To study the UC performance, we measured the EQEs using the EQE calculation method reported by Izawa, et al. and Hu, et al. (see the Methods section for more details). We varied the thickness of the TCA by spin coating solutions with varying concentrations of TCA in acetonitrile (1-6 mM, see Figure SI 8 for TCA optimization). Additionally, we tuned the PbS thickness by varying the PbS solution concentration between 5 and 50 mg/mL to deposit films which absorb between 0.3 and 6.2% of the incident 808 nm laser (see SI Note 1 and 2 for percent absorption calculations). As shown by the orange trace in Figure , there was a steady decline in EQE for the control devices with increasing PbS thickness (no TCA). While thicker PbS films are expected to absorb more NIR photons, they also suffer from more parasitic BT of singlet excitons from rubrene/DBP. Additionally, the energy transfer between QDs is limited due to the insulating shell created by the oleic acid ligands. This shell also limits the energy transfer from PbS to rubrene/DBP. So far, this decline in EQE with increasing PbS thickness has presented a crucial bottleneck to unlocking higher EQEs in this system.