Photon upconversion (UC) processes result in the emission of photons at higher energies than those absorbed. Among the several recent novel applications of UC, the most widely studied is its use with photovoltaic (PV) cells. Photon UC can sensitize PV cells to portions of the solar spectrum at lower energy than the band gap, which are wasted in a normal single-junction cell, and so begins to address one of the major sources of PV cell efficiency loss. Developing a rigorous but practical method of quantifying upconversion efficiency is therefore an important objective. This task is complicated by the nonlinearity of upconversion efficiency at application-relevant light intensities, meaning the excitation conditions under which efficiency is measured must also be specified. A first-principles approach to determining upconversion efficiency, based on the quantum yields of the underlying photochemical processes, is rigorous in principle but difficult in practice. Absolute photometric measurements that treat the upconverter as a black box are similarly difficult, and measure optical losses alongside the photochemical performance. The widely-utilized relative actinometry method, based on comparisons to a known fluorescence standard, fails as a rigorous method without explicit consideration of the generation profile and reabsorption. In response to these issues, we report an upconverter action spectrum experiment, which is based on continuous-wave photoluminescence techniques. The experiment is used to determine the upconversion efficiency of a photochemical upconverter employing triplet–triplet annihilation (TTA). Full specification of the excitation conditions is made, allowing the efficiency measurement to be linked to well- defined solar excitation conditions. We measure the TTA–UC performance of the PQ4PdNA : rubrene system over a range of excitation conditions corresponding to 0.09–3.22 multiples of AM1.5G solar illumination. At 1 sun, we obtain a TTA yield of 1.1%.
