Appl. Phys. Lett. 99, 163301 (2011)

An intimate and random mixture of two organic semiconductors is used in many of the best-performing organic solar cells. These bulk heterojunction devices benefit from a large active surface area and are easy to make. However, the disordered nature of the mixture makes it difficult to extract charges, which hinders power-conversion efficiencies. Now Charles Black and colleagues at Brookhaven National Laboratory and State University of New York, Stony Brook, have improved charge extraction from a bulk heterojunction device by reducing disorder.

The researchers mixed the hole-conducting polymer P3HT and the electron-conducting polymer PCBM into a templated array of vertically aligned cylindrical pores, each with a diameter of approximately 75 nm. Previous work had demonstrated that P3HT was a better hole-conductor when confined to nanoscale volumes because individual polymer chains were better aligned. Black and colleagues confirmed that their confined P3HT had improved alignment, and measured hole mobilities ∼500 times larger than in unconfined blends. As a result, they found that, relative to an untemplated device of the same thickness, their device absorbed only 38% of the light, but produced 80% of the photocurrent. Normalized to volume, the photocurrent density was double that of the untemplated device.

Further performance improvements may result from increasing the electron mobility of PCBM, and increasing the areal fraction accounted for by the cylinders.