Fabrication and characterization of Al-BSF bifacial solar cell

Abstract

Crystalline silicon, in its single crystalline or multi-crystalline (mc) format, dominates the photovoltaic (PV) industry. However, PV energy generation cost is still higher than energy-conversion costs of carbon-based fossil fuels. Since the price of silicon wafer accounts for almost 50% of the energy conversion cost, historically cost reduction through reducing Si wafer thickness has been successful approach. This approach is now reaching its limits due to yield reduction of thinner wafers, thermal expansion mismatch between Al and thin Si wafer, and reduced optical absorption. In recent years, bifacial solar cells have attracted attention due to their several attractive features including: (a) applicability to thinner wafers, (b) superior high temperature performance, (c) elimination of wafer warpage, (d) lower metal usage, (e) symmetric structure irrespective of n or p wafer, (f) enhanced power generation, and (g) simple processing. A bifacial solar cell structure consists of bulk (p or n-type semiconductor), emitter, back surface field (BSF), anti reflective coatings (ARC) and identical metal grids on both sides. In this study, a new combination method of emitter and BSF layer for npp+ bifacial structure has been investigated. The npp+ structure has been chosen due to its inherent simplicity and process similarity to industrial monofacial solar cell manufacturing. The new process relies on POCl3 diffusion for emitter formation on the front side; and screen printed Aluminum (Al) for BSF on the rear surface. A screen-printed process is used to apply Al to the wafer followed by high temperature firing process to form Al back surface field and contact. In this case, excess Al is removed from the rear using wet-chemical etching resulting in an Al-doped p+ surface. LIV, surface photovoltage, and EDAX techniques were employed to characterized solar cell performance; PC1D simulations were applied to determine front and rear surface efficiencies. Poor rear surface performance of such bifacial solar cells has been attributed to inadequate passivation, higher reflection, and ineffective back surface field.