Back contact solar cells have been investigated for many years with different device designs and for various applications. In the first part of this section a short overview is given on high efficiency back contact solar cells. Since low cost devices for applications in the mass market are investigated in this work, different designs of this category are described in the following.
4.1.1 High efficiency devices
In conventional solar cells, the metal coverage on the front side is a compromise between shadowing and series resistance losses. High efficiency back contact solar cells allow the decoupling of these two factors: the front surface is optimised for optical performance as well as low surface recombination and the rear surface for low series resistance losses. Therefore back contact solar cells are especially suitable for concentrator applications which require a low series resistance due to a high I2R power loss.
In general, high efficiency back contact solar cells have a collecting junction only on the rear surface whereas the front surface is well passivated (see Figure 4.1). The minority charge carriers mainly generated in the front surface region have to diffuse a long way to the
rear junctions. Hence, these devices require a high ratio of bulk diffusion length to cell thickness.
The first design of an Interdigitated Back Contact (IBC) solar cell (see left picture in Figure 4.1) was investigated by Schwartz and Lammert [Schw75], [Lam77]. (For a review of the early research on back contact solar cells for concentrator applications see [Schw82]).
Swanson reduced the rear surface recombination leading to the point-contact silicon solar cell (see right picture in Figure 4.1) [Swa84]. The manufacturing process for all of these devices were rather complex. Sinton [Sin90] introduced a simplified manufacturing process for the IBC cell design referred to as the trench mesa design. This process involves only one photolithography step and no alignment steps at all. SunPower Corp. [Ver94] and Amonix are currently commercialising IBC solar cells.
Figure 4.1: Schematic drawing of (left) Interdigitated Back Contact (IBC) solar cell [Schw82] and (left) point contact solar cell [Swa84].
4.1.2 Advantages of low cost back contact solar cells
The research on low cost approaches was motivated by several aspects. First of all, the device designs should lead to efficiencies in the range of conventional cells on materials with a lower bulk diffusion length. The technology involved in the processing of high efficiency back contact solar cells is too expensive to be produced for the mass market of one sun applications. In this work the term “low cost industrial type solar cells” is used for devices for which the costs per Wp are comparable to conventional industrial solar cells.
The materials to be used for solar cell processing are the standard materials of the PV industry: solar grade Cz-Si and mc-Si.
The research on back contact solar cells is motivated by the following points:
• Easier module assembly
• High aesthetics
• Potential for higher solar cell efficiency
• Easier module assembly for thin solar cells with thickness well below 200 µm
• Reduced series resistance for large area solar cells (>12.5x12.5 cm2)
Easier module assembly and high aesthetics
Currently, about 85% of the total PV-shipment consists of crystalline silicon solar cells [Pho02]. In the modules the cells are interconnected in series with solder tabs running from the front to the rear surface of the next solar cells. These solder tabs need to be highly conductive for minimum series resistance losses and are therefore rather wide (around 1.5 to 2.5 mm). For architects these busbars are disturbing with respect to the optical appearance of PV modules. This point is especially important for building integrated PV which is a fast growing market segment in the current PV industry. By eliminating the busbar solder tabs or the complete metallisation on the front surface, this new generation of crystalline Si modules has a higher optical appearance and opens new markets for PV.
The above mentioned procedure for series interconnection of conventional solar cells has several disadvantages. The process itself is difficult to automate and therefore labour-intensive [Gee97]. The solder tabs will stress the wafer at the edges unless the cells are generously spaced. Back contact solar cells allow novel schemes for module interconnection (see section 5). Also back contact cells can be closer spaced allowing a higher packing density within the module.
Potential for higher solar cell efficiency
Due to the reduction or even absence of grid shadowing losses the active light absorbing front surface is increased. Some designs of back contact solar cells also have a large fraction of a second carrier collecting junction at the rear leading to higher currents densities. This is especially beneficial for crystalline silicon materials with a lower ratio of bulk diffusion length to cell thickness.
Advantages for future crystalline silicon modules: thinner wafers and larger substrates The induced stress at the wafer edges during module fabrication might be too high for thin wafers leading to an enhanced breakage rate. If the wafer size is enhanced towards 15x15 cm2 or larger, very high currents above 8 A will be created under one sun illumination. Since these currents have to be transported in the busbar tabs, the series resistance loss I2R will be enhanced. The trade-off between resistive and optical shading losses in module fabrication is omitted for most back contact designs. In these designs, the high currents are transported in the interconnections on the rear which can be optimised for a low series resistance.
4.1.3 Device designs of low cost back contact solar cells Metallisation Wrap Around Solar Cells
MWA solar cells can be described as conventional cells in which the busbars on the front are moved towards the edge regions on the rear surface (see Figure 4.2). In these devices there is still finger metallisation present on the front side. Compared to other rear contact solar cells, it is advantageous that these devices do not require holes or vias for the electrical interconnection of front and rear.
The main technological problems to be solved for MWA solar cells are:
• Sufficient and reliable metal deposition at the edges
• Contact separation of the p- and n-type regions on the rear
p-contact
Figure 4.2: Characteristic regions of three different back contact designs. (left) Metallisation Wrap Around (MWA), (middle) Metallisation Wrap Through (MWT), (right) Emitter Wrap Through (EWT).
• Excellent finger conductivity in conjunction with low shadowing losses which is especially important for large area MWA solar cells
The research on MWA solar cells already started in the eighties [Ami82], [Mic81], [Cav84]. These devices were considered for space application and rather complex processing sequences were applied, which are not seen as low cost approaches in the PV industry nowadays. Applying thick film metallisation, MWA solar cells have also been investigated in [Gab00] and [Ker00a].
In this work, the advantages of highly conducting fingers accomplished in BCSCs and the advantages of cell metallisation by electroless plating were combined to fabricate MWA cells by low cost production techniques. This device design and its manufacturing process was patented [Joo00a].
Metallisation Wrap Through
Very close to the design of conventional solar cells is the Metallisation Wrap Through concept (see Figure 4.2). Similar to the MWA concept, the busbars are moved to the rear surface leaving the finger metallisation on the front. In these devices holes are introduced into the wafer which transport, after diffusion and metallisation, the current from the front fingers to the rear side busbar. The device design has been investigated in [Ker98], [Ker00c]
applying screen printed metallisation.
Emitter Wrap Through
The Emitter Wrap Through (EWT) solar cell concept [Gee93] evolved from the polka-dot solar cell [Hal80]. The basic idea is to leave all metal contacts on the rear but to use the front side emitter for current collection. The electrical interconnection between the front side emitter and the rear side emitter contact is accomplished by laser drilled holes (see Figure 4.2). These holes obtain a (heavy) phosphorus diffusion and, if possible, get metallised for effective charge carrier transport. The number of vias required for EWT solar cells is relatively large and is in the range of several thousand on a cell area of 10x10 cm2. Efficiencies up to 21.4% have been obtained for EWT solar cells using FZ-Si and photolithography (cell area 2x2 cm2) [Glu02]. Applying industrial process technologies, highest efficiencies of 16.1% were reported on Cz-Si (cell area 100 cm2) with screen printed metallisation [Kre01].
POWER back contact solar cell
Different device designs of the POWER (POlycrystalline Wafer Engineering Result) have been suggested [Wil95], [Fat95]. Amongst them is a back contact variant shown in Figure 4.3. In the POWER cell concept perpendicular grooves are introduced from the front and rear by mechanical abrasion at a depth exceeding half of the wafer thickness. This leads to small holes at the cross-over point. Besides an optical semitransparency these holes serve as electrical interconnection of the front side emitter to the rear side emitter contact in back contact POWER cells.
Figure 4.3: Schematic drawing of (left) back contact POWER cell [Wil95] and (right) Pin-Up Module (PUM) [Bul01].
PUM module
The Pin-Up Module (PUM) is a new solar cell and module design. The cell itself can be described as a conventional solar cell with a minor number of holes. This limited number of holes (9 or 16 for large area solar cells) serves as vias for mechanical and electrical connection to the interconnection material [Bul00], [Bul01]. The gridlines on the front conduct the current towards the holes. A pin, placed on the interconnection ribbon, is introduced into the vias and soldered or glued to the front contact. By this procedure rear contacting is achieved on module level.
Interdigitated back contact
In this concept the device structure of the high efficiency IBC solar cell is applied to lower quality material. Solar cells are currently investigated using thin ribbon silicon material processed with the dendritic web technique. As a special feature of this process self-doping pastes are used to define the interdigitated contact pattern on the rear [Mei98].
Other low cost approaches of back contact solar cells include the VEST cell [Ham97]
and cells with a triode structure [War92]. A review on different back contact solar cell designs for low cost applications can be found in [Smi00].