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-γ 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.