Rear Side Of Solar Cells Research Articles

In situ phosphorus-doped polycrystalline silicon films by low pressure chemical vapor deposition for contact passivation of silicon solar cells

In situ phosphorus (P)-doped polycrystalline silicon (poly-Si) films by low pressure chemical vapor deposition (LPCVD) were studied in this work for the fabrication of poly-Si passivating contacts. In situ doping was targeted for enabling the full potential of the high-throughput LPCVD technique, as it could allow leaner fabrication of industrial solar cells featuring poly-Si passivating contacts than the more common ex situ doping routes. By careful optimization of the deposition temperature and the flows of the carrier gas (H2) and the dopant precursor (PH3), high doping in the poly-Si layers was achieved with active P concentrations up to 1.3⋅1020 cm−3. While reduction in the deposition rate (rdep) and thus in the throughput is a known problem when growing in situ P-doped films by LPCVD, this reduction could be limited, and the resulting rdep was equal to 0.078 nm/s. The developed poly-Si films were characterized both structurally and in terms of their passivation potential in poly-Si contacts. The latter yielded recombination current densities down to 1.5 fA/cm2 in passivated (J0,p) and 25.6 fA/cm2 in screen-printing metallized (J0,m) regions on saw-damage removed (SDR) Cz-Si surfaces, accompanied by a contact resistivity (ρc,m) of 4.9 mΩ⋅cm2. On textured Cz-Si surfaces, the corresponding values were J0,p = 3.5 fA/cm2, J0,m = 56.7 fA/cm2, and ρc,m = 1.8 mΩ⋅cm2. Optical impact of the developed poly-Si films was also assessed and a short circuit density loss of 0.41 mA/cm2 is predicted per each 100 nm of poly-Si applied at the rear side of solar cells.

Metallization of crystalline silicon solar cells for shingled photovoltaic module application

The shingled photovoltaic (PV) module is a high-power PV module technology that is manufactured cells dividing and bonding with an electrically conductive adhesive (ECA). Here, the ECA is composed of 70–80% of silver, an acrylic, and a solvent. We focused on the formation of solar cell metallization suitable for the shingled PV module. Generally, the Ag pads on the rear side of the crystalline Si solar cells don’t printed on the Al surface because of the peeling problem. Therefore, a back surface field (BSF) layer is not formed at this area. The characteristics of the solar cell can be improved by applying a rear Ag pad together with the BSF layer in the entire area of the back surface of the solar cell. The formation of the Al-BSF layer and the peeling of the Ag pad off from the interface between Al-BSF and Al were analyzed using by a scanning electron microscope. The peeling phenomenon could be removed by the adjustment of the elevation rate to the peak temperature during co-firing process. The external quantum efficiency (EQE) analysis revealed the influence of the full Al-BSF layer on the rear side of the solar cells showing higher values in the long-wavelength region. Additionally, the divided cell strips were interconnected via ECA bonding, and the characteristics were investigated. The interconnected cell exhibited the efficiency of 18.302% while the efficiencies of two cell strips were 18.039% and 18.162%, respectively.

Internal quantum efficiency mapping for evaluation of rear surface of passivated emitter and rear cell

An internal quantum efficiency (IQE) mapping method that can emphasize the effects of local structures on the rear side of solar cells is demonstrated. The IQE of crystalline silicon passivated emitter and rear cells was mapped at 1000 nm and revealed periodic patterns with lower IQE in the local aluminum back surface fields on the rear surface. A larger-scale map indicated an effect that is apparently related to the spin-etching process on the rear surface. The proposed IQE mapping method can provide information that will help identify the factors that limit the efficiency.

Optimal Design of Buried Emitter of EWT Silicon Solar Cells Type by Numerical Simulation

The growth of the photovoltaic industry is based on the reduction of manufacturing costs to make the energy more competitive with fossil fuels. To achieve this aim, we must reduce the production cost ratio on cell efficiency. One way is to make all contacts on rear side of solar cells to reduce the shadowing effects. One of the most recent proposed technologies is the emitter wrap through. This structure has an emitter on the front but all contacts are on the rear side. The interconnection of the emitter between the front and the rear is accomplished through holes in the substrate. These holes, usually buried with a laser, are diffused and thus allow ensuring the collection of the holders from the front face until to the rear one. This concept increases also the lateral surface of the junction.In this paper, the simulation under Atlas/Silvaco environment was used to optimize the most important geometrical and physical parameters of EWT silicon solar cells. The main difficulty was the large number of parameters affecting the performance of this type of structure. However, our study focuses on the substrate, the emitter, the back surface field (BSF) and the metal contacts. Our numerical simulation code allows to monitoring the optimal design of EWT structure. The conversion efficiency can reach a value of 18.6% by considering a p-type multi- crystalline silicon substrate with a minority carrier lifetime in order of 10-6s.

Laser-welded Interconnection of Screen-printed Si Solar Cells

We demonstrate the laser welding of Al interconnects to the BSF rear-side of screen-printed two-side-contacted solar cells. The Al paste on the rear side of solar cell is laser-welded to an Al foil. This reduces the silver consumption of the solar cells by making silver pads on the rear side obsolete. Our proof-of-concept modules are free of laser damage. A 3-cell-module from 6” solar cells shows no change in fill factor within the statistical measurement uncertainty after artificial aging in 500 humidity-freeze cycles.

Hexagonal sphere gratings for enhanced light trapping in crystalline silicon solar cells.

Enhanced absorption of near infrared light in silicon solar cells is important for achieving high conversion efficiencies while reducing the solar cell's thickness. Hexagonal gratings on the rear side of solar cells can achieve such absorption enhancement. Our wave optical simulations show photocurrent density gains of up to 3 mA/cm2 for solar cells with a thickness of 40 µm and a planar front side. Hexagonal sphere gratings have been fabricated and optical measurements confirm the predicted absorption enhancement. The measured absorption enhancement corresponds to a photocurrent density gain of 1.04 mA/cm2 for planar wafers with a thickness of 250 µm and 1.49 mA/cm2 for 100 µm.

Role of a‐Si:H bulk in surface passivation of c‐Si wafers

AbstractThe low thermal stability of hydrogenated amorphous silicon (a‐Si:H) thin films limits their widespread use for surface passivation of c‐Si wafers on the rear side of solar cells. We show that the thermal stability of a‐Si:H surface passivation is increased significantly by a hydrogen rich a‐Si:H bulk, which acts as a hydrogen reservoir for the a‐Si:H/c‐Si interface. Based on this mechanism, an excellent lifetime of 5.1 ms (at injection level of 1015 cm–3) is achieved after annealing at 450 °C for 10 min. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)