Revealing loss and degradation mechanisms in metal halide perovskite solar cells: The role of defects and trap states

For centuries we have relied on fossil fuels to produce energy for our needs, causing significant damage to the environment and our own health. To make an energy transition possible, technology has to step up, providing solutions for cleaner and cheaper energy production. In the field of solar energy, perovskite-based devices can offer a feasible alternative to conventional technologies, involving less energy intensive and cheaper manufacturing processes. Despite the great technological advancements of the past years, open circuit voltage losses and especially poor long-term stability are two of the main bottlenecks that still have to be overcome in order to bring the technology to market. In this thesis we have addressed such issues by investigating the origin and impact of electronic trap states on charge carrier dynamics in perovskite thin films of different composition...ChemE/Opto-electronic Material

Quantifying Charge Carrier Recombination Losses in MAPbI₃/C60 and MAPbI₃/Spiro-OMeTAD with and without Bias Illumination

To increase the open-circuit voltage in perovskite-based solar cells, recombination processes at the interface with transport layers (TLs) should be identified and reduced. We investigated the charge carrier dynamics in bilayers of methylammonium lead iodide (MAPbI3) with C60 or Spiro-OMeTAD using time-resolved microwave conductance (TRMC) measurements with and without bias illumination (BI). By modeling the results, we quantified recombination losses in bare MAPbI3 and extraction into the TLs. Only under BI did we find that the density of deep traps increases in bare MAPbI3, substantially enhancing trap-mediated losses. This reversible process is prevented in a bilayer with C60 but not with Spiro-OMeTAD. While under BI extraction rates reduce significantly in both bilayers, only in MAPbI3/Spiro-OMeTAD does interfacial recombination also increases, substantially reducing the quasi Fermi level splitting. This work demonstrates the impact of BI on charge dynamics and shows that adjusting the Fermi level of TLs is imperative to reduce interfacial recombination losses.ChemE/Opto-electronic Material

Comparing the Calculated Fermi Level Splitting with the Open-Circuit Voltage in Various Perovskite Cells

While the power conversion efficiency of metal halide perovskite (MHP) solar cells has increased enormously, the open-circuit voltage, V oc , is still below the conceivable limit. Here, we derive the Fermi level splitting, μ F , for various types of noncontacted MHPs, which sets a limit for their achievable V oc , using rate constants and mobilities obtained from time-resolved photoconductivity measurements. Interestingly, we find that for vacuum-evaporated MAPbI 3 and K + -doped (MA,FA,Cs)Pb(I/Br) 3 , the μ F /e values are close to the reported V oc values. This implies that for an improvement of the V oc , charge carrier recombination within the bare perovskite has to be reduced. On the other hand, for MHPs with Cs + and/or Rb + addition, the experimental V oc is still below μ F /e, suggesting that higher voltages are feasible by optimizing the transport layers. The presented approach will help to select which techniques and transport layers are beneficial to improve the efficiency of MHP solar cells. ChemE/Opto-electronic Material

Interplay between Charge Carrier Mobility, Exciton Diffusion, Crystal Packing, and Charge Separation in Perylene Diimide-Based Heterojunctions

Two of the key parameters that characterize the usefulness of organic semiconductors for organic or hybrid organic/inorganic solar cells are the mobility of charges and the diffusion length of excitons. Both parameters are strongly related to the supramolecular organization in the material. In this work we have investigated the relation between the solid-state molecular packing and the exciton diffusion length, charge carrier mobility, and charge carrier separation yield using two perylene diimide (PDI) derivatives which differ in their substitution. We have used the time-resolved microwave photoconductivity technique and measured charge carrier mobilities of 0.32 and 0.02 cm2/(Vs) and determined exciton diffusion lengths of 60 and 18 nm for octyl- and bulky hexylheptyl-imide substituted PDIs, respectively. This diffusion length is independent of substrate type and aggregate domain size. The differences in charge carrier mobility and exciton diffusion length clearly reflect the effect of solid-state packing of PDIs on their optoelectronic properties and show that significant improvements can be obtained by effectively controlling the solid-state packing.ChemE/Opto-electronic Material

How Deep Hole Traps Affect the Charge Dynamics and Collection in Bare and Bilayers of Methylammonium Lead Bromide

Wide-band-gap perovskites such as methylammonium lead bromide (MAPB) are promising materials for tandem solar cells because of their potentially high open-circuit voltage, which is yet still far below the maximum limit. The relatively short charge-carrier lifetimes deduced from time-resolved photoluminescence (TRPL) measurements seem in strong contrast with the long lifetimes observed with time-resolved photoconductance measurements. This is explained by a large amount of hole defect states, NT > 1016 cm-3, in spin-coated layers of MAPB residing at or near the grain boundaries. The introduction of hypophosphorous acid (HPA) increases the average grain size by a factor of 3 and reduces the total concentration of the trap states by a factor of 10. The introduction of HPA also increases the fraction of initially generated holes that undergo charge transfer to the selective contact, Spiro-OMeTAD (SO), by an order of magnitude. In contrast to methylammonium lead iodide (MAPI)/SO bilayers, a reduction of the carrier lifetime is observed in MAPB/SO bilayers, which is attributed to the fact that injected holes undergo interfacial recombination via these trap states. Our findings provide valuable insight into the optoelectronic properties of bromide-containing lead halide perovskites essential for designing efficient tandem solar cells.ChemE/Opto-electronic MaterialsChemE/O&O groe

Charge Carrier Dynamics upon Sub-bandgap Excitation in Methylammonium Lead Iodide Thin Films: Effects of Urbach Tail, Deep Defects, and Two-Photon Absorption

To further understand the optoelectronic properties of metal halide perovskites, we investigate sub-bandgap absorption in methylammonium lead iodide (MAPbI3) films. Charge carrier dynamics are studied using time-resolved microwave conductivity measurements using sub-bandgap excitation. From changes in the decay dynamics as a function of excitation energy and intensity, we have identified three regimes: (i) Band-like charge transport at photon energies above 1.48 eV; (ii) a transitional regime between 1.48 and 1.40 eV; and (iii) below 1.40 eV localized optically active defects (8 × 1013 cm-3) dominate the absorption at low intensities, while two-photon absorption is observed at high intensities. We determined an Urbach energy of approximately 11.3 meV, indicative of a low structural and/or thermal disorder. Surprisingly, even excitation 120 meV below the bandgap leads to efficient charge transfer into electron (C60) or hole (spiro-OMeTAD) transport layers. Therefore, we conclude that for MAPbI3, the band tail states do not lead to nonradiative losses. ChemE/Opto-electronic Material

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.ChemE/Opto-electronic MaterialsChemE/O&O groe

Traps in the spotlight: How traps affect the charge carrier dynamics in Cs₂AgBiBr₆ perovskite

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs2AgBiBr6. However, efficiencies of Cs2AgBiBr6-based photovoltaics are still modest. To elucidate the loss mechanisms, in this report, we investigate charge dynamics in Cs2AgBiBr6 films by double-pulse excitation time-resolved microwave conductivity (DPE-TRMC). By exciting the sample with two laser pulses with identical wavelengths, we found a clear photoconductance enhancement induced by the second pulse even 30 μs after the first laser pulse. Modeling the DPE-TRMC results, complemented by photoluminescence and transient absorption, we reveal the presence of deep emissive electron traps, while shallow hole trapping is responsible for the long-lived transient absorption signals. These long-lived carriers offer interesting possibilities for X-ray detectors or photocatalysis. The DPE-TRMC methodology offers unique insight into the times involved in charge trapping and depopulation in Cs2AgBiBr6.ChemE/Opto-electronic MaterialsChemE/O&O groepLarge Scale Energy Storag

Recombination and localization: Unfolding the pathways behind conductivity losses in Cs₂AgBiBr₆ thin films

Cs2AgBiBr6 (CABB) has been proposed as a promising nontoxic alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this work, we study the optoelectronic properties of CABB thin films using steady state and transient absorption and reflectance spectroscopy. We find that optical measurements on such thin films are distorted as a consequence of multiple reflections within the film. Moreover, we discuss the pathways behind conductivity loss in these thin films, using a combination of microsecond transient absorption spectroscopy and time-resolved microwave conductivity measurements. We demonstrate that a combined effect of carrier loss and localization results in the conductivity loss in CABB thin films. Moreover, we find that the charge carrier diffusion length and grain size are of the same order of magnitude. This suggests that the material's surface is an important contributor to charge-carrier loss.ChemE/Opto-electronic Material