Application Of The Monomolecular Layer With Strong Dipole Moment In Organic Solar Cells

Organic solar cells （OSC） devices have attracted tremendous attentions for their inherentadvantages of low cost, large area, light weight and flexibility, etc. However, the efficiency ofpolymer solar cells is still significantly low for practical applications in large scale. To furtherimprove the photovoltaic performance of the OSCs, various efforts have been tried such asdeveloping new photovoltaic materials, thermal annealing, choice of solvent, etc. And thepower conversion efficiency （PCE） has been reported to reach6%-7%, which indicates apromising future of the OSCs for the widely commercial use. In this paper, the main pointwill be proposed by the following two aspects.（1） Because of the low energy difference between HOMO （of P3HT） and LUMO (ofPC₆₁BM), the open circuit voltage （Voc） is only around0.6V, which has been a mainbottleneck for the P3HT:PC₆₁BM based OSCs development. In this paper, the authorsproposed a effective method to fabricate the highly efficient OSCs based on poly[N-9’’-hepta-decanyl-2,7-carbazole-alt-5,5-（4’,7’-di-2-thienyl-2’,1’,3’-b-enzothiadiazole）:[6,6]-phenyl C₇₁-butyric acid methyl ester (PCDTBTT:PC₇₁BM), by introducing PCDTBTand PC₇₁BM as donor and acceptor respectively, and as high as0.898V of open circuitvoltage was achieved. The fill factor （FF） and PCE was improved dramatically by4%and6%by employing LiF as the cathode modification. Additionally, compare with TiOx extensivelyused as optical spacer in the OSCs, LiF possessed an enormous cost advantage. The influenceof the thermal treatment on the surface morphology of the active layer was also investigated.（2） ITO, as the anode of the OSCs, can be easily eroded by poly（3,4-ethylenedioxythiophene）-polystyrene sulfonic acid （PEDOT:PSS） due to the acidicnature. Besides, the In+will accelerate diffusing into the PEDOT:PSS buffer layer, leading toa dramatic degradation of device performance, as the PEDOT:PSS coated ITO was exposed toair. In this article the autors used a1nm thickness of Al₂O₃, as well as LiF to modify theinterface between ITO and PEDOT:PSS. Because of the strong dipole moments of LiF andAl₂O₃, either can enhance the built-in electric field, which increases the probability of thecarriers reaching the corresponding electrode. In addition, the low work function ofPEDOT:PSS can decrease the energy barrier for carrier transmission. A21.7%improvementin the power conversion efficiency of experimental devices was achieved, from2.76%to3.36%, mainly because the short circuit current was enhanced by almost30%.