Cell Processing

Silicon heterojunction (SHJ) solar cell technology is an attractive technology for large-scale production of solar cells with a high conversion efficiency beyond 24%. One key element of SHJ solar cells, contrasting with today’s widespread passivated emitter and rear contact (PERC) cell technology, is the use of transparent conductive oxide (TCO), which poses challenges in performance and costs but also presents opportunities. This paper discusses these aspects and shows the potential for improving cell efficiency at reduced cost by using new TCOs deposited by direct current (DC) sputtering.

In this paper the situation of solar cell production in China is summarized, and an attempt is made to answer the question of whether passivated contacts could replace PERC technology, which will eventually reach its efficiency limit in the future.

Passivated-contact solar cell designs, such as TOPCon or silicon heterojunction solar cells (SHJs), enable cell efficiencies greater than 24%, and are promising candidates for the next revolution in mass production after the passivated emitter and rear cell (PERC). Plated metallization (Ni/Cu/Ag or Cu/Ag) fits well with new constraints on low-temperature processing and the combination of low material costs and highly conductive bifacial metal grids for these types of solar cell.

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.