Fourier-transform Photocurrent Spectroscopy Research Articles

Evolution of interfacial defects and energy losses during aging of organic photovoltaics

As the power conversion efficiencies of Organic Solar Cells (OSCs) approach 20 %, the stability of the device becomes an increasingly urgent issue. To enhance device stability, it is crucial to identify potential loss mechanisms. In this study, we investigated the trap-state-dependent degradation mechanism of OSCs by directly comparing devices with different hole transport layers (HTLs) that introduce distinct interfacial defect distributions. Employing electrochemical impedance spectroscopy (EIS), Fourier transform photocurrent spectroscopy (FTPS), electroluminescence quantum efficiency (EQEEL), and temperature-dependent J-V techniques, we unraveled the relationship between device degradation and interfacial trap states and energy loss in PM6:Y6 devices. A lower density of interfacial deep traps is evidently correlated with smaller non-radiative recombination losses during the aging of devices based on PEDOT:PSS. Conversely, a higher density of deep traps in aged devices with WS2 interlayers and HTL-free configurations is presumed to be responsible for a significant increase in non-radiative recombination losses. The escalating deep-trap-state density in aged devices is observed to elevate carrier recombination, consequently deteriorating device performance. Beyond the scope of the energy balance theory, an additional factor, probably attributed to the change in the work function of ITO, was found to contribute significantly to energy loss in aged cells, particularly in HTL-free devices. These results highlight the potential for improving device stability via interface engineering.

The native and metastable defects and their joint density of states in hydrogenated amorphous silicon obtained from the improved dual beam photoconductivity method

In this study, undoped hydrogenated amorphous silicon (a-Si:H) thin films deposited under moderate dilution ratios of silane by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) have been investigated using steady-state photoconductivity and improved dual beam photoconductivity (DBP) methods to identify changes in multiple gap states in annealed and light-soaked states. Four different gap states were identified in annealed state named as A, B, C, and X states. The peak energy positions of these Gaussian distributions are consistent with those recently identified by Fourier transform photocurrent spectroscopy (FTPS). After in situ light soaking, their density increases with different rates as peak energy positions and half-widths remain unaffected. The electron-occupied A and B states located below the dark Fermi level and their density and ratios in the annealed and light-soaked states correlate well with those defects detected by time-domain pulsed electron paramagnetic resonance (EPR) experiments. The A, B, and X states located closer to the middle of the bandgap anneal out at room temperature in dark and define the “fast” states. However, the C states show no sign of room temperature annealing such that they must define the “slow” states in undoped a-Si:H. The results found in this study indicate that the anisotropic disordered network is a more appropriate model than previously proposed defect models based on the continuous random network to define the nanostructure of undoped a-Si:H, where multiple defects, D0 and non-D0 defects, can be identified by using the improved DBP method.

Halogen-Free Donor Polymers Based on Dicyanobenzotriazole with Low Energy Loss and High Efficiency in Organic Solar Cells.

Halogenation of organic semiconductors is an efficient strategy for improving the performance of organic solar cells (OSCs), while the introduction of halogens usually involves complex synthetic process and serious environment pollution problems. Herein, three halogen-free ternary copolymer donors (PCNx, x= 3, 4, 5) based on electron-withdrawing dicyanobenzotriazole are reported. When blended with the Y6, PCN3 with strong interchain interactions results in appropriate crystallinity and thermodynamic miscibility of the blend film. Grazing-incidence wide-angle X-ray scattering measurements indicate that PCN3 has more ordered arrangement and stronger π-π stacking than previous PCN2. Fourier-transform photocurrent spectroscopy and external quantum efficiency of electroluminescence measurements show that PCN3-based OSCs have lower energy loss than PCN2, which leads to their higher open-circuit voltage (0.873V). The device based on PCN3 reaches power conversion efficiency (PCE) of 15.33% in binary OSCs, one of the highest values for OSCs with halogen-free donor polymers. The PCE of 17.80% and 18.10% are obtained in PM6:PCN3:Y6 and PM6:PCN3:BTP-eC9 ternary devices, much higher than those of PM6:Y6 (16.31%) and PM6:BTP-eC9 (17.33%) devices. Additionally, this ternary OSCs exhibit superior stability compared to binary host system. This work gives a promising path for halogen-free donor polymers to achieve low energy loss and high PCE.

Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)y thin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI3 solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV–vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag–Bi–I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3–6.6 × 1017 cm–3 and well approximated by a monoexponential decay with a rate constant krec in the range of 2.0–4.3 × 108 s–1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)y compounds, thereby suggesting strong electron–phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7 provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for “AgI-rich” compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.

Quantum criticality of excitonic Mott metal-insulator transitions in black phosphorus

Quantum phase transition refers to the abrupt change of ground states of many-body systems driven by quantum fluctuations. It hosts various intriguing exotic states around its quantum critical points approaching zero temperature. Here we report the spectroscopic and transport evidences of quantum critical phenomena of an exciton Mott metal-insulator-transition in black phosphorus. Continuously tuning the interplay of electron-hole pairs by photo-excitation and using Fourier-transform photo-current spectroscopy as a probe, we measure a comprehensive phase diagram of electron-hole states in temperature and electron-hole pair density parameter space. We characterize an evolution from optical insulator with sharp excitonic transition to metallic electron-hole plasma phases featured by broad absorption and population inversion. We also observe strange metal behavior that resistivity is linear in temperature near the Mott transition boundaries. Our results exemplify an ideal platform to investigating strongly-correlated physics in semiconductors, such as crossover between superconductivity and superfluity of exciton condensation.

Evaluation of Hydrogenated Amorphous Silicon Oxide Photo-Absorber from Quantum Efficiency in Thin-Film Solar Cell by Fourier Transform Photocurrent Spectroscopy

Evaluation of Hydrogenated Amorphous Silicon Oxide Photo-Absorber from Quantum Efficiency in Thin-Film Solar Cell by Fourier Transform Photocurrent Spectroscopy

1,10-Phenanthroline as an Efficient Bifunctional Passivating Agent for MAPbI3 Perovskite Solar Cells.

Passivation is one of the most promising concepts to heal defects created at the surface and grain boundaries of polycrystalline perovskite thin films, which significantly deteriorate the photovoltaic performance and stability of corresponding devices. Here, 1,10-phenanthroline, known as a bidentate chelating ligand, is implemented between the methylammonium lead iodide (MAPbI3) film and the hole-transport layer for both passivating the lead-based surface defects (undercoordinated lead ions) and converting the excess/unreacted lead iodide (PbI2) buried at interfaces, which is problematic for the long-term stability, into "neutralized" and beneficial species (PbI2(1,10-phen)x, x = 1, 2) for efficient hole transfer at the modified interface. The defect healing ability of 1,10-phenanthroline is verified with a set of complementary techniques including photoluminescence (steady-state and time-resolved), space-charge-limited current (SCLC) measurements, light intensity dependent JV measurements, and Fourier-transform photocurrent spectroscopy (FTPS). In addition to these analytical methods, we employ advanced X-ray scattering techniques, nano-Fourier transform infrared (nano-FTIR) spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to further analyze the structure and chemical composition at the perovskite surface after treatment at nanoscale spatial resolution. On the basis of our experimental results, we conclude that 1,10-phenanthroline treatment induces the formation of different morphologies with distinct chemical compositions on the surface of the perovskite film such that surface defects are effectively passivated, and excess/unreacted PbI2 is converted into beneficial complex species at the modified interface. As a result, an improved power conversion efficiency (20.16%) and significantly more stable unencapsulated perovskite solar cells are obtained with the 1,10-phenanthroline treatment compared to the MAPbI3 reference device (18.03%).

Towards Quantitative Interpretation of Fourier-Transform Photocurrent Spectroscopy on Thin-Film Solar Cells

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation.

Study of transport parameters and defect states in thin film perovskites under different environments − air or vacuum − and after light-soaking

We present some advanced characterization techniques developed to investigate on the opto-electronic properties of thin film semiconductors and apply them to perovskite layers. These techniques are the steady state photocarrier grating (SSPG) and the Fourier transform photocurrent spectroscopy (FTPS). The SSPG was developed to study the ambipolar diffusion length of carriers and the FTPS was imagined to measure the variations of the below gap absorption coefficient with the light energy, giving information on the defect densities of the gap responsible for this absorption. The potentialities of these techniques are first detailed and then exemplified by their application to thin film perovskites. To study their stability, these films were exposed to different environments, air or vacuum, and in their as-deposited state or after light-soaking with heavy light. We find that the diffusion length and density of states are quite stable, even after light-soaking, and suggest that the degradation of devices exposed to 1 sun mainly comes from the evolution of the contacts instead of the perovkite itself.

Fine-tuning of the chemical structure of photoactive materials for highly efficient organic photovoltaics

The performance of organic photovoltaics is largely dependent on the balance of short-circuit current density (JSC) and open-circuit voltage (VOC). For instance, the reduction of the active materials’ optical bandgap, which increases the JSC, would inevitably lead to a concomitant reduction in VOC. Here, we demonstrate that careful tuning of the chemical structure of photoactive materials can enhance both JSC and VOC simultaneously. Non-fullerene organic photovoltaics based on a well-matched materials combination exhibit a certified high power conversion efficiency of 12.25% on a device area of 1 cm2. By combining Fourier-transform photocurrent spectroscopy and electroluminescence, we show the existence of a low but non-negligible charge transfer state as the possible origin of VOC loss. This study highlights that the reduction of the bandgap to improve the efficiency requires a careful materials design to minimize non-radiative VOC losses. Materials design rules play a key role in enabling high performance in organic photovoltaics. Here the authors achieve 12.25% efficiency on 1 cm2 non-fullerene solar cells by tuning the side chains’ branching point and the fluorine substitutions in donor and acceptor materials.

Suppression of Recombination Energy Losses by Decreasing the Energetic Offsets in Perylene Diimide-Based Nonfullerene Organic Solar Cells

In this work, a range of nonfullerene organic solar cells comprising two perylene diimide (PDI)-based small molecule acceptors in combination with four representative polymer donors have been investigated and compared. In addition to significant differences in the power conversion efficiency, the energy losses of photovoltaic devices vary widely for these two PDI-based acceptors when paired with different donors. The sensitive Fourier-transform photocurrent spectroscopy (FTPS) and electroluminescence (EL) measurements have been performed to quantify their respective energetic offsets (ΔEoffset) and energy losses, with the aim of understanding the distinct energy losses in the studied organic blends. By comparing these results, we find that with decreasing ΔEoffset both nonradiative recombination loss and radiative recombination loss due to the charge-transfer state absorption are suppressed; as a result, the total energy loss is decreased. These observations offer a deep understanding of how the energetic...

Efficient Nonfullerene Organic Solar Cells with Small Driving Forces for Both Hole and Electron Transfer.

State-of-the-art organic solar cells (OSCs) typically suffer from large voltage loss (Vloss ) compared to their inorganic and perovskite counterparts. There are some successful attempts to reduce the Vloss by decreasing the energy offsets between the donor and acceptor materials, and the OSC community has demonstrated efficient systems with either small highest occupied molecular orbital (HOMO) offset or negligible lowest unoccupied molecular orbital (LUMO) offset between donors and acceptors. However, efficient OSCs based on a donor/acceptor system with both small HOMO and LUMO offsets have not been demonstrated simultaneously. In this work, an efficient nonfullerene OSC is reported based on a donor polymer named PffBT2T-TT and a small-molecular acceptor (O-IDTBR), which have identical bandgaps and close energy levels. The Fourier-transform photocurrent spectroscopy external quantum efficiency (FTPS-EQE) spectrum of the blend overlaps with those of neat PffBT2T-TT and O-IDTBR, indicating the small driving forces for both hole and electron transfer. Meanwhile, the OSCs exhibit a high electroluminescence quantum efficiency (EQEEL ) of ≈1 × 10-4 , which leads to a significantly minimized nonradiative Vloss of 0.24 V. Despite the small driving forces and a low Vloss , a maximum EQE of 67% and a high power conversion efficiency of 10.4% can still be achieved.

Suppressing the Surface Recombination and Tuning the Open-Circuit Voltage of Polymer/Fullerene Solar Cells by Implementing an Aggregative Ternary Compound.

In this work, we present a novel small molecule based on dithienylthienothiadiazole units (named SM1) acting as an efficient component in ternary blend organic solar cells to modify the hole extraction at the interface. Our findings show that the SM1 suppresses the surface recombination and enhances the open-circuit voltage ( Voc). By introducing SM1 in a host system composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl- C61-butyric acid methyl ester (PCBM), we obtained Voc values of up to 0.75 V and fill factors larger than 70% for the ternary blends. As a consequence, the power conversion efficiency is improved by about 30% compared to P3HT:PCBM binary devices. Interestingly, external quantum efficiency and absorption spectra in the near-infrared region do not show any contribution of SM1 in dried films. Instead, the addition of the small molecule improves the Voc by reducing the surface recombination losses. To shed light on the recombination processes, we carried out Fourier-transform photocurrent spectroscopy and impedance spectroscopy measurements. This work shows that the ternary concept can also have functionalities other than photosensitization and can even act as a morphology-directing agent or an interface modifier.

Optical absorption spectra of Cu2ZnSn(S,Se)4 thin-film solar cells by Fourier transform photocurrent spectroscopy

We demonstrated the potential of Fourier transform photocurrent spectroscopy (FTPS) in investigating the optical absorption and quality of a Cu2ZnSn(S,Se)4 (CZTSSe) absorber within the cell structure. We fabricated thin-film solar cell structures with a CZTSSe absorber, which was prepared by a nanoparticle coating technique and a postannealing process. The optical absorption spectra of CZTSSe absorbers prepared at different annealing temperatures were measured by FTPS. We successfully evaluated the optical band-gap energy from the spectra. An exponential tail was found in the spectra. Urbach energy increased with increasing annealing temperature. The deterioration of photovoltaic performance with increasing Urbach energy might be due to potential fluctuation rather than to structural randomness.

Optical absorption spectra of perovskite thin films for defect estimation by photothermal bending spectroscopy

The measurement of the optical absorption coefficient spectra of perovskite thin films of CH3NH3PbI3 by resonant photothermal bending spectroscopy (resonant-PBS) and Fourier-transform photocurrent spectroscopy (FTPS) for the defect estimation of the material has been attempted. A fundamental absorption edge was observed in the photon energy ħω region of 1.5–1.6 eV by both techniques. In addition, extra absorption of about 102 cm−1 was detected at ħω < 1.5 eV only in the spectra measured by resonant-PBS. This extra absorption seems to indicate the existence of localized states in the bandgap of CH3NH3PbI3 films. The difference between resonant-PBS and FTPS profiles below the absorption edge is caused by the difference in measurement mechanism, that is, resonant-PBS utilizes a temperature rise of the sample by the nonradiative recombination of photoexcited carriers, while FTPS measures photocurrent arriving at electrodes by the electric field after the photoexcitation. These results seem to indicate that resonant-PBS is a useful tool for the defect estimation of CH3NH3PbI3.

A simple method to measure intermolecular charge-transfer absorption of organic films

Abstract Intermolecular charge transfer (CT) complexes of organic molecules are actively researched due to their wide use and strong influence on organic optoelectronic device performance. In previous studies, exciplexes, excited-state intermolecular CT complexes, failed to show absorption in ultraviolet-visible-near-infrared absorption spectroscopy measurements. Recently, however, direct exciplex CT absorption from the ground state of donor-acceptor mixed films was detected in the sub-bandgap region by Fourier transform photocurrent spectroscopy (FTPS) and photothermal deflection spectroscopy (PDS) measurements. Here, we report a simple method to measure intermolecular CT absorption that combines simple reflection–transmission and ellipsometry measurements of thick films to determine the extinction coefficient for the small-molecular CT state. The extinction coefficients extracted from different film thicknesses were consistent, up to 10−4 in the sub-bandgap wavelength region, comparable to that obtained with PDS executed on thin films.

Local Observation of Phase Segregation in Mixed-Halide Perovskite

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

Comparison of FTPS Performed on Thin Films and Solar Cells

Fourier Transform Photocurrent Spectroscopy (FTPS) has been used to investigate on the absorption coefficient versus photon energy (α(hν)) spectroscopy of thin film semiconductors. We have developed an FTPS experiment to study the α(hν) spectrum of hydrogenated amorphous silicon (a-Si:H) thin films, to subsequently deduce information on the density of states in the sub bandgap region. In this work we studied whether the same α(hν) spectroscopy could be found both for coplanar a-Si:H films deposited on glass and for NIP diodes incorporating the same material as the intrinsic layer. The back and front contacts of the diodes are made of transparent conductive oxides, either ITO or ZnO. The results show that the Urbach tails of both coplanar and NIP diodes are very similar but that α at low photon energy is slightly lower for the diodes than for the coplanar samples. In addition, we noticed that the diode with ZnO contacts presents a rapid increase of α below 0.95 eV. Transmission-reflection measurements performed on the diodes show that the diode with ZnO contacts exhibits a weaker transmission in the same wavelength range where we observed an increase of α, though the reflection with both types of contacts remains the same, indicating a parasitic absorption of the back ZnO layer. These results show that FTPS measurements performed on diodes must be taken very cautiously and that one has to take into account the optical properties (e.g., absorption) of the contacts to properly deduce reliable α(hν) spectroscopy.

Role of the Fermi level in the formation of electronic band-tails and mid-gap states of hydrogenated amorphous silicon in thin-film solar cells

Hydrogenated amorphous silicon solar cells in p-i-n and n-i-p configurations were made with the intrinsic absorber layer deposited at different temperatures, between 200 and 350 °C. Using Fourier-transform photocurrent spectroscopy, the sub-gap absorption was measured, allowing the evaluation of the band-tail width and mid-gap defect quantity of the intrinsic absorber layer of the working device. When deposited at high temperature (&amp;gt;200 °C), p-i-n cells showed a larger performance decrease than n-i-p cells, along with broader band-tails as well as a larger number of defects created in the absorber layer. Hydrogen content measurements showed that for high temperature deposition (&amp;gt;200 °C), the Si-H bond becomes markedly less stable if the Fermi level of the intrinsic layer is shifted toward the valence band by an adjacent p-layer. Furthermore, by annealing samples at different stages of their layer stack deposition, the impact of the band-tail and mid-gap defect states on the open-circuit voltage and on the fill factor was evaluated. Based on these insights, we propose a model to predict the losses of solar cell parameters.

Photophysics of Molecular‐Weight‐Induced Losses in Indacenodithienothiophene‐Based Solar Cells

The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3‐bis(3‐(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number‐average molecular weight (Mn). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo‐CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier‐transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10−4 cm2 V−1 s−1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.