Research On Inverted Bulk Heterojunction Polymer Solar Cells

With energy prices and concerns over global climate change rising, there is great interest in using clean energy such as solar cells to capture sunlight and generate electricity. If solar cells were price-competitive with grid electricity, it would radically change the way the world's population obtain its energy. Polymer solar cells (PSCs) based on a blend of conjugated polymers and fullerenes have attracted much attention because of their light weight, low cost, and promising application in future. Some researchers hope to reduce solar cell costs by depositing solar cells in roll-to-roll coaters, similar to the ones used to print newspapers or photographic film. The performance of PSCs improves remarkably with the introduction of"bulk heterojunction"consisting of an interpenetrating network of electron donor and acceptor materials. The power conversion efficiency (PCE) of PSCs based on a blend of PBT1 and highly soluble fullerene derivative 6,6-phenyl C61 butyric acid methyl ester (PCBM) has reached up to 7.4%. In these reported devices, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is typically spin-cast on ITO surfaces to prevent electron leakage and to aid in hole extraction. However, recent researches have shown that PEDOT:PSS degraded the performance of device due to its corrosion to ITO. In order to solve this problem, one of feasible approches is to reverse the device structure by using a less air sensitive and high work function metal as the back hole-collecting electrode. This stucture can avoid the use of PEDOT:PSS.We fabricated a simple inverted PSC with a structure of ITO/nc-TiO₂/P3HT:PCBM/Metal. First, TiO₂ thin films were prepared by tetrabutyl titanate through a sol-gel method. The TiO₂-sol was spin cast on ITO-coated glass substrates and annealed at 450℃for 2 h to form a stable anatase nano-crystalline TiO₂ (nc-TiO₂) layer. Here, nc-TiO₂ serves as an electron-selective layer as well as a hole-blocking layer. It would prevent the eletron-hole recombination at the interface between ITO and P3HT:PCBM.However, device performances with different metal electrodes are poor. We deduce that various surface states and defects are formed at the interface between the metal and the active layer during the metal thermal-evaporation. Meanwhile, it is possible for the PCBM to transport electrons to the metal, thereby compromising the external quantum efficiency and device efficiency. Because the LUMO level of PCBM is higher than the Femi level of metal. Thus, we inserted a transition metal oxide layer, such as MoO₃ and WO₃, as a hole extraction layer between the active layer and top electrode. Consequently, the device performances are greatly improved and the open-circuit voltage is almost the same despite of the difference of work fuction of metals.The PCE of device ITO/nc-TiO₂/P3HT:PCBM/MoO₃ (1 nm)/Ag is 2.57% with Jsc of 6.57 mA/cm2, Voc of 0.628 V, and FF of 62.3% under 100 mW/cm2 white light illumination. For devices with Au or Ag as top electrode, the optimal thickness of MoO₃ is 1 nm, while the optimal thickness of MoO₃ is 5 nm in devices with Al as top electrode. Prior to metal deposition, the fabrication conditions for devices are held constant. The sole differential is the interface between MoO₃ and electrode metals. We investigate the contact interface by XPS. The results show that the chemical reaction between MoO₃ of Al occurs during thermal evaporation of Al. However, contact of MoO₃ with Au or Ag is physical. The PCE of device ITO/nc-TiO₂/P3HT:PCBM/WO₃ (10 nm)/Ag is 2.58% with Jsc of 7.20 mA/cm2, Voc of 0.597 V, and FF of 60.0% under 100 mW/cm2 white light illumination. The dependence of the device performances on WO₃ film thickness and different top metal electrodes is investigated. The optimal thickness of WO₃ is 5-10 nm.The PCE has not reached the commercialized level due to the mismatch of the absorption spectrum of the active layer and the solar spectrum. In order to improve PCE of PSCs, one possible way is to lower the band-gap of polymers to match the solar spectrum. Another possible way is to fabricate tandem or stacked PSCs, thereby absorbing more solar photons by the multiple active layers in the multiple-device structure. Thus, it is necessary to do some research on semitransparent devices, which are parts of tandem or stacked PSCs. Furthurmore, PSCs with high transparency could be also applied onto other interesting applications, such as power-generating windows. Here, we introduced semitransparent inverted PSCs with a structure of ITO/nc-TiO₂/P3HT:PCBM/MoO₃(WO₃)/Metal/MoO₃ (WO₃). It should be noticed that the hole-extraction layer and the light-coupling layer are the same material which would easy the fabrication complexity.Semitransparent inverted PSCs with MoO₃/Ag/MoO₃ are investigated. The inner MoO₃ serves as a hole extraction layer, while the outer MoO₃ is added as a light coupling layer to enhance optical transmittance of the device. The device performance of device ITO/nc-TiO₂/P3HT:PCBM/MoO₃ (1 nm)/Ag (10 nm)/MoO₃ is better than that of ITO/nc-TiO₂/P3HT:PCBM/MoO₃ (1 nm)/Ag (10 nm). The transmittance also inproves. Device performance dependence on the thickness of the light coupling layer is investigated. The results shows that Jsc and PCE increase with the increase of the thickness of the outer MoO₃ layer when illuminated from ITO side. However, the performance decreased when illuminated from MoO₃/Ag/MoO₃ side. Additionally, the sheet resistance of MoO₃/Ag/MoO₃ is independent of the thickness of the outer MoO₃ layer.Semitransparent inverted PSCs with WO₃/Ag/WO₃ are also investigated. The inner WO₃ serves as a hole extraction layer, while the outer WO₃ was added as a light coupling layer to enhance optical transmittance of the device. Device performance dependence on the thickness of the light coupling layer was investigated. The results shows that device with WO₃/Ag/WO₃ (40 nm) appears the lowest Jsc when illuminated from ITO side, while it exhibits the largest Jsc when illuminated from WO₃/Ag/WO₃. Additionally, the effect of the thickness of the outer WO₃ layer on Jsc was investigated by simulating the optical field distribution.