Photoinduced absorption spectroscopy on MDMO-PPV:PCBM solar cells under operation

Near-steady-state photoinduced absorption (PIA) spectroscopy is employed to study the charge carrier dynamics in polymer:fullerene (poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene]: 1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61; MDMO-PPV:PCBM) solar cells under operating conditions. Artifacts in the PIA spectra originating from thermal heating of the aluminum electrode and from interference of incoming and reflected light waves are analyzed. PIA signals from functioning devices are studied as a function of applied bias and temperature and complemented with photocurrent measurements under identical conditions. Comparison between photocurrent and PIA measurements shows that PIA mainly probes trapped cationic charge carriers while the photocurrent samples mobile carriers. The number density of trap sites is estimated at 1017 cm−3 and trapped carriers can be stabilized by applying a reverse bias potential over the device.