**5. Conclusion**

332 Solar Cells – Thin-Film Technologies

The optical simulations were performed using the Sun*Shine* simulator (Krč et al., 2003) which takes as an input a layered structure with the wavelength dependent complex refraction index coefficients, which comprise the real part *n*(*λ*), called refractive index, and the complex part *k*(*λ*) known as the extinction coefficient. Both are defined in for each layer. For the monograin material we used the complex refraction index coefficients as obtained by Paulson (Paulson et al., 2003) for the thin film Cu(In1-*x*Ga*x*)Se2 alloy with the *x* = 0.66. This corresponds to the energy gap of 1.41 eV. The layer's interfaces were described using the roughness coefficient – *σrms*. In our case we set the *σrms* equal to 100 nm at all interfaces. Simulation of the external quantum efficiency (Fig. 13) shows a good agreement between the measured *QE* and the simulated *QE* in the shorter wavelengths region, while in the middle wavelengths there seems to exist some discrepancy – most probably due to the discrepancy between the measured and modelled *μh,CZTSSe*. The cut-off wavelengths are well pronounced at both temperatures and correspond to the band-gap of 1.4 eV. In the long wavelength region (*λ* > 900 nm) the non-vanishing plateau of the simulated *QE* points to a mismatch in the absorption properties of the thin film CIGS and the monograin layer CZTSSe materials.

> Wavelength [nm] 400 500 600 700 800 900 1000

Fig. 13. Comparison of the measured and simulated external quantum efficiency of the

In Fig. 13 dashed lines represent the simulation and the arrow indicates temperature decrement. The non-vanishing plateau of the simulation originates from the mismatch in the absorption properties of the thin film CIGS (used in the simulation) and the monograin layer

Both, measured and simulated *QE* show that the temperature change does not affect their shape, which inclines us to a conclusion that most of the photogenerated carriers recombine in the SDL and at the SDL/CZTSSe interface. This fact can as well be observed from the

The absorptance simulations show that if all photogenerated carriers originating from the photon flux absorbed in the CZTSSe layer were extracted, the *Jsc* would equal to 37.7 mA/cm2. Taking into account the *SFF* the latter would reduce to a 29.4 mA/cm2. This value

meas., *T* = 310 K sim., *T* = 310 K meas., *T* = 210 K sim., *T* = 210 K

Decreasing *T*

External quantum efficiency

0.0

CZTSSe materials.

AM1.5 illuminated CZTSSe monograin solar cell.

cumulative recombination profile (not shown here).

0.1

0.2

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 [/] We have set up the baseline model of the Cu2SnZn(Se,S)4 monograin layer solar cell, which is able to predict the *J-V* characteristics and the external *QE* of the AM1.5 illuminated MGL solar cell in the temperature range from 310 K to 210 K. The model comprises following material properties:

i) in between the CdS and CZTSSe layers, the highly defective region called the surface defect layer – SDL, comprising a high concentrations of the mid-gap donor defects and a lower concentration of the mid-gap acceptor defects;

ii) in the CZTSSe monograin layer the narrow Gaussian distribution of shallow acceptor traps at 0.05 eV above the valence band and the wider distribution of the compensatory donor traps extending at least 0.3 eV deep into the energy band, relative to the valence band; iii) energy gap of the CZTSSe monograin material equals to 1.4 eV, width of the SCR at 310 K equals to 180–200 nm and the concentration of the apparent doping *pSCR* is in the range from 1x1016 cm-3 to 2x1016 cm-3.

Low *FF* can be attributed to the low CZTSSe hole mobility, which equals to 1.5 cm2/Vs, and to the low apparent doping *pSCR*, which originates from the compensatory effect of the shallow acceptors and deeper donors. Comparison of the flux absorbed in the CZTSSe monograin absorber and the three times lower actual current density of the extracted carriers shows us that further possibilities may reside in the shaping of the collection efficiency of the monograin absorber and/or in the additional passivation of the CdS/CZTSSe interface. Since the former is mainly attributed to the SCR this might not be an easy technological task. Whether these limiting properties are the result of the necessary surface engineering prior to the formation of the CdS/CZTSSe monograin heterojunction or they simply originate from the physical properties of the structure's materials, we were be not able to determine at this point.
