Optimization of an Electron Transport Layer to Enhance the Power Conversion Efficiency of Flexible Inverted Organic Solar Cells
© The Author(s) 2010
Received: 24 June 2010
Accepted: 17 August 2010
Published: 31 August 2010
The photovoltaic (PV) performance of flexible inverted organic solar cells (IOSCs) with an active layer consisting of a blend of poly(3-hexylthiophene) and [6, 6]-phenyl C61-butlyric acid methyl ester was investigated by varying the thicknesses of ZnO seed layers and introducing ZnO nanorods (NRs). A ZnO seed layer or ZnO NRs grown on the seed layer were used as an electron transport layer and pathway to optimize PV performance. ZnO seed layers were deposited using spin coating at 3,000 rpm for 30 s onto indium tin oxide (ITO)-coated polyethersulphone (PES) substrates. The ZnO NRs were grown using an aqueous solution method at a low temperature (90°C). The optimized device with ZnO NRs exhibited a threefold increase in PV performance compared with that of a device consisting of a ZnO seed layer without ZnO NRs. Flexible IOSCs fabricated using ZnO NRs with improved PV performance may pave the way for the development of PV devices with larger interface areas for effective exciton dissociation and continuous carrier transport paths.
Organic solar cells (OSCs) have been widely investigated in the past decade due to their numerous potential advantages including relatively inexpensive and light-weight materials, compatibility with flexible plastic substrates, and ease of fabrication [1–3]. However, the short exciton-diffusion length and inefficient exciton dissociation in a polymeric matrix of OSCs results in low quantum efficiency, which limits their use in many potential applications [4, 5]. Moreover, the lifetimes of OSC devices are short, and thus careful encapsulation strategies should be developed for use in practical working environments [6–8]. For the efficient dissociation of excitons, a bulk hetero-junction (BHJ) blend of donors and acceptors is generally used, leading to nanoscale morphology and facilitating charge transport in interpenetrating networks [8–10].
In the conventional structure of an OSC based on BHJ, indium tin oxide (ITO) modified with p-type poly(3, 4-ethylene dioxythiophene):(polystyrene sulfonic acid) (PEDOT:PSS) is used as an anode . However, PEDOT:PSS is an acidic water-based solution, which causes interface instability in the photoactive layer and corrosion of the ITO [11, 12]. To improve the interface stability and prevent device degradation, an alternative is to use as an inverted configuration , with ITO serving as the cathode and a high work function metal as the anode. It should be pointed out that only modified ITO can serve as the cathode for electron extraction, and thus the functional layers for modifying ITO mainly focus on metal oxides. ZnO is one of the applicable functional metal oxides for use in this application due to its high electron mobility and high degree of transparency in the visible wavelength range. Moreover, its crystal structure allows it to be grown anisotropically, making possible the production of highly efficient OSCs based on vertically oriented ZnO nanorods (NRs) for use as continuous electron transport pathways [14, 15].
Herein, we report a dramatic increase in the power conversion efficiencies (PCE) of flexible inverted OSCs (IOSCs) fabricated with ZnO NRs/ZnO seed layers acting as direct electron transport pathways. This method involves the growth of ZnO NRs via an aqueous solution route at the low temperature of 90°C. This is a simple and effective process for improving the PCE of IOSCs. Compared with ZnO seed layer-based IOSCs, the PCE of the IOSCs fabricated with ZnO NRs was increased by approximately threefold using simulated air mass (AM) 1.5 global full-sun (1.5G, 100 mW/cm2) illumination. This work suggests a method to fabricate efficient photovoltaic (PV) devices with larger interface areas for effective exciton dissociation area and optimum continuous carrier transport paths, which should be useful for future applications.
We first prepared a ZnO seed solution using zinc acetate dihydrate [Zn(CH3COO)2·2H2O] as a source and ethanol as a solvent. Briefly, zinc acetate dihydrate (final concentration 30 mM) was stirred for 30 min in ethanol at 60°C. ZnO seed layers were then deposited using spin coating at 3,000 rpm for 30 s onto ITO-coated polyethersulphone (PES) substrates. Spin coating was repeated between 12 and 18 times in order to control ZnO seed layer thickness and density. The deposited seed layers were thermally treated at 150°C for 10 min after each deposition. After seed layer formation, the substrates were maintained in a solution consisting of deionized water, 25 mM zinc nitrate hexahydrate [Zn(NO3)2·6H2O], and 25 mM hexamethylenetetramine [C6H12N4] (HMT) for 30 min at 90°C to prepare the ZnO NRs.
We prepared a P3HT: PCBM-blended solution for active layer deposition using spin coating at 2,000 rpm for 120 s. Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were dissolved in chlorobenzene at a weight ratio of 1:1 for 1 day, resulting in a P3HT:PCBM blend. After annealing an active layer for 30 min at 150°C, a 20-nm-thick MoOx electron blocking layer and a 100-nm Au layer were deposited via thermal evaporation through a shadow mask.
The structure of the IOSCs consisted of Au/MoOx/P3HT:PCBM/ZnO, a NR/ZNO seed layer, and ITO/PES stacked from bottom to top. Investigation into surface morphology and thickness was carried out using field emission scanning electron microscopy (FE-SEM). PV performance was evaluated by measuring short circuit current density (J sc ), open circuit voltage (V oc ), fill factor (FF), power conversion efficiency (PCE) and series resistance(R s ) using a solar simulator under AM 1.5G conditions (100 mW/cm2).
Results and Discussion
Summary of device performance
ZnO seed layer thickness (nm)
ZnO seed layer 12 times
ZnO seed layer 15 times
ZnO seed layer 18 times
ZnO NRs/ZnO seed layer 12 times
ZnO NRs/ZnO seed layer 15 times
ZnO NRs/ZnO seed layer 18 times
We also compared IOSCs with a ZnO NRs/ZnO seed layer with IOSCs consisting of a ZnO seed layer only. As summarized in Table 1, the PV performance of the IOSCs fabricated with a ZnO NRs/ZnO seed layer was improved about threefold compared with that of IOSCs fabricated with a ZnO seed layer. This substantial improvement in PV performance can be explained in two ways. First, improvement in PV performance was the result of an increased exciton dissociation interface area between ZnO and the active layer using ZnO NRs. The energy level diagram in Fig. 2 shows the position of the lowest unoccupied molecular orbital (LUMO) level of ZnO at—4.2 eV , which suggests that electrons from P3HT with a LUMO of—2.7 eV  can be injected into the ZnO NRs. Therefore, a larger area between ZnO and the active layer is favorable for increased exciton diffusion and separation events. For IOSCs with a ZnO seed layer, the exciton dissociation interface between the ZnO seed layer and organic material was planar, and most of the photo generated excitons were unable to reach the interface. This resulted in a large recombination probability in locations distant from the interface due to low exciton-diffusion length. Conversely, for the IOSCs with a ZnO NRs/ZnO seed layer, upon filling the space between the ZnO NRs with organic materials, the exciton dissociation interface area was greatly increased, and most of the photogene rated excitons were able to reach the interface before recombination.
PV performance of flexible IOSCs with an active layer consisting of a blend of P3HT:PCBM was investigated by varying the thicknesses of ZnO seed layers and by introducing ZnO NRs. A ZnO seed layer or ZnO NRs grown on the seed layer were used as an electron transport layer and pathway to optimize PV performance. The optimized device with ZnO NRs exhibited a threefold increase in PV performance compared with that of a device consisting of a ZnO seed layer without ZnO NRs. The optimization of the electron transport layer in the present work is one of the most important aspects for further improvement on solar cell efficiency.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0015035 and 2009-0077682) and also by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2009T100100614).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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