Effect of device geometry on the performance of TiO2 nanotube array-organic semiconductor double heterojunction solar cells

TiO2 nanotube arrays are used as the electron accepting network in hybrid heterojunction excitonic photovoltaic devices. The performance of the photovoltaic devices is studied using two configurations, namely frontside illumination where the illumination occurs from the cathodic side of the device and backside illumination where the illumination is from the anodic side. A blend of regioregular poly(3-hexylthiophene) and a methanofullerene (Phenyl C71-butyic acid methyl ester) is infiltrated into the nanotubes to form the devices. Non-transparent nanotube arrays fabricated on Ti foil substrates were used in backside illuminated solar cells while transparent nanotube arrays fabricated on conducting glass were used to form the frontside illuminated cells. The frontside illumination geometry was found to be superior to the backside geometry due to ease of forming uniform contacts to devices and lower photonic losses due to absorption. In addition to the P3HT–PCBM interface, the P3HT–TiO2 nanotubes interface provides an additional heterojunction for charge separation. Typical backside illuminated solid state solar cells show a short-circuit current density of 3.91 mA/cm2, 324 mV open circuit potential and a 0.43 fill factor, while typical frontside illuminated solar cells show a short-circuit current density of 12.4 mA/cm2, 641 mV open circuit potential and a 0.51 fill factor yielding power conversion efficiencies of 4.1% under AM 1.5 sun.