrate that this is not the case, with both of electron transfer from SSMs and hole transfer from [70]PCBM being equally efficient independently on the SSM chemical structure. However, the position of the polaron absorption band varies substantially, from 1.6 micrometers (TPA/DCV molecule) to 2.2 micrometers (TPA/Rh molecule) which highlights the effect of the donor/acceptor groups on the photophysical properties. The efficiency of charge separation depends on the [70]PCBM acceptor concentration with the optimal SSM:[70]PCBM ratio strongly dependent on the SSM structure. At the most favorable [70]PCBM contents the blends provide up to 60% of long-lived separated charges, which matches well the external quantum efficiencies of the OSCs. Importantly, for all SSM molecules, the optimized blend composition for ultrafast charge separation directly correlates with that for device operation. This indicates a great influence of ultrafast sub-ns processes on ultraslow timescale at which the device operates, and proves ultrafast spectroscopy as a valuable tool for OSCs optimization.

[1] Y. Liu et al., Sci. Rep., 3 (2013), 3356
[2] J. Min et al., Adv. Energy Mater., 4 (2014), 1400816
[3] J. Min et al., J. Mater. Chem. C, 2 (2014), 7614
[4] Z. Li et al., Adv. Energy Mater., 2, (2012), 74
[5] O. Kozlov et al., Adv. Energy Mater., 5 (2015), 1401657

Improvements in the maximum attainable efficiency of dye sensitized solar cells (DSSCs) compared to other solar cell technologies have been marginal, with the current best efficiency at 13%, which is still below the value reported for CIGS or perovskite-based cells. The key to improving the efficiency of DSSC well beyond 15% is to improve the spectral absorption range of the sensitizing dye. Various strategies have been employed in the past such as incorporation of co-sensitizers, energy relay donors (ERDs), plasmonic metal nanoparticles and up-conversion phosphors. Most ERDs have been based on organic dyes that transfer energy to sensitizing dyes through Forster resonance energy transfer (FRET). Here, we report a new inorganic energy relay donor with high spectral overlap with the absorption range of N719 dye. Both steady state and time resolved fluorescence spectroscopy show that the mode of energy transfer is radiative as opposed to FRET observed in organic dyes. Solar cells employing these energy relay donors in the TiO₂ matrix show a high excitation energy transfer to the dye. As a result, an 87% increase in the photocurrent density along with 47% increase in power conversion efficiency when compared to conventional TiO₂-dye cells was observed.

The major roles of mesoporous semiconductor photoanodes in dye sensitized solar cells (DSSCs) are dye adsorption, collection of photoexcited electrons from the dye (charge injection) and electron transport. In order to understand their material properties and their effects on the associated charge transfer processes in DSSCs, specifically recombination processes, various types of photoanode materials need to be investigated. Mesoporous titanium oxide (TiO₂) films are often used as DSSC photoanodes. Though various other photoanode materials like ZnO, Nb₂O₅, SnO₂, SrTiO₃ etc. have been attempted, their material properties have not been systematically studied. Herein we present a detailed study of band edge position and electron distribution in the mesoporous strontium titanate (SrTiO₃) photoanode. The band edge energy of SrTiO₃ is determined by a spectroelectrochemical method, developed in our lab, and is found to be significantly higher than the often used TiO₂. Electrochemical impedance spectroscopy, cyclic voltammetry and charge extraction method show that SrTiO₃ has lower trap state density than TiO₂. Significantly higher photovoltage is observed while using SrTiO₃ as photoanode instead of TiO₂. In order to properly realize the pros and cons of using SrTiO₃ photoanodes a systematic study of the recombination processes and electron transport properties will be presented. Specifically, the recombination kinetics from various energy states (conduction band and trap states) of the photoanodes are studied using one-electron, outer sphere redox shuttles. The higher conduction band (CB) energy of SrTiO₃ provides higher photovoltage but also hinders the electron injection from the dye. Dyes having higher excited state energies and longer lifetimes would be beneficial to optimize the current and hence the overall efficiency of the DSSC.

We fabricated nearly 100% transparent, high performance catalyst on FTO with annealing ethyl cellulose in vacuum. DSSC of C/FTO (8.74%) showed higher PCE than traditional counter electrode Pt/FTO (8.45%) with good stability. And the device with C/FTO counter electrode work in back illumination shows PCE of 5.91%, short circuit current (Jsc) of 11.92 mA/cm², open circuit voltage (Voc) of 0.739 V, and fill factor (FF) of 0.671. EIS, CV curves and Tafel plot were collected to investigate the characteristics of different counter electrodes. And the higher performance of C/FTO can be attributed to the better catalystic ability of I₃^- to I^-, which resulted in higher Jsc and Voc of the solar cell. However, FF of device made of C/FTO is lower than that made of Pt/FTO. This is due to larger overall resistance of device made from C/FTO.

In summary, we have fabricated nearly 100% transparent catalyst on FTO for high performance DSSC. And it is cheap, environmentally friendly and easy to fabricate, comparing to traditional FTO/Pt counter electrode. In addition, C/FTO can be produced in a large scale for industrial DSSC fabrication, as it is fabricated with blade coating method.

The DFTB (Density Functional Tight Binding) method is a very enticing approach for the modeling of interfaces. The three orders of magnitude advantage in computational costs permits routine modeling of large (of the order of 10³-10⁴ atoms) systems and, specifically, allows modeling of large dyes adsorbed on semiconductor surfaces with model sizes providing converged properties. How accurate is the DFTB approach for modeling of this kind of interfaces where molecular states are located by design in the band gap and where energy level matching as well as adsorption mode and strength are key for solar cell operation?
We present a comparative DFTB – DFT (Density Functional Theory) study of the adsorption of 2-anthroic acid on the (101) surface of anatase TiO₂. Several adsorption modes are studied: bidentate bridging (BB), bidentate chelating (BC), and two monodentate modes (M1 and M2). Two parameterizations of DFTB were used which have previously been used for TiO₂ interfaces including dye adsorption. DFTB predicts adsorption energies which differ from those computed by DFT not only in magnitude (by about 0.5 eV) but also in the order among different configurations. The electronic structure, and specifically dye-semiconductor band alignment computed with DFTB are not consistent with DFT results and with experimental data. DFT correctly puts the molecular HOMO level in the bandgap and LUMO (which is strongly hybridized with TiO₂) in the conduction band, which is consistent with experimentally observed interfacial charge transfer band formed by 2-anthroic acid adsorption on titania. In contrast, DFTB puts the molecular LUMO level (which remains unoccupied upon adsorption) in the bandgap and HOMO in the valence band. We also show that the strategy of geometry optimization with DFTB followed by single-point DFT calculations also does not necessarily result in plausible adsorption energies and should be used with care.

The solar to energy conversion in dye-sensitized solar cells (DSSCs)^[1] as well as the solar fuel generation by intramolecular photocatalysis^[2] are promising approaches towards a more sustainable energy generation. One of the key materials in these systems is a light harvesting complex, e.g., a ruthenium(II) polypyridyl complex, which enables visible-light driven electron transfer to an acceptor unit, e.g., a photoanode of a DSSC or a bridging ligand of an intramolecular photocatalyst. For driving a chemical reaction like the generation of hydrogen at least two electrons have to be accumulated near or at the catalytic unit.^[2] In the model systems available today, this is accomplished by a sacrificial electron donor, which reduces the formerly oxidized primary electron donor and gives rise to a single reduced intermediate. Initiated by a second light absorption event a second charge transfer may take place, preferably accumulating both charges at the molecular bridge near the catalytic unit.

The electron transfer processes both in DSSCs and in intramolecular photocatalytic systems lead to transiently populated yet long-lived oxidized / reduced species. Although these species should be of functional importance in the overall solar-energy conversion their excited-state properties are only sparsely studied. Thus, in this contribution UV-Vis absorption and resonance Raman spectroelectrochemistry is employed as a spectroscopic tool to characterize the electronic transitions in electrochemically generated oxidized / reduced species. For example, the charge transfer character of a single-oxidized 4H-imidazole-ruthenium dye anchored to TiO₂ shows a pronounced wavelength-dependent charge transfer, indicating that these complexes are potential multi-photoelectron donors in DSSCs.^[3] Resonance Raman spectroelectrochemistry furthermore allows for the investigation of photoinduced electron transfer in the single-reduced species of intramolecular photocatalysts^[4] and to study the ability of complexes to store multiple electrons,^[5] which is one requirement for the generation of solar fuels.

Acknowledgements
This work was supported by the Fonds der Chemischen Industrie (J.S., B.D.), the DAAD (Y.Z.) and the COST Action CM1202, PERSPECT-H2O.

References
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Multinary semiconducting compounds with crystal structures of chalcopyrite CuFeS₂, kesterite Cu₂(Zn, Fe)SnS₄, and stannite Cu₂FeSnS₄ are at the focus of current research as absorbing materials in solar cells. To obtain these materials with optimal physical characteristics it is necessary to know the processes of phase formation during preparation of such materials from chemical elements, especially for the binary system Fe-S where compounds Fe_1-xS (pyrrhotite) and pyrite FeS₂ (in structure of pyrite and marcasite) exist. The goal of this paper is to study the phase formation in the Fe-S system by means of combined DTA and Mössbauer studies. To achieve this goal, the mixtures of the elements were heated in DTA apparatus, quenched following the corresponding thermal peaks, and then investigated using XRPD and Mössbauer studies.

The X-ray studies were carried out using monochromatic Cu K_a-radiation (1.5406 Å, step size 0.01° or 0.04°, counting time 10 s). The DTA measurements were performed using special carriers within a high-temperature steel clamp. The temperatures of the structural phase transitions were determined using Pt/90%Pt-10%Rh thermocouples with the heating rates being 2-3 K/min (accuracy of ±2 K). Reproducible results were obtained by placing powder samples of 1g in evacuated (1.3×10^-2 Pa) quartz capsules using Al₂O₃ as a reference material. Room-temperature ⁵⁷Fe Mössbauer spectra were taken in transmission geometry using a ⁵⁷Co/Rh g–ray source. The velocity scale was calibrated relative to ⁵⁷Fe in Rh.

The mixtures of Fe and S with the atomic ratios Fe:S = 1:1; 1:2, and 2:3 were investigated. It was found that up to the melting point of sulphur there is no essential formation of iron sulphides. The formation of FeS compound takes place after melting of sulphur in the temperature range from 490 to 590 K. The thermal peak is divided into two subpeaks with maximums at 522 and 576 K. Such shape is formed because simultaneously with the formation of iron sulphides (exothermic peak) crystallographic transitions (endothermic peaks) occur. Mössbauer parameters of the quenched samples allowed attributing the thermal peak at temperatures 590-620 K to formation of FeS₂, which at the subsequent heating decomposes on peritectics at 1015 K. A monotectic reaction in the range of composition FeS-S was confirmed at temperature 1380 K. A full description of the alloys by DTA, XRPD, and Mössbauer studies allowed to make recommendations for the technology of preparation of multinary compounds with crystal structures of chalcopyrite CuFeS₂, kesterite Cu₂(Zn, Fe)SnS₄, stannite Cu₂FeSnS₄, and compounds existing in the Cu-Fe-S system.

Acknowledgment. V.R. Sobol would like to thank the Belarusian Republican Foundation for Fundamental Research for financial support of the studies under project F15MLD-025.

Copper aluminum diselenide (CuAlSe₂) and copper gallium diselenide (CuGaSe₂) belong to the I-III-VI₂ group compounds and are extensively studied as active elements in optical filters and absorbing materials in solar cells. To optimize their properties, it is necessary to characterize and develop the technology of the growth of alloys based on these materials. The goal of the present paper is to grow single crystals of the CuAlSe₂ – CuGaSe₂ system and study them using Raman spectroscopy.

The initial elements for the preparation of CuAlSe₂ and CuGaSe₂ ternary compounds and their solid solutions were 99.9998% copper, 99.9997% aluminum, 99.999% gallium, and 99.9999% selenium. Synthesis of the initial ternary compounds CuAlSe₂ and CuGaSe₂ was performed in quartz ampules by melting the elements at a temperature that exceeds the melting point of the compound by 20K. After preparation of the initial compounds, single crystals of seven alloys of the (CuGaSe₂)_1-x (CuAlSe₂)_x system with molar part of CuAlSe₂ (x) equaling to 0.20, 0.40, 0.50, 0.60, 0.70, 0.80, and 0.90 were grown by chemical vapor transport in an evacuated quartz tube using iodine as a transporting agent. Typical dimensions of the grown plate-like single crystals were about 10 x 5 x 0.5 mm³ with a well developed surface (112).

Phase relations in the CuAlSe₂ - CuGaSe₂ system were investigated by means of X-ray powder diffraction, optical microscopy, scanning electron microscopy, and differential thermal analysis. The XRPD and microstructure studies showed that all of the prepared alloys of the CuAlSe₂ - CuGaSe₂ system were one-phased.

Raman measurements in backscattering configuration were performed at room temperature using a WITec alpha 300R confocal Raman imaging system with an excitation Nd:YAG laser at a wavelength of 532 nm and a 20x objective lens. Spectra were obtained in the wave number region from 80 cm^-1 to 3000 cm^-1 with spectral resolution of 0.9 cm^-1. The wave number accuracy was controlled by calibrating the spectra against the line of laser. It was established that all spectra were dominated by the total symmetric mode of A₁ symmetry at 186 cm^-1. The compositional dependencies of frequencies of modes for the CuAlSe₂ – CuGaSe₂ system were constructed, and the nature of modes will be discussed. Shifts in frequencies and changes in relative intensities of some modes in different cluster regions of the samples were found. These can be explained by the variation of chemical composition inside of the homogeneity region of the corresponding solid solutions.

Acknowledgment. V.R. Sobol would like to thank the Belarusian Republican Foundation for Fundamental Research for financial support of the studies under project F15MLD-025.

Graphene shows great potential as a transparent conductive electrode to replace conventional indium tin oxide (ITO) for optoelectronic applications. However, the large amount of residues generated during graphene transfer can lead to large short circuit currents and degradation of the device performance. We hereby report a new method to achieve the clean and efficient transfer of graphene onto active substrates such as BaFeO₂ and InGaN applications. Unlike the conventional poly(methyl methacrylate) (PMMA)-supported transfer technique, a buffer layer was inserted between PMMA and graphene, forming PMMA/buffer/graphene/BFO and PMMA/buffer/graphene/InGaN structures. In this method, the PMMA is not in direct contact with graphene and the buffer layer has a large solubility in clean solvent. These two conditions result in a substantial reduction of the detrimental residues between graphene and the substrate material. Consistent with this result, the BFO and InGaN devices that received the graphene electrode via the new PMMA/buffer layer transfer technique exhibited a significantly improved performance compared with previous devices made with the conventional PMMA transfer method.

The Ta-N system has a remarkably rich phase diagram ranging from the solid solution Ta(N) to the highly nitrided Ta₃N₅ phase. Applications are equally wide-spread and some typical examples are TaN mechanical coatings [Sah97] or diffusion barriers for microelectrronics. Lately, Ta₃N₅ andTaNO as well, has attracted interest for bias-free photoelectrolysis of water [Ish04] in the view of the energy transition challenge. For this, the material is well suited due its conduction and valence band position with respect to the to the half-cell potentials of the H₃O⁺/H₂ and the O₂/H₂O electrochemical couple.
Much effort in the past has been devoted to master sputtering of pure metals under various process conditions. The transfer of knowledge to reactive species on the other hand is not readily possible as the incorporation of non-metallic atoms in a film follows a completely different physics: 1) The precursors are diatomic gas (N₂,O₂) that needs to undergo physical reactions to be chemically react with the growing film. 2) These reactive species do not have the same kinetic distribution as the metallic species which gain their initial energy upon sputtering from the target. 3) The incorporation of reactive species into the film can follow more than one pathway. 4) Depending on the type of metal/non-metal system used, the film can undergo crystallographic changes purely depending on the metal/non-metal composition with strong influences on film density, electrical and optical properties.
The objective of this presentation is to elucidate the pathways from gaseous N₂ in presence or lack of oxygen, in order to elaborate Ta-N-O films with appropriate optical properties, nano-cristalline structure and morphology for solar energy conversion. The incidence of the substrate temparature on the films quality will also be discussed. The results are interpreted with regard to principal differences and similarities of direct current (DC) and high-power impulse magnetron (HIPIMS) sputtering. Technical approach is using optical emission spectroscopy (OES), Rutherford backscattering spectrometry (RBS) and nuclear reaction analysis (NRA), Scanning Electron Microscopy (SEM), Grazing incidence RX scattering (GIWAXS) and Photoelectron Spectroscopy (XPS)..

There is a growing need for efficient and cheap technology that can utilise renewable energy resources to generate power for a sustainable future. Solar energy, among all, has the highest potential for cost effective power generation. To tap this potential there are growing advancements in materials that can cater to this need. Lead halide perovskite is a very exciting cost-effective and high-throughput material which in a short span of 4 years since its inception has achieved a certified solar to electric conversion efficiency of 20.1% This halide perovskite has been experimented with varying device structures. However, lead being highly toxic needs further research attention in development a stable lead free alternative without compromising with the photoconversion efficiency (PCE). In this work we have presented a comparative study of diffenent alternative of lead halide perovskite such as partially Sn substituted CH₃NHPbI₃ and CsSnI₃. Sn is a group 14 element with comparable ionic radius has obviously became the first choice of replacement but it also suffers from oxidative instability as it converts from Sn²⁺ to Sn⁴⁺ leading to degradation to the cell efficiency. Moreover, complete replacement of Pb by Sn leads to narrowing of the bandgap and shows metallic conductivity. To address these issues different organic cations and mixed halide based compositions has been investigated to supress the Sn²⁺ oxidation as well as broadening the bandgap. Other multivalent transition metals like Mn, Cu, Fe, Co and Ni has been explored as alternative cation for Pb²⁺. These perovskite metarials are known to show ambipolar properties and therefore ideally may not need any costly organic holeconductor separately hence both hole-conductor free and organic hole-conductor incorporated configurations has been evaluated.

Carbon nanotubes show great potential to improve the overall efficiency of blends of conjugated polymer-fullerene derivatives and are envisaged to improve the charge separation and/or transport in this blends typically used for organic photovoltaic cells. In this work we investigated the effect of adding semiconducting single walled carbon nanotubes (6,5) (SWNTs (6,5)) to blends of P3HT:PC60BM. We used femtosecond transient absorption spectroscopy in order to understand the charge transfer processes in the active layer of the solar cells. For the study we placed the SWNTs in three different configurations, 1-glass/P3HT:PCBM/SWNT, 2-glass/SWNT/P3HT:PCBM and 3- P3HT:PCBM:SWNT (bulk heterojunction), to comprehend the influence of having the SWNTs in diverse positions inside the active layer. We found that in our SWNT sample the employed batch contains not only (6,5) SWNTs, but also a significative amount of SWNTs with (7,5) chirality. For the structures in a bilayer configuration, 1 and 2, in the spectral region resonant with the E11 transition of the SWNTs, as expected, we found the photobleach (PB) signature of the (6,5) SWNTs, nevertheless a photoinduced absorption (PA) signal at shorter wavelengths was only observed when the SWNTs where placed near the glass substrate and not on top of the P3HT:PCBM blend. This PA feature was also visible when the SWNTs were added in a bulk heterojunction composition (structure 3). The kinetics of the bare (6,5) SWNTs probed at 1025 nm (maximum of the bleach peak for our sample) when compared with the rest of the different structures show a really similar behavior with the structure 1, while a much slower decay and broad signal is observed for the ones where the PA signal is present. When we set the probe in the region of the E22 transition for the SWNTs, we see the bleaching of the second exciton for the (6,5) and the (7,5) tubes. For all our three structures at around 600 nm a strong bleach signal is observed, as we pump selectively into the SWNTs, we do not expect to see a bleach from P3HT or PCBM, which absorbed in the visible region. We think this might be an indication for a photoinduced charge transfer to P3HT, leading to a bleach signal of P3HT. We attribute the differences observed in the transient spectra for the structures 1 and 2 to dissimilarities in the morphology of the two samples produces by the degree of interaction between the P3HT:PCBM blend and the SWNT layers, that are achieved when the layers are deposed in the two different configurations, this make the samples act more closely to only one of the layers present in the structures when excited to selected spectral regions. We think that for the structure 2 a stronger interaction between the SWNTs and the P3HT:PCBM blend and a more close behavior to a bulk heterojunction structure is achieved. We consider that the charge transfer process that occurs between polymer and SWNTs could be really helpful for the light harvesting of solar cells.

Lead perovskite(PbCH₃NH₂I₃) has been established itself has highly photoactive material with ambipolar carrier transport properties leading to recod efficiency 20.1% solar cells. On the otherhand, MoS₂ is another intersting 2D material with tunable electric transport properties as well as has exceptional chemical and mechanical stability. Specifically when it can be transformed to its metastable 1T form from stable 2H form it shows metallic conductivity compared to its standard direct bandgap semicondicting nature. Therefore, a hybrid material of nanowire lead perovskite and 1T MoS₂ can be an ideal candidate for ultra high efficiency transistor based photodetector upto single electron detection capability at room temperature leveraging the unique directional charge seperation and high electronic two dimensional conductivity of single layer 1T MoS₂. In this work single layer MoS₂ has been synthesized in CVD from thermal decomposition of single molecular ammonium tetrathiomolydate precursor along with insitu reduced gold nanoparticles. Presence of gold nanoparticle affects a plasmon and stress induced transformation of 2H single layer MoS₂ to 1T MoS_2. Lead perovskite nanowires has been prepared by slip-casting technique. A transistor has been prepared with organic holeconductor . device shows excellent photodetection behaviour at room temperature and ultra low intensity light in the visible range.

High-efficiency and low-cost crystalline silicon (c-Si) solar cells are expected to be promising renewable energy resources in the near future. However, despite the significant manufacturing capacity growth and cost reduction due to the economies of scale, the increase of c-Si solar cell efficiency has been relatively slow, as the world record has only grown from 24.7% to 25.6% in the past 15 years¹. Theoretical work predicted the efficiency limit to be 29%², which requires both high short circuit current density (Jsc) and high open circuit voltage (Voc). Current c-Si solar cells have achieved a Jsc larger than 40mA/cm², but the Voc is still far below the theoretical limit. On the other hand, high Voc could be achieved by concentrating more carriers into a smaller volume by thinning the cells. If the cell thickness is reduced by 10-100 times to below 10μm, the Voc could potentially be enhanced by 120mV, as shown in the simulation results to be presented in this work. In addition, though a thinner cell typically means lower light absorption, recent work³ on photon management has shown that ultra-thin c-Si solar cells with 2. What’s more, reducing the thickness of c-Si solar cells means lower silicon material cost, which is 57% of the total cell cost currently⁴. However, though much effort has been rigorously put into this area, no previous experimental work has shown a Voc enhancement by decreasing the thickness of ultra-thin c-Si solar cells, which is contradictory to the theoretical prediction.

In this work, we investigate the Voc enhancement in ultra-thin c-Si solar cells through a series of simulations and experimental work. For the first time, the enhancement of Voc by reducing the thickness of ultra-thin c-Si cells is demonstrated experimentally. Multiple recombination mechanisms have been studied, and the corresponding key design principles are also demonstrated. With the carefully passivated cell structure, a 5-μm-thick cell achieved a Voc of 649mV, which is 9mV higher than for a 50-μm-thick control sample and is also the highest Voc among all recently reported sub-25-μm-thick cells. The 5-μm-thick cell also has an efficiency of 11.7%, which is the highest for c-Si solar cells with ≤5μm thickness. In addition, our results show that high Voc and high efficiency could be potentially achieved in ultra-thin c-Si solar cells with relatively poor material quality. To summarize, our work shows a promising way to realize both low material cost and high efficiency in c-Si solar cells.

[1] Green MA et. al.,. Solar cell efficiency tables (Version 45).
[2] Tiedje T, et. al., IEEE Trans on Electron Devices 31, 711-716 (1984).
[3] Brongersma, M., et. al., Nature materials 13.5 (2014): 451-460.
[4] International Technology Roadmap for Photovoltaic (ITRPV) 2014.

Organic solar cells offer sustainable solar energy conversion by means of mechanical flexible, light-weight, cost effective printable devices. Despite considerable efforts during the past several decades, organic solar cells still have lots of chance for performance enhancement. The principal energy harvesting steps, including photoexcitation of excitons, exciton dissociation into separate charges, and charge transport to electrodes, commonly suffer from low efficiencies in organic active layers. Photoexcitation of an organic semiconductor is inherently confined by the specific light absorption spectra given by the chemical structures. The diffusion of excitons and separated charges is retarded in the low crystalline organic active layers. To date, many different approaches have been exploited to enhance the device performance of organic solar cells, introducing new organic semiconductors, device structure optimization, nanoscale additives and so on. Unfortunately, delicate interplay among the chemical structures of organic semiconductors, processing parameters, and device structures, makes it challenging to effectively improve the different energy harvesting steps in a concurrent way.

Plasmonic nanoparticle provides an effective route to enhance the exciton generation and separation of charge carriers in organic bulk-heterojunction solar cells. However, metallic plasmon components commonly trap charge carriers and thereby deteriorate eventual charge harvest at electrodes. Here we present metal-carbon hybrid nanomaterials randomly incorporated in the active layer of organic solar cell surpass such intrinsic limitation and effectively improve solar cell performance. Plasmonic Au nanoparticles are decorated at N- and B-doped carbon nanotubes by electrostastic and coordinate interactions, respectively. While plasmonic nanoparticles promote charge generation and dissociation in the organic active layer, B- and N-doped carbon nanotubes promote charge transport without quenching. Such concurrent enhancement of principal energy harvest steps greatly improves device efficiency. Single-junction solution-processed solar cell produced from commercially available organic semiconductor attains the device efficiency of 9.98 %. This synergistic combination of plasmonic component with carbon materials is generally useful for organic optoelectronics and electronics suffering from poor controllability of charge generation and transport.

Implementing photovoltaic (PV) generation on the terawatt scale necessitates reductions in cost without compromising efficiency. Achieving high efficiency requires both low recombination rates at charge collection interfaces and optically transparent contacts.¹ HIT cells are one example of this type of approaches in silicon PV technology.² However, this and other high-efficiency Si technologies require high process temperature and vacuum-based deposition, both of which add cost.
Here, we demonstrate that a very low cost and scalable method can be used to produce n-Si heterojunction solar cells with high open circuit voltage. For the transparent hole collecting contact, we used p-type transparent, highly conducting (CuS)x:(ZnS)1-x nanocomposite thin films synthesized by facile, rapid, low temperature chemical bath deposition from earth-abundant elements in aqueous solution. The CBD process produces nanocomposite phases of ZnS (sphalerite) and CuS (covellite) within the films. Control of the p-type conductivity is achieved by varying the amount of CuS, which is the hole conducting phase in this system. Control of transparency (50-75% in the visible for a 50 nm film) is achieved by adjusting the concentration of complexing agent to tune the nanocrystal sizes. Optimal films have a nanocrystal size of ca. 5 nm and hole conductivities up to 1000 S cm^-1, which is similar to the electron conductivity of sputtered ITO at the same process temperature.
Heterojunction solar cells were fabricated by direct CBD growth of p-(CuS)_x:(ZnS)_1-x on n-Si. Under illumination of 1 Sun, an open circuit voltage (Voc) of 500 mV and a short circuit current density of 16 mA cm^-2 is achieved. The V_oc is comparable to those of other emerging low cost heterojunction Si solar cells, such as single-walled carbon nanotube / Si (370-530mV)³ and graphene / Si (360-552mV) ^4,5 PV devices.

Keywords: transparent conducting materials, p-type, chemical bath deposition, heterojunction photovoltaics.

(1) Wurfel, U.; Cuevas, A.; Wurfel, P. IEEE J. Photovoltaics 2015, 5 (1), 461–469.
(2) De Wolf, S.; Descoeudres, A.; Holman, Z. C.; Ballif, C. green 2012, 2 (1).
(3) Jung, Y.; Li, X.; Rajan, N. K.; Reed, Nano. Lett. 2013, 10–14.
(4) Zhang, Y.; Zu, F.; Lee, S.-T.; Liao, L.; Zhao, N.; Sun, B. Adv. Energy Mater. 2014, 4 (2).
(5) Bullock, J.; Yan, D.; Cuevas, A.; Wan, Y.; Samundsett, C. Energy Procedia 2015, 77, 446–450.

Polycrystalline cubic-AgSbS₂ (a = 0.5652 nm) is obtained by heating amorphous thin film of Ag-Sb-S, deposited at 10 ^oC from a solution mixture of antimony salt, thiosulfate and silver nitrate. A film of thickness 700 nm is obtained via sequential deposition. The amorphous Ag-Sb-S film has an optical band gap (E_g) of 2.03 eV. This value decreases to 1.8 eV when the film has been heated at 280 ^oC, when more than 95% of the material has been transformed to crystalline matter with cubic structure similar to that of the mineral cuboargyrite. The polycrystalline thin films are photoconductive, with electrical conductivity (σ_ph) 10^-5 (Ω cm)^-1 under 1000 W/m² tungsten-halogen light. A typical solar cell using this absorber shows open circuit voltage (V_oc) 626 mV, short circuit current density (J_sc) 1.35 mA/cm², fill factor (FF) 0.64 and conversion efficiency (η) of 0.54 %. Improvement of the overall cell parameters due to controlled incorporation of Se in the AgSbS₂ film will be presented. For this, a thin film with 80 nm thickness of Se is deposited on a glass substrate from a solution of sodium selenosulfate at pH 4.5. This film is kept in contact with the Ag-Sb-S film to serve as a Se-source, and the pair is heated at 180 ^oC. Colloidal graphite paint is applied on this converted film and heated further at 280 ^oC. The overall cell performance improved when this absorber is used. While V_oc is slightly reduced to 527 mV, J_sc increased up to 2.07 mA/cm²; a high FF of 0.6 is maintained and η is 0.65 %. Optimizing the Se content in AgSbS₂:Se might improve the results further.

The ternary alloy GaAsBi is a potentially important material for infrared light emitting devices and multijunction photovoltaics, but its use has been limited by its poor optical quality. We will discuss the growth of these alloys using light-assisted MBE, where the resulting epi-layers exhibit intense, narrow, band edge photoluminescence similar to other ternary GaAs based alloys, such as InGaAs. The measured spectral linewidths are as low as 14meV and 37meV at low temperature (10K) and room temperature, respectively. These values are less than half of previously reported GaAsBi PL linewidths. The improved optical quality is attributed to the use of incident UV irradiation of the epitaxial surface and the presence of a partial surface coverage of bismuth in a surfactant layer during epitaxy. Comparisons of samples grown under illuminated and dark conditions provide insight into possible surface processes that may be altered by the incident UV light. The improved optical quality now opens up possibilities for the practical use of GaAsBi in optoelectronic devices.

Tungsten selenide (WSe₂) is a crystalline semiconductor material that exhibits a layered hexagonal crystal structure, giving the material mechanical and electronic properties that show promising applications ranging from solid lubricants to solar cell absorbers. With its low mineral extraction costs, a reported direct bandgap of 1.35-1.4 eV and high minority carrier mobility, the WSe₂ material system is seeing renewed interest in next-generation solar cell material development. Nanotechnology is making possible new low-cost, non-vacuum methods for the deposition of photovoltaic thin films. In our studies, we have discovered that at temperatures as low as 110 ^oC, the reaction between W(CO)₆ and Se in a hydrocarbon or chlorinated hydrocarbon solvent forms a solid product consisting of a mixture of trigonal crystalline Se and an amorphous tungsten-containing species forms. After being drop cast from a solution suspension as a thin layer on an inert substrate, this precursor can be annealed under an argon atmosphere at 550 ^oC to yield an exceptionally well-ordered WSe₂ thin film of (002) orientation. This process temperature is over 300 ^oC lower than that needed to produce high quality films via selenization of a tungsten thin film, allowing low-cost soda lime glass to be employed as a substrate. The microstructural, optical and electronic properties of these films have been characterized by XRD, Raman microscopy, and absorption spectroscopy and will be compared to films grown by the high temperature (875 ^oC) reaction of tungsten films with selenium.

Among many available photovoltaic technologies at present, gallium arsenide (GaAs) is one of the recognized leaders for performance and reliability; however, it is still a great challenge to achieve cost-effective GaAs solar cells for smart systems such as transparent and flexible photovoltaics. In this study, highly crystalline long GaAs nanowires (NWs) with minimal crystal defects are synthesized economically by chemical vapor deposition and configured into novel Schottky photovoltaic structures by simply using asymmetric Au−Al contacts. Without any doping profiles such as p−n junction and complicated coaxial junction structures, the single NW Schottky device shows a record high apparent energy conversion efficiency of 16% under air mass 1.5 global illumination by normalizing to the projection area of the NW. The corresponding photovoltaic output can be further enhanced by connecting individual cells in series and in parallel as well as by fabricating NW array solar cells via contact printing showing an overall efficiency of 1.6%. Importantly, these Schottky cells can be easily integrated on the glass and plastic substrates for transparent and flexible photovoltaics, which explicitly demonstrate the outstanding versatility and promising perspective of these GaAs NW Schottky photovoltaics for next-generation smart solar energy harvesting devices.

A growing interest in transparent conducting oxides (TCOs) stems from limitations of a wider use of indium tin oxide (ITO) films as transparent electrodes. As a very promising alternative material to ITO, zinc oxide films doped with group III element (ZnO:Al, ZnO:Ga) have been investigated. Novel applications of alternative TCOs include flexible electronics. Here, possibly high stability of electrical properties of the TCO films under bending conditions is required.
In this work, we studied ZnO:Al/organic hybrid films grown on polyethylene terephthalate (PET) substrates by combined atomic and molecular layer deposition (ALD/MLD) techniques. ALD and MLD methods offer uniform film deposition over substrates as large as several m². The investigated films varied with respect to organic content. The organic constituent was obtained from the alternate doses of hydroquinone and diethylzinc precursors. Structural and electrical investigations of the hybrid films were carried out. Bending tests revealed piezoresistive behavior of the ZnO:Al/organic films. The piezoresistive response lowers for the films with higher organic content. Applications of the ZnO:Al/organic films as transparent electrodes in flexible thin film structures will be shown.
The work was partially supported by The Polish Ministry of Science and Education program “Iuventus Plus” (2015-2017) under decision No. 0267/IP2/2015/73, and by the National Science Centre (NCN, Poland) under decision No. DEC-2012/06/A/ST7/00398.

Tin selenide thin films of orthorhombic (OR) or of cubic structure (CUB) produced epitaxially on cubic tin sulfide have distinct properties: optical band gap of 1 eV and 1.4 eV and p-type electrical conductivities of 1 and 0.001 Ω^-1 cm^-1, respectively. The films are stable upon heating at temperatures of up to 300 ^oC. When the heating is done in a Se-ambient at 300-350 ^oC , the films are converted to n-type SnSe₂ with band gap 1.2 eV and electrical conductivity, 2 Ω^-1 cm^-1. The structural, electrical and optical modifications in tin selenide thin films offers many possibilities in developing solar cells as well as thermoelectric devices. A simple device is a solar cell structure: CdS/SnSe-CUB in which the open circuit voltage is 0.37 V and current density, 1. 2 mA/cm². A thermoelectric p-SnSe/n-SnSe₂ couple offers 0.5 mV/K. With the availability of SnS thin films of orthorhombic or CUB structure with optical band gap of 1.2 and 1.74 eV respectively, many new device structures become possible. While the cubic phase tin chalcogenide so far is possible only by chemical deposition, thin films of SnSe and SnS are produced by thermal evaporation as well chemical deposition.

The quest for low cost, non-toxic and earth abundant photo-absorbers as an alternative to toxic Cd in cadmium telluride (CdTe) or scarce In and Ga in copper indium gallium (di)selenide (CIGS) have spurted research in new materials systems for thin film solar cells. Current favorite is Cu₂ZnSnS₄ (CZTS), but due to composition dependent narrow stability range for photovoltaic properties, simple binary chalcogenide semiconductors have generated renewed interest. Tin sulfide (SnS), a potential photo-absorber among others FeS₂ and ZnSnP₂, is being actively investigated. SnS is p-type semiconductor (p~10¹⁵ -10¹⁷ cm^-3), has a band gap of 1.3 eV and high (>10⁴cm^-1) absorption coefficient utilizing near optimum solar radiation with thin absorber in solar cell. Commonly, SnS films are deposited by direct methods like electrodeposition, aerosol spray and vacuum evaporation. Inclusion of secondary Sn₂S₃, SnS₂ phases due to lack of deposition controls in these methods have constrained realization of device specific optoelectronic properties. Furthermore, interface defect mediated carrier recombination with n-type CdS or ZnO junctions is a bottleneck for attaining high photo-conversion efficiency.
This research provides a new perspective on indirect synthesis of photosensitive monophasic SnS films by organic chemical vapor sulfurization of Sn thin film. S-radicals formed by closed space pyrolysis of di-tert-butyl disulfide (TBDS) diffusively react with Sn to produce SnS film. SnS being an amphoteric semiconductor converts to n-type by trivalent Sb and Bi dopants. The organic vapor sulfurization method described in this research facilitates single-step synthesis of buried junction structures and thus SnS solar cells in a p-n homojunction or p-i-n structures.
In this work sputter deposited 100 nm Sn thin film was converted to SnS by sulfurization under 100 sccm flow of TBDS vapor preheated to 100°C and structural phase evolution and film growth kinetics were investigated for sulfurization at 300, 350 and 400°C for periods 45, 60 and 90 min. X-Ray diffraction studies establish single phase highly crystalline film in orthorhombic crystal structure forms at 350°C. Diffusive sulfurization mechanism was observed as continued sulfurization for longer periods causes secondary reactions with SnS to form SnS₂ or Sn₂S₃. Raman scattering results confirm SnS formation with the identification of 2Ag, 2B2g optical phonons modes. Optical bandgap studies confirm a low energy 1-1.1eV indirect bandgap and a strong absorption threshold between 1.4 to 1.6 eV depending on the sulfurization conditions correlating with intrinsic defects and phase structure of the film. Sulfurization conditions for bilayer Sn (100 nm)/ ZnO (75 nm) thin film structures were investigated to form graded bandgap heterojunction structure in a single step and photovoltaic performance of device was evaluated. This paper will report the results of these investigations.

In this work, ZnSnSe₂ (ZTSe) thin films were deposited by using single crystalline powder of ZnSnSe₂ grown by vertical Bridgman-Stockbarger technique. The deposition process were carried out by means of e-beam evaporation on the well-cleaned soda lime glass substrates and keeping them at the substrate temperature of 200°C. The structural, optical and electrical properties of ternary ZTSe thin films were investigated depending on the annealing temperature at 300-500°C. X-ray diffraction (XRD) analysis showed that as-grown films were in amorphous structure, however annealing at 300°C triggered the crystallization on the preferred ternary structure and annealing at 500°C resulted in the single phase polycrystalline structure. From the compositional analysis with the energy dispersive X-ray spectroscopy (EDS), the detail information about the stoichiometry and the segregation mechanisms of the constituent elements in the structure were determined for both as-grown and annealed samples. In addition, they were morphologically characterized using scanning electron microscopy (SEM). In the interest of window layer for possible photovoltaic applications with chalcopyrite and kesterite structures, the optical properties for the ZTSe thin films were studied depending on the structural changes with annealing. Analysis of the absorption behavior of the films revealed the existence of allowed direct transition, and the corresponding band gap values were calculated from their Tauc plots. The electrical properties were analyzed using temperature dependent dark- and photo-conductivity, and room temperature Hall effect measurements. From the variation of electrical conductivity as a function of the ambient temperature, the current transport mechanisms and corresponding activation energies at specific temperature intervals for each sample were determined.

An innovative and scalable synthesis approach to the formation of phase-pure Cu₂ZnSnS₄ (CZTS) nanocrystals has been developed using aerosol spray pyrolysis. This material is a potential replacement for commercialized semiconductors (such as CdTe and CIGS) that are used in photovoltaic devices and whose long-term viability is threatened by sustainability and environmental issues. Based upon earth abundant constituents and low chemical toxicity, CZTS, with a reported bandgap of ~1.5 eV^[1], has the potential to be a superior alternative to these other materials. Ongoing research is focused on increasing the efficiency of CZTS-based cells from the current record (12.6% by Wang et al.^[2] for a CZTSSe cell) to the >18% necessary to be considered economically viable. Hages et al. ^[3] have demonstrated that the incorporation of column-IV elements — specifically germanium — improves photovoltaic characteristics of the material, similar to those improvements observed and utilized in the CIGS material system. In our work, we demonstrate the effects of incorporating silicon, rather than germanium, into the CZTS lattice. We demonstrate a controllable, cost-effective, and reproducible synthesis of high-quality CZTS and CZTSiS nanoparticles and films. A modified spray pyrolysis method involving decomposition of copper, zinc, and tin diethyldithiocarbamate precursors allows uniform incorporation of dopants (such as sodium) that are known to increase crystal grain growth during nanoparticle sintering. Once formed, the nanoparticles are deposited onto a substrate from a methanol dispersion using an “ink-spray” process with an argon-driven airbrush. To form an efficient absorber layer in a photovoltaic device, the coating is then annealed in a sulfur-vapor atmosphere resulting in a thin film with uniformly large crystal grain morphology throughout the film thickness (~1-2 µm). Silicon can either be incorporated directly into the CZTS through inclusion of Si-nanoparticles in the precursor solution, or by depositing a solution containing CZTS and Si nanoparticles onto a substrate and annealing.

^[1] H. Wang. “Progress in Thin Film Solar Cells Based on Cu₂ZnSnS₄,” International Journal of Photoenergy 2011 (2011).
^[2] Wang, Wei, Mark T. Winkler, et al. “Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency.” Advanced Energy Materials 4, no. 7 (2014).
^[3] Hages, Charles J., Sergej Levcenco, et al. “Improved Performance of Ge-Alloyed CZTGeSSe Thin-Film Solar Cells through Control of Elemental Losses.” Progress in Photovoltaics: Research and Applications 23, no. 3 (2015).

Development of novel techniques for study of copper zinc tin sulfide Cu₂ZnSnS₄ (CZTS) phase formation and decomposition is a critical step toward high quality CZTS thin films for solar cell applications. In the present work, we report the results of in situ Raman monitoring of CZTS phase under relevant processing conditions at elevated temperatures for the first time. CZTS thin films are prepared from simple sol-gel method followed by heat treatment, which, for some recipes, may involve sulfurization at 550°C for 1.0 hour using 5.0 vol. % hydrogen sulfide (H₂S) balanced by nitrogen (N₂). All samples are characterized using ex situ techniques such as XRD, SEM, and EDS to confirm phase formation and purity. XRD and Raman spectroscopy showed successful preparation of CZTS phase. Raman micro-spectroscopy measurements were carried out using 514.5 nm excitation laser on CZTS samples at room temperature and under in situ conditions in sealed chamber or in open air at elevated temperatures (300°C to 650°C) for different periods of time (1.0 to 3.0 hours). Preliminary results of in situ Raman monitoring showed that CZTS phase is Raman active at elevated temperatures up to ~ 600°C. As expected, Raman peaks are observed to shift to lower frequency values and broaden as the temperature goes higher. In addition, important information regarding CZTS phase decomposition and oxidation at high temperatures are revealed. Coupled with XRD analysis of the in situ Raman examined CZTS samples in air, a two-step oxidation reaction is suggested. CZTS phase oxidizes first at ~ 400°C to form tin oxide (SnO₂) and other binary sulfides such as copper sulfide (Cu_2-xS) and zinc sulfide (ZnS). Then, at temperatures higher than 400°C, the remaining sulfides oxidize to form zinc oxide (ZnO) and copper sulfate (CuSO₄). The EDS analysis of in situ Raman examined CZTS samples indicates copper loss for all samples examined at different temperatures. The study demonstrates that in situ Raman, when combined with other ex situ characterization techniques could provide valuable insights that help understand the complex processes involving phase formation and degradation for CZTS and other related multi-component chalcogenides phase as solar absorber materials.

As an earth abundant, low cost, and non-toxic material, Copper Zinc Tin Sulfide Cu₂ZnSnS₄ (CZTS) could be an excellent candidate for the next generation of eco-friendly materials for thin film solar cell applications. In this study, we report successful preparation of CZTS thin films using very low ppm level of hydrogen sulfide (H₂S) gas at relatively low temperature. Sol gel technique is adopted for the preparation of metal oxide based films from precursor solution that contains Cu, Zn, and Sn metal ions. Sulfurization process was carried out in tube furnace using 100 ppm H₂S balanced by hydrogen for 1.0 and 2.0 hours at different temperatures. The dependence of thin film microstructure, sulfur content, CZTS phase purity as well as chemical homogeneity on the sulfurization temperature (350°C, 450°C and 550°C) was investigated. Samples sulfurized at 350°C for 1.0 hour showed successful formation of relatively dense films of CZTS phase as confirmed by XRD, Raman micro-spectroscopy, SEM, and EDS. EDS analysis confirmed that CZTS films prepared at 350°C have near stoichiometric composition except for zinc metal as samples sulfurized at different temperatures showed zinc rich composition. Moreover, EDS line scan profiling of samples sulfurized at 350°C showed uniform metal composition across the CZTS film thickness. However, samples sulfurized at higher temperatures (450°C and 550°C) experienced sulfur loss as confirmed by EDS results that showed strong dependence of sulfur content on sulfurization temperature; sulfur content decreased from 46.0% to 22.0% for samples prepared at 350°C and 550°C respectively. Moreover, metal alloys such as Cu₃Sn are observed to form at high sulfurization temperature (550°C) due to the significant sulfur loss. As the sulfurization time increased from 1.0 to 2.0 hours, CZTS films observed to have multiple-layered structure of larger grains with approximately 1 µm size on top layer while the layer close to substrate surface maintained grains with submicron size range. Further study to understand the formation of CZTS using low ppm level H₂S sulfurization may enable new, low cost and environmental friendly methods of preparing high quality CZTS films for solar cell applications.

Lately, due to the growing demand on new classes of two-dimensional materials in broadband optoelectronic device applications, LEDs and photovoltaics, accurate calculation of their electronic and optical properties has gained great importance. The structural, electronic and optical properties of few members of Group VA (such as Arsenene and Antimonene) were predicted using first-principles approaches. We have utilized hybrid functionals on top of the standard density functional theory, as well as the G₀W₀ approximation within the framework of many-body perturbation theory. Beyond the random-phase approximation, the excitonic effects were also accounted for by undertaking the Bethe-Salpeter formalism. The buckled and (symmetric/asymmetric) washboard structures of both As and Sb were considered for all calculations. The prominent optical absorption for Arsenene’s buckled and washboard configurations occur similarly and near the visible light region, extending further to the ultra-violet regime. Specifically, an onset of interband absorption is observed near 2.5 eV, spanning an optically active range up to 7 eV. On the other hand, B-Sb displays a slightly earlier onset of absorption; approximately at 1.5 eV, whereas the a-W structure reveals prominent peaks also within the infrared regime. We have also investigated the influence of the thickness of 2-D systems and biaxial tensile strain, which were observed to tune the spectral (optical) properties of Group VA materials; hence could be noteworthy parameters for their solar cell applications. We belive that this study highlights the significance of the prospective realization of new generation layered (2-D) forms of Group VA members aimed for optoelectronic applications, especially within the solar energy regime. This research was supported by TUBITAK (project no. 115F088).

High quality thin films of Bismuth ferrite BiFeO₃ (BFO) were grown by atomic layer deposition (ALD). The solid precursors were Bi(TMHD)3 and Fe(TMHD)3, both with 99.9% purity, and deionized water was used as oxygen source. The thickness of BFO thin films was controlled by self-limiting molecular size of precursors and chemical absorption between precursors and hydroxyl groups of Bi-O and Fe-O layers. X-ray photoelectron spectroscopy (XPS) analysis of the BFO layers confirmed that the film is stoichiometric with a Bi³⁺:Fe³⁺ atomic ratio of 1:1 and no diffusion of Bi atoms were found at low deposition temperature (~250 °C) employed in the ALD process. The phase formation of the films was confirmed by X-ray diffractometry and Raman spectroscopy. The uniformity and roughness of films were characterized by Atomic Force Microscope (AFM). The ferroelectric nature of the films was characterized by utilizing Piezoresponse Force Microscopy (PFM) and hysteresis loop measurement using a Sawyer-Tower configuration. Further, the evolution of grains size, ferroelectric and leakage properties of BFO thin films was studied as a function of post deposition annealing temperature in the range of 450-750 °C in preselected gas atmosphere. The photovoltaic properties of ALD-grown BFO thin film capacitor structures were evaluated under white light illumination. The encouraging results demonstrate the possibility of fabricating ferroelectric devices for light harvesting as well as its integration into silicon technology for logic and memory devices.

Organic photovoltaics (OPVs) are an attractive area of solar cell research due to their inexpensive nature, ease of large-scale fabrication, flexibility, and low-weight. However, low levels of light absorption, exciton diffusion lengths, and charge carrier motilities ultimately restrict the maximum power conversion efficiency (PCE) of OPVs, limiting them as currently insufficient for commercial viability. The incorporation of noble metallic nanoparticles (NMPs) into OPVs has been shown as an effective method for the improvement of the PCEs, through localized surface plasmon resonance (LSPR) and scattering enhancements to essentially increase the total light absorption into OPVs. Silver nanoparticles (Ag NPs) are of the greatest interest for this application, because of their higher scattering power and lower cost, in relation to other NMPs. In particular, the anisotropy of Ag nanoprisms allows for greater plasmonic effects in comparison to the Ag nanospheres counterparts, which further increases their practicality as an additive to OPVs. Unfortunately, bare Ag nanoprisms lack stability in acidic environments that are typical in hole transport layer materials, such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). We have found that coating the edges of the Ag nanoprisms with gold dramatically augments their stability in acidic solutions and films. In this study, we integrate gold-coated Ag nanoprisms into PEDOT:PSS to improve the PCE of OPV devices, constructed from readily available donor and acceptor materials, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The goal of the study is to determine the optimized edge-length and concentration of the Ag nanoprisms in PEDOT:PSS for the maximal increases in the PCE. We study the J-V curves, external quantum efficiency (EQE), and absorbance enhancements with the addition of the Ag nanoprisms to the OPV devices. The morphology, work function, and conductivity variations by the addition of the Ag nanoprisms to PEDOT:PSS were also characterized. We also quantify the LSPR and scattering effects of the Ag nanoprisms within the OPV devices to pinpoint the cause for PCE improvements.

Organic photovoltaic cells (OPVs) are a promising technology for low cost, light weight, large-area, and high-throughput energy generation. In this study, two novel semiconducting polymers based on benzodithiophene and dithienophosphole oxide (DTP) units are designed and synthesized. Interestingly, the introduction of DTP units brings highly polarizable characteristics, which is beneficial for the photocurrent in solar cells. Thus, the donor–acceptor type of conjugated polymers based on this new acceptor has superior charge transfer properties and highly efficient PL quenching efficiencies. As a result, polymer solar cells (PSCs) with high power conversion efficiencies of 6.10% and 7.08% are obtained from poly(3,5-didodecyl-4-phenylphospholo[3,2-b:4,5-b']dithiophene–4-oxide-alt-4,8-bis(5-decylthiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene) (PDTP–BDTT) and PDTP–4-oxide-alt-4,8-bis(5-decylselenophen-2-yl)benzo[1,2-b:4,5-b']dithiophene) (PDTP–BDTSe), respectively. The PDTP–BDTSe copolymer is now the best performing DTP-based material for PSCs. Using the polarizable unit strategy determined in this study for the molecular design of conjugated polymers is expected to greatly advance the development of organic electronic devices.

Cu(In,Ga)Se₂ (CIGSe) is the most common photo-absorption layer material for thin-film solar cells; however, Ga and In are relatively scarce elements in earth and the preparation of high-efficiency CIGSe device usually requires the utilization of highly toxic H₂Se gas. With the physical properties similar to those of CIGSe, Cu₂ZnSnS₄ (CZTS) is hence proposed since it contains relatively inexpensive, non-toxic and earth-abundant Zn and Sn elements. Moreover, selenium (Se) may further add into CZTS to form the CZTSSe so as to modulate the bandgap (E_g) of photo-absorption layer and improve the conversion efficiency (η) of solar cell.
This work prepares the CZTSSe thin films by sputtering deposition of single-phase CZTS target and post annealing treatment at 570°C in Se/S vapor ambient. The CZTSSe solar cell samples with the Mo/CZTSSe/CdS/i-ZnO/IZO/Al structure were also prepared and their performances were evaluated. By varying the ratio of Se/S elements during the annealing, we achieved the Cu₂ZnSn(S_xSe_1-x)₄ layers with x in the range of 0.2 to 1 and all CZTSSe layers possessed the composition with Cu-poor/Zn-rich feature. Depending on the Se/S ratio, the CZTSSe layers are mixture of CZTS and CZTSe kesterite phases at various ratios as revealed by x-ray diffraction and Raman spectroscopy analyses. UV-Vis spectroscopy indicated the E_g of CZTSSe samples are in the range of 1.06 to 1.45 eV when x varies from 0.2 to 1. Hall measurement observed the best transport property with p-type carrier concentration of 2.17×10¹⁵ cm^-3 and mobility of 8.9 cm²×V^-1×sec^-1 in CZTSSe layer with x = 0.46. Under the AM1.5 illumination condition, the CZTSSe thin-film solar cell sample exhibited the best performance with open-circuit voltage of 0.502 V, short-circuit current density of 27 mA/cm², fill factor of 50% and η of 6.9%.
Keywords: CZTSSe thin-film solar cell, Sputtering deposition, Se/S vapor annealing treatment.

CZTS (Cu₂ZnSnS₄ ) is a promising material for solar cell absorbers and consists of abundant and environmentally friendly elements. The efficiency of a cell based on this material has increased from 2% in 2001 to 8.8 % in 2015 [1]. CTS (Cu₂SnS₃) is a similar material, which recently has reached an efficiency of 4.3 % [2].
Pulsed laser deposition (PLD) is considered as a technique, by which a material with a complicated stoichiometry can be transferred from a target to a growing film in a vacuum chamber. It is a versatile technique, particularly suited to mixtures of elements, e.g. metal oxides and alloys. However, if a volatile component in the target material, e.g. oxygen, is lost by evaporation from the target or the film during the laser irradiation, the stoichiometry can be controlled by performing the deposition in a background gas, e.g. also oxygen.
However, for sulfides the deposition needs to be carried out in a background gas of H₂S which is explosive and toxic. Alternatively, the sulfurization can be performed by a post-(deposition) annealing treatment in sulfur atmosphere. This treatment is time-consuming and involves a careful optimization procedure.
We investigate a new approach to prevent the loss of sulfur in CZTS, which is by using a reactive sulfur beam during PLD of CZTS in vacuum. The sulfur cracker source provides a beam with a high concentration of reactive sulfur dimers (S₂) and sulfur trimers (S₃) during the growth by cracking long rings and non-reactive chains of S₈ molecules. We will show the first preliminary results from a simultaneous operation with a PLD set-up and a sulfur cracker in a vacuum chamber.
The physics behind the PLD process of chalcogenide materials CZTS and CTS is partly determined by the composition of the target materials (which are typically made sintered powders of binary compounds). Our results show that the deposition process is a combination of evaporation of the most volatile phases/elements and ablation of the most stable ones. In our case at low laser fluence ( F2 ) the dominant process is laser-induced evaporation: films are very smooth and no (or very little) copper is transferred from the target. Indeed, the most volatile elements in the target are S and Zn (and SnS as a compound). On the other hand, at high fluence (F> 1.5J/cm²) films are characterized by an over-stoichiometric Cu-content (with respect to the target) and severe sulfur loss. The region of stoichiometric transfer is found between these two values of the laser fluence. We have obtained an efficiency of the CZTS cells exceeding 2 % with the most appropriate film composition and expect to reach a higher efficiency with the sulfur cracker.
[1]S. Tajima et al., Appl. Phys. Express 2015, 8, 082302
[2] A. Kanai et al. Jap. J. Appl. Phys. 54, 08KC06 (2015)
[3] L.-C. Chen, in: Pulsed laser deposition of thin films, Wiley (1994) p. 167.

A conversion efficiency of 22.3% on Cu(In,Ga)(Se,S)₂ (CIS-based) cell has been achieved. This is the highest efficiency in not only all thin-film PV but also multi crystalline Si PV. The efficiency improvement encourages expansion of production volume. In the future, tens of GW era on the CIS-based PV could be realized. In the case, the material cost is considered to be one of the biggest factors in the cost structure. In order to reduce the material cost, we are focusing on In-free and Se-free chalcogenide solar cells as next generation solar cells. For the In-free cells, the efficiency of Cu₂ZnSn(Se,S)₄ cell and submodule have been reached 12.7% and 11.8%, respectively. For the Se-free cells, the efficiency of pure-sulfide Cu(In,Ga)S₂ cell and submodule have been reached 15.5% and 13.2%, respectively. In this presentation, we would review our recent progresses on our In-free and Se-free approaches.

This presentation will detail experimental progress in developing a new thin-film photovoltaic compound, Ag₂ZnSnSe₄ (AZTSe). This material is a variant of the widely-studied Cu-based kesterite Cu₂ZnSnSe₄ (CZTSe), where Ag is substituted for Cu in an effort to minimize disorder on the I-II cation sublattice. We will present a comparison of the optical properties of AZTSe and CZTSe, which indicate that the substitution of Cu with Ag induces a clear reduction in absorber band tailing and sub-Eg states. The radiative defects in AZTSe are found to be relatively shallow with respect to the band edges as judged from preliminary photoluminescence characterization. Additionally, we find that the dielectric constant of AZTSe is ~ 12.6, which is almost identical to the value for CIGS. This is in contrast to the value of 8.6 in CZTSe. The higher dielectric constant is a promising material parameter, indicating that the charged defects which are present in the structure are likely to be screened more effectively. We will present a study of the electronic properties of AZTSe as a function of stoichiometry and various dopants. Finally, we will discuss some of the implications for device formation as well as some preliminary device results.

The information, data, or work presented herein was funded in part by the U.S. Department of Energy, Energy Efficiency and Renewable Energy Program, under Award Number DE- EE0006334.

One of the reasons for Cu(In,Ga)Se₂ based thin film solar cells sucess is the remarkable flexibility of its chalcopyrite crystal structure [1,2]. This flexibility is a key point also for the quaternary kesterite type compounds like Cu₂ZnSnS₄/Se₄ (CZTS, CZTSe) because the thin film growth is in fact a non-equilibrium process. Nevertheless the absorber layers exhibits an overall off-stoichiometric composition, thus the existence of intrinsic point defects is strongly correlated with the chemical potential and therefore dependent on the composition of the material. Finally these structural defects influence the electronic properties of the final device, sensitively.
Both compounds, CZTS and CZTSe, crystallize in the kesterite type structure (space group I-4) [3]. The structure can be described by a stacking sequence of cation layers Cu/Sn – Cu/Zn – Sn/Cu – Cu/Zn – Cu/Sn perpendicular to the crystallographic z-direction. An off-stoichiometric composition, for instance Cu-poor, originates from the prospensity of the structure to stabilze copper vacancies, the charge balancees beeing commonly insured by appropriate substitutions on the cationic sites. If the oxidation states of the cations and anions are retained going from stoichiometric CZTS/Se to off-stoichiometric CZTS/Se, a number of specific substitutions can be envisioned to account for the charge balance in the off-stoichiometric material. These substitutions lead to the formation of point defects in the crystal structure of the material. For a Cu-poor and Zn-rich composition as well as a Cu-rich and Zn-poor composition certain substitutions were suggested by Lafond et al. [4]. For Cu-poor and Zn-poor as well as Cu-rich and Zn-rich CZTS/Se possible cationic substitutions were introduced by our group [5]. Each of these substitutions is connected with the formation of special point defects.
The presentation will give an overview of the cation substitutions and the connected point defects, which are considered to form off-stoichiometric kesterites by keeping the charge balance. The basis for our studies is a number of CZTS and CZTSe powder samples, synthesized by solid state reaction. The chemical (by WDX spectroscopy) and structural analysis (by neutron diffraction) results in the determination of defect types and concentrations, which can be correlated with the compositional ratios Cu/(Zn+Sn) and Zn/Sn.
[1] C. Stephan, S. Schorr, H.-W. Schock, M. Tovar, Appl. Phys. Let. 98 (2011) 091906
[2] C. Stephan, T. Scherb, C, Kaufmann, S. Schorr, H.-W. Schock, Appl. Phys. Let. 101 (2012) 101907
[3] S. Schorr, Solar Energy Materials and Solar Cells 95 (2011) 1482-1488
[4] A. Lafond et al., Z. Anorg. Allg. Chem. 638 (2012) 2571–2577
[5] L.E. Valle-Rios, K. Neldner, G.Gurieva, S. Schorr, J. Alloys Comp. (2015) doi:10.1016/j.jallcom.2015. 09.198

Polycrystalline Cu₂ZnSn(S,Se)₄ (CZTSSe) compounds have received wide research interest due to their potential as inexpensive absorber materials composed of earth-abundant elements. Photovoltaic devices fabricated on CZTS,Se have reached conversion efficiencies of 12.6 %. However, higher density of I-II exchange sites can strongly limit the maximum efficiency that can be achieved by this material. A potential earth-abundant substitute for CZTSSe is Ag₂ZnSnSe₄ (AZTSe) where the large covalent radius of Ag compared to Zn can limit the density of exchange sites and potentially lead to higher open-circuit-voltages. One of the key parameters to optimize the photovoltaic performance of polycrystalline solar cells is to control the concentration of recombination sites at the grain boundaries as well as the bulk of the film. To determine the composition and charge at the grain boundaries, this work has employed Auger nanoprobe microscopy (NanoAuger) with 8nm lateral resolution combined with high resolution ambient Kelvin Probe Force Microscopy (KPFM) with dual-lock-in amplifier setup. In order to characterize the grain boundaries as a function of depth within the films, grazing angle cryo-Focused Ion Beam (at -180 °C to -190 °C) is employed. Solution-deposited CZTS,Se grain boundaries showed negative charge (upward band bending) that is associated with the presence of SnOx phase formed during the film’s air anneal. In contrast, thermally-evaporated Cu(In,Ga)Se₂ (CIGSe) had positively-charged grain boundaries (downward band bending) despite the absence of a distinct compositional difference between the grains and grain boundaries. These results are compared with NanoAuger and KPFM measurements on grazing cross-sections of thermally-evaporated AZTSe. Various grain boundary passivation methods were explored for AZTSe.

Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic material could be the earth-abundant (i.e low cost) and low toxicity alternative compounds for the commercialized Cu(In,Ga)(S,Se)₂ thin-film solar cells. To achieve this possible, specific efforts applied to the absorbers, front and back interfaces/contacts must be performed for improving CZTSSe performance. [1,2]
In this work, we demonstrated the high efficiency CZTSSe solar cells by introducing an interfacial Ge doping layer between the absorber (i.e., CZTSSe) and the buffer layer (i.e., CdS) before selenization/sulfurization processes. In the statistically studies, the Ge layers increase short circuit current density (Jsc) from 24.6 mA/cm² to 29.5 mA/cm² and open circuit voltage (V_oc) from 455 mV to 492 mV, possibly resulting from the suppression of Sn⁺² state formation by small quantities of Ge (Ge/(Sn+Ge)