Study On Interfacial Modification And Device Performance Of Inverted Polymer Solar Cells

Polymer solar cells?PSCs?have copious advantages,such as their light weight,low-cost,solution processability and mechanical flexibility,which attract extensive research interest.Recently,bulkheterojuntion PSCs have achieved rapid development benefiting from the exploitation of new donor and acceptor materials,creation of new device structures and interfacial engineering.However,polymer solar cells still have enormous untapped potential in the search for higher power conversion efficiency?PCE?and long-term device lifetime.Taking these goals into consideration,inverted polymer solar cells were fabricated based on low-temperature processed TiO₂ electron transport layers?ETL?,and it was found that device performance could be further improved with bulk or surface modification to the TiO₂ ETL.Investigations into the interfacial charge transfer properties were then carried out to explore the charge generation mechanism,providing a route to eliminate the light-soaking instability in inverted polymer solar cells.Furthermore,we constructed an interdigitated interface between the active layer the electron transport layer by an in-situ templating method to improve the device performance.All of these efforts are expected to provide theoretical and practical guidance to process stable and efficient inverted polymer solar cells.The main conclusions are listed as follows:?1?Highly crystalline anatase TiO₂ nanoparticles were synthesized by a low-temperature route?80 ^oC?and the TiO₂ dispersion was directly casted to form the TiO₂ETL.Inverted polymer solar cells?i-PSCs?were fabricated based on the low-bandgap donor material poly[4,8-bis?5-?2-ethylhexyl?thiophen-2-yl?benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-?4-?2-ethylhexyl?-3-fluorothieno[3,4-b]thiophene-?-2-carboxylate-2-6 diyl)?PTB7-Th?and fullerene derivative acceptor PC71BM.We further added a titanium chelate?TOPD?interlayer to increase the compactness of the TiO₂ ETL film,which boosted the device efficiency from 8.73%to 9.82%.With further modifications of TiO₂:TOPD film by using UV+EA treatments,we could substantially increase the PCE to 10.55%and also eliminate the light-soaking issues in these i-PSCs.Compared to direct devices,i-PSCs based on compositional and surface modified TiO₂ ETL also exhibited much better stability and retained 95%PCE after 600 h storage under ambient conditions.We then compared the changes of interfacial properties before and after surface treatments using X-ray photoelectron spectroscopy?XPS?,conductivity and electrochemical impedance spectroscopy?EIS?measurements.The results indicated that conductivity increased after surface treatments,and polar solvent treatment resulted in favorable interfacial dipoles,which would dramatically reduce the transfer resistance on the active layer/electrode interface.All of these modifications contributed to the enhancement of efficiency and the elimination of light-soaking.?2?Two different kinds of water/alcohol soluble polyelectrolytes were selected as the surface or bulk modifiers of TiO₂,namely polyethylenimine?PEI?and poly[9,9-bis?6'-?N,N-diethylamino?propyl?-fluorene-alt-9,9-bis?3-ethyl?oxetane-3-ethyloxy?-hexyl?fluorene]?PFN-OX?,which have different insulation and viscoelastic behaviors at room temperature.Inverted polymer solar cells were then fabricated based on PTB7-Th:PC₇₁BM to systemically investigate the relationship between interfacial properties and device performance.XPS,EIS and scanning the kelvin probe microscope?SKPM?results indicated that PEI,with its large amount of polar amino groups,could notably reduce the work function?WF?of TiO₂ compared to PFN-OX,leading to the enhanced charge selectivity.Finally,light-soaking-free i-PSC solar cells with efficiencies over10%were obtained.Excitingly,PEI modification gives great versatility to device architecture,and by adding PEI either in the bulk or as an adjacent layer below or above the TiO₂ ETL we were able to obtain high efficiency and light-soaking-free inverted PTB7-Th:PC71BM solar cells.?3?An interdigitated interface can be formed in-situ during the solution casting process of the photoactive layer on the blend films made of electron-transporting PFN-OX and a templating material,n-type semiconductor 3,9-bis?2-methylene-?3-?1,1-dicyanomethylene?-indanone??-5,5,11,11-tetrakis?4-hexylphenyl?-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene)?ITIC?.Compared to the devices with a conventional planar interface between the active layer and PFN-OX ETL,devices with the ITIC-templated interdigitated interface increased the efficiency of PTB7-Th:PC₇₁BM solar cells from 9.27%to 10.42%.Futhermore,devices with a PS-templated interdigitated interface have an improved a PCE of 9.94%,which indicated the validity of this construction method.To explore the effect of the interdigitated interface on device performance,we deeply investigated the interfacial electrical doping and charge transport properties.The results indicated that favorable electrical doping of PFN-OX to ITIC as well as charge transfer between PTB7-Th and ITIC contribute to this superior efficiency enhancement in devices employing an ITIC-templated interdigitated PFN-OX ETL.?4?Polymer solar cells based on the conjugated donor polymer poly[?5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl?-alt-?3,3???-di?2-octyldodecyl?-2,2?;5?,2??;5??,2???-quate-thiophen-5,5???diyl?]?PffBT4T-2OD?were fabricated,and the effect of three solvent extraction methods on molecular orientation and phase separation in both lateral and vertical directions were investigated.The three techniques were vacuum extraction?VA?,thermal annealing?TA?,and methanol-rinsing extraction?MA?.The results indicated that TA would increase the crystallinity of the polymer and induced the aggregation of fullerene.However,it had less influence on the vertical separation of the active layer,which still showed a polymer-rich surface containing⁸0%polymer at the film surface.Methanol washing would decrease the crystallinity of the polymer and promoted the migration of PC₇₁BM to the donor/acceptor interface and the upper surface of the active layer.In a conventional configuration,the VA-treated devices exhibits a relatively low PCE?9.00%?.Although the performance can be slightly improved to 9.33%by TA-treatments,devices still have low fill factors?FF?.However,MA-treated devices exhibited enhanced efficiency?10.25%?.In an inverted configuration,PCEs of 10.7%were obtained in both VA-and MA-treated devices,while TA-treated devices had a slightly lower PCE of 10.2%.Based on the analysis of lateral and vertical phase separation,device performance and charge blocking properties,we can conclude that if inhomogeneous distribution of the donor and acceptor in the active layer occurs,leading to an enrichment layer of a component adjacent to the electrode surface,it will have little influence on device performance unless the charge transport channel is completely destroyed by the enrichment layer.