Budi Tjahjono

Rapid Stabilization of High-Performance Multicrystalline P-type Silicon PERC Cells

Light-induced or, more broadly, carrier-induced degradation (CID) in high-performance multicrystalline silicon (TIP mc-Si) solar cells remains a serious issue for many manufacturers, and the root cause of the degradation is still unknown. In this paper, the impact of firing temperature on the stability of lifetime test structures is investigated, and it is found that substantial CID can be triggered if peak temperatures exceed approximately 700 °C. We then investigate two pathways to stabilize the performance of industrially produced TIP mc-Si passivated emitter rear contact cells which have been fired at CID-activating temperatures (~740 °C-800 °C) currently required for silver contact formation. The first is a fast-firing approach, whereby it is demonstrated that an additional firing step at a reduced temperature after cell metallization can suppress the extent of Voc degradation by up to 80%. The second approach is the accelerated degradation and subsequent recovery of carrier lifetime through the use of high-intensity illumination during annealing at elevated temperatures. A 30 s process is found to suppress the maximum extent of degradation in Voc by up to 60% and up to 80% for longer processes. Ultimately, the results suggest that a combined approach of fast-firing and a high-intensity-illuminated anneal could achieve the best results in terms of Voc, stability.

18.5% laser-doped solar cell on CZ p-type silicon

For many years, the selective emitter approach has been well-known to yield cell efficiencies well above those achieved by conventional screen-printed cells. A simple and effective way of forming a selective emitter can be achieved by laser doping to simultaneously pattern the dielectric with openings as narrow as 8 µm, and create heavy doping beneath the metal contacts. In conjunction with laser doping, light-induced plating (LIP) is seen as an attractive approach for forming metal contacts on the laser-doped regions, without the need for aligning masks or other expensive, long laboratory processes. As laser-doping is gaining increasing interests in the PV industry, selection of the most appropriate laser and processing conditions is important to ensure high yields in a production environment. In this work, we have identified a suitable laser that enables good ohmic contacts for a wide range of laser scan speeds. Sheet resistances of laser-doped lines as low as 2 ohms/sq was achieved at a scan speeds of <1 m/s, while a sufficiently high doping (∼20 ohms/sq) is still achievable at scan speeds up to 6 m/s. Optimization of the laser parameters in this work lead to a cell efficiency of 18.5% being achieved with the laser-doped selective emitter (LDSE) structure. The cell also has an excellent pseudo fill factor (pFF) of 82.3% and a local ideality factor n nearing unity. This indicates there is minimal laser-induced damage and junction recombination as a result of the laser doping process.

Photoluminescence imaging for determining the spatially resolved implied open circuit voltage of silicon solar cells

Photoluminescence imaging has widely been used as a characterisation tool for the development of silicon solar cells. However, photoluminescence images typically only give qualitative information due to the presence of an unknown calibration constant. In this work, quasi-steady-state photoconductance measurements on partially processed solar cells and I-V measurements on finished solar cells are used to determine the calibration constants to yield spatially resolved implied open circuit voltage images. This technique is then applied to determine the implied open circuit voltage of laser doped selective emitter solar cells at various stages of cell fabrication after the formation of the full area aluminium back surface field when other characterisation techniques such as photoconductance cannot be used.

Rear junction laser doped solar cells on CZ n-type silicon

N-type silicon (Si) has been shown to have generally higher bulk lifetimes and far better post illumination performance stability compared to boron doped p-type materials of similar crystallographic quality. In particular, the high minority carrier diffusion lengths in n-type wafers makes the rear emitter n+np+ structure an attractive option, especially when incorporated with screen printing as a simple and cost effective way to create an Al-alloyed junction on the back surface. However, when screen printing is used to apply the front contacts, its wide metal lines and its requirement for a heavy Phosphorus-doped front surface significantly reduces the performance of this simple device with the latter limiting the blue wavelength response and surface passivation quality. Recently, the laser doping process has been shown capable of overcoming these major drawbacks due to its ability to produce a selective emitter. In this present work, an innovative application of the laser doping process in the fabrication of such rear Al-alloyed emitter n+np+ device enables an excellent energy conversion efficiency of 18.2% to be achieved on commercial grade n-type CZ wafers (148.6cm2).

19.2% efficiency n-type laser-grooved silicon solar cells

N-type silicon wafers have been found to have higher bulk lifetimes compared to those of boron-doped p-type silicon wafers with the same resistivity, and proved to have no light-induced degradation associated with the boron-oxygen complex. In this paper, the laser-grooved buried contact (BC) technology was used to fabricate interdigitated backside contact (IBC) solar cells on the phosphorus-doped silicon wafers. Three obstacles that hindered the performance of n-type interdigitated backside buried contact (IBBC) solar cells - parasitic shunt resistance, metallization issues, and optimization of the diffused regions - are discussed. 19.2% efficiency has been achieved on the n-type IBBC solar cell, and all BC solar cells made on the phosphorus-doped n-type silicon wafers, regardless of the crystalline growth techniques, show no light-induced degradation.

New Emitter Design and Metal Contact for Screen-Printed Solar Cell Front Surfaces

A new emitter design incorporating semiconductor fingers has been developed for use with conventional screen-printing technology. The primary advantage of this new design is that it alleviates the need for a heavy top surface diffusion and therefore overcomes the major weaknesses traditionally associated with screen-printed solar cells, namely a poor response to short wavelengths of light and a high top surface metal shading loss. Importantly, the new technology is compatible with all existing screen-printing equipment and infrastructure. Direct comparison between conventional screen-printed cells and those incorporating semiconductor fingers shows the latter to have about a 10% performance advantage when fabricated on a large scale manufacturers production line. Efficiencies about 18% have been demonstrated on 150cm2 devices with the new technology planned for large scale manufacturing by the end of 2006

Investigation of Al-Doped Emitter on N-Type Rear Junction Solar Cells

This brief models the junction discontinuities of a rear Al-doped p⁺ emitter (np⁺) formed by screen printing and firing. Theoretical fitting of the suns-<i>V</i> _oc data to the circuit model shows that not only do the junction discontinuities deteriorate cell <i>V</i> _oc, for the case of p-type cells, but they also reduce cell fill factor on n-type cells through increased junction recombination and nonlinear shunts.

A method to characterize the sheet resistance of a laser doped line on crystalline silicon wafers for photovoltaic applications

A theory is presented that correlates the different sheet resistance (Rsh) values of the same phosphorus laser doped (LD) line approximated by two different methods: the LD box and transfer length measurement (TLM) methods. By modeling the LD line junction profile, an effective Rsh value using the LD box method is obtained and used to derive the Rsh upper limit (Rsh.UL) of the LD line. This value matches within ±10% of the Rsh.UL value obtained using the TLM method across four lasing speeds. Subsequently, a LD box method is introduced to determine the LD line Rsh.UL easily without modeling work.

The influence of silicon nitride layer parameters on the implied Voc of CZ silicon wafers after annealing

The passivation potential of PECVD SiNx deposited on undiffused p-type Si surfaces is investigated. The influence of post-deposition annealing temperature and the film parameters (refractive index and thickness) on the implied Voc of textured, commercial grade, p-type CZ wafers was studied. Improvement in the implied Voc values of SiNx passivated CZ wafers was observed for two different SiNx films for all annealing temperatures in the range of 600–820°C. Excellent implied VOC values above 700 mV achieved on these wafers indicate that this process can be potentially used as rear passivation for various commercial cell technologies. Initial results on the use of this process in the fabrication of the new Double Sided Laser Doped solar cells structure demonstrate this potential.

Improved adhesion for plated Solar cell metallisation

Experts predict that copper plated metal contacts will eventually become the dominant metallisation for silicon wafer-based technologies once several key issues are solved. Of particular importance is the adhesion strength and hence durability of such plated contacts with many of the largest cell manufacturers currently nervous about considering such metallisation for large-scale manufacturing due to concerns in this area. A new approach for enhancing the adhesion strength for plated contacts involves establishing laser-machined anchor points in the silicon surface which when plated act to enhance both the ohmic contact and the mechanical adhesion strength for the metallisation. Cells manufactured on a production line using this innovation typically lose 0.1% in efficiency in absolute terms relative to identically manufactured cells that do not use the anchor points. BP Solars Saturn cells using a similar approach have demonstrated that such plated contacts are at least as durable if not more durable than screen-printed contacts installed at the same time 20 years ago.