Tesis doctoral de Delfina Muñoz Cervantes
The cost of high efficiency crystalline silicon (c-si) modules is hindering the progress of the pv industry as a viable alternative for clean energy production. Therefore, cell manufacturers are searching different approaches that could allow the desired cost reduction. For instance, considering that c-si wafers represent about 30-50% of the module price, the final cost would be significantly reduced by using very thin c-si wafers (< 200 ¿m). However, when decreasing the c-si thickness the rear surface recombination becomes important. Although thermal oxidation very effectively passivates the c-si surface, thin wafers tend to warp at the high temperatures (~1000 °c) involved in the process. On the other hand, low cost multicrystalline silicon wafers are not compatible with high temperature steps due to strong lifetime degradation. Therefore, low temperature surface passivation schemes have gained special interest due to their compatibility with both thin and low quality c-si substrates. heterojunction solar cells with thin hydrogenated amorphous silicon (a-si:h) films deposited at low temperature on c-si wafers have attracted the interest of the photovoltaic community due to their high-efficiency and cost-effective fabrication process. Sanyo electric co. Has reported conversion efficiencies (¿) over 19% for mass produced solar cells with the so-called heterojunction with intrinsic thin-layer (hit) structure. In this device a very thin (5 nm) intrinsic a-si:h buffer reduces interface recombination, which leads to impressing open-circuit voltages (voc) over 700 mv. Most groups, included sanyo, use the plasma-enhanced cvd technique to grow the a-si:h films. Recently, the hot-wire cvd (hwcvd) technique has also demonstrated its potential to fabricate high-efficiency heterojunction silicon solar cells. In the hwcvd technique, besides some technological advantages, the absence of ion bombardment reduces the damage to the c-si surface. in this work, we have concentrated our effort in optimizing all the fabrication steps to obtain heterojuction solar cells by hwcvd in a completely low temperature process (200ºc). First, we have optimized the material deposited by hwcvd varying the different deposition parametres (pressure, filament temperature, hydrogen and silane flows, doping level, substrate temperature) in order to obtain good quality a-si:h and c-si:h layers. Intrinsic and doped materials with good structural, electrical and transport properties have been obtained. Second, we have concentrated our effort in optimizing the heterojunction emitter on p-type c-si substrates. In particular, the importance of the thin intrinsic a-si:h buffer and the influence of hydrogen pre-treatments was deeply studied by means of the spectroscopic ellipsometry and quasy-steady-state photoconductance techniques. These studies have allowed to obtain solar cell precursors with implicit open circuit voltages higher than 690 mv. Later on, the optimization of the front contact was performed. On the one hand, the indium-tin-oxide transparent conductive coating was optimized to obtain a very low resistivity (<4í--10-4 ·cm) and good antireflection properties. Furthermore, the front metal grid was also optimized in terms of the series resistance and shadowing. Last, complete solar cells with conversion efficiency up to 15.4% have been fabricated on flat p-type (14 ¿·cm) cz silicon wafers with a high temperature aluminum back-surface-field (al-bsf) contact. finally, we have proceeded to investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by hot-wire cvd to replace traditional high temperature al-bsf contacts. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. To date, double-side heterojunction solar cells by hwcvd with conversion efficiencies of 14.5% have been already obtained in a fully low temperature process (<200ºc). Considering the preliminary character of these devices, this is a very promising starting point for a further increase in the conversion efficiency of bifacial heterojunction solar cells fabricated by hwcvd. Ptthe cost of high efficiency crystalline silicon (c-si) modules is hindering the progress of the pv industry as a viable alternative for clean energy production. Therefore, cell manufacturers are searching different approaches that could allow the desired cost reduction. For instance, considering that c-si wafers represent about 30-50% of the module price, the final cost would be significantly reduced by using very thin c-si wafers (< 200 ¿m). However, when decreasing the c-si thickness the rear surface recombination becomes important. Although thermal oxidation very effectively passivates the c-si surface, thin wafers tend to warp at the high temperatures (~1000 °c) involved in the process. On the other hand, low cost multicrystalline silicon wafers are not compatible with high temperature steps due to strong lifetime degradation. Therefore, low temperature surface passivation schemes have gained special interest due to their compatibility with both thin and low quality c-si substrates. heterojunction solar cells with thin hydrogenated amorphous silicon (a-si:h) films deposited at low temperature on c-si wafers have attracted the interest of the photovoltaic community due to their high-efficiency and cost-effective fabrication process. Sanyo electric co. Has reported conversion efficiencies (¿) over 19% for mass produced solar cells with the so-called heterojunction with intrinsic thin-layer (hit) structure. In this device a very thin (5 nm) intrinsic a-si:h buffer reduces interface recombination, which leads to impressing open-circuit voltages (voc) over 700 mv. Most groups, included sanyo, use the plasma-enhanced cvd technique to grow the a-si:h films. Recently, the hot-wire cvd (hwcvd) technique has also demonstrated its potential to fabricate high-efficiency heterojunction silicon solar cells. In the hwcvd technique, besides some technological advantages, the absence of ion bombardment reduces the damage to the c-si surface. in this work, we have concentrated our effort in optimizing all the fabrication steps to obtain heterojuction solar cells by hwcvd in a completely low temperature process (200ºc). First, we have optimized the material deposited by hwcvd varying the different deposition parametres (pressure, filament temperature, hydrogen and silane flows, doping level, substrate temperature) in order to obtain good quality a-si:h and c-si:h layers. Intrinsic and doped materials with good structural, electrical and transport properties have been obtained. Second, we have concentrated our effort in optimizing the heterojunction emitter on p-type c-si substrates. In particular, the importance of the thin intrinsic a-si:h buffer and the influence of hydrogen pre-treatments was deeply studied by means of the spectroscopic ellipsometry and quasy-steady-state photoconductance techniques. These studies have allowed to obtain solar cell precursors with implicit open circuit voltages higher than 690 mv. Later on, the optimization of the front contact was performed. On the one hand, the indium-tin-oxide transparent conductive coating was optimized to obtain a very low resistivity (<4í--10-4 ·cm) and good antireflection properties. Furthermore, the front metal grid was also optimized in terms of the series resistance and shadowing. Last, complete solar cells with conversion efficiency up to 15.4% have been fabricated on flat p-type (14 ¿·cm) cz silicon wafers with a high temperature aluminum back-surface-field (al-bsf) contact. finally, we have proceeded to investigate low temperature deposited back contacts based on boron-doped amorphous silicon films obtained by hot-wire cvd to replace traditional high temperature al-bsf contacts. The influence of the deposition parameters and the use of an intrinsic buffer layer have been considered. To date, double-side heterojunction solar cells by hwcvd with conversion efficiencies of 14.5% have been already obtained in a fully low temperature process (<200ºc). Considering the preliminary character of these devices, this is a very promising starting point for a further increase in the conversion efficiency of bifacial heterojunction solar cells fabricated by hwcvd. Pt
Datos académicos de la tesis doctoral «Silicon heterojunction solar cells fabricated by hot ¿wire cvd«
- Título de la tesis: Silicon heterojunction solar cells fabricated by hot ¿wire cvd
- Autor: Delfina Muñoz Cervantes
- Universidad: Politécnica de catalunya
- Fecha de lectura de la tesis: 21/07/2008
Dirección y tribunal
- Director de la tesis
- Cristóbal Voz Sánchez
- Tribunal
- Presidente del tribunal: Luis Castañer muñoz
- pere Roca i cabarrocas (vocal)
- david Soler vilamitjana (vocal)
- jordi Andreu batallé (vocal)