Study on the Photovoltaic Performance and Mechanism of Thick-Film Polymer Solar Cells

Author:Wang Zuo

Supervisor:chen jun wu


Degree Year:2019





Solution-processed polymer photovoltaic cells(PSCs)have attracted wide attention due to their light weight,flexibility and potential low cost.High-speed roll-to-roll(R2R)printing is the most promising way to realize the commercial application of PSCs.However,the thickness of active layer would have a large variation during the high-speed"roll-to-roll"printing.For this reason,it is very important for PSCs to keep the excellent photovoltaic performance when the active layer thickness varies in a wide range.At present,most active layer materials used in PSCs often show great sensitivity to thickness.The optimal energy conversion efficiency(PCE)is usually achieved in the thickness of 100?150 nm,which would restrict the yield control during the high-speed"roll-to-roll"printing.The active layer with average thickness above 200 nm is not only more suitable for high-speed printing,but also can reduce pinholes in the active layer.In the first chapter,the background and recent development of PSCs were described.We compared the photovoltaic performances of some novel photovoltaic materials and tried to reveal some working mechanism related to thick-film devices.More importantly,it was found that some active layer systems could show the optimal efficiency at the 300 or even 400 nm thickness level,which would promote the development of highly efficient thick-film PSCs towards the application in the high-speed"roll-to-roll"printing.In the second chapter,we introduced a high hole mobility conjugated polymer(Si25)as polymer donor in combination with a nonfullerene acceptor(O-IDTBR)to construct active layers of PSCs and studied the performances of non-fullerene photovoltaic cells under the varying thickness of active layer.Although Si25 and O-IDTBR have similar optical band gaps(both 1.61 eV),there is complementary absorption intensity in the short wavelength region and further enhanced absorption under the thick film.At the same time,energy level offsets of HOMO and LUMO between the donor and acceptor are very small,of 0.11eV and 0.17eV,respectively.However,high exciton separation efficiency can still be achieved under such small offsets.High open-circuit voltage(1.03 V)and high short-circuit current density(21.11mA/cm2)are obtained simultaneously in PSCs based on 400 nm active layer,which greatly suppresses trade-off between Jsc-Voc.In addition,high power conversion efficiency(PCE)from 10.2%to 11.54%can be achieved with thick blend films from 210 to 560 nm,a thickness range beneficial to pin-hole free printing.The related fill factors(FF)are between51.59%and 53.33%,which indicates FF is insensitive to film thickness.The inferior FF is due to very low electron/hole mobility ratio.The FF may be improved if O-IDTBR is replaced by a high electron mobility receptor.In the third chapter,we choose IEICO-4F,a non-fullerene acceptor with higher electron mobility,to pair three Si25 polymers(Si25-L,Si25-H1 and Si25-H2)with different molecular weight(Mn)and molecular weight distribution(PDI),so as to explore the effects of Mn and PDI on device performance.The Si25-H2:IEICO-4F-based PSCs exhibit the best device performance,whose the PCE can reach 13.2%based on 320 nm thick active layer and the FF is over 70%.As far as we know,this is one of the highest PCEs in PSCs with active layer thickness over 300 nm.Moreover,the Si25-H2:IEICO-4F-based PSCs based on active layer thicknesses from 210 to 460 nm display PCEs more than 11%,supplying a wide processing window.Interestingly,the electron mobility of the Si25-H2:IEICO-4F blend films is 8 times higher than that of the IEICO-4F pure film,suggesting that the Si25-H2 can induce the acceptor phase with a higher electron mobility,which is also confirmed by GIWAXS analysis.In the fourth chapter,we tried to solve the problem that PBODT:ITIC-based non-fullerene PSCs exhibited a rapid decrease in PCE for active layer thickness of 200 or 300nm.This is related to the low electron mobility of ITIC.1D/2A PBODT:PC71BM:ITIC-based ternary PSC was fabricated by addition of a fullerene receptor PC71BM into the PBODT:ITIC system.The PCE of the ternary PSC can reach 8.42%at 300 nm thick active layer,significantly higher than those for PBODT:PC71BM and PBODT:ITIC-based PSCs.Further studies indicated that the ternary blend films could show higher electron mobility and better morphology.In addition,we also compared the thermal stability of the binary and ternary PSCs,where the thermal stability of the ternary PSC was the best.In the fifth chapter,2D/1A ternary blend films were proposed,based on two polymer donors of PTB7-Th and PBDB-T with comparable HOMO levels in combination with a nonfullerene O-IDTBR acceptor.The ternary PSCs showed high open-circuit voltage of 1.02V,one of the highest values among 2D/1A ternary devices.A maximum PCE of 11.58%was achieved for the ternary PSCs based on PTB7-Th:PBDB-T:O-IDTBR=0.7:0.3:1.5 blend film,higher than 9.74%and 6.99%for PTB7-Th:O-IDTBR and PBDB-T:O-IDTBR binary blend film,respectively.The improved efficiency with the ternary PSCs is due to the significantly elevated short-circuit current density.Higher charge dissociation probabilities,enhanced hole and electron transports,and favorable morphology were found for the ternary blend film.However,with 210 nm thick active layer,the PCE of the ternary PSCs would decrease significantly,which was due to the decreased electron mobility of the active layer.After a long thermal annealing at 85°C for 168 h,a good PCE of 9.37%was still retained by the ternary PSCs,much higher PTB7-Th:O-IDTBR and PBDB-T:O-IDTBR binary PSCs.The ternary PSCs based on a large area of 0.91 cm2 also showed a high efficiency of 9.01%.