Organic photovoltaics (OPV) has gained much attention as a possible candidate for the next generation of clean electricity due to organic semiconductors’ high absorption coefficients, light weight and flexibility, and low-cost, high throughput fabrication methods ^{[1] [2]} .  In optoelectronics devices, indium tin oxide (ITO) has been widely used as transparent conducting electrodes.  However, the need for a substitute for ITO is ever increasing due to the limited availability of indium on earth; furthermore, device issues like susceptible ion diffusion into the organic films ^[3] and mechanical brittleness ^[4] limit the applicability of ITO in OPVs. Therefore, an ITO-substitute needs to be developed with these characteristics: low-cost, mechanically robust, transparent, electrically conductive, and ultimately capable of demonstrating performance comparable to ITO-based photovoltaics.

In the past, we have synthesized graphene sheets using Ni thin film (300 nm) as a catalyst layer via atmospheric pressure chemical vapor deposition (APCVD): either single to few-layer graphene sheets or multi-layer graphene sheets (>10 layers). The sheet resistance and optical transmittance obtained from the multi-layer graphene were around 500~1000Ω/sq and 75%, respectively. We further improved the synthesis conditions using copper foil (25 µm) as metal catalyst via low-pressure chemical vapor deposition (LPCVD). This method enabled us to synthesize large area, uniform monolayer graphene (>90%) with improved electrical conductivity (400~500Ω/sq) and optical transmittance (~97%). By transferring several times, we could further improve the quality of graphene electrodes (e.g., 3-layer graphene sheet: 300-400 Ω/sq with >90% transmittance) (Figure 1). We then successfully integrated these graphene sheets into the OPV with overall performance comparable, but slightly inferior, to ITO counterparts, possibly due to the relatively higher sheet resistance. Moreover, due to the hydrophobicity of graphene’s surface, uniform coverage of PEDOT:PSS layer was challenging, which was detrimental to device success rates. Various PEDOT:PSS alternatives were investigated, and it was found that AuCl₃ doping significantly improves the graphene OPV device performances, possibly due to the improved conductivity and the work function tuning of graphene electrodes as well as the PEDOT wettability (Figure 2).

1. C. W. Tang, “2-layer organic photovoltaic cell,” Applied Physics Letters, vol. 48, pp. 183-185, Jan. 1986.
2. P. Peumans, A. Yakimov, and S. R. Forrest, “Small molecular weight organic thin-film photodetectors and solar cells,” Journal of Applied Physics, vol. 93, pp. 3693-3723, 2003.
3. A. R. Schlatmann, D. W. Floet, A. Hilberer, F. Garten, P. J. M. Smulders, T. M. Klapwijk, and G. Hadziioannou, “Indium contamination from the indium-tin-oxide electrode in polymer light-emitting diodes,” Applied Physics Letters, vol. 69, pp. 1764-1766, Sep. 1996.
4. Z. Chen, B. Cotterell, W. Wang, E. Guenther, and S. J. Chua, “A mechanical assessment of flexible optoelectronic devices,” Thin Solid Films, vol. 394, pp. 202-206, Aug. 2001.

Figure 1: (a-d) Transmission electron microscopy images of one to two layers of hexagonal boron nitride (hBN) film, (e) Selective electron diffraction pattern of hBN film, (f) Electron energy loss spectra of hBN film.

Hexagonal boron nitride (hBN) is very attractive for a variety of applications, particularly as deep ultraviolet emitter, transparent membrane, dielectric layer/substrate, or protective coating. In this work, we carried out detailed investigation of hBN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP).  Previously, few-layer hBN was synthesized via CVD under atmospheric pressure (AP) on metallic substrates ^{[1] [2]} .  In contrast, one- or two-layer hBN is synthesized under LPCVD. Ammonia borane, which is easier accessible and more stable in the atmosphere than borazine, is used as a BN precursor. These mono- or bi-layer hBN films are characterized by atomic force microscopy, transmission electron microscopy and electron energy loss spectroscopy analyses.  Our results suggest that the growth here occurs via surface-mediation, which is similar to graphene growth on Cu under low pressure. These atomically thin layers are particularly attractive for atomic membranes or dielectric layer/substrates for graphene devices.

1. Y. M. Shi, C. Hamsen, X. T. Jia, K. K. Kim, A. Reina, M. Hofmann, A. L. Hsu, K. Zhang, H. N. Li, Z. Y. Juang, M. S. Dresselhaus, L. J. Li, and J. Kong, “Synthesis of few-layer hexagonal boron nitride thin film by chemical vapor deposition,” Nano Letters, vol. 10, pp. 4134-4139, Oct. 2010.
2. L. Song, L. Ci, H. Lu, P. B. Sorokin, C. Jin, J. Ni, A. G. Kvashnin, D. G. Kvashnin, J. Lou, B. I. Yakobson, and P. M. Ajayan, “Large scale growth and characterization of atomic hexagonal boron nitride layers,” Nano Letters, vol. 10, pp. 3209-15, Aug. 2010.