Barbara Terheiden

Thermal stability of amorphous silicon/silicon nitride stacks for passivating crystalline silicon solar cells

The thermal stability of amorphous silicon/silicon nitride double layer surface passivation of p-type and n+-type crystalline surfaces is investigated for different deposition temperatures of the silicon nitride capping layer. An increase from 300to400°C results in a significant improvement of the thermal stability of the surface passivation. The minimum surface recombination velocity achieved on p-type (1.5Ωcm) silicon wafers is 0.75±0.6cm∕s and remains at 10±0.5cm∕s after 30min annealing at 500°C.

Mesoporous Germanium Formation by Electrochemical Etching

Weight reduction of multi-junction III-V semiconductor solar cells is an important budget issue for space applications. Typically, space solar cells are epitaxially formed on a Ge or GaAs substrate wafer. The substrate material determines the lattice constant of the stack, provides mechanical stability during the cell process, and serves as bottom cell. The substrate wafer is typically more than 100 µm thick for reasons of mechanical stability during cell processing, whereas a few µm thickness are sufficient for the bottom cell to match the photogenerated currents in the top and middle cells and not to be current limiting. Unnecessarily heavy substrate wafers hence reduce the available payload for satellite missions. There are several techniques that permit the production of very-thin lightweight highly-efficient space solar cells. Ge or GaAs substrates are commonly removed by chemical wet etching, which reduces weight but has the disadvantage that the substrate wafer is lost for further use. Separating the electrically active solar cells from their substrates by a lift-off process, could save the substrate and reduce costs. The application of a layer transfer process for multi-junction III-V semiconductor space solar cells is hence of main interest for all space agencies. Lift-off processes based on epitaxial growth of the absorber layer onto a porous etched substrate already exist for the fabrication of monocrystalline silicon solar cells. Brendel demonstrated the so-called Porous Silicon (PSI) process for the production of monocrystalline thin-film Si solar cells. This method uses a double layer of mesoporous Si formed by means of electrochemical etching: A mesoporous layer with low porosity at the surface of the substrate is used as a seed layer for the Si epitaxy, while a buried high porosity layer is used as a pre-determined breaking-point. The formation of porous germanium (PGe) has been not intensively studied. This doctoral work focuses on the fabrication and characterization of porous germanium layers by means of electrochemical etching. This thesis evaluates the potential applications of porous Ge layers for the fabrication of very-thin space solar cells. Additionally, the formation of mesoporous GaAs and mesoporous Si layers with miscut orientations is investigated.

Enabling dielectric rear side passivation for industrial mass production by developing lean printing-based solar cell processes

Al<inf>2</inf>O<inf>3</inf> rear-passivated large-area silicon solar cells with screen-printed metallization are demonstrated for the first time.

Staebler-Wronski-like formation of defects at the amorphous-Silicon-crystalline silicon interface during illumination

The effective surface recombination velocity of amorphous-silicon-coated crystalline silicon wafers is measured after illumination for various durations to investigate the stability of the surface passivation. We develop a defect model to determine the densities of dangling bond states at the a-Si:H/c-Si interface from fitting the experimental lifetime data. The surface recombination velocity of both p-type and n-type substrates is Seff=3±1 cm/s at τn=1015 cm−3 in the as-deposited state. Illumination induces an increase to Seff=16±5 cm/s due to an increase in the dangling bond density by one order of magnitude. This increase is reversible by annealing at 300 °C for 5 min.

Passivating boron silicate glasses for co-diffused high-efficiency n-type silicon solar cell application

Doping layers commonly have but one function: supplying the dopants to form a doped region within a substrate. This work presents B doping layers/stacks, which at the same time supply dopant atoms, passivate the B-doped crystalline Si surface sufficiently well (j0E < 50 fA/cm2), and show optical properties suitable for anti-reflective coating. Furthermore, these boron silicate glasses can act as a barrier against parasitic P in-diffusion during a co-diffusion step. The boron emitters diffused from the inductively coupled plasma plasma-enhanced chemical vapor-deposited B containing SiOx layers are investigated and optimized concerning passivation quality and contact properties for high-efficiency n-type solar Si cell designs. It is shown that even 10 nm thin SiOx:B films already allow for suitable emitter sheet resistance for screen-printed contacts. Furthermore, SiOx:B layers presented here allow for iVOC values of 675 mV and contact resistivity of 1 mΩcm2 for commercial Ag instead of Ag/Al pastes on the ...

Back contact monocrystalline thin-film silicon solar cells from the porous silicon process

We develop a back contact monocrystalline thin-film silicon solar cell using the porous silicon process. Laser processes are applied for all structuring steps. Thus no photolithography or other masking techniques are required. A single evaporation step is used to metallize the cell. Laser scribing is used for contact separation. The cell has a planar front surface, an area of 79.2 cm2 and a cell thickness of 30 µm. We reach an efficiency of 13.5 %. The open-circuit voltage is 633 mV and the short-circuit current density is 28.7 mA/cm2.

Light scattering and diffuse light propagation in sintered porous silicon

The scattering coefficient and the refractive index of sintered porous silicon are deduced from measurements on 1–4μm thick freestanding films. Mie’s theory is applied to describe the light scattering by the spherical pores. Using a reduced effective refractive index for the host medium in Mie’s theory accounts for the close spacing of the pores and results in an agreement between the measured and calculated scattering coefficients. A coherent calculation for the specular nonscattered radiation is combined with a model that describes the propagation of the scattered diffuse light flux. For this diffuse model two approaches, the Kubelka Munk theory and a Lambertian model developed in this work, are compared. The combined model reproduces both, the specular as well as the diffuse component of the measured reflection and transmission.

Quantitative evaluation of grain boundary activity in multicrystalline semiconductors by light beam induced current : an advanced model

We present an advanced analytical model which applies to light beam induced current contrast profiles to determine reliably the effective surface recombination velocities (Seff) of grain boundaries (GBs) and diffusion lengths (Ldiff) in the grains, in cases where a GB is close to the studied one or when Ldiff of the neighboring grain differs. We introduce additionally a new method for a very accurate determination of the plateau value of the investigated linescan and make use of simultaneously fitting GB profiles measured at various laser wavelengths both aiming at increasing the accuracy of the Ldiff determination. Through several special case investigations, the various applications and limitations of the model are demonstrated. We discuss the influence of the electrical parameters of the semiconductor on the various zones of the profile as well as the influence of measurement technique parameters on the experimental profile and point out the need of an accurately determined small laser beam radius to e...

Analytical model for the optimization of locally contacted solar cells

An empiric analytical model is presented that describes both, the diode saturation current j/sub 0/ and the dark series resistance R/sub s/ of the passivated and locally contacted rear side of solar cells. The model is experimentally verified for both, point and stripe contacts. Metallization fractions from 1.0% to 50% are realized in the experiments. The measured values of j/sub 0/ and R/sub s/ fit the theory within the error of the measurement. We use the modelled j/sub 0/ and R/sub s/ in a one-dimensional simulation of the solar cell performance, to optimize the rear contact geometry for maximum efficiency.

N-type bi-facial solar cells with boron emitters from doped PECVD layers

This work is mainly focused on an alternative method for emitter formation by means of boron diffusion from a boron-doped plasma-enhanced chemical vapor deposition (PECVD) doping source. With this approach only one high temperature process is necessary for emitter and BSF/FSF formation (co-diffusion), without depletion of surface doping concentration. This enables time and cost-efficient fabrication of solar cells with high conversion efficiencies, as shown in this work, on large area (156.25 cm) bi-facial devices with conversion efficiencies up to 19.7% measured with white back sheet. Furthermore, the contact formation with screen-printing of silver/aluminum (Ag/Al) pastes and its emitter shunting behavior due to Ag/Al spikes, varying with the firing conditions in a belt furnace, are of major interest. Low contact resistance values below 4 mΩcm can be realized with screen-printed Ag/Al contacts on 55 –70 Ω/sq PECVD boron emitters. In addition, Ag/Al spikes with a depth of around 1 – 3 μm could be detected with SEM measurements.