*3.2. Optical and Electronic Characterizations*

The thickness of CCO and Mg:CCO nanoparticle films are controlled by the number of coating cycles that were performed during deposition [27]. Figure 7a–c shows SEM images for CCO, 5% Mg:CCO, and 10% Mg:CCO films; no regions of bare substrate are seen for all films. Figure 8a shows the UV-vis absorbance and transmission (inset) spectra of well-covered CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) films. The absorbance values at 300 nm lie between 0.21 and 0.22 for all films, which are highly transparent (transmission > 90%) in the visible region. All three films are 18 nm thick as determined by ellipsometry. The direct band gap (*Eg*) is extrapolated from the Tauc plot (Figure 8b). The average values of the direct *Eg* are 3.27 ± 0.02 eV, 3.25 ± 0.03 eV, and 3.27 ± 0.03 eV for CCO, 5% Mg:CCO, and 10% Mg:CCO, respectively (Table 3). These values are the same within the uncertainty of the measurement (~0.03 eV). Thus, Mg doping does not affect the direct *Eg* of CCO. The *Eg* for pure CCO is consistent with our previous result [27].

**Figure 7.** SEM images of (**a**) CCO, (**b**) 5% Mg:CCO, and (**c**) 10% Mg:CCO films on ITO substrates.

**Figure 8.** (**a**) Ultraviolet–visible (UV-vis) absorbance and transmission (inset) spectra and (**b**) Tauc plots and linear fits of the band edge (straight lines) for CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) films.

**Table 3.** Thickness and direct *Eg* of CCO, 5% Mg:CCO, and 10% Mg:CCO films with ~0.22 absorbance at 300 nm wavelength.


<sup>1</sup> Direct *Eg* is averaged over three measurements.

Figure 9a shows box plots of the work function (*WF*) for CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) films. The median *WF* values of CCO, 5% Mg:CCO, and 10% Mg:CCO films are 5.19, 5.17, and 5.22 eV, respectively. The 50th percentile *WF* value for the 10% Mg:CCO is higher than that of CCO and 5% Mg:CCO. The 5% Mg:CCO films exhibit the largest spread with a long tail to the large *WF* than CCO films. Thus, Mg:CCO films generally appear to have higher *WF* values than CCO films, although the difference is below the level of statistical significance. Figure 9b shows box plots of the ionization energy (*IE*) for CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) films. The median *IE* values of CCO, 5% Mg:CCO, and 10% Mg:CCO films are 5.11, 5.08, and 5.06 eV, respectively. The energy step for the *IE* measurement is 0.05 eV. The 50-percentile *IE* value decreases monotonically with increasing Mg concentration. Furthermore, the measured *IE* value is consistent with the *IE* of 5.1 eV from previous band structure calculations [27]. As shown in Figure 9, the overall *WF* values are larger than *IE* values, especially for Mg:CCO films. Thus, these films are *p*-type degenerately doped. The difference between *WF* and *IE* values (*WF – IE*) increases with Mg concentration from 0.08 eV for CCO to 0.16 eV for 10% Mg:CCO (Table 3), indicating that Mg:CCO films may have higher conductivity, consistent with previous results [35,36].

**Figure 9.** Box plots of (**a**) work function and (**b**) ionization energy for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) films. The percentiles are set to 90% whisker top, 75% box top, 25% box bottom, and 10% whisker bottom for each data set. In (a), 20 batches of *WF* measurements are used in each data set. In (b), 13 batches of *IE* measurements are used in each data set.

#### *3.3. CCO and Mg:CCO as HTLs in OSCs and PSCs*

Figure 10a and Table 4 show the results of P3HT:PC61BM devices with CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs. The average *Jsc* of P3HT:PC61BM devices is higher with Mg:CCO HTL (from 6.94 mA cm−<sup>2</sup> for CCO to ~7.05 mA cm−<sup>2</sup> for Mg:CCO). The average *Voc* of Mg:CCO is also higher than that of undoped CCO HTL (~0.582 V versus 0.570 V). However, both Mg:CCO devices exhibit lower average *FF* (0.642 for 5% doping and 0.666 for 10% doping versus 0.685 for no doping). The tradeoff of the three parameters leads to similar *PCE* values for all devices independent of Mg doping. We note that the variation among different diodes is larger in *Jsc* than *Voc* or *FF*, which is typical of OPV devices. Nonetheless, there is a systematic trend of average *Jsc* increase with Mg doping.

Figure 10b and Table 4 show the results of PFBT2Se2Th:PC71BM devices with CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs. The average *Voc* and *FF* are similar in all devices with values of ~0.665 V and ~0.685, respectively. The average *Jsc* of the devices increases monotonically from 10.50 mA cm−<sup>2</sup> for undoped CCO HTL to 10.88 mA cm−<sup>2</sup> for 10% Mg:CCO HTL. Thus, the *PCE* values of PFBT2Se2Th:PC71BM devices are higher when Mg:CCO, instead of undoped CCO, is used as the HTL. Among different diodes, the variation of *Jsc* is larger than that of *Voc* or *FF*. Moreover, Mg:CCO devices have even larger variation of *Jsc*. However, a similar systematic trend of increasing average *Jsc* as P3HT:PC61BM devices is observed in PFBT2Se2Th:PC71BM devices.

Figure 10c and Table 4 show the results of PTB7-Th:ITIC devices with CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs. The average *Jsc* of the devices increases monotonically from 11.55 mA cm−<sup>2</sup> for undoped CCO HTL to 12.02 mA cm−<sup>2</sup> for 10% Mg:CCO HTL. The average *Voc* and *FF* are highest for the 5% Mg:CCO in this batch of devices, but they do not depend on Mg doping in other batches. Generally, the *PCE* values of PTB7-Th:ITIC devices are higher when using Mg:CCO as the HTL due to the increase in *Jsc*. This work is the first using CCO and Mg:CCO as HTL for BHJ OSCs with a non-fullerene acceptor.

Figure 10d and Table 4 show the results of MAPbI3 PSCs with CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs under forward (solid lines) and reverse scans (dashed lines). Only slight hysteresis is seen, indicating minimal trap states at the CCO or Mg:CCO/perovskite interface. Under forward scan, the average *Voc* increases monotonically from 0.985 V for the undoped CCO HTL to 1.007 V for the 10% Mg:CCO HTL. Similar trends are observed in the *Jsc* (from 18.91 mA cm−<sup>2</sup> for the undoped CCO HTL to 19.40 mA cm−<sup>2</sup> for the 10% Mg:CCO HTL) and *FF* (from 0.678 for the undoped CCO HTL to 0.703 for the 10% Mg:CCO HTL). Overall, the *PCE* of the devices improves monotonically from 12.64% for the undoped CCO HTL to 13.73% for the 10% Mg:CCO HTL. We note that the variation among different diodes is large for all the parameters. However, there are systematic trends of increases among the average *Jsc*, *Voc*, and *FF* with Mg doping. Under reverse scan, the average *Jsc* increases monotonically from 18.70 mA cm−<sup>2</sup> for the undoped CCO HTL to 19.37 mA cm−<sup>2</sup> for the 10% Mg:CCO HTL. The *FF* of the devices using the 5% Mg:CCO and 10% Mg:CCO HTLs were similar,

0.719, but for the undoped CCO HTL, a lower *FF* (0.697 versus 0.719) was observed. The *Voc* is similar in all devices with values of ~1.01 V. Overall, the *PCE* of the devices improves monotonically from 13.19% for the undoped CCO HTL to 14.12% for the 10% Mg:CCO HTL. As in the forward scan data, despite the variation among different diodes, there is a systematic trend of increasing average *Jsc* with Mg doping. Jeong et al. observed that Mg:CCO produces PSCs with a slightly higher *Jsc* and *Voc*, but a lower *FF*, resulting in no improvement in the *PCE*; however, they did not report Mg concentration [40]. The inset in Figure 8d shows the external quantum efficiency (EQE) at wavelength ranging from 300 to 800 nm for CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs. A broadband increase in EQE is observed with Mg doping, consistent with the increases of average *Jsc* in forward and reverse *J-V* scans.

**Figure 10.** Average *J-V* curves (number of devices for each system is given in the footer of Table 4) of (**a**) P3HT:PC61BM OSCs, (**b**) PFBT2Se2Th:PC71BM OSCs, (**c**) PTB7-Th:ITIC OSCs, and (**d**) MAPbI3 PSCs measured in AM 1.5G 100 mW cm−<sup>2</sup> illumination with CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) hole transport layers (HTLs). In (d), solid *J-V* curves are measured under forward scan, dashed *J-V* curves are measured under reverse scan, and the inset is the external quantum efficiency (EQE) measurements of representative MAPbI3 cells with CCO (black curve), 5% Mg:CCO (red curve), and 10% Mg:CCO (blue curve) HTLs.


**Table 4.** The device parameters of OSCs and MAPbI3 PSCs with CCO, 5% Mg:CCO, and 10% Mg:CCO HTLs.

<sup>1</sup> For P3HT:PC61BM OSCs, 12, nine, and eight devices were measured for CCO, 5% Mg:CCO, and 10% Mg:CCO HTL, respectively. For PFBT2Se2Th:PC71BM OSCs, eight, seven, and 11 devices were measured for CCO, 5% Mg:CCO, and 10% Mg:CCO HTL, respectively. For PTB7-Th:ITIC OSCs, 10, eight, and nine devices were measured for CCO, 5% Mg:CCO, and 10% Mg:CCO HTL, respectively. For MAPbI3 PSCs, 10, 11, and 10 devices were measured for CCO, 5% Mg:CCO, and 10% Mg:CCO HTL, respectively, under both forward and reverse scans.

Several groups have reported elemental diffusion from inorganic transport layer into MAPbI3 when using CdS ETL [58], CrO*x* [59], and CuI [60] HTLs. In order to examine this possibility, we performed XPS studies on the surfaces of MAPbI3 films on top of ITO/HTL. Figure 11 shows the normalized XPS spectra of (a) survey, (b) Cu 2p, (c) Cr 2p, and (d) Mg 2p core levels for MAPbI3 films processed on top of CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue) HTLs. For all HTLs, all peaks in the survey spectra are indexed as the component elements (C, N, Pb, and I) of MAPbI3, consistent with our previous result [27]. The Cu 2p and Cr 2p spectral ranges are free of any peaks corresponding to Cu 2p or Cr 2p (dashed lines), indicating no presence of Cu and Cr elements at the surface of MAPbI3 layer. The Mg 2p spectrum shows two peaks at 48.2 eV and 46.6 eV corresponding to the binding energy of the I 4d orbitals [61]. No Mg 2p peak at 50.8 eV (dashed line) was detected, indicating no presence of Mg element. Thus, if metal diffusion from CCO and Mg:CCO HTLs into MAPbI3 occurs, it does so at a level below the sensitivity of XPS. This result is consistent with thermodynamic calculation: the calculated formation enthalpy of CCO is -6.0 eV [22], significantly lower compared to that of CdS (−1.5 eV) [62] and CuI (−0.3 eV) [63] and slightly lower compared to that of Cr2O3 (−5.9 eV) [64]. Mg:CCO has the same crystalline structure as CCO and the Mg doping content is small, thus, the formation enthalpy of Mg:CCO is expected to be similar to that of CCO. Thus, CCO and Mg:CCO are more stable and less likely to decompose or react than the aforementioned HTLs. Nevertheless, additional experimentation is warranted to explore the possibility of reactivity between CCO/Mg:CCO and perovskite phases.

**Figure 11.** Normalized XPS (**a**) survey, (**b**) Cu 2p, (**c**) Cr 2p, and (**d**) Mg 2p spectra at the surfaces of MAPbI3 films on top of ITO/HTL. HTLs are CCO (black), 5% Mg:CCO (red), and 10% Mg:CCO (blue). In (**a**), all peaks are indexed by the component elements (C, N, Pb, and I) of MAPbI3. In (**b**–**d**), the dotted lines show binding energies for the Cu 2p, Cr 2p, and Mg 2p core levels of CCO and Mg:CCO. The positions of Cu 2p1/2, Cu 2p3/2, Cr 2p1/2, and Cr 2p3/<sup>2</sup> peaks are indexed according to our previous CCO reports [24,27]. The position of Mg 2p peak is indexed according to the report from Hoogewijs et al. [65]. In (**d**), the peaks correspond to the I 4d orbitals; peaks due to the Mg 2p orbitals are not observed.

In order to explore charge transport at the CCO and Mg:CCO/MAPbI3 interface, we performed TRPL measurements. Figure 12a shows the PL emission spectrum for ITO/MAPbI3 (green), ITO/CCO/MAPbI3 (black), ITO/5% Mg:CCO/MAPbI3 (red), and ITO/10% Mg:CCO/MAPbI3 (blue). For all samples, the main PL emission peaks are at ~750 nm, consistent with the literature [66]. PL intensities for MAPbI3 on top of CCO and Mg:CCO HTLs are lower compared to that of MAPbI3 on ITO, indicating CCO and Mg:CCO HTLs are effective in promoting charge transfer. However, PL intensities are similar among MAPbI3 on top of CCO and Mg:CCO HTLs. Figure 12b shows the normalized TRPL decay kinetics for ITO/MAPbI3 (green), ITO/CCO/MAPbI3 (black), ITO/5% Mg:CCO/MAPbI3 (red), and ITO/10% Mg:CCO/MAPbI3 (blue). With the addition of CCO and Mg:CCO HTLs, a faster PL decay is observed relative to ITO/MAPbI3. The inset table in Figure 12b shows the PL lifetimes extracted from three exponential fits in all samples (lines in Figure 12b). The τ*1*, τ*2*, and τ*<sup>3</sup>* lifetimes of the ITO/MAPbI3 structure are 1.5 ns, 4.9 ns, and 16.3 ns, respectively. After adding CCO and Mg:CCO HTLs, the τ*1*, τ*2*, and τ*<sup>3</sup>* decreases to 0.7 ns, ~3.0 ns, and ~11.0 ns, respectively, indicating enhanced charge extraction and consistent with the literature result [66]. Again, there are no significant differences in PL lifetimes among films of MAPbI3 on CCO and Mg:CCO HTLs. Mg doping in CCO is expected to lead lower PL intensity and shorter lifetimes. However, these effects are not discernable in our TRPL results, presumably because they may be confounded by factors besides charge transfer, such as surface recombination [67].

**Figure 12.** (**a**) Photoluminescence (PL) emission spectra and (**b**) time-resolved photoluminescence (TRPL) decay for ITO/MAPbI3 (green), ITO/CCO/MAPbI3 (black), ITO/5% Mg:CCO/MAPbI3 (red), and ITO/10% Mg:CCO/MAPbI3 (blue). In (**b**), the lines are fits to three exponential decays: dotted green line for ITO/MAPbI3, solid grey line for ITO/CCO/MAPbI3, dashed brown line for ITO/5% Mg:CCO/MAPbI3, and dotted-dashed blue line for ITO/10% Mg:CCO/MAPbI3. The inset table in (**b**) shows the fitted PL lifetimes for all samples.

Figure 13 shows the stabilized photocurrents and efficiencies for representative MAPbI3 cells with CCO, 5% Mg:CCO, and 10% Mg:CCO HTLs. Time-dependent photocurrent measurements are taken at a bias of ~0.8 V with stabilized photocurrent values of 17.35 mA cm−2, 18.05 mA cm−2, and 17.81 mA cm−<sup>2</sup> for CCO, 5% Mg:CCO, and 10% Mg:CCO HTLs, respectively. The values of the stabilized photocurrent are higher with Mg doping, reflecting the increases in *Jsc* and *FF* observed in forward and reverse scans for Mg:CCO MAPbI3 PSCs. The stabilized efficiencies for CCO, 5% Mg:CCO, and 10% Mg:CCO HTLs are 13.89%, 14.43%, and 14.26%, respectively.

**Figure 13.** The stabilized photocurrents and efficiencies for the representative MAPbI3 cells with (**a**) CCO, (**b**) 5% Mg:CCO, and (**c**) 10% Mg:CCO HTLs. In (**a**,**b**), the applied bias is at 0.8 V. In (**c**), the applied bias is initially at 0.85 V. After 50 s, it switches to 0.8 V.

Figure 14 shows the bar charts of average *Jsc* for P3HT:PC61BM OSCs, PFBT2Se2Th:PC71BM OSCs, PTB7-Th:ITIC OSCs, and MAPbI3 PSCs under forward and reverse scans for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) HTLs. The average *Jsc* of all OSCs and MAPbI3 PSCs are higher with Mg:CCO HTLs. The small average *Jsc* increases in all systems may be partially attributed to the better conductivity of Mg:CCO HTLs resulting from the increased *WF* with respect to *IE* with Mg doping. Additionally, the broadband increase with Mg doping content in the EQE spectra of PSCs (Figure 10d inset) signifies that the increased HTL work function contributes to a stronger electric field within the device, more efficiently extracting photoexcited carriers regardless of the depth at which the generating photons are absorbed. If *Voc* and *FF* are independent of Mg doping, the *PCE* may be expected to increase due to the boost in *Jsc*. However, they do not show a consistent trend from batch to batch. *Voc* and *FF* are more susceptible to film roughness, which can vary due to aggregation

of the nanoparticles in the suspensions and variation in spin coating conditions. The tradeoff between *Jsc* and *Voc*/*FF* results in little or no statistical *PCE* improvement (Table 4).

**Figure 14.** The average *Jsc* barcharts with error bars for P3HT:PC61BM OSCs, PFBT2Se2Th:PC71BM OSCs, PTB7-Th:ITIC OSCs, and MAPbI3 PSCs under forward and reverse scans for CCO (black color), 5% Mg:CCO (red color), and 10% Mg:CCO (blue color) HTLs.
