Ligang Yuan

Annealing of cuprous oxide (Cu 2O) thin films in vacuum without phase conversion for subsequent inclusion as the channel layer in p-type thin film transistors (TFTs) has been demonstrated. This is based on a systematic study of vacuum annealing effects on the sputtered p-type Cu 2O as well as the performance of TFTs on the basis of the crystallographic, optical, and electrical characteristics. It was previously believed that high-temperature annealing of Cu 2O thin films would lead to phase conversion. In this work, it was observed that an increase in vacuum annealing temperature leads to an improvement in film crystallinity and a reduction in band tail states based on the X-ray diffraction patterns and a reduction in the Urbach tail, respectively. This gave rise to a considerable increase in the Hall mobility from 0.14 cm²/V·s of an as-deposited film to 28 cm²/V·s. It was also observed that intrinsic carrier density reduces significantly from 1.8 × 10¹⁶ to 1.7 × 10¹³ cm⁻³ as annealing temperature increases. It was found that the TFT performance enhanced significantly, resulting from the improvement in the film quality of the Cu 2O active layer: enhancement in the field-effect mobility and the on/off current ratio, and a reduction in the off-state current. Finally, the bottom-gate staggered p-type TFTs using Cu 2O annealed at 700 °C showed a field-effect mobility of ∼0.9 cm²/V·s and an on/off current ratio of ∼3.4 × 10².

External quantum efficiency and transient photocapacitance (TPC) spectra were obtained for perovskite solar cells with methylammonium lead triiodide perovskite absorbers formed by either dip or vapor conversion. These measurements reveal an extended band of sub-gap states in all of the devices studied. The defect band is best fit by a pair of defects, and the appearance of the defect signal in the transient photocapacitance spectra indicates that at least one of the observed defects is in the perovskite absorber. The cells with the largest density of defect states show the lowest short-circuit current density and open-circuit voltage for slow, quasi-steady-state, current density-voltage sweeps and the largest hysteresis in short-circuit current density for fast sweeps. This suggests that defect states in the perovskite absorber limit steady-state device performance, and that these defects or associated mobile charges play a role in the hysteresis observed in current density-voltage measurements.

Ion migration and accumulation in perovskite and interface have recently attracted considerable research interest because it is closely related to carrier extraction and hence to the performance of perovskite solar cells. Here using specific optical probe techniques and perovskite, the authors investigate the effect of light illumination (soaking) at the CH₃NH₃PbI₃/spiro-OMeTAD and CH₃NH₃PbI₃/Phenyl-C61-butyric-acid-methyl ester interfaces, focusing on the dynamics of mobile ions and photoexcited carriers. Time dependent photoluminescence (PL) intensity, electron–hole recombination and optical microscopy images are used to monitor the illumination effects at the interface as a function of light illumination time and intensity. Under continuous illumination, the PL intensity exhibits dynamic quenching in the timescale of seconds to minutes. The authors attribute this PL quenching to the accumulation of mobile ions at the interface during light soaking. Only negative ions cause such PL quenching and the rate at which PL intensity decreases depends on the illumination intensity. The authors found that the accumulated ions also impede the extraction of photogenerated holes from the perovskite layer into spiro-OMeTAD, which increases electron–hole recombination. This investigation provides novel insight into the ion migration mechanism by light soaking and therefore its impact on the operation of a perovskite solar cell.

Light soaking effect at the CH₃NH₃PbI₃/spiro-OMeTAD and CH₃NH₃PbI₃/PCBM interfaces is investigated using time dependent photoluminescence (PL) intensity, time-resolved PL, and optical microscopy images, focusing on the dynamics of mobile ions and photoexcited carriers. Negative ion accumulation is found to result in PL quenching at the interface of perovskite and spiro, which impedes the extraction of photogenerated holes.

Organometallic halide perovskites solar cells are fabricated on nano-scaled corrugated substrates using a sequential deposition method. The corrugated substrates are fabricated using colloidal lithography followed by reactive ion etching. The corrugated structure is found to accelerate the chemical reaction between the sequentially deposited lead iodide (PbI₂) and methyl ammonium iodide layers to form stoichiometric perovskite films, and the corrugated morphology is preserved at the interface of the hole transport layer (HTL) and the perovskite layer. The shunt resistance of the corrugated devices is found to be higher than that of the planar devices, leading to a higher open circuit voltage (V_OC) and fill factor (FF) in the corrugated devices. Finite-difference time-domain simulation is carried out on both planar and corrugated devices. The results revealed that light absorption is enhanced in the corrugated devices due to the corrugated HTL/perovskite interface, resulting in a significantly higher short circuit current (J_SC) observed in the corrugated devices. As a result, the average power conversion efficiency increases from 8.7% for the planar devices to 13% for the corrugated devices.

A simple and scalable method to fabricate nano-scaled corrugations on substrate of perovskite solar cells is demonstrated. Faster chemical reaction between lead iodide and methyl ammonium iodide is observed in the corrugated devices. Optical field enhancements in the corrugated devices are confirmed with the finite-difference time-domain modeling and simulation. As a result, the perovskite solar cells fabricated on corrugated substrates show a 50% enhancement of power conversion efficiency compared to planar devices.