| Fossil fuels such as coal,oil and natural gas are the major sources of energy generation worldwide,the continual dependence of energy to the fossil fuels increases the climate change,pollution and ecological damage problems all over the world.In the past decades,scientific and industrial field are actively exploring new ways of clean energy utilization,which includes wave,tide and solar energy.Statistically,the energy which radiate on earth's surface by sun each year is more than 30 times of all the coal energy reserves in our planet.As a representative of the third generation solar cells,organic polymer soar cells(OSCs)have the advantages of light weight,low-cost and easy fabrication over large areas by roll-to-roll processes.Thanks to the deep exploration of relevant optical,device physics,design of new structure and materials,many novel structure and high performance devices are developed.Meanwhile,the adjustment and optimization of active and interfacial layer is also a main way to increase the charge generation in active layer and charge extraction capability at interface.So,the power-conversion efficiencies and stability got considerable development.OSCS are generate tightly-bound holes and electrons(excitons)in the photoactive layer under illumination,which can only be dissociated at the donor-acceptor(D-A)interface,diffused and extracted at respective electrodes.So,the donor materials in active layer need high mobility to ensure the charge transfer effectively.But,it is well known that electrons usually transport faster than holes in most fullerene-based bulk hetero-junction(BHJ)active layers,which leads to the unbalanced charge transport and thereby degrades the device performance.Meanwhile,in the charge transport process,there are always exit electronic barrier between electron transfer layer(ETL)and fullerene acceptor materials.Additionally,in the ZnO ETL,the existence of interfacial defect states which can trap photoelectron and induce charge recombination losses,resulting in reduced carriers that are effectively transit to the external circuit thus limiting efficiency enhancement.Inspired by the guidelines mentioned above,we propose a novel P-type material to dope active layer and ZnO ETL surface passivation strategies to adjustment and optimization of active and interfacial layer respectively.We use various testing methods to investigate the micro-morphology,optical properties,energy levels,charge mobility and defect states before and after modification of active and interfacial layer.Both optimization schemes are completed in the fullerene systems.After optimization,relatively high power conversion efficiencies(PCEs)were achieved when we just used an easy-processed method.In the aspect of active layer optimization,to solve the problem of unbalance transport between electron and hole in fullerene active layer,we reported in detail an easy-processed method of increasing OSCs performance by doping Pentacene(Pc)into the active layer.Since Pc molecules has high crystallinity and hole mobility(?h),it will balance the charge transport in P3HT:PC71BM-based inverted organic solar cells.We investigated the active layer morphology,optical properties,electrical properties and carrier recombination mechanism after doping.It is demonstrated that very low doping level at parts per thousand(ppt)concentrations can increase the ?h from 1.12×10-5 cm2V-1S-1 to 4.88×10-5 cm2V-1S-1,corresponding decrease the accumulation of space charge.UV-vis absorption spectra showed the improvement of light absorption capacity in the range of 300?475 nm.Additionally,the lowest unoccupied molecular orbital(LUMO)and the highest occupied molecular orbital(HOMO)of Pc are located at-3.21 eV and-4.99 eV,respectively.This energy structure can forms the energy cascade between P3HT and PC71BM.Time-resolved photoluminescence(TRPL)spectrum reveal that the life time of exciton with in the blend films decreases in the presence of Pc,indicating that the Pc dopant may participate in the charge transfer process and facilitate the exciton dissociation.The light intensity dependence of the VOC and Jsc reveals that the introduction of Pc does not introduce trap states or acts as recombination centers.Instead,it reduced bimolecular recombination rate.As a result,a light-doping level of 0.2 wt%significantly improves the PCEs by 38%,from 3.46%to 4.79%in P3HT/PC71BM-based solar cells.In the aspect of interfacial layer optimization,on account of conventional metal oxides such as ZnO,however,not just suffers from the high work function which does not match the energy level of the acceptor component in the blend active layer,but also introduces trap states as charge carrier recombination centers.To solve these disadvantages,the ZnO/In nano-injunction was introduced as interfacial modification layer in PTB7-Th:PC71BM-based inverted solar cells.After ultra-thin In metal deposited on ZnO ETL,there is nearly no influence on micro-morphology,optical and active layer phase separation properties.XPS spectra shows that the In2O3 in the surface of the In metal,which is participate in the charge transfer process by forming the energy cascade because of the lower WF of it.In addition,it also reduces the work function to a more appropriate level to match the acceptor component in the blend active layer,as characterized by the UPS set up.The photoluminescence(PL)and TRPL spectral study shows that the introduction of indium thin film can effectively suppress the ZnO defects emission,which is attributed to the electrons provide by In and transfer to ZnO matrix and fill their trap states,suppressing the interfacial carrier recombination.All of these improvements accompanying the introduction of indium thin film contribute to electrons transport,charge extraction and reduce carrier recombination.The cells with ZnO/In nano-junction exhibited a higher Jsc and FF which we attribute to reasons mentioned above.The maximum PCEs of the OSCs based on PTB7-Th:PC71BM blending is 10.05%,and average efficiency of 20 devices boost from 8.13%to 9.84%. |
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