Record Efficiency of PhosTop Solar Cells from n-type Cz UMG Silicon Wafers

AbstractHighly purified n-type Upgraded Metallurgical Grade (UMG) silicon carries a large potential for high efficiency low cost solar cells. In this study, the industrially producible “PhosTop” solar cell concept is employed to manufacture large-area n-type rear junction solar cells from such a Si material with a screen-printed Al-alloyed full-area emitter featuring a selective phosphorous front surface field (FSF) and a SiO2/SiNx:H passivation on the front.Since resistivity at the seed end is about seven times as high as at the tail end of the UMG Si ingot and carrier lifetime decreases from seed to tail end, a clear dependence of the solar cells’ IV characteristics on the original position of the corresponding wafers in the UMG Si ingot is observable. Maximum conversion efficiency is reached (on a wafer which has been taken out at about one fifth of the ingot's length distant from the seed end) by η = 19.0% being, to the authors’ knowledge, the highest efficiency so far reported on industrial type solar cells manufactured from 100% UMG Si

Contacting and recombination analysis of boron emitters via etch-back for advanced n-Type Si solar cells

In p-type c-Si solar cells, selective emitter structures generated by emitter etch-back (EEB) have been introduced in recent years in order to minimize electrical losses in the phosphorous emitter being one of the dominant factors limiting the performance of standard screen-printed p-type c-Si solar cells. In this work, a homogeneously or selectively etched-back boron emitter is demonstrated to provide additional benefits yielding an enhanced conversion efficiency in n-type Si solar cells. By means of subsequent B-EEB, contacting and recombination properties of B emitters dependent on their sheet resistance, surface concentration, and profile depth are studied indicating the latter to be the crucial parameter. Based on this, the characteristics of the optimal B emitter regarding low saturation current density and low specific contact resistivity are determined for the cases of homogeneous and selective etch-back. By employing the selectively etched-back B emitter in initial solar cells, a VOC gain of 5 mV and a significant shunting reduction compared with homogeneously doped devices is achieved

Investigation of radiation damage to the Al₂O₃/Si wafer interface during electron beam evaporation by means of C-V and lifetime measurements

For the purpose of reducing recombination activity in crystalline silicon solar cells, atomic layer deposited aluminum oxide (Al2O3) has proven a promising candidate. Its excellent surface passivation is effected by an exceptionally high density of negative fixed charges Qf generating a strong field-effect along with a chemical passivation reducing the density of interface traps Dit. The dependence of these two measures on the temperature and the duration of the post-deposition anneal activating the passivation of Al2O3 is investigated by measuring the capacitance-voltage (C-V) characteristics. To directly correlate Qf and Dit with the effective minority carrier lifetime eff, a new kind of sample structure is developed, whereby both measurement types can be conducted on the same test sample. The interface properties of samples with thermal and electron beam evaporated metal contacts are compared and a correlation with the obtained passivation quality quantified by eff is identified in order to investigate the influence of the radiation damage. It is found that Qf, Dit and eff of Al2O3 passivated p-doped Si wafers exhibit a correlation when annealing parameters are varied and that an electron beam evaporation of Al damages the Al2O3/Si interface and significantly reduces eff. Finally, a method to restore the effective lifetime is developed and investigated which yields a recovery rate of 65% corresponding to a reduction of Dit and an increase of Qf

Electrical and optical analysis of dielectric layers for advanced passivation of BBr3 diffused p+ emitters in n-type c-Si solar cells

Due to a different surface concentration and type of minority charge carriers compared to n+ emitters (on p-type substrates), novel surface passivation methods are required when using p+ emitters on n-type silicon for which a number of efficiency records have been achieved in recent years [1,2,3]. In this study emitter saturation current densities j0e of two BBr3 diffused emitters passivated by a variety of dielectric stacks are compared using following processes: SiOx by dry thermal oxidation, direct and remote plasma enhanced chemical vapor deposited SiNx, atomic layer deposited Al2O3, SiOx remaining from the RCA standard cleaning process [4] yielding the lowest j0e of 17.5 fA/cm² for a sample passivated by an Al2O3/SiNx stack. Furthermore, the j0BSF values of POCl3 diffused n+ surfaces passivated by several passivation layers are determined as they are applied at the solar cell rear side. The resulting combined j0e and j0BSF values j0combined are compared with IV characteristics of solar cells passivated by the corresponding passivation of emitter and BSF on front- and rear side. As passivation layers are additionally employed as antireflection coatings also the spectral reflectance of the investigated passivation stacks with optimized thickness was determined

Etch-back of p⁺ structures for selective boron emitters in n-type c-Si solar cells

AbstractIn order to minimize electrical losses in the phosphorous emitter being one of the dominant factors limiting the performance of standard screen-printed p-type c-Si solar cells, selective emitter structures have been introduced to advanced standard p-type solar cells in recent years. A selective emitter is expected to yield various benefits for many different kinds of n-type solar cell concepts as well. The technical implementation of such a selective p+ diffused Si region by wet chemical etch-back of the heavily doped Si wafer surface via porous Si (por-Si) formation is developed into a well controllable process using a new etching solution adapted for p+ doped Si layers in respect of their higher concentration of valence band holes. As an initial proof of concept, integrated into 100 μm thin n-type bifacial large-area Si solar cells, the selectively etched-back B emitter yields a VOC gain of 5 mV and an Rshunt increase by a factor of 20

Comparison of bifacial and monofacial large-area n-type Si solar cells from 100 μm thin wire-sawn wafers

publishe

Screen-printed Al-alloyed rear junction solar cell concept applied to very thin (100 μm) large-area n-type Si wafers

Reducing the thickness of crystalline Si wafers processed to solar cells returns two significant benefits. Firstly, processing cost is reduced by saving cost- and energy-intensive Si material. Secondly, the required diffusion length of minority carriers is smaller, thus, wafers with a smaller carrier lifetime (e.g. due to higher base doping) can be utilized. In this work, the industrially feasible "PhosTop" cell concept is employed by manufacturing large-area n-type rear junction solar cells with a screen-printed Al-alloyed emitter featuring a selective phosphorous front surface field and a SiO2/SiNx passivation on the front.PC1D simulations for substrates with different base doping concentrations show that the range of base resistivities utilizable for those PhosTop solar cells is extended towards higher doping concentrations with decreasing wafer thickness. PC1D forecasts a conversion efficiency of the chosen 2.8 Ωcm n-type Czochralski-Si wafers of 19.2% for 100 μm thickness, merely 0.1% less than for standard thickness but saving ∼25% of the Si material. The manufactured thin large-area solar cells achieve a maximum efficiency of 19.0%