High efficiency PTB7-based inverted organic photovoltaics on nano-ridged and planar zinc oxide electron transport layers

Abstract

Inverted organic photovoltaics (OPVs) have gained considerable attention as they offer increased device stability and processing advantages. ZnO serves as an effective electron transport layer (ETL) in inverted OPVs while efficiently blocking holes. In this work, the sol gel precursor concentration, spin coating speed and baking conditions were used to simultaneously control the film thickness and morphology of ZnO ETLs in inverted devices using the active layer of poly thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC 71 BM) bulk heterojunction (BHJ). Nano-ridged and planar structures of ZnO films were obtained simply by dynamically and statically baking the precursor films, respectively. Consistently high FF's exceeding 70% and PCEs averaging 7.65-8.01% with a maximum PCE reaching 8.32% were achieved on both nano-ridged and planar ZnO films when the active layer was maintained at ~94 nm. The effects of ZnO film thickness and morphology, along with the active layer thickness, on the device performance were further investigated using both experimental and transfer matrix optical modeling methods. The results demonstrated that high efficiency PTB7-based inverted OPVs can be achieved on both nano-ridged and planar ZnO ETLs as long as the optimal active layer thickness is maintained. devices with thinner active layers but is still within ~0.5 mA/cm 2 . The thickness of the planar ZnO films in the devices is in the range of approximately 42 to 62 nm. Within this range, Figure 4 shows the calculated J SC varies from 14.7 to 15.1 mA/cm 2 for a 94 nm active layer (i.e. the approximate PTB7:PC 71 BM thickness of devices in Fig. 3), which is close to the experimental J SC . Figure 4 shows that both the ZnO and the active layer thicknesses require simultaneous optimization to achieve maximum photocurrent.Device performance independent of film thickness for planar ZnO films was also observed in the study of inverted BHJ devices with the P3HT:PC 61 BM active layer. 14 As an ETL, the ZnO film should completely cover the ITO electrode to prevent current leakage. Once complete coverage is achieved, the thickness of the film has little effect on performance regardless of whether the active layer is P3HT:PC 61 BM or PTB7:PC 71 BM. However, the surface roughness of ZnO films affects the device performance in different ways when the active layer is P3HT:PC 61 BM versus PTB7:PC 71 BM. For P3HT:PC 61 BM BHJ devices, the increase of surface roughness from 2.86 to 4.02 nm resulted in the decrease of FF from 56 to 44%, accompanied by the increase of series resistance (R S ) from 10.7 to 31.7 Ω cm 2 . 14 It was suggested that a rougher ZnO film surface could induce more small voids between the P3HT:PC 61 BM active layer and the ZnO layer. In contrast, a consistently high FF of 72.3-73.4% was displayed in our devices regardless of the roughness variation from 2.59 to 4.16 nm among the four statically baked ZnO films. This may be due to differences in the microstructure and other properties of ...

High‐Performance Organic‐Silicon Heterojunction Solar Cells by Using Al‐Doped ZnO as Cathode Interlayer

“…Firstly, the Fourier transform infrared spectroscopy (FTIR) and X‐ray photoelectron spectroscopy (XPS) measurements were carried out. As shown in Figure a, the peak at 1130 cm −1 corresponds to a stretching vibration mode of SiOSi bonds that comes from the native silicon oxide present between the AZO film and the substrate in all films . According to references, the peaks at 430, 480, 510, and 540 cm −1 all can be ascribed to the stretching vibrations of ZnO bonds .…”

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“…The n-type metal oxides, particularly titanium dioxide (TiO 2 ), tin oxide (SnO 2 ), and zinc oxide (ZnO), have been used as CIL materials in the inverted device architecture due to their low-energy levels, adequate optical transmittance, and acceptable electronic conductivity. 213–215 Our group introduced ZnO as interfacial layer in inverted organic solar cell as early as 2008. 216 ZnO nanoparticles (ZnO-NP) were another effective CIL for OSCs (Fig.…”

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“…For this reason, kinds of ETLs tend to include conjugated polymers [11][12][13], transition metal oxides [14] and hybrid materials [15]. Therefore, solution-processed ZnO is applied as ETL, due to it shows low temperature processability and high mobility, low work function (WF), high transparency and offers a convenience, simple device fabrication method [14][15][16][17][18][19][20]. However, many defects on the surface of ZnO nanoparticles prepared by sol-gel method are unavoidable, leading to the recombination of electron-hole pairs [18].…”

“…Therefore, solution-processed ZnO is applied as ETL, due to it shows low temperature processability and high mobility, low work function (WF), high transparency and offers a convenience, simple device fabrication method [14][15][16][17][18][19][20]. However, many defects on the surface of ZnO nanoparticles prepared by sol-gel method are unavoidable, leading to the recombination of electron-hole pairs [18]. Internal doping of ZnO nanoparticles is better choice, both restraining surface charge recombination and improving electron extraction.…”