05 Fakultät Informatik, Elektrotechnik und Informationstechnik
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Item Open Access Top‐down approach to study chemical and electronic properties of perovskite solar cells : sputtered depth profiling versus tapered cross‐sectional photoelectron spectroscopies(2021) Das, Chittaranjan; Zia, Waqas; Mortan, Claudiu; Hussain, Navid; Saliba, Michael; Ingo Flege, Jan; Kot, MałgorzataA study of the chemical and electronic properties of various layers across perovskite solar cell (PSC) stacks is challenging. Depth‐profiling photoemission spectroscopy can be used to study the surface, interface, and bulk properties of different layers in PSCs, which influence the overall performance of these devices. Herein, sputter depth profiling (SDP) and tapered cross‐sectional (TCS) photoelectron spectroscopies (PESs) are used to study highly efficient mixed halide PSCs. It is found that the most used SDP‐PES technique degrades the organic and deforms the inorganic materials during sputtering of the PSCs while the TCS‐PES method is less destructive and can determine the chemical and electronic properties of all layers precisely. The SDP‐PES dissociates the chemical bonding in the spiro‐MeOTAD and perovskite layer and reduces the TiO2, which causes the chemical analysis to be unreliable. The TCS‐PES revealed a band bending only at the spiro‐MeOTAD/perovskite interface of about 0.7 eV. Both the TCS and SDP‐PES show that the perovskite layer is inhomogeneous and has a higher amount of bromine at the perovskite/TiO2 interface.Item Open Access Mobility and homogeneity effects on the power conversion efficiency of solar cells(2008) Mattheis, Julian; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)The thesis on hand investigates the interplay between detailed radiation balances and charge carrier transport. The first part analyzes the role of limited carrier transport for the efficiency limits of $pn$-junction solar cells. The second part points out the influence of transport on the absorption and emission of light in inhomogeneous semiconductors. By incorporating an integral term that accounts for the repeated internal emission and reabsorption of photons (the so-called photon recycling) into the diffusion equation for the minority carriers, the first part of the thesis develops a self-consistent model that is capable of describing the power conversion efficiencies of existing devices as well as of devices in the radiative recombination limit. It is shown that the classical diode theory without the inclusion of photon recycling produces accurate results only if the minority carrier lifetime is at least ten times smaller than the radiative lifetime. The thesis shows that even in the radiative recombination limit, charge carrier transport is extremely important. The thesis thus presents a universal criterion that needs to be fulfilled by any photovoltaic material in order to obtain high power conversion efficiency. The numerical results are analyzed and compared to an analytical approximation. The thesis applies the developed model to solar cells made of crystalline silicon, amorphous silicon and Cu(In,Ga)Se$_2$ (CIGS). It shows that crystalline silicon solar cells neither have transport problems in the radiative recombination limit nor in existing devices. In Cu(In,Ga)Se$_2$ solar cells, mobilities are at most two orders of magnitude above the critical mobility and guarantee complete carrier collection only close to the radiative limit. The second part of the thesis investigates the role of carrier transport for the absorption and emission of light in semiconductors with band gap fluctuations. The chapter develops an analytical statistical model to describe the absorption and emission spectra of such inhomogeneous semiconductors. Particular emphasis is placed on the role of the length-scale of the band gap fluctuations. As it turns out, the crucial quantity with respect to the emission spectrum is the ratio of the charge carrier transport length and the length-scale of the band gap fluctuations. Both, absorption edge and emission peak are broadened by band gap fluctuations. Comparison with numerical simulations underlines the importance of the fluctuation length in relation to the diffusion length. The model is applied to experimental absorption and photoluminescence data of Cu(In,Ga)Se$_2$ thin films with varying gallium content. The ternary compounds CuInSe$_2$ and CuGaSe$_2$ exhibit the smallest magnitude of fluctuations with standard deviations in the range of $20-40 \meV$. The fact that the quaternary compounds show standard deviations of up to $65 \meV$ points to alloy disorder as one possible source of band gap fluctuations. All observed fluctuations occur on a very small length scale that is at least ten times smaller than the electron diffusion length of approximately $1 \mum$.Item Open Access Boron partitioning coefficient above unity in laser crystallized silicon(2017) Lill, Patrick C.; Dahlinger, Morris; Köhler, Jürgen R.Item Open Access Amorphous silicon based solar cells(2007) Al Tarabsheh, Anas; Werner, Jürgen H. (Prof. Dr. rer. nat. habil.)This thesis focuses on the deposition of hydrogenated amorphous silicon (a-Si:H) films bymeans of plasma enhanced chemical vapour deposition (PECVD). This technique allows the growth of device quality a-Si:H at relatively low deposition temperatures, below 140 °C and, therefore, enables the use of low-cost substrates, e.g. plastic foils. The maximum efficiencies of a-Si:H solar cells in this work are η= 6.8 % at a deposition temperature Tdep = 180 °C and η = 4.9 % at a deposition temperature Tdep = 135 °C. Decreasing the deposition temperature deteriorates the structural and electronic quality of a-Si:H films. Therefore, the deposition conditions are carefully optimized at low temperatures. The mismatch in the mechanical properties of the plastic foils and the inorganic semiconductor layers have less effect on the a-Si:H films at low deposition temperatures. As a result, the deposition temperatures should be decreased to minimize mechanical deterioration of the films but without losing too much of the electronic properties of the films. A novel analytical description of the current density/voltage (J/V) characteristics of p-i-n solar cells well represents experimental J/V curves of a-Si:H solar cells. The extended model solves the continuity and transport equations for electrons and holes, and fully accounts for the contributions of the drift and the diffusion currents. Many analytical models neglect the contribution of the diffusion current in describing the a-Si:H solar cells. Other existing models assume the diffusion lengths of electrons and holes to be equal, resulting in a symmetric distribution of carrier concentrations around the center of the intrinsic layer of the p-i-n solar cells. Both restrictions strongly limit the ability of these analytical models to accurately reproduce the J/V-characteristics of real solar cells. In contrast to existing analytical models, the new analytical description solves the continuity and transport equations of carriers at each location within the i-layer for the whole range of applied voltages. The peculiar extension of this model over previous ones enables a more realistic description of solar cells. My novel analytical model implements i) different values of the diffusion lengths, or mobility-lifetime products, of electrons and holes, and ii) realistic wavelength and depth dependencies of the photogeneration rate of charge carriers. The results of the model demonstrate that the location of the main recombination path of the photogenerated carriers inside the i-layer is voltage dependent, rather than being fixed at the middle of the i-layer as existing models assume. For a realistic description of the solar cell optics in calculating the J/V-characteristics, I fully account for the reflection of photons at the back contact. The model proves that the performance of a-Si:H solar cells which are illuminated through the p-layer is better than the one of cells illuminated through the n-layer. Testing corresponding J/V-characteristics from this model against experimental data of bifacial a-Si:H solar cells with transparent front and backside contacts, reveals that this extended analytical model well describes the output characteristics of real a-Si:H p-i-n solar cells. The model proves that the current collection of bifacial p-i-n solar cells is larger if the light enters through the p-layer because the mobility μn of electrons is larger than the mobility μp of holes. This thesis also investigates the dependence of the electrical and optical properties of a-Si:H films on the deposition conditions, and how those properties are enhanced by optimizing the deposition conditions. I apply the optimized layers to solar cells deposited on glass and on polyethylene terephtalate (PET) substrates. The incorporation of a buffer layer or a microcrystalline layer enhances the performance of the cells.Item Open Access Sheet conductance of laser-doped layers using a Gaussian laser beam : an effective depth approximation(2024) Hassan, Mohamed; Werner, Jürgen H.Laser doping of silicon with pulsed and scanned laser beams is now well-established to obtain defect-free, doping profile tailored, and locally selectively doped regions with a high spatial resolution. Picking the correct laser parameters (pulse power, pulse shape, and scanning speed) impacts the depth and uniformity of the melted region geometry. This work performs laser doping on the surface of single crystalline silicon, using a pulsed and scanned laser profile with a Gaussian intensity distribution. A deposited boron oxide precursor layer serves as a doping source. Increasing the local inter-pulse distance xirrbetween subsequent pulses causes a quadratic decrease of the sheet conductance Gshof the doped surface layer. Here, we present a simple geometric model that explains all experimental findings. The quadratic dependence stems from the approximately parabolic shape of the individual melted regions directly after the laser beam has hit the Si surface. The sheet resistance depends critically on the intersection depth dchand the distance xirrof overlap between two subsequent, neighboring pulses. The intersection depth dchquadratically depends on the pulse distance xirrand therefore also on the scanning speed vscanof the laser. Finally, we present a simple model that reduces the complicated three dimensional, laterally inhomogeneous doping profile to an effective two-dimensional, homogeneously doped layer which varies its thickness with the scanning speed.