Methylammonium-tin-iodide (MASnxI3, 0.9 ≤ x ≤ 1.4) systems were prepared by self assembly process in aqueous solutions. The “as-prepared” MASnxI3 systems exhibit a crystalline tetragonal structure (space group I4cm) with polyhedral-shaped crystallites. The as-prepared samples were annealed at T = 150 °C, t = 8 h under nitrogen and synthetic air. Under nitrogen, the CH3NH3SnxI3 systems adopted a cubic crystalline structure (space group P4mm) with crystallites of 2–4 μm length, whereas under air, the formation of noncrystalline phases was observed. The optical absorption spectra displayed absorption edges at 1107.0 nm (x = 0.9), 1098.6 nm (x = 1.0), and 1073.2 nm (x = 1.1), respectively, whereas at higher Sn-content (x ≥ 1.2), a broad tail of the absorbance profile was observed. The photoluminescence (PL) emission spectra (RT, λexc = 500 nm) showed major PL-events over 1 µm range and the appearance of additional bands at increasing the Sn-content. The fabrication of layers with a semiconducting behavior was demonstrated.

Thin film silicon tandem junction solar cells based on amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) were developed with focus on high open-circuit voltages for the application as photocathodes in integrated photoelectrochemical cells for water electrolysis. By adjusting various parameters in the plasma enhanced chemical vapor deposition process of the individual µc-Si:H single junction solar cells, we showed that a-Si:H/µc-Si:H tandem junction solar cells exhibit open-circuit voltage over 1.5 V with solar energy conversion efficiency of 11% at a total silicon layer thickness below 1 µm. Our approach included thickness reduction, controlled SiH4 profiling, and incorporation of intrinsic interface buffer layers. The applicability of the tandem devices as photocathodes was evaluated in a photoelectrochemical cell. The a-Si:H/µc-Si:H based photocathodes exhibit a photocurrent onset potential of 1.3 V versus RHE and a short-circuit photocurrent of 10.0 mA/cm2. The presented approach may provide an efficient and low-cost pathway to solar hydrogen production.

We have studied Cu2S absorber layers prepared by physical vapor deposition (PVD) by calibrated spectral photoluminescence (PL) and by confocal PL as function of temperature T and excitation fluxes to obtain the absolute PL-yield at an excitation flux equivalent to the AM1.5 spectrum and to calculate the splitting of the quasi-Fermi levels (QFL) µ = Ef,n-Ef,p and the absorption coefficient α(E), both in the temperature range 20 K ≤ T ≤ 400 K. The PL-spectra reveal two peaks at E1 = 1.17 eV and E2 = 1.3 eV, of which the low energy peak is only detectable at temperatures T < 200 K. The samples show an impressive QFL-splitting of µ > 700 meV at 300 K associated with a pseudo band gap of Eg = 1.25 eV. The high energy peak shows an unexpected temperature behavior, namely an increase of the PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T from extrapolated T = 0K value of µ = 1.3 eV. The PL-yield versus temperature will be discussed in terms of different defect states in the band gap. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of the opto-electronic properties by confocal PL shows a strong correlation between the QFL-splitting, the Urbach energy EU and the optical band gap Eopt, respectively.

In thin film solar cells interfaces between lattice mismatched or dissimilar materials are used for the front and the back contact. A p-i-n device structure should be possible as most simple but ideally suited thin film solar cell. In contrast the interfaces in CdTe solar cells are found to be much more complex containing interdiffused phase boundaries at the front as well as at the back contact. By comparison to non-interdiffused interfaces using contact phases of adapted work functions it can be shown that the contact potentials of the front contact but also of the back contact are dominated by Fermi level pinning. The pinning states are evidently due to dislocation defects at the boundary of CdTe to the contact phases. Based on these results it is concluded that interdiffused phase boundaries or appropriate passivation layers are a precondition for efficient solar cells whenever strongly lattice mismatched or dissimilar materials are combined.

CdTe thin film solar cells still suffer from problems related to back contacts which provide stable ohmic contacts without electrical losses. In previous studies metal/CdTe contacts have shown significant limitations. A promising option for the realization of ohmic back contacts is the use of a highly p-doped ZnTe interlayer on top of the CdTe absorber, in particular since metal/ZnTe contacts have shown very good electrical properties. In this work we studied the electronic und chemical properties of nitrogen doped p-ZnTe films. p-ZnTe:N films were prepared by using reactive RF magnetron sputtering with N2/Ar gas mixtures or by thermal evaporation with an additional nitrogen plasma source. Samples and their contacts have been prepared in DAISY-SOL (DArmstadt Integrated SYstem for SOLar energy research) which combines a full vacuum production with an in-situ photoelectron spectroscopy (XPS/UPS) analysis. The results of XPS/UPS investigations and electrical measurements will be discussed in comparison to previous results on metal/CdTe contacts. As ZnTe forms an interlayer in the CdTe thin film solar cell, the ZnTe/CdTe interface properties must be also taken into consideration. Our experiments have shown in agreement to previous studies that the valence band offset is nearly ideal for hole transport across the interface. Also electrical measurements have been carried out to investigate the metal/ZnTe/CdTe layer sequences in their contact properties.