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38th European Photovoltaic Solar Energy Conference and Exhibition; 508-510, To overcome the efficiency limitation of single-junction Si solar cells, tandem structures can be made with a top cell based on perovskite materials, benefitting from their low processing cost, versatility in bandgap tuning and high performance. In 2-terminal (2T) silicon/perovskite (Si/PK) tandem devices controlling the recombination between the two sub-cells is critical to the overall performance of the tandem. Here, we present improved open circuit voltage for a PTAA based recombination junction via double-layered hole transporting stack. Tandem devices based on a dual-cation (2C) perovskite Csy(MAxFA1-x)1-yPb(I1-xBrx)3 with 1.6 eV bandgap in the top cell and silicon heterojunction bottom cell with open-circuit voltages above 1.8 V and 24.6 % power conversion efficiency with 1 cm2 monolithic active area have been obtained with reproducible results.

36th European Photovoltaic Solar Energy Conference and Exhibition; 516-521, Degradation of the Fill factor (FF) of p+/n silicon heterojunction solar cells (SHJ) is mainly caused by the presence of two thermionic barriers in the cell. The first barrier occurs at the hetero-interface between intrinsic a-Si:H(i) spacer and the n-type crystalline silicon substrate (c-Si(n)). The second is the Schottky barrier that occurs at the front contact interface between the p-type doped amorphous silicon (a-Si:H(p+)) and the transparent conducting oxide (TCO). It is shown that it is possible to distinguish which of either barrier is responsible for the cell performance degradation by carefully inspecting the I-V characteristics under illumination. A clear electro-physical interpretation is given for each situation for cells with different doping levels in a-Si:H supported by the results of numerical simulations carried out by means of AFORS-HET device simulator. An experimental case study of real SHJ cells with different a-Si:H doping levels is used to confirm the proposed interpretations.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 765-767, We demonstrate solar cell fabrication on large area (125mm 125mm) epitaxial wafers of 50μm thickness. During handling and transport no mechanical support was used for the wafers, and the process was identical to the process that was used for 50μm and 160μm Czochralski wafers. Due to the wafer preparation procedure, thin epitaxial wafers were found to be mechanically weaker than Czochralski wafers. This resulted in wafer breakage especially during wet processing. The efficiency of epitaxial cells reached 17%, which was lower than the 19% and 20% efficiencies obtained with 50μm and 160μm Czochralski wafers. The cell results were thus found to be a good metric for substrate quality.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 328-330, In imec’s i2-module concept, silicon heterojunction interdigitated back-contacted (SHJ-IBC) solar cells are fabricated on monocrystalline foils bonded to glass. The proposed technology allows for cell processing on thin wafers mechanically supported by the glass, increasing the yield of processing such thin wafers. A process sequence for SHJ-IBC cell fabrication that can be applied to bonded thin foils is described. We investigated and optimized individual process steps on thick wafers. Then the developed steps were integrated into a process flow to fabricate solar cells on wafers with different thicknesses and bonding agents. On wafers with a thickness of 190 μm, functional cells with efficiencies of 22.6% and 21.7% were made on freestanding and silicone bonded wafers, respectively. On thin wafers of 57 μm, our best SHJ-IBC cell on an EVA bonded wafer exhibits excellent Voc of 740 mV and efficiency of 20.0%, which demonstrates the high potential of the i2-module concept.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 313-316, Layer transfer process based on a porous silicon template is one of the promising replacements for the conventional wafer sawing technologies. For the process to be cost-effective, one challenge is the multiple recycling of the Si substrate used as a template for epitaxial growth. The Si substrate after detachment has different types of defects. Therefore, reusing the substrate without surface reconditioning induces low detachment yields and generates low quality epitaxial Si-foils. The current work presents the first steps taken into the realization of successful substrate re-use. Firstly, possible defects and undesirable features on the surface after the detachment process are identified. Secondly, a wet-chemical reconditioning process is developed. Finally, the effectiveness of this reconditioning process is evaluated by monitoring the evolution of the foil quality through different generations. Until now, 5 generations of epitaxial foils have been demonstrated at imec.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 639-642, A modification is proposed to the standard HIT cell structure in which the interfacial inversion layer at the a-Si:H/c-Si interface is prone to collapse. It consists of substituting the p+ inversion layer emitter by a very shallow p+ doped layer at the surface of the c-Si prior to a-Si:H deposition. This layer could be created by p+ epitaxial growth, p+ recrystallized polysilicon, or simply by front p+ thermal diffusion. The aim is to maintain a permanent p+ emitter irrespective of the conditions of a-Si:H and cell voltage, and hence to avoid the problems encountered with the collapse of the inversion layer associated to the a-Si:H quality, TCO and operating conditions. Simulations are carried out using Silvaco tools and AFORS-HET device simulators to determine and compare the I-V characteristics of standard HIT cells suffering from inversion layer collapse, and those of similar structures implementing the proposed modification, with the characteristics of a standard well performing reference HIT cell. Restitution of the normal I-V characteristics in the dark and under illumination after applying the proposed modification is confirmed.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1967-1970, ITRPV roadmaps predict that the thickness of wafers will continuously decrease. For thicknesses below 100 μm, alternatives for wafer sawing need to be developed. IMEC proposes a foil creation technique by lifting off a thin epitaxially grown layer as a substitute for wafer sawing to create substrates with a thickness around 40 μm. To enable integration of those foils into solar cells, with sufficiently high yield, an ‘i2-module’ concept is proposed. This is an interdigitated back-contact solar module process that can be applied on very thin foils, and where those foils are supported by either the parent substrate or the module glass at all times. To define a process flow adapted to the requirements and limitations of the epitaxial foils, the stability of the foils is tested during a wide range of front-side processes. It is found that epitaxial foils are not sufficiently stable during high temperature processes. Therefore, a process sequence based on low-temperature processes is defined for front-side processing. The yield of each step in the front-side process flow is evaluated. The constrains for front-side processing, together with the constrains defined by the silicone bond, lead to the choice of a back-contact heterojunction (HJ) cell with a PECVD a-Si:H emitter.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1024-1027, In this paper, the electrical and optical characteristics of the Indium-Doped Tin Oxide (ITO) are tuned in order to design an ITO-based spacer between the a-Si:H and the metal contact suited for Amorphous / Crystalline Heterojunction interdigitated Back Contact Solar Cells (a-Si:H / c-Si HJ i-BC). ITO carrier concentration, mobility, resistivity, extinction coefficient, and work function, as well as the ITO / a-Si:H specific contact resistances are evaluated as a function of O2 relative flow rate during ITO deposition. An ITO having carrier concentration of 4.5x1019 cm-3, mobility of 20.2 cm2/(V.s) and resistivity of 6.24 mΩ.cm is found to be an optimum leading to ITO / a-Si:H(p/i) and ITO / a-Si:H(n/i) specific contact resistances values of 0.6 Ω.cm2 and 0.1 Ω.cm2 respectively and a RS reflectance of 58% at 1200nm when integrated in Silver / ITO / a-Si:H / c-Si(textured) test structures.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1186-1190, Two dimensional solar cell simulations using Silvaco educational tools are carried out to investigate the possible analogy between the a-Si:H(p+)/c-Si(n) silicon solar cell with a hetero-interface (HIT) and the inversion layer MIS silicon solar cell, both cells having an inversion layer emitter. The p+ inversion layer emitter in the HIT cell is systematically induced by the valence band offset at the a-Si:H(p+)/c-Si(n) interface while the inversion layer in the MIS cell has been intentionally induced. It is shown that if both cells have similar lateral p+ doped emitter contact regions their dark currents match and are similarly affected by the series resistance due to lateral current flow. The I-V characteristics of the MIS cell with lateral contacts under illumination show a strong degradation in the Fill Factor and the open circuit voltage due to recombination in the p+ diffused contact regions. By introducing a full emitter TCO metallization layer to the top side of the HIT cell, these losses were removed and the high efficiency of the HIT cell was restored.

28th European Photovoltaic Solar Energy Conference and Exhibition; 822-826, Valence band offset and valence band bending at the p+ hydrogenated amorphous silicon/n-type crystalline silicon (a-Si:H(p+)/n_c-Si) interface in heterojunction silicon solar cells (HIT) induce a p+ inversion layer which pushes the pn junction 20-25 nm deep in the c-Si side. The heavily hole populated interfacial inversion layer acts as the HIT cell emitter while the ultrathin amorphous silicon layer is the front contact. The impact of doping level and dangling bond defects in the a-Si:H layer and of interface states on the inversion layer properties is investigated by means of AFORS-HET simulations. Apart from the electric field created by the conduction band offset, the HIT cell performs like a c-Si solar cell which has an inversion layer emitter matching that of the HIT cell, with an open circuit voltage of 733 mV. The latter rolls off at relatively large interface (front surface) recombination velocity. Increasing the a-Si:H(p+) doping level modifies the peak hole concentration in the inversion layer and increases the front surface field. On the other hand, c-Si cell with an ultrathin dopant diffused emitter having also the same emitter hole profile sees its performance degraded at relatively low surface reombination because it suffers from bandgap narrowing and from doping dependent mobility degradation and has no front surface field. Inserting an ultrathin intrinsic or lowly doped a-Si:H spacer between a-Si:H(p+) and c-Si provides less hole population in the inversion layer which reduces Auger recombination and improves the open circuit voltage at low surface recombination to 735 mV despite the less effective front surface field.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2416-2419, We propose a structure to enhance the output current of thin silicon solar cell by placing islands of silver particles on the backside of the cell. The silver/silicon interfaces support Plasmon waves at long optical wavelengths; longer than about 620 nm. Thus the energy of such wavelengths, which are not efficiently absorbed in the thin silicon cell, are transformed into Plasmon waves that add to the absorption in the silicon. Rigorous analysis of Plasmon waves lead to closed forms for the lateral attenuation and in depth attenuation in both silver and silicon sides. The fraction of Plasmon energy absorbed on the silicon side is derived, hence the Plasmon generated current is obtained. Numerical examples confirm noticeable enhancement of the electric current due to the presence of the backside silver particles.