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Eén manier om de kosten van elektriciteit opgewekt door PV systemen verder te verlagen, is het verhogen van het zonnecel- en modulerendement. De zeer efficiënte n-Pasha (bifacial n-type) zonnecel is ontwikkeld door ECN en Tempress Systems (beide uit Nederland) en andere partners. N-Pasha werd gedurende 2009/2010 in proefproductie gebracht door Yingli Green Energy, China, ECN en Tempress, en vervolgens sinds 2010 in productie genomen door Yingli. In 2014 werd een productielijn voor tweede generatie n-Pasha zonnecellen geïnstalleerd en gestart bij Mission Solar Energy in de VS. Inmiddels is een 3e generatie n-Pasha in ontwikkeling, en ook achterzijdecontact-varianten van n-Pasha zijn in pilot-productie getest. Al deze generaties n-Pasha hebben als gemeenschappelijke noemer dat productierijpheid en kosteneffectiviteit essentieel waren (en zijn) voor succes

35th European Photovoltaic Solar Energy Conference and Exhibition; 676-680, This paper models and compares the energy yield to be expected from bifacial 2-terminal (2T) and 4-terminal (4T) hybrid perovskite/crystalline silicon tandem modules. Following the work by Asadpour et al. the comparison includes 2T tandem cells employing a thick perovskite absorber layer with standard band gap, which in monofacial operation have significant mismatch of short circuit currents of top and bottom cell, and therefore low efficiency. The perovskite stack properties are taken from the current Solliance high-performance semi-transparent perovskite cells, and the c-Si model represents current high-efficiency industrial nPERT cells. As shown by Asadpour et al., the combination of thick absorber and standard band gap with bifacial operation is a promising approach for 2T bfiacial tandem modules, resulting in higher performance than the c-Si bottom cells. We find that such a 2T bifacial tandem module should be able to outperform even an optimized 2T module with wide band gap perovskite. Additionally, employing a planar c-Si front side in combination with textured module glass can potentially perform nearly as well as employing a textured c-Si front side. These results support the feasibility of important simplifications for production of 2T tandem modules, as long as these will be operated bifacially. The dependence of energy yield of 2T bifacial tandems on albedo is qualitatively different from that of 4T and single junction c-Si bifacial modules. This reduces the competitiveness of 2T bifacial tandems, compared to 4T and SJ c-Si, with increase of the albedo of the environment.

The purpose of the work is to investigate high-efficient and stable 4-terminal perovskite/c-Si tandem devices. Hybrid tandems of c-Si bottom cells and perovskite top cells are perhaps the most promising candidates to reduce the levelized cost of electricity by increasing the overall tandem cell efficiency. To turn this promise in a commercial success, perovskite/c-Si tandem devices need to fulfill at least the following requirements: matching lifetimes between the perovskite top cell and the bottom c-Si cell, i.e. 25 years; high near-infrared transparency of top cell, scalable process steps and low cost of materials

Two factors have coincided to stimulate the recent spur in interest for hybrid tandem PV technology based on crystalline silicon – the fact that balance-of-system (BOS) costs are increasingly dominating turnkey system costs, which strengthens the effect of high efficiency in reducing Wp costs, and the discovery of perovskite solar cells as a promising low-cost wide-band-gap partner for crystalline silicon (c-Si). This paper presents the progress and analysis of four-terminal (4T) perovskite/c-Si tandem technology at ECN part of TNO, with perovskite technology development carried out within the Solliance research organization. Tandem cell optimization and optical loss analysis are presented, with the combination of high efficiency and high near-infrared (NIR) transmittance of the perovskite cell resulting in an experimental tandem efficiency of 26.3%. An outlook is offered for loss reduction and efficiency increase. Upscaling and interconnection of the perovskite cell are crucial aspects for industrialization, and the status at Solliance is briefly described. High-end c-Si bottom cells are then compared with mainstream industrial cells; how industrial PERC and nPERT cells can be optimized for tandem application is described, as well as even making them suitable for two-terminal tandem application, through the application of polysilicon (polySi) passivated contacts. Finally, this paper looks at the cost of tandem versus single-junction (SJ) c-Si systems, and shows that the recent literature on perovskite module manufacturing cost is consistent with a potential cost advantage of tandem devices.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 1176-1180, The combination of a semi-transparent, wide band gap perovskite solar cell (ST PSC) with a crystalline silicon (c-Si) bottom cell in a tandem configuration is considered a promising approach to enhance solar cell efficiencies over the current single junction c-Si cell efficiencies. In this study, we use a simple model based on experimental EQEs and IV-parameters, to predict the 4-terminal tandem efficiency of advanced industrial c-Si cells of different architectures, combined with an experimental 15% ST PSC with a 1.55eV band gap. In addition, we use this model to study the expected effect of the variation of the band gap of the perovskite absorber on the 4-terminal efficiency which results in a positive outlook towards > 25% tandem efficiencies. Furthermore, we demonstrate an experimental 4-terminal tandem efficiency of 24.1% with a MWT-HIT bottom cell.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 1171-1175, The requirements to achieve cost reduction with perovskite/c-Si tandem PV systems relative to singlejunction (SJ) perovskite or c-Si PV systems are modeled. Specific cases assume a perovskite band gap of 1.7eV in tandem use and 1.55 eV in SJ use, both having the same module efficiency expressed as a fraction of the Shockley- Queisser limit for the band gap. Compared to SJ perovskite PV systems, current and foreseen c-Si cell manufacturing costs are low enough to achieve cost reduction when adding c-Si cells to a perovskite module, if, in particular, additional loss (other than related to SQ limit as a function of band gap) of power conversion efficiency (PCE) of the perovskite module from SJ to tandem can be mostly avoided (it should be less than about 10-20%). Compared to SJ c-Si PV systems, requirements for cost-effective modification to tandem structure are, not suprisingly, a high PCE and low cost for the added perovskite stack. For example, for reasonable parameter assumptions, a semi-transparent perovskite PCE of 16.4% and a stack fabrication cost of about 35 €/m2 would result in 10% cost reduction, the same PCE with a stack fabrication cost of 17€/m2 in 16% cost reduction, for tandem PV systems relative to the c-Si technology. In both tandem-SJ comparisons, if there is a significant increase of module manufacturing cost for, e.g., more expensive junction box, power optimizer, additional encapsulant materials, etc., or balance-of-system costs these should be included in the c-Si cell or perovskite stack costs (translated to the m2 cost), respectively.

Uitgangspunt van dit project was dat achterzijde-gecontacteerde zonnecellen (Fig. 1) zeer geschikt zijn voor toepassing in gebouw-geïntegreerde fotovoltaïsche (BIPV) toepassingen. Dit heeft te maken met o.a. het uniforme aanzicht van zulke modules (Fig. 2), het hoge rendement (meer vermogen per oppervlakte-eenheid), en het mogelijk betere gedrag onder gedeeltelijke beschaduwing, betere temperatuurcoefficient, en nog andere mogelijke voordelen. De projectdoelstellingen waren als volgt: 1. Ontwikkeling van achterzijde-gecontacteerde zonnecellen (zowel metal-wrap-through (MWT) type, als interdigitated back contact (IBC) type) voor hoog rendement en industriële toepassing. Rendementsdoelstelling voor de cel 22%. 2. PV moduletechnologie (interconnectie en laminatie) voor 0% cell-to-module vermogensverlies en gereduceerde kosten. Rendementsdoelstelling voor de module 20.5%. 3. Ontwikkeling van gehard dun glas (dunner dan 2.2mm) voor moduleproductie. 4. Ontwikkeling, test, en evaluatie (met terugkoppeling vanuit de markt) van BIPV elementen op basis van achterzijde-gecontacteerde zonnecellen.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 256-261, Simulation results are presented that elucidate the role of the interfacial oxide layer in the passivation and carrier selectivity exhibited by doped poly-Si contact layers. An approach is introduced by which the oxide is represented as a layer with limited charge carrier mobility. This approach is easy to implement in numerical simulations and directly affects the conductivity of minority as well as of majority charge carriers, which determine the carrier selectivity. Our results show that for optimal passivation and carrier selectivity the oxide should not be a perfect tunnel oxide but should show higher effective charge carrier mobility. This means that poly-Si/oxide layers with substantial in-diffusion tails can be used, which has benefits when fire-through metallization is applied.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 460-465, We present a high-performance bifacial n-type solar cell with n+ polysilicon (polySi) back side passivating contacts and fire-through screen-printed metallization, processed on 6" Cz wafers. The cells were manufactured with low-cost industrial process steps yielding a best efficiency of 20.7%, and an average Voc of 674 mV. We analysed effects of variation of doping level, thickness, and oxide properties of the n-type polySi/SiOx layers, as well as hydrogenation from a PECVD SiNx:H coating, which led to recombination current densities down to ~2 fA/cm2 and ~4 fA/cm2 on planar and textured surface, respectively. The results are novel in four aspects: the cells are bifacial, they are full 6" size while employing LPCVD for the polySi deposition, the polySi passivating contact is metallized by fire-through screen printing paste, and hydrogenation is done by PECVD-deposited and fired SiNx:H. Analysis shows that the wafer bulk lifetime in the cell is high and that the Voc of the cell is limited by the Jo of the uniform diffused boron emitter and its contacts. Ways to improve the efficiency of the cell to > 22% are indicated.

31st European Photovoltaic Solar Energy Conference and Exhibition; 829-834, Metal wrap through (MWT) cells and modules offer technology that combines a front junction cell with an all back contact module interconnection offering potential for higher efficiency, improved cost to power ratio, lower electrical cell to module losses, and improved module aesthetics. For the best conversion efficiency, isolation of the vias for the emitter contacts is key to reduce shunting pathways. However, in reverse bias conditions, the vias are also a controlled current pathway for homogenous dissipation of heat, offering new design opportunities for diode free, shade tolerant modules. We have developed a process for via isolation in high efficiency n-MWT cells where the behavior under reverse bias can be tuned with little impact on illuminated I-V curve and efficiency. We find that after the first exposure to reverse bias, the efficiency of these cells stabilizes and further reverse bias soaking or testing has little impact on performance. Further, these cells were processed into single cell laminates for simulated hot spot testing. The laminates with the highest reverse current performed best under all shading conditions (20%-100%) and were in fact the only laminates that survived all shading conditions without breakage, hot spot development, or significant conversion efficiency reduction.

31st European Photovoltaic Solar Energy Conference and Exhibition; 2545-2548, In this paper we have presented an overview of the recent reliability results obtained at ECN for foilbased p-MWT, and novel high-efficiency n-MWT and IBC back-contact modules. As deduced from extensive TC and DH testing on p-MWT modules, two alternative low-cost conductive back sheet foils were shown to improve the module reliability. Exposure to DH of the foil-based modules can cause Cu discoloration. This is observed for EVA and a number of alternative encapsulants, with moisture ingress and some specific interactions between encapsulant and Cu substrate playing a role. The n-MWT modules showed improved resistance to DH exposure as compared to n- Pasha (front-to-back tabbing) modules. TC300 and DH2000 tests were passed for frameless IBC Mercury 2x2 modules built using standard foil-based module manufacture process and standard module materials.

31st European Photovoltaic Solar Energy Conference and Exhibition; 1925-1929, In this paper we investigate new approaches to enhance recovery of valuable materials during the recycling of crystalline Si (cSi) PV modules. The recycling of out-of-specs, damaged or end-of-life cSi PV modules will gradually become more important for PV suppliers and recyclers. Also recycling can help to further reduce carbon and environmental footprint of cSi PV. We tested two approaches to enhance recyclability of frame, glass, and silicon. The research was based on ECN’s conductive back-contact module technology. First, alternative edge sealants, easy to release from the module, were tested on their protection against air and moisture ingress into the module. Several alternatives were established which show comparable protection as the state-of-the-art silicone-based or double-sided adhesive tape edge sealants, but are much more easy to remove. Second, thermoplastic encapsulant was investigated as a method to improve recyclability of PV modules. The thermoplastic encapsulant used in this study results in PV modules with improved resistance to damp heat (DH) conditions, compared to EVA-based modules. The separation of the components (cells, glass, backsheet) in PV laminates with this thermoplastic encapsulant, using a wire saw device at temperatures around 200 degrees C, was demonstrated. This method may allow recovering of intact solar cells out of end-of-life, out-of-specs or damaged PV modules.

29th European Photovoltaic Solar Energy Conference and Exhibition; 642-647, We report on our high efficiency n-type metal-wrap-through (MWT) cell and module technology. In this work, bifacial n-type MWT cells are produced by industrial processes in industrial full-scale and pilot-scale process equipment. The n-type cells benefit from high recombination lifetime in the wafer and bifaciality. When combined with MWT technology, high-power back-contact modules result, which can employ also very thin cells. We will discuss optimization of the MWT contact layout and impact on paste consumption, showing how MWT technology, like multi-busbar technology, can support very low paste consumption while increasing efficiencies at the same time. We report a cell conversion efficiency of 21% (in-house measurement on a not conducting, but reflecting chuck), a significant gain compared to our earlier work. Full-size modules (60 cells) have been made with power exceeding 300Wp and cell-to-module ratios of the different I-V parameters are discussed. Modules from cells with average efficiency of 20.9% are pending. This work shows that low-cost n-type bifacial cells are suitable for industrial high efficiency back-contact technology.

29th European Photovoltaic Solar Energy Conference and Exhibition; 523-527, For cost reduction as well as reduction of environmental footprint of solar cells, it is important to reduce the use of silver (Ag) in metallisation. In this work we report on large reduction of Ag consumption for industrial high-efficiency bifacial n-type cells with front and rear Ag grid, by modest process changes. Both frontand- rear contacted, as well as metal-wrap-through cells, are considered. The approaches explored are, for the front, seeding with a small amount of Ag fire-through ink or paste, and plate the thus formed contact grid with copper (Cu). For the rear we use the same approach, or replace the Ag grid by a Al layer deposted by physical vapour deposition (PVD). Front Ag consumption of 10mg per wafer and rear Ag consumption of 20mg per wafer were demonstrated for inkjet seeding. The front finger width after inkjet seed&plate is about 55m. With screen print or stencil print seeding a significant Ag reduction can be obtained too, but not to the same level as with inkjet printing. The replacement of a rear Ag grid by an Al contact layer deposited by PVD requires optimisation of the rear dielectric in order to obtain good rear internal reflection. We model and experimentally demonstrate that a small modification of the rear dielectric already yields an acceptable reflection. In all experiments we find approximately equal or better efficiency than in the reference process; and also the cost of ownership estimates are favorable. This shows that a better environmental footprint can be well combined with improved production cost.