Device physics of inverted all-polymer solar cells

The device physics of inverted all-polymer solar cells based on a blend of the polymers poly(3-hexylthiophene) and poly(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)2,1,3-benzothiadiazole]-2',2('')-diyl) is investigated. In particular, the influence of active layer thickness on device parameters is investigated and compared to that of devices with a standard geometry. Increasing the active layer thickness from 70 to 480 nm is found to increase the open circuit voltage from 0.1 to 0.71 V and the external quantum efficiency (EQE) from 7% to 24%. In contrast, an optimum EQE of about 25% for the standard geometry is found for a film thickness of 70 nm, which decreases sharply with increasing active layer thickness. The shape of the EQE spectra of standard geometry devices also become severely distorted with increasing active layer thickness, with a minimum in EQE coinciding with the wavelength corresponding to maximum light absorption. In contrast, the shape of the EQE spectra of inverted devices remains essentially unchanged with increasing active layer thickness. Optical simulations of light absorption in the active layer have also been performed and demonstrate that the distortion in the EQE spectra of thicker standard geometry devices is consistent with photoexcitations created in the back half of these devices being more efficiently harvested than those in the first 100 nm of the inverted device. Furthermore, the fact that the EQE spectrum of inverted devices does not significantly broaden with increasing thickness suggests that harvesting of photoexcitations remains efficient in the front half of the device where most of the light is absorbed. Device modeling is employed to demonstrate that the lower mobility of electrons (and electron trapping) causes a favorable redistribution of the internal electric field in the inverted device with electric field increasing near the transparent electrode coinciding with the region of maximum light absorption. In contrast, in the standard device the internal electric significantly decreases near the transparent electrode causing a reduction in field-dependent charge separation and increased bimolecular recombination. Our results demonstrate that inverted devices may be an effective way to overcome losses in organic solar cells where electron mobility is typically lower than hole mobility. (C) 2010 American Institute of Physics. [doi:10.1063/1.3371364]