A detailed loss analysis is presented for a 15.9% large area ultrathin silicon (UTSi) solar cell. The loss analysis is based on a comprehensive study of the electrical and optical parameters of the champion solar cell. The results indicate that the UTSi solar cell has an efficiency potential of 19.9% using currently available technologies and is capable of achieving 22.2% efficiency in the long run.

An 18 μm thin crystalline silicon solar cell was demonstrated, and its best open circuit voltage is 642.3 mV. However, this value is far from the cell’s theoretical upper limit in an ideal case. This paper explores the open circuit voltage losses of the thin silicon solar cell, starting from the ideal case, through first principle calculation and experiments. The open circuit voltage losses come from the introduced recombination due to the non-ideal surface passivation and contacts integration on front and rear surfaces, and edge isolation. This paper presents a roadmap of the open circuit voltage reduction from an ideal case of 767.0 mV to the best measured value of 642.3 mV.

This paper presents the characterization and analysis of a 16.8%-efficient 18 um silicon solar cell on steel substrate. Photoluminescence (PL) is used to monitor the solar cell’s fabrication step by step, and a correlation between PL intensity and implied open circuit voltage is established. PL images yield spatially resolved maps of key electrical parameters such as dark reverse saturation current density, open circuit voltage, series resistance, short circuit current density at maximum power point and efficiency. Light IV/dark IV curve fitting and local ideality factor curve fitting are used to extract electrical parameters. Keywords—18 um, curve fitting, photoluminescence, solar cell, steel substrate, thin silicon.

Thin crystalline silicon solar cells have the potential to achieve high efficiency due to the potential for increased voltage. Thin silicon wafers are fragile; therefore, means of support must be provided. This paper reports the design, development, and analysis of an 18-mu m crystalline silicon solar cell electrically integrated with a steel alloy substrate. This ultrathin silicon is epitaxially grown on porous silicon and then transferred onto the steel substrate. This method allows the independent processing of each surface. The steel substrate enables robust handling and provides a conductive back plane. Three groups of cells with planar and textured structures are compared; significant improvements in J(sc), V-oc, and fill factor (FF) are achieved. The best cell shows an efficiency of 16.8% with an open-circuit voltage of 632 mV and a short-circuit current density of 34.5 mA/cm(2).

A method to laminate a thin monocrystalline Si layer to a conductive and fracture-resistant carrier such as steel has been developed, resulting in a practical design for high volume production of robust ultra-thin (10-20 μm) “kerfless” Si wafers. With this technology front and rear cell features based on the world-record PERL cell design have been integrated. A confirmed efficiency of 15.1% has been achieved on a 20-micron thick one-cm2 solar cell. This 15.1% is believed to be the highest confirmed efficiency achieved with ultra-thin silicon integrated with a conducting substrate.

A Ge:Si solar cell under a silicon solar cell can lead to as much as a 5.5% absolute efficiency gain for a multi-junction solar module at 30× concentration. This work demonstrated short circuit current densities that were 93% of the model prediction and open circuit voltages that were 92% of the model predictions for 88% Ge content, Ge:Si solar cells below Si at 30 suns. Silicon solar cells can absorb few photons in the wavelength range above 1150nm due to the effect of the absorption coefficient. One possible method to enhance the absorption of long wavelength photons is to apply a Ge solar cell below Si. However, this method is industrially impractical due to the high cost of Ge substrates. In this work, a low cost Ge:Si solar cell grown on silicon with strong long wavelength light sensitivity will be demonstrated. This work starts with an all epitaxial growth design, analyzes the performance limits, examines the trade-offs between solar cell performance, Ge composition and material quality and concludes with the pathways to higher efficiency. The high quality Ge:Si layers with Ge content above 85% were achieved on Si substrates using reduced pressure chemical vapor deposition (RPCVD) technology. Three high Ge content Ge:Si solar cells were designed, fabricated and analyzed. The encouraging results experimentally prove that low cost Ge:Si solar cells grown on Si can have high performance below Si. This has been achieved as a direct result of low dislocation density step graded Ge:Si buffers developed in this research. In this paper, the pathway to achieve low cost and high efficiency Ge:Si low band gap solar cells grown on silicon is described.

Low band gap germanium:silicon (Ge:Si) solar cells for operation with a silicon solar cell in a multi-junction concentrator system was designed, fabricated, characterized and analyzed. First principle simulations show that an efficiency of 2.3% can be achieved for 88% Ge concentration Ge:Si solar cells below Si at 30 suns. Through solving critical shunting and open circuit voltage (Voc) problems, an efficiency of 0.79% with a Voc of 350 mV and a fill factor (FF) of 66% was achieved for our third generation Ge:Si solar cells below Si at 30 suns.

A 3-terminal Si-Si:Ge tandem solar cell, which consists of a top silicon solar cell and a bottom Si:Ge solar cell sharing a common base, has been designed and fabricated. This solar cell is designed to replace standalone Si solar cell and Si:Ge solar cell in a multi-junction solar cell system in order to simplify the module, improve its performance and reduce its cost. Initial results show that both Si solar cells and Si:Ge solar cells in this structure can achieve good performance. The best Si solar cell shows a Voc 598.5mV, Jsc 22.9mA/cm 2 and FF 77.3% under one sun, best Si:Ge solar cell shows a Voc 189mV and Jsc 3.26mA/cm 2 below silicon filter. None of these solar cells have anti-reflection coatings.