Silicon heterojunction solar cells toward higher fill factor

Abstract

One of the most limiting factors in the record conversion efficiency of amorphous/crystalline silicon heterojunction solar cells is the not impressive fill factor value. In this work, with the aid of a numerical model, the ways to enhance the cell fill factor up to 85% are investigated in detail, considering the properties of conventional amorphous‐doped films, wider Energy gap layers, and transparent conductive oxide films. The band alignment among the various materials composing the heterojunction is the key to high efficiency but becomes an issue for the solar cell fill factor, if not well addressed. One of the most interesting outcomes of this work is the evidence of hidden barriers arising between the transparent conductive oxide and both selective contacts, due to the mismatch between their work functions. The measurement of light current‐voltage characteristics performed at low temperature is proposed as a way to identify the presence of these barriers in efficient solar cells that do not possess high fill factor values. Experimental J‐V characteristics compared with numerical simulations demonstrated that the sometimes neglected cell base contact needs instead a more careful consideration. To this aim, a model to predict the presence of a hidden barrier at the base contact that limits the cell fill factor is proposed.

Selective contacts and fill factor limitations in heterojunction solar cells

“…The models presented in this work have been obtained from numerical simulations of solar cells J-V characteristics at different temperatures and with different characteristics of the constituting materials, similarly to what already discussed in a previous work. 14 We have followed a monodimensional description of the device along its thickness based on the homogeneity of the crystalline silicon absorber along the other two dimensions under the practical hypothesis of high-quality monocrystalline silicon. Among all the numerical simulators available to describe the carrier transport along a solar cell, intended as a stacked structure, we have preferred to use a homemade numerical simulator, 19 for the sake of familiarity and good description of amorphous materials.…”

“…19 To simplify the description, the density of states at the interface between the a-Si:H buffer layer and c-Si is integrated along the thickness of the buffer layer and then reported as D it . More details can be found in Martini et al 14 In this work, the same numerical simulator has been used to describe the emitter side behavior as a function of temperature for different doping concentration of the (p) a-Si:H layer. The most relevant properties of any material used in the device simulations, such as E g , μ, χ, optical absorptions and refractive indexes are deduced from experimental measurements and are listed in Table 1.…”

“…In this structure, the main FF limitations come from series resistance, arising from TCO lateral transport, by the specific contact resistivity between the TCO and the silver screen printed grid, and of course by the metal grid conductivity, which is usually one order of magnitude lower than that of the conventionally achieved in c-Si homojunctions, due to temperature sintering below 200 C of the screen printable silver pastes. 13 Besides these issues, other parameters can limit the HJ FF, and they have been well addressed one by one in a previous work, 14 showing how a realistic upper FF limit for this technology could be 85%. A key factor for the FF limitation is represented by the presence of barriers to charge transport which are not easily identified at Room Temperature (RT).…”

“…In a previous work, 14 it has been shown how the optimal TCO work function on the base contact must range below 4.25 eV in order to avoid, at RT, any FF limitation coming from the presence of the barrier. We have also illustrated that a high doping of the a-Si:H layer can help in reducing the impact of a higher Φ TCO on the FF.…”

“…The analysis of the measured curves permits to define an activation energy (E act ) for this transition. Then we have F I G U R E 1 Sketch of a bifacial a-Si:H/c-Si heterojunction solar cell (top) and its band structure under dark, short circuit, and room temperature conditions (bottom) [Colour figure can be viewed at wileyonlinelibrary.com] extrapolated a theoretical curve, 14 reported as the solid line in Figure 3, which correlates E act to Φ TCO .…”