The Relation Between Open‐Circuit Voltage and the Onset of Photocurrent Generation by Charge‐Transfer Absorption in Polymer : Fullerene Bulk Heterojunction Solar Cells

Photocurrent generation by charge‐transfer (CT) absorption is detected in a range of conjugated polymer–6,6]‐phenyl C₆₁ butyric acid methyl ester (PCBM) based solar cells. The low intensity CT absorption bands are observed using a highly sensitive measurement of the external quantum efficiency (EQE) spectrum by means of Fourier‐transform photocurrent spectroscopy (FTPS). The presence of these CT bands implies the formation of weak ground‐state charge‐transfer complexes in the studied polymer–fullerene blends. The effective band gap (E_g) of the material blends used in these photovoltaic devices is determined from the energetic onset of the photocurrent generated by CT absorption. It is shown that for all devices, under various preparation conditions, the open‐circuit voltage (V_oc) scales linearly with E_g. The redshift of the CT band upon thermal annealing of regioregular poly(3‐hexylthiophene):PCBM and thermal aging of poly(phenylenevinylene)(PPV):PCBM photovoltaic devices correlates with the observed drop in open‐circuit voltage of high‐temperature treated versus untreated devices. Increasing the weight fraction of PCBM also results in a redshift of E_g, proportional with the observed changes in V_oc for different PPV:PCBM ratios. As E_g corresponds with the effective bandgap of the material blends, a measurement of the EQE spectrum by FTPS allows us to measure this energy directly on photovoltaic devices, and makes it a valuable technique in the study of organic bulk heterojunction solar cells.