Characterization of diffusion length degradation in Czochralski silicon solar cells

Abstract

Commercially produced, unencapsulated, Czochralski (CZ) silicon solar cells can lose 3%–4% (relative) of their initial efficiency after exposure to light, after minority-carrier injection during dark forward bias, or after thermal treatment at 100–400 °C. All three degradation methods reduce the minority-carrier diffusion length in the cell substrate. Under light, the decrease in efficiency is rapid (<30 min at 1 sun), but the cell power remains stable thereafter. The effect is completely reversible and the cell performance recovers in <12 h in the dark at room temperature. The various conditions under which CZ silicon cells degrade, and the reverse process, annealing, are characterized by spectral response and current–voltage (I–V) measurements. Iron impurities are a possible cause of this effect.

Formation rates of iron-acceptor pairs in crystalline silicon

“…charged Fe i and the negatively charged dopant atoms. Although such an attraction must exist, its effective range is of the order of 4 -5 nm, 10,18 significantly less than the dopant separation even in the most heavily doped samples used here ͑around 22 nm for N A =10 17 cm −3 ͒. However, for doping concentrations higher than approximately 5 ϫ 10 18 cm −3 , the range of Coulombic attraction becomes comparable to or exceeds the dopant separation.…”

“…This can be achieved by measuring the characteristic time constant of Fe-acceptor pair formation after dissociation by either illumination or thermal activation. 2 Such association times have been measured for FeB pairs in numerous studies, [5][6][7][8][9][10][11][12] the most widely used results being those of Zoth and Bergholz, 2 who determined an expression for the FeB pair association time constant as a function of boron concentration and temperature based on a model of Reiss et al 13 If the pair-formation process is limited by diffusion of Fe i , as is suggested by measurements of the activation energy for pair formation, the same expression should be valid for other p-type dopant species also. However, this has not been confirmed experimentally.…”

Recombination and radiation damage in crystalline silicon solar cell material

“…This is a topic that has been very extensively studied and there is very strong evidence that it is due to the formation of a complex of boron and oxygen [13][14][15][16][17]. Essentially when solar cells made from boron doped Czochralski silicon are operated the performance is often observed to degrade by about 10% in the first few days of operation.…”

“…What actually happens in the solar cell in practice is that the efficiency is degraded quite rapidly when exposed to sunlight due to the dissociation of the iron boron pairs and the formation of interstitial iron as a more powerful re-combination centre. However, during the hours of darkness the iron boron pairs reassociate and the efficiency of the cell recovers [13]. The time that this takes depends strongly on the ambient temperature but at 300 K a typical association time constant would be 3 hours.…”

Contaminação de lâminas de silício por metais durante a imersão em solução diluída de ácido fluorídrico ou de água deionizada contendo cloretos metálicos

“…Co, Ni e Cu difundem muito rapidamente mesmo à temperatura ambiente no silício, não são estáveis nos estados intersticiais e precipitam ou formam complexos ou aglomerados durante tratamentos térmicos (8,9) . Cobre e níquel podem causar ruptura nos óxidos de silício empregados em portas MOS (9,11,12) ao passo que Ferro-Boro tem um efeito prejudicial na eficiência de células solares em concentração ao redor de 10 13 cm -3 (7,8,9) e mesmo na escala (13) de 10 11 cm -3 . O ferro é uma das mais importantes impurezas já que é o principal constituinte dos muitos materiais tecnicamente importantes e, portanto, é uma impureza muito comum (10,14) .…”