The front transparent conductive oxide layer is a source of significant optical and electrical losses in silicon heterojunction solar cells because of the trade-off between free-carrier absorption ...and sheet resistance. We demonstrate that hydrogen-doped indium oxide (IO:H), which has an electron mobility of over 100cm2/Vs, reduces these losses compared to traditional, low-mobility transparent conductive oxides, but suffers from high contact resistance at the interface of the IO:H layer and the silver front electrode grid. This problem is avoided by inserting a thin indium tin oxide (ITO) layer at the IO:H/silver interface. Such IO:H/ITO bilayers have low contact resistance, sheet resistance, and free-carrier absorption, and outperform IO:H-only or ITO-only layers in solar cells. We report a certified efficiency of 22.1% for a 4-cm2 screen-printed silicon heterojunction solar cell employing an IO:H/ITO bilayer as the front transparent conductive oxide.
•High-mobility indium oxide (IO:H) is used in silicon heterojunction solar cells.•Cells with IO:H have excellent EQE but low FF because the IO:H/Ag contact is poor.•We propose IO:H/ITO bilayers for low contact resistance and high mobility.•Solar cells with IO:H/ITO bilayers outperform cells with pure IO:H or ITO layers.•With an IO:H/ITO bilayer, our best cell achieves a certified efficiency of 22.1%.
•State-of-the-art passivation obtained with a-Si:H-based passivated contacts.•SHJ cell precursor lifetimes up to 18 ms with buffer layers as thin as 4 nm.•Finger widths down to 16 µm with ...screen-printing of low-temperature Ag pastes.•Cu electroplating process for 6″ SHJ cells established in R&D production line.•SHJ cell efficiencies up to 23.9% with industrial fabrication processes.
This paper reviews recent progress made at CSEM on the development of low-temperature processes for the fabrication of amorphous silicon-based passivated contacts and for the metallization of high-efficiency silicon heterojunction (SHJ) solar cells. Intrinsic a-Si:H passivation layers were optimized by trying to minimize the drop in lifetime usually observed after the deposition of the p-doped a-Si:H layer on top. State-of-the-art passivation levels are obtained, demonstrated by minority carrier lifetimes above 50 ms on lowly doped wafers, and close to 18 ms on actual SHJ cell precursors with buffer layers as thin as 4 nm. Regarding cell metallization, the screen-printing process of low-temperature Ag pastes has been optimized, resulting in finger width as low as 16 µm. Alternatively, a photolithography-free copper electroplating process has been developed. Using inkjet printing of hotmelt for patterning, 25-µm-wide and highly conductive fingers can be deposited. This process was tested in SHJ cell pilot production conditions, showing high cell performance (22.3% median efficiency) and good reproducibility. Finally, using the developed passivated contacts and screen-printing process, SHJ solar cells fabricated with industry-compatible processes showed efficiencies up to 23.1% on large-area devices and up to 23.9% on 4 cm2 devices.
This work presents the upscaling of the tunnel IBC technology on large area, Czochralski (Cz) n-type wafers. At the junction level, a self-aligned PECVD masking technology has been developed for the ...deposition of hydrogenated nano-crystalline silicon (nc-Si:H) layers on industrial 6-inch pseudo-square wafers. This damage free patterning technology allows state-of-the-art passivation with a minority carrier lifetime of 9 ms at an injection level of 1015 cm-3, thus enabling extremely long diffusion lengths up to several millimetres. The use of indium-free, cost effective aluminium-doped zinc oxide strongly reduces the materials bill of the tunnel-IBC technology while maintaining very low contact resistance for both the electron and the hole contacts. Remarkably, these tunnel-IBC devices demonstrated a conversion efficiency of 25% on large area (90.25 cm2) industrial wafer with a thickness of 155 μm. Series resistance analysis points out probable losses from the hole contact and the base. The limitation of the Transfer Length Method is discussed when used to extract the hole contact resistance.
Investigation of the dc vacuum breakdown mechanism Descoeudres, A.; Levinsen, Y.; Calatroni, S. ...
Physical review special topics. Accelerators and beams,
09/2009, Volume:
12, Issue:
9
Journal Article
Peer reviewed
Open access
Breakdowns occurring in rf accelerating structures will limit the ultimate performance of future linear colliders such as the Compact Linear Collider (CLIC). Because of the similarity of many aspects ...of dc and rf breakdown, a dc breakdown study is underway at CERN to better understand the vacuum breakdown mechanism in a simple setup. Measurements of the field enhancement factor β show that the local breakdown field is constant and depends only on the electrode material. With copper electrodes, the local breakdown field is around 10.8GV/m , independent of the gap distance. The β value characterizes the electrode surface state, and the next macroscopic breakdown field can be well predicted. In breakdown rate experiments, where a constant field is applied to the electrodes, clusters of consecutive breakdowns alternate with quiet periods. The occurrence and lengths of these clusters and quiet periods depend on the evolution of β . The application of a high field can even modify the electrode surface in the absence of breakdown. Measurements of time delays to breakdown show two distinct populations, immediate and delayed breakdowns, indicating that two different mechanisms could exist. The ratio of these two populations depends on the conditioning state of the electrodes and on material. Gas release during breakdown is dominated by H2 and CO. This degassing is mainly due to electron-stimulated desorption. During the quiet periods without breakdown, gases are also released but the quantities are much smaller. All the measurements presented here emphasize the crucial role of field emission in the breakdown triggering.
The rf accelerating structures of the Compact Linear Collider (CLIC) require a material capable of sustaining high electric field with a low breakdown rate and low induced damage. Because of the ...similarity of many aspects of dc and rf breakdown, a dc breakdown study is underway at CERN in order to test candidate materials and surface preparations, and have a better understanding of the breakdown mechanism under ultrahigh vacuum in a simple setup. Conditioning speeds and breakdown fields of several metals and alloys have been measured. The average breakdown field after conditioning ranges from 100MV/m for Al to 850MV/m for stainless steel, and is around 170MV/m for Cu which is the present base-line material for CLIC structures. The results indicate clearly that the breakdown field is limited by the cathode. The presence of a thin cuprous oxide film at the surface of copper electrodes significantly increases the breakdown field. On the other hand, the conditioning speed of Mo is improved by removing oxides at the surface with a vacuum heat treatment, typically at 875°C for 2 hours. Surface finishing treatments of Cu samples only affect the very first breakdowns. More generally, surface treatments have an effect on the conditioning process itself, but not on the average breakdown field reached after the conditioning phase. In analogy to rf, the breakdown probability has been measured in dc with Cu and Mo electrodes. The dc data show similar behavior as rf as a function of the applied electric field.
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