Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in-depth numerical device simulation and advanced cost modelling. As references, Al-BSF (aluminium back-surface field) and PERC (passivated emitter and rear) cells with local rear-side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single-sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double-sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost-competitive under monofacial operation compared with an 18% efficient (≙ pMPP = 18 mW/cm2) multicrystalline silicon (mc-Si) Al-BSF cell and a 19% mc-Si PERC cell for maximum output power densities of pMPP ≥ 17.3 mW/cm2 and pMPP ≥ 18.1 mW/cm2, respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al-BSF and PERC cells. For the PERT cell, corresponding values are pMPP ≥ 21.7 mW/cm2 and pMPP ≥ 22.7 mW/cm2, respectively, assuming the current price offset (≈50%, at the time of October 2014) of n-type Czochralski-grown silicon (Cz-Si) compared with mc-Si wafers. The material price offset of n-type to p-type Cz-Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm2, which correlates to a conversion efficiency difference of 1%abs for monofacial illumination with 1 sun. From p-type mc-Si to p-type Cz-Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm2 for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear-side illumination intensities for arbitrary front-side efficiencies ηfront by means of the performed numerical simulations. For a BOSCO cell with ηfront = 18%, minimum rear-side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n-type Cz-Si PERT cell with ηfront = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n-type to p-type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE

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This article introduces a postmetallization "passivated edge technology" (PET) treatment for separated silicon solar cells consisting of aluminum oxide deposition with subsequent annealing. We present our work on bifacial shingle solar cells that are based on the passivated emitter and rear cell concept. To separate the shingle devices after metallization and firing, we use either a conventional laser scribing mechanical cleaving (LSMC) process or a thermal laser separation (TLS) process. Both separation processes show similar pseudo fill factor (pFF) drops of − 1.2% abs from the host wafer to the separated state. The pFF of the TLSseparated cells increases by up to +0.7% abs from the as-separated state after PET treatment due to edge passivation, while the pFF of LSMC-separated cells increases by up to +0.3% abs. On cell level, the combination of TLS and PET allows for a designated area output power density of p out = 23.5 mW/cm², taking into account an additional 10% rear side irradiance.

Bifacial applications are a promising way to increase the performance of photovoltaic systems. Two silicon solar cell concepts suitable for bifacial operation are the passivated emitter, rear totally diffused (PERT) and the both sides collecting and contacted (BOSCO) cell concepts. This work investigates the bifacial potential of these concepts by means of in-depth numerical device simulation and experiment with a focus on the impact of varying material quality. It is shown that the PERT cell concept (representing a structure with front-side emitter only) requires high-minority-carrier-diffusion-length substrates with L-bulk > 3 x W (with cell thickness W) to exploit its bifacial potential, while the BOSCO cell (representing a structure with double-sided emitter) can already utilise its bifacial potential on substrates with significantly lower diffusion lengths down to L-bulk approximate to 0.5 x W. Experimentally, BOSCO cells with and without activated rear-side emitter are compared. For rear-side illumination, the activated rear-side emitter is measured to increase internal quantum efficiency at wavelengths lambda < 850 nm by up to 45%(abs) (factor of 9) and 30%(abs) (factor of 2) for cells processed on p-type multicrystalline silicon substrates with L-bulk approximate to 0.3 x W and L-bulk approximate to 2.6 x W, respectively. For PERT cells processed on n-type Czochralski-grown silicon substrates, an according increase in internal quantum efficiency for rear-side illumination of more than 20%(abs) (factor of 1.3) is measured when changing from a substrate with L-bulk approximate to 3.0 to 10.0 x W. The performed simulations and experiments demonstrate that the BOSCO cell concept is a promising candidate to successfully exploit bifacial gain also on low- to medium-diffusion-length substrates such as p-type multicrystalline silicon, while PERT cells require a high-diffusion- length substrate to utilise their bifacial potential. Furthermore, the BOSCO cell concept is shown to be a promising option to achieve highest output power densities, even when using lower quality and therefore possibly more cost-effective silicon substrates

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