Low‐cost hybrid solar cells have made tremendous steps forward during the past decade owing to the implementation of extremely thin inorganic coatings as absorber layers, typically in combination with organic hole transporters. Using only extremely thin films of these absorbers reduces the requirement of single crystalline high‐quality materials and paves the way for low‐cost solution processing compatible with roll‐to‐roll fabrication processes. To date, the most efficient absorber material, except for the recently introduced organic–inorganic lead halide perovskites, has been Sb2S3, which can be implemented in hybrid photovoltaics using a simple chemical bath deposition. Current high‐efficiency Sb2S3 devices utilize absorber coatings on nanostructured TiO2 electrodes in combination with polymeric hole transporters. This geometry has so far been the state of the art, even though flat junction devices would be conceptually simpler with the additional potential of higher open circuit voltages due to reduced charge carrier recombination. Besides, the role of the hole transporter is not completely clarified yet. In particular, additional photocurrent contribution from the polymers has not been directly shown, which points toward detrimental parasitic light absorption in the polymers. This study presents a fine‐tuned chemical bath deposition method that allows fabricating solution‐processed low‐cost flat junction Sb2S3 solar cells with the highest open circuit voltage reported so far for chemical bath devices and efficiencies exceeding 4%. Characterization of back‐illuminated solar cells in combination with transfer matrix‐based simulations further allows to address the issue of absorption losses in the hole transport material and outline a pathway toward more efficient future devices.

Hydrogenated amorphous silicon oxynitride (a-SiO x N y :H) films, which are deposited by the plasma decomposition of silane and nitrous oxide at low temperatures (T dep o 300 1C), are investigated in order to evaluate the potential of these films for photovoltaic applications. In this work both, intrinsic and doped a-SiO x N y :H films are investigated in terms of their electrical, optical and structural properties using Fourier-transform infra-red (FTIR) and secondary-ion mass-spectroscopy (SIMS), as well as highresolution transmission electron microscopy (HRTEM), photo-conductance decay (PCD), spectral ellipsometry and temperature-dependent conductivity measurements. The plasma deposition parameters are optimized in terms of effective minority carrier lifetime, dark conductivity and low absorbance (i.e. high optical band gap). The optical band gap of the a-SiO x N y :H films can be widened up to 2.2 eV compared to a-Si:H due to the incorporation of oxygen and nitrogen into the amorphous network. Not only the optical band gap but also the passivation quality and the dark conductivity of the films are well correlated with the oxygen and nitrogen concentration, which are monitored by means of SIMS measurements. When applying an a-SiO x N y :H film with an optical band gap of 2.0 eV, a very high effective minority carrier lifetime of 2.5 ms is measured. In case of doped films, conductivities up to s dark ¼ 4.5 Â 10 À 3 S/cm for the n-type doping and s dark ¼ 3.9 Â 10 À 4 S/cm for the p-type doping are achieved. Combining the intrinsic and doped a-SiO x N y :H films to heteroemitter stacks on a crystalline Si base, a very high implied open circuit voltage of up to 733 mV is demonstrated. FTIR and HRTEM measurements reveal a homogenous distribution of Si-Si and Si-O-Si bonds in the a-SiO x N y :H films.

a b s t r a c t a r t i c l e i n f oAmorphous silicon (a Si) is common in the production of technical devices and can be deposited by several techniques. In this study intrinsic and doped, hydrogen less amorphous silicon films are RF magnetron sputter deposited and post hydrogenated in a remote hydrogen plasma reactor at a temperature of 370°C. Secondary ion mass spectrometry of a boron doped (p) a Si layer shows that the concentration of dopants in the sputtered layer becomes the same as present in the sputter target. Improved surface passivation of phosphorous doped 5 Ω cm, FZ, (n) c Si can be achieved by post hydrogenation yielding a minority carrier lifetime of~360 μs finding an optimum for~40 nm thin films, deposited at 325°C. This relatively low minority carrier lifetime indicates high disorder of the hydrogen less sputter deposited amorphous network. Post hydrogenation leads to a decrease of the number of localized states within the band gap. Optical band gaps (Taucs gab as well as E 04 ) can be determined to~1.88 eV after post hydrogenation. High resolution transmission electron microscopy and optical Raman investigations show that the sputtered layers are amorphous and stay like this during post hydrogenation. As a consequence of the missing hydrogen during deposition, sputtered a Si forms a rough surface compared to CVD a Si. Atomic force microscopy points out that the roughness decreases by up to 25% during post hydrogenation. Nuclear resonant reaction analysis permits the investigation of hydrogen depth profiles and allows determining the diffusion coefficients of several post hydrogenated samples from of a model developed within this work. A dependency of diffusion coefficients on the duration of post hydrogenation indicates trapping diffusion as the main diffusion mechanism. Additional Fourier transform infrared spectroscopy measurements show that hardly any interstitial hydrogen exists in the post hydrogenated a Si layers. The results of this study open the way for further hydrogen diffusion experiments which require an initially unhydrogenated drain layer.

The impact on the performance of GaN HEMTs of in situ ammonia (NH3) pre-treatment prior to the deposition of silicon nitride (SiN) passivation with low-pressure chemical vapor deposition is investigated. Three different NH3 pre-treatment durations (0, 3, and 10 minutes) were compared in terms of interface properties and device performance. A reduction of oxygen at the interface between SiN and epi-structure is detected by Scanning Transmission Electron Microscopy-Electron Energy Loss Spectroscopy measurements in the sample subjected to 10 minutes of pre-treatment. The samples subjected to NH3 pre-treatment show a reduced surface-related current dispersion of 9 % (compared to 16% for the untreated sample), which is attributed to the reduction of oxygen at the SiN/epi interface. Furthermore, NH3 pre-treatment for 10 minutes significantly improves the current dispersion uniformity from 14.5 % to 1.9 %. The reduced trapping effects result in a high output power of 3.4 W/mm at 3 GHz (compared to 2.6 W/mm for the untreated sample). These results demonstrate that the in situ NH3 pre-treatment before low-pressure chemical vapor deposition of SiN passivation is critical and can effectively improves the large-signal microwave performance of GaN HEMTs.

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