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Open access publications by faculty, staff, postdocs, and graduate students from the Institute of Energy Conversion.
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Item Two-Step Close-Space Vapor Transport of MAPbI3 Solar Cells: Effects of Electron Transport Layers and Residual PbI2(ACS Applied Energy Materials, 2022-09-11) Kuba, Austin G.; Harding, Alexander J.; Richardson, Raphael J.; McCandless, Brian E.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.The effect of the electron transport layer (ETL) on the growth of methylammonium lead iodide (MAPbI3) thin films by two-step close-space vapor transport (CSVT) is reported. Nanocrystalline CdS, as well as amorphous SnO2 and C60, were selected as ETLs on indium tin oxide-coated glass substrates prior to two-step CSVT. The ETL affected the PbI2 growth, leading to different morphological and crystallographic properties. These differences carried over through the methylammonium iodide reaction to the MAPbI3 phase, but compact films with a reasonable morphology could be made on each ETL. The ETL also affected the PbI2-to-MAPbI3 reaction rate. Solar cells processed on each ETL showed a low level of residual PbI2 was important for good photovoltaic conversion efficiency (PCE). The PCEs were similar on average, but trade-offs in J–V parameters depended on the ETL selection. When films on each ETL were reacted beyond an optimal PbI2 residual content, solar cells had lower performance driven by decreases in different J–V parameters. The ETL also had practical effects on J–V performance, namely, hysteresis and air stability. The hysteresis of solar cells on C60 was much less than on SnO2 and CdS. However, the solar cells with C60 ETLs were not stable in air, exhibiting FF and Jsc losses in as little as 15 min of air exposure, while those made on the other ETLs were stable for hours. Thus, the choice of ETL for two-step CSVT affects the growth of PbI2 and its reaction to MAPbI3, but interfacial chemistry considerations and effects on current and atmospheric stability appear to be more important for device performance and yield.Item Air-Induced Conductivity Loss in Fullerene ETLs Can Drive Charge Extraction Losses in Vapor-Deposited Perovskite Solar Cells(ACS Applied Energy Materials, 2024-12-04) Kuba, Austin G.; Santiwipharat, Chaiwarut; Richardson, Raphael J.; Das, Ujjwal K.; Dobson, Kevin D.; Shafarman, William N.The effect of air exposure on all-vapor processed perovskite solar cells using C60 fullerene electron transport layers (ETLs) was investigated. C60 is used in lead halide perovskite solar cells as an ETL to decrease hysteresis and improve stabilized power output. However, air exposure to n-i-p solar cells using C60 ETLs without encapsulation or doping can result in performance degradation due to FF loss and the onset of s-shaped J–V curves. This is correlated to orders of magnitude increase in C60 resistivity upon air exposure. Drift-diffusion simulations provide evidence that a change in the C60 carrier concentration or mobility can lead to the FF loss and s-shaped J–V curve. The degradation does not occur when using inorganic ETLs but does occur in p-i-n architecture using C60 ETLs, further confirming that the C60 layer is the source of the degradation. This is an additional pathway for perovskite solar cell degradation upon air exposure beyond the instability of the perovskite itself. The loss of efficiency can be reduced in p-i-n solar cells using a LiF interlayer, and a better combination of hysteresis and air stability can be achieved in n-i-p solar cells using a C60/SnO2 bilayer ETL.Item Flexible Cation Exchange Environment via Ligand-Free Metal Chalcogenide Thin Films(ACS Nanoscience Au, 2024-11-07) Lacey, Hannah R.; Dobson, Kevin D.; Hernández- Pagán, Emil A.Cation exchange (CE) has emerged as a premier postsynthetic method to carefully tune the chemical composition and properties of nanocrystals with excellent morphology retention. However, reaction conditions are typically dictated by the ubiquitous ligands bound to their surface, limiting their solubility and influencing the thermodynamics/kinetics of the reaction. To bypass these challenges, we report on CE reactions with Cu+, Ag+, Cu2+, Cd2+, Zn2+, and Mn2+ utilizing ligand-free CdS and CuxSey thin films as host templates. The exchange reactions could be performed sequentially or simultaneously (i.e., two guest cations) to access compositionally diverse products. The incorporation of cations on the host films was confirmed using SEM-EDS, XPS, and ICP-MS analyses, as well as tracking wavelength shifts in the UV–vis absorption spectra. The flexibility of this approach was demonstrated as reactions were carried out using an array of different guest precursor salts and solvents with a range of polarities. Moreover, the reactions were generalizable among selenide and sulfide films and proceeded under milder conditions in comparison with reported nanocrystal reactions. A ligand-free environment with flexible reaction conditions, as the work herein, could aid in deconvoluting the different factors involved in CE reactions and further expand its use for fundamental research and applications like photovoltaics, optoelectronics, and catalysis.Item Hydrogen Sulfide Passivation for p-Type Passivated Emitter and Rear Contact Solar Cells(IEEE Journal of Photovoltaics, 2023-12-11) Mouri, Tasnim Kamal; Upadhyaya, Ajay; Rohatgi, Ajeet; Ok, Young-Woo; Upadhyaya, Vijaykumar; Rounsaville, Brian; Hua, Amandee; Hauschild, Dirk; Weinhardt, Lothar; Heske, Clemens; Das, Ujjwal K.This work reports on the application of sulfur (S)-passivation to passivated emitter and rear contact (PERC) solar cells. The emitter surface was passivated by hydrogen sulfide (H 2 S) gas phase reaction and capped by a hydrogenated amorphous silicon nitride (a-SiN x :H) layer. The sulfur passivation on a symmetrically n + diffused emitter is shown to lead to an emitter saturation current density (J 0n+ ) of 30 fA/cm 2 at R sheet,n+ ≈ 100 Ω/sq. The application of S-passivation to the emitter surface in the PERC cell structure, with the rear surface passivated by an aluminum oxide (Al 2 O 3 )/a-SiN x :H stack, showed a promising implied open-circuit voltage (iV OC ) of 686 mV before metallization. This iV OC was higher than that for the a-SiN x :H or SiO 2 /a-SiN x :H passivated emitter surfaces (675 and 674 mV, respectively) on PERC cells processed in the same run. However, a significant drop in cell V OC is observed for the S-passivated PERC cell after the completion of device fabrication with laser patterning, screen-printed metal contact deposition, and firing. Nonetheless, an efficiency of ∼20% and a V OC of ∼650 mV was achieved with an emitter surface passivated by sulfur. We identified that the 760 °C contact firing process degrades the S-passivation quality. The surface morphology was studied, and a detailed surface analysis was performed to study the causes of the S-passivated surface degradation.Item Smart PV Inverter Cyberattack Detection Using Hardware-in-the-Loop Test Facility(IEEE Access, 2023-08-23) Kaewnukultorn, Thunchanok; Sepúlveda-Mora, Sergio B.; Broadwater, Robert; Zhu, Dan; Tsoutsos, Nektarios G.; Hegedus, StevenThis paper evaluates residential smart photovoltaic (PV) inverters’ responses to cyberattacks and assesses the performance of an intrusion detection strategy for smart grid devices by comparing time-series power flow results from a simulation application called Faster Than Real-Time (FTRT) Simulator to measurements from a Power Hardware-in-the-Loop (P-HIL) laboratory as a testbed. Twenty different cyberattacks from three classes - Denial of Service (DoS), Intermittent attack, and Modification - were designed and tested with grid-tied smart inverters in order to study the inverters’ responses to malicious activities. The intrusion detection strategy was developed using a comparison between the predicted PV power output from FTRT and the power flows measured from P-HIL laboratory through the API interface. Real and reactive power thresholds were assigned based on a number of repeated experiments to ensure the applicability of the thresholds. The results showed that inverters from different manufacturers have their own unique responses which could be detected by the power flow measurements. Our detection method could identify over 94% of actual malicious actions and 7.4% of no-attack hours are detected as false positives. Out of 38 under-attack hours, 2 undetected hours are due to the intermittent attacks. Different attacks can be detected based on the targeted components of the complex power that attackers are aiming to cause disturbances. Our findings additionally show that DoS can be noticed immediately after the devices have been sabotaged, and they can be detected from the active power analysis. However, modification attack detection will depend more on the reactive power measurements, while intermittent attacks remain the most challenging for the proposed detection method since the objective of intermittent attacks is to create an oscillation of the complex power components which need a relatively high time resolution for the measurement.Item Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se2 Solar Cells Fabricated using Different Methods(Solar RRL, 2023-06-06) Bothwell, Alexandra M.; Wands, Jake; Miller, Michael F.; Kanevce, Ana; Paetel, Stefan; Tsoulka, Polyxeni; Lepetit, Thomas; Barreau, Nicolas; Valdes, Nicholas; Shafarman, William; Rockett, Angus; Arehart, Aaron R.; Kuciauskas, DariusVoltage losses reduce the photovoltaic conversion efficiency of thin-film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se2. Herein, voltage loss analysis of Cu(In,Ga)Se2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition-driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.Item Inner shell excitation by strong field laser rescattering: optimal laser conditions for high energy recollision(Journal of the Optical Society of America B, 2021-11-15) Kelley, L.; Germain, Z.; Jones, E. C.; Milliken, D.; Walker, Barry C.We address the challenge of finding the optimal laser intensity and wavelength to drive high-energy, strong field rescattering and report the maximum yields of K-shell and LI-shell hole creation. Surprisingly, our results show laser-driven rescattering is able to create inner shell holes in all atoms from lithium to uranium with the interaction spanning from the deep IR to x-ray free electron laser sources. The calculated peak rescattering follows a simple scaling with the atomic number and laser wavelength. The results show it is possible to describe the ideal laser intensity and wavelength for general high-energy laser rescattering processes.