M. Brinza

Designing amorphous silicon based solar cell structure on plastic

Fabrication of solar cells on cheap plastics, due to its demand for a low temperature processing (∼100 °C) needs adaptation of the cell structure of the state-of-the-art high efficiency thin film silicon solar cells made at high temperature (200 °C). Use of a double n-layer (n-µc-Si/n-a-Si) instead of a single n-µc-Si layer improves Voc from a low 0.66V to 0.82V in the as deposited state of an n-i-p type a-Si cell made at 100 °C. In a high temperature deposited cell, where a p-type µc-Si (band gap Eg =1.1 eV) is used in combination with a-Si i-layer (Eg =1.8 eV), a thin buffer a-Si layer made at 100 °C acts as a barrier for electron back diffusion from the i-layer. For the low temperature deposited i-layers this type of buffer layer loses its advantage of band offset and fails to stop the electron back diffusion. To verify this hypothesis, a-Si n-i-p cells were made with a heavily doped amorphous silicon p-layer at high hydrogen dilution (leaving out the ineffective buffer layer). We obtained indeed a high Voc of 0.89 V in as deposited state and after annealing at 100 °C the Voc reached 0.92 V. An efficiency of 6.6% was obtained for a cell made on a flat Ag/ZnO:Al back reflector (BR) on a glass substrate. A low current density of 12.5 mA/cm2 is attributed to the fact that it was a smooth BR. The high Voc of 0.92V was reproduced in a similar cell on stainless steel foil substrate. Cells with similar structure and deposition condition were fabricated on plastic substrates, namely polyethylene naphthalene (.PEN) and polyethylene terpthalate (PET). Deposition conditions of ZnO:Al by magnetron sputtering were adapted to reduce the deposition related stress on the plastic foil while the Ag layer was thermally evaporated. A comparable Voc of 0.89 V is obtained in as deposited state and 0.92 V after annealing. The cells on PEN and PET showed initial efficiencies of 6.3% and 5.9% respectively.

Gas phase considerations for the deposition of thin film silicon solar cells by VHF-PECVD at low substrate temperatures

Fabrication of thin film silicon solar cells on cheap plastics or paper-like substrate requires deposition process at very low substrate temperature, typically ≤ 100 °C. In a chemical vapor deposition process, low growth temperatures lead to materials with low density, high porosity, high disorder and high defect density. This can be partly attributed to the small diffusion length of precursors on the growing surface and to the temperature range below the glass transition temperature of the deposited films. Plasma enhanced deposition technology helps improving material quality at low deposition temperatures by providing extra energy to the growing surface by ion bombardment. In this paper we have explored the gas phase conditions in a very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) process at low temperatures. Using a retarding field ion energy analyzer installed on the substrate holder in a deposition system called ATLAS, we measured ion flux and ion energy at various substrate temperatures (39, 100 and 200°C). The pressure was fixed at 0.16 mbar and the silane part (silane/(silane+hydrogen)) at 0.166. We observed that with a decrease in temperature, the ion flux hardly changes, whereas the ion energy decreases. Similar results were obtained from a simulation of a 2D plasma discharge. As the deposition rate does not vary significantly with temperature, we infer that the average ion energy per deposited atom decreases with decreasing temperature. This explains the high porosity of the materials deposited at low temperatures. To obtain device quality material at 100 °C, a lower silane part (0.063) was therefore needed than in the case of 200 °C (silane part of 0.5). We have achieved 5.3% efficiency for an amorphous silicon test cell, deposited at 100 °C on a smooth Ag/ZnO coated stainless steel foil.

Transient Photoconductivity Study of the Distribution of Gap States in 100°C VHF-deposited Hydrogenated Silicon Layers

The energy distribution of gap states has been examined by means of transient photocurrent measurements in a series of 100°C VHF-deposited Si:H samples that spans the amorphous to microcrystalline transition. The ‘amorphous’ distribution, consisting of a continuous background and a prominent dangling-bond-induced peak, remains largely intact across the transition. The transport path located at the conduction band edge in a-Si:H, some 0.63 eV above the dangling bond D − energy, moves down to ∼0.55 eV above the corresponding D − level in the microcrystalline samples.