T. Walter

Bio: T. Walter is an academic researcher. The author has contributed to research in topics: Band gap & Heterojunction. The author has an hindex of 3, co-authored 3 publications receiving 684 citations.

Determination of defect distributions from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions

Abstract: A method to deduce energy distributions of defects in the band gap of a semiconductor by measuring the complex admittance of a junction is proposed. It consists of calculating the derivative of the junction capacitance with respect to the angular frequency of the ac signal corrected by a factor taking into account the band bending and the drop of the ac signal over the space charge region of the junction. Numerical modeling demonstrates that defect distributions in energy can be reconstructed by this method with high accuracy. Defect distributions of polycrystalline Cu(In,Ga)Se2 thin films are determined by this method from temperature dependent admittance measurements on heterojunctions of Cu(In,Ga)Se2 with ZnO that are used as efficient thin film solar cells.

Meyer–Neldel behavior of deep level parameters in heterojunctions to Cu(In,Ga)(S,Se)2

Abstract: Admittance spectroscopy was measured on Cu(In,Ga)(S,Se)2 thin film and single crystal heterojunctions. The emission rates of defects for various near‐stoichiometric compositions follow a Meyer–Neldel rule, showing increasing attempt‐to‐escape frequencies with increasing defect depth. Defects in highly (In,Ga)‐rich material showed lower attempt‐to‐escape frequencies and follow a separate Meyer–Neldel relation. Repetitive air annealing of a CuInSe2 heterojunction revealed a shift of the depth and capture cross section of an observed defect.

Density of states in CuIn(SSe)2 thin films from modulated photocurrent measurements

Abstract: Results of modulated photocurrent measurements in the temperature range between 180 and 300 K on polycrystalline CuIn(S,Se)2 thin films are presented. Modeling of the obtained phase shifts implies a continuous energetic distribution of traps in the band gap. A superposition of an exponential energetic distribution of traps with a characteristic energy of 60 meV and a Gaussian peak with a depth of 250 meV can explain the obtained phase shift and amplitudes between 250 and 300 K. For lower temperatures a change of the transport path, reducing the effective depth of the peak in the density of states and reducing the ‘‘attempt to escape frequency’’ is likely. At low temperatures the phase shift depends on the photon flux due to the separation of the demarcation levels whereas in the case of the higher temperatures no significant dependence on the light intensity could be detected.

Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells

Abstract: The large photocurrent hysteresis observed in many organometal trihalide perovskite solar cells has become a major hindrance impairing the ultimate performance and stability of these devices, while its origin was unknown. Here we demonstrate the trap states on the surface and grain boundaries of the perovskite materials to be the origin of photocurrent hysteresis and that the fullerene layers deposited on perovskites can effectively passivate these charge trap states and eliminate the notorious photocurrent hysteresis. Fullerenes deposited on the top of the perovskites reduce the trap density by two orders of magnitude and double the power conversion efficiency of CH(3)NH(3)PbI(3) solar cells. The elucidation of the origin of photocurrent hysteresis and its elimination by trap passivation in perovskite solar cells provides important directions for future enhancements to device efficiency.

Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations

Abstract: The ionic defects at the surfaces and grain boundaries of organic–inorganic halide perovskite films are detrimental to both the efficiency and stability of perovskite solar cells. Here, we show that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskite with their negative- and positive-charged components. The efficient defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, which is supported by density-function-theory calculation. The defect passivation reduces the open-circuit-voltage deficit of the p–i–n-structured device to 0.39 V, and boosts the efficiency to a certified value of 20.59 ± 0.45%. Moreover, the defect healing also significantly enhances the stability of films in ambient conditions. Our findings provide an avenue for defect passivation to further improve both the efficiency and stability of solar cells. Losses in solar cells can be caused by material defects in the bulk or at interfaces. Here, Zheng et al. use quaternary ammonium halides to passivate various perovskite absorbers and prepare solar cells with certified efficiency above 20%, suggesting that both anionic and cation defects are affected.

Solvent annealing of perovskite-induced crystal growth for photovoltaic-device efficiency enhancement.

Abstract: Solvent-annealing is found to be an effective method to increase the grain size and carrier diffusion lengths of trihalide perovskite materials. The carrier diffusion length of MAPbI3 is increased to over 1 μm. The efficiency remains above 14.5% when the MAPbI3 thickness changes from 250 nm to 1 μm, with the highest efficiency reaching 15.6%.

Hybrid passivated colloidal quantum dot solids

Abstract: Improved performance in a photovoltaic device made of colloidal quantum dots is achieved through a combination of passivation by halide anions and organic crosslinking.

Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

Abstract: We review the application of impedance spectroscopy in dye-sensitized solar cells, quantum dot-sensitized solar cells and organic bulk heterojunction solar cells. We emphasize the interpretation of the impedance parameters for determining the internal features of the device, concerning the carrier distribution, materials properties such as the density of states and/or doping of the semiconductors, and the match of energy levels for photoinduced charge generation and separation. Another central task is the determination of recombination mechanisms from the measured resistances, and the factors governing the device performance by combined analysis of resistances as a function of voltage and current–voltage curves.