## 3 Results
It is known that contact resistance between the active layer and electrode and the charge recombination process are affected by the values of Rs and Rsh [20; 21; 22; 23]. Low values of shunt resistance led to the increased leakage of some current, and high values of series resistance led to decreased current density values. In the case of a solar cells device, the series resistance played a dominant role in determining the power conversion efficiency of solar cells [24; 25]. A small value of Rs contributed to an increase in the charge carriers' mobility in the photoactive layer, which led to higher efficiency. The Rs and Rsh resistance of the devices, with and without inserting the rGO layer, were measured from the solar simulator. The highest value of series resistance, Rs = 21.3 \(\Omega\) cm\({}^{2}\), and \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Device** & \(\mathbf{J_{sc}}\) & \(\mathbf{V_{oc}}\) & FF & **Eff** \\
**Structure** & **(mA/cm\({}^{2}\))** & **(V)** & **(\%)** & **(\%)** \\ \hline With & 3.41 \(\pm\) 0.02 & 0.67 \(\pm\) 0.03 & 62 \(\pm\) 4 & 1.52 \(\pm\) 0.02 \\ PEDOT:PSS & 4.58 \(\pm\) 0.01 & 0.70 \(\pm\) 0.02 & 67 \(\pm\) 2 & 2.14 \(\pm\) 0.01 \\ With & & & & \\ rGO/PEDOT:PSS & 6.69 \(\pm\) 0.01 & 0.70 \(\pm\) 0.00 & 70 \(\pm\) 2 & 3.27 \(\pm\) 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Device parameters as open-circuit voltage (V\({}_{oc}\)), short-circuit current density (J\({}_{sc}\)), fill factor (FF) and power conversion efficiency (PCE) from J-V characteristics for different hole transport layers; with PEDOT:PSS, rGO and rGO/PEDOT:PSS under A.M 1.5G illumination at 100 mW/cm\({}^{2}\). Figure 3: (**i**) (**a**) Current density-voltage curve of photovoltaic devices based on different hole transport layers and (**b**) the energy level alignment of devices with PEDOT:PSS as the HTL and rGO as the buffer layer. (**ii**) Device structure (**a**) ITO/PEDOT:PSS/PbS:PCBM/E-GaIn, (**b**) ITO/rGO/PbS:PCBM/E-GaIn, (**c**) ITO/rGO/PEDOT:PSS/PbS:PCBM/E-GaIn. the smallest value of current density, \(\rm J_{\rm sc}=3.41\) mA/cm\({}^{2}\), led to low efficiency, PCE = 1.52%. When the rGO layer was inserted as a buffer layer, the series resistance value decreased and the value of current density increased. The lowest value of series resistance, Rs = 9.8 \(\rm\Omega cm^{2}\), and the highest value of current density, \(\rm J_{\rm sc}=6.69\) mA/cm\({}^{2}\), of the device with rGO/PEDOT:PSS as the hole transport layer obtained the highest power conversion efficiency, PCE = 3.27 %. Figure 3i,b depicts the energy level of the device with PEDOT:PSS as an HTL and rGO as a buffer layer. Based on the literature, the work function of ITO, PEDOT:PSS and E-GaIn are -4.7 eV, 5.0 eV and 4.2 eV, respectively [26; 27; 28]. The work function of rGO is 4.8 eV [29]. It was reported that the HOMO and LUMO levels of PbS are \(-5.0\) and \(-4.0\) eV and the HOMO and LUMO levels of PCBM are \(-6.0\) and \(-4.0\) eV [30]. This indicates that the reduced rGO buffer layer is capable of overcoming the energy barrier between ITO and PEDOT:PSS, which leads to improving the hole transportation from the p-n junction to the anode (ITO), resulting in the device's efficiency increasing. To improve the efficiency of the device, the morphology of the photoactive layer should be optimized. PEO is a good material to use as the buffer layer because PEO solubilizes well in water and many other organic solvents. Depositing PEO film without damaging the surface roughness of the active layer, and making it smoother, leads to improved device performance [31]. The PEO buffer layer with different spin-speeds (round per minute (rpm)) was inserted between the photoactive layer and back contact (E-GaIn) as an interface layer. The surface morphologies of the photoactive layer with spin-speeds of PEO were observed using atomic force microscopy (AFM) and video contact angle (VCA) techniques, as shown in Figure 4. The contact angle value is related to the film's surface roughness and hydrophilic nature.