Nickel plating on p+ silicon : a characterization of contact resistivity and line resistance
2012, Seren, Sabine, Braun, Stefan, Schiele, Yvonne, Hahn, Giso, Terheiden, Barbara
Nickel plating on p+ Si is a promising approach for the metallization of n-type Si solar cells. Ni acts as diffusion barrier for copper, which is used for thickening of the Ni contacts. In this work the adhesion and the contact resistivities of Ni plated lines on different boron emitters as well as the line resistances are evaluated. For that purpose, boron emitters with different sheet resistances on Czochralski n-type Si wafers are used. The dielectric passivation layer (SiNx) on top of the emitter is locally opened by photolithography or by laser ablation to perform a line structure, which is afterwards electroless Ni plated. During a sintering step, nickel silicide is formed to achieve the required adherence and contact resistivity to the Si. To improve the adherence of the Ni plated layer and therefore to decrease the contact resistivity, a plating process with two separated Ni plating steps, named “two-step Ni plating”, is introduced. With this process narrow and sharp lines (15-80 μm) with contact resistivities of about 0.6 mΩcm² are demonstrated. Using electrodeposition of Cu, line resistances of 0.45 Ω/cm are measured with line widths below 50 μm. This work demonstrates that the introduced plating technique is well suited for high efficient solar cell.
Emitter optimization for mono- and multicrystalline silicon : a study of emitter saturation currents
2010, Seren, Sabine, Junge, Johannes, Seren, Sven, Hahn, Giso
In this work the influence of varied diffusion parameters for an industrial open tube POCl3 diffusion furnace upon the emitter saturation current density on monocrystalline silicon is investigated. Further on, the effect of phosphorus gettering on multicrystalline silicon and on the sheet resistance on both mono- and multicrystalline silicon is under investigation. In addition, diffusion profiles are determined using the ECV (Electrochemical Capacitance Voltage) technique. Aim of this work is to enhance the performance of lowly doped emitters (80- 140 Ω/sq) applied in a photolithography based high efficiency solar cell process with special respect to defect-rich block cast multicrystalline silicon material. Understanding the influence of temperature, time and gas flow variations during the diffusion process is very important to enhance solar cell performance especially for mc silicon. For such materials the POCl3 gettering effect and the defect kinetics during the diffusion and the cool down phase after the diffusion are of major interest besides the reliable contact formation and low emitter saturation currents resulting in a good blue response of solar cells. The experiments performed in this work demonstrate that different mono- and multicrystalline silicon materials can benefit from adapted diffusion recipes in terms of significantly reduced emitter saturation currents and increased bulk lifetimes resulting in enhanced solar cell efficiencies.
The European project 20Plµs : 20 percent efficiency on less than 100µm thick industrially feasible crystalline-Si solar cells
2012, Terheiden, Barbara, Horbelt, Renate, Schiele, Yvonne, Seren, Sabine, Ebser, Jan, Hahn, Giso, Morrison, D., Heathman, K., Devenport, S., Holman, Z.
The European project 20plμs is developing Si wafer solar cells with efficiencies above 20% on wafers less than 100 μm thick. Three principal solar cell process routes are investigated. The three approaches are distinguished by the doping type and maximum process temperature: p-type monocrystalline Cz-Si and multicrystalline Si solar cells subjected to high temperature processes are called pht, and n-type Cz-Si cells fabricated with low and high temperature processes are called nlt and nht, respectively. Already at the project’s midterm, a particular pht solar cell process was transferred to pilot line production. Key issues such as wafering, surface passivation, light trapping, metallisation and life cycle analysis were tackled to determine which process should be transferred. To date, by integrating the processes investigated within the project into full solar cells, efficiencies up to 18.7% (pht), 19.0% (nht) and 20.8% (nlt, 4 cm2) have been achieved on 100 μm thick large area Si wafers.
Evaluating the efficiency limits of low cost mc Si materials using advanced solar cell processes
2010, Junge, Johannes, Ebser, Jan, Seren, Sabine, Terheiden, Barbara, Seren, Sven, Hahn, Giso, Kaes, Martin
The evaluation of the efficiency potential of Si materials for solar cell production is one key aspect for strategic decisions in today’s photovoltaic business. In this work a flexible photolithography-based cell process is presented which is in particular well-suited for defect-rich multicrystalline Si material. One decisive feature is the low overall thermal budget of the process since it is based on only one longer high-temperature step (the P diffusion) and a short firing step to obtain a decent hydrogen passivation from a hydrogen-rich PECVD (Plasma-Enhanced Chemical Vapor Deposition) SiNx:H layer. A further MIRHP (Microwave Induced Remote Hydrogen Plasma) step at a temperature below 400°C completes the hydrogen passivation of bulk defects. The process is derived from the standard photolithography based process at the University of Konstanz (UKN) and can easily be adapted to all kinds of dielectric rear side passivation patterns like a-Si, SiO2, SiCx and Al2O3 or stack systems. The rear side contact in this approach is established by Laser Fired Contacts (LFCs). Results presented in this work originate from a process based on an Al2O3 rear side passivation which is deposited at less than 200°C and subsequently annealed at about 400°C. Efficiencies above 18% on EFG and Calisolar polysilicon material, above 14% on RGS and above 20% on FZ reference material are demonstrated on 2 x 2 cm2 solar cells. For all mc-Si materials these efficiencies are very close to the highest efficiencies ever obtained by applying other already established high efficiency processes.