Intensity and wavelength dependence of bimolecular recombination in P3HT:PCBM solar cells: A white-light biased external quantum efficiency study

Bimolecular recombination is often a major photogenerated charge carrier loss mechanism in organic photovoltaic (OPV) devices, resulting in lower fill factor (FF) compared to inorganic devices. The recombination parameter α can be obtained from the power law fitting of short-circuit current (J_sc) on illumination intensity (I), $Jsc∝Iα$⁠, with α values less than unity taken as an indication of reduced photon-to-electron extraction efficiency and the presence of bimolecular recombination in OPV. Here, we show that this intensity-averaged measurement is inadequate. An external quantum efficiency (EQE) apparatus under constant white-light bias can be used to measure the recombination parameter (α_EQE*) as a function of wavelength and carrier density (white-light intensity). Examining the dependence of α on background white-light bias intensity and excitation wavelength provides further understanding of photon-to-electron conversion loss mechanisms in P3HT:PCBM bulk heterojunction devices in standard and inverted architectures. In order to compare EQE and current-voltage (JV) measurements, we discuss the special case of devices exhibiting sub-linear intensity response (α < 1). Furthermore, we demonstrate several important advantages of the white-light biased EQE method of measuring bimolecular recombination compared to existing methods, including sensitivity in probing intensity-dependent recombination compared to steady-state JV measurements, the correlation of α_EQE* and FF in devices, elucidation of recombination mechanisms through spectral dependence of carrier loss, and the robustness of α_EQE* obtained via integration over the entire absorption region. Furthermore, this technique for measuring recombination is immediately accessible to the vast majority of researchers as the EQE apparatus is ubiquitous in PV research laboratories.