Advanced Rear-Side Contact Schemes on i-PERC (Industrial Passivated Emitter and Rear Cell) Solar Cells (Geavanceerde achterzijde contacterings- en passivatieschema's voor hoog-efficiënte industriële kristallijn Si-zonneceltechnologie)

For many years, the photovoltaic industry has been using Al to form the back contact of crystalline silicon solar cells. The objective of this PhD is to reach a better understanding of the contact formation at the rear side of the Passivated Emitter and Rear Cell (PERC) type solar cell. This type of cell features at the rear side a stack of dielectrics as a passivation layer which is pre-opened in order to allow the subsequent metallization step. The way this dielectric stack is opened, by laser ablation, the deposition of the metal, with various techniques, and the high temperature step required for the formation of the Al-Si alloy which serves as a contact, will be investigated in this work.It will be demonstrated that Si dissolution in Al plays an important role in the formation of the contacts, giving on one hand the needed dopants to create an effective Back Surface Field (BSF) layer, and on the other hand forming a deep pyramidal shape in the contact region which might affect the reflection of the light inside in the cell, and increase the surface recombination velocity of the device.A novel method to characterize the local Al-Si alloy formation is introduced, allowing the in-situ observation of the high temperature step for the contact formation. By doing this, details of the process which cannot be taken into account once the device is finished, are studied and explained through hypotheses involving the phase diagram between both elements.The performance at cell level of the different combination of parameters affecting the formation of the BSF region has been evaluated. For this, changes in contact pitch, firing temperature and profile, Al thickness and composition are studied for the PERC cells.At the end, the rear reflectance loss mechanisms are also investigated by altering the dielectric layers used for passivation, together with the study of the effect of Si incorporation during the firing step, showing that this Si presence at the back side is the main responsible for the loss observed.Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i Samenvatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii List of abbreviations and symbols . . . . . . . . . . . . . . . . . . v 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Working principle of a solar cell . . . . . . . . . . . . . . . . . 4 1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Recombination mechanisms in solar cells . . . . . . . . . . 7 1.3.1 Radiative recombination . . . . . . . . . . . . . . . . . . . . 8 1.3.2 Auger recombination . . . . . . . . . . . . . . . . . . . . . . 9 1.3.3 Bulk recombination through defects . . . . . . . . . . . . 9 1.3.4 Surface recombination through defects . . . . . . . . . . 11 1.3.5 Emitter recombination . . . . . . . . . . . . . . . . . . . . . 12 1.4 PERC cells and back surface field principle . . . . . . . . 13 1.4.1 Local Al-BSF (PERC) . . . . . . . . . . . . . . . . . . . . . . 13 1.4.2 Full Al-BSF . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1.5 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 2 Al-Si alloys and BSF formation . . . . . . . . . . . . . . . 21 2.1 Introduction to Al-Si alloys . . . . . . . . . . . . . . . . . . . . . . . 21 2.2 Phase diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.3 Laser ablation process . . . . . . . . . . . . . . . . . . . . . . . . . . 27 2.4 Alloy process formation . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.4.1 Full Al-BSF . . . . . . . . . . . . . . . . . . . . . . . . 31 2.4.2 Local Al-BSF . . . . . . . . . . . . . . . . . . . . . . . 35 2.5 Si distribution in Al after firing . . . . . . . . . . . . . . . . 43 2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3 Local Al-BSF formation by in-situ observation . . . . . . . . . . . . . . . 47 3.1 In-situ observation of the contact formation . . . . . . . . . 47 3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.3 Experimental details . . . . . . . . . . . . . . . . . . . . . . . 50 3.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 4 Impact of the BSF on the performance of PERC cells 59 4.1 Metallized contact fraction . . . . . . . . . . . . . . . . . . . 59 4.1.1 Pyramidal shaped contacts . . . . . . . . . . . . . . 59 4.1.2 Irregular-shape contacts . . . . . . . . . . . . . . . . 70 4.2 The effect of the temperature . . . . . . . . . . . . . . . . . 74 4.3 Behavior upon cooling . . . . . . . . . . . . . . . . . . . . . . 79 4.4 Impact of the metallization material . . . . . . . . . . . . . 82 4.4.1 Al-Si eutectic PVD . . . . . . . . . . . . . . . . . . . 82 4.4.2 Screen printed Al . . . . . . . . . . . . . . . . . . . . 85 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5 Dielectric degradation. Impact on reflectance . . . . . . . . . . . . 89 5.1 Reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.2 Effect of Al on the reflectance . . . . . . . . . . . . . . . . . 91 5.3 Transmission electron microscopy (TEM) . . . . . . . . . . 94 5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 6 Conclusions and Outlook 99 6.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 6.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Appendix A 103 Appendix B 107nrpages: 149status: publishe

Reduced overhead for intra-cluster and inter-cluster sensor-to-actor communications in IEEE 802.15.4 networks

International audienceThis paper proposes a novel way to enable sensor-to-actor communications in wireless sensor and actor networks (WSANs) that only requires a simple modification to the IEEE 802.15.4 header. Although this standard includes the definition of sensor-to-actor communication, it does not address how to provide it. Our proposal is able to provide this sensor-to-actor communication at the 802.15.4 MAC layer, thereby reducing the overhead of the additional network header, and achieving an important reduction of the energy consumption. In addition, avoiding the need of a network layer with full routing capabilities in the sensors provides further memory, processing and communications saving. Moreover, this paper considers two scenarios for sensor-to-actor communications: intra-cluster and inter-cluster. The former enables the communication among sensors/actors at the same wireless sensor and actor network, and the latter allows sensors/actors located in geographically separated cluster to communicate transparently as if they were in the same WSA

Large-area n-Type PERT solar cells featuring rear p+ emitter passivated by ALD Al2O3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al2O3 and PECVD SiOx. After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al2O3/SiOx stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm2, provided that the Al2O3 thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al2O3/SiOx passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and Voc above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation

The impact of silicon solar cell architecture and cell interconnection on energy yield in hot & sunny climates

Extensive knowledge of the dependence of solar cell and module performance on temperature and irradiance is essential for their optimal application in the field. Here we study such dependencies in the most common high-efficiency silicon solar cell architectures, including so-called Aluminum back-surface-field (BSF), passivated emitter and rear cell (PERC), passivated emitter rear totally diffused (PERT), and silicon heterojunction (SHJ) solar cells. We compare measured temperature coefficients (TC) of the different electrical parameters with values collected from commercial module data sheets. While similar TC values of the open-circuit voltage and the short circuit current density are obtained for cells and modules of a given technology, we systematically find that the TC under maximum power-point (MPP) conditions is lower in the modules. We attribute this discrepancy to additional series resistance in the modules from solar cell interconnections. This detrimental effect can be reduced by using a cell design that exhibits a high characteristic load resistance (defined by its voltage-over-current ratio at MPP), such as the SHJ architecture. We calculate the energy yield for moderate and hot climate conditions for each cell architecture, taking into account ohmic cell-to-module losses caused by cell interconnections. Our calculations allow us to conclude that maximizing energy production in hot and sunny environments requires not only a high open-circuit voltage, but also a minimal series-to-load-resistance ratio