Study On The Mechanism Of Improving Performance Of Polymer Solar Cells By Doping Technology Used In Active Layer

Polymer solar cells (PSCs) are attractive because of several advantages, including lowcost, light weight, semi-transparency and mechanical flexibility etc. However, PSCs are stillfar from commercial production because of the low PCE, and the photoelectric performancehas become the main direction of research work. Much work has been done to improve theperformance of PSCs, including thermal annealing, morphology control using solventmixtures, inverting the structure of PSCs and introducing metal nanostructures into theorganic matrix etc. In this article, I studied and analyzed the improved mechanism of deviceperformance by the active layer doping technology, it is based on polymer solar cellmanufacturing technology and Combined experimental and simulations. The work aims toexplore the methods to optimize the performance and improve the PCE of PSCs.In the second chapter of the article, the author fabricated PSCs based on blends ofpoly{[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]}(PBDTTT-C-T) as electron donors and[6,6]-phenyl-C71-butyric acid methyl ester(PC71BM) as electron acceptor in organic solvents.Added3%(v/v)1,8-diiodooctane (DIO) into host solvent as the additive, and then addednitrobenzene to the blend solutions at various volume ratios. Experiments show that when thenitrobenzene volume ratio was12%, the performance of device was improved mostsignificantly, the short circuit current density (Jsc) obtained was19.66mA/cm2, the powerconversion efficiency (PCE) improved from7.49%to8.88%. The author measured the photonabsorption property, surface morphology and external quantum efficiency (EQE) of active layerfilms and device and found that adding nitrobenzene into blend solution helps to optimize thesurface morphology of the film, form good phase separation. The light absorption capacity andthe charge transport capability of active layer film have been optimized significantly to improvethe Jscand PCE. However, the optimal performance of the device cannot be attributed solely to changes of film morphology and the intensity of absorption.Based on the research of experiment contents in the previous chapter, nitrobenzene addedinto cative layer blend solution to optimize the device performance has more major mechanism.In the third chapter, the author measured the transient absorption spectrum and fluorescenceemission spectrum of active layer films combined simulations, which shows that nitrobenzeneand PBDTTT-C-T form stable coplanar charge transfer complexes through hydrogen bonds.Formation of the PBDTTT-C-T-C6H5NO2complex simultaneously increases the externalquantum efficiency (EQE). The underlying mechanisms of increased EQE are attributed to (1)higher Lowest Unoccupied Molecular Orbital (LUMO) of PBDTTT-C-T-C6H5NO2for moreefficient photoinduced electron transfer to the LUMO of PC71BM and (2) efficient quenching offluorescence in the active layers due to formation of the PBDTTT-C-T-C6H5NO2complex. Thisdiscovery clearly illustrates the potential of hydrogen bonded complexes as a new route forefficient polymer based on photovoltaic devices.There are so many methods to optimize the performance of PSCS, in the fourth chapter,the author fabricated solar cells based on blends of poly (3-hexylthiophene)(P3HT) aselectron donors and [6,6]-phenyl C60-butyric acid methyl eater (PCBM) as electron acceptorin organic solvents, and dual optimization power conversion efficiency of polymer solar cellsby spin-coating a P3HT layer between Poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonate)（PEDOT:PSS）and active layer and incorporate Ag nanoparticles in theactive layers. The thickness of P3HT layer was45nm and mass ratio of Ag nanoparticles was5%introduced by fabricate solar cell, leading to surface morphology and internal structure ofactive layers. The UV-Vis absorption spectrum and external quantum efficiency of solar cellsimproved significantly and shew a red-shift. The author obtained a short circuit currentdensity of11.21mA/cm2and power conversion efficiency of3.79%.