A recent study associate carbon with single photon emitters (SPEs) in hexagonal boron nitride (h-BN). This observation, together with the high mobility of carbon in h-BN suggest the existence of SPEs based on carbon clusters. Here, by means of density-functional theory calculations we studied clusters of substitutional carbon atoms up to tetramers in hexagonal boron nitride. Two different conformations of neutral carbon trimers have zero-point line energies and shifts of the phonon sideband compatible with typical photoluminescence spectra. Moreover, some conformations of two small C clusters next to each other result in photoluminescence spectra similar to those found in experiments. We also showed that vacancies are unable to reproduce the typical features of the phonon sideband observed in most measurements due to the large spectral weight of low-energy breathing modes, ubiquitous in such defects.

Cu-chalcopyrite semiconductors are commonly used as light absorbing materials on solar cell devices. The study of the heterointerfaces between the absorbent and the contact materials is crucial to understand their operation. In this study, band alignments of the heterojunctions between CuGaS 2 chalcopyrite and different semiconductors have been theoretically obtained using density functional theory and more advanced techniques. Band alignments have been determined using the average electrostatic potential as reference level. We have found that the strain in the heterointerfaces plays an important role in the electronic properties of the semiconductors employed here. In this work CuAlSe 2 /CuGaS 2 and CuGaS 2 /ZnSe heterointerfaces show band alignments where holes and electrons are selectively transferred through the respective heterojunctions to the external contacts. This condition is necessary for their application on photovoltaic devices.

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Cuprous oxide has been conceived as a potential alternative to traditional organic hole-transport layers in hybrid halide perovskite-based solar cells. Device simulations predict record efficiencies using this semiconductor, but experimental resultsdo not yet show this trend. More detailed knowledge about the Cu 2 O/perovskite interface is mandatory to improve the photoconversion efficiency. Using density functional theory calculations, here, we study the interfaces of CH 3 NH 3 PbI 3 with Cu 2 O to assess their influence on device performance. Several atomistic models of these interfaces are provided for the first time, considering different compositions of the interface atomic planes. The interface electronic properties are discussed on the basis of the optimal theoretical situation, but in connection with the experimental realizations and device simulations. It is shown that the formation of vacancies in the Cu 2 O terminating planes is essential to eliminate dangling bonds and trap states. The four interface models that fulfill this condition present a band alignment favorable for photovoltaic conversion. Energy of adhesion and charge transfer across the interfaces are also studied. The termination of CH 3 NH 3 PbI 3 in PbI 2 atomic planes seems optimal to maximize the photoconversion efficiency.