(34e) Improved Performance of Earth-Abundant Cu2ZnSn(SxSe1-x)4 Solar Cells Through Ge Incorporation

Charles Hages and  Rakesh Agrawal

 School Of Chemical Engineering, Purdue University, West Lafayette, IN, USA

Advancements in thin film Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) solar cells have recently achieved power conversion efficiencies of >10%, indicating the potential of this low cost, earth abundant material system as a viable alternative to copper indium gallium diselenide (CIGSe) and CdTe absorbers¹.  The initial appeal of CZTSSe as a viable solar p-type material comes from its ideal band gap of 1.0 to 1.5 eV.  Band gap tuning in current high efficiency devices in literature has been optimized mainly through control of the S/(S+Se) ratio in CZTSSe.  However, current research has recently shown the additional ability for tuning  band gap of CZTSSe through partial substitution of Sn for Ge into the crystal lattice forming Cu₂Zn(Sn_yGe_1-y)(S_xSe_1-x)₄ (CZTGeSSe)^2,3.  This presentation will introduce material and electrical characterization of both CZTGeS synthesized nanoparticles as well as CZTGeSSe sintered films as a function of Ge incorporation.

Powder x-ray diffraction (PXRD) has shown that control of the reaction synthesis procedure allows synthesized CZTGeS nanoparticles to maintain tetragonal structure (kesterite/stannite) throughout all ratios of Ge incorporation.  Current work suggests the importance of the tetragonal (kesterite) crystal structure for enhanced photovoltaic device performance, including CZGeS where the wurtzite phase has been calculated to have increased stability.  Raman spectroscopy has also been measured for CZTGeSSe to understand changes in the main Raman shift mode of CZTSSe as Ge is incorporated into the crystal lattice.  In addition, Raman spectroscopy has been used to help understand and identify the formation of binary and ternary phases in CZTGeSSe nanoparticles and sintered films.

Electrical characterization of CZTGeSSe devices has shown a linear increase in open-circuit voltage (Voc) as the Ge content in increased.  The current optimization at 30% Ge/(Sn+Ge) has been achieved through a balance of increased Voc while maintaining a sufficiently high photo-generated current (Jph) as the band gap in increased with Ge content.  Further analysis is under way using photoluminescence to quantify changes in optical absorption and band gap as a function of Ge incorporation.

Further analysis of temperature dependent Voc has shown the CdS/CZTGeSSe heterojunction interface as a source of limiting recombination in the measured devices.  In addition to this, elemental losses of Ge at the interface have been found to significantly contribute to measured Voc limitations in CZTGeSSe devices, further suggesting the importance of the buffer/absorber interface for improvements in device performance.  Changes in the reaction synthesis procedure as well device fabrication techniques have proven the ability for decreased elemental losses at the interface, with direct positive improvements seen in Voc measurements.

In addition to band gap optimization, Ge incorporation into CZTSSe has been used to minimize the unfavorable conduction band offset between CdS/CZTSSe.  External quantum efficiency measurements have shown increased carrier collection efficiency due to a decrease in the conduction band offset as Ge incorporation is increased to 30%.  This work demonstrates the importance of both S/Se control as well as Ge/Sn control to optimize the band gap as well achieve favorable band alignments.

[1] D. A. R. Barkhouse et al., “Device characteristics of a 10.1% hydrazine-processed Cu2ZnSn(Se,S)4 solar cell”, Prog. Photovolt: Res. Appl., 20, 2012, pp. 6-11.

[2] G. M. Ford et al., “Earth Abundant Element Cu2Zn(Sn1-xGe)S4 Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication”, Chem. Mater., 23, 2011, pp. 2626-2629. 3.

[3] Q. Guo et al., “A generalized and robust method for efficient thin film photovoltaic devices from multinary sulfide nanocrysatal inks”, Photovoltaic Specialists Conference (PVSC), 2011 37^th IEEE. pp. 003522-003526.