Engineered Interfaces Using Surface and Contact Passivation in Silicon Solar Cells

Kristopher O. Davis, Winston V. Schoenfeld
2018 The Electrochemical Society Interface  
S ilicon solar cells have enjoyed significant advancement in the past decade with manufacturing costs dropping considerably in tandem with measurable increases in absolute cell efficiency. As cell efficiencies have climbed, surface passivation has quickly become a key factor in realizing higher efficiencies. The first silicon solar cells to exceed efficiencies above 24% relied on covering the majority of the front and rear surfaces with insulating dielectrics, primarily silicon oxide (SiO 2 ),
more » ... o reduce surface recombination, only placing the metal contacts required for carrier collection at a small fractional area of the total surface. 1 Figure 1 provides a cross-section of an advanced cell architecture that has recently transitioned into highvolume manufacturing, the passivated emitter rear contact (PERC) cell. As noted in the figure, the top surface includes a front contact grid that contacts the emitter in specific regions while the backside is comprised of a full metal contact that makes contact to the wafer backside at specific sites where the passivation is omitted. More recently, researchers have realized the potential for engineering the passivation/silicon and contact/silicon interfaces using fixed charge. This article provides an overview of the role of surface passivation in silicon solar cells and presents current advanced passivation approaches that enable designed engineering of the cell interfaces for increased efficiency. Recombination Loss and the Role of Surface Passivation The role of surface passivation is best understood by assessing recombination loss in a silicon solar cell. Recombination loss is characterized by the total saturation current density (J 0 ) of the cell, roughly approximated as the sum of the individual J 0 values of the front surface (J 0f ), bulk (J 0b ), and rear surface (J 0r ). Current values of J 0b are roughly 50 fA·cm -2 for high quality p-type Czochralski (Cz) wafers and even lower for n-type Cz wafers, as compared to 200 fA·cm -2 for J 0f of PERC cells today. 2 Thus, recent silicon wafer advancements that have increased the bulk lifetime of the wafers resulting in a lower J 0b , have made recombination loss at the silicon/ passivation interface a limiting factor. While J 0b is primarily driven by silicon wafer quality, the J 0 of the front and rear surfaces are more complex and can be approximated as the weighted average of the surface passivated regions and contacted regions, 2 denoted as Regions I and II in Fig. 1 : where J 0fp and J 0rp are the front and rear recombination parameters for the passivated regions, J 0fc and J 0rc are the front and rear contact recombination parameters, and A fc and A rc are the fractional area of the front and rear surface covered by metal contacts. Typical values for the insulating, passivated regions (Region I) of industrial cells (J 0fp and J 0rp ) are below 10-20 fA·cm -2 for undiffused surfaces and in the range of 50-100 fA·cm -2 for diffused surfaces. 2,3 Recombination at metal/silicon interfaces (Region II) is normally much higher, with values for industrial cells ranging greatly from 500 to over 10,000 fA·cm -2 for both p + and n + surfaces depending on the doping concentration, depth and microstructure of the metal contacts. 2,4,5 While Region I passivation is sufficient to reach the J 0b values of typical wafers, the high saturation current density, J 0c , of the contacted regions increases the net surface J 0 for the front and back surfaces of the cell to much higher values. Thus, it is desirable to identify ways in which to further increase passivation quality in Region I, and more importantly Region II under the contacts. The following paragraphs present recent approaches towards further reducing J 0 values at the cell surface through engineered interfaces, often utilizing fixed charge. Surface Passivation As noted earlier, with rising cell efficiencies surface passivation has become increasingly more important to reduce carrier loss at the cell surface that would otherwise limit absolute efficiencies. Extremely low surface recombination levels can be achieved when both chemical and field effect passivation are utilized effectively at the surface. Chemical passivation refers to the traditional passivation of surface states via saturation of dangling bonds, often measured by the interface defect density near midgap (D it-midgap , or D it for short), while field effect passivation refers to a deliberate reduction in the concentration of electrons or holes, preferably the minority carrier for highly doped silicon surfaces. 3,6 It is the latter, electronic passivation, that has enabled engineered interfaces at the silicon solar cell surface. A thermally grown SiO 2 layer accompanied by a hydrogenation step (e.g., forming gas anneal) can drastically reduce D it , and lightly doped surfaces, called front and back surface fields, can be used to manipulate carrier concentrations near the surface. More recently, the deposition of thin dielectric films with inherent fixed charges Fig. 1. Illustration of a PERC solar cell with front and rear surface passivation. Regions I and II refer to the passivation and contact regions for the front surface, respectively. . © ECS 2018 address. See Downloaded on 2018-07-20 to IP
doi:10.1149/2.f07181if fatcat:jnw34dlckbbatdtpcfzivdsliy