Rhett Evans

Lifetime limiting recombination pathway in thin-film polycrystalline silicon on glass solar cells

The minority carrier lifetimes of a variety of polycrystalline silicon solar cells are estimated from temperature-dependent quantum efficiency data. In most cases the lifetimes have Arrhenius temperature dependences with activation energies of 0.17–0.21 eV near room temperature. There is also a rough inverse relationship between lifetime and the base dopant concentration. Judging by this inverse law, the activation energies of the lifetimes, and the absence of plateau behavior in the lifetimes of the higher doped cells at low temperatures, it is inferred that the dominant recombination pathway involves the electronic transition between shallow states which are 0.05–0.07 eV below the conduction band and 0.06–0.09 eV above the valence band, respectively, consistent with the shallow bands in silicon dislocations. The modeled recombination behavior implies that deep levels do not significantly affect the lifetimes for most of the cells at and below room temperature.

High efficiency at module level with almost no cell metallisation: Multiple wire interconnection of reduced metal solar cells

Perpendicular multiple busbar wires have proven an effective way of interconnecting standard screen printed solar cells with high reliability and low cell to module loss. The technology is also an effective way to interconnect plated solar cells, where conventional soldered interconnects may be problematic. However, the full benefits of this interconnection technology can be fully realized on plated cell structures with drastically reduced plated metallization. This paper presents a new selective emitter plated cell structure with metallization reduced to around 1 μm thickness, interconnected using perpendicular multiple busbar wires. Metal usage on the cell is reduced by more than 90% compared to conventional plated or screen print cells and the use of Ag almost eliminated, while high efficiency at the module level is achieved along with environmental durability.

A multivariate approach to utilizing mid-sequence process control data

While the measurement of cell efficiency is still considered one of the primary assessments of a cells quality, a modern photovoltaic manufacturing facility will also include a range of metrology to assess the performance at various steps during the manufacturing sequence. These measurements can be used to control individual processes and ensure reliable process interactions, but they are at their most powerful where they can be correlated to the final performance of the cell. Such a relationship is not always easy to establish, particularly when the data collected during the process cannot be parametrized entirely with a single variable. This paper shows how two multivariate approaches can be used to form a relationship between cell lifetime data collected early in a fabrication sequence, and the final cell Voc. While building a model with a high level of predictive accuracy is rarely feasible, it is possible to identify a higher proportion of under-performing product and provide insight into how material type interacts with the manufacturing sequence.

Data mining photovoltaic cell manufacturing data

So called “data mining” techniques comprise a broad family of statistical investigative and analysis techniques from informal exploratory and graphical methods through to sophisticated multivariate analysis. Data mining of photovoltaic (PV) cell manufacturing data can be used with an understanding of cell performance to isolate the variance in production associated with wafer material quality or other time based changes. This can lead to new insights for SPC and for understanding process variance.

A techno-economic analysis method for guiding research and investment directions for c-Si photovoltaics and its application to Al-BSF, PERC, LDSE and advanced hydrogenation

A techno-economic analysis method is used to analyse industry standard mono crystalline silicon photovoltaic technologies – Aluminium Back Surface Field (Al-BSF) and Passivated Emitter and Rear Cell (PERC), together with promising process variations – Laser Doped Selective Emitter (LDSE) and three implementations of advanced hydrogenation. Building on a previously reported manufacturing cost and uncertainty analysis method, the impact of uncertainty in module performance and market price is added to estimate the manufacturers gross margin. Two additional interpretation methods are described – (i) simultaneous Monte Carlo and (ii) contribution to variation – that help distinguish the impact of small differences between sequences, and identify the most important factors (cost, performance or market) affecting commercial viability. Combining these methods allows a rapid commercial viability assessment without requiring exact data on all the inputs. The analysis indicates that PERC is more commercially attractive than Al-BSF, with a median improvement in manufacturers margin >5%. Al-BSF + LDSE is found to reduce manufacturers margin. The PERC + LDSE and PERC + advanced hydrogenation sequences are estimated to provide a median margin improvement >2% compared to PERC alone. The advantage of PERC + LDSE depends strongly on delivering the expected 0.9%abs cell efficiency gain with high production yields and receiving a selling price premium for higher power modules; less important is the production cost of the LDSE process steps. The advantage of advanced hydrogenation depends strongly on achieving the expected 0.2%abs as-produced efficiency gain with high production yield, as well as realising a 0.5% selling price premium for being “CID-Free”; these factors outweigh the production costs of the alternative hydrogenation processes.

High performance plasma hydrogenation for large-grained polycrystalline silicon thin film solar cells

Polycrystalline silicon thin-film solar cells with large grains up to 1 mm wide and 10 mm long created by diode laser crystallization require an enhanced high performance plasma hydrogenation process for better cell performance. Higher process temperature, longer process time and higher plasma power (800°C, 16 minutes and 3.9 kW, respectively) than those used for smaller grain polycrystalline silicon films were applied and their effects on the cell performance were evaluated. It is found that open circuit voltage increases from 417 mV to 451 mV for 0.1-suns and from 518 mV to 529 mV for 1-sun due to lower the diode saturation currents from 3.1e-5 A/cm2 to 1.2e-5 A/cm2 after such a hydrogenation process. This result shows that enhanced hydrogenation has a potential for application to various polycrystalline silicon films with wide range of grain size and defect density.

Material characteristics of crystalline Si thin-film solar cells on glass fabricated by diode laser crystallization

Diode laser crystallization was performed silicon thin film on glass. Large linear grains along the laser scanning direction were formed when the laser scanning speed of 150-1000 mm/min was used. First order □3 twin boundaries were found to be dominating grain boundaries. Pole figure measurement showed very uniform (100) texture can be formed when SiOx layer capping layer was used. Promising bulk resistivity of the as-crystallized films was resulted. Emitter was formed using spin on diffusion and subsequent RTP Suns Voc results after emitter formation exhibited n=1 recombination. Hydrogen plasma passivation effectively passivated grain boundaries.