Local Back Surface Field Research Articles

Recombination at local aluminum-alloyed silicon solar cell base contacts by dynamic infrared lifetime mapping

The application of local aluminum (Al)-alloyed contacts to the p-type base of silicon solar cells reduces minority charge carrier recombination due to the formation of a local back surface field (LBSF). We study the recombination properties and formation of base contacts, which are realized by local laser ablation of a dielectric stack (laser contact opening - LCO) and subsequent full area screen printing of Al paste. Based on charge carrier lifetime measurements using the camera-based and calibration-free dynamic infrared lifetime mapping (ILM) technique, we determine contact recombination velocities at the contacts as low as Scont = 65 cm/s on 200 Ωcm float-zone silicon (FZ-Si) and corresponding reverse saturation current densities of J0,cont = 900 fA/cm² on 1.5 Ωcm FZ-Si. As a result we show that local contact geometries with point contact radii r > 100 µm and line contact widths a > 80 µm are appropriate for lowest contact recombination employing local Al alloyed contacts. Furthermore, complete and high quality laser ablation of the dielectric stack is necessary for the formation of a sufficiently thick LBSF. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SiliconPV 2011. local back surface field; carrier lifetime; laser ablation; silicon solar cells

Analysis of local Al-doped back surface fields for high efficiency screen-printed solar cells

In this paper, we investigate the surface recombination of local screen-printed aluminum contacts applied to rear passivated solar cells. We measure the surface recombination velocity by microwave-detected photoconductance decay measurements on test wafers with various contact geometries and compare two different aluminum pastes. The aluminum paste which is optimized for local contacts shows a deep and uniform local back surface field that results in Smet = 600 cm/s on 1.5 Ωcm p-type silicon. In contrast, a standard Al paste for full-area metallization shows a nonuniform back surface field and a Smet of 2000 cm/s on the same material. We achieve an area-averaged rear surface recombination velocity Srear = (65 ± 20) cm/s for line contacts with a pitch of 2 mm. The application of the optimized paste to screen-printed solar cells with dielectric surface passivation results in efficiencies of up to 19.2 % with a Voc = 655 mV and a Jsc = 38.4 mA/cm² on 125×125 mm² p-type Cz silicon wafers. The internal quantum efficiency analysis reveals Srear = (70 ± 30) cm/s which is in agreement with our lifetime results. Applying fine line screenprinting, efficiencies up to 19.4 % are demonstrated.

Physical properties of industrial 19% rear side passivated Al-LBSFR- solar cells

Abstract With the objective of high efficiency solar cells rear side passivation with local aluminium back surface field and dielectric reflection (Al-LBSFR) were developed at Centrotherm and cell efficiencies of 19.4% were achieved. In this paper major physical properties of the developed technology, named centaurus technology, are presented. A low reflection on the solar cell front side is achieved by a textured surface, which enables in particular an inclined light entry at the solar cell front side, which is mandatory for a total light reflection on the solar cell dielectric rear side. Likewise a smooth instead of a textured rear side surface was more suitable for a high rear reflection and also more appropriate for better rear surface passivation. In addition, the influence of the rear side local BSF and metallization fraction was identified as a possible Voc limitation in combination with the front emitter saturation currents. Lower temperature coefficients of the cell power output or cell efficiency and of the open circuit voltage were evaluated for Al-LBSFR cells.

Amorphous silicon carbide heterojunction solar cells on p-type substrates

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO 2 passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow ( PH 3 ) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current–voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH 3 -flow as pure a-Si:H(n).

Industrially Feasible Rear Passivation and Contacting Scheme for High-Efficiency n-Type Solar Cells Yielding a $V_{\rm oc}$ of 700 mV

n-Type solar cells with passivated rear surface and point contacts have been proven to have an enormous efficiency potential. However, an industrially feasible process for the realization of the passivated locally contacted rear side of this solar cell type is still missing. Therefore, a rear passivation scheme based on doped amorphous silicon carbide was investigated. The newly developed PassDop layer results in excellent surface passivation and, at the same time, acts as a doping source. After the PECVD of the PassDop layer, contact points are locally opened by a laser pulse, and simultaneously, a local back surface field is formed using the phosphorus contained in the layer. In the last step, the rear side is contacted by the evaporation of aluminum. Due to the very effective passivation of the rear side by the doped passivation layer as well as the excellent contact formation by the laser process, the best cell (aperture area of 4 cm2) exhibits an open-circuit voltage of 701 mV and a fill factor of 80.1%, resulting in a confirmed solar cell efficiency of 22.4%.

Laser fired back contact for silicon solar cells

To get high efficiency c-Si solar cells reduction of surface recombination losses and good surface passivation and/or Back Surface Field (BSF) formation are needed. Most industrial solar cells are made covering the back area with screen-printed Al, forming an Al–BSF upon firing step, with a Back Reflectance of 65% and a Back Surface Recombination Velocity (BSRV) of 1000 cm/s on 1 Ωcm Si wafer. Simulations reveal that PV efficiency can increase up to 18% after improving the BSRV to ≤ 200 cm/s and the BR to > 95%. The aim of this work is to get these goals by a laser fired back contact with low temperature passivation of the remainder of the back. This can be obtained by a double layer of PECVD Amorphous Silicon and Silicon Nitride, on which a spin-on Boron dopant layer is deposited. The structure is completed by 2 µm thick e-beam evaporated Al. The formation of an improved local BSF is obtained using a Nd:YAG pulsed laser, which promotes an Al & B simultaneous diffusion trough the passivation layers. Several cells, using this structure, have been fabricated on different substrates. By fitting procedure of cell Internal Quantum Efficiency we have extracted several parameters as surface recombination velocity, diffusion length and internal reflection that are comparable with the state of art of the cells having effective back surface field.

Study of electrical properties of oxidized porous silicon for back surface passivation of silicon solar cells

Back surface passivation becomes a key issue for the silicon solar cells made with thin wafers. The high surface recombination due to the metal contacts can be lowered by reducing the back contact area and forming local back surface field (LBSF) in conjunction with the passivation with dielectric layer. About 3 × 10 - 7 m thick porous silicon (PS) layer with pore diameter mostly of 1 × 10 - 8 – 5 × 10 - 8 m was formed by chemical etching of silicon using the acidic solution containing hydrofluoric acid (HF), nitric acid (HNO 3) and De-ionized water in the volume ratio 1:3:5 at 298 K for which etching time was kept constant for 360 s. Electrical properties of oxidized PS was studied through the current–voltage ( I – V ) and capacitance–voltage ( C – V ) characteristics of the metal–insulator–semiconductor (MIS) device in which the oxidized PS was used as an insulating layer and the results were further analyzed. The C – V curves of all the studies MIS devices showed the negative flatband voltage varying from - 2 to - 5 V , confirming that the oxidized layer of PS has fixed positive charge.

Gettering by heat thermal processing: application in crystalline silicon solar cells

The aim of this work is to getter unwanted impurities from solar grade crystalline silicon (Si) wafers and then to enhance their electronic properties. This was done by forming a sacrificial porous silicon (PS) layer on both sides of the Si wafers and by performing infrared (IR) thermal annealing treatments (at around 950 °C) in a SiCl 4/N 2 controlled atmosphere. The process allows concentrating unwanted impurities in the PS layer and near the PS/silicon interface. These treatments reduce the resistivity by about two orders of magnitude at a depth of about 40 μm and improve the minority carrier diffusion length from 75 to 210 μm. This gettering method was also tested on silicon wafers where grooved fingers and back contacts were achieved using a chemical vapor etching (CVE) method. Front buried metallic contacts and small holes for local back surface field were then achieved after the gettering stage in order to realize silicon solar cells. It was shown that the photovoltaic parameters of gettered silicon solar cells were improved as regard to ungettered ones.

Optimization of the rear contact pattern of high‐efficiency silicon solar cells with and without local back surface field

AbstractThe influence of the point spacing and size on the cell efficiency is studied for different silicon solar cell structures with local rear contacts: the PERC (passivated emitter and rear cell) with its high recombination at the rear contacts and the LBSF (local back surface field) or PERL (passivated emitter and rear locally diffused) cell with reduced combination at the rear contacts due to a diffused high‐low junction (or LBSF) beneath the contacts. Float zone materials of different resistivities have been investigated.The experimental results are explained by three‐dimensional finite difference simulations for the open‐circuit voltage, the short‐circuit current and the fill factor.