Talia S. Gershon

Perovskite-kesterite monolithic tandem solar cells with high open-circuit voltage

We report a monolithic tandem photovoltaic device with earth-abundant solution processed absorbers. Kesterite Cu2ZnSn(S,Se)4 and perovskite CH3NH3PbI3 solar cells were fabricated monolithically on a single substrate without layer transfer. The resulting devices exhibited a high open circuit voltage (Voc) of 1350 mV, close to the sum of single-absorber reference cells voltages and outperforms any monolithic tandem chalcogenide device (including Cu(In,Ga)Se2) reported to date. Ongoing optimization of several device elements including the severely limiting top contact electrode is expected to yield superior currents and efficiency. Importantly, our device architecture demonstrates the compatibility and synergistic potential of two of the most promising emerging photovoltaic materials and provides a path for optimization towards >20% efficiency.

Photoluminescence characterization of a high-efficiency Cu2ZnSnS4 device

We report on low-temperature (4 K) photoluminescence of an 8.3% efficient Cu2ZnSnS4 photovoltaic device. Measurements were recorded as a function of excitation intensity, and the evolution of the resulting spectra is discussed. The spectra indicate that the radiative recombination is characteristic of heavily compensated material with a high quasi donor-acceptor pair density, as determined by the relationship between peak height, peak position, and excitation intensity, as well as the carrier lifetimes at different wavelengths. The blue-shift of the defect-derived peak position is used to estimate the quasi donor-acceptor pair spacing and density. The data indicate an average pair spacing of roughly 3.3 nm, yielding an overall total radiative-defect density of ∼1.3 × 1019 cm−3.

Roadmap on optical energy conversion

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

Ag2ZnSn(S,Se)4: A highly promising absorber for thin film photovoltaics

The growth in efficiency of earth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has slowed, due in part to the intrinsic limitations imposed by the band tailing attributed primarily to I-II antisite exchange. In this study, density functional theory simulations show that when Ag is substituted for Cu to form kesterite Ag2ZnSnSe4 (AZTSe), the I-II isolated antisite formation energy becomes 3.7 times greater than in CZTSSe, resulting in at least an order of magnitude reduction in I-II antisite density. Experimental evidence of an optoelectronically improved material is also provided. Comparison of the low-temperature photoluminescence (PL) structure of Cu(In,Ga)Se2 (CIGSe), CZTSSe, and AZTSe shows that AZTSe has a shallow defect structure with emission significantly closer to the band edge than CZTSe. Existence of suppressed band tailing is found in the proximity of the room-temperature PL peak of AZTSe to its measured band gap. The results are consistent with AZTSe being a promising alternative to CZTSSe and CIGSe for thin film photovoltaics.

The impact of sodium on the sub-bandgap states in CZTSe and CZTS

We compare the optically active sub-bandgap states in polycrystalline Cu2ZnSnSe4 (CZTSe) and Cu2ZnSnS4 (CZTS) thin films as a function of sodium content. In all samples studied, we find that CZTSe has a lower concentration of radiative defect-derived states compared to CZTS and that the states are also shallower in CZTSe compared to CZTS. Further, we find that sodium impacts the relative ratios of two sub-bandgap peaks in the 4 K photoluminescence (PL) spectra of CZTSe (one at ∼0.85 eV and another at ∼0.92 eV). We propose that both of these sub-bandgap peaks stem from intrinsic point defects in CZTSe rather than from electronic states introduced by sodium; this is supported by a measurement on a sodium-free single-crystal of CZTSe. We also show that films with stronger emission through the shallower sub-bandgap states at 4 K display room-temperature PL closer to the bandgap energy. For all sodium quantities studied, one broad PL peak is observed in the 4 K PL spectrum of CZTS which also shifts towards the...

Relationship between Cu2ZnSnS4 quasi donor-acceptor pair density and solar cell efficiency

We examined the 4 K photoluminescence spectra of over a dozen Cu2ZnSnS4 films and eight devices. We show that samples deficient in zinc show on average a higher quasi donor-acceptor pair (QDAP) density. However, the QDAP density in samples with the same metal composition also varies widely. Devices prepared with similar metal compositions show different open-circuit voltages and fill factors. These metrics are correlated with the concentration of QDAPs in the absorbers. One additional device with insufficient zinc showed the empirically observed low-efficiency expected for this composition. This sample also showed the highest quasi donor-acceptor pair density of all the devices measured.

Examination of electronic structure differences between CIGSSe and CZTSSe by photoluminescence study

In this paper, we elaborate on the interpretation and use of photoluminescence (PL) measurements as they relate to the “donor/acceptor” and “electrostatic potential fluctuations” models for compensated semiconductors. Low-temperature (7 K) PL measurements were performed on high-efficiency Cu(In,Ga)(S,Se)2 and two Cu2ZnSn(S,Se)4 solar cells with high- and low-S/(S + Se) ratio, all fabricated by a hydrazine solution-processing method. From excitation-dependent PL, the total defect density (which include radiative and non-radiative defects) within the band gap (Eg) was estimated for each material and the consequent depth of the electrostatic potential fluctuation (γ) was calculated. The quasi-donor-acceptor pair (QDAP) density was estimated from the blue-shift magnitude of the QDAP PL peak position in power-dependent PL spectra. As a further verification, we show that the slope of the lifetime as a function of photon energies (dτ/dE) is consistent with our estimate for the magnitude of γ. Lastly, the energet...

Nanoscale Characterization of Back Surfaces and Interfaces in Thin-Film Kesterite Solar Cells

Combinations of sub 1 μm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine-methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO3/Cu2ZnSn(S,Se)4 (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO3, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO3 demonstrated the presence of large voids within MoO3 enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved.

Modification of defects and potential fluctuations in slow-cooled and quenched Cu2ZnSnSe4 single crystals

Recent literature reports have shown the ability to manipulate Cu-Zn cation ordering for Cu2ZnSnSe4 (CZTSe) via low temperature treatments. Theoretical arguments suggest that one of the major roadblocks to higher VOC—significant band tailing—could be improved with increased cation order; however, few direct measurements have been reported and significant device improvements have not yet been realized. This report investigates electrical properties, defects, and devices from quenched and slow-cooled single crystals of CZTSe. The extent of disorder was characterized by Raman spectroscopy as well as x-ray diffraction, where the change in Cu-Zn order can be detected by a changing c/a ratio. Quenched samples show higher acceptor concentrations, lower hole mobilities, and a lower-energy photoluminescence (PL) peak than crystals cooled at slower rates, consistent with a reduction in the bandgap. In addition, samples quenched at the highest temperatures showed lower PL yield consistent with higher quantities of d...

Flexible kesterite solar cells on ceramic substrates for advanced thermal processing

Flexible CZTSSe solar cells were fabricated on 40-micron thick zirconia substrates compatible with R2R fabrication. Lower thermal budgets by a factor of 25 - 150 and superior temperature stability could not only drastically reduce the energy budget and processing time but also enable the study of radically new annealing profiles such as ultrafast ramping and cooling rates that are impossible on glass. Higher purity than soda lime glass also offers improved control of critical elements such as sodium. Initial tests achieved efficiency of 11.5 % with Voc =452mV, FF =66 % and Jsc = 38 mA/cm2.