O. Kunz

Effective light trapping in polycrystalline silicon thin-film solar cells by means of rear localized surface plasmons

Significant photocurrent enhancement has been achieved for evaporated solid-phase-crystallized polycrystalline silicon thin-film solar cells on glass, due to light trapping provided by Ag nanoparticles located on the rear silicon surface of the cells. This configuration takes advantage of the high scattering cross-section and coupling efficiency of rear-located particles formed directly on the optically dense silicon layer. We report short-circuit current enhancement of 29% due to Ag nanoparticles, increasing to 38% when combined with a detached back surface reflector. Compared to conventional light trapping schemes for these cells, this method achieves 1/3 higher short-circuit current.

Lifetime limiting recombination pathway in thin-film polycrystalline silicon on glass solar cells

The minority carrier lifetimes of a variety of polycrystalline silicon solar cells are estimated from temperature-dependent quantum efficiency data. In most cases the lifetimes have Arrhenius temperature dependences with activation energies of 0.17–0.21 eV near room temperature. There is also a rough inverse relationship between lifetime and the base dopant concentration. Judging by this inverse law, the activation energies of the lifetimes, and the absence of plateau behavior in the lifetimes of the higher doped cells at low temperatures, it is inferred that the dominant recombination pathway involves the electronic transition between shallow states which are 0.05–0.07 eV below the conduction band and 0.06–0.09 eV above the valence band, respectively, consistent with the shallow bands in silicon dislocations. The modeled recombination behavior implies that deep levels do not significantly affect the lifetimes for most of the cells at and below room temperature.

Advances in evaporated solid-phase-crystallized poly-Si thin-film solar cells on glass (EVA)

Polycrystalline silicon thin-film solar cells on glass obtained by solid-phase crystallization (SPC) of PECVD-deposited a-Si precursor diodes are capable of producing large-area devices with respectable photovoltaic efficiency. This has not yet been shown for equivalent devices made from evaporated Si precursor diodes (“EVA” solar cells). We demonstrate that there are two main problems for the metallization of EVA solar cells: (i) shunting of the p-n junction when the air-side metal contact is deposited; (ii) formation of the glass-side contact with low contact resistance and without shunting. We present a working metallization scheme and first current-voltage and quantum efficiency results of 2 cm2 EVA solar cells. The best planar EVA solar cells produced so far achieved fill factors up to 64%, series resistance values in the range of 4-5 Ωcm2, short-circuit current densities of up to 15.6 mA/cm2, and efficiencies of up to 4.25%. Using numerical device simulation, a diffusion length of about 4 𝜇m is demonstrated for such devices. These promising results confirm that the device fabrication scheme presented in this paper is well suited for the metallization of EVA solar cells and that the electronic properties of evaporated SPC poly-Si materials are sufficient for PV applications.

Influence of the absorber doping for p-type polycrystalline silicon thin-film solar cells on glass prepared by electron beam evaporation and solid-phase crystallization

A systematic investigation of the influence of the absorber doping on the performance of planar, p-type, evaporated, solid-phase crystallized polycrystalline silicon thin-film solar cells on glass is presented. It is found that the optimum Suns-Voc parameters (open-circuit voltage and pseudo fill factor) are achieved at intermediate absorber doping of Nabs∼1–2 × 1017 cm−3, while high short-circuit currents are achieved at the lowest absorber doping of Nabs ≤ 6 × 1015 cm−3. Since the short-circuit current is the dominating factor to achieve high conversion efficiencies for evaporated polycrystalline silicon cells, the maximum pseudo efficiencies are achieved at very low absorber doping. The Suns-Voc characteristics of lightly doped cells can be adequately described by a modified two-diode model with n1=1 and n2≈1.5, which is in contrast to the value of 2 for n2 commonly quoted in the literature. PC1D modeling demonstrates that such a low ideality factor for space charge region recombination can be modeled ...

Anomalous temperature dependence of diode saturation currents in polycrystalline silicon thin-film solar cells on glass

Temperature dependent Suns-Voc measurements are performed on four types of polycrystalline silicon thin-film solar cells on glass substrates, all of which are made by solid phase crystallization∕epitaxy of amorphous silicon from plasma enhanced chemical vapor deposition or e-beam evaporation. Under the two-diode model, the diode saturation currents corresponding to n=1 recombination processes for these polycrystalline silicon p‐n junction cells follow an Arrhenius law with activation energies about 0.15–0.18eV lower than that of single-crystal silicon p‐n diodes of 1.206eV, regardless of whether the cells have an n- or p-type base. This discrepancy manifests itself unambiguously in a reduced temperature sensitivity of the open-circuit voltage in thin-film polycrystalline silicon solar cells compared to single-crystal silicon cells with similar voltages. The physical origin of the lowered activation energy is attributed to subgap levels acting either as minority carrier traps or shallow recombination centers.

Recent advances in polycrystalline silicon thin-film solar cells on glass at UNSW

Polycrystalline Si (pc-Si) thin-film solar cells on glass are a very promising approach for lowering the cost of photovoltaic solar electricity. This paper reports on the status of three distinctly different pc-Si thin-film solar cells on glass under development at the University of New South Wales (UNSW). The cells are termed EVA, ALICE and ALICIA, are less than 3 microns thick, and are made by vacuum evaporation, a fast and inexpensive Si deposition method. EVA cells are made on non-seeded glass, whereas ALICE and ALICIA are both made on a thin large-grained pc-Si seed layer formed on glass by metal-induced crystallisation. All three solar cells seem to be capable of voltages of over 500 mV and, owing to their potentially inexpensive and scalable fabrication process, have significant industrial appeal.

Modelling the effects of distributed series resistance on Suns-V oc , m-V oc and J sc -Suns curves of solar cells

A simple model based on the simulation of distributed series resistance effects in solar cells is presented. This model overcomes limitations of the standard two-diode model representation of solar cells in respect to fitting Suns-Voc (m-Voc) and Jsc-Suns curves. The model results include (i) an improved understanding of two-dimensional current flows in shunted solar cells, (ii) the prediction of shunt types from m-Voc curves and corresponding distributed resistance model fits, (iii) the insight that lateral current flows in solar cells are responsible for deviations between Jsc-Suns data and two-diode model fits in the high illumination range. The presented approach extends the application range of Suns-Voc measurements to solar cells which cannot adequately be described with the two-diode model.

Comparative Study of Solid-Phase Crystallization of Amorphous Silicon Deposited by Hot-Wire CVD, Plasma-Enhanced CVD, and Electron-Beam Evaporation

Solid-phase crystallization (SPC) rates are compared in amorphous silicon films prepared by three different methods: hot-wire chemical vapor deposition (HWCVD), plasma-enhanced chemical vapor deposition (PECVD), and electron-beam physical vapor deposition (e-beam). Random SPC proceeds approximately 5 and 13 times slower in PECVD and e-beam films, respectively, as compared to HWCVD films. Doping accelerates random SPC in e-beam films but has little effect on the SPC rate of HWCVD films. In contrast, the crystalline growth front in solid-phase epitaxy experiments propagates at similar speed in HWCVD, PECVD, and e-beam amorphous Si films. This strongly suggests that the observed large differences in random SPC rates originate from different nucleation rates in these materials while the grain growth rates are relatively similar. The larger grain sizes observed for films that exhibit slower random SPC support this suggestion.

Device fabrication scheme for evaporated SPC poly-Si thin-film solar cells on glass (EVA)

Polycrystalline silicon thin-film solar cells on glass obtained by solid phase crystallisation (SPC) of PECVD-deposited a-Si have good photovoltaic efficiency. In this paper we present a functioning device fabrication scheme for equivalent solar cells made from evaporated SPC poly-Si films (EVA). Respectable efficiencies of above 4% are achieved for both p-type and n-type EVA solar cells on planar glass superstrates. The various hurdles that had to be passed to achieve these results are described.

Elimination of severe shunting problems due to air-side electrode formation on evaporated poly-Si thin-film solar cells on glass

Recent progress in the metallization of poly-silicon thin-film solar cells on glass, created by solid phase crystallization (SPC) of evaporated amorphous silicon (EVA), revealed that this type of solar cell suffers from severe shunting problems when the air-side (i.e., rear) metal contact is deposited. Pinholes of sizes between 5 and 20 microns exist in this material at moderate density (0.1–1 mm−2), and at first glance these pinholes appear to be the reason for the shunting. However, experiments with pinhole filling prior to the metallization process and attempts to find a correlation between the magnitude of the shunt and the density of the pinholes both lead to the conclusion that (i) there has to be a different shunting path and (ii) the pinholes visible in the optical microscope are unproblematic for the metallization of the rear surface. The shunting is shown to be of a spatially distributed nature and to happen at a rather high shunt density of at least several shunts per mm2. Contacting only a small fraction of the rear Si surface via a point contacting scheme, whereby the Al/poly-Si contact area must not exceed ∼ 5%, is shown to work well on EVA solar cells.