9:10 AM - EN07.11.04
Towards 1V Open-Circuit Voltage and Beyond—Reducing Bulk and Interface Losses in Wide Bandgap Chalcopyrite CuInGaS2 Solar Cell
Mohit Sood1,Shukla Shukla1,Damilola Adeleye1,Michele Melchiorre1,Susanne Siebentritt1
University of Luxembourg1
Bandgap tunability of copper indium gallium disulfide Cu(In,Ga)S2 from 1.55eV (CuInS2) to 2.5eV (CuGaS2) makes it an excellent top cell option for use in a tandem devices. An open-circuit voltage (VOC) of 973 mV and a power conversion efficiency of 15.5% Cu(In,Ga)S2 demonstrates its promises for tandem solar cells in combination with silicon or Cu(In,Ga)Se2. [2,3] However, even the best devices still suffer from a significant VOC deficit (~600 mV) compared to its bandgap owing to non-radiative recombination in the device. Further advancements require a better comprehension of the recombination channels limiting the VOC of the device, whether they lie in the bulk or at the interface.
We present investigation on a composition series of Cu(In,Ga)S2 absorbers grown under different Cu concentration i.e. different [Cu]/[In+Ga] (CGI) ratio with bandgap (Eg) around ~1.6 eV, suitable for combination with record efficiency bottom cells. We have shown in the past that a variation in CGI of CuInS2 drastically influences the recombination in the bulk.  Here we study higher band gap films with a band gap gradient. The opto-electrical properties are probed with photoluminescence measurements, and the electrical properties by current-voltage and capacitance measurements on solar cells fabricated with either CdS or Zn(O,S) buffer. The results of low temperature photoluminescence demonstrate suppression of deep defects as the CGI is decreased from 1.02 to 0.93, consequently leading to a maximum quasi-Fermi level splitting (qFLs) of 972 meV. Current-voltage measurements of devices prepared with CdS and Zn(O,S) buffer layer exhibit efficiencies close to 13 % with a Voc deficit of 650 mV in Cu-poor (CGI<1) Cu(In,Ga)S2 devices. Temperature dependent VOC measurements show the presence of strong front interface recombinations in all devices, independent of buffer layer used, except one. The device prepared from absorber with CGI < 1 using Zn(O,S) buffer layer, temperature dependent VOC measurements show VOC extrapolation to the bandgap of the absorber at 0 Kelvin, demonstrating front interface passivation in the device. Recently front interface passivated Cu(In,Ga)S2 device with high VOC have been demonstrated, although with a lower bandgap. Our study with higher bandgap allows differentiating between the influence of a defective surface layer and unfavorable band alignment. The findings demonstrate that high qFls and VOC is achieved only with an unetched, i.e. undamaged surface and suitable band alignment. In this case, VOC is limited by bulk recombination. We investigate the deep defects responsible. Finally, capacitance-voltage measurements conclude rather low doping values in Cu poor Cu(In,Ga)S2, an increase in which could lead to further improvement in qFLs and hence the VOC of the device and take it to beyond 1 V.
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