Jan Lossen

Fully Ion Implanted and Coactivated Industrial n-Type Cells With 20.5% Efficiency

We present our progress in fully implanted n-type cell for industrial manufacturing. Screen-printed n-type cells with boron emitters are a viable contender to succeed p-type cell in industrial production. A major open topic is the availability of a robust manufacturing flow. The synergistic qualities of ion implantation allow for a very efficient and potentially robust process sequence. We review some of the challenges arising from this approach, such as activation of the implanted boron emitter and the reverse current characteristics. Or current process allows manufacturing of 156 × 156 mm 2 cells with efficiencies up to 20.5% with a single coactivation anneal.

Bifacial n-Type Cells With >20% Front-Side Efficiency for Industrial Production

We present our progress in the development of monocrystalline n-type cell for industrial manufacturing. Our cell uses a homogeneous boron front-side emitter and a phosphorous back-surface field, which are contacted with screen-printed grids on both sides of the cell. Our current process allows the stable manufacturing of 156 × 156 mm2 cells with a front-side efficiency of around 20% on industrial equipment. Besides the inherent immunity to light induced degradation of the n-type substrate, this cell type offers the opportunity to significantly enhance the energy yield by the employment of a bifacial module technology. First field tests indicate the potential of more than 10% additional current gain, challenging the energy yield ofmore complex monofacial cells of even higher efficiency.

Pilot-line processing of screen-printed Cz-Si MWT solar cells exceeding 17% efficiency

On the way to higher efficiencies, back contact solar cells are a promising alternative to conventional screen-printed solar cells. Especially, the MWT (metal wrap through) solar cell concept with only a few additional process steps compared to the conventional cell process is appropriate for a fast transfer to industry. The focus of this work is to change the conventional cell process as little as possible and to reduce the time-to-market. Hence, a MWT process with only three screen-printing steps and without efficiency losses is realized by combining the via and the solder pad metallization steps. In comparison with the conventional cell process the number of screen-printing steps is not increased and thus the costs for metallization rise only marginally due to somewhat increased paste consumption. In comparison to a previous approach the additional costs of the MWT process can be significantly reduced. A successful MWT process transfer from Fraunhofer ISEs Photovoltaic Technology Evaluation Center (PV-TEC) pilot-line to the pilot-line of ersol Solar Energy AG is shown. Mean cell efficiencies above 17% and maximum efficiencies of 17.3% are achieved for Cz-Si MWT solar cells within pilot-line production. Moreover, an efficiency gain compared to simultaneously processed, conventionally screen-printed solar cells of 0.3% absolute has already been reached. Furthermore, the influence of p-contact solder pads on the cell efficiency is analyzed in detail by current voltage (IV-) and photoluminescence (PL-) measurements within this work.

Fully screen-printed PERC cells with laser-fired contacts — An industrial cell concept with 19.5% efficiency

The introduction of a dielectric rear side passivation with local contacts (PERC) opens a path for improvements in the cost of crystalline silicon photovoltaics. First, the cell efficiency is increased by higher internal rear side reflectance and better passivation. Second, the elimination of the cell bow allows the use of thinner wafers to reduce the silicon consumption. While the PERC cell concept is well known, there is still no favorable industrial solution for the rear side metallization. Currently, there are two candidates for industrially relevant processes: The i-PERC process, where the dielectric is opened before the application of the rear-side metallization, and laser-fired contacts (LFC) [1], where the contacts are laser-fused after metallization. LFC promises good process robustness. However, so far, state-of-the-art efficiencies were only reported with evaporated Al rear-side metallization (PVD) [2]. Nevertheless, near-term introduction of the PERC concept would favor a fully screen-printed (SP) metallization to maintain compatibility with established process equipment.

All screen-printed industrial n-type Czochralski silicon solar cells with aluminium rear emitter and selective front surface field

The influence of the base dopand on the cell performance in a cell type with selective front phosphorus diffusion and an alloyed aluminum rear doping is investigated in this work. First this was done by using two dimensional device simulations to vary the doping species (n- or p-type) and the concentration over a broad range. For n-type base material we found that for a given front side system (diffusion and passivation) the fill factor tends to increase while the short circuit current decreases with increasing base doping concentration. The ideal base doping is therefore a function of the quality of the front side diffusion and passivation. In comparison to the n-type cells the dependence of the p-type cell results on the base doping concentration is much weaker. Based on the simulations a base doping concentration of 8 □cm was chosen to process n-type solar cells, which were then compared with identically processed p-type cells. For our front and rear screen-printed large-area n- as well as for our p-type cells we have achieved efficiencies up to 17.6 %.

Development of a high-throughput fine line metallization process using CFD-simulation

In order to enhance dispensing technology towards an industrial application in Silicon Photovoltaics, in particular throughput rate has to be increased. For this reason, a novel parallel high precision fine line dispensing unit is currently being developed at Fraunhofer ISE providing one nozzle per contact finger and a central Paste supply. In order to ensure a homogeneous paste distribution to all nozzles, the influence of paste rheology on the flow profile of the dispensing nozzles was analyzed. An analytical comparison of two different dispensing pastes with water gave a good insight on the influence of paste rheology on flow patterns inside the dispensing nozzles. Furthermore, numerical CFD-simulation (CFD: Computational Fluid Dynamics) was used to investigate different nozzle geometries and finally print head designs. In various iteration steps, the influence of fabrication tolerances especially concerning the nozzle geometry was isolated and print head designs were optimized based on CFD towards maximum process stability. In the meantime, process optimization using a single nozzle approach led to an average finger width below 35 μm, confirmed by several characterization methods.

Local Doping Profiles for Height-Selective Emitters Determined by Scanning Spreading Resistance Microscopy (SSRM)

In order to confirm the existence of a height-selective emitter, we compared local doping profiles of such emitters with profiles of a standard homogeneous emitter. The concept of a height-selective emitter is based on the classical selective emitter but in a smaller scale. The pyramid tips are highly doped. The sides and valleys are lowly doped. The comparison of the doping profiles was addressed by using a high-spatial-resolution analysis method: scanning spreading resistance microscopy. The measurement was performed on fully produced cells with a height-selective emitter and a standard homogeneous emitter, both with random pyramid textured surfaces. We prepared samples from these cells and investigated the cross sections. A representative pyramid of each type of emitter was selected. We found that for height-selective emitters the surface concentration can strongly vary depending on the measured position of the selected pyramid. The tip of the pyramid is heavily doped, while the bottom is lowly doped. For standard cells with a homogeneous emitter, the doping profiles do not differ dramatically as for the sample with a height-selective emitter. We calculated the local sheet resistance by using the measured local emitter profiles and a doping-dependent mobility model for phosphorus-doped silicon.

Bifacial screen-printed n-type passivated emitter rear totally diffused rear junction solar cells

We present the n-type passivated emitter rear totally diffused (n-PERT) rear junction (RJ) silicon solar cell concept as an industrially viable and cost effective alternative to passivated emitter and rear cells (PERC). In this work, we focus on a bifacial version of the cell type, featuring an H-pattern grid design on the rear side, and investigate the dependence of cell parameters on base doping concentration. With the software Quokka3, we performed a series of simulations to understand the cells electrical performance and its bifacial behavior. We varied front and rear side finger pitch, rear side irradiance, bulk minority-carrier lifetime (τb) and base resistivity (ρb). We measured the bifacial power generation of our best cell according to the equivalent irradiance method (GE) and compared it with the simulations.