Technical Papers

This paper presents the main features of imec’s n-PERT (passivated emitter rear totally diffused) cells, which have achieved independently confirmed efficiencies of 22%. A special focus is given to the selective front-surface field formation by laser doping, which – combined with imec’s front-plating sequence and the excellent rear-surface passivation by Al2O3 on the boron-diffused emitters – has enabled very high voltages (close to 685mV) to be realized on large-area n-type Cz material.

Ion implantation offers significant process simplification potential for the fabrication of back-junction back-contact (BJBC) solar cells. First, the number of high-temperature steps can be reduced to one when applying a co-annealing process which includes an in situ growth of a silicon oxide passivation layer. Second, the implanted regions can be patterned in situ by utilizing shadow masks. ISFH's results from evaluating both aspects are reported in this paper. With fully ion-implanted, co-annealed and laser-structured small- area cells, efficiencies of up to 23.41% (20mm x 20mm designated area) have now been achieved. It is shown that the excellent recombination behaviour of 156mm x 156mm BJBC cells patterned in situ implies a potential for realizing efficiencies greater than 23%; however, back-end issues have so far limited the efficiency to 22.1% (full-area measurement). Ion implantation can also be utilized for the doping of BJBC cells with carrier-selective junctions based on polycrystalline silicon. The current status of ISFH's work in this direction is presented.

This paper reports on the status of large-area, 156mm, bifacial, n-type passivated emitter and rear totally diffused (n-PERT) solar cells, which feature full-area homogeneous doped regions on the front and rear sides. The fabrication process includes either two separate gas-phase diffusion processes with sacrificial diffusion barrier layers, or a sophisticated co-diffusion approach, in which a deposited stack of borosilicate glass (BSG) and silicon oxide acts as a dopant source during back-surface field (BSF) formation in a tube furnace. Thus, the co-diffusion approach reduces the number of required high-temperature processes to one, which significantly streamlines the process sequence. It is shown that by implementing two deposition phases during the BSF diffusion process, it is possible to separately control both the depth and the surface concentration of the BSF. The use of a tailored BSG source allows low recombination and specific contact resistance values on both the front and rear sides, resulting in peak conversion efficiencies of 19.9%. A discussion on the recombination at the emitter -metal interface completes the paper, and several paths to driving the conversion efficiency towards 22% are outlined.

In the last few years PV technology has seen continuous improvements, with significant enhancements at the cell and module levels. In addition to the requirement of high efficiency, the long-term reliability of PV modules leads to proposals for innovative module concepts and designs. Meyer Burger has developed a low-temperature wire-bonding technology, known as SmartWire Connection Technology (SWCT), with the aim of offering a cost-effective solution for high-efficiency solar cells while minimizing cell-to-module losses. The introduction of this interconnection design immediately brings new challenges, especially in the selection of an appropriate encapsulant, which must ensure a good processability as well as the required long-term module reliability. The compatibility of the most cost-effective types of encapsulant currently available on the market was analysed in the study reported in this paper. Thermoplastic polyolefin encapsulants with water absorption less than 0.1% and no (or few) cross-linking additives have proved to be the best option for long-lasting PV modules in a glass-glass (GG) configuration. The development of a laminator having a symmetrical structure (two heating plates without any vacuum membrane) has also opened the door to fast lamination processes with cycle times under eight minutes.

The current standards (IEC 61646 and IEC 61730-2, and IEC 62804 draft for c-Si only) are clearly insufficient to guarantee satisfactory long-term stability and energy yield for thin-film modules, given that reports from the field, as well as from laboratory test results (beyond IEC testing), in some cases show significant degradation of IEC-certified modules. Accordingly, thin-film modules can also exhibit degradation effects, such as TCO corrosion and power degradation, because of potential-induced degradation (PID). This paper presents the results obtained for thin-film modules subjected to bias and damp-heat (BDH) conditions in both indoor and outdoor tests. In order to assess module lifetimes for different thin-film technologies with respect to PID, indoor- and outdoor-determined leakage currents are compared and analysed, taking into account weather data and results from accelerated ageing tests. Finally, on the basis of simulations and investigations for different installation locations, module lifetimes are estimated and discussed.

Sophia, a four-year European Commission-funded project to promote coordination across the EU's PV research community came to an end in January. With 20 partners drawn from industry and academia, the project appears to have fulfilled its aims of fostering greater collaboration. But with Europe's PV manufacturing industry facing a dire predicament in the face of competition from Asia, is it too little, too late?

The combination of storage technology with larger scale solar projects is an emerging science. Andy Colthorpe speaks to PV power plant pioneers, Belectric, about a project completed at the end of 2014 to install battery storage at a utility-scale solar installation in Brandenburg.

The effective operation and maintenance of large PV power plants is critical to ensuring these facilities perform optimally. Sara Ver-Bruggen looks at the latest O&M technologies and strategies emerging as plant owners seek to maximise their investment returns.

Maximising production from a PV system is critical, since nearly all of the investment is made prior to system activation. Monitoring of PV systems allows operators to identify any performance or safety problems early so that they can be repaired quickly, thus minimising energy losses. Joshua Stein of Sandia National Laboratories and Mike Green of M.G. Lightning Electrical Engineering discuss some new monitoring strategies that are necessary for expeditiously identifying and locating system faults.

Project name: Jasper solar power project; Location: Near Kimberly; Northern Cape, Province, South Africa; Capacity: 96MWp; Annual Generation: 180,000MWh.