• Energy Research

AbstractThin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were deposited at a high growth rate of 5 nm/s by Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PECVD). A single deposition yielded 30 working cells out of 30 test cells. After characterization of J-V characteristics under AM1.5 conditions, spectral response and Fourier Transform Photocurrent Spectroscopy (FTPS) for defect density, the cells were subjected to a beam of 1 MeV protons, with different irradiation times for a series of cells, up to a maximum fluence of 1015 protons per cm2, to investigate the feasibility of using this type of solar cells in space. After degradation, the cells were characterized again. The highest fluence reduced the conversion efficiency of the solar cells by a factor of 10. These values are compared to a similar experiment in literature for thin film amorphous silicon cells. The spectral response shows that the quantum efficiency for low energy photons is drastically reduced, suggesting that the damage is mainly inflicted to the bulk of the absorber material. This is strongly supported by the FTPS results, which show a clear trend of increasing defect density with increasing fluence. The optical absorption coefficient at 0.8 eV increased by a factor of 20. After characterization, the cells were isochronally annealed, up to the deposition temperatures, to investigate if the original performance of the cell could be restored. The mid-gap absorption decreased by a factor of 5 after annealing.

AbstractThin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were deposited at a high growth rate of 5 nm/s by Very High Frequency Plasma Enhanced Chemical Vapor Deposition (VHF PECVD). A single deposition yielded 30 working cells out of 30 test cells. After characterization of J-V characteristics under AM1.5 conditions, spectral response and Fourier Transform Photocurrent Spectroscopy (FTPS) for defect density, the cells were subjected to a beam of 1 MeV protons, with different irradiation times for a series of cells, up to a maximum fluence of 1015 protons per cm2, to investigate the feasibility of using this type of solar cells in space. After degradation, the cells were characterized again. The highest fluence reduced the conversion efficiency of the solar cells by a factor of 10. These values are compared to a similar experiment in literature for thin film amorphous silicon cells. The spectral response shows that the quantum efficiency for low energy photons is drastically reduced, suggesting that the damage is mainly inflicted to the bulk of the absorber material. This is strongly supported by the FTPS results, which show a clear trend of increasing defect density with increasing fluence. The optical absorption coefficient at 0.8 eV increased by a factor of 20. After characterization, the cells were isochronally annealed, up to the deposition temperatures, to investigate if the original performance of the cell could be restored. The mid-gap absorption decreased by a factor of 5 after annealing.

Upconversion of subbandgap light of thin film single junction amorphous silicon solar cells may enhance their performance in the near infrared (NIR). In this paper we report on the application of the NIR–vis upconverter β-NaYF4:Yb3+(18%) Er3+(2%) at the back of an amorphous silicon solar cell in combination with a white back reflector and its response to infrared irradiation. Current–voltage measurements and spectral response measurements were done on experimental solar cells. An enhancement of 10 μA/cm2 was measured under illumination with a 980 nm diode laser (10 mW). A part of this was due to defect absorption in localized states of the amorphous silicon

Upconversion of subbandgap light of thin film single junction amorphous silicon solar cells may enhance their performance in the near infrared (NIR). In this paper we report on the application of the NIR–vis upconverter β-NaYF4:Yb3+(18%) Er3+(2%) at the back of an amorphous silicon solar cell in combination with a white back reflector and its response to infrared irradiation. Current–voltage measurements and spectral response measurements were done on experimental solar cells. An enhancement of 10 μA/cm2 was measured under illumination with a 980 nm diode laser (10 mW). A part of this was due to defect absorption in localized states of the amorphous silicon

Abstract The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs has to be taken into consideration also in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied Intensity Modulated Photocurrent Spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the DC quantum yield and the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can occur due to the presence of dangling bond defect centers in the band gap, which have an amphoteric character and can enhance the photocurrent due to trapping and thermal emission processes of photogenerated carriers.

Abstract The experimental observation of internal quantum efficiencies above unity in crystalline silicon solar cells has brought up the question whether the generation of multiple electron/hole pairs has to be taken into consideration also in solar cells based on direct gap amorphous semiconductors. To study photogenerated carrier dynamics, we have applied Intensity Modulated Photocurrent Spectroscopy (IMPS) to hydrogenated amorphous silicon p-i-n solar cells. In the reverse voltage bias region at low illumination intensities it has been observed that the DC quantum yield and the low frequency limit of the AC quantum yield Y increases significantly above unity with decreasing light intensity, indicating that more than one electron per photon is detected in the external circuit. This phenomenon can occur due to the presence of dangling bond defect centers in the band gap, which have an amphoteric character and can enhance the photocurrent due to trapping and thermal emission processes of photogenerated carriers.

Abstract Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10 −5 /Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm 2 has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 A/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).

Abstract Microcrystalline silicon germanium films showing excellent opto-electronic properties have been prepared at a substrate temperature of 195°C by radio frequency plasma enhanced chemical vapor deposition at 13.56 MHz. A white light (AM 1.5) photoconductivity of 5×10 −5 /Ω cm and ambipolar diffusion length of 114 nm (from SSPG) established the device quality. Films are intrinsic (Fermi level near midgap; activation energy E a (0.49 eV) is approximately half the band gap (1.01 eV)). Performance of preliminary n–i–p solar cells (with μc-SiGe:H i-layer) on stainless steel and molybdenum substrates justify their photosensitivities. A current density of 9.44 mA/cm 2 has been generated in an i-layer of only 150 nm thick without any back-reflector. A deposition rate of 0.75 A/s for such a thin layer gives this material much advantage than a μc-Si cell, where a thickness of >2 μm is needed. A high V oc of 0.43 eV has been achieved for such a low mobility gap cell (Ge fraction 60%).