Nanoscale Structure And Electical Properties Of Polymersolar Cell Studied By Electric Scanning Probe Microscope

Polymer Solar Cell (PSC) has drawn much attention in the energy field for itslow-cost and promising application. At present, the research mainly focused on thedesign and synthesis of new electron donor and accept materials, and the optimizationof the active layer.However, the local electric properties such as charge separation atthe interface of the BHJ and the degradation of the device have receive little attention.Electric force microscopy (EFM) is a powerful tool to study the nanoscale morphologyand electric properties of PSC with high resolution. In this paper, we take BHJ polymersolar cell as model, and P3HT:PCBM active layer thin film as the research object. Weuse EFM to analysis the electric property of PSC as follows:1. The influence of the weight ratio of P3HT:PCBM to the PCE of the PSC withthe same processing condition (thermal annealing at110℃for10min), the ratio is3:1,2:1,1:1,1:2,1:3. The ratio of1:1demonstrates the highest performance with PCE of4.01%. We use AFM to investigate the morphology of the active layer at differentratios, from3:1to1:3, the RMS(root-mean-square) changed from15.6nm to1.48nm,additionally, we use EFM to characterize the phase separation and surface potentialof the active layer. At3:1and2:1ratio, the phase contrast is clear, while at1:1ratio,the phase separation is small, at1:2and1:3, there is almost no phase separation Itindicate that1:1ratio will form a better interpenetrating network for the generation andtransport of the charge carrier. We observed the variation of the surface potential withand without light, when the ratio is3:1,2:1,1:1, the surface potential decrease,indicate the accumulation of negative charge, when the ratio is1:2and1:3, there isalmost no change of the surface potential.3. The influence of thermal annealing with different temperature and time to thePCE of the PSC with the1:1ratio of P3HT:PCBM. Take70℃，110℃，150℃，190℃ and10min,30min,60min,90min as the parameter, The result show that after thermalannealing the Isc increased obviously, when70℃90min and150℃30min the PCEof the PSC reached4.4%and4.6%respectively. With the temperature of thermalannealing rise from70℃to190℃，the roughness of the film increased, and the PCEof the device show a process of firstly increased and then decreased. And with the timeof thermal annealing rise, it shows the same regular. Compared with the untreatedactive layer film, the absorption spectra show a red-shift. EFM measurement can beused to investigate the phase separation. For the untreated P3HT:PCBM film, P3HTarrange randomly. After a thermal annealing at150℃for30min, the PCBMnanocrystal will disper around P3HT, the two form a good cross interpenetratingnetwork for efficient carrier transfer pass, the PCE of the device will be greatlyimproved. The morphology and surface potential of the P3HT:PCBM BHJ film underthree different annealing process is compared. Without annealing, the roughness issmall, the film is smooth, the surface potential distribute uniformly. After annealing at150℃for30min, a better structure formed, there are a large number of clusterformation. When the annealing temperature reach190℃， the roughness greatlyincreased and show large aggregation, resulting in the reduction of their interface, andunltimately lead to the reduction of the PCE of the device.4. With a long variation temperature condition, we explore the process of thermaldegradation of PSC. Firstly, we take two optimum conditions of annealing at70℃for90min and at150℃for30min and compare their stability. We get their degradationcurve at room temperature and70℃. The morphology of the active layer werecharacterized. The structure of the interpenetrating network can easily be destoryedwith annealing at70℃for90min., thus reduce the PCE of the device.At last we investigate the process of thermal degradation under the ISOSstandardized procedure at25℃，85℃and25℃-85℃thermal cycling condition.Comaring the morphology of the film of P3HT:PCBM annealing at150℃for30minbefore and after2h at85℃condition,we find the nanoscale of the phase separationis almost the same, the surface of the small protrusions showed aggregationcharacteristics. These results provide a guideline for the study of the failuremechanismsof polymer solar cell.