Charles J. Hages

Generalized current-voltage analysis and efficiency limitations in non-ideal solar cells: Case of Cu2ZnSn(SxSe1−x)4 and Cu2Zn(SnyGe1−y)(SxSe1−x)4

Detailed electrical characterization of nanoparticle based Cu2ZnSn(SxSe1−x)4 (CZTSSe) and Cu2Zn(SnyGe1−y)(SxSe1−x)4 (CZTGeSSe) solar cells has been conducted to understand the origin of device limitations in this material system. Specifically, temperature dependent current-voltage analysis has been considered, with particular application to the characterization of solar cells with non-ideal device behavior. Due to the presence of such non-ideal device behavior, typical analysis techniques—commonly applied to kesterite-type solar cells—are found to be insufficient to understand performance limitations, and an analysis methodology is presented to account for the non-idealities. Here, the origin of non-ideal device behavior is chiefly considered in terms of electrostatic and band gap potential fluctuations, low minority carrier lifetimes, temperature dependent band edges, high surface/bulk recombination rates, and tunneling enhanced recombination. For CZTSSe and CZTGeSSe, the main limitations to improved dev...

Generalized quantum efficiency analysis for non-ideal solar cells: Case of Cu2ZnSnSe4

Detailed quantum efficiency (QE) analysis of a nanoparticle-based Cu2ZnSnSe4 (CZTSe) solar cell has been conducted to understand photogenerated carrier collection in the device. Specifically, voltage-dependent analysis has been considered to characterize both diffusion limitations and recombination limitations to carrier collection. Application of a generalized QE model and corresponding experimental and analytical procedures are presented to account for non-ideal device behavior, with specific consideration of photogenerated charge trapping, finite absorber thickness, back-surface recombination, and recombination of photogenerated carriers via interface, space-charge-region limited, and/or band tail limited recombination mechanisms. Analysis of diffusion limited collection results in extraction of the minority carrier diffusion length, mobility, back surface recombination velocity, and absorption coefficient. Additionally, forward bias QE measurements afford analysis of the dominant recombination mechanism for photogenerated carriers. For the analyzed CZTSe device, diffusion limitations are not expected to play a significant role in carrier collection in forward bias. However, voltage-dependent carrier collection, previously identified to contribute to open-circuit voltage limitations, is attributed to high recombination rates via band tail states/potential fluctuations in forward bias. A consideration of the assumptions commonly applied to diffusion length, band gap, and band tail extraction is also discussed.

The importance of band tail recombination on current collection and open-circuit voltage in CZTSSe solar cells

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells typically exhibit high short-circuit current density (Jsc), but have reduced cell efficiencies relative to other thin film technologies due to a deficit in the open-circuit voltage (Voc), which prevent these devices from becoming commercially competitive. Recent research has attributed the low Voc in CZTSSe devices to small scale disorder that creates band tail states within the absorber band gap, but the physical processes responsible for this Voc reduction have not been elucidated. In this paper, we show that carrier recombination through non-mobile band tail states has a strong voltage dependence and is a significant performance-limiting factor, and including these effects in simulation allows us to simultaneously explain the Voc deficit, reduced fill factor, and voltage-dependent quantum efficiency with a self-consistent set of material parameters. Comparisons of numerical simulations to measured data show that reasonable values for the band tail parameters (charact...

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)Se2 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 300 K and 350 K. We show that heating the Cu(In, Ga)Se2 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 T0.6.

Device comparison of champion nanocrystal-ink based CZTSSe and CIGSSe solar cells: Capacitance spectroscopy

Capacitance spectroscopy has been used to compare charge carrier and defect properties of champion nanocrystal-ink based CZTSSe and CIGSSe solar cells, with efficiencies reported here at 9.2% and 14.2%, respectively. Differences in energy level, frequency/temperature response, and contributions to bulk conductivity have been identified for the different materials. Due to these differences, contributions to the free carrier density have been associated with a single defect for CIGSSe, while associated with two defects in CZTSSe. Additionally, carrier freeze-out out at low temperatures has been identified for both devices, contributing to increasing series resistance at low temperatures as determined from the bulk conductivity. In addition to differences in defect formation, CZTSSe has been characterized with a reduced Vbi when compared to CIGSSe.

The role of interparticle heterogeneities in the selenization pathway of Cu–Zn–Sn–S nanoparticle thin films: a real-time study

Real-time energy dispersive X-ray diffraction (EDXRD) analysis has been utilized to observe the selenization of Cu–Zn–Sn–S nanoparticle films coated from three nanoparticle populations: Cu- and Sn-rich particles roughly 5 nm in size, Zn-rich nanoparticles ranging from 10 to 20 nm in diameter, and a mixture of both types of nanoparticles (roughly 1 : 1 by mass), which corresponds to a synthesis recipe yielding CZTSSe solar cells with reported total-area efficiencies as high as 7.9%. The EDXRD studies presented herein show that the formation of copper selenide intermediates during the selenization of mixed-particle films can be primarily attributed to the small, Cu- and Sn-rich particles. Moreover, the formation of these copper selenide phases represents the first stage of the CZTSSe grain growth mechanism. The large, Zn-rich particles subsequently contribute their composition to form micrometer-sized CZTSSe grains. These findings enable further development of a previously proposed selenization pathway to account for the roles of interparticle heterogeneities, which in turn provides a valuable guide for future optimization of processes to synthesize high quality CZTSSe absorber layers.

Analysis of temperature-dependent current-voltage characteristics for CIGSSe and CZTSSe thin film solar cells from nanocrystal inks

Thin film solar cells with CIGSSe and CZTSSe absorber layers fabricated from nanocrystal inks represent economically scalable technologies for alternative sources of energy. Although these two materials share similar properties important to functioning as a photovoltaic absorber, lab scale CIGSSe devices have achieved power conversion efficiencies 1.5 to 2 times higher than their CZTSSe counterparts. In the current work, CIGSSe and CZTSSe devices similarly processed from nanocrystal inks and exhibiting efficiencies of 14.2% and 9.2%, respectively, are characterized by temperature-dependent current-voltage (IVT) analysis to reveal limitations to CZTSSe device performance compared to CIGSSe.

Device limitations and light-soaking effects in CZTSSe and CZTGeSSe

Advancements in thin film Cu2ZnSn(SxSe1-x)4 (CZTSSe) solar cells have recently achieved power conversion efficiencies >;10%, indicating the potential of this low cost, earth abundant material system as a viable alternative to CIGS and CdTe absorbers [1]. Understanding the limitations in this material system is essential for further advancements in device performance. This work demonstrates the importance of the CdS/CZTSSe interface and conduction band alignment for improving the performance of CZTSSe solar cells. Through incorporation of germanium into CZTSSe, a material with tunable band gap and conduction band level can be achieved. Comparing Cu2Zn(SnyGe1-y)(SxSe1-x) (CZTGeSSe) with standard CZTSSe devices allows for characterization of device limitations as a function of band gap and the conduction band offset at the CdS/CZTSSe interface. This work characterizes the effect of Ge incorporation on recombination, series resistance, and quantum efficiency in CZTSSe.

Characterization of nanocrystal-ink based CZTSSe and CIGSSe solar cells using voltage-dependent admittance spectroscopy

Voltage-dependent admittance spectroscopy has been applied nanocrystal-ink based CZTSSe and CIGSSe solar cells to understand the origins of admittance signatures characterized for devices fabricated from this method; devices considered here have demonstrated champion device efficiencies of 9.0% and 14.2%, respectively. Two admittance signatures characterized for CZTSSe have been attributed to two bulk defect levels; however, a single admittance signature characterized for CIGSSe is expected to be due to a Schottky barrier at the back contact. Additionally, punch-through due to the free carrier freeze-out at low temperatures only occurs in CZTSSe, which suggests a shallower defect contributing to the carrier density in CIGSSe than CZTSSe. Results are compared with that reported for CIGSSe and CZTSSe devices fabricated from other processing techniques.

The physics of V bi -related IV crossover in thin film solar cells: Applications to ink deposited CZTSSe

IV measurements of thin film solar cells often show a crossover between the illuminated and dark curves. Crossover can occur for several different reasons. In this paper, we explore crossover in CZTSSe solar cells fabricated using nanocrystalline ink deposition with selenization and compare it to crossover in ink-based CIGSSe solar cells. Crossover in CIGSSe appears to be related to traps, as is commonly observed, but crossover in CZTSSe appears to be due to a different mechanism. Using numerical simulation, we show that crossover can arise from a simple explanation that is common to solar cells with different structures and closely related to the built-in potential of the device. Using IVT and CV measurements, we show that both simulations and experimental analysis point to a cross-over voltage in our CZTSSe cells that is directly related to the built-in voltage of the device, and which may play a role in limiting the open-circuit voltage.