–Solution-processed flexible organic photovoltaics (OPVs) have gained tremendous popularity worldwide during the last two decades. Indium tin oxide (ITO) is commonly used as a transparent conductive electrode in OPVs due to its large conductivity and good transmittance but it is fragile, rigid, and quite expensive. In this paper, flexible solar cells, with Poly (3, 4-ethylenedioxythiophene): poly(styrene-sulfonic acid) (PEDOT: PSS) transparent conductive electrodes are successfully demonstrated on a Polyethylene Naphthalate (PEN) substrate. Prior to PEDOT: PSS electrode, a buffer layer of polyvinyl alcohol (PVA) is added to PEN substrate. The OPV with PEDOT:PSS anode produced power conversion efficiency (PCE) average values of (12.0 % ± 0.1%) while the reference device with ITO exhibited PCE values of (8.0 % ± 0.0%) under a 500-lux-light emitting diode (LED) lamp. Besides PCE improvement, a flexible OPV device also exhibits superior flexibility over ITO as it can retain its PCE up to 85% even after 500 bending cycles in air while the PCE of ITO-based device declined to 15 % of its original value during a similar bending condition. To our knowledge, this excellent bending flexibility and PCE value are the highest recorded, under a 500-lux LED lamp on a PEN substrate. Keywords— organic photovoltaics, ITO-free, solution-processed, flexibility, dim lighting conditions. INTRODUCTION Organic photovoltaic has become one of the promising sustainable economic solutions as a renewable energy source over the last few decades [1]. The solar cell has been widely used in modern applications like robotics, greenhouses, portable electronics, consumer products, and electronic textiles due to its enormous advantages of low-cost, high-throughput, easy fabrication processes for large area solar cells, lightweight and suitable for flexible substrates [2]. Apart from the use of solar cells in outdoor environments, many low-power applications such as sensors, chargers, actuators, and wireless sensor nodes (WSN) etc. in houses and offices are also operating under low light conditions [3]. The irradiation spectrum and intensity of the light sources used for indoor applications are different from that of under 1-sun [4]. Various artificial sources like a light emitting diode (LED), fluorescent lamp, halogen lamp and incandescent lamps are usually used for dim light applications [5]. Flexibility is one of the important factors for the commercialization of organic photovoltaic technology in consumer applications. Flexible organic devices need a flexible transparent bottom electrode, which should exhibit high conductivity, better mechanical flexibility and transmittance. Currently, Indium Tin Oxide (ITO) is a standard transparent bottom electrode widely used in photovoltaic devices but its high cost, low conductivity on plastic substrates, and poor bending capabilities hinder its use for flexible optoelectronic systems. There are alternative transparent conductive electrodes such as carbon nanotubes, conductive polymers, silver nanowires, graphene and many others that have been used on different flexible substrates. Single-walled carbon nanotube network has been used as transparent electrode with average transmittance and sheet resistance of 85% of 200 Ω/sq respectively. Metal Nanowires such as Cu and Ag are also good alternatives of ITO substrate. Yang et al. with PCE of 2.5 % using conventional geometry have fabricated AgNw on PET substrate. Their device presented transmittance of 80% with a comparatively lower sheet resistance of 30.8 Ω/sq. In 2016, Wang et al., increased transmission of Cu NWs electrode to 90% by applying hydrogen plasma treatment at room temperature while sheet resistance declined to 19 Ω/sq. Graphene is also an ideal contender for next-generation flexible electrodes due to its mechanical stability, high carrier mobility (2 × 105 cm2/Vs), low cost, large specific area (2.6 × 103 m2/g) and better transmittance. Conductive polymer like PEDOT: PSS is extensively used polymer for fabrication of solar cells. Different strategies including surface treatment by acids, spin-rinsing, and dry transfer of PEDOT: PSS, has been proposed to increase the electrical conductivity, and optical transmittance of flexible electrode employed by PEDOT: PSS. Zhou et al. used a combination of PH500 and PEDOT (EL) as a conductive electrode. The structure of their device was PET/polymer anode/APFO-3: PCBM/LiF/Al, which demonstrated PCE of 2.2 %. All the previously described transparent electrodes are investigated under 1-sun conditions while few issues for example high surface roughness of the electrodes, costly and complex fabrication methods, still exists for the use of these electrodes under low light conditions. Recently, we reported ITO-free flexible organic solar cell employing ZnO/Ag/ZnO as the transparent conducting anode for indoor light applications with high optical transmittance up to 92% and very low sheet resistance of 4.8 Ω/sq. The inverted structure (PET/ZnO/Ag/ZnO/PEIE/P3HT: ICBA/MoOx/Ag). In the current study, ITO free flexible organic solar cell is fabricated using PEDOT: PSS (PH1000) as transparent anode on a PEN substrate. The flexible device with P3HT: ICBA active layer showed higher power conversion efficiency (PCE) of ~ 11% under 500-lux-LED approximately 25% higher than the reference ITO based device, which exhibits 8% under similar conditions. Furthermore, it shows superior mechanical flexibility over ITO up to 500 bending cycles and can withstand 85% of its PCE at a bending radius of 7.99 mm without mechanical deformation. WORKING MECHANISM AND ANALYSIS The work function values of ITO and PEDOT: PSS electrodes with and without PEIE layer were calculated using Kelvin probe (KP) in ambient air. Highly oriented pyrolytic graphite (HOPG) and ITO_glass was used as a reference which displayed WF values of (5.14 ± 0.058 eV) and (4.87 ± 0.027 eV) respectively. The schematic of the OPV device with an inverted structure is shown in Figure 1. Fig. 1. Schematic of various participating layers of organic photovoltaic (OPV) device. The WF value of the reference PEN_ITO was found to be (4.82 ± 0.074 eV) while the anode PEDOT: PSS (PH1000) on PEN substrate exhibited higher WF value of approximately (5.02 ± 0.043 eV). There was a huge difference between the WF values of anodes and electron affinity (EA) (3.57eV) of the acceptor ICBA to provide suitable energy-level alignment. Therefore, to resolve this issue, a buffer layer of PEIE for surface modification was added on both anodes. The inclusion of this PEIE layer, resulted in an improved energy-level alignment of anodes with EA of the ICBA by decreasing the WF values of PEN_ITO and PEN_PEDOT: PSS films to (4.05 ± 0.039 eV) and (4.12 ± 0.054 eV) respectively. The optical transparency of the OPVs presented in Figure 2. Transmittance for PEN_PEDOT: PSS and PEN_ITO in visible spectral range (390-700 nm) is found to be ~ 75 and ~ 85 % respectively. PEN_PEDOT: PSS presented slightly higher transmittance (~70 %) compared to that of PEN_ITO (~65%) in 400-450 nm range. Fig. 2. Transmittance spectra of electrodes. The J-V curve of both devices under 1-sun and in the dark is depicted in Figure 3. Under 1-sun, PEN-ITO reference device exhibited PCE of (2.3 ± 0.3%) with Jsc (6.9 ± 0.8 mA/cm2) and Voc (826 ± 2 mV). Whereas, the device PEN_PEDOT: PSS showed PCE (1.5 ± 0.1%) and comparatively smaller Jsc (6.3 ± 0.5 mA/cm2). Voc also drops to (638 ± 24 mV). Voc of PEN_PEDOT: PSS device is significantly smaller than that of PEN_ITO device under 1-sun, which could be the reason of inferior PCE. Voc of fullerene based OPVs is generally determined by difference between HOMO and LUMO energy levels of donor and acceptor respectively. In this study, the difference in energy levels of PEN_ITO_PEIE (4.05 eV) and PEN_PEDOT:PSS_PEIE (4.12 eV) with ICBA (3.57 eV) is 0.48 and 0.57 eV respectively. Relatively higher difference in energy level of PEN_PEDOT: PSS device compared with that of PEN_ITO, results in smaller Voc followed by decline in PCE under 1-sun. Another reason for smaller Voc could be the higher sheet resistance (RCH) of PEN_PEDOT: PSS because higher sheet resistance (132 Ω/cm2) could induced the surface recombination instead of drift of charge carriers which results in higher leakage current and smaller RshA values. CONCLUSION In this study, solution-processed BHJ organic solar cells with flexible ITO and PEDOT: PSS (PH 1000) as bottom electrodes are fabricated on PEN substrate successfully. PEN_PEDOT: PSS device exhibited superior performance over PEN_ITO in terms of power conversion efficiency and mechanical durability up to 500 bending cycles under indoor light conditions at different bending curvatures. The optimized PCE values for both PEN_PEDOT: PSS and PEN_ITO devices are (11 % ± 0.1%) and (8% ± 0.1%) under LED respectively. On top of that, PEN_PEDOT:PSS OPV demonstrated excellent mechanical stability compared to PEN_ITO device as it retained its PCE and FF values up to ~85 and ~ 90% respectively even after 500 bending cycles. These results provide an effective pathway toward the use of low-cost ITO-free flexible optoelectronic devices for indoor light applications.