Inverted p-i-n perovskite solar cells (PSCs) are promising candidates for monolithic silicon/perovskite tandem solar cells due to their superior operational stability under various stressor conditions compared with n-i-p structures. To date, p-i-n PSCs have achieved efficiencies exceeding 27% in single-junction devices and 35% in silicon/perovskite tandem solar cells. Despite these achievements, nonradiative recombination at the perovskite/C₆₀ interface remains a major bottleneck preventing p-i-n PSCs from approaching their theoretical Shockley-Queisser limit, primarily through V_OC losses. To mitigate these losses, extensive interfacial engineering strategies have been explored, including the incorporation of alkylammonium salts and the insertion of mild dipole layers at the perovskite/C₆₀ interface. The resulting improvements in V_OC are typically attributed to more favorable energy level alignment, enhanced charge extraction, and reduced interfacial trap densities. However, the underlying physical mechanisms and their quantitative analysis remain insufficiently understood.

To address this knowledge gap, we combine widely used time-correlated single-photon counting (TCSPC) with high dynamic range gated charge-coupled device (CCD) to perform transient photoluminescence (TrPL) measurements spanning more than ten orders of magnitude in intensity on state-of-the-art Cs_0.05MA_0.10FA_0.85Pb(I_0.97Br_0.03)₃ (CsMAFA) perovskite films interfaced with different charge selective layers. The use of these complementary techniques enables cross-validation of TrPL data and minimizes potential measurements artifacts. Analysis of the differential decay time (τ_diff) as a function of Fermi-level splitting ΔE_F reveals that recombination in bare CsMAFA layer is dominated by shallow traps,^1,2 while incorporation of the hole transport layer MeO-2PACz effectively passivates these traps, resulting in prolonged decay times. In contrast, the introduction of C₆₀ significantly shortens the decay time and leads to a distinct plateau in τ_diff at high ΔE_F, followed by an exponential increase at low ΔE_F, observed in both CsMAFA/C₆₀ bilayers and complete device stacks.

These behaviors can be rationalized by a transition from surface recombination dominated decay at high ΔE_F to a shallow trap dominated regime at lower ΔE_F. To decouple the different physical mechanisms and quantitatively capture this interplay, we develop a semi-analytical model and perform drift-diffusion simulations that account for surface recombination, shallow trap states, and electron exchange (j_exc) between the perovskite and the transport layer. Beyond explaining the experimental observations, this framework provides a general platform for understanding how transport layer thickness and interfacial band offset influence carrier decay dynamics in perovskite/TL bilayer systems.