Yu-Che Hsiao

Fundamental physics behind high-efficiency organo-metal halide perovskite solar cells

Organo-metal halide perovskite solar cells have shown remarkable progress in power conversion efficiencies in the past five years due to some amazing intrinsic properties such as long-range ambipolar transport characteristics, high dielectric constants, low exciton binding energies, and intrinsic ferroelectric polarizations. This review article discusses recent results with the focus on fundamental physics involved in internal photovoltaic processes in perovskite solar cells. The discussion includes charge transport, photoexcited carriers versus excitons, exciton binding energies, ferroelectric properties, and magnetic field effects. The objective of this review article is to provide the critical understanding for materials synthesis and device engineering to further advance photovoltaic actions in the state-of-the-art organo-metal halide perovskite solar cells.

Magneto‐Optical Studies on Spin‐Dependent Charge Recombination and Dissociation in Perovskite Solar Cells

DOI: 10.1002/adma.201405946 high photoexcitation intensity can lead to room-temperature magneto-photoluminescence and magneto-photocurrent with negative and positive signs, respectively, below the fi eld of 200 mT. Our results provide evidence that the charge recombination and dissociation are spin dependent at room temperature in OMHPs. Essentially, our results indicate that applying a magnetic fi eld can suppress the spin mixing between antiparallel and parallel spin states in electron–hole pairs and consequently decreases the antiparallel spin states but increases the parallel spin states. The change between different spin states in electron–hole pairs can eventually modify the exciton formation when the electron–hole pairs relax into excitons. Because of Pauli Exclusion Principle applied onto excitonic states, the singlet and triplet excitons can have high and low annihilation rates. As a result, decreasing the antiparallel spin states in electron–hole pairs by suppressing the spin mixing can weaken the singlet exciton formation, consequently leading to a decrease on photoluminescence toward the development of negative magneto-photoluminescence. On the contrary, decreasing the exciton formation can slow down the consumption on the electron–hole pairs. This can lead to more electron–hole pairs ready for charge dissociation to generate a photocurrent, generating a positive magneto-photocurrent. Clearly, the spin polarizations can be used as a new approach to control the charge recombination and dissociation in OMHPs. Furthermore, by using the observed magneto-photoluminescence and magneto-photocurrent, we investigate the dissociation effects in electron–hole pair states at different excitation intensities to further understand charge recombination and dissociation at different densities at device-operating condition. We fi nd that the critical bias required to completely quench the magneto-photocurrent and magneto-photoluminescence signals is increased with increasing photoexcitation intensity in OMHPs. This implies that, as the charge density increases with increasing the photoexcitation intensity, the formation of electron–hole pairs is enhanced, requiring a stronger fi eld to complete the charge dissociation toward generating photocurrent. Simultaneously, the line-shape on magneto-photocurrent and magnetophotoluminescence shows a narrowing phenomenon with increasing photoexcitation intensity. This indicates that the electron–hole pairs experience mutual inter-pair interactions and consequently changes the internal interactions within each electron–hole pairs. Clearly, our magneto-optical studies can provide critical understanding on controlling spin-dependent charge recombination and dissociation toward improving the photovoltaic actions in perovskite solar cells. Figure 1 a shows the magneto-photocurrents at different excitation intensities from the continuous wave (CW) laser beam of 532 nm in OMHP solar cells with the device architecture of ITO/PEDOT:PSS/CH 3 NH 3 PbI 3– x Cl x /PC 71 BM/TiO x /Al. We Organometal halide perovskites (OMHPs) have recently become extremely attractive photovoltaic candidates to develop higheffi ciency solar cells. The power conversion effi ciencies have quickly increased from 3% up to 20% through materials processing and device engineering efforts. [ 1–8 ] These recent developments have brought a tremendous demand on revealing the intrinsic properties of photoexcited states to further enhance the photovoltaic performance in perovskite solar cells. The OMHPs can be considered as ferroelectrically semiconducting materials with dual spontaneous electric polarizations and photoexcited states. [ 9,10 ] In such ferroelectric semiconducting materials the photoexcited excitons have very low binding energies ranging from 37 to 98 meV, [ 11–13 ] which can cause a quick charge dissociation in picoseconds at room temperature. [ 14,15 ]

Control of morphology and function of low band gap polymer–bis-fullerene mixed heterojunctions in organic photovoltaics with selective solvent vapor annealing

Replacing PCBM with a bis-adduct fullerene (i.e. ICBA) has been reported to significantly improve the open circuit voltage (VOC) and power conversion efficiency (PCE) in P3HT bulk heterojunctions. However, for the most promising low band-gap polymer (LBP) systems, replacing PCBM with ICBA results in very poor short-circuit current (JSC) and PCE although the VOC is significantly improved. Therefore, in this work, we have completed small angle neutron scattering and neutron reflectometry experiments to study the impact of post-deposition solvent annealing (SA) with control of solvent quality on the morphology and performance of LBP–bis-fullerene BHJ photovoltaics. The results show that SA in a solvent that is selective for the LBP results in a depletion of bis-fullerene near the air surface, which limits device performance. SA in a solvent vapor which has similar solubility for polymer and bis-fullerene results in a higher degree of polymer ordering, bis-fullerene phase separation, and segregation of the bis-fullerene to the air surface, which facilitates charge transport and increases power conversion efficiency (PCE) by 100%. The highest degree of polymer ordering combined with significant bis-fullerene phase separation and segregation of bis-fullerene to the air surface is obtained by SA in a solvent vapor that is selective for the bis-fullerene. The resultant morphology increases PCE by 190%. These results indicate that solvent annealing with judicious solvent choice provides a unique tool to tune the morphology of LBP–bis-fullerene BHJ system, providing sufficient polymer ordering, formation of a bis-fullerene pure phase, and segregation of bis-fullerene to the air surface to optimize the morphology of the active layer. Moreover, this process is broadly applicable to improving current “disappointing” LBP–bis-fullerene systems to optimize their morphology and OPV performance post-deposition, including higher VOC and power conversion efficiency.

The impact of selective solvents on the evolution of structure and function in solvent annealed organic photovoltaics

This study examines the development of structure and performance in an organic photovoltaic (OPV) thin film comprised of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Specifically, the influence of P3HT and PCBM solubility in the solvents utilized for vapor annealing on the morphological properties and OPV performance of the resultant active layer is examined. The evolution of P3HT crystallinity as well as the growth of PCBM aggregates in the solvent annealed thin films were examined by Grazing Incidence Wide Angle X-ray Scattering (GIWAXS), Atomic Force Microscopy (AFM), and Energy Filtered Transmission Electron Microscopy (EFTEM). It is shown that P3HT crystallinity increases initially, then decreases with time for solvents that have a finite P3HT solubility. Alternatively, PCBM aggregates grow continuously with SVA, but are modulated by the PCBM solubility. High P3HT crystallinity and moderate PCBM phase separation correlates to improved power conversion efficiency (PCE). Hence, the relative P3HT and PCBM solubility plays a crucial role in choosing the best SVA time of different annealing solvents. Specifically, for samples annealed using solvents that prefer P3HT, PCE benefits from further SVA after the peak P3HT crystallinity time, which is ascribed to additional PCBM phase separation. On the other hand, solvents that prefer PCBM induce excess PCBM phase separation at longer SVA times, which limits exciton dissociation and PCE. EFTEM cross section images indicate that PCBM is distributed toward the bottom of the film, whereas SVA in a solvent with high PCBM solubility may induce PCBM to segregate towards the air surface, which benefits charge transport processes by preventing electron–hole recombination.

Dielectric Interface Effects on Surface Charge Accumulation and Collection towards High-Efficiency Organic Solar Cells

This paper reports the experimental studies on the effects of dielectric thin-film on surface-charge accumulation and collection by using capacitance-voltage (C-V) measurements under photoexcitation. The dielectric thin-films with different surface polarizations are used with inverted device architecture based on the common photovoltaic PTB7:PC71BM film. In the C-V measurements, the peak-voltage shift with light intensity, namely, Vpeak shift, is particularly used to determine the surface-charge accumulation. We find that the Vpeak shows a smaller shift with light intensity when a higher surface polarization of dielectric thin-film is used. This means that a higher surface polarization of dielectric thin-film can decrease the surface-charge accumulation at electrode interface. However, a lower surface polarization of dielectric thin-film leads to a larger shift with light intensity. This implies that a lower surface polarization of dielectric thin-film corresponds to a larger surface-charge accumulation. ...

Addressing dynamic photovoltaic processes at electrode:active layer and donor:acceptor interfaces in organic solar cells under device-operating conditions

This article presents our experimental studies to unravel the dynamic photovoltaic processes occurring at donor:acceptor (D:A) and electrode:active layer (E:A) interfaces under device-operating conditions by using two unique magneto-optical measurements, namely photo-induced capacitance and magnetic field effect measurement. First, we have found that a higher surface polarization of dielectric thin film can decrease the surface charge accumulation at E:A interface. The photo-induced capacitance results indicate that dielectric thin film plays a crucial role in the charge collection in generating photocurrent in organic solar cells. Second, our experimental results from magnetic field effect show that the binding energies of charge transfer (CT) states at D:A interface can be evaluated by using the critical bias required to completely dissociate the CT states. This is the first experimental demonstration that the binding energies of CT states can be measured under deviceoperating conditions. Furthermore, we use our measurement of magnetic field effect to investigate the most popular organic photovoltaic solar cells, organometal halide perovskite photovoltaic devices. The results of magneto-photoluminescence show that the photogenerated electrons and holes are inevitably recombined into electron-hole pairs through a spin-dependent process in the perovskites. Therefore, using spin polarizations can present a new design to control the photovoltaic loss in perovskites-based photovoltaic devices. Also, we found that introducing D:A interface can largely affect the bulk charge dissociation and recombination in perovskite solar cells. This indicates that the interfacial and bulk photovoltaic processes are internally coupled in developing photovoltaic actions in perovskite devices. Clearly, these magneto-optical measurements show a great potential to unravel the deeper photovoltaic processes occurring at D:A and E:A interfaces in both organic bulk-heterojunction and perovskite solar cells under device-operating conditions.

Effects of a ferroelectric interface on thermionic injection-induced cooling in single-heterojunction devices based on thin-film electrode/medium/electrode design

This paper reports experimental studies on the effects of a ferroelectric interface on thermionic cooling in single-heterojunction electrode/medium/electrode thin-film devices by using high-dielectric MoO3 oxide and a ferroelectric P(VDF-TrFE) polymer. We observe a thermionic cooling of 0.10 °C from the single Au/MoO3/ITO device at a low injection current of 0.50 mA cm−2. The experimental studies at different film thicknesses and current densities suggest that this cooling effect is determined by three competing processes: phonon absorption from injected carriers through a thermionic process via charge–phonon interactions, Joule heating from the electrical transport of injected carriers, and heat transfer between charge-injecting and charge-collecting electrodes through phonon conduction. Furthermore, we find that inserting a ferroelectric polymer [P(VDF-TrFE)] interface can largely enhance the thermionic cooling from 0.10 °C to 0.20 °C by a factor of 2 in the Au/P(VDF-TrFE)/MoO3/ITO device at a very low injection current of 0.15 mA cm−2, as compared with the Au/MoO3/ITO device without a ferroelectric interface. Our analysis indicates that the ferroelectric P(VDF-TrFE) interface can decrease the heat transfer between charge-injecting and charge-collecting electrodes due to its low thermal conductivity but still allow a thermionic injection due to its ferroelectric polarization to enhance the cooling effect. Therefore, our work presents a new approach to enhance the thermionic cooling effect by using a ferroelectric interface in organic heterojunction thin-film electronic devices.