Figure 2. (a) External quantum efficiency (EQE) spectra for PM6 and PYF-T-o based all-PSC devices; (b) EQE spectra for PM6 and PJ1-γ based devices; (c)J SC versus light intensity of different devices; (d) FTPS-EQE spectra measured from 500 to 1800 nm; inset: a zoom-in view of the EQE spectra in the low energy region; (e) TPV decay curves for devices; (f) TPC decay curves for devices.
The incorporation of SA, along with the utilization of the SqP method, effectively improves the device performance, particularly in the PJ1 system. To understand the charge carrier transport and recombination, we first measured the variation ofJ SC relative to light intensity (plotted on a logarithmic scale) in Figure 2c , where the slope S of each data set is fitted and listed. Typically, S falls within the range of 0.8-1.0, and the variation in the numerical value of Swithin this range is mainly attributed to the loss of charge carriers in bimolecular recombination. Overall, BC devices have slightly less bimolecular recombination than SqP devices, but the difference is small, and the best device in SqP has bimolecular recombination similar to that in BC devices. For BC devices, the S value for (BC)PM6:PYF-T-o is 0.998, for (BC)PM6:PYF-T-o+ SA is 0.994, for (BC)PM6:PJ1-γ is 0.997, and for (BC)PM6:PJ1-γ +SA is 0.994. For SqP devices, the S value for (SqP)PM6/PYF-T-ois 0.953, for (SqP)PM6/PYF-T-o+ SA is 0.951, for PM6/PJ1-γis 0.953, and for PM6/PJ1-γ +SA is 0.992. We also investigated the variation of V OC under different light intensities, and the results, reflected by the ideality factor,n id,l (Figure S2 ), showed two trends: Firstly, the SqP method significantly reduces the trap-assisted recombination as the n id,l values are much lower than those of the BC devices; Secondly, the addition of the SA further decreases the trap-assisted recombination in the SqP devices, but not so strongly as it does in the BC devices. To summarize the combined effects of SqP and SA: SqP does not show improvements in bimolecular recombination without the aid of SA, but significantly reduces trap-assisted recombination, which is further reduced when SA is incorporated.
High-sensitivity EQE measurements were employed to investigate trap states in this study: Sub-bandgap EQE was measured via Fourier-transformed photocurrent spectroscopy (FTPS), and the resulting photocurrent spectra for the BC devices and SqP devices are presented inFigure 2d . Due to noise, the low-energy signals were truncated. The inset of Figure 2d displays a clear quantum efficiency for all devices in the low-energy region (e.g., ~1.20-1.28 eV), which is likely attributed to the presence of deep trap states within the bandgap, resulting in relatively poor FF values for the devices. Notably, devices prepared via the SqP method exhibited fewer states in the low-energy region overall compared to those prepared using the BC method. The trend observed for the devices in the low-energy region of the FTPS test aligns with that of then id values. In order to gain insights into charge recombination and extraction, transient photovoltage (TPV) and transient photocurrent (TPC) experiments were conducted, the results of which are shown in Figure 2e and 2f , respectively. Compared to devices prepared using the BC method, devices prepared using the SqP method display a slower TPV decay and a faster TPC decay. The decay constants are obtained by fitting the TPV and TPC decay curves with a single exponential function. For BC devices, the TPV decay constants and TPC decay constants are 1.02µs, 1.13µs, 1.09µs, and 1.72µs, and 0.053µs, 0.028µs, 0.052µs, and 0.033µs for (BC)PM6:PYF-T-o , (BC)PM6:PYF-T-o +SA, (BC)PM6:PJ1-γ , and (BC)PM6:PJ1-γ +SA. For SqP devices, the TPV decay constants and TPC decay constants are 1.75µs, 2.80µs, 2.59µs, and 3.20µs, and 0.046µs, 0.041µs, 0.040µs, and 0.027µs for (SqP)PM6/PYF-T-o , (SqP)PM6/PYF-T-o +SA, (SqP)PM6/PJ1-γ , and (SqP)PM6/PJ1-γ +SA. We note that the TPV decay constant is referred to as the carrier “lifetime” in some literature, but it has been shown that the constant is affected by charge carrier mobilities and interfacial properties of the device in the TPV measurement, so we choose to stay with the term “TPV decay constant” in this work, which could have contributions from the bulk lifetime and can only be used as an indication of it. Overall, the TPV and TPC results seems to suggest that both the SqP method and the incorporation of the SA may make the charge carriers live longer and be extracted easier compared to the BC method or the SA-free devices.
To assess the charge carrier transport property, we employed the space-charge limited current (SCLC) model to calculate the electron and hole mobilities. The details of SCLC device fabrication and data analysis can be found in the experimental section of the Supplementary Information and in Figure S3 , and the calculated mobilities are summarized in Table S3 . As we have demonstrated in our previous reports,[47, 49] due to the low degree of polymerization of the PSMAs compared to the high degree of polymerization of the donor polymers such as PM6, and their relatively weaker crystallization/aggregation tendency compared to small molecule acceptors, the electron mobility in the recent PSMA-based all-PSC device is generally lower than the hole mobility in the same BC device. For instance, the electron mobility of the (BC)PM6:PYF-T-o device is 2.40×10-4 cm2 V-1s-1 while the hole mobility is 4.14×10-4 cm2 V-1s-1. From the SCLC results in Table S3 , we found that the electron mobility can be improved by both the SqP method and the incorporation of the SA, particularly in the PYF-T-obased devices. The enhanced electron mobility is crucial for the devices with a conventional architecture where more excitons are generated in the bottom part (near the anode) that we will discuss in details later.