Innovation Trends in Solar Module Technology

Over the past decades, spectacular achievements in both the manufacturing chain as well as R&D efforts have made crystalline silicon, a low-cost source of electricity. While, the previous years were dominated by multi-crystalline technology and Mono-PERC, the current direction is moving towards N-type technologies like Tunnel-Oxide Passivated Contacts (TOPCon) with increased wafer sizes, predominantly at M10. (See Figure 1)
 

With TOPCon touted to promise for higher efficiency, it can offer negligible Light induced degradation (LID), Light and elevated Temperature induced degradation (LeTID), higher Bifaciality factor (~80 percent) and lower temperature of coefficients than PERC, it’s evident that TOPCon will become the mainstream workhorse in 2023. The comparison study shows the TOPCon bifacial module¹ has the higher energy yield performance to 13.31percent on average compared to PERC bifacial modules on flat white roof, and up to 20.94percent yield gain is possible for a realistic system, the majority of the contribution from its high bifaciality, low degradation, optimized temperature coefficient and low-light performance.

While there had been great efforts in R&D of cell manufacturing, only very recentlyAiko solar has achieved 23.5percent (with All Back contact modules) as reported by TaiyangNews in March 2023².This is by far the highest module efficiencies achieved in production settings.

The PERC Modules have increased their efficiency, focussing mainly on half cut cells, higher number of busbars, bifaciality factor, glass-glass configuration and some manufacturers even working on shingling concept as their main leverages. However, the advancement of TOPCon modules, mainly focuses on glass-glass, bifaciality factor, half cut cells, Multi-busbars (MBB) and reduced gap or even zero gap in some cases.

In order to reduce the Cell-To-Module (CTM) losses, most advanced module technologies primarily focus on reducing the light reflectivity and to reduce the resistance losses. In order to improve the light transmittance, several approaches including, single layer and in some cases double layer/multi-layer anti-reflection coating on glass is being preferred, cell placement optimization with narrow or even zero gap, usage of reflective backsheets or white EVA on the rear side, reduced optical shadowing using reflective ribbons and textured ribbons, etc. In order to reduce the resistance losses, these days multi-busbars are preferred, using half-cut or strip cells for shingling, improvised junction box and cable design, etc.

For Tier 1 players, multi-busbar was the easy adoption and a low hanging fruit to reduce the electric losses and thus all manufacturers took a big step in going towards 9 or 10 or even to16 busbar configuration recently, which has become the industry standard these days. Next to that, implementation of circular copper wires instead of flat ribbons have become the new norms now. The Implementation of Multi-busbar configuration at module level also assisted to achieve lower shading losses at cell level, which reduced silver paste consumption as well.

During the transition from 166mm to 182mm or 210mm, some of the Tier1 players, apart from half-cut cells were exploring with one-third cut cells. The cell’s current, which greatly influences resistance losses, gets reduced proportionately to total number of cuts involved in a cell, thus reducing the overall losses involved. However, using this configuration requires doubling the stringer capacity to match the module production capacity and needed a laser tool to slice the cell. With increasing wafer sizes, which correspondingly increases cell currents, the half-cell is more or less becoming inevitable. On the negative note, half-cell configuration causes edge losses, which becomes extremely sensitive in advanced cell configurations like HJT. Thus, equipment manufacturers came up with the option of non-destructive laser cutting tools (The latest 3D Micromac tool with thermal laser separation along with water-cooling technology to dissipate the heat faster). Next to this, manufacturers have already started exploring Passivation Edge Technology (PET) to reduce the edge recombination effects.

The solar cells in a module was placed in such a way that there will be some amount of buffer to the mechanical stress getting developed during the stringer operations. However, with the advancements of equipment’s and automation handlers has paved the way to reduce the cell-to-cell gap to minimal and heading towards zero gap. This zero gap technology is technically called as shingling which gains on active module area. In shingling, the cells are cut into several strips and connected to each other to a shingle structure of tiles placed on roofs using conductive adhesives. The technology has gained huge interest with many manufacturers who wants to be in a premium segment, however this technology is protected by several patents and promoted mainly by Maxeon and Solaria Corporation.

PERC was the first technology, where bifaciality played a huge role, which became so successful that the industry trend wants to follow it for every other technology adopted. In a PERC module, going from monofacial to bifacial involved the usage of transparent (instead of opaque) or glass-glass configuration for rear side light management. The second change was to use 2mm glasses on both sides of the module against using 3.2mm glass on the front for Monofacial modules. With many manufacturers to reduce the cost without compromising on the quality, the glass thickness for both front and rear is being reduced to 1.8mm or even 1.6mm in rare cases. Also, the current practice is to use POE (instead of EVA) on the rear side to provide extra protection against Potential Induced Degradation (PID) effect.

A transparent backsheet as the rear cover of a bifacial module seems to be a good alternative to glass. Modules with transparent polymer rear covers naturally weigh the same as standard modules with opaque backsheet, and do not require any extra care in handling while the manufacturing process remains the same. The bifacial technology has one main disadvantage, that, it reduces the front side power due to the loss of sunlight that hits cell gaps. First glass makers, then followed by backsheet makers, came-up with a solution, where-in they print a reflective film to fill in the empty spaces, while the areas occupied by the cells remain transparent.

In 2019, when the industry was moving from 158.75mm cells to 161mm to 166mm to 182mm to 210mm wafer sizes, there were lot of chaos happened as the status quo was challenged against the standard wafer formats with many non-standard sizes. The introduction of large size formats came from Tier 1 players to differentiate from what others can offer, which ultimately lead to the birth of 600Wp or even 700Wp+ modules today. The primary motivation is to cut down on balance of system (BOS) reaching very high module powers through larger wafer size formats, with 182mm size being the most preferred wafer size format.

With an unquenchable thirst and hunger towards producing high power modules by Tier 1 manufacturers, the industry is investing largely on cell/module R&D to reap more benefit out of the module. With respect to this, while some players in India are still conservative in their approach in going from Multi-crystalline to Mono-PERC technology, the other notable big players who are new to this segment are taking big leaps directly investing on TOPCon or HJT cell/module manufacturing and will definitely be the forerunners in setting record efficiencies in the near future. The government through Production Linked Initiatives (PLI) initiative has already given huge incentives, mainly for the Integrated PV manufacturers who will set-up polysilicon-ingot-wafer-cell-module manufacturing shortly, and will ultimately consolidate this market in terms of cost as well as capacity in this country and more importantly looked upon as an alternate supplier to the whole world.
 

- Dr. Balachander Krishnan, Chief Operating Officer (COO), Indosol Solar Pvt. Ltd., SPV of Shirdi Sai Electricals Ltd.