Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells

Vaněček, M.; Poruba, A.

doi:10.1063/1.1446207

Cited by 176 publications

(132 citation statements)

References 7 publications

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“…Micro-Raman experiments were performed on the solar cells in two ways: with focused excitation light arriving either through the TCO on the top, last-deposited silicon layer of the device (i.e., on the n-layer for the p-i-n devices), or through the glass substrate and the TCO on the bottom, first deposited silicon layer of the device. FTPS was performed with a Nicolet 8700 to analyze the material quality of the solar cells [30]. A sample was prepared as cross-section for TEM observation on a Philips CM200 microscope operated at 200 kV.…”

Section: Sample Fabrication Characterization and Methods For Countinmentioning

confidence: 99%

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post‐oxidation

Python

Dominé

Söderström

et al. 2010

Progress in Photovoltaics

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Microcrystalline silicon (mc-Si:H) cells can reach efficiencies up to typically 10% and are usually incorporated in tandem micromorph devices. When cells are grown on rough substrates, ''cracks'' can appear in the mc-Si:H layers. Previous works have demonstrated that these cracks have mainly detrimental effects on the fill factor and open-circuit voltage, and act as bad diodes with a high reverse saturation current. In this paper, we clarify the nature of the cracks, their role in postoxidation processes, and indicate how their density can be reduced. Regular secondary ion mass spectrometry (SIMS) and local nano-SIMS measurements show that these cracks are prone to local post-oxidation and lead to apparent high oxygen content in the layer. Usually the number of cracks can be decreased with an appropriate modification of the substrate surface morphology, but then, the required light scattering effect is reduced due to a lower roughness. This study presents an alternative/complementary way to decrease the crack density by increasing the substrate temperature during deposition. These results, also obtained when performing numerical simulation of the growth process, are attributed to the enhanced surface diffusion of the adatoms at higher deposition temperature. We evaluate the cracks density by introducing a fast method to count cracks with good statistics over approximately 4000 mm of sample cross-section. This method is proven to be useful to quickly visualize the impact of substrate morphology on the density of cracks in microcrystalline and in micromorph devices, which is an important issue in the manufacturing process of modules.

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Section: Sample Fabrication Characterization and Methods For Countinmentioning

confidence: 99%

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post‐oxidation

Python

Dominé

Söderström

et al. 2010

Progress in Photovoltaics

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“…The resulting photovoltaic devices delivered a sufficient amount of photocurrent at short circuit in order to perform highly sensitive measurements by Fouriertransform photocurrent spectroscopy (FTPS). 47 Briefly, FTPS uses the external light beam of an FTIR with external beam output option, equipped with a halogen lamp and quartz beam splitter in order to obtain spectra in the visible and near-infrared spectral region. The FTIRs modulated output light beam illuminates the organic photovoltaic devices under investigation.…”

Section: B Photocurrent Anisotropymentioning

confidence: 99%

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

2012

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An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E CT ) and energy of the donor exciton (E D * ) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D + A − ) and of the local polymer excited state (D * A). We show that the D * A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer:fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E D * − E CT difference. We find that, upon decreasing E D * − E CT , the local excitonic D * A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer:fullerene systems, this effect is expected to become detrimental for device performance if E D * − E CT < 0.1 eV. This however, depends on the electronic coupling between D * A and D + A − , which we experimentally estimate to be ∼ 6 meV for the TQ1:PCBM system.

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“…16,17 Smirnov et al 18 showed that the decrease of photoconductivity observed in c-Si: H films under light soaking is associated with the creation of metastable defects, similarly as in the case of a-Si: H. We made further observations on light-soaked single-junction c-Si: H solar cells: 13,19 we noted an increase of defect-related absorption, as monitored by the value of the absorption coefficient at 0.8 eV ͓␣͑0.8 eV͔͒ evaluated by Fourier transform photocurrent spectroscopy ͑FTPS͒. 20 We suggested that light-induced defects could be spatially located at the surface of the nanocrystals. 19 Indeed, based on well-known observations made on a-Si/ c-Si heterojunction solar cells, 21,22 we proposed that the amorphous phase is necessary in the intrinsic layer in order to passivate the defects present at the surface of the nanocrystals.…”

Section: Introductionmentioning

confidence: 84%

“…20 The spectra were calibrated at 1.35 eV by setting the absorption coefficient of the c-Si: H cells equal to that of crystalline silicon ͑=235 cm −1 ͒. The implicit assumption underlying this calibration procedure is that defectrelated absorption ␣͑0.8 eV͒ originates from the crystalline phase of the mixed-phase microcrystalline material.…”

Section: Methodsmentioning

confidence: 99%

Kinetics of creation and of thermal annealing of light-induced defects in microcrystalline silicon solar cells

Meillaud

Vallat-Sauvain

Shah

et al. 2008

Journal of Applied Physics

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Single-junction microcrystalline silicon ͑ c-Si: H͒ solar cells of selected i-layer crystalline volume fractions were light soaked ͑AM1.5, 1000 h at 50°C͒ and subsequently annealed at increasing temperatures. The variations of subbandgap absorption during light soaking and during thermal annealing were monitored by Fourier transform photocurrent spectroscopy. The kinetics were shown to follow stretched exponential functions over long times such as 1000 h. The effective time constants appearing in the stretched exponential function decrease with decreasing crystalline volume fraction as well with increasing annealing temperature. Their Arrhenius-like dependence on temperature is characterized by a unique value of the activation energy. Furthermore, we demonstrate that the configuration of the solar cells ͑p-i-n or n-i-p͒ does not influence the degradation kinetics, as long as the average crystallinity of the intrinsic layer is of comparable value.

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Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells

Cited by 176 publications

References 7 publications

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post‐oxidation

Microcrystalline silicon solar cells: effect of substrate temperature on cracks and their role in post‐oxidation

Polarization anisotropy of charge transfer absorption and emission of aligned polymer:fullerene blend films

Kinetics of creation and of thermal annealing of light-induced defects in microcrystalline silicon solar cells

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