J. Phys. D: Appl. Phys. 31 (1998) 2326–2330. Printed in the UK PII: S0022-3727(98)92417-2 Nanoporous n-TiO 2 /selenium/p-CuCNS photovoltaic cell K Tennakone, G R R A Kumara, I R M Kottegoda, V P S Perera and G M L P Aponsu Institute of Fundamental Studies, Hanthana Road, Kandy, Sri Lanka Received 11 March 1997, in final form 9 June 1998 Abstract. On illumination, selenium deposited on nanoporous n-TiO 2 transfers photogenerated electrons into TiO 2 . When p-CuCNS is coated on top of the selenium deposited on nanoporous n-TiO 2 , holes are directed into the p-CuCNS. A photovoltaic cell of nanoporous n-TiO 2 /selenium/p-CuCNS based on the above charge transfer process generates a photocurrent of ∼3.0 mA cm −2 and a photovoltage of ∼600 mV at 800 W m −2 simulated sunlight. The efficiency of the cell seems to be limited by surface recombination and the presence of voids in the TiO 2 film. Photoelectrochemical experiments also indicate that when selenium is deposited on nanoporous n-TiO 2 photogenerated electrons in selenium are efficiently transferred to TiO 2 . 1. Introduction Porous films of high-bandgap semiconductors consisting of nanocrystallites exhibit novel properties as a result of the enormous effective surface area and fineness of the crystallite size [1–5]. Because of the above properties such films have been extensively studied for potential applications in photovoltaic and photocatalytic systems [6–8]. Photoelectrochemical and fully solid state solar cells have been fabricated by sensitizing nanoporous films of TiO 2 with pigments and dyes [1–10]. Their performance depends on the ability of photoexcited pigment molecules absorbed at the surface of semiconductor crystallites to inject carriers into the bands of the semiconductor. It is also known that a low-bandgap semiconductor deposited on the nanoporous film is also capable of injecting carriers into high-bandgap porous film when the former is illuminated with visible light [11–16]. In this work we describe the construction of a photovoltaic cell by depositing a thin film of selenium on a nanoporous film of TiO 2 , followed by a thin film of p-CuCNS on top of the selenium film. Other photovoltaic structures have also been reported with selenium (Schottky barrier or heterojunction with n-type material) [17, 18]. Cadmium sulphide appears to be a suitable material for heterojunction action with selenium since it has the same crystal structure (hexagonal) with reasonable lattice mismatch [19–22]. In the present work selenium has been used as a sensitizer for a high-bandgap material (n-TiO 2 ). The functioning of the cell depends on efficient transfer of photogenerated electrons in selenium to nanoporous n-TiO 2 and holes to p-CuCNS. The sandwich structured cell, n-TiO 2 /Se/p-CuCNS, generates a photovoltage of ∼600 mV and photocurrent of 3.0 mA cm −2 at 800 W m −2 simulated sunlight. Photoelectrochemical experiments also indicate that when selenium coated on nano- porous TiO 2 is illuminated, photogenerated elections are efficiently transferred to TiO 2 . Selenium, being a p-type semiconductor, shows a cathodic photoresponse in an electrolytic medium. However, when selenium is deposited on nanoporous n-TiO 2 the observed photoresponse is anodic. A cathodic photocurrent is again observed when selenium is deposited on a thin polycrystalline film of TiO 2 (coated on conducting glass), indicating that the nanoporous structure of TiO 2 film is essential for transfer of photogenerated electrons. 2. Experiment Nanoporous films of TiO 2 were coated on fluorine doped conducting tin oxide (CTO) glass (1.0 × 2.0 cm 2 , sheet resistance ∼10   −1 ) by the following method. Titanium isopropoxide (1 ml) and glacial acetic acid (5 ml) were added to isopropanol (15 ml) and 5 ml of water added drop by drop to the mixture which was kept stirred. Colloidal gel consisting of fine crystallites of TiO 2 is produced by the hydrolysis of titanium isopropoxide and the above procedure prevents their agglomeration. The CTO glass plate was placed on a hot plate (surface temperature ∼125 ◦ C) and the solution was evenly spread (with help of a dropper and a glass rod) on the surface and allowed to dry. The plate was then sintered at 450 ◦ C for 10 min, repeating the process until a semitransparent film was 0022-3727/98/182326+05$19.50 c  1998 IOP Publishing Ltd