Michel Frei

On the Nature of Shunt Leakage in Amorphous Silicon p-i-n Solar Cells

In this letter, we investigate the nature of shunt leakage currents in large-area (on the order of square centimeters) thin-film a-Si:H p-i-n solar cells and show that it is characterized by following universal features: (1) voltage symmetry; (2) power-law voltage dependence; and (3) weak temperature dependence. The voltage symmetry offers a robust empirical method to isolate the diode current from measured “shunt-contaminated” forward dark IV. We find that space-charge-limited current provides the best qualitative explanation for the observed features of the shunt current. Finally, we discuss the possible physical origin of localized shunt paths in the light of experimental observations from literature.

Understanding the Impact of the Doping Profiles on Selective Emitter Solar Cell by Two-Dimensional Numerical Simulation

The selective emitter (SE) design, featuring lower doped areas between the front contact fingers and higher doped areas underneath the front metallization, is crucial to improve the performance at the front side of a monocrystalline (c-Si) silicon solar cell. One of the most interesting and promising low-cost SE process consists of the screen printing of a phosphorus-doped paste, allowing a separate optimization of the doping profiles in the metallized and nonmetallized front-side areas. By referring to this kind of process, this paper presents a simulation study with a decoupled analysis on the effect of the lowly doped and highly doped profiles on the performance of an SE solar cell, by means of 2-D electro-optical numerical device simulations. Moreover, by exploiting the 2-D modeling, the effect of the alignment tolerance used in the SE diffusion process for the subsequent metallization process has been also investigated. Numerical results show that the adoption of an optimized design for the SE cell can lead to an efficiency improvement above 0.4%abs compared with the 75 Ω/sq homogeneous emitter reference cell.

Identification, characterization, and implications of shadow degradation in thin film solar cells

We describe a comprehensive study of intrinsic reliability issue arising from partial shadowing of photovoltaic panels (e.g., a leaf fallen on it, a nearby tree casting a shadow, etc.). This can cause the shaded cells to be reverse biased, causing dark current degradation. In this paper, (1) we calculate the statistical distribution of reverse bias stress arising from various shading configurations, (2) identify the components of dark current, and provide a scheme to isolate them, (3) characterize the effect of reverse stress on the dark current of a-Si:H p-i-n cells, and (4) finally, combine these features of degradation process with shadowing statistics, to project ‘shadow-degradation’ (SD) over the operating lifetime of solar cells. Our results establish shadow degradation as an important intrinsic reliability concern for thin film solar cell.

2-D numerical simulation and modeling of monocrystalline selective emitter solar cells

This paper presents a detailed analysis of the dependence of the performance of crystalline silicon (c-Si) selective emitter solar cells on geometrical parameters and doping profiles. Based on two dimensional drift-diffusion TCAD simulations, we report the effects of the front contact pitch and doping profiles on the most important output parameters of solar cells. Simulations show that a significant gain in terms of output power of the cell may arise compared to the homogeneous emitter solar cell. We also present an analysis of the main loss mechanisms and trade-offs for this solar cell.

Numerical Simulation on the Influence of Via and Rear Emitters in MWT Solar Cells

In this paper, we analyze, by means of numerical simulations, metal wrap through (MWT) silicon solar cells without a rear emitter and/or via an emitter that feature a Schottky contact between the Ag metal and the p-base. We show how the effective Schottky barrier height affects both dark and illuminated properties of the cell. An equivalent electrical model for the dark analysis is proposed, which accounts for the shunting effects and the thermionic-emission current at Ag/p-base contact. We investigate the figures of merit of MWT solar cells for different via configurations, highlighting the influence of the Ag/p-base barrier height. Moreover, the influence of the rear busbar width, as well as of the operating temperature, is analyzed.

Numerical Simulation and Modeling of Resistive and Recombination Losses in MWT Solar Cells

This study analyzes the impact of resistive and recombination losses in metal wrap through (MWT) solar cells through technology computer aided design (TCAD) numerical simulations. Two types of MWT architectures are considered in this study: “point busbar,” featuring one circular tabbing contact for each via at the back side, and “continuous busbar,” in which the rear busbar connects all the vias along a line. A comparison with conventional, H-pattern, front contact (FC) solar cells is performed by adopting the surface recombination velocity at the rear-contact isolation region as a parameter representative of possible passivation options. The differences under dark and light conditions are highlighted. Moreover, the following resistive losses in MWT cells are investigated: via resistance, shunting effect, and lateral conduction of charge carriers above rear busbar. An analytical model to account for the lateral conduction of charge carriers is proposed and validated by means of numerical simulations. While the advantage of MWT over FC cells is confirmed by simulation, we quantitatively show how the resistive and recombination losses limit the efficiency of MWT cells.

Understanding the impact of double screen-printing on silicon solar cells by 2-D numerical simulations

In this work we investigate c-Si solar cells in which a two steps (double) screen-printing is implemented for the front contact in order to increase the aspect ratio of the fingers. To this purpose numerical simulations are performed on a solar cell and metallization properties are defined according to experimental data. The analysis of solar cells with conventional single screen-printing is also performed for comparison. In agreement with experiments we show the efficiency improvement of double printing over single printing. The analysis of electrical parameters (fill factor, short-circuit current and open-circuit voltage) allows to understand the source of the enhanced efficiency.

2-D Numerical analysis of the impact of the highly-doped profile on selective emitter solar cell performance

Two-dimensional (2-D) numerical simulations have been performed to investigate the impact of the doping profile in the metal-contacted highly-doped regions for c-Si selective emitter (SE) solar cells. Numerical results show that the doping profile under the metallization significantly influences the recombination effects on the front-side of the SE cell and consequently its performance. A strong impact on the short-circuit current density of the solar cell has been seen. This is mainly due to the inclusion of large alignment tolerances used in the SE diffusion for the subsequent metallization process, leading to broad highly-doped areas. In this regard, a quantitative analysis of this effect has been carried out. The results reveal that an improved alignment, allowing a reduction of the alignment tolerances, leads to a wider process window of doping profiles and hence better SE cell performance.

Numerical simulation and modeling of rear point contact solar cells

High efficiency silicon monocrystalline solar cells commonly adopt point contacted rear surfaces to reduce the recombination losses in the rear side of the device. However, the reduction of the rear contact surface leads to an increase of series resistance losses. Modeling and analysis of rear point contact solar cells is strategic to optimize the device design by taking into account several competing physical mechanisms. Owing to their complicated geometries, the analysis of these devices requires three-dimensional (3-D) numerical simulation. In this work we analyze the influence of the most important geometrical and electrical parameters on the conversion efficiency of rear point contact solar cells.

Intrinisic reliability of amorphous silicon thin film solar cells

In this paper, we have discussed three intrinsic reliability issues of thin-film -Si∶H solar cells; space charge limited shunt conduction through localized metal-semiconductor-metal structures; shadow degradation in series connected cells in a module, and light induced degradation. Despite their distinct external manifestation, these intrinsic reliability issues appear to share common physical phenomena. For example, the light induced and the shadow degradation may be related because they are described by very similar time-exponents (see Fig. 4c and 6a). While the physics of G are different (e.g. photon induced dissociation for LID and (possibly) electron-hole recombination induced dissociation for shadow degradation), it is likely that they both break SiH bonds and are subsequently follow similar diffusive kinetics. Finally, analogies to CMOS reliability; e.g., shunt conduction related to non uniform conduction through oxides, shadow degradation to bulk defect generation and TDDB in gate dielectric, and light induced degradation to NBTI in PMOS transistors; may help illuminate many aspects of the degradation processes.