Inkjet Printing of Isolation Layers for Back-Contacted Silicon-Heterojunction Solar Cells

For wafer based silicon solar cells, the combination of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction emitters (SHJ) [1] and back-contacted back-junction solar cell concepts (BCBJ) [2] offer a very high efficiency potential of around 24%. Stangl et al. proposed a relatively simple and therefore attractive cell concept comprising a two level metallization isolated by an insulation layer. The emitter layer consisting of doped amorphous silicon with a thickness of several nm and the emitter metallization layer comprise circular openings where the back surface field layers and the respective metallization establish contact to the absorber. In this work the potential of inkjet printing for the deposition of the isolation layer with photoresists or other polymeric fluids is evaluated. Challenges are the required placement precision and the feature size. In order to produce circular openings of the order 10 μm, the drop formation has to be optimized, and the ink spreading on both surfaces on the aluminum emitter and on the silicon wafer substrate have to be controlled. Introduction For wafer based silicon solar cells, the combination of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction emitters (SHJ) [1] and back-contacted back-junction solar cell concepts (BCBJ) [2] offer a very high efficiency potential of around 24% [3] due to the high Voc values enabled by silicon heterojunctions in conjunction with the excellent short circuit current of BCBJ solar cells. In addition, one-sided a-Si:H/c-Si hetero contact systems can be relevant in crystalline thin-film Si photovoltaics if the bulk carrier lifetime in the thin film absorber material is high enough, such that the cell benefits from the reduced surface recombination at the heterojunction. Figure 1 displays schematic cross sections of the solar cell investigated in this approach as proposed by Stangl et al (Point Rear Contacted Amorphous-crystalline Silicon Heterojunction). The front side is coated with an anti-reflection coating which also serves as passivation layer. The crystalline silicon substrate (here n-type, polarities are interchangeable) is textured with random pyramids on the front side in order to further enhance the absorption of the incoming light. The emitter layer consists of (p/n)-doped amorphous silicon with a thickness of several nm. Amorphous silicon layers provide an excellent surface passivation of the crystal and a good selectivity regarding charge carrier collection, enabling the aforementioned high Voc values. The emitter is contacted with a first metal layer. Emitter and metallization layer comprise circle openings where the back surface field layer, consisting of n-doped amorphous silicon and the respective metallization establish contact to the absorber (see horizontal cross section on the left of Figure 1). Thus the two polarities must be isolated in order to avoid parasitic current paths from the emitter metallization to the BSF metallization, which significantly decrease the solar cell performance. The fabrication process is shown schematically in Figure 2. After a single sided texturization, realized by masking one side against the wet-chemical texturization bath consisting of potassium hydroxide and isopropanole the front side is coated with a SiNx antireflection coating for passivation and antireflection purposes. anti-reflection coating and passivation layer n-type c-Si wafer emitter