Chapter 4 Using Graphene and Graphene oxide as a hole transport layer for
4.2 Graphene
4.2.3 Experimental step
In this section, we introduced using a single layer graphene to substitute for PEDOT:PSS as a hole transport layer in inverted polymer solar cells as shown in Figure 4.15. After the deposition of ZnO electron transport layer and the P3HT:PCBM blend film, the following transferred process was carried out in the glove box. We used a thermal released process, which attached the CVD graphene grown on a copper foil to a thermal released tape, and then etched the copper by immersing the tape with graphene in the Fe(NO3)3. After the etching process, we got a single layer graphene on a thermal released tape. Then, we attached the tape with graphene onto the top of the device and rolled by the roller. Then, the whole device was put on the hot plate at 120°C. After a minute, a device with a single layer graphene as a hole transport layer was prepared.
Different metal top electrodes were deposited by thermal evaporation for comparison.
Figure 4.15 Device architecture of an inverted polymer solar cell with a single layer graphene as a hole transport layer
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4.2.4 Device performance and discussion
Electrical transport characteristic of single layer graphene with and without oxygen doping is shown in Figure 4.16. The two devices were measured in air and in vacuum respectively. After exposed to air ambient, the Dirac point of the device shifted to higher voltage as illustrated by the curve with circle points in Figure 4.16, indicating increased hole doping in the graphene device due to the adsorption of H2O and O2. This phenomenon makes graphene another candidate to replace PEDOT:PSS for hole transporting. Figure 4.17 is AFM image of single layer graphene transferred on a glass.
The continuity and uniformity of graphene on the substrate give graphene a great possibility to overcome the inhomogeneity of PEDOT:PSS which leads to the poor electron blocking property.
Figure 4.16 Electrical properties of SLG with and without oxygen doping.
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Figure 4.17 AFM image of graphene deposited on a glass substrate.
The effect of single layer graphene hole transport layer on the performance of inverted P3HT:PCBM PSCs using metals with different work functions as the top anodes (Ag, Al) was investigated. The current density-voltage curves under illumination are shown in Figure 4.18(a) and (b). Devices without graphene single layer between BHJ and the metal electrodes showed quite poor performance with low short circuit currents (Jsc) and open circuit voltages (Voc). Since the upper bound of Voc is governed by the difference between the work functions of the electrodes, a clear trend of increased Voc with increasing anode work function was observed. However, when a graphene HTL was introduced, the two devices showed improved performance with a Voc of ∼0.5V in two kinds of metal electrodes.
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Figure 4.18 (a)(b) The current density–voltage (J-V) characteristics of inverted polymer solar cells (a) without graphene HTL and (b) with graphene HTL using different top metal electrodes.
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The J-V curves of devices with no HTL and with different HTLs including PEDOT:PSS, GO and Graphene are shown in Figure 4.19. Although efficiency of the device with graphene HTL is still lower compared to that of PEDOT:PSS and GO, it still has a great improvement of the performance compared to the device without HTL.
The possible reason for the lower Jsc and fill factor of the device with graphene may be the residual organic impurity between the graphene and the top electrode which leads to the poor contact between the graphene layer and the top anode.
Figure 4.19 Current-voltage characteristics of the inverted polymer solar cells consisting of PEDOT:PSS, GO and Graphene as a hole transport layer.(A.M. 1.5 illumination, 100 mW cm-2). The device performance of the device without HTL was also shown.
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Table 4.3 Summaries of the device performances of the inverted polymer solar cells under illumination (A.M.1.5, 100 mW cm-2) using PEDOT:PSS, GO and Graphene as a hole transport layer. The device performance of the device without HTL was also shown.
In summary, we have demonstrated that GO and Graphene can also be used as an hole transport layer (HTL) in inverted type polymer solar cell to improve the device performance as shown in Table 4.3. The optimized device performance with GO HTL is comparable to those using PEDOT:PSS as the HTL. However, graphene is another promising material to replace PEDOT:PSS. To overcome the impurity during transferred procedure for better performance is the first priority for further optimization.
HTL Voc (V) Jsc (mAcm-2) FF PCE (%)
PEDOT:PSS 0.52 11.9 54.3 3.37
GO 0.54 11.7 50.8 3.22
Graphene 0.54 9.7 46.2 2.42
None 0.28 6.03 41.5 0.70
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5
Bifacial semitransparent inverted OPV with graphene/GO top electrode
Abstract
In this section, we demonstrated a semitransparent inverted polymer solar cell using a top laminated graphene electrode without damaging the underlying organic photoactive layer. The lamination process involves the simultaneous thermal releasing deposition of the graphene top electrode during thermal annealing of the photoactive layer. The resulting semitransparent polymer solar cell exhibits a promising power conversion efficiency of approximately 76 % of that of the standard opaque device using an Ag metal electrode. The asymmetric photovoltaic performances of the semitransparent solar cell while illuminated from two respective sides were further analyzed using optical simulation and photocarrier recombination measurement. The devices consisting of the top laminated transparent graphene electrode enable the feasible roll-to-roll manufacturing of low-cost semitransparent polymer solar cells and can be utilized in new applications such as power-generated windows, or multi-junction or bifacial photovoltaic devices.
Chapter 5 Bifacial semitransparent inverted OPV