## 3 Results
A small contact angle value indicates that the film is highly hydrophilic and has low surface roughness. Figure 4: (**a–c**) Topography and (**d–f**) 3D AFM images of ITO/rGO/PEDOT:PSS/PbS:PCBM layer without and with the PEO buffer layer; (**g–k**) video contact angle images. The topography and three-dimensional (3D) images of the surface morphology of the ITO/rGO/PEDOT:PSS/PbS + PCBM without PEO layer are displayed in Figure 4a,d. The surface roughness is about \(\sim\)8.6 nm and the contact angle is about 32.5\({}^{\circ}\). Figure 4b,e and Figure 4c,f show the topography and 3D images of the ITO/rGO/PEDOT:PSS/PbS + PCBM with PEO of 3000 rpm (SS-3000) and 4000 rpm (SS-4000), respectively. The results show that when inserting the PEO layer between the active layer and E-GaIn layer, the surface roughness and contact angle of the device's active layers decrease. The lowest value of roughness and contact angle are 4.7 nm and 13.5\({}^{\circ}\) for the sample (shown in Figure 4c,f,k, which was coated with PEO film (SS-3000)). Adding the PEO film makes the surface roughness of the active layer smoother, which leads to the improvement of the contact between the active layer and electrode. The device's fabrication process and the complete device structure are presented in Figure 5a. The effects of PEO buffer layer on the electrical property of the device were investigated by considering the parameters of the solar cell, as shown in Figure 5b and Table 2. The device without PEO film shows an efficiency of 3.27% (J\({}_{\text{sc}}\): 6.69 (mA/cm\({}^{2}\)); V\({}_{\text{oc}}\):0.7(V); FF:70). The device with PEO (SS-3000) film has an efficiency of 3.78% (J\({}_{\text{sc}}\): 7.42 (mA/cm\({}^{2}\)); V\({}_{\text{oc}}\): 0.75 (V); FF:65) and the device with PEO (SS-4000) film has an efficiency of 4.34% (J\({}_{\text{sc}}\): 7.61 (mA/cm\({}^{2}\)); V\({}_{\text{oc}}\): 0.75 (V); FF:72). The device's active layer morphology was controlled by adding PEO as a buffer layer, which led to an increase in the device's performance. Increasing short-circuit current is related to the improvement of the contact between the  
Figure 5: (**a**) Device fabrication process and (**b**) the current density-voltage (J-V) curves of the device without and with the PEO buffer layer. electrode and the active layer. More charge carriers transported to the electrode lead to an increase in the device's efficiency. In this study, based on PbS quantum dots and PCBM, the parameters of vacuum-free quantum dot planar hybrid solar cells were investigated by inserting reduced rGO and PEO into the fabrication process. The device's efficiency is still low compared with the inverted structure of quantum dot PbS solar cells and thin-film structure (PCE-10%) [32]. However, the highest current density circuit, open-circuit voltage and maximum power conversion efficiency of about 4.34% were demonstrated using a simple and vacuum-free process. This efficiency is over two times the performance of the previously reported planar hybrid PbS-organic polymer solar cells [13; 30]. This value is low compared with the recorded value of the conventional invert device structure, but it can present a new structure to improve the efficiency of the quantum dot PbS solar cell, as well as the hybrid solar cells based on it.