Tandem organic solar cells (OSCs), consisting of more than one (usually two) sub-cells, stacked and connected by a charge recombination layer (i.e., interconnecting layer (ICL)), are highly promising to boost OSC efficiency. The ICL plays a critical role in regulating the performance of tandem device. Here, we report ZnO or C$_{60}$ as a simple ICL, modified then, by a PEDOT:PSS layer, can act as an efficient ICLs in tandem device to achieve a significant increase in performance compared to single sub-cell. After optimized optical properties of MEH-PPV:PC$_{70}$BM and P3HT:PC$_{70}$BM sub-cells by varying the annealing temperature, these ICLs are used to connect photoactive layers. For all tandem devices, we showed an improvement in optical properties, especially in the ground and excited states compared to optimized single sub-cells. The degree of improved performance in tandem devices is attributed to the connection which provides each ICL. For fully optimized tandem, based on C$_{60}$ ICL, reached ${\sim}$78% enhancement in charge transfer compared to the bottom sub-cell and 82% compared to the top sub-cell, obtained for ${\lambda}_{emission}$ = 650 nm.

The solution-processable small organic molecules have turned into one of the most promising semiconductors due to their exclusive properties in the inexpensive optoelectronic field. In this work, we report the study of the incorporation effect of the small organic molecules of p-nitro-benzylidenemalonotrile (N$O_2$-BMN) into conjugated polymer poly(3-hexylthiophene) (P3HT) on the optical and electrical properties. The UV–visible and photoluminescence (PL) responses were performed to confirm the enhancement of the optical properties. PL results showed that P3HT: N$O_2$- BMN composite with (1:1.25) ratio has the best charge transfer efficiency that occurs between both compounds. The current–voltage characteristics through dark J (V) revealed that the ITO/P3HT: N$O_2$ BMN/Al structure had a diode behaviour with a relatively low threshold voltage ($V_{th}$). We have estimated the different electrical parameters such as the saturation current, barrier high ${\Phi}_b$ and the ideality factor (n). Moreover, we have evaluated the different charge transport mechanisms by the analyses of J–V characteristics in a double logarithmic scale under dark conditions. The obtained results proved that the incorporation of N$O_2$-BMN enhances the electrical properties of the composite.