Impact of Photoactive Layer & Cathodic Buffer Layers on Power-Conversion-Efficiency Enhancement for Organic Photovoltaic Cells

Abstract

Photovoltaic (PV) cells have recently attracted attention as a renewable, sustainable source of electricity, because they are a clean energy source, have a low fabrication cost, and can be processed on a flexible substrate. Therefore, in this study, we investigated the power conversion efficiency (PCE) enhancement for organic solar cell implemented with photoactive and cathodic buffer layers. First, P3HT weight varied, the dependence of the power-conversion efficiency (PCE) on the weight of P3HT was similar to that of the short-circuit current (JSC), but not that of to the open-circuit voltage (VOC) and the fill factor (FF). That trend is associated with the fact that the absorbance of light and the carrier transport resistance strongly depend on the weight P3HT. In particular, the PCE and JSC of a cell with a specific weight ratio of P3HT to PCBM, i.e., P3HT (0.0266 g):PCBM (0.0133 g), were maximized, 3.48 % and 8.43 mA/cm2 , which were improvements of 676 and 532%, respectively. We, therefore, investigated in detail how the characteristics of a P3HT:PCBM-based PV cell depend on the weight of P3HT for a fixed PCBM weight in the solvent. In particular, we discuss the mechanism by investigating the light absorbance and the carrier transport resistance in a polymer PV cell with different P3HT and fixed PCBM weights. Second, we suggest new photovoltaic cell structure to be able to have the tandem effect in a view point of the light absorption enhancement to replace the materials used in complicated tandem process. In particular, we used CuPc as a small-molecular donating material and P3HT as a polymer donating material to expand the wavelength range of a light absorption. In addition, by applying the CuPc:P3HT:PCBM blended layer using spin-cast process in photovoltaic cells, we investigated the performance of photovoltaic cells fabricated with a CuPc:P3HT:PCBM blended layer. By fabricating the photovoltaic cells of ITO / PEDOT:PSS / CuPc:P3HT:PCBM / BCP / Al with small-molecular and polymer donating materials blended layer, we demonstrated a high PCE of 4.20% with high Jsc of 10.05 mA/cm2. This performance of photovoltaic cell with the blended layer of small-molecular and polymer can be competitive with that of tandem cells. Finally, We, investigated the effect of cathode buffer layer co-evaporated Alq3:Liq on the performance of polymer photovoltaic cell. In particular, to finding an optimal Alq3:Liq thickness and characterizing a binding energy for several cathode buffer materials, we investigated how much the performance of the photovoltaic (PV) cell with cathode buffer layer co-evaporated Alq3:Liq is enhanced, compared with the PV cells with mono and bi- layers. The inserting Alq3:Liq layer into the PV cell affects strongly the power conversion efficiency (PCE) and fill factor (FF) of the PV cell, i.e., 1-nm-thick Alq3:Liq layer increased those of the PV cell by 55.22 and 7.15%, respectively, compared with the cell without Alq3:Liq layer. In particular, the PCE and the JSC of the PV cell with Alq3:Liq layer were 4.14 and 6.2% higher than those of BCP/LiF bi-layer cell, respectively. This was attributed to that the Liq deposited on Alq3 reduced the electron injection barrier in-between the work-function of aluminum electrode and LUMO level of organic material, resulted in that the peak of binding energy moved toward a high HOMO level.