**3. Results and Discussion**

Figure 1A–C respectively correspond to XRD, FT-IR, and Raman analyses to explore the structural properties of the as-prepared materials [14–16]. As shown in Figure 1A, an intense and sharp peak centered at 10.65◦ in the (GO) curve, which corresponds to the (001) crystal surface of the GO nanoflakes. The pattern of pure PEDOT depicts a broad peak in the region of 25.82◦, which corresponds to the polymer chain structure of PEDOT. However, neither of the two peaks appeared in the composite samples. This is due to the influence of conjugation and the coating effect of PEDOT on GO sheets, indicating that the in-situ polymerization changes the growth state of the polymer chain [14,17].

**Figure 1.** (**A**) XRD of GO, PEDOT and PEDOT-GO (**B**) FTIR spectra of GO, PEDOT and PEDOT-GO (**B**) Raman spectra of GO, PEDOT and PEDOT-GO.

In Figure 1B, the PEDOT curve shows two peaks at 981 cm−<sup>1</sup> and 836 cm−<sup>1</sup> are duo to C–S–C bond stretching of thiophene ring. The tensile vibration of the C–O–C bond at 1199 cm−<sup>1</sup> was detected. The peak at 1338 cm−<sup>1</sup> is due to C=C and C–C in the thiophene ring indicating that PEDOT was successfully synthesized. Combining three curves, the characteristic peaks of GO and PEDOT are all reflected in PEDOT-GO. Furthermore, two peaks at 1199 cm−<sup>1</sup> and 1338 cm−<sup>1</sup> on PEDOT skewed to 1214 cm−<sup>1</sup> and 1401 cm−<sup>1</sup> on the curve PEDOT-GO. This redshift phenomenon was due to the π–π stacking interaction between GO and PEDOT [13].

In Raman spectra, the three characteristic peaks of 441 cm<sup>−</sup>1, 1434 cm−1, and 1505 cm−<sup>1</sup> in the red curve indicate the successful synthesis of PEDOT [18]. Meanwhile, the characteristic peaks of 1343 cm−<sup>1</sup> and 1590 cm−<sup>1</sup> (black curve) correspond to the respiratory vibration peaks of SP2 hybrid carbon atoms and the symmetric stretching motion peaks of SP2 hybrid atoms in the carbon ring, respectively, which are the characteristic peaks (D and G) of GO. Meanwhile, the characteristic peaks at 1434 cm−<sup>1</sup> (red curve) are assigned to C<sup>α</sup> = C<sup>β</sup> symmetric stretching vibration in PEDOT, which moves to 1427 cm−<sup>1</sup> in the Raman spectra of the PEDOT-GO sample (Figure 1C). This redshift phenomenon demonstrates that the PEDOT polymer changed to the quinoid form, and thus enabled the increase of conductivity [19]. Moreover, the characteristic peaks of GO and PEDOT are reflected in the curve of PEDOT-GO, confirming the in-situ polymerization of PEDOT-GO nanocomposites.

Figure 2a depicts an SEM cross-sectional view of the device. The cell structure is clearly displayed, and the thickness of the composite film is about 50 nm. Figure 2b presents the top view of the device [20–22]. The entire composite film is thin and evenly covered on the perovskite layer. Figure 2c,d are the TEM images of the GO and PEDOT-GO composite material, respectively. It can be seen that the surface of the graphene oxide sheet is smooth and transparent (Figure 2c). By comparison, the surface of the graphene oxide sheet is coated with a large amount of PEDOT nanoparticles in the PEDOT-GO film (Figure 2d), which is mutually confirmed with the previous analysis. This tight combination comes

from the presence of conjugated heterocyclic structures and electronegative oxygen atoms. Simultaneously, PEDOT rich in free electrons and GO rich in carboxyl group form a good conjugated structure.

**Figure 2.** (**a**,**b**) SEM images of the device (**c**) TEM images of GO (**d**) TEM images of PEDOT-GO.

In order to investigate the optimum component, samples with PEDOT/GO ratio as 0, 0.5, 0.75, 1 were prepared and tested, respectively. Figure 3a is the J–V curves of the mesoporous PSCs with different mass ratio PEDOT-GO composite films. The FTO/cp-TiO2/MAPbI3/C structure of PSC was fabricated as a control group compared with FTO/cp-TiO2/MAPbI3/PEDOT-GO (or spiro-OMeTAD)/C structure. Figure 3b shows the Nyquist plots and the equivalent circuit model of the PSCs [23]. The high frequency arc is reflected to the hole transport and extraction between the PEDOT-GO and the carbon cathode; the low frequency arc shows charge recombination of PSCs [24].

The corresponding photovoltaic parameters are shown in Table 1. The device exhibits the highest performance when the PEDOT-GO mass ratio is 0.75, the PCE reached the 14.09% with the voltage (**Voc**) as 1.10 V, the short-circuit current (**Jsc**), as 20.36 mA/cm2, and the fill factor (FF) as 0.63. The PCE of the modified sample was increased by 26.6% compared to the HTL-free one. It is worth noting that, the FTO/TiO2/MAPbI3/spiro-OMeTAD/C sample with the PCE of 13.49%, Voc of 1.10 V, Jsc of 20.70 mA/cm2, and FF of 0.59 shows similar performance to the PEDOT-GO (0.75) sample. Meanwhile, the lowest Rtr value of the PEDOT-GO (0.75) sample as 23.9 Ω indicates the excellent charge transfer performance, and the highest Rrec value as 187.4 Ω indicates its best anti-recombination property among all cells [25]. When the PEDOT/GO mass ratio is lower than 0.75, the **Jsc** increases with the PEDOT content, but the **Voc** remains unchanged, indicating that the hole transport performance of the composite material is effectively optimized and enhanced. However, when the PEDOT/GO mass ratio gets higher than 0.75, the dispersion of the composite material in the solvent becomes worse, suggesting the insufficient addition of GO, which leads to the deterioration of the film quality and negatively affects both the **Jsc** and **Voc**. Finally, the mass ratio of PEDOT / GO is determined to be 0.75, the film quality and hole transport performance of the composites reach a balance, and the highest PCE is obtained. In addition, the hysteresis in the J-V curve of PEDOT-GO based devices is significantly reduced compared with the HTL-free devices (Figure 3c) [26]. PEDOT-GO composite material effectively optimized the hole extraction and transfer ability of

the device, and reduces the built-in electric field at the interface of perovskite and HTL. Figure 3d shows the corresponding incident photon-to-electron conversion efficiency (IPCE) curves: the integrated current value for PEDOT-GO (0.75) sample was 19.69 mA cm−2, which is consistent with the **Jsc** values extracted from the J-V curves.

**Figure 3.** (**a**) J-V curves of control (HTL-free) sample and mass ratio PEDOT/GO: 0/1, 0.5/1, 0.75/1, 1/1, spiro-OMeTAD samples (**b**) Nyquist plots of resistance for the above samples (**c**) current-voltage characteristics with forward and reverse scans of PEDOT-GO and control sample (**d**) IPCE spectra of the PEDOT-GO (0.75) devices (**e**) steady-state PL spectra of PSCs of HTL-free sample and PSCs with PEDOT-GO (0.75) (**f**) time-resolved PL spectra of HTL-free sample and PSCs with PEDOT-GO (0.75).

The steady-state photoluminescence (PL) and the time-resolved photoluminescence (TRPL) tests were also conducted to evaluate the hole extraction ability of the HTL [27]. In Figure 3e, with the introduction of PEDOT-GO as HTL, the intense fluorescence at 790 nm is obviously suppressed. As shown in Figure 3f, for the control sample, the fast decay time (τ1) was 133.44 ns, and the slow decay time (τ2) was 63.08 ns, with an amplitude τave (τave = ΣAiτ<sup>i</sup> 2/ΣAiτi, where A1 and A2 are pre-exponential factors) of 103.72 ns. For the PEDOT-GO sample, τ<sup>1</sup> was 99.63 ns, and τ<sup>2</sup> was 45.71 ns, derived in an amplitude τave of 66.34 ns. Obviously, the sharp decrease in the average fluorescence lifetime indicates that the PEDOT-GO film effectively inhibits the charge recombination. This is consistent with the analysis of the polymer structure obtained by Raman. The above experiments further verify the positive effect of the PEDOT-GO film, which dramatically promote the separation and directional transmission of electrons and holes, thus explaining the increased PCE in the device [28].

**Table 1.** Photovoltaic parameters of PSCs (Sweep speed of 0.25 V s<sup>−</sup>1, voltage range 0 V–1.2 V, electrode area of 0.06 cm2).


The stability of solar cells samples, without encapsulation, was further evaluated and compared in the air with the humidity of ~35% [29]. As shown in Figure 4, after ten-days placement, the PCE of spiro-OMeTAD-based solar cells decreased to 75% of the initial value, while the PCE of PEDOT-GO-based solar cells still maintained 90% of the initial value. As an approximation, the time at which the efficiency has degraded to 80% of the initial value was denoted as Ts80 [30]. We can observe that the Ts80 of spiro-OMeTAD-based solar cells was about 5 days, while it needed more than 10 days for that of PEDOT-GO-based solar cells. The device with PEDOT-GO HTL takes twice as long to fall to the same level of spiro-OMeTAD-based solar cells, which suggests that better durability can be realized by the introduction of PEDOT-GO composite films.

**Figure 4.** The durability test of spiro-OMeTAD-based solar cells and PEDOT-GO-based solar cells.
