Solution‐Based Silicon in Thin‐Film Solar Cells

Abstract

factor of 7. This shows that solution-based silicon is a highly promising candidate for industrial-grade applications of solutionbased semiconductors. Evaluation of Precursors NPS and CPSIn literature, most groups reporting silicon fi lms fabricated from a liquid precursor use a cyclic hydridosilane, namely cyclopentasilane (CPS). We decided to use a branched molecule instead, namely neopentasilane (NPS). The molecular structures of CPS and NPS, as well as the process charts for obtaining solid amorphous silicon (a-Si) layers, are shown in Figure 1 . We characterized the NPS used in our process chain by NMR and by mass spectroscopy, showing the expected fi ngerprints mentioned in literature. [ 4 ] Employing NPS over CPS yields major advantages in processing effi ciency as well as in material quality. In general, branched molecules have a considerably better solubility in organic solvents, because the branches act as spacers, preventing strong interactions between the molecules and enabling better intercalation of solvent molecules. [ 5 ] The NPS material is therefore better soluble than CPS, which leads to improved fi lm homogeneity and uniformity. Moreover, in NMR measurements, we found that the NPS-oligomer bears 70% SiH 3 end groups, in contrast to 1.0% for the CPS-oligomer. Such end groups facilitate the cross-linking of the material to a solid network. Since this process is responsible for the formation of silicon-silicon bonds, we expect a positive effect on the coordination of silicon atoms, resulting in less dangling bonds and improved electronic properties. Until now, we have however not been able to demonstrate differences in nanoscopic amorphous silicon structure between CPS and NPS.Another major advantage of employing NPS instead of CPS lies in the differences in material synthesis. The synthesis of the CPS monomer involves a coupling reaction and subsequent chlorination of diphenyldichlorosilane to obtain decachlorocyclopentasilane. This process produces a large amount of various by-products, which are diffi cult to separate and recycle. However, in the synthesis of NPS, we use catalytic rearrangement of octachlorotrisilane to obtain dodecachloroneopentasilane,

Liquid hydridosilane precursor prepared from cyclopentasilane via sonication at low temperatures without the action of light

“…The spectra exhibit absorption peaks in the two expected regions associated with a-Si:H, namely the Si-H and Si-H 2 stretching modes in the range 2000-2100 cm À1 and the Si-Si wagging mode at about 630 cm À1 (not shown). The stretching mode at 2000 cm À1 is associated with Si-H bonds where the hydrogen passivates a silicon dangling bond located in a dense amorphous matrix [5]. On the other hand, modes between 2075 and 2100 cm À1 are attributed to Si-H/Si-H 2 vibrational modes at the internal surface of hollow regions or microvoids [17].…”

“…From these measurements, the light-to-dark photosensitivity ratio or photoresponse defined as r PR = r ph /r d is calculated and used as a figure of merit for the optoelectronic quality of the material. A higher r PR indicates a lower defect density in the film, which in turn means better suitability for application in optoelectronic devices [5].…”

“…In addition, the liquid phase processing of the silicon precursor allows for high deposition rates using well-established techniques such as ink-jet printing, slot-die casting and aerosol coating. Applications include optoelectronic devices such as thin-film transistors [2,3], solar cells [4][5][6], and more recently, passivation layers in silicon heterojunction solar cells [7].…”

Sonophotolytically Synthesized Silicon Nanoparticle‐Polymer Composite Ink from a Commercially Available Lower Silane

“…The fabrication of amorphous and microcrystalline silicon films from printed or sprayed layers presents a low‐cost alternative to conventional high‐vacuum techniques such as plasma enhanced chemical vapor deposition (PECVD). Such inks have already been used in a variety of different optoelectronic applications, including thin‐film solar cells, TFTs, and surface passivation layers for Si wafers …”

Solution-processed amorphous silicon surface passivation layers

“…First experiments with liquid polysilane precursors such as cyclopentasilane 6 , neopentasilane 7 or trisilane 8 were conducted to manufacture a variety of structures and devices. Amorphous silicon (a-Si:H) thin films were applied in solar cells 9,10 , light-emitting devices 11 and thin-film transistors 6 . Furthermore crystalline silicon nanowires were used as anode material in lithium batteries 12 .…”