Polyethylenimine (PEI) and ethoxylated polyethylenimine (PEIE) are known to reduce the work function of a wide array of electrode materials and consequently improve charge extraction in inverted photovoltaic devices. To this point, PEIE has received the majority of research interest but recent evidence suggests that PEI may provide a larger work function reduction (Δφ). In this study, we investigated both PEI and PEIE as the cathode-modifying interlayer in inverted photovoltaic devices. A series of PEIs with varying molecular weights was studied to uncover any molecular weight dependencies. TEM images demonstrated that the bulk morphology of the PEI and PEIE polymers are very different. Photoelectron spectroscopy results showed that the work function reduction of each of the PEIs (Δφ = 0.73 – 1.09 eV) was larger than that of PEIE (Δφ = 0.56 eV). To characterize the effect of the larger work function reduction, inverted photovoltaic devices were fabricated and evaluated. A PEI interlayer led to a 36% increase in power conversion efficiency (PCE) compared to the control device without an interlayer (PCE = 6.06 %). However, a PEIE interlayer produced a 22% increase in device efficiency, not as large as PEI. Finally, the effect of the PEI and PEIE interlayers on temporal stability was studied. It was found that device stability increased with increasing molecular weight with the device containing the highest molecular weight PEI retaining the largest fraction of its initial performance after a three month period in air. These data show that PEI provides superior performance as an interlayer in inverted photovoltaic devices compared to PEIE due to a larger work function reduction. This observation, along with the molecular weight effect on stability, could assist in the rational design of new cathode-modifying interlayers.