Sebastian Schaefer

Low damage reactive ion etching for photovoltaic applications

New concepts in silicon solar cell design require dry processing technologies. For this reason two reactive ion etching (RIE) processes have been developed: one for surface cleaning and one for the removal of phosphorous glass (PSG). However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF6 RIE cleaning has been investigated by means of secondary ion mass spectroscopy, positron annihilation, and minority charge carrier lifetime measurements. Particles contained in the etch gas can be detected up to a depth of 50–80 nm in the silicon sample. A two layer model of vacancy distribution has been established: A layer of high vacancy concentration (1019 cm−3) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 μm with a vacancy concentration of 1016 cm−3. Effective minority charge carrier lifetimes decrease to about 10% of the lifetime of the wet etched control during RIE. If a heavily damaged layer of 20 nm i...

Plasma surface texturization for multicrystalline silicon solar cells

Textured front surfaces improve the efficiencies of solar cells due to reduced reflection and light trapping. Most methods of wet chemical surface texturing depend on crystal orientation. The authors have therefore developed a SF/sub 6//O/sub 2/ reactive ion etch process for the mask-less texturing of silicon independently from grain orientation. Following the black silicon method, they have optimised a surface texture with a typical lateral dimension in the order of 100 nm and with very good homogeneity. Since the texturing sensitivity depends on the process parameters as well as on the condition of the reactor and the wafer surface, pre-conditioning is a crucial step. The weighted reflection of an optimised process is less than 3% without any additional antireflection coating. However, the texturing enlarges the surface drastically and shallow diffusions result in high sheet resistance. Multicrystalline silicon solar cells with black silicon texturing therefore outperform planar cells, but still suffer from bad short-wavelength response due to high surface recombination and internal resistance.

New simplified methods for patterning the rear contact of RP-PERC high-efficiency solar cells

New processing schemes for fabricating the rear contact pattern of the PERC-structure (passivated emitter and rear cell) are demonstrated. Both, thermally-grown silicon oxide (SiO/sub 2/) and plasma-deposited silicon nitride (SiN/sub x/) are used as the passivating rear layer. The first processing scheme utilizes plasma etching of the dielectric layer through a mask. The plasma process was optimized in order to reduce the damage in the silicon base of the cell. Efficiencies of 21.5% and 21.7% have been achieved for SiN/sub x/ and SiO/sub 2/ rear layers, respectively. The second approach uses a laser beam to remove the dielectric layer for the rear contact pattern. Efficiencies of 19.7% and 21.3% have been achieved for SiN/sub x/ and SiO/sub 2/ rear layers, respectively. Reference cells with the same front structure but conventionally processed rear (photo resist, wet-chemical etching) show only a slightly higher efficiency of 22.0% on cells with a SiO/sub 2/ passivation layer. This proves that both approaches have a very high potential.

Self-aligned metallization and reactive ion etched buried base contact solar cells

Damage-free reactive ion etch processes have been developed that allow the formation of grooves with intentional under-etching of the mask, e.g. photo resist. In a succeeding metal evaporation step this very same mask obstructs the deposition of metal such that the metal is only deposited on the bottom of the groove. In a lift-off process the surplus metal is removed from the wafer surface. This metallization technique allows simple fabrication of single-side contacted solar cells. It has been used to form a buried base contact through the homogeneous emitter of a rear contact solar cell. Only one mask was used to define emitter and base areas and to contact the base. After depositing the metal on the bottom of the groove, it was electroplated. No significant shunts, i.e. no short circuits between emitter and base, are observed even though during electroplating the metal has grown out of the grooves. This rear contact solar cell with buried-base contacts achieves an efficiency of 16·3% under front side illumination (neither n++ nor p++ doping under the contact fingers). The cell still lacks open-circuit voltage (595 mV) and fill factor (70·9%), probably due to the lack of side-wall passivation in the grooves and of a sealing of the open pn-junction. Copyright

An overview of plasma sources suitable for dry etching of solar cells

The characteristic feature of a plasma system is the method of plasma excitation. A parallel plate system, i.e. reactive ion etching (RIE), an atmospheric downstream plasma (ADP) source and two types of microwave plasma sources (slot and linear antenna) are tested regarding their suitability for solar cell processing in terms of high throughput and plasma-induced damage. Possible damage in silicon etching is analysed by means of solar cells. Although damage can be minimized in RIE, a decrease of cell parameters can only be avoided completely if ion bombardment is avoided (slot and linear antenna plasma source, ADP). The parallel plate system and the slot antenna source show insufficient etch rates (<1 /spl mu/m/min) while those of linear antennas and ADP are extremely high (>10 /spl mu/m/min). Therefore, of the analysed plasma sources, the latter represent the most promising systems for solar cell processing.