We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4 % and a photon indistinguishability of up to 94 ± 1 % by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

In this section the governing equations of the drift-diffusion model implemented in Setfos 4.6 1 are explained. The quantities are described in the next section.

of development, 15 with a record power conversion efficiency (PCE) now exceeding 25%. 16 An important property of perovskites is the possibility of readily tune their bandgap, 17-20 making them a suitable candidate for applications in single-junction as well as multijunction solar cells, 21-24 e.g. in combination with narrow-bandgap absorbers such as Cu(In,Ga)Se2, 25-28 silicon, 29-35 or by using complementary perovskites. 36-42 Perovskite alloys of the type ASn1-xPbxI3 (where A is an organic or inorganic cation, or a mixture of them) have bandgaps in the 1.20-1.25 eV range for lead content 0.25 ≤ x ≤ 0.5. 43,44 This requires perovskite compositions with wide bandgaps in the 1.75-1.85 eV range in order to aim at perovskite-perovskite tandem devices that can exceed the theoretical efficiency limit of single junction solar cells. 21-24 Perovskite films with wide bandgaps suitable for perovskiteperovskite tandems can be readily obtained by using mixed iodide/bromide formulations, 17 and mixed A-site cations are also employed to improve the photo-and thermal stability of the compounds. 45-47 The study of wide-bandgap perovskite materials and solar cells is a booming field of research, well summarized in recent reviews 23,48 and in research articles containing some of the best performing devices to date. 49-51 In comparison with narrower-bandgap materials, 52 wide-bandgap perovskite solar cells suffer from a larger open-circuit voltage (Voc) deficit, i.e. the Voc does not scale linearly with the bandgap as predicted by the Shockley-Queisser (SQ) limit. This deviation is due to non-radiative recombination in the perovskite bulk and at the interface with the transport layers. 51,53-55 For this reason, a large number of studies aimed at developing bulk and surface passivation strategies, as well as to identify suitable transport layers and contacts. 56-60 The vast majority of studies on wide-bandgap perovskite solar cells relied on solution-processed perovskite thin-films. Vacuum deposition is an alternative method with superior control over the film thickness and composition; it is compatible with large areas and eliminates the processing concerns related with the use of solvents. 61-63 This is especially relevant for the fabrication of complex multilayer architectures, necessary for tandem solar cells. 37,64 Moreover, vacuum deposition allows the deposition of pinhole-free, uniform and smooth films. 65-68 Early reports on vacuum-deposited widebandgap perovskites used the simplest formulation, methylammonium lead iodide-bromide, MAPb(I1-xBrx)3. We showed that these type of compounds with bandgap (Eg) up to 1.7 eV (x ≈ 0.2), are stable even at high irradiance levels, and the corresponding perovskite solar cells exhibited PCE up to 15.9%. 69 When the amount of bromide is increased (x ≥ 0.3), the perovskite demixes into iodide-and bromide-rich phases in a process known as "halide segregation", 45,70,71 which can be readily monitored from the red-shifted perovskite photoluminescence (PL) spectrum. 69,72 The iodide-rich, nar...