| AVS 60th International Symposium and Exhibition | |
| Energy Frontiers Focus Topic | Monday Sessions |
| Session EN+AS+NS+SS-MoA |
| Session: | Interfacial Challenges in Nanostructured Solar Cells |
| Presenter: | K. Sharma, Eindhoven University of Technology, Netherlands |
| Authors: | K. Sharma, Eindhoven University of Technology, Netherlands D. Deligiannis, Delft University of Technology, Netherlands J. Willem Weber, Eindhoven University of Technology, Netherlands S. Smit, Eindhoven University of Technology, Netherlands A.A. Bol, Eindhoven University of Technology, Netherlands M.C.M. van de Sanden, Dutch Institute for Fundamental Energy Research (DIFFER), Netherlands M. Creatore, Eindhoven University of Technology, Netherlands R.A.C.M.M. van Swaaij, Delft University of Technology, Netherlands W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Silicon heterojunction solar cells (SHJ) have achieved excellent conversion efficiencies both at lab (~25%) and industrial scale (>20%).1 Further enhancement towards theoretical efficiencies (~28%) is expected to occur when novel concepts and/or new functional materials are implemented to address the present optical and resistive losses in ITO and doped layers utilized in SHJ solar cells. Recently, graphene on silicon has been successfully tested as a Schottky barrier junction.2 Innovative solar cell structures are required to realize the true potential of graphene. To this end, we are reporting a novel device structure where undoped graphene was tested as a transparent conductive layer. Conversion efficiencies of about 10% have been achieved, which is an order of magnitude higher than reported so far in the literature for undoped graphene. CVD grown graphene was transferred onto SHJ device structures in an advanced multi-step procedure and the following solar cell layouts were used: stack A-graphene/(i)a-Si:H/(n)c-Si/(i)a-Si:H/(n)a-Si:H; and stack B-graphene/(p)a-Si:H/(i)a-Si:H/(n)c-Si/(i)a-Si:H/(n)a-Si:H. The amorphous silicon layers were deposited by means of RF-PECVD in a cluster tool. Current-voltage (J-V) and external quantum efficiency (EQE) measurements were carried out to characterize the devices. A minority carrier lifetime as high as 2 ms was achieved for the investigated stacks, largely due to the excellent surface passivation induced by (i)a-Si:H layers. Open-circuit voltage, short-circuit current density, fill-factor, and conversion efficiency values of 384 mV, 25 mA/cm2, 60%, and 5.8%, respectively, were achieved for stack A. Simulations of the band structure have shown that graphene, due to its self-adaptive nature2 acts as an energy barrier for charge transport, which could be further enhanced if the graphene is doped. For stack B, with the graphene applied onto a p-type a-Si:H layer, the open-circuit voltage was significantly enhanced up to 569 mV, while the fill-factor increased to 74%. However, in this case the short-circuit current density dropped to 24 mA/cm2 due to enhanced absorption losses in the (p)a-Si:H. The conversion efficiency of10.1% was achieved. These initial results suggest that graphene can potentially be used to decouple electrical and optical properties of the front contact in SHJ solar cells to achieve higher conversion efficiencies.
1 A. Descoeudres, Z.C. Holman, L. Barraud, S. Morel, S. De Wolf, and C. Ballif, IEEE Journal of Photovoltaics 3, 83 (2013).
2 H. Zhong, Z. Liu, G. Xu, Y. Fan, J. Wang, X. Zhang, L. Liu, K. Xu, and H. Yang, Applied Physics Letters 100, 122108 (2012).(ref. 7-9 therein)