Franz-Josef Haug

Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance

Solar cells based on organometallic halide perovskite absorber layers are emerging as a high-performance photovoltaic technology. Using highly sensitive photothermal deflection and photocurrent spectroscopy, we measure the absorption spectrum of CH3NH3PbI3 perovskite thin films at room temperature. We find a high absorption coefficient with particularly sharp onset. Below the bandgap, the absorption is exponential over more than four decades with an Urbach energy as small as 15 meV, which suggests a well-ordered microstructure. No deep states are found down to the detection limit of ∼1 cm(-1). These results confirm the excellent electronic properties of perovskite thin films, enabling the very high open-circuit voltages reported for perovskite solar cells. Following intentional moisture ingress, we find that the absorption at photon energies below 2.4 eV is strongly reduced, pointing to a compositional change of the material.

Light trapping in solar cells: can periodic beat random?

Theory predicts that periodic photonic nanostructures should outperform their random counterparts in trapping light in solar cells. However, the current certified world-record conversion efficiency for amorphous silicon thin-film solar cells, which strongly rely on light trapping, was achieved on the random pyramidal morphology of transparent zinc oxide electrodes. Based on insights from waveguide theory, we develop tailored periodic arrays of nanocavities on glass fabricated by nanosphere lithography, which enable a cell with a remarkable short-circuit current density of 17.1 mA/cm(2) and a high initial efficiency of 10.9%. A direct comparison with a cell deposited on the random pyramidal morphology of state-of-the-art zinc oxide electrodes, replicated onto glass using nanoimprint lithography, demonstrates unambiguously that periodic structures rival random textures.

Nanoimprint lithography for high-efficiency thin-film silicon solar cells.

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm(2) for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance.

Modeling of light scattering from micro- and nanotextured surfaces

We present a calculation routine for the angular and spectral dependence of scattered light after transmission through textured interfaces. Based on a modified Rayleigh–Sommerfeld integral, the treatment requires only measured surface profiles, and the refractive indices of the two materials adjacent to the textured interface but no fitting parameter. For typical surface morphologies used in solar cell fabrication, the calculations correctly reproduce the angle resolved scattering at 543 nm and the total scattered light intensity in the spectral range from 400 to 2000 nm. The model is then applied to predict the behavior of the interface between ZnO and silicon in a thin film solar cell which is not experimentally accessible.

Comparison and optimization of randomly textured surfaces in thin-film solar cells

Using rigorous diffraction theory we investigate the scattering properties of various random textures currently used for photon management in thin-film solar cells. We relate the haze and the angularly resolved scattering function of these cells to the enhancement of light absorption. A simple criterion is derived that provides an explanation why certain textures operate more beneficially than others. Using this criterion we propose a generic surface profile that outperforms the available substrates. This work facilitates the understanding of the effect of randomly textured surfaces and provides guidelines towards their optimization.

Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler

Angle resolved measurements of the external quantum efficiency of n-i-p single junction amorphous solar cell deposited on a grating structure show clearly defined peaks of enhanced photocurrent in the weakly absorbing region between 1.6 and 2.15 eV. We explain these absorption phenomena and their angular variation with the excitation of guided modes via grating coupling. Calculation using an equivalent flat multilayer system permits to relate the theoretical values with the experimental data.

Organic-Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells

We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

Asymmetric intermediate reflector for tandem micromorph thin film silicon solar cells

The micromorph solar cell (stack of amorphous and microcrystalline cells) concept is the key for achieving high efficiency stabilized thin film silicon solar cells. We introduce a device structure that allows a better control of the light in-coupling into the two subcell components. It is based on an asymmetric intermediate reflector, which increases the effective thickness of the a-Si:H by a factor of more than three. Hence, the a- Si:H thickness reduction diminishes the light induced degradation, and micromorph tandem cells with 11.2% initial and 9.8% stabilized efficiencies (1000 h, 50 °C, and 100 mW/cm2) are made on plastic substrates with Tg<180 °C.

Resonances and absorption enhancement in thin film silicon solar cells with periodic interface texture

We study absorption enhancement by light scattering at periodically textured interfaces in thin film silicon solar cells. We show that the periodicity establishes resonant coupling to propagating waveguide modes. Ideally, such modes propagate in the high index silicon film where they are eventually absorbed, but waveguide modes exist also in the transparent front contact layer if the product of its refractive index and thickness exceeds half the wavelength. Taking into account that the absorption coefficient of realistic transparent conducing films exceeds the one of silicon close to its band gap, certain waveguide modes will enhance parasitic absorption in the transparent front contact. From an analysis based on the statistic distribution of energy among the available waveguide and radiation modes, we conclude that conventional thin film silicon solar cells with thick and nonideal contacts may fail to reach the previously noted bulk limit of 4nSi2; instead, a more conservative limit of 4(nSi2-nTCO2) applies.

Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells

The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-based polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of CdS with different bath temperatures (60 and 80 °C) are compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber layer. This precipitation cannot be avoided and seems to be a principal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning the interaction with the absorber layer surface. For bath temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe2 interface region consists of a continuous transition zone with low defect ...