Modification Of PEDOT:PSS Films Toward Applications In Polymer Solar Cells

During the past two decades, polymer solar cells (PSCs) have been attracting increasing attention as an emerging and promising renewable energy source, and showing advantageous potential in low-cost manufacturing, flexible and easy roll-to-roll fabrication. In recent years, power conversion efficiency (PCE) of PSCs has been improved gradually, attributing to the complicated synthesis of novel conjugated polymer donors and fullerene acceptors and/or interface engineering, while improving further the PCE is the key issue of current PSC research. In this dissertation, we focused on improving the conductivity of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) by its modification and consequently applying the modified PEDOT:PSS as hole transport layer (HTL), and carried out the following works:(1)Copper(Ⅱ) bromide (CuBr2) salt was applied to dope PEDOT:PSS as the HTL in bulk heterojunction (BHJ) PSCs, improving dramatically the conductivity of PEDOT:PSS film and consequently the PCE of BHJ-PSC device. Under the optimized doping concentration of CuBr2of10mmol·L-1, the conductivity of CuBr2:PEDOT:PSS film reaches5.6×10-2S·cm-1, which is improved by about300times compared to that of the pristine PED(?)(?)PSS film. Upon applying CuBr2:PEDOT:PSS as HTL in BHJ-PSC device based on poly[N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadia zole)(PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM)(PCDTBT:PC71BM), PCE reaches7.05%, which is improved by～20.7%compared to that of the reference device based on pristine PEDOT:PSS HTL (5.84%) and represents the highest PCE for PCDTBT:PC71BM-based PSC devices without electron transport layer (ETL) reported so far. The dramatic improvement of the conductivity of PEDOT:PSSfilm is interpreted by the weakening of the Coulombic attractions between PEDOT and PSS components. According to scanning Kelvin probe microscopy (SKPM) measurements, the work function of CuBr2:PEDOT:PSS slightly increases compared to that of the undoped PEDOT:PSS, enabling the improvement of the hole transport ability of PEDOT:PSS.(2) PEDOT:PSS was doped by a novel zwitterion,3-(N-morpholino)-2-hydroxypropanesulfonic acid (MOPSO), leading to a dramatic improvement of its conductivity and consequently efficiency enhancement in BHJ-PSCs based on PEDOT:PSS HTL and versatile photoactive systems. Under the optimized MOPSO doping concentration of20mmol·L-1, the conductivity of PEDOT:PSS film increased by about two orders of magnitude, and this is interpreted by the weakening of the Coulombic attractions between PEDOT and PSS components induced by MOPSO. MOPSO doped PEDOT:PSS was applied as HTL of BHJ-PSC devices based on different photoactive layers including poly(3-hexylthiophene-2,5-diyl)(P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PC6iBM),poly[N-9"-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,l’,3’-be nzothiadiazole)(PCDTBT)/[6,6]-phenyl C71-butyric acid methyl ester (PCDTBT:PC71BM) and thieno[3,4-b]-thiophene/benzodithiophene (PTB7):PC71BM, leading to the best power conversion efficiency (PCE) of3.62%,7.03%and7.56%, respectively, which are obviously enhanced relative to those of the corresponding reference devices based on pristine PEDOT:PSS HTL. The efficiency enhancement upon MOPSO doping is found to result from the increase of short-circuit current density (Jsc), which could be ascribed to the increase of the photoabsorption of the photoactive layer, the decrease of the work function of PEDOT:PSS and the improved conductivity of PEDOT:PSS HTL.(3) A PEDOTPSS/HPSA bilayer transparent electrode was fabricated by spin-coating a hydroxypropanesulfonic acid (HPSA) layer onto PEDOT:PSS thin film, and was applied as an alternation to ITO anode. The highest conductivity of the PEDOTPSS/HPSA films reached1020S·cm-1, and the transparency of the PEDOT.PSS/HPSA films in the visible range was over80%. According to AFM, UV-vis and Raman spectoscopic measurements, the conductivity improvement was proposed to result from two reasons. One is the removal of PSS moiety by methanol, and the other is that HPSA-induced segregation of PSS chains and the conformational change of the conductive PEDOT chains within PEDOTPSS. When HPSA in methanol was spin-coated onto PEDOT.PSS film, HPSA molecules could diffuse into the PEDOTPSS film with the swelling effect of methanol and interact with PEDOTPSS, leading to phase separation between PEDOT and PSS compounds. As a result, PEDOT chains would take reorientation by getting away from the PSS chains, allowing more inter-chain interactions among the PEDOT components. Thus, the energy barrier in the PEDOT:PSS film would be lowered and charge transfer among the PEDOT chains would become easier, leading to a tremendous enhancement of tte conductivity of PEDOT.PSS film. Using PEDOT:PSS/HPSA bilayer films as the transparent anodes alternating ITO, ITO-free BHJ-PSC devices based on PCDTBT:PC71BM systems exhibit a PCE of5.52%, which is comparable to the corresponding devices based on the traditional ITO anode. The obtained Voc is lower than that of the ITO-based devices, and this is mainly attributed to the decrease of of the work function of PEDOT:PSS.