J. Knobloch

Degradation of carrier lifetime in Cz silicon solar cells

Abstract The lifetime degradation induced by light illumination or carrier injection which is observed in Czochralski-grown silicon (Cz-Si) leads to a significant decrease of solar cell efficiency. Thus, the reduction of this effect has a high potential for the improvement of Cz-Si solar cells. In the present work both, the analysis of the underlying defect and its technological reduction are discussed. A clear correlation of the Cz-specific metastable defect with the oxygen and boron concentration in Cz-Si has been observed. Especially, recently performed lifetime measurements on oxygen-free boron-doped p-type MCz silicon and gallium-doped oxygen-contaminated Cz-silicon, both of which show no degradation, confirm this hypothesis. While the quantitative correlation between the defect concentration and boron is linear, the increase of the defect concentration induced by the interstitial oxygen concentration is superlinear, i.e. it follows a potential law of power approximately 5. Beyond the defect analysis, two different ways to reduce the metastable defect concentration are discussed. A proper material choice by substituting or reducing one of the major components of the metastable defect can completely avoid the degradation effect. The excellent performance of oxygen-free MCz-Si and gallium-doped Cz-Si is reflected in the achieved record efficiencies of 22.7% and 22.5%, respectively. In standard boron-doped oxygen-contaminated p-type Cz-Si a strong reduction of the metastable defect concentration can be achieved by a high-temperature process step resulting in an improvement of the stable bulk lifetime by a factor of 2–4.

Solar cells with efficiencies above 21% processed from Czochralski grown silicon

Czochralski-Si (Cz-Si) of several manufacturers and with resistivities ranging from 1 to 13 /spl Omega/cm were processed into solar cells with efficiencies higher than 20% (AM1.5) using the LBSF/PERL processing sequence. The highest efficiency was 21.7%. The investigation of high efficiency Cz-Si solar cells was augmented by computer simulation and a study of the carrier lifetime before and after processing. A small degradation of solar cell performance in the lower resistivity material is discussed. Furthermore, a much simpler processing sequence is presented revealing efficiencies well above 19% on Cz-silicon and 21% on float zone-silicon.

High-efficiency silicon solar cells for low-illumination applications

At Fraunhofer ISE the fabrication of high-efficiency solar cells was extended from a laboratory scale to a small pilot-line production. Primarily, the fabricated cells are used in small high-efficiency modules integrated in prototypes of solar-powered portable electronic devices such as cellular phones, handheld computers etc. Compared to other applications of high-efficiency cells such as solar cars and planes, the illumination densities found in these mainly indoor applications are significantly below 1 sun. Thus, special care was taken to keep the cell efficiency level high even at very low illumination levels. For this reason, particularly the cell border was analyzed and optimized carefully. The excellent cell characteristics achieved at low illumination densities increase the benefit of a solar power supply for such devices by an order of magnitude if compared to standard solar cells.

Surface passivation of high efficiency silicon solar cells

Theoretically and experimentally determined design guides for significantly reduced recombination at the emitter and rear surfaces of full-area Al-BSF (back-surface region) and oxide-passivated bifacial cells are given. The impact of emitter thickness and surface dopant concentration on emitter saturation current and solar cell efficiency is outlined. A modified emitter structure (locally deep diffused below the metal contacts) is predicted to have superior performance. Measured V/sub oc/ values reveal the potential of deep emitter cells to achieve efficiencies above 20% in spite of high metallization factors. Experimentally, a strong dependence of passivation quality on oxide thickness and base doping concentration is found. The BSF quality of a diffused aluminium layer decreases strongly with increasing drive-in time. For SiO/sub 2/-passivated rear surfaces of bifacial cells, measurements of the dependence of the surface recombination velocity on the excess carrier concentration are presented.<<ETX>>

100 cm2 solar cells on Czochralski silicon with an efficiency of 20·2%

A solar cell process optimized for oxygen-contaminated silicon was used to fabricate 10 × 10 cm 2 cells on gallium-doped p-type Czochralski (Cz) silicon. An independently confirmed record efficiency of 20.2% was achieved. Although the material used contains a significant concentration of interstitial oxygen, no illumination-induced degradation of the cell parameters was observed. This is in excellent agreement with the observation that the metastable defect underlying the minority carrier lifetime is correlated with oxygen and boron. Thus, using gallium instead of boron as the dopant for p-type Cz silicon is an appropriate way to avoid the carrier lifetime degradation which is observed in boron-doped oxygen-contaminated Cz-Si.

Silicon thin-film solar cells on insulating intermediate layers

Abstract An interdigitated front grid structure for both the emitter and base was simulated and realized. This contact design is suitable for thin-film solar cells on insulating substrates or insulating intermediate layers. Confirmed efficiencies of up to 18.2% were achieved on a 46 μm thick epitaxial silicon layer which was grown on a SIMOX wafer with an implanted compact SiO 2 intermediate layer. Samples with and without a highly doped back surface field were prepared to study the influence of the back-side recombination velocity. L eff values of 250 and 52 μm, corresponding to S back values of 800 and 10 5 cm/s, respectively, were measured, thus, underlining the importance of a low back-side recombination velocity. The optical confinement properties of the SiO 2 intermediate layers were calculated depending on the angle of the incident rays. An angle from the plane normal which is larger than 23° is necessary in order to achieve the condition of total internal reflection. Future work will focus on recrystallized Si layers on foreign substrates [1]. Since the surface of the silicon layer is fairly rough after the recrystallisation process, another set of masks was designed which is more tolerant to aligning accuracy. This is mainly relevant for the area where the base contacts are located between the locally diffused emitter. The technology for CVD Si-layer deposition, zone melting recrystallization (ZMR), as well as for a simplified solar cell process is under investigation.

Solar cells with mesh-structured emitter

The mesh-structured emitter solar cell (MESC) is introduced as a novel solar cell processing scheme. By the formation of inverted pyramids or microgrooves on a wafer with a homogeneous heavy phosphorus diffusion, a mesh of highly conducting emitter lines is formed. Using this technique, the lateral conductivity of the emitter can be increased, keeping the emitter dark saturation current at a low level. The high phosphorus surface concentration results in a low contact resistance even for screen-printed contacts. Thus, this technique is ideal for solar cells with screen-printed contacts, because the finger spacing of the front contact can be extended, resulting in smaller shadowing losses. Also the processing scheme of high-efficiency solar cells can be simplified, because the formation of the surface texturization and the locally deep diffused emitter can be combined in one step. The first cells with a mesh-structured emitter, evaporated front contacts and local ohmic rear contacts have shown efficien ies up to 21.1%. Lifetime test structures have been used to determine a low dark saturation current of 58 fA cm−2 for the mesh-structured emitter, although the structure is not yet optimized.

High-efficiency solar cells from FZ, CZ and MC silicon material

At ISE, a research project has been carried out which aims at the development of single crystalline solar cells of high efficiency. For this task a clean room laboratory for solar cell processing has been established, various measuring stations for the characterization of processes and cells have been set up, and a processing sequence for the manufacturing of such cells has been established. All systems and all processes are working well. The highest efficiencies (AM1.5) obtained to date are 21.1% for FZ-, 19.3% for CZ-, and 16.2% for MC (multicrystalline)-silicon.<<ETX>>

Highly efficient 115-μm-thick solar cells on industrial Czochralski silicon

Thin solar cells on industrial Czochralski-material were produced and analyzed. For the Random Pyramid-Passivated Emitter and Rear Cell (RP-PERC) with planar rear surface, a maximum efficiency of near 20% was achieved for 115 as well as 165 μm thickness. In comparison, cells textured at the rear surface with a standard industrial process reached about a 2% (absolute) lower efficiency. This difference is consistently explained by an increased rear surface recombination, whereas light trapping properties are excellent for both rear surface treatments. The Cz-specific light-induced degradation of the thin cells is investigated.

Impact of metallization techniques on 20% efficient silicon solar cells

The damage to the Si-SiO/sub 2/ interface by electron-beam in comparison to thermal (i.e. resistive heating) evaporation of Al and Ti is investigated for oxide thicknesses ranging from 14 to 105 nm. C-V and PCD measurements on MOS test structures fabricated on 1 to 100 Omega -cm p-type FZ-silicon reveal: (1) severe damage to the interface is caused by e-beam, but not by thermal evaporation, (2) in terms of midgap interface state densities and PCD time constants the electron-beam damage is removed by a postmetallization anneal in forming gas for all oxide thicknesses, (3) a photoresist or Al layer of up to 1.6- mu m yields no effective shielding. Solar cells with thick passivating oxides (105 nm) gave comparably high efficiencies above 20% for both metallization techniques.<<ETX>>