HYBRID, CHEMICALLY PASSIVATED N-TYPE SILICON / PEDOT:PSS SEMICONDUCTOR–INSULATOR-SEMICONDUCTOR SOLAR CELL Rotem Har-Lavan, Pranav Joshi, Omer Yaffe, Igal Levine, and David Cahen 1 1 Department of Materials and Interfaces, Weizmann Institute of Science, Rehovoth 76100, Israel ABSTRACT We describe a hybrid inorganic-organic solar cell, wherein the n-Si absorber interface is chemically passivated and electrically contacted by a conductive polymer, PEDOT:PSS. In this structure, which is completely fabricated from its components at room temperature, the Si is type-inverted at the hybrid interface with the polymer, thus effectively creating an SIS type solar cell without any significant insulating film. For moderately doped Si, the surface is strongly inverted and photogenerated current is being collected from the entire area of the solar cell. The good lateral conduction of minority carriers in the inversion layer helps to mitigate a major limitation of PEDOT:PSS, viz. its high sheet resistance. INTRODUCTION Inversion layer solar cells, also known as Schottky barrier or MIS solar cells, were introduced in the the 70's of the previous millennium to reduce (mainly Si) solar cell fabrication costs, without significant adverse effects on their efficiency.[1] [2] Ideally, if a metal makes electronic contact with a semiconductor, an energy barrier,  Bn , equal to the difference between the work function of the metal,  m , and the semiconductor's electron affinity, X SC , (for n-type) forms within the semiconductor. Combining a high work function metal with n-Si, or a low work function metal with p-Si should, in principle, lead to type inversion in the space-charge region (SCR) of the Si. Such type conversion creates, de facto, an n-p + junction (or p-n + , if p-Si is used), as shown in Figure 1b, without the need for a high-temperature diffusion process step. It was shown experimentally that if there is direct metal semiconductor contact, the open-circuit voltage (VOC) is only a small fraction of the potential one, due to Fermi level pinning. Introduction of an ultra-thin insulating layer between the metal and the semiconductor, was demonstrated to unpin the Fermi level of the semiconductor – thus increasing the VOC of MIS solar cell to values comparable with their p-n junction analogs.[2] For decades it was believed that at least 10 Å of an insulation layer, usually a thermally grown oxide, is essential as tunneling barrier to diminish majority carrier transport and allow the device to be dominated by minority carrier currents.[3] Furthermore, thinner interfacial layer did not seem to prevent the direct metal-semiconductor reaction that is known to introduce surface states, and act to pin the semiconductor's Fermi level.[4] Wittmer and Freeouf showed that with a non-interacting metal, such as Hg, as contact to H-terminated Si, the full difference between the semiconductor's ionization potential and the metal's work function is expressed in the resulting energy barrier height.[5] Lately we introduced a self-assembled monolayer of organic molecules of different lengths as insulator in a Metal-Insulator- Semiconductor junction, and showed that the interfacial layer in such structures can be so thin (down to 2 Å), so as not to present a significant insulating barrier, while still leading to strong inversion of the Si.[6] Based on these findings, the requirements that an interfacial layer in an inversion layer solar cell ought to fulfill are: 1. prevent direct metal-semiconductor chemical interaction and formation of metal-induced gap states, MIGS (induced density of interface states, IDIS, in an SIS cell) 2. passivate the Si surface chemically to minimize dangling bonds, and electrically, to reduce the surface state density, and, specifically, to minimize potential mid-gap recombination- generation sites. 3. Optionally, the layer can introduce an interface dipole to alter the Si electron affinity to allow Si type inversion, if the initial potential difference is not sufficient. The efficiency of SIS solar cells, where the metal is replaced by a degenerate wide bandgap semiconductor, should in principle exceed that of its parallel MIS since the absorber's majority carrier transport is suppressed by the forbidden gap of the window semiconductor.[7] Here we present a Si inversion layer solar cell structure, where the monocrystalline n-Si absorber is chemically etched to produce an H-terminated surface, known for its superior passivation properties. The soft deposition of the PEDOT:PSS top electrode via spin-coating serves to preserve this passivation,[8] and protect the surface when a metallic grid current-collector is deposited by thermal evaporation. Spin-coated PEDOT:PSS is > 90% transparent over the entire visible spectrum into the near-IR. This makes it a possible top contact, because in the MIS solar cell configuration incident light passes through the metal contact to reach the Si absorber. The PEDOT:PSS work function (5.1-5.3 eV) should strongly invert the underlying n-Si surface, thus assuring sufficient lateral conduction of minority carriers within the SCR to compensate for the