Electricity produced by cadmium telluride (CdTe) photovoltaic modules is the lowest cost in the solar industry, and now undercuts fossil fuel-based sources in many regions of the world. This is due to recent efficiency gains brought about by alloying selenium into the CdTe absorber, which has taken cell efficiency from 19.5% to its current record of 22.1%. While the addition of selenium is known to reduce the bandgap of the absorber material and hence increase cell short-circuit current, this effect alone does not explain the performance improvement. Here, by means of cathodoluminescence (CL) and secondary ion mass spectrometry (SIMS), we show that selenium enables higher luminescence efficiency and longer diffusion lengths in the alloyed material, indicating that selenium passivates critical defects in the bulk of the absorber layer. This passivation effect explains the recordbreaking performance of selenium-alloyed CdTe devices, and provides a route for further efficiency improvement that can result in even lower costs for solar generated electricity.

. (2011) 'Stable polymorphs crystallized directly under thermodynamic control in three-dimensional nanocon nement : a generic methodology.', Crystal growth design., 11 (2). pp. 363-366. Further information on publisher's website:https://doi.org/10.1021/cg101200fPublisher's copyright statement:This document is the Accepted Manuscript version of a Published Work that appeared in nal form in Crystal growth design, copyright c American Chemical Society after peer review and technical editing by the publisher. To access the nal edited and published work see https://doi.org/10.1021/cg101200fAdditional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbstractThermodynamic control of crystallization has been achieved to produce stable polymorphs directly by using 3D nano-confinement in microemulsions. The theoretical basis for thermodynamic control of crystallization using 3D nano-confinement is outlined. Our approach leap-frogs the usual metastable polymorph pathway because crystallization becomes governed by the ability to form stable nuclei, rather than critical nuclei. The generality of this approach is demonstrated by crystallizing the stable polymorph of three 'problem' compounds from microemulsions under conditions yielding metastable forms in bulk solution. The polymorphic compounds are mefenamic acid (2-[(2,3-(dimethylphenyl)amino] benzoic acid), glycine (aminoethanoic acid) and the highly polymorphic 5-methyl-2-[(2-nitrophenyl) amino]-3-thiophenecarbonitrile, commonly known as ROY because of its red, orange and yellow polymorphs. Application of this methodology should prevent another Ritonavir-type disaster, whereby a marketed drug transforms into a more stable form, reducing its bioavailability and effectiveness. The lowest energy nuclei selectively grow in our approach. Consequently this also provides a generic method for producing higher crystallinity materials, which may prove beneficial for crystallizing proteins and inorganic nanocrystals.Statement of urgency and brief summary of significant findings. We believe the paper fulfills the requirements of urgency for a Communication because it details for the first time a generic method to obtain thermodynamic control of crystallization. This enables stable polymorphs to be crystallized directly, to prevent another Ritonavir-type disaster. The methodology used selectively grows the lowest energy crystal nuclei, so it can also produce materials with higher crystallinity, which may prove of use for a wide range of crystalline materials, including potential...

Chemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu 2 ZnSnS 4 . Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor-acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of Zn Cu + antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are 'potential wells' for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes 'trapped' within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu 2 ZnSnS 4 crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices.

Cathodoluminescence is used to measure the recombination velocity of the heterointerfaces between Cu2ZnSnS4 (CZTS) and CuxSnySz, SnS secondary phases precipitated along the grain boundaries as well as ZnS precipitated within the CZTS grain interiors. The CZTS/CuxSnySz and CZTS/ZnS heterointerfaces had recombination velocities smaller than the bulk carrier diffusion velocity while the opposite is true for the CZTS/SnS heterointerface. Secondary phases having crystal structures compatible with CZTS (e.g., ZnS, Cu2SnS3) are likely to form heterointerfaces with small misfit strain and hence low interfacial recombination velocity. The precipitation of such secondary phases along grain boundaries in CZTS provides a novel mechanism for grain boundary passivation. However, it is not known if grain boundary passivating secondary phases would necessarily increase the overall photovoltaic device efficiency since other factors, such as the band gap of the secondary phase compared to the Shockley-Queisser ideal value and the nature of the heterointerface between CZTS (i.e., type-I vs type-II), also affect device operation and must therefore be taken into consideration.

The crystal structure of Sb2Se3 gives rise to unique properties that cannot otherwise be achieved with conventional thin-film photovoltaic materials, such as CdTe or Cu(In,Ga)Se2.It has previously been asserted that, grain boundaries can be made benign provided only the weak van der Waals forces between the (Sb4Se6)n ribbons are disrupted. Here it is shown that non-radiative recombination is suppressed even for grain boundaries cutting across the (Sb4Se6)n ribbons. This is due to a remarkable self-healing process whereby atoms at the grain boundary can relax to remove any electronic defect states within the band gap. Grain boundaries can however impede charge transport due to the fact that carriers have a higher mobility along the (Sb4Se6)n ribbons. Because of the ribbon misorientation certain grain boundaries can effectively block charge collection. Furthermore, it is shown that CdS is not a suitable emitter to partner Sb2Se3 due to Sb and Se inter-diffusion. As a result a highly defective Sb2Se3 interfacial layer is formed that potentially reduces device efficiency through interface recombination.