Time resolved photoluminescence on Cu(In, Ga)Se-2 absorbers: Distinguishing degradation and trap states

Recent reports have suggested that the long decay times in time resolved photoluminescence (TRPL), often measured in Cu(In, Ga)Se-2 absorbers may be a result of detrapping from sub-bandgap defects. In this work, we show via temperature dependent measurements, that long lifetimes >50 ns can be observed that reflect the true minority carrier lifetime not related to deep trapping. Temperature dependent time resolved photoluminescence and steady state photoluminescence imaging measurements are used to analyze the effect of annealing in air and in a nitrogen atmosphere between 300K and 350K. We show that heating the Cu(In, Ga)Se-2 absorber in air can irreversibly decrease the TRPL decay time, likely due to a deterioration of the absorber surface. Annealing in an oxygen-free environment yields a temperature dependence of the TRPL decay times in accordance with Schockley Read Hall recombination kinetics and weakly varying capture cross sections according to T-0.6. Published by AIP Publishing

Chemistry and Dynamics of Ge in Kesterite : Toward Band-Gap- Graded Absorbers

peer reviewe

Development and characterization of nanoparticle-based kesterite solar cells

Kesterite solar cells based off of the Cu2ZnSn(S,Se) 4 (CZTSSe) material system are a promising technology for earth-abundant photovoltaic applications. Thin-film absorbers fabricated from kesterite materials demonstrate a tunable band gap ideal for photovoltaic applications, high absorption coefficient ideal for low material usage, and success with low-cost, scalable, solution-based processing techniques. However, CZTSSe solar cells achieve low power-conversion efficiencies relative to other successful solar absorber technologies with cubic/tetragonal crystal structure, such as c-Si, GaAs, CdTe, and Cu(In,Ga)Se2. In this work, the development of kesterite-based solar cells from nanocrystal inks is discussed in terms of understanding and improving device performance for this material system. Herein, an overview of CZTSSe solar technology, nanoparticle synthesis, thin film formation, and optoelectronic characterization is presented, illustrating the challenges associated with the formation of high-quality quaternary semiconductors. Detailed optoelectronic and material characterization is used to develop an understanding of the fundamental limitations to improved device performance, with general applicability to the CZTSSe material system; namely, detailed analysis of the origin of voltage limitations commonly reported for CZTSSe solar cells is considered. Fundamental device performance limitations are associated here with a high propensity for detrimental defect formation in CZTSSe absorbers. Ultimately, an improved understanding of these device performance limitations and their association with the absorber material properties is used to develop novel thermal processing techniques to improve the absorber transport properties. Optimized absorber processing has resulted in the fabrication of record performance nanocrystal-based CZTSSe solar cells. In addition to novel CZTSSe absorber processing, modification of the absorber properties is also presented in terms of alloyed-kesterite materials based off of the CZTSSe material system. Namely, the development of Ge-alloyed Cu2Zn(Sn,Ge)(S,Se)4 (CZTGeSSe) and Ag-alloyed (Ag,Cu)ZnSnSe4 (ACZTSe) absorbers is described. CZTGeSSe and ACZTSe are shown to demonstrate promise as alternative kesterite material absorbers due to improved optoelectronic properties and device performance relative to CZTSSe. Record device performance is reported here for CZTGeSSe and ACZTSe solar cells. Lastly, detailed optoelectronic characterization—including current-voltage, quantum efficiency, and capacitance spectroscopy—is used to develop novel analysis methodologies for non-ideal diodes. This work demonstrates how traditional optoelectronic characterization analysis techniques, commonly applied to CZTSSe solar cells, can result in misinterpretation of device limitations if non-ideal diode behavior is not accounted for in the diode analysis. Here, a consideration of the complex material properties in the developed optoelectronic characterization analysis methods has resulted in improved accuracy in understanding fundamental device performance limitations in kesterite solar cells

Evaluation of recombination losses in thin film solar cells using an LED sun simulator − the effect of RbF post-deposition on CIGS solar cells

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses

Atomic Scale Structure of (Ag,Cu)2_{2}ZnSnSe4_{4} and Cu2_{2}Zn(Sn,Ge)Se4_{4} Kesterite Thin Films

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both (Ag,Cu)$_{2}$ZnSnSe$_{4}$ and Cu$_{2}$Zn(Sn,Ge)Se$_{4}$ based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X-ray diffraction and low temperature X-ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu$_{2}$Zn(Sn,Ge)Se$_{4}$, the thin film absorbers show a very similar behavior as Cu$_{2}$Zn(Sn,Ge)Se$_{4}$ powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn-S bonds instead of Ge-S bonds. In (Ag,Cu)$_{2}$ZnSnSe$_{4}$, the dependence of the Ag-Se and Cu-Se bond lengths on Ag/(Ag+Cu) might indicate an energetic advantage in the formation of certain local configurations

Evaluation of recombination losses in thin film solar cells using an LED sun simulator − the effect of RbF post-deposition on CIGS solar cells

Distinguishing among different electrical loss mechanisms − such as interface and bulk recombination − is a common problem in thin film solar cells. In this work, we report a J–V measurement technique using different illuminating spectra to distinguish between these two recombination losses. The basic idea is to change the relative contribution of bulk recombination to the total losses of photo-generated charge carriers by generating them in different depths within the absorber layer using different spectral regions of the illuminating light. The use of modern LED sun-simulators allows an almost free design of illumination spectra at intensities close to 1 sun. The comparison of two simple J–V measurements, one recorded with illumination near the absorber's band-gap energy and one with light of higher energy, in combination with supporting measurements of the absorber properties, as well as device modeling, enables the extraction of the diffusion length and the interface recombination velocity. Using this technique, we show that in CIGS solar cells, an RbF post-deposition treatment does not only reduce interface recombination losses, as often reported, but also reduces bulk recombination in the CIGS absorber. Furthermore, we find that both cells, with and without RbF treatment, are dominantly affected by interface recombination losses