Philipp Keller

Bio: Philipp Keller is an academic researcher from University of Konstanz. The author has contributed to research in topics: Solar cell & Silicon. The author has an hindex of 5, co-authored 13 publications receiving 56 citations.

Influence of temperature on light induced phenomena in multicrystalline silicon

Abstract: The changes in effective lifetime for mc-Si under the influence of elevated temperatures ranging from 160°C to 400°C and illumination of 2 suns were investigated. It is shown that Light and elevated Temperature Induced Degradation (LeTID) and its subsequent curing can be observed up to a temperature of ~280°C, but vanishes above. Furthermore, it is shown that lifetimes exceed the initial value during prolonged illumination which is interpreted as advanced hydrogenation. However, even though lifetimes exceed the initial value due to advanced hydrogenation at temperatures >280°C, these lifetimes prove to be at least partially instable at low temperatures under illumination suggesting LeTID is not or at least not completely cured. It is concluded that LeTID and advanced hydrogenation are related to different defect systems. In addition, it is shown that both LeTID and advanced hydrogenation only occur above a peak set firing temperature of 720°C thus indicating that lower peak firing temperatures can successfully suppress LeTID, however, only at the price of losing the benefit from advanced hydrogenation as well. Furthermore, it is shown that increasing illumination intensity speeds up the dynamics of LeTID and advanced hydrogenation, respectively.The changes in effective lifetime for mc-Si under the influence of elevated temperatures ranging from 160°C to 400°C and illumination of 2 suns were investigated. It is shown that Light and elevated Temperature Induced Degradation (LeTID) and its subsequent curing can be observed up to a temperature of ~280°C, but vanishes above. Furthermore, it is shown that lifetimes exceed the initial value during prolonged illumination which is interpreted as advanced hydrogenation. However, even though lifetimes exceed the initial value due to advanced hydrogenation at temperatures >280°C, these lifetimes prove to be at least partially instable at low temperatures under illumination suggesting LeTID is not or at least not completely cured. It is concluded that LeTID and advanced hydrogenation are related to different defect systems. In addition, it is shown that both LeTID and advanced hydrogenation only occur above a peak set firing temperature of 720°C thus indicating that lower peak firing temperatures can success...

Multicrystalline silicon solar cells from RST ribbon process

Abstract: The aim of Solarforce is the achievement of high efficiency solar cells made out of ≤100 µm thick multicrystalline silicon wafers produced by the Ribbon on Sacrificial Template (RST) process. For a first evaluation of the RST material, solar cells were fabricated on p-type RST wafers with different processes. The first conclusion is that the density of silicon carbide inclusions has to be lowered to obtain better FF values, and in turn a better homogeneity in cell performances. Secondly, conversion efficiency is found to be limited around 14% by low bulk minority carrier lifetime values, limiting both Voc and Leff. ICP-MS analyses on RST material show that this lifetime may be improved with the possible achievement of low metallic impurity content in the wafers, under 1 ppb wt. Further enhancement is also shown to be possible with the n doping of RST material, a well controlled process in ribbon growth (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Passivated emitter and rear cell—Devices, technology, and modeling

Abstract: Current studies reveal the expectation that photovoltaic (PV) energy conversion will become the front-runner technology to stem against the extent of global warming by the middle of this century. In 2019, the passivated emitter and rear cell (PERC) design has taken over the majority of global photovoltaic solar cell production. The objective of this paper is to review the fundamental physics of the underlying cell architecture, its development over the past few decades to an industry main stream product, as well as an in-depth characterization of current cells and the future potential of the device structure. The early development of PERCs was set by an intriguing series of improvements starting in 1989 and resulting in a long-standing energy conversion efficiency record of 25.0% set up in 1999. It took a decade of intense technological development to implement this structure as an upgrade to existing production lines and another decade to increase the efficiency of industrially manufactured cells to over 22%. Our analysis of state-of-the-art large-area screen-printed PERCs is based on the pilot-line technology in the Photovoltaic Technology Evaluation Center at the Fraunhofer ISE, which is assumed to be representative of current state-of-the art cell processing. The main recent cell efficiency improvements have been achieved thanks to fine line metallization taking advantage of the high quality emitter formation and passivation and to improvements in material quality. In order to enhance the energy yield of the PV modules, innovations in interconnection technology like multibusbar and shingling technology as well as bifaciality are supported by PERC developments. Over the years, ongoing improvements have been made in the understanding of PERCs by analytical and numerical modeling of these devices. We show a study based on 3D numerical modeling and an extrapolation of the PERC device structure and technology to achieve an efficiency of 26%. This result surpasses earlier investigations due to the combination of technology components, as further improved front contact and emitter design as well as rear passivation and mirrors. We expect that PERCs can also play a strong role at the bottom of multijunction solar cells and will defend a strong position in global PV production beyond the end of the now starting decade.

Underdense a-Si:H film capped by a dense film as the passivation layer of a silicon heterojunction solar cell

Abstract: Underdense hydrogenated amorphous silicon (a-Si:H) prepared by plasma-enhanced chemical vapor deposition was used as a passivation layer in silicon heterojunction (SHJ) solar cells. By reducing the thickness of the underdense a-Si:H passivation layer from 15 nm to 5 nm, the open circuit voltage (Voc) of the corresponding SHJ solar cell increased significantly from 724.3 mV to 738.6 mV. For comparison, a widely used transition-zone a-Si:H passivation layer was also examined, but reducing its thickness from 15 nm to 5 nm resulted in a continuous Voc reduction, from 724.1 mV to 704.3 mV. The highest efficiency was achieved using a 5-nm-thick underdense a-Si:H passivation layer. We propose that this advantageous property of underdense a-Si:H reflects its microstructural characteristics. While the porosity of a-Si:H layer enables H penetration into the amorphous network and the a-Si:H/c-Si interface, a high degree of disorder inhibits the formation of the epitaxial layer at the a-Si:H/c-Si interface during pos...