Solar Energy, Vol.174, 1078-1084, 2018
Experimental investigation of charge transfer, charge extraction, and charge carrier concentration in P3HT:PBD-DT-DPP:PC70BM ternary blend photovoltaics
A sensitizer consisting of D-A structured polymer (PBD-DT-DPP) is assessed with P3HT:PC70BM solar cell. This polymer possesses a narrow energy band gap, which complements in absorption of P3HT:PC70BM binary blend. Use of PBD-DT-DPP as a sensitizer in the ternary blend OPV is reported earlier, which showed enhanced power conversion efficiency (PCE) in the conventional device structure. In this work, charge transfer and charge transport properties of the ternary blend (inverted device) for its improved performance are discussed in detail. Ternary blend system demonstrated an improved device performance (similar to 3.35%) as compared with the binary of P3HT:PC70BM devices (similar to 2.58%). Ternary blend devices showed enhanced short circuit current density (J(sc)) of similar to 4 mA/cm(2) relative to P3HT:PC70BM. To understand the reason for the decrease in fill factor (FF), mobility analysis using charge extraction with increasing linear voltage (CELIV) measurements were carried out. However, there is not much change in the open circuit voltage (V-oc). Results indicate increased bimolecular recombination for the ternary blend, suggesting increased disorder in the morphology of ternary blend films. Time-resolved microwave conductivity (TRMC) measurement was utilized to understand the photoconductance properties of the ternary blend films. Interestingly, TRMC results showed binary PBD-DT-DPP:P3HT and PBD-DT-DPP:PC70BM mixtures possess similar transient photo-conductance to that of P3HT:PC70BM. Further, photoluminescence (PL) measurements were carried out to probe the charge transfer process in the blend systems. Capacitance-voltage results showed enhanced carrier concentration in ternary system. The detailed analysis showed that ternary system helps in improving the charge generation and charge transport and hence improved device performance.