Doping Of The Organic Semiconductors And Applications In Photovoltaic Devices

Doping is an effcient strategy to increase carrier density,shift Fermi level position,and fill traps of organic semiconductors.Although it has been widely reported in the past decades,doping to the organic semiconductor still faces series of problems.For example:solution-processed n-dopants are few;n-dopants are unstable in air;doping techniques are not compatiable with the preparation of large-area flexible electronics.To tackle these issues,this thesis focuses on the study of developing novel n-dopant,doping process（p-type）and application of doping in organic optoelectronic devices.First,an amidine of 1,8-Diazabicyclo[5.4.0]undec-7-ene（DBU）is developed as an effective n-dopant that can universally dope electron acceptors,including PC₆₁BM and N2200,by mixing the dopant with the acceptors in organic solvents or exposing the acceptor films in the dopant vapor.The DBU doping considerably increases the conductivity(from7.80×10^-8 S cm^-1 to 3.62×10^-4 S cm^-1)and shifts the Fermi levels up（from-4.8 eV to-4.4eV）of the PC₆₁BM films,indicating a remarkable doping effect.The increased conductivity as well as a shift of Fermi level is benefical to the carriers transportation,extraction and collection in the devices.Second,the DBU-doped PC₆₁BM is employed as an electron transorting layer in perovskite solar cells.The n-doping removes the“S-shape”of J-V characteristics,which leads to an enhancement of the device performance.When the doping concentration reaches0.1 wt.%,the device yields an optimum power conversion efficiency（PCE）.The PCE of the cells increases from 10.51%to 16.69%,with a notable improvement of the fill factor from 0.54 to 0.76.The results from PC₆₁BM based electron-only device indicate the charge injection barrier between the PC₆₁BM layer and the Ag electrode was effectively reduced.Moreover,PC₆₁BM based OFETs also shows the electron mobility of the doped PC₆₁BM film is increased by orders of magnitude.In addition,it is found the doped PC₆₁BM electron transport layer can passivate the perovskite surface defects due to the DBU molecule with a strong ability to donate electrons.Moreover,when DBU as additive is added into the perovskite prescusors,generation of iodine impurity in the perovskite film can be suppressed that improves the device performance.Third,inverted non-fullerene organic solar cell with p-doped surface of active layer is realized via immersing the active layer PTB7-Th:IEICO-4F into a polyoxometalate（PMA）isopropanol solution.A high PCE of 11.37%is obtained without evaporated MoO₃ as the hole-collecting layer.The PMA molecule could oxidize（dope）the polymer donor PTB7-Th during the immersion process.The PMA-doped film shows increased electrical conductivity and work function,which will promote hole extraction in the device.An efficient printable non-fullerene organic solar cell is then fabricated via transfer-printing or doctor-blading the conducting polymer poly（3,4-ethylenedioxythiophene）:poly（styrenesulfonate）（PH1000）electrode directly on top of the PMA immersed active layer,yielding a PCE of 10.37%.