ZiQi Sun

The work functions for charge transport layers in perovskite solar cells affect device performance significantly. In this work, the regular poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is modified by adding a polymer electrolyte PSS-Na to improve its HTL function in perovskite solar cells. The modified PEDOT:PSS films (called m-PEDOT:PSS) possess higher work function than the regular one. Its energy level matches the valence band of perovskite very well, leading to enhanced V_oc and PCE (power conversion efficiency). When CH₃NH₃PbI₃ is used as the light absorber, the cell with PEDOT:PSS HTL gives a V_oc of 0.96 V and a PCE of 12.35%, while the cell with m-PEDOT:PSS layer gives a V_oc of 1.11 V and a PCE of 15.56%. Enhanced V_oc and PCE are also achieved when CH₃NH₃PbI₂Br or CH₃NH₃PbBr₃ is used as the light absorber. The m-PEDOT:PSS/CH₃NH₃PbBr₃/PC₆₁BM solar cells demonstrate an outstanding V_oc of 1.52 V.

A modified poly(3,4-ethylenedioxythiophene) (PEDOT) layer is developed and used as the HTL for perovskite solar cells, leading to enhanced performance. Using m-PEDOT:PSS (1:2) as the HTL and CH₃NH₃PbI₃ as the light absorber, a V_oc of 1.11 V and a power conversion efficiency of 15.56% are achieved. A V_oc of 1.52 V is obtained from CH₃NH₃PbBr₃ solar cells, which is the highest V_oc for perovskite/PCBM solar cells.

Scanning nanofocus X-ray diffraction (nXRD) performed at a synchrotron is used to simultaneously probe the morphology and the structural properties of spin-coated CH₃NH₃PbI₃ (MAPI) perovskite films for photovoltaic devices. MAPI films are spin-coated on a Si/SiO₂/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) substrate held at different temperatures during the deposition in order to tune the perovskite film coverage. The films are then investigated using nXRD and scanning electron microscopy (SEM). The advantages of nXRD over SEM and other techniques are discussed. A method to visualize, selectively isolate, and structurally characterize single perovskite grains buried within a complex, polycrystalline film is developed. The results of nXRD measurements are correlated with solar cell device measurements, and it is shown that spin-coating the perovskite precursor solution at elevated temperatures leads to improved surface coverage and enhanced solar cell performance.

Scanning nanofocus X-ray diffraction is used to simultaneously probe the morphology and the structural properties of spin-coated CH₃NH₃PbI₃ perovskite films. The technique allows for perovskite grain segmentation based on a variety of structural properties and is used to understand why spin-coating the perovskite precursor solution at elevated temperature leads to improved surface coverage and enhanced solar cell performance.

A simple yet general swelling–deswelling microencapsulation strategy has been developed to achieve well dispersed and intimately passivated crystalline organic–inorganic perovskites nanoparticles within polymer matrixes and results in a series of highly luminescent CH₃NH₃PbBr₃ (MAPbBr₃)–polymer composite films with unprecedented water and thermal stabilities and superior color purity.

Despite the rich experience gained in controlling the microstructure of perovskite films over the past several years, little is known about how the microstructure affects the device properties of perovskite solar cells (HPSCs). In this work, the effects of the perovskite film microstructure on the charge recombination and light-soaking phenomenon in mixed halide HPSCs are investigated. Devices with noncompact perovskite morphology show a severe light soaking effect, with the power conversion efficiency (PCE) improved from 3.7% to 11.6% after light soaking. Devices with compact perovskite morphology show a negligible light soaking effect, with PCE slightly increased from 11.4% to 11.9% after light soaking. From device investigations, photoluminescence, and impedance spectroscopy measurements, it is demonstrated that interface electron traps at the grain boundaries as well as at the crystal surface dominate the light soaking effect. Severe trap-assisted recombination takes place in HPSCs using noncompact films, while it is effectively eliminated in devices with compact films. Moreover, how the grain size of the perovskite film affects the light soaking phenomenon is investigated. In the case of compact perovskite films, the size of the grains has a limited effect on the light soaking. In these compact films, grains are fused and trap states are effectively reduced.

The effects of the perovskite film microstructure on the charge recombination and light-soaking phenomenon are investigated in hybrid perovskite solar cells (HPSCs). It is demonstrated that interface electron traps at the grain boundaries as well as at the crystal surface dominate the light soaking effect.

The role of excitons on the amplifications of lead halide perovskites has been explored. Unlike the photoluminescence, the intensity of amplified spontaneous emission is partially suppressed at low temperature. The detailed analysis and experiments show that the inhibition is attributed to the existence of exciton and a quantitative model has been built to explain the experimental observations.

The molecular structure of pyridine derivatives is critical to perovskite solar cell performance, especially stability. Most of the pyridine additives easily form complexes with perovskite. A new pyridine additive with a long alkyl chain substituted at its o-position does not corrode perovskite. The stability of devices containing this additive is the highest among the investigated cells.

The coordination effects of additives during perovskite crystal growth are investigated, and a novel technique to fabricate high-quality perovskite thin films by introduction of weak coordination additives (e.g., acetonitrile) in the precursors is demonstrated.

Influence of light exposure on cesium lead halide nanostructures has been explored. A discovery of photon driven transformation (PDT) in 2D CsPbBr₃ nanoplatelets is reported, in which the quantum-confined few-monolayer nanoplatelets will convert to bulk phase under very low irradiation intensity (≈20 mW cm⁻²). Benefiting from the remarkable emission color change during PDT, the multicolor luminescence photopatterns and facile information photo-encoding are established.

A highly conductive, air stable and scalable poly(3,4-ethylenedioxythiophene) (PEDOT): poly(4-styrenesulfonate) (PEDOT:PSS) are prepared by using mass production ultrafiltration. By effectively removing excess PSS and various reaction impurities using repeated 100 nm pore membrane filtration, purified PEDOT:PSS exhibit conductivity as high as 2000 S cm⁻¹.

Benzylamine is introduced as a surface passivation molecule that improves the moisture-resistance of the perovskites while simultaneously enhancing their electronic properties. Solar cells based on benzylamine-modified formamidinium lead iodide perovskite films exhibit a champion efficiency of 19.2% and an open-circuit voltage of 1.12 V. The modified FAPbI₃ films exhibit no degradation after >2800 h air exposure.

The stability of single-crystalline/monocrystalline-like perovskite film is expected to be better than its microcrystalline counterparts. In the present work, highly orientated perovskite thin films (CH₃NH₃PbI_3–xCl_x) are prepared by means of aquointermediates assisted solution process. It displays super-duper preferred-orientation along <110> direction that is close to the single crystal, and its diffraction intensity ratio of (110)/(310) is nearly two orders of magnitude higher in contrast to the films that prepared by traditional way. Owing to its superior performances, e.g., highly crystallized quality, stress-free inside films, longer electron lifetime, faster temporal response time, etc., the highly orientated perovskite-based solar cells accordingly allow realizing high efficiency while improving its thermal stability.

Highly orientated perovskite thin films (CH₃NH₃PbI_3–xCl_x) are prepared by means of aquointermediate assisted one-step solution process. It demonstrates monocrystalline-like performances, e.g., extremely high preferred-orientation, stress-free inside films, longer electron lifetime, lower defect density, and faster temporal response time. The highly orientated perovskite-based solar cells allow realizing the efficiency as high as 16.9% while improving its thermal stability.