Effect of Low Temperature on Charge Transport in Operational Planar and Mesoporous Perovskite Solar Cells

Low-temperature optoelectrical studies of perovskite solar cells using MAPbI₃ and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to the overall recombination process should not be overlooked because the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction, and its mean value is independent of the applied illumination intensity. In planar devices, charge extraction declines upon MAPbI₃ transition from a tetragonal to an orthorhombic phase, indicating a connection between the trapping/detrapping mechanism and temperature. Studies on charge extraction by linearly increasing voltage further support this assertion, as charge carrier mobility dependence on temperature follows multiple-trapping predictions for both device structures. The monotonously increasing trend following the rise in temperature opposes the behavior observed in neat perovskite films and indicates the importance of transporting layers and the effect they have on charge transport in fully assembled solar cells. Low-temperature phase transition shows no pattern of influence on thermally activated electron/hole transport

Charge Transport through Electrospun SnO₂ Nanoflowers and Nanofibers: Role of Surface Trap Density on Electron Transport Dynamics

A larger amount of tin precursor was dispersed in electrospun polyvinyl acetate fibers than that required for SnO₂ fiber formation upon annealing, thereby creating a constraint such that all nuclei formed during annealing could not be accommodated within the fiber, which leads to enhanced reaction kinetics and formation of highly crystalline–cum–higher surface area SnO₂ flowers. The flowers are shown to have a lower density of surface trap states than fibers by combining absorption spectra and open circuit voltage decay (OCVD) measurements. Charge transport through the SnO₂ flowers in the presence of the iodide/triiodide electrolyte was studied by OCVD, electrochemical impedance spectroscopy, and transient photodecay techniques. The study shows that the flowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with fibers. Photocurrent transients were used to extract the effective electron diffusion coefficient and mobility which were an order of magnitude higher for the flowers than that for the fibers. The flowers are also shown to have an enhanced Fermi energy, on account of which as well as higher electron mobility, dye-sensitized solar cells fabricated using the SnO₂ flowers gave <i>V</i>_OC ∼700 mV and one of the highest photoelectric conversion efficiencies achieved using pure SnO₂

Enhanced Performance Using an SU‑8 Dielectric Interlayer in a Bulk Heterojunction Organic Solar Cell

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility–lifetime (μ–τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability

Estimation performance of the adopted oscillator ensemble models applied on 100 samples of type B films, picked randomly from the semi-synthetic dataset.

Note that the spectral data are in consistent with linear spectrophotometric measurements.</p

Fig 1 -

A flow diagram of the forward and inverse processes: (a) The forward process (yellow panel) involves an UV-Vis-NIR Spectrometer to measure the reflectance & transmittance {R(λ), T(λ)} resulting from a film of thickness d nm and complex refractive index: n(λ) + ik(λ), (b) The inverse problem (green panel) is to find the desired optical parameters {d, n(λ), k(λ)} from the measured data {R(λ), T(λ)}.</p

Obtaining reflectance and transmittance spectra for different data types.

The film thickness in experimental data are measured using combinations of the process variables (M, rpm). Further details are available in the Data Curation section.</p

Electrospun ZnO Nanowire Plantations in the Electron Transport Layer for High-Efficiency Inverted Organic Solar Cells

Inverted bulk heterojunction organic solar cells having device structure ITO/ZnO/poly­(3-hexylthiophene) (P3HT):[6,6]-phenyl C61 butyric acid methyl ester (PCBM) /MoO₃/Ag were fabricated with high photoelectric conversion efficiency and stability. Three types of devices were developed with varying electron transporting layer (ETL) ZnO architecture. The ETL in the first type was a sol–gel-derived particulate film of ZnO, which in the second and third type contained additional ZnO nanowires of varying concentrations. The length of the ZnO nanowires, which were developed by the electrospinning technique, extended up to the bulk of the photoactive layer in the device. The devices those employed a higher loading of ZnO nanowires showed 20% higher photoelectric conversion efficiency (PCE), which mainly resulted from an enhancement in its fill factor (FF). Charge transport characteristic of the device were studied by transient photovoltage decay and charge extraction by linearly increasing voltage techniques. Results show that higher PCE and FF in the devices employed ZnO nanowire plantations resulted from improved charge collection efficiency and reduced recombination rate

Fig 3 -

Inverse solutions obtained using the synthetic spectral data of metal-oxide films: (a) Actual and estimated spectra with optimized TLO ensemble; total estimation loss = 0.0260, (b) Actual and estimated optical constants with TLO; actual and estimated thickness = 60.0, 60.26 nm, (c) Actual and estimated spectra with optimized GO ensemble; total estimation loss = 0.1011, (d) Actual and estimated optical constants with GO; actual and estimated thickness = 477, 479.71 nm, (e) Actual and estimated spectra with optimized TLO ensemble; total estimation loss = 0.0493, (f) Actual and estimated optical constants with TLO; actual and estimated thickness = 1230, 1233.54 nm.</p

Fig 4 -

Inverse solutions obtained using the semi-synthetic spectral data of MAPbI3 films: (a) Actual and estimated spectra with optimized GO ensemble; total estimation loss = 0.0530, (b) Actual and estimated optical constants with GO; actual and estimated thickness = 62.0, 59.44 nm, (c) Actual and estimated spectra with optimized GO ensemble; total estimation loss = 0.0454, (d) Actual and estimated optical constants with GO; actual and estimated thickness = 169.0, 166.53 nm, (e) Actual and estimated spectra with optimized TLO ensemble; total estimation loss = 0.0335, (f) Actual and estimated optical constants with TLO; actual and estimated thickness = 1350.0, 1359.86 nm.</p

Estimation performance of various methods on experimental data.

In the present spectrophotometry, light beams fall straight (normally) on the films.</p