Extremely stable all solution processed organic tandem solar cells with TiO2/GO recombination layer under continuous light illumination

Abstract

One approach to harvest a wide solar spectral solar energy is to stack two solar cells with different absorption characteristics in a tandem cell architecture. Herein, solution processed tandem solar cells, with highly transparent titanium oxide (TiO₂) and graphene oxide (GO) as an efficient recombination layer, were designed, fabricated and characterized. We have adopted poly[(4,4′-bis(3-ethylhexyl)dithieno[3,2-b:′′3′-d]silole)-2,6-diyl-alt-(2,5-(3-(2-ethylhexyl)thiophen-2-yl)thiazolo[5,4-d]thiazole]:indene-C₆₀ bisadduct (PSEHTT:ICBA) and poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl]:[6,6]-phenyl-C₇₀ butyric acid methyl ester (PSBTBT:PC₇₀BM) as the active layers for the front and rear cells, respectively. The TiO₂/GO serves as an electron and hole collecting and recombination layer. The tandem solar cells showed a high open circuit voltage (V_OC) 1.62 V, a moderate short circuit current density (J_SC) 8.23 mA cm⁻², fill factor (FF) 62.98%, leading to the power conversion efficiency of 8.40%. The obtained V_OC value of tandem solar cells is ideal for the summation of V_OCs attained from front and rear cells and it is evident that our tandem solar cells are well connected in series. Moreover, this tandem cell exhibits a 20% drop in conversion efficiency under continuous AM illumination for 2880 h.