A detailed comparative study on electrical and photovoltaic characteristics of Al/p-Si photodiodes with coumarin-doped PVA interfacial layer: the effect of doping concentration

In this study, three different poly(vinyl alcohol) (PVA) films doped with weight percentages of 0.05, 0.10 and 0.20 coumarin were coated on p-Si wafer via spin-coating method for the purpose of investigating the interaction of coumarin dopant with polymer host at molecular level. Therefore, metal–polymer–semiconductor (MPS) structures were formed and their current–voltage (I–V) and admittance measurements were taken to compare the main electrical parameters of the MPS structures with different film thicknesses. The values of ideality factor (n), barrier height (Φ_b), rectification ratio (RR = I_F/I_R), series resistance (R_s) and energy-dependent profiles of surface states (N_ss) were calculated using the forward bias I–V data. There exists increasing trend for N_ss values from mid-gap energy of Si toward the bottom of conductance band. The highest values of RR and photosensitivity (I_photo/I_dark) were found as 4.62 × 10⁴ at ± 4 V for the MPS structure with 0.10 wt% coumarin doping level, respectively. The photoresponse of the structures was also analyzed using $I_{\text{ph}}=A{P}^m$ relation, and the value of m was obtained from the slope of ln(I_ph)–Ln(P) plot for each diode as 1.48, 1.27 and 1.57, respectively. Experimental results suggest that 0.10% coumarin-doped PVA caused MPS structure to reveal better performance considering higher RR and lowest N_ss, and so it can be considered as an alternative interfacial layer material for replacing traditional insulators.