Page - 6 - in Photovoltaic Materials and Electronic Devices

Table 2. ThesummaryofphotovoltaicparametersofPTB7:PC71BMsystemsolar cellswithoutorwith3v%DIOand/or1wt%PS. Voc (V) Jsc (mA/cm2) FF (%) PCE(%) Rsh(Ωcm2) Rs (Ωcm2) Device1 0.79˘0.01 10.47˘0.013 49.65˘0.02 4.11˘0.03 302.8 18.2 Device2 0.79˘0.01 10.60˘0.012 54.50˘0.09 4.56˘0.02 311.5 8.56 Device3 0.75˘0.01 14.23˘0.09 71.31˘0.05 7.61˘0.02 757.6 5.44 Device4 0.76˘0.01 16.37˘0.08 71.68˘0.04 8.92˘0.02 915.8 4.24 Under the same spin-coating condition, the thicknesses of films 1 and 3 are almost the same. When doped with PS, the thicknesses of films 2 and 4 increases and are almost the same. This shows that doping PS encourages an increase the thickness of the active layer without decreasing other electronic properties; for example,RshandRsofcorrespondingdevicesdopedwithPSareimprovedasshown in Table 2. In order to understand the effect of high-molecular-weight insulating polymers-PS in the mixture of PTB7 and PC71BM on the optics properties and the surfacemorphology, furthercharacterizationhasbeencarriedout. Theabsorption spectra of the neat PC71BM and PTB7 films are shown in Figure 2a. PC71BM has two apparent absorption peaks at 375 nm and 480 nm. The absorption spectra of PTB7 show an apparent complementary absorption in the range from 550 nm to 750 nm. Two broad absorption peaks at around 624 and 682 nm are attributed to thecharacteristicpi-pi* transitionof thePTB7polymer [26,27]. Incomparisonto the absorption spectra of the films prepared with and without additives, there is no obviouspeakshiftobservedinthefilmpreparedwithPSandinthefilmprepared withpristineCB,asshowninFigure2b. It can be found there is no significant difference between Film 1 and Film 3, while there is an increase of obvious relative absorption intensity in the region of 300–800 nm for Film 2 and Film 4. This result indicates that under the same conditions,filmswithPSadditivescanharvest solarphotonsmoreeffectively than the films prepared from CB and CB: DIO. The more light absorbed, the higher photocurrentgenerated[28]. Thisisthereasonwhythe Jsc increasesaftertheaddition ofhigh-molecular-weight insulatingpolymerPS. 6