Silicon/Organic Heterojunction for Photoelectrochemical Energy Conversion Photoanode with a Record Photovoltage Wei Cui, Shan Wu, Fengjiao Chen, Zhouhui Xia, Yanguang Li, Xiao-Hong Zhang, Tao Song,* Shuit-Tong Lee, and Baoquan Sun* Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou, Jiangsu 215123, China S Supporting Information *
ABSTRACT: Silicon (Si) is a good photon absorption material for photoelectrochemical (PEC) conversion. Recently, the relatively low photovoltage of Si-based PEC anode is one of the most significant factors limiting its performance. To achieve a high photovoltage in PEC electrode, both a large barrier height and high-quality surface passivation of Si are indispensable. However, it is still challenging to induce a large band bending and passivate Si surface simultaneously in Sibased PEC photoanodes so far, which hinders their performance. Here, we develop a simple Si/poly(3,4ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) heterojunction with large band banding and excellent surface passiviation for efficient PEC conversion. A chemically modified PEDOT:PSS film acts as both a surface passiviation layer and an effective catalyst simultaneously without sacrificing band bending level. A record photovoltage for Si-based PEC photoanodes as high as 657 mV is achieved via optimizing the PEDOT:PSS film fabrication process. The density of electron state (DOS) measurement is utilized to probe the passivation quality of the organic/inorganic heterojunction, and a low DOS is found in the Si/PEDOT:PSS heterojunction, which is in accordance with the photovoltage results. The low-temperature solution-processed Si/organic heterojunction photoanode provides a high photovoltage, exhibiting the potential to be the next-generation economical photoanode in PEC applications. KEYWORDS: photoelectrochemical, silicon/organic heterojunction, surface passiviation
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performance, a functional layer on the Si electrode is necessary to induce a large barrier height and passivate its surface defects simultaneously.11 On one hand, increasing the interfacial band bending could enhance the barrier heights in semiconductors and improve the photovoltage in PEC process.12 On the other hand, decrease of surface defects can reduce the recombination rate of light-induced charge carriers effectively, improving the performance of a photoelectrode.4,11,13
hotoelectrochemical (PEC) conversion is an effective approach to produce renewable environmentally friendly energy by converting sunlight into energy stored in chemical fuel, such as hydrogen and oxygen.1−5 Among those semiconducting PEC electrodes, silicon (Si) is one of the potential candidate materials for light absorption in PEC conversion due to its high light harvest efficiency, long carrier diffusion length, mature processing techniques, and so on.1,4,6−9 In a Si-based PEC electrode, photogenerated electron−hole pairs are dissociated in Si depletion region and then participate in PEC reactions via a certain catalyst to enhance the kinetics of reactions in electrolytes.8,10 In order to boost the PEC © 2016 American Chemical Society
Received: July 3, 2016 Accepted: September 12, 2016 Published: September 12, 2016 9411
DOI: 10.1021/acsnano.6b04385 ACS Nano 2016, 10, 9411−9419
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ACS Nano Buried n−p+(p−n+) Si junction is usually applied to introduce band bending in Si-based PEC photoanode (photocathode) for water oxidation (reduction).4,14−17 Alternatively, Schottky junctions, either metal/semiconductor (MS) or metal/insulator/semiconductor (MIS) structure, are also utilized in PEC electrodes, in which the barrier height is mainly determined by the absolute value difference between the conduction (valence) band edge of semiconductors and the work function of metals.12,18,19 With the MIS structure, high performance and stable photoanodes could be simply acquired by merely deposition of a thin layer of Ni or Co on n-Si.1,20 Metal oxides such as TiOx layers fabricated by atomic layer deposition (ALD) have been used to protect the photoanode from corrosion and reduce the surface recombination.4 However, the low work function of TiOx layers may reduce band bending level on n-Si surface in photoanodes for water oxidation, although the conductivity is improved, as reported previously.4,12 Amorphous Si has also been incorporated on the surface of crystal Si,21,22 which could not only passivate defects but also form a p−i−n junction on Si substrates. So far, the critical issue that hinders the performance of a Si-based PEC electrode is still the low photovoltage.12,20 Most of the PEC electrodes could only provide a photovoltage
