Development of advanced hydrogenation processes for silicon solar cells via an improved understanding of the behaviour of hydrogen in silicon

Abstract

The understanding and development of advanced hydrogenation processes for silicon solar cells are presented. Hydrogen passivation is incorporated into virtually all silicon solar cells, yet the properties of hydrogen in silicon are still poorly understood. This is largely due to the complex behaviour of hydrogen in silicon and its ability to exist in many different forms in the lattice. For commercial solar cells, hydrogen is introduced into the device through the deposition of hydrogen-containing dielectric layers and the subsequent metallisation firing process. This process can readily passivate structural defects such as grain boundaries but is ineffective at passivating numerous defects in silicon solar cells such as the boron-oxygen complex, responsible for light-induced degradation in p-type Czochralski silicon. This difficulty is due to the need to first form the boron-oxygen defect and also due to atomic hydrogen naturally occupying low-mobility and low-reactivity charge states. However, these challenges can be overcome using advanced hydrogenation processes incorporating excess carrier generation from illumination or current injection that increase the concentration of the highly mobile and reactive neutral charge state. As a result, after fast firing, additional low-temperature advanced hydrogenation processes incorporating illumination can be implemented to enable the passivation of difficult defects like the boron-oxygen complex. With the implementation of such processes for industrial silicon solar cells, efficiency improvements of 1.1% absolute can be obtained.

Stability Study of Silicon Heterojunction Solar Cells Fabricated with Gallium‐ and Boron‐Doped Silicon Wafers

“…During the treatment, excess carriers are generated via illumination to manipulate hydrogen already introduced into the solar cell during the SHJ solar cell fabrication sequence. [13] The whole solar cell area was illuminated simultaneously for 30 s using a 960 nm continuous-wave solid state diode laser with monochromatic illumination intensity of %55 kW m À2 . During the treatment, the solar cells were vacuum contacted on a hot plate with a set temperature of 200 C. The solar cells temperature was monitored with an infrared thermometer and a peak temperature of 255 C was registered after 18 s under illumination.…”

Temporary Recovery of the Defect Responsible for Light- and Elevated Temperature-Induced Degradation: Insights Into the Physical Mechanisms Behind LeTID

“…For example, it is known that hydrogen passivates defects by tying up dangling bonds. For a recent review of hydrogen passivation, please see [40]. The following model is, therefore, mainly intended to provide a basis for the investigation of the physical nature of the LeTID defect.…”

“…As pointed out by Herguth [54], the normalized defect density depends on the excess carrier density. Therefore, we corrected the calculated N t * for Δn using the equations of [54] by assuming a mid-gap defect level with a capture cross section ratio of ∼30 [7], [39], [40].…”

Hydrogen-related defects measured by infrared spectroscopy in multicrystalline silicon wafers throughout an illuminated annealing process

“…Hydrogen (H) can passivate and neutralize a wide range of defects and impurities in crystalline silicon, such as metallic impurities, grain boundaries, and dislocations. [1][2][3][4][5][6][7] Thus, hydrogen introduction, also called hydrogenation, and the corresponding beneficial effects on the minority carrier lifetimes in silicon wafers is important for the performance of crystalline silicon solar cells. Hydrogen can be introduced into the bulk silicon in several ways; high temperature processing in a hydrogen atmosphere, 8 ion implantation, 9 plasma treatment, 10 and release of hydrogen from a dielectric layer.…”