Analysis of Cell to Module Loss Factor for Shingled PV Module

2.1 Performances and Challenges

PV module manufacturing industries is becoming the primary resource of clean energy. This report provides emphasis on the technological developments in field of commercialized PV solar modules. A shingled module can generate more energy compared to other modules due to improved methodology with the same power rating under the standard test conditions. The generated energy mainly depends on factors (i) installation of the solar module and (ii) the light reflectance of the surface. Implementing glass in a module protects it from environmental as well as gives mechanical strength to a module. The manufactures use the glass-glass module at an industrial scale, the new shingled product also profits, in the same way forming a mature technological process. The shingling technology allowed to further boost P_out by employing the overlapping of adjacent cells. HIP MWT superimposed on its neighbored cells active area using an interconnection technique. With this new field of technology, it is possible to further reduce the cost of energy generated by photovoltaic modules for all applications.

2.2 Losses in Shingled cell to module

Researchers are targeting to reduce the loss factors during Module fabrication to enhance output power as well as efficiency. Accurate measurement using a simulation tool (sun simulator) for PV Solar cell to module is important to understand the loss mechanisms and improve efficiencies^[10,11]. Due to the shingling technology, applied to the solar cells and modules, there are additional CTM gains added to the module. This CTM loss mainly focus on the optical and resistive losses arising from cell spacing and interconnection^[12,13]. Using $$ {EQE}_{cell}^r$$, the light absorbed in the cells for different module structure, at standard testing condition the change in current concerning double glass structure can be calculated as shown in eq. (1)^[14].

Where $$ {EQE}_{cell}^r\left(\lambda \right)$$ is the external quantum efficiency of the mono-facial cells used to develop module. In mono facial cells, generally current flows concerning the potential gradient. For example, in the mono-facial cells with “H” pattern, the current flows almost to the nearest finger, then to the nearest bus bar and then along the corresponding ribbon. The CTM resistive losses can be reduced by using some advanced HIP MWT technique with strong increase in reverse current at the front contact.

2.3 Approaches for Loss Reduction

There are several technologies at the module level which play a vital role in improving the cell to module (CTM) power ratio. Making use of innovative technologies like cell interconnection methods, the number of strips and spacing between cells plays an important role to increase the module power ratings. This technique started recently with an increase in the number of bus bars reaching multi bus bar level (HIP MWT) while researchers already started doing without ribbon by mass-producing shingle type of cells^[15]. The module manufacturers need to take care of the longevity of the module which gradually increases the production cost. The technology thus requires innovations to reduce the cost without compromising in module quality. Recent developments saw the encapsulation and back sheet field choosing double glass on both sides for packaging the module. The module processing steps like interconnection, stringing and laminations play an important role in reducing the optical losses. Optical losses mainly occur due to reflection at various interfaces of the module layers “air-glass - encapsulant - cell”. The absorption of light from the front cover glass and EVA adds up to this. Light scattering from the cell gap in the module and the metal contacts covered with ribbons also contribute to the plus side. The new technologies are assumed to reach a CTM power gain of 100% despite the various loss mechanism in today’s PV modules^[16].

2.4 Shingled Cell design

Shingled cell is designed based on overlapping the front side of the busbar of a cell to the rear side bus bar of the neighboring cell leading to a metallization of bus bar less front and rear side shingled module. Complete shingled cell with overlap covering the entire bus bar not only minimizes the cell area but also the designated area which is defined as the difference between the total cell area and the overlap cell area responsible for increasing the output power by reducing inactive cell area^[17]. Overlapping of the cell provides mechanical and electrical junction to adjacent cells which further minimizes the series resistance on the interconnection level. The power loss and equivalent resistance of the shingled module is given by eq. (2) and eq. (3)

Where R_e, R_ec, R_f are emitter resistance, emitter contact resistance, finger resistance as given in eq. (4-6) respectively. The w_pcs and l_pcs are respectively the width and length of the solar cell stripes, O_pcs is the overlapping between cells defined as the busbar width at tge edge. P-type silicon shingled solar cells are designed based on Czochralski-grown silicon (CZ-Si) passivized emitter and rear contact cell (PERC) technology. Different overlap scenarios using PERC cell is taken into consideration for shingled module integration. It is crucial to examine the MWT area to determine the distance needed to maximize the interconnection area and short-circuit current simultaneously.

2.5 Losses in Cell cut process and shingled interconnection

The design of shingled module technology can increase the efficiency and power up to 33 W_p and 1.86%_abs as compared to ribbon-based interconnection. CTM ratio for efficiency and power is improved mainly with ribbon or wire cell interconnection^[18]. In the commercial process of Shingled technology manufacturing, the power losses can be measured as given in eq. (7).

The shingled module is having higher impact in recent days due to the loss factors are of nearly zero value. Initially, commercial cell efficiency was 21.6% and the CTM ratio for power efficiency was 93.5%. Similarly, the output power of the module reached approximately 335.8 Wp and the CTM power ratio of 99.4%^[19~21]. Shingled cells are interconnected using different materials like electrically conductive adhesive (ECA). Basically, ECA depends on time-pressure, auger or jetting. The adhesive can resist more stress absorbing than solder and hence can withstand rigors of thermal cycling and process at low temperatures. Cost of ECA is half of pure silver filled conducive adhesives. Additional feature is that the conductive adhesives is designed to cure in seconds at 150°C to 180°C to enable fast filled conductive adhesives. ECA’s are used to electrically interconnect solar cells using ribbons or direct cell to cell contact. The p-n junction material bulk resistivity ρ_bulk and contact resistivity ρ_contact needs to be taken into consideration. From the Fig. 1, the electrical resistance R of an ECA-interconnection can be expressed as given in eq. (8)

Fig. 1.

Introduction of a shingled solar cell using ribbon

A shingled module is developed with the important features with 66 shingled cell per module having string distance of 2 mm using 6 strings. The following features are (i) low-iron glass with anti-reflective coating, (ii) 3.2 mm thickness, (iii) Encapsulant foil thickness of 0.45 mm with a low UV cut-off (iv) white TPT. It is assumed for a shingled module, the thickness of interconnection material (ECA) is approximately 50 μm having specific resistance of 0.1 μΩ/m. Size of a monocrystalline shingled cell is 156.75 × 26 mm² and efficiency of 23.6% (0.88W_p). Module dimensions are 1,667 × 998 mm (1.66 m²) with margins of 33 mm (top, bottom) and 23.75 mm (left, right) assuming a cell overlap of 1 mm.

Eq. (9) and (10) gives the expression for cell efficiency and module efficiency. Where K₁ and K₂ are the loss factors related to the inactive module areas, mainly the border and cell spacing area.

Figure 2 shows the shingled cell overlap for ModuleCTM fabrication. Shingling of cell into 6 stripes are produced with 5 cuts.

Fig. 2.

Shingling cell overlap and and reduce shading loss

3.1 Methodologies to analyze the efficiency using different technologies

Every manufacturing industry is aiming to bring out solutions for the technology which can give maximum module efficiency at a lower cost. Previously, studies were done on several factors that affect P_mpp and module area. The use of PERC cell technology can reduce the cell production to $0.06. Compared to multi crystalline (η ~ 20-21%) PERC cells, monocrystalline PERC cells are having an efficiency slightly higher of 21-22% around 2015 midway. Though technology progressed more and more in last few years. This research paper shows the recent progress in this aspect and it shows the future possibilities of further progress^[23-27]. The performance of commercial PERC cells is much better compared to others in respect of V_oc as determined by cell J₀ and resistive losses^[28]. The estimated additional manufacturing cost of PERC cell including the wafer cost leveraged by higher efficiency is around 6%/W and at the module level the leveraging cost reduced to 2%/W. The factors on which the research work focus is to implement the PERC technology, to make highly efficient, increase and cost effective shingled cell.

However, this fabrication technique has some practical limit and further gain in efficiency is limited. The search for maximum efficiency has led the industry to adopt PERC technology. The Shingling approach can be applied to any cell architecture as long interconnections are on the side of the cell. Using BSF, PERC, bifacial-PERC, n-PERT/PERL, HJ and others. The shingling technology shows different advantages in comparison to other PV modules i.e lower ohm losses, better area utilization, lower temperature processing, lower operating temperature and being made better aesthetics. In the shingled module, there is no cell gap so denser packing can be obtained. Because of small area utilization for shingled connected in series, it minimizes the ohm losses. When the cells are cut into strips, the photo generated current (I_ph,cell) and the series resistance (R_s) is affected. The other parameters like diode saturation current (I_OC), shunt resistance (R_sh) and ideality factor of the diode (i_idc) are kept constant. ECA’s curing temperature is less than ribbon soldering, so the cells experience less amount of stress and bowing. Low operating temperature is related to increase in energy generation. Most of the features of shingled technologies giving advanced benefits leads to increase in an efficiency of the module.

Properties of SiN_x as AR coating having thickness of 80 nm, processed by plasma enhanced chemical vapor deposition, acts as an anti-reflection coating and can be utilized as excellent surface passivation layer, good metallization mask, diffusion barrier, chemical resistance, high temperature stability, low cost and throughput and good overall rating. AR layer structure is created with an oxide layer that provides efficient absorption of light and the efficiency of the structure increased to 24.4%. The cell parameters such as short circuit current (I_SC) and fill factor (FF) vary with screen printing processing conditions mainly the aspect ratio and alignment. It depends on parameters like (i) wire dimension, (ii) mesh count and (iii) emulsion. Therefore, it is important to modify the metallization process to improve the performance of the solar cell.

3.2 ECA Material for cell interconnection

Conventional PV solar module technology are based on H-pattern cells, ribbons and soldered interconnections. This configuration has some disadvantages: (i) high shading loss (ii) expensive metallization (iii) high resistivity loss (iv) incompatible with very thin wafers and (v) load containing solder.

From Figure 4 it can be examined that for each cell stripe width it has an optimized amount of metallization finger metal line varies with respect to the maximum power of a shingled cell. From the simulated result it is concluded that even a lower amount of metallization line on the front side is enough to reach the maximum power point. Adhesion between joint material and metallization material needs to have excellent interconnection for Shingled technology which can be classified as (i) solder and (ii) ECA. Different types of ECA can be further classified as (i) Epoxy based, (ii) Acrylate based and (iii) Silicone based. Power degradation of the module is related to the loss in FF and I_SC based on increase in series resistance of module. This review article showed the most improved alternative of the interconnection material to reduce the CTM loss^[30]. Table 1 shows the some solutions optimized for using in interconnection.

Fig. 4.

Pmpp of full cell for variation width of cell strip assuming no further losses due to interconnection[29]

Table 1.

Optimization of material solutions

Graphene material has significant (α=2.3%) fraction of incident white light having properties like zero bandgap semiconductor with a small overlap between valance and conduction band, excellent electrical conductivity and posing to a major challenge for graphene electronics. The graphene material has a fermi level of 0.16eV and dielectric grating with periodicity (d=130 nm). The resistance of the ECA material is compared with silver and aluminum paste showing a low resistance value for ECA with silver paste.

Figure 5 explains the change in resistance with the change in length for two different pastes. Eutectic gallium-Indium binary alloy (EGaIn) is a combination of organic and inorganic material having non-toxicity, deformability, superior electrical properties, flexible and reconfigurable. The use of this material in the interconnection of shingled technology is quite advantageous as it is highly efficient with stretchable interconnections. The mechanical strength is 22% and electrical conductivity (α=24,100 S/cm). EGaIn shows advance properties over molted solders (used in the set of methodologies called micro solids) which need heating and cooling steps for the fabrication process and incompatible with heat-sensitive materials such as organics. Laser scanning microscope is used to show different ECA images. Electrical losses in shingled module depend on factors like (i) Cell Interconnection and (ii) String Interconnection. The paste is applied to pneumatic dispenser on the MWT. The ECA paste is laminated using liquid silicone followed by curing which is done using liquid silicone at the standard process condition.

Fig. 5.

Change in resistance with the change in length for ECA-AgAl and ECA-Ag paste

It can be seen, the results based on the EGaIn based interconnections maintain high electrical conductivity under severe elongation up to 95%, while silver pastes loose conductivity at an elongation of 0.6% and 60%, respectively. The contact resistance of EGaIn almost remains unchanged at a bending radius of ~1.5 mm, at an angle of 360°, while in case of silver paste shows a large variation. The lifetime of a module can be increased by selecting proper interconnection material. Below Fig. 6 is showing a shingled module assembled with different layers.

Fig. 6.

Schematic diagram showing shingled module

3.3 EVA Material

The power loss due to encapsulant material is approximately 0.8 W which can be reduced by using excellent material such as EVA, back sheet and glasses are laminated by applying heat on both sends for protection. The module is finally completed with a frame. Different material combinations are used for module design such as Polyolefin (POE), Polyvinyl fluoride (TPT) and Ethylene Vinyl Acetate (EVA).

The elasticity of EVA and toughness of TPT together have wonderful property and protections. Now a days, conventional solar cells generally use TPT as back-sheet substrate material. TPT as back-sheet material showed excellent durability, impermeability, insolubility, erosion durability, mechanical stability, and hydrophobic property. Thickness of the material is 0.32 mm and the breakdown voltage is approximately 17 KV. This encapsulant material can be fabricated between the double glass layer of thickness 22 mm for better absorption and prevention from mechanical stress.

3.4 Shingled Module design

To complete the study of shingled cell to module loss analysis, different module with 5 and 6 cell strips are optimized and found that with 6 strips the area utilization is improved. The two main area on which it is focused are the module area and maximum power point of a module. For design purpose, selection of wafer, cutting into strips, aligning them to form shingled cell to module, anti-reflection coating, shingled interconnection and module encapsulation are the key factors required. There are other factors like solar cell area, exposed module area, packing factor, electrical power output, module weight, encapsulation thickness, back sheet having white TPT, and thickness of interconnection contributes to the module design. Module dimension can be set according to the cell size, margins and overlap between shingled. The above- mentioned Si-wafer and 6 stripes per cell having inactive area of 0.117 m² and overlap of 0.8 mm the dimension of shingled module can be designed as 6 stripes × 96 wafers in a module. The module can be designed using series parallel combination of cells i.e 12 cells in series and 8 cells in parallel. Conventional modules are having glass and back sheet as encapsulant material finally framing and substrate material. The above assemble the module size can be defined as 1,8811,254 mm² having area 2.358 m², margin equal to 30 mm (top and bottom) 20 mm (left and right). As the interconnection loss optimization depends on the cell arrangement so the architecture of module is important. Also, with 96 cells, the module size will increase accommodating more shingled cells in the same area. The loss mechanism from cell to module depends on several factors as explained below. Cell spacing, (ii) Front to back cell Interconnection, (iii) Encapsulant cover, (iv) Module margin, (v) Interconnection coupling, (vi) String Interconnection, (vii) Electrical mismatch, (viii) Interconnection shading loss and (ix) Junction box and cabling. For module optimization these key factors play critical role in CTM loss.

Fig. 7.

Results showing overlap depth on CTM-power loss & CTM-ratio in Shingled modules

Shingled module uses no tabs between the cells (only between the strings). The series connection can be realized by stacking one edge over another like a roof shingled. These losses are calculated in percentage from the raw data after testing of module power and efficiency. Applying different material and technique some losses have been optimized in this paper and a proper conclusion has been drawn.