In this article we report on the latest update of our double-side contacted n-type Cz Si based back junction solar cell development. The solar cells have been manufactured by adopting a new low cost emitter formation process instead of a high-temperature boron diffusion tube furnace process. This approach allows for the reduction of the process complexity and therefore costs which are driven to a level similar to those of a standard p-type Si PERC process. The impact of the rear contact design of the n-Si solar cell is studied by using line and point contacts and various metal fractions of the point contact (7 % to 29 %). The n-Si solar cell structure with line contacts (fraction of 7 %) resulted in an efficiency of 19.2 % with an open circuit voltage of 647 mV and a fill factor of 74.2 %. The use of a point contact pattern (fraction of 7 %) improved the efficiency to 21.3 % with an open circuit voltage of 663 mV and a fill factor of 79.1 %. The gain of the open circuit voltage results in an efficiency gain of 0.5 %abs. The main efficiency gain of 1.2 %abs is related to an improvement of the fill factor which can be mainly attributed to a reduction of the series resistance losses. After exposing the solar cells to 25 °C and 75 °C at 1 sun illumination for 24 hours no degradation of the performance is observed which proofs the expected stability of n-type Cz Si based solar cells.
Al 2 O 3 /SiN x stacks that are prepared at low temperatures in chemical vapor deposition processes excel in terms of surface passivation applicable in industrial p-type Si solar cells. The conversion efficiencies that are feasible for solar cells with Al 2 O 3 /SiN x rear dielectric stacks, have been investigated by numerical simulations, including the optical performance of the stacks, which was considered for various Al 2 O 3 and SiN x film thicknesses. The optically optimized film thicknesses were found to be 15-30 nm for Al 2 O 3 and 100-120 nm for the SiN x films. Experimentally, the surface passivation was found to be similar for annealed Al 2 O 3 /SiN x stacks and single-layer Al 2 O 3 films with an almost equal level of field-effect and chemical passivation, as determined by optical second harmonic generation and corona charging experiments.
Degradation and regeneration of recombination parameters can occur in the bulk and at the surfaces of silicon solar cells. This article focuses on the time-resolved analysis of the recombination properties of textured 1.7 Ω cm boron-doped p-type Cz-Si and 5 Ω cm phosphorus-doped n-type Cz-Si wafers, where the surfaces are passivated by n + poly-Si on interfacial oxide layers exposed to a rapid thermal annealing (RTA) step in a conventional firing furnace. We observe a thermally activated instability in the lifetime over the entire examined injection range. Our experiments show that minority carrier injection (e.g., by illumination) is not required. Degradation in the surface passivation quality of the poly-Si on oxide layer-corresponding to an increase of the saturation current density J 0 by up to a factor of five- causes the degradation of the effective lifetime. Interestingly, the surface passivation fully regenerates under prolonged annealing and finally improves even beyond the initial state. Both the extent of the lifetime degradation and the change in J 0 depend on the postprocessing treatment temperature which we varied between 80 and 400 °C. Our results indicate that two different processes are responsible for the degradation and the regeneration. Reference samples which did not receive an RTA treatment show no degradation of the surface passivation quality. The RTA treatment applied therefore triggers the degradation effect. A large improvement of the surface passivation quality under prolonged annealing (e.g., at 400 °C) is observed for all samples examined in this study.
The effect of the deposition and annealing temperature on the surface passivation of atomic layer deposited Al2O3 films was investigated on n-type Cz silicon wafers. The deposition temperature was varied between 200 and 500̊C and the annealing temperature between 300 and 450̊C, respectively. Films prepared at 200 and 300̊C showed an improvement of surface passivation with increasing anneal temperature. The Al2O3 films grown at 400 and 500̊C did not improve by annealing. By corona charging experiments it was revealed that the improvement in surface passivation with increasing anneal temperature of films grown at 300̊C can be attributed to a significant increase in chemical passivation with a minor increase in field-effect passivation. For Cz and FZ wafers an identical surface passivation was achieved with the chemical passivation being lower for Cz wafers due to the surface morphology and the field-effect passivation being quite similar. Consequently the field-effect passivation was found to be the more important passivation mechanism.
Summary form only given. In the ALBA-II project, Q-Cells SE and ISFH are developing high-efficiency emitter-wrap-through (EWT) solar cells on n-type silicon wafers. N-type silicon grown by the Czochralsky method (Cz) forms the basis of this high-efficiency solar cell development as it offers high charge carrier lifetimes. The EWT solar cell concept nevertheless does not impose the same strict requirement onto the bulk material and front surface passivation quality than interdigitated back-contact back-junction (IBC-BJ) high-efficiency cell concepts. Thus, the relaxed front surface passivation requirement of our EWT cells allows us to employ a - compared to IBC-BJ solar cell concepts - relatively simple device structure and process sequence with only two dopant diffusion processes (phosphorus and boron). EWT solar cells are known for very high current collection efficiency. We achieve high open-circuit voltages of our cells by passivating the front and rear boron-diffused p-type emitter by a stack of aluminum oxide and silicon nitride (Al 2 O 3 -SiN). In order not to offset these advantages by current transport losses (such as the VIRE effect) we use relatively low Si wafer resistivities of 1.5 Ωcm and include a POCl 3 -diffusion process for the formation of a back-surface field (BSF). We passivate the BSF by a thermally grown oxide, which gets covered during the subsequent cell process by the Al 2 O 3 -SiN emitter passivation stack. We use pico-second laser ablation for the formation of contact openings through the passivation layers and employ nano-second laser ablation for all other structuring steps, including the aluminum rear contact structuring. With this approach we achieve on our small area (4 cm 2 ) cells a short-circuit current density (J sc ) of 40.4 mA/cm 2 , an open-circuit voltage (V oc ) of 661 mV, fill factors (FF) well above 80% and thus cell efficiencies exceeding 21%.
The surface passivation performance of Al2O3 films attracted attention in the field of solar cells and semiconductor devices and depends on the conditions of the applied post-deposition annealing step. The effect of annealing temperature and different annealing atmospheres on the surface passivation quality of atomic layer deposited Al2O3 films was investigated on n-type float-zone Si wafers. Photoconductance decay measurements were carried out to characterize recombination velocities and carrier lifetimes. The chemical and field-effect passivation mechanism, i.e. the interface trap density and the fixed charge density, respectively, were studied by capacitance-voltage experiments. Low surface recombination velocities of Seff,max ∼1 cm/s corresponding to a carrier lifetime of 9.0 ms were achieved for samples annealed in O2 atmosphere whereas annealing in H2 and N2 led to slightly higher Seff,max-values ∼2 cm/s. The annealing temperature was found to affect both the fixed charge density and the interface trap density whereas in contrast the annealing atmosphere affected only the interface trap density, i.e. the chemical passivation. According to the expectations the highest surface passivation quality is based on a high fixed charge density and a low interface trap density.
The surface passivation of SiO 2 /Al 2 O 3 stacks prepared at low process temperatures was investigated on phosphorous diffused n + -type Si surfaces with a broad range of sheet resistances. Two kinds of SiO 2 films were prepared, the first with plasma-enhanced chemical vapor deposition (PECVD) and the second in a wet chemical process. After atomic layer deposition of the Al 2 O 3 capping layer, the resulting SiO 2 /Al 2 O 3 stacks differ in the polarity of their fixed charge density, i.e., the PECVD SiO 2 stacks had a positive and the wet chemically grown SiO 2 stacks a negative fixed charge density. The PECVD SiO 2 /Al 2 O 3 stacks resulted in a high surface passivation over a broad range of sheet resistances whereas the wet chemically grown SiO 2 stacks were only feasible for diffused surfaces with low sheet resistances (<; 100 Ω/□). By corona charging experiments, it was established that the field effect based on a negative fixed charge density was the reason for the loss in surface passivation in the specific range of diffused surfaces.
