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Hydrogenated microcrystalline silicon (μc-Si:H) has become a material of increasing interest these last years mainly for its use in cost-effective production of tandem and triple junction thin film silicon based solar cells. Lately, the use of novel doped silicon oxide (SiO x ) layers were shown to be very promising for increasing the solar cells efficiency [1,2]. We present in this study a detailed analysis on the possible reasons behind this significant increase of electrical performances. Complete solar cells were developed in an industrial type reactor with their intrinsic layer (i-layer) deposited at a high growth rate of 1 nm/s by VHF-PECVD. Different i-layer material quality and substrate roughness were systematically evaluated during this investigation. We demonstrate conversion efficiency increase of up to 29% when both these p-type and n-type doped SiO x layers are used instead of the regular microcrystalline ones, while keeping the bulk of intrinsic material unchanged and efficiencies over 8% are achieved for a wider range of plasma parameters and substrate roughness. Extensive material analysis is presented hereafter to understand the physical origins for the improvements observed. XRD, Raman and FTIR spectroscopy, intrinsic stress, FTPS and SIMS measurements were done along with SEM images of the solar cells. It is found that devices with very different efficiencies can lead to similar FTIR and FTPS spectrum. We show that the integration of doped SiO x layers reduces to some extent the influence of porous regions, i.e. microcracks, on the electrical properties of the solar cells, and the possible physical reasons for this improvement are discussed. The development of these extrinsic defects, not detected by FTPS and FTIR, is becoming especially detrimental on highly textured substrates, required for increased light trapping. This highlights the fundamental nature difference of intrinsic and extrinsic defects which can both drive the cells performances.

We report here on the latest research developments of tandem micromorph (amorphous/ microcrystalline) silicon solar cells in our laboratory. High conversion efficiency for micromorph cells requires both a dedicated light management to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. We will show that an improved cell design based on the use of silicon oxide doped layers permits to achieve high efficiencies on substrates that are usually considered as inappropriate for μc-Si:H because of their roughness. Furthermore, recently, new front contacts based e.g. on bi-layers or Ultraviolet nanoimprint lithography were developed, leading to very promising results in micromorph solar cells. While efficiencies of 12.7% initial and 11.3% stable could be achieved with only 1.1 μm of bottom cell on a front rough Low Pressure Chemical Vapor Deposited (LPCVD) ZnO, a remarkable 12% initial was also reached on textured replica (as on the master). Emphasis is also laid in our lab on increasing the deposition rate of mc-Si:H while maintaining high quality material. This is done by reducing the interelectrode gap while working at high deposition pressure, in "powder free" processes at 40 MHz. We could observe that high pressure-low hydrogen dilution process conditions lead to dense high quality material. So far, conversion efficiencies up to 8.5% have been achieved at 1 nm/s for single junction μc-Si:H solar cells with 1.8 μm thick absorber layer. We also report a promising micromorph tandem initial efficiency of 11.9% with the μc-Si:H i-layer at 0.9 nm/s. High efficiency micromorph solar cells could thus be fabricated under conditions that are favorable to industrial, low-cost, fabrication of micromorph modules. Recent results of tandems combining original substrates and improved deposition processes suggest that stabilised efficiencies close to 13% can be expected in a near future.

Improving micromorph devices performances nowadays requires a current density increase: good devices typically exhibit an open circuit voltage times fill factor product of one (V oc ×FF∼1.4V×0.71=1). Their short-circuit current density (J sc ) value thus dictates their efficiency (expressed in %). Maximizing it with reasonable cells thicknesses necessitates therefore combining robust cell design with adequate light management through transparent electrodes and intermediate reflectors engineering. We will first show how record micromorph devices (13.5% initial and >11.5% stabilized efficiencies) are prepared on optimized single-layer ZnO electrodes. Such electrodes requirements will be discussed: 1) Strong and wide light scattering is needed on the entire useful wavelength range. Large features grant high total currents (>26mA/cm2) while sharp ones allow for high top cell currents (>13mA/cm2). 2) Sufficiently small or smooth substrate features permits high quality cell growth, providing good cell design (typically V oc over 1.4V). 3) Good conduction and transparency for electrodes (requiring ∼50cm2/V/s TCO mobility) should preserve sheet resistance close to 20Ω/□ (for FF>70%) with low absorption. We will then focus on pushing further micromorph devices potential. Either textured intermediate reflectors can fulfill the bottom cell needs, or double-texture substrates can be implemented: light scattering at large wavelengths is here achieved via nanoimprint lithography (a versatile approach to glass-texturing), topped by small and sharp ZnO features guaranteeing high top cell current. By combining excellent TCO with smart under-structures, thin devices delivering high currents with excellent efficiencies are within reach.