Factors affecting the photovoltaic behavior of inverted polymer solar cells using various indium tin oxide electrodes modified by amines with simple chemical structures

• Surface-modification of ITO electrode with low molecular weight amines
• Ionization potential of ITO was decreased by forming an electrical double layer.
• Light-soaking effect has been observed by irradiating white light.
• The light-soaking effect mainly improved the open-circuit photovoltage.
• Open-circuit photovoltage was limited by ionization potential of amine-modified ITO.

In a glass–indium tin oxide (ITO)/amine/regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au cell, which uses small molecule amine-modified ITO as the electron collection electrode, a light-soaking effect under irradiation of simulated sunlight was restrained considerably compared with in an ITO/P3HT:PCBM/PEDOT:PSS/Au cell containing bare ITO. That is, the time taken to arrive at a saturated Voc from the initial Voc became short when the ionization potential (IP) of ITO reduced by the amine modification, and consequently both of its saturated Voc and power conversion efficiency (PCE) improved. The IP decreased with an increase in the number (N) of amino groups in a single amine molecule, because the basic amino groups can efficiently neutralize any acidic hydroxyl groups on ITO through a multipoint interaction. The superior performance of the cell containing the amine-modified electrode with large N was perhaps because the energy mismatch formed by a contact between ITO and acceptor PCBM reduced, and consequently the rate of electron collection at ITO increased.