Using Super Thin Finger+ (STF+) technology, JTPV's MoNo 2 cell offers a gain in FF by up to 0.3% and 0.1% in efficiency compared to a baseline cell
Using half-cell edge passivation technology, MoNo 2 sees an increase in FF of 0.5 to 0.8 and a cell efficiency gain of 0.2% and 0.25%, compared to an HEP-free MoNo 1 cell
MoNo 2 uses WBSF technology to improve cell level bifaciality by up to 90%, while internal test data from JTPV showed that a 0.15 percentage point increase in cell efficiency is achievable
The increasing demands of PV module makers for higher power, efficiency, and long-term reliability largely depend on the performance and quality of the PV cells. Additionally, the persistent decline in PV prices, driven by overcapacity, has increased the demand for high-quality PV cells with technological upgrades, whether for in-house or third-party use.
At the recent TaiyangNews Annual Flagship Conference – High-Efficiency Solar Technologies 2024 – Xinrui An, R&D Manager at JTPV, provided an overview of the latest upgraded MoNo 2 series n-type PV cell for module makers (see Jietai Solar presentation here). Jietai Solar, or JTPV, is a Chinese PV cell manufacturer with production bases in Chuzhou and Huai'an with a cumulative annual capacity of 40 GW, and offers high-efficiency n-type PV cells.
The MoNo 2 series n-type PV cell – an enhanced version of the MoNo 1 series – incorporates advanced features and improvements, says An. The MoNo 1 version, with 4 different wafer sizes, features a range of advanced technologies including stack wafer deposition process, emitter metal contact optimization, super thin fingers, and super multi busbars (SMBB).
In the stacked wafer deposition process, when wafers are placed back-to-back or face-to-face, depending on the desired processing side, a slight wraparound layer forms on the opposite side. This can be removed with a single-side wet chemical etch. An also noted that the double-sided deposition process requires careful control of parameters, such as ramp rate and gas flow, as the wafers may experience thermal stress due to the asymmetric deposition of films. Additionally, optimizing the emitter metal contact allows a less corrosive metallization paste with fewer silver-aluminum (Ag-Al) spikes, reducing contact recombination. This results in an additional 5 mV in Voc and a 0.3% relative efficiency gain, without significantly affecting contact resistance, shared An.
Moreover, improved metallization techniques, such as super-thin fingers (STF) and SMBB, which reduce grid spacing to enhance fill factor (FF), have been made possible by the development of silver paste that can be routinely printed with 20 µm narrow finger grid lines.
Harnessing the key features of the MoNo 1 series, the company said it has enhanced the cell structure in the MoNo 2 series by incorporating multiple technological advancements – Super Thin Finger + (STF+), Half-Cut Edge Passivation (HEP), and Wave Back-Surface Field (W-BSF). STF+, a novel fine-line printing technique, pushes the limit down to 15 µm fingers with smaller spacing, reducing resistive losses in the metal and emitter while minimizing shading on the cell. According to JTPV, an increase in finger aspect ratio to up to 70% due to the STF+ metallization, leads to a gain of 0.3% FF and 0.1% efficiency compared to baseline cell with a finger aspect ratio of up to 55%. The company also noted that the incorporation of STF+ technology, along with adjusting the emitter sheet resistance to prioritize higher Voc over higher FF, can lead to a higher CTM value for module suppliers, without affecting efficiency gains. Furthermore, the improved finger aspect ratios enable a tradeoff between cell efficiency and silver paste consumption.
The Half-Cut Edge Passivation (HEP) technology passivates the edges of laser-cut half cells by depositing an Al2O3 or silicon nitride layer, reducing the risk of Si dangling bonds and other defects at the cell edges, which could have decreased minority carrier lifetime and increase microcrack formation. Depositing the passivation film layer on the affected cell edge during a stacked wafer deposition process can reduce or even repair laser damage. This also allows for modules made from half-cut cells with more consistent characteristics, leading to higher power output. JTPV's in-house testing results between non-HEP-treated and HEP-treated cells showed an increase in FF of 0.5 to 0.8, translating to a cell efficiency gain of 0.2% and 0.25%. Annealing during thermal film deposition can improve both surface and bulk passivation, suggesting the potential for increased power with the HEP process, noted the cell maker. JTPV's tests revealed that an average gain of 5W in power output is achievable for both single-glass and double-glass 182-72 modules featuring HEP-processed MoNo 2 cells.
The MoNo 2 cell’s improved rear optics, achieved by optimizing the properties of the rear passivation layer (W-BSF), involves modulating the layer thickness between rear fingers to create an optically inhomogeneous structure that reduces parasitic absorption. This has resulted in improved bifaciality of up to 90% at the cell level, while internal test data from JTPV show that a 0.15 percentage point increase in cell efficiency is achievable, with a boost in Isc that compensates for FF loss due to the increased rear sheet resistance.
JTPV offers all major wafer sizes, including 182 x 182 mm, 182 x 183 mm, 182 x 210 mm, and 210 x 210 mm, for both the MoNo 1 and MoNo 2 series cell portfolios. Each wafer size has a 10 W power difference between the MoNo 1 and MoNo 2 types.
However, the cell maker has validated the superior performance of its latest MoNo 2 n-type cell at the module level through in-house testing, demonstrating higher bifaciality, greater power generation, a lower temperature coefficient, lower operating temperature, and reduced long-term performance degradation.