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 crystallogra