Kesterites

The most efficient thin film solar cells are based on Cu(In,Ga)(S,Se)2 (CIGSSe) and CdTe compounds, known as second generation polycrystalline thin films. The challenge of these materials is to reduce the cost per watt of solar energy conversion, but they are actually formed by expensive and/or scanty elements in the earth’s crust such as In, Ga, Te and other that present toxicity issues like Cd. In the last years, new materials with properties of interest for photovoltaic applications and formed by non toxic and relatively abundant elements, have been suggested as alternatives to the main second generation solar cells based on CdTe and CIGSSe. Semiconductor compounds with kesterite structure (Cu2ZnSn(SxSe1−x)4, Cu2ZnSnS4, Cu2ZnSnSe4) and other like In2S3, all of them Cadmium-free have been proposed as new candidates for thin film solar cells. However, reported solar cell efficiencies for these compounds have not yet reached the expected values. In this work, we present a review of the limiting factors for achieving high efficiency in thin film solar cells, related to deposition methods as well as the different mechanisms that limit cell performance. Significant results in the processing of solar cells using some of these compounds and preliminary results of the In2S3 deposition with an overview to its use as buffer layer are presented.

One of the most important issues in kesterite Cu2ZnSnS4 (CZTS)-based thin film solar cells is low open circuit voltage, which is mainly related to loss mechanisms that take place in both CZTS bulk material and CdS/CZTS interface. A device model for CZTS/CdS solar cell which takes into account loss mechanisms influence on solar cell performance is presented. The simulation results showed that our model is able to
reproduce experimental observations reported for CZTS/CdS based solar cells with the highest conversion efficiencies, measured under room temperature and AM1.5 intensity. The comparison of simulation results to experimental observations demonstrated that among the different loss mechanisms,
trap-assisted tunneling losses are the major hurdle to boost open circuit voltage. Under this loss mechanism, a solar cell efficiency enhancement up to 10.2% with CdS donor concentration decrease was reached. Finally, the possible path toward a further solar cell efficiency improvement is discussed.

Low open circuit voltage (Voc) values have been widely reported in kesterite Cu2ZnSnSe4 (CZTSe)-based thin film solar cells. So far, a complete understanding of the main sources of these low performances is far from clear. In this work, a theoretical model for CZTSe solar cell with record efficiency is presented. Among the different device loss mechanisms, trap-assisted tunneling recombination is introduced as the major hurdle to boost Voc values. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations. Finally, it is found that a further solar cell efficiency enhancement of up to 19.4% with an open circuit voltage close to 708 mV can be achieved by using more resistive CdS layers which is in contradiction to p-n junction behavior. In this way, a MIS performance is proposed to promote Voc and efficiency values. As a result, this approach could help to solve at least one of the main issues of this technology.

In this work, a device model for Cu2ZnSnS4 (CZTS) solar cell with certified world record efficiency is presented. A study of the most important loss mechanisms and its effect on solar cell
performance was carried out. The trap-assisted tunneling and CdS/CZTS interface recombination are introduced as the most important loss mechanisms. Detailed comparison of the simulation results to the measured device parameters shows that our model is able to reproduce the experimental observations (quantum efficiency, efficiency, Jsc, FF, and Voc) reported under normal operating conditions. Finally, a discussion about a further solar cell efficiency improvement is addressed.

In this paper, we elaborate on the interpretation and use of photoluminescence (PL) measurements
as they relate to the “donor/acceptor” and “electrostatic potential fluctuations” models for
compensated semiconductors. Low-temperature (7 K) PL measurements were performed on highefficiency
Cu(In,Ga)(S,Se)2 and two Cu2ZnSn(S,Se)4 solar cells with high- and low-S/(SþSe)
ratio, all fabricated by a hydrazine solution-processing method. From excitation-dependent PL, the
total defect density (which include radiative and non-radiative defects) within the band gap (Eg)
was estimated for each material and the consequent depth of the electrostatic potential fluctuation
(c) was calculated. The quasi-donor-acceptor pair (QDAP) density was estimated from the blueshift
magnitude of the QDAP PL peak position in power-dependent PL spectra. As a further verification,
we show that the slope of the lifetime as a function of photon energies (ds/dE) is consistent
with our estimate for the magnitude of c. Lastly, the energetic depth of the QDAP defects is examined
by studying the spectral evolution of the PL as a function of temperature. The shallow defect
levels in CIGSSe resulted in a significant blue-shift of the PL peak with temperature, whereas no
obvious shift was observed for either CZTSSe sample, indicating an increase in the depth of the
defects. Further improvement on Cu2ZnSn(S,Se)4 solar cell should focus on reducing the sub-Eg
defect density and avoiding the formation of deep defects.