With a global market share of about 90%, crystalline silicon is by far the most important photovoltaic technology today. This article reviews the dynamic field of crystalline silicon photovoltaics from a device-engineering perspective. First, it discusses key factors responsible for the success of the classic dopant-diffused silicon homojunction solar cell. Next it analyzes two archetypal high-efficiency device architectures – the interdigitated back-contact silicon cell and the silicon heterojunction cell – both of which have demonstrated power conversion efficiencies greater than 25%. Last, it gives an up-to-date summary of promising recent pathways for further efficiency improvements and cost reduction employing novel carrier-selective passivating contact schemes, as well as tandem multi-junction architectures, in particular those that combine silicon absorbers with organic–inorganic perovskite materials.

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High-efficiency crystalline silicon solar cells: status and perspectives

n-Type Si solar cells with passivating electron contact: Identifying sources for efficiency limitations by wafer thickness and resistivity variation

Laser contact openings for local poly-Si-metal contacts enabling 26.1%-efficient POLO-IBC solar cells

The global photovoltaic (PV) market is dominated by crystalline silicon (c-Si) based technologies with heavily doped, directly metallized contacts. Recombination of photo-generated electrons and holes at the contact regions is increasingly constraining the power conversion efficiencies of these devices as other performance-limiting energy losses are overcome. To move forward, c-Si PV technologies must implement alternative contacting approaches. Passivating contacts, which incorporate thin films within the contact structure that simultaneously supress recombination and promote charge-carrier selectivity, are a promising next step for the mainstream c-Si PV industry. In this work, we review the fundamental physical processes governing contact formation in c-Si. In doing so we identify the role passivating contacts play in increasing c-Si solar cell efficiencies beyond the limitations imposed by heavy doping and direct metallization. Strategies towards the implementation of passivating contacts in industrial environments are discussed. The development of passivating contacts holds great potential for enhancing the power conversion efficiency of silicon photovoltaics. Here, De Wolf et al. review recent advances in material design and device architecture, and discuss technical challenges to industrial fabrication.

/pdf/passivating-contacts-for-crystalline-silicon-solar-cells-66twad5vaz.pdf

Passivating contacts for crystalline silicon solar cells

This letter examines the application of transparent MoOx (x < 3) films deposited by thermal evaporation directly onto crystalline silicon (c-Si) to create hole-conducting contacts for silicon solar cells. The carrier-selectivity of MoOx based contacts on both n- and p-type surfaces is evaluated via simultaneous consideration of the contact recombination parameter J0c and the contact resistivity ρc. Contacts made to p-type wafers and p+ diffused regions achieve optimum ρc values of 1 and 0.2 mΩ·cm2, respectively, and both result in a J0c of ∼200 fA/cm2. These values suggest that significant gains can be made over conventional hole contacts to p-type material. Similar MoOx contacts made to n-type silicon result in higher J0c and ρc with optimum values of ∼300 fA/cm2 and 30 mΩ·cm2 but still offer significant advantages over conventional approaches in terms of contact passivation, optical properties, and device fabrication.

/pdf/molybdenum-oxide-moox-a-versatile-hole-contact-for-silicon-1mdxpra2cv.pdf

Molybdenum oxide MoOx: A versatile hole contact for silicon solar cells

Recombination behavior and contact resistance of n+ and p+ poly-crystalline Si/mono-crystalline Si junctions

We study ion implantation for patterned doping of back-junction back-contacted solar cells with polycrystalline–monocrystalline Si junctions. In particular, we investigate the concept of counterdoping, that is, a process of first implanting a blanket emitter and afterward locally overcompensating the emitter by applying masked ion implantation for the back surface field (BSF) species. On planar test structures with blanket implants, we measure saturation current densities $J_{0,{\rm poly}}$ of down to $1.0 \pm 1.1$ fA/cm2 for wafers passivated with phosphorus-implanted poly-Si layers and $4.4 \pm 1.1$ fA/cm 2 for wafers passivated with boron-implanted poly-Si layers. The corresponding implied pseudofill factors $pFF_{{\rm impl}}$ . are 87.3% and 84.6%, respectively. Test structures fabricated with the counterdoping process applied on a full area also exhibit excellent recombination behavior ( $J_{0,{\rm poly}} = 0.9 \pm 1.1$ fA/cm 2, $pFF_{{\rm impl}.} = 84.7$ %). By contrast, the samples with patterned counterdoped regions exhibit a far worse recombination behavior dominated by a recombination mechanism with an ideality factor $n > 1$ . A comparison with the blanket-implanted test structures points to recombination in the space charge region inside the highly defective poly-Si layer. Consequently, we suggest introducing an undoped region between emitter and BSF in order to avoid the formation of ${\rm p}^{+}/{\rm n}^{+}$ junctions in poly-Si.

Ion Implantation for Poly-Si Passivated Back-Junction Back-Contacted Solar Cells

Boron diffusion is commonly associated with the formation of an undesirable boron-rich layer (BRL), which is often made responsible for degradation of the carrier lifetime in the bulk. We investigate the phenomenology of the BRL formation, which results from BBr3 boron diffusion processes, and its impact on sheet resistance and bulk lifetime. Our measurements show that boron silicate glass (BSG) and BRL thicknesses vary between 50 and 600 nm and 0 and 80 nm respectively within the two-dimensional wafer surface of one sample for one diffusion process. Both thicknesses strongly depend on the gas composition during composition and deposition time. Further results show that BRL formation is favored by high concentrations of BBr3 vapor and of oxygen during B2O3 deposition. Also, high drive-in temperatures promote the growth of the BRL. We find that a BRL of more than 10 nm thickness causes a degradation of the carrier lifetime in the bulk of the silicon wafer. In particular, we show that this bulk lifetime degradation occurs during the cool-down ramp after the diffusion process. We show that carrier lifetime degradation can be avoided either by limiting the process temperature to 850 °C and thus preventing BRL formation or through reconverting the BRL by a drive-in step in oxidizing atmosphere at 920 °C.

https://iopscience.iop.org/article/10.1088/0268-1242/25/5/055001/pdf

Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer during BBr3 diffusion processes

The performance of passivated emitter and rear (PERC) solar cells made of p-type Si wafers is often limited by recombination in the phosphorus-doped emitter. To overcome this limitation, a realistic PERC solar cell is simulated, whereby the conventional phosphorus-doped emitter is replaced by a thin, crystalline gallium phosphide (GaP) layer. The resulting GaP/Si PERC cell is compared to Si PERC cells, which have (i) a standard POCl3 diffused emitter, (ii) a solid-state diffused emitter, or (iii) a high efficiency ion-implanted emitter. The maximum efficiencies for these realistic PERC cells are between 20.5% and 21.2% for the phosphorus-doped emitters (i)–(iii), and up to 21.6% for the GaP emitter. The major advantage of this GaP hetero-emitter is a significantly reduced recombination loss, resulting in a higher Voc. This is so because the high valence band offset between GaP and Si acts as a nearly ideal minority carrier blocker. This effect is comparable to amorphous Si. However, the GaP layer can be c...

Tobias Ohrdes

Papers

Recombination behavior and contact resistance of n+ and p+ poly-crystalline Si/mono-crystalline Si junctions

Ion Implantation for Poly-Si Passivated Back-Junction Back-Contacted Solar Cells

Charge carrier lifetime degradation in Cz silicon through the formation of a boron-rich layer during BBr3 diffusion processes

A numerical simulation study of gallium-phosphide/silicon heterojunction passivated emitter and rear solar cells

Injection dependence of the effective lifetime of n-type Si passivated by Al2O3: An edge effect?