Seite - 4 - in Photovoltaic Materials and Electronic Devices

characteristics of the OPVs were measured using a Keithley 4200 semiconductor characterization system under a simulated AM 1.5G spectrum with power of 100 mW/cm2 generated by ABET Sun 2000 solar simulator. The corresponding J-V curves were recorded from´1 V to 1 V with an interval of 0.01 V. An incident photon to current conversion efficiency (IPCE) spectrum was measured on Zolix SolarCellScan100. Themorphologyof thefilmswas investigatedbyatomic force microscopy (AFM) using a multimode Nanoscope IIIa operated in tapping mode. All the samples were measured with a scan size of 5ˆ5µm2. The hole mobility and electron mobility of PTB7: PC71BM blend films were measured by space charge limitedcurrent (SCLC)method. All the testswere inambientair conditions. 3. ResultsandDiscussions The J-V characteristic curves of the OPVs with different PS ratios are shown in Figure 1a. The PV performances of the OPVs are summarized according to the J-V curves and listed in Table 1. Among all the different ratios, it can be found that the device with 1 wt % of PS demonstrates the highest median PCE of 4.56% along with a short-circuit current (Jsc) of 10.60 mA/cm2, an open-circuit voltage (Voc) of 0.79 V, and a fill factor (FF) of 54.50%. The data in Table 1 shows that the PCEimprovement ismainlyattributedto theenhancement in Jsc andFF.Tofurther investigate themechanismresponsible for theenhancedperformanceof theOPVs with thePSadditions, theoptimizedvolumeratioof1%wasused. It is reported that DIO can improve the morphology of the active layer and enhance the performance of organic solar cells [24]. Consequently, organic solar cells based on PTB7:PC71BM with two additives DIO and PS were prepared to improve photovoltaic properties. The concentration of DIO is 3 wt % according the reference [18], and that of PS is 1 wt % according to the above results. The J-V curvesof theOPVswithdifferentadditivesunder illuminationofsimulatedAM1.5G (100 mW/cm2) are shown in Figure 1b and summarized in Table 2. Device 1 demonstrates a PCE of 4.11% with a Jsc of 10.47 mA/cm2, aVoc of 0.79 V, and a FFof49.65%. As shown in Table 2, with the addition of 1 wt % PS (weight fraction of the BHJ components) inDevice2, Jsc increases to10.60mA/cm2 andFFincreases to54.50%, which results in a PCE of 4.56%. If both 3.0 v% DIO and 1 wt % PS are added to the solution prior to spin casting, the PCE of Device 4 is further increased to 8.92 along with aVocof 0.76 V, a Jscof 16.37 mA/cm2, and a FF of 71.68%. The improved Jscvalue isconfirmedbymeasuringEQE(Figure1c). ThemaximumEQEvalueof Device1 is43.72%andit is increasedto63.37%forDevice4. Thesingle logarithmic dark current curves show that Device 4, Device 3 and Device 2 have smaller leakage current compared with Device 1, as shown in Figure 1d. It is well-known that the leakage current is determined by the shunt resistance (Rsh) [25]. The larger Rsh 4