University Center of Excellence for Photovoltaics
University Center of Excellence for Photovoltaics
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ItemComparison of Front and Back Surface Passivation Schemes for Silicon Solar Cells(Georgia Institute of Technology, 1998-07) Moschner, J. D. ; Doshi, P. ; Ruby, D. S. ; Lauinger, T. ; Aberle, A. G. ; Rohatgi, AjeetThis work presents a comprehensive study on fast, low-cost methods for the electronic passivation of the phosphorus-diffused front surface and the non-diffused p-type rear surface of crystalline Si solar cells. Titanium dioxide is compared with rapidly-grown thermal oxide (RTO) and PECVD silicon nitrides from three different laboratories. Double layers of RTO and Ti02 or SiN are also investigated. We demonstrate that SiN and RTO single layers can provide very good passivation on both the front and back surface of solar cells. It is also shown that double layers consisting of a thin RTO film and silicon nitride can improve the passivation quality of most SiN layers, and enhance the stability under thermal treatment. With the proper choice of RTO, SiN, and thermal treatment, excellent surface recombination velocities on the back as well as very low emitter saturation currents can he reached using these fast, industrially feasible methods. All films used also provide or are compatible with a good antireflection coating of the cell surface.
ItemSelf-Doping Contacts and Associated Silicon Solar Cell Structures(Georgia Institute of Technology, 1998-07) Meier, D. L. ; Davis, H. P. ; Shibata, A. ; Abe, T. ; Kinoshita, K. ; Bishop, C. ; Mahajan, S. ; Rohatgi, Ajeet ; Doshi, P. ; Finnegan, M.Contacts to <111> Si which are self-doping and self-aligning were investigated. Such contacts are applicable both to conventional cell structures as selective emitters and to more demanding structures such as interdigitated back contact cells. Emphasis was placed on alloyed contacts of AI for providing a self-doping p-type contact and of Ag-Sb for a self-doping n-type contact. Alloying at 900°C of 1.1% (wt.) Sb in Ag doped Si to a value of 2 x 10 (18) Sb/cm(3), suggesting a 5% (wt.) Sb is needed for ohmic contact. An AI alloy p-n junction was found to be suitable for a solar cell if placed at the back of the cell, with 13.2% efficiency and good IQE demonstrated for a fully screen-printed dendritic web cell. A prototype interdigitated back contact cell was fabricated by screen printing (AI and Ag) with tight alignment (100 11m lines and spaces) on a dendritic web substrate with an efficiency of 10.4%.
ItemEffective Passivation of the Low Resistivity Silicon Surface by a Rapid Thermal Oxide/PECVD Silicon Nitride Stack and Its Application to Passivated Rear and Bifacial Si Solar Cells(Georgia Institute of Technology, 1998-07) Rohatgi, Ajeet ; Narasimha, S. ; Ruby, D. S.A novel stack passivation scheme, in which plasma silicon nitride (SiN) is stacked on top of a rapid thermal SiO(2) (RTO) layer, is developed to attain a surface recombination velocity (S) approaching 10 em/s at the 1.3 Ω-cm p-type (l00) silicon surface. Such low S is achieved by the stack even when the RTO and SiN films individually yield considerably poorer surface passivation. Critical to achieving low S by the stack is the use of a short, moderae temperature anneal (in this study 730°C for 30 seconds) after film growth and deposition. This anneal is believed to enhance the release and delivery of atomic hydrogen from the SiN film to the Si-Si0(2) interface, thereby reducing the density of interface traps at the surface. Compatibility with this post-deposition anneal makes the stack passivation scheme attractive for cost-effective solar cell production since a similar anneal is required to fire screen-printed contacts. Application of the stack to passivated rear screen-printed solar cells has resulted in V(oc)'s of 641 mV and 633 mV on 0.65 Ω-cm and 1.3 Ω-cm FZ Si substrates, respectively. These V(oc) values are roughly 20 mV higher than for cells with untreated, highly recombinative back surfaces. The stack passivation has also been used to form fully screen-printed bifacial solar cells which exhibit rear-illuminated efficiency as high as 11.6% with a single layer AR coating.
ItemDetermining the Sufficiency of Standard Protective Relaying for Islanding Prevention in Grid-Connected PV Systems(Georgia Institute of Technology, 1998-07) Begović, Miroslav ; Ropp, M. E. ; Rohatgi, Ajeet ; Pregelj, AleksandarRecently there has been a resurgence of concern about islanding of grid-connected photovoltaic (PV) systems. This condition occurs when the PV system continues to energize a section of the grid after that section has been isolated from the main utility voltage source. Generally, islanding is undesirable for a number of reasons, and it is therefore important that PV systems incorporate methods to prevent islanding. However, PV systems are in general already required to be equipped with standard protective relays, namely over/undervoltage relays (OYR/UYR) and over/underfrequency rclays (OFR/UFR), which disconnect the PV system from the utility system in the event that either the magnitude or frequency of the PV system's terminal voltage deviates beyond certain thresholds. These relays will prevent islanding in the majority of cases. In fact, it has been argued that the probability of an islanding event that would not be detected by these relays is insignificant. The purpose of this paper is to investigate the probability of occurrence of the conditions which could lead to failure of the OVR/UVR and OFR/UFR to detect islanding using three methods, and thereby to determine whether the standard protective relays provide sufficient islanding protection.