A new approach for alkali incorporation in Cu2ZnSnS4 solar cells

The addition of alkali elements has become mandatory for boosting solar cell performance in chalcogenide thin films based on kesterites (Cu2ZnSnS4, CZTS). A novel doping process is presented here, that consists in the incorporation of sodium or lithium during the deposition of the CdS buffer layer, followed by a post-deposition annealing (PDA). As the doping route leads to more efficient devices in comparison with the undoped reference sample, the influence of PDA temperature was also investigated. Compositional profiling techniques, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and glow discharge optical mission spectroscopy (GDOES), revealed a dependence of the alkaline distribution in kesterites with the PDA temperature. Although the doping process is effective in that it increases the alkaline concentration compared to the undoped sample, the compositional profiles indicate that a significant proportion of Li and Na remains 'trapped' within the CdS layer. In the 200 °C–300 °C range the alkali profiles registered the higher concentration inside the kesterite. Despite this, an additional alkali accumulation close to the molybdenum/fluorine doped tin oxide substrate was found for all the samples, which is frequently related to alkali segregation at interfaces. The addition of both, lithium and sodium, improves the photovoltaic response compared to the undoped reference device. This is mainly explained by a substantial improvement in the open-circuit potential (Voc) of the cells, with best devices achieving efficiencies of 4.5% and 3% for lithium and sodium, respectively. Scanning-electron microscopy images depicted a 'bilayer structure' with larger grains at the top and small grains at the bottom in all samples. Moreover, the calculated bandgap energies of the CZTS films account for changes in the crystallographic order-disorder of the kesterites, more related to the PDA treatment rather than alkali incorporation. Even if further optimization of the absorber synthesis and doping process will be required, this investigation allowed the evaluation of a novel strategy for alkali incorporation in kesterite based solar cells.Peer ReviewedPostprint (published version

Reducing non-radiative recombination in solution-processed kesterite thin-film solar cells

The growing energy demand cannot be further satisfied by burning fossil fuels otherwise, the increase in average temperature above 1.5 °C compared to pre-industrial levels will be inevi-table. A transition to renewable energy sources like solar, wind, water, and geothermal can guarantee sustainable human development. Photovoltaic technologies, enabling direct con-version of light into electricity by means of a device called solar cell, play a crucial role thanks to the abundance of solar energy radiating on the continents of planet earth. Silicon-based solar cells constitute the largest part of the photovoltaic market, but thin-film photovoltaics al-lows for large-area processing and flexible applications while offering a high throughput manufacturing process. Thin-film technologies based on compound semiconductors, such as Cu(In,Ga)Se2 and CdTe, already demonstrate efficiencies higher than 20% at laboratory scale and large-area solar modules are available in the market, but they include elements of limited abundance (In, Ga and Te). In this context, kesterite Cu2ZnSn(S,Se)4 offers a valid alternative as it is based on earth-abundant non-toxic elements, hence it can meet the requirements for large-scale, low-cost production to meet the energy demand. So far, the maximum achieved efficiency for kesterite solar cells amounts to less than 13% due to the low open-circuit voltage (Voc) caused by a large amount of non-radiative recombination losses (∆Voc,nrad). This limitation is also known as Voc-deficit or Voc,SQ-deficit. While the former is defined as the difference between the absorber bandgap (Eg) and the Voc, the latter is the difference between the maximum achievable Voc with respect to the Shockley-Queisser theoretical limit (Voc,SQ) and the Voc. In a typical kesterite solar cell, the kesterite layer interfaces at the bottom with the Mo back electrical contact and at the front with the CdS buffer layer to form the so-called p-n junction followed by a transparent electrical front-contact. While multiple sources of recombination can be found at both interfaces and in the absorber bulk, recent studies help define the mechanisms that predominantly contribute to increasing the ∆Voc,nrad loss. Both interfaces are affected by anion dissociation and consequently by an inhomogeneous chalcogen distribution. This leads to an altered composition at the front interface compared to the absorber bulk, which correlates with high surface recombination. At the back interface, the dissociative reaction between Mo back contact and the kesterite absorber layer leads to the formation of secondary phases, voids, and consequently an inhomogenous absorber mor-phology. The main issues in the bulk are the bandgap fluctuations caused by Cu/Zn sublattice disorder and the presence of defects originating from the multiple oxidation states of Sn and their in-terplay with chalcogen vacancies. The objective of the thesis work was to investigate these recombination mechanisms in kes-terite solar cells and find ways to reduce them for increasing the device efficiency. To reduce the Cu/Zn sublattice disorder, a possible approach is the substitution of either Cu or Zn with an isoelectronic element. In this thesis, Li incorporation was investigated with this purpose. In fact, contrary to heavier alkalis, Li does not segregate along the grain boundaries, but alloys with the kesterite host material by replacing Cu. With the increase in Li content, a variation in lattice parameters, indicating an influence on Cu/Zn disorder, was measured. Con-currently, the minority carrier concentration and consequently the Voc increased. This is corre-lated with lower non-radiative recombination as measured through photoluminescence quan-tum yield (PLQY). Through additional optimizations of the annealing process, a solar cell with 11.8% efficiency was obtained with a Voc,SQ-deficit of 0.365 V. To further reduce the Voc,SQ-deficit, the high surface recombination at the front interface can be reduced by implementing a compositional gradient. For this purpose, H2S post-annealing was carried out on bare CZTSSe absorbers. The procedure successfully incorporated S and led to grain densification close to the surface. However, measurements of the elemental dis-tribution did not show the presence of a compositional gradient, but rather a homogenous in-crease in S throughout the absorber thickness. As a consequence, the influence on Voc,SQ-deficit was negligible. The formation of chalcogen vacancies mentioned above can be hindered by incorporating Te in the absorber layer since it has a lower vapor pressure compared to S/Se. To promote Te alloying with the kesterite material, the following three approaches were investigated: addi-tion of Te pellets in the annealing chamber, evaporation of a Te thin-film on the kesterite pre-cursor before the crystallization process, and spin-coating of a TeCl4 solution during the kes-terite precursor deposition. While few devices obtained with these methods had a lower Voc,SQ-deficit than the baseline, the other PV parameters were always adversely affected lead-ing to a lower efficiency compared to the baseline. The cause is an inferior crystalline quality of the absorber, which influences fill factor and current-density, as well as the squareness of the external quantum efficiency (EQE) curve, which is linked to the collection of the photo-generated charge carriers. This eventually leads to unreliable Eg estimation, thus misleading conclusions about the Voc,SQ-deficit as it depends on the Eg value. The presence and interplay of multiple recombination pathways are a source of uncertainty in the Eg extraction, which is an essential parameter for voltage deficit quantification. Theoretical-ly, a good absorber is also a good emitter, meaning that the radiative and non-radiative re-combination can be assessed without relying on Eg. The combination of electroluminescence with temperature-dependent characterizations of the solar cells allowed to investigate the correlation between Sn content and device performance. The measured increase in efficiency was correlated with an improvement of the interface quality between absorber and buffer layer (CZTSSe/CdS), while the amount of the non-radiative recombination in the absorber in-creased. Another technique applied for the extraction of ∆Voc,nrad was absolute photoluminescence, which allows the estimation of the non-radiative recombination on bare absorbers. The tech-nique was used to evaluate the influence on suppressing the chalcogen dissociation reaction between the Mo back contact and the absorber layer with an Al2O3 intermediate layer for sur-face interface passivation. The investigation was carried out on a special sample design, which allowed the direct comparison of three rear interfaces (glass/CZTSSe, Mo/CZTSSe, Al2O3/CZTSSe) on a single absorber optimized for a device efficiency >10%. The lowest non-radiative recombination loss of about 290 mV was measured for kesterite grown on Mo, while Al2O3 increased the loss up to 350 mV. It appears that the typical fabrication procedure of kesterite absorbers, which requires two separate steps for deposition and crystallization, has a negative influence on the passivating properties of the Al2O3 layer. In the literature, the addi-tion of an Al2O3 intermediate layer is commonly reported to be beneficial, but its influence is assessed through device performance, rather than by direct extraction of ∆Voc,nrad. These find-ings suggest that the Mo back contact is not limiting the performance of the current state-of-the-art kesterite solar cells. From the results presented in the thesis, it appears that the quality of the kesterite bulk mate-rial has the largest influence on the amount of non-radiative recombination. For future work to enhance the efficiency of kesterite solar cells, unambiguous identification of kesterite struc-tural defects and direct quantification of their influence on ∆Voc,nrad should be prioritized over pursuing the approaches of replacing back electrodes and front buffer layer, since the influ-ence of interfaces and layers in the solar cell device structure can overshadow the limitations originating from the absorber bulk

Synaptic transistors with aluminum oxide dielectrics enabling full audio frequency range signal processing

The rapid evolution of the neuromorphic computing stimulates the search for novel brain-inspired electronic devices. Synaptic transistors are three-terminal devices that can mimic the chemical synapses while consuming low power, whereby an insulating dielectric layer physically separates output and input signals from each other. Appropriate choice of the dielectric is crucial in achieving a wide range of operation frequencies in these devices. Here we report synaptic transistors with printed aluminum oxide dielectrics, improving the operation frequency of solution-processed synaptic transistors by almost two orders of magnitude to 50 kHz. Fabricated devices, yielding synaptic response for all audio frequencies (20 Hz to 20 kHz), are employed in an acoustic response system to show the potential for future research in neuro-acoustic signal processing with printed oxide electronics

Influence of the Rear Interface on Composition and Photoluminescence Yield of CZTSSe Absorbers: A Case for an Al2O3 Intermediate Layer

The rear interface of kesterite absorbers with Mo back contact represents one of the possible sources of nonradiative voltage losses (Delta V-oc,V-nrad) because of the reported decomposition reactions, an uncontrolled growth of MoSe2, or a nonoptimal electrical contact with high recombination. Several intermediate layers (IL), such as MoO3, TiN, and ZnO, have been tested to mitigate these issues, and efficiency improvements have been reported. However, the introduction of IL also triggers other effects such as changes in alkali diffusion, altered morphology, and modifications in the absorber composition, all factors that can also influence Delta V-oc,V-nrad. In this study, the different effects are decoupled by designing a special sample that directly compares four rear structures (SLG, SLG/Mo, SLG/Al2O3, and SLG/Mo/Al2O3) with a Na-doped kesterite absorber optimized for a device efficiency >10%. The IL of choice is Al2O3 because of its reported beneficial effect to reduce the surface recombination velocity at the rear interface of solar cell absorbers. Identical annealing conditions and alkali distribution in the kesterite absorber are preserved, as measured by time-of-flight secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy. The lowest Delta V-oc,V-nrad of 290 mV is measured for kesterite grown on Mo, whereas the kesterite absorber on Al2O3 exhibits higher nonradiative losses up to 350 mV. The anticipated field-effect passivation from Al2O3 at the rear interface could not be observed for the kesterite absorbers prepared by the two-step process, further confirmed by an additional experiment with air annealing. Our results suggest that Mo with an in situ formed MoSe2 remains a suitable back contact for high-efficiency kesterite devices