Philipp Löper

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.

Enhancement of silicon solar cell efficiency by upconversion: Optical and electrical characterization

Upconversion (UC) of subband-gap photons is a promising possibility to enhance solar cell efficiency by making also the subband-gap photons useful. For this application, we investigate the material system of trivalent erbium doped sodium yttrium fluoride (NaYF4:20%Er3+), which shows efficient UC suitable for silicon solar cells. We determine the optical UC efficiency by calibrated photoluminescence measurements. Because these data are free from any influence of losses associated with the application of the upconverter to the solar cell, the obtained values constitute the upper limit that can be achieved with an optimized device. Subsequently, we compare the results of the optical measurements with the results obtained by using solar cells as detectors on which the upconverter material is applied. We find an optical UC quantum efficiency of 5.1% at a monochromatic irradiance of 1880 W m−2 (0.27 cm2 W−1) at 1523 nm. The device of silicon solar cell and applied upconverter showed an external quantum efficien...

Complex Refractive Index Spectra of CH3NH3PbI3 Perovskite Thin Films Determined by Spectroscopic Ellipsometry and Spectrophotometry.

The complex refractive index (dielectric function) of planar CH3NH3PbI3 thin films at room temperature is investigated by variable angle spectroscopic ellipsometry and spectrophotometry. Knowledge of the complex refractive index is essential for designing photonic devices based on CH3NH3PbI3 thin films such as solar cells, light-emitting diodes, or lasers. Because the directly measured quantities (reflectance, transmittance, and ellipsometric spectra) are inherently affected by multiple reflections, the complex refractive index has to be determined indirectly by fitting a model dielectric function to the experimental spectra. We model the dielectric function according to the Forouhi-Bloomer formulation with oscillators positioned at 1.597, 2.418, and 3.392 eV and achieve excellent agreement with the experimental spectra. Our results agree well with previously reported data of the absorption coefficient and are consistent with Kramers-Kronig transformations. The real part of the refractive index assumes a value of 2.611 at 633 nm, implying that CH3NH3PbI3-based solar cells are ideally suited for the top cell in monolithic silicon-based tandem solar cells.

CH 3 NH 3 PbI 3 perovskite / silicon tandem solar cells: characterization based optical simulations

In this study we analyze and discuss the optical properties of various tandem architectures: mechanically stacked (four-terminal) and monolithically integrated (two-terminal) tandem devices, consisting of a methyl ammonium lead triiodide (CH(3)NH(3)PbI(3)) perovskite top solar cell and a crystalline silicon bottom solar cell. We provide layer thickness optimization guidelines and give estimates of the maximum tandem efficiencies based on state-of-the-art sub cells. We use experimental complex refractive index spectra for all involved materials as input data for an in-house developed optical simulator CROWM. Our characterization based simulations forecast that with optimized layer thicknesses the four-terminal configuration enables efficiencies over 30%, well above the current single-junction crystalline silicon cell record of 25.6%. Efficiencies over 30% can also be achieved with a two-terminal monolithic integration of the sub-cells, combined with proper selection of layer thicknesses.

Raman Spectroscopy of Organic–Inorganic Halide Perovskites

Micro-Raman spectroscopy provides laterally resolved microstructural information for a broad range of materials. In this Letter, we apply this technique to tri-iodide (CH3NH3PbI3), tribromide (CH3NH3PbBr3), and mixed iodide-bromide (CH3NH3PbI3-xBrx) organic-inorganic halide perovskite thin films and discuss necessary conditions to obtain reliable data. We explain how to measure Raman spectra of pristine CH3NH3PbI3 layers and discuss the distinct Raman bands that develop during moisture-induced degradation. We also prove unambiguously that the final degradation products contain pure PbI2. Moreover, we describe CH3NH3PbI3-xBrx Raman spectra and discuss how the perovskite crystallographic symmetries affect the Raman band intensities and spectral shapes. On the basis of the dependence of the Raman shift on the iodide-to-bromide ratio, we show that Raman spectroscopy is a fast and nondestructive method for the evaluation of the relative iodide-to-bromide ratio.

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.

Modeling upconversion of erbium doped microcrystals based on experimentally determined Einstein coefficients

The upconversion of infrared photons is a promising possibility to enhance solar cell efficiency by producing electricity from otherwise unused sub-band-gap photons. We present a rate equation model and the relevant processes in order to describe the upconversion of near-infrared photons. The model considers stimulated and spontaneous processes, multi-phonon relaxation, and energy transfer between neighboring ions. The input parameters for the model are experimentally determined for the material system, β-NaEr0.2Y0.8F4. The determination of the transition probabilities, also known as the Einstein coefficients, is the focus of the parameterization. The influence of multi-phonon relaxation and energy transfer on the upconversion are evaluated and discussed in detail. Since upconversion is a non-linear process, the irradiance dependence of the simulations is investigated and compared to the experimental data of quantum efficiency measurements. The results are very promising and indicate that upconversion is ...

The effect of photonic structures on the light guiding efficiency of fluorescent concentrators

It is possible to increase the efficiency of fluorescent concentrator systems with photonic structures. This is achieved by reducing the losses caused by the loss cone of total internal reflection. Examples of fluorescent concentrators we are currently working with are given and different photonic structures designed for the application on these fluorescent concentrators are presented. We discuss the optical characteristics of the photonic structures and their effects on the light guiding efficiency of the fluorescent concentrators. An analytical model is established to analyze and quantify the effects of these filters on the light guiding efficiency theoretically. This model is used to analyze the given photonic structures in detail. We show that with a real photonic structure the loss cone losses can be reduced by more than 75%.

A Membrane Device for Substrate‐Free Photovoltaic Characterization of Quantum Dot Based p‐i‐n Solar Cells

Silicon nanocrystals (Si NCs) embedded in Si-based high band gap matrices show promise as building blocks for all crystalline silicon (c-Si) tandem solar cells. [ 1–3 ] Careful control of the Si nanocrystal size permits a high and adjustable effective band gap due to quantum confi nement. [ 4 , 5 ] The effi ciency limit of a silicon solar cell could be increased from 29% to 42.5% with the addition of a 1.7 eV band gap top solar cell to the 1.12 eV bulk Si bottom solar cell. [ 6 ] Several other types of tandem solar cells have already been realized, but either suffer from degradation and low overall conversion effi ciencies [ 7 , 8 ] or can be used only in concentrator systems due to their expensive material and process costs. [ 9 ] Si NCs embedded in a high band gap matrix provide a material class which is nontoxic, abundant and compatible with Si technology. Size controlled homo genously distributed [ 4 ] and highly luminescent nanocrystals have been achieved by a superlattice approach and high-temperature annealing. Evidence for quantum confi ned states in Si NC embedded in silicon dioxide (SiO 2 ) has been given by photoluminescence [ 10 ] and luminescence quantum yields of up to 25% were already obtained. [ 11 ]

Silicon nanocrystals embedded in silicon carbide: Investigation of charge carrier transport and recombination

An illumination-dependent analysis of silicon nanocrystal p-i-n solar cells is presented within the framework of the constant field approximation. Silicon nanocrystals in silicon carbide were prepared by solid-phase crystallization and contacted with doped a-SixC1−x:H. This paper aims at determining the fundamental transport and recombination properties, i.e., the effective mobility lifetime product, of the nanocrystal layer at device level. Illumination-dependent current-voltage curves are modelled with a voltage-dependent collection function with only two free parameters, and excellent agreement is found between theory and experiment. An effective mobility lifetime product of 10−10 cm2/V is derived and confirmed independently from an alternative method.