Frank Feldmann

Carrier-selective contacts for Si solar cells

Carrier-selective contacts (i.e., minority carrier mirrors) are one of the last remaining obstacles to approaching the theoretical efficiency limit of silicon solar cells. In the 1980s, it was already demonstrated that n-type polysilicon and semi-insulating polycrystalline silicon emitters form carrier-selective emitters which enabled open-circuit voltages (Voc) of up to 720 mV. Albeit promising, to date a polysilicon emitter solar cell having a high fill factor (FF) has not been demonstrated yet. In this work, we report a polysilicon emitter related solar cell achieving both a high Voc = 694 mV and FF = 81%. The passivation mechanism of these so-called tunnel oxide passivated contacts will be outlined and the impact of TCO (transparent conductive oxide) deposition on the injection-dependent lifetime characteristic of the emitter as well as its implications on FF will be discussed. Finally, possible transport paths across the tunnel oxide barrier will be discussed and it will be shown that the passivating...

The Irresistible Charm of a Simple Current Flow Pattern – 25% with a Solar Cell Featuring a Full-Area Back Contact

Screen-printed Al-BSF silicon solar cells have dominated the PV market for decades. Their long-term success is based on a low-complexity cell architecture and a robust production sequence. The full-area rear contact allows a simple and effective one-dimensional current flow pattern in the base resulting in high fill factors. Some of the successor technologies of this simple but yet successful cell architecture, i.e. partial rear contact (PRC) and interdigitated back contact (IBC) solar cells, have a significantly higher process and pattern complexity. This paper discusses a cell structure with an architecture very similar to the classical screen-printed aluminium back surface field (Al-BSF) solar cell but with a higher efficiency potential. This is achieved by substituting the full-area doped back surface region by a passivated contact scheme consisting of a tunnel oxide covered by a heavily doped silicon film, called TOPCon. The champion efficiency of 25.1% on n-type silicon shows that this structure has a high potential while keeping the process effort low and the current flow pattern simple. The very high open circuit voltage of 718 mV and fill factor of 83.2% results from both, the very low recombination and transport losses caused by this contact scheme and cell architecture.

Doped Layer Optimization for Silicon Heterojunctions by Injection-Level-Dependent Open-Circuit Voltage Measurements

Besides passivation of the c-Si absorber, provided mainly by the undoped buffer layer, the net doping of the silicon thin films plays a major role in the performance of silicon-based heterojunction (SHJ) solar cells. However, junction engineering is complex as high net doping often interferes with the interface passivation and the optical properties of the silicon thin films. We show that injection-level-dependent open-circuit voltage (Suns- Voc) measurements are a simple and valuable method for the characterization and optimization of the doped amorphous silicon (a-Si:H) layers. It is shown by experiment and device simulations that at high illumination intensities the Suns- Voc characteristic exhibits a strong signature of defect recombination within the a-Si:H, which is determined by the a-Si:H doping and the interfacial transparent conducting oxide (TCO) properties. This fact is exploited for a qualitative interpretation of the interplay between a-Si:H and the interfacial TCO properties. As a clear correlation between the Suns- Voc characteristic and the maximum power point conditions of the devices exists, fill factor (FF) losses attributed to the doped a-Si:H and the interfacial TCO properties can 1) be easily predicted in the early stage of device optimization on simple test structures, or 2) these FF losses can be identified and distinguished from other FF losses in the final device.

Numerical Simulation of Carrier-Selective Electron Contacts Featuring Tunnel Oxides

Recently, n-type Si solar cells featuring tunnel-oxide-passivated contacts have achieved remarkable conversion efficiencies of up to 24.9%. Different approaches concerning the doped Si layer, which can be amorphous, polycrystalline, or partially crystalline, have been presented over the past few years. In this paper, carrier-selective electron contacts featuring tunnel oxides are investigated by means of numerical device simulation. The influence of 1) the Si layer material, 2) the Si layer doping, 3) an additional in-diffusion in the absorber, 4) the surface recombination velocity at the oxide interface, and 5) the oxide thickness and the tunneling mass are investigated by means of an open-circuit voltage analysis, as well as a fill factor (FF) analysis. With the fundamental understanding generated in this paper, we are able to explain the excellent device performance of solar cells with carrier-selective contacts featuring tunnel oxides.

High-Efficiency n-Type HP mc Silicon Solar Cells

Silicon solar cells featuring the highest conversion efficiencies are made from monocrystalline n-type silicon. The superior crystal quality of high-performance multicrystalline silicon (HP mc) in combination with the inherent benefits of n-type doping (higher tolerance to common impurities) should allow the fabrication of high-efficiency solar cells also on mc silicon. In this paper, we address high-efficiency n-type HP mc solar cells with diffused boron front emitter and full-area passivating rear contact (TOPCon). n-type HP mc silicon was crystallized at Fraunhofer ISE featuring a very high average lifetime in the range of 600 <italic>μ</italic>s (i.e., diffusion length >800 <italic>μ</italic>m) after application of all high-temperature steps necessary for cell fabrication. Using a “black silicon” front texture we have achieved a weighted reflectance of ∼1% and simultaneously a very good electrical performance, i.e., <italic>J</italic>₀<italic>_e</italic> values of ≤ 60 fA/cm² for a 90 Ω/sq emitter. The resulting n-type mc silicon solar cells show certified conversion efficiencies up to 21.9%, representing the current world record for mc silicon solar cells.

Tunnel oxide passivated contacts formed by ion implantation for applications in silicon solar cells

Passivated contacts (poly-Si/SiOx/c-Si) doped by shallow ion implantation are an appealing technology for high efficiency silicon solar cells, especially for interdigitated back contact (IBC) solar cells where a masked ion implantation facilitates their fabrication. This paper presents a study on tunnel oxide passivated contacts formed by low-energy ion implantation into amorphous silicon (a-Si) layers and examines the influence of the ion species (P, B, or BF2), the ion implantation dose (5 × 1014 cm−2 to 1 × 1016 cm−2), and the subsequent high-temperature anneal (800 °C or 900 °C) on the passivation quality and junction characteristics using double-sided contacted silicon solar cells. Excellent passivation quality is achieved for n-type passivated contacts by P implantations into either intrinsic (undoped) or in-situ B-doped a-Si layers with implied open-circuit voltages (iVoc) of 725 and 720 mV, respectively. For p-type passivated contacts, BF2 implantations into intrinsic a-Si yield well passivated co...

Approaching efficiencies above 25% with both sides-contacted silicon solar cells

A high cell performance is one of the key drivers to reduce the levelized cost of electricity for PV. Nevertheless, the production costs of a solar cell technology have to be low, to allow a market entry. A both sides-contacted solar cell can potentially fulfill both requirements. In this work we present the latest results of our DOE FPACE project: A both sides-contacted n-type solar cell with a passivated rear contact and an efficiency of 24.9 %. Furthermore, we present preliminary results for the next solar cell improvements and the path to reach efficiencies well above 25 %.

High-Efficiency Multicrystalline Silicon Solar Cells: Potential of n-Type Doping

By quantifying the role of dopants, impurities and crystal structure, we present guidelines for the fabrication of highly efficient multicrystalline (mc) silicon solar cells. Processed mc n-type wafers feature higher charge carrier diffusion lengths and thus a significantly larger efficiency potential compared with identically produced mc p-type wafers. Still, metal impurities limit the charge carrier lifetime in mc n-type wafers. We identify the main metal impurities in mc n-type silicon and quantify the resulting recombination losses. Attributing the main losses to precipitates and decorated crystal defects, the optimal efficiency potential of mc silicon is exploited by combining n-type high-performance multicrystalline silicon (HPM-Si) with a high efficiency cell concept featuring a full area passivated rear contact (TOPCon). The record cell features an efficiency of 19.6%, which is the highest efficiency reported for an mc n-type silicon solar cell. By reducing series resistance losses and improving the optics of the front surface, efficiencies of 21-22% should be attainable on n-type HPM-Si TOPCon solar cells.

Influence of the transition region between p- and n-type polycrystalline silicon passivating contacts on the performance of interdigitated back contact silicon solar cells

Passivating contacts based on thin tunneling oxides (SiOx) and n- and p-type semi-crystalline or polycrystalline silicon (poly-Si) enable high passivation quality and low contact resistivity, but the integration of these p+/n emitter and n+/n back surface field junctions into interdigitated back contact silicon solar cells poses a challenge due to high recombination at the transition region from p-type to n-type poly-Si. Here, the transition region was created in different configurations—(a) p+ and n+ poly-Si regions are in direct contact with each other (“pn-junction”), using a local overcompensation (counterdoping) as a self-aligning process, (b) undoped (intrinsic) poly-Si remains between the p+ and n+ poly-Si regions (“pin-junction”), and (c) etched trenches separate the p+ and n+ poly-Si regions (“trench”)—in order to investigate the recombination characteristics and the reverse breakdown behavior of these solar cells. Illumination- and injection-dependent quasi-steady state photoluminescence (suns-P...

Si solar cells with top/rear poly-Si contacts

Passivated contacts based on low-pressure chemical vapor deposited (LPCVD) heavily-doped poly-Si and a thin SiOx layer are explored for the application in an interdigitated back contact (IBC) solar cell. The poly-Si/SiOx contacts are realized by applying wet-chemically grown SiOx tunnel layers and amorphous Si (a-Si) layers doped via ion implantation that are subsequently transformed into poly-Si/SiOx contacts by a high temperature step. The impact of doping species, ion dose, and poly-Si thickness on the surface passivation of such contacts is studied. Excellent J0 values down to 4.5 fA/cm2 were measured for n+ poly-Si contacts, while J0 values of 22 fA/cm2 were obtained for p+-poly-Si contacts. Solar cells with top/rear poly-Si contacts were processed and Voc values up to 709 mV and FF values above 81% were measured. Furthermore, the upper bound for the parasitic absorption losses in 10-40 nm thick poly-Si films was quantified to be about 0.5 mA/cm2 per 10 nm poly-Si layer thickness.