State-of-the-art triple cation, mixed halide perovskites have been extensively studied in perovskite solar cells in both n-i-p and p-i-n configurations showing very promising performances and stabilities. To gain more knowledge regarding the collection efficiencies of excess charge carriers in Cs_0.05FA_0.85MA_0.10Pb(I_0.97Br_0.03)₃ (CsMAFA) by a hole (PTAA/Spiro-OMeTAD, Spiro) or electron (C60) transport layer (TL), electrodeless microwave photoconductivity measurements (TRMC) were carried out on bilayers of PTAA/CsMAFA, CsMAFA/C60, CsMAFA/Spiro and C60/CsMAFA. First of all, the TRMC signal heights of all bilayers were reduced to about half of the bare CsMAFA layer. We attribute this to the efficient charge transfer of one type of carrier from CsMAFA to a selective TL. Since the mobility in all TLs is more than an order of magnitude lower than that in CsMAFA, the TRMC signal in the bilayer originates mainly from the carrier residing in the perovskite layer. From the reduction, we come to balanced electron and hole mobilities for CsMAFA of 16 and 12 cm²/Vs independent of the investigated configuration. Moreover, the charge carrier lifetime was elongated in comparison with the bare CsMAFA layer, indicating that both electron and hole transfer is efficient and that interfacial recombination occurs on a longer timescale than the decay in the single layer.

In addition, trilayers of PTAA/CsMAFA/C60 and C60/CsMAFA/Spiro were studied without and with bias illumination (BI) resembling AM1.5, yielding hugely different TRMC signals. Without BI both TLs pull out excess electrons and holes from the CsMAFA layer, yielding only a very small TRMC signal. In contrast, on BI saturation of the TLs occurs, blocking any additional charge extraction of laser-induced excess charges, yielding a much higher and longer-lived TRMC signal. Moreover, the latter signal compares well with the TRMC signal of a bare CsMAFA layer measured under identical conditions. On basis of these observations, we propose that an internal electric field is formed in PTAA/CsMAFA/C60 trilayer under BI, impeding any further efficient charge collection from CsMAFA to the TLs. These results imply that for efficient collection, charges in TLs should be removed rapidly to prevent the build-up of an internal electric field opposing efficient charge collection. This implies that both TLs should have minimal conductivity or more specifically mobility to warrant rapid removal of light-induced charges.