{"id":3236,"date":"2011-06-29T14:35:18","date_gmt":"2011-06-29T14:35:18","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/?p=3236"},"modified":"2011-07-21T14:36:35","modified_gmt":"2011-07-21T14:36:35","slug":"graphene-photovoltaics-2","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/graphene-photovoltaics-2\/","title":{"rendered":"Graphene Photovoltaics"},"content":{"rendered":"

Organic photovoltaics (OPV) has gained much attention as a possible candidate for the next generation of clean electricity due to organic semiconductors\u2019 high absorption coefficients, light weight and flexibility, and low-cost, high throughput fabrication methods [1<\/a>] <\/sup> [2<\/a>] <\/sup>.\u00a0 In optoelectronics devices, indium tin oxide (ITO) has been widely used as transparent conducting electrodes.\u00a0 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<\/a>] <\/sup> and mechanical brittleness [4<\/a>] <\/sup> 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.<\/p>\n

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 500~1000\u03a9\/sq and 75%, respectively. We further improved the synthesis conditions using copper foil (25 \u00b5m) 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 (400~500\u03a9\/sq) and optical transmittance (~97%). By transferring several times, we could further improve the quality of graphene electrodes (e.g.,<\/em> 3-layer graphene sheet: 300-400 \u03a9\/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\u2019s 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<\/sub> 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).<\/p>\n\n\t\t