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. 2011 Mar 10;98(10):103305. doi: 10.1063/1.3554381

Thickness dependence of the MoO3 blocking layers on ZnO nanorod-inverted organic photovoltaic devices

Mingjun Wang 1,2, Yuan Li 1, Huihui Huang 1,2, Eric D Peterson 1, Wanyi Nie 1, Wei Zhou 1, Wei Zeng 2, Wenxiao Huang 1, Guojia Fang 2,a), Nanhai Sun 2, Xingzhong Zhao 2, David L Carroll 1,a)
PMCID: PMC3069983  PMID: 21464889

Abstract

Organic solar cells based on vertically aligned zinc oxide nanorod arrays (ZNR) in an inverted structure of indium tin oxide (ITO)∕ZNR∕poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester(P3HT:PCBM)∕MoO3∕aluminum(Al) were studied. We found that the optimum MoO3 layer thickness condition of 20 nm, the MoO3 can effectively decrease the probability of bimolecular recombination either at the Al interface or within the active layer itself. For this optimum condition we get a power conversion efficiency of 2.15%, a short-circuit current density of 9.02 mA∕cm2, an open-circuit voltage of 0.55V, and a fill factor of 0.44 under 100 mW∕cm2 irradiation. Our investigations also show that the highly crystallized ZNR can create short and continuous pathways for electron transport and increase the contact area between the ZNR and the organic materials.

Photovoltaic (PV) devices based on conjugated polymers such as poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM) have attracted a lot of attention in recent years because they are easily fabricated. This offers the possibility of solar cells that are cost-effective, flexible, and of large area. However, the strong acidic nature of the poly(3,4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) modified layer, can lead to corrosion of the indium tin oxide (ITO) electrode1 and degradation of the conjugated polymer in the active layer. Several groups have constructed inverted structure PV devices with highly transparent metal oxides, i.e., TiOx (Refs. 2, 3) or ZnO (Refs. 4, 5) on the transparent electrode as the cathode buffer layer. ZnO is a good candidate for this application because it is a cheap and environmentally friendly material that can be synthesized with high purity and crystallinity at low temperature. Its high electron mobility and high transparency in the visible-wavelength range allow it to be both an effective electron transporter and excellent wave guide. Olson et al. have reported efficient ZnO∕polymer devices with different ZnO morphologies.6, 7 Vertically oriented ZnO nanorods arrays (ZNR) can provide continuous electron transport pathways. Moreover, the high surface-to-volume of the vertically aligned ZNR can ensure three-dimensional contact with the polymer and provide many direct electrical pathways to transport electrons to the cathode directly. Vishal Shrotriya et al. have verified that MoO3 can replace PEDOT:PSS as the hole selective layer, which can be conveniently deposited by evaporation before depositing the anode metal.8 The thickness of the MoO3 hole selective layer is a vital parameter in the inverted solar cell structure. To further understand the effect of MoO3 thickness on the ZNR-based invert structures, layers of various thicknesses were deposited onto a series of inverted ZNR solar cells by thermal evaporation. We found the thickness of the MoO3 layers had a significant effect on the performance of the inverted ZNR solar cells.

The ZNR-based inverted solar cell was fabricated by the normative process as illustrated in Fig. 1. Devices were fabricated on precleaned and etched conducting ITO [Delta Technologies, Rs=10 Ω sq−1 (Delta Technologies Ltd, Stillwater, Minnesota)] coated glass substrates. A 130 nm P3HT:PCBM (1:0.8 Weight ratio, 15 mg∕ml in chlorobenzene) layer was spin-cast on top of pregrown ZNR9 substrates. Different thicknesses of MoO3 were deposited and capped with a 100 nm Al by thermal evaporation (5×10−6 torr) with a shadow mask. The devices were then annealed under N2 ambient at 150 °C for 7 min. The PV characteristics were measured with a Keithley 236 source-measurement unit under 100 mW∕cm2 using an AM 1.5G standard Newport #96000 Solar Simulator. The external quantum efficiency (EQE) was measured using a Newport Cornerstone 260 Monochromator in conjunction with a Newport 300W Xenon light source, and a Merlin Lock-In amplifier. The morphology analysis of the ZNRs and solar cell composite structure were done with a scanning electron microscope [JEOL, JSM-6330F (JEOL Ltd, Tokyo, Japan)].

Figure 1.

Figure 1

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Schematic illustration of the fabrication process of ZNR based bulk heterojunction (BHJ) solar cells.

Figure 2a shows ZNRs approximately 120 nm in length that are grown on a 100 nm ZnO seed layer. The ZNRs form a highly organized structure with the rods forming perpendicular to the seed layer in closely packed formation, leading to a rough surface. From Fig. 2b we can see that P3HT: PCBM formed an even coating on top of the ZNRs, and the infiltrated well between the rods.

Figure 2.

Figure 2

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(a) The side view SEM images of the ZNRs grown by the hydrothermal method and (b) cross sectional views of the P3HT: PCBM-coated ZNRs.

Figure 3a shows the J-V curve of devices without (0 nm: device A) and with 5, 10, 15, 20, and 25 nm MoO3 blocking layer (devices B, C, D, E, and F, respectively), we can see that with the increased thickness of the MoO3 layer, the short-circuit current density (Jsc) increased from 5.25 to 9.02 mA∕cm2. It should be noted that the surface roughness of the ZNR array is much larger than a spin coated sol gel based ZnO film. This led us to use very slow spinning speeds to ensure a reasonably even coating of polymer-fullerene solution on the rods. However, the surface of the active layer was still not smooth enough for a 15 nm or less MoO3 layer to form a consistent film. For devices B and C, the MoO3 layer can only cover the top of each rod and the bottom well between two ZnO nanorods, leaving the aluminum capping layer in direct contact with the polymer at the sides of the rods. This means that the MoO3 is only partially effective as an hole transport layer. Due to the similar conduction band edge of ZnO and Al, the electron can jump from the lowest unoccupied molecular orbital of PCBM to either the conduction band of the Al or the ZnO. This decrease in electrode selectivity can effectively increase the probability of bimolecular recombination (either at the aluminum interface or within the active layer itself). For thicknesses between 15 and 20 nm of MoO3 more and more of the surface of these rods were covered until there was no direct connection between the active layer and the cathode. The MoO3 could then act as a full-time hole transporting, electron and exciton blocking layer, which led to a significant increase in Jsc. The open-circuit voltage (Voc) of the 20 nm MoO3 device is increased from 0.223 to 0.555 V, providing further evidence for removal of shunts from the active layer to the aluminum cathode. For device F, a thicker MoO3 layer will reduce the shunt resistance and caused the mismatch of optical field distribution in active layer,10, 11 which resulted in a decrease in the performance of the device.

Figure 3.

Figure 3

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(a) J-V curve of the ZNR based inverted solar cell with different MoO3 thickness and (b) the dependence of FF on a Rsh. (c) J-V curve of ZNR and ZnO film based inverted solar with 20 nm MoO3 layer. (d) The EQE of both the ZNR with different MoO3 thickness (ZNR devices) and ZnO film with 20 nm MoO3 layer devices (ZnO device).

The extracted device parameters are summarized in Table 1. We can see that the fill factor (FF) of the device increased with the increase of MoO3 thickness. However, the FF is still very low compared with reported,12 that is because we get relatively low shunt resistance (Rsh) due to the presence of electrical shunt paths created by the ZnO nanorod arrays and the voltage dependent carrier recombination created by the rough surface of the ZNR. An extreme drop in Rsh can cause a drop in the Voc of a solar cell. The fill factor of solar cells can be influenced by the nonzero Rs and by the finite shunt resistance (Rsh). According to literature,13, 14Rsh and Rs can be independently derived from the slope of the I-V curve of a solar cell at I=Isc (V=0) and V=Voc (I=0). In this case, as we can see in the Table 1, all the devices have an approximate Rs around 30 Ω∕cm2, so we propose that the main influence to the performance of these cells is the Rsh, which can be seen in Fig. 3b. Increasing the Rsh results in an increase in FF. We should note that even though, in our device, the electron transport layer contains both a 100 nm ZnO seed layer and a 120 nm nanorod array layer, we still can get a higher Jsc than the flat ZnO film device reported in literature12 that may ascribe to the highly ordered ZNR providing a more direct electron pathway.

Table 1.

The summary of device performance.

  EF (%) Voc (V) Jsc (mA∕cm2) FF EQE (%) Rsh (Ω cm2) Rs (Ω cm2)
A 0.203 0.188 4.29 0.260 21.7 48.08 30.35
B 0.394 0.223 6.12 0.288 36.4 58.28 30.26
C 0.367 0.277 5.43 0.307 41.1 78.11 30.29
D 1.06 0.478 6.99 0.326 43.4 176.99 46.87
E 2.15 0.555 9.02 0.442 57.6 387.88 22.17
F 0.343 0.191 6.03 0.298 40.6 49.02 22.44
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A device with a 200 nm ZnO seed layer as the electron selective layer was fabricated, for comparison. The other conditions are same with the 20 nm MoO3 ZNR device. The J-V curve is shown in Fig. 3c. We can see that the Voc of the two devices are similar, but the Jsc with only the ZnO film is much lower than the ZNR device, it is reported that the diffusion length of excitons in the conjugated polymer is typically on the order of ∼10 nm,15 so the photo generated charge carriers in the 130 nm polymer layer in the ZnO film device are mostly recombined before collection and transport to the electrode in the polymer. Because the gap between two rods are on the order of 10 nm, which is suitable for efficient charge transport, each ZnO nanorod can create a short and continuous pathways for electron transport and increase the contact area between the ZnO nanorod arrays and the organic materials, resulting in a higher photocurrent. Our experimental results confirm this explanation.

The EQE of both the ZNR with different MoO3 thickness (ZNR devices) and ZnO film with 20 nm MoO3 layer devices (ZnO device) are shown in Fig. 3d. The ZNR devices show a maximum EQE of 57.6% at 515 nm. The peak in the EQE spectrum at about 350 nm results from the absorption of the ZnO. With the increased thickness of the MoO3 layer, a dramatic increase in the EQE is observed of around 36.4%. This increase is likely the result of enhanced charge collection in the device. All of the ZNR devices have a higher EQE than the ZnO device. The EQE spectrum is broader for the ZNR devices in comparison to the ZnO device.

In summary, we studied the effect of the MoO3 thickness on a ITO∕ZNR∕P3HT:PCBM∕MoO3∕Al inverted solar cell. We get a maximum efficiency of 2.15% with a 20 nm MoO3 layer which can effectively improve the interface between the active layer and the Al electrode. Our investigations also show that the vertically oriented, high electron mobility ZNR arrays can provide continuous electron transport pathways and increase the contact area between the ZNR and the organic materials making the ZNR based solar cell a promising candidate for high efficiency PV devices.

Acknowledgments

This research was supported by a grant from the U.S. Dept. of Energy under Grant No. DE-FG02-07ER46428 and the AFOSR through Grant No. FA9550-04-1-0161.

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