Role of hole transport layers in normal and inverted bulk heterojunction organic solar cells: A light intensity dependent capacitance-voltage analysis

In organic solar cells (OSC) hole transport layers (HTL) are essential for boosting the power conversion efficiency (PCE); therefore, it is crucial to determine the underlying mechanisms. In the present manuscript, we report a study that examines the effects of two HTL such as PEDOT:PSS and MoO₃, on the electrical characteristics of P3HT:PC₆₁BM (active layer) based bulk heterojunction (BHJ) in normal (ITO/PEDOT:PSS/P3HT:PCBM/Al) and inverted (ITO/ZnO/P3HT:PCBM/MoO₃/Ag) configurations. Current density-voltage (J-V) and illumination intensity dependent capacitance-voltage (C-V) measurements have been performed and compared for with and without HTL (PEDOT:PSS in normal and MoO₃ in inverted) devices. Results suggest that the extracted photovoltaic parameters (short circuit current density (J_sc), open circuit voltage (V_oc) and fill factor (FF)) of HTL containing devices are superior to without HTL devices in both normal and inverted OSC, due to unfavourable charge carrier accumulation and band bending in HTL free devices. Such charge build-up at active layer/anode interface generates photovoltage contributing as loss factor to PCE and has been investigated in C-V characteristics measured under varied illumination intensities (0/dark, 50 and 100 mWcm⁻²). Parameters such as built-in-voltage (V_bi), effective built-in-voltage (V_{bi, eff}), accumulated carrier concentration and surface photovoltage (V_spv = V_{bi, eff} - V_bi) are extracted as a function of illumination intensity. Our findings reveal that HTL free devices, in both conventional and inverted OSCs structures, marked with low J_sc, V_oc and FF values generate higher SPV. The SPV calculation method used here has been proven to be crucial in recommending a suitable HTL for OSC device architectures.