The development of interface engineering for improving stability and efficiency of perovskite solar cells and understanding meta-stability of perovskite solar cells

The power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has increased from 3.8 to 25.2% in the last decade, making perovskite the most promising material for future solar cells. However, further PCE and stability improvement are important for successful commercialization. Therefore, the aim of this thesis is to investigate ways of increasing PCE and addressing instability of PSCs. Initial PCE increase has been observed during ambient storage for many PSCs. Through a series of experiments, the origin of the storage effect was attributed to a combination of i) defect reduction in perovskite, ii) conductivity increase, and iii) evolution of the highest occupied molecular orbital (HOMO) in spiro-OMeTAD. In particular, the HOMO level change was revealed to play a significant role in PCE improvement. In terms of strategy for improving PCE, a novel passivation technique was developed by forming 2D/3D perovskite thin layer using a mixture of formamidinium iodide and iso-butylammonium iodide on the perovskite layer. This technique achieved a maximum PCE of 21.7%, while simultaneously enhancing device light and moisture stability. The defect density reduction, the uniform surface coverage of the passivation material and the suppressed ion migration by bulky organic cation were found to be the key parameters for PCE and stability improvement. Storage effect was also studied for these passivated PSCs. It is found that the changed conduction band of passivated perovskite influenced the initial temporal change of PCE, suggesting the importance of interface band alignment by passivation and conductive materials. Also, despite significantly suppressed non-ideal recombination at the surface/interface by passivation, analysis of the dominant recombination revealed the need for defect reduction in bulk perovskite. Consequently, by engineering the composition of bulk perovskite layers to decrease defects, PCE of 22.2% was achieved. Finally, the effects of removing one of hole transport material (HTM) additives, 4-tert-butylpyridine (tBP) (via HTM solvent engineering) on device performance and thermal stability were investigated. The suppressed morphological change at high temperature for tBP-free HTM was the reason for thermal stability improvement. This work shows that comparable efficiencies can still be achieved without the use of the thermally unstable HTM dopant

Elucidating Mechanisms behind Ambient Storage-Induced Efficiency Improvements in Perovskite Solar Cells

ペロブスカイト太陽電池の常温熟成機構の解明. 京都大学プレスリリース. 2021-02-17.Initial improvement in power conversion efficiency (PCE) during ambient storage is often seen in perovskite solar cells (PSCs). In this work, we studied the origin of PCE enhancement by ambient storage on typical n-i-p PSCs. We found improvements in both fill factor and open-circuit voltage during the first 2 days of storage. By analyzing temperature and light intensity-dependent VOC, we found that the charge recombination mechanism changed from surface- to bulk-dominated because of defect passivation at the perovskite surface upon storage. In addition, we found that storage improves the conductivity and lowers the highest occupied molecular orbital level of the spiro-OMeTAD, improving charge extraction. These results show that there is more than one factor causing the storage-induced improvements in perovskite solar cells

Carbon Nanotube/Biomolecule Composite Yarn for Wearable Thermoelectric Applications

In this work, yarn made of a hybrid material of carbon nanotubes (CNTs) and artificial biomolecules is created for the wearable thermoelectric (TE) module. Among the limited methods due to the sensitivity of proteins, the co-use of the ionic liquid and polymeric surfactant with the dialysis method is found to be effective for the dispersion of the CNT/biomolecule composite with a low CNT loss rate and high coverage by biomolecules on the CNT. This new method improved the TE performance by decreasing the bundle diameter of the CNT/C-Dps nanocomposite and better tensile strength. The incorporation of a biomolecule, in particular, significantly reduced the thermal conductivity of CNT yarns, demonstrating that the hybrid composite is advantageous for wearable device applications. This method also outperformed the conventional dispersion against the pristine CNT yarn (without protein), demonstrating the application’s generality. Finally, a low-density testing of the TE module using the CNT/biomolecule composite is demonstrated, exhibiting the output power of 4.37 μW m–2 with a thermoelectric voltage of 4.5 mV at a temperature difference of 20 K. The output power density and voltage can be easily increased 500-fold by increasing the density of the yarn and the number of series connections. This study proposes a practical method for producing an environment-friendly CNT/biomolecule hybrid yarn, which has the potential to be useful in future wearable TE applications

The Effect of 4-tert-Butylpyridine Removal on Efficiency and Thermal Stability in Perovskite Solar Cells

Perovskite solar cells (PSCs) have shown a significant improvement in power conversion efficiency (PCE) in the last few years. However, instability of PSCs is still a barrier for successful industrialization. In particular, spiro-OMeTAD with additives, despite a popular choice for hole transport material (HTM) in PSCs, is one of the causes for device thermal instability. In this work, one of additives in HTM, 4-tert-butylpyridine (tBP) is proved to be a factor of device instability under thermal treatment. Simple solution engineering which excludes the use of tBP in the HTM results in better device stability. The origin of thermal stability improvement shown in this work is attributed to the suppression of morphological change of HTM. Further researches towards thermal stability of perovskite material and dopant-free HTM should be essential since tBP removal was not able to solve the thermal stability issue

Unidirectionally Aligned Donor–Acceptor Semiconducting Polymers in Floating Films for High‐Performance Unipolar n‐Channel Organic Transistors

Abstract The orientational control of semiconducting polymers (SCPs) in floating films offers several advantages over conventional solution‐processing methods for device fabrication, and the unidirectional floating film transfer method (UFTM) has been applied to fabricate large‐area oriented p‐type SCP films. Here, UFTM can be used to prepare high‐performance n‐type SCP films of P(NDI2OD‐T2) is reported. A strong correlation between the degree of polymer orientation and the solvent used for the preparation of P(NDI2OD‐T2) floating films is observed. In particular, the size of the nanofibers aligned along the orientation direction prepared using chloroform is dramatically increased by adding a small amount of chlorobenzene. Microstructural characterization reveals that the P(NDI2OD‐T2) floating films are uniaxially aligned with edge‐on orientation, whereas the spin‐coated films are isotropic with face‐on orientation. Notably, the floating films of P(NDI2OD‐T2) prepared with solvent blending have higher electron mobility with ambipolar‐like device characteristics and high threshold voltage (VTH) of ≈25 V. Finally, using the one‐step immersion process, the introduction of an interlayer of hexa(ethylene glycol)‐dithiol between the source/drain contacts and P(NDI2OD‐T2) film results in a drastic improvement in device, giving unipolar n‐channel transistor characteristics with a VTH of ≈0 V, on/off ratio of >105, and mobility reliability factor of almost 100%

Role of Second Polymer Donor in Polymer–Fullerene Ternary Blend Solar Cells Evaluated by Photoconductive Atomic Force Microscopy

The introduction of a third component to a donor–acceptor blend binary device can boost the performance of organic solar cells. This study investigated the role of a second polymer donor (D2, the third component) in the operation of polymer–fullerene blend solar cells in terms of the spatial distribution of D2 in the ternary active layer, which was visualized using photoconductive atomic force microscopy (PC-AFM). Ternary devices that consisted of a low-bandgap polymer, poly[(4,4-bis(2-ethylhexyl)-dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7 diyl] (PSBTBT), serving as D2, a wide-bandgap polymer donor, poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(2,2-ethyl-3(or4)-carboxylate-thiophene)] (PTO2), and a fullerene acceptor, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), utilized as host materials, were prepared at different PSBTBT loading amounts. The photocurrent images obtained using PC-AFM with selective illumination of PSBTBT and nonselective white-light illumination were compared to distinguish the photocurrents of PSBTBT from those of all the components. The comparison indicated that the photogenerated hole transport channels of PSBTBT were formed without a distinct phase separation from PTO2. The functional nanomorphology visualized by PC-AFM provides insights into the evolution of the open-circuit voltage and fill factor of the PSBTBT:PTO2:PCBM ternary device due to the increased PSBTBT loading

Immediate and Temporal Enhancement of Power Conversion Efficiency in Surface-Passivated Perovskite Solar Cells

This work reports strategies for improving the power conversion efficiency (PCE) by capitalizing on temporal changes through the storage effect and immediate improvements by interface passivation. It is demonstrated that both strategies can be combined as shown by PCE improvement in passivated perovskite solar cells (PSCs) upon ambient storage because of trap density reduction. By analyzing the dominant charge recombination process, we find that lead-related traps in perovskite bulk, rather than at the surface, are the recombination centers in both as-fabricated and ambient-stored passivated PSCs. This emphasizes the necessity to reduce intrinsic defects in the perovskite bulk. Furthermore, storage causes temporal changes in band alignment even in passivated PSCs, contributing to PCE improvement. Building on these findings, composition engineering was employed to produce further immediate PCE improvements because of defect reduction in the bulk, achieving a PCE of 22.2%. These results show that understanding the dominant recombination mechanisms within a PSC is important to inform strategies for producing immediate and temporal PCE enhancements either by interface passivation, storage, composition engineering, or a combination of them all to fabricate highly efficient PSCs

Complementary bulk and surface passivations for highly efficient perovskite solar cells by gas quenching

The power conversion efficiency (PCE) of metal halide perovskite solar cells (PSCs) has improved dramatically from 3.8% to 25.5% in only a decade. Gas quenching is a desirable method for fabricating high-efficiency cells as it does not consume antisolvents and is compatible with large-area deposition methods such as doctor blading and slot-die coating. To further improve PCEs for gas-quenched PSCs, here, we develop complementary bulk and surface passivation strategies by incorporating potassium iodide (KI) in the perovskite precursor and applying n-hexylammonium bromide (HABr) to the perovskite surface. We show that (1) KI induces a spatial-compositional change, improving grain boundary properties; (2) KI and HABr reduce traps, especially at levels close to the mid-gap; and (3) HABr greatly improves the built-in potential of the device, thereby improving voltage output. The champion device achieves a steady-state PCE of 23.6% with a VOC of 1.23V, which is, to the best of our knowledge, the highest for PSC by gas quenching to date