Exceptional Surface Passivation and Carrier Transport in Pyramidically Textured N-Type Silicon by Organic Molecules with Sulfonic Acid Head Groups and Perfluoroalkyl Tail Groups

Improving the passivation quality of the surface in silicon (Si) solar cell is one of the main strategies for enabling higher performance. Excellent passivation schemes have been recently developed, with lifetimes reaching 0.5 sec using, for example, Al₂O₃ and SiN_x dielectric layers with proper (i.e., negative and positive for p- and n-type Si, respectively) fixed charges. However, one of the main disadvantages of such dielectrics is the deposition methods requiring vacuum and, possibly, high-temperature annealing steps. Eliminating these steps has the potential to reduce the fabrication costs of silicon solar cells considerably. One family of materials that potentially can provide high passivation of silicon is organic materials. Due to the simplicity of their deposition methods (often fabricated in the room ambiance without subsequent thermal annealing), and the capacity to inherently possess high dipole moments and fixed charges, organic materials might deliver the same level of performance as inorganic dielectrics but at a reduced cost. In this regard, we present an investigative study of organic materials with sulfonic acid head groups and perfluoroalkyl tail groups.<br/>We demonstrate minority carrier lifetime values of over 8 ms (an implied open circuit voltage (iV_oc) of &gt; 730 mV) with organic materials on textured Czochralski n-type Si wafers, comparable to the passivation level of high-performance inorganic dielectrics. Moreover, we show that only 5% of organic material dissolved in a solution of deionized water (DI) and isopropanol (IPA) is sufficient to passivate the silicon surface. IPA/DI water provides much higher performance compared to alternative solvents (DMSO, ACN, Ethanol, and IPA without water). The critical role of DI water is elucidated.<br/>Si surfaces coated with organic materials were investigated using Attenuated Total Reflectance (ATR), Fourier Transform Infrared spectroscopy (FTIR), and X-ray Photoelectron Spectroscopy. Initial results indicate that passivation is mainly provided by the field effect, resulting in a favorable energy band bend bending at the surface of Si. Due to the chemical structure of organic materials and their similarity to those of self-assembling monolayers, we hypothesize that these organic materials form dipoles at the interface generating an electric field and resulting in a favorable band bending. This hypothesis is also supported by contact angle measurements, which reveal high hydrophobicity for surfaces treated with organic materials.<br/>These organic materials provide good passivation together with good carrier transport properties. A contact resistance as low as 106 mΩ/cm² is achieved through n-Si – organic materials – aluminum stacks, which can be further reduced to 15 mΩ/cm² with a thin layer of LiF_x deposited under Al metal.<br/>Lastly, we discuss the stability of organic materials and demonstrate that keeping the samples under nitrogen ambient, at low temperatures (2 ^oC), or adding PCBM ([6,6]-Phenyl C₆₁ butyric acid methyl ester) into the solutions improves the stability considerably. A very first proof-of-concept solar cell with organic materials deposited at the rear of the solar cell demonstrated an efficiency of ~17%, only 3% lower than the PERC type silicon solar cell produced in the same batch.<br/>Our results prove that organic materials with a head group that anchors to pyramidally textured Si and a tail group that provides a high dipole with a cost-effective, low-temperature fabrication can provide good passivation and charge transport and thus have great potential for providing high-performance solar cells.