Cell Processing

For the work reported in this paper, a new model of the screen-printing process was set up in order to improve the understanding of the screenprinting process, with a focus on the interaction between Ag paste and the screen.

Organic/inorganic lead halide perovskite solar cells (PSCs) have received global attention because of their excellent photovoltaic performance and ease of fabrication. PSC’s have reached over 24% power conversion efficiency demonstrating that the lead halide perovskites are the most promising class of materials for next-generation thin-film photovoltaics. The unprecedented increase in the device performance from 3.8% to 24% in less than 10 years is mostly due to compositional engineering of mixed cations, and anions, as well as improved processing protocols has made PSC the fastest development of a new material in the PV field. Though the efficiencies on Lab scale are staggering, the full potential of this burgeoning technology cannot be realized without addressing the following challenges: fighting the degradation of the material is the highest focus for the moment and has several fronts for improvements including within larger scale cells or modules. The other main challenge of the new class of material is the toxicity risks due to the presence of lead. The research community is actively working on the mitigation and reduction of the associated risks. The exceptional properties of this material combined alongside its inherent relative lower costs have already triggered the interests of industries and start-up worldwide while on a European regional level, EPKI for European Perovskite Initiative was formed gathering all the significant players in the field.

This paper presents the calibration of solar cells, in accordance with the IEC 60904 standards, carried out at the solar cell calibration laboratory of the Calibration and Test Center (CalTeC) at the Institute of Solar Energy Research Hamelin (ISFH). For the calibration of a solar cell, the cell area, the spectral responsivity (SR) and the current–voltage (I–V) curve have to be determined. The I–V curve then yields the characteristic parameters, including the power conversion efficiency, fill factor, short-circuit current and opencircuit voltage. The required measurement facilities and contacting stages are explained in detail; in addition, the measurement procedures are introduced. The precision and accuracy of the resulting characteristic parameters and curves are demonstrated by recent intercomparisons between different international calibration laboratories.

Passivated emitter and rear cell (PERC) solar cell design is the industry standard for high-volume solar cell manufacturing today. The next challenge for the PV industry is to find a low-cost cell upgrade technology platform that can be easily retrofitted in existing production lines to modify the front side and enhance the rear. The monoPolyTM technology platform, developed at SERIS together with its strategic industry partners, offers an attractive solution and paves the way for the adoption of passivating contacts in large-scale manufacturing. This platform requires only one tool upgrade for most PERC/T production lines, has one less process step than a standard PERC production process, and yields a +1%abs. efficiency boost over a standard PERC process. The authors believe that monoPoly will enable the PV industry to mass produce cells with efficiencies exceeding 24% in their existing lines in the near future.

Solar simulators are among the most important and fundamental measurement tools in photovoltaic production facilities as well as in R&D labs. Two major solar simulator technologies can be distinguished: xenon light sources and, more recently, light sources using light-emitting diodes (LEDs). While xenon solar simulators are a well-established technology, LED-based systems appear to be promising candidates for future applications, as they provide a higher flexibility with regard to the flash times, spectral light composition and intensity. Measurement recipes for power quantification under standard test conditions (STC) can be adapted to high-efficiency cells, which require longer flash times. Furthermore, fast inline spectral testing, such as a rapid external quantum efficiency (EQE) test or a rapid reflectivity test, becomes feasible. However, the development of LED-based systems requires well-designed optical and electronic components to ensure high-precision measurements on the basis of a laterally uniform and temporally stable light field.

Today’s industry-standard B-doped monocrystalline silicon still suffers from light-induced degradation (LID) of the carrier lifetime. Illumination at elevated temperatures leads to a so-called regeneration, i.e. a recovery of both the carrier lifetime and the solar cell efficiency. However, even though the carrier lifetime on test wafers increases from about 1ms after processing to 3ms after regeneration, the corresponding PERC+ cell efficiencies in both states are identical; possible reasons for this discrepancy are discussed in this paper.

Silicon heterojunction (SHJ) solar cells are the archetypes of ‘fullsurface passivating contact’ solar cells; such contacts are required in order to achieve typical open-circuit voltages of up to 730–750mV. Although SHJ technology has fewer manufacturing steps and enables higher efficiencies than standard passivated emitter and rear cell (PERC) technology, the market has been slow in taking it up. This paper discusses some of the obstacles that have been overcome in the last 10 years, and shows why the technology is now readier than ever for a competitive mass-market launch.

Stable high voltages in solar cells and modules are becoming increasingly important as large PV systems are being set up in desert regions and are therefore exposed to high temperatures. High-voltage solar cells have lower temperature coefficients and thus produce a higher energy yield for such PV systems. Standard passivated emitter rear cell (PERC) devices have moderate voltages below 680mV, and also have the risk of degrading in such regions, because of light and elevated-temperature induced degradation (LeTID) effects and, in more recent observations, passivation degradation. This paper presents a solution for PERC producers to easily make the switch to n-type passivated emitter, rear totally diffused (nPERT) solar cells, which are capable of stable efficiencies above 22% and voltages close to 700mV, at almost no additional cost.

SolarWorld has played a pioneering role in triggering and implementing the shift from p-type multicrystalline aluminium backsurface field (Al-BSF) to p-type monocrystalline passivated emitter and rear cell (PERC) as the next mainstream solar cell technology, and recognized PERC to be the door opener to an extremely simple and cost-effective implementation of a bifacial solar cell. This paper reviews PERC technology development at SolarWorld, featuring an industrial baseline process for monocrystalline five-busbar (5BB) p-type PERC solar cells exceeding 22.0% median (22.5% maximum) cell efficiency by May 2018, before operations at SolarWorld came to a final halt.

Silicon heterojunction (SHJ) solar cells demonstrate a high conversion efficiency, reaching up to 25.1% using a simple and lean process flow for both-sides-contacted devices, and achieving a record silicon solar cell efficiency of 26.7% in back-contacted configuration. In addition, the field advantages of SHJ cell technology are a native bifaciality and low thermal coefficient providing impressive energy yield. Finally, the technology demonstrates potential cost reduction as it is perfectly suited for thin wafers integration. The SHJ technology is therefore today triggering strong interest in the PV industry, appearing on the roadmap of different cell manufacturers, with several production lines and pilot lines being installed worldwide. One limiting factor of the technology is related to the metallization: due to temperature restrictions on heterocontacts, the standard firing through silver paste needs to be replaced by low curing temperature paste. This type of pastes yield fingers with higher bulk resistivity (two to three times the one obtained with high temperature cured silver pastes) and lower adhesion after soldering. In this paper, materials, processes and costs figures will be reviewed for the metallization and module integration of SHJ solar cells, with a focus on copper plating benchmarked to silver screen-printing, for varying module interconnection technologies.