Page - 115 - in Photovoltaic Materials and Electronic Devices

transparency [8,9]. In the case of small cells, this hurdle can be masked using small dimensions and addition of a patterned metallic grid to the TCO, thereby compensating for its low conductivity. Such a combination of materials can increase the efficiency by creating a dramatic shift in conductivity at the expense of only a small loss in transmittance [10,11]. Classic wafer based cells do not have a TCO and fully rely on metallic grids. Therefore, the cell layout and ink requirements are highly different from the desired characteristicsofmonolithically interconnectedthinfilmcells,whichrequiressmaller feature sizes and, in the case of thin film CIGS cells, limited annealing temperatures below 200˝C. A few studies of grid on TCO were performed, but these reflected the statusof ink jetprinting, resulting in low(<1µm)and100µmwidegrids. Because of these low and wide grid dimensions, there was hardly an advantage of this TCO + metal grid approach compared to the TCO only approach [12–15]. Therefore, in solar panel production, this solution has not been adopted for monolithically interconnectedsolarcells. Recently, however, the full potential of the application of metallic grids with optimized finger and cell dimensions (i.e., lower width and larger height) was reportedtogiveasignificantboost in thinfilmsolarefficiencies [16]. Becausesuch approachwouldaddcomplexity in themanufacturingprocess, theefficiencygain should be determined and evaluated with respect to manufacturing issues. Previous study [16,17] was performed on cells with efficiencies of 15.5% and many aspects such as cell layout and absorber material band gap, were not discussed in depth. Moreover, previous designs did not take into account the material interface issue of contact resistance. The investigation of contact resistance in solar cells has only briefly been touched [18,19] and its impact on design of monolithically integrated solarpanelsstillneeds tobeaddressed. Moreover, thepreviouscasewas limitedto lowefficiencyCIGSororganicPV[15,16]andthecase forhighefficiencythinfilm solar cells, spanning a wide range of band gaps should be investigated. In short, there isa lackofknowledgeof the impactof thecell layoutandspecificmetal-TCO interaction (e.g., contact resistance) on the preferred grid design and the expected efficiencybenefit. This work focuses on the introduction of metal finger grid to enhance the performance of thin film solar panels with up-to-date cell efficiencies of 19%. The effects of cell length and interconnection area, as well as the band gap of the absorber material and the contact resistance are modeled. In contrast to previous work reflecting a rather idealized situation, specific issues such as the losses due to the specific contact resistance and the impact of reduced irradiation intensity arediscussed. Thecalculatedcell efficienciesgiveguidelinesoverawiderangeof (non-ideal) circumstances foruseful frontcontact technologies thataimtoenhance the thinfilmsolarpanelefficiency. 115