Interface band gap narrowing behind open circuit voltage losses in Cu₂ZnSnS₄ solar cells

We present evidence that band gap narrowing at the heterointerface may be a major cause of the large open circuit voltage deficit of Cu$_2$ZnSnS$_4$/CdS solar cells. Band gap narrowing is caused by surface states that extend the Cu$_2$ZnSnS$_4$ valence band into the forbidden gap. Those surface states are consistently found in Cu$_2$ZnSnS$_4$, but not in Cu$_2$ZnSnSe$_4$, by first-principles calculations. They do not simply arise from defects at surfaces but are an intrinsic feature of Cu$_2$ZnSnS$_4$ surfaces. By including those states in a device model, the outcome of previously published temperature-dependent open circuit voltage measurements on Cu$_2$ZnSnS$_4$ solar cells can be reproduced quantitatively without necessarily assuming a cliff-like conduction band offset with the CdS buffer layer. Our first-principles calculations indicate that Zn-based alternative buffer layers are advantageous due to the ability of Zn to passivate those surface states. Focusing future research on Zn-based buffers is expected to significantly improve the open circuit voltage and efficiency of pure-sulfide Cu$_2$ZnSnS$_4$ solar cells.Comment: Accepted at Applied Physics Letter

Stacked Janus Device Concepts: Abrupt pn-Junctions and Cross-Plane Channels

Janus transition metal dichalcogenides with a built-in structural cross-plane (cp) asymmetry have recently emerged as a new class of two-dimensional materials with a large cp dipole. Using first-principles calculations, and a tailored transport method, we demonstrate that stacking graphene and MoSSe Janus structures result in record high homogeneous doping of graphene and abrupt, atomically thin, cross-plane pn-junctions. We show how graphene in contrast to metals can act as electrodes to Janus stacks without screening the cp dipole and predict a large photocurrent response dominated by a cp transport channel in a few-layer stacked device. The photocurrent is above that of a corresponding thin-film silicon device illustrating the great potential of Janus stacks, for example, in photovoltaic devices

Giant tunnel-electron injection in nitrogen-doped graphene

International audienceScanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on SiC(000¯1) and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunneling channel. At nitrogen sites, this feature vanishes due to an increase of the elastic channel that is allowed because of symmetry breaking induced by the nitrogen atoms. A large conductance enhancement by a factor of up to 500 was measured at the Fermi level by comparing local spectroscopy at nitrogen sites and at carbon sites. Nitrogen doping can therefore be proposed as a way to improve tunnel-electron injection in graphene

The paediatric change laboratory:optimising postgraduate learning in the outpatient clinic

BACKGROUND: This study aimed to analyse and redesign the outpatient clinic in a paediatric department. The study was a joint collaboration with the doctors of the department (paediatric residents and specialists) using the Change Laboratory intervention method as a means to model and implement change in the outpatient clinic. This study was motivated by a perceived failure to integrate the activities of the outpatient clinic, patient care and training of residents. The ultimate goal of the intervention was to create improved care for patients through resident learning and development. METHODS: We combined the Change Laboratory intervention with an already established innovative process for residents, 3-h meetings. The Change Laboratory intervention method consists of a well-defined theory (Cultural-historical activity theory) and concrete actions where participants construct a new theoretical model of the activity, which in this case was paediatric doctors’ workplace learning modelled in order to improve medical social practice. The notion of expansive learning was used during the intervention in conjunction with thematic analysis of data in order to fuel the process of analysis and intervention. RESULTS: The activity system of the outpatient clinic can meaningfully be analysed in terms of the objects of patient care and training residents. The Change Laboratory sessions resulted in a joint action plan for the outpatient clinic structured around three themes: (1) Before: Preparation, expectations, and introduction; (2) During: Structural context and resources; (3) After: Follow-up and feedback. The participants found the Change Laboratory method to be a successful way of sharing reflections on how to optimise the organisation of work and training with patient care in mind. CONCLUSIONS: The Change Laboratory approach outlined in this study succeeded to change practices and to help medical doctors redesigning their work. Participating doctors must be motivated to uncover inherent contradictions in their medical activity systems of which care and learning are both part. Facilitators must be willing to spend time analysing both historical paediatric practice, current data on practice, and steer clear of organisational issues that might hamper a transformative learning environment. To ensure long-term success, economical and organisational resources, participant buy-in and department leadership support play a major role. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12909-016-0563-y) contains supplementary material, which is available to authorized users