Search

A method for analyzing the power losses of solar cells is presented, supplying a complete balance of the incident power, the optical, thermodynamic, and electrical power losses and the electrical output power. The involved quantities have the dimension of a power density (units: W/m(2)), which permits their direct comparison. In order to avoid the over-representation of losses arising from the ultraviolet part of the solar spectrum, a method for the analysis of the electrical free energy losses is extended to include optical losses. This extended analysis does not focus on the incident solar power of, e. g., 1000 W/m(2) and does not explicitly include the thermalization losses and losses due to the generation of entropy. Instead, the usable power, i.e., the free energy or electro-chemical potential of the electron-hole pairs is set as reference value, thereby, overcoming the ambiguities of the power balance. Both methods, the power balance and the free energy loss analysis, are carried out exemplarily for a monocrystalline p-type silicon metal wrap through solar cell with passivated emitter and rear (MWT-PERC) based on optical and electrical measurements and numerical modeling. The methods give interesting insights in photovoltaic (PV) energy conversion, provide quantitative analyses of all loss mechanisms, and supply the basis for the systematic technological improvement of the device.

Minority carrier mobility is a crucial transport property affecting the performance of semiconductor devices such as solar cells. Compensation of dopant species and novel multicrystalline materials call for accurate knowledge of minority carrier mobility for device simulation and characterization. Yet, measurement techniques of minority carrier mobility are scarce, and published data scatter significantly even on monocrystalline material. In this paper, the determination of minority carrier mobility from self-consistent quasi-steady-state photoluminescence measurements of effective carrier lifetime is presented. The measurement design is distinguished by a limitation of carrier recombination through minority carrier transport—with excess carrier generation and recombination confined to opposite interfaces, respectively. Minority carrier mobility is inferred from the minority carrier diffusion coefficient via the Einstein relation. An experimental proof of concept on monocrystalline p-type material is provided, showing good agreement with state-of-the-art data and models. Considerations for the applicability of the method to compensated and multicrystalline silicon materials are discussed.

Knowledge of the carrier mobility in silicon is of utmost importance for photovoltaic applications, as it directly influences the diffusion length and thereby the cell efficiency. Moreover, its value is needed for a correct quantitative evaluation of a variety of lifetime measurements. However, models that describe the carrier mobility in silicon are based on theoretical calculations or fits to experimental data in monocrystalline silicon. Multicrystalline (mc) silicon features crystal defects such as dislocations and grain boundaries, with the latter possibly leading to potential barriers through the trapping of charge carriers and thereby influencing the mobility, as shown, for example, by Maruska [Appl. Phys. Lett. 36, 381 (1980)]. To quantify the mobilities in multicrystalline silicon, we performed Hall measurements in p-type mc-Si samples of various resistivities and different crystal structures and compared the data to majority carrier Hall mobilities in p-type mo nocrystalline floatzone (FZ) silicon. For lack of a model that provides reliable values of the Hall mobility in silicon, an empirical fit similar to existing models for conductivity mobilities is proposed based on Hall measurements of monocrystalline p-type FZ silicon. By comparing the measured Hall mobilities obtained from mc silicon with the corresponding Hall mobilities in monocrystalline silicon of the same resistivity, we found that the mobility reduction due to the presence of crystal defects in mc-Si ranges between 0 and 5 only. Mobility decreases of up to 30 as reported by Peter [Proceedings of the 23rd European Photovoltaic Solar Energy Conference, Valencia, Spain, 1-5 September 2008], or even of a factor of 2 to 3 as detected by Palais [Mater. Sci. Eng. B 102, 184 (2003)], in multicrystalline silicon were not observed.

In this study we present an advanced method for precise modeling of highly aluminum-doped p+ silicon, leading to excellent agreement of numerical and experimental data within a broad range. We analyze the influence of different recombination mechanisms on the saturation current densities of Al-doped Si surfaces for different Al profiles. Lateral doping inhomogeneities and the effect of incomplete ionization have been examined in detail. We demonstrate that incomplete ionization affects the profile characteristics significantly and, therefore, has to be accounted for in accurate modeling of highly Al-doped silicon.

Extensive investigations on industrial multicrystalline silicon solar cells have shown that, for standard 1 Omega cm material, acid-etched texturization, and in absence of strong ohmic shunts, there are three different types of breakdown appearing in different reverse bias ranges. Between -4 and -9 V there is early breakdown (type 1), which is due to Al contamination of the surface. Between -9 and -13 V defect-induced breakdown (type 2) dominates, which is due to metal-containing precipitates lying within recombination-active grain boundaries. Beyond -13 V we may find in addition avalanche breakdown (type 3) at etch pits, which is characterized by a steep slope of the I-V characteristic, avalanche carrier multiplication by impact ionization, and a negative temperature coefficient of the reverse current. If instead of acid-etching alkaline-etching is used, all these breakdown classes also appear, but their onset voltage is enlarged by several volts. Also for cells made from upgraded metallurgical grade material these classes can be distinguished. However, due to the higher net doping concentration of this material, their onset voltage is considerably reduced here. (C) 2011 American Institute of Physics. [doi:10.1063/1.3562200]

Excellent surface passivation of crystalline silicon (c-Si) is desired for a number of c-Si based applications ranging from microelectronics to photovoltaics. A plasma-enhanced chemical vapor deposition double layer of amorphous silicon-rich oxynitride and amorphous silicon nitride (SiN x) can provide a nearly perfect passivation after subsequent rapid thermal process (RTP) and light soaking. The resulting effective minority carriers' lifetime (eff) is close to the modeled maximum on p-type as well as on n-type c-Si. Restrictions on the RTP of passivated surfaces, typical of other common passivation schemes (e.g., amorphous Si), are relieved by this double layer. Harsher thermal treatments can be adopted while still obtaining salient passivation. Furthermore, characterization of the same, such as, surface photovoltage, capacitance voltage, and electron paramagnetic resonance, enables the reproducibility and the understanding of the passivation scheme under test. It is s hown that the strong quality of surface passivation is ensured by a mechanism that emits electrons from shallow donor states in the passivation layer system and therefore creates a positive field effect.