Rolf Stangl

Interface recombination in amorphous/crystalline silicon solar cells, a simulation study

The paper presents a numerical simulation of the behavior of a-Si:H/c-Si heterojunction solar cells. The simulations address in particular the question of the role of interface recombination for the device performance. It is shown that the critical parameters are the density of interface states at the a-Si:H/c-Si heterojunction and the band bending which is determined by the band offsets and the front contact work function. It is shown that due to the more favorable band bending the structure with the p-type emitter on an n-type c-Si absorber has an intrinsic advantage over the inverse structure. The role of an undoped a-Si:H buffer layer is discussed and it is shown that the front contact TCO/a-Si:H has considerable influence on the band bending in the c-Si wafer and therefore is of crucial importance for the cell performance.

Planar rear emitter back contact amorphous/crystalline silicon heterojunction solar cells (RECASH / PRECASH)

Point / stripe contacted, planar rear emitter back contact amorphous/crystalline Silicon, a-Si:H/c-Si, heterojunction solar cells are presented (RECASH / PRECASH solar cells), combining the high efficiency concepts of silicon heterojunctions (high VOC potential) and back contacts (high ISC potential). Electrically insulated point or stripe contacts to the solar cell absorber are embedded within a low temperature deposited rear side planar amorphous silicon emitter layer. The new contacting schemes for back contacted a-Si:H/c-Si heterojunction solar cells require less structuring and enable the use of low cost patterning technologies which result in a large structure size (i.e. inkjet printing, screen printing). The efficiency potential of back contacted a-Si:H/c-Si heterojunction solar cells (> 24 %) is discussed by means of numerical computer simulation. First RECASH and PRECASH solar cells have been realized and are compared to a conventional front contacted a-Si:H/c-Si heterojunction solar cell (SHJ). The predicted higher short circuit current potential of back contacted a-Si:H/c-Si heterojunction solar cells could be proofed.

Physical and Technological Aspects of a-Si:H/c-Si Hetero-Junction Solar Cells

We report on the basic properties of a-Si:H/c-Si hetero-junctions, their effects on the recombination of excess carriers and its influence on the a-Si:H/c-Si hetero-junction solar cells. For this purpose we measured the gap state density distribution in thin a-Si:H layers, determined its dependence on deposition temperature and doping by an improved version of near UV-photoelectron emission spectroscopy. Furthermore, the Fermi level position in the a-Si:H and the valence band offset were directly measured. In combination with interface specific methods such as surface photovoltage analysis and our numerical simulation program AFORS-HET, we are able to find out the optimum in wafer pretreatment, doping and deposition temperature for efficient a-Si:H/c-Si solar cells without an i-type a-Si:H buffer layer. By a deposition at 210degC with an emitter doping of 2000 ppm of B2 H6 on a well cleaned pyramidal structured c-Si(p) wafer we reached 19.8 % certified efficiency

Basic electronic properties and optimization of TCO/a-Si:H(n)/c-Si(p) hetero solar cells

We report on a detailed analysis of the basic electronic properties and the optimization of amorphous/crystalline silicon heterojunction solar cells (a-Si:H(n)/c-Si(p)). The gap states density of the ultrathin a-Si:H emitter on c-Si was determined by photoelectron yield spectroscopy. By varying the a-Si:H film thickness the valence band offset was determined to about 0.45 eV. The density of states at the a-Si:H/c-Si interface amounts to about 2/spl times/10/sup 11/cm/sup -2/eV/sup -1/ at midgap. This result was obtained by field dependent surface photovoltage measurements. In addition, photoluminescence measurements were performed to investigate the recombination at the a-Si:H/c-Si interface. To gain an optimized solar cell performance the deposition temperature of the a-Si:H and the gas phase doping concentration was varied. These optimizations lead to a maximum efficiency of 17.2% for a TCO/a-Si:H(n)/c-Si(p)/a-Si:H(p) solar cell fabricated using low temperature processes only.

AFORS-HET, an open-source on demand numerical PC program for simulation of (thin film) heterojunction solar cells, version 1.2

We offer the open-source on demand version 1.2 of AFORS-HET, a numerical computer simulation program for modelling (thin film) heterojunction solar cells. It is distributed free of charge by download via internet: www.hmi.de/bereiche/SE/SE1/projects/aSicSi/AFORS-HET.